RF Site Survey
This section reviews the reasons that an RF site survey is used in wireless network design, and the process to conduct such a survey. An RF site survey is the first step in the design and deployment of a wireless network, and the most important step to ensure desired operation. A site survey is a process by which the surveyor studies the facility to understand the RF characteristics in the environment, plans and reviews RF coverage areas, checks for RF interference, and determines the appropriate placement of wireless infrastructure devices.
In a wireless network, issues that can prevent the RF signal from reaching all parts of the facility include multipath distortion, hidden node problems, and near/far issues. To address these issues, the regions where these issues occur must be found. A site survey helps by defining the contours of RF coverage in a particular facility, discovering regions where multipath distortion can occur and areas where RF interference is high, and finding solutions to eliminate such issues.
Although the WCS can detect RF interference and optimize around it, it cannot analyze the RF interference to provide the information necessary to identify and locate the source. A spectrum analysis tool, such as Cognio Spectrum Expert, can classify the interference, determine its impact on the network, and enable the administrator to physically locate the source of interference and take action.
NOTE Information on Cognio Spectrum Expert is available at http://www.cognio.com/.
A site survey that determines the RF coverage area in a facility also helps to determine the number of wireless infrastructure devices required and where they should be deployed such that an organization's business requirements are met.
RF Site Survey Process
Typical steps in an RF site survey process include the following:
Step 1 Define customer requirements: This includes the number and type of wireless devices to support, the sites where such devices will be located, and the service levels expected. Peak requirements, such as support for conference rooms, should also be identified. APs should be placed to support the locations and numbers of WLAN clients.
Step 2 Identify coverage areas and user density: Obtain a facility diagram, and visually inspect the facility to identify the potential RF obstacles. Identify areas that might have a large number of users, such as conference rooms, and the areas that are not used as heavily, such as stairwells.
Step 3 Determine preliminary AP locations: AP location information includes the availability of power, wired network access, cell coverage and overlap, channel selection, and mounting locations and antenna type.
Step 4 Perform the actual survey: The actual survey verifies the AP locations. Be sure to use the same AP model for the survey that is in use or will be used in the network. During the survey, relocate APs as needed, and retest.
Step 5 Document the findings: Record the locations and log signal readings and data rates at the outer boundaries of the WLAN.
These steps are expanded on in the following sections.
Define the Customer Requirements
As part of defining the customer requirements, the following questions should be asked:
1. What type and number of wireless devices need to be supported? Here are some considerations:
a. Is existing WLAN or RF equipment in place? Existing WLAN equipment will need to be integrated, and the impact of existing RF devices should be mitigated.
b. Will the WLAN be used only for data, or will wireless phones also be supported? If wireless phones will be used, the wireless network needs to be built with better receive signal strength to support the required QoS.
c. Are there peak periods to support? For example, use of wireless services in a conference room will peak during meetings.
2. Will users be stationary or on the move while using the WLAN? If users are mobile, high-quality hand-offs during client roaming are required.
3. Where should wireless coverage be supported? How many buildings, floors, and areas should be supported? Are outdoor bridged links needed?
4. What level of support should be provided? What level of redundancy and self-healing is needed in the wireless infrastructure?
The answers to these questions are important to defining the scope of the wireless infrastructure design.
Identify Coverage Areas and User Density
Part of the site survey report should include a floor plan showing coverage areas and areas that the customer has defined as no-coverage areas. A sample floor plan is shown in Figure 9-23. The floor plan provides the customer, installer, and troubleshooter with some indication of what coverage each AP should be providing. The expected density of the wireless devices, which might be one or two per office and 20 or more per conference room, should be identified.
Figure 9-23 Identify Coverage Areas on a Floor Plan
Elevator
Shafts Office
File Room or Supply Room: Large Filing or Metal Cabinets
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