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This blog post expands upon a recent question I posted in the CWNP’s CWAP/CWDP study forum relating to the practice of providing coverage overlap for voice grade Wi-Fi deployments:- Why is specifying 2 or more APs with Sig’ Strength of -6xdBm best practice for Voice over Wi-Fi?
It was sparked off by a the suggestion that as a design guide, -6x dBm coverage from 2 or more APs is required over the space you are looking to cover with voice grade Wi-Fi. (Where x can be -65/67 depending who you talk to).

Note: I wrote a whole ream on the subject exploring different roaming scenarios ; this post is a cut down version of that ream and makes a whole bunch of assumptions in order to get to the core of the argument… I’ll try and polish-and-post the original soon!

From the The Ideal – Wi-Fi Coverage In Theory.

The Ideal

The image above shows how these things are often represented, and her you can quickly see the logic for suggesting 2 APs at -67 dBm. As shown, anywhere where the cells ‘touch’ will have to APs at about -67dBm. I’d call this “zero overlap” and would be met by a primary design specification of “coverage at -67 dBm over 95% of the internal area” or something like that.

To the The Real – Wi-Fi Coverage In Practice.

But what happens when we try to do this in the real world.
More often than not we have a scenario like the one pictured below:-

Simple Real World

Meet our actual (simulated) room, with concrete walls and a single door leading out into a corridor.

Ekahau here is showing anything covered with -65dBm or above.

To meet our “2 APs at -65dBm or above” specification to support voice, we’d have to install another AP somewhere in that room.

Anywhere outside is going to fail to cover some part of that – even an AP put outside the door (shown as blue) does not cover that room entirely:-
AP outside door

So What’s the point of 2 x -67dBm?

So, why would we want to do this?

Replicate it for each room in a facility and soon you’ve got loads of APs which IMO are mostly just serving to add Co-Channel Interference.
There is only one way out of that room (not even a window for the truly adventurous) so there isn’t anywhere for a client to go except into the corridor. If we’ve designed to -6x dBm there will be an AP for us to roam to as soon as we leave the room to provide coverage in the corridor. (Even if it’s far down the corridor, as long as coverage meets the spec’, it should be more than adequate.

(Sure, i’m making some assumptions here – that APs are pretty sturdy and don’t fail often; that there is plenty of capacity available, but hopefully the scenario i’ve outlined is enough to illustrate my point)

If you’re reading this and have an opinion, I’d really like to know!