In the example, at slot t, stations A and B
start with a backoff counter equal to 2 and 3 respectively. Hence, we might expect them to
transmit in consecutive slots, namely slot t+2 and slot t+3.
STAA transmits STAA
DIFS
busy medium STAB
DIFS
b frozen STAB transmits
busy medium
b=2
b:2 1 b:1 0
b:2 1 b:3 2 b:1 b:1 0
slot t slot t+1 slot t+2 slot t+3 slot t+4
b=3
Figure 4.8: The slot immediately following a transmission can be accessed only by the
transmitting station.
As expected, station A decrements the backoff counter to 0 at the end of slot t+1, thus
transmitting a frame in slot t+2. It can then schedule the transmission for the next frame.
With probability 1/(CWmin+1), it will extract 0 as the new backoff counter, and hence it will
immediately transmit the next frame in the first slot available, incurring a DIFS interval
after the end of the transmission (slot t+3, in the figure). Let us now focus on station B. In
slot t+2, it will see station A??™s transmission on the channel and will freeze the backoff
counter. It will thus start slot t+3 with a backoff counter value equal to 1, and (assuming
slot t+3 to be empty) it will ultimately transmit only in slot t+4.
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