Abstract
寬頻整合服務數位網路的交換方式主要是採用非同步傳輸模式，它主要特性是吸收資料的變化。也就是說，我們可以用可變位元率的方式傳送。因為多個可變位元率的資料源可以共享網路資源，所以大大提高了傳輸效率。但是當多個資料源產生大量資料，會使得網路擁擠，而造成大量資料封包遺失。所以本篇論文主要討論在以非同步傳輸模式為基礎的影像傳輸系統中，資料封包的遺失和保護方法。首先，我們先介紹基本的非同步傳輸網路及資料封包的遺失和保護方法，其中雙層影像編碼是最適合的編碼方式。此方法是將影像資料分成兩部份︰第一部份包含基本的影像資料，人眼所能接受的最低影像品質；第二部份資料只是增加影像品質，若這些資料遺失了，並不會使影像品質變得太差。我們提出一個新的雙層影像編碼系統，並和以前的三種雙層影像編碼系統比較。實驗結果顯示，此方法在遺失率相同的情況下有較低的位元率和較高的信號雜訊比。同時，我們也介紹資料交錯傳送及適應性錯誤隱藏的方法，這些方法也是有效的封包遺失補償方法。有一些服務如高畫質數位電視，它需要影像品質維持在高畫質，這可以藉著限制封包遺失率來達成。我們可以用許可控制選擇一條滿足使用者要求的路徑，所以本篇論文最後建立一個可變位元率模型可用來達成許可控制。因為過去所提出的可變位元率模型大都只包含一個影像序列，或者位元率變化不大的影像，而我們提出的模型可以包含位元率變化較大的範圍。Asynchronous Transfer Mode(ATM) is a switching mechanism whichhas been accepted for Broadband Integrated ServicesDigital Network(BISDN). The principal characteristics of ATMnetworks are that they can absorb bit rate variations so thatlinks are suited to cell traffic generated by variable ratesources and that they can offer high efficiency becausenumerous variable bit-rate sources can share networkresources. But variable bit-rate transmission may lose largepackets when the network is congested This thesis discussespacket loss and its protection in an ATM based videodisstribution system. At first basic configuration ofthe ATM-based video transmission system and its packet lossprotection schemes are discussed. The two-layer codingscheme is the best suitable coding scheme for packet lossprotection in the ATM based video distribution system. Thismethod divides the video data into two part, the firstpart generates the guaranteed packets and the second partgenerates enhancement packets. Image quality won't bedegraded seriously when the enhancement packets are lost .Wepropose a new two-layer coding with adaptive priorityassignment and compare it with other methods. Simulationresults show that it generates less bit-rate and higher PSNRunder the same loss-ratio. Block interleaving andadaptive error concealment is also proposed as aneffective packet loss protection scheme. Some services, forexample, HDTV must keep image quality all the time. Thiscan be implemented by keeping the cell loss probabilitywithin a certain limit. An admission control can do this jobwhen the network path is set up. So we propose a VBR modeland an admission control scheme which can be easilyimplemented.