Research On Congestion Control Algorithm Of TCP Westwood In Wireless Networks

In today’s information age, because of it’s flexibility, convenience and other advantages, wireless networks have been more and more widely used, so the service quality of wireless networks becomes very important.TCP protocol which based on the traditional wired environment can effectively improve efficiency of network transmission. The arithmetic of congestion control in TCP assumes that the transmission of network middle node is reliable. However, the wireless networks has high error rate, bandwidth changes, and frequent movement, large time delay and other characteristics, thus the link quality is poor. Those characteristics make TCP expose many problems and poor performance in wireless network environment. Obviously, TCP used well in wired network can’t adapt to the wireless networks which link quality is poor. So the research on how to alleviate the wireless networks congestion, and improve the performance of the wireless networks has become increasingly important issue. It must improve congestion control protocols used before or put forward new protocols in order to remain high efficiency of communication in the wireless networks.At present, a variety of enhanced algorithms of TCP are designed. They can be classified into four broad categories:link-layer protocols; split-connection protocols; end-to-end protocols and integrated protocols. TCP Westwood (for short TCPW) algorithm is an ideal end-to-end congestion control algorithms in wireless networks, its basic idea is TCP sender side by observing the rate of returning ACKs to estimate the end-to-end available bandwidth on the link. The congestion bandwidth estimation of BEW is then used to adjust congestion window and slow start threshold when a congestion episode. Thus the cyber source can be fully used after retransmission. TCPW largely eliminates the random loss of negative effect to the network bandwidth utilization rate. But TCPW exists the following problems:the estimation algorithms proposed for TCP Westwood reveals an overestimation of the available bandwidth, leading to aggressive and unfair behavior; congestion window increments during slow-start phase and congestion avoidance phase remain same as Reno, with this approach enables the congestion loss become inevitable and reduces the network utilization rate; TCPW can not distinguish reasons of packet loss, the wireless packet losses is often mistaken for the congestion losses, so that the congestion processing mechanism is called frequently, resulting in the throughput of link is declining. The purpose of this paper is to solve the two problems exist behind TCPW.This paper summarizes the algorithm of network congestion control, introduces the characteristics of wireless networks as well as the challenges it is faced. Then describes the current status of wireless network congestion and discuss the direction of further research. Then facing the two problems of TCPW, this paper put forward two kinds of improved algorithm, the research work are as follows:Firstly, In TCP Westwood, congestion window increments during congestion avoidance remain the same as in Reno, that is linear increase blindly which result in network congestion easily. And it can not distinguish loss types in a wireless network so that the retransmission mechanism is called frequently at sender when wireless packet losses happened resulting in the decline in throughput, In order to solve the problems mentioned above, an improved algorithm termed TCP Westwood-J which introduces the relative time delay、throughput and combining the advantages of TCP Vegas was proposed. The simulation experiments result is good and the algorithm completely follow the end to end thought, does not need the support of intermediate nodes.Secondly, When TCPW protocol in high bit error rate wireless network,it can not distinguish wireless packet losses and congestion losses effectively. A mixed improved algorithm of network congestion control named TCPW PM was presented, which unites the algorithms of network layer and transmit layer into one. In TCPW PM, router make the network congestion status and the transmit layer calculate throughput so that it can take appropriate congestion control measures. The experiments show that in high bit error rate wireless networks, the improved algorithm TCPW PM significantly improves throughput and maintains a good fairness and friendliness with respect to other TCP flows.Finally, give a summary of this paper and pointed out the next research work, in order to further improve the performance of TCPW, recommendations is given to distinguish loss types in a wireless network.