Detecting the RF Noise Floor

In preparation of the CWAP exam I came across a major misconception of how the RF noise floor is detected and measured that warrants being shared. Before I start let’s define what RF noise floor is.

Noise Floor

The RF noise floor is defined as all the background RF signal that is received in the frequency range that your device is operating in. This could be generated from any type of device whether an intentional radiator, such as wireless cameras, or an unintentional radiator, such as lights or motors. Determining the noise floor is important in in site surveys and troubleshooting because with it you can determine the signal to noise ratio (SNR), which is the measurement in dB of how high your received signal from access points is above the noise floor. Or in simpler terms, how much of the signal from the APs can actually be heard.

RF Noise Detection in Wireless NICs

The wireless NIC is not a spectrum analyzer, and though it can transmit and receive data at impressive rates, the only thing getting past its encoding filter is bits, it cannot see raw ambient RF signals.

So how does the wireless NIC report the noise floor? After all you have seen many different screens from various 802.11 devices displaying either noise or signal to noise ratios (SNR). Well, various vendors have come up with unique ways to guess the noise floor but again; since wireless NICs can only process bits they cannot see ambient RF signals. What’s worse? Each vendor that manufactures 802.11 devices calculates noise in a different way. Each of them have developed sophisticated algorithms for calculating noise based on bit errors and other factors. Some have even figured out how to turn off the bit encoding filters and use RF signals coming through the antennas much like a spectrum analyzer, however they can never encode bits at the same time.

Scenario

Imagine that you are in a room surrounded by a Faraday cage and absolutely no RF signals from outside the room are audible. You turn on your wireless NIC based protocol analyzer. Obviously, there will be no noise to measure.

Now you bring a microwave oven into the room and turn it on. Guess what…. still no noise is detected. Why? There are still no modulated signals and thus no bits to corrupt or harm.

After that, you bring in an AP. Finally you will see noise, but the values will be very low because the AP will only be beaconing meaning that there is only a small amount of modulated bits riding on the RF. The microwave will harm those bits and the wireless NIC interprets the broken and harmed modulated bits as noise.

Finally, you add 4 wireless clients to the room. You get them connected to the AP and start generating data traffic. Now you can see a high level of noise because numerous bits are now transmitted across the RF medium in the form of frames. These frames are all hurt by the microwave oven, with lots of corrupted bits causing the wireless card to report high levels of noise.

Site Survey

So, when you do a site survey you are affectively measuring the signal strength of your access point as well an estimated SNR only the channel you are measuring in.

This causes problems because you do not know which channel a wireless controller will place the access point after the installation. You could of course measure each AP location on each possible channel but you could imagine the amount of time it would take and therefore making the site survey unaffordable.

Luckily, now a day, RF site survey engineers are most likely to be equipped with spectrum analyzers that are capable of seeing raw RF energy from any device in the frequency band.

Bottom Line

A spectrum analysis is the only way to see the real floor noise (or interference) in a certain environment and you cannot depend on the interpretation of the noise floor from wireless NICs.

Source: Certified Wireless Analysis Professional Official Study Guide, Cisco Spectrum Expert User Guide