Beating Signal Loss in WLANs

Unfortunately when deploying wireless LANs, we have to abide by the laws of
physics. Wireless signals propagating through the air lose strength while encountering
natural and manmade obstacles. It would be nice if RF signals would propagate
without an bounds, but that simply doesn’t occur on Earth.

In order to deploy an effective wireless LAN solution, installers must have
a good understanding of the causes of signal loss (attenuation) and how to implement
applicable countermeasures. This knowledge becomes extremely important when
performing an RF
site survey
, which technicians use to determine the optimum location of
access points to provide necessary range. With familiarity of RF attenuation,
you’ll accomplish RF site surveys more efficiently and get higher performing
wireless network installations as a result.

Attenuation basics

Attenuation is simply a reduction of signal strength during transmission. You
represent attenuation in decibels (dB), which is ten times the logarithm
of the signal power at a particular input divided by the signal power at an
output of a specified medium. For example, an office wall (i.e., medium) that
changes the propagation of an RF signal from a power level of 200 milliwatts
(the input) to 100 milliwatts (the output) represents 3 dB of attenuation. Consequently,
positive attenuation causes signals to become weaker when traveling through
the medium.

When signal power decreases to relatively low values, the receiving 802.11
radio will likely encounter bit errors when decoding the signal. This problem
worsens when significant RF
interference
is present. The occurrence of bits errors causes the receiving
802.11 station to refrain from sending an acknowledgement to the source station.
After a short period of time, the sending station will retransmit the frame,
possibly at a lower data rate with hopes of extending the range of the transmission.

Excessive attenuation causes the network’s throughput to decrease because of
operation at a lower data rate and the additional overhead necessary to retransmit
the frames. Generally, this means that the user is operating within the outer
bounds of an access point’s range. There’s enough attenuation present to decrease
signal power below acceptable values. At worst case, signal power loss due to
attenuation becomes so low that affected users will lose connectivity to the
network.

Causes of attenuation

Both signal frequency and range between the end points of the medium affect
the amount of attenuation. As either frequency or range increases, attenuation
increases. Unlike open outdoor applications based on straightforward free
space loss formulas
, attenuation for indoor systems is very complex to calculate.
The main reason for this difficulty is that the indoor signals bounce off obstacles
and penetrate a variety of materials that offer varying effects on attenuation.

Discussion of the various algorithms to estimate indoor path loss is beyond
the scope of this article. As a general rule of thumb, however, expect to encounter
approximately 100dB of attenuation over distances of 200 feet when using 802.11b
radios operating at 11Mbps. Keep in mind also that attenuation is not linear–it
grows exponentially as range increases.

Typical office obstacles such as doors, windows and walls offer fairly known
levels of attenuation. These values of attenuation are in addition to the path
loss mentioned earlier. The following provides some examples of the attenuation
values of common office construction:

Plasterboard wall

3dB

Glass wall with metal frame

6dB

Cinder block wall

4dB

Office window

3dB

Metal door

6dB

Metal door in brick wall

12.4dB

As a result, a typical small office could have several plasterboard walls equating
to an additional 9 to 12dB of attenuation. Metal doors and glass walls could
sometimes be in the way of the propagation of the signal, causing even larger
amounts of attenuation. Of course this decreases the operating range of the
access points.

Counteracting attenuation

The main goal of combating attenuation is to avoid having signal power within
the area where users operate to fall below the sensitivity of the 802.11 radio
receivers. You need to ensure that the receiver is always able to hear the transmissions.
Bear in mind also that higher levels of RF interference, such as that caused
by 2.4GHz cordless phones or Bluetooth devices,
will negatively impact the ability for the receiver to decode the signal. As
RF interference signal levels become higher than 802.11 signals, an 802.11 receiver
will encounter considerable bit errors when trying to demodulate the 802.11
signals.

How much attenuation is acceptable? The mathematical method for determining
this is to take into account EIRP
(equivalent isotropically radiated power) and receiver sensitivity. Receiver
sensitivity is different depending on whether you’re using 802.11a or 802.11b
and the data rate that users are operating. The higher the data rate, the lower
the receiver sensitivity requirements. In other words, a receiver must be more
sensitive to detect higher data rate signals.

For example, the EIRP of the source station could be 200 milliwatts (23dBm)
and the receiver sensitivity would be -76dBm for 802.11b at 11Mbps. Thus, you
can only afford to have 99dB of attenuation [23dBm — (-76dBm)] before the signal
drops below the receiver’s ability to hear the signal. Thus at 200 feet from
the access point, the user’s 802.11b receiver will probably barely notice signals
from the access point. If obstructions such as walls are present, then operating
range will be less.

You can use these concepts to help with planning the location of access points.
When setting up access points to operate near their maximum range, be aware
that obstacles such as walls will offer additional amounts of attenuation that
could cause loss of connectivity. For example if you’re planning the range of
a particular access point to be 200 feet, then having a few walls in between
the access point and users will cause an additional 9dB or more of attenuation,
which could likely be enough to push the signal power down below the receiver’s
sensitivity. As a result, place your access points closer together to ensure
adequate coverage.

It’s nearly impossible to accurately determine the range of wireless signals
through indoor facilities without performing some live testing. As a result,
be sure to accomplish an RF
site survey
to verify location estimates. The use of an 802.11 radio along
with site survey software with successful test results proves that signal levels
are above minimum requirements. Also consider using a wireless LAN analyzer,
such as AirMagnet
or AiroPeek
to measure signal power at various points throughout the facility to ensure
signal power levels are well above the receiver sensitivity.

Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book,
Wireless LANs
(SAMs, 2001), and regularly instructs workshops on wireless LANs.

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