近年來，IEEE 802.11e無線區域網路已成為寬頻無線接取盛行技術之一環。無線區域網路之所以普及，是因為具備容易建置與無線用戶端設備多元化等優點。而在無線區域網路的應用上，趨勢顯示在無線網路上使用網路電話等的即時服務的需求將與日俱增。有鑑於愈多人利用無線區域網路做為即時語音通話的傳輸技術，如何提供具有對時間延遲敏感度高的即時語音通話服務使其有良好的通訊品質就成為一個很重要的研究議題。
本研究試圖提出一個加強IEEE 802.11e競爭視窗的機制，來改善在無線區域網路上傳輸即時語音訊務的效能，我們稱這個方法為適應性動態調整競爭視窗機制。本方法使用預設競爭視窗的最小值及最大值(CWmin[AC]及CWmax[AC])、存取點所連線工作站的數量及週期性計算的平均碰撞率等做為參數，結合三個程序「初始選擇CWmin[AC]及CWmax[AC]的處理」、「傳輸發生碰撞後競爭視窗(CW[AC])的處理」及「傳送成功後競爭視窗(CW[AC])的處理」來適應網路的負載情況而動態調整競爭視窗的大小。
本研究使用NCTUns 6.0來模擬與驗證所提出的方法，並與標準的增強分散式通道存取機制就傳送語音服務做效能的評估比較。結果顯示所提出的方法於網路呈現高負載的情況時，效能評估項目均優於標準的增強分散式通道存取機制，傳輸量平均可提升26%，存取延遲時間平均可降低49%，碰撞率平均可降低59%，通道使用率平均可增加26%。故本研究所提出的方法能充份地反應網路的負載現況，減少傳輸碰撞的機會，有效地提升無線網路的傳輸效能。

In recent years, IEEE 802.11e Wireless Local Area Networks (WLANs) have emerged as one of the prevailing technologies for the broadband wireless access. WLANs have become popular due to ease of installation and the increase popularity of wireless clients. WLANs are increasing finding applications that include real time traffic like Voice over IP . The emerging usage of WLANs for real-time applications like voice transmission services calls for intensive research efforts to address the Quality of Services (QoS) issues to cope with the delay-sensitive needs of real-time voice transmission services.
This research seeks to sketch the enhancement of contention window mechanism of IEEE 802.11e to improve the efficiency of real-time voice transmission over WLANs. We propose the mechanism of adaptively dynamic tuning of the contention windows for enhanced voice services over IEEE 802.11e WLANs. The parameters used in our proposed method include CWmin[AC], CWmax[AC], the amount of QoS station with associated access point, and the periodically calculated average collision rate of QoS station during transmission. We propose three mechanisms to dynamically adjust the size of contention window to cope with the traffic loads including the mechanism of initial selection of CWmin[AC] and CWmax[AC], the mechanism of CW[AC] after the collision, and the mechanism of CW[AC] after the success of transmission.
We use NCTUns 6.0 to simulate and validate our proposed method. We compare the performance of voice transmission over WLANs of our proposed method with the standard enhanced distribution channel access (EDCA) mechanism. The results indicate that during high traffic loads, the performance of our proposed method is better than EDCA by 26% higher in throughput, 49% lower in access delay time, 59% lower in collision rate, and 26% higher in channel utilization. We conclude that our proposed method is found to enhance the network performance by increasing the overall system throughput and achieving more acceptable quality in terms of delay time and collision rate.

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