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Collisions & Virtual collisions in IEEE 802.11 networks

Collisions & Virtual collisions in IEEE 802.11 networks. Libin Jiang EE228a Communication Networks. Outline. Problem of Carrier-Sensing in 802.11 Some Symptoms The New Design Performance Evaluation Throughput-Collision Tradeoff Conclusion. T2. R1. R2. T1. PCS. 1. 2. Range.

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Collisions & Virtual collisions in IEEE 802.11 networks

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  1. Collisions & Virtual collisions in IEEE 802.11 networks Libin Jiang EE228a Communication Networks

  2. Outline • Problem of Carrier-Sensing in 802.11 • Some Symptoms • The New Design • Performance Evaluation • Throughput-Collision Tradeoff • Conclusion

  3. T2 R1 R2 T1 PCS 1 2 Range Basic Mode Link contention Graph |T2-R1|<IR; (Link: a Transmitter-Receiver pair) |T1-T2|<PCSRange RTS/CTS mode R1 T1 R2 T2 |T2-R1|<IR; VCS VCSRange>IR Range VCS Range Problem of Carrier-Sensing in 802.11 • Hidden-terminal problem (causing collisions) • RTS/CTS IR: Interference Range PCSRange: Physical Carrier Sensing Range VCSRange: Virtual Carrier Sensing Range

  4. Collisions & unfairness still exist if a receiver can sense “busy” channel but the transmitter can’t Transmitter does not know when to transmit Collisions Virtual collisions T2 can send RTS to R2, but R2 does not reply with a CTS (May not be a real collision) Information asymmetry T1 knows Link 3 T3 does not know Link 1, resulting in collisions Link 3 gets a much lower throughput than Link 1 Cannot be solved by justusing a large CS Range R2 T3 R3 R1 T1 VCS T2 Range VCS Range RTS/CTS Mode VCSRange>IR Problem of Carrier-Sensing in 802.11 Virtual Collision Collision

  5. Packet T1->R1 T2 R2 R1 Preamble, length MAC Data T1 PCS Preamble, length MAC Data (T2->R2) Range Packet T2->R2 If |T1-R2|<PCSRange, R2 can miss the packet T2->R2 Basic Mode Packets on Link 2 are often lost, for any PCSRange. Problem of Carrier-Sensing in 802.11 • Similar situation exists in "Basic Mode“, if the receiver cannot “restart” to receive a stronger packet Packets arriving at R2

  6. Outline • Problem of Carrier-Sensing in 802.11 • Some Symptoms • The New Design • Performance Evaluation • Throughput-Collision Tradeoff • Conclusion

  7. Symptoms • Frequent packet collisions cause many problems [1] • Throughput Unfairness • Routing Instability [1] Xu, S.; Saadawi, T., “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks?”, Communications Magazine, IEEE, Volume: 39, Issue: 6, June 2001, Pages:130 - 137

  8. TCP 1: node 1  node 3, starts earlier at 3.0 sec TCP 2: node 6  node 4. starts at 10.0 sec. Symptoms Throughput Unfairness 1 2 3 4 5 6 Tool: Network Simulator 2 • Nodes are spaced by 140m • No RTS/CTS, PCSRange = 550m. • 3 hops<PCSRange<4 hops • Data rate: 11Mbps • Packet size: 1460 Bytes • Routing protocol: AODV (Ad-hoc On-demand Distance Vector Routing)

  9. A UDP flow: node 1  node 12 (11 hops). Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability Symptoms Routing Instability ... 1 2 3 4 5 6 11 12

  10. Outline • Problem of Carrier-Sensing in 802.11 • Some Symptoms • The New Design • Performance Evaluation • Throughput-Collision Tradeoff • Conclusion

  11. The New Design--for IEEE 802.11 Basic Mode • Definitions • Range Requirement: Transmitter must sense the interfering link(s) • Receiver Requirement: Receiver assumes no role in Carrier-Sensing • “Restart”: If a stronger packet arrives later, the receiver switches to receiving the packet • In any case, return ACK if receiving a DATA packet

  12. The New Design--for IEEE 802.11 RTS/CTS Mode • Range Requirement: transmitter must receive the RTS or CTS from interfering link(s) • Receiver Requirement: Receiver assumes no role in Carrier-Sensing • Same as before, except… • In any case, return CTS/ACK if receiving a RTS/DATA packet

  13. Outline • Problem of Carrier-Sensing in 802.11 • Some Symptoms • The New Design • Performance Evaluation • Throughput-Collision Tradeoff • Conclusion

  14. TCP 1: node 1  node 3, starts earlier at 3.0 sec TCP 2: node 6  node 4. starts at 10.0 sec. Performance Evaluation TCP unfairness 1 2 3 4 5 6

  15. A UDP flow: node 1  node 12 (11 hops). Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability Performance Evaluation Routing Instability ... 1 2 3 4 5 6 11 12

  16. Outline • Problem of Carrier-Sensing in 802.11 • Some Symptoms • The New Design • Performance Evaluation • Throughput-Collision Tradeoff • Conclusion

  17. In the design, CSRange/dmax seems to be large: A smaller PCSRange can not remove hidden-terminals, but may give a higher throughput To study the tradeoff, consider a random network M=4 16 APs, 64 randomly located clients D/M=175m dmax=175/root(2) PCSRange>468m satisfies Range Requirement Throughput-collision tradeoff

  18. Throughput-collision tradeoff Range Requirement met Collision Probability vs. PCS Range Total throughput vs. PCS Range

  19. The tradeoff always exists The tradeoff is improved by meeting the Receiver Requirement Throughput-collision tradeoff Throughput-collision tradeoff

  20. Conclusion • 802.11 does not avoid hidden-terminal-induced collisions & virtual collisions • It is the root of many problems (symptoms) • 2 requirements (Range Requirement and Receiver Requirement) is sufficient to solve the problem • Tradeoff between throughput & collisions

  21. Thank you!

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