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Sidevõrgud IRT 0020
loeng 8 06. nov. 2006
Avo Ots
telekommunikatsiooni õppetoolraadio- ja sidetehnika instituut
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Market
End-Users
Content and Service Providers
Service operators/Telecommunications Networking Solutions
Physical Telecommunication Network
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Architectures
Infrastructure-less/ distributed/ad-hoc mode
Infrastructure mode
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Hidden Terminal Problem• Node B can communicate with A and C both• A and C cannot hear each other• When A transmits to B, C cannot detect the
transmission using the carrier sense mechanism• If C transmits, collision will occur at node B
A B C
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Multiple Access with Collision Avoidance
• When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to A
• On receiving RTS, node A responds by sending Clear-to-Send (CTS), provided node A is able to receive the packet
• When a node (such as C) overhears a CTS, it keeps quiet for the duration of the transfer– Transfer duration is included in RTS and CTS both
A B C
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Reliability
• Wireless links are prone to errors. High packet loss rate detrimental to transport-layer performance.
• Mechanisms needed to reduce packet loss rate experienced by upper layers
• When node B receives a data packet from node A, node B sends an Acknowledgement (Ack).
• If node A fails to receive an Ack, it will retransmit the packet
A B C
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IEEE 802.11 DCF
• Uses RTS-CTS exchange to avoid hidden terminal problem– Any node overhearing a CTS cannot transmit for the duration of
the transfer
• Uses ACK to achieve reliability• Any node receiving the RTS cannot transmit for the
duration of the transfer– To prevent collision with ACK when it arrives at the sender
– When B is sending data to C, node A will keep quite
A B C
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Collision Avoidance
• With half-duplex radios, collision detection is not possible
• CSMA/CA: Wireless MAC protocols often use collision avoidance techniques, in conjunction with a (physical or virtual) carrier sense mechanism– Carrier sense: When a node wishes to transmit a packet, it
first waits until the channel is idle
– Collision avoidance: Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit
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Congestion Avoidance
• When transmitting a packet, choose a backoff interval in the range [0,cw]– cw is contention window
• Count down the backoff interval when medium is idle– Count-down is suspended if medium becomes busy
• When backoff interval reaches 0, transmit RTS
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IEEE 802.11 PCF
• Purpose: contention-free data transmission
• System components– Access Point (AP): a coordinator controlling the medium
access in a poll-and-response manner
– Stations: transmit only when being polled
• A LAN operates in PCF or DCF mode– The duration in which PCF operates is called contention-
free period (CFP)– Before/after a CFP, the network operates in DCF.
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PCF• Starting
– AP seizes the medium by using “priority inter-frame space” (PIFS)
– AP sends out a beacon packet to announce the beginning of a CFP (the packet contains the duration of the CFP)
• In a CFP– AP may transmit data packets to any station– AP may send a polling packet to a station
• The polled station replies with a data packet or a NULL packet (when nothing to send)
• Ending– AP sends out an END packert.
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Priority Assignment Methods (1)
• Strict Priority QueuingQueue
Queue 0
Queue 1
Queue 2
Flow 0
Flow 1
Flow 2
If packets in queue
If packets in queue
else
else
• FIFO
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Queue 0
Queue 1
Queue 2
Flow 0
Flow 1
Flow 2
Probability 0.1
Probability 0.2
Probability 0.7
Priority Assignment Methods (2)
• Weighted Fair Queuing
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MAC Management
• Synchronization– finding and staying with a WLAN.
– Synchronization functions
• Power management– sleeping without missing any messages
– power management functions, e.g., periodic sleep, frame buffering, traffic indication map
• Association and Re-association– joining a network, roaming, moving from one AP to another,
scanning
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MANET (Mobile Ad Hoc Networks• Formed by wireless hosts which may be mobile• No pre-existing infrastructure• Routes between nodes may potentially contain multiple hops
– Nodes act as routers to forward packets for each other– Node mobility may cause the routes change
AB
C
D
AB
C D
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Why MANET?• Advantages: low-cost, flexibility
– Ease & Speed of deployment
– Decreased dependence on infrastructure
• Applications– Military environments
• soldiers, tanks, planes
– Civilian environments• vehicle networks
• conferences / stadiums
• outside activities
– Emergency operations• search-and-rescue / policing and fire fighting
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Challenges• Collaboration
– Collaborations are necessary to maintain a MANET and its functionality.
– How to collaborate effectively and efficiently?– How to motivate/enforce nodes to collaborate?
• Dynamic topology– Nodes mobility– Interference in wireless communications
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Routing Protocols: Overview• Proactive protocols
– Determine routes independent of traffic pattern
– Traditional link-state and distance-vector routing protocols are proactive
– Examples: • DSDV (Dynamic sequenced distance-vector)
• OLSR (Optimized Link State Routing)
• Reactive protocols– Maintain routes only if needed
– Examples: • DSR (Dynamic source routing)
• AODV (on-demand distance vector)
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Routing Protocols: Tradeoff• Latency of route discovery
– Proactive protocols may have lower latency since routes are maintained at all times
– Reactive protocols may have higher latency because a route from X to Y may be found only when X attempts to send to Y
• Overhead of route discovery/maintenance– Reactive protocols may have lower overhead since
routes are determined only if needed
– Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating
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Route in DSR
B
A
S E
F
H
J
D
C
G
IK
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
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Dynamic Source Routing• When node S wants to send a packet to
node D, but does not know a route to D, node S initiates a routing process
• Runs in three phases– Route Discovery Route Reply Path
Establishment
• Route Discovery– Source node S floods Route Request (RREQ) – Each node appends own identifier when
forwarding RREQ
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Dynamic Source Routing (DSR)• Each packet header contains a route, which
is represented as a complete sequence of nodes between a source-destination pair
• Protocol consists of two phases – route discovery– route maintenance
• Optimizations for efficiency– Route cache– Piggybacking– Error handling
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DSR Route Discovery• Source broadcasts route request (id, target)
• Intermediate node action– Discard if id is in <initiator, request id> or node
is in route record– If node is the target, route record contains the
full route to the target; return a route reply– Else append address in route record;
rebroadcast
• Use existing routes to source to send route reply; else piggyback
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DSR Route Maintenance• Use acknowledgements or a layer-2 scheme
to detect broken links; inform sender via route error packet
• If no route to the source exists– Use piggybacking– Send out a route request and buffer route error
• Sender truncates all routes which use nodes mentioned in route error
• Initiate route discovery
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Optimizations for efficiency• Route Cache
– Use cached entries for during route discovery
– Promiscuous mode to add more routes
– Use hop based delays for local congestion
– Must be careful to avoid loop formation
– Non propagating RREQs
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Route Discovery in DSR
B
A
S E
F
H
J
D
C
G
IK
Z
Y
Represents a node that has received RREQ for D from S
M
N
L
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B
A
S E
F
H
J
D
C
G
IK
Represents transmission of RREQ
Z
YBroadcast transmission
M
N
L
[S]
[X,Y] Represents list of identifiers appended to RREQ
Route Discovery in DSR
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B
A
S E
F
H
J
D
C
G
IK
Z
Y
M
N
L
[S,E]
[S,C]
Route Discovery in DSR
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B
A
S E
F
H
J
D
C
G
IK
Z
Y
M
N
L
[S,C,G,K]
[S,E,F,J]
Route Discovery in DSR
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Route Reply in DSR• Destination D on receiving the first RREQ,
sends a Route Reply (RREP)
• RREP is sent on a route obtained by reversing the route appended to received RREQ
• RREP includes the route from S to D on which RREQ was received by node D
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B
A
S E
F
H
J
D
C
G
IK
Z
Y
M
N
L
RREP [S,E,F,J,D]
Represents RREP control message
Route Reply in DSR
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Route Reply in DSR• Node S on receiving RREP, caches the
route included in the RREP
• When node S sends a data packet to D, the entire route is included in the packet header– Hence the name source routing
• Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded
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B
A
S E
F
H
J
D
C
G
IK
Z
Y
M
N
L
DATA [S,E,F,J,D]
Packet header size grows with route length
Data Delivery in DSR
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Lingid• J. Jun and M. L. Sichitiu, “The nominal capacity of wireless mesh networks,”
IEEE Wireless Communications, Oct., pp. 8-14, 2003.• C. Zhu, M. J. Lee, and T. Saadawi, “On the route discovery latency of wireless
mesh networks,” in Proc. IEEE CCNC ’05, 2005, pp. 19-23.• J-H Huang, L-C Wang, and C-J Chang, “Coverage and capacity of a wireless
mesh network,” in 2005 International Conference on Wireless Networks, Communications, and Mobile Computing, 2005, vol. 1 pp. 458-463.
• I. Akyildiz and X. Wang, “A survey on wireless mesh networks,” IEEE Radio Communications, Sep., pp. S23-S30, 2005.
• The Internet Engineering Task Force, Ad hoc On-Demand Distance Vector (AODV) Routing. [Online] Jul. 2003, [2006 Apr. 21], Available at HTTP: http://www.ietf.org/rfc/rfc3561.txt
• The Internet Engineering Task Force, The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks (DSR). [Online] Jul. 2004, [2006 Apr. 21], Available at HTTP: http://www.ietf.org/internet-drafts/draft-ietf-manet-dsr-10.txt.
• The Internet Engineering Task Force, Optimized Link State Routing Protocol (OLSR). [Online] Oct. 2003, [2006 Apr. 21], Available at HTTP: http://www.ietf.org/rfc/rfc3626.txt.
• S. Naghian and J. Tervonen, “Semi-infrastructured mobile ad-hoc mesh networking,” in Proc. IEEE PIMRC ‘03, 2003, vol. 2 pp. 1069-1073.
• T-J Tsai and J-W Chen, “IEEE 802.11 protocol over wireless mesh networks: problems and perspectives,” in Proc. IEEE AINA ’05, 2005, vol. 2, pp. 60-63.
