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Two ways of designing Wireless LANs

By Keith Parsons, Contributor
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The first, and perhaps the easiest, is to look at designing your Wireless network like a big calculation. This begins to get quite complex with many variables, yet the idea behind it is simple enough.

Build your design by looking at a variety of factors that go into how many access points you might need. If you look at enough variables, it starts to feel like you might have a good handle on getting the proper result.

Let's go through a simple example.

First, you have to pick the Access Point of choice. This will determine the capabilities and capacities that will be used further on in the calculations. Things like single band vs dual-band, supported channel widths in 5GHz, number of supported associations per radio, how many concurrent SSIDs, what you Basic Data Rates will be. This will give you available airtime to support client needs.

Next, you take into consideration detailed information about your client base. How many, what type, what 5GHz channels might be supported, channel width, supported spatial streams, supported MCS rates, etc.

Finally, you add into your monster calculation the types of applications and services that will need to be supported by your clients. TCP data rates needed, RTLS, VoIP, video conferencing, streaming video, etc.

Put all of this in a big massive spreadsheet – along with a bunch of assumptions and out drops a number of Access Points you'll need in order to support those assumptions.

Don't get me wrong, this is a great way to get a quick estimate and get in the ballpark with a Bill of Materials and a starting point for Access Points.

But it is still JUST A CALCULATION – it has nothing at all to do with the real world. Especially with the RF situation you are designing for.

The REAL way to design for Wi-Fi that just works, is to design around RF Contention areas. The total capacity of any high-density WLAN is totally dependent on being able to get the absolute most Frequency Reuse as possible.

Again, to reiterate: It is all about Frequency Reuse!

No matter what your calculation process yields in the way of Access Point counts… it all comes to naught if you can't manage to get frequency reuse.

Now what do I mean by 'Frequency Reuse'. In any given area, how many times can I reuse a specific frequency? Think of a large circular amphitheater, perhaps 100m across. (imagine about the size of an American Football field only in a circle). With all the seats arranged in concentric circles around a tennis court in the center.

How many Access Points will you need for Coverage in this arena? Think hard for a minute…

OK, you've had enough time. The answer is ONE!

You'd only need a single access point in such a facility and with an Omni antenna and high power setting (still below FCC limits) you could have all seats covered with say -70dBm of signal. Meaning, any seat in the arena could have someone sitting with a smartphone and have Wi-Fi access from a single Access Point.

That's the way omni antennas work. And with nothing in the way between the Access Point on center court, and each and every seat. You'd have fantastic coverage. No seat would be more than 60m-70m from the AP and life would be great.

Well… not totally great… because you would be limited by the association count capability of the access point. Perhaps 250 total clients spread across two radios and you'd be at association max. This is nowhere near the amount of seats in the arena.

So the answer for Coverage is ONE… but that doesn't meet the capacity requirements at all.

So, you triple the count and put three Access Points on center court. You've now got a radio on channels 1, 6, and 11 and you've tripled, the capacity on 2.4GHz and tripled the capacity in 5GHz with radios on channels 36, 44, 52.

But you are still nowhere near the capacity goals for number of access points that came from your monster calculation project.

Thus we run into the REAL design issue. How to get the most number of Access Points without causing co-channel interference. Because as soon as you have two AP's on the same channel, they both see each other, and the clients see each other… because of how 802.11 works – they will all defer to each other in a large contention domain. Thus you added an extra access point, but didn't gain any additional capacity – all the access points and clients on the same channel will share the bandwidth together.

This is why I try to emphasize it is all about Frequency Reuse. The goal of Wi-Fi design is to get the most number of AP's you can without causing Co-Channel Interference and contention areas.

Thus, instead of using only calculation tools, you need to have in your toolkit arsenal, at least one tool that can do predictive RF analysis. You can use these 'survey'- type tools to predict where your RF is going, and how to move AP's around, choose different antennas, different power settings, and learn to use walls, floors, and other RF dense obstacles to design IN RF frequency reuse!

The next time you are thinking about Wireless LAN design remember… It's not about the calculation as much as it is how to design IN Frequency Reuse.