A Study Of TCP Congestion Control Over Wireless Network

With the fast development and widely application of wireless networking technologies, The next generation mobile wireless Internet will be expected to guarantee people enjoying interactive multimedia services "with anybody, at anytime, in anyplace". However, compare with the wired environment, the characters of wireless networks are high bit error rates, low bandwidth, long time delay, frequency moving and so on, so there are high error rate in wireless network. In wireless networks, congestion is not the only reason for data losing. If we use the traditional congestion control technique in wireless networks, it can make the sender decrease the sending rate unnecessarily and results the decrease of the network performance. Therefore, it is an important research project that how to improve the performance of TCP in wireless environment.The thesis includes three aspects to research the wireless network congestion control.First, the TCP theory and the key technology of TCP congestion control algorithm are researched in the thesis. There have analyzed the TCP message format, TCP link, link to terminate, link to reset and TCP state machine;At the same time we compare a few TCP congestion control algorithm (such as Tahoe, Reno, NewReno, SACK and Vegas) and discuss the key technology of TCP congestion control algorithm .Second, the TCP control strategies and the performance of TCP in the wireless network are analusised. The content is mainly about the TCP congestion control strategies which are the wave properties of the slacken data transmit rate, separating congestion control and error control, the relativity of the congestion control strategies and its application. And then there is the mathematical anaysis about the enfluence on the TCP throughput which is from the error of wireless transmission;And summerizes the congesion decision methods and analyses the research progress of the TCP congestion control in the wireless nerwork. We compare the performance of the transitional TCP in the wireless network using the NS-2 simulator, in the basis of the simulator experiment conclude the causes enfluence the performance of the TCP.In the last, a new congestion control algorithm based on double filtering bandwidth estimation over wireless network is proposed. At first section presents a thoroughly analytic evaluation for bandwidth estimate scheme, such as the throughput formula, fairness. Even though its fairness andeffectiveness can be proved, the bandwidth estimate scheme will have poor performance when the sender estimates the incorrect bandwidth or the random packet loss rate exceeds a few percent. The thesis presents TCP DFBE (Double Filtering Bandwidth Estimation) which calculates the average bandwidth by filtering average data packet length and average time interval, and then gets the more accuracy bandwidth value by double filtering the average bandwidth. The thesis proposes the algorithm thinking and implementation of the DFBE. The algorithm has been compared with transitional TCP protocols in the way of congestion windows changing, throughput and the fairness in NS-2 wireless environment. The simulation results show that the algorithm can reduce the influence brought by link error and improve the TCP throughput over wireless network and have the better fairness.