Lessons learned on reliable transmission of real-time MPEG-2 streams over ATM

Abstract

This paper describes a system that has been designed and built at AT&T Bell Labs for studying transmission of real- time MPEG-2 video over ATM networks for multi-cast applications. The set-up comprises a hardware real-time MPEG-2 video, audio and system encoder, an ATM network adaptation module for MPEG-2 transport over AAL-5, and ATM switch, a software system decoder and a hardware elementary stream decoder. The MPEG-2 transport stream has been characterized in terms of robustness to errors. This preliminary study showed the higher importance of the structural information of the stream (PES packet headers TS headers, sequence, picture headers, etc.) with respect to the coded video data (motion vectors, DCT coefficients, etc.). A brief study of the current MPEG-2 hardware decoding architectures allowed us to better understand the effects of bit-stream errors on the resulting video quality. In our experiments, while the loss of some structural data such as picture start codes led the hardware decoder to loose synchronization or to freeze, the loss of video data only affected the image quality. Furthermore the recovery times from a loss of synchronization were orders of magnitude higher than the recovery from some video data loss. An error-resilient real-time software transport stream decoder has been developed. In multiplex-wide operations (i.e. operations on the entire transport stream) it takes advantage of ring buffers and manages the timing information appropriately. In video-stream specific operations it uses resynchronization mechanisms at the picture level which exploit the redundancy of the PES and transport stream syntax. Furthermore time data transfers between the system decoder and the elementary stream decoder are employed. Experiments show that proper use of these methods can significantly improve the system performance.