S. Katti, H. Rahul, W. Hu, D. Katabi, M. Medard, J. Crowcroft, “XORs in the Air: Practical Wireless Network Coding,” ACM SIGCOMM Conference, (September 2006). [PDF]

Summary

This paper present COPE, a new forwarding architecture for wireless mesh networks that significantly improves the overall throughput by exploiting the shared nature of wireless medium using network coding. All the nodes communicate using broadcast in COPE. Each node stores overheard packets from its neighbors in a buffer pool for a short amount of time and also tells its neighbors what packets it has in its own pool using a bitmap. When a node transmits any packet, it uses the knowledge about its neighbor (if it knows) or intelligently guesses to XOR multiple packets together and transmits them as a single packet. Receiving node in the next hop, decodes the packet by re-XORing it with the packets in its pool. Packets are encoded only if they can be extracted in the next hop.

The authors explain through examples and prove theorems to show Coding gain and Coding+MAC gain (gain due to resource allocation less skewed toward hub-type nodes) of using COPE over traditional options. They have three results:

1. In the absence of opportunistic listening, COPE’s maximum coding gain is 2, and it is achievable;
2. In the absence of opportunistic listening, COPE’s maximum Coding+MAC gain is 2, and it is achievable; and
3. In the presence of opportunistic listening, COPE’s maximum coding gain is unbounded.

There are several implementation related gotchas:

• Packets are never delayed to avoid unnecessary queue development.
• COPE prefers XORing packets of same length and keeps multiple virtual queues per neighbor based one for each packet size!
• It also tries to avoid TCP packet reordering and use hop-by-hop ACKs and retransmissions.
• While guessing whether or not to XOR one more packet to the mix, COPE calculates the probability based on some metric (ETX/ETT), and if the probability goes below a threshold (0.8 in the paper), it does not XOR that packet.
• Moreover, COPE implements a pseudo-broadcast consisting of multiple unicasts to work-around default broadcasts poor reliability and lack of backoff.

COPE was evaluated on a 20 node 802.11a wireless testbed with a bit rate of 6 Mbps. Main results are:

• For toy topologies, TCP flows get lower (no MAC gain) boost than UDP flows.
• For a congested wireless medium, UDP flows gets 3-4x boost in overall throughput and approaches Coding+MAC gain. But with increasing load, UDP decreases and seems to have been stabilizing around 2x range.
• But TCP suffers from hidden-terminal induced collision related backoff and cannot gain much using COPE. Without any hidden terminal, the maximum goodput shows 38% improvement.
• Almost half the coded packets are XOR of only 2 packets and trailing off at a maximum of 5 packets.
• Finally, if COPE based access network is connected to the Internet through a gateway, throughput gain varies from 5% to 70% based on uplink to downlink ratio.

Comments

This is an overall excellent paper that combines theory with practice and backs up the claims with wide range of experimental evaluation. The paper is also very well written and easy to follow. However, there are several issues that could use more explanation. COPE requires as many virtual queues as there are packet sizes (which they say essentially to be 2 for the Internet) for each neighbor within its hearing distance, and will have to look through all of them to find optimal XOR strategy. The paper does not provide any evaluation of how big they grow or how long its takes to optimally pick an XORing strategy.

The authors say that the pseudo-broadcast mechanism will make sure that every node gets more than once chance to overhear. However, they avoid to mention and evaluate that this might also result in a higher number of collisions and link layer retransmits.