The access point
An access point (AP) is a bridge between radio and cable. One side is a cable to your home router. The other side is a radio that talks to laptops and phones. Their frames reach the wired network through it.
About ten times a second (every 102.4 ms) the AP broadcasts a beacon to everyone in range. It says the network name, called the SSID ("HomeNet"), the channel (6) and the security it needs (WPA3).
Wi-Fi uses three radio bands. In 2.4 GHz, channels sit 5 MHz apart but each signal is about 20 MHz wide, so neighbours overlap; only 1, 6 and 11 stay clear. 5 and 6 GHz offer many more channels but reach less far.
Joining the network
Your laptop scans: it tunes to each channel in turn and listens for beacons, or sends a probe request asking "who is here?". It hears HomeNet on channel 6 and tunes its radio there.
Two short exchanges follow. Authentication is a first hello. Association is the laptop asking "please add me"; the AP answers with an association ID, AID 1. Now the AP will pass the laptop's frames to the wired LAN.
On a WPA2 or WPA3 network, AP and laptop then swap four key messages. They prove both hold the same secret from the password, and make fresh encryption keys. The password itself is never sent. The security lesson covers the details.
Taking turns
Laptop and phone share channel 6, and only one radio can send on it at a time. A sending radio can't hear a crash, so each listens first and talks only when the air is quiet. This is CSMA/CA: listen, then avoid collisions.
Both have a frame ready. When the air goes quiet, each waits 28 µs, then counts down a random number of 9 µs slots. The laptop drew 3, the phone drew 7. Random draws make it unlikely both finish together.
The laptop's count hits 0 first, so it sends its data frame to the AP. The phone hears the channel go busy and freezes its count at 4, ready to carry on later.
10 µs after the frame ends, the AP sends a tiny acknowledgement (ACK). No ACK means the frame was lost, maybe in a collision, so the laptop resends after a longer random wait. The AP passes the data on to the wired router.
The air is quiet again. After 28 µs the phone carries on from 4, not a fresh draw, so it goes next. Its frame reaches the AP and gets its own ACK. Turn by turn, everyone shares one channel.
Generations
Wi-Fi generations: Wi-Fi 4 is 802.11n, 5 is 802.11ac, 6 is 802.11ax, and 6E is 802.11ax on the 6 GHz band. Wi-Fi 7 is 802.11be. Each adds wider channels and smarter sharing; listen, wait and ACK stay.
The frames, field by field
;; step 1/12 · the-ap · local ;; at access point + home router access point radio MAC (BSSID) 00:00:5e:00:53:01 radio side 2.4 GHz, channel 6, network name "HomeNet" wired side Ethernet cable to the home router 192.168.1.1 (00:00:5e:00:53:02) home LAN 192.168.1.0/24 (private, RFC 1918) the AP is a layer 2 bridge: it moves frames between Wi-Fi (IEEE 802.11) and Ethernet (IEEE 802.3) many home routers have an AP built in; here it is drawn as its own box
Channels 1, 6 and 11 are the classic choice where channels 1 to 11 are allowed (as in the US). Countries that allow 1 to 13 can also fit 1, 5, 9 and 13 for 20 MHz-wide signals. Timing values are for 802.11n in the 2.4 GHz band with the short 9 µs slot: SIFS 10 µs, DIFS 28 µs, first contention window 0 to 15 slots (IEEE 802.11-2020). The 5 GHz band uses SIFS 16 µs and DIFS 34 µs.