Alvarion BreezeACCESS VL 5.4, System Manual

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BreezeACCESS VL System Manual
5-52
Performance Parameters
The Performance Parameters menu enables defining a series of parameters
that control the method by which traffic is transmitted through the
BreezeACCESS VL wireless access network.
The Performance Parameters menu provides the following parameters:
RTS Threshold:
The RTS Threshold parameter defines the minimum frame size that
requires an RTS/CTS (Request To Send/Clear To Send) handshake.
Frames whose size is smaller than the RTS Threshold value are
transmitted directly to the wireless link without being preceded with RTS
frames. Setting this parameter to a value larger than the maximum frame
size eliminates the RTS/CTS handshake for frames transmitted by this
unit.
The available values range from 20 to 1600 bytes.
The default value is
60 bytes for SUs and 1600 for AUs. It is recommended
that these values be used to ensure that RTS/CTS is never used in the AU.
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, and wait for this number of time slots before accessing the
media, always checking if a different unit has access 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.