The Research Of Backoff Algorithm On MAC Protocol For IEEE802.11

Wireless Local Area Network(WLAN) is a network which communicates through shared wireless channels. Media Access Control layer is an important part of IEEE802.11 protocol and is the direct sender and receiver of the packets from the channel. Whether the MAC protocol can efficiently use the finite channel bandwidth makes a crucial role to improve the performance of WLAN.In order to improve the throughput and reduce the contension probability of the channel, MAC protocol uses the backoff algorithm to produce a backoff intervals before transmission which is randomly selected from a specified range of 0 and CW to reduce the contention. This dissertation firstly has deeply analysed the different backoff algorithms of MAC protocol based on IEEE 802.11 DCF model and then has done the following work: At first, we propose a new method to improve the copy mechanism of MILD algorithm and simulate the improved mechanism. Secondly, we propose a new backoff algorithm,namely, Fairness Backoff Algorithm(FBA) based on the improved mechanism.FBA derives the expression for the optimum backoff intervals(Bopt) through analyzing all kinds of channel states,estimates the parameters of the FBA and simulates output performance of MAC protocol with different parameters. Thirdly, we stimulate three bakoff algorithms BEB,MILD,FBA and analyse their performance in light of channel throughput, average packet delay and fairness.As IEEE802.11 requires nodes competes for access to the channel after waiting for a "backoff"intervals, those selfish nodes may wait for smaller backoff intervals than well-behaved nodes. It is unfair to those well-behaved nodes. Therefore, we make some modifications to the IEEE802.11 protocol to detect and punish the unfair behaviors at the last part of this dissertation.