近年來，由於無線網路技術的蓬勃發展，加上多媒體應用快速普及與資料編解碼技術成熟，使得無線網路結合影音多媒體以提供各式多樣化服務已是通訊技術中一項重要的必然趨勢。但由於無線通道的傳送品質具有明顯的不可靠性，使得影音多媒體服務在無線網路上的傳送充滿挑戰性，而使用過小或過大的封包長度均會降低頻寬使用率。此外，由於視訊編碼的特性而造成不同型態之視訊封包具不同重要性，因此必須分別加以保護以提升多媒體接收品質。 為解決上述的問題，本論文之研究共分兩部分：第一部分提出在無線網路環境下，多個使用者使用視訊服務的封包最佳傳輸機制。其依據無線網路的通道狀況、檔頭額外負荷、輪詢排程延遲、傳輸延遲與使用人數等因素，利用數學分析而推導出最佳封包長度的封閉解，以有效提升頻寬使用率；第二部分提出一套具優先權考量的共享式自動重送機制(S-ARQ)，以保護不同型態與重要性之視訊封包。S-ARQ則依據無線環境的通道現況，利用ARQ權杖分享的觀念，動態而有彈性地分配重送次數給不同重要性之視訊封包，以達到較佳的接收品質。模擬結果顯示，在IPv4/IPv6 IEEE 802.11 b/a/g/e等網路的各種不同錯誤狀況中，利用本論文所決定之最佳封包長度進行資料的封裝時，均使網路端頻寬使用量達到最大。以此最佳封包長度為基礎，再配合使用S-ARQ機制，將可有效提升視訊接收品質1至2.5 dB。 Video applications over wireless networks exhibit great potential due to the tremendous progress of wireless network development and video compression techniques. However, the variant wireless channel conditions significantly increase the challenge of providing effective packetization and unequal error protection. Basically, the packetization work is a tradeoff between the header overhead and the packet error rate for determining an efficient packet size. On the other hand, the property that video packets with different types in the encoding process have different significance should be considered to provide an effective unequal error protection. This thesis proposes an optimal transmission strategy for delivering video data over WLAN environment in the case of multi-users. The proposed framework includes two major parts: an optimal packet size determination method and a shared ARQ mechanism. The optimal packet size is obtained based on the current error situation of WLAN, header overhead, scheduling delay, transmission delay, and the number of users, for achieving the maximum bandwidth utilization. A mathematic closed form of the optimal packet size is derived. The shared ARQ mechanism introduces a share concept for the retransmission opportunities of different video frames and dynamically allocates these retransmission opportunities based on their importance. Simulation results show that using the proposed mechanisms can achieve the maximum bandwidth utilization in various WLAN environments with different IP implementations, including IPv4/IPv6, over IEEE 802.11b/a/g/e networks. Moreover, integrating with the optimal transmission strategy, the proposed shared ARQ mechanism can improve the received PSNR up to 2.5dB.
