Ad Hoc Network Congestion Control Performance Analysis And Optimization Research

Ad Hoc self-organizing network is formed by a group of independent wireless nodes cooperating with each other,which does not need fixed infrastructure and adopts distributed management network.It is a self-creation,self-organization and self-management network.The traditional TCP congestion control mechanism is designed for wired network,which considers network congestion as the only cause of packet loss or timeout.However,packet loss in Ad Hoc network may be caused by random error code,route reconstruction and connection disconnect,link layer competition failure and other reasons.At present,there is no uniform and stable scheme for congestion control in Ad Hoc network.This thesis studies the congestion control scheme in Ad Hoc network.Firstly,we compared the congestion control mechanism under the traditional wired scene,including Tahoe,Reno,New Reno,SACK,Vegas between wired scene and the Ad Hoc scene based on the NS2.Simulation results show that the performance of traditional TCP algorithm in Ad Hoc network decreases significantly,so it is necessary to design congestion control scheme in Ad Hoc network specially.Secondly,the main problems faced by congestion control in Ad Hoc network are simulated,including hidden nodes and exposed nodes,high bit error rate,and high mobility.These problems are the fundamental differences between Ad Hoc network and traditional wired network,and also the starting point of the designing of Ad Hoc congestion mechanism.Then,the existing congestion control scheme of Ad Hoc network is introduced and evaluated including the improved scheme based on link layer,the improved scheme based on network layer,the improved scheme based on transport layer and the cross-layer network scheme.Two of the end-to-end improvement schemes based on transport layer,TCP Westwood and ADTCP,are studied in more detail.Finally,ADTCP was improved in this thesis based on the simulation of the main problems faced by congestion control in Ad Hoc network and the assessment of the existing congestion control scheme of Ad Hoc network.This thesis optimizes the calculation method of IDD and STT and proposes to suppress the cupidity of congestion windows to reduce the volatility in wireless networks.The ACK merging mechanism is proposed to reduce the competition of reverse data streams.Simulation based on NS2 shows that the improved scheme can significantly improve the congestion control performance of Ad Hoc network.The improved scheme has higher throughput than the unimproved scheme in an Ad Hoc environment with bit error rate,strong mobility and multi-tcp stream competition.Under the competitive conditions of bit error rate of 0.05,mobile rate of 5m/s and 15 TCP streams,the throughput of the original ADTCP scheme was improved by 19.2%,14.9% and 27.3%,respectively.