The Wifi Stack (802.11)

As a low-level engineer, you might deal with drivers (Linux `mac80211`), firmware, or embedded IoT. Wifi isn't just "Ethernet without wires"; it is a complex state machine built to manage a shared, noisy medium.

1. The Layers

The 802.11 standard maps primarily to the Physical (PHY) and Data Link (MAC) layers of the OSI model.

2. Anatomy of a Frame

Unlike Ethernet frames which are simple destination/source structures, Wifi frames are heavily packed with management info.

Frame Types

• Management (Type 00): Beacons, Probes, Auth, Assoc. These manage the connection.
• Control (Type 01): RTS, CTS, ACK. These manage the medium (prevent collisions).
• Data (Type 10): The actual IP packets wrapped in 802.11 headers.

3. The Connection Dance

Connecting to Wifi is a rigorous 4-step process. If any step fails, you get "Unable to connect."

1. Scanning: The client sends Probe Request frames. APs respond with Probe Response containing capabilities (Rates, SSID).
2. Authentication (802.11): A legacy artifact. Usually just "Open System" authentication saying "Hello, I exist."
3. Association: The client asks to join the BSS (Basic Service Set). The AP assigns an Association ID (AID).
4. 4-Way Handshake (Security): The real authentication (WPA2/WPA3).
   • PTK (Pairwise Transient Key) is derived.
   • GTK (Group Temporal Key) for broadcasts is exchanged.
   • If this fails, it's usually a "Wrong Password" error.