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WISPs: Beware of Traffic Lights

Although the immediate culprit is a poorly configured radio, the real problem lies in local governments designing systems to fail by hiring the cheapest consultants available—and failing to consult local WISPs who have the knowledge.

[December 13, 2005]

On the ISP-Wireless list in November, RF reported:

I have an interesting situation here. Have a few motels in a nearby area that I provide 80211b APs, Nomadix HSGs, and authentication for.

This afternoon one of the motel's guests calls the shop describing that he would at first get a good signal, then within a few mins it would completely drop. After a lengthy troubleshooting session with him, and being able to access the AP through the HSG and things seeming normal I assumed it was his laptop having the problem.

A half hour later another call, a different motel, same type of problem, similar hardware, but better APs: Proxim AP700's compared to Belkin pre Ns.

I go to the motel and try to connect with my laptop, at first I would have a good signal strength, then a minute or so later nothing. I check my Atheros client utility and find a bunch of CRC errors and frame recieved errors. Open up net stumbler, same thing was happening with every AP in the area. Tried with Kismet, saw the same thing. At first things would be normal then every AP would drop, as if no signal existed, even with me right underneath the AP.

One thing new in the area is a traffic light with what looks like an omni on the light itself with another omni on a small metal shed approx 75' from the light.

I wonder and scratch my head, then dismiss the thought, but then I watch the light change and the 802.11 signal loss, and there it is, every time the light changes the APs are dropped—and I do mean every AP in the area. One motel seems to be unfaffected, maybe due to the construction (brick and concrete instead of wood) but it happens in their parking lot.

I come home. I live about 1 mile from the area, and the same thing is happening here. I can't connect to my own ap.

Has anyone heard of this, what could the city have installed that is affecting everything in the area? I did a quick google search on the issue and didn't come up with anything, but will try somemore.

[BG replied] "We've recently started to see something similar on one of our APs. Every tenth ping lost. Tons of deferred transmissions—more than the number of fragments transmitted. "Max retries exceeded" errors on about 5 percent of all packets. Worst on channel 1, which just happened to be where the AP was. Using the AP radio to scan for interfering Wi-Fi sources did not turn anything up, which means that it's either not 802.11b or not beaconing.

Fortunately, it's not everywhere—just on one of our APs. Shifting the AP up a channel from Channel 1 (where it normally is) to Channel 2 helps a bit, but still lots of deferred transmissions and retries.

Don't know who this is.. It might be the City, might be a ham, might be someone with a cloaked 802.11b AP. But it doesn't matter—the point is that we WISPs really, really, really need some spectrum of our own."

By the way, here are some links on the use of 2.4 GHz for traffic lights. Note that most of the systems mentioned are slow hoppers. Bandwidth is not an issue (how much bandwidth do you need to control a traffic light), so they transmit massive amounts of energy per bit. Many are slow hoppers—meaning that they can sit on your channel for up to .4 second, which is more than enough time to disrupt it. Some are mesh nodes. See, for example,

http://www.nowwireless.com/nm/pdf/trafficlight.pdf

and

http://www.imsasafety.org/journal/marapr04/7.pdf

Also see this more general comment from Andy Seybold on the 2.4 GHz band:

http://www.muniwireless.com/archives/guests/603

[RF thanked BG] "Thanks, those were informative.

I wanted to lay low in this community, just do my job, pay my taxes, and so on, but if the city or whomever feels that they can install with impunity a device that interferes with the commerce of it's businesses, and home users (taxpayers) of 802.11 then they have a fight on their hands. Sure some of the signals from my APs are out there, but I carefully watch which channels other ap's are on and set mine to not interfere with the existing ones.

Don't know as of yet whether I'll approach this as an employee of the company I work for or as a concerned citizen.

I am just wondering how many calls the shop will get on this tomorrow...

How has your battle gone? The one with the water company.

[GW suggested] "The problem may be, that the City doesn't realize what they've done or that these new street lights cause problems. It's not unusual that a 3rd party company specs the product and it gets used.

Hopefully with the right approach and some finesse the City will be open to discussing this issue and working towards a resolution with you.

Will be curious to hear what happens."

RF found some answers and reported back to the list:

Made quite a few calls this AM. It turned out that it is the state highway dept putting in the new lights.

Spoke with the person in charge of our area and he flatly denied that their equipment was responsible for what was happening. However he did admit to using channel hopping equipment, then went on to say that he thought they had been assigned their own frequency to operate on by the government.

Well, that ended the call, then approx an hour later everything began working again, on its own, I happened to be at the location, but all I was doing was monitoring the "interference" then suddenly the "hopper" stopped. I could see APs again.

I tried to connect to an AP, it worked, and thus far all has been well. I wish I could say I fixed it, and in a round about way perhaps I did, but only by calling the correct person.

[RA guessed] "Hmmm, Guessing here but I'll bet that whoever set the system up, set it up with default frequencies on the system, which was probably in the 2.4 spectrum. One call, the engineer logs in, sets the correct freq, and awayyyyy they go... Let us know if it comes back?"

[JS said] "Sounds just like every call to SBC for a T1 being down.

'Oh, everything is good here, must be your equipment there.'

'But we have 16 T-1s, and only one of them is down, and they're all plugged into the same MUX, same config, nothing has changed. And if we move the cable to another port it follows the cable, not the port, which means it is on your end.'

'We'll send out a tech.'

30 minutes later it comes back, and 30 minutes after that a tech shows up and says, 'I can't find anything wrong, must have been your equipment.'"

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