Study On Modified Algorithm Of TCP Protocol In Wireless LAN

Transmission Control Protocol (TCP) is a connection-oriented reliable transport layer protocol that provides a byte-stream service to its clients. TCP provides reliable data transmission service for application layer on top of unreliable network layer, which is the dominant transport layer protocol. The traditional TCP assume that the loss of segments and ACK is caused by network congestion. This assumption is suitable for low error and wide band wired channel. With the rapid development of computer technology and wireless communication technology, people require more high-speed services. IEEE 802.11 wireless local area network (WLAN) is applied extensively for its convenience, flexibility and mobility of terminal. When TCP is applied in WLAN, the traditional TCP based on wired network can not adapt to the bad channel quality of wireless channel because that wireless channel has higher error rate, lower bandwidth and larger delay. Various fading, interference and mobility of terminals will bring Doppler frequency shift and hand-off problems, which will affect the performance of TCP seriously. The assumption that segment loss is caused by congestion fails.This paper summarizes the form and limitations of improvement scheme in existing wireless TCP. According to the characteristics of wireless channel, we proposed a kind of end-to-end modified TCP protocol which is called TCP-C. The works in this paper are as follows:Firstly, most of researches on TCP performance are based on the two diameter propagation channel model. This model is an ideal model, that is to say, it can not reflect burst error of wireless channel which is caused by fading and crossing talk. In order to simulate wireless channel more correctly, we add Markov model into NS-2 simulation software. We simulate the wireless fading channel with a two order Markov model.Secondly, this paper propose a reasonable, simple and effective end-to-end TCP improvement algorithm TCP-C that estimates queue length with RTT measurement method to forecast congestion degree of links. In order to discriminate congestion loss and wireless random loss, a dynamic threshold N is designed. When wireless random loss is detected, we adopt the improved fast recovery algorithm. TCP does not decrease congestion window and slow start threshold. Moreover, in the stage of congestion avoidance, congestion window will increase one segment in every two RTTs if queue length is larger than threshold N, which can slow down the increasing rate of congestion window and maintain TCP connection around larger window to enhance the throughput of TCP protocol.Finally, we established IEEE802.11b WLAN based on NS-2 simulation software, and then compared simulation performance of TCP-C protocol with that of TCP reno, TCP Nwereno and TCP Vegas. The results show that TCP-C protocol can distinguish congestion lost package and wireless random lost package according to the state of the links, and then take different congestion control mechanism to different causes of lost package. The throughput of TCP-C protocol has significantly improvement than that of TCP reno, TCP Newreno and TCP Vegas. we analyze the performance anomaly phenomenon in WLAN and improve this phenomenon by declining the length of TCP segment and increasing the width of competition window simultaneously.