Tài liệu Wireless Networks: The Definitive Guide tập 2

After frame transmission has completed and the DIFS has elapsed, stations may attempt
to transmit congestion-based data. A period called the contention window or backoff
window follows the DIFS. This window is divided into slots. Slot length is mediumdependent;
higher-speed physical layers use shorter slot times. Stations pick a random
slot and wait for that slot before attempting to access the medium; all slots are equally
likely selections. When several stations are attempting to transmit, the station that picks
the first slot (the station with the lowest random number) wins.
As in Ethernet, the backoff time is selected from a larger range each time a transmission
fails. Figure 3-7 illustrates the growth of the contention window as the number of
transmissions increases, using the numbers from the direct-sequence spread-spectrum
(DSSS) physical layer. Other physical layers use different sizes, but the principle is
identical. Contention window sizes are always 1 less than a power of 2 (e.g., 31, 63, 127,
255). Each time the retry counter increases, the contention window moves to the next
greatest power of two. The size of the contention window is limited by the physical layer.
For example, the DS physical layer limits the contention window to 1023 transmission
slots.
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