How do currently available commercial wireless topology mappers do this? On Wed, Apr 2, 2008 at 7:59 PM, <[EMAIL PROTECTED]> wrote:
> On Wed, 2 Apr 2008, Hal Murray wrote: > > > Date: Wed, 02 Apr 2008 15:49:38 -0700 > > From: Hal Murray <[EMAIL PROTECTED]> > > To: Martin Langhoff <[EMAIL PROTECTED]> > > Cc: [EMAIL PROTECTED], [EMAIL PROTECTED], [EMAIL PROTECTED], > > [EMAIL PROTECTED], [email protected], > > Ryan Crawford Comeaux <[EMAIL PROTECTED]>, > > Hal Murray <[EMAIL PROTECTED]> > > 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 > _______________________________________________ > Server-devel mailing list > [email protected] > http://lists.laptop.org/listinfo/server-devel > -- Aaron Huslage - 503.860.1634 http://blog.hact.net IM: AIM - ahuslage; Yahoo - ahuslage; MSN - [EMAIL PROTECTED]; GTalk - [EMAIL PROTECTED]
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