1. Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks? Shugong Xu Tark Saadawi June, 2001 IEEE Communications Magazine

2. Prolog The IEEE 802.11 MAC protocol is the standard for wireless LANs; it is widely used in testbeds and simulations for wireless multihop ad hoc networks research. Although it can support some ad hoc network architectures, it is not intended to support the wireless mobile ad hoc networks

3. Prolog In wireless mobile ad hoc networks, multihop connectivity is one of the most prominent features. So, we will ask: Can the IEEE 802.11 MAC Protocol Function Well in Multihop Networks ?

4. Prolog By presenting some problems encountered in an IEEE 802.11-based multihop networks, the author concludes that: The current version of this wireless LAN protocol does not function well in multihop ad hoc networks.

5. Outline Introduction The TCP Instability Problem and Analysis The Serious Unfairness and Analysis Conclusion

6. Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks? Introduction

7. Intro. IEEE 802.11 MAC Protocol CSMA / CA Combined network allocation vector (NAV) with clear channel assessment (CCA) to indicate the busy state of the medium. Using RTS/CTS scheme to reduce the probability of two stations colliding due to not hearing each other.

8. Intro. Hidden terminal problem: This will cause collision on data transmission. AC B

9. Intro. Exposed terminal problem: If these terminals are not minimized, the available bandwidth is underutilized. A C B D

10. Intro. The serious problems encountered in an IEEE 802.11-based multihop ad hoc networks: The TCP Instability Problem. The Serious Unfairness Problem.

11. Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks? The TCP Instability Problem and Analysis

12. TCP Instability Problem The network topology: 123456 Interfering Range of node 4 100 m 250 m

13. TCP Instability Problem The TCP session is the only traffic in the network with four hops from source 1 to destination 5. 123456 SourceDestination

14. TCP Instability Problem If the network condition does not vary, the TCP throughput should stay stable within some range.

15. TCP Instability Problem

17. Collision Collision will occur in node 2 when nodes 1 and 4 is sending at the same time. Exposed Station Node 2 has to defer when node 4 is sending. 123456 SourceDestination

18. TCP Instability Problem Collision Exposed Terminal

19. TCP Instability Problem Collision Node2 cannot receive RTS when node4 is sending (hidden terminal problem) Exposed Station Problem Node2 cannot send back CTS even if it receives the RTS from node1 correctly. After failing to receive CTS from node2 seven times, a route failure event occurs. 123456 SourceDestination

20. TCP Instability Problem By adjusting the Window size:

21. TCP Instability Problem Discussion: The maximum number for possible back- to-back sending is four, greatly reduces possible that other nodes might fail to access the channel in seven tries. 123456 SourceDestination

22. Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks? Serious Unfairness and Analysis

23. Serious Unfairness 2 TCP Connections First session starts at 10.0s ( 6 4 ) Second session starts 20.0s later ( 2 3 ) 123456 SourceDestination Source

24. Serious Unfairness First session startSecond session start

25. Serious Unfairness The throughput of the first session is zero in most of its lifetime after the second session starts. There is not even a chance for it to restart. The loser session is completely shutdown even if it starts much earlier.

26. Serious Unfairness

28. Discussion: Node5 cannot reach node4 when Node2 is sending (collision) Node3 is sending ACK (defer) 123456 SourceDestination Source

29. Serious Unfairness Discussion: TCP Instability / Unfairness Problem. These MAC layer problem appear when the traffic load becomes large enough, even if the traffic is not from TCP. 123456

30. Does 802.11 Work Well in Multihop Wireless Ad Hoc Networks? Conclusion

31. The hidden terminal problem still exists in multihop networks. The exposed terminal problem will be more harmful in a multihop network and there is no scheme in IEEE 802.11 standard to deal with this problem. The binary exponential backoff scheme always favors the latest successful node. It will course unfairness.