[maemo-users] Power consumption and WLAN APs

From: Kalle Valo kalle.valo at nokia.com
Date: Mon Jul 21 12:53:13 EEST 2008
"ext Andrew Daviel" <advax at triumf.ca> writes:

> On Mon, 21 Jul 2008, Kalle Valo wrote:
>>> 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?
> I guess. I was using the "battery-status" application which reports
> percentage of battery left, and measuring the slope.

Ok, I'm not aware of that application. I hope the percentage is
linear, so that we can trust that.

>> 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.
> I tried doing a broadcast ping every 200ms at home (the tablet sees it
> if I run tcpdump) but it had no noticable effect on the battery
> consumption graph.

Ok, the result was as expected.

>> 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
> I'll have a look at the AP config tomorrow and see if I can find
> anything.

I doubt that you will find anything which will fix your problem, I'm
afraid. These are usually software bugs in the AP firmware and which
can be only fixed by a firmware update :/

> My linksys at home has beacon interval 100ms, DTIM=1 (i.e. default
> values). The Linksys docs suggest that DTIM data is only sent if
> there are broadcast packets waiting, so perhaps setting this larger
> would have little effect if I don't have much broadcast traffic.

Actually DTIM makes a difference, because N8x0 will wake up only for
DTIM beacons. Let's take an example: if beacon interval is 100 ms and
DTIM is 10, N8x0 will wake up for beacons every 1000 ms. But if DTIM
would be 3, N8x0 would wake up for beacons every 300 ms.

> Besides, the problem is at work, not home.

That's right. Optimising DTIM might get you, for example, from 5 days
to 6 days of standby time with correctly working APs. But, if at your
work the standby time is 8 hours, optimising DTIM doesn't do any good
because the chip is obviously sleeping very litte, if any.

Kalle Valo

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