AP Internal and Module Radios Interfere Each Other

[tresdodi]

I have a lightweight access point AIR-CAP3602I-A-K9 with an AIR-RM3000AC-A-K9 ac radio module installed. The AP has a couple of built-in radios, one for 2.4 GHz and the other for 5 GHz that supports WiFi n at most. All works fine except that it appears that the 5 GHz radios interfere each other. Graphing stats collected over SNMP it is observable that they report other's traffic as interference. See below how interference parallels traffic:

      Built-in radio                           Module radio

        Built-in radio                           Module radio

The module radio adds ac support and 160 MHz channel width but functionally it's an extension of the internal radio. Some settings can be specified separately, but many others on the module are linked to the internal radio, like the channel and operational status (enabled/disabled). So, I can't set them on different channels. And if I turn off the internal radio the external one also turns off.

Is this mutual interference expected when using a radio module or am I missing some configuration that would solve this?

[Rich R]

I've never used that module so can't say what's normal but I don't see how they can work on the same channel as that will definitely cause interference!
Note that module is not an "extension of the internal radio" - it's an independent self-contained third radio.
Note also max channel width it supports is 80MHz.
https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/aironet-access-point-module-802-11ac/data_sheet_c78-727794.html

What WLC and version of software are you using?

[tresdodi]

Hello Rich. You are right, it only gets to 80 MHz. The GUI letting me select 160 MHz misled me.

The WLC is an AIR-CT2504-5-K9 V03 with software v8.5.164.0. Under Wireless > Access Points > Radios > 802.11a/n/ac I can set the channel for the internal radio:

However, the same setting for the module is not available and the channel will be set to the same as the internal radio:

Also, if I disable the internal radio the module shuts down together:

[Rich R]

I'd call that a bug in the GUI!  Only things I can suggest:
- Update to 8.5.182.7 (below) to make sure you're on the latest available code with all available bug fixes.
- Try configuring from CLI rather than GUI.

If neither of those work then you're on your own as the product is end of support so Cisco won't even consider opening a bug for it.

[tresdodi · Accepted solution]

Doing further digging I found Cisco Wireless Controller Configuration Guide, Release 8.5, which says that "The 802.11ac radio in slot 2 is a subordinate radio [...]. Because the 802.11ac is a subordinate radio, it inherits many properties from the main 802.11a/n radio on slot 1." Furthermore, under section Restrictions for 802.11ac Support the GUI restrictions that I see are plainly stated:

• The 802.11ac module is turned off if the built-in 5-GHz radio is turned off.
• The 802.11ac module main channel cannot be changed individually.

Fair enough. Still, these limitations surprise me because they mean that I'm stuck with the radios interfering each other, unless there's some WLC handling occurring under the hood to mitigate this.

[Rich R]

Well spotted - I was thinking to go looking through the docs but you got there first!
Yes, it makes no sense at all...