Local Phy + Global Routing: A Fundamental Layering Principle for Wireless Networks

Engineering design of wireless networks typically involves "layering": reliable communication over the wireless medium -- the physical (phy) layer -- is used to create bit pipes between the nodes, over which the data is routed. The optimality of such an approach, as well as the fuzziness in where to draw the boundary between physical and network layers, is often unclear. In this talk we derive a fundamental, information-theoretic, layering principle: "local" physical layer schemes combined with global routing are near optimal for general multiple unicast traffic. We show this in a broad context: networks with both wireless and wireline components (eg: cellular networks) and under different channel models: packet erasures and additive Gaussian noise. As part of this design principle we derive rules of thumb for where to draw the line between physical and network layers. Feedback is seen to play a critical role in enabling the separation between physical and network layers. The key technical contribution is an approximate max flow, min cut theorem in polymatroidal wireline networks.