Re: [Roll] ETX metric

["Anders Brandt" <abr at sdesigns.dk>]

Henning Rogge wrote:

>... 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". 

Good pragmatic approach to reality.

>... 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.

I have seen other people on this thread indicating that RSSI
measurements tell a rather polluted truth.
This is also our experience.
Translated to the real world, ETX information based on the number of
received Acks from lower layers is a cheap and often sufficient metric.

ETX actually could be a good candidate for a default metric working
_FAIRLY_GOOD_ in most environments on most hardware platforms.

- Anders

-----Original Message-----
From: roll-bounces at ietf.org [mailto:roll-bounces at ietf.org] On Behalf Of
Henning Rogge
Sent: 13. oktober 2009 10:11
To: roll at ietf.org
Subject: Re: [Roll] ETX metric

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