Re: Offtopic? Windows 7 Virtual Wifi adapter.

[Dan Williams <dcbw redhat com>]

On Mon, 2009-05-18 at 11:44 -0400, Darren Albers wrote:
> On Mon, May 18, 2009 at 11:00 AM, John W. Linville
> <linville tuxdriver com> wrote:
> > On Mon, May 18, 2009 at 10:57:05AM -0400, Tom Sutherland wrote:
> >> Ran across this article today:
> >>
> >> http://www.engadget.com/2009/05/18/microsofts-virtual-wifi-will-make-windows-7-wireless-adapters-d/
> >>
> >>
> >> >From the article...
> >> "The tech lets one piece of WiFi hardware be represented in Windows as
> >> two separate adapters, meaning you can connect to two hotspots
> >> simultaneously if you like, or turn your virtual device into an access
> >> point that others can connect to."
> >>
> >> In the long term, would something like that be relevant to
> >> networkmanager?
> >
> > You have to make it work in the kernel first...
> >
> > John
> > --
> > John W. Linville                Someday the world will need a hero, and you
> > linville tuxdriver com                  might be all we have.  Be ready.
> 
> Don't some wireless drivers support this already with multiple
> subinterfaces that correspond to a VLAN?

They dont' really correspond to a VLAN, they appear as separate
"wiphy" (aka wireless phy) devices which can be configured separately.

Note that by doing this "splitting", you're effectively time-sharing the
adapter, and your transfer rate will go down by more than half, since
the adapter has to be on one channel for 1/2 the time, and the other
channel for the other 1/2 the time.  It's actually more than 50% because
there's channel switch time involved, and probably null-packet exchanges
when jumping to a channel to notify the AP that you're coming back from
sleep.

Yes, it can work, and yes, some mac80211 drivers support this type of
functionality, but something that *also* should be done is figuring out
how to tell users that little Elves didn't magically replace the single
radio in their wifi card with two separate radios, and about the
inherent limitations thereof.

You could of course do a STA + AP with the same card on the same
channel, where for instance the STA was connected to an existing AP, and
you were connection-sharing the traffic out over the your own
just-created AP to friends or something.  But of course there you'll
also be increasing contention of the medium (since you're effectively
broadcasting the same traffic on the same channel twice, once from
friend -> you, then you -> real AP).  There is no silver bullet here of
course, unless you really do have two wifi cards in your machine.

Dan