IEEE 802.11通訊標準的鏈路速率, 競爭視窗,及頻道參數交互作用對效能影響之研究與系統實作

Abstract

在多重速率與多通道的IEEE 802.11無線網路中，鏈路速率、競爭視窗和頻道參數間的交互作用對系統效能的改進扮演很重要的角色。為了利用頻道的多樣性達成平行通訊，之前的研究提出一個能夠支援目前普遍使用的IEEE 802.11無線網路的通道跳頻機制(CHS)，而也設計相對應的通道路由協定(CDR)使節點在多跳階的情況下能夠做更有效率的傳輸。此外，在如此多樣性的通道下，每個通道也各自擁有不同的特性，而在利用不同速率傳輸時，值得注意的是為了能成功成解碼收到的封包，我們所需要訊號源與雜訊的比值(SINR)的門檻值也會不同。一旦無法成功解碼封包，802.11 Distributed coordination function (DCF) 與速率調整機制會同時作用大大抑制傳輸的意願；當成功解碼封包時，802.11 DCF 會將競爭視窗設為最小值並同時調升傳輸速率鼓勵通訊的進行。因此，將競爭視窗與傳輸速率的調整機制分開考量會對802.11系統效能造成損害，所以先前的研究中，提出了一套同時考慮競爭視窗與傳輸速率的適應性速率調整機制(EARC)。而在本論文中，我們使用一套開放原始碼的路由器軟體在真實多重速率、多通道的無線網路環境中實現我們上述所提到的EARC、CHS和CDR協定。我們在室內無線網路環境下實際操作並以吞吐量來評估我們系統，而實驗結果顯示，我們結合EARC、CHS和CDR所實作出的無線網路系統會優於其他基於 IEEE 802.11 b/g多重速率、多通道無線網路系統，也證實多通道、多重速率路由實際的無線環境下能改善系統效能。

In IEEE 802.11 multi-rate wireless networks with multiple orthogonal (non-overlapping) channels available, interoperability of link rate, contention window and channel parameters plays an important role in terms of system capacity. In order to achieve better spatial diversity, a multi-radio channel-hopping scheme (CHS) has been devised to utilize multiple orthogonal channels available in widespread IEEE 802.11-based wireless systems. A corresponding channel-diverse routing (CDR) protocol has been proposed to realize efficient multi-hop communications. Furthermore, radio channels possess varying transmission characteristics. When the channel condition is good, we tend to encourage the transmission by increasing the link rate while setting a smaller contention window, and vice versa. Link rate is associated with a certain required Signal-to-Interference-and-Noise Ratio (SINR) threshold for successfully decoding received packets. On transmission failure, both rate reduction and 802.11 DCF binary exponential backoff represent double penalties for this wireless link. On the other hand, once transmission succeeds, 802.11 DCF resets the backoff contention window to the minimum value. At the same time, traditional link adaptation may also decide to increase the data rate. We observe that the separate consideration of the link rate and backoff mechanism harms the 802.11 system performance. Thus, a new mechanism which jointly considers link rate and contention window together entitled Enhanced Adaptation of link Rate and Contention window, abbreviated as EARC, has been proposed. In this thesis, we further implement the aforementioned EARC, CHS and CDR mechanisms in a real multi-rate, multi-hop wireless networking testbed based on an open-source router software. Through extensive indoor experiments, we evaluate the operational performance in terms of data throughput. Experimental results show that the combination of EARC, CHS and CDR outperforms other strategies in static IEEE 802.11b/g multi-rate, multi-hop wireless environments.