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Sidevõrgud IRT 0020

loeng 8 06. nov. 2006

Avo Ots

telekommunikatsiooni õppetoolraadio- ja sidetehnika instituut

avots@lr.ttu.ee

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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.