For a signal to be heard and understood by a receiver, it must be loud enough to overcome the noise floor, or background RF noise.
If the signal isn't significantly louder than the noise floor, the receiver won't be able to understand it, much like when you can't understand what another person is saying in a noisy room.
In 2.4 GHz, the noise floor is typically around -105 dBm or so. 5 GHz noise floors are generally a bit higher, averaging approximately -95 dBm. Every environment will have a slightly different noise floor.
In most 2.4 GHz environments, the noise floor is not visible with Wi-Spy.
A 2.4 GHz environment where the noise floor isn't visible.
A temporarily heightened noise floor is usually caused by interference from an out-of-band interferer, or a transmitter or electric device that is unintentionally causing interference in a band other than it's own.
A 2.4 GHz environment where the noise floor has been elevated.
Out-of-band interference can be caused by:
- Heavy industrial equipment
- HVAC equipment
- Large electric motors
- Nearby cell towers
A Verizon or other CDMA cell phone a few inches from a Wi-Spy receiving a phone call will cause a tiny bit of out-of-band interference. Typically, we don't see the same effect with a GSM phone.
Out-of-band interference isn't always constant. A transmitter sendings bursts or an electric motor turning on and off will cause the interference to come and go. You can track when and how often it happens with the waterfall view. Color indicates amplitude, so the black portions are where the noise floor was normal, with the blue timespans showing where activity occurred.
The first (lower) half of this waterfall view shows where the noise floor was elevated.
Locating an Out-of-Band Interferer
Try to find the source by walking around, looking for the point at which the noise floor is the highest. The higher the noise floor, the closer you are to the out-of-band interferer. You can watch the height of the Density View or the intensity of the color (blue is quiet, closer to red is louder) in the waterfall view to find where the noise floor is the loudest.