802.11 Protocol Architecture
When the station needs to transmit a frame, the DCF determines
whether the wireless medium is idle by monitoring the station’s
receiver. If the medium is not idle, which is the case when another
station is transmitting or RF interference is present, then the station
wanting to transmit the frame will defer (wait). Once the medium is
idle, however, the station may transmit the frame provided its NAV is
equal to zero and its backoff timer has expired.
If the transmitting station learns that its frame transmission was not
successful (due the absence of an acknowledgement frame within a
specific amount of time), then the station will execute the medium
access process again in order to retransmit the frame. A station reduces
the probability of collisions among stations sharing the medium by
using a random backoff time. The period of time immediately
following a busy medium is when the highest probability of collisions
occurs, especially under high utilization. The reason for this
occurrence is that many stations may be waiting for the medium to
become idle and may transmit at the same time. Once the medium is
idle, a random backoff time defers a station from transmitting a frame,
minimizing the chance that stations will collide.
The MAC entity calculates the random backoff time using the
following formula:
Backoff Time = Random() X aSlotTime
Random( ) is a pseudo-random integer drawn from a uniform
distribution over the interval [0,CW], in which CW (contention
window) is an integer within the range of values of the MIB
(management information base) attributes aCWmin and aCWmax. The
random number drawn from this interval should be statistically
independent among stations. aSlotTime equals a constant value found
in the station's MIB.
FIGURE 2.13
Contention Window and Slot Times
