Study On MAC Layer Protocol Of Wireless Lan And Analysis Of QoS Performance

The wireless LAN is the combination product of computer network and wireless communication technology in the 1990's, it provide the access to network via wireless channel, thus it could be a potential method for the future telecommunication: mobility, personality and multimedia application, it also comes to be an efficient broad-band wireless access mechanism. This paper mainly investigates the wireless protocol: IEEE 802.11 MAC layer, and also do some research about QoS mechanism.Firstly, on the basis of the background knowledge of wireless LAN, this paper gives a detailed research of IEEE 802.11 MAC layer protocol. The wireless LAN concerns only the two bottom layers about the OSI seven-layer model, they are physical layer and MAC layer thus have a simple network construct. The MAC layer profile include media access scheme, authentication and encryption, it also describe the frame format. Various kinds of network services are also include in this part. DCF mode is the basic wireless media access method, which is based on CSMA/CA (channel sense multiple access/ contention avoidance), if a station prepares to send packet, it firstly listen to the channel using the virtual channel sense mechanism to decide if it is idle for transmit. If it is busy the station has to choose a random time to backoff, and waiting for another round of contention.The traditional wireless LAN 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, analyse several types of QoS schemes, the original protocol describe the IPS mode and RTS/CTS mode, according to contention window and backoff arithmetic, this paper give another two modified arithmetic which can be testified by simulation.In order to analyse the difference QoS mechanism applied in different environment home use or office use, we set up several simulation environment, the compare focus on three basic characters: throughput, delay and packet drop rate. From the simulation result we can see the modified backoff arithmetic can generally promote the network performance, and different network environment need different QoS mechanism.This paper also give simple instruction to the simulation tools such as ns2, GNUplot, Xgraph and movement scenario generator, etc.