trilby
Posts: 1
Joined: Thu Apr 26, 2018 7:58 pm

5GHz wifi on 3B+ triggering DFS radar detection

Thu Apr 26, 2018 8:26 pm

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?

epoch1970
Posts: 3894
Joined: Thu May 05, 2016 9:33 am
Location: Paris, France

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

Sun Apr 29, 2018 10:14 am

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?
"S'il n'y a pas de solution, c'est qu'il n'y a pas de problème." Les Shadoks, J. Rouxel

mushu999
Posts: 29
Joined: Sun Aug 20, 2017 11:24 pm

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

Sun Apr 29, 2018 3:15 pm

Interesting. I have a 3B+ and a Netgear R7000 Nighthawk router with factory firmware and my logs don't show anything like that.

CasioJones
Posts: 2
Joined: Sun Jun 11, 2017 10:54 pm

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

Mon Apr 30, 2018 11:35 am

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

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