Layering of Communication Networks and a Forward-Backward Duality
  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. 
Michael Cyran, Birgit Schotsch, Johannes B. Huber, Robert F.H.   Fischer, Vahid Forutan
cs.IT math.IT