QoS, WMM and U-APSD

For the terminals to achieve a long talk time and high standby times with one battery charge, both the terminals and the access points need to support relevant power saving mechanisms. U-APSD (Unscheduled Automatic Power Save Delivery) ensures that the terminal only transmits when necessary. During the terminal's sleep phase, the access point ensures that data packets which are to be sent to the telephone are temporarily stored and that the telephone is also woken up on time. Whether U-APSD functions correctly depends on the QoS class that is signalled, because U-APSD always has a reference to the QoS class. When doing the configuration, therefore, the manufacturer's recommendations should be complied with.

The voice data is transmitted as RTP (real-time transport protocol) data. To achieve the transmitting of the RTP voice data packets between the IP PABX and the VoWLAN telephone with low latency times, the voice data needs to be given priority over the normal data.

This chapter describes how the prioritising of the voice data works in the LAN and the WLAN, and what relationship there is with the U-APSD power saving mechanism.