The PS-POLL Mechanism is the Legacy Power save mechanism. The following article tries to explain the same. As the name suggests PS-POLL stands for Power Save Polling. The Access point uses the TIM information element to indicate to the station that there is unicast data buffered for the WLAN station at the Access Point.
The station wakes up at Listen Interval/DTIM time to receive the beacon from the Access Point.
It then checks the TIM Information element to check whether its Association ID is set in The TIM Information element.
If the Association ID of the Station is set – then it sends a specific frame to retrieve data from the AP.
This Frame is termed as a PS-Poll frame.
The PS-Poll frame is a control frame and the frame format is given below from the 802.11 standard
AID – Association ID of the station sending the PS-Poll frame to the Access Point – the two MSB bits of AID in a PS-Poll frame will be 1 (Duration ID field is used as AID field in PS-POLL)
BSSID – Basic Service Set Identifier
TA – Transmitter address of the station (Station MAC address)
FCS – Frame control sequence
On receiving a PS-POLL frame from the 802.11 station, the AP would ACK the PS-Poll frame and then sends a single data packet to the Station. If there are more data packets queued for the 802.11 Station at the Access Point, the Access Point sets the “More Data bit” in the frame control to true.
The WLAN (802.11) station processes the received data frame and on processing the frame control field realizes that it has more data queued at the Access point. The WLAN (802.11) station will send another PS-Poll frame to retrieve another data packet. When the “More Data bit” is set to zero – the Access Point has no more data uffered for the WLAN station and the station can go to sleep.
In legacy Power save devices, for transmitting each PS-Poll frame- the 802.11 station has to contend for the medium.
The PS-Poll frame transaction is provided pictorially below
The following article looks at enhancements to the legacy power save mechanism that was introduced in by the Wi-Fi Organization. It was termed as WMM Power save and we will look at the Unscheduled Automatic Power Save Delivery (U-APSD) mode as it was implemented widely rather than WMM Scheduled Automatic Power Save Delivery (S-APSD).