Re: Setting up a temporary long-distance wifi for a science experiment

IQC Peter <IQC.Peter.34u5uc@no-mx.wirelessforums.org> hath wroth:

>Good Day,

Well, it was until you asked an interesting question. Please don't do
that.

>I am trying to do an experiment on Bell's Inequality, which is a part
>of Quantum Mechanics.

<http://en.wikipedia.org/wiki/Bell's_theorem>

>In order to conduct the experiment we need to
>have two observation stations each several kilometers away from a
>sending station, with a stable internet connection between them.

How "stable"? I presume you mean that you're able to do accurate
differential timing measurements between the two stations. That's not
going to happen with wireless. The transmission and reception delays
are NOT constant or stable and will vary substantially. Attach some
numbers to "stable" and I'll offer some suggestions.

>We will have the sending station on the fourth floor of our building,
>and the receiving stations will be in opposite directions from the
>building. This is not a permanent set-up: we will assemble and align the
>receivers each time we wish to do a test, and then pack it up.

1. Do you have line of sight between the two endpoints?
2. Do you have Fresnel Zone clearance? You didn't specify the number
of kilometers, so I can't calculate it for you. See:
<http://www.terabeam.com/support/calculations/fresnel-zone.php>
3. Is there any local interference on 2.4GHz along the path?

>Currently I'm planning on using a D-Link DIR-655 (it has three
>removable antennas). I'm going to connect two of the antenna jacks to
>directional Yagis ('WiFi Antenna | 14 Element WiFi Yagi Antenna'
>(http://www.radiolabs.com/products/an.../14eleyagi.php), and
>point the Yagis in different directions towards the receivers.

Won't work. You will NOT get a MIMO based high speed connection at a
distance of several kilometers. At best, you'll get perhaps about a
12Mbit/sec connection, which will yield about half or 6Mbits/sec in
TCP thruput. You only need one antenna at each end as the MIMO
wireless router will revert to 802.11g when confronted with long time
delays (at long ranges).

Also, Yagi antennas are convenient but just don't have the gain that
you can get with a dish. The typical yagi is 14-16dBi, while the dish
goes up to 24dBi.

When you figure out how far are "several kilometers", plug them into a
path loss calculator to see how big an antenna you need and what
speeds you can expect:
<http://wireless.wikia.com/wiki/Wi-Fi#Link_Calculations>

>On each
>receiver ends I'm going to have another Yagi, which will be plugged into
>a Buffalo Technology WLICBG54HP wireless adapter which in turn is
>connected to a laptop.

Good unit. I like Buffalo.

>Will this setup work?

Probably not for what I'm guessing you are attempting. You can
probably time code your packets from a GPS source, but you did't
mention that. You just wanted a "stable internet" connection,
whatever that means. More clues needed.

>I'm worried about how the router will function.
>In order for a computer to connect does the computer need to be in range
>of all three antennas?

Yes. No line of sight, no communications. Actually, with the Fresnel
Zone problem, you'll need MORE than line of sight at 2.4GHz.

>Because obviously in my setup each computer would
>be in range of one only. D-Link does not seem to know the answer
>either.
>
>If this setup will not work, could someone please recommend some
>alternate hardware? My budget is less than $500 Canadian, but other than
>that there are no hard restrictions. We'd like a range as long as
>possible, but don't have any real target range.

Then find a target range. If you must guess, assign a worst case
distance. Otherwise, calculations are impossible. You also need to
put some numbers on:
1. Expected bandwidth (download speed).
2. Expected reliability (outage hrs per day)
3. Line of sight issues.

I strongly suggest you forget about MIMO and all the acronyms that
claim to be better than 802.11g. The problem is that none of these
improve range. They generally improve speed, not range. Details on
request (I'm late for lunch).


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558