TCP Improvement Over Wireless Networks Based On TCPW

Transmission Control Protocol (TCP), which is a transport layer protocol, is used by many standard data application in current wired networks. It provides reliable end-to-end data transmission service. Congestion control mechanism is a hot field in researching on TCP, which has proven to work well in traditional networks where packets losses are mainly due to network congestion. On the other hand, a totally different scenario is happened in the wireless link. Due to the high bit error rate (BER) and other wireless related characteristics, congestion is no longer the reason of packet losses. In addition, traditional TCP is not able to distinguish the causes of packet losses. It is totally comply with the principle of Additive Increase/Multiplicative Decrease (AIMD). Hence, TCP performance is affected. Research on TCP congestion control in wireless becomes essential with great applicable significance.There are many related proposals concerning about TCP improvement over wireless networks. The research focuses on one of them, TCP Westwood (TCPW). The rationale behind is that the TCP sender uses bandwidth estimation (BWE) to drive congestion window (cwnd) and slow-start threshold (ssthresh). The philosophy is called Additive Increase/Adaptive Decrease (AIAD). This mechanism is different from traditional congestion control after packet loss occurs and it has much bandwidth utilization.In the thesis, two algorithms based on TCP Westwood are proposed.The first one is a variant of bandwidth estimate based on network stability. The gap between two adjacent bandwidth samples is estimated in a filter to represent stability of the current network as the stability factor. The maximum factor is calculated during a certain number of factors at most. The ration of the current factor and the maximum one is used to adjust the filter gain of the EWMA filter of TCPW. The bandwidth estimate is then used to drive cwnd and ssthresh after packet losses occur.In addition, the other algorithm is a simple sender side modification of slow start which is based on TCP Westwood. The philosophy is to use bandwidth estimation to adaptively adjust the ssthresh in slow start phase. This would enable slow start exits into congestion avoidance phase at a more reasonable time instead that the standard slow start uses an arbitrary ssthresh. If ssthresh is set too high, the exponential increase of cwnd will generate too many packets too quickly. If the initial ssthresh is set too low, the sender does not effectively utilize the exponential increase of cwnd. The utilization of a large bandwidth is not effective when the startup is poor.The goal of the research is to optimize bandwidth utilization and improve throughput performance of TCP over wireless networks. Experiments are performed via wired/wireless mixed networks. The simulation results illustrate that the two algorithms have more throughput than others. On the other hand, the first algorithm provides comparatively better fairness and friendliness.