[OLPC Networking] RSSI value questions

[Polychronis Ypodimatopoulos]

Ryan,

Like Ben said, inducing the physical layout of the network from metrics 
such as RSSI will give you poor results for various reasons. What 
"Space" did was to average arrival rates from direct neighbors over a 
"long" period of time (anywhere between 1 and 10 seconds) to avoid 
highly temporal effects like multipath and noise. Even so, the result is 
only a rough layout of the network. If you'd like to achieve better 
accuracy I thing you should combine other ideas like sound measurements, 
as Ben suggested.

Pol


Benjamin M. Schwartz wrote:
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> Ryan Crawford Comeaux wrote:
> | I'm looking to build an application through Google's Summer of Code for the
> | school server that uses wireless location detection methods to monitor and
> | approximate the physical location of all nodes within the mesh.
> | Unfortunately, I can't seem to find any information on whether or not the
> | network allows the server to poll nodes within the network for RSSI
> | measurements the nodes have made between themselves and all others within
> | their range.
>
> I was very interested in this problem as well.  I was told that it was
> impossible for two reasons:
>
> | Is this something that the networking firmware/drivers even allow?
>
> 1. The firmware dynamically varies the transmit and receive gain to
> minimize power usage and interference, but this information is not
> available from userspace.
> 2. Signal intensity is a terribly inaccurate measure of distance, due to
> the complex interference patterns typical of 2.4GHz waves in buildings.
>
> I am no longer so sure that either of these things is true, but neither am
> I optimistic.  I hope someone else on the networking list can provide a
> better answer.
>
> | If not,
> | is it functionality that could be requested of Marvell to provide?  If so,
> | how accurate are the RSSI measurements and to what decimal precision are
> | they available?
> |
> | Also, what kind of interest within the community is there for this kind of
> | application, if any?  I think the idea at least has interesting uses as far
> | as securing the network goes, but I'd really like to know what everyone
> | actively using and/or developing for the systems thinks.
>
> There's definitely a great deal of interest.  The closest thing so far is
> "Space": http://web.media.mit.edu/~ypod/mesh/ .  Instead of the analog
> distance measure of RSSI, Space uses the binary measure of whether two
> nodes have a direct connection in the mesh.  I am not sure whether Space
> is still working in recent builds.
>
> I have worked on http://wiki.laptop.org/go/Distance , an Activity that
> uses sound propagation delay to measure distance between two XOs.
> Distance achieves accuracy on the order of 1 cm, but it is clearly limited
> by its inability to measure through walls.  Also, due to the complicated
> way in which sound propagates, it is unlikely that Distance will ever be a
> good tool for measuring the entire position constellation of a group of XOs.
>
> If we had control of the wireless firmware, there is perhaps a chance that
> we could use radio propagation delay for distance measurement.  Accuracy
> would be limited by receive jitter, so the minimum expected error would
> probably be at least 3 meters.
>
> - --Ben
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-- 
Polychronis Ypodimatopoulos
Graduate student
Viral Communications
MIT Media Lab
Tel: +1 (617) 459-6058
http://www.mit.edu/~ypod/