在 IEEE 802.16(d) 網狀網路上利用網路編碼技術提升一對一資料流的傳輸效能

Abstract

網路編碼技術利用對網路封包作加密及解密的動作來提升網路傳輸的效能。而網路編碼技術可以簡略分成兩個方向：intra-session及inter-session coding，在這篇論文裡，我們將重點放在inter-session coding，由於inter-session coding是針對不同的一對一資料流來做編碼，所以比起intra-session coding的困難度相對提高。 這篇論文與之前網路編碼的理論分析研究大不相同，我們提出一個容易實作及佈建的機會式網路編碼架構並建構在IEEE 802.16(d) 網狀網路上。不同於其他研究者提出的實作架構，我們的系統只需要路由層及媒體存取控制層的相關資訊即可發現編碼機會，而不需要額外的網路協定。由於這項優勢，我們的系統可以輕易的架構在真實世界的網路上而不需大幅的修改。 除此之外，我們指出了一項新的“延伸的隱藏終端點問題”，這個問題在使用網路編碼的網路中會很容易發生，然而，在目前常用的無線網路標準裡無法避免這個問題，所以我們提出一個頻寬保留的機制去避免此問題的發生，而這個機制是由原本IEEE 802.16(d) 網狀模式的三方交握機制去延伸而得。我們的模擬結果展現了在應用層效能上網路編碼技術比起原本的路由機制大幅提昇，而且我們提出的頻寬保留機制可以顯著地減少延伸的隱藏終端點問題的發生及封包碰撞的次數。

Network coding is a packet-encoding-decoding mechanism that aims to increase the data transmission efficiency of a network. The development of network coding can be roughly classified into two categories: intra-session and inter-session coding. In this thesis, we focus on the inter-session network coding because it is more challenging to improve the performances of multiple unicast flows. Unlike other theoretical studies, we propose an easy-to-implement and easy-to-deploy network coding scheme that is based on the opportunistic approach over IEEE 802.16(d) mesh networks. As compared with previous opportunistic coding scheme, our proposed scheme need not employ an additional protocol to find coding opportunities or coding structures. Instead, only the routing and MAC-layer information are needed in our coding scheme. Using this advantage, it is easier to be realized in a real-life network as compared with the previously-proposed opportunistic coding schemes. In addition, in this thesis we point out a new “extended hidden terminal problem,” which can frequently occur in a wireless network using network coding. We will explain why the EHT problem can not be solved by currently-existing wireless network standards and propose a bandwidth reservation mechanism that is extended from the IEEE 802.16(d) mesh-mode three-way handshake procedure to prevent the EHT problem from occurring. Our simulation results show that the throughput performance of a network using our network coding scheme is better than that of a network using the traditional routing. Furthermore, our extended bandwidth reservation mechanism can reduce the EHT problem and decrease the collision occurrences largely.