（1）Legacy 802.11 Power Save
Power Save mode：客户端在两个信标帧之间进入休眠状态以节电，可选模式。
（2）WMM Power Save、
客户端和AP都要通过Wi-Fi CERTIFIED WMM-PS认证。
（3）802.11n Power Save
1. Legacy Power Save Mode (Pre 802.11e, now rolled into 802.11-2007)There are only two power save modes.
• Active mode
o This is when the STA is awake and the AP isn’t
buffering data for the STA. This can be referred to
as Constant Aware Mode. In this state the STA
device saves no battery power.
• Power save mode
o When in power save mode, it doesn’t mean that the
STA is asleep. It just means that it is in a mode to
conserve battery life.
• The STA can send and receive frames.
• The STA radio cannot send or receive frames.
注意：关于传统节电方式legacy Power Save Mode
• The Power Management bit（电源
管理比特位）, found in the header of every
Wi-Fi frame is used to notify the AP of the STA’s power
management state. If the bit is set to 1, the STA is in
Power Save Mode and the AP will buffer data. If the bit is
set to 0, the STA is not in Power Save Mode.
• The More Data bit（更多
数据比特位）, found in the header of every Wi-Fi
frame, is used only by the AP (it’s always 0 when sent by a
STA). When the AP is delivering buffered data to a STA,
the AP sets the More Data bit to 1 until the buffer is empty.
When the buffer is empty, the More Data bit is set to 0.
• Inside of every Becon is a Traffic Indication Map (TIM)（流量指示图）.
The TIM is a list of STA’s that have data buffered. Each
STA is referred to not by MAC address in the Beacon, but
by Association Identifier (AID). This is because MAC
addresses are too large and would make for large Beacon
frames. AID’s are much smaller and more efficient.
• ATIMs are TIMs for an Ad Hoc network.（用于adhoc的特殊TIM，ATIM）
• Power Save Poll (PS-Poll) – Used in legacy power save
mode. When a STA sees its AID in a Beacon frame, it will
Tx a PS-Poll frame to the AP, indicating it is ready to
2. 802.11e (802.11-2007) Power Save Mode (WMM-PS)
Automatic Power Save Delivery (APSD)
• STAs do not use PS-Poll frames to retrieve buffered data
• STAs switch from Power Save Mode to Active Mode when
they want to retrieve buffered data. This is done by
sending a frame (Any data frame or Null Data frame) to flip
the Power Management bit. When the STA is awake, the
AP sends all data in the buffer.
3. HT Power Save Modes
Spatial Multiplexing Power Save (SMPS)
• 802.11n (HT-OFDM) uses multiple data streams to
increase throughput. One form of 11n power save, SMPS,
can selectively shut down all but one radio. This conserves
battery life so that 11n STAs can mimic the battery
performance of legacy single radio devices in active mode.
Power Save Multi Poll (PSMP)
• Similar to 802.11-2007’s APSD