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

Good Day,

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

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.

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'

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and point the Yagis in different directions towards the receivers. 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.

Will this setup work? 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? 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.

Thank you, Peter

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Reply to
IQC Peter
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Do you really mean an /Internet/ connection between the devices? (In which case why not just use a local Internet connection and run some sort of VPN across the resulting link. OpenVPN works well for me.) Or do you mean that you require a direct /network/ connection?

Chris

Reply to
Chris Davies

IQC Peter hath wroth:

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

We will have the sending station on the fourth floor of our building,

  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:
  3. Is there any local interference on 2.4GHz along the path?

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:

Good unit. I like Buffalo.

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.

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

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).

Reply to
Jeff Liebermann

To Chris: As long as the computers can communicate, the experiment will work. Either a network or an internet connection is fine. However we need line of sight, and where we're planning on during the experiments (in a field) there is no local internet connection.

To Jeff: By stable, I just mean that the connection is constant, and it won't disconnect regularly (losing the connection makes our program crash). We have GPS-synched time-stampers for the incident photons, so we don't have to worry about network delays.

We will have line of sight (otherwise our photons wouldn't make it, which would kill the experiment). I'd set the minimum distance at 2km from each receiver station to the sending station, for a total of 4km between the receiving stations. More distance would be great, but that should be a good start. There will be the occasional tree branch in the Fresnel zone, but it will be mostly clear. There is mostly likely some 2.4GHz background radiation, but we're doing this in a field so it should be minimal.

As far as the MIMO, I think I must have explained myself poorly. Each receiver station will have only one antenna. However, the central sending station will need two directional antennas, one pointed at each receiving station. I just wasn't sure if this would screw up the router, since each computer would be connecting via only one antenna, and the router may require all antennas to be in range of the computer. I know that a satellite dish would offer higher gain, however we feel that it would be overly cumbersome since we will have to move the equipment every time we wish to do the experiment. We thought that Yagis were a good compromise.

We don't need a high speed connection, we're only sending text. Say 50 kB/s. We will be running the experiment exclusively at night (during the day, the sunlight overpowers our signal photons). We would run the experiment for around 4 hours a night, say 3 nights a week. It would be nice if the connection was solid for that time - a disconnection crashes the program, and it is a bit of a pain to have to restart it.

Thanks for your help.

Reply to
pgforbes

On Mon, 18 Feb 2008 10:12:53 -0800 (PST), snipped-for-privacy@gmail.com wrote in :

You'll always have some disruption percentage. To keep it low, you'll need decent signal-to-noise. See calculations in the main wiki below.

That won't work. Standard wireless units only use one antenna at a time. You'll need two, each with its own antenna, one for each remote location.

Not a "satellite" dish -- high-gain dish antennas aren't much more cumbersome than yagis, with higher gain and easier to work with.

Can't fix the program to make it more robust?

Reply to
John Navas

Are you running the Quarknet cosmic ray experiments? If so, where did you get the PMT paddles?

Michael

Reply to
msg

I'll look into the dishes then, and be sure to get two routers/wifi access points. I'll be sure to look into the program as well. And no, I'm not doing any Quarknet cosmic ray experiments at this time. Thanks for the assistance.

Reply to
pgforbes

snipped-for-privacy@gmail.com hath wroth:

That's not going to happen with any form of wireless. For example, if you have a 30dB fade margin in the wireless links:

That translates to a 99.9% reliability, which is about 9 hours of downtime per year. For your 4 hour test period, that's 2.5 minutes of downtime. Wireless is just not a guaranteed delivery mechanism. That's why start/stop/resume protocols were invented for downloads on dialup connections, where the connection might be lost due to someone picking up the phone, and where starting over makes little sense. Work on the protocol problem.

Good because packet loss will wreck that.

Do the Fresnel Zone calculations:

The usual goof is having the antennas too low, where the ground enters the Fresnel Zone and mangles reliability. At 2km range, you'll need 8 meters clearance at midpoint around the line of sight. That's a nice way of saying that your link antenna must be 8 meters off the ground (assuming a flat earth model). Are they?

Fine, but realize that the link calculations in the aformentioned URL are the ideal and best case. Everything you throw at your link will result in a reduction in signal strenth, SNR, and/or reliability. It only gets worse, not better.

Ahem. See list of possible sources of intereference at:

The plague of municipal wireless networks and home systems are the major sources of interference. Over a pair of 2km links, presumably in a metropolitan area, the chances of encountering such interference is VERY high. It will come and go, so again, your delivery protocol is important.

Can I pass on explaining how MIMO (802.11n Draft 2) works and simply state that MIMO is about speed, not range. It won't guy you anything except at close range. Details later if you want them.

It doesn't matter. Yagi, dish, panel, coffee can, or paper clip antennas are not going to make MIMO work at 2km range. The crude explanation is that range and speed are inversely proportional (at a given power level and bandwidth). See range chart at:

Roughly, if you double the distance, you cut the speed to 1/4th. That includes MIMO (with a few exceptions that hog more bandwidth). By the time you get past about 25 meters range, all the "enhanced" modes beyond 54Mbits/sec crap out. Forget about MIMO.

I explained why a yagi sucks as compared to a dish in my previous rant. Please re-read.

Then you might be using the wrong hardware and technology. Wi-Fi is all about speed. At low speeds, there are 900MHz point to point radios, that will work much better. Some vendors:

etc. Search for "900Mhz RS-232" or "900MHz RF modem".

These are more expensive but will work much better. At 900Mhz, the yagi is the antenna of choice. I'm not sure what you're using for interface, but they come in ethernet or serial i/o. They are also commonly used by surveyors for Diff GPS correction links. I have a few of the Freewave radios sending telemetry (weather, etc) over an

8km link with lousy line of sight.

If you know anyone around that does wireless data collection or telemetry, they probably have at least one pair of link radios that can be borrowed.

Yep. Science is best done under cover of darkness.

Fix the program. There's no reason for that to happen and there's no way you're going to get a disconnect free, or zero packet loss wireless link without protocol assistance.

Best of luck. Sounds like an interesting experiment.

Reply to
Jeff Liebermann

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