10/22/2010
Wireless LAN – Interference Issues
When it comes to WLAN interference issues the worst offenders are analog video senders. These devices come in a few different flavors; security cameras, baby monitors and A/V transmitters are the most common. The key to their destructive way is that they are analog. Unlike digital device, analog transmitters are always transmitting, even when they have no new information to send. This equates to heavy band utilization on the channel(s) the device is operating on and make for very difficult Wi-Fi operation.
The first time I found one of these devices "in the wild" it really threw me. The effect of the device (later determined to be multiple devices) was devastating. This was before the era of portable PCMCIA or USB spectrum analyzers and the only option was to drag out a 25lb. HP bench-top spectrum analyzer. Needless to say, fewer spectrum samples were taken back then.
I was onsite surveying the building for an 802.11b deployment and first noticed that my survey tool was recording almost no coverage based on a minimum receive level even with the AP directly in front of me. Thinking that I had a hardware or configuration problem I reviewed the AP configuration, swapped out my wireless adapter, changed channels and finally reinstalled my survey software. Feeling somewhat defeated I packed up and went for some lunch. While I ate, I decided to fire up my laptop and try again. Sure enough the signal recorded was good. Later, back onsite, the problem remained and I knew I was likely dealing with some serious interference, the worst I had ever seen.
The next day I enlisted the help of another engineer and that bench top spectrum analyzer. Within 30 minutes we had located the first offender. It was a small black and white wireless security camera with its own integrated receiver and monitor. Once we located one, the rest were easy to find. I believe there were six or seven of these devices in the one building and they were spread out pretty well.
I was able to get the users of these devices to set them all on the same channel (of 4 available) while I surveyed the building for coverage. Video quality on each monitor took a nosedive, but I was able to get my work done. My recommendation was to move these devices to 900MHz to avoid interference with the new 802.11b system. These days, these cameras could likely be adopted into the WLAN by using 802.11 wireless cameras and monitoring them through a web browser.
One quick note: if you want somebody to swap out their equipment for something more compatible and there is no existing policy regarding the use of RF devices in your organization – they will generally want YOU (your department anyway) to pay for it! Mission critical applications running on unlicensed radio are on the rise – think about getting an RF usage policy together for your organization if you don’t already have one!
SideBand can help you develop such a policy.
M Tarr, Senior WLAN Engineer
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