|
Canopy Wireless Solutions offers RF Surveys including an optional
Spectral Analysis study as one of its available Professional Services.
Why conduct a Site Survey ?
For new construction Site Surveys are generally
performed to gather information needed as input to the design
process for a project.
For Wireless systems a RF Coverage Study is
included as part of the overall survey to determine RF propagation
characteristics for the location being surveyed to determine
placement, and antenna requirements for Access Points, Client units,
and other radio frequency equipment.
For existing systems a RF Frequency/Spectrum Analysis may
be called for to determine sources of radio frequency and electrical interference to the
desired signals, and perhaps to identify the location of Rouge
Access Points.
RF Site
Survey Steps
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:
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
|