Analysis And Improvement Strategy Of TCP Performance For Ad Hoc Networks

As a particular network of wireless networks, Ad Hoc network has a good prospect of application because of its advantages of quick and easy networking, simple communication protocols, without any infrastructure, survivability capability, distributed topology and less energy consumption. The available study results show that TCP protocol, which was widely used in wired networks, can not be efficiently used in wireless networks. The reason is that packet loss is the only indication of congestion in TCP congestion control of wired networks, while in wireless network, besides the network congestion, the high error rate, interference, channel competition and the routing break due to mobility of the wireless environment can also cause packet loss. In Ad Hoc networks TCP can not distinguish these specific reasons for packet loss and deals with it as congestion, resulting in the performance degrade dramatically.Abundant Ad Hoc network scenes are modeled and simulated on the OPNET simulation platform, and varieties of throughput, stability and fairness of TCP in all kind of simulation scenes are inspected and analyzed intensively in this thesis. The main reasons that affect the performance of Ad Hoc networks is that inequality of channel competition caused by IEEE 802.11 DCF used in MAC layer, hidden terminal, exposed terminal problems, route handoff and route break caused by node mobility. The performance of stability, fairness and throughput of TCP becomes unsatisfactory duo to these effect factors of lower layers and the inadaptability of mechanism of TCP itself.Then, a novel algorithm, named TCP-D(Dynamic), which based on extensive experimental results and available algorithms and adopts new delay parameters dynamically under the premise of limiting CWND and adjusts the real time length of MAC competition window according to the queue length of MAC layer, is proposed. This new algorithm can improve the throughput, stability and fairness of TCP simultaneously by amending parameters. Finally, aiming at the problem that TCP performance declined after rebuilding route because of node mobility in mobile Ad Hoc networks, TCP-DM(Dynamic Mobile) strategy that modifies the maximum CWND and certain delay parameters of TCP dynamically based on the route unilateral hops that are rebuilt every time is presented. The simulation results demonstrate that the proposed algorithms can improve the performances of stability, fairness and throughput of TCP well in Ad Hoc networks.