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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.
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Interactive Activity
Interactive Activity: 802.11 MAC Frame Format
This activity displays the Media Access Control frame format
plus descriptions.
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