Recently

the problem of packet loss and errors in image communication over erasure channels

such as Internet and wireless networks

becomes an important topic. This creates the need for new approaches to image coding

which combine high compression efficiency and robustness. As a new approach

multiple description coding (MDC) is attractive in this field

which separates the source into several equally important bit streams(descriptions) and then transmitted over different channels. Based on many related literatures

an overview on the state of the art of MDC is given in this paper. Firstly

MDC's history and background in information theory are introduced. And then MDC is classified into several categories

including quantizion based

transform based

space expansion based and so on. Each is described respectively from the mathematic model and its applied conditions. Comparisons between MDSQ and MDPCT are made followed. At the end

some conclusions are given

including problems in MDC and what will MDC be in the future.