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I've had instructors and students get confused on this topic repeatedly, so I thought I'd throw it out here on the forum.

802.11-1999 (R2003), table 23

There are two MODES:

Mode 1 - Active Mode (AM)

Mode 2 - Power Save (PS)

In PS mode, there are two STATES:

State 1 - Awake

State 2 - Doze

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When a STA flips the PM bit, it's telling the AP that it's going into PS MODE, NOT into the Doze STATE. A STA goes in and out of the Doze STATE according to a predefined set of parameters. One such parameter is the DTIM interval. A STA will awaken just prior to a beacon containing a DTIM so that it may get BOTH MC/BC and Unicast traffic. If there is BC/MC data, that data will be sent immediately following the DTIM-carrying beacon, and the beacon will tell the client whether any BC/MC OR Unicast data is forthcoming for it. It will stay awake long enough to get its queued data (through PS-Polling).

Now, how the PM bit flipping scenario (like with Atheros cards) works is that instead of STAYING in PS MODE like it's supposed to, the STA starts going into and out of PS MODE. When the STA tells the AP that it's no longer in PS MODE, all queued traffic will be flooded directly to the client right then. What the STA is "supposed" to do is to go into PS MODE just one time and STAY there. It then goes in and out of Doze STATE. Don't confuse states and modes.

The way the AP knows to buffer unicast traffic for the client is through the client going into PS MODE. As long as it remains in PS MODE, traffic will be queued and must be received through PS-Poll frame exchanges.

Devinator

Unscheduled APSD (the one used in EDCA mode) does almost exactly what Devin described above for the Atheros adapter. The only difference is that a TXOP may be used to send a CFB rather than having the AP forward frames one at a time after the Power Management subfield is flipped to 0.

On Dell Wireless adapters you can choose between PSP mode and this APSD-like alternative. Choose "Enabled" if you want PSP mode and choose "Fast" if you want the APSD-like alternative.