Implementing Power Management

Implementing Power Management
The power management function of an IEEE 802.11 network enables stations to go
into sleep mode to conserve power for long periods of time without losing information.
The 802.11 power management is supported with the use of access points;
therefore, it’s not available when implementing ad hoc networks. You should definitely
consider implementing this feature if the conservation of batteries powering
the radio card and appliance is a concern.
You implement power management on IEEE 802.11 LANs by first setting the access
points and radio cards to power-save mode via the vendor’s parameter initialization
routines. The access points and radio cards will then carry out the power management
functions automatically. As part of the power management routine, the access
points will maintain a record of the stations currently working in power-save mode
by monitoring the single-bit power management field in the Frame Control field of
the MAC header for frames sent on the network. The access points will buffer packets
addressed to the stations in power-save mode.  The access points will forward the buffered packets to the applicable stations when
they return to active state (awake state) or when the stations request them. The
access point knows when a station awakens because the station will indicate its active
state by toggling the power management bit in the Frame Control field of the MAC
frames.
A station can learn that it has frames buffered at the access point by listening to the
beacons sent periodically by the access point. The beacons will have a list (called a
traffic indication map) of stations having buffered frames at the access point. A station
uses a power-save poll frame to notify the access point to send the buffered
packets.