Each
arrow represents the probability that a station belonging to a given priority class transmits
on the channel. Only two classes are considered in the figure. Since each class employs a
different AIFS value (in the example, the difference between the two values is equal to two
backoff slots), some slots can be accessed by only one class (in the example, slots labelled
with index 0 and 1). We define these slots, which are shaded and pointed by a single arrow,
as protected. Note that protected slots occur after each busy slot, and therefore the
percentage of protected slots grows as the network congestion increases (this is an
immediate consequence of the fact that, with a high number of competing stations, the
average number of idle backoff slots, between two consecutive frames transmitted on the
channel clearly reduces, and as such the relative amount of protected slots increases).
At the end of each channel access, the stations contend for acquiring the right of the
next transmission grant. The contention is based on the comparison of the backoff counter
values of each contending station, since the station with the lowest backoff expiration time
acquires the right to initiate the next transmission.
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