[Olsr-dev] Seeking comments: OLSR+ETX v/s DSR+ETX
Henning Rogge
(spam-protected)
Thu Jan 24 10:40:20 CET 2008
Am Mittwoch 23 Januar 2008 19:35:31 schrieb sebastian sauer:
> > So the value of the formula should be between zero and 1.
> well, given the constraints you gave, i also never disagreed
They are not my constraints, they are constraints of the code we have at the
moment.
> "slow start feature" could be to start with a small measurement
> intervall in the beginning and gradually make it bigger.
That's exactly what the formula does... instead of starting with a window size
of 1, it starts with lq_wsize/2 and slowly grows to lq_wsize.
> > That's a "builtin" problem of all distributed routing protocolls... if
> > the local instances use different configurations you get "strange
> > results".
>
> no, the link-quality concept as proposed in the original MIT paper clearly
> does not have this problem.
> (what's also why the ETX path-metric has all this nice features, like
> being monotonic, etc.)
See "3.1 Metric":
(http://pdos.csail.mit.edu/papers/grid:mobicom03/paper.pdf)
> ETX = 1 / (df * dr)
>
> The delivery ratios df and dr are measured using deticated link probe
> packets. Each node broadcasts link probes of a fixed size, at an average
> period r (one second in the implementation). To avoid accidental
> synchronization, r is jittered by up to +-0.1r per probe. Because the probes
> are broadcast, 802.11b does not acknowledge or retransmit them. Every node
> remembers the probes it receives during the last w seconds (ten seconds in
> our implementation), allowing it to calculate the delivery ratio from the
> sender at any time t as:
>
> r(t) = count(t-w,t) / (w/r)
As you can see even the original ETX paper use the same aproach as the Olsrd
implementation. It assumes that every node use a fixed window to calculate
the delivery ratio
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