From: mathew <mathew@mantis.co.uk>
Newsgroups: comp.sys.mac.misc
Subject: Re: Difference between 2400, 2400 v.42, and 9600 baud?
Date: Mon, 11 Jan 93 16:38:37 GMT
Organization: Mantis Consultants, Cambridge. UK.
X-Newsreader: rusnews v0.98

jelmore@nyx.cs.du.edu (John Elmore) writes:
> Okay, I'll admit it. I'm still using a 1200 baud modem. But I'm looking to
> buy a new modem. However, I'm puzzled about the difference between a 2400
> baud modem, a 2400 baud modem with v.32 or v.42 compression, and a 9600
> baud modem. All the ads I read say that "effective throughput is 9600 baud
> with v.42 compression". What does this mean?

It's marketing bullshit, ignore it.  I'll explain why below.

First, here's my very own list of all the modem jargon you need to know
about:

- - - cut here - - -

Connection Protocols and Miscellaneous Standards:
-------------------------------------------------

These all specify the basic speed of the modem and how you connect it to your
computer.

V.21     110 or 300 bps.
V.22     1200 bps connection.
V.22bis  2400 bps connection.
V.23     600, R1200/T75 or R75/T1200 bps connection.
V.24     Our old friend the RS-232C interface.
V.26bis  2400 or 1200 bps half-duplex connection. (Not the same as V.22bis)
V.27ter  4800 bps half-duplex connection. (Not the same as V.32)
V.32     4800 or 9600 bps.
V.32bis  14400 bps.
PEP      Proprietary technique used when Trailblazers talk to each other.
V.Fast   Something faster than V.32bis.  Not finalized yet, but lots of
         impatient manufacturers are coming out with their own mutually
         incompatible interpretations of what they think the eventual
         standard will say.

V.26bis and V.27ter are believed to be used by Fax machines.


Error-control Techniques:
-------------------------

These specify whether the modem has a system for detecting and correcting
errors.  These only work if the modem at the other end has the same system.

LAPB      Uses link layer portion of X.25. Also available in HDX (half
          duplex) and AFT (Hayes Asynchronous Framing Technique) flavours.
V.42 \    V.42 == LAPM, and is compatible with MNP levels 2, 3, and 4.
LAPM /    Also in HDX and AFT flavours. MNP compatibility is also referred to
          as LAPM/ALT (Alternative protocol).
X.25      Standard for point-to-multi-point connections through X.25
          packet-switched networks.
Ping-pong Proprietary technique used by older Hayes modems on 4800 and 9600
          bps connections.


Compression Techniques:
-----------------------

These specify whether the modem has a system for squeezing more data down in
the same number of bits or baud (state changes).  These only work if the modem
at the other end has the same system.

MNP 5     Uses Lempel-Ziv for 2:1 compression.
ADC       Proprietary Hayes protocol, gives 2:1 compression.
V.42bis   Layered on LAPM connections only; gives 4:1 compression.  Switches
          off automatically if its use would not be advantageous (e.g. if the
          data is already compressed).

Acronyms:
---------
DTE       Data terminal equipment -- the computer or terminal attached to the
          modem.
DCD       Data Carrier Detect -- the modem switches this on when it detects
          a carrier signal.
DTR       Data Terminal Ready -- used by the DTE to signal to the modem that
          it is ready to receive data.
DSR       Data Set Ready -- used by the modem to signal to the DTE that it is
          ready to receive data.
RTS/CTS   Request To Send / Clear To Send -- signals used for flow control.
          The DTE and modem each use one of the signals to indicate their
          readiness to receive more data.
XON/XOFF  An alternative to RTS/CTS for flow control.  Two characters XON and
          XOFF (^S and ^Q) are used to pause and resume the flow of data.
          A refinement, Transparent XON/XOFF allows ^S and ^Q characters to
          be sent because it automatically 'escapes' them.
bps       Bits per second

- - - cut here - - -

Baud rate is a technical term which isn't terribly useful -- bits per second
is what you'll actually see coming out of the modem and into your Mac.

Now, to answer your questions:

A 2400 baud modem is one which provides up to 2400 bps of data to your Mac
when connected to another 2400 baud modem.

A V.32 modem is one which provides 4800 or 9600 bps of data when connected to
another V.32 modem.  "2400 with V.32" is a misunderstanding.

A V.42 modem is one which will detect most errors and correct them, but only
if it is connected to another V.42 modem.

You might be thinking of a V.42bis modem, which is a modem with compression. 
A 2400 baud modem with V.42bis compression will provide you with up to 9600
bps, but only on files which can be compressed efficiently, and only when
you're talking to another V.42bis modem.  Any V.42bis modem will also have
V.42 error-correction.

The chances are most large downloads you'll be doing will be things like .sit
files, which V.42bis won't compress much.  So a 2400 modem with V.42bis
probably won't give you anything like the performance of a plain 9600 modem
in actual use.

Any ad which says "effective throughput is 9600 baud with v.42 compression"
is talking crap.  Firstly, it's bps that's the relevant measurement, not
baud.  Secondly, effective throughput will be significantly less than 9600
for most large files you're likely to be transferring, as explained above.

>                                                 Can I hook up a 2400 baud
> with compression to a plain 9600 baud modem and get 9600 throughput on my
> end?

No.  The plain 9600 modem won't have V.42bis, so you'll get a plain 2400 baud
connection.  If the 9600 modem had V.42bis, you'd get a 2400+V.42bis
connection -- that is, at most 9600 bps, and usually less than that.

>  Do I need a modem (either 2400 or 9600 baud) with compression on the
> other end to use compression?

Yes.  Don't worry, though; if you're transferring compressed files -- like
.sit archives -- compression at modem level won't help much.

> I want 9600 baud speed, but if I can get it with a 2400 baud with
> compression, that saves me $$$. Please, someone with more knowledge about
> these things help me!

My advice is that you won't get 9600 speed from a 2400 modem even if it has
V.42bis compression.  I don't have the money for a modem, but if I did I
wouldn't even consider a 2400 one.  I'd sooner have a plain 9600 than a
2400+V.42bis.

Here at Mantis we use Hayes V.32/V.42bis modems running as plain V.32 half
the time, plus a couple of 2400/V.42 and 2400/V.42bis modems.  Oh, and ISDN.

I hope the above information helps you.  Any corrections are welcomed.  No
flames, please, just technical data.

Unfortunately, modems are something of a black art, and there's lots of
confusing terminology.  There are also a lot of cowboys who will sell you a
slow modem on the assurance that because it has V.something, it must be as
good as a fast modem.

Another trick I've seen used is to sell a 2400 baud modem, but to put in the
phrase "9600 bps DTE transfer speed".  All that actually means is that it
could theoretically talk to the computer at 9600 bps; DTE speed is the speed
at which it talks to the computer.  The data will still go down the phone
lines at 2400, and hence you'll only get 2400 bps of actual throughput.

If you have any other questions, feel free to mail me.  I promise not to try
and sell you a modem. :-)


mathew
Newsgroups: comp.sys.mac.misc,comp.sys.mac.system