5GHz Site Survey sometimes doesn't work on EU routers

[ColinTaylor]

As the title says, this is something that has bugged me for a couple of years now. So I just thought I'd throw it out there and see if "it's just me".

As far as I can tell this problem doesn't effect US routers, so I'm particularly interested in responses from fellow EU owners.

The problem: If you have your 5GHz band operational and currently using one of the DFS channels (52 - 140) doing a Wireless Site Survey will fail to show any 5GHz results. Reconfiguring the 5GHz radio to use a non-DFS channel (36 - 48) and rerunning the Site Survey will now show nearby 5GHz access points (irrespective of the channel they are operating on).

The cause: When the 5GHz radio is configured for DFS the scan is rejected.

# wl -i eth2 country
DE (EU/13) GERMANY
# wl -i eth2 chanspec
100/80 (0xe06a)
# wl -i eth2 scan -t passive
wl: Scan Rejected
# wl -i eth2 scan -t active
wl: Scan Rejected

# wl -i eth2 chanspec
36/80 (0xe02a)
# wl -i eth2 scan -t passive
# wl -i eth2 scanresults | grep ^SSID
SSID: "TP-LINK_5DD3_5G"
SSID: "VM5692010"
# wl -i eth2 scan -t active
# wl -i eth2 scanresults | grep ^SSID
SSID: "TP-LINK_5DD3_5G"
SSID: "VM5692010"

Side note: I wonder whether some of the problems reported by people trying to setup a "Media Bridge" might be because of the client's inability to scan the 5GHz band (assuming the client device was currently using a DFS channel).

Comments welcome.

 

[JDB]

I’d noticed this. And manually configured my Media Bridge as it wouldn’t find the router.
It is mildly annoying.


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[ColinTaylor]

Thanks for the feedback @JDB. It's nice to know I'm not imagining it .

I've never tried setting up a Media Bridge so didn't know whether there was a "manual" option. I'd speculate that out-of-the-box or after a factory reset the 5GHz channel would default to a non-DFS one, in which case people probably wouldn't even be aware of this. Just guessing.

 

[JDB]

My AC88U acting as a bridge connected as a client to 5Ghz ch100 does scan OK. It seems the issue is only when the wireless driver is in AP mode and a DFS ch (which it would be when you run through the Media Bridge setup wizard whilst in Router mode).


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[sfx2000]

The problem: If you have your 5GHz band operational and currently using one of the DFS channels (52 - 140) doing a Wireless Site Survey will fail to show any 5GHz results. Reconfiguring the 5GHz radio to use a non-DFS channel (36 - 48) and rerunning the Site Survey will now show nearby 5GHz access points (irrespective of the channel they are operating on).

The cause: When the 5GHz radio is configured for DFS the scan is rejected

That is correct behavior for DFS channels - since many site survey tools use Probe Request messages at the 802.11 layer, this requires the radio to transmit on the channel - with DFS, there is a period of time needed for the radio to detect incumbent radars in the area prior to transmitting on the channel...

 

[JDB]

Surely passive scan should still work on DFS channels though?


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