ExOR: Opportunistic Multi-Hop Routing for Wireless Networks

Summary

This paper proposes ExOR, an integrated routing and MAC technique that uses cooperative diversity to increase throughput of large unicast transfers in multi-hop wireless networks.

ExOR takes advantage of broadcast transmission to send data to multiple nodes at once. The furthest (to source and closest to destination) node that heard the broadcast will then broadcast it out again, until it reaches the destination. This results in a higher throughput because ExOR makes use of transmissions that reach unexpectedly far or fall short. It also increases network capacity since it needs fewer retransmissions than traditional routing.

Data is sent in batches and is stored by each intermediate node until all the data is sent before deciding which node is closest to destination. ExOR schedules when each node sends its fragments so that only one node sends at a time. Once 90% of the packets have been sent, the remaining 10% get sent using traditional routing.

ExOR was implemented on Roofnet. It was found to outperform traditional routing by a factor of 2. It was also found to have the most performance gains on the longest routes.

Criticism & Questions

I think cooperative diversity routing is a very interesting idea. I think it makes sense intuitively why it would have good performance. I am however concerned about its incompatibility with regular TCP, a hindrance to its use in most practical settings. In addition, when doing the experiment, the authors spend about 10 minutes initially just setting up the nodes and then every 20 minutes, stop the experiment to update their link loss measurements. I'm not sure how practical that is and how it would affect overall throughput.