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In multimedia communications, compressed video streams need to be transmitted over networks that have inconsistent and time-varying bandwidth requirements. To make the best use of available network resources at any time and guarantee a maximum level of perceptual video quality from the end-user’s perspective, a certain flow control mechanism must be introduced into the video communication system (Cote et al., 1998; Wang, 2000). | Compressed Video Communications Abdul Sadka Copyright 2002 John Wiley Sons Ltd ISBNs 0-470-84312-8 Hardback 0-470-84671-2 Electronic 3 Flow Control in Compressed Video Communications Introduction In multimedia communications compressed video streams need to be transmitted over networks that have inconsistent and time-varying bandwidth requirements. To make the best use of available network resources at any time and guarantee a maximum level of perceptual video quality from the end-user s perspective a certain flow control mechanism must be introduced into the video communication system Cote et al. 1998 Wang 2000 . Over-rating the output of a video coder can cause an undesirable traffic explosion and lead to congested networks. On the other hand uncontrolled reduction of the output bit rate of a video coder leads to unnecessary quality degradation and inefficient use of available bandwidth resources. Flow control techniques must then be employed to regulate and control the output bit rates of video sources in the network to achieve the best trade-off between quality and bandwidth utilisation Girod 1993 . One of the main challenges of video communications is to provide a guaranteed quality of service when the network is swamped with excessive delays and information loss rates Kurose 1993 . Network congestion could be avoided by using preventive instead of reactive remedies. Congestion avoidance techniques in video communications must consist of an efficient flow control mechanism that regulates the rates of active video sources Jacobson 1988 . In a bit rate regulation scheme the video source might sometimes be required to decrease its output flow due to high traffic load across the network. This reduction in bit rate could certainly lead to quality degradation since the quantisation distortion becomes more noticeable at lower bit rates. However the quality degradation resulting from a coarser quantisation process is far less detrimental to the video quality than

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