description source codes are source codes designed for systems where
source reconstruction based on only a portion of the original data
stream may be required. Multiple
description codes are useful for applications like packet-based networks
(this includes wireline systems like the internet as well as wireless
systems including the majority of the wireless data systems), diversity
communication systems (e.g., communications systems using antenna
diversity), and distributed data storage systems.
For example, in a packet-based communication system, where data is
broken into small segments, each of which is separately sent through
the channel, we would like to be able to reconstruct our signal under
a full range of packet-loss scenarios. Similarly, in diversity channel
systems, where a signal is sent from source to destination over a
collection of channels, we would like to be able to reconstruct some
version of our data string even when one or more of the channels in
our collection fails.
in distributed data storage systems, where data storage is distributed
across a number of hosts, we would like to be able to reconstruct
the stored data even when some subset of the hosts is unavailable.
In each of these applications, a multiple description code could be
designed to allow reconstruction based on the received partial information.
That is, we could design multiple description
codes to allow source reconstruction under any packet-loss scenario
in a packet-based network, under any channel failure condition in
a diversity systems, and under any host availability situation in
a distributed data storage system. (Multi-resolution source
codes, which allow for reconstruction under a subset of these conditions,
may be thought of as a special case of multiple description codes.)
a multiple description code, we would like for the distortion achieved
in a source reconstruction based on any possible subset of packets
to be as close as possible to the lowest possible distortion at the
received rate. Unfortunately, these best possible performances are
not simultaneously achievable, as can be seen immediately from the
multi-resolution source coding example. The goal in the theoretical
research into the mutliple description problem is to completely describe
the space of received rates and distortions achievable by a multiple
description source code. Current results in this area include bounds
on this space, but existing bounds are not yet tight.