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Computer NetworkingComputer Networking A Top-Down Approach Featuring the Internet
计算机网络 - 自顶向下方法与 Internet 特色
Chapter5 The Link Layer and Local Area Networks
School of Computer Science amp Technology22
Chapter Goals
understand principlesprinciples behind data link layer serviceserror detectionerror detection amp correctioncorrectionsharing a broadcast channel multiplemultiple accessaccesslink layer addressingaddressingreliable data transfer flow control donedone
implementation of various link layer technologiesEthernetEthernet broadcast channelPPPPPP point to point channel
School of Computer Science amp Technology33
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology44
A link-layer protocol is used to move a datagram over an individualindividual linkLinksLinks
communication channelscommunication channels that connect adjacent nodes
The PDU PDU of a link-layer protocol is called FrameFrame
ldquo linkrdquo
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology55
Datagrams may be transferred by differentdifferent linklink protocolsprotocols over different linksdifferent links
eg EthernetEthernet on first link frameframe relayrelay on intermediate links 8021180211 on the last link
Different link protocol may provide differentdifferent servicesservices
eg may or may not provide rdt (reliable data transfer) over a link
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology66
1 Link Layer Services
Possible Services
FramingFraming
Link AccessLink Access
Reliable deliveryReliable delivery
Flow ControlFlow Control
Error detection and correctionError detection and correction
HalfFull duplexHalfFull duplex
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology22
Chapter Goals
understand principlesprinciples behind data link layer serviceserror detectionerror detection amp correctioncorrectionsharing a broadcast channel multiplemultiple accessaccesslink layer addressingaddressingreliable data transfer flow control donedone
implementation of various link layer technologiesEthernetEthernet broadcast channelPPPPPP point to point channel
School of Computer Science amp Technology33
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology44
A link-layer protocol is used to move a datagram over an individualindividual linkLinksLinks
communication channelscommunication channels that connect adjacent nodes
The PDU PDU of a link-layer protocol is called FrameFrame
ldquo linkrdquo
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology55
Datagrams may be transferred by differentdifferent linklink protocolsprotocols over different linksdifferent links
eg EthernetEthernet on first link frameframe relayrelay on intermediate links 8021180211 on the last link
Different link protocol may provide differentdifferent servicesservices
eg may or may not provide rdt (reliable data transfer) over a link
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology66
1 Link Layer Services
Possible Services
FramingFraming
Link AccessLink Access
Reliable deliveryReliable delivery
Flow ControlFlow Control
Error detection and correctionError detection and correction
HalfFull duplexHalfFull duplex
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology33
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology44
A link-layer protocol is used to move a datagram over an individualindividual linkLinksLinks
communication channelscommunication channels that connect adjacent nodes
The PDU PDU of a link-layer protocol is called FrameFrame
ldquo linkrdquo
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology55
Datagrams may be transferred by differentdifferent linklink protocolsprotocols over different linksdifferent links
eg EthernetEthernet on first link frameframe relayrelay on intermediate links 8021180211 on the last link
Different link protocol may provide differentdifferent servicesservices
eg may or may not provide rdt (reliable data transfer) over a link
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology66
1 Link Layer Services
Possible Services
FramingFraming
Link AccessLink Access
Reliable deliveryReliable delivery
Flow ControlFlow Control
Error detection and correctionError detection and correction
HalfFull duplexHalfFull duplex
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology44
A link-layer protocol is used to move a datagram over an individualindividual linkLinksLinks
communication channelscommunication channels that connect adjacent nodes
The PDU PDU of a link-layer protocol is called FrameFrame
ldquo linkrdquo
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology55
Datagrams may be transferred by differentdifferent linklink protocolsprotocols over different linksdifferent links
eg EthernetEthernet on first link frameframe relayrelay on intermediate links 8021180211 on the last link
Different link protocol may provide differentdifferent servicesservices
eg may or may not provide rdt (reliable data transfer) over a link
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology66
1 Link Layer Services
Possible Services
FramingFraming
Link AccessLink Access
Reliable deliveryReliable delivery
Flow ControlFlow Control
Error detection and correctionError detection and correction
HalfFull duplexHalfFull duplex
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology55
Datagrams may be transferred by differentdifferent linklink protocolsprotocols over different linksdifferent links
eg EthernetEthernet on first link frameframe relayrelay on intermediate links 8021180211 on the last link
Different link protocol may provide differentdifferent servicesservices
eg may or may not provide rdt (reliable data transfer) over a link
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology66
1 Link Layer Services
Possible Services
FramingFraming
Link AccessLink Access
Reliable deliveryReliable delivery
Flow ControlFlow Control
Error detection and correctionError detection and correction
HalfFull duplexHalfFull duplex
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology66
1 Link Layer Services
Possible Services
FramingFraming
Link AccessLink Access
Reliable deliveryReliable delivery
Flow ControlFlow Control
Error detection and correctionError detection and correction
HalfFull duplexHalfFull duplex
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology77
FramingFramingencapsulate a datagramdatagram into a frameframe adding
headerheader and trailertrailerThe structurestructure of a the frameframe is specified by
the link layer protocolLinkLink AccessAccess
How to share a medium Medium Access ControlMedium Access Control (MACMAC) protocol serves to
coordinate the frame transmission of the many nodes
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology88
ReliableReliable deliverydelivery between adjacentadjacent nodesUsing ack ack and retransmissionretransmission as TCP
we have learned how to do this already
seldom used on low bit error link Such as fiber some twisted pair media
wireless links high error rates
Q why both link-levellink-level and end-endend-end reliability
1 Link Layer Services
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology99
Flow ControlFlow Control pacingpacing adjacent sending and receiving nodes
Error Detection and Correction Error Detection and Correction errors caused by signal attenuationattenuation noisenoise receiver detectsdetects presence of errors
informsinforms sender for retransmission or dropsdrops frame receiver identifiesidentifies and correctscorrects bit error (s) without
resorting to retransmission
Full-duplex Half-duplex SimplexFull-duplex Half-duplex Simplexwith half duplexhalf duplex nodes at both ends of a link can
transmit but notnot at same time
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
1 Link Layer Services
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1010
link layer protocol implementedimplemented in an adapteradapter
aka Network Interface Card (NICNIC)
Ethernet card PCMCIA card 80211 card
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1111
sending sidesending sideencapsulates datagramdatagram in a frameframeaddsadds errorerror checkingchecking bits rdtrdt flowflow controlcontrol etc
receiving sidereceiving sidecheckingchecking errorserrors rdtrdt flowflow controlcontrol etcextractsextracts datagram from frame and passespasses it to receiving node
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
2 Adapters Communicating
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1212
2 Adapters Communicating
adapter is a semi-autonomous semi-autonomous system
It can receivereceive a frame checkcheck errorserrors and discarddiscard the corrupted frames withoutwithout notifying other componentsIt interruptsinterrupts the parent node onlyonly ifif it wants to pass
datagram up the protocol stack
The parent node fullyfully delegateddelegated to the adapter the task of transmitting the datagram across the link
The adapter is typically housedhoused in the box as the rest of the node
51 Introd
uction
and
51 In
trodu
ction an
d
Services
Services
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1414
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1515
EDCEDC= ErrorError DetectionDetection and CorrectionCorrection bits (redundancy)
D D = Data protected by error checking may include header fields
bull Error detection not 100 reliable
bull protocol may miss some errors but rarely
bull larger EDC field yields better detection and correction
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
Error Detection and Correction
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1616
Error-Detection amp -Correction Techniques
Parity Checks
Internet Checksumming
Cyclic redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1717
1Single Bit Parity Single Bit Parity Detect single bit errors
- Can detect odd number bits errorodd number bits error
even
odd
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1818
22 Two Dimensional Bit ParityTwo Dimensional Bit Parity - can detectdetect single bit errors - can correct correct single bit errors
0 0
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
1 Parity Checks
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology1919
FECFEC The ability of the receiverreceiver to both detectdetect and correctcorrect errors
0 0
1 Parity Checks
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2020
Sendertreat segmentsegment contents as sequencesequence of 16-bit 16-bit
integersintegerschecksumchecksum addition (1rsquos complement sum) of
segment contentssender puts checksumchecksum value into checksumchecksum field
Receiverthe sum of the received data (including the
checksum) = all 1 bitsNO -NO - error detectedYESYES - no error detected
2 Internet Checksum
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2121
Checksumming Checksumming methods require require relatively littlelittle overhead but provideprovide relatively weakweak protection against errors
Why is checksummingchecksumming used at the transporttransport layerTransport layer is typically implemented in
softsoft in a host as a part of OSOS
2 Checksumming
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2222
The data (DD) ) are viewed as a d-bit d-bit binary number
The SenderSender chooses rr additional bits (R CRC bitsR CRC bits) and appends them to the DD such that d+r d+r bits is exactly divisible by GG using modulo-2 modulo-2 arithmetic Sender and receiver both know the (r+1)-bitr+1)-bit generator (G)(G)
The receiver receiver divides (d+r)-bit(d+r)-bit (DRDR) by G G If non-zeronon-zero remainder error detectederror detected
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2323
Modulo-2 algorithmNo addition carriesaddition carries
No subtraction borrows subtraction borrows The addition (+)addition (+) and the subtraction (-)subtraction (-) are identical identical
and both are equivalentare equivalent to the bitwisebitwise exclusive-orexclusive-or (XORXOR)
For example1011 XOR 0101 = 1110
1011 mdash 0101 = 1110
1011 + 0101 = 1110
3 Cyclic Redundancy Check
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2424
How the Sender computes the How the Sender computes the RRWant Want DD22rr XOR RR = nGnG
equivalentlyequivalently DD22rr XOR RR XOR RR = nG nG XOR RR
DD22rr = nG nG XOR RR
equivalentlyequivalently if we divide D2r by G want remainderremainder R
R = remainder[ ]
D2r
G
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2525
52 Error D
etection an
d
52 Error D
etection an
d
Correction
Correction
3 Cyclic Redundancy Check
ExampleExampleDD=101110 GG = 1001rr =3 and TT
(D000D000)=101110000000TT (DRDR)=101110011011
CRC can detect all burst errors less thanless than r+1 bits r+1 bitsCRC-32( international standard ) =100000100110000010001110110110111
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2626
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2727
Two types of linkslinkspoint-to-pointpoint-to-point
Only Only a sender and a receivera receiver sits at each end PPP for dial-up access
broadcast (shared wire or medium)broadcast (shared wire or medium)Many Many senders and receivers shareshare the link
traditional EthernetWLANUpstream of HFC
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2828
single shared broadcast channel two or more simultaneoussimultaneous transmissions by
nodes will cause interference CollisionCollision-if node receives twotwo oror moremore
signals at the same time
Multiple Access ProblemHow to coordinatecoordinate the access of a
sharedshared broadcastbroadcast channelchannel
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology2929
multiple access protocolmultiple access protocoldistributed algorithmdistributed algorithm that determines how
nodes share the channel ie determine whenwhen and whichwhich node can transmit
communication about channel sharing must use channel itself
no out-of-bandout-of-band channel for coordination
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3030
The requirements to multiple access multiple access protocolprotocol for a broadcast channelbroadcast channel of rate RR bpsWhen one node can transmit it send at rate RRWhen M M nodes want to transmit over the channel
each can send at average rate RMRM bpsFully decentralized
no special nodespecial node to coordinate transmissionsno synchronizationsynchronization of clocks slots
Simple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3131
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive useRandom AccessRandom Access
channel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3232
TDM (Time Division MultiplexingTime Division Multiplexing)Time divided into FramesFrames and further divides each
Frame into N slotsslots each slot is then assigned to one of the N N nodes
Nodes access to channel in rounds unused slots go idleidle
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3333
TDMA eliminates collisions collisions and is perfectly fairfair but has two drawbacksdrawbacks
each node gets a dedicateddedicated transmission rate of RNRN and the average rate is limited to RNRN
a node mustmust always wait for its turnwait for its turn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
11 TDMA
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3434
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
The spectrumspectrum is divided into frequencyfrequency bandsbandseach station assigned fixedfixed frequency bandunused transmission time in each frequency
bands goes idleidle
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3535
advantagesadvantages and drawbacksdrawbacks
can avoids collisionscollisions
shareshare the channel fairlyfairly
a node is limitedlimited to a bandwidthbandwidth of RNRN
a node needneed notnot waitswaits forfor its turnturn
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
12 FDMA
frequ
ency
bands
time
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3636
TDM (Time Division MultiplexingTime Division Multiplexing)
FDM (Frequency Division MultiplexingFrequency Division Multiplexing)
CDMA (Code Division Multiplexing AccessCode Division Multiplexing Access)Assign a different codecode to each node
Each node encodesencodes the data bits using its unique code
Different nodes can transmit simultaneously
Usually used in military systems in early time
Details discussed in wireless networking of Advanced Advanced Computer NetworksComputer Networks
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
1 Channel Partitioning Protocols
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
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53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
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dressin
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ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
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ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
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ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
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ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
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dressin
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-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
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dressin
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-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
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dressin
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-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3737
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Accesschannel not divided allow collisionsldquorecoverrdquo from collisions
Taking turnsTaking turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3838
When a node has a packet to send it transmit the packet at fullfull channelchannel datadata raterate R R bpsno a prioripriori coordination coordination among nodes
may cause collisioncollision
Each node involved in the collision repeatedlyrepeatedly retransmitsretransmits its packet until the packet gets through withoutwithout anyany collisioncollisiona node waits some time choosed randomly randomly
before retransmission
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
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ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
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ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology3939
random access protocol specifies how to detect collisionshow to recover from collisions
Examples of random access protocolsslottedslotted ALOHAALOHA
Pure ALOHAPure ALOHA
CSMACSMA CSMACDCSMACD CSMACACSMACA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
2 Random Access Protocols
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4040
Assumptionsall framesframes have same sizesame size L bitstimetime is divided into equal size slotsequal size slots
time to transmit 1 frame LR
nodes start to transmit frames onlyonly at beginningbeginning of slotsslotsnodes are synchronizedsynchronized
if there 2 or more2 or more nodes transmitting in a slot all nodes can detectdetect the collisioncollision
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4141
Operationwhen node obtains fresh framefresh frame it transmits
in the next slotnext slotIf there is not a collisioncollision node can send new new
frameframe in next slotnext slotThe sending node can detect the collision before
the end of the slot
If there is a collision collision node retransmitsretransmits frame in each subsequentsubsequent slotslot with probability pp until successsuccess
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4242
Advantages single active nodesingle active node can continuously transmit at fullfull
raterate of channelhighly decentralized only slots in nodesnodes needneed to be to be
inin synchronizationsynchronizationsimplesimple
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4343
Drawbacksmany wasted slotswasted slots because of collisioncollision
successful slotsuccessful slot The slot which only one node transmitidle slotsidle slots because of the probabilistic policy probabilistic policy
nodes must must detect the collision in less than time to transmit a packet
clock synchronizationsynchronization used to determine the startend of one slot
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
21 Slotted ALOHA
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4444
21 Slotted ALOHA
Suppose NN nodes with many frames to send each transmits in each slot with probability p pProb that any node has a success = Np(1-p)Np(1-p)N-1N-1
EfficiencyEfficiency of slotted ALOHA is the long-long-run fractionrun fraction of successful slots when there are many nodes each with many frames to send
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol 3701)1( 1lim
ePNP N
N
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4545
1)1()( NpNppE21 )1)(1()1()( NN pNNppNpE
))1()1(()1( 2 NpppN N
NppE
10)(
N
NNNN
NpE
N
NN
11
)1
1()
11()
11(
1)( 11
1)1
1(lim NN
eNN
N
1)
11(lim
epE
N
1)(lim
21 Slotted ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4646
unslotted ALOHAunslotted ALOHA - no synchronizationsynchronizationwhen frame first arrives transmit immediately collision probability increasesincreases
frame sent at t0 collides with other frames sent in [t0-Lrt0+LR]
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4747
P(success by given node) =
P(node transmits) P(no other node transmits in [t0-LRt0] P(no other node transmits in [t0t0+ L R ] = p (1-p)N-1 (1-p)N-1= p (1-p)2(N-1)
)1(2)1()( NpNppE)3(2)2(2 )1)(1(2)1()( NN pNNppNpE
))1(2)1(()1( )3(2 NpppN N
12
10)(
NppE
)1(2)12
11(
12)(
N
NN
NpE
eepE
N 2
11
2
1)(lim
22 Pure (un-Slotted) ALOHA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4848
CSMACSMA CCarrier SSense MMultiple AAccess
listenlisten beforebefore transmittingtransmitting
If channel sensed idleidle then transmits entire frame immediately
If channel sensed busybusy then waits waits a random amount of time time and then sensessenses the channel
23 CSMA
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology4949
If all nodes perform If all nodes perform carrier sensingcarrier sensing do do collisions occur all the samecollisions occur all the same
collision entire packet transmission time wasted
note the role of distance amp propagation delay in determining collision probability
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
spatial layout of nodes collisions can still occur
propagation delaypropagation delay means two nodes may not hear each otherrsquos transmission
23 CSMA
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5050
CSMACD carrier sensingsensing and deferraldeferral as in CSMAcollisions detectedcollisions detected within short timecolliding transmissions abortedcolliding transmissions aborted
reducing channel wastage
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5151
collision detection easyeasy in wired LANs
measure signal strengths compare transmitted received signals
but difficultdifficult in wireless LANsreceiver shut off while transmitting for saving
energy
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5252
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
24 CSMACD (collision detection)
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5353
CategoriesCategories of Multiple Access ProtocolMultiple Access ProtocolChannel PartitioningChannel Partitioning
Random AccessRandom Access
Taking turnsTaking turnsNodes take turns but nodes with more to send
can take longer turns
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
Introduction
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5454
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
3 Taking-Turns Protocols
PollingPolling master node ldquoinvitesinvitesrdquo slave nodes to transmit in turnconcerns
polling overheadoverhead and polling latencylatencysingle point of failuresingle point of failure (master)
Token passingToken passing control tokentoken passed from one node to next sequentially concerns
token overheadoverhead and latencylatencysingle point of failure (token)single point of failure (token)
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5555
What do you do with a shared mediashared mediaChannel PartitioningChannel Partitioning by time frequency or
codeRandom partitioningRandom partitioning (dynamic)
ALOHA S-ALOHA CSMA CSMACDCSMACD used in EthernetCSMACA used in 80211 WLAN (Wireless LAN)
carrier sensingcarrier sensing easy in some technologies (wire) hard in others (wireless)
Taking TurnsTaking Turnspolling from a central site or token passing
53 Mu
ltiple A
ccess Protocol
53 Mu
ltiple A
ccess Protocol
4 Summary
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5656
LAN technologies
Categories of NetworksLAN-Local Area NetworkWAN-Wide Area NetworkMAN-Metropolitan Area Network
Data Link layer so farservices error detectioncorrection multiple access
Next LAN technologiesaddressingEthernethubs switchesPPP
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5757
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
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ayer Ad
dressin
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-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5858
1 MAC Addresses32-bit IP address
network-layernetwork-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to another
physically-connectedphysically-connected interface (same network)48 bit48 bit MAC address MAC address (for most LANs) is burned burned in the
ROM ROM of NICof NICfor example 1A-2F-BB-76-09-AD
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology5959
1 MAC Addresses
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6262
1 MAC Addresses (more)MAC flatflat address portability
can move LAN card from one LAN to another
Like an ID cardIP hierarchicalhierarchical address NOT portable
depends on IP subnet to which node is attached
Like a postal address
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6363
QuestionQuestion how to determine MAC address of B if a node knows Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6464
Each IP node on LAN has an ARP ARP table
ARP Table IPMAC address mappings
for some nodes sited on the same LAN
lt IP address MAC address TTLgt
TTL typically 20 min
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6565
2 ARP Address Resolution Protocol
AA wants to send datagram to BB and BBrsquos MAC address not in AArsquos ARP tableARP table
AA broadcastsbroadcasts ARP request ARP request packet packet is sent which containing Bs IP address ARP Packet=src_ip src_mac des_ipdes_macARP packet is encapsulated in a frame whose dest MAC
address = FF-FF-FF-FF-FF-FFall machines on LAN receive the ARP queryARP query request
B receives ARP frameARP frame replies to A with its (Bs) MAC addressframe sent to Arsquos MAC address (unicast)why
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6666
ARP Packet
2 ARP Address Resolution Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6767
A caches (saves) IP-to-MAC address pair in its
ARP table until information becomes oldbecomes old (times
out)
soft state information that times out (goes away) unless
refreshed
ARP is ldquoplug-and-playplug-and-playrdquo
nodes create their ARP tables without
intervention from net administrator
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
2 ARP Address Resolution Protocol
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6868
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
Two ARP tables in router R one for each IP network (LAN)
A
RB
2 Routing to another LAN
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology6969
A creates datagram with source A destination B A uses ARP to get Rrsquos Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees itrsquos
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
2 Routing to another LAN
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7070
3 Dynamic Host Configuration Protocol
DHCP is a ClientServerClientServer protocolThe clientclient is typically a newly arriving host
wanting to obtain network configuration information
each subnetsubnet will have a DHCP server at least
runs over UDPUDP
plug and play
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7171
Operationserver discovery server discovery msg
Dest IP255255255255 Source IP0000Dest MACFF-FF-FF-FF-FF-FF
server offer server offer msgDHCP offer msg including IP address Subnet Mask
Address lease time DNS Server Default GatewayRouter etc
DHCP request DHCP request msgchoose one from multiple offers may be come from
different DHCP serversDHCP ACK DHCP ACK msg
server ack
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7272
3 Dynamic Host Configuration Protocol
54 Lin
k-L
ayer Ad
dressin
g54 L
ink
-Layer A
dd
ressing
UDP Port Number
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7373
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7474
Introductionldquodominantdominantrdquo wired LAN technology
cheap $200 for 100Mbs
first widely used LAN technology
Simpler cheaper than token LANs and ATM
Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7575
Bus topologyBus topology popular through mid 90s
Now starstar topologytopology prevails
Connection choices hubhub or switchswitch (more later)
hub orswitch
Introduction
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7676
1 Ethernet Frame Structure
Sending adapter encapsulatesencapsulates IP datagram (or other network layer protocol packet) in Ethernet frameEthernet frame
Preamble(8 bytes) 7 bytes with pattern 1010101010101010 followed by one byte with
pattern 1010101110101011
used to synchronize receiver sender clock rates
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7777
Addresses 6 bytesmatching destination MAC address or with broadcast
address (eg ARP packet) to net-layer protocolotherwise adapter discards frame
Typeindicates the higher layer protocol (mostly IP but Novell
IPX and AppleTalk)CRC
checked at receiver if error is detected the frame is simply dropped
55 Eth
ernet
55 Eth
ernet
1 Ethernet Frame Structure
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7878
11 Manchester Encoding
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology7979
12 Unreliable and connectionless service
Connectionlessno handshakinghandshaking between sending and receiving
adapter
Unreliablereceiving adapter doesnrsquot send ACKsNAKs to the
sending adapterstream of datagrams passed to network layer can
have gapsgaps will be filled if app is using TCPotherwise app will see the gaps
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8080
2 Ethernet uses CSMACD (P460)
No slotsadapter doesnrsquot transmit if it senses that
some other adapter is transmitting that is carrier sensecarrier sense
transmitting adapter abortsaborts when it senses that another adapter is transmitting that is collision detectioncollision detection
Before attempting a retransmission adapter waits a random time that is random accessrandom access
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8181
2 Ethernetlsquos CSMACD (P461)
1 Adapter receives datagram from net layer amp creates framecreates frame
2 If an adapter sensessenses channel idleidle it starts to transmit frame If it senses channel busybusy waitswaits untiluntil channelchannel idleidle and then transmits(96-bit times96-bit times)
3 If adapter transmits entire frame withoutwithout detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting abortsaborts and sendssends jamjam signal(48-bit stream)signal(48-bit stream)
5 After aborting adapter enters exponentialexponential backoffbackoff after the mmthth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8282
2 Ethernetlsquos CSMACD (P461-462)
Exponential BackoffExponential Backoff GoalGoal
adapt retransmission attempts to estimated current load
heavy load random wait will be longerfirst collisionfirst collision choose K from 01 after second collisionsecond collision choose K from 0123hellipafter tententhth or more or more collisionscollisions choose K from
01234hellip1023delay is K512 bit-time
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8383
2 Ethernetlsquos CSMACD
Bit time Bit time 1 microsec for 10 Mbps Ethernet
for K=1023 wait time is about 50 msec
Jam SignalJam Signal make sure all other transmitters are aware of
collision 48 bits
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8484
CSMACD Example
Ethernet 网络中的只有只有两个节点 A 和 B 活动相距 225bit-time 假设 A 和 B 同时发送Frame 造成冲突并且 A 和 B 选择不同的 K 值退后重传两节点重传的 Frame 会不会再一次造成冲突 t=0 时 A 和 B 同时开始发送 Frame t=225bit-time 时两者均检测到冲突 A 和 B 在 t=225+48 = 273bit-time 结束传输 jam 信号 假设 KA=0 KB=1
何时 A 开始重传何时 A 重传信号到达 B
何时 B 计划开始重传B 计划的重传会不会延后进行
55 Eth
ernet
55 Eth
ernet
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8585
CSMACD Example (contdhellip)
55 Eth
ernet
55 Eth
ernet
时间 t (bit-time) 事 件0 A 和 B 均开始发送 Frame
225 A 和 B 均检测到冲突225 + 48 = 273 A 和 B 结束传输 Jam 信号273+225 = 498 498+ 96 = 594
B 的最后一位 Jam 信道到达 A A 检测到信道空闲
273+512 = 785785+ 96 = 881
B 计划重新侦测信道是否空闲B 计划开始重传 Frame
594+225 = 819 881gt819
A 重传 Frame 的第一位到达 BB 将自己的重传计划推后进行
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8787
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8888
1 Hubs (P465) Hubs are essentially physical-layer physical-layer repeaters
bits bits coming from one link go out all all other links at the same rate
no frame bufferingbufferingno CSMACDCSMACD at hub adapters detect collisionsmay provide net management functionality
twisted pair
hub
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology8989
Backbone hubBackbone hub interconnects LAN segmentssegments ExtendsExtends max distance between nodes But individual segment collisioncollision domainsdomains become one large
collisioncollision domaindomain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
backbone hub
1 Hubs (P465)
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9090
2 Switch (P467)
Link layer deviceLink layer devicestores and forwardsstores and forwards Ethernet framesexaminesexamines frame header and selectively
forwardsforwards frame based on dest MAC addresswhen frame is to be forwarded on segment
uses CSMACDCSMACD to access segmenttransparenttransparent
hosts are unaware of presence of switches
plug-and-play self-learningplug-and-play self-learning
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9292
21 Forwarding Filtering (P468)
A switch has a switch tableswitch table(MAC Address Interface Time Stamp)(MAC Address Interface Time Stamp)
stalestale entries in table dropped (TTL can be 60 min)
switch learnslearns which hosts can be reached through which interfaceswhen frame received switch ldquolearnslearnsrdquo
location of sender incoming LAN segment
records senderlocation pair in switch table
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9393
Switch example (P469)
Suppose C sends a frame to D
Switch receives frame from Cnotes in switch table that C is on interface 1because D is not in table switch forwards frame into interfaces 2
and 3 (broadcasting)(broadcasting)frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
address interface
ABEGC
11231
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9494
Switch example (P469)
Suppose D replies back with frame to C
Switch receives frame from Dnotes in switch table that D is on interface 2because C is in table switch forwards frame only to interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
12 3
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9595
22 Self-Learning (P471)
Switch Table is built automaticallyautomatically dynamicallydynamically autonomously mdash autonomously mdash without any intervention from a network administrator or or from a configuration protocol
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9696
22 Self-Learning (P471)
switch table is initially initially emptyempty if framersquos dest address is not in the Switch table
switch forwardedforwarded thethe frameframe toto allall otherother interfacesinterfaces otherwise the frameframe is forwardedforwarded toto thethe interfaceinterface
for each incoming frame switch stores in its tableMAC address in framersquos source address field interface that the frame comes fromarrival time
switch deletes deletes an address in the table if no frames are received with that address as the source address after a period of time
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9797
23 Switch traffic isolationswitch installation breaks subnet into LAN segmentsbreaks subnet into LAN segmentsswitch can filter can filter packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9898
24 Switches dedicated access
Switch with many interfacesHosts have direct connection
to switchNo collisions full duplex
SwitchingSwitching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology9999
More on Switches (P473)
cut-through switchingcut-through switching
frame forwarded from input to output port
without first collecting entire frame
slight reduction in latency
combinations of shareddedicated
101001000 Mbps interfaces
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology100100
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology101101
Switches vs Routers (P475)
both store-and-forwardstore-and-forward devicesrouters network layer devices switches link layer devices
routers maintain routing tablesrouting tables implement routingrouting algorithmsalgorithms switches maintain switchingswitching tablestables implement filtering filtering self-self-
learninglearning algorithmsalgorithms
switch
56 Intercon
nection
s56 In
terconn
ections
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology102102
Summary comparison (P476)
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology103103
Roadmap
51 Link Layer Introduction and Services52 Error-Detection and -Correction 53 Multiple Access Protocols54 Link-Layer Addressing55 Ethernet56 Hubs and Switches57 PPP58 Link Virtualization ATM and MPLS
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
School of Computer Science amp Technology114114
Homeworks
P495
Problem 141112
Ch
apter 5
Ch
apter 5
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