User's Manual

Menus and Parameters
Manual Revision 1.0
4-77
Minimum Contention Window
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
media 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
media. 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
media. The unit will attempt to access the media only after a time equal
to DIFS (for more details refer to Arbitration Inter-Frame Spacing
on
page 4-56) plus N time slots, always checking if a different unit has
accessed the media before. Each time the unit tried to transmit and a
collision happened; 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.
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 media 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 Arbitration Inter-Frame Spacing on page 4-56.