Performance Issues With IEEE802.11 in Ad Hoc Networking

1. Performance Issues With IEEE802.11 in Ad Hoc Networking 汇报人：曾苗祥 作者：Claude Chaudet

2. introduction • Background: • IEEE802.11 has become the standard for wireless network. But most of the protocols designed for ad hoc networks assume that IEEE802.11 is used for lowest-layer communications. And till now there is no work that summarizes all encountered performance issues. • Purpose： • 1)present an overview of all known performance problems with the 802.11DCF that can appear in ad hoc networks; • 2)discuss possible and proposed solutions to these issues.

3. IEEE802.11 DCF DIFS(Distributed interframe space):the aim is to insure that the channel is idle.

4. Performance issues The author classified the presented performance into 3 categories: • Configuration that lead to long-term fairness issues in which some flows suffer from starvation while other flows capture the whole channel bandwidth; • Configuration that result in short-term fairness issues where the frames are emitted in bursts; • Configuration that result in overall throughput decrease, where a part of the network capacity is not used and thus wasted. The 802.11 DCF mechanisms that prevent an emitter from accessing the radio medium in normal conditions can be classified in three categories: • The emitter cannot transmit because the medium is busy for a long time; • Its random wait phase(given by the back off) is long due to its contention window increase after repeated collisions; • Its fixed wait phase is longer than in usual conditions since it uses an EIFS in place of a DIFS due to the presence of distant transmissions.

5. The long-term fairness issue A and E are totally independent. This can be happen when flows are emitted at a low throughput or there is a obstacles block the line of sight between A and E. a: Result: the pair (C ,D)expect no more than 15% of the medium capicity.While the (A,B)and (E,F) share the others. Throughput for the three pair configuration with different packet size

6. b: A and C are independent but both communicate with B This can be happen either when flows are emitted at a low throughput and emitters are distant enough or if obstacles block the line of sight between A and C. Result: c Result: the throughput of each emitter increases with the packet size, but the ratio between the two emitters’s throughout remains the same: B sends around seven times more packets than A. Large EIFS problem

7. The short-term fairness issue The RTS-CTS exchange can solve hidden node situation and result in fair medium access in the long term , but the behavior of the medium access is unfair in the short term This can be happen either when flows are emitted at a low throughput and emitters are distant enough , or if obstacles block the line of sight between the emitters. d: a: Hidden node scenario

8. Low aggregate throughput The overall used bandwidth decreases with low-speed communication links or high-speed communication links with obstacles making A and D independent. e: Small EIFS problem f: This configuration happens as soon as two mobiles not in communication range wish to communicate and routing is involved

9. Throughput for chain configuration : different chain length Throughput for chain configuration : six hops,with and without RTS/CTS

10. POSSIBLE SOLUTIONS • Bring the nodes knowledge of congestion level of neighbor emitter; • Manages the contention window size in order not favor the terminal that has just won the contention for the subsequent transmission; • Allow a CTS reply only if the RTS is received with a power larger than a given threshold; • Use multiple channels in terms of time , frequency or code • Use directional antennas or/and power control in order to reduce interference and increase spatial reuse.

11. 谢谢大家