Target wake time
Target wakeup time (TWT) is a power-saving mechanism that
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allows an AP to schedule specific times for devices or stations (STAs) to wake up and communicate
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enables those devices to remain in a power-saving state for longer periods, and
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reduces overall network congestion.
Feature history
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Feature name |
Release information |
Feature description |
|---|---|---|
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Target wake time |
Cisco IOS XE 17.2.1 |
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. |
Delivery traffic indication map
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.
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).
A Delivery Traffic Indication Map (DTIM) is a bit-map that
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is contained within beacons
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indicates an AP has buffered downlink traffic, and
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specifies which particular clients are awaiting transmission.
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.
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.
Optimize power-saving with target wake time
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 can be used to exchange frames with other STAs.
Any frame exchanges that occur during a TWT service period can use any Physical Protocol Data Unit (PPDU) format that is supported by both the AP and STA involved in that TWT agreement. This includes formats such as High Efficiency Multi-User PPDU (HE MU PPDU) and High Efficiency Trigger-Based PPDU (HE TB PPDU).
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Individual TWT: An individual TWT session is established through negotiation between the AP and a single STA. This arrangement provides a dedicated service period for both downlink and uplink communication between the AP and that specific STA. The expected traffic is highly concentrated within the negotiated service period, achieving up to 99% accuracy. Each service period begins at a defined offset from the Target Beacon Transmission Time (TBTT), lasts for the agreed-upon duration, and then repeats at regular intervals.
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Solicited TWT: In a solicited TWT agreement, the STA initiates the TWT session by sending a request to the AP.
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Unsolicited TWT : In an unsolicited TWT agreement, the AP initiates the TWT agreement with the STA. The AP sends a TWT response specifying the service period, which the STA then accepts.
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Broadcast TWT: In a broadcast TWT agreement, a High-Efficiency AP invites the STA to join a broadcast TWT operation, which may involve participating in an existing broadcast service period or a newly established service period.
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.
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