Distributed Coordination Function (DCF)

Distributed coordination function (DCF) is a mandatory technique used to prevent collisions in IEEE 802.11-based WLAN standard (Wi-Fi). It is a medium access control (MAC) sublayer technique used in areas where carrier-sense multiple access with collision avoidance (CSMA/CA) is used.

Technique

The basic technique of DCF is as follows −

• When a station has a frame to transmit, it waits for a random backoff time. The random backoff time is defined by a contention window having a random number of time slots. The backoff time is given by the following equation −

  Time_backoff = random()×Time_slots

  Here, the function generates a random number and is the time period for one slot.

• If the station senses that the channel is busy during the contention period, it pauses its timer till the channel is clear.

• At the end of the backoff period, if the channel is clear, the station will wait for an amount of time equal to DIFS (Distributed Inter-Frame Space) and sense the channel again.

• If the channel is still clear, the station transmits a RTS (request to send) frame.

• The destination station responds using a CTS (clear to send) frame if it is available.

• Then the transmitting station sends the data frames.

• After the frames are sent, the transmitting station waits for a time equal to SIFS (Short Inter-Frame Space) for the acknowledgement.

• At the end of this transmission process, the station again waits for the backoff time before the next transmission.

The technique is shown in the following diagram −