[OLPC Networking] RSSI value questions

[Oliver Mattos]

>
> the commercial products are able to do it with unmodified laptops, so it
> should not require a change to the over-the-air signal (unless this
> generation of active antenna hardware isn't up to the task)
>

I'd be very innterested to know how they work - I can't see any way it could
work using speed of light triangulation using any old off the shelf
hardware, although there are quite a lot of other tricks I can see it could
do:

   1. If you can pursuade two or more laptops to transmit simultaniously,
   you could adjust the phase of the signal to see at what point you get most
   constructive interference.  Possibly that could work without any way to
   adjust the phase, instead relying on the inaccuracy in the clock generator
   of each radio.  It might be possible to repeat that process on a couple of
   similar wireless channels to calculate a total distance that is much longer
   than the wavelength.  I suspect this method would need many more than 3
   laptops - I'm just trying to visualize how it could work since chances are
   no laptop is going to know the phase it's transmitting on with respect to
   any other, so you need at least multiple observors.
   2. Another method that would look like "transmission time" to the
   software layer would be to just time, in microsecs, the amount of time to
   ping a laptop - the ping time here is far more affected by the nominal
   bitrate than by the distance, but in general since the auto bit rate
   selection is dependant on signal, and signal is dependant on distance, it
   could give meaningful results.

 the problem with trying to use sound is that it requres a clear path from
> the servers to the laptops, something I would not expect to see very much.
> it's also very sensitive to the direction the laptops are pointing.


Doesn't need to be the server - any path from any laptop to any other can be
used, and the multiple path/no lign of sight path will exist in a similar
way in wireless.  I suspect that if you've got a year project ahead of you,
it might be best to start on sound to understand the issues involved, and
then move to wireless, since at the physics level, they're both waves and
have many similar behaviours.  (reflection, refraction, diffraction etc.)

For a finished product, sound within or near audible frequencies will not be
suitable for many uses - so that means nothing below 25kHz - don't be
tempted to use 15-22kHz because those of us with good ears get very angry at
devices that pollute those frequencies - yes I'm looking at all those
Electronic Engineers who make switch mode power supplies with low tolerence
timing circuits!! (nokia chargers, XO charger, laptops when on standby, set
top boxes) (/rant)

Thanks
Oliver


>  On Wed, Apr 2, 2008 at 3:14 PM, Martin Langhoff <
> > martin.langhoff at gmail.com> wrote:
> >
> > > On Wed, Apr 2, 2008 at 4:27 AM,  <david at lang.hm> wrote:
> > >
> > > >  trying to work from signal strength won't work well, but you may be
> > > >
> > > >
> > > able
> > >
> > > >  to triangulate based on the arrival time of the signal at various
> > > >  locations.
> > > >   there are companies that do this commercialy with 3+ access points
> > > >
> > > The recommended configuration for mid-to-high-end school servers has 3
> > > active antennaes attached, and our recommendation is that they are
> > > placed well apart. They can be up to 10m apart due to USB cable lenght
> > > limits, and Wad mentioned 2m minimum recommended distance. If the
> > > distance is enough (in relation to the granularity of timers in the
> > > antennaes) then telling the XS about relative location of the
> > > antennaes could provide enough info.
> > >  Having said that, I suspect that being able to do any of the above is
> > > somewhat far ahead in time ;-)
> > >  cheers,
> > >   m
> > > --
> > >  martin.langhoff at gmail.com
> > >  martin at laptop.org -- School Server Architect
> > >  - ask interesting questions
> > >  - don't get distracted with shiny stuff - working code first
> > >  - http://wiki.laptop.org/go/User:Martinlanghoff
> > >  _______________________________________________
> > > Networking mailing list
> > > Networking at lists.laptop.org
> > > http://lists.laptop.org/listinfo/networking
> > >
> >
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