IEEE TRANSACTIONS ON BROADCASTING, VOL. 45, NO. 1, MARCH 1999 zyxwvutsrq 129 zyx An Efficient Protocol Architecture for Error-Resilient MPEG-2 Video Communications over ATM Networks Pedro Cuenca, Antonio Garrido, Francisco Quiles and Luis Orozco-Barbosa Abstract-MPEG-2 video communications over ATM networks is one of the most active research areas in the field of computer communications. In the transmission process of a variable bit rate video signal over an ATM network, cells are inevitably exposed to delays, errors and losses due to the statistical multiplexing used in these networks. These phenomena affect the quality of the video signal and without adequate measures to control the propagation of the impairments the quality of the service may fall below acceptable levels. In this paper, we study the impact of cell losses on the quality of a MPEG-2 video sequence encoded in a variable bit rate mode. We introduce a set of control mechanisms at different levels of the protocol architecture to be used in MPEG-2- based video communications systems using ATM networks as their underlying transmission mechanism. Our results (using different video sequences) show the effectiveness to improve the video quality by using a structured set of control mechanisms to overcome for the loss of cells carrying VBR MPEG-2 video streams. We argue that in order to be able to create video systems able to cope with cell losses encountered in computer communications systems, a structured set of error-resilient protocol mechanisms is needed. Index zyxwvutsrqponm Terms- MPEG-2 Video, Video Quality, ATM Networks, Cell Losses, Error Resilient Mechanisms. I. INTRODUCTION Recent developments on the areas of video coding and compression techniques are enabling the deployment of computer-based video communications systems. However, one of the main challenges to overcome remains the design and deployment of protocol architectures able to cope with the stringent requirements of video communications. In order to prove effective a video communication system will not only require the use of powerful processors and high-speed networks, but the use of resource control and error recovery mechanisms able to properly manage the system resources and cope with system errors. The design of an integrated protocol architecture regrouping the different control mechanisms has to take into account the characteristics of the various system elements: from the application down to the transmission mechanism. A clear understanding of the characteristics and functions of the different layers is essential for the design of effective integrated protocol architectures. Pedro Cuenca. Antonio Garrido and Francisco Quiles are with the Departamento de Infordtica, Universidad de Castilla-La Mancha. 02071, Albacete, SPAIN (e-maiklpcuenca. antonio, paco] @info-ah.uclm.es) Luis Orozco-Barbosa is with School of Information Technology and Engineering, University of Ottawa, 161 Louis Paster St, Ottawa, Ontario K1N 6N5, CANADA (e-mail : Ibarhosa@uottawa.ca) Publisher Item Identifier: S 0018-9316(99)02628- I In this work, we describe a set of protocol mechanisms to be used in the design and development of MPEG-2 video communications system using ATM networks as the underlying communications support (figure 1). PHYSICAL w Enor zyxw Concerlmrnl Video De~odiiie HierarLhical I I I I Figure 1. Protocol archtecture. We organize this paper by introducing one by one the control mechanisms following the path from the encoder to the decoder via the ATM network. In section 11, we start by briefly describing the characteristics of the MPEG-2 video coding scheme. A trace of a video stream is given to illustrate the statistical characteristics of a typical MPEG-2 video stream. In section 111, we present an analysis of the perceptual impact of cell losses over the quality of the MPEG-2 video sequences. In section IV, we introduce an zyxwv Adaptive Hierarchical Coding Scheme. This scheme is fully compatible with the MPEG-2 specifications [14]. In section V, we propose a novel Hierarchical Packing Scheme for the transmission of MPEG-2 video streams over ATM. Other packing schemes have been proposed in the literature [lo], but none of them considers the use of different priority levels for the transport of ATM cells. In section VI, we explore the effect of the MPEG-2 VBR video traffic on the performance of an ATM network model in terms of cell loss rate and cell loss distributions. In particular, we explore how the temporal properties of MPEG-2 VBR video traffic affect network performance when many traffic streams are multiplexed onto a single channel. The use of Cell Discarding Mechanisms is also considered to manage the buffer occupancy of the switch in order to increase the error-resilient of the MPEG-2 video communications system. Finally, in 0018-9316/99$10.00 zyxwvut 0 1999 IEEE