Study On TCP Fairness In Wireless Ad Hoc Network

Wireless ad hoc network is an important subfield of computer communicationnetwork, and is an indispensable part of the Next Generation Network, hence has beenbecome a popular research filed recent years. TCP is the most prevalent transportprotocol over Internet. IEEE802.11MAC can achieve fairness of accessing channelinter-nodes while can provide little fairness inter-flows because the interaction of TCPwith DCF within a node. In this thesis, we conduct studies on TCP fairness in wirelessad hoc network to analyze the fairness mechanism between TCP flows interacted byDCF and to make some advices on improving fairness among TCP flows by modifyingMAC algorithm directed by the features of IEEE802.11MAC and the properties ofwireless ad hoc network.The thesis discusses the reasons of causing TCP fairness, those are, and BinaryExponential Backoff algorithm employed by IEEE802.11MAC, hidden/exposureterminal, and location of node in network. Moreover, we analyze that the congestioncontrol mechanism can further enhance the fairness. We theoretically analyze thefairness arises from the interaction of TCP congestion control algorithm with IEEE82.11MAC protocol, and we validate the analysis by simulation in NS-2. With these,we mathematically model the fairness among TCP flows in wireless ad hoc network, inwhich we model the probability of channel access in single node in a chain topologyfirst, and then we extend the mathematic model to a general topology of wireless ad hocnetwork. We further extend the model to multi-hop network topology to make a generalconclusion. Our analysis results show that the fairness among TCP flows is related tosome factors, i.e., the number of neighbor nodes, the number of hidden nodes, and thenumber of hops, along the TCP routing path. We simulate the mathematic model inNS-2and the simulation results show that the model can describe the fairness amongTCP flows in wireless ad hoc network. We also make some modification on IEEE802.11MAC based on our mathematic model to achieve good fairness among TCPflows in wireless ad hoc network. The modification is set on the backoff window of anode from a random mode to a new setting mode that the backoff window is decided by the number of neighbor nodes with one hop of a node and the number of hidden nodes.We evaluate our proposed algorithm in NS-2by simulation in various typical topologyof wireless ad hoc network, and the simulation results show that the modified MACalgorithm can outperform IEEE MAC protocol.