Distributed Coordination Function
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Distributed Coordination Function (DCF) is the fundamental MAC technique of the IEEE 802.11 wireless LAN standard. DCF employs a distributed CSMA/CA distributed algorithm and an optional virtual carrier sense using RTS and CTS control frames.
DCF mandates a station wishing to transmit to listen for the channel status for a DIFS interval. If the channel is found busy during the DIFS interval, the station defers its transmission or proceeds otherwise. In a network that a number of stations contend for the multi-access channel, if multiple stations sense the channel busy and defer their access, they will also find that the channel is released virtually simultansously and then try to seize the channel. As a result, collisions may occur. In order to avoid such collisions, DCF also specifies random backoff, which is to force a station to defer its access to the channel for an extra period. The length of the backoff period is determined by the following equation:
DCF also has an optional virtual carrier sense mechanism that exchanges short Request-to-send (RTS) and Clear-to-send (CTS) frames between the source and destination stations between the data frame is transmitted.
DCF also includes a positive acknowledge scheme, which means that if a frame is successfully received by the destination it is addressed to, the destination needs to send an ACK frame to notify the source of the successful reception.
DCF is defined in subclause 9.2 of the IEEE 802.11 standard and is de-facto default setting for Wi-Fi hardware.
IEEE 802.11 standard also has an optional access method using a Point Coordination Function (PCF). PCF allows the Access Point (PC) acting as the network coordinator to manage channel access.
