TCP Fairness In Multihop Wireless Ad Hoc Networks

As wireless networks are becoming increasingly prevalent, the problem of ensuring TCP fairness is an increasingly important issue. In this dissertation, we investigate the problem of TCP fairness in multi-hop wireless Ad Hoc networks. The causes of TCP unfairness are studied and the multi-hop MAC delay is analyzed. We propose a MAC delay based Neighborhood RED scheme (D_NRED) to improve the TCP fairness. Moreover, a dynamic distributed scheme—Adaptive_Pause is proposed.Much traffic in Wireless Ad Hoc networks is very sensitive to delay, and delay is an important metric of fairness. We analyze the average packet delay on IEEE 802.11 DCF under finite load traffic in multi-hop Ad Hoc networks. We employ a Markov chain model to analyze the channel access delay. We model each node using an M/G/1 queue and derive the queueing delay. Thus the average packet delay in single-hop scenario can be obtained. The model is also extended to evaluate the end-to-end packet delay in multi-hop Ad Hoc networks. To validate our analytic results, we have done extensive simulation. The results from analysis and simulation match closely.As it is known, the packet delay is large if the wireless channel is very busy and the overall traffic load exceeds the capacity of the channel. Thus the packet delay can reflect whether or not the channel is busy. In this dissatation, we propose D_NRED scheme to detect channel congestion, tune the dropping probability, and improve TCP fairness. The scheme detects congestion according to MAC delay and notifies the nodes which use too much channel capacity to drop their packets. Thus the expressed node can get a chance to transmit. This scheme includes three parts: how to find out the congestion center correctly and effectively; how to inform its neighbors about the congestion; and how the neighbors to calculate their local packet drop probability. The MAC delay can be measured by modifies the driver instead of the hardware.Therefore, D_NRED scheme has a good practical implement. Simulation results show that D_NRED scheme can improve the TCP fairness.We propose a simple but efficient fairness mechanism: Adaptive_Pause, which is distributed and low cost at implementation. Each node monitors the occupation of the channel due to its emissions and determines whether it should pause an interval in order to avoid channel capture. The characteristic of the scheme is that the nodes which consume too much bandwidth share will actively pause their emission. Comparing to the passive scheme in which the node controlling their transmission when received a message packet from the node at the expressed location, the proposed scheme is more promptly and with less overhead. By using a two dimensional Markov model, the optimal parameters are found. These parameters can be used by the nodes to dynamically determine their fair share. Simulation results validate the correction of our analysis and the scheme's effect in TCP fairness improvement.