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Fair Sharing of MAC under TCP in Wireless Ad Hoc Networks

Fair Sharing of MAC under TCP in Wireless Ad Hoc Networks. Mario Gerla Computer Science Department University of California, Los Angeles Los Angeles, CA 90095 http://www.cs.ucla.edu/NRL/wireless. Outline. Overview of ad hoc networks. GloMoSim network simulator.

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Fair Sharing of MAC under TCP in Wireless Ad Hoc Networks

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  1. Fair Sharing of MAC under TCP in Wireless Ad Hoc Networks Mario Gerla Computer Science Department University of California, Los Angeles Los Angeles, CA 90095 http://www.cs.ucla.edu/NRL/wireless

  2. Outline • Overview of ad hoc networks. • GloMoSim network simulator. • Overview of CSMA, FAMA and IEEE 802.11. • MAC performance with TCP. • Variable Hop Length Experiments. • Hidden Terminal Experiments. • Ring Experiments. • Grid Experiments. • Static. • Mobility.

  3. Ad-Hoc Network Characteristics • Instantly deployable, reconfigurable infrastructure • Node mobility • Heterogeneous nodes (big/small; fast/slow etc) • Heterogeneous traffic (voice, image, video, data) • Limited battery power • Multihopping ( to save power, overcome obstacles, enhance spatial spectrum reuse, etc.)

  4. Cellular vs Multihop Standard Base-Station Cellular Networks Instant Infrastructure, Multihop wireless Networks

  5. Ad Hoc Network Applications • Disaster Recovery (flood, fire, earthquakes etc) • Law enforcement (crowd control, border patrol etc) • Search and rescue in remote areas • Sport events, festivals • Ad hoc nomadic, collaborative computing • Indoor network appliances • Sensor networks • Battlefield

  6. Simulation Using GloMoSim • Detailed model of the protocol stack. • Allows investigation of TCP and MAC layer interactions. • Capability to simulate large number of nodes. • GloMoSim web page. • http://pcl.cs.ucla.edu/projects/domains/glomosim.html

  7. MAC Layer Protocols • CSMA • Requires carrier sensing before transmission. • If the channel is free, the packet is transmitted immediately. • Otherwise, it is rescheduled after a random timeout. • FAMA • Builds on CSMA. • Uses the RTS (Request To Send) and CTS (Clear To Send) exchange to prepare the floor for data transmission. • 802.11 • Uses carrier sensing and RTS/CTS, similar to FAMA. • Utilizes link-level ACKs. • Collision Avoidance scheme.

  8. 0 1 2 3 4 5 Variable Hop Length Experiments Configuration • Each node is 10 meters apart from its neighbors. • Each node has a radio power range of 10 meters. • 2Mbps channel bandwidth. • FTP traffic. • TCP window size varies from 1 to 16 packet size. • Variable number of hops (single connection). • i.e., FTP connection 0-1, 0-2, 0-3, 0-4, 0-5 (one at a time).

  9. Variable Hop Length Experiments Results

  10. Variable Hop Length Experiments Results (Cont’d)

  11. Variable Hop Length Experiments Results (Cont’d) • CSMA and FAMA degrades with window size > 1 pkt. • Collisions between TCP data and ACKs. • 802.11 performs the same no matter the window size. • Link-level ACKs combat collisions.

  12. 2 0 1 Hidden Terminal Experiments Configuration • FTP traffic • Connections from node 0 to node 1 and from node 2 to node 1. • Node 0 and node 2 cannot hear each other.

  13. Hidden Terminal Experiments Results • CSMA suffers from hidden terminal. • FAMA and 802.11 performs well due to RTS/CTS exchange.

  14. 0 1 7 6 2 5 3 4 Ring Experiment Configuration • FTP traffic • Multiple single hop connections. • i.e., 0-1, 1-2, 2-3, 3-4, 4-5, 5-6, 6-7, 7-0 (at the same time).

  15. Ring Experiment Results • FAMA best in both fairness and aggregate throughput. • 802.11 unfairness due to timers.

  16. 802.11 Fairness • Yield time. • Time a node yields after transmitting a frame.

  17. Grid Experiment Configuration 8 3 6 7 4 5 0 1 2 12 17 15 16 13 14 9 10 11 24 22 21 26 19 25 23 18 20 35 30 29 33 34 31 32 27 28 39 44 42 43 40 41 36 37 38 51 49 48 53 46 52 50 45 47 62 57 60 61 58 59 54 55 56 66 71 69 70 67 68 63 64 65 79 77 80 75 74 72 78 76 73 • Each node is 10 meters apart from its horizontal and vertical neighbors. • Each node has a radio power range of 30 meters. • FTP connections are established between node 18 to node 26, node 36 to node 44, node 54 to node 62, node 2 to node 74, node 4 to node 76 and node 6 to node 78.

  18. Grid Experiment Configuration (Cont’d) • 2Mbps channel bandwidth. • Nodes move at a rate of 10 meters per second in a random direction with a probability of 0.5. • When mobility is not considered, static routing is used. • When mobility is introduced, Bellman-Ford routing is utilized with routing table updates occurring once every second.

  19. Grid Experiments Results (No Mobility) • Without mobility • CSMA performs poorly due to interference by neighboring streams and by intersecting streams. • FAMA fair due to RTS/CTS and less aggressive yield time. • 802.11 exhibits capture.

  20. Grid Experiments Results (With Mobility) • CSMA and FAMA collapse with mobility due to lack of fast loss recovery facilities. • 802.11 still operational. • Link level ACKs help recover from loss caused by transient nodes. • Capture exists.

  21. Conclusion • RTS/CTS exchange improves fairness. • Link-level ACKs important to combat packet loss in wireless ad-hoc environment. • Impact of MAC layer timers need further study.

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