40MHZ and Channel Planning

[Erik2]

Hello

Quick question.  I have manually set the 5GHZ channel width on our system to 40MHz.  I have noticed that the channels being selected are overlapping. 

I would assume that if I defined 40MHz channel it would pick from one of the 14 non overlapping channels.  That is what I would like it to do.  With our density 14 channels would work perfectly in terms on non-overlapping etc.

When I go into the channel planning area I am only allowed to select 20MHz channels as allowed.  Selecting these fills in the 40MHz area.

The problem as stated above is that even though my channel width is 40 only it is still picking a mixture of 20 40 and even 80 MHz channel numbers causing overlaps that are even showing as high interference in the AP neighbors area.

Is there any way to force the 40MHz channel APs to only pick from the "40MHz" channels?

Or am I think wrongly.  Hope this all makes sense.

Thanks 

[RWelch]

You can go to Wireless > Configure > Radio Settings and select the RF profile (tab) and select UPDATE AUTO CHANNELS to refresh the channels.  In my experience that has fixed close proximity AP overlap issues. 

[Erik2]

Shouldn't it do that itself?

[RWelch]

Auto Channel dynamically adjusts the channels of the client-serving radios to avoid RF interference (both 802.11 and non-802.11) and develops a channel plan for the wireless network. 

Auto Channel is a good fit for most wireless networks, providing a baseline channel configuration that can then be adjusted manually if needed. 

https://documentation.meraki.com/MR/Monitoring_and_Reporting/Location_Analytics/Meraki_Auto_RF%3A__W...

[RWelch]

Pressing the Update Auto Channels button on the radio settings page in the dashboard will force a one-time optimization of channels used by all APs within the dashboard network.

This will usually result in a minute or two of downtime as the APs adjust their channel. 

[GIdenJoe]

It has to do with the way the channels are displayed.
When you select your channels in the RF profile you get like "in between" channels if you look at the 40 MHz and the 80 MHz lines.
However what happens in effect that your have a primary 20 MHz channel + a secondary channels.  So 36+40, 44+48 etc etc.  Primary channels are used for all mgmt and ctrl frames and only the full 40 MHz is only used during datatransfers if your client supports it.

The problem you do have to look at is that you don't have any overlapping areas where the primary channel is swapped.  You would risk more collisions when you have 2 BSS'es where one is 44+48 and the other is 48+44.  This will cause issues because channel looks clear during mgmt and ctrl frames and then collide during the data frame transmission.

I also found that the auto-rf sometimes still uses a channel that has been used even within range so while it is working it is not perfect and needs checking into.  I'm hoping the new AI-RRM with the use of NDP will improve this.

[Erik2]

Thanks guys for your response so far.

Take a look at this graph from AP neighbors.

It is choosing channel 40 for the channel.  40 isn't even considered one of the non overlapping 40MHZ channels.

I would really like it if I could have my all 40MHZ width APs choose only from the non overlapping.

EDIT:  Also, we still have a profile that uses 20MHZ channel widths as you can see from the diagram.  So couldn't this be a problem?  The 40 channel is looking at the 40 area for clear to send and then when data is transferring the 44 could be in use without it knowing since it is a 20MHZ channel and cause issues?

[RWelch]

Are you assigning one RF profile across the entire network or assigning a separate RF profile to each AP that is or would be different RF profile for each AP?  I tend to create a separate RF profile for each AP that I manage in order to reduce the channel overlap - it's kind of a cumbersome task up front but alleviates problems long term.  Just a thought or idea if you aren't doing so already.

https://documentation.meraki.com/MR/Radio_Settings/RF_Profiles

[Erik2]

How many APs do you usually have?

[GIdenJoe]

Jep that does look bad :s
It does not make sense an AP would choose 40+44 or 44+40 since that effectively causes problems for channel 36 and 48.  This would only make sense if channel 36 is disabled in the RF profile but this does look like bad behavior in the algorithm.

[Erik2]

I am going to move down to 20MHZ overnight here and see if that makes a difference.  We have an AP per classroom.  I am nervous about the bandwidth loss but hopefully should be ok.

Any opinions?

[GIdenJoe]

You should be able to have a rough estimate how much bandwidth you could have using 20 MHz by using a taking the average PHY rate of all clients you expect to be connected to the AP and then taking about 2/3 to 1/2 of that as actual goodput.

However the best thing you can do is allow for a few testing days of normal usage and just check the AP's of interest what their average airtime is.  If you keep below 50% in heavy use you should be fine using 20 MHz.