  In layered communication networks there are only connections between
intermediate nodes in adjacent layers. Applying network coding to such networks
provides a number of benefits in theory as well as in practice. We propose a
"layering procedure" to transform an arbitrary network into a layered
structure. Furthermore, we derive a "forward-backward duality" for linear
network codes, which can be seen as an analogon to the "uplink-downlink
duality" in MIMO communication systems.
