Hi all,

I have a Vigor2927 at home and I have noticed that the 5GHz wifi defaults to Channel 40, even though I set it to a different one.

To see that I use the Wifi Analyzser app on my phone. When changing this to 60 from the previous setting I have seen that the channel did hop to Channel 60, but it seems to have reverted to its default setting.

Firmware version is 4.5.2.1

With what channel bandwidth set ?

And is it an AC or AX model ?

Is it a DFS (Dynamic Frequency Selection) problem?

When wi-fi is using any 5GHz channel from 52 to 144 (in the UK - can be slightly different in other regions) it must periodically check for radar.  If it detects any radar activity on its channel then it switches to another channel.  It might choose sensibly or it might just move to a channel where DFS is not required (32 - 48). I used to have a Netgear router which would always end up at the bottom of the band, along with almost every other router and access point in my block of flats.  Sometimes it would do it within a day, sometimes it would take many weeks, but eventually it would end up at the bottom of the band with everyone else.  My Draytek 2866Vac and AP905 seem much better than my old Netgear stuff at sticking to their assigned their 5GHz wi-fi channels.  Maybe the rules are slightly less strict now?

There's another block of non-DFS channels at the top of the band but these are only allowed to transmit at a reduced power.  Try using a channel in the 149 - 165 range if you're not limited by signal strength.  I have noticed that some wi-fi equipment won't let you set up on the higher channel numbers so you should find that very few of your neighbours are there.

On Wikipedia there's a table of 5GHz wi-fi channels showing which ones use DFS.

Hence my question about the bandwidth setting - you could still be on DFS space if the bandwidth is large.
You'll find 36 is fine with any bandwidth setting - but it's likely to be the one your neighbours are using - which will only be an issue if they're really close anyway.

149 would be a good suggestion but it's not supported on your 2927 (needs to be a DrayOS 5 model to do channels above 140) but even then only if your own clients can 'see' it there - Google Home devices - at least the earlier ones - cannot.

Some of my neighbours' 5GHz WiFi signals from adjacent flats are stronger than my own from the adjacent room.  I can see about a dozen SSIDs and almost all of them are 80MHz wide and in the bottom block of non-DFS channels.  So, for me, getting out of that 32 - 48 channel range is essential in order to get a reliable radio link.  Surprisingly, it actually works without noticeable degradation most of the time even with the interference.

I hadn't noticed that my DrayOS 4 device (2866Vac router) doesn't allow the highest channel numbers.  The one thing that I put there is a DrayOS 5 device (AP905 access point).  I'm not sure if DrayOS 4 devices' hardware prevents the use of these radio channels, maybe because the frequency range is limited by the RF filters fitted.  Or perhaps it's an old software setting or the transmitter not supporting the required transmit power reduction (for SRD).  Perhaps a firmware update could fix it...

Even if firmware could fix it they're unlikely to at this point.

I'd not heard of SRD restrictions until you mentioned it. It's interesting that this (admittedly quite old) Draytek page makes no mention of it and shows the 149+ channels as having the same max power ratings as 36-64.

https://www.draytek.co.uk/support/guides/the-5ghz-wireless-band

If you read further down the wiki article you linked to it does say this :

"United Kingdom
The UK's Ofcom regulations for unlicensed use of the 5 GHz band is similar to Europe, except that DFS is not required for the frequency range 5.725–5.850 GHz and the SRD maximum mean e.i.r.p is 200 mW instead of 25 mW.[61]"

So there is no lower power requirement here in the UK anyway - it's exactly the same as the lower channels (which will probably be why I've never noticed any difference even though I do use 149 on one of my AP805's)