>> Yep. My current interest is working on frequency diversity -- >> something that Benjamin Henrion has shown can dramatically improve >> the throughput in a mesh network. It's far from trivial, either from >> a theoretical or implementation point of view, which makes it >> particularly interesting for me.
> My solution to this was rather drastic - switch to the 5.8ghz band, use > 802.11n - which has way more channels available than 2.4 - and move to > a country with a non-existent regulatory regime for this spectrum. I'm > hoping your solution is more elegant. Well, the two are pretty much orthogonal. I'm working on using channel information in route selection, while yours is about increasing the number of channels. One doesn't prevent the other. There are basically two topologies that I'm considering: - a network with transit at one frequency and host access at another one (think 2.4 GHz edge routers with a 5 GHz backbone); - a mesh composed of multi-radio routers (think putting three radios tuned on non-interfering channels in all of your mesh routers). Normal Babel (ETX) will make sub-optimal decisions in either case, since it ignores frequency information. While it is possible to work around that with manual filtering and/or biasing of metrics, it should be possible to make this sort of decisions automatically. There's a fair amount of published litterature on this subject (my desk is currently littered with papers speaking about diversity routing), but I find a lot of it highly suspect. First, all of the results I'm seeing are either derived analytically or come from simulations, while radio interference is something that we don't understand well, and I don't trust the simulations. Second, every paper introduces a different metric, often with little or no justification, and they only ever compare their diversity-enabled protocols to basic ETX or ETT -- I'm not finding any comparisons between different diversity metrics. So the first theoretical task is to define a diversity metric that is sufficiently easy to compute, and understand how it relates to the metrics in the litterature. I think I've finished that -- I have 3 metrics that I know how to implement cheaply, and I understand how they relate both to each other and to a number of metrics found in the litterature. The other theoretical issue is that these metrics violate a standard property of routing metrics known as "isotonicity". The usual opinion is that non-isotonic metrics don't work, so people typically go to a lot of trouble to work around the lack of isotonicity. While I don't have proof of this fact, my intuition is that Babel is sufficiently robust to give non-pessimal results with a non-isotonic metric, something that will need to be checked both experimentally and theoretically. (As an additional bit of circumstantial evidence: Cisco's EIGRP, which has similar properties to Babel, uses a non-isotonic metric by default.) I should have a diversity branch of the Babel daemon in time for the meeting in Italy, where we'll hopefully be able to do enough testing to know for sure whether this approach makes sense. Juliusz _______________________________________________ Babel-users mailing list Babel-users@lists.alioth.debian.org http://lists.alioth.debian.org/mailman/listinfo/babel-users