How WiFi works.
Your laptop and the AP run a small dance every time they exchange a frame. Beacons advertise. Probes discover. Auth and Associate get you on the BSS. Then CSMA/CA listens before each transmission to avoid collisions on a shared half-duplex channel.
The access point shouts "I exist, my SSID is HomeNet, my BSSID is aa:bb:cc:dd:ee:ff, here are my supported rates" 10 times a second. Beacons let clients discover the network without actively scanning. Your phone's WiFi list is built from beacons it has heard.
- BSSID
- The AP's unique MAC address. SSID is the human-friendly name (HomeNet); BSSID identifies the specific radio.
- Beacon interval
- Default 100 TUs (102.4 ms). Shorter = faster discovery, more airtime overhead. Tunable per AP.
Why WiFi can\'t use full Ethernet-style collision detection
Ethernet uses CSMA/CD — Collision Detect. It can sense voltages on the wire while transmitting; if it sees garbled signal, it knows someone else is talking. WiFi can\'t do this because a radio can\'t simultaneously transmit at a few hundred milliwatts and receive at a few microwatts (the desired signal is buried under your own transmission). So it does CA — Collision Avoidance. Listen first, randomized backoff, hope for the best, retry on lost ACK.
Why your WiFi is slow at the coffee shop
Three reasons that compound. (1) Airtime is shared. 50 clients all trying to talk = exponentially growing collision rate. (2) Slow clients drag down fast ones: 802.11 schedules in airtime, not bytes, so a client transmitting at 6 Mbps eats 100× the airtime per byte of one at 600 Mbps. (3) Interference from neighbouring networks on the same channel. Cure: dual-band APs that move clients to less crowded bands, MU-MIMO, and OFDMA (in WiFi 6) that let an AP talk to multiple clients simultaneously.
WiFi 6 / 6E / 7 in one breath
WiFi 6 (802.11ax) brought OFDMA — divide a 20 MHz channel into many sub-channels and serve many clients at once instead of taking turns. 6E added the 6 GHz band (vastly less congested than 2.4/5). WiFi 7 (802.11be) ups channel widths to 320 MHz and adds Multi-Link Operation — a client can use 5 GHz and 6 GHz simultaneously on different streams. Same CSMA/CA underneath; everything new is about getting more useful work out of each microsecond of airtime.
802.11 internals →
Frame structure, RTS/CTS, power-save modes, fast roaming (802.11r/k/v), WPA3 SAE handshake, the airtime accounting math.
Open the Codex →