📦 802.11 Frame Format

Not all fields are present in every frame — it depends on the 802.11 version and message type

Field	Description
Frame Control	indicates message type and subtype
Duration/ID	either indicates channel-reservation time (microseconds) or serves as an association(connection) identifier, depending on message type
Addresses (up to 4)	Which addresses appear, and in what order, depends on the message type
Sequence Control	Used to reassemble fragments and eliminate duplicate frames
QoS Control	Used to prioritize certain traffic (QoS)
HT Control (High Throughput control)	Added in 802.11n to enable high-throughput operations
Frame Body	The encapsulated packet
FCS (Frame Check Sequence)	Trailer field used for error checking (same purpose as in Ethernet frames)

The Four Possible Address Fields

Address	Meaning
DA (Destination Address)	The final recipient of the frame
SA (Source Address)	The original sender
RA (Receiver Address)	The immediate recipient (not necessarily the final destination)
TA (Transmitter Address)	The immediate sender (not necessarily the original source)

📌 Wireless networks need up to 4 address fields (vs. 2 in Ethernet) because of the unique requirements of 802.11 communication (e.g. frames often being relayed through an AP rather than sent directly between the original source and final destination).

ℹ️ Wi-Fi generation nicknames: 802.11n (Wi-Fi 4) is also called High Throughput (HT) Wi-Fi; 802.11ac (Wi-Fi 5) is also called Very High Throughput (VHT) Wi-Fi.

🤝 802.11 Association Process

For a wireless station to send traffic through an AP, it must go through an association process, moving through three connection states:

Not authenticated, not associated
Authenticated, not yet associated
Authenticated and associated — only now can the station send traffic through the AP

Process Steps

Scanning — the station learns about available APs/BSSs via:
- Active scanning: station sends a probe request, AP replies with a probe response
- Passive scanning: station listens for periodic beacon messages sent by the AP to advertise its BSS
Authentication exchange — e.g. station provides a password; if successful, reaches state 2 (authenticated, not associated)
Association request/response — if successful, reaches state 3 (authenticated and associated) — now the station can send traffic through the AP