The IEEE 802.11 standard
discusses, in Clause 9.2.5.2, how the backoff counter is decremented. Here, it specifies that
if the medium is determined to be busy at any time during a backoff slot, then the backoff
procedure is suspended (meaning that the backoff timer shall not decrement for that slot).
Let us assume that a station has a backoff counter equal to a value b at the beginning of a
slot-time. If the current slot-time is idle, at the end of the slot-time, the backoff counter is
duly decremented and the station will start the next slot-time with backoff value b-1.
Conversely, if the current slot-time is busy (because another station starts transmitting in
the considered slot), the station freezes the backoff counter to the value b. This implies that
Performance Study of IEEE 802.11 DCF and IEEE 802.11e EDCA 81
the station starts the slot immediately following a busy one with the same backoff value b.
In other words, the backoff counter is decremented only during idle slots.
The implications of this operation is not immediately evident on the modelling
framework described until now. Figure 4.8 illustrates what happens when two stations
access the channel with different backoff values.
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