Saturday, November 30, 2013

Outdoor WLAN site survey


Last year I wrote a blog post because I thought I was going to be doing a lot of outdoor WLAN site surveying.  That post went through all the details of how to set your PC up, step by step, to get you up and running with outdoor surveying.  Fast forward to this month, when we were tasked with designing and installing some outdoor WLAN coverage for a manufacturing plant.


Since the area was not too large, I decided to survey it twice.  The first survey was the regular walkabout – the way you would site survey an indoor area.  As you can see, I went with the default settings for signal propagation on the walkabut – and if I were to do it all over again, I would set it at fifty feet or so.


There are four Cisco 1552 series mesh access points in this design, and they’re not meshed together.  They are all in local mode, which means they are not using backhaul radios – both radios are for client access.  One thing I want to point out is that one is pole mounted, one is mounted to building with wood siding, and the other two mounted to metal skinned buildings.  All four access points have the dynamite stick antennas, too.  Can you tell which ones are which?



I really should have changed the survey propagation when I did this walkabout(above), since it makes it look like we provided coverage inside those buildings.  By the way, those buildings are in the 200,000 – 400,000 square foot range.


By looking at the same survey (below) at -50dBm, you can see where the access points are.  My guess is that you can also make a better guess as to which ones are mounted to metal buildings and which ones are not.





Here’s the same area(below) – the only changes are that I am doing a GPS survey this time, and a few days later.  I’m letting RRM control the power, just like the walkabout survey above.  The power on three of the APs was set at three, and one of them was set to full power – which is where the controller thought they needed to be.  The original site survey was done at power level three for -75dBm 2.4 GHz coverage.  I surveyed it that way because I wanted to see the coverage area that the controller thought I needed.  All of the buildings in this coverage area have Wi-Fi inside them – however the metal skinned buildings tend to block most of the signals from escaping.   I recently did a WLAN survey in a Quonset building (corrugated metal building in the shape of a 55 gallon drum cut in half length-wise) and when we walked out of that building, all signals were blocked entirely.  If I ever need a Faraday cage, I will remember to ask them if I can borrow their building.


Here’s -75dBm 2.4 GHz coverage.  Three APs power level three, one AP power level of one:




After changing the power to level three on the access point that was set to level one (and statically setting the other three so they would not change) I surveyed again.  It’s amazing how your feet don’t get tired when you drive around with a laptop on your dashboard and a GPS receiver taped to the roof of your car.


You might think that dropping the power level on the AP set to power level one would make a big difference in the -75dBm coverage.  Each power level is a 3dB change in the WLAN controller.  Dropping the power level two notches effectively cuts the power in half, and then cuts that power in half again.  The AP that was on full power was the AP closest to the bottom of the screenshot.  I expected something a little more dramatic, actually.  We were aiming to cover that parking lot directly below the access point.  The controller must have thought we wanted to cover both parking lots.  You can see the coverage area decrease slightly in the survey below.


Here’s -75dBm 2.4 GHz coverage.  All APs set to power level one:



All in all, we covered the two parking lots we were aiming to cover, along with the area between the buildings so that WLAN client devices could roam from one building to another when the human did.  One thing to note – see that “main drag” where I drove from the top of the map to the bottom -  the signal seems to drop off immediately for some reason?  That street is lined with very large trees that stay green all year long.  I  could cover it with Wi-Fi, but what’s the point?  You should be looking at those gorgeous trees!











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