The Research Of Backoff Algorithm In IEEE 802.11 DCF Cooperative MAC

IEEE 802.11 is the main and the most popular access technology in wireless local area network, the backoff mechanism in its MAC layer has been a hot spot in current research. This thesis studies the cooperative MAC protocol in wireless local area network. We focus on the backoff algorithm in cooperative MAC protocol.The main job of this thesis is to study the backoff algorithm in IEEE 802.11WLAN DCF mechanism. Analysis shows that there are some problems in the binary exponential backoff algorithm. Firstly, the BEB algorithm results in"capture" effect and also this algorithm brings to unsteadiness in the throughout. Secondly, the BEB algorithm was originally designed for the signal rate, and did not consider multi-rate situation, therefore, it results the unfairness in access time. According to these two problems, this thesis makes the correlative research. Adjusted schemes are proposed, and simulation results show that the schemes can improve the system performance, improve the unfairness.Aiming at the existing retreats in the binary exponential backoff algorithm, this thesis makes some improvement in the binary exponential backoff algorithm in cooperative MAC protocol. We adopt a novel backoff mechanism to improve the "capture" effect. The stations adopt fixed competition window to compete the channel, meanwhile, when a collision happens, we use the random-queue way for data transmission. As for the unfairness in access time in the IEEE 802.11b multi-rate case, the stations with different rate get different backoff algorithms. In addition, we improve the classic two-dimensional Markov chain model so as to model the new backoff algorithm. At last, we analyze the system throughout of the new backoff algorithm.Theoretical analysis and simulation show that the adjusted algorithm solved the existing two problems in the binary exponential backoff algorithm. The new algorithm improves the unfairness phenomenon, improve the system performance.