[Babel-users] Unscheduled Unicast Hellos: some background on 802.11 link quality estimation

[Juliusz Chroboczek]

>> 2. 802.11 uses adaptative rate control on unicast frames, but not on
>> multicast frames.

> This is not entirely true, since hostapd is able to send multicast as
> unicast-frames.

And this will cause Babel's link-quality estimation to yield incorrect values.

Dynamically computed metrics are an open area of research, and one that is
sadly neglected by the scientific community.  I believe that it is
important, but it is very difficult to publish in that area (FWIW, our
paper about the RTT metric was rejected twice).

ETX is an elegant hack that uses the fact that 802.11 multicast isn't
subject to ARQ.  As you justly note, Ruben, this assumption can fail in
edge cases, and there's nothing that can be done about it.  The obvious
solution would be to have explicit cross-layer signalling, as in the case
of Lamparter's variant of IS-IS.  Doing that right is not easy, and might
(or might not) require changes to the link-layer.

I'm personally interested in that area of research, and I'll probably have
a fair amount of funding in the near future.  However, since the area is
not fashionable, I cannot promise that we'll be able to publish.

-- Juliusz