Interference that looks like Wi-Fi but is not (Televic Conference System)

Performing a spectrum analysis for one of my customer last week, I discovered a funny interference I wanted to share with you. The interference was found in a training classroom of a typical office environment.

As you can see, the interference was about 20MHz wide and kind of looked like what a Wi-Fi signal would generate. The only thing is that is was on channel 3. I found it strange because the customer had his whole infrastructure deployed on channel 1, 6 and 11 on the 2.4GHz band. So I started to look for other Wi-Fi networks in the list of Wi-Fi network that Chanalyzer was detecting but I couldn't find any... (See picture below)

So, with the customer, we started to look for a potential source of interference. We first started to look at some wireless microphones but they were using lower frequencies (in the 500MHz). Then, we found a device that looked like an Access Point:

Funny enough, the name of this device is the Televic Conference Confidea Wireless Control Access Point. It works as an access point for these following wireless microphones called Confidea DD/CD G3 Wireless Discussion Unit:

And there were about 10 of these units in the training classroom.

Performing some testing, we found out that turning these units on was adding more activity on the channel used. The more units were in use, the more the channel appeared to be used.

So to make sure that we have had found the interference, we unplug the Wireless Control Access Point. And the channel cleared up. We were now sure that this conferencing system was the interference detected.
So we plugged the system back on, and surprised, it started to use another channel (channel 8)!

So the system appeared to be choosing a channel randomly before using it. Later on, and after a visit on Televic website, I found out that the Wireless Control Access Point was not choosing the channel randomly. Here is what was stated:

Apparently, by default, it can use any channels on the 2.4GHz band, which can be problematic if you have your WLAN infrastructure deployed over channel 1, 6 and 11.
I was not able to find more details about what the 4 bands were but I have seen the Televic system use the 2.4GHz band as well as the UNII-3 band.

How to deal with it

There is a way to block certain frequencies on the Confidea system management interface. This way, you have some sort of control on what channels it will be using. You can avoid having the system using channels overlapping with your Wi-Fi infrastructure.

Here is what the documentation says:

So in our case, we have 3 of these systems deployed in 3 training classrooms. Taking into consideration the customer context and environment, here is what I have recommended:

• Configure the 3 Confidea systems to use frequencies in the 5GHz band. Each system will have to be using a different channel to avoid interference. Do not use the 2.4GHz band.
• Create a RF profile on the controller excluding the use of these 3 5GHz channels.
• Use this RF profile for the access point deployed in and around the training centre.

Interference Summary

The following table summaries the characteristics of this interference:

It is always fun (for me) to find sources of interference and find a way to mitigate them as best as possible!

Cheers'


Written by François Vergès

Sources:
 - http://www.televic-conference.com/en/confidea-wireless-conference-system
 - http://www.televic-conference.com/cms_files/N-71-Foto2.jpg
 - ​http://www.televic-conference.com/cms_files/N-300-enImgImage_1.jpg
 - http://www.televic-conference.com/en/confidea-wcap
 - http://www.televic-conference.com/en/confidea-DD-CD