Power wi fi devices

Wi-Fi Power Modes

This document provides details on the different Wi-Fi power modes which can be used by the host to optimize the power efficiency of its application. In addition to providing information on the different Wi-Fi power modes available, the document describes how they should be enabled using the WF(M)200 FMAC or LMAC APIs. To optimize even further the power consumption, the WF200 can make use of its different device power modes (i.e., active, sleep, reset and shutdown). The device power modes are documented here.

Low Power Wi-Fi Background

Wi-Fi power save mechanisms were implemented first in the 802.11 specification. However, at this time (1999) there was almost no battery powered Wi-Fi devices on market so Power Save (PS) was rarely used. Current day IoT devices have a set of specific, well-defined, low-power requirements:

1. Power Save Mode (PSM) (1999)
2. Automatic Power Save Delivery (APSD) as a part of WMM (Wi-Fi Multimedia) related to QoS (Quality-of-Service), with two modes:
   • Scheduled (S-APSD), targeting large data flow such as video and audio streaming. Rarely used as it is simpler to leave power save.
   • Un-scheduled (U-APSD), using a trigger frame sent by the Station, targeting less demanding bidirectional communication such as VoIP

The WF(M)200 provides everything required to save power by turning the RF and other parts of the chip off whenever possible (i.e., when not active or receiving beacons) while maintaining the connection to the AP, as well as allowing the host processor to enter suspend mode in the absence of relevant Wi-Fi traffic.

WF(M)200 Low Power Wi-Fi

Silicon Labs WF(M)200 allows applications to achieve optimal power consumption by providing a highly configurable Wi-Fi power mode selection API.

Wi-Fi Protocol Power Modes

The WF(M)200 supports four Wi-Fi power modes.

• Active mode
• Legacy 802.11 PS Mode (PS-Poll)
• Fast-PS mode
• U-APSD mode (WMM-PS)

Below are cases for each mode where the station is connected to an Access Point and configured in the power mode.

To indicate that it will go to sleep, the Station sends a QoS NULL data frame with the Frame Control Field’s ‘P’ (Power Management) bit set to 1 .

Active Mode

This is the standard and default mode where the Station is not in power save.

• 2 unicast packets received by the AP for the Station.
• The delivery is done immediately.
• An acknowledgement packet is sent to the AP after every reception.

Legacy 802.11 PS Mode (PS-Poll)

• 2 unicast packets received by the AP for the Station.
• The delivery is postponed until next the Station wake-up.
• The AP indicates data availability using the AID (Association IDentifier) in the TIM (Traffic Indication Map).
• On the next wake-up, the Station sends a PS-Poll to retrieve one packet.
  • The Station has to send a PS-Poll to receive one single packet.
  • The PS-Poll frame sent by the Station contains its AID.

  

  Fast-PS Mode

  Fast-PS is not defined in the 802.11 specification but is a de-facto popular method to handle power save by having the STA indicate it enables/disables Power Management. When out of power save, less messages are required conpared to PS-Poll to retrieve all the buffered frames.

  • 2 unicast packets received by the AP for the Station.
  • The delivery is postponed until next the Station wake-up.
  • The AP indicates data availability using the AID (Association IDentifier) in the TIM (Traffic Indication Map).
  • On the next wake-up, the Station disables power save mode. The Station sends a QoS Null frame with the ‘P’ bit set to 0 to indicate it leaves power save mode.
  • The AP sends packets.
  • The Station re-enables power save mode after a certain amount of time without activity (Timeout).

  

  U-APSD Mode

  Introduced with the WMM-PS specification (Wi-Fi Multimedia-Power Save), U-APSD (Un-scheduled Automatic Power Save Delivery) is part of the latest protocols introduced in the Wi-Fi specification.

  • The AP indicates U-APSD support in beacon frames via a WMM/WME Information Element (IE), in the WME QoS Info/U-APSD bit.
  • The Station indicates U-APSD support in (re) Association frames via a WMM/WME Information Element (IE), in the WME QoS Info/AC_BE/AC_BK/AC_VI/AC_VO bits, corresponding to ‘Best Effort’/’BacKground’/’VIdeo’/’VOice’.
  • 2 unicast packets received by the AP for the Station.
  • The delivery is postponed until next the Station wake-up.
  • The AP indicates data availability using the AID (Association IDentifier) in the TIM.
  • On the next wake-up, the Station sends a QoS trigger frame to start a Service Period.
  • The AP sends packets with EOSP bit (End Of Service Period) at 0 as long as there is still data available.
  • The last available packet comes with EOSP bit at 1 ending the Service Period.
  • The Station goes back in power save.

  

  Choosing the Most Efficient Wi-Fi Power Mode

  You can refer to the table below to select the most appropriated Wi-Fi power mode for your application.

  Configuring the Wi-Fi Power Mode in your Application

  FMAC Wi-Fi Power Mode API

  In sl_wfx_set_power_mode() , set strategy to the Wi-Fi power mode which fits your application the best.

  // Initialize the Wi-Fi FMAC driver sl_wfx_init(); // Connect to a Wi-Fi access point sl_wfx_send_join_command(); // Wait for the succesful connection indication /**************************************************************************//** * @brief Set the power mode used as a station. The Wi-Fi chip will enable the * Wi-Fi power mode feature. * * @param mode is the power mode to be used by the connection * @arg WFM_PM_MODE_ACTIVE * @arg WFM_PM_MODE_BEACON * @arg WFM_PM_MODE_DTIM * @param strategy is the device power save polling strategy * @arg WFM_PM_POLL_UAPSD * @arg WFM_PM_POLL_FAST_PS * @param interval is the number of beacons/DTIMs to skip while sleeping * @returns SL_STATUS_OK if the command has been sent correctly, * SL_STATUS_FAIL otherwise * * @note the power mode has to be set once the connection with the AP is * established *****************************************************************************/ sl_wfx_set_power_mode(mode, strategy, interval); 

  • The default connection mode is always active.
  • If the host requests a U-APSD or Fast-PS power mode, the WF(M)200 applies the request if the mode is supported by the AP. If this is not the case, the WF(M)200 can default to PS-Poll. The host is informed via a «power save error indication».
  • If an interoperability issue is detected, the WF(M)200 can take the initiative to switch power mode (U-APSD -> Fast-PS, Fast-PS -> PS-Poll). The host is informed via a «power save error indication».

  In addition to the power mode, the host can configured how many beacons/DTIMs should be skipped by the WF(M)200.

  The broadcast frames are sent right after a beacon with DTIM. If a Station uses DTIM skipping (The Station does not wake up for every DTIMs), it can miss part of the broadcast traffic. If the application relies on receving important broadcast messages, it is recommended to wake up on every DTIMs.

  LMAC Wi-Fi Power Mode API

  The Wi-Fi power save mode can be enabled on a Linux platform with the command below.

  # Activate Wi-Fi power save iw wlan0 set power_save on

  Not all Linux kernels allow U-APSD. To enable it, follow these intructions. The WF(M)200 handles the Wi-Fi power save mode based on the AP and MAC80211 settings:

  • The LMAC firmware defaults to U-APSD if supported by the AP and enabled by MAC80211 (see above).
  • If U-APSD is not supported, it uses Fast-PS.
  • In both cases, if the firmware detects an issue to retrieve data when using U-APSD, it switches to Fast-PS.

  The LMAC API defaults to configuring the WF(M)200 to wake up on every DTIMs.

  Источник

  Low-Power Wi-Fi Ideal for Energy Efficient IoT Devices

  Wi-Fi may not be the first wireless technology you think of when considering a low-power application, but it should be.

  The IoT is one of the fastest-growing sectors in the world, with more devices added to the Internet every day. We need access to information and knowledge in the most efficient and secure way possible, and with minimal delay. Consumer electronics, especially wearable devices, are probably the most visible proof of this with more than a billion devices expected to be in use by the end of this year. It’s hard to attribute this proliferation to any one specific protocol. Take a smartwatch, for instance, which might use a Wi-Fi network to sync fitness data, classic Bluetooth to stream audio to earphones, ANT to transmit heart rate sensor data, and Bluetooth Low Energy (LE) to send notifications to a smartphone.

  Why Wi-Fi Protocol for IoT Devices

  Wi-Fi is a widely deployed protocol with more than 22 billion devices, which means that it’s virtually ubiquitous and available in most homes and commercial environments. It has a significantly higher data throughput — almost 10-100x higher than most IoT protocols, which enables audio and video streaming with almost no delay. The high data rates, as well as the range, enable many different applications. Finally, Wi-Fi offers constant cloud connectivity, making gateways unnecessary. With these benefits, Wi-Fi is emerging as an obvious choice for embedded solutions.

  

  Wi-Fi 4 Versus Wi-Fi 5 Versus Wi-Fi 6 for IoT Devices

  Looking at the evolution of the Wi-Fi specification, 802.11n (Wi-Fi 4) is better suited for IoT applications than 802.11ac (Wi-Fi 5). One reason is because 802.11n is dual band (2.4 GHz and 5 GHz) versus 802.11ac which is single band based on 5 GHz. For embedded applications, the range and better object penetration of 2.4 GHz comes in handy compared to the 5 GHz frequency. Also, the cost and power consumption of 802.11ac-based systems is higher due to the higher protocol complexity. While 11ac does provide enhanced throughput, the data rates provided by 802.11n are more than sufficient for most battery-operated IoT applications. 802.11ax (Wi-Fi 6) is the latest version of the specification, promising a maximum throughput speed of 9.6 Gbps, compared to 3.5 Gbps on Wi-Fi 5 and 600 Mbps on Wi-Fi 4. Wi-Fi 6 was introduced mainly to address the rapid increase in the number of devices on Wi-Fi networks fueled by the growing demand for IoT devices.

  Low Power Wi-Fi Design Decisions for Unique IoT Applications

  The low power features available in the latest Wi-Fi standards mean that a lot of today’s IoT devices are «always-on» and connected, with extended battery life due to ultra-low power consumption. For example, a Wi-Fi/Bluetooth LE smart-lock application connects to the cloud via Wi-Fi for remote access and provisions the lock onto the Wi-Fi network using Bluetooth LE. Keep in mind that operations like securing cloud connectivity (TLS certificate exchange), network communication (MQTT to communicate with the cloud), and over-the-air (OTA) updates happen in the background further impacting the system’s current consumption. By optimizing power consumption, the typical battery life for the smart lock staying in an «always connected» mode is three years for a low congestion environment and two years for a dense and congested wireless environment.

  Smaller devices like smartwatches pose additional challenges with space and battery life. Small form factor SiP modules are ideal for sleek designs with quick time to market. Wearables often require GPS connectivity for location tracking, Wi-Fi for cloud connectivity, Bluetooth A2DP for music streaming, and Bluetooth LE to connect to sensors. All these operations need to be performed in addition to OTA updates, while extending the battery life as long as possible on a single charge. Wi-Fi client devices in listening mode take as much power as when they are receiving data. For this reason, the «always on» feature of Silicon Labs’ RS9116 family of SoCs is essential in providing high performance with ultra-low power consumption.

  With widespread technology like Wi-Fi gaining traction with IoT devices, standalone or in a multiprotocol mode, it provides the immense potential to transform industries. For a more in-depth look at designing low-power Wi-Fi applications, see our whitepaper, “The Future of Wi-Fi in Low-Power IoT Devices.»

  Источник

  Читайте также:  Вай фай директ передача данных