Alvarion SW VERSION 5.1 system manual 203

Menus and Parameters

„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

Operation and Administration

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