DCF or Distributed Co-ordination Function was the initial Access mechanism introduced by the 802.11 standard’s body. The mechanism defined how different 802.11 stations need to compete in order to gain access to the transmission (air) medium for transmitting a packet. The DCF mechanism is a carrier sense multiple access and collision avoidance (CSMA/CA) mechanism and it introduced the following aspects

• Physical Carrier sense – to check if the physical medium was active – indicated to the MAC layer by the PHY layer
• Virtual Carrier sense – to further have a virtual mechanism (Network Allocation Vector) to avoid interfering with an on-going transmission
• Time-frames to wait before transmitting a packet (DIFS/Slot times as well as SIFS)
• Exponential Back-off algorithm

Points 1 and 2 define the carrier sense mechanism

Points 3 and 4 define the wait duration in accordance with points 1 and 2 – which an 802.11 station needs to wait before transmitting

The different Time intervals are

Slot-time – basic time interval for contention-window. Slot Time differs as per different versions of standard

DIFS – (Distributed Inter-frame Space) = SIFS Time + 2 Slot Time

PIFS – (Point Co-ordination Inter-frame Space) = SIFS Time + Slot Time

SIFS – Short Inter-Frame Space – differs as per different versions standard (a/b/g/n)

The DCF basic access mechanism is depicted pictorially below

FIG COURTESY: IEEE Std 802.11-2012^TM (section 9.3.4.2)

• The DCF mechanism entails all 802.11 Stations to wait a DIFS time period after it senses the medium to be idle and the internal Network Allocation vector to have counted down to zero
• If the medium is again sensed to be busy – the Physical carrier sense will notify the MAC layer to not transmit
• Once the medium is sensed to be free for a DIFS time duration, the 802.11 station has to wait a further time-period (contention window back-off – multiple of slot times)
• If the medium is still seen to be free after the contention window wait time – the next frame can be transmitted

The Back-off mechanism employed in DCF is discussed next.

Back-off Mechanism in WLAN