Summary
ExOR is a protocol for multi-hop wireless networks where data is sent and opportunistically heard by intermediate nodes. The routing decisions are made based on which nodes happen to hear the packet rather than a preconceived route.
Specifically, the sender creates a prioritized list of intermediate receivers which is sent with the data. The prioritization is done by computing the ETX metric from each node to the destination. A schedule is setup such that each node in the prioritized list has a chance to broadcast its received data. All nodes listen to broadcasts and if they overhear some packets being sent they avoid re-sending those packets. In the end most of the packets (~90%) reach the final destination. The remaining 10% can be sent by traditional routing methods.
The authors evaluate the system using the RoofNet deployment of 38 nodes distributed over six square kilometers in Cambridge, Massachusetts. They show that they achieve two to four times the throughput of traditional TCP.
Thoughts
Overall this is a cleaver way to use the broadcast nature of wireless communication to improve throughput. It would be interesting to look at forms of forward error correction to encode the 10% of the data that is lost in the process.
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If you take a peek at slide deck from class today, you'll see a design for system that combines XOR and ExOR--essentially using coding gains from combining packets to do a better job of delivering batches to destinations. Check it out.
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