多重跳躍與多重通道下之無線網路產出效能研究

Abstract

多重跳躍下IEEE 802.11無線網路的產出效能跟傳送速率及從起點到終點的路徑長度有著相當高的關係。也就是說，路徑越長，產出的效能越低。不同的路徑長度會有各自不同的最佳傳送速率，而這個傳送速率產生的結果，也就是產出效能在這個路徑長度裡是最好的。但是如果傳送的速率稍微超過這個最佳值，會造成嚴重的效能損失。所以如果我們知道路徑的長度，我們就可能知道最佳的傳送速率。另外IEEE 802.11的標準支援多個不互相干擾的通道，而資料可以在這些通道下同時傳輸出去。這個多通道的特性可以改善多重跳躍的產出效能。 在本篇碩士論文裡，我們經由模擬找出在多重跳躍多重通道下的無線網路產出效能，尤其是在不同的通道給定方法以及在不同的網路拓撲之下的產出效能。我們討論了不同路徑長度下的通道使用率，接著分析通道使用率的上界及下界，然後提出了一個簡單的流量控制的機制來改善在鍊狀及隨機網路拓樸下的產出效能及傳送率。我們使用ns-2這個軟體來驗證我們的分析，模擬結果也證明了我們所提出的流量控制機制明顯地增加了產出效能以及改善了封包的傳輸率。

The throughput in multi-hop IEEE 802.11 wireless network is highly dependent on the sending rate and the route length from the source to the destination. The longer the route length is, the fewer the throughput is. Also there exists a sending rate which achieves the optimal throughput at each given route length. Moreover, to increase the sending rate slightly over the optimal value will decrease throughput significantly. This means that if we know the route length, we may know the optimal sending rate results the optimal throughput. In the IEEE 802.11 standard, there are multiple non-interfering channels which could be used to transmit packets simultaneously. This multi-channel nature of IEEE 802.11 surely could help to improve the throughput performance. In this thesis, we show how the throughput performance is in the multi-hop multi-channel wireless network by simulations, especially with different channel assignments and in different network topologies. We discuss the channel utilization in node chains and then propose a simple flow control mechanism to improve the throughput and also the packet delivery ratio in both node chains and random topologies. The simulations results by the network simulator 2 (ns-2) verify our analysis of channel utilization of node chains and also show improvements of flow control mechanisms.