Roll With The Punches; Wi-Fi’s One-Two Combo

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Addressing the explosive growth of mobility is as easy as hitting a moving target. Consider the moving target of mobility in general, combined with the fact that mobile users actually move, and the equation compounds exponentially.

Keeping up takes a quick step.

More often than not those seeking new Wi-Fi networks break their investigation down into two elements: the APs (feeds and speeds) and the solution elements (management, BYOD/Guest on-boarding, and analytics). While the solution elements are often considered the front-runner when selecting one vendor from another, there remains a large group of buyers whose selection criterion is heavily dependent on AP feeds and speeds.

It’s always an interesting conversation, for me, chatting with prospective or existing customers.

I’m often asked, “How fast are your access points?” usually followed with “how many devices per radio can you support?” The old one-two combo that attempts to knock me back on my feet towards the ropes.

And don’t get me wrong there is an answer to both of the questions, but my slow response is rooted in a lack of general understanding – the punches certainly did not create a momentary stupor. I know the always-popular “AP Bakeoff” is usually a few steps away coming to attempt a knockout blow.

The questions aren’t as flawed as I suggest; what is flawed is anyone who answers these questions with a simple number as opposed to exposing the variables required to actually calculate.

Let’s look at both questions a bit deeper.

Question #1 – How fast are your access points?

Answer #1 – there is always a data sheet, regardless of vendor, that will prescribe a maximum data rate. As experts we are quick to point out the variables in this equation such as band in use, channel widths, protocol deployed and overall environment quality. We then blurt out a rate such as 1.3Gbps on 5GHz, assuming 80MHz channel width, 256 QAM achieved with near proximity to an AP and high SNR.

If that is your answer you are only half done in responding to the question. A correct answer should include the semi-rhetorical question: “How fast are your devices?”

As we should help educate: achieved data rates and overall throughput is governed by device capability, the quantity and mix of said devices, and the applications that are in use. For illustrative purposes I supply some “variables” in table format (this by no means is exhaustive):

Devices        Sample applications

The variables above help to illuminate the flaw in the question related to speed. And note, it doesn’t directly answer the question either as every environment will have different device types, different mix of devices, different ratio of concurrent use, and certainly a different mix of applications. Of course there is no way to know exactly what data rates will be achieved, in the real-world, so we usually take a leap of faith and assume best case (at least for the sake of providing a reasonable calculation).

Adding to the complexity are large environments that are variables onto themselves. Take for example a higher education campus comprised of: auditoriums, dorms, labs, administrative buildings, hospitals, arenas, stadiums, ect. In each one of these sub-environments you are bound to find different numbers of users, different types and mix of devices, and different types of applications in use.

All of this is highly subjective, but must be accounted for before supplying an answer to the question.

So what about question #2 – How many clients can your radio support?

Answer #2 – again, we’ll find ourselves digging into a realm of variables. But, to this set of variables we must add one very important dimension to base calculations – cell capacity.

In terms of pure physics, radio frequency is well understood and limitations imposed are based in the natural world. Another way to say it – we can’t magically make a given Wi-Fi channel produce “extra” bandwidth. A single radio cell will prescribe a limited amount of bandwidth in total.

General guideline, looking at single 20MHz and 40MHz channel cells with round numbers:

General Rates

Looking at these bandwidth totals we can start to understand what a full calculation would require; again taking into account all the variables mentioned in answer #1 and answer #2 to derive how many devices per radio a given AP can support. The devices, application in use and each’s required bandwidth, are the key factors to conclude how 30Mbpbs, 45Mbps, 50Mbps or 70Mbps of total airtime bandwidth would be consumed.

So… with all that said, we circle back to the top about moving targets in our wireless world.

How do I answer the questions without going down for a TKO?

I express wireless estimations one AP at a time, and by thinking about each radio one cell at time. One cell at a time, in any scenario/environment, we can think towards breaking down a massively simplified question like “how many clients per radio” into a digestible answer rooted in an analytical approach that can offer real value compared to inaccurate guesswork.

I cringe to think that answers to question #1 and #2 are responded to with an actual number, based on a datasheet, in quick response during real-world meetings.

Hopefully my breakdown starts to help you understand how to address these two questions next time you are jabbed with them.

I’ll leave you with an example to help illuminate my point on the analytical approach – this comes from a job interview question made famous by Google:

How many golf balls does it take to fill a 747 airplane?

If you guess a number as opposed to using an analytical approach breaking the problem into smaller parts you are surely wrong – and more than likely you are not getting the job.

Wi-Fi cells are the golf balls; the environment you are planning for is the 747 airplane. There isn’t a direct answer to the question; think twice before supplying one!



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