061
Daily Dose of Curious
HOW IT WORKS
It is why a busy channel feels slow rather than broken. Every extra device adds waiting, not errors, and two devices that cannot hear each other but can both hear the access point are the classic cause of the ones that do go missing.
Mostly by taking turns politely. Each device listens before it transmits, waits a random time if the channel is busy, and expects an acknowledgement for every frame. Silence afterwards is treated as a collision and the frame is sent again.
What actually happens
Listen first
A device with something to send checks whether anybody else is already transmitting on the channel.
Wait a random time
If the channel is busy, or has only just gone quiet, it counts down a random backoff so that two waiting devices are unlikely to start together.
Send, then expect a reply
The receiver confirms each frame with a short acknowledgement. No acknowledgement means the frame is assumed lost.
Back off further
Each failed attempt widens the range the random wait is drawn from, so a crowded channel slows everyone down rather than jamming.
The clever bit
A radio cannot hear a collision while it is transmitting, so Wi-Fi cannot detect one the way wired Ethernet once did. It avoids them instead, and infers the ones it could not avoid from an acknowledgement that never arrived.
The words
- CSMA/CA
- Carrier sense multiple access with collision avoidance: listen, wait, then talk.
- Hidden node
- Two devices that cannot hear each other, so both transmit to the access point at once.
Where this comes from
- IEEE 802.11 — CSMA/CA and the distributed coordination function
- Exponential backoff in shared-medium networks
This one is not an opinion. It is how the thing works, and these are what that rests on.