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Chapter 4 - Operation and Administration
BreezeACCESS VL System Manual
184
The Minimum Contention Window parameter determines the time that a unit
waits from the time it has concluded that there are no detectable transmissions
by other units until it attempts to transmit. The BreezeACCESS VL system uses a
special mechanism based on detecting the presence of a carrier signal and
analyzing the information contained in the transmissions of the AU to estimate
the activity of other SUs served by the AU. The target is to minimize collisions in
the wireless medium resulting from attempts of more than one unit to transmit at
the same time.
The system uses an exponential Back-off algorithm to resolve contention between
several units that want to access the wireless medium. The method requires each
station to choose a random number N between 0 and a given number C each time
it wants to access the medium. The unit will attempt to access the medium only
after a time equal to DIFS (for more details refer to section 4.2.6.2.11
) plus N time
slots, always checking if a different unit has accessed the medium before. Each
time the unit tries to transmit and a collision occurs; the maximum number C
used for the random number selection will be increased to the next available
value. The available values are 7, 15, 31, 63, 127, 255, 511 and 1023.
The Minimum Contention Window parameter is the first maximum number C
used in the back-off algorithm. The higher the number of SUs served by the same
AU, the higher the Minimum Contention Window for each SU should be. In
addition, when the Wireless Link Prioritization Option is enabled, the Minimum
and Maximum Contention Window parameters can be configured to provide
certain units with an advantage over other units.
The available values are 0, 7, 15, 31, 63, 127, 255, 511 and 1023. A value of 0
means that the contention window algorithm is not used and that the unit will
attempt to access the medium immediately after a time equal to DIFS.
The default value is 15.
CAUTION
A value of 0 disables the contention window back-off algorithm. It should only be used in point-to-
point applications. For more details on configuring units in a point-to-point link refer to section
4.2.6.2.11.
4.2.6.5.2 Maximum Contention Window
The Maximum Contention Window parameter defines the upper limit for the
maximum number C used in the back-off algorithm as described in Minimum
Contention Window above.
The available values are 7, 15, 31, 63, 127, 255, 511 and 1023.
The default value is 1023.