From: oahvenla@snakemail.hut.fi (Osma Ahvenlampi)
Newsgroups: comp.sys.amiga.datacomm
Subject: Re: Modem Terminology is Confusing
Date: 23 Dec 92 16:17:15 GMT
Organization: Helsinki University of Technology, Finland
Distribution: world
NNTP-Posting-Host: lk-hp-4.hut.fi
In-reply-to: rbharding@yuggoth.org's message of 22 Dec 92 20:52:40 EST

In article <rbharding.038j@yuggoth.org> rbharding@yuggoth.org (Ron Harding) writes:
>V.32    : method for transmitting raw data at 9600 bps full-duplex (both
>          directions at once.)

More or less right.

>V.32bis : method for transmitting raw data at 14400 bps full-duplex.

This too. V.32bis standard also tells how the modem can adapt to changing
line conditions by raising or dropping the speed.

>HST 9600    : method for transmitting raw data at 9600 bps in one direction,
>              and 2400 bps in the other direction.  A short delay is required
>              to the turn the line around.  This is usually not a problem
>              in most terminal sessions or while downloading, since data flow
>              is quite lopsided in either case.  Unfortunately, it's
>              proprietary to US Robotics, and only found in their modems.
>              It's quite common now, but with the increasing popularity of
>              V.32xxx modems, I think HST's days are numbered.

There might have been a HST 9600, I don't know, but the HST 14400 was what
made it popular.

>V.42    : error-control protocol.  Should simulate an error-free connection.
>          Also known as LAPM (Link Access Procedure for Modems).

V.42 standard describes LAPM, but LAPM is, strictly speaking, not the same.

>V.42bis : data compression protocol.  Achieves up to 4x compression, giving
>          a max throughput of 57600 bps using V.32bis.  Also known as BTLZ 
>          (British Telecom Lempel Ziv).

BTLZ can give you more than 4x compression. Try sending a file of all zeros
on a 2400 bps line, with 57600 bps DTE speed, and you'll notice it. That 4x
figure comes from the fact that the highest DTE speed normally supported is
57600 bps, which is 4 times the 14400 speed. You can not send data from the
computer to the modem faster than that.

>MNP2-4  : Various error-control protocols.  I don't know the differences
>          between them.  "Microcom Network Protocol"

The important point is, that MNP4, like V.42, gives you more than "bps rate
divided by ten" transfer rate (in cps), because most of the start and stop
bits are dropped. You can achieve speeds nearing "bps rate divided by eight"
cps using these advanced error correction protocols.

>MNP5    : data compression protocol.  Achieves up to 2x compression.
>          Unfortunately it hurts your throughput if you try to send
>          uncompressible data through it.

This, too can theoretically achieve more than 2x compression.

>In many cases, you don't need to put all your eggs in one protocol basket.
>Hayes Optima 144 supports everything up there except the HST standards, and
>optionally FAX protocols..  The US Robotics HST Dual Standard supports at 
>least HST and the V.32 suite, and probably others but I don't know for
>certain.

Most current high speed modems offer V.21, Bell 103, V.22, Bell 212A, V.22bis,
V.32 and V.32bis besides fax compatibility, MNP2-4, MNP5, V.42 and V.42bis.
Some (such as USR HST and ZyXEL) offer also a proprietary 16800 bps protocol.
Hayes Optima 144 may be a fine modem, and probably is, but it definitely is
not alone. And it definitely does NOT support "everything there is except HST".

>Since we're on the subject, I'll also talk a bit about modem escape sequences.
>Escape sequences are sent to the modem to change it from data-transmission
>mode to command mode, usually to issue an "ATH" (hang up) sequence.

>Hayes Improved Escape Sequence with Guard Time (what a mouthful):
>    Consists of short delay, followed by three special characters, usually
>    '+', followed by another short delay.  Patented (and vigorously enforced)
>    by Hayes.

>TIES    : "Time-independant Escape Sequence"  Used by Hayes-pseudo-compatible
>          modems.  The sequence consists of the five characters: "+++AT", 
>          that is, the Hayes sequence, but without the guard times, followed
>          by the "AT" characters that must preceed any Hayes-compatible
>          command sequence.

Bull. I have not seen a single modem that would react to "+++AT", every one
of them have used the "delay, three characters, delay" escape sequence. In all
of the cases the delay lenght and escape code have also been user definable, 
usually with S registers 12 and 2, respectively.
This is of course with modems using AT command set, extended or otherwise.

The rest of the post has been deleted as it was discussion of why the guard
tone escape is better than TIES escape, and is irrelevant.

Even if it might seem so at points, this post is not intended as a flame. I
felt it was necessary to correct especially the escape sequence part, and gave
some comments about other parts, while on the subject.
--
Osma Ahvenlampi  -  oahvenla@snakemail.hut.fi   * Workstation power for micro-
All my opinions are not necessarily really mine * computer price: Amiga := FUN