[Server-devel] [OLPC Networking] RSSI value questions

[Aaron Huslage]

How do currently available commercial wireless topology mappers do this?

On Wed, Apr 2, 2008 at 7:59 PM, <david at lang.hm> wrote:

> On Wed, 2 Apr 2008, Hal Murray wrote:
>
> > Date: Wed, 02 Apr 2008 15:49:38 -0700
> > From: Hal Murray <hmurray at megapathdsl.net>
> > To: Martin Langhoff <martin.langhoff at gmail.com>
> > Cc: david at lang.hm, networking at lists.laptop.org, devel at lists.laptop.org,
> >     bens at alum.mit.edu, server-devel at lists.laptop.org,
> >     Ryan Crawford Comeaux <crawford.comeaux at gmail.com>,
> >     Hal Murray <hmurray at megapathdsl.net>
> > Subject: Re: [OLPC Networking] RSSI value questions
> >
> >
> >> 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.
> >
> > I assume the 10 m above is 5m for each antenna.  5m is the nominal limit
> on
> > USB cables.  I think you can get longer than that by using
> hubs/repeaters.
> > I've got some 1 port hubs that are built into the connector blob on a 5m
> > cable.  I found a web page that said there is a limit of 5 hubs but I
> haven't
> > tried it.
> >
> >
> > What sort of timer and/or time stamper does the active antenna and/or
> WiFi
> > gear in the XO have?
> >
> >
> > I think there are two approaches that might be interesting.
> >
> > If all you have is 2 antennas listening to the same packet, then you
> need
> > more than good granularity on the timers.  You also need to synchronize
> the
> > timers.
> >
> > If you have the relative time of arrival of the signal at 2 antennas,
> you can
> > compute the direction the signal came from.  The scale factor is the
> speed of
> > light between the two antennas.  That's 1 ft/ns in air.  10 m is
> (rounding)
> > 50 ft, so we need time stamps accurate to a (small) fraction of 50 ns.
> > That's the right ball park.
> >
> > That gives you direction, no distance.
>
> from two antennas you get just direction. with more antennas you get
> direction from different points and can then triangulate to get location.
>
> you may not be able to do this just with the three active antennas
> connected to a single school server.
>
> you may need an additional active partner (either active antennas
> connected to a different school server, or a laptop in a known position
>
> >
> > The other approach requires help from the XOs.
> >
> > Take a pair of systems.  Exchange a pair of packets.  Grab the time
> stamps,
> > both transmit and receive.  That's enough information so you can
> calculate
> > the time/distance between the units and the clock offsets.  That pattern
> and
> > calculation is the core of NTP.  I'll say more if anybody wants.
> >
> > That gives you distance, no direction.
> >
> >
> > If you had a handful or systems and lots of distance measurement pairs,
> you
> > might be able to make a map.  I think you need to know the location of a
> > couple of units.  Without that, flips of the map over X or Y (or any
> other)
> > axis also give you a valid answer.  The other antennas on the XS might
> be
> > good enough.
> >
> > This needs timestamps with the granularity of how good you want the
> location
> > to be.  If you want the locations within 10 feet you need (handwave) 10
> ns.
> > You might get some more info by averaging several samples.
>
> > Is this a 2D or 3D problem?
>
> it can be either, but lets start with 2D
>
> David Lang
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