在IEEE 802.11e實驗平台上的強化視訊串流

Abstract

近幾年來，隨著有線和無線網路的普及，多樣化的資訊在網路上傳輸。除了傳統的資料傳輸之外，一些多媒體應用也深受大家喜愛，例如視訊會議、即時影像串流等。另一方面，為了因應多媒體資訊傳輸的需求，目前相當熱門的IEEE 802.11無線網路標準也提出能提供多媒體服務品質（Quality of Service, QoS）保證的延伸標準－－802.11e。在這篇論文中，我們實做出一個在802.11e架構下的跨網路階層（cross-layer）設計，來提高H.264/AVC影像標準的影片在無線網路中的傳輸品質。我們結合了跨層動態影像優先權分配（Cross-layer Adaptive Video Prioritization，CAVP）和軟性服務品質 （SoftQoS）。其中跨層動態影像優先權分配提供了應用層的動態影像畫格優先權分配（Video Frame Prioritization，VFP）和媒體存取層的動態優先權分配（MAC-layer Adaptive Prioritization，MAP）。在應用層決定封包的重要性及優先權後，接著在媒體存取層預測每個存取種類（Access Category，AC）的延遲時間，並決定封包實際歸屬的存取種類。軟性服務品質則是藉由對競爭窗口（Contention Window，CW）的控制，達到提高及時性和高重要性封包的傳輸成功率。在真實平台上的測量結果顯示，我們提出的架構的確能提高無線網路傳輸後的動態影像的品質，尤其在網路較為擁塞的環境下。

In recent years, wireless communication are rapidly developed. IEEE 802.11 based WLAN is one of the most popular networks. More users use wireless network to access multimedia contents from internet. That is to say, the traffic in the air include traditional data packets and multimedia packets. Therefore, an extension of IEEE 802.11 standard called 802.11e is developed to provide QoS support for different types of traffic. In this thesis paper, we implement a cross-layer architecture to enhance the QoS transmission of H.264/AVC video stream in IEEE 802.11e wireless environment. We integrate Cross-layer Adaptive Video Prioritization (CAVP) and SoftQoS. CAVP provides Application layer Video Frame Prioritization (VFP), which prioritizes packet according to PSNR influence level, and MAC-layer Adaptive Prioritization (MAP), which estimates the delay time of each access category (AC) and chooses the better one. SoftQoS, on the other hand, controls contention window (CW) size to improve the successful transmission rate of important information. We also shows the results of the experiments on real testbed. Our cross-layer architecture has better performance than the works before, especially when the channel is congested.