| Summary: | 碩士 === 國立臺灣大學 === 電機工程學研究所 === 88 === As real-time video transmission is becoming common over the Internet, error control is particularly indispensable in the delivery of video streams since transmission errors are unavoidable. Video communication over IP networks involves more issues than that over ATM networks in terms of error control. First, IP networks provide neither prioritized transmission nor guaranteed QOS for data packets. Second, video streams need to be packetized well before being transmitted over IP networks in order to have good error resilience characteristics. Finally, the mitigation of spatial and temporal error propagation in IP networks is very different from that in ATM networks upon the loss of IP packets. In this thesis, we develop an error-resilient video communication system based on the H.323 family of ITU-T recommendations. Error concealment methods are employed at the decoder to limit spatial error propagation with low computational complexity. With a low-delay backward channel for packet receipt reports, bi-directional error resilience mechanisms are employed to avoid error spreading in the temporal domain. Two operation modes, i.e., ACK and NACK, are devised for two communicating parties. Using the mechanism developed, our system can eliminate error propagation after either one frame lag or one round trip delay. Experimental results show that PSNR performance can be maintained to a certain level without consuming much additional bandwidth as compared to simple INTRA updates. Our system can also adaptively choose an operation mode to maintain a satisfactory video quality at the decoder under varying channel error conditions. It is shown by experimental results that our error control scheme is suitable for video transmission over IP-based LANs at packet loss rates up to 20%.
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