How do currently available commercial wireless topology mappers do this?<br><br>
<div class="gmail_quote">On Wed, Apr 2, 2008 at 7:59 PM, <<a href="mailto:david@lang.hm">david@lang.hm</a>> wrote:<br>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">On Wed, 2 Apr 2008, Hal Murray wrote:<br><br>> Date: Wed, 02 Apr 2008 15:49:38 -0700<br>> From: Hal Murray <<a href="mailto:hmurray@megapathdsl.net">hmurray@megapathdsl.net</a>><br>
> To: Martin Langhoff <<a href="mailto:martin.langhoff@gmail.com">martin.langhoff@gmail.com</a>><br>> Cc: <a href="mailto:david@lang.hm">david@lang.hm</a>, <a href="mailto:networking@lists.laptop.org">networking@lists.laptop.org</a>, <a href="mailto:devel@lists.laptop.org">devel@lists.laptop.org</a>,<br>
> <a href="mailto:bens@alum.mit.edu">bens@alum.mit.edu</a>, <a href="mailto:server-devel@lists.laptop.org">server-devel@lists.laptop.org</a>,<br>> Ryan Crawford Comeaux <<a href="mailto:crawford.comeaux@gmail.com">crawford.comeaux@gmail.com</a>>,<br>
> Hal Murray <<a href="mailto:hmurray@megapathdsl.net">hmurray@megapathdsl.net</a>><br>> Subject: Re: [OLPC Networking] RSSI value questions<br>
<div class="Ih2E3d">><br>><br>>> The recommended configuration for mid-to-high-end school servers has 3<br>>> active antennaes attached, and our recommendation is that they are<br>>> placed well apart. They can be up to 10m apart due to USB cable lenght<br>
>> limits, and Wad mentioned 2m minimum recommended distance. If the<br>>> distance is enough (in relation to the granularity of timers in the<br>>> antennaes) then telling the XS about relative location of the<br>
>> antennaes could provide enough info.<br>><br>> I assume the 10 m above is 5m for each antenna. 5m is the nominal limit on<br>> USB cables. I think you can get longer than that by using hubs/repeaters.<br>
> I've got some 1 port hubs that are built into the connector blob on a 5m<br>> cable. I found a web page that said there is a limit of 5 hubs but I haven't<br>> tried it.<br>><br>><br>> What sort of timer and/or time stamper does the active antenna and/or WiFi<br>
> gear in the XO have?<br>><br>><br>> I think there are two approaches that might be interesting.<br>><br>> If all you have is 2 antennas listening to the same packet, then you need<br>> more than good granularity on the timers. You also need to synchronize the<br>
> timers.<br>><br>> If you have the relative time of arrival of the signal at 2 antennas, you can<br>> compute the direction the signal came from. The scale factor is the speed of<br>> light between the two antennas. That's 1 ft/ns in air. 10 m is (rounding)<br>
> 50 ft, so we need time stamps accurate to a (small) fraction of 50 ns.<br>> That's the right ball park.<br>><br>> That gives you direction, no distance.<br><br></div>from two antennas you get just direction. with more antennas you get<br>
direction from different points and can then triangulate to get location.<br><br>you may not be able to do this just with the three active antennas<br>connected to a single school server.<br><br>you may need an additional active partner (either active antennas<br>
connected to a different school server, or a laptop in a known position<br>
<div class="Ih2E3d"><br>><br>> The other approach requires help from the XOs.<br>><br>> Take a pair of systems. Exchange a pair of packets. Grab the time stamps,<br>> both transmit and receive. That's enough information so you can calculate<br>
> the time/distance between the units and the clock offsets. That pattern and<br>> calculation is the core of NTP. I'll say more if anybody wants.<br>><br>> That gives you distance, no direction.<br>><br>
><br>> If you had a handful or systems and lots of distance measurement pairs, you<br>> might be able to make a map. I think you need to know the location of a<br>> couple of units. Without that, flips of the map over X or Y (or any other)<br>
> axis also give you a valid answer. The other antennas on the XS might be<br>> good enough.<br>><br>> This needs timestamps with the granularity of how good you want the location<br>> to be. If you want the locations within 10 feet you need (handwave) 10 ns.<br>
> You might get some more info by averaging several samples.<br><br>> Is this a 2D or 3D problem?<br><br></div>it can be either, but lets start with 2D<br><br>David Lang<br>
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