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
Wednesday, June 20, 2012
3 important steps to make you're WLAN project successful
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
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About Us
Skyline networks is specialized in a professional range of wireless LAN services worldwide. We offer customized services for every aspect of your WLAN.
The Skyline Blog Authors:
Joeri De Winter
Justin Cetko