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

3 important steps to make you're WLAN project successful

Throughout the many years that we have been in the business, we have seen many different approaches on how WLANs are deployed. Sometimes they were successful and sometimes they were not. If you are involved in the WLAN project from A to Z the flow is transparent. Otherwise, depending from where you step into the story, you are always dependent on third party information which, is sometimes contradictional with what you are doing onsite. Our part of the story was mostly the pre-installation site survey which is a crucial part of the whole but somewhat useless if, for example, the information regarding the WLAN applications that will be deployed on the WLAN is not correct.

Another thing that we noticed was that after the site survey was executed and documented, the WLAN infrastructure would be installed and no fine tuning or verification site survey would be performed before the deployment of the WLAN applications which, from our point of view, is another crucial part of the whole. For example, during the site survey measurements were taken while the access points are transmitting at a certain static tx power level and after installation the radio management algorithm of the controller (if architecture is controller based) will feel the access points and adjust channel and tx power settings automatically. Depending on the building infrastructure and the access point density, the radio management algorithm could lower down the tx power of the access points below the threshold used during the site survey. This could cause coverage holes in the coverage area. Another example is that the access points are installed incorrectly according to the site survey recommendations and the radio pattern will be different than as foreseen.

With all this in mind, we want to stress out the following 3 important steps to achieve a successful WLAN project.

Step1: Information gathering

I guess all WLAN engineers who are doing site surveys have heard this one "I want a site survey, for everything" ;-).

Step 1 is probably the most important step. Gathering information about the WLAN project, such as defining coverage areas, which type of applications will be used now and in the nearby future and in which frequency, high capacity areas, high utilization areas, WLAN vendor and type of access points, aesthetic restrictions for the placement of access points, and so on. I know….. it is a true hassle to collect all the information and sometimes the customer does not know what you're talking about.

The best way to do this is to fill out the Request For Information document. All of the questions to get the information are in there. You just need to fill in the check boxes.

Step2: The Pre-installation (onsite) site survey

Once all of the information gathered from the RFI you can actually start doing the onsite measurements. A small kick-off meeting with the customer onsite to double check the information won't do any harm.

The goal of the onsite site survey is to pinpoint the amount of access points and/or antennas needed to meet the requested WLAN requirements, as well as how and where the APs should be installed.

A spectrum analyses is also required to check if there is any non Wi-Fi interference onsite that could disrupt the performance of the new WLAN infrastructure.

Rogue access points/signals detection can be an additional option to the WLAN site survey.

All the results will be documented.

Step3: The verification site survey (post installation site survey)

Most of the times Step3 is not foreseen in the project due to budgetary or other reasons or there might be a time gap between the pre-installation site survey an and the installation of the WLAN infrastructure. And I agree, most of the deployments will have no problems running their WLAN applications certainly if they are only data applications, but that does not mean that you're WLAN is performing in the most optimal conditions. When adding more and more applications, performance problems can arise. Anyway, we highly recommend adding Step3 in the process before the project will go live.

The goal of the verification site survey is to optimize and fine-tune the overall WLAN to the requested WLAN requirements. Coverage, retry percentage, tx power settings, data rates, channel reuse and AP installation should all be taken into consideration.

All results will be documented.

Cheers,

Joeri De Winter