Re: bmAttribute NO_REMOTE_WAKEUP

On Saturday 17 March 2007 3:32 pm, Jon Smirl wrote:

> > > Two devices I have that also have NO_REMOTE_WAKEUP are both Wifi
> > > adapters. Wifi has it's own power saving scheme that allows it to poll
> > > for activity without being fully powered.

Right, but that still consumes power.  And WiFi isn't really all
that power-efficient.


> > What use is polling for activity if the devices have no way to report the
> > activity to the computer?
> 
> I was wondering if this was a hint that the wifi devices aren't
> supposed to be suspended either. 

As Alan pointed out:  no.  It just means that when they're
suspended, they aren't active enough to be able do provide
Wake-On-WLAN capability.


> Most wifi devices have an embedded 
> processor that would need to keep running.

Yes, but ... remember that USB bus powered devices (like WiFi
adapters) have a low ceiling on power usage when they're suspended.

Like, 0.5 mA averaged over each second ... or at most 2.5 mA for
high-current devices.

There are certainly CPUs that get work done with that kind of
power level, but most of them are 8-bit microcontrollers that
don't have enough power to run a WiFi stack.  And many of those
would still consume too much of that power budget to operate a
radio along with the CPU, anyway...

It's much more typical that those processors have a power state
analagous to suspend-to-RAM, in which they stay during such low
power states.  So WiFi is unavailable during suspend.

ISTR that for example a PXA255 processor needs more than 0.5 mA
when suspended, but an OMAP1710 doesn't.  That's part of why the
latter gets used in lots of cell phones, and the former doesn't.


> 		Wifi has a mode where it 
> says: I'm going to shut down for xx ms and then wake up so if you have
> any traffic for me send it in the window where I'm awake.  Wifi is 
> pretty optimized for power savings since it is designed for battery
> operation. If you suspend the device I don't know if this wake in a
> window mode will keep working.

I suspect you're thinking about ZigBee instead ... much lower data
rate, but designed to run for a couple years on some AA batteries.
(Used for distributed sensor applications.)

Both of them can operate in such slotted modes.  But WiFi is much
less power-efficient.

- Dave

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