[mythtv-users] Wireless Pre-N for Linux?

Bill Williamson bill at bbqninja.com
Tue Mar 27 23:32:47 UTC 2007

On 3/28/07, James Oltman <cnlibmyth at gmail.com> wrote:
> > >
> > > However due to interference you can generally take the theoretical
> > > throughput and chop it in half: not many people will get more than
> > > about
> > > 25-27Mbps out of an 802.11g link.
> >
> > And then you need to chop it in half again to get the usable bandwidth
> > for data. once all the framing overhead is taken into account.
> >
> > Cheers,
> >    Steve
> > _______________________________________________
> >
> I always understood the 54Mbps to be a half duplex rated speed.  Thus,
> 26Mbps is the actual Full Duplex speed.  Am I wrong?  The of course you have
> to take into account the noise etc in your environment.

You are wrong.  54Mbps is the raw bitrate at the physical layer.
There is no such thing as "duplex" in the wireless world.  With no
interferance (plug the transmit antenna plug into the receive antenna
port) you can get a full 54mbps of physical layer transport.  However,
about 20% of that is lost immediatly to 802.11g protocol overheads
(packet structure etc).  You CAN get a 1 way data stream of about
40Mbps using UDP with a perfect signal.  It's not split in "half" for
duplex, if you're the only one transmitting then you have the entire
channel (excepting the 802.11 overhead).

Now if you're using TCP things get even slower due to the receiver
having to send ACK packets back, using more of that channel (and the
more "client switching" there is, the slower the protocol is), which
slows you down to about ~30mbps max.

In fact, the more clients you have, the slower your OVERALL speed is.
With an FTP transfer and perfect signal a single client can pull down
about 30Mbps from an access point.  Two clients will drop down to
about 12-13Mbps each.  6 clients can only get maybe 3Mbps each.  10
clients will get 1-2Mbps each.

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