What does “half-duplex” mean anyway? Simply put, communication can only happen in one direction at a time.
Wi-Fi devices are much like people. We talk by generating noise, with our mouth. We listen to generated noise with our ears. Yet, because we all use the same frequency range for talking, we cannot comprehend if multiple people talk at the same time. Wi-Fi devices work similarly. If two devices try to transmit data at the same time, a collision will occur and neither device will comprehend the other.
How do we avoid collisions? Simple, again: don’t talk while someone else is talking! It is easy to take turn, though, with few people. How do you choose who gets to speak when there are many people? We could all just go in a circle, talking when it is our turn. What if I have something really important to say (voice/video data)? What if I just need to say something quickly, but someone far away is talking slowly and yelling (airtime fairness)?
Luckily, there is an organization constantly coming up with rules for how to dole out time to wireless devices in a network.
Recently, I was asked, “What are AIFS used for?” On this version of the Wi-Fi Rabbit Hole, I wanted to answer that question for others who may ask their wirelessly inclined friends.
To start, I discussed earlier, when participating in a wireless conversation devices can’t talk at the same time; therefore, our devices should listen before talking, to make sure the air is clear. This is known as carrier sense. (Further down: there are both physical carrier sense and virtual carrier sense.)
Next, devices need to make sure not to start talking at the same time. Imagine waiting for a lull in conversation only to start talking just as your friend does, bad times. Here is where we get our first run in with interframe spaces: DIFS.
The DCF Interframe Space is an interval after a frame that devices must wait before transmitting. This is accompanied by a random backoff timer where devices choose a random amount of time to wait in addition to the DIFS. The theory goes, if everyone chooses different random numbers, someone will talk first instead of everyone starting at the same time. A DIFS is defined as SIFS + (2*slot time). What is a SIFS?!
A Short Interframe Space is a smaller waiting time. Why would you need it? Well remember due to the nature of Wi-Fi, devices can’t know that their transmissions made it to their destination; thus, every frame transmission requires an Ack(nowledgement).
If you an I are already having a conversation, we don’t have to wait normal times to affirm that we heard each other. To make sure you get my ack, without having to wait around, and to make sure you don’t have to say your sentence over again, I only have to wait a SIFS before acknowledging your sentence.
SIFS ensure that an ack is always the first frame to follow a transmission. This helps avoid needless retransmissions (read: me getting antsy, thinking you didn’t hear me, and repeating myself).
As shown above the DIFS and the SIFS, most wireless communication can occur, no problem. But earlier I asked, “What if I have something important to say (voice/video data)?”
This brings us to the Arbitration Interframe Space.
Anyone who has ever tried to video chat over a shoddy connection can tell you, “Delays, even slight ones, to voice or video are easily noticeable.” Therefore, networks have built in mechanisms for handling priority data: quality of service. In Wi-Fi, this is handled by forcing low priority data to wait longer before talking.
When QoS is enabled, AIFS are used in place of DIFS in order to help high priority data have a better chance at talking first. AIFS lengths are variable. Therefore, higher priority data can be given a shorter interframe space length, allowing it to access the medium earlier.
So next time your friends ask you about those AIFS settings in their Wi-Fi configuration, you tell them, “AIFS are part of the QoS mechanisms for Wi-Fi. They are longer than or equal to a DIFS. By changing their length, you can change the priority of certain data on your network.” Simple, right?
See you next time!
Further down: There was an earlier further down that starts you on the track of CCA vs NAV, for those looking for more – CWNP IFS white paper.