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Due to the expansion and diversity of multimedia applications and the underlying networking platforms with their associated communication protocols, there has been a growing need for inter-network communications and media gateways. Eventually, these applications will encounter compatibility problems. Not only will asymmetric networks run different set of communication protocols, but they will also operate various kinds of incompatible source coding algorithms that are characterised by different target bit rates and compression techniques. . | Compressed Video Communications Abdul Sadka Copyright 2002 John Wiley Sons Ltd ISBNs 0-470-84312-8 Hardback 0-470-84671-2 Electronic 6 Video Transcoding for Inter-network Communications S. Dogan A. H. Sadka 6.1 Introduction Due to the expansion and diversity of multimedia applications and the underlying networking platforms with their associated communication protocols there has been a growing need for inter-network communications and media gateways. Eventually these applications will encounter compatibility problems. Not only will asymmetric networks run different set of communication protocols but they will also operate various kinds of incompatible source coding algorithms that are characterised by different target bit rates and compression techniques. Therefore the interoperability of these source coders necessitates the presence of a control unit which acts as a media traffic gateway lying on the borders of the underlying networking platforms. This chapter is dedicated to the investigation of various methods which achieve the interoperability of compressed video streams while taking into consideration the application-driven constraints and the varying network conditions. The video transcoding algorithms are examined and analysed and their performances are evaluated using both subjective and objective methods. 6.2 What is Transcoding Video transcoding comprises the necessary operations for the conversion of a compressed video stream from one syntax to another one for inter-network communications. Thus the tool that makes use of this algorithm to perform the necessary conversions is called a video transcoder. The original idea behind video transcoding was the scaleability of video coding techniques Ghanbari 1989 Radha and Chen 1999 . These techniques comprise a 216 VIDEO TRANSCODING FOR INTER-NETWORK COMMUNICATIONS layered video encoder structure that provides different layers of compressed video with each layer coded at a different bit rate. Scaleability allows