Alvarion BreezeAccess II System Manual page 217

Menus and Parameters
! Minimum Contention Window – The BreezeACCESS system uses a
special mechanism based on detecting the presence of a carrier
signal (Carrier Sense Multiple Access-CSMA) and analyzing the
information contained in the transmissions of the AU to estimate
the activity of other SUs served by the same AU. The target is to
minimize collisions in the wireless media resulting from trials of
more than one unit to transmit at the same time. The contention
window is the time that a unit will wait from the time it has decided
that there are no transmissions by other units until it will attempt
to transmit. The contention window is a random number whose
maximum value is determined by the value of the Minimum
Contention Window parameter and the number of previous trials
to transmit the frame. The higher the number of SUs served by the
same AU, the higher the Minimum Contention Window for each SU
should be.
The Valid values are from 7 to 255.
The default value is 31.
! Maximum Multicast Rate (AU only)– Determines the maximum
rate of multicast and broadcast transmissions. Multicast and
broadcast transmissions are not acknowledged; therefore there is a
chance that such transmissions will not be properly received
without the possibility of using the acknowledgement mechanism
for retransmission. Therefore, it is recommended to use a lower rate
for transmission of broadcast, multicast and control frames, to
increase the probability that they will be received without errors.
The available selections are 1, 2, and 3 Mbps.
The default is the minimum possible rate, 1Mbps.
! Multi-Rate – Link quality dynamically changes, due to various
environmental conditions. Dynamically switching between the
possible transmission rates increases the probability of using the
maximum rate for the current radio link quality at any given
moment. Decreasing the rate one step improves the receiver
sensitivity by 6-8dB. When enabled, the transmission rate decisions
are made separately for each unit. This algorithm is completely
separated from the retransmission mechanism defined by Number
of Retransmission and Number of Dwells to Retransmit
parameters. The algorithm provides Access Units with
simultaneous, adaptive support for multiple Subscriber Units at
different rates. The decision on the rate of each transmission (first
attempt) is based on counting transmissions, retransmissions and
successful windows. A window is defined as N consecutive
transmission or retransmission attempts, defined by the Multi-Rate
Decision Window Size parameter. A successful window is a window
in which the number of failed transmissions is not higher than a
defined threshold. A transmission is defined as failed if an ACK was
not received after the first transmission of a frame. If the current
rate is lower than the maximum available rate, the transmission
rate will be increased to the next available rate following a number
of successful windows. In order to minimize fluctuations, the
BreezeACCESS Version 4.5 System Manual
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