Wednesday, February 21, 2007

MAC-Layer Anycasting in Ad Hoc Networks.

MAC-Layer Anycasting in Ad Hoc Networks Romit Roy Choudhury and Nitin Vaidya. ACM SIGMCOMM Computer Communications Review, Vol. 34, number 1, Jan. 2004.

This is a paper I should have read a long time ago about multi-hop wireless mesh networks. It is deceiptively simple: the idea is to create a set of equivalent paths from the source to the destination using the route table information (the paper assumes a table-driven routing protocol) using a hop-count metric. If the metric is such tat no routes are strictly equivalent, routes could also be ordered and the set created from the list of routes satisfying a given criteria, say within some factor of the best path according to that metric.

Anyhow, once the route set is created at the network layer, the packet is given to the MAC layer with a list of potential candidates. The MAC layer then chooses which candidate to use based on the information available at the MAC layer. So if there are two possible candidates to send the packets to, the MAC chooses the one with the best condition (for example, availability, if one has sent a CTS and the other has not; or link rate and link conditions).

So the packet's route is not set a priori, but it is evolving as the packet moves along. The packet is not duplicated, there is still only one copy forwarded at each hop, but it makes use of the diversity available at each hop at this point in time.

A few notes: the benefit is not evaluated at all in this paper, which is a huge surprise to me. It is typically a requirement from any reviewer I have encountered. I am sure there is a subsequent performance evaluation somewhere, I just haven't looked for it.

But it would actually be interesting to see the performance improvement. On a short path, there would not be any. And the performance of long path is degraded by the fact that the multiple path are long. It might improve on a basic routing protocol, but it would improve on poor performance in the first place. Anyhow, I should look for that performance evaluation follow-up which must be somewhere.

The paper discusses using reactive protocols, such as DSR, but the solution is actually rather convoluted: instead of collecting a route, the RREQ-RREP exchange must collect a whole tree routed at the destination (for the uplink) so that each node can know the place of its neighbor in the relaying structure toward the destination. So that means that each RREQ-RREP must be broadcast more than strictly necessary, and extra-flooding is usually viewed as a bad thing.

Anyhow, this is really light reading. It is only the description of the idea, with no analysis nor performance evaluation, so it is a quick and easy and interesting read.

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