Lesson 12 · Connections: wires and wireless

How Wi-Fi works

An access point joins Wi-Fi devices to your wired home network. Follow a laptop as it hears the access point's beacons, scans, joins and gets its keys. Then watch the laptop and a phone share one radio channel: each listens first, waits a random number of slots, and every frame is answered with an ACK.

10 min12 steps5 questionsBeginner

Fig 12 · Interactive

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5 quick questions

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01What does an access point's beacon tell nearby devices?

A beacon is a short broadcast frame, sent about ten times a second (every 102.4 ms), that announces the network name, channel and supported security. The password is never broadcast.

02Why are channels 1, 6 and 11 the usual choice in the 2.4 GHz band?

Neighbouring channels overlap and interfere. Centres 25 MHz apart (2412, 2437 and 2462 MHz) keep the classic 22 MHz-wide signals apart.

03Your laptop drew a backoff of 3 slots and the phone drew 7. What happens?

Both count down after the air goes quiet. The laptop hits 0 first and sends. The phone hears the channel busy, freezes at 7 − 3 = 4, and resumes from 4 when the air is quiet again.

04Your laptop sends a data frame but no ACK comes back. What does it conclude and do?

A radio cannot hear a collision while it is sending, so the ACK is its only proof of delivery. Without one it retries, and its contention window doubles (0–15 slots, then 0–31, ...).

05Which pairing of Wi-Fi generation and IEEE standard is right?

Wi-Fi 4 is 802.11n, Wi-Fi 5 is 802.11ac, Wi-Fi 6 is 802.11ax (6E adds the 6 GHz band) and Wi-Fi 7 is 802.11be.

Key terms

Glossary

Access point (AP): The box that joins Wi-Fi devices to the wired network. It has a radio on one side and an Ethernet cable on the other, and passes frames between them.
SSID: Service Set Identifier: the network name, such as "HomeNet", that the access point announces in its beacons.
Beacon: A short broadcast frame an access point sends about ten times a second to announce its network name, channel and security.
Channel: A slice of a radio band that one network uses. In 2.4 GHz the channels are 5 MHz apart but about 20 MHz wide, so neighbours overlap.
Association: The step where a device asks to join an access point and is given an association ID (AID). After it, the AP forwards the device's frames.
4-way handshake: Four key messages between the access point and a device on a WPA2 or WPA3 network that prove both hold the same secret and create fresh encryption keys.
CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance: listen before talking, wait a random number of slots, and expect an ACK for every frame.
ACK: Acknowledgement: a tiny frame the receiver sends 10 µs (in 2.4 GHz) after a data frame to say it arrived. No ACK means send again.

Fine print

Notes and sources

MAC addresses are documentation values from RFC 7042 (00:00:5e:00:53:xx); ff:ff:ff:ff:ff:ff is the real broadcast address. The home LAN uses the private range 192.168.1.0/24 (RFC 1918). "HomeNet" is a made-up name.
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.
WPA2 and WPA3 differ in how the password is checked (WPA3 uses SAE), but both finish with the same 4-way handshake. The Wi-Fi security lesson covers both.

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