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Wireless Networking Handbook
(Publisher: Macmillan Computer Publishing)
Author(s): Jim Geier
ISBN: 156205631x
Publication Date: 09/01/96

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You should do the following when performing the site survey:

1.  Verify the accuracy of the facility blueprints. Initial blueprints are drawn by an architect before the building is constructed. Changes are not always made to the drawings as the building is modified, especially for the relocation of walls and office partitions. You should walk through the facility before running tests to be sure walls are where they are supposed to be—if not, update the drawings.
2.  Mark permanent user locations. On the blueprints, mark the location of users who will be operating from a fixed location.
3.  Mark potential user roaming areas. In addition to the permanent users, outline potential user roaming areas within the building. In some cases, the roaming areas may be the entire facility. However, there may be some areas where users will never roam.
4.  Identify obstacles that may offer significant attenuation to the radio waves. Observe the construction of the facility, and mark the location of obstacles that may cause a hindrance to radio wave propagation.
5.  Identify potential sources of interference. You should have done this when reviewing the environment as part of the definition of requirements. If not, determine what other RF devices are present, and assess their effect on the wireless LAN. You can do this by talking to someone at the facility who manages existing RF devices, or you can use a spectrum analyzer to record RF transmissions that fall within the frequency band your wireless LAN will operate. Outline the areas on the blueprints that the sources of interference will affect.
This step is important! A company in Washington D.C. purchased 200 wireless LAN cards, installed them, and later discovered radio interference from a nearby military base blocked the operation of half the users. A proper verification of coverage or even the use of a spectrum analyzer would have saved this company a great deal of money and frustration.
6.  Identify the preliminary location of access points. Based on the wireless LAN vendor’s range specifications and information gained from steps 4 and 5, identify the preliminary location(s) of access points and wireless servers. The goal is to ensure all permanent and roaming users can maintain access to applicable network resources via access points. For small areas (less than 1,000 feet in diameter), no access points may be necessary; however, larger areas will require access points to produce a multi-cell system. Mark the presumed locations on the blueprints.
7.  Verify the location of access points. This is best done by installing an access point or master station at each of the locations identified in step 6, and then testing the signal strengths at each of the permanent and potential user locations. You need to configure a portable computer with the applicable wireless LAN adapter and site survey software supplied by the vendor. Proxim, for example, ships a site survey tool with their RangeLAN products that loads on the portable computer and broadcasts messages to all other units with the specified domain. Each unit responds to these broadcasts, and the survey tool, after ten broadcasts, displays a Link Quality number that represents the percentage of packets to which the tool received a response. The tool displays a Link Quality of 5 for 100% packet acknowledgment. A Link Quality of 4 represents 80%, Link Quality of 3 represents 60%, and so on. A Link Quality of 5 is optimum, but operation is still possible at lower quality levels. (Users will experience some delay though.). With the appropriate tool loaded, walk with the portable computer and record the signal qualities at all applicable locations. If the signal quality falls below suggested values supplied by the vendor, then consider relocating the access point.

Verifying the Design

Design verification ensures that the solution you have chosen will support requirements. Actually, the verification of access points covered in the preceding section was a form of design verification that tested the wireless network portion of a network architecture—the physical and data link layers. But you might also need to verify higher layer architectural elements, such as applications, communications protocols, and system interfaces as well. This prevents the purchase of inappropriate network components and hours spent working out bugs just before the system needs to be operational.

The following are methods you can use to verify the design:

  Physical prototyping
  Simulation
  Design review


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