無線網狀網路下TCP效能提升之研究

Abstract

無線網狀網路(Wireless Mesh Network)因為具有佈建容易跟成本低廉的優點，因此成為近年來最熱門的廣域無線網路接取技術。而無線網狀網路上最為使用者所廣泛使用的傳輸協定就是TCP，然而TCP因為無線網狀網路上節點互相干擾的特性，效能表現並不理想。因此，我們想要研究TCP在無線網狀網路上的行為，並提出方法來提升TCP的效能。 在本篇論文中，我們清楚定義無線網狀網路中著名的隱藏終端問題(hidden terminal problem)與暴露終端問題(exposed terminal problem)，及提出其檢驗方法；並利用模擬的方式具體呈現這兩個問題對於TCP效能的影響。再者，我們依據隱藏終端問題(hidden terminal problem)與暴露終端問題(exposed terminal problem)定義了非傳輸重疊性質與no-HTP子系統，且依此設計了no-HTP頻道指派演算法與Two-Alternative-Link subsystem頻道指派演算法。這兩個演算法可以解決或大幅減輕hidden terminal problem與exposed terminal problem，藉此提升TCP在無線網狀網路上的效能表現。 最後我們藉由模擬實驗呈現我們提出的演算法在各種不同情境中效能的表現。實驗結果證明我們的演算法可以有效地減少節點間互相干擾的情形，提升TCP的效能，而且在不同情境中的效能表現都優於先前的頻道指派方法(TwoHop頻道指派方法)。

Wireless Mesh Networks (WMNs) have become the most popular wireless access technology in wide area networks by taking the advantage of easy deployment and low cost. In wireless mesh networks, most data is transmitted by TCP. However, due to the interference between mesh nodes, performance of TCP degrades severely. Therefore we study the behavior of TCP in WMNs and propose approaches to improve TCP performance. In this paper, we conduct extensive simulations to observe the behavior of TCP in WMNs. First, we clearly define the well-known hidden terminal problem and exposed terminal problem. Then we propose two methods to test these problems and demonstrate the TCP performance degradation caused by these problems. Based on previous analyses, we propose the non-transmission overlapping property and no-HTP subsystem. Moreover, we design the no-HTP channel assignment algorithm and the Two-Alternative-Link subsystem channel assignment algorithm to solve or alleviate hidden terminal problem and exposed terminal problem. With these two problems reduced, the TCP performance in WMNs is substantially improved. We simulate our algorithms in various scenarios to evaluate the performance. The results show that our proposed algorithms greatly alleviate interference between nodes and outperform previous channel assignment algorithm (TowHop channel assignment algorithm) in all scenarios.