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Menus and Parameters
Operation and Administration
203
SIFS (Short Inter-Frame Spacing) is used for certain frames that should be
transmitted immediately, such as ACK and CTS frames. The value of SIFS is
16 microseconds.
DIFS (Distributed coordination function Inter-Frame Spacing) is typically
used for other frame types when the medium is free. If the unit decides that
the medium is not free, it will defer transmission by DIFS plus a number of
time slots as determined by the Contention Window back-off algorithm after
reaching a decision that the medium has become free. DIFS equal SIFS plus
AIFS, where AIFS is a configurable number of time slots.
Under regular conditions, AIFS is configured to two time slots. To support
prioritization in the wireless link, we can configure a higher AIFS for low priority
traffic (AIFS of two time slots will always be used for high priority traffic as well as
AU’s transmissions of broadcasts/multicasts and beacons). This will give
advantage to units that need to transmit high priority traffic (depending also on
the configured values for the Contention Window parameters).
Other parameters related to transmission to the wireless media that can be
configured separately for high/low priority packets are the Number of HW Retries
and Burst Duration.
Typically, a lower value of Number of HW Retries should be configured for traffic
such as VoIP, which on the one hand is sensitive to delays and on the other hand
is less sensitive to missing packets than data traffic.
The Burst Duration, which defines the maximum duration of a burst, should be
set to a lower value for delay sensitive traffic. Typically the Burst Duration of the
AU should be set to higher value than that of the SUs, because of the higher
number of packets that should be transmitted by the AU.
When the Wireless Link Prioritization feature is enabled, the following parameters
are not applicable:
Arbitration Inter-Frame Spacing (AIFS)
Number of HW Retries
Burst Mode Option
Burst Mode Time Interval
When an SU with a SW version below 4.0 tries to associate with an AU that has
the Wireless Link Prioritization feature enable, the AU will generate a trap that
will include information about this SU. In this way the system administrator can
be alerted that the SU should be upgraded. This is necessary because otherwise