Analyzing And Improving The TCP Under Wireless Multi-path Fading Channel

TCP/IP (Transmission Control Protocol/Internet Protocol) have developed rapidly. Now most Internet services such as WWW, FTP,E-mail and Telnet are based on the TCP. TCP provides a reliable schedule for the transmission, which is based on Congestion Control Mechanism. The Congestion Control Congestion considers the network congestion as the sole reason of data loss, the original TCP is designed for the wired channel, it can do a good performance in the wired environment.Along with the rapid development of wireless communications, particularly the upcoming 3G business, a growing number of mobile terminals have access to internet. In the future, the combination of wireless communications and internet communications is a major tendency. when the traditional TCP is used in the wireless link, the performance is seriously reduced. In the wireless environment, compared with the wired channel, there are more complex features. First, the wireless environment caused the signal interfered each other, such as multi-path fading, the Doppler frequency shift, the shadow effect. It will result in the wireless channel bandwidth dynamicly; In addition, due to the time-varying channel and the different transmission power, there are different characteristics of the channel in the different locations. These are the unique nature of wireless networks. It is due to have the characteristics of wireless channel. The wireless link will lead to TCP inefficiencies and deterioration.Therefore, how to effectively improve the wireless TCP become an important research topic. Most of the research in wireless TCP is in the TwoRay large-scale propagation model.This model could not accurately simulate the propagation of Rayleigh fading process and Ricean fading process. Therefore,this paper provides a good reference for further study the wireless TCP under fading channel.At present, there are three kinds of solutions for the wireless TCP: end-to-end scheme, split connection scheme, link-layer scheme. The end-to-end scheme amends the TCP end directly. The transmitter or receiver can make a distinction between precision Congestion-related packet loss and other forms of wireless environment packet loss. Only when the network congestion occurs, the TCP congestion control process was activated;For other forms of packet loss ,the recovery process is implemented for other mistakes. The split connection scheme terminate TCP in the base station connected to hide wireless link for the wired network. Such programs divided the source and the end into two separate parts by TCP connection. One TCP is connected between the base station and wired networks. The other TCP is connected between the base station and mobile equipment. The first is the traditional TCP connection; The other TCP is the modified TCP for the wireless link. The aim of the link-layer scheme is to correct errors by local retransmission or forward error correction in the link layer.It can hide the packet loss of wireless link to the TCP source.In order to propose a more suitable TCP under the wireless fading channel,this paper first outlines the current status of the development of wireless networks and wireless TCP facing problems. Wireless networks include WLAN and WAN. WLAN include IEEE 802.11, Bluetooth, HomeRF. WAN include GSM,GPRS,WCDMA, CDMA2000, TD-SCDMA, etc. We emphasis on the IEEE 802.11. because we establish a simulation model according to the most widely used IEEE802.11-b.On this basis, we outline the basic principle of TCP, the message format of TCP, TCP connection establishment, Data transmission, TCP termination process,and flow control mechanism, congestion control mechanism,error control mechanism. We focuse on the process of slow start ,congestion avoidance,fast retransmit,and fast recovery in the congestion control mechanism for the follow-up improvements.We analyze the character of the wireless fading channel, large-scale tansmission model and small-scale fading model. Focusing on the wireless Rayleigh fading model and Ricean fading model.We add Ricean model into NS-2 successfully to imitate Ricean fading and Rayleigh fading channels. We analyze the character of the wireless channels by adjusting the parameter K and V.The parameter K stands for the degree of the multi-path multipath fading.when K is equal to 0, it represents the Rayleigh fading.The parameter V represents the maximum speed of the movement of objects in the wireless environment.The speed is not necessarily refered to the receiving terminal. It can be the velocity of any object in the propagation environment.Through the analysis of TCP in the wireless environment,the increased function for wireless TCP,the solutions for wireless TCP,we propose a new bandwidth estimation algorithm. The algorithm is based on end-to-end scheme, it refers to the network Measurement solution and the intelligent strategy of the source.The source can control the congestion window size intelligently.The algorithm only modify the source code.It is modified on the version of Reno.The dynamic characteristics of network bandwidth provide the basis for the source.We adopt different strategies to control network congestion with measuring network state information, thereby improving the performance.We analyze the improved TCP in different simulation scenes. Firstly, setting up the same parameter for the multipath fading channel,we use different TCP such as TCP-reno, TCP-Sack. TCP-Vegas and the improved TCP.The only difference is TCP, the results show that the improved TCP have a better transmission throughput.Secondly, we simulate the propagation delay caused by wired network to analyze the TCP throughput.When the delay is set 150ms,the improved TCP have achieved the maximum throughput. 17.8% throughput in the improved algorithm is improved than TCP-Sack. , 12.5% throughput in the improved algorithm is improved than TCP-Reno.Finally, we analyze the performance of the congestion control mechanism by congestion window and slow start threshold of different TCP.The results show that the improved TCP can dynamically set its slow start threshold and congestion window by the situation of network.Rather than other algorithms,when it happened to timeout or dropped, blindly reduce the threshold to half of the current window.The slow start threshold and congestion window size of the improved TCP is significantly higher than other TCP, it is also why other TCP transmission performance is lower than the improved TCP.