Active vs. predictive site surveys: pros and cons

Despite all the advances in Wi-Fi, it is surprising how often the stumbling block for successful wireless deployment is the site survey.

The goal of the site survey is to determine the number and location of access points (APs) required to meet the Wi-Fi project’s design goals, such as coverage, performance and user capacity.

An accurate site survey is just as important as selecting the proper hardware. If the site survey is too optimistic and calls for too few APs, the final installation will yield poor coverage, poor performance, or both. This in turn will require a follow-up survey to determine what changes may be required to the number or location of deployed APs.

The end result is increased costs for additional APs and costs for more cable, equipment, contractors and installation effort. If the survey is too conservative, more APs than needed will be specified and money is wasted on unneeded equipment and labor costs.

Site surveys can be performed by a number of methods but in general fall into two groups: active and predictive. An active survey (sometimes called a live survey) uses an actively transmitting AP and a Wi-Fi-enabled laptop to measure Wi-Fi signal strength from the AP in the actual environment to determine coverage, performance, how many APs are required, and where they will be installed.

The pros and cons

Which wireless LAN site survey method you use depends on the type of project your company is working on. Some of the key considerations between active and predictive surveys include:
 Active (live) surveyPredictive survey
Survey EffortPotentially higherPotentially lower
Access Point PlacementExact locations chosen

Final locations may

need to be moved
Wireless CoverageMeasured and knownEstimated: May require more APs
Wireless PerformanceMeasured and known

Estimated: May require

more APs
Cabling and EquipmentTypically accurateMay require cables to be added or relocated
InstallationUsually a one-time eventMay require moving APs around or adding more APs after installation
Best Suited For

Larger Wi-Fi projects

and 802.11n
Smaller Wi-Fi projects

A predictive site survey uses a software package to simulate the construction of the building. Floor plans of the site are imported into the tool and allow the user to assign attenuation values (how much RF energy is absorbed) for walls, floors, cubicles, windows and other objects in the building (most tools typically have a database of materials to choose from). The application will then predict the number and location of APs required to meet the desired coverage and performance goals.

Each method has advantages and disadvantages: Active site surveys measure the real radio-frequency performance of each AP location and, because they measure real signal propagation, they innately take into account all physical characteristics and contents of the building. There is no need to estimate the thickness or density of walls or floors and provide for things like books on bookshelves, file cabinets, mirrors in the bathroom, contents of closets, insulation in walls, and ducts and piping between floors.

This type of survey provides more accurate performance information for each AP and allows for the physical inspection of the chosen locations to assure the AP can be mounted there. An active site survey also can measure sources of interference or other Wi-Fi networks that may be operating nearby before the network is installed.

The downside of an active survey is it is more labor intensive and requires physical access to the building. What’s more, some of today’s thin APs won’t work without being connected to their controller, which can make surveying more cumbersome as there is more equipment to lug around, and is one of the reasons some integrators may try to push for a predictive site survey.

Predictive site surveys may take less time but still require “seat time” in front of a computer vs. the “leg time” needed for an active survey.

A predictive survey is advantageous when travel to the location is not feasible or access to the building is not possible. The construction of the building still must be known and input into the program. It is typically difficult to accurately model all the contents and makeup of a building without physically seeing it, and is especially hard for buildings that have changed or have been remodeled over time and don’t match the floor plan used.

Predictive coverage and performance is an estimate, and so will be the number and location of APs the tool calls for. APs may not even be physically mountable at the predicted location unless it is visually inspected. One other note, with 802.11n it will be even more difficult to predict performance as most tools cannot model RF multipath (reflections) which plays a greater role in 802.11n performance.

Getting the survey correct becomes more important with larger scale projects. Dan McCarriar of Carnegie Mellon University, who is upgrading one of the oldest and largest campus-wide Wi-Fi networks to 802.11n, says: “We are using and requiring active site surveys. Rolling out Wi-Fi for a large campus such as ours mandates accurate site surveys. We do not want to go back and redo things later. It is unacceptable from a cost, equipment, cabling, and manpower perspective for a project of this size to be off by even a small percentage”.

In summary, predictive site surveys may be acceptable for smaller, less complex Wi-Fi deployments, or can be used for a budgetary estimate that is then followed up with an active survey. The larger and more business critical your Wi-Fi project is, the more essential the accuracy of an sctive site survey becomes. Predictive tools can be off by more than 50% in terms of the actual numbers of AP required.

Most vendors are capable of doing both active and predictive surveys. To ensure the success of a Wi-Fi project, require vendors to provide an active site survey, especially on large Wi-Fi projects.

Sauter is director of product marketing at Xirrus and also participates in IEEE and Wi-Fi Alliance task groups. He can be reached at kurt.sauter@xirrus.com.

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