An IEEE 802.11-based Wireless Local Area Network (WLAN) system provides packet based data communications among Stations (STAs) over a wireless medium. At the Medium Access Control (MAC) sublayer, the MAC Service Data Units (MSDUs) are received from or delivered to the upper layer; and MAC Protocol Data Units (MPDUs) are formed and transported between MAC peer STAs. MPDUs are also called MAC frames in the IEEE 802.11 standard.
A MAC frame type is identified by a combination of a 2-bit Type field and a 4-byte Subtype field in the Frame Control field of the MAC header. There are three frame types defined in the IEEE 802.11-2012 specification, including the Management frame, the Control frame, and the Data frame. For each frame type, multiple Subtypes have been defined, as shown in Table 8-1 in the IEEE 802.11-2012 specification. In the IEEE 802.11ad draft standard, another frame type is defined, called an Extension frame. Two Subtype values have been defined for the Extension frame type, a directional multi-gigabit (DMG) Beacon subtype and a short beacon frame subtype.
A MAC frame generally consists of three basic components: A MAC header, which comprises a frame control field, a duration field, an address field, optional sequence control information, optional quality of service (QoS) Control information (QoS data frames only), and optional HT Control fields (+HTC frames only); a variable-length frame body, which contains information specific to the frame type and subtype; and a frame check sequence (FCS), which contains an IEEE 32-bit cyclic redundancy check (CRC).