[maemo-users] Power consumption and WLAN APs

[From: Kalle Valo]

"ext Andrew Daviel" <advax at triumf.ca> writes:

> On Mon, Apr 21, 2008 at 4:58 PM, Eero Tamminen <eero.tamminen at nokia.com> 
> wrote:
>>  One possibility is a WLAN AP with broken power management
>
> I recently noticed that my tablet battery runs down faster at work than 
> at home, with the WiFi enabled but otherwise sitting idle (12%/hour vs 
> 1%/hour).

What do you mean by 12%/hour? That the battery will run out in eight
hours?

My rule of thumb is basically this:

o battery lasts 6-8 hours -> PSM not working at all
o battery lasts 6-8 days -> PSM working well

This is on a N810 with clean install (no badly behaving applications
so that CPU can sleep), display turned off and no data traffic in the
network. ping needs to be run occasionally (from network, not from
tablet!) to be sure that the device is really associated to the AP.

> I thought the issue might be that there's more broadcast/multicast 
> traffic at work, which would get through the filters in the ethernet chip 
> and into the kernel, but then I found a reference to your post on the 
> list.

Yes, multicast and broadcast traffic will increase WLAN power
consumption. But there has to be a lot of traffic going on to really
notice it.

If the AP is behaving correctly, the power consumption in PSM on a
idle N800/N810 are mostly affected by these:

o beacon interval (longer has smaller power consumption)
o DTIM interval (same as above)
o broadcast/multicast traffic

If the AP is violating the spec then, well, everything goes.

Disclaimer: written under influence of jet lag at 4am. Reader
discretion is advised.

-- 
Kalle Valo