| RF SITE SURVEY |
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 in case of 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. A RF site survey also detects the presence of interference coming from other sources that could degrade the performance of the wireless LAN.
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.
- Verify access point locations. This is when the real testing begins.
Install an access point at each preliminary location. Take note of data rates and signal readings at different points as you move to the outer bounds of the access point coverage. 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.
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