Studies On Congestion Control Of TCP In Wireless Network

With the rapid development of network technology and wireless communication inrecent two decades, wireless network has been spread into the daily work and life, fromwhich can be foreseeable that people would claim more for the network performance asthe mobile wireless network expends its way on development. Wireless ad hoc networkis a type of wireless network and is the important part of the Next Generation Network.TCP is the most important transport protocol over the Internet and became theindispensable part of network stack, in which its congestion control algorithm is thecore of the TCP protocol. TCP was initially designed for wired network thereforecannot work normally if it is transplanted into wireless network because of the differentpropagation media. So many packets are flowed into wireless network exceeding itsprocessing ability if the TCP sender sends packets indicated by the congestion controlalgorithm in wired network, which results in congestion, and therefore make the spatialsaturation. The congestion control in wireless ad hoc network has already been anattractive research field in network, and there still are many difficulties in this researchfield. Therefore, we conduct studies on TCP congestion control in IEEE802.11MACbased wireless ad hoc network, and thus we make some modifications on TCPcongestion control algorithm to enhance TCP performance in wireless ad hoc network.In this thesis, we discuss the reason causing TCP congestion in wireless ad hocnetwork, especially focuses on four aspects, which are wireless transport error, thehidden/exposure terminal in wireless network, asymmetry of wireless link and networkrestructuring. We also investigate the increasing pattern of congestion window in TCP,and simulation results show that the throughput is decreased as the hop increased alongthe TCP routing path because of MAC contention and spatial reusing. Based on this, wediscuss the state transforming of TCP when congestion occurs, and we further discussthe difference of computing BDP between wireless ad hoc network and wired network.Based on the discussion, we find that congestion in wireless ad hoc network arises fromthat packets flowed into network exceeds the BDP upper threshold. We design a newalgorithm to review the BDP upper threshold, and based on the algorithm, we modify the TCP congestion control algorithm by tuning its congestion window. We conductsimulation in NS-2to evaluate the proposed congestion control algorithm in wireless adhoc network, and the simulation results show that, in various typical wireless ad hocnetwork topology, the modified TCP congestion control algorithm can outperformtraditional TCP by decreasing the times number of zero setting when timeout occurs,and therefore smooth the sending rate of the TCP sender. The proposed TCP congestioncontrol algorithm can also decrease the end-to-end delay and thereby wirelessbandwidth utilization.