From: jim@n5ial.chi.il.us (Jim Graham)
Newsgroups: comp.os.msdos.misc
Subject: 14,400 bps 
Distribution: world
Date: Fri, 02 Oct 92 20:05:46 GMT
Organization: Me? Organized? Hah! :-)

In article <1992Oct1.192449.6264@panix.com> schuster@panix.com writes:
> In article <1992Sep29.061505.2299@wixer.cactus.org> roadhog@wixer.cactus.org
>  (Lindsay Haisley) writes:
>> Do I need new telecomm software to support 14,400 bps, or is this speed
>> spec just a euphamism for an "actual" speed of 9600 bps with data
>> compression?

> [....] But this doesn't matter. You simply set your term software
> to 19200 bps or above and the modem will buffer the difference using
> flow control. 

and there's more to it than that, too....  besides, with a V.32bis
modem with V.42/V.42bis, you really want at least 38.4 kb, and not
19.2 kb on the serial port.

oh, before I forget --- use *ONLY* bi-directional hardware flow control
(RTS high == computer ready to receive data from modem if modem has data
to send, and similarly, CTS high == modem ready to receive data from
computer).  anything else, especially (ACK! THPPPT!) software (Xon/Xoff)
flow control, is just inviting disaster at high speeds.  any half-decent
comm program will support this, as will every high-speed modem I've ever
seen.

you may also find that you need a 16550AFN UART in your serial port to
handle the speeds.  just like the flow control, any half-decent comm
program will provide full support for the 16550.

let's say you're using V.32bis, along with V.42 (error control) and
V.42bis (data compression).  there are a number of things involved here
that can increase your throughput substantially, but you need the higher
port speeds to take advantage of them.

first, V.42 (like MNP3) strips off the start/stop bits used on the async
serial link.  thus, instead of 10 bits/character, you're only using the
actual 8 bits/character that have meaning.  if you were to set your port
speed on the serial link at exactly 14,400 bps, you would be limited to
a maximum throughput of 1440 chars/second (14,400 bps / 10 bits/char),
but between modems, you would be capable of 1800 cps before overhead,
or about 1724 cps after overhead, except you would not be able to use
that extra 284 cps since 1440 cps (between your compter and your modem)
is your bottleneck.  of course, this also applies if the other end
doesn't set things right.....

second, if you're transmitting *TEXT*, you can see a very substantial
increase in performance from V.42bis data compression --- usually on
the order of from 2:1 to 2.5:1, maybe (if you're lucky) even as high
as 3:1 at times.  of course, like the above, the slowest link in the
chain drags the rest down with it, so you always want to set your port
speed as high as possible, to ensure that *IT* isn't the bottleneck.

did that make sense?  sorry, but my mind is kinda on something else
right now (off in another state, and so on).  feel free to e-mail me
for a better description.
   --jim

PS:  this question really belonged in comp.dcom.modems, not here.

--
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