WLAN power saving: brain dump

WLAN power saving: brain dump

[Werner Almesberger]

This is just a brain dump, so that the ideas are out in public.
Some day, I might actually implement this, unless somebody else
beats me to it :-)

These reflections are my own. I made them while reading 
specifications of WLAN cards with growing horror.

Looking at WLAN cards, they uniformly seem to have excessive
power consumption even if just laying in wait. This is
particularly annoying for devices with otherwise very low
power consumption, such as PDAs.

It would seem to me that the typical use of such a device would
allow for completely powering down the WLAN card if not needed.
In particular, all packets reaching a normal PDA are in response
to some previous and comparably recent emission from that device.

Furthermore, we generally have a fairly good idea of when we
still want to send something, and, although to a lesser extent,
when we're expecting incoming data.

So actively powering down a WLAN card while idle could use the
following inputs:

- an indication of whether any socket has a non-empty send queue
- the time since the last time all send queues were empty (we want
  to have a certain cool-off period, for various reasons, including
  connectionless activity like ARP or UDP)
- an indication if whether any sockets have a read or a poll or
  select for reading pending (this is very unreliable, but may
  provide useful input with some appliations)
- time since the last successful reception
- an indication of whether any sockets have pending out-of-order
  data
- a manual override

All this should be keyed by interface. Things to consider:

- the time and power it takes to bring a WLAN card back online
- the time we regularly have to wait for something to happen
  even if we seem to be able, e.g. the delay between sending an
  HTTP GET and a response (if we can get hints from the Web
  client, no guessing is needed here)
- the correlation between idle time and future idleness from the
  user

There's probably more. For a proof of concept implementation, it
should be sufficient to consider ARP, DNS, SSH/SCP, and HTTP(S).

- Werner

-- 
  _________________________________________________________________________
 / Werner Almesberger, Buenos Aires, Argentina         wa@almesberger.net /
/_http://www.almesberger.net/____________________________________________/

Re: WLAN power saving: brain dump

[Harald Welte]

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Hi Werner!

On Tue, Nov 16, 2004 at 11:30:27AM -0300, Werner Almesberger wrote:
> This is just a brain dump, so that the ideas are out in public.
> Some day, I might actually implement this, unless somebody else
> beats me to it :-)

Mh.  Don't recent 802.11 chipsets already have a number of power-saving
features, even going as far as having special provisions in the 802.11
protocol for special power saving modes?

Sorry for that indefinitive and blurry response, but power saving is the
about the single part of 802.11 that I'm actually not really interested
in (and thus haven't read up with the docs).

The problem when you just 'stupidly' power down a card is that you will
loose the association [and thus authentication] with the AP.  Thus you
have to re-associate and re-authenticate every time you come up from
power down mode.  Especially when you rune more advanced authentication
mechanisms like WPA/WPA2 this process can take quite some time, and
involve latency-prone operations such as contacting a radius server from
the AP, etc.

-- 
- Harald Welte <laforge@gnumonks.org>               http://www.gnumonks.org/
============================================================================
Programming is like sex: One mistake and you have to support it your lifetime

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Re: WLAN power saving: brain dump

[Werner Almesberger]

Harald Welte wrote:
> Mh.  Don't recent 802.11 chipsets already have a number of power-saving
> features, even going as far as having special provisions in the 802.11
> protocol for special power saving modes?

They do, but they're still very power-hungry. (Well above 100 mW,
it seems.)

But meanwhile I've been pointed to new technology that should do
a lot better in this regard:
http://www.semiconductors.philips.com/technologies/wirelessconnectivity/80211b/

So maybe active power management from the host would only be
needed for obsolete hardware in the not too distant future.
Still, there's a lot of this around ...

> Sorry for that indefinitive and blurry response, but power saving is the
> about the single part of 802.11 that I'm actually not really interested
> in (and thus haven't read up with the docs).

With a PDA, it's be about the only thing you think about ;-)

> The problem when you just 'stupidly' power down a card is that you will
> loose the association [and thus authentication] with the AP.

Yes, that's what worries me a bit. A few hundred ms of delay
should be fine, but if it takes longer, that might be a problem.

- Werner

-- 
  _________________________________________________________________________
 / Werner Almesberger, Buenos Aires, Argentina         wa@almesberger.net /
/_http://www.almesberger.net/____________________________________________/