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Sidevõrgud IRT 0020 loeng 8 06. nov. 2006

Sidevõrgud IRT 0020 loeng 8 06. nov. 2006

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Sidevõrgud IRT 0020 loeng 8 06. nov. 2006

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  1. SidevõrgudIRT 0020loeng 8 06. nov. 2006 Avo Ots telekommunikatsiooni õppetoolraadio- ja sidetehnika instituut avots@lr.ttu.ee

  2. Market End-Users Content and Service Providers Service operators/ Telecommunications Networking Solutions Physical Telecommunication Network

  3. Architectures Infrastructure mode Infrastructure-less/ distributed/ad-hoc mode

  4. 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 Hidden Terminal Problem A B C

  5. 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 Multiple Access with Collision Avoidance

  6. 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 Reliability

  7. 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 IEEE 802.11 DCF

  8. 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 Collision Avoidance

  9. 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 Congestion Avoidance

  10. 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. IEEE 802.11 PCF

  11. 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. PCF

  12. Queue Queue 0 Queue 1 Queue 2 Flow 0 If packets in queue else Flow 1 If packets in queue else Flow 2 Priority Assignment Methods (1) • Strict Priority Queuing • FIFO

  13. Queue 0 Queue 1 Queue 2 Flow 0 Probability 0.1 Flow 1 Probability 0.2 Flow 2 Probability 0.7 Priority Assignment Methods (2) • Weighted Fair Queuing

  14. 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 MAC Management

  15. 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 MANET (Mobile Ad Hoc Networks B A A B C C D D

  16. 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

  17. 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

  18. 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)

  19. 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

  20. Route in DSR Y Z S E F B C M L J A G H D K I N Represents a node that has received RREQ for D from S

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. Route Discovery in DSR Y Z S E F B C M L J A G H D K I N Represents a node that has received RREQ for D from S

  27. Route Discovery in DSR Y Broadcast transmission Z [S] S E F B C M L J A G H D K I N Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ

  28. Route Discovery in DSR Y Z S [S,E] E F B C M L J A G [S,C] H D K I N

  29. Route Discovery in DSR Y Z S E F [S,E,F,J] B C M L J A G H D K I N [S,C,G,K]

  30. 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

  31. Route Reply in DSR Y Z S RREP [S,E,F,J,D] E F B C M L J A G H D K I N Represents RREP control message

  32. 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

  33. Data Delivery in DSR Y Z DATA [S,E,F,J,D] S E F B C M L J A G H D K I N Packet header size grows with route length

  34. 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.