How many Wireless Access Points do I need?

This is one of the more frustrating questions I get asked…  how long is a piece of string?!

I understand why people want to know – is it even worth doing a Survey if my budget will get blown, can I stretch to the better model of Access Point, how many PoE Switchports will I need to provide – but it’s not a quick or easy question to answer.

Before you ask this, or before you trust anybody who tries to answer, there are several things to consider;

  • What does the network need to do? (Data, Voice, Video, Location Tracking, RFID, etc)
  • On what scale will the network operate? (1 User or 10,000? How many devices per User?)
  • How will you quantify that the network is working well? (SNR? Bandwidth? Packet loss? User reports?)
  • What are the physical characteristics of your environment? (Open plan? low ceilings? Thick walls? Congested RF?)
  • What are the capabilities of the Clients using the network? (frequencies, transmit powers, phy, security capabilities, battery powered?)

I find it best to start with the desired target User experience and work back from there, via environmental considerations and device capability considerations, to arrive at an answer to the question.  Here’s a story of how requests like this sometimes go;

Let’s say you want a good, high performance network with Clients that are generally just browsing the Internet and sending e-mails – short bursty traffic flows.  Let’s further suppose you have a modern-ish fairly open plan office across three floors of a building with 200 people per floor.  You’ve recently splashed out on laptops and tablets so everybody has a device that supports 802.11AC and you want a ‘Wireless First’ network connectivity model because none of the Users can be bothered to drag around the brick that goes with their shiny new Surface Pro.

You’ll want to develop a target contention ratio of Users per Access Point to ensure everybody gets a fair amount of bandwidth to supported the anticipated usage; let’s say 20 Users per Access Point.  So, without even seeing any floorplans, 600 Users divided by 20 Users per Access Point gives us 30 Access Points as an initial estimate.  Let’s assume all of the floorplans are identical, so 30 Access Points across three floors leaves us with an initial estimate of 10 Access Points per floor.  With a bit of effort you also research the devices your Clients are using and you determine that you need a particular Signal Strength to let them hit the data rate they want; let’s say -67dBm at 5GHz.

Consider the layout of the floor, will ten Access Points realistically provide the coverage you want – at least -67dBm Signal Strength / +25dB Signal Strength @5GHz everywhere?  With lift shafts, stairwells, storage cupboards and areas that have particularly high or low user densities, you may end up needing 12 Access Points per floor, that’s 36 Access Points in total as a basic initial estimate.  Any experienced WiFi person will be good at estimating this off plan.  Many will give you “heatmaps” to show the coverage you can have, but if these are produced offline and without having gone to site, they’re just educated guesswork and must not be relied upon, and no, it doesn’t matter if they used Ekahau, AirMagnet, WCS, or whatever… it’s all guesswork.  No matter how impressive they look, they are not reliable.  If you want reliable results, do a proper site survey.

“You’ll need 36 Access Points” is the message that goes back to the customer, “but we’ll do a site survey just to check”.  An engineer goes to site and starts surveying for -67dBm at 5GHz with a target 20:1 contention ratio, but soon spots a problem.  Your office is in Zone 1, central London.  You’re surround above, below and on all sides by other Corporations all operating their own WiFi and the noise floor is much higher than expected.  To meet or exceed the minimum Signal Strength and Signal to Noise requirements, the coverage area of each Access Point has to be reduced.  This gives you a better User to AP contention ratio, but it increases the number of Access Points you need – let’s say you need an extra two per floor, so we’re now at 42 Access Points.

The survey continues and when the surveyor gets to the last floor, it becomes apparent the floorplan doesn’t represent the actual layout at all.  Instead of an open plan office, it has been divided up in to small meeting rooms with lots of floor to ceiling glass walls. The architect has used nice expensive, heavy glass that is hard for 5GHz to penetrate so to keep hitting your SS and SNR requirements, another two Access Points are needed on that floor and we’re now at 44 Access Points from our initial uninformed estimate of just 30.

You’ve read that 802.11AC can provide some really fast data rates – 2.6Gbps or so – can your 44 shiny new Access Points do that please?  In short, no.  With 14-16 Access Points on a relatively small, open plan floor, you’re likely limited to 40MHz channels to avoid channel re-use issues.  Rather than the sexy 160MHz channel 2.6Gbps rates you’ve been reading about from the Manufacturer’s marketing team, you’ll have peak datarates more like 600Mbps.

44 Access Points, 600 Mbps peak rate, fine.  Let’s get them installed.  Wait, why don’t they work?  Did you audit your LAN properly?  Newer 802.11AC Access Points need PoE+ or even UPoE to work properly.  Older switches might only support 802.3af and the one switch you have that does support 802.3at is at its power limit.  Access Layer LAN upgrade please…

The morale of the story?  When you ask one simple question – how many Access Points will I need – but you get 50 questions back with no sign of your question being answered, take the time to answer thoroughly, it will be time well spent in the long run.

 

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