Methods for matching compressed video to ATM networks

Abstract

Over the last few years three technologies have reached the stage of maturation where then can become synergistic. These are wideband, high speed networking, high quality video compression (MPEG-I and 11), and high capacity affordable digital storage media.This paper addresses the interaction of these three technologies. In particular, it examines the problem of taking a coTapressed video data stream that is stored on a server, and h n s m i t t i n g it over an ATM channel which has a capacity smaller than that required by the data stream. The conventional approach to this problem would be to transcode by decoding the video data, and then re-encoding so as to meet the channel constraints. Currently this is not a cost effective solution since, while MPEG decoders are relatively inexpensive, encoders are not.Our approach to this problem is to partially decompress the video bitstream. Then, perform the transcoding in the quantized data domain. Finally, a valid bitstream is reassembled and transmitted. This approach has the advantage providing nearly identical quality as the traditional transcoding approach, at a fraction of the hardware cost. 1. MATCHING PREVIOUSLY COMPRESSED VIDEO TO A TRANSMISSION CHANNEL CHANNEL MATCHINO ENCODER YIDEO MEDIA Figure 1: Overview of Channel Matching ProcessAn interesting application that is starting to be examined is the delivery of stored compressed video over varying transmission media. In examining this application, a potential problem becomes apparent quite quickly; namely: what if the transmission media has a lower rate capacity than the compressed video requires? The traditional approach to this problem would be to decode the video, and then recompress to the lower bitrate ( Figure 1). Unfortunately, MPEG is a highly asymmetric codec. The encoder requires dramatically mort processing power then the decoder. So much so, that this approach is not an economically viable solution. Another approach would be to use a scalable encoder where the bitstream consists of multiple layers. The base layer will provide a minimal quality version of the program, while the enhancement layer(s) provide addition quality. This approach has the disadvantage of providing only a small number of possible transmission rates. An application where it is desirable to have finer control over the transmission rate than is achievable by layered codec is variable bitrate video transport over ATM (Asynchronous Transport Mode) networks. Since these type of networks do not have infinite capacity, a congestion control policy is agreed to when a connection is set up. An example of one such policy is leaky bucket control. In this control policy, the Idrammitter agrees to a peak transmission rate and a burst interval. The burst interval specifies the maximum time that the transmitter can stay a t the peak transmission rate.Reibman and Ifaskell [6] have shown the constraints an encoder must meet to satisfy such a control policy and give examples of suboptimal rate control algorithms that satisfy those constraints. Thi...

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