Crash Course: Wireless Site Surveys

Site-survey planning for wireless LANs has never been simple. And with new wireless features like multiband operation (802.11b/g-2.4 GHz, 802.11a-5 GHz and so on), wireless intrusion-detection and -prevention systems, voice over WLAN and location services, it's gotten even tougher.

The site-survey process, which helps determine the placement and configuration of wireless APs (access points) in your WLAN, has been used primarily for gauging adequate coverage and avoiding channel overlap. But today's WLANs must meet specific requirements, such as providing high-performance bandwidth for each user, reliability and fault tolerance, voice support using ubiquitous high-signal-strength coverage, and overlapping coverage of three or more APs.

Site surveys also help measure performance, troubleshoot problems and mitigate interference, so you'll be conducting them regularly to support changes made to your wireless environment. Conventional manual site surveys, done by installers armed with pen and paper, are too time-consuming and grueling for many enterprises, but there are three types of software-based WLAN site survey tools to simplify the job.The first are those integrated into your AP infrastructure that provide initial estimations of WLAN coverage and real-time dynamic radio management. The second type run on laptops to assist in the on-site survey walk-through. The third are full-blown modeling applications that provide intensive WLAN coverage calculations.

These tools offer interfaces that let you run visualizations of your WLAN and design it for ubiquitous coverage. With visualization, you can virtually manipulate or relocate access points, sensors and antennas without having to physically move them by trial-and-error. Site-survey tools also let you test "what if" scenarios (the addition of access points, hardware failures, a swell in the number of users); identify and avoid sources of interference; and support a performance baseline for your WLAN.

You may decide not to perform a site survey if your initial WLAN deployment is small and you're building out your WLAN incrementally. But once that WLAN starts to expand, a site survey is inevitable.

Built-Ins

If you're building a WLAN at a site with no existing wireless infrastructure, the big question is how many APs you'll need and where they should go. Depending on the AP's architecture--whether it's an autonomous AP or a thin AP with a WLAN switch--you may be able to hold off on buying them and use an AP estimation tool first.Autonomous APs are useful in small-scale deployments but lack the centralized configuration management needed for a large-scale enterprise WLAN deployment. And they aren't available with built-in site survey tools. If you run a WLAN switched infrastructure, site-survey tools are typically built into the WLAN switches, so before you even purchase an AP you can use those estimation tools to gauge the number of APs you'll need and where to place them.

Aruba Networks, Cisco and other vendors offer integrated site-survey estimation tools in their WLAN switches. These tools vary slightly in their functionality but, generally speaking, work like this: After you scan in your floor plan as an image file (JPEG, GIF or PNG, for instance), the tool prompts you for your per-user bandwidth requirements; whether the WLAN will be 802.11a or 802.11b/g or both; the number of users per access point; whether voice over WLAN is required; and the areas of the floor plan that won't be wireless.

The tools then perform calculations to come up with recommendations, such as optimal placement of APs, RF channel and expected coverage. You can manipulate the results to further customize your WLAN design.

Aruba's offering is not a standalone product, but rather a "license-free" feature of the management interface for Aruba's Mobility Controller Web-based wireless LAN switch. Cisco offers a application called Wireless Control System that runs on a Windows-based server platform and must be purchased separately (WCS is a popular, $4,000 infrastructure add-on for WLAN management).Neither tool is comprehensive, however, when it comes to predicting propagation through physical obstacles and WLAN coverage. Aruba's tool doesn't take into account propagation through a variety of building materials, but Cisco's tool lets you assign to up to seven categories of wall types, so it's a little more helpful. Both the Aruba and Cisco tools generate suggested AP placement, RF channel assignment, RF power settings and expected data rates, all of which are depicted with heat maps on the floor plan (see screen, left).

The visualization feature in these tools is customizable, so you can virtually add APs or move around APs generated by the tool, for example. But beware that these tools are vendor-specific--Aruba's tool can only be used with an Aruba WLAN--and their estimations are not precise.

Most APs continuously scan their WLAN environments, conducting 24x7 site surveys and providing things such as automatic channel assignment, dynamic power adjustment, collision avoidance through automated stand-by operation and the ability to feed data to real-time visualization tools. The visualizations are updated and supported by actual readings gathered by the APs, and offer real-time site surveys of your WLAN RF environment. The APs, meanwhile, continuously adjust themselves to avoid channel overlap and problems in the wake of adds, moves and changes.

Laptops and Walk-Throughs

A manual site survey makes sense if your site is too small to justify an expensive enterprise WLAN switch architecture and instead will have autonomous APs, or if your WLAN environment is too specialized--think warehouses, hangars, ships and outdoor WLANs--for the cookie-cutter estimation tools. But you can put away your markers and floor plans because there are some helpful tools priced around $3,000 for manual site surveys, too.AirMagnet and Ekahau, for instance, offer laptop-based tools for a walk-through site survey. AirMagnet's Survey PRO and Ekahau's Site Survey let you "visualize" your WLAN against your floor plan: You place an AP that's part of the site-survey kit in a desired location and then walk through the site with the tool. The tool records the path traversed, giving the software numerous points of reference so it can understand the propagation of the site-survey AP. The data is then used to develop an accurate visualization.

You can use these tools for outdoor WLANs as well. They integrate with a GPS receiver and use GPS coordinates as points of reference you can plot on the site map. These tools provide real signal-strength readings taken at specific locations rather than estimations based on statistical models.

They also collect data points such as nearby access points, received signal strength, AP performance, interference sources and spectrum analysis. And they use your existing Wi-Fi laptop card software and modify it for rapid channel scanning, so you're using the same WLAN NICs that provide your users access to the WLAN.

Once the walk-through is complete with the site survey AP at one location, the survey results are saved, and the AP is moved to a new location. Then, the walk-through process is repeated. These tools let you use a single site survey AP multiple times (it's configured to emulate multiple APs). Once you've gathered all measurements, you can use the tool in simulation mode and move APs around on the floor plan virtually. These simulations are helpful, but sometimes less accurate than those provided by RF modeling tools.

Note that estimation and prediction tools don't provide insight on how interference impacts WLAN performance, though they typically provide spectrum analysis, so you can sweep the site for interference. Spectrum analysis is a must for conducting a site survey for WLANs that require high availability or for congested, unlicensed Wi-Fi bands.

Heavy Lifting

High-powered wireless site-survey tools--full-blown modeling applications, such as Motorola's Wireless Valley and AirTight Networks' Planner--let you model the entire WLAN. They will cost you a little more too, from $4,000 to $6,000. They import your floor plan and "model" the environment with details such as building materials and precise distances. You get an accuracy of 85 percent out of the box with a 2 dB to 3 dB standard deviation. These tools are best for large, complicated WLAN environments, if you're planning a repeatable WLAN design or if you want to test "what-if" scenarios.

Trapeze's Ringmaster is an integrated tool that models the RF environment and automatically adjusts APs as needed. Once you've imported your floor plan, you can customize it with specifics such as drywall, cement wall, metal studs and so on. APs can be moved or added virtually, and walls can be torn down to test and plan alternative WLAN designs.

AirTight also offers a site-survey planning service ($500 per 50,000 feet) where it models your WLAN so you don't need to purchase the vendor's modeling application (which costs $4,000). And the company includes a one-time offer of free planning services with the purchase of its AirTight wireless IDS starter kit. Motorola's Wireless Valley LAN Planner, meanwhile, is a component of Wireless Valley, a suite of RF design tools that let you model different types of RF systems.

Decision Time

So which tool is best for your WLAN site survey? That depends on the size and scope of your planned WLAN. For medium-sized and large installations (more than 20 APs) where you plan to purchase a WLAN switched infrastructure, it's best to use the planning tool included in the WLAN switch product. These tools also make sense if you're trying to build a self-healing, robust WLAN that can adapt automatically to environmental factors and reconfigure itself for optimum performance. If you want to further augment your site survey, you can add RF modeling tools for more comprehensive planning and spectrum-analysis tools to ensure WLAN coverage.

For small installations (without a WLAN switched infrastructure) or for specialized environments like outdoor WLAN deployments, go with the laptop-based tools for your manual site survey.

And if your organization is medium to large with a specialized environment or with applications that require large WLANs or the planning and support of a repeatable enterprise-wide WLAN, a site-survey modeling tool is best. This tool is also useful if you need an accurate preinstallation estimation of how many APs you'll need and where they should sit, for example, or if you want to execute "what-if" testing without affecting the production WLAN.

Cornell W. Robinson III is an associate and wireless security consultant for Booz Allen Hamilton and a contributing freelance reviewer for Network Computing. He previously was an adjunct professor at Syracuse University and a manager at the Center for Emerging Network Technologies there. Write to him at [email protected].