Presentation given by John Aldington, University of Kent at the Jisc Wireless Mobility Day on 21 April 2021

1. Lessons Learned from Wi-Fi Surveys
Jon Aldington
University of Kent
I took this picture on a particularly lovely December morning. This is the “south slopes”
of the University leading down from the campus to the city of Canterbury. Canterbury
cathedral can just be seen sticking up through the mist and to the left of trees on the
right. The area in the foreground has good Wi-Fi coverage.
My email: J.P.Aldington@kent.ac.uk
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2. Notes
• If you are viewing this presentation there are notes explaining the
pictures on the PowerPoint note pages
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3. The University of Kent’s main site is a large campus university on the outskirts of
Canterbury. Founded in 1965, it has expanded greatly since then, particularly in the last
20 years or so. University buildings stretch from the foreground towards the far distance
(approx. 1.5km)
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4. Disclaimer
• Many of the slides show heat maps generated by Ekahau Pro
• Ekahau Pro is our key Wi-Fi survey (and modelling) tool
• We use the Ekahau Sidekick to capture Wi-Fi data in the field
• The University has no relationship to Ekahau other than as a fully paid up customer
• I have no relationship to Ekahau other than being a satisfied user
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5. Picture: A random University of Kent building on a nice day
The project:
• “Improve student satisfaction with Wi-Fi”. That’s what I like - a project with clearly
defined and easily measurable goals.
• Significant investment available
• First of all, understand what you already have, so:
• Step 1: Attend Ekahau training course
• Step 2: Get busy with Wi-Fi surveys
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6. Signal strength ‘heatmap’ scale
Number of visible access points /
interference scale
The colour scales are chosen to be more easily visible for viewers with colour vision
deficiency
-67dBm is university’s minimum Wi-Fi signal strength. This signal will usually work very
well for a laptop. Might be a bit marginal for a smart phone where -63 or -64dBm would
be better, particularly on 5GHz
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7. Case Study 1
– Pembroke Campus Drill Hall Library
The Pembroke Campus is in Chatham. It is a shared campus with the University of
Greenwich and Canterbury Christ Church University.
The University of Kent runs the wireless network on behalf of the partnership in some of
the shared buildings.
The Drill Hall Library is the very long building in the upper middle of the picture (yes,
that’s all one building)
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8. Exterior view of the Drill Hall Library. The building is about 185m long.
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9. The interior consists of the main library space which runs the full length of the building.
There are teaching rooms and offices at the rear on 2 floors.
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10. We are a Cisco house. Cisco’s Radio Resource Management had been left to do its thing.
After coverage problems on 2.4GHz, the configuration had been changed to ensure that
2.4GHz radios were not turned off.
However, 5GHz channel widths and power levels on both bands were left to Cisco’s
good(?) judgement.
Here’s the signal strength picture on 5GHz
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11. Using Ekahau Pro you can view the coverage from a single access point (or a couple of
access points). This picture shows the signal strength from a single access point on
5GHz.
A single AP will reach much of the 185m length of the library
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12. This shows the number of access points that are visible on a single channel (the channel
to which you are most likely to be associated). Most locations can see at least 2 access
points, with 3 or 4 common and even up to 6 visible in some places.
On 5GHz, RRM had mostly allocated 80MHz channels. This meant a *lot* of contention
on 5GHz. Unless you live in a cave, or have no neighbours within 200 metres, you are
almost invariably way better off with 20MHz channels (and in a busy, contended
environment, an 80MHz channel will likely give you less throughput than a 20. It has the
potential to clash with 4 different 20MHz channels instead of one; an 80MHz channel
only gives significantly more throughput for large sequential transfers (e.g. file
download, speed test!). In a mixed environment with a number of users, the difference
between 80MHz and 20MHz channels will barely be noticeable in terms of throughput,
even ignoring the far worse contention).
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13. On 2.4GHz the picture’s not so pretty. Cisco Radio Resource Management has seen the
number of access points and lowered the power to an unhealthy extent
Note, however, that we’ve found that most devices select a Wi-Fi channel and more
importantly a band based on signal strength. In areas where 5GHz is several dB stronger
than 2.4GHz, you get 90-95% of devices on the 5GHz band. This is usually a very good
thing, as 2.4GHz is usually congested and suffers from interference from microwave
ovens, Bluetooth, security cameras etc.
However, in the Drill Hall Library, the power had been lowered more than we’d like with
a number of marginal areas (yellow) and some areas below the university’s minimum
standard (grey). White areas were not surveyed.
The good news is we could fix all the issues for free. All that was needed was some
changes to the settings on the Wi-Fi controllers.
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14. This is the 2.4GHz signal strength after we made the changes.
Note (from the icons) that we’d turned off 2.4GHz on half the radios in the main library
space. We still have good coverage, and turning off some of the radios will reduce co-
channel interference / contention.
There’s an area in the middle that’s a bit marginal, but still above the minimum. We took
the decision that marginal signal is probably better than too much co-channel
interference.
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15. This is the 5GHz channel interference picture after we made the change to 20MHz
channels. The picture is vastly better with just one access point visible in most locations
and at most two in a few areas. Performance will be much more consistent (even if a
speed test run at 5am will show lower peak speeds).
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16. Case Study 2 – Clocktower Building
An historic building on the Chatham Historic Dockyard campus. The site has been a Royal
Naval dockyard for several hundred years. Now a museum and very popular location for
film and television. It is where they film “Call the Midwife” among others. Several
buildings are leased to other organisations, including the University of Kent.
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17. These screen captures are taken from Cisco Prime (but the problem I’m about to show
you was identified following an Ekahau survey)
This shows the number of client devices connected on the 2.4GHz band – we’re still in
Covid lockdown and the building is empty.
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18. The picture shows channel utilisation. What the heck – the channel is 52% used with no
connections! Why?
Turns out there’s an office building next door. They rent out small offices to customers.
They have taken the approach of putting one access point in each small office, even
though the building is pretty transparent to Wi-Fi. There are about 30 offices and the
access points are spread across 3 channels, so around 10 access points per channel.
There are three SSIDs – main, guest and printers. Their data rate minimum is set to
1Mbps.
Wi-Fi beacons are sent out by the access points: 10, per second, per SSID, per access
point. At 1Mbps, do the sums, allow for some collisions, and you end up with your
2.4GHz channel 50-60% full – of beacon frames.
Attempts to engage our neighbours with an offer of free consultancy failed. However,
they have since done something and the background beacon traffic is now reduced by
about half (I’ll need to visit the site with the Ekahau equipment to find out exactly what
they’ve done).
(my solution is to up the minimum data rate to 12Mbps, turn off at least 2/3 of the
2.4GHz access points and remove an SSID or two if possible, you’ll then be a factor of at
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19. least 36 better off and using more like 2-3% air time rather than 50-60%).
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20. Case Study 3 – Rutherford & Eliot Colleges
Two of the original 1960’s colleges, essentially the same design. Corridors of 8 rooms.
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21. This shows an example of one of the blocks, but where to put the access points?
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22. This shows an example of one of the blocks, but where to put the access points?
This is option 1
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23. This shows an example of one of the blocks, but where to put the access points?
… and this is option 2
Both had been used. Option 1 in Eliot, Option 2 in Rutherford.
However, the annual student “Internet in your accommodation” satisfaction survey
showed that both options scored similarly. If anything, option 2 was marginally preferred
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24. The pair of APs left a gap in the middle of the coverage
This map shows the coverage from just the pair of APs on the corridor
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25. We’re now looking at ‘option 2’ with the access points in the middle of the corridors on
alternative floors.
This heat map shows the signal from the AP on the floor above the corridor.
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26. This map shows the combined coverage from all the APs in the building.
It actually works pretty well.
In contrast, the buildings with 2 APs per corridor in some cases had a slight gap in
coverage in the middle rooms.
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27. Case Study 4 – Tyler Court A
A long accommodation block. They layout is such that central corridors run the entire
length of the building on upper floors. The building is 100m from end to end
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28. A single access point provides useful signal along 1/2 of the length of the corridor and
virtually along the full length of the building.
Note, however, that it only usefully covers a few rooms.
Do NOT survey a corridor and assume this reflects the coverage in the rooms – it will not
and will give you a very misleading picture. Ekahau will extrapolate the signal strength
into the rooms if you let it and you’ll assume everything is fine (and your management
will refuse to let you spend money, because they have a map to prove the signal is good,
despite the complaining students).
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29. Look at the shopping list of access points visible at a point on the corridor.
This causes two problems.
Firstly contention: Access points are fighting another 10+ access points that are visible
on the same channel.
There’s also the so-called ‘hidden node’ problem. Wi-Fi will wait until it can see that a
channel is clear. An end user’s device in their study bedroom will see the nearest one or
two access points. Once it thinks the channel is clear, it will transmit. However, the
access points in the corridors can see 10+ other access points. If another access point is
transmitting when the end user’s device transmits, there will be a collision requiring a
retransmission. It’s a miracle there’s any throughput at all on 2.4GHz.
Lesson: Putting access points in corridors is a bad idea; Putting access points in really
long corridors in a building with several storeys is a really bad idea.
The solution, and high on the priority list is to move the access points into study
bedrooms.
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30. Case Study 5 – Library Shelving areas
When we put Wi-Fi in the library, we didn’t worry about areas that were just
bookshelves. Some were covered pretty well by bleed, others did not have much
coverage
It was agreed that shelving areas needed to be covered
… but how does Wi-Fi propagate amongst bookshelves?
There’s only one way to find out – enter the access point on a stick!
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31. D Block ground floor. Relatively wide shelves with a good gap between the shelves and
the ceiling.
The area needing coverage was modest in size (around 20m x 15m).
How many APs would we need?
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32. Here’s the AP on a stick…
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33. And the resulting coverage map – there’s some attenuation from the bookshelves, but
combined with existing access points, a single AP in the position of the APoS will provide
very good coverage.
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34. D block second floor
Taller shelves and a higher ceiling…
We need to cover a larger area (around 40m x 20, with many more bookshelves.
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35. Here’s the APoS
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36. … and the resulting coverage.
The AP covers a decent portion of the area. Three new APs along with the existing APs
will cover the area well.
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37. B block first floor.
The area to be covered is approx. 28m x 15m
Note the bookshelves are metal and go right up to the ceiling.
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38. Here’s the APoS
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39. … and here’s the very limited coverage you get in this area. The metal shelves are clearly
reducing the coverage and look like they’re decreasing the signal strength even at very
short range.
We’re going to need at least 6 APs to cover this relatively smaller area.
Metal shelving to ceiling level obviously has a much bigger effect.
Lesson1: Beware metal shelving (or probably any shelving) to ceiling level
Lesson2: APoS surveys are crucial if you want to understand what coverage you’ll get
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40. Key findings from doing many surveys
• Coverage maps are full of little surprises
• Gaps in coverage you wouldn’t have expected one
• Areas well covered where you wouldn’t have expected it
• With the odd big surprise thrown in
• Amazement that Wi-Fi even works when you can ‘see’ 10+ access
points on the same channel
• Wi-Fi goes a long way outdoors. The image shows the signal from a single AP with a
13dBi antenna on a length of footpath (200m from the AP to the building on the left)
• You can make a lot of improvements for just the cost of a survey plus some time to
analyse the results
• You can turn off 2.4GHz on quite a number of APs and still have good coverage,
particularly in big open areas
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41. Key Lessons applied to all locations:
• Putting access points in corridors is a bad idea
• Putting access points in really long corridors is a really bad idea
• Limit RRM, ARM or equivalent
• Lock it to 20MHz wide channels on 5GHz (don’t be tempted to go wider unless
you *really* know what you’re doing and why)
• Ensure it doesn’t turn off 2.4GHz radios
• Ensure it doesn’t lower transmit power too much
• Ensure 5GHz is stronger than 2.4GHz where possible
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42. Don’t guess – measure!
Questions?
The map is the full picture of one of the floors of the library building. The gap in
coverage on the right hand side is the second area of bookshelves we looked at in the
presentation.
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