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 (Adapted from mnet.cs.nthu.edu.tw/paper/jbb/010704.pps)

2. Overview IEEE 802.11 MAC Protocol The TCP Instability Problem and Analysis The Serious Unfairness and Analysis Conclusion

3. IEEE 802.11 MAC Protocol – (1) IEEE 802.11 MAC Protocol  is the standard for wireless LANs  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.

4. IEEE 802.11 MAC Protocol – (2) Hidden terminal problem:  This will cause collision on data transmission. AC B

5. IEEE 802.11 MAC Protocol – (3) Exposed terminal problem:  If these terminals are not minimized, the available bandwidth is underutilized. A C B D

6. IEEE 802.11 MAC Protocol – (4) The IEEE 802.11 MAC protocol in wireless mobile ad hoc networks, multihop connectivity is one of the most prominent features. Can the IEEE 802.11 MAC Protocol Function Well in Multihop Networks?

7. IEEE 802.11 MAC Protocol – (5) The serious problems encountered in an IEEE 802.11-based multihop ad hoc networks:  The TCP Instability Problem.  The Serious Unfairness Problem. Authors concludes that the current version of this wireless LAN protocol does not function well in multihop ad hoc networks.

8. TCP Instability Problem – (1) The network topology: 123456 Interfering Range of node 4 100 m 250 m

9. TCP Instability Problem – (2) The TCP session is the only traffic in the network with four hops from source 1 to destination 5. 123456 SourceDestination

10. TCP Instability Problem – (3) If the network condition does not vary, the TCP throughput should stay stable within some range.

11. TCP Instability Problem – (4)

12. TCP Instability Problem – (5)

13. TCP Instability Problem – (6) 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

14. TCP Instability Problem – (7) Collision Exposed Terminal

15. TCP Instability Problem – (8)  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

16. TCP Instability Problem – (9) By adjusting the Window size:

17. TCP Instability Problem – (10) 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

18. Serious Unfairness – (1) 2 TCP Connections  First session starts at 10.0s ( 6  4 )  Second session starts 20.0s later ( 2  3 ) 123456 SourceDestination Source

19. Serious Unfairness – (2) First session startSecond session start

20. Serious Unfairness – (3) 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.

21. Serious Unfairness – (4)

22. Serious Unfairness – (5)

23. Serious Unfairness – (6) Discussion:  Node5 cannot reach node4 when Node2 is sending (collision) Node3 is sending ACK (defer) 123456 SourceDestination Source

24. Serious Unfairness – (7) 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

25. Conclusion 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 cause unfairness.