Improvement And Performance Analysis For DCF Mechanism In IEEE802.11 MAC Layer

In recent years, with the rapid development of wireless communication technology, the Wireless Local Area Network (WLAN) has been widely deployed, and become a hot topic in the literature. As the the mainstream standards of WLAN, the IEEE802.11 series standards have attracted a lot of attention all over the world. Due to the vital role of the protocol design of Media Access Control (MAC) layer on the performance of the WLAN. It is of great interest to investigate the IEEE802.11 MAC layer.The basic access control method of the IEEE802.11 MAC protocol is the Distributed Coordination Function (DCF) which utilizes the backoff mechanism such that each site independently selects the backoff time to avoid collisions. This thesis studies the DCF, improves its backoff method and then analyzes its performance. As a result, the system throughput is improves by reducing the probability of the collision between each station.Firstly, this thesis theoretically analyzes the Binary Exponential Backoff (BEB) of the DCF protocol, establishes a two-dimensional Markov model, and deduces the formula of the system saturate throughput as well as the parameters which influence the performance of the system throughput. Secondly, we analyze the crucial factor that affects the system throughput. In particular, we find out that the crucial factor which affects the system throughput is the size of the initial backoff window closely related to the network size. Finally we carry on the simulation on the NS2 platform.Based on the above analysis and the known improved algorithm, in this thesis, a new adaptive algorithm is proposed. The algorithm sets the channel congestion level estimation parameter L to evaluate the network size in real time. According to the value of L, the algorithm then designs the initial backoff window size, which results in reducing the collision probability and improving the performance. Finally, the proposed algorithm is simulated on NS2 platform. The simulation result shows that it indeed improves the system throughput and reduces the delay and the packet loss rate compared with the original protocol.