Teaching WiFi Airtime – Part 1

What is Airtime?

Airtime is a measure of the time it takes for a WiFi device (i.e. AP, wireless router, laptop, smartphone, etc.) to transmit data. And the total amount of airtime available is fixed, meaning it can and will run out (or reach 100% utilization). Physics tells us that the slower the device, the longer it takes to transmit data. Thus, slow devices use up more airtime, leaving less airtime for all the other devices to share.

So, the question we should be asking here is, what makes a WiFi device slow? Turns out, there are quite a few things. Read on.

“One at a Time Please”

WiFi is a shared (and half-duplex) medium, meaning that all devices operating on the same channel must compete (or contend) for the same limited airtime, and only one device can “talk” at any given moment. This reminds me of a school classroom. The teacher is the AP, and the students are the connected devices. No one else speaks when the teacher speaks. And students comment or respond one at a time, by first raising their hand and then waiting to be called upon. This way, the lesson is communicated to the entire class, and questions can be asked and answered in an efficient manner.

There are many parallels between WiFi and this classroom example, and we’ll get into these below. Even the enforcement of students raising their hands and waiting to be called upon, exist in WiFi as well (this is a bit more advanced [CSMA-CA] and not specifically covered in this blog post – perhaps later).

“Wait Your Turn”

Knowing that WiFi is a shared medium, where only one device “talks” at a time, it’s understandable that devices must contend with each other for their opportunity to transmit. Obviously, the more devices you have connecting to the WiFi, the more contention there will be. What many may not realize though, is that if there is another AP operating on the same channel, whose signal is strong enough to be “heard” by our AP and its connected devices, then we must also wait for that AP and its devices to talk when it’s their turn – increasing the contention much, much more. The more contention that exists, the longer a device must wait to transmit.

Returning to the classroom example, remember, the classroom has a strict “raise your hand” policy. And the more students attending the class, the longer they must wait to be called upon. In my freshman year of high school, my science class shared a wall and a door with another classroom; and often, that door was left open. Now occasionally, that neighboring classroom got pretty noisy, as did ours, which forced both classes to do one of few things:

  • Wait until the neighboring noise died down enough to not interfere (or contend) with the other classroom (thus increasing the contention and overall wait time)
  • Close the door, creating enough physical interference to eliminate the need for our class to contend with the other (ensuring that our AP and devices can’t “hear” the neighboring AP and devices)
  • Move to another classroom (change the channel upon which you are operating)

One might think that another option would be to speak louder (increase the AP Tx power level). However, if only one person is allowed to speak at a time, then it doesn’t gain anything to speak more loudly. But, it can have an adverse effect. Speaking more loudly may cause other classrooms to hear you, causing them to have to wait for you to finish, ultimately increasing the contention. That said, what if we speak in a quieter tone (lower the AP Tx power level)? This may work; however, students in the back of the room may no longer be able to hear the teacher.

*** continue reading in part 2 ***

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