Wmm Power Save - Polycom SpectraLink 8020 Deployment Manual

Deploying SpectraLink 8020/8030
Wireless Telephones
July 2009
Best Effort
(AC_BE)
Background lowest
(AC_BK)
4.2.2

WMM Power Save

The second component of Wi-Fi Standard QoS, WMM Power Save, is based on the IEEE 802.11e
Unscheduled-Automatic Power Save Delivery (U-APSD) mechanism and is an enhancement over the
legacy 802.11 power save mechanism used pre-R3.0. The application-based approach used in WMM
Power Save enables individual applications to decide how often the client needs to communicate with the
access point and how long it can remain in a „restful‟ state. In addition, WMM Power Save increases
transmission efficiency because the same amount of data can be transmitted in a shorter time and using
fewer frames. Two benefits of WMM Power Save for the SpectraLink 8020/8030 are to conserve battery
life and to make AP handoff decisions without the risk of missing packets.
Power save behavior is negotiated during the association of a handset with an AP. WMM Power Save or
legacy power save is set for each WMM access category transmit queue separately, as shown in Figure
10. For each access category queue, the AP will transmit all the data using either WMM Power Save or
legacy power save, using the assigned WMM QoS mechanism.
AC_VO
WMM
Power Save
Applications that do not initiate power save can still coexist with WMM Power Save enabled applications
on the same device. In this case, data from the other applications will be delivered with legacy power
save.
The transmission process begins with the handset sending a trigger frame on any of the WMM access
categories using WMM Power Save to indicate that it is awake and ready to download any data frame
that the AP may have buffered (Figure 11). After the AP receives the trigger frame, it sends an
acknowledgement frame (Ack) to indicate it is ready to send the data. Each frame transmitted by the AP
indicates whether more data for the handset is buffered (more data = 1). When the AP is ready to stop
sending downlink data it sends an EOSP (End of Service Period) message to the handset. If the handset
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0, 3 3 + 0 to 15
2, 1 7 + 0 to 15
Table 6 - WMM Access Categories
Handset data
AC_VI
(audio) (call control)
WMM
Power Save
Transmit to AP
Figure 10 – WMM Queues Using WMM Power Save
31
Best Practices Guide
Other (PTT, OAI,
RTLS)
Not used
AC_BE AC_BK
(PTT, OAI, RTLS) (no uplink traffic)
WMM WMM
Power Save Power Save
Other (PTT, OAI,
RTLS)
Not used