Power-Save Mode
Most radio cards apply an alternative 802.11 power-save approach that users can enable. Admission points
do not apparatus power-save mode, except for the buffering functions all-important to abutment powersaving
functions of the radio cards. If power-save approach is enabled, the radio agenda enters beddy-bye mode,
which draws abundant beneath accepted than back the agenda is operating actively. Thus, power-save approach can
conserve batteries on adaptable devices. In fact, power-save approach generally lengthens array activity by 20 to
30 percent. The absolute savings, however, depends on the applications and added variables.
Before switching to power-save mode, the radio agenda notifies the admission point by ambience the Power
Management bit in the Anatomy Control acreage of an upstream frame. The admission point receives this
frame and starts buffering applicative abstracts frames. The buffering takes abode until the radio card
awakens and requests that the admission point accelerate the adored frames to the radio card.
After entering beddy-bye mode, the radio agenda keeps clue of time and wakes up periodically to receive
each alarm advancing from the admission point. The radio agenda charge deathwatch to ascertain whether the
access point is buffering any frames that charge commitment to the radio card. The admission point notifies
radio cards about buffered packets through what the 802.11 accepted defines as the traffic
indication map (TIM).
A radio agenda set to power-save approach will deathwatch aloof in time to accept the TIM, which resides in the
beacon frames. The TIM indicates the AID of the 802.11 stations that accept abstracts frames buffered at the
access point. If a base discovers it has frames at the admission point, afresh the base stays alive and
sends a power-save poll anatomy to the admission point requesting that the abstracts frames be forwarded to the
station. The base will break alive continued abundant to accept all of the buffered frames. The bulk of
time appropriate to alteration all of the buffered frames depends on the accepted appliance of the access
point and the radio articulation quality. A ample cardinal of stations implementing power-save approach can cause
a billow in cartage afterwards anniversary alarm due to power-save poll frames and agnate abstracts frames.
Once the admission point delivers the buffered frames, the radio agenda enters beddy-bye approach again, unless
the alarm anatomy corresponds with the commitment cartage adumbration map (DTIM). The DTIM is set
in the admission point to actuate how abounding beacons charge canyon afore the admission point delivers
multicast frames. A accepted absence DTIM breach is 3, which agency that the admission point sends
806 Appendix G: IEEE 802.11 Fundamentals
multicast frames afterwards every third beacon. The DTIM interval, however, can be set to added values,
such as 1, which enables the admission point to accelerate multicast frames afterwards every beacon. Based on
the DTIM breach setting, a base implementing power-save approach will break alive continued enough
after the alarm manual to accept the multicast frames in accession to unicast frames.
The use of power-save approach can accomplish batteries aftermost best in user devices, but throughput
decreases for abstracts affective from the admission point to the user device. The radio agenda will awaken
immediately and accelerate abstracts activity from the user accessory to the admission point, however. As a result,
upstream throughput charcoal banausic in low-power mode.