The Research & Improvement Of TCP Congestion Control Algorithm Over Wierless Network

Many kinds of wireless networks, such as Wireless LAN (Wi-Fi), Wireless MAN (WiMax) and cellular network (WCDMA, GPRS, etc.), which have the advantages of flexible bandwidth allocation, easy access and rapid recovery, have been deployed and have become integral parts of the Internet. The scope of the network extends from a single wired network to a variety of wired-cum-wireless network. However, due to high bit error of wireless link, sudden packet loss severe, easy to interfere, and handoff, the performance of traditional TCP (Transport Communication Protocol) which was designed for wired link decreased in wireless networks. Therefore, the research of wireless TCP has been one of the hot topics in recent years.This paper outlines TCP and its congestion control mechanisms, wireless networks and their characteristics, and analyzes the deficiencies and defects of traditional TCP in wireless networks. Then, we use NS2 (Network Simulation Version 2) to simulator traditional TCP in wireless networks with different wireless packet loss rates, and get the result that traditional TCP frequently reduced congestion window due to regard the wireless packet loss as congestion packet loss, resulting in the bad performance of TCP. Focusing on the disability of the traditional TCP on distinguishing between congestion loss and wireless loss, error congestion control mechanisms and error recovery mechanism in wireless networks, this paper presents a new protocol, that is TCP BR. TCP BR divides network status into different levels according to the value of RTT (Round Trip Time). Packet loss occurs at different levels of state, TCP BR adopts different measures to adjust the congestion window to regulate the transmission rate and to improve TCP performance. This new protocol increases the congestion window at a lower speed in order to to ensure network stability and avoid premature timeout or congestion. And in reducing the congeston window, the fine reductions of the window are used to try to recover from the congestion. If the attempt fails, the congeston window is reduced at a faster speed to recover from network congestion and at the same time to achieve TCP throughput and network stability.We use NS 2 to simulator TCP BR. By simulating network environment at different wireless packet loss rates, we have proved that TCP BR performs well in wireless networks. It has high throughput with low window jitter. At the same time, TCP BR has a good agreement with other TCP, that is to say, it is fairness.