Wireless devices are commonly small in size and powered by batteries. Because the devices are mobile and carried around, it is not very practical to stop and charge the batteries. To maximize the battery life, the device should conserve as much power as possible.
By default, the radio (both transmitter and receiver) is powered on all the time, so that the device is always ready to send and receive data. That might be good for performance, but applies a constant drain on the battery. Fortunately, the 802.11 standard defines some methods to save power by putting the radio to sleep when it is not needed.
Be aware that a device’s radio sleeping is different than the whole device sleeping, as when you close the lid on a laptop. ...