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Assisted Site Surveys

In an assisted site survey, you first need to determine how big a cell needs to be for the user density defined in the design. This might vary in different locations throughput the site, and this test will need to be performed in all the similar areas. An assisted site survey helps to ensure that an AP can cover the amount of area needed, without any dead areas. In most cases, you use an assisted survey when the power of the AP provides cell coverage greater than desired for the user density.

Performing a User-Density Test

Using the facility diagram survey method, locate the center (if using an omniantenna) of a common type of area in the facility, such as a group of cubicles. Place an AP at that location, in a similar position to how it would be installed. Define the number of users, and their locations should be serviced by this one AP, and note this information on the facility diagram (see Figure 11-17).

Then perform a walkabout test for that one AP, using the values defined earlier in this chapter, and determine where the cell boundaries are. Compare this cell boundary to the desired coverage areas defined on the site map. Reduce the power of the AP, and perform the walkabout test again. Once more, compare the cell boundary to that of the desired coverage. When the coverage area is at the desired range, note the power setting on the facility diagram.

After completing this step for all identified area types, you can use this information to estimate on the facility diagram where APs should be placed (see Figure 11-18).

If the area requires a unique antenna such as a Yagi, patch, or other "nondefault" antenna, you might need to survey it entirely using the walkabout method. (Such a need sometimes arises because many assisted or automated survey tools do not account for anything other than omnidirectional antennas.) The same holds true for areas in which the desired coverage is the maximum size possible with the AP and antenna combination at full power. This would normally occur in a facility such as a warehouse with only a handful of wireless bar code scanners, or an outdoor site where the system is used by a limited number of security guards for wireless VoIP and the coverage area is very large.

Using the Assisted Site Survey Tool

After completing the density test, you can progress to the initial installation of APs. Using the estimated location of the AP (see Figure 11-18), install the AP and set to full power.

Assisted site survey methodologies vary from vendor to vendor. In general, however, you complete the following steps after performing a density test:

Install the APs based on the density test and floor plan.

Perform an AP scan.

Perform a client walkabout.

Generate the radio parameters based on the AP scan and walkabout.

In an AP scan, you use one AP's receiver to hear the surrounding APs. The system records all surrounding AP signal strengths and determines what the power setting of the AP should be. In many cases, this will provide coverage in all areas, but there might still be some dead spots. One key point to remember about this method is that the APs are typically mounted in the ceiling. Therefore, the path loss between APs is typically less than the loss occurring between the AP and a client, which is located much lower, and possibly behind cube walls, desks, and so on. This is exactly why it is vital to perform a verification walkabout, looking for dead spots, RF shadows, and so on.

The Cisco WLAN products enable the client to participate in the assisted survey process (via a client walkabout). The client sends data from a walkabout up to the management system, providing not only AP-to-AP signal strengths but also client-to-AP signal strengths.

RF Configuration Parameters

After completing the site survey, you must select the proper channels for the APs so that channel reuse is properly implemented (to reduce same-channel overlap and improve performance). Some systems allow automatic channel selections. In such a case, you must verify that there are no adjacent cells on overlapping channels after you have installed and configured the system.

Any other parameters that need to be set, such as packet size, diversity antennas, or RTS/CTS, should be identified and documented as part of the survey. It is important to configure and verify the coverage after the installation.

Some products offer the feature to have the AP set power and channel schemes as an ongoing task. Although this might seem like a desirable feature, it can sometimes cause a network to become unstable. If RF interference or a network glitch causes an AP to drop off line for any length of time, the network will start to reconfigure the APs, for both power and channel scheme. When the "down" AP comes back on line, the reconfiguration takes place again.

Although this automatic reconfiguration feature might sound great for a failed AP, if the AP is intermittent, or the interference that is causing the problem is intermittent, the systems will go through constant reconfiguration, disrupting normal usage. To prevent the constant changes to configurations, take care to ensure reconfiguration parameters filter out these fluctuations and temporary situations.