[maemo-users] WLAN Power Save Mode timeout

From: Kalle Valo Kalle.Valo at nokia.com
Date: Tue Feb 20 15:49:54 EET 2007
"Paul Klapperich" <maemo.org at bobpaul.org> writes:

> Could you maybe provide a  quick layperson's explanation of what setting PSM
> Timeout, ie how it saves battery power[1]? A technical explanation or link
> to a more technical explanation would also be neat, but the simple
> explanation should satisfy most of us and provide a research starting point
> if we desire to go more in depth. Google didn't seem to know a whole lot
> about it off hand, but I figure you probably do.

Sure. Here's a small introduction, I hope it helps:

Basically WLAN Power Save Mode means that we turn off the radios
completely and only wakeup for the beacons sent by the AP. This wakeup
period can be configured by the administrator of the AP. The wakeup
period is DTIM count multiplied by beacon period. For example, if DTIM
is 3 and beacon period is 100 ms, N800 wakes up every 300ms (3*100

But because using PSM creates latency (especially if the network
administrator has configured long wakeup periods), using PSM would be
too slow for the users. So what N800 does it that it enables PSM only
after certain time after the last data frame is sent. And that's the
time wlan_sleep_timeout gconf key is used for.

For example, if the timeout is 1000 ms, the device will wait a total
of one second after the last data frame sent until PSM is enabled.
That's a long time to waste power just to keep radios running just in
case. But if the timeout is reduced to 200 ms, radios are turned on
only fifth of the original time.

Kalle Valo

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