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. Problem of Carrier-Sensing in 802.11 Hidden-terminal problem (causing collisions) T1 R1 T2 Basic Mode |T2-R1|<IR; |T1-T2|<PCSRange PCS Range R2 12 Link contention Graph (Link: a Transmitter-Receiver pair) T1 R1 T2 VCS Range VCS Range R2 RTS/CTS mode |T2-R1|<IR; VCSRange>IR IR: Interference Range PCSRange: Physical Carrier Sensing Range VCSRange: Virtual Carrier Sensing Range RTS/CTS

4. Problem of Carrier-Sensing in 802.11 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 just using a large CS Range Collision Virtual Collision T1 R1 R3 RTS/CTS Mode VCSRange>IR VCS Range VCS Range T3 T2 R2

5. Problem of Carrier-Sensing in 802.11 Similar situation exists in "Basic Mode“, if the receiver cannot “restart” to receive a stronger packet T1 R1 T2 PCS Range R2 Preamble, lengthMAC Data Preamble, lengthMAC Data (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. Packet T1->R1 Packet T2->R2 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. Symptoms Throughput Unfairness TCP 1: node 1  node 3, starts earlier at 3.0 sec TCP 2: node 6  node 4. starts at 10.0 sec. 123456 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. Symptoms Routing Instability A UDP flow: node 1  node 12 (11 hops). Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability 123456 1112...

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 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 Definitions

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. Performance Evaluation TCP unfairness TCP 1: node 1  node 3, starts earlier at 3.0 sec TCP 2: node 6  node 4. starts at 10.0 sec. 123456

15. Performance Evaluation Routing Instability A UDP flow: node 1  node 12 (11 hops). Routing instability is triggered by excessive packet collisions, which is mistaken for route unavailability 123456 1112...

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

17. Throughput-collision tradeoff In the design, CSRange/d max 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  d max =175/root(2)  PCSRange>468m satisfies Range Requirement

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

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