視訊封包封裝與調適性自動重送視訊封包封裝與調適性自動重送

摘要

隨著視訊資料編碼和網路技術的日益精進，在IEEE 802.11無線區域網路上資料的種類與特性已逐漸從過去的非即時性數據資料演進至現階段的視訊多媒體服務，然而，由於IEEE 802.11無線網路易受干擾的特性和無法提供服務品質保証的現況，使得多媒體服務品質遭受嚴重的影響。

在本論文中，同時提出一套可依照視訊資料重要性而彈性封裝的視訊封包封裝機制，以及一套具有優先權考量的調適性自動重送機制來解決目前的問題。在編碼端，具優先權考量的視訊封裝機制可依據視訊封包的不同重要性，彈性地調整封包封裝的最大允許長度，使較重要的視訊封包可以因封包封裝長度的適度縮小，而在較高錯誤率的無線通道下不容易發生漏失。具有優先權考量的調適性自動重送機制則可依據無線通道的現況和視訊封包的重要性，動態地調整可重送次數，以同時達較好的通道使用效率和視訊品質。模擬實驗結果顯示，在五種不同錯誤環境以及四種不同視訊來源的各種交叉組合下，調適性自動重送機制和具優先權考量的視訊封裝機制均能有效地保護較重要的封包，減少資料漏失率並且符合視訊服務的延遲限制。在平均錯誤位元長度10位元和位元錯誤率為3×10^-4至5×10^-4的環境中，本機制可比靜態自動重送和優先權自動重送多出平均3dB的視訊品質。

 

Video Packetization and Adaptive ARQ for Video Transmission over Wireless LAN

Abstract

With the technology advances in digital compression and wireless networks, real-time streaming of video content over IEEE 802.11 wireless networks is a multimedia application full of potentials. However, unlike wired packet switched networks, IEEE 802.11 wireless networks are relatively unreliable due to an error prone physical channel and the lack of QoS guarantee. Therefore, the video quality transmitting over wireless networks will possibly be degraded.

In the thesis, we propose a priority video packetization mechanism and an adaptive ARQ scheme according to video packet priorities to reduce the degradation in wireless networks. At the video encoder, the priority video packetization mechanism sets the video packet size base on the video types. Namely, the packet size is reduced for important video frames to reduce packet losses when transmitting over severely interfered wireless channels. The adaptive ARQ in the IEEE 802.11 point coordinator adapts the repeat limits depend on the wireless channel condition and video packet priorities to achieve both high channel utilization and good video quality. Simulation results show that with five error patterns and four video sequences, the proposed mechanism indeed achieves reduction of lost video packets and protection of important video packets within the delay constraint. The proposed mechanisms can improve the PSNR by 3dB on average under wireless channels with a mean burst error length 10 bits at the range of bit error rate from 3×10^-4 to 5×10^-4.