XORs in The Air: Practical Wireless Network Coding

Summary

This paper proposes COPE, a new architecture for wireless mesh networks. It uses packet coding in order to reduce the number of required transmissions. Like ExOR, it takes advantage of the broadcast nature of the wireless channel.

The packet coding algorithm makes sure to never delay a packet in order to code a packet. It gives preference to XORing packets of similar length. It will never XOR packets headed to the same nexthop. It tries to limit reordering packets from the same flow. Finally, it ensures that each neighbor who gets a coded packet has a high probability of being able to decode it.

They use "coding gain" as the performance metric for COPE. Coding game is the ratio of the number of transmissions required by the current non-coding approach, to the minimum number of transmissions used b COPE to deliver the same set of packets.

COPE was implemented on a 20-node wireless testbed over 3 topologies. It was found to have about a 1.33 coding gain. The cross topology over TCP was found to be a little lower than expected which they attributed to header overhead, imperfect overhearing and an asymmetry in throughputs of the 4 flows.

Finally, they mention fairness and claim that when packets are coded, increasing fairness will increase the overall throughput of the network.

Criticism & Questions

I thought this paper was interesting, definitely another interesting way to make use of the broadcast nature of wireless. I'm not sure if COPE could be used together with ExOR, maybe if ExOR were modified to accommodate multiple flows at once. But, if not, I would really like to see a side-by-side comparison of the two to determine which has greater performance.