WNM Sleep Motivation
Features > WNM Sleep
For some use cases it is desirable for a device to remain asleep for an extended period of time (i.e., longer than the DTIM period) without being disassociated by the AP. This allows such a device to achieve greater power saving and thus longer battery life than would be possible using the normal 802.11 power saving mechanisms. WNM Sleep is an extended power saving mode to support such a use case.
A device may enter WNM Sleep mode, during which it is able to turn its radio off for an extended period of time. During this time, it is effectively deaf and will therefore not receive any traffic directed to it. As such, WNM Sleep is best suited to such applications where the device is the originator of traffic and does not need to be woken by network traffic. An example of such a use case may be a motion activated camera that uploads an image when it detects movement.
WNM Sleep and BSS Max Idle Period
Wireless Network Management (WNM) Sleep mode is an extended sleep mode that allows a STA to sleep for an extended period of time. When in WNM Sleep the STA does not need to wake up to receive every DTIM beacon. While a STA is in WNM Sleep, the AP will pause group key updates (GTK, IGTK, BIGTK) for that STA until it leaves WNM Sleep.
While a STA is asleep it will not receive any traffic. In the case of unicast traffic, this will be buffered by the AP until either the STA wakes up to receive it or until the AP discards it at its discretion. The STA will miss any group addressed frames sent while it is sleeping.
A key constraint when using WNM Sleep is that the STA should not remain in WNM Sleep for longer than the BSS Max Idle Period. The BSS Max Idle Period is the period of time that a STA may be idle (i.e., during which the AP does not receive any frames from it) without being disassociated by the AP.
The BSS Max Idle Period is a configurable option for both AP and STA. A STA may include its preferred BSS Max Idle Period in a BSS Max Idle Period element in its (Re) Association Request and the AP provides the chosen BSS Max Idle Period in its (Re)Association Response. Setting a longer BSS Max Idle Period allows STAs to sleep for longer while maintaining a connection to the AP. The trade off is that a longer BSS Max Idle Period means that the AP must wait longer to reclaim resources if a STA disappears without a clean disassociation.