Research On Contention Protocol Of MAC Layer In Wireless Local Area Networks

In recent years, with the rapid development of mobile internet and internet of things, there are more and more intelligent terminals that are used. Wi Fi, as the popular way of wireless internet access, becomes a daily life to us. As the standard of WLAN(wireless Local Access Network), IEEE802.11 is developing continuously with new technologies and new manufacturing process. The MAC layer contention protocol becomes more and more important with the expansion of network scale. This paper is focus on the DCF access mode in MAC layer.Firstly, the development history and present situation of IEEE802.11 protocol is briefly reviewed. Then, the widely accepted DCF access mode, including its core protocol: CSMA/CA protocol and two key technologies: carrier sense and BEB backoff algorithm is analyzed in detail. In addition, the classical Bianchi's two-dimensional Markov model is also well introduced in this paper. Finally, several important performance parameters are defined or formulized.According to the simulation, BEB backoff algorithm performs unsatisfactory in all aspects. The main work of this paper is to make an improvement on backoff mechanism in CSMA/CA protocol. A new backoff algorithm called Fixed Idle Interval is proposed after detailed analysis on some improved backoff algorithm in recent years. FII algorithm is able to obtain the best contention window by using the number of idle interval happened in last contention window. After phenomena observation, theory analysis, formula derivation and simulation, a conclusion is drew that the best idle interval in basic access mechanism is 8 to 10 whereas in RTS/CTS access mechanism, the best one is 4. The pseudo code of FII algorithm is given in chapter 4. The simulation shows that FII algorithm has a dramatic improvement over BEB algorithm in throughput and delay, which can be in actual application.The fairness performance is not improved so well in FII backoff algorithm, thus,in chapter 5 of this paper, a new MAC protocol, Semi-DCF protocol, is proposed. This protocol is based on the CSMA/CN transmission process and uses signature recognition technology. In Semi-DCF, the random transmission is changed into sequential transmission in retransmission, which greatly increases the fairness performance. Throughput and delay performance of Semi-DCF are also improved in comparison to DCF. In the future, the Semi-DCF protocol has a very broad application prospect.