[OLPC Networking] RSSI value questions
Benjamin M. Schwartz
bmschwar at fas.harvard.edu
Tue Apr 1 20:01:52 EDT 2008
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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
| 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.
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