Research On The Fairness Of MAC Protocols In Wireless Ad Hoc Networks

Wireless Ad Hoc network is a distributed self-organized network which is composed of a collection of peer wireless mobile or stationary nodes. This kind of network doesn't rely on any pre-existing fixed network infrastructure and can be deployed quickly. It is especially attractive to military tactical cases, disastrous rescue, and remote regions.Currently, the IEEE 802.11 MAC protocol is the defacto standard for wireless ad hoc networks. However, this protocol was not originally designed for multihop networks. When it is used in wireless ad hoc networks, there are unfair phenomenons for nodes to access channel. The serious condition that even one single node or data flow exclusive use the channel resources and other nodes or data flow is in the complete "hunger" state may arise. So it is very meaningful to study IEEE802.11 standard and improve it.IEEE 802.11 standard specifies both the physical layer (PHY) and the medium access control (MAC) layer. In our research work, we emphasize on the distributed coordination function (DCF) which is the dominating medium access control mechanism of IEEE 802.11. We conduct the performance ananlysis and fairness improvement of IEEE 802.11 DCF in wireless ad hoc networks.In this paper, we first introduce the characteristics, application environments, and research hotspots of wireless ad hoc networks. Then we use two kinds of Markov model to analyze the saturated throughput performance of IEEE 802.11 DCF in ad hoc Networks.This dissertation focuses on the research of MAC layer fairness in wireless ad hoc networks based on IEEE802.11 standard. With simulation experiment, it has made a deeply analysis on the factors that trigger fairness problems. Then On the basis of current study, it proposes an improved channel access mechanism named EATC (Enhanced Adaptive Transmission Control).which can improve MAC layer fairness. In this mechanism, based on active neighbor number and current bandwidth ratio, each node calculates whether its current bandwidth access is fair. Contention window is adjusted for binary exponential backoff process according to the results. The improved mechanism also uses the sender and receiver mutually collaborative scheme in order to avoid unfairness issues caused by the hidden terminal. The simulation results show that compared with the IEEE 802.11 backoff algorithm, this improved mechanism has a good improvement in the channel access fairness and achieves a better compromise between improving throughput and Channel Access fairness performance.