Wi-Fi – Circles Are Bad

Let’s start with the classic design guideline, the trope that you need overlapping circles. “Make sure they overlap 20%, that’s a proper design.” At least that’s what they say.

The queuestion is, how do you achieve that? How do you know you have the percentage overlap that you need? Ok, maybe we account for dB loss? Well then how do you properly calculate that loss? That’s easy, find a document that gives you the loss over the distance you need, correlate the two. Call it a day! Well…

Let’s take a look at the design guide by Cisco.

Source : Mobility Design Guide 8.1

Great! So I’ve got my answer! Overlapping channels, with a 19dB separation. That shouldn’t be too hard. Right? Right?!?

Well let’s see here, let me put this AP down right here in the hallway.

That’s odd. It’s not a circle. It’s like the hallway reflected the signal down the hall instead of letting it spread evenly.

This is why your wifi isn’t performing as expected. A long enough hallway will require you to place multiple APs in it to get the coverage you need. The instant you have co-channel interference you just reduced your AP count from 2 to 1.

How do we solve for this, so the circles actually behave more like circles? You place the AP in rooms.

This allows your design to reduce the chance of co-channel interference. And allows for you to account for the amount of overlap you need. But look carefully, we still don’t have circles. It’s very critical to find out the dB loss your walls will create, this circle won’t look the same in a different office.

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