I was at a meeting today with a large Mechanical/Electrical Engineering firm who was in need of some wireless expertise. More, and more they are getting asked to include wireless "designs" for building projects and are finding (as many do) that it's not as simple as it seems.
The discussion took many turns, but often came back to something like, "So, if we have a school with say 35 students per classroom how many APs do we need?" My answer would be, "it depends." What does it depend on? Their requirements.
How many clients (not users, but devices)? What type of clients (1 stream, 2 stream, 3 stream)? What applications will they be using (e-mail & web, video streaming vs. YouTube caching, voice, etc) What are the bandwidth requirements for their State testing? And more.
The point was - just like they could not just "make up" an electrical, or engineering design out of the blue (How many people need to be in the space? What's the total power consumption required? Do we need HVAC in all locations?) - one could not just "make up" a WLAN "design" (Well, one could, but then you get what you get). That made total sense to them which was good.
I love explaining how wireless works and seeing their eyes light up. I love how it makes sense to them when I explain why they're not going to see 1.3GBs throughput, or adding more APs is not a default answer to a problem, how coverage & capacity are different things, how having a bunch of low-end single stream devices is not as efficient as have a bunch of 2, or 3 stream devices, etc.
The FIRST step to wireless network design, and the best way to avoid the BAD-FI, is to determine the REQUIREMENTS and EXPECTATIONS of the customer. Here just a few of things you should consider:
How many clients will be using the WLAN?
What are the types/capabilities of the devices? (# of streams, 5GHz support, DFS support, 802.11r/k/v support, etc.)
What applications will be using the WLAN and what are the requirements of those applications?
Is there a budget for the project?
Are there accurate, scale floor plans available?
What security and authentication types are you looking to support?
What the total bandwidth coming into your facility?
What is the time-frame for the project?
Aesthetics: Are external antennas ok? Do LEDs need to be off Should APs be inconspicuous?
Cable lengths: where are/is the MDF/IDFs located? More than 300ft from the APs?
These are a few off the top of my head, but you get the gist. DEFINE your requirements and expectations BEFORE you design a solution.
Anything else and you're just guessing.