Teaching WiFi Airtime – Part 2

*** please read part 1 before continuing on with part 2 below ***

“Fill the Front Seats First”

Physics tells us that the closer a device is to the AP, the stronger the received signal (RSSI) should be. And, in the world of WiFi, the stronger the signal is compared to the nearby noise (SNR), the higher the connected data rate will be. The higher the data rate, the quicker the data can be sent to the receiver, and the less time other devices must wait to transmit. The speed in which a device can transmit has a direct relation to the wait times of contending devices, thus impacting the overall available airtime (aka utilization).

Furthermore, when the signal drops too low, devices may not receive all of the data being sent, or some of the data be become corrupt. In either case, the data will need to be resent to the receiving device (increasing airtime utilization) – and often, in an effort to ensure delivery, this data gets resent at a lower data rate (further increasing airtime utilization).

In our classroom example, when students sit near the front of the class, they are closer to the teacher, and better able to hear and understand what is being taught. Students further away may miss certain details and are more likely to ask the teacher to repeat what was said. This slows the learning process for the entire class.

“You’re Too Smart. I’m Moving You to the Advanced Placement Class”

There are several generations of WiFi (we’re entering the 6th generation now with 802.11ax, or WiFi-6). With each generation comes improved capabilities that ultimately increase the speed at which devices can transmit. That said, the WiFi powers-that-be decided that each generation of WiFi would be backwards-compatible with all previous generations (assuming they operate on the same frequency). So, my trusty old WiFi-2 device (which tops out at 54 Mbps) can connect to my WiFi-5 AP (which tops out at 1000+ Mbps). Well, isn’t that nice? No, not necessarily.

Remember, transmit speed impacts airtime. When an older gen (and therefore slower) device is transmitting, all the new faster devices must still wait for the older device to finish. However, if you have a WLAN that only allows the newer and faster devices to connect, then that WLAN will be able to communicate and process data much more quickly and efficiently. This can be especially beneficial when dealing with video and large file transfers. 

Returning to our classroom, the smarter (faster) students can help the class as a whole learn more quickly because they require less time process what’s being taught. However, the other students are slowing the learning process for the more capable and smarter students. This is why many schools offer advanced placement classes. Group the more capable students together in their own class, and these students will learn more quickly, and therefore acquire more knowledge in the same amount of time as the non-advanced class.

“And Thus It Is With WiFi”

I hope my classroom example made sense and helped illustrate how WiFi airtime works. It’s not the easiest of concepts to grasp, but it is one of the most important. And just like a teacher has several techniques he/she can implement in the classroom to improve learning efficiency, so does a WiFi engineer with their WLAN. Some of these include:

  • Limit data rates
  • Use more channels
    • Use 20 MHz wide channels
  • Limit the number of SSIDs
  • Do not throttle the bandwidth
  • Implement Airtime Fairness (assuming the infrastructure supports it and device applications aren’t negatively impacted)

Which of these, if any, you choose to use is up to you. Always do your research before making these kinds of config changes, or you may end up booting client devices from your WLAN unintentionally – which would be bad, especially if one of them is your boss’.

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