【作者】 尤维；

【导师】 康健；

【作者基本信息】 吉林大学 ， 通信与信息系统， 2011， 硕士

【摘要】 传输控制协议(TCP)是一种面向连接的可靠的基于字节流的传输层协议,TCP在网络层上向应用层提供可靠的数据传输服务,TCP协议是目前因特网上主要的传输层协议。在TCP协议中,假设报文段和确认的丢失是由于网络拥塞造成的,这一假设适用于低误码率、高带宽的有线信道。随着计算机网络技术和无线通信技术的飞速发展,人们对无线高速数据业务的要求越来越高,IEEE802.11无线局域网以其方便、灵活和终端移动性的特点得到广泛应用。TCP协议应用于无线局域网时,由于无线信道误码率高、带宽低和频繁移动等特性,使得基于有线网络的TCP协议无法适应链路质量相对较差的无线网络,无线信道中各种衰落和干扰以及终端移动所带来的多普勒频移和越区切换等问题严重影响着TCP协议的性能,为了提高TCP协议在无线网络中的性能,须对TCP协议进行改进。本文在分析无线网络TCP协议改进方案特点的基础上,针对无线信道特点,提出了一种端到端的TCP协议改进方案TCP-C协议。论文主要工作如下：首先,目前大多数研究无线网络TCP协议性能使用的是二径传播模型,这种模型较为理想,不能模拟无线信道因衰落和干扰引起的突发错误。为了更为准确的模拟无线信道,本文在NS-2仿真软件中添加Markov模型,用二态Markov模型来模拟无线衰落信道。其次,本文提出一种合理、简单而有效的端到端的TCP改进协议TCP-C,该方案利用RTT测量方法估计链路的队列长度,从而估计链路的拥塞状况。为了区分拥塞丢包和无线信道高误码率造成的随机丢包,设计动态的门限值N。当检测到无线随机丢包时,改进拥塞控制机制,TCP协议并不减小拥塞窗口和慢启动门限；另外,在拥塞避免阶段,当链路的队列长度大于门限值N时,拥塞窗口在每两个RTT内增加1个报文段,减慢拥塞窗口的增加速率,使TCP连接更长时间地保持在较大的窗口值,从而提高TCP协议的吞吐量。最后,用NS-2仿真软件建立IEEE802.11b无线局域网,仿真TCP-C协议的性能,并与TCP Reno、TCP Newreno、TCP Vegas和、TCP WestwoodNR等版本协议进行比较。仿真结果表明,改进后的TCP-C协议能够根据链路的状况有效地区分拥塞丢包和无线信道高误码率造成的随机丢包,对不同原因造成的丢包采取不同的拥塞控制机制,TCP-C协议吞吐量比TCP Reno、TCP Newreno和TCP Vegas有明显提高。另外,本文对无线局域网中的效果异常现象进行了分析,并提出了同时减小TCP报文段长度和加大竞争窗口的方法来改善效果异常现象。更多还原

【Abstract】 Transmission Control Protocol (TCP) is a connection-oriented reliable transport layer protocol that provides a byte-stream service to its clients. TCP provides reliable data transmission service for application layer on top of unreliable network layer, which is the dominant transport layer protocol. The traditional TCP assume that the loss of segments and ACK is caused by network congestion. This assumption is suitable for low error and wide band wired channel. With the rapid development of computer technology and wireless communication technology, people require more high-speed services. IEEE 802.11 wireless local area network (WLAN) is applied extensively for its convenience, flexibility and mobility of terminal. When TCP is applied in WLAN, the traditional TCP based on wired network can not adapt to the bad channel quality of wireless channel because that wireless channel has higher error rate, lower bandwidth and larger delay. Various fading, interference and mobility of terminals will bring Doppler frequency shift and hand-off problems, which will affect the performance of TCP seriously. The assumption that segment loss is caused by congestion fails.This paper summarizes the form and limitations of improvement scheme in existing wireless TCP. According to the characteristics of wireless channel, we proposed a kind of end-to-end modified TCP protocol which is called TCP-C. The works in this paper are as follows:Firstly, most of researches on TCP performance are based on the two diameter propagation channel model. This model is an ideal model, that is to say, it can not reflect burst error of wireless channel which is caused by fading and crossing talk. In order to simulate wireless channel more correctly, we add Markov model into NS-2 simulation software. We simulate the wireless fading channel with a two order Markov model.Secondly, this paper propose a reasonable, simple and effective end-to-end TCP improvement algorithm TCP-C that estimates queue length with RTT measurement method to forecast congestion degree of links. In order to discriminate congestion loss and wireless random loss, a dynamic threshold N is designed. When wireless random loss is detected, we adopt the improved fast recovery algorithm. TCP does not decrease congestion window and slow start threshold. Moreover, in the stage of congestion avoidance, congestion window will increase one segment in every two RTTs if queue length is larger than threshold N, which can slow down the increasing rate of congestion window and maintain TCP connection around larger window to enhance the throughput of TCP protocol.Finally, we established IEEE802.11b WLAN based on NS-2 simulation software, and then compared simulation performance of TCP-C protocol with that of TCP reno, TCP Nwereno and TCP Vegas. The results show that TCP-C protocol can distinguish congestion lost package and wireless random lost package according to the state of the links, and then take different congestion control mechanism to different causes of lost package. The throughput of TCP-C protocol has significantly improvement than that of TCP reno, TCP Newreno and TCP Vegas. we analyze the performance anomaly phenomenon in WLAN and improve this phenomenon by declining the length of TCP segment and increasing the width of competition window simultaneously.更多还原