Capability Information Cap

Sixteen bits advertising what the BSS supports and requires.

Reading a capture

Read the Privacy bit first: it says whether the network is encrypted at all, without needing to look at any element. Several other bits are reserved now but were meaningful in older amendments, which is why a modern beacon has gaps here.

Subfields

• ESS — Set by an access point.
• IBSS — Set by a station in an independent BSS. Mutually exclusive with ESS — both set is malformed.
• Privacy — Set when data frames in this BSS are encrypted.
• Short Preamble — Whether the short DSSS preamble is allowed — 96 microseconds instead of 192.
• QoS — Whether the BSS supports the QoS facility, and therefore whether QoS Data frames will appear.
• Short Slot Time — A 9 µs slot instead of 20 µs.

Where it sits

Beacon MPDU > Frame Body > Capability Information

• Open in the explorer — Association Request
• Open in the explorer — Association Response
• Open in the explorer — Beacon
• Open in the explorer — Probe Response

Accuracy

Clause

802.11-2024 §9.4.1.4

Byte order

little-endian — least significant octet first

Verified

Not verified against the standard. Treat this as a claim, not a fact.