Performance Analyze And Optimize On IEEE 802.11 DCF

With the rising popularity of Wireless Local Area Networks(WLAN),the requirement of transmitting real-time multimedia traffic(such as video,audio) in the wireless networks becomes more and more important.Supporting Quality of Service(QoS) in WLAN has been an important challenging task.In comparison with the wire networks,the performance and QoS of wireless networks has great disparity.Real-time applications impose stricter requirements on service parameters,such data rate,delay and delay jitter.To provide reliable and guaranteed services for these applications,the medium access control(MAC) protocol is one of the important factors in performance and QoS problem of wireless networks.So it is significant to the performance of IEEE 802.11 MAC protocol,and the QoS problem that relate to MAC protocol in t\wireless networds.In this thesis the author introduces briefly the background, concepts, advantages, application future and many standards of WLAN,and the Distributed Coordination Function (DCF) mechanisms provided in IEEE 802.11.The author also introduced in detail the technique of transmission,basic network topology and protocol system.Then various DCF based QoS schemes are discussed.Our research work bases on IEEE 802.11 standard.Through presenting the details of the DCF mechanisms,we can get the conclusion that the traditional DCF mechanisms can only provide the"best effort"service,those time-sensitive traffic such as video and audio require better quality of service,but it is because wireless LAN have the high mobility,high bit error rate and channel access scheme based on contention mode,to realize QoS mechanism becomes difficult.Then this paper research on the backoff arithmetic,and propose scheme to reduce DIFS overhead.To avoid reducing the transmission efficiency for the sake of frequent change of contention window,we modification the mechanism of sensing the channel for node with successful transmission of a packet and node involved in collision.Whenever a node sense the channel and if it finds the channel busy,the node don't choose the random backoff and it will wait for Ts before sensing the channel again for DIFS period. we reduce the chance of two nodes selecting the same backoff slot. So we get higher throughput and reduce the average delay.At the same time, to supporting QoS for real-time traffic,we proposed a priority-based Algorithmetic.The priority is based on frame space.In view of the fact that badwidth is allocated unfairly between high priority traffics and low priority traffics,we provide dynamic priority to prevent starvation in low priority traffics and offer better service for low priority traffics.The simulation results verify that the modified scheme is not only provide higher throughput and lower average delay,but also fair.This paper also give simple instruction to the simulation tools such as ns2 and its simulation method.Finally we prospect the future of the DCF in IEEE802.11.