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Have you ever had to conduct a predictive wireless survey? Not really a wireless dude, but, somebody in the corner office thought it needed to be done for the new building? Next thing you know, an Ekahau Sidekick shows up on your desk, with the Pro software, and an email from the boss. Sweat is pouring from your head because you’re not only excited, but horrified at the potential of screwing up an entire office.

Relax. It’s not as crazy as you think and you don’t have to be a Wi-Fi Nostradamus to create an accurate predictive survey.

Where to Start?

Once you have launched Ekahau Pro, you need to start a new project and import a floorplan. The in’s and out’s of Ekahau Pro is beyond the scope of this article, but, if you’re looking for some help, check out the Ekahau University site for more details. You will need to know how to add furniture, walls, and multi-story buildings into your plan. You’ll also need to verify any dB loss if building construction is uncertain.

Planning a wireless deployment isn’t just adding walls, dB loss (though important), or sucking in a floorplan to your project. There is a lot of art and science when it comes to adding enough wireless access points (WAP) for proper coverage, roaming considerations, and limiting the deployed WAPs so as not to induce wireless problems. It’s one thing to automatically create a channel and power plan with your wireless software, but, you should understand that once deployed, APs may modify these things regularly to adjust to user demand and changing airwaves.

Where To Put Your APs?

Channel Interference - 5GHz
Channel Interference – 5GHz

If you’re creating a predictive survey there are a handful of things to consider. You need to understand how clients associate to an AP especially with regard to roaming. When making a predictive heat map, user requirements are critical. Ask architecture questions like:

  • Do the users roam, or are the stationary?
  • Do they use soft-phones?
  • Are location services needed?
  • Where is the office located? Is it in the middle of a multi-tenant mall or is it the anchor store?
  • What are the support requirements for 2.4/5GHz spectrums? What adjacent RF broadcasters that may cause interference?

Think about the number of users the wireless environment should support. A school is much different than a municipality, which is different than a commercial enterprise, which is different than a local pub.

That “spectrum” question is a big one. The 2.4GHz spectrum is susceptible to co-channel interference from surrounding wireless devices. You only have a handful of channels available so get ready for problems that may not be addressable. Channels 1, 6, and 11 are available but if your neighbor is using them too—and they are—you’re going to have channel interference issues.

Image shows primary signal strength with minimal overlap
Primary Signal Strength

After placing your WAPs on your Ekahau project, there are some stats in addition to signal strength to help predict a successful wireless deployment. WAP density is important for roaming and overall coverage. If you use the minimum number of WAPs to give better than a -65dB coverage value, you may find that clients will associate to the WAP at the end of the hall rather than roam to the closer one. However, too many can cause channel overlap, even on the 5GHz spectrum. Too few won’t work, and too many has diminishing returns.

Proper WAP count, channel plan, and placement is key for your primary signal strength, but there’s another design principle often overlooked. Secondary signal strength. Some engineers only plan for the primary signal strength in their predictive surveys. This focused design approach of the primary coverage area considers only main factors of good wireless connectivity for all corners of the floor plan; RSSI at -65dBm, SNR around 25dB, and signal strength no worse than -67dB. While that may be a reasonable goal, it doesn’t account for the times when an AP goes offline in a certain area.

Secondary Signal Strength with maximum overlap
Secondary Signal Strength

If you only plan for the primary signal strength, the section of the building with the offline WAP becomes a black hole and service desk tickets will be opened. By including the secondary signal in your plans you can supplement coverage areas during an outage or maintenance. What is secondary coverage? This is the coverage area serviced by the second strongest AP at a given RSSI value, measured in dBm or mW.

In the signal strength images (above and right) the overlap of RF from the WAPs would be considered secondary and optional for clients when the primary RSSI falls to unacceptable values. It’s even more pronounced when you change perspectives from a one dimensional image, to multi-spatial RF “clouds” in your office facilities. These balloons of radio energy move through walls, around obstacles, and between floors. Tracking the RF is enough to make the most senior of network engineers bang their head on the desk in frustration.

So what should you do? How do you plan?

Wi-Fi is like having a bunch of people with megaphones walk into a warehouse and start yelling at everyone else so they can be heard. The science is understanding noise controls and the art is figuring out how best to deploy those controls. To do that, you really need a solid understanding of your wireless requirements. Education facilities may need more access points, placed in all the classrooms, with power properly adjusted because most students normally aren’t roaming but are concentrated. For commercial offices, overlapping signals in hallways and offices may be required to support users moving between meetings or working in common spaces. And lets not forget most mobile phones use Wi-Fi calling and can get a little wonky if signal is good enough to maintain a connection to the AP but not good enough to carry voice data.

Ultimately having good requirements understanding prior to starting your predictive analysis is your bedrock upon which you build your report. Make sure you understand the building floorplan and construction so your prediction will be correct. Only with this information will you be able to provide accurate prediction of your airwaves when you deploy the WLAN to production.

Brian Gleason

I am a full-time Sr. Network Consultant for a leading Value Added Reseller, SHI. My career in the IT industry has stretched nearly 25 years primarily in the enterprise space; financial, biomedical, and high tech sectors.