Wi-Fi Airtime

Airtime and contention domains can be a difficult concepts to grasp when studying Wi-Fi. We often think of Wi-Fi in cells or strict zones surrounding an access point and fail to think about the transmit radius of each client associated to that access point.

Because of the way the 802.11 medium is accessed by a station (Access Point or client) we must consider the entire area of influence surrounding each station which transmits.  It's not just simple circular patterns surrounding each AP. CSMA/CA is a very importants starting point for any learner of Wi-Fi, it is covered well in the Certified Wireless Network Administrator (CWNA) course. But there is much much more to learn...

The best source I have found that expands on these concepts in detail is the Very High Density 802.11ac Networks Validate Reference Design Guide, written by Chuck Lukaszewski (CWNE #112) from Aruba Networks. The Theory Guide is a fantastic read but may take a few goes to understand if you are just starting out. When ever I present on Wi-Fi design I use some of the concepts out of this guide.

Especially fitting is the reference to the Matrix in this guide. You really are jumping in to a whole new dimension of understanding by working through it.

You're here because you know something. What you know you can't explain, but you feel it. You've felt it your entire life, that there's something wrong with the world. You don't know what it is, but it's there, like a splinter in your mind.
Morpheus to Neo, The Matrix (1999)

The Theory Guide explains that the contention domain extends as far as a frame can be detected by another station. The range is a physical distance from a transmitter where the the legacy preamble can still be decoded. That happens to be a very long way.

This graphic of two distinct Collision Domains is a very rare occurance. Unless these two Access Points operate on sufficiently seperate channels they would likely be very far apart. In most environments, especially high density environments collision domains of the same frequency overlap  somewhat.

It's more likely that the collision domain of two APs on the same frenquecy in essence extend the collision domain to cover the detectable distance of both of the APs. The guide delves further to show that even this is rudimentary. Clients must be considered also! Futhermore the time domain which is shown as the x-axis is also vital.

This is your last chance. After this, there is no turning back. You take the blue pill - the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill - you stay in Wonderland and I show you how deep the rabbit-hole goes.
Morpheus to Neo, The Matrix (1999)

Once you are at this point of the document you begin to realise how simplistic some of the planning tools we use in the Wi-Fi industry are and some of their shortcomings. I suspect the shotcomings will take a very long time to solve.

In your Wi-Fi learning journey I highly recommend you take a wander down this path. Even if you don't work with Aruba products the Theory Guide profiles fantastic lessons for us all.

The images in this blog are borrowed from here: https://www.arubanetworks.com/vrd/VHD_VRD_Collection/wwhelp/wwhimpl/js/html/wwhelp.htm#href=ChapT2.html#1045569 they are used for basic primers for the topics that are extensively covered in the Aruba Validated Reference Design Guide from where they originate.