Monday, July 9, 2012
Beamforming and How to survey for it?
Beamforming
and How to survey for it?
Beamforming is popular these days with certain WiFi
venders, this technology is maybe not a big mystery for the geeks in the WiFi
industry, although I see and hear a lot of misconceptions around beamforming.
What
is it?
Beamforming allows an access point to
concentrate RF energy in the direction of the WLAN client/receiver in order to
improve the Signal to Noise Ratio at the WLAN client/receiver and thereby
improving WLAN performance (Increased SNR = increased data rates & decreased
retransmissions).
So basically you could say beamforming affects the
radiation pattern of the access point, so with this in mind anything that
affects the RF pattern should be taken in consideration when doing a
pre-installation onsite site survey, correct?
Before we elaborate on this further let's dig
into beamforming first to grab a good understanding what it actually is (anyway
how I see it ;-)).
Beamforming comes in 3 versions, static beamforming, transmit beamforming and dynamic beamforming (sometimes they are
called different but for this blog we use these names to differentiate).
Static
Beamforming:
Under static beamforming we can understand that an
access point will use a fixed directional antenna to focus the RF energy (beam)
in a certain direction, the coverage area in this direction will extend in
comparison with a traditional omnidirectional antenna. Almost every vendor can
do static beamforming when their access point supports external (directional)
antennas. The directional antenna will be aimed (fixed) towards a certain area
to improve SNR for that area, keep in mind that all mobile WLAN clients have
small omnidirectional antennas on board that also need to send traffic back
towards the directional antenna of the access point (more on that later). So
static beamforming will be mostly used to improve SNR for a certain location or
to extend a coverage area.
Some vendors like Xirrus, use an array of static
directional antennas to improve SNR in 360 degrees, by assigning a different
channel to each beam they can be packed together within a single array.
Transmit
Beamforming (TxBF)
Under transmit beamforming we can understand that
an access point will transmit 2 or more phase-shifted signals towards a WLAN
client/receiver in order to achieve an constructive signal at the WLAN
client/receiver. The phase-shifted signals as they travel through the air
create constructive signal (in phase) points at various locations in time and
space, the goal here is to achieve this at close as possible to the WLAN
client/receiver and thereby increase SNR.
To do this correct, possible feedback is required
coming from the WLAN client/receiver to help pinpoint the location of the WLAN
client/receiver.
Explicit and implicit transmit beamforming are 2 forms
of optional components of the 802.11n standard and require feedback from
802.11n clients, as till now 802.11n chipsets do not support either form of
transmit beamforming.
Although Cisco has his own proprietary form of TxBF
called ClientLink, version 1.0 works only with legacy 802.11ag WLAN clients without
requiring feedback. Optimizing SNR values on the 802.11ag clients in a mixed
environment (802.11agn) will increase airtime for all 802.11 WLAN clients and
is beneficial for overall WLAN performance.
ClientLink 2.0 works for all 802.11agn clients
(release 7.2 on the 3600 series access points).
Dynamic
Beamforming
Under dynamic beamforming we can understand that an
access point will focus RF energy (beam) towards a certain WLAN client/receiver
in a way it's kinda like static beamforming but with a big difference that it
is not static and uses an dynamic antenna array that can change it's radiation
pattern on a frame by frame basis. It selects the optimum antenna pattern for
each communicating device in real time, while actively avoiding interference
and minimizing noise to nearby networks and devices.
Test results showed that dynamic beamforming had the highest SNR increase in an overall WLAN deployment of the 3 beamforming technologies as described above.
The only WiFi vendor currently offering dynamic beamforming technology is Ruckus Wireless.
Site
survey considerations
By understanding the 3 different types of beamforming, what do we need to take in consideration while doing a onsite pre-installation site survey?
This is how we see it.
Note that these are only additional considerations for the onsite site survey concerning beamforming technologies, during the actual site survey other considerations like capacity, throughput and so on need to be addressed also.
Static
beamforming:
Because static beamforming will cover only one
certain coverage area following points need to be taken in consideration.
• Always measure with the antenna type that will be
proposed in the bill of materials.
• Adapt your TX power settings before you measure
(see additional note at the end).
Transmit
beamforming:
In this case only cisco has a proprietary form of
TxBF, we recommend to follow their recommendations (I really appreciate all the
documentation Cisco put's out there)
• "Although ClientLink increases the SNR and
data rates of clients at the edges of the cell, it does not extend the maximum
range of a cell. This is because certain packets that must be heard by all
clients (such as beacons) are sent as broadcasts, which ClientLink cannot optimize.
These broadcast packets effectively become the limiting factor on the cell
size. Therefore cell spacing should not be increased when using
ClientLink. In fact, to be conservative, the ClientLink feature can be
turned off during a site survey." this makes sense to me I would
say ClientLink feature must be turned off during a site
survey.
• Take note that ClientLink 2.0 is enabled by
default, and needs to disabled by a cli command.
Dynamic
Beamforming:
In this case only Ruckus supports this technology, I
have a tiny issue with their recommendation to do onsite site surveys. I've got
this information from their technical guy at a Big Dog seminar.
• Ruckus recommends that onsite measurements need to
be done with the WLAN client/receiver in the beam and with the access point at
full tx power, note that the beacons are not broadcasted in the directional
pattern but in a omnidirectional pattern to communicate with other WLAN
clients/receivers. Also sending out traffic in a directional pattern at full
power (20dBm EIRP in Europe) raised an big question mark in my mind that
is related towards WiFi math and physics, certainly if you want to support
smart mobile devices (see additional note at the end).
• I would recommend doing the site survey with the
access point tx power settings at 14dBm IR.
Additional Note: this note refers to the
"antenna reciprocity theorem" (damm
that’s a hard one if you're native tongue is not English ;-)) which I refer to
when it comes to tx power settings and antenna options.
Basically what this theorem says is the following, If
an electromagnetic force of some particular magnitude is applied to the
terminals of antenna "A" and the received current is measured at some
other antenna "B" then an equal current (in both amplitude and phase)
will be obtained at the terminals of antenna "A" if the same
electromagnetic force is applied to the terminals of antenna "B".
In human terms, no matter with type of antenna is
used on both sides if the input power on both ends is the same then, if you can
hear me, I can hear you.
Explained in a real world scenario: if you're
laptop has a wireless NIC with a lousy omnidirectional antenna on board and is
sending at a tx power input of 17dBm (50mw), and I have the same laptop with
the same setup sitting 1km away from each other. In this case we would probably
not be able to set up a wireless communication because we don't hear each
other, if I connect a high gain directional antenna to my wireless NIC and aim
it towards you're direction whenever I can hear you, we will be able to set up
a wireless link even if you're lousy omnidirectional antenna don't reach me, as
long as we have the same tx power input on both laptops (tell this to all the
IT managers that for security reasons are eager to block WiFi signals so that
they don't bleed into the parking lot).
So lets put this in an access point and a mobile
wireless device relationship.
Consider what happens if an access point has a tx
power input of 17dBm with a directional antenna and the mobile
wireless device has an tx power input of 14dBm with a low gain omnidirectional
antenna, due to the fact that the access point is more powerful then the mobile
wireless device, then "reciprocity" is broken and around the edge of
the cell the mobile wireless device will attempt to associate with the access
point that it will never reach.
Consider what happens if an access point has a tx
power input of 14dBm with a directional antenna and the mobile wireless device
has an tx power input of 17dBm with a low gain omnidirectional antenna, due to
the fact that the access point is less powerful then the mobile wireless
device, also "reciprocity" is broken, but there is a big but here
;-), reciprocity is broken in a way that augments, rather than detracts.
So taking all of this in consideration, and if
you're WLAN needs to support smart mobile devices (BYOD, which have a limited
power input) it's better to have your access points on limited tx power
settings, so take this in account when doing the onsite site survey.
Please reply, if there are any remarks or somebody
has additional information regarding this topic.
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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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
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