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DFS Channels Problem

Hi All,

In my WiFi network, I have 10 1850 series APs and 3500 series WLC. The site survey was done 2 years ago before me by one of the IT companies and each out of 10 APs was assigned 40mhz channel going sequentially from 36 to 132.

The problem is that last weeks I started to get a lot of complaints and after investigation I noticed that APs changes their channel even though DCA is turned off on them. I noticed that half of APs hears a lot of radars (even though I am not close to any radar station and I think they are false positives) in most of the channels and move to some interfered channel which creates a problem. I was advised in this forum to move all APs to non-DFS channels but 36 40 44 48 are not enough for my AP count considering they are not very far away from each other and I am using bonded channels.

What should I do?

Any help is appreciated!

Thanks in advance!

Rising star

First thing is to determine which channels are seeing the DFS events, and remove them from your channel plan.

Depending on the number that are seeing it you might need to go back to 20MHz wide channels to reduce CCI.


Even if they are false positives due to regulations if the AP thinks it's seeing a DFS event it has to by law move off that channel.


I have seen other things cause DFS events like infrared/ thermal light sensors, and other things. So a site survey to try and identify the source might be required.


There are extreme measures you can go to like coating the windows in RF blocking film - but be aware that this could also take out the 3/4/5G cell signal as well

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Hall of Fame Master

There isn’t much you can do. DFS channels from all enterprise vendors, will change channels no matter if DCA is disabled. If you can determine what channels are affected, remove those channel(s) from the DCA list.
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If you haven't already - ask what changed in those offices. Did they install new motion detection equipment for lighting/security/electric doors etc? New equipment which radiates microwaves (like microwave oven). Any new electrical/equipment installations or changes which might be generating interference. Otherwise has anything like that been installed outside recently? In the UK many speed and red-light cameras use radar detection which is quite strong although usually a fairly focused beam. As Haydn said using 20MHz channels reduces the likelihood of false radar detections and will still provide enough bandwidth for most users.
VIP Advisor

There were various DFS bugs in older software releases. Make sure you run a fairly current version on the WLC.


Actually I use the recommended image by Cisco (8.5.151 something like that which I do not remember and cannot check right now :D). But I kinda fixed the problem by disabling all DFS channels from DCA leaving only 4 channels and using DCA in APs with 20Mhz channels which helped. 


Yeah, if you indeed have that much DFS activity, then that would be the second solution.


The AP's you use lack something what Cisco calls Dual-DFS logic. Due to this the AP only uses the main chipset to determine if the detected "energy spike" is indeed DFS which results in a lot of false DFS hits. The workaround is to only use UNII-1 and UNII-3 channels (when allowed), as you already discovered :-) Sadly this can't be done on outdoor access points like the 1530...


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