legacy power save and triggered power save

The WME method does not allow nonperiodic traffic to use triggered power save; it must use legacy power save instead.

Legacy power save is less efficient than triggered power save

A single trigger frame can retrieve multiple frames in triggered power save
One cannot tell which is the right legacy option to use because TIMs do not indicate how much traffic is buffered, leading to greater inefficiency
PS polls are inefficient if long packet bursts are buffered, as a PS poll is needed for every buffered frame
PM bit (exiting and returning to power save mode) is inefficient for independent frames, as it involves two extra frames for each buffered frame
If the wrong legacy option is chosen, the inefficiency of legacy power save is even greater

By enabling all applications (periodic and nonperiodic) to use the new power save, battery life is extended, as in 802.11e