The Analysis And Improvement Of TCP Vegas Performance In MANET

Mobile Ad-hoc Network (MANET) is composed of a group of mobile nodes that dynamicly constitutes distributed autonomous network through wireless links, featuring no central node, multi-hops, self-organization and dynamicly changes in network topology, and it is widely used in military and civilian services. The research of TCP protocol in MANET is now a hot spot. TCP is originally designed for wired networks, but TCP attributes the packet loss to network congestion. However, due to the frequent route changing, the high bit error rate and the channel conflicts in MANET, the reason of packet loss is complex, and it has a significant impact on TCP performance. When TCP congestion control mechanism is simply deployed in MANET it will cause dramaticly decline in TCP performance.This thesis describes the facing problems when TCP applied in MANET and the current related research status, and presents the relevant techniques of MANET and TCP congestion control. It analyzes and compares several typical TCP congestion control algorithms, and makes in-depth analysis to the major factors impacting on the performance of TCP in MANET. The thesis lists several typical schemes for improvements, and then focuses on the research of TCP Vegas congestion control algorithm in MANET. After analyzing the problems when TCP Vegas is applied in MANET, a new algorithm named TCP Vegas-HA (TCP Vegas-Hops Adaptive) has been proposed to improve its performance.TCP Vegas-HA scheme tests and obtains a group of optimal threshold values of different hops and node caches under specific network environment, takes them as the empirical values, and develops a new congestion control algorithm for the common condition of multi TCP streams mixed transmission on this basis. To resolve the problem of pseudo routing failures caused by conflcts in MANET wireless channels and the problem that minimum round-trip time (BaseRTT) cannot be accurately measured in MANET due to changes of route hops, this thesis proposes a corresponding scheme for improvements and a new measurement method based on empirical values. NS2 simulation results show that the scheme of TCP Vegas-HA can increase TCP throughputs effectively in MANET, and has great suitability both to static and dynamic mobile ad hoc networks.