DFS and scan list + auto frequency

pe1chl · Thu Nov 05, 2020 5:55 pm

In another attempt to get DFS working on a newly installed wAP ac (before I throw in the towel and disable DFS, as it usually goes...) I tried this setup:

set [ find default-name=wlan2 ] band=5ghz-a/n/ac \
    channel-width=20/40mhz-Ce country=netherlands disabled=no distance=\
    indoors frequency=auto installation=indoor mode=ap-bridge scan-list=DFS

where the scan-list DFS contains:

add band=5ghz-a/n/ac extension-channel=Ce frequency=5540 list=DFS name=ch108 \
    width=20
add band=5ghz-a/n/ac extension-channel=Ce frequency=5500 list=DFS name=ch100 \
    width=20
add band=5ghz-a/n/ac extension-channel=Ce frequency=5660 list=DFS name=ch132 \
    width=20

I would have hoped that it would automatically pick a channel from the list (100,108,132) and
check if radar is on the frequency, and of so it would search again and land on another channel.

However, it always selects 5540 and after:
wlan2: radar detected on 5540000
it just disables the interface for some time (detecting radar) and after it finds nothing it continues
to operate on 5540. It never switches to one of the other channels.

Is my understanding of the DFS process wrong? When I use a scanlist on 2 GHz (I normally make
one with channels 1-6-11 and select auto) it finds a random channel from these 3. Of course there
is no DFS on that band, but it appears that the auto frequency on 5 GHz does not even try to
find a suitable channel but always selects the first one from the list. When I open the status tab
of a 2 GHz interface and then disable/enable it I see it quickly look at the 3 frequencies and then
stick on one. At 5 GHz I do not see this happen, it always selects the first one and never moves.

Is this a bug or is it intended functionality? I run 6.47.7 on it.

bpwl · Thu Nov 05, 2020 6:49 pm

This works for me on SXTsq ac (not set to "auto", just had set the frequency 5540). By radar detect the DFS list is used top down. It's not 100% clear if the list continues or not, with the second radar detection later on. (It's too long ago to reconstruct. Reboot and any modification restarts everything. I removed the source of false posities (wAP ac on 5220) in my setup, so no recent incidents available)

Differences are: ROS 6.45.6, SXTsq, frequency set to 5540, installation = any
The 5540 will not be checked as possible solution on radar detect, but another from the list. (next? can't tell for sure)
Is "auto" just checking every freq in the channel list, and your first one (5540) passes the radar test ?????
In my list 5500 is the first , 5660 is the next after 5540. List is 5500,5540, 5660, 5580

Feb/20/2020 22:18:31 dude,event syslog: 192.168.20.5: wireless,info wlan1: radar detected on 5540000
Feb/20/2020 22:18:31 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:7B@wlan1: disconnected, disabling
Feb/20/2020 22:18:31 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:A5@wlan1: disconnected, disabling
Feb/20/2020 22:18:31 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:B3@wlan1: disconnected, disabling
Feb/20/2020 22:18:33 dude,event syslog: 192.168.20.5: wireless,debug wlan1: must select channel
Feb/20/2020 22:18:33 dude,event syslog: 192.168.20.5: wireless,debug wlan1: radar reported on 5540000
Feb/20/2020 22:18:33 dude,event syslog: 192.168.20.5: wireless,debug wlan1: selected channel 5660000
Feb/20/2020 22:18:33 dude,event syslog: 192.168.20.5: wireless,debug wlan1: search for radars on 5660000
Feb/20/2020 22:19:33 dude,event syslog: 192.168.20.5: wireless,debug wlan1: no radar detetected, start network
...
Feb/12/2020 18:42:55 dude,event 
Feb/12/2020 20:11:54 dude,event syslog: 192.168.20.5: wireless,info wlan1: radar detected on 5540000
Feb/12/2020 20:11:54 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:7B@wlan1: disconnected, disabling
Feb/12/2020 20:11:54 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:A5@wlan1: disconnected, disabling
Feb/12/2020 20:11:54 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:B3@wlan1: disconnected, disabling
Feb/12/2020 20:11:56 dude,event syslog: 192.168.20.5: wireless,debug wlan1: must select channel
Feb/12/2020 20:11:56 dude,event syslog: 192.168.20.5: wireless,debug wlan1: radar reported on 5540000
Feb/12/2020 20:11:56 dude,event syslog: 192.168.20.5: wireless,debug wlan1: selected channel 5660000
Feb/12/2020 20:11:56 dude,event syslog: 192.168.20.5: wireless,debug wlan1: search for radars on 5660000
Feb/12/2020 20:12:56 dude,event syslog: 192.168.20.5: wireless,debug wlan1: no radar detetected, start network
...
but I also have this
...
Jun/19/2020 13:24:28 dude,event syslog: 192.168.20.5: wireless,info wlan1: radar detected on 5540000
Jun/19/2020 13:24:28 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:7B@wlan1: disconnected, disabling
Jun/19/2020 13:24:28 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:A5@wlan1: disconnected, disabling
Jun/19/2020 13:24:28 dude,event syslog: 192.168.20.5: wireless,info xx:xx:xx:A7:EB:B3@wlan1: disconnected, disabling
Jun/19/2020 13:24:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: must select channel
Jun/19/2020 13:24:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: radar reported on 5540000
Jun/19/2020 13:24:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: selected channel 5500000
Jun/19/2020 13:24:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: search for radars on 5500000
Jun/19/2020 13:25:22 dude,event syslog: 192.168.20.5: wireless,info wlan1: radar detected on 5500000
Jun/19/2020 13:25:25 dude,event syslog: 192.168.20.5: wireless,debug wlan1: must select channel
Jun/19/2020 13:25:25 dude,event syslog: 192.168.20.5: wireless,debug wlan1: radar reported on 5540000
Jun/19/2020 13:25:25 dude,event syslog: 192.168.20.5: wireless,debug wlan1: selected channel 5500000
Jun/19/2020 13:25:25 dude,event syslog: 192.168.20.5: wireless,debug wlan1: search for radars on 5500000
Jun/19/2020 13:25:27 dude,event syslog: 192.168.20.5: wireless,info wlan1: radar detected on 5500000
Jun/19/2020 13:25:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: must select channel
Jun/19/2020 13:25:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: radar reported on 5540000
Jun/19/2020 13:25:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: selected channel 5500000
Jun/19/2020 13:25:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: search for radars on 5500000
Jun/19/2020 13:26:30 dude,event syslog: 192.168.20.5: wireless,debug wlan1: no radar detetected, start network

...
other device, different list
...

Jun/28/2020 19:09:05 dude,event syslog: 192.168.20.12: wireless,info debug: wlan1: radar detected on 5540000
Jun/28/2020 19:09:05 dude,event syslog: 192.168.20.12: wireless,info debug: xx:xx:xx:A7:EB:73@wlan1: disconnected, disabling
Jun/28/2020 19:09:09 dude,event syslog: 192.168.20.12: wireless,debug debug: wlan1: must select channel
Jun/28/2020 19:09:09 dude,event syslog: 192.168.20.12: wireless,debug debug: wlan1: radar reported on 5540000
Jun/28/2020 19:09:09 dude,event syslog: 192.168.20.12: wireless,debug debug: wlan1: selected channel 5580000
Jun/28/2020 19:09:09 dude,event syslog: 192.168.20.12: wireless,debug debug: wlan1: search for radars on 5580000
Jun/28/2020 19:10:09 dude,event syslog: 192.168.20.12: wireless,debug debug: wlan1: no radar detetected, start network

pe1chl · Thu Nov 05, 2020 7:15 pm

Ok thanks for info. It is a different chip and a different RouterOS version, maybe that has some influence as well.
I am sure that those channels I put in the list (100,108,132) do not have radar, but there occasionally is misdetection due to nearby phones transmitting on 3G/4G.
Best solution is probably to disable DFS. As it is now, it just isn't a workable system (not only for MikroTik, other manufacturers have similar misdetection issues)...

bpwl · Thu Nov 05, 2020 8:01 pm

Well outdoor you have no choice in Europe (ETSI). There are no U-NII-3 ( 5.8GHz) channels allowed. And channels 36-48 (+DFS 52-64) are indoor-only.

First device was SXT sa5 ac, second SXTsq ac.

pe1chl · Fri Nov 06, 2020 12:37 pm

Of course we all want a DFS system that does what it is supposed to do: detect the presence of other band users (weather radar) and switch to another frequency when they are detected.
But it seems the manufacturers are unable to make it. What we get is a system that makes false detections (affected by nearby mobile phones transmitting on 2G/3G/4G) and even worse: when it makes such a false detection it does not exhibit reasonable behavior. e.g. in my case I want it to try only those 3 channels, not leave it free to choose another channel and then select ch116 or ch124 where the weather radar is actually present, so it will get a DFS hit AGAIN and waste more time. But the combination of auto frequency selection, a scan list, and DFS enabled does not seem to work correctly.

Faced with that situation, and without any help from the manufacturer, the only recourse is to disable DFS and operate it on the channels where there is no radar.
It is not that the users are not willing to operate with DFS, it just is that the manufacturers do not provide a working DFS solution.

johnmoore · Tue Dec 01, 2020 6:25 pm

How can one completly disable the DFS feature? I was under the impression that one can not!

PackElend · Tue Aug 03, 2021 10:12 pm

Hi there,
it looks like that I'm not the only one having difficulties understanding MikroTik wireless implementation in regard to frequency selection.

As there are many guides on how to optimize 2 GHz selection, I checked what is out there for 5 GHz and came across the inconsistent DFS-mode implementation.

My understanding is that, in 2 GHz, the device boots up, checks the frequency listed in the scan-list and select the most suitable one out of those listed in frequency.
In that case, I leave scan-list blank, so default is active, which means all hardware supported frequencies are check, as I don't know if ROS also takes into account occupation of neighbouring channels due to possible adjacent channel interference. In addition

In the case of 5 GHz, I would expect that only Channel Availability Check (CAC) and In-Service Monitoring (ISM) is added that's it but:
In Wireless Local Area Networking - RouterOS - MikroTik Documentation is stated that:

frequency (integer [0..4294967295]; Default: )
Channel frequency value in MHz on which AP will operate.

Allowed values depend on the selected band, and are restricted by country setting and wireless card capabilities. This setting has no effect if the interface is in any of the station modes, or in wds-slave mode, or if DFS is active.

as DFS is active for any frequency beyond Channel 48 (see List of WLAN channels - Wikipedia for details) it is pretty much useless to set anything in frequency for 5 GHz, the only options to avoid DFS-channels as discussed in HAP AC2 5ghz aces point starting up slow? : mikrotik (reddit.com)
It also suggests that one can deactivate the DFS, which is not allowed and should be technically possible.

Preventing deactivating this is done by removing this setting from version 6.37, see Mikrotik Changelog 6.37 | Greg Sowell Saves The World

— IMPORTANT! DFS CHANGES:
DFS configuration in RouterOS has been redesigned, now device looks at specified country settings (/interface wireless info country-info), and applies corresponding DFS mode for each frequency range automatically, making dfs-mode setting unnecessary.

!) wireless – DFS option is removed, corresponding DFS mode for each frequency range applies automatically;

much more details and background info in this regard is given in Radar Detection and DFS on MikroTik | MikroTik User Meeting London 2016

Last but not least, I'm not sure if the scan-list may limit the frequencies scanned by /interface wireless scanner and /interface wireless frequency-monitor.

What do you think about the above said?

PackElend · Wed Aug 04, 2021 10:14 am

Last but not least, I'm not sure if the scan-list may limit the frequencies scanned by /interface wireless scanner and /interface wireless frequency-monitor.

If found hints in Scan - Manual:Interface/Wireless - MikroTik Wiki

Scan command allows to see available AP in the frequency range defined in the scan-list

but nothing mentioned in regard to Snooper - Manual:Interface/Wireless - MikroTik Wiki and interface wireless frequency-monitor.

PackElend · Fri Aug 06, 2021 3:18 pm

FYI:
I'm in touch with support, first improvement, wiki/help pages are merged.
Instead of
https://help.mikrotik.com/docs/display/ ... +Interface
https://help.mikrotik.com/docs/display/ ... Networking
there is only
https://help.mikrotik.com/docs/display/ ... +Interface

I'll keep you posted

DFS and scan list + auto frequency

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