Research On TCP Protocol Optimization Based On Cross-layer Design In Wireless Ad Hoc Networks

Wireless ad hoc network is a multi-hop network system which consists of a set of mobile or static terminals with both sending and receiving functions. It doesn’t need any support of fixed infrastructure and is easy to be deployed. Meanwhile, it fits the communication requirements of users about flexibility and mobility, and thus it has been widely studied and developed.As a classical transport protocol for providing reliable service, transmission control protocol (TCP) is also applied in wireless ad hoc networks. However, severe contention collisions, wireless random errors and frequent route changes are seriously affecting the TCP performance and TCP-based applications. Therefore, focusing on the TCP performance optimization over wireless ad hoc networks has a strong practical significance. Although many researches have devoted in this area, the TCP in the wireless ad hoc networks still faces many challenges and is worthy of further research and thinking.TCP adapts to the changes in the actual networks mainly through controlling its sending window and retransmission time-out (RTO). In this paper, from these two perspectives, we develop two corresponding performance optimization strategies by using cross-layer design ideas.Firstly, we design a novel window adaptation mechanism based on capacity usage indication through cross-layer feedback. In IEEE802.11-based wireless ad hoc networks, contentions and interference always cause segment losses before queue overflows and cut down the contribution of buffer to the capacity. As a result, conventional TCP may be easily over-injection and lead to poor throughput. In this paper, we first analyze the relationship between the offered traffic and the obtained throughput. From the analysis, we investigate the negative impact of contention on the optimal throughput obtainment, and obtain the synchronous relationship between the channel occupancy and capacity consumption. Then we reveal the hidden terminal problem is the main manifestation of contention. The presence of hidden terminal offers the early sign of the network overload. However, the TCP may continue to increase the window due to the hidden function of link-layer mechanism, thus leading to the overshooting problem. Based on the above analysis, we consider two effective metric to indicate the connection capacity usage and further propose a novel window adaptation mechanism, which can make full use of network resource and meanwhile avoid the window overshooting from bringing the performance degradation.Secondly, we design a novel TCP timing mechanism based on contention detection through cross-layer feedback. In wireless ad hoc networks, the round-trip time (RTT) fluctuates seriously due to dynamic network environment. And the values of RTT samples may not be able to represent the actual network status, thus leading to the inaccuracy of traditional RTO estimation which is only based on the values of RTT samples and each sample is given same weight. In this paper, we first investigate the RTT fluctuations and its impact on RTO estimation through simulations. Then, we analyze the main causes of RTT fluctuations and reveal the positive correlation between network contention level and RTT fluctuations. Based on the above analysis, we propose a TCP timing mechanism based on the variance of contention delay. By comparing the RTT fluctuations with the contention level, the mechanism gives different weight to each RTT sample, thus updating RTO and TCP sending state more reasonably.