支援無線區域網路服務品質之改良型公平排程策略

Abstract

隨著無線區域網路越來越受歡迎，對於支援多媒體和確保服務品質的應用程式需求變得比以前更重要。雖然有很多改進原本IEEE 802.11 MAC的機制被提出來，以支援服務品質，但是他們對於高優先權和低優先權的流量會有頻寬分配不公平的現象。我們提出一個分散式加強型公平排程策略來解決上述問題。基於在後退計數器遞減狀態的快速後退機制和依網路負載情況來動態修改後退區間的機制，我們可以強化加強型公平排程策略的效能。我們經由模擬來評估本加強型公平排程策略的效能。實驗結果顯示我們所提出的加強型公平排程策略比分散式公平排程策略高出13%的效能和比分散式公平排程策略低6%的平均MAC延遲時間，而且兩者有幾乎相等的公平指標。雖然IEEE 802.11e的進階分散式頻道存取機制比加強型公平排程策略和分散式公平排程策略有較好的效能，但是它非常不公平。進階分散式頻道存取機制的免競爭機制是導致高效能、低平均MAC延遲時間和低公平性的主因。我們的加強型公平排程策略非常適合於對頻寬公平分配要求很高的應用程式，如付費應用服務。

As wireless LANs are gaining popularity, the demand for supporting multimedia and QoS-sensitive applications becomes more important than before. Although enhancements to the legacy IEEE 802.11 MAC to support QoS mechanisms have been proposed, they suffer from unfair allocation of bandwidth between high and low priority traffic. We propose a distributed enhanced fair scheduling (EFS) scheme that can conquer the above problem. With a fast backoff mechanism in the backoff timer decrement state and by dynamically adjusting backoff intervals according to the network load, we can enhance the performance of EFS. We evaluate the performance of EFS through simulation. Experimental results show that the proposed EFS has better throughput performance than DFS by 13%, lower average MAC delay than DFS by 6% and the two have nearly equal fairness. Although the enhanced distributed channel access (EDCA) in IEEE 802.11e has better throughput and delay performance than EFS and DFS, it has very poor fairness. The contention free burst (CFB) mechanism in EDCA is the main factor that results in good throughput performance, lower average MAC delay and poor fairness. Our EFS is very suitable for applications that needs strict fair bandwidth allocation, such as pay services.