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Wireless interference investigation - burglar alarms implicated!

Running one of the oldest WiFi mesh systems in the UK, sometimes means you have to discover things all by yourself; and we have now had an interesting case of interference, which I was finally able to track down.

For several months, a friend’s mesh network had been experiencing vastly increased noise between the hours of 5.30pm and 7am, and most of the weekend. It was almost like clockwork, and caused parts of the mesh to drop out. We spent ages looking for video senders, WiFi cameras and the like.The weirdest thing was that the noise was only at the gateway, and not on any of the neighbouring nodes, yet it was picking up ~30db of noise.

So, we took a Wi-Spy spectrum analyser, and started to investigate. We switched the meshnodes to channel 1 - away from channel 11- and saw a weird signal at channel 9.

This was a constant signal, which revealed itself because the average was close to the peak. After some moving around, we homed in on a Mazda 6 parked nearby which was emitting the signal.

Turning the car alarm off eliminated the noise. Turning it on brought it back. Turning the alarm on and disabling the internal motion sensors gave no problem.

Culprit found. Most cars use ultrasonic motion detectors for their alarms. Mazda 6s use channel 9. Which stomps on 11 *and* 6. Great!

Anyway, we decided to make a video of the experience, which you can find here or here

Whilst making this video, we also discovered that recent Mitsubishi Shoguns and possibly Chrysler Crossfires have the same problem.

If you want to download the WiSpy recordings, they are here, here and here

Joel blogs regularly http://www.pateley.net/blog/?p=5 here

Tags: RFI, WiFi, mesh network, LocustWorld