A large fraction of the information that flows across today's networks is useful even in a degraded condition. Examples include still images, speech, audio, and video. When this information is subject to packet losses or retransmission is impossible due to real-time constraints, superior performance with respect to total transmitted rate, distortion, and delay may sometimes be achieved by adding redundancy to the bit stream rather than repeating lost packets.
In multiple description coding, a joint source-channel method, the data is broken into several streams with some redundancy among the streams. When all the streams are received, one can guarantee low distortion at the expense of having a slightly higher bit rate than a system designed purely for compression. On the other hand, when only some of the streams are received, the quality of the reconstruction degrades gracefully, which is very unlikely to happen with a system designed purely for compression.
We will motivate the use of multiple descriptions and review some of its information theoretical beginnings. We will then discuss two practical approaches: one quantizer based and the other transform based. Finally, we will outline possible applications.