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Unscheduled Automatic Power Save Delivery (U-APSD)
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Unscheduled Automatic Power Save Delivery (U-APSD)

  • The main focus of the 802.11e amendment, which is now part of the 802.11-2012 standard, is quality of service.
  • IEEE 802.11e amendment also introduced an enhanced power management method called automatic power save delivery (APSD):
  1. The two APSD methods that are defined are scheduled automatic power save delivery (S-APSD)
  2. Unscheduled automatic power save delivery (U-APSD).
  • Wi-Fi Alliance’s WMM Power Save (WMM-PS) certification is based on U-APSD
  • WMM-PS is an enhancement over the legacy power saving mechanisms already discussed.
  • The goal of WMM-PS is to have client devices spend more time in a doze state and consume less power.
  • WMM-PS is also designed to minimize latency for time-sensitive applications such as voice during the power-management process.
  • The legacy power-management methods have several limitations.
  • As shown in Figure 9.12, a client using legacy power management must first wait for a beacon with a TIM before the client can request buffered unicast frames.
  • The client must also send a unique PS-Poll frame to the AP to request every single buffered unicast frame.
  • This ping-pong power-management method increases the latency of timesensitive
    applications such as voice.
  • The clients must also stay awake during the ping-pong process, which results in reduced battery life.
  • In addition, the amount of time that the clients spend dozing is determined by the vendor’s driver and not by the application traffic.
Picture
Figure 9.12 Legacy power management

Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.

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