TWT Theory of Operation
Features > Target Wake Time (TWT)
The target wake time (TWT) feature allows STAs to go to sleep for longer periods and ignore DTIM beacons, as well as to dynamically negotiate wakeup periods with the AP . These wake up times are known as the TWT service period(SP). It also allows the AP to spread out client traffic so that when a STA device wakes up it won’t need to contend with other devices. This allows the STA device to reduce the required amount of time that it needs to be awake and improves the power utilization.
Figure 1 - TWT Service Periods are independent of the AP’s beacon and can be dynamically renegotiated on the fly.
There are two types of TWT scheduling, and two types of TWT 'flows'.
TWT scheduling options:
- Implicit - Where the TWT requesting STA calculates the next TWT by adding a fixed value to the current TWT value. This results in a periodic service period.
- Explicit - Where the TWT requesting STA wakes and performs a frame exchange and receives the next TWT information in a response from the TWT responder (typically the AP). This allows the interval between service periods to be dynamically set.
TWT ‘flow’ options:
- Announced - TWT Responders have to wait for QOS Null frames from STAs to deliver buffered data.
- Unannounced - TWT Responders don't need to wait for QoS Null frames from STAs to deliver buffered data
Negotiation of TWT agreements between a requester and responder occurs through the TWT IE (see 9.4.2.199 TWT element) which can be appended to association frames and/or TWT information frames.
Once the STA has successfully negotiated a TWT flow with the AP, it may sleep until the next service period begins.
Note: Currently there is support only for implicit TWT scheduling with announced intervals, and this is negotiated at association/re-association. Only individual TWT agreements are supported (between 1 AP and 1 STA), and only one TWT flow per AP/STA pair.