Error Resilient Two-Layer H.263-based Codecs for ATM Environments T he introduction of new multi-media services and the enhancement of the existing services forced the existing networks to their performance limits. Most of the networks today face the problem of bandwidth limitation. ATM networks solve the problem of bandwidth bottleneck by the introduction of statistical multiplexing and priority switching. Many video coders/decoders (codecs) have been designed to suit this new technology [1–6]. Some of these codecs are designed to increase the resilience of the codec to cell loss as well as the sharing ability of the assigned bandwidth by implementing various layering techniques. Other multi-media services designers prefer to implement cell concealment techniques at the decoder side to combat cell loss [7]. This paper presents new two-layer error resilient video codecs based on the classical TWIN-H.261 and the Inter-Enhance H.261 video codecs [2–4]. A study of the performance of the proposed codecs is included in this paper with a comparison with the other classical two-layer codecs in terms of the reconstructed image quality, the resilience to cell loss, the bit rate produced and the codec complexity. # 2001 Academic Press T. Nikolaidis, M. Hasan and F. Marvasti Department of Electronic Engineering, King’s College London, University of London, U.K. E-mail: mohd.hasan@kcl.ac.uk Introduction The increased demand on the available bandwidth—due to the tremendous advances in the development of the existing multimedia services as well as the introduction of new services—has led to the development of new data communication techniques which have introduced new concepts in the world of data switching and multi- plexing. Asynchronous Transfer Mode (ATM) Net- works came to solve many of the problems encountered during the design and development of the multimedia services in which most of these problems are due to the bandwidth limitations of the existing networks. ATM networks are capable of transporting data and time- sensitive traffic such as video and audio on the same channel at very high bit rates by making efficient use of the available bandwidth and implementing very ad- vanced switching and multiplexing techniques. One of the main drawbacks of the ATM technology is the degradation of the service quality when traffic congestion occurs. This is due to the cell discarding algorithms implemented in the Segmentation and Re- assembly (SAR) sublayer of the ATM Adaptation Layers (AAL). Cells are discarded due to input, central or output buffer overflow or in case of excessive delays for time- sensitive services. The video bitstream packetization standard ISO/IEC 13818-1 has provisions for two-layer techniques. Layer- ing is a simple form of Joint Source-Channel Coding and a lot of work has been carried out on this field [2–4] with encouraging results. The organization of the paper is as follows: In the next section, we present a background of Layered Coders/Decoders (Codecs). To follow, we present some 1077-2014/01/060275+11 $35.00/0 # 2001 Academic Press Real-Time Imaging 7, 275–285 (2001) doi:10.1006/rtim.2000.0227, available online at http://www.idealibrary.com on