[maemo-users] Power consumption and WLAN APs

From: Kalle Valo kalle.valo at nokia.com
Date: Wed Jul 23 22:34:14 EEST 2008
"ext John Holmblad" <jholmblad at acadiasecurenets.com> writes:

> Kalle.

Hi John,

> thanks for sharing your deep, if sleep deprived,  insights re 802.11
> power management.

I'm glad that it was helpful.

> I have read them out of both general interest in the workings of
> 802.11 but also as a result of the fact that just this weekend I
> upgraded my N800 to OS2008 (~20 minutes-yeah) and
> reinstalled/upgraded/tested most of my apps (~7 hours-boo). 
> As you know, the 802.11g standard  which uses OFDM modulation supports
> backward compatibility with 802.11b which uses DSSS modulation. Does
> the PSM method you describe work the same way for 802.11g irrespective
> of which modulation method is being used by the radio? I should think
> so.

Yes, PSM does not depend on the modulation (DSSS, OFDM) or the band
(2.4 or 5 GHz).

> However, the reason I ask this question relates to the following
> observation:
> After the OS upgrade I started getting low battery warnings when my
> N800 was not connected to AC power for "a while" (less than a few
> hours).  I do not remember having this problem when my N800 was
> running OS2007.

I would first recommend to see if you have badly behaving application
which is waking up CPU too often. Other option is to disable WLAN
altogether using Offline mode and try to test if it's really WLAN

> Noteworthy here is the fact that both before and after the OS upgrade,
> my method of network access for the N800 was the same, that is, 802.11
> wireless to a Proxim (formerly Lucent)  Orinoco AP-2000. Now this
> AP-2000 is a dual radio (802.11a and 802.11g) system with both radios
> active at the same time, albeit in their respective frequency bands
> (5.7 ghz and 2.4 ghz respectively).

Like I said, the band and modulation shouldn't affect here in any way.

> Furthermore, I am fairly certain that at least one of the client
> devices associated with the 802.11g radio on the AP-2000 is operating
> in 802.11b (DSSS) mode (I can, but did not verify this), which, of
> course, means that any other 802.11g clients associated with that
> radio will "downshift" to 802.11b mode.
> One thing that is different is that, prior to the upgrade, my N800
> while at my desk was less than ~1 meter from the AP-2000. Now, because
> I have moved my desk to a different location,  it is ~5 meters from
> the AP-2000 and the signal has to penetrate a ceiling/floor to get to
> the AP.  I will do some more testing today of the N800 while detached
> from its AC power source to see how long it can go before the battery
> runs down.

There's also the background scan which is enabled if the RSSI value
goes below a certain threshold. But the threshold is nowadays quite
low and it doesn't increase power consumption that much, even with
background scanning enabled you should still get days of standby time.

I bet my money on a badly behaving application. But as usually with
gambling, I'm just guessing here.

Kalle Valo

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