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Low-power system design with AIROC™ Wi-Fi & Bluetooth® combo
chip and PSoC™ 6 MCU
WLAN power optimization techniques
3
WLAN power optimization techniques
3.1
IEEE 802.11 (Wi-Fi) power saving
Topics in this section are partly based on information contained in IEEE
802.11 standard includes several parameters that allow stations to save power, although power saving is
accomplished at the expense of throughput or latency to the station.
This section only provides an overview of power-saving methods and associated terms from the IEEE
specification. If you are already familiar with the topic and interested in the implementation, see
save
implementation.
3.1.1
Beacon interval
Beacon frames are periodic frames broadcast by the Wi-Fi access points (APs) to communicate throughout the
serviced area the characteristics of the connection offered to the Wi-Fi stations (STAs). This information is used
by STAs trying to connect to the network as well as STAs already associated with the Basic Service Set (BSS).
The period at which beacon frames are transmitted is called "Target Beacon Transmission Time" (TBTT) or
simply "Beacon Interval". even though the beacon interval is configurable for a given AP, it is typically set to
102.4 ms (100-time units, where the 1-time unit is 1024 µs per 802.11 specifications). The beacon interval
information is available as part of the beacon frame itself. See the
each beacon includes a Traffic Indication Map (TIM) that contains information regarding packets buffered for
STAs. It is the responsibility of the STAs to read the TIM and retrieve the packets destined for them.
3.1.2
Listen interval
To save power, STAs sleep by powering down most of the Wi-Fi subsystem. While the STAs are asleep, APs
buffer frames for them and indicate the availability through beacon TIMs. The sleeping STAs periodically wake
up to listen to traffic announcements (TIMs) and determine whether the AP has any buffered frames. When
STAs associate with an AP, part of the data in the Association Request frame is the listen interval. The listen
interval indicates to the AP the frequency at which a power save station will wake up to listen to beacon frames.
An AP may use the listen interval as a guide to determine the duration it should retain buffered frames for a
power save station. Larger listen intervals require more AP memory for frame buffering.
In reality, APs generally do not consider the listen interval requested by an STA. If an STA sets the listen interval
to a value, there is no guarantee that the AP will buffer all the packets received for the station while the station
is asleep. In addition, if the AP supports Delivery Traffic Indication Map (DTIM), that setting may override the
listen interval requested by STAs.
Most APs enforce an association timeout on stations, i.e., if the AP has not received a frame from a station
within the association timeout (usually 60 seconds) or the station has not returned an ACK to a keep-alive
frame, the station will be disassociated. The association timeout cannot be negotiated by the station.
3.1.3
DTIM period
The DTIM period is a parameter associated with an infrastructure network and is advertised in the AP Beacon
frame. The polled approach using standard TIMs is not suitable for multicast and broadcast frames, because it
takes too much capacity to transmit multicast and broadcast frames multiple times. Instead of the polled
approach, broadcast and multicast frames are delivered after every DTIM interval.
Application note
802.11 specification
802.11 specification
10
document. The
Wi-Fi power
for details. In addition,
002-27910 Rev. *C
2023-05-29
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