802.11 Power Management

With transmit and receive currents on the order of 80-200mA, it's important to understand what options exist both within and beyond the 802.11 standard to power down the transceiver and extend battery life.

Active Mode

  • Transmitting or Receiving function is ON
  • Few hours battery life

802.11 PS-Poll Mode

  • This is the only power-saving mode specified in 802.11
  • Transceiver function is DUTY-CYCLED
  • Provides virtual "always on" power saving capability
    • In PS-Poll mode, the station always maintains a connection to the network
      • Station periodically POLLs the airwaves for the presence of DTIM packets sent by the AP in it's beacon transmissions
    • ~200mS response latency
  • Requires PS supported Access Point to buffer the packets
  • Few days/weeks battery life

This example shows a best case (lowest latency) DTIM period (n=1 – i.e. every beacon from the AP has a DTIM message).

  • Radio automatically wakes just prior to DTIM beacon
  • DTIM packet is read
  • Bit position is checked to see if there is buffered data at Access Point
  • If no data, radio reverts back to sleep

In this example, the duty cycle = 4/96. So, the 85mA active RX current drops to 3.5mA

power-management-DTIM-example.png
Click image to enlarge.

Hibernate

  • Radio function is turned OFF
  • Disconnected from the AP (session lost)
    • Need to reconnect when turned on (Associate + DHCP)
    • Takes 1-10s
  • TCP/IP stack needs to also be restarted
  • Few months/years battery life (depending on power profile)

Saved State

A custom power management technique available on Roving Networks RNxxx modules as sleep mode

  • Transmit function is DUTY-CYCLED. Receive function is turned OFF.
  • Transmit-only after first connection, saves configuration data in non-volatile memory
  • Provides very low power fast on/off activity
  • Very low latency (100ms) to restart, send turn off
  • Many routers will not allow indefinite sleep
    • Best for sending data more often than every minute
  • Years of battery life for hourly connection with dynamic IP

The table on this page summarizes the current consumption and power modes available for Microchip's current Wi-Fi® product offering.

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