A Transmission Control Framework For Continuous Media

Terry M. Talley, Ph.D. 1997

Technical Report TR97-02
University of North Carolina at Chapel Hill
Department of Computer Science
Chapel Hill, NC
May, 1997


Desktop video conferencing allows people to simulate face-to-face conversations by integrating real-time two-way audio and video with the computer system. Unfortunately, the quality of video conferences carried over current networks such as the Internet is often inadequate for effective communication. Network congestion can cause video conferences to experience high latencies and poor fidelity. We claim that in many cases we can sustain low-latency, high-fidelity conferences over current networks even when the networks are highly congested if we carefully manage the transmission of the audio and video streams at the endpoints of the conference.

Network congestion is caused by two distinct types of network constraints: capacity constraints and access constraints. Capacity constraints limit the bit rate that can be supported by the network. Access constraints limit the message rate that can be supported by the network. We claim conferences can heuristically identify the type of network constraint causing congestion and reduce the effects of the congestion by carefully selecting the bit and message rates associated with each of the conference media streams. We explain and empirically demonstrate why addressing capacity and access constraints requires two complementary transmission adaptations: scaling and packaging. Scaling increases or decreases the bit rate associated with a media stream by controlling the generation and compression of the media stream. Packaging increases or decreases the conference message rate by controlling the type and number of media data units, or frames, placed into each message. We describe a transmission control framework that shows how scaling and packaging can be used to preserve conference quality when the network has capacity constraints, access constraints, or a combination of capacity and access constraints. We develop a transmission control algorithm based on this framework and demonstrate that the algorithm can deliver low-latency, high-fidelity conferences even on heavily congested networks. We also show that the algorithm delivers conferences with lower latency and higher fidelity than those delivered by non-adaptive transmission algorithms or by algorithms that only scale the video bit rate.

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Last revised Tue May 5 19:55:08 EDT 1998 by jeffay at cs.unc.edu.