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5GHz wifi on 3B+ triggering DFS radar detection

Posted: Thu Apr 26, 2018 8:26 pm
by trilby
Something of a weird problem... As you might know, all 5GHz wifi channels above channel 48 require the use of Dynamic Frequency Switching (DFS) which basically means that if the access point (i.e. the wifi router) detects radar signals then it automatically switches to a different wifi channel. My problem is that my Raspberry Pi 3B+ seems to be causing my wifi router to think that it's detecting radar whenever the Pi is connected to it using a 5GHz channel.

I'm using a Netgear R7800 router running the latest OpenWrt (open source firmware) and the Pi is configured to connect to its 5GHz 802.11ac network. With the Pi turned off, the router works fine using any DFS channel and with multiple 5GHz devices connected to it. However, the moment I turn the PI on, something like this appears in the router log:

Code: Select all

Thu Apr 26 20:17:14 2018 daemon.notice hostapd: wlan0: DFS-RADAR-DETECTED freq=5500 ht_enabled=0 chan_offset=0 chan_width=3 cf1=5530 cf2=0
Thu Apr 26 20:17:14 2018 daemon.notice hostapd: wlan0: DFS-NEW-CHANNEL freq=5180 chan=36 sec_chan=1
That indicates that the router has detected radar signals and moved to a new wifi channel. If the new channel is also a DFS channel, this process repeats until it has switched to a non-DFS channel (48 or below). This seems to happen no matter which DFS channel the router is using to start with.

Does anyone have any idea what the 3B+ might be doing that makes the router think that it's emitting radar signals?

Re: 5GHz wifi on 3B+ triggering DFS radar detection

Posted: Sun Apr 29, 2018 10:14 am
by epoch1970
AFAIK DFS is an AP-side function, nothing is required from a regular STA to comply with DFS.
Do you get the same false positives with a stock firmware on the AP?

Re: 5GHz wifi on 3B+ triggering DFS radar detection

Posted: Sun Apr 29, 2018 3:15 pm
by mushu999
Interesting. I have a 3B+ and a Netgear R7000 Nighthawk router with factory firmware and my logs don't show anything like that.

Re: 5GHz wifi on 3B+ triggering DFS radar detection

Posted: Mon Apr 30, 2018 11:35 am
by CasioJones
There are definite issues with the B+ & the netgear r7800 with wifi.

I had stock firmware last week & could not connect with the pi from my windows machine, although it would work from the pi.
So a couple of days ago i switch from stock to dd-wrt (Kong) , and now i have less issues- but i cold not duplicate your problem.
However saying that i dont know wifi very well - apart from telling when it doesn't work.

I would suggest trying another router just to see if it's the B+ or not.

Chris