最近幾年，高速無線區域網路被廣泛架設及應用在機場，車站，飯店和街角。未來對於無線區域網路所支援之多媒體的應用更為重要及被重視。差異性服務及輸出量之提升在多媒體無線網路的領域是為主要所關注的問題。在這一篇論文裏，我們延伸了原有IEEE 802.11e的分散式存取模式(Distribution Coordination Function, DCF)，針對多媒體視訊及語音服務，在有干擾及錯誤發生(error-prone)的無線環境下提供差異性服務及輸出量之提升。我們提出two-stage backoff counter來減少backoff counter被凍結(freeze)的負荷(overhead)。此外，我們提出封包傳送前的丟棄策略(drop policy)以減少視訊及語音服務在無線環境不良時做不必要的重傳封包之動作。我們利用二維(two-dimensional) markov chain來分析two-stage backoff演算法之效能。除此之外，我們透過ns-2電腦模擬來評估我們所提出的演算法之網路效能。相較於原來的802.11e，我們所提出的方法在多媒體的輸出量、延遲時間及差異性服務上可以達到更好的效能。

In recent years, high-speed wirelesses LANs are widely deployed in airports, stations, hotels and street corners. Support multimedia applications over wireless LANs will becomes an important popular in the future. Service differentiation and throughput enhancement are major concerns in multimedia wireless LANs. In this thesis, we extend IEEE 802.11e Distributed Coordinated Function (DCF) to further provide service differentiation and improve throughput for live video and audio over error-prone wireless LAN. We propose an adaptive two-stage backoff counter to reduce the overheads in blocking backoff counter. Furthermore, we explore the proactive frame drop policy to eliminate unnecessary retransmissions of video and audio frames under bad channel conditions. To investigate the performance of the proposed schemes, we develop an analytic model for two-stage backoff counter based on two-dimensional markov chain. Simultaneously, we run intensive computer simulation using ns-2. The results demonstrate the superior of proposed schemes in terms of system throughput and service differentiation.

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http://www.tkn.tu-berlin.de/research/802.11e_ns2/