Wireless LANs need full radio coverage: survey your site!

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So, you're ready to implement a wireless LAN to support mobile applications, like price marking, inventory control, or hospital bedside monitoring? Before installing wireless LAN access points, you need to ensure that there will be adequate radio wave coverage throughout the facility and that no unforeseen RF interference will wreak havoc on the system. Here's a step-by-step procedure (often called an RF site survey) you can follow to determine the optimum number and location of access points for a particular environment:

  1. Obtain a copy of the facility blueprints and verify their accuracy. Blueprints are often outdated because structural changes, such new construction or the removal of walls, are not always reflected in the drawings. As a result, you should walk through the facility before running tests, and update your copy of the blueprints as necessary.
  2. Mark user locations. On the blueprints, mark the location of users who will be operating from fixed locations. Outline potential user roaming areas within the building as well. For some shops, the roaming areas may cover the entire facility; however, there may be some areas in your facility where users will never roam.
  3. Identify obstacles that may attenuate the radio waves. Office walls and the high metal shelves typically found in warehouses cause a great deal of attenuation.
  4. Identify potential sources of RF interference. Discuss sources of interference with the facility's frequency manager, if one exists. If there's doubt about the nature of potential interference, utilize a spectrum analyzer (such as Tektronix's U3661 or Berkeley Varitronics Systems' Champ) to record RF transmissions that fall within the frequency band your wireless LAN will use. On your blueprints, outline the areas that the sources of interference may affect.
  5. Identify preliminary locations of access points based on the wireless LAN vendor's range specifications and the information you gained from steps 2 through 4. Your goal is to ensure that all stationary and roaming users can maintain access to the wireless LAN via an access point.
  6. Verify the location of access points. This is best done by setting up access points at each location identified in step 5 and testing the signal strength at all corresponding user locations. If possible, utilize a client adapter and antenna type that will be part of the eventual system. This will provide the most accurate results. Most wireless LAN vendors have utilities that run on a client PC and record signal levels and packet-transmission statistics. With the appropriate tool loaded, walk with a portable computer and record the signal qualities at all applicable locations. If the signal quality falls below the values suggested by the vendor, consider relocating some access points or adding more. Plan on having an extension cord available to supply power to the access point at each test location. Also, be sure to wear comfortable shoes, because you're going to do a lot of walking!

I can't overemphasize the importance of performing RF site surveys before installing a wireless LAN. These steps will ensure a much more successful implementation.

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