A surprising culprit

I have often blogged about the effect of interfering devices with regards to wireless networks.  As I have stated, there are many "common" sources of interference.  Examples include Bluetooth devices, microwave ovens, and cordless phones.  

From time to time, however, I am surprised by other devices that can interfere with the performance of Wi-Fi networks.   This post investigates one such device.

Actually, I can't even claim credit for discovering this one.  Some colleagues of mine (Micah Frisby and Phillip Field) found this while troubleshooting a situation at a university.  A specific area of the university was experiencing an issue where laptops could not connect to the wireless network.  My colleagues performed a spectrum analysis of the area. Spectrum analysis uses a special tool to look at the airwaves at Layer 1 of the OSI model.   A snapshot of their analysis looked like this:

In this case, the tool being used was the Wi-Spy DBx.  It's a great low cost tool for performing such analysis.  The top two portions of the graph can be thought of as "heat maps" where warmer colors (such as red & yellow) indicate strong signal and cooler colors (such as green & blue) indicate weaker signal. 

The fact there is a spike in the middle graph indicates a source of interference.   The trouble is, you can't necessarily tell what the source of interference is just by looking at the graph (although you can compare it to the "signatures" of common devices, such as those along the right-hand side of this screenshot). 

In this case, my colleagues had to walk around the area searching for the source of interference.  The top graph in the spectrum analysis assisted them with their search.  The top graph is a "waterfall" graph, which graphs the signal strength over time, where time is graphed across the vertical axis.  Therefore, the two red streaks in the top graph represent the time my colleagues were closest to the interference.  Using this information, they were able to determine the interfering signal was coming from the motion detector pictured below.  

Traditionally, motion detectors and security cameras are hard wired into the back-end security system.  However, we are moving into an ever increasingly un-tethered world.  Some newer security systems use wireless detectors, cameras, etc. that send their information to the back-end system via Wi-Fi.  It's a great application of wireless technology, but unfortunately can also interfere with the wireless adapter in your laptop.  Ultimately, this can affect not only connectivity, but cuts back the amount of available throughput. 

To make matters worse in this instance, the device was emitting on channel 9.  Due to the spread spectrum nature of Wi-Fi, operating on channel 9 affects wireless devices operating on both channel 6 and channel 11. Remember, there are only three non-overlapping channels in 2.4 GHz.  Therefore, this motion detector limited the wireless network in this area to a single channel.

Have YOU ever been surprised about a device that interferes with your wireless network?   If so, please let me know by submitting a comment below! 

Douglas J. Haider is a Wireless Consulting Engineer with Xirrus. He hosts a personal blog at WiFiJedi.com, and micro-blogs on Twitter @wifijedi