摘要
Ad Hoc网络是一种有别于传统网络的新型网络。由于某些新特点的出现,当传统网络中提供端到端可靠性的TCP协议运行于Ad Hoc网络时,会出现严重的不公平性。公平性是网络正常运行的基本要求,解决不公平性有着重要的意义。本文旨在通过量化的手段对Ad Hoc网络TCP流的公平性问题做比较深入的分析和研究,进而提出改进方案以提升Ad Hoc网络TCP流的公平性。
文章对多跳环境下引起TCP流不公平性的因素做了分析、总结。指出了导致Ad Hoc网络TCP流不公平性的根本因素。在此基础上对TCP流的公平性问题进行了数学分析,建立了描述多跳TCP流公平性的数学模型。模型定量地阐明了导致TCP流不公平性的根本因素对公平性的作用和影响。
在上述工作的基础上,文章提出了改进802.11Ad Hoc网络TCP流公平性的解决方案——F-MAC,该方案通过探测影响TCP流公平性的因素,来控制节点MAC层的退避窗口的变化,以此来提高TCP流的公平性。
文章基于NS2仿真平台,对建立的数学模型的合理性和提出的F-MAC算法的有效性进行了仿真验证。仿真结果表明:(1)文中所建立的基于802.11Ad Hoc网络TCP流公平性的数学模型具有一定的普适性;(2)文中所提出的F-MAC算法能有效地提高802.11Ad Hoc网络TCP流的公平性。
Ad Hoc network is a kind of new network which is different from all other traditional networks.Unfairness will be arised when TCP is running over the Ad Hoc network. Fairness is the basic demand for networks, so it is important to solve the unfairness problem.The TCP flow's unfairness problem was deeply analyzed by quantitive manner, and then a new scheme was proposed to improve the TCP flow's fairness in the paper.
The factors that lead to TCP unfairness were analyzed and concluded, and the basic factors were pointed out. Then a mathematical model was built to analyze TCP fairness quantitively.
A new scheme F-MAC with low overhead cost was proposed to improve TCP fairness. In this scheme, the backoff window at MAC layer was controlled by detecting the basic factors that lead to TCP unfairness ,so that the TCP fairness was improved.
Fianally, the rationality of the model and the validity of the F-MAC was validated by simulation under NS2. The result shows that:(l) the mathematical model is universal;(2)F-MAC can improve the TCP fairness remarkably in 802.11 based Ad Hoc networks.
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