432 Configuring quality of service

NN47250-500 (Version 03.01)

U-APSD support

WMM clients that use powersave mode can more efficiently request buffered unicast packets from AP radios by using
U-APSD.
When U-APSD support is enabled in WSS Software, a client can retrieve buffered unicast packets for a traffic priority
enabled for U-APSD by sending a QoS data or QoS-Null frame for that priority. U-APSD can be enabled for individual
traffic priorities, for individual clients, based on the client’s request. A client enables U-APSD for a traffic priority by
indicating this preference when (re)associating with the AP radio.
A client can but is not required to request U-APSD for all four traffic priorities. The AP radio still buffers packets for all
traffic priorities even if the client does not request U-APSD for them. However, to retrieve buffered packets for priorities
that are not using U-APSD, a client must send a separate PSpoll for each buffered packet.
U-APSD is supported only for QoS mode WMM.
(To enable U-APSD support, see “Enabling U-APSD support” on page 434.)

Call admission control

Call Admission Control (CAC) is an optional feature that helps ensure that high-priority clients have adequate band-
width, by limiting the number of active sessions AP radios can have for an SSID. For example, you can limit the number
of active sessions on a VoIP SSID to ensure that each call receives the bandwidth required for quality voice service.
You can use CAC with either QoS mode (WMM or SVP).
CAC is disabled by default. You can enable session-based CAC on a service-profile basis. When enabled, CAC limits
the number of active sessions a radio can have to 14 by default. You can change the maximum number of sessions to a
value from 0 to 100.
(To configure CAC, see “Configuring call admission control” on page 434.)
Note. CAC is configured on a service profile basis and limits association to radios only
for the service profile’s SSID. Association to the radios by clients on other SSIDs is not
limited. To ensure voice quality, do not map other service profiles to the radio profile you
plan to use for voice traffic.