The importance of being overheard: throughput gains in wireless mesh networks

A flurry of recent work has focused on the performance gains that may be achieved by leveraging the broadcast nature of the wireless channel. In particular, researchers have observed that nodes other than the intended recipient of a packet may overhear the transmission in certain settings. Systems have been proposed to leverage this so-called overhearing phenomena by opportunistically adjusting forwarding paths, suppressing similar transmissions, and superimposing packet transmissions using network coding. The effectiveness of such approaches in practice depends greatly on the empirical overhearing rate, which is a function not only of the particular network and its environment, but also upon individual nodes' transmission rates. Most existing opportunistic routing systems use a single, fixed bitrate throughout the network, leaving open significant opportunity for increased performance. We present modrate, a mechanism to jointly optimize rate selection and overhearing opportunities to maximize overall network throughput. We implement modrate in ExOR, an integrated routing and MAC protocol that leverages overhearing to improve bulk-data transfers, and compare its performance in a 48-node wireless mesh network testbed to ExOR, MORE, and traditional routing. While modrate increases the number of profitable overhearing instances in the network, we discover that ExOR extracts far less value from overhearing than might be expected. Instead, the majority of ExOR's performance improvement in many instances is due to its bulk-acknowledgment scheme.