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

 

[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

 

[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.

 

[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.

 

[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.