Configuration Options and Protection Mechanisms
Power Management
802.11 network interface cards consume significant amounts of energy
and drain batteries fast, especially in smaller handheld devices. To
prolong battery life, the 802.11 standard defines an optional "power-
save mode." End users can activate power-save mode via the radio
card’s vendor-supplied configuration tool (client utilities) or operating
system interface. With power-save mode disabled, the 802.11 network
card is generally in receive mode listening for packets and occasionally
in transmit mode when sending packets. These modes require the
client station to keep most circuits powered-up and ready for operation.
General Operation
Stations that have their client utilities configured for power-save mode
will send all of their frames to the access point with the power
management bit in the frame control field of each 802.11 MAC frame
header set to 1. This indicates the station’s desire to remain in power-
save mode, and it informs the access point that it should buffer unicast
data frames for the station until polled by the station. This continues to
be the case until such time that the station’s client utility is
reconfigured for fully awake mode. At this time, the station will send
its frames to the access point with the power management bit set to 0 to
indicate that it is fully awake and the access point should not buffer
frames on its behalf.
When dozing, the station consumes much less power than normal by
shutting off power to nearly everything except for a timing circuit.
This enables the station to consume very little power and still be able to
wake up periodically (at a predetermined time) to receive regular
beacon transmissions coming from the access point. Each beacon
frame contains a traffic indication map (TIM) that identifies which
dozing stations have unicast frames buffered at the access point. These
buffered frames are awaiting delivery to their respective destinations.
The dozing station will wake up to view the TIM in the first beacon it
hears. A station may doze at its leisure once in power-save mode.
When the station discovers frames are buffered at the access point the
station will send PS-Poll frames to the access point until the access
