2. The reference model
The reference model covered in the standard emphasizes the separation of the system into two major parts: the MAC of the
data link layer and the PHY. The layers described in the standard are shown in Figure 7.2.
The MAC sublayer architecture includes the distributed coordination function (DCF), the point coordination function (PCF),
and their coexistence in an IEEE 802.11 LAN. The DCF is the fundamental access method of the IEEE 802.11 MAC and it is
known as carrier sense multiple access with collision avoidance (CSMA/CA). The DCF is implemented in all STAs. An optional
access method is called the point coordination function (PCF). The operation of PCF is essentially that of polling, with
one station (point coordinator) performing the role of the polling master.
The DCF and the PCF operate concurrently within the same BSS. The two access methods alternate, with a contention-free
period (CFP) followed by a contention period (CP).
Fig. 7.2. The 802.11 reference model.