一個無線區域網路上支援即時影音傳輸的適應性輪詢機制

Abstract

IEEE 802.11e標準主要是為了提供無線區域網路上的服務品質(QoS)。然而它卻依然無法做到即時媒體傳輸的品質保證，同時也會產生不少頻寬的浪費。本篇論文提出一種新的輪詢機制以達到更好的服務品質，像是改進時間顫動(jitter)、減少存取延遲(access delay)，以及提高頻寬的使用率。我們提出一個適應性的輪詢機制，此機制基於不同的工作站需要不同的服務區間，計算出每個工作站更合適的輪詢時間，以避免被輪詢到的工作站沒有訊框可以傳送，而造成浪費。不同的影音編碼方式，也會在這個新的輪詢機制考量範圍內。此外，為了偵測出更合適的輪詢時間以減少存取延遲，我們會在短暫的時間內，使用較短的服務區間來輪詢。同時我們會動態地增減語音傳輸資料流的服務區間，來適應此語音類型資料流的特徵。我們利用NS-2進行模擬實驗，數據結果顯示，我們提出的方法比起慣用的輪詢機制，可以減少更多存取延遲，也能降低時間顫動的標準差，同時提高整個無線頻寬的使用率。

The main goal of IEEE 802.11e standard is to support Quality of Service (QoS) in wireless local area networks (WLANs). However, it makes a lot of unnecessary overheads and is still unable to guarantee QoS for real-time audio/video services. The motivation of this thesis is to provide better QoS, such as reducing jitter and access delay, and to improve the channel utilization. We propose an adaptive polling scheme which minimizes QoS-Null replies by calculating much appropriate polling time for all QoS traffic streams, based on much accurate service intervals for different QoS traffic streams. Also different codecs are taken into consideration in our proposed scheme. Besides, in order to reduce access delay and find a much precise polling time, we reduce polling interval to poll some traffic streams. Further, we dynamically increase or decrease service interval for voice transmission based on its talk-spurt and silence alternation characteristic. The simulation results through NS-2 show that our proposed scheme can reduce access delay and standard deviation of jitter as well as enhance channel throughput comparing with the commonly used round-robin polling scheme