> Date: Wed, 23 Sep 1998 18:25:10 +0200
> From: "Ulrich Hausmann" <ulrich.hausmann@a2e.hp.shuttle.de>
> X-Priority: 3
> Precedence: list
> Delivered-To: port-mac68k@netbsd.org
> 
> Bill,
> 
> a question: how it comes I'm getting frequently/regularily connect
> speeds when dialing to my ISP (in MacOS OT/PPP) of between 42.000 -
> 48.000 bps? Using a Q630 with Lasat 560 Voice . . .
> 
> Regards, Ulrich

In V28 protocol your modem goes through a 'line analysis' during
handshaking and decides the best speed to use (up to the available
maximum) depending on line quality.  That's what the strange 'boinggg'
noise is doing as your modem connects.  If your modem is good quality,
and compatible with the modem at the other end, the modems may well
dynamically adjust this speed up or down if the line quality changes
significantly during the connection.  Slower working allows a
reduction in error rate which increases overall useful throughput if
it is done correctly.

In general modems will work at any line speed up to and including the
interface speed, which seems to be a maximum of 57600 bps for MacBSD.
If line speed (the data transmission rate down the phone line between
the two modems) is less than interface speed (the data transmission
rate between the modem and the computer) then the modem - if set up
correctly - uses data compression techniques to increase its
throughput.  Advertising copy abuses the maximum compression available
to give the impression that huge throughput is available, in reality
compression by the modem only helps with certain types of file and if
your modem is smart enough it doesn't try and re-compress gzip and
other compressed files as doing so makes them bigger.

Unless you have a really exceptional ISP, or you are running a modem
that is very much faster than 57600 (unlikely unless you are using a
cable modem) it is improbable that the difference between a NOMINAL
56k modem running at an interface speed of 57600 and the same modem
with the interface set to double that is going to be more than
marginal.  In your case, your line speed is well below the interface
speed so the only time your interface speed could possibly hold you up
is if you are transmitting large and highly compressible files.  In
which case a dose of compression before sending the file will do you
more good anyway as gzip compresses better than modems.

In reality, for most people and except for connections at dead of
night, the limiting factor is likely to be the other end of your
internet connection getting slow due to user demand.

The way to really find out how fast a connection you have is to send
or receive a large file and time how long it takes.  (If you want to
play, try setting your interface speed down a bit then repeating with
the same file and see how much your interface speed reduction really
costs in overall data rate).

Hope this helps.  Any corrections always welcome.

Russell.