Distributed coordination Function (DCF)
When a node (station) receives a packet to transmit, if the node senses the
medium has been idle for a period of time longer than or equal to a Distributed Inter
Frame Space (DIFS), the packet transmission may begin at the beginning of the
immediately following slot. Otherwise, the node should defer the packet transmission as
follows. The node waits until the medium is idle for a DIFS and then sets its backoff
timer. The backoff timer is set to a value which is randomly selected between 0 to CW-1
with equal probability, where CW represents contention window size, see Figure 1.
Initially, contention window size is set to its minimum CWmin for the first transmission
attempt of a packet. Every time the packet is involved in a collision, contention window
size is doubled for the next transmission attempt. However, the contention window size
cannot exceed its maximum CWmax.