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Performing Wireless Site Surveys

In my previous article on ways to improve WiFi performance, I listed wireless surveys as the second step. Wireless site surveys are important for planning and validating proper WLAN operation. They help confirm that you’ve met the requirements for optimal performance. In this post, I’ll cover two types of site surveys performed in different phases of a WLAN lifecycle: pre-deployment and post-deployment.

Pre-deployment wireless site survey

Before deploying a new wireless network, you should conduct a pre-deployment site survey to begin developing a wireless network design based on requirements gathered from company stakeholders. You can conduct a pre-deployment site survey in one of two ways: AP-on-a-stick or predictive modeling.

AP-on-a-stick in the most time consumting. It involves configuring an access poitn and WLAN to associate with a laptop. The laptop has the wireless site survey software installed on it and is used to gather information such as signal strength, data rate, and channel overlap. It's called AP-on-a-stick simply because an access point is mounted on a pole and is raised to the height of where the access point will be installed. The wireless engineer then walks around the access point, gathering the data needed to determine the cell size, signal propagation and other information.

In the site survey software, the engineer freezes the access point on the floor plan so that he or she can then move the access point to another location and survey a new area.  That way, an engineer can find the edge of each cell and plan proper overlap of cells for roaming and secondary coverage and other requirements.

While still used by wireless engineers, the AP-on-a-stick method isn’t performed very often today. It takes too much time for an engineer to perform and for some companies that cost is a high hourly rate. However, some scenarios, such as outdoor deployments, lend themselves well to the method.

A faster and more efficient way to design a wireless network using site survey software is to create a predictive model. The output is a floor plan with simulated access points strategically placed with a proper channel plan according to coverage and capacity requirements. 

The site survey software performs all these calculations with special formulas depending on the vendor access point and antenna chosen for the predictive model. The engineer imports a floor plan into the software and traces wall materials across the floor plan in accordance to the expected dB loss. 

Going back to coverage requirements, the software will consider configured minimum requirements such as:

  • Co-channel interference or channel overlap
  • Signal strength
  • Data rate
  • Packet loss

The software also can consider capacity requirements by configuring the number of devices and their types expected on the wireless network. The devices are determined during the requirements gathering phase and added into the site survey software. 

As a reminder, this method is exactly what it’s called: predictive. It’s the process of predicting signal propagation, data rates, channels, access point transmit power, and other data based on math and attenuation.

Post-deployment wireless site surveys

Post-deployment surveys also are often called validation surveys. The purpose of a validation survey is to ensure the wireless network deployed meets customer or client requirements. You should compare the validation survey to the predictive model previously performed. In some scenarios, there may not have been a pre-deployment survey performed prior to an installation. A post-deployment survey can be conducted at any time and also is part of the iterative process of a wireless deployment.

The process of performing a validation survey begins with importing a floor plan. The next step is to measure a long wall, which is required to crunch numbers used for calculating RF characteristics. One benefit of validation surveys is skipping the tracing of walls on the floor plan.

With the validation survey ready, it’s time to take the laptop, start the survey, and walk at a slow and steady pace. It’s important to walk at a slow and steady pace for the external adapter to passively gather wireless information from all the necessary channels. The adapter would be scanning on all frequencies of the 2.4 GHz spectrum and as well as for all the channels in the 5 GHz spectrum. To conserve performance, it’s possible to select channels on which to scan.

After walking and gathering this data, it is time to analyze and validate. At a minimum, determine if the cell size of an access point is within specification. Identify if the transmit power level is configured correctly.  It will be critical to have secondary coverage from other access points to validate that the roaming requirements are met, if needed. This can be visualized with site survey software, such as Ekahau Site Survey. The secondary coverage will visualize overlapping cells.

Verify the channel widths are appropriate. In most environments, 20 MHz change widths will provide the most efficiency. Anything larger that will result in poor channel re-use and may result in channel overlap. If your validation survey indicates 40 MHz channel widths are useable then by all means, do it. But I highly discourage the use of 80 MHz or 160 MHz channel widths.

Speaking of overlap, another critical measurement is channel overlap. While it is crucial to have secondary coverage, channel overlap should be kept at a minimum. Having two or more access points operating on the same channel in the same area will create the capacity of one access point. Not only will the devices be sharing the same contention domain, the access points will also be contending for air time.

Comb through the validation survey and determine if any optimization needs to occur. If so, revalidate the design with another survey and compare with customer or client requirements. Only when those requirements are met can the deployment can be deemed completed. Keep in mind that wireless environments and requirements change, which can trigger the whole wireless survey cycle again.

Wireless site survey software 

You may be wondering what software is available for conducting predictive and validation surveys. Luckily, there are a handful of options available; the list below isn’t exhaustive. Perform your due diligence and ask for a demo to find out which one works best with your workflow.

  • Ekahau Site Survey
  • iBwave
  • Fluke Networks AirMagnet
  • Acrylic WiFi
  • TamoGraph
  • VisiWave

Learn more about wireless infrastructure and supporting the mobile enterprise in the Wireless & Mobility Track at Interop Las Vegas this spring. May 2-6. Don't miss out -- Register now!