This series of blogs (Defining Wi-Fi) will likely stretch to infinity. The blogs will focus on defining terms related to Wi-Fi at a level between the dictionary and a concise encyclopedia, but not quite matching either. Hopefully, the community finds them helpful over time.
NOTE: Entry created August 26, 2016.
The noise floor, in wireless networking, is the RF energy in the receiver space from other intentional and unintentional radiators nearby or at a distance as well as natural phenomena that results in the existence of electromagnetic energy at some measurable level. Defined differently it is the sum of all those signals or energy generators that you aren’t trying to receive. It is a moment-by-moment factor in RF signal reception. The following capture from AirMagnet Spectrum XT shows the noise floor related to channels 1 and 6 in 2.4 GHz.
Two common myths are believed about the noise floor.
- The noise floor is the same on all channels in a band.
- The noise floor can be measured at a moment and that is the constant noise floor.
The first myth is very important as the noise floor may well be several dB higher in some channels than in others (remember, -95 dBm is higher than -100 dBm when measuring RF energy). This will impact SNR (read my definition of SNR here) and cause variance in data rates available on those channels if not considered. While the noise floor may be constant across channels in what we sometimes call a “clean” environment, it is not uncommon to see channel 1 with a noise floor of say -97 dBm and channel 6 with a noise floor of say -95 dBm (these numbers are just for example purposes). This variance is a difference of 60% in signal strength. Depending on the received signal strength, it can easily result in a data rate 2-3 levels (or more) lower in the channel with a higher noise floor.
The second myth assumes that there are no intermittent radiators (a term used instead of transmitters to include unintentional radiators) present. Such radiators may only generate RF energy periodically and can be missed with a quick measurement. Additionally, such devices may cause reception problems after the WLAN is operational because of their manual use. That is, a human turns them on when they want to use them and, when at rest, they do not cause interference at other times. For example, microwave ovens.
We usually use the term interference (instead of noise floor), which I will define in detail in a later post, to reference nearby radiators that cause significant RF energy in a channel at levels greater than what the noise floor would be without them, such as the previously mentioned microwave oven. This differentiation is important because we can often do something about such components (remove them, change the channels, shield them, etc.). However, when considering the noise floor on a moment-by-moment basis, one could argue that these devices raise the noise floor. Why? Because even when they are present, a lower data rate Wi-Fi signal may be able to get through, if sufficient SNR can still be achieved.
However, if the other device is a transmitting device and not simply a radiating device, such a design decision may result in interference caused by the Wi-Fi device against the non-Wi-Fi device. Additionally, the Wi-Fi device is not likely to change its data rate based on one or even two frame retries. Therefore, the raised noise floor (interference in this case) results in higher retries instead of data rate shifts when the interference is on a low duty cycle (does not communicate a large percentage of the time). Yes, it can get complicated.
Here’s a great analogy when considering the noise floor. Many people like to sleep with a fan on. Why do they do this? They are raising the noise floor (of course, related to sound waves instead of RF electromagnetic waves). When the noise floor is raised around them, distant noises do not have as much sound to noise ratio and they are less likely to alert the sleeper. They are intentionally making it more difficult to receive audible signals by raising the noise floor.
The RF/electromagnetic noise floor is an important consideration in design. In an environment with a higher noise floor, the APs must be placed and configured with this in mind. Many vendor recommendations for AP placement and hardware specifications assume a particular noise floor (that they seldom communicate). If your environment presents a very different noise floor, their recommendations and receiver sensitivity ratings may not prove true.