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Motion Detection System creating Interesting Interference

9/7/2016

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If you are like me and you love interference stories, this is one of them! I came across this interesting interference while I was performing an assessment for one client.
Picture
At first I had no idea what it could be and I didn't know if the 2 pics were part of the same signal or not. So I didn't know if I was looking for 1 or 2 sources. After a while walking around with the spectrum analyzer open, trying to figure out where these interferences could come from, I still couldn't figure out where this was coming from.

I was working with my good friend Cédric Terrier on this one and he told me he had seen similar interferences coming from motion detection sensor. BINGO, after that it took us a few seconds to find them. Indeed, two of them were installed in two adjacent rooms.

Here is what they looked like:
Picture

The Technology behind these sensors

These sensors use an interesting technology to detect motion. It uses 2 directional antennas to send signals on the 2450MHz frequency (according to the specifications. In reality, we see that different frequencies are in use). Their technology allows a more precise measurement of the motion and is event able to detect the speed of the intruder!

Here is their antenna pattern:
Picture
And here are the technical specifications:
Picture
All the information related to this product can be found in the following specification sheet document:
detecteur_aritech_dd400.pdf
File Size: 51 kb
File Type: pdf
Download File

Mitigation

In this case, the mitigation was easy since we found out that the system was not used anymore. So they simply disconnected them.

Interference Summary

The following table summaries the characteristics of this interference:
Picture
Thank you for reading!

Cheers'

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

5/24/2016

2 Comments

 
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.
Picture
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)
Picture
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:
Picture
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:
Picture
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)!
Picture
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:
Picture
Picture
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:
Picture
Picture
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:
Picture
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
2 Comments
    Picture

    François Vergès

    François Vergès is the founder of SemFio Networks. As a Network Engineer, he has a real passion for Wi-Fi.

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