WiFi Channel interference on the 2.4GHz band

[paulypuk]

Hi, I’ve got a wifi network at a school, all working well except for an issue that I’ve noticed recently.
I added a couple of new wifi Access Points and during setup, needed to check the nearest in-use channels, so that I could set the new ones up appropriately.
A feature on the Draytek Vigor APs allows me to also check for interference across channels.

I saw that Channels 7, 8, 9, and 10 were showing 94% Channels Utilisation and 94% Channel Energy.
All other channels were showing more usual activity and useage.

I immediately took the new APs offline but they weren’t the cause as the high interference persisted.
I’m not sure how long this has been like this.
Been around school turning off IP telephone system, Microwave ovens, Lights, TVs, Computers, iPads, printers, smart speakers, alexa things, smart plugs, anything else I couild find and other APs - but I can’t find the cause of this interference.
It's Term Break just now so it's easy to turn stuff off.

The 2.4GHz channels affected are
Channel 7 (2442 MHz)
Channel 8 (2447 MHz)
Channel 9 (2452 MHz)
Channel 10 (2457 MHz)

The 5GHz band isn’t affected by this issue at all.

I wonder if anyone has any ideas as to the possible cause here?

I’ve listed the channels affected in case they point to anything in particular that perhaps you’ve come across.

Any suggestions gratefully received.

Thanks,
Paul

 

[ChrisD.]

Those channels will overlap each other. On 2.4 GHz you only really have 1, 6, and 11 which are completely separate from each other. If you have neighbouring APs using 9 and 10 as an example, they would interfere with one another.

I confess I have never done any large scale AP deployments so not 100% sure how important that is (someone like @WJA96 is one of the resident Wi-Fi gurus who will no doubt correct me), but personally I would try to put all APs on 1, 6, and 11 and avoid having neighbouring APs using the same channel.

 

[paulypuk]

Hi Chris, Thanks for taking the trouble to reply.
All APs are already using either 1, 6, or 11 on the 2.4 band.
Any APs that do use the same channel - which is inevitable - are far enough apart on site so as not to cause any problems.
If my scans ever show interference, then the level is usually no higher than 20% or so. I've never seen it up at 94% anywhere.
We're also lucky enough to not have any close "neighbour" networks nearby.
Thanks, Paul

 

[MissChief]

Hi Chris, Thanks for taking the trouble to reply.
All APs are already using either 1, 6, or 11 on the 2.4 band.
Any APs that do use the same channel - which is inevitable - are far enough apart on site so as not to cause any problems.
If my scans ever show interference, then the level is usually no higher than 20% or so. I've never seen it up at 94% anywhere.
We're also lucky enough to not have any close "neighbour" networks nearby.
Thanks, Paul

No old wireless boosters or older cordless phones or anything that can broadcast a signal?

 

[paulypuk]

Certainly no boosters at all. It all changed to IP phones a couple of years ago and the only other ones aren't in use now. There's a fixed phone in a lift come to think of it - I shall go and check that out and disconnect it for testing purposes. Mischievous suggestions though, thanks. I turned all of the phones off last week along with their router but made no difference to the problem. They should have been on my list on the first post.

 

[Orcvader]

Another thing to check, are all APs set to 20MHz width? 40 would cause it to overlap even if they're using 1/6/11.

 

[paulypuk]

Hi, yes, all APs are fixed to 20MHz only on the 2.4 band. No auto 20/40 enabled on any of them either.

 

[ChrisD.]

If you have an Android phone there are Wi-Fi scanning apps which would show any broadcasting AP names.

 

[paulypuk]

Thanks Chris. I have scanning apps on my iPhone and on laptop. All school APs are named and easily located.
There are no other APs on site except ours, and no local wifi ssids close enough to interfere with ours as we're on our own parcel of land really.
The issue is coming from unidentified equipment within premises. I've even been through my scanning app and ID'd everything (that's turned on) by MAC address if the IP address doesn't ID it.
Hostnames often show up so I can discount everything showing on scans. My app would show me any hidden ssids and there are none of those nearby either.
I'm in school tomorrow so will try to find anything else that might be the source of this noise. Probably turn off sections of the network and see if I can at least narrow down the area.

 

[ChrisD.]

Could be anything then, without expensive equipment it's going to be difficult to narrow down depending on the size of the school. As pointed out above and as you've mentioned check for the obvious items, most items are 'smart' these days such as boilers, fridges, ovens, Hue, Hive, anything that might talk to its own hub, Sky, Sonos, IC systems, walkie talkie/CB radio, personal hotspot devices (3 mifi) etc etc.

 

[Pulseammo]

There was a somewhat recent LTT video (I know... groan...) where he was trying to find the source of some interference and he had some kind of expensive scanner, but he also mentioned some cheaper hardware, perhaps you can look into this?

I don't know if the scanning app you have is similar but perhaps walking closer to the source of the interference would boost the signal levels and so you'll be able to close in on it just by walking about the school and taking readings?