Coverage & capacity planning, before going wireless

The pre-installation site survey has long been a core element of wireless-LAN deployments. The concept is simple: set up an access point, and, running a site-survey application on a mobile computer, walk around, noting signal strength in various locations in order to build a coverage map.

Then use the data to deploy production access points for optimal coverage.

While early deployments were indeed optimized for coverage because of high capital-equipment costs, today's WLAN deployments need to be much more concerned with capacity.

That's because WLANs must support potentially large numbers of users with a diverse application base requiring ever-greater throughput over time. Plus, dramatically larger deployments in more challenging physical layouts, and the mission-critical nature of today's enterprise WLANs add to the complexity.

All enterprise-class WLAN products today include some form of site-survey capabilities. However, at least a partial installation of the system to be purchased is required before these can be used. For this reason, and because a wide range of site-survey strategies can be applied, a number of third-party site-survey tools have appeared in the market.

There are four types of site survey tools: spectral sweep, predictive, analytical and production monitoring. This review focuses on predictive and analytical tools. Predictive tools use sophisticated RF modeling techniques to simulate the performance of a hypothetical WLAN system without the requirement for actually installing any real equipment.

Analytical tools reside on a mobile computer, with real-world readings taken and recorded as the user walks around in an environment where access points are installed and powered on, and often connected to the remainder of the infrastructure.

Keep in mind that these two classes of products cannot be directly compared against each other. Indeed, in complex environments, the use of both might be desirable. In addition, some of the products tested are actually hybrids of the two.

Two strategies: Predictive and analytical tools

The general idea behind using a predictive tool is to create (or import, via, for example a .dxf file) a floor plan of the area to be deployed, assign properties to objects in the drawing (walls, floors), and then to place access points (in some cases automatically) and simulate the propagation of RF signals through this virtual environment.

Radio-wave propagation is a highly non-linear, essentially statistical phenomenon, but good estimates can be obtained if data values are carefully considered. The best results are obtained by engineers with a detailed background in RF, and/or those who have been trained to use a given predictive tool and had sufficient time to experiment with it before using it on a production task.

Analytical tools use real-world measurements to plot RF propagation via a combination of the site-survey tool running on a mobile computer and APs installed in the area to be covered. These can be successfully applied by those with a less-detailed knowledge of RF and with only minimal training and usage experience, although the feature-rich nature of the products we tried may imply a longer learning curve for some.

All of these tools encompass functionality well beyond what has traditionally been the definition of a site survey, incorporating a form of WLAN assurance that aids in verifying configuration, operation, performance, and even detecting rogue access points.

Most Wi-Fi systems vendors also offer planning and survey tools; if you've already picked a vendor, these might be the best place to start, as they will already be optimized for the equipment you'll be using. And while many site survey tools can produce detailed RF predictions and analysis, enterprise-class Wi-Fi management systems also will set (and change) such parameters as channel assignments and transmit power levels, as, for example, RF conditions or building configuration change.

Thus a post-installation survey will often have even more value than a pre-installation planning exercise.

A few concluding remarks

We decided not to pick a winner in this review. Anyone contemplating a WLAN installation should carefully evaluate throughput and coverage requirements, as well as perform a RF sweep for interference. If a WLAN system vendor has already been selected, check to see if they have a predictive or analytical tool that will meet your needs.

Be prepared to do a lot of walking (don't forget to scan outside to check for signal leakage) and to invest some time in carefully considering the analyzed results in light of throughput and capacity requirements in addition to coverage. Multiple survey passes may be required for a complete picture. Beyond that, as we discovered here, there are many fine – properly used, anyway – site-survey tools to choose from.

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