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I live a small, low-density town. Within 50m radius, there are 10 houses, no offices. Normally, I can see ~8 APs on 2.4GHz, one of which is ours. They're well spread across the band. It also functions at 5GHz, but obviously, I can only see ours within the house.

All in all, a fairly standard, low interference scenario.

But for the past couple of months, every so often 2.4GHz dies. And I mean dead. Our 2.4GHz-only devices drop off the network. When I scan from laptops, phones, dongles, I can only see our 5GHz network. Even the neighbours —who between them normally cover the entire spectrum— have dropped off the face of the earth. This lasts for minutes... And then everything Just Works™ again.

This is low-density population. I don't think our 80yo or middle-aged family neighbours are likely to be using a for-purpose Wi-Fi-blocker but that's really how this presents. Like a high-interference sustained fuzzing of the band.

The only change to note is a set of roadworks that has slowly been moving past our house. Temporary traffic lights might have causes some interference and I'd have bought this when they were right outside but they're about 100m away now.

So I'm stumped. Any ideas? Assuming this is interference, is there anything I can do to track it (without spending billions)? Even if that's just registering that there is interference coming from somewhere.

  • Wireless traffic lights (at least in Europe) are usually around 430 MHz, so they would be unlikely to cause the issue. Are there maybe other 2.4GHz transmitters in the area? A badly shielded microwave, radio amateur doing experiments, etc. – mtak Jan 4 '18 at 11:56
  • Thanks for the heads up about traffic lights. There may be transmitters. Given the blanket knock-out it seems likely, but I don't know where they are, how to confirm them, or locate them. That's what the question is ☺ – Oli Jan 4 '18 at 12:09
  • Does it die at predictable times? (Keep a log.) – user1686 Jan 4 '18 at 12:15
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As you don't see any networks when this issue happens, the source of the interference is likely to be a non-WiFi protocol. This could be any device that works on 2.4GHz.

To find the source of these transmissions, you would need a receiver tuned for 2.4GHz. Preferably this would be a spectrum analyzer, so you can see the bandwidth of the interference. Connect the spectrum analyzer to a directional antenna tuned for 2.4GHz (Yagi design for example) and you will be able to "home in" on the source of the interference.

Decent spectrum analyzers are quite expensive. You might be able to find something usable in the "hacker/maker" community for cheap SDR receivers (RTLSDR with a downconverter or a HackRF). Another idea is to find a radio amateur or hackerspace in your area, they could have that kind of equipment and a usually happy to help.

  • Wi-Spy seems only slightly expensive... Various prosumer APs have a basic "spectrum analyzer" feature built in and can be obtained for around $100 (e.g. Ubiquiti's airMAX M series or Mikrotik's RouterBoards or in fact even my old laptop's ath9k-based adapter). I assume some of those would do the job as well? – user1686 Jan 4 '18 at 12:39
  • I don't know for those specific products, but at least for Ubiquity Unify AP and ath9k adapters you can only see interference that is using WiFi "protocol". It won't display non-WiFi interference. – mtak Jan 4 '18 at 12:58
  • Wi-Spy is a powerful dual-band spectrum analyzer that measures WiFi and non-WiFi activity in both the 2.4 GHz and 5 GHz bands. so it seems Wi-Spy is a suitable product for this! – mtak Jan 4 '18 at 13:00
  • that's certainly not true in my experience – both the airView program in the screenshot and ath9k's "spectral scan" debug tool have several times shown Bluetooth, microwave ovens, and such. – user1686 Jan 4 '18 at 13:16
  • Both my Ubiquity Unify AP and Unify Controller do not show non-WiFi interference. As I said, I don't know for airView or Mikrotik. If you say they do this, I believe you. Although it might not be the right form-factor to walk around the neighborhood with :) – mtak Jan 4 '18 at 13:23
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Found the likely culprit: Google Chromecast.

Our closest next door neighbour recently bought a couple of Chromecasts. These little apparently have a bug that floods their network with hundreds of thousands of MDNS discovery packets. The reports do only suggest a DOS on their own networks, but it's still 2.4GHz data being blurted out. It's going to cause some interference at that sort of rate.

I expect time to fix this now it's being confirmed but in short order, I've asked them to unplug the one nearest us and we've not had any drop-outs since.

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