IEEE 802.11 分散式協調功能非飽和環境下之競爭視窗大小最佳化及效能分析

Abstract

在本論文中，我們以近幾十年來廣泛採用IEEE 802.11在無線區域網路在媒介存取控制層中主要的分散式協調功能為基礎，並討論在分散式協調功能中採取的載波感測多重存取和碰撞避免中倒退時間機制的競爭視窗大小最佳化。而現行中已有研究討論此競爭區間大小的問題，但是多為在飽和情況之下，本論文則是以趨於現實網路的非飽和環境中做討論。 我們利用簡化後的一維馬可夫鏈作非飽和的模型分析，加入新的空閒狀態做機率分析，並導出在效能最佳化下的傳送機率，最後利用等式得到的最佳競爭視窗大小視窗來做模擬分析，可以得到在簡化分析為基礎下非飽和環境的效能確實比飽和環境下更好。

In this thesis, we discuss the issue of optimizing the contention window size in the Backoff time scheme under the CSMA/CA mechanism of Distributed Coordination Function (DCF) in Medium Access Control layer of IEEE 802.11 Wireless LANs. Although some prior research discussing the contention window size optimization problem could be found in the literature, most of them assumed the underlying network to operate under the saturated environment. This thesis addresses the issue for networks under the unsaturated environment, which comes more naturally in the real world. We use a simplified one-dimension Markov Chain model to analyze the unsaturated model. Following a detailed transmission probability analysis, our result shows that the estimated contention window size under the unsaturated model is better than that in the saturated model.