  Increasing network utilization is often considered as the holy grail of
communications. In this article, the concept of sub-rate coding and decoding in
the framework of linear network coding (LNC) is discussed for single-source
multiple-sinks finite acyclic networks. Sub-rate coding offers an add-on to
existing LNC. It allows sinks whose max-flow is smaller than the source
message-rate, termed \emph{sub-rate sinks}, to decode a portion of the
transmitted message without degrading the maximum achievable rate of LNC sinks
whose max-flow is equal (or greater) than the rate of the source node. The
article studies theoretical aspects of sub-rate coding by formulating the
conditions a node (and indeed the network) must fulfill so as to qualify as a
legitimate sub-rate sink.
