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The fundamental access method of IEEE 802.11 MAC for contention service is called Distributed Coordination Function (DCF), and is also known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). If a node in a network utilizing DCF sends a packet, it draws a random exponential backoff if at that time the medium is sensed to be busy with other node transmission. This backoff is decremented as soon as the medium is idle for the Distributed InterFrame Space (DIFS) period and it freezes as soon as the medium becomes busy again. If this backoff reaches zero and continues at zero for the DIFS period, then the node starts its transmission.
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