Whenever, a Wi-Fi station intends to sleep, it will set the power management bit of the last frame that it is transmitting to 1. This indicates to the AP the client station’s intention to sleep and AP will start buffering the data for the client.
AP includes Association IDs of all the associated stations in the TIM field of it’s beacon frame that has data buffered since they have gone to sleep mode. TIM is used for unicast data buffer and DTIM is used for multicast/broadcast data buffer. ATIM is the field used in Adhoc mode to store the AIDs of the stations whose data need to be buffered.
Legacy Power save
This traditional method will prompt the sleeping client to send the PS-POLL control frame when they wake up every listening interval to get the buffered data from the AP. For every data frame, client has to send a PS-POLL frame first to get the data followed by ACK from the client. This mechanism will continue till the More Data field in the Data frame from the AP = 0; indicating there is no more buffered data for the client.
WMM Power save – Trigger and Delivery Mechanisms
This method focuses in reducing the time delay caused by the legacy power save mechanisms. This is done by sending a trigger frame like a NULL frame to indicate to the AP that client is awake and is ready to receive the buffered data frames from the AP. PS-POLL frames are completely avoided in this situation that will speed up the process. WMM Power Save is based on U-APSD (Unscheduled- Automatic Power Save Delivery). However, 802.11e includes S-APSD (Scheduled – Automatic Power Save Delivery) for scheduled transmissions and data streaming.
SMP (Spatial Multiplexing Power Save)
- 802.11n Power save mechanisms
- Wi-Fi station turns off the all the radios except one radio for transmission
- Uses static and dynamic methods for power save
- Clients use SM power save action frames to inform the AP that only one radio is active and hence utilizing only one spatial stream. Same frame is sent to inform the AP when all radios are turned ON. This is the static method
- In dynamic method, clients can use same SM Power save action frame like static method. In addition, AP can also trigger the client to wake up the radios by sending RTS frame. This will activate the sleeping radios by the client and sends a CTS frame back to the AP
PSMP (Power Save Multi-Poll)
- Similar to APSD mechanisms in 802.11e for HT clause 20 radios.
References:
CWAP Official Study Guide
CWNA Official Study Guide
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