User Manual

Table Of Contents
Chapter 4 - Operation and Administration
BreezeACCESS VL System Manual
196
4.2.6.6.3.5 Wireless Link Prioritization Parameters (AU)
To better support delay-sensitive and other high-priority traffic, a set of Wireless
Link Prioritization parameters enables configuring parameters that affect the
processes of gaining access to the wireless media and of transmitting high/low
priority packets.
The Wireless Link Prioritization feature is a licensed feature and is available only
in units with the suitable Feature License.
The time interval between two consecutive transmissions of frames is called Inter-
Frame Spacing (IFS). This is the time during which the unit determines whether
the medium is idle using the carrier sense mechanism. The IFS depends on the
type of the next frame to be transmitted, as follows:
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: