E-DOC-CTC-20060609-0001 v2.0
Chapter 2
802.11 Standards
13
2.1 MAC Sublayer
MAC architecture
The MAC architecture uses the following two access methods:
> Distributed Coordination Function (DCF): this is the fundamental access method of the MAC sublayer.
The DCF is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). This
access method is implemented in all wireless stations, for use within both ad-hoc network and
infrastructure network configurations.
> Point Coordination Function (PCF): this is an optional access method of the MAC sublayer. The PCF can
only be used within infrastructure network configurations. This access method uses a point coordinator
(PC), which operates at the AP of the BSS, to determine which station has the right to transmit at a given
time.
The DCF and the PCF can coexist in a way that allows both to operate simultaneously within the same BSS.
When a PC is operating in a BSS, the two access methods alternate.
Carrier Sense Multiple Access (CSMA)
CSMA is a listen before talk mechanism. A wireless station that wants to transmit a frame must first sense the
medium. The wireless station senses the medium using two mechanisms:
> A physical carrier sense mechanism: this mechanism is provided by the physical layer. A station can
check the physical layer and detect whether the medium is in use. The wireless medium is in use if
another station is transmitting.
> A virtual carrier sense mechanism: this mechanism is provided by the MAC sublayer. Even if none of the
stations is transmitting, the medium might still be reserved by a station via the Network Allocation Vector
(NAV). The NAV of a station gives a prediction of future transmissions on the medium. It is based on the
duration information in the 802.11 frames. The NAV is a timer that is decremented at a uniform rate. A
station will not try to transmit until the NAV has decremented to 0.
If one of the mechanisms indicates that the medium is in use, then a station must postpone its transmission.
If the medium is not in use, then a station is allowed to transmit.
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