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Wireless Local Area Network (WLAN) 405

The client device may enter PS mode, for example, if its transmission buffer is empty
and no data has been received from the AP for some time. To inform the AP that it will
enter PS mode, the client device sends an empty frame to the AP with the PS bit set in
the MAC header. When the AP receives such a frame, it will buffer all incoming frames
for the client device for a certain time. During this time, the client device can power
down the receiver. The time between reception of the last frame and activation of the
PS mode is controlled by the client device. A typical idle time before power save mode
is activated is half a second.
If a client device wants to resume data transfer, it simply activates its transceiver again
and sends an empty frame containing a MAC header with the PS bit deactivated.
Subsequently, data transfer can resume immediately (see Figure 6.10).
For most applications used on mobile devices, such as web browsing, data will only be
delivered in rare cases once PS mode has been activated. So that frames are not lost,
they are buffered on the AP. Thus, a device in PS mode has to periodically activate its
transceiver so that it can be notified of buffered frames by the AP. This is done via the
Traffic Indication Map (TIM) IE, which the AP includes in every beacon frame. Each
device has its own bit in the TIM, which indicates whether buffered frames are waiting.
The client device identifies its bit in the TIM via its association identity (AID), which is
assigned by the AP to the client device during the association procedure. Up to 2007
AIDs can be assigned by each AP. Therefore, the maximum size of the TIM IE is 2007
bits. To keep the beacon frames as small as possible, not all bits of the TIM are sent. The
TIM, therefore, contains a length and offset indicator. This makes sense as in practice
only a few devices are in PS mode and therefore only a few bits are required.
As beacon frames are sent at regular intervals (e.g. every 100 milliseconds), the AP
and client device agree during the association procedure on a listen interval, after which
the TIM has to be read. To negotiate the listen interval, the client device proposes an

Access
Terminal
point

Data frames

No data transfer for,
e.g. 500 milliseconds
Empty frame
(Power save bit = 1)

New Terminal deactivates
data Power save mode transceiver and just
receives beacon frames
Empty frame
(Power save bit = 0)

Data frames

Figure 6.10 Activation and deactivation of PS mode (acknowledgment frames not shown).