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KEY In general, an AP can support approximately seven to eight wireless phones or about 20 POINT data-only devices.
The facility should be visually inspected to identify potential issues, such as metal racks, elevator shafts, stairwells, and microwave equipment.
Determine Preliminary AP Locations
The next step in the RF site survey process is to identify preliminary AP locations based on the planned coverage area and user density. This step can be supported with several tools. For example, you can import real floor plans and assign RF characteristics to building components in the Cisco WCS to increase design accuracy. WCS has a planning mode feature that estimates how many APs are needed for a given floor in a building, based on the floor plan; an example is provided in Figure 9-24. When predicting range and coverage, the planning mode feature takes into account the following:
■ Coverage or capacity
■ Throughput required
■ Number of square feet
Figure 9-24 Preliminary AP Locations Are Based on the Floor Plan
• Default access point placement
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NOTE Recall that, after implementation, the Cisco UWN can adjust power and channels so that APs create the least amount of interference with each other.
The WCS can be used to estimate the quantity and preliminary locations of APs at the start of a site survey. This approach is most successful in simple carpeted office environments that are less RF-challenging than other environments, such as warehouses.
The Cisco WCS also provides an integrated RF prediction tool that can be used to visualize the detailed WLAN design, including lightweight AP placement, configuration, and performance/ coverage estimates. Graphical heat maps, as illustrated in Figure 9-25, help IT staff visualize anticipated WLAN behavior for easier planning and faster rollout. A heat map diagrammatically represents signal strength—the warmer the color, the stronger the signal.
Figure 9-25 Heat Maps Help Visualize Coverage
Figure 9-25 Heat Maps Help Visualize Coverage
NOTE The accuracy of this heat map greatly depends on the effort spent adjusting RF loss characteristics of building components such as walls, solid doors, and stairways.
Perform the Actual Survey
The next step in the process is to conduct the actual survey to determine the coverage based on the planned AP locations. The process to determine the coverage characteristics of an enterprise office site includes the following:
1. Measure the radius of the coverage area for a given data rate.
2. Move from the corner to the edge of the coverage area, and measure the data rate.
3. Determine the coverage range behind stairwells, offices, supply rooms, cubicles, and so on.
4. With as many APs as available, build the planned wireless coverage.
5. Establish nonoverlapping channels as often as possible to reduce contention.
6. Repeat this process until all the required coverage areas are set up.
A tool, such as AirMagnet Survey PRO, can be used to perform a manual site survey; results include the following:
■ Signal strength
■ Signal-to-noise ratio
■ Channel interference
NOTE AirMagnet Survey PRO information is available at http://www.airmagnet.com/.
A free tool available for Cisco 802.11a/b/g client cards is the Cisco Aironet Site Survey Utility. The following information is provided by the tool:
■ Signal strength
■ Signal-to-noise ratio
NOTE Information about the Aironet Site Survey Utility is available at http://www.cisco.com/ en/US/products/hw/wireless/ps4555/
products_installation_and_configuration_guide_chapter09186a0080796b85.html.
Coverage holes are areas where clients cannot receive a signal from the wireless network. When deploying a network, there is a trade-off between the cost of the initial network deployment and the percentage of coverage hole areas. A reasonable goal for launch is to have between 2 and 10 percent coverage holes; in other words, between two and ten test locations out of 100 random test locations might receive marginal service. After launch, RRM identifies these coverage areas and reports them to the WCS, allowing the network manager to correct the holes, based on user demand.
Document the Findings
After completing the site survey, the final step in the process is to document the findings. A proper site survey report provides detailed information that includes customer requirements, AP coverage, interference sources, equipment placement, power considerations, and wiring requirements. The site survey documentation serves as a guide for the wireless network design, and for the installation and verification of the wireless communication infrastructure. The site survey report should also contain a list of the parts that will be needed, including the following:
■ The total number of APs, and a recommendation that a spare be kept on hand in case of emergency
■ The total number and type of antennas required and how they will be mounted
■ Any proposed accessories and network components
The site survey report should include diagrams showing the facility, AP locations and coverage, and proposed cable runs. Covered areas, as well as those not needing coverage, should be indicated. Whenever possible, include photographs of the planned AP location or proposed antenna installation to make it very clear how and where the equipment should be installed. The tools and methods used for the site survey should be described.
If wireless voice support is required, the site survey methodology needs to be enhanced to plan for voice coverage and capacity. For example, wireless data is less susceptible to disruption than wireless voice when it comes to cell overlap, RF noise, and packet delay. Therefore, surveying for wireless voice coverage requires more effort and time than for data-only coverage at the same site.
NOTE Further discussion of site surveys to support voice over WLANs is available in Design Principles for Voice over WLAN at http://www.cisco.com/en/US/netsol/ns340/ns394/ns348/ networking_solutions_white_paper0900aecd804f1a46.shtml.
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