802.11 Protocol Architecture
Medium Access
As part of transmitting a frame, the station must first gain access to the
wireless medium, which all stations within the BSS share.
Distributed Coordination Function (DCF)
The DCF is a mandatory 802.11 access method and offers a distributed
mechanism for sharing access to the common wireless medium. DCF
makes use of a carrier-sense multiple access/collision avoidance
(CSMA/CA) protocol, which is similar to what 802.3 (Ethernet) LANs
utilize.
Carrier Sense
The DCF includes a combination of both virtual and physical carrier
sense mechanisms and attempts to allow only one station to transmit on
the wireless medium at any given time.
FIGURE 2.10
The 802.11 DCF access method
The DCF carries out a virtual carrier sense protocol based on
reservation information found in the duration field of all frames’ MAC
headers. This process helps improve performance by reducing
collisions and allowing stations to efficiently complete frame exchange
sequences. The content of the duration field, which is a time value
measured in microseconds (µs), announces impending use of the
medium for the remaining frames in a frame exchange sequence.
