Possibilities - Taking the Buzz Out of Buzz Words - Part 2

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TAKING THE BUZZ OUT OF BUZZ WORDS - PART 2
------------------------------------------

*** From August 1990 Possibilities Newsletter ***
*** Copyright 1990 by eSoft, Inc.  All Rights Reserved ***

Taking The "Buzz" Out of Buzz Words
Part 2: Modem Standards
by Alan D. Bryant

The world of computers is riddled with buzz words -- technical jargon for 
the various parts of computers, their functions, and applications.  In 
telecommunications it's the same thing.  Terms like Baud, Bits, Parity, 
MNP, Half Duplex, and Full Duplex can make a TBBS system designer's life
seem more complex than it really is.  The problem is, these buzz words are 
attached to many of the components and concepts that a TBBS system designer 
must grasp to make the most of online system implementation and even to 
explain a system's operation to its users.

Fortunately, most telecommunications terminology isn't hard to understand -- 
once it's been explained by someone who knows what the terms mean and can 
speak English clearly enough to break them down in understandable language.  
Alan Bryant is just such a person and we at eSoft are lucky enough to 
have him on our technical support staff.

In the following special three-part series, Alan will tackle many of the 
common telecommunications buzz words you'll encounter as a TBBS system 
designer and bring them a lot closer to home with straightforward, plain-
English definitions and step-by-step explanations. In this installment, he 
demystifies the CCITT and the international telecommunications standards 
that are set by this prestigious committee then explains each of the MNP 
modem standards -- Ed.

What is the CCITT?

The CCITT is the acronym for the Consultative Committee on International 
Telephone and Telegraph.  This is an international body of technical experts 
responsible for developing data communications standards for the world.  The 
group falls under the organizational umbrella of the United Nations and its 
members include representatives from major modem manufacturers, common 
carriers (such as AT&T), and governmental bodies.

CCITT Modulation Standards

The CCITT establishes standards for modulation -- actual modem signaling 
methods.  It also determines standards for error correction and data 
compression (See part 1 of this series for a full description of these modem 
layers).  For this reason, it is possible (and likely) that one modem might 
adhere to several CCITT standards, depending on the various features and 
capabilities the modem offers.

All modems signal one another at a variety of speeds, so CCITT standards for 
modulation are utilized by virtually every modem manufacturer.  Some of the 
standards which are primarily modulation do include some of the higher 
layers (such as negotiation) as well.  Multi-speed modems may use several of 
these standards, which include:

V.21

V.21 is a data transmission standard at 300 bps.  This standard is used 
primarily outside of the United States.  (300 bps transmissions in the 
United States primarily use the BELL 103 standard).

V.22

V.22 is a data transmission standard at 1200 bps.  This standard is also 
used primarily outside of the United States.  (1200 bps transmissions in the 
United States primarily use the BELL 212A standard).

V.22bis

V.22bis is a data transmission standard at 2400 bps.  This is the 
international standard for 2400 bps, and is used both inside and outside the 
United States.

V.23

V.23 is a split data transmission standard, operating at 1200 bps in one 
direction and 75 bps in the reverse direction.  Therefore, the modem is only 
"pseudo-full-duplex," meaning that it is capable of transmitting data in 
both directions simultaneously, but not at the maximum data rate.  This 
standard was developed to lower the cost of 1200 bps modem technology, which 
was still very costly in the early 1980s, when such modems were designed.  
This standard is still in use, but primarily in Europe.

V.29

V.29 is a data transmission standard at 9600 bps which defines a half duplex 
(one-way) modulation technique.  Although modems do exist which implement 
this standard, it has generally only seen extensive use in Group III 
facsimile (FAX) transmissions.  Since it is a half-duplex method, it is 
substantially easier to implement this high speed standard than it would be 
to implement a high speed full-duplex standard.  V.29 is not a complete 
standard for modems, so V.29-capable modems from different manufacturers 
will not necessarily communicate with one another.  

V.32

V.32 is also a data transmission standard at 9600 bps, but V.32 defines a 
full-duplex (two-way) modulation technique.  It is a full modem standard, 
and also includes forward error correcting and negotiation standards as 
well.  Many modem manufacturers already have or will be introducing V.32-
compatible modems.  This is generally considered "the" standard for high-
speed modems today.

V.32 is expensive to implement, since the technology required for it is 
complex.  As this standard becomes more common and manufacturing techniques 
are refined, the pricing for V.32 modems should go steadily downward.  At 
this writing, V.32 capable modems are selling for between $500 and $1000 
each.

Some manufacturers have created modems that can use both their own 
proprietary high speed standard and the V.32 standard, for compatibility 
with their older non-V.32 modems.  The new Hayes Ultra and U. S. Robotics 
HST Dual Standard are examples of the new "dual personality" modems that are 
now on the market.

V.32bis

This is a developing high speed standard.  When fully defined (likely by 
early 1991), V.32bis will operate at 14,400 bps and, like V.32, will be a 
full-duplex method.  The CCITT has not yet defined this standard, so no 
modems currently use it (although some new modems have implemented what is 
expected to be the standard and may claim V.32bis compatibility).

Error Correcting and Data Compression

The CCITT also has adopted formal standards for the higher layers of Error 
Correction and Data compression (See Part 1 of this series for a full 
description of these layers).  In order for any error correction or data 
compression protocol to work, modems on BOTH ends of the connection must 
support it.  Once two modems are connected, they automatically negotiate 
between themselves to determine the best mutual protocols they both support.

V.42

V.42 is a CCITT error-correction standard that's similar to MNP Class 4 (See 
"What is MNP" below).  In fact, because the V.42 standard includes MNP 
compatibility through Class 4, all MNP 4-compatible modems can establish 
error-controlled connections with V.42 modems.  This standard, however, 
prefers to use its own better performing protocol -- LAPM (Link Access 
Procedure for Modems).  LAPM, like MNP, copes with phone line impairments by 
automatically re-transmitting data that is corrupted during transmission 
assuring that only error free data passes through the modems.  Many modem 
manufacturers make MNP Class 4-compatible modems, and some offer V.42- 
compatible modems as well.

V.42bis

V.42bis is a CCITT data compression standard similar to MNP Class 5, but 
providing about 35% better compression.  Of course, this also means it 
provides better throughput.  V.42bis only compresses data that needs 
compression.  Each block of data is analyzed, and if it can benefit from 
compression, compression is enabled.  Files on bulletin board systems are 
often compressed already (using ARC, PKZIP, and similar programs).  While 
MNP Class 5 can actually decrease throughput on this type of data, V.42bis 
will not -- compression is only added when a benefit will be realized.

To negotiate a standard connection using V.42bis, V.42 must also be present. 
Thus, a modem with V.42bis data compression is assumed to include V.42 error 
correction.  Some modem manufacturers already make V.42bis compatible 
modems, and more are on the way.

V.42bis is NOT compatible with MNP Class 5.  A V.42bis modem will establish 
an error-free connection with MNP-capable modems (since V.42bis includes 
V.42), but only up to MNP Class 4.  

What is MNP?

MNP stands for "Microcom Networking Protocol" and was created by Microcom, 
Inc., a modem manufacturer.  MNP offers end-to-end error correction, meaning 
that the modems are capable of detecting transmission errors and requesting 
re-transmission of corrupted data.  Some levels of MNP also provide data 
compression.

As MNP evolved over time, different classes of the standard were defined, 
describing the extent that a given MNP implementation supports the protocol.  
Most current implementations support Classes 1 through 5.  There are higher 
classes, but are usually unique to modems manufactured by Microcom, Inc. 
since they are still proprietary.

MNP is generally used for its error correction capabilities, but MNP Classes 
4 and 5 also provide performance increases, with Class 5 offering real-time 
data compression.  The lower classes of MNP are not usually important to you 
as a modem user, but they are included here for completeness.

MNP Class 1

MNP Class 1 is referred to as Block Mode.  It uses asynchronous, byte- 
oriented, half-duplex (one way) transmission.  This method provides only 
about 70% efficiency.  It provides error correction only, and is rarely used 
today.

MNP Class 2

MNP Class 2 is called Stream Mode, and uses asynchronous, byte-oriented, 
full-duplex (two way) transmission.  This class also provides error 
correction only.  Because of protocol overhead (the time it takes to 
establish the protocol and operate it), throughput at Class 2 is actually 
only about 84% of that for a connection without MNP, delivering about 202 
cps (characters per second) at 2400 bps (240 cps is the theoretical 
maximum).  Class 2 is rarely used today.

MNP Class 3

MNP Class 3 incorporates Class 2, and is more efficient.  It uses a 
synchronous, bit-oriented, full-duplex method.  The improved procedure 
yields throughput about 108% of that of a modem without MNP, delivering 
about 254 cps at 2400 bps.

MNP Class 4

MNP Class 4 is a performance enhancement class that uses Adaptive Packet 
Assembly and Optimized Data Phase techniques.  Class 4 improves throughput 
and performance by about 5%, although actual increases depend on the type of 
call (local or long-distance, noisy or clean connection), and can be as high 
as 25% to 50% on some links.

MNP Class 5

MNP Class 5 is a Data Compression protocol which  uses a real-time adaptive 
algorithm.  It can give an increase of up to 50% in throughput, but the 
actual performance of Class 5 is very dependent on the type of data being 
sent.  Raw text files will allow the highest increase, while program files 
cannot be compressed as much and the increase will be less.  On pre-
compressed data (files already compressed with ARC, PKZIP, etc.), MNP 5 can 
actually expand the data and performance can actually decrease.  For this 
reason, MNP 5 is often disabled on BBS systems.

MNP Class 7

MNP Class 7 is the other major MNP protocol you are likely to encounter.  
MNP 7 provides Enhanced Data Compression.  When combined with Class 4, it 
can obtain about a 300% improvement in performance.  It is designed 
primarily for use with V.22bis (2400 bps) modem.  This class is currently 
unique to Microcom modems.  Since it requires much more hardware and is 
usually inferior to V.42bis, it is not likely to proliferate.

What does it all mean?

Despite the fact that they can seem quite confusing, all of these standards 
exist to benefit you the modem user.  You want to be able to compare modems 
on price, reliability, performance, and support.  You also want to be able 
to know that modems from different manufacturers will communicate with each 
other.

The past couple of years in the high speed modem arena has shown what 
happens when market demand occurs faster than associated standards.  You are 
forced to pick a single manufacturer and become locked in to gain the 
capabilities you want.  The purpose of standards is to prevent this 
situation.

When standards are widely adopted, you get the best of technology and 
competition.  However, you need to know what the standards mean to be able 
to be an informed consumer.

Next month we'll wrap up this discussion with explanations of most of the 
rest of the various terminology common to the modem world, but not always 
fully understood.  Don't miss it!

- END -
PS0890-4
Rev. 8/90

Copyright (C) 1994 eSoft, Inc., All Rights Reserved.  Permission granted
to distribute this file in its entirety, without modification, to any
interested party.  Any other use requires the written permission of
eSoft, Inc.

IMPORTANT:  The information herein is subject to change without notice.
Please call or write to confirm factual information of importance to you
or your organization.

