[Olsr-dev] ETX Calculation

[Henning Rogge]

Am Donnerstag 18 März 2010 20:54:11 schrieb Vincent, Michael - 0665 - MITLL:
> We're using OLSR (0.5.6-r7) in a testbed where a vehicle moves away from a
> fixed point in a straight line.  As the signal weakens (LQ increases) we
> add a relay node directly between the fixed point and moving vehicle (on
> the straight line path).  The issue is that the ETX for the 2-hop path
> (via the relay) doesn't register better than the direct path until the
> packet loss on the direct path is way up.

> I've read the Link Quality README
> (http://www.olsr.org/docs/README-Link-Quality.html) and it seems to
> indicate that the ETX for a multi-hop path is the sum of the individual
> paths (ETX1 + ETX2 ...).  Looking at the source code - this seems to be
> the case also.
Yes, all metrics used in OLSRd are additive metrics.

> We'd like to use the relay node quicker - that is, before the direct path
> suffers from horrible packet loss and the ETX for the multi-hop path takes
> over.  I think the summing of ETX without some 'scaling factor' is the
> issue.
Normally you should not suffer from unicast loss when ETX starts increasing 
because of linklayer retransmissions (at least that's one of the assumptions 
behind ETX I think).

One problem you have is that each hop in your network (even a perfect hop) 
costs 1. So the two-hop path is only considered if the single-hop path has ETX 
2.0 (this problem can only be solved by changing the sourcecode.

Another problem might be that the metric does not change fast enough. You can 
change this for etx_fpm and etx_float by using a larger lq_aging parameter 
(for example 0.1). Set the Hello-Interval to a smaller value will help too.

Add the following lines to your config file to test it:

LinkQualityAging 0.1
LinkQualityAlgorithm    "etx_fpm"

Henning Rogge
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