4. MAC protocols and LANs - University of Surreyinfo.ee.surrey.ac.uk/Teaching/Courses/ee2.cdn/sun/cdn_slides_4.pdf · „ The adaptive tree walk protocol: dynamically allocate time

Computer and Data Networks, 4. MAC protocols and LANs 1 ©Dr.Z.Sun

4. MAC protocols and LANs


Outline

� MAC protocols and sublayers, � LANs: Ethernet, Token ring and Token bus � Logic Link Control (LLC) sublayer protocol� Bridges: transparent (spanning tree), source routing and

remote bridges


Multiple Access Control (MAC) Protocols

� There are two categories of networks:• One uses point to point connections: computer dial-up

links, using SLIP, PPP, HDLC• The other uses broadcast channels: more than one

stations share the same channel, such as LANs using random access or token

� MAC protocols is used to determine which station goes next to access the shared channels


Random Access: Aloha and Slotted Aloha

• User transmit whenever they have data

• Listen to the channel to see if the frame is OK

• Contention system


Channel efficiency

� Throughput S = GP0

� Poisson distribution:P[k] = Gk e-G / k!

In 2 frame interval, the number of frames generated is 2G, thus P0 = e -2G

=> S = G e -2G

� Max. throughputS = 1/2e, when G=1/2

� For slotted Alohathe vulnerable period is 1 frame period

(halved), thus P0 = e -G

=> S = G e -G


Carrier Sense Multiple Access (CSMA)

� 1-persistent: the station listens before sending. if the channel is busy, it waits until it idle. Transmit when the channel is idle. if collision, the station waits a random amount of time and start all over again

� non-persistent: If busy, the station does not continually sense. Instead, waiting for a random period, then repeating the algorithm

� p-persistent: It applies to slotted channel. If it is idle, it transmits with probability of p.


CSMA/CD

� Further improvement than persistent and non-persistent over Aloha, by aborting transmission as soon as stations detect a collision

� Contention period is 2τ where τ is propagation delay

� Example: for a 1 km cable, the τ is about 5 µseconds

� Ethernet is one of this version� No MAC-sublayer protocol

guarantees reliable delivery


Collision free protocols

� A bit map protocol: it is also called reservation protocol. Each contention period consists of exactly N slots for N stations. Efficiency = d/(d+1)

� Binary countdown: each station has a binary address, start to broadcast with the high order bit. It stops as soon as a high order position 0 is overwritten with a 1. Efficiency = d/(d+log2N)


Limited contention protocols

� Two important performance measures: delay at low load and channel efficiency at high load. At low load, contention is preferable due to low delay. At high load, reservation is preferable due to high efficiency.

� The adaptive tree walk protocol: dynamically allocate time slots. If a collision occurs during slot 1, the entire tree is searched, depth first to allocate all the ready stations.

1/e = 0.368


IEEE 802.3 and Ethernet

� Bus Topology � Whole family of 1-persistent

CSMA/CD� From 1 - 10 Mbit/s on different

media� Switched Ethernet


Ethernet MAC Frame

For a 10-Mbit/s LAN� Max. 2500 meters and 4 repeaters (500 m/segment)� Min. allowed frame must take 51.2 µ seconds

(corresponds to 64 bytes)If operating at 1 Gbit/s� Max. 250 meters, and Min. frame size 640 bytes


IEEE 802.5 Token Ring

• Ring topology, suitable for real time• Token holding time is 10 ms• Speed 1 and 4 Mbit/s• Delimiter, access control, frame

control • sources and destination address,

checksum are the same as the 802.3


IEEE 802.4 Token Bus

• Logical ring, suitable for real time• For priorities: 0, 2, 4, 6• Speed 1, 5, and 10 Mbit/s• Preamble, delimiter, control, • Sources and destination address,

checksum are the same as the 802.3


IEEE 802.2 Logical link control

� Hide difference between the various 802 networks by providing a single format and interface to the network layer

� Based on HDLC, provide 3 service options as the link layer

� Error control using acknowledgment

� Flow control using a slide window

� All 802 LANs and MAN offer best-efforts service


Bridges

• Why using bridges?1. Different department

have different LANs initially;

2. geographical separated; 3. Accommodate the load; 4. Physical distance limit; 5. Reliability, 6. Security

• A bridge connecting k different LANs will have k different MAC layers and k different physical layers, one for each type.


Bridges from 802.x to 802.y

Operation of a LAN bridge from 802.11 to 802.3.


Bridges from 802.x to 802.y (frame format)

• Problems with bridging different LANs:1. Different frame format, 2. Different rate, 3. Different maximum frame length


Transparent (spanning tree) bridgeForwarding (Filtering)• If destination & source address is the

same, discard the frame• If different, forward the frame• Destination unknown, use flooding• Spanning tree solve loop problemAddress learning• Initialise the forwarding database to

empty • Update the entry in the routing table

(make one if it does not exit) with the frame address and arrival time

• Periodically scan the routing table and purge all old entries (more than a few minutes.


Spanning tree algorithm

� Exchange bridge protocol data units (BPDUs) to elect root bridge with highest priority and smallest bridge identifier

� Each bridge selects root portwith minimum root path cost(RPC) from the root to the port

� Bridges connected to the same segment elect a designated bridge with a port having minimum path cost from the port to the root

� Port identifiers are used as tie-breakers

Root bridge

RPC = 1, 4 RPC = 2, 5

X

X

X

RPC = 1, 4 RPC = 2, 3 RPC = 3, 4RPC = 2, 3

RPC = 2, 5

RPC = 3, 4

RPC = 4,3,4


Source routeing bridges

� Transparent bridges use only subset of the topology (the tree)

� Source routeing assumes that the sender knows whether or not the destination is on its own LAN.

� Implicit in that every machine can find the best path to the other machine

� Discovery frame flooding is used if destination unknown

� Frame explosion is solved by flooding along the spanning tree


Comparison of 802 bridges


Remote bridges

� It connect two or more distant LANs

� Put a bridge in each LAN and connect the bridge pairwise with point to point lines (such as leased telephone line)

� Various protocols can be used on the point to point lines (such as data link protocol) putting complete MAC frame in the payload

� Or strip off the MAC header and trailer at the sources and put it back at the destination


Repeaters, Hubs, Bridges and Switches (1/2)

(a) Which device is in which layer.(b) Frames, packets, and headers.


Repeaters, Hubs, Bridges, Switches (2/2)

(a) A hub. (b) A bridge. (c) a switch.


Virtual LANs

(a) Four physical LANs organized into two VLANs, by two bridges. (b) The same LANs organized into two VLANs by switches.


The IEEE 802.1Q Standard

Transition from legacy Ethernet to VLAN-aware Ethernet. The shaded symbols are VLAN aware. The empty ones are not.


The IEEE 802.1Q Standard (2)

The 802.3 (legacy) and 802.1Q Ethernet frame formats.


Summary

� MAC protocols and sublayers, � LANs:

• Ethernet, • Token ring• Token bus

� Logic Link Control (LLC)� LAN interconnections:

• Transparent (spanning tree) bridge• Source routing bridge• VLAN

Documents

4. MAC protocols and LANs - University of Surreyinfo.ee.surrey.ac.uk/Teaching/Courses/ee2.cdn/sun/cdn_slides_4.pdf · „ The adaptive tree walk protocol: dynamically allocate time