Clearing up your channels

Simply wanting to listen to my iPod in the car turned out to be a lesson in co-channel interference.  You see, I have a vehicle made in 2005.  Like most cars manufactured that year, it has neither a cassette deck nor an auxiliary port.  Therefore, I was forced to purchase an FM transmitter. 

You know what I'm talking about, right?  You plug it into your iPod, and then tune your car stereo to a specific frequency.  The model I purchased was a Belkin TuneCast Auto LIVE.   The problem with these devices is that the sound quality is poor - often, you hear not only whatever you are trying to play, but the underlying radio station as well.  This is because there is a radio station broadcasting at the same frequency as the FM transmitter plugged into your iPod.  These two competing signals make it difficult to listen to either one. 

This is similar to what we would call "co-channel interference" in the Wi-Fi world.  Instead of two competing FM signals, you would have two competing Wi-Fi signals from neighboring Access Points, which are operating on the same channel.   

This problem is especially prevalent in the 2.4 GHz frequency band, where there are only 3 non-overlapping channels (see graph below).  Each channel is 22 MHz wide, but the channels are spaced only 5 MHz apart.  Therefore, if you have one AP operating on channel 1 and another AP operating on channel 2, they will interfere with each other.  

 

In the radio world, this is analogous to where there are two competing stations in close proximity on the dial -- for instance, at 101.5 and 101.3 FM.  In a case like that, you encounter "adjacent channel interference".  The answer in both the radio and Wi-Fi realms is to create enough channel separation for multiple signals operating in the same physical space.  

In the case of my Belkin device, it has a feature called "Clear Scan LIVE" (with an associated iPhone application).  From the box, "ClearScan technology quickly scans and finds the best FM frequency - all with one push of a button."   Basically, ClearScan is trying to avoid co-channel interference by scanning the air and choosing a frequency (channel) that is not in use. 

A similar feature is available in most all enterprise-class wireless LAN gear.  In the Wi-Fi world, this feature is often referred to as "auto-channel" or "radio management".  In my opinion, it's a "must-have" feature when you are doing your vendor selection.   I would also like to see this technology built into more consumer-grade Access Points moving forward. 

So, the next time you are rocking out to the radio in your car and hear two competing stations, take it as your opportunity to teach those around you about co-channel interference.

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