All stations must construct frames for transmission and decode frames upon
reception, based on a standard frame format. The MAC protocol data units
(MPDUs), or frames, are described as a sequence of fields in a specific order,
as shown in the activity below.
Each frame consists of the following basic components:
- A MAC header, which consists of frame control, duration, address, and
sequence control information
- A variable length frame body, which contains information specific to the
frame type. For example, in data frames, this would contain upper layer data.
- A frame check sequence (FCS), which contains an IEEE 32-bit cyclic
redundancy check (CRC)
Frame Types
The three main types of frames used in the MAC
layer are as follows:
- Data frames
- Control frames
- Management frames
Data frames are used for data transmission. Control frames, such as
Request To Send (RTS), Clear to Send (CTS), and Acknowledgment (ACK), control
access to the medium. Management frames, such as beacon frames, are transmitted
in the same manner as data frames to exchange management information, but are
not forwarded to upper layers.
MAC Architecture
Before
transmitting a frame, a STA must gain access to the medium using one of two
methods, which are shown in Figure
:
- The fundamental access method of the IEEE 802.11 MAC, carrier sense
multiple access with collision avoidance (CSMA/CA), is called the Distributed
Coordination Function (DCF). The DCF is implemented in all STAs, for use within
both ad hoc and infrastructure network configurations.
- The IEEE 802.11 MAC may also incorporate an optional access method, called
the Point Coordination Function (PCF), which creates contention-free (CF)
access. The PCF can only be used on infrastructure network configurations.
Coexistence of DCF and PCF
The DCF and the PCF can both
operate concurrently within the same BSS. When this is the case, the two access
methods alternate, with a CF period followed by a contention period. In
addition, all frame transmissions under the PCF may use an Interframe Space
(IFS) that is smaller than that used for frames transmitted by way of the DCF.
The use of smaller IFS implies that point-coordinated traffic shall have
priority access to the medium over STAs operating in DCF mode.