Model Analysis And Protocol Design Of TCP Congestion Control In Wireless Networks

Multihop wireless networks are distributed systems that consist of wireless terminals that canfreely and dynamically self-organized. The nodes in the network expand the range of wirelesscommunication through a multihop way to establish a long distance connection. After that, thenetwork has a certain flexibility. To enable congestion control in multihop wireless networks,Transport Control Protocol (TCP) congestion control scheme seems to be the natural choice. TCP wasoriginally designed for Wide Area Network (WAN). It is difficult for TCP to adapt to the lowbandwidth, frequent topology changes, and contention between nodes which access the channel inmultihop wireless networks. The performance of TCP significantly degrades in such networks. Withthe rapid application in battlefield, disaster rescue and vehicular communication, TCP congestioncontrol has become the bottleneck of global multihop wireless communication. Thus, it is one of themost important things to work with TCP and improve its performance in multihop wireless network.The main target of our work is designing a scheme to perform well in multihop wireless networkthrough further modeling congestion control algorithm and proposing a cross-layer solution, theinnovations points of our work are summarized as follows.(1) Based on the analyzing of the difficulty of saturated throughput model of TCP Reno inwireless networks, we have found that prior work on TCP Reno steady state throughput modelsignored fast recovery state, thus leading to the deviation of calculating congestion window. Moreover,prior models did not consider of the slow start state of TCP timeout behavior. Therefore, we furtherdevelop a model which introduces a characterization of fast recovery algorithm for the saturatedthroughput of TCP Reno based on its loss indications and calculate congestion window values as afunction of loss rate and round trip time. Finally, extensive measurements demonstrate that our modelis able to estimate congestion window effectively and predict the saturated throughput of TCP Reno.(2) According to the detailed analysis of the problems when traditional TCP congestion controlmechanism encounters in multihop wireless networks, we find that medium contention at the mediunaccess control (MAC) layer: hidden terminals and exposed terminals cause server medium contentionthus leading to starvation problems; moreover, intra-flow and inter-flow interferences trigger datasegments congestion. Besides, overshooting congestion window in transport layer is also the mainreason why TCP performance degradates. We propose a new cross layer solution called TCP based oncollision ratio protocol. Based on this mechanism, nodes can get link layer condition by piggybacking extra field to declare the channel’s collision ratio in the head of RTS/CTS frames and feed backcollision ratio to its source node periodically through its chain at MAC layer. Thus source node canadjust its congestion window from the feedback information. Simulation results show that ourproposed protocol can effectively adjust the amount of outstanding data based on the level of currentmedium contention and significantly improve the throughput of the network.