Jouni Malinen wrote :
>
> On Thu, Mar 23, 2006 at 08:16:04PM -0500, Dan Williams wrote:
> > That's why wireless-tools and Wireless Extensions switched to
> > passive scanning.
>
> Can you please point to some documentation/email thread/etc. describing
> this preference to use passive scanning? I was not aware of such a
> preference and have assumed that active scanning would be the preferred
> default because it is quite a bit faster and provides more complete
> results.
I was scratchig my head over that one, I'm glad I'm not the
only one.
Prior to WE-18, the API did not specify anything, so it was
left to the discretion of the driver/card. With WE-18, you can specify
what you want, but most apps don't, so effectively we are in the
pre-WE-18 situation.
I think that active scanning is preferred is most case because
it's faster. The power issue is likely overblown, as scanning doesn't
occur that frequently and generate a minimal number of packets. If
scanning had a real impact on power, it would have the same impact on
throughput, and people would complain about it (I almost wrote a paper
on the parallel between power and throughput optimisation).
The first type of card are cards that do scanning in the
background. Thos cards need to interrupt traffic on a periodic basis
to perform scanning, the less time they spend scanning, the less
impact it will have on throughput and latency. If the card detect that
there is no traffic, it could use passice scanning.
The second type of card are cards that don't do scanning in
the background. In that case, scanning is triggered at the time of the
request, and both the user and the app are waiting for the result
(well, except those wasteful apps that do periodic scans). You don't
want the user to wait.
Jouni Malinen wrote :
>
> On Fri, Mar 24, 2006 at 12:38:40PM -0500, Dan Williams wrote:
>
> > However, the problem with active scanning is that you have to power up
> > the transmit components of the radio, while passive scanning, even
> > though it takes a bit longer, doesn't necessarily require that. Active
> > scanning takes more power.
>
> For some class of devices, that is certainly correct. For other classes,
> it does not really matter much (e.g., my laptop has large enough battery
> for me not to care about active scan).
Thinking about it some more...
In most cards, the difference of power consumption between Tx
and Rx is not that much, less than an order of
magnitude. Unfortunately, my link to number is 404. If you do active
scanning, you are done much faster and thefore can switch your
receiver off and go back to full power saving much earlier. Which
means that you may actually consume less power...
Let pull hypothetical numbers, for the sake of the argument,
let say Tx = 1W, Rx = 200mW, idle = 20mW. Let say active scanning is
20ms per channel (2ms Tx, 18ms Rx), passive scanning is 200ms per
channel. Let say we scan 10 channels.
Over a 2 sec period, active scanning is 72mW.
Over a 2 sec period, passive scanning is 200mW.
Actually, it's a well known fact that in wireless networking,
it's not Tx that kill your battery, but Rx.
Have fun...
Jean
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