Wake on Wireless is a feature to allow the system to go into a low-power state (e.g. ACPI S3 suspend) while the wireless NIC remains active and does varying things for the host, e.g. staying connected to an AP or searching for networks.
Here's a non-exhaustive list of triggers:
Use iw to configure WoWLAN before going to sleep.
When the system is going to suspend, cfg80211/mac80211 will inform the driver if WoWLAN is enabled. The driver then sets up the hardware correctly according to the required triggers and when the bus suspend is later invoked sets up the device as a wakeup source.
When the wireless NIC detects a wakeup event, it will wake up the host through the bus-specific methods:
The platform will then wake up the system and eventually the device driver will be notified through the different resume callbacks to wake up the device. The driver can then inform the upper layers of the reason why it was woken up (if it received such trigger).
Non-WoW frames which are received are ignored (dropped), but frames received should be ACKed by the STA wireless hardware without any help from the host CPU.
There might be issues with multicast and broadcast frames when using WPA/RSN if the device is not capable of rekeying – in this case the host must be woken up to handle the rekeying.
802.11 powersave and possibly SMPS will usually be used while suspended to save power.
Some devices support a keep-alive frame to send to the AP, which might be configurable. If supported, keep-alive frames will (at least for ath9k) be sent right after a TBTT, it can be configured to be sent in the order of 10 to 60 seconds.