March 2004 — Features
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Finding Waves
Once the access point is in place, simply plug the device into an electrical outlet; a data connection is not necessary at this step. Then, use the signal measurement software included with your wireless card to begin measuring the signal propagation of the access point. The software will typically display a bar or line graph to indicate statistics such as signal strength, interfering signals ("noise") and signal-to-noise ratio (SNR). In addition, most of the software quantifies signal strength with general ratings such as "Good," "Fair" and "Poor." For educational use, signal strength of at least "Fair" is needed to ensure a sufficient signal.
To begin e-witching, use the laptop with the signal measuring software to map the perimeter's usable signal. Begin by walking away from the access point while viewing the signal strength on the laptop. When the signal degrades to a point where it is fluctuating between "Poor" and "Fair," mark that location as being the perimeter of signal propagation from the first access point. Remember, the access point's signal d'es not follow a flat plane, and the signal from the access point can usually be measured on the floors both above and below where the access point is placed.
After the perimeter for the first access point's signal has been marked, examine the perimeter line and determine if coverage is adequate for the space that needs wireless coverage. If not, find the most sensible location to place the next access point that is beyond the perimeter line and move the access point to this location. Perform the same site survey with this new location and mark the perimeter of this access point location. Continue following these steps until enough access point locations have been surveyed to provide complete coverage.
Once the site survey is complete, the surveyor knows the exact number of necessary access points as well as their locations. The final step is to purchase the access points for installation and configuration into the existing hard-wired network. E-witching is the simplest technique for performing a site survey. Depending on the size of the facility, this technique can typically be completed in just a few hours; however, very little documentation is delivered in this method. Another drawback with e-witching is that there is a tendency to "overnetwork" or place an inflated number of access points in the facility.
Scientific Divining
Performing a site survey using the scientific divining method overcomes the pitfalls of e-witching. Documentation is generated through the use of coverage maps specifying signal strength in any location. A complete scientific divining survey will also help eliminate redundant access points using a scientific method to determine the best locations for access points. The scientific divining method requires identical equipment as the e-witching method, with the addition of a fire-escape map, which is basically a quick drawing of the layout of floors that are to be covered by the WLAN. If these diagrams are not readily available, they can be drawn by hand.
The first step in scientific divining is to subdivide the map into measurement zones (each zone is a logical room breakdown). For example, a small office may be designated as Zone 1A, while a larger room may be split into Zones 1B and 1C. For each floor of a building, a spreadsheet should be created that lists all of the zones in the top row and allows for written entries of signal strength in the row immediately beneath. Once the map has been subdivided into specific zones and the spreadsheet created, it is time to begin the actual site survey.
