Target Wake Time
The existing Wi-Fi client power-saving mechanisms have been in use since 802.11b, where the client devices sleep between AP beacons or multiple beacons, waking up only when they have data to transmit (they can transmit at any time, as AP does not sleep), and beacons containing the Delivery Traffic Indication Map (DTIM), a bit-map, indicates that the AP has downlink traffic buffered for transmission to particular clients.
If a client has a DTIM bit set, it can retrieve data from the AP by sending a Power-Save Poll (PS-Poll) frame to the AP. This power-save scheme is effective but only allows clients to doze for a small beacon interval. Clients still need to wake up several times per second to read DTIM from the beacon frame of the AP.
With 802.11e, the new power-saving mechanism was introduced that helps voice-capable Wi-Fi devices, as voice packets are transmitted at short time intervals, typically 20 ms/sec. Unscheduled automatic power-save delivery (U-APSD) allows a power-save client to sleep at intervals within a beacon period. AP buffers the downlink traffic until the client wakes up and requests its delivery.
Extended Power-Savings Using Target Wake Time
Target wake time (TWT) allows an AP to manage activity in the Wi-Fi network, in order to minimize medium contention between Stations (STAs), and to reduce the required amount of time that an STA in the power-save mode needs to be awake. This is achieved by allocating STAs to operate at non-overlapping times, and/or frequencies, and concentrate the frame exchanges in predefined service periods.
TWT capable STA can either negotiate an individual TWT agreement with TWT-scheduling AP, or it can elect to be part or member of Broadcast TWT agreement existing on the AP. An STA does not need to be aware that a TWT service period (SP) can be used to exchange frames with other STAs. Frames transmitted during a TWT SP can be carried in any PPDU format supported by the pair of STAs that have established the TWT agreement corresponding to that TWT SP, including High Efficiency Multi-User Physical Protocol Data Unit (HE MU PPDU), High Efficiency Trigger-Based Physical Protocol Data Unit (HE TB PPDU), and so on.
Following are the TWT Agreement Types:
Individual TWT
Single TWT session is negotiated between AP and an STA. This ensures a specific service period of DL and UL between AP and STA with expected traffic to be limited within the negotiated SP of 99% accuracy. The service period starts at specific offset from the target beacon transmission time (TBTT) and runs for the SP duration and repeats every SP interval.
TWT Requesting STA communicates the Wake Scheduling information to its TWT responding AP, which then devises a schedule and delivers the TWT values to the TWT requesting STA when a TWT agreement has been established between them.
Solicited TWT
STA initiates the TWT session with the AP.
Unsolicited TWT
AP initiates TWT setup with STA. AP sends TWT response with service period which is accepted by STA.
Broadcast TWT
High-Efficiency AP requests the STA to participate in the broadcast TWT operation, either on-going broadcast SP or new SP.
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