With wireless systems, it’s very difficult to predict the propagation of radio
waves and detect the presence of interfering signals without the use of test
equipment. Even if you’re using omni-directional antennas, radio waves don’t
really travel the same distance in all directions. Instead walls, doors, elevator
shafts, people, and other obstacles offer varying degrees of attenuation, which
cause the Radio Frequency (RF) radiation pattern to be irregular and unpredictable.
As a result, it’s often necessary to perform a RF site survey to fully understand
the behavior of radio waves within a facility before installing wireless network
access points.
The ultimate goal of a RF site survey is to supply enough information to determine
the number and placement of access points that provides adequate coverage throughout
the facility. In most implementations, "adequate coverage" means support
of a minimum data rate. A RF site survey also detects the presence of interference
coming from other sources that could degrade the performance of the wireless
LAN.
The need and complexity of a RF site survey will vary depending on the facility.
For example, a small three room office may not require a site survey. This scenario
can probably get by with a single access point located anywhere within the office
and still maintain adequate coverage. If this access point encounters RF interference
from another nearby wireless LAN, you can likely choose a different channel
and eliminate the problem.
A larger facility, such as an office complex, apartment building, hospital,
or warehouse, generally requires an extensive RF site survey. Without a survey,
users will probably end up with inadequate coverage and suffer from low performance
in some areas. You certainly wouldn’t want to relocate and add access points
to the facility after installing and interconnecting 20 access points or more.
When conducting an RF site survey, consider these general steps:
- Obtain a facility diagram. Before getting too far with the site survey,
locate a set of building blueprints. If none are available, prepare a floor
plan drawing that depicts the location of walls, walkways, etc. - Visually inspect the facility. Be sure to walk through the facility
before performing any tests to verify the accuracy of the facility diagram.
This is a good time to note any potential barriers that may affect the propagation
of RF signals. For example, a visual inspection will uncover obstacles to
RF such as metal racks and partitions, items that blueprints generally don’t
show. - Identify user areas. On the facility diagram, mark the areas of fixed
and mobile users. In addition to illustrating where mobile users may roam,
indicate where they will not go. You might get by with fewer access points
if you can limit the roaming areas. - Determine preliminary access point locations. By considering the
location of wireless users and range estimations of the wireless LAN products
you’re using, approximate the locations of access points that will provide
adequate coverage throughout the user areas. Plan for some propagation overlap
among adjacent access points, but keep in mind that channel assignments for
access points will need to be far enough apart to avoid inter-access point
interference.Be certain to consider mounting locations, which could be vertical posts or
metal supports above ceiling tiles. Be sure to recognize suitable locations
for installing the access point, antenna, data cable, and power line. Also
think about different antenna types when deciding where to position access
points. An access point mounted near an outside wall, for example, could be
a good location if you use a patch antenna with relatively high gain oriented
within the facility. - Verify access point locations. This is when the real testing begins.
Many wireless LAN vendors, including Cisco,
Symbol, and Proxim,
provide free RF site survey tools that identifies the associated access point, data
rate, signal strength, and signal quality. You can load this software on a
laptop or PocketPC and test the coverage of each preliminary access point
location. Alternately, you could use a handheld site survey tool available
from several different companies. For example, Berkeley
Varitronics Systems offers a line of handheld devices, such as Grasshopper
and Scorpion, that provide advanced site survey functions.Install an access point at each preliminary location, and monitor the site
survey software readings by walking varying distances away from the access
point. There’s no need to connect the access point to the distribution system
because the tests merely ping the access point; however, you’ll need AC power.
So be sure to take along an extension cord, and learn where AC outlets exist.Take note of data rates and signal readings at different points as you move
to the outer bounds of the access point coverage. In a multi-floor facility,
perform tests on the floor above and below the access point. Keep in mind
that a poor signal quality reading likely indicates that RF interference is
affecting the wireless LAN. This would warrant the use of a spectrum analyzer
to characterize the interference, especially if there are no other indications
of its source. Based on the results of the testing, you might need to reconsider
the location of some access points and redo the affected tests. - Document findings. Once you’re satisfied that the planned location
of access points will provide adequate coverage, identify on the facility
diagrams recommended mounting locations. Of course the installers will need
this information. Also, provide a log of signal readings and supported data
rates near the outer propagation boundary of each access point as a basis
for future redesign efforts.
These steps will point you in the right direction, but experience really pays
off. If you’re new to wireless LANs, you’ll begin to build an odd intuition
about the propagation of radio waves after accomplishing several RF site surveys.
Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs (2nd Edition), and regularly instructs workshops on wireless LANs.
