Re: [Roll] ETX metric

[Henning Rogge <hrogge at googlemail.com>]

Am Dienstag 13 Oktober 2009 04:48:34 schrieb JP Vasseur:
> > I am saying that when using wireless links, a node should be aware
> > that his communication could interfere with other communication.
> > Less powered is the transmission signal, less noise is seen by
> > neighbours.
> > A low powered node will also need less energy to communicate in a
> > less noisy environnement.
> 
> Bear in mind, that you do not know this environment so this may lead
> you to an incorrect conclusion.
> The real metric is the link reliability, which reflect the properties
> of the environment.
We have a similar discussion about metrics in the Manet-WG. Different kind of 
metrics seem to be useful in different enviroments. Signal strength seem to be 
a good indicator for a "bad" link, but in praxis (especially with incorrect 
signal strengt estimation of consumer hardware) sometimes links with low 
signal strength are better than others with higher signal strength. On the 
other side ETX might be easy to implement on any kind of layer-2 hardware, but 
is only a rough estimation of the "link quality" which does not consider long 
time link statistics (stability, variance, ...) or transmission speed.

Metrics for wireless networks are still being researched without "final 
perfect metric" in sight, so ROLL should keep metric IDs for futher metric 
types. But you should put at least ONE easy to implement metric into the basic 
WG document. The problem of OLSRv1 (as an example) was that it did NOT include 
any metric except hopcount in it's basic document, so most people using OLSR 
for their projects (who do NOT research metrics for mesh networks) just use 
hopcount metrics and think that OLSR does not work.

So the question about a metric for ROLL should not only be "what's the best 
metric for our networks" but "what metric is simple enough that everyone can 
implement it on any kind of hardware".

> > As said somewhere else, the final metric should be a computation of
> > different metrics.
> 
> This is the objective function.
Yes, a good metric "post processing" (including long term statistical analysis 
of a links different metrics) can be a good thing to enhance the quality of 
the "link cost" estimation.

Henning Rogge

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