Study On TCP Over MAC Protocol In Wireless Ad Hoc Network

With the rapid development of computing technology and wireless communication,wireless network has been spread over our routine work and daily life by its application.Wireless ad hoc network is a type of wireless network and is the important part of theNext Generation Network. TCP is the most important transport protocol in Internet andis the core of the network stack. However, wireless ad hoc network is deperformed byits MAC protocol which employs IEEE802.11standard as its protocol and conducts apoor efficiency on TCP taking on a bad performance compared with wired network.In this thesis, we will take careful studies on the interaction of IEEE802.11MACprotocol and TCP layer, and thereby make some useful modification on the MAC toenhance TCP performance in wireless ad hoc network.The thesis analyzes the related details of IEEE802.11MAC protocol andconcludes that the Binary Exponential Backoff algorithm decreases the throughput ofTCP severely, and in wireless ad hoc network the hidden/exposure terminal can causeunfair between TCP flows hence result in very low throughput in some flows. Moreover,the thesis analyzes the congestion control mechanism of TCP and point out thatcongestion control mechanism interacted by IEEE802.11MAC protocol can furtherdeperform the TCP on the throughput in wireless ad hoc network. Based on the analysis,the thesis makes some modifications on the IEEE802.11MAC protocol and TCPcongestion control mechanism to enhance the TCP performance in wireless ad hocnetwork. First, we design a new algorithm to estimate the active node in the network toincrease the throughput of the MAC layer, and then we design an optimizing algorithmfor initial contention window in MAC layer by decreasing the maximum backoff value,which is coupled with the estimating active node algorithm. We conduct simulations toevaluate the evaluate the algorithm performance in NS-2simulator, and the simulationresults show that the modified MAC algorithm can outperform the IEEE802.11MACprotocol both on estimating the active node and on enhancing the TCP throughput, andthus increases the TCP performance in wireless ad hoc network. Second, we modify thecongestion control mechanism of TCP based on cross-layer method, which is independent from the network stack layers by reading/writing message or passingnetwork information, to improve the efficiency of the node in network. We also modifythe routing breakage detection method in TCP when we improve the TCP congestioncontrol algorithm. We simulate the modified algorithms in NS-2network simulator, andthe simulation results show that the modified TCP congestion control algorithmoutperforms TCP NewReno on throughput by affected with the modified MACalgorithm, moreover, it decreases the MAC delay in the node.