RF prediction vs. site surveys

Q: What’s the difference between an RF prediction and a site survey? Are different wireless LAN deployments better suited for one or the other?

–Chris, Cleveland

A: An RF prediction estimates the coverage of your access points through a computer simulation of your RF environment, and the transmit power of the access point antennae. Most available systems use a two-part process to give you a detailed map for placement of access points with performance estimates. First, you modify a graphic of your floor plan with editing software to create a map that accurately describes the RF characteristics of your building’s walls. Then you start your prediction by virtually placing access points on the map and generating the graphical coverage plot of the expected average signal strength intensity experienced by users in various building locations.

A site survey measures actual signal strength intensity of physically placed access points across your coverage area to verify assumptions made in the planning phase. There are two different site survey approaches, which both require that you first arrange your network of access points based on your planning findings. One method automatically conducts the survey across all access points. But for deployments with high coverage requirements, we believe there is no substitute for the second method, which requires that you walk through your network area with a client (laptop) containing survey software. After data has been collected, good systems will have tools that ensure that the number, placement, and channel configuration of your access points provide your required network quality.

As for which method is best for your company, consider your performance and coverage requirements. If you have a typical building with open cubicles and offices where you can live with 5% coverage holes, or can afford to over-deploy access points, you should consider a common sense approach. This estimates the average coverage radius of your access points based on your building type and information from existing wireless networks.  This can be quite accurate if your WLAN solution uses dynamic RF intelligence to optimally adjust access point controls.

RF prediction is useful when your requirements become more demanding, such as 100% coverage with a minimum of 5M bit/sec throughput (the basics for VoIP applications). This method is also helpful when your building characteristics are somewhat heterogeneous because of a variety of materials throughout the building. A full site survey is needed for only the most demanding requirements, including atypical building styles, such as sports arenas, and stringent coverage requirements approaching 0% coverage holes or hard coverage borders. However, a minimal site survey can also be helpful to verify the assumptions of other methods.