Power Saving

 
In an infrastructure network, the AP is the heart of the power management
system [8]. If a STA wants to switch off the radio part for some period, it
warns the AP through the frame control field of transmitted frames. In this
case, the AP will buffer frames to STAs in power saving (PS) mode and then,
during the transmission of the beacon frame, the AP broadcasts a traffic indication
map (TIM) that contains an indication about the STAs that have
frames buffered.
Thus, STAs listen for beacons after they wake up. If there are some
buffered frames, they request the packets from the AP with a PS-poll frame
and stay awake to receive data. If there are no buffered frames, the STAs
revert to the sleeping mode. This is the case of unicast transmission.
On the other hand, for multicast/broadcast transmission, a special
TIM called delivery TIM (DTIM) is a multiple of TIM period, after which
the AP transmits multicast/broadcast packets. The PS mechanism is illustrated
in Figure 4.13.
In ad hoc mode, every STA can transmit a beacon frame. After a beacon
interval, every STA competes to transmit beacon frames with a backoff
algorithm. The first STA wins and the others cancel their beacon and adjust
their local time with the one included in the beacon frame that was
transmitted.
The PS mechanism in ad hoc mode is similar to that described earlier,
but in this case a special TIM is used, called ad hoc TIM (ATIM). ATIM is
transmitted during the ATIM window, in which all of the STAs, including
those operating in a PS mode, are awake. ATIMs are unicast frames that must
be acknowledged by the receiver. After this, the receiver must be awake to
wait for the announced packet. The mechanism is described in Figure 4.14.