無線網路（WLAN）不若有線網路是在實體線路上做傳輸，它是以空氣當介質，利用廣播方式作傳輸，所以在傳送上容易遭遇網路壅塞（congestion）或雜訊干擾（interference）而導致封包遺失需要重送，對一般資料而言影響不大，但對一些延遲敏感的影音串流服務，卻會導致影像傳輸品質下降，所以我們研究視訊編碼的特性，在不需要更動任何接收端的情況下，利用跨層（Cross Layer）的方式，在MAC層和應用層設計出一個速率與錯誤結合控制器（Error and Rate Joint Controller），希望在無線網路中能讓影像傳輸得以改善。我們提出NCE（Network Condition Estimator）模組，是用來區別無線網路中的錯誤，是由壅塞所造成或者是雜訊干擾造成。藉由上層跨到底層所得到的影像資訊，和NCE分析出的結果，PSRC（Packet-Selective Rate Controller）和ATEC（Adaptive TXOP Error Controller）控制器，能讓AP作出適當的調整以達到視訊傳輸的改善。除此之外，AP能夠，傳送目前無線網路的狀況給上層，讓影音伺服器能夠因應目前的網路情況而作出最適當的調整，如：使用FEC（Forward Error Correction）保護重要的資料，藉由調整影音編碼的量化參數（Quantization），或採用ULP（Unequal Loss Protection）的方法降低資料傳送率以減緩壅塞的發生。此外，為了讓影像即時到達，將這些上層方法整合成一個即時傳輸的RT-ERJC（Real-Time Transport Error and Rate Joint Controller）控制器；最後，將上下兩部份的方法整合起來；由實驗結果顯示，提出的方法比其它跨層設計方法都要好，在各種網路狀況下都能得到較佳的視訊品質。

The transmission of WLAN (Wireless Local Area Network) could easily suffer from network congestion or noise interference, which in turn leads to packet loss and the subsequent need of retransmission; moreover, these phenomena could easily lead to the decrease in video transmission quality of some delay-sensitive multimedia applications; therefore, how to improve the video transmission in WLAN thus becomes a very important issue. In this article, a MAC-centric and Application-centric cross layer design is proposed to improve video transmission over WLAN using error and rate joint control. The NCE (Network Condition Estimator) is designed to distinguish network errors caused by congestion or interference. Based on the video information provided from application layer and the analysis result from NCE, the PSRC (Packet-Selective Rate Controller) and ATEC (Adaptive TXOP Error Controller) are proposed to let AP adjust properly in order to achieve the improvement of video transmission. In addition, we also let AP to feedback some of the current status of wireless network, so that video streaming server will respond to the current situation and make the most appropriate adjustment, such as: the use of FEC (Forward Error Correction) to protect the important data, and to adjust the quantitative value of video or use ULP (Unequal Loss Protection) approach in order to reduce the data sending rate in order to reduce the occurrence of congestion. Additionally, in order to let the video real-time reach to receiver, we integrated these upper layer approaches into a real-time transmission controller, named RT-ERJC. Finally, we also integrated the both side approaches. And simulation results reveal that our cross layer design is capable of reacting to the varying channel condition with less and smoother quality degradation than several distinct cross layer schemes.

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