From: jim@n5ial.chi.il.us (Jim Graham)
Newsgroups: comp.dcom.modems
Subject: What to set COMM speed for V32bis Modems
Distribution: world
Date: Sat, 25 Jul 92 01:44:58 GMT
Organization: Me? Organized? Hah! :-)

In article <BrtBxD.Ks1@animal.gcs.co.nz> lchiu@animal.gcs.co.nz writes:

> This is probably a simple question but here goes...

yep, a simple question.  and one that, outside of comp.dcom.modems
regulars, most people don't seem to know about.

> If you have a v32bis modem and you wish to dial another one, what do you
> set your comms speed to? Using Procomm+ I could not find a setting for
> 14.4 but 9600 19.2. In fact the modem responded to AT command at 19.2
> even though I have never been able to connect at any speed yet.

well, if you set it at 9600, your modem won't connect at 14.4, period.
that's the first thing to keep in mind.  :-)

but, here's something else to consider --- let's say you set your port
speed at EXACTLY 14.4 kb (just pretend for a minute that your software
allows this), and you're using either MNP4 or V.42 error control.

now, between your modem and the computer, on the async serial link, you
are actually using 10 bits per character (8 data bits, plus 1 start bit
and 1 stop bit).  so, the absolute maximum data rate you could get (in
characters/second, which is what you're really concerned with) will be

    14,400 bits/second
    ------------------  == 1440 characters/second (cps)
    10 bits/character

thing is, if you're using either MNP4 or V.42 error control, the
start/stop bits are stripped off, so between the modems, you're only
using 8 bits/character.  guess what...that amounts to 1800 cps between
the modems (before error control overhead, which brings it down to
around 1700 to 1750 cps). see a problem here? your serial link is now
the bottleneck.  not good!

on the other hand, let's say you set your port speed at 19,200 bps.
now, on your serial link, you are capable of 1920 cps, and the link
between the modems is once again the bottleneck.

now, assuming you're using V.42bis data compression (don't use MNP5...)
between the modems.  if so, for *SOME DATA*, mostly for text, you can
see anywhere between 2:1 to 2.5:1 on average, sometimes even 3:1 (and
in extreme cases, which I've yet to really see, even 4:1).

oops, now your serial port is the bottleneck again.....  if you're using
V.32bis, V.42, and V.42bis, I wouldn't suggest setting the port speed
anywhere below 38.4 kb, assuming your computer, serial port, and
software can handle 38.4 (see the ongoing thread regarding the 16550
UART for more details there).  of course, this has to be the case on
BOTH ends....the weakest link in the chain will still drag everything
else down with it.

as has been mentioned in another, related thread, you *MUST* use flow
control between the modem and computer.  you really want to use
bi-directional hardware flow control (if anything, usually referred to
in modem documentation as RTS/CTS flow...but this term is also used in
some modem docs for something else).  the key thing you want to see
in the docs is an indicator that CTS being high indicates that the
modem is ready to receive data from the computer, and RTS being high
indicates that the computer is ready to receive data from the modem,
and that these are independent of each other.

a long answer to a short question, I know, but hopefully it provides
some information that will help you better understand what's behind
wanting to set the port speed higher.  feel free to e-mail me for more
info, clarifications, etc.
   --jim

--
Standard disclaimer....Ever since my cat learned to type, there's no telling
whose thoughts these really are....         73 DE N5IAL (/9)
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