While every network node only relays messages in a traditional communication system, the recent network coding (NC) paradigm proposes to implement simple in-network processing with packet combinations in the nodes. NC extends the concept of "encoding" a message beyond source coding (for compression) and channel coding (for protection against errors and losses). It has been shown to increase network throughput compared to traditional networks implementation, to reduce delay and to provide robustness to transmission errors and network dynamics. These features are so appealing for multimedia applications that they have spurred a large research effort towards the development of multimedia-specific NC techniques. This paper reviews the recent work in NC for multimedia applications and focuses on the techniques that fill the gap between NC theory and practical applications. It outlines the benefits of NC and presents the open challenges in this area. The paper initially focuses on multimedia-specific aspects of network coding, in particular delay, in-network error control, and media-specific error control. These aspects permit to handle varying network conditions as well as client heterogeneity, which are critical to the design and deployment of multimedia systems. After introducing these general concepts, the paper reviews in detail two applications that lend themselves naturally to NC via the cooperation and broadcast models, namely peer-to-peer multimedia streaming and wireless networking. I. INTRODUCTION Network coding (NC) is one of the recent breakthroughs in communications research. It has first been proposed in [1] and potentially impacts all areas of communications and networking. In a nutshell, the conventional networking paradigm is based on routing, where packets are forwarded towards the destination by each network node. NC is based on the idea that intermediate network nodes process packets in addition to relaying them. Processing typically involves generating and sending new output packets as linear combinations of the incoming ones. It can be done at different levels of the classical protocol stack (transport/application, MAC/IP, physical layer, and joint processing involving multiple layers). This new paradigm entails a radical change in the design of communication networks and applications. Multimedia delivery techniques heavily rely on the transport and network layers of the communication protocol stack. Multimedia communications have been optimized to fully exploit the traditional protocols and maximize the quality of service. With the new NC paradigm in networks with source, path and peer diversity, the traditional coding and communication methods have to be rethought in terms of the way packets are created, combined and delivered. In particular, multimedia applications could clearly benefit from the improvements offered by NC in terms of augmented throughput, reduced vulnerability to packet erasures and ease of deployment in large scale distributed systems. The NC concept has been initially devised in the information theory community. The gap between the information theoretic models and the models of real multimedia applications is fairly large. This has generated a lot of interest in developing NC schemes that yield significant benefits in multimedia applications, especially in peer-to-peer (P2P) streaming and wireless communication systems. In particular, multimedia applications pose unique and important challenges depending on the application scenario, such as large amounts of data, stringent timing constraints, heterogeneity in the packet importance as well as in the client set. Therefore, practical NC algorithms have to be specifically designed to cope with these challenges under several additional constraints from realistic settings. First, the decoding complexity has to stay reasonable for realtime decoding. Second, the algorithms need to be scalable and robust to handle the dynamics of the sources, the intermediate nodes and the paths in a large network infrastructure. Finally, the NC solutions must be adaptive to accommodate various timing constraints in delivering multimedia content, as these are among the most fundamental issues to be addressed in a realistic multimedia system. All these issues call for an appropriate design of the distributed communication protocol and effective coding strategies. Similarly to the traditional multimedia communication framework, the most promising solutions in NC systems certainly rely on end-to-end system optimization based on a joint design of the source and network code, where timing or complexity constraints are added to the data communication problem. The objective of this paper is to review recent work in NC-based multimedia communication and to provide a comprehensive survey of the problems that have been fully or partially solved, along with the open research questions. The paper guides the readers through these problems in the following way. In Section II, we briefly review the theory of NC, we summarize the main theoretical results, and we put them in the context of practical multimedia communications applications. In Section III, we review the specific NC techniques that have been developed to tackle the issues mentioned above. Our analysis covers in-network error control, as well as media-specific error control. The former refers to NC techniques that have the objective of maximizing network performance from a "multimedia" point of view, e.g. minimizing delay. Media-specific error control introduces distortion into the NC setting, employing network-oriented source coders based on layered and multiple description coding. Low-complexity NC techniques are also considered. Sections IV and V describe two specific application domains of NC principles, namely P2P and wireless networks. These are important because they naturally lend themselves to NC due the cooperation and broadcast models. In particular, Section IV is concerned with the use of NC for P2P media streaming. It describes in detail how NC can radically change the way P2P streaming systems are designed, leveraging a coded representation that facilitates content distribution and streaming. It further includes a discussion of NC for both live and on-demand streaming. Section V is dedicated to the applications of NC in wireless networks, considering various topologies,
With the recent advances in network coding research, the potentials of network coding have developed in the framework of P2P and overlay data delivery networks REF .
Network Coding Meets Multimedia: a Review
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