Thursday, June 21, 2012
Detecting the RF Noise Floor
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
Source: Certified Wireless Analysis Professional Official Study Guide, Cisco Spectrum Expert User Guide
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