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Beginner

Not all 5 GHz channels being used

Hi All,

I've got a deployment of 1142N APs using a pair of 5508 controllers running 7.3.101.0.  We're in the United States and the country is set to "US".  I've added the UNII-2 channels to Auto-RF on the controllers via "config advanced 802.11a channel add 100" and so forth.  According to the output of "

show advanced 802.11a channel", I've enabled every 5 GHz channel that's legal for use in the US:

  802.11a 5 GHz Auto-RF Channel List

    Allowed Channel List......................... 36,40,44,48,52,56,60,64,100,

                                                   104,108,112,116,132,136,140,

                                                  149,153,157,161,165

My confusion comes because the automatic assignment of 5 GHz channels is heavily biased toward just a few of them, with most of the available space being ignored.  Here are the 5 GHz channel assignments from a large, single-story building with 31 APs:

(qty 2) 36

(qty 6) 40

(qty 5) 44

(qty 7) 48

(qty 1) 64

(qty 1) 149

(qty 3) 153

(qty 3) 161

(qty 3) 165

A total of nine channels are used (two of them only once) while twelve other channels are ignored completely.  Many of these APs are close enough to hear one another, so assigning seven of them to ch. 48 makes no sense.

Looking at my campus as a whole, I've got numerous APs using channels 36, 40, 44, 48, 149, 153, 157, 161, and 165.  I've got at least one each using channels 52, 56, 60, 64, 112, 116, and 132.  Nothing at all is on 100, 104, 108, 136, and 140.  I am using 20 MHz wide channels, btw, so all are independent.

Can anyone shed light on A) why my distribution is so heavily skewed toward the first nine channels, and B) how I can learn whether certain channels are being avoided due to the presence of radar, etc, on the frequency?  I'd like to take full advantage of 5 GHz by having as little channel re-use as possible.

Thanks,

Allen

9 REPLIES 9
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Enthusiast

Hi

Agreed in the US you have 21 channels available for reuse. However you are limited to 8 channels because most clients cant support DFS in the UNII-2  and UNII-2e bands. Hence unless all your clients are DFS capable, I would suggest you use the UNII-1 and UNII-3 bands only.

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Hall of Fame Master

I use unii-1, unii-2 & unii-3, with no issues. unii-2 extended isn't supported by a lot of clients like mentioned in the other post. You do tend to see more unii1 & unii-3 than unii-2 unless you have high density deployments. I would worry about the distribution on the 5ghz.

Sent from Cisco Technical Support iPad App

-Scott
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Scott, can you clarify what you mean by "worry about the distribution on the 5 ghz"?  Thanks!

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I wouldn't worry about how many ap's are on what 5ghz channel... the algorithm will handle the best channel for that AP.

Thanks,

Scott

Help out other by using the rating system and marking answered questions as "Answered"

-Scott
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I have to say I try and stay away from UNII2 and UNII2E because of DFS. We get hit from time to time with radar alerts and it causes some havoc on our voice network.

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin

"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
___________________________________________________________
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I've only had one customer in which we ran into DFS issues, but maybe ist just in the Chicagoland, midwest area.  Good point though.

Thanks,

Scott

Help out other by using the rating system and marking answered questions as "Answered"

-Scott
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I see about 4 - 10 alerts a week in WCS ..

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin

"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
___________________________________________________________
Highlighted

In addition to my comments above, VoWifi phones implement off-channel passive or active scanning to find APs for potential roaming and the more channels a client has to scan, the longer it takes to find a suitable AP.  This also affects battery life.

Highlighted

Yup...

__________________________________________________________________________________________
"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin

"Satisfaction does not come from knowing the solution, it comes from knowing why." - Rosalind Franklin
___________________________________________________________
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