AOH :: V_42.TXT|
Describes modem standards including v.42
Taking The "Buzz" Out of Buzz Words
by Alan D. Applegate
Copyright 1990 by eSoft, Incorporated.
All Rights Reserved
Note: The following three part series on modem fundamentals is
reprinted with permission from the eSoft possibilities newsletter
June, July, and August 1990 issues. Possibilities is a monthly
customer support publication of:
15200 E. Girard Avenue
Aurora, Co 80014
This series of articles may not be reproduced in any form except
by inclusion of the above copyright notice. This file is
authorized for distribution without charge only if it is
unchanged in any way. Any use of this information in any other
way must include proper credit to its source.
Part 1: The Basics of Modems
[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 Applegate 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
No doubt you've wondered at one time or another about modem
standards. There are currently several active standards, and
they involve more than just the modem's actual operating speed.
Without these standards, modems from one manufacturer most likely
couldn't "talk" to modems made by another manufacturer.
Consequently, at least a basic understanding of modem standards
is also necessary if you want to make the right choices when
selecting modems for use on your TBBS system.
Generally speaking, 300, 1200 and 2400 bps modems each use a
different standard that is adhered to by all modems and modem
makers. (It should be noted that standards for 300 and 1200 bps
are different in the United States than they are in Europe.)
Standards for 9600 bps transmission have been established for
some time, but the technology to implement those standards was,
until recently, expensive. To get around the high cost of using
the existing standard, modem manufacturers have created several
of their own proprietary high-speed modem standards. This is why
so many high-speed modems will only "talk" to another high-speed
modem of the same brand.
Data transmission speeds, however, are not the only type of modem
standard. Actually, modem standards are grouped into four
distinct areas or "layers." These are shown in the illustration
Modulation is the starting (or bottom) layer for all modems
("modem" means MOdulator - DEModulator). Each layer builds upon
Modulation refers to the signaling method that is used by the
modem. Two modems must use the same modulation method in order
to understand each other. Each data rate uses a different
modulation method, and sometimes there is more than one method
for a particular rate. An example of this is the Bell 212A and
V.22 modulation standards (described below); they both specify
1200 bps modulation, but they work differently, and are not
Negotiation refers to the manner in which two modems establish
which modulation method will be used during a connection. Modems
"listen" to the tones sent by a remote modem to determine what
modulation method will be used. Since different modulation
methods often use different answer tones, these can be used by
the calling modem to determine which method to use. Negotiation
standards have been created to make the process easier. These
standards dictate the sequence of events that will occur when a
modem answers the phone, eliminating the guesswork associated
with the "listen to the tones" method. Negotiation is part of
many modem standards.
Error correction refers to an ability that some modems have to
identify errors during a transmission, and to automatically re-
send data that appears to have been damaged in transit. If error
correction is used, both modems must adhere to the same error
correction standard to make it work. Fortunately, there are
error correction standards which are followed by most modem
Data compression refers to a built-in ability in some modems to
compress the data they're sending, automatically "squeezing" data
to a smaller size as it is sent. This, of course, saves time and
can result in considerable money saved by long-distance modem
users. Depending on the type of files that are sent, data can be
compressed by as much as 50% of its original size, effectively
doubling the speed of the modem.
In this scenario, a 2400 bps modem with data compression is
capable of sending some files as quickly as a 4800 bps modem
WITHOUT data compression. Not all types of data can be compressed
by 50%, but gains can nearly always be realized.
We'll take a look at each of the various data compression
standards later in this series, but first let's examine those
modem standards that are associated directly with the
transmission speed of the modem.
Standards for 300 and 1200 Bps
Most 300 bps modems follow the standard created initially by
AT&T, called Bell 103, and are common in the United States. Most
modems manufactured for use outside the United States support the
CCITT V.21 standard instead, and are not compatible with Bell 103
modems. Some modems can be set to follow either standard.
AT&T also created the Bell 212A standard for 1200 bps modems.
It's become the common standard in the United States. Most
modems manufactured for use outside the United States support the
CCITT V.22 standard instead, and are not compatible with the Bell
212A modems. Some modems can be set to follow either standard.
Most modems manufactured since 1985 are capable of
differentiating between the two standards, and can effectively
handle either one.
2400 Bps Standards
The international standard for 2400 bps communications is CCITT
V.22bis. This is used by modems manufactured for use both inside
and outside the United States. Most 2400 bps modems include
automatic detection of the data rate fall back, if a data rate
lower than 2400 bps is detected at the other end of the
9600 Bps Modems -- Are There Standards?
Contrary to what might be believed, standards for high speed data
transmission have been in place for some time. Acknowledged
standards came in two forms -- a half duplex standard, commonly
used in fax machines and called V.29, and a full duplex standard
called V.32 (we'll take a look at half and full duplex later in
the series). The technology required to implement the V.32
standard remained prohibitively expensive for many years. This
forced most modem manufacturers to create their own less-
expensive proprietary transmission methods.
U.S. Robotics, for example, created the Courier HST, ("High Speed
Technology"). This design is not full duplex, meaning that it
does not support high speed transmission in BOTH directions.
Current HST modems send data at 14,400 bps in one direction, and
450 bps in the other direction. The high speed channel changes
direction depending on which side of the transmission has the
most data to send. HST modems can only talk at high speed with
other HST modems, although they also adhere to existing standards
for 300, 1200 and 2400 bps operation.
Telebit, another modem manufacturer, created PEP ("Packetized
Ensemble Protocol"), which is used in their Trailblazer modem
series. Like the HST, PEP modems will only connect at high speed
with other PEP modems.
Hayes also developed their own technology for high speed
transmission, in the absence of an inexpensive standard. Like
the others, Hayes high speed modems only talk high speed to other
Fortunately, the cost of V.32 high speed transmission technology
has come down drastically in recent years, and is displacing
other high speed proprietary protocols in popularity. This means
that, finally, high speed modems are starting to communicate with
a common standard. U.S. Robotics' new Courier HST Dual Standard
is one example of a new high speed modem utilizing both U.S.
Robotics' own HST transmission standard and the V.32 high speed
standard. The new Hayes V-series Ultra Smartmodem 9600 is
another "multiple-standard" high speed modem that utilizes the
Next month we'll discuss the CCITT and the international
telecommunications standards that are set by this prestigious
committee. We'll even de-mystify the whole family of MNP
standards. Also we'll examine the data compression standards.
What works, what doesn't and what is realistic to expect from
data compression in a modem? MNP vs. V.42bis -- don't
Part 2: Modem Standards
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 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
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
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 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 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 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
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.
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 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 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
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
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(tm) and Optimized Data Phase(tm) 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
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!
Part 3: Communication Terminology
Of Bits and Parity...
In parts 1 and 2, we took a closer look at the most common and
often least understood terms and standards in the world of the
modems we use. There are, however, several other
telecommunications terms that can be confusing. Though they
don't necessarily relate to modem-buying decisions specifically,
understanding these terms can add important additional power to
your communications dealings. They also will help you understand
how to set up the terminal programs your users will have to
configure to call your TBBS system. Among the most commonly
faced (and least understood) are the concepts of Data Bits,
Parity, and Stop Bits.
The American Standard Code for Information Interchange - ASCII -
is a standard that defines 128 different characters that can be
used for data transmission. These include control characters,
letters of the alphabet (in both upper and lower case), numbers,
and a full set of punctuation characters. Because there are only
128 ASCII characters, only 7 binary digits (bits) are required to
form each of the 128 possibilities.
Many computer makers have extended the ASCII character set by
adding 128 more characters. This was accomplished by simply
adding one more binary digit, resulting in a total of 256
transmittable data characters. Each manufacturer, however,
created their own set of 128 additional characters. All extended
character sets are NOT the same.
In the case of the IBM PC and compatibles, the extended
characters include international alphabet, graphics and
mathematics characters. These are commonly known as IBM Graphics
In communications, common settings are either for 7-bit or 8-bit
data. Generally, both ends of the connection must be set the
same way. If one end is set to 7-bit data and the other end is
set to 8-bit data, reliable communication cannot usually be
established. This is because one end interprets the 8th data bit
as a parity bit (explained in a moment), and the other end tries
to interpret it as a part of the current character. On a
connection like this, some characters will display properly,
while others will appear as "garbage," depending on which
direction the data is traveling.
If the communications link is set to transmit only 7-bit data,
the sendable characters are limited to the 128 defined ASCII
characters. The extended character set, such as the PC's single-
and double-line boxes and foreign characters, CANNOT be sent
unless the link is first set to allow the transmission of 8-bit
Some systems have even 5-bit and 6-bit data, and use character
sets such as Baudot and Selectric, but these systems are uncommon
When you establish communications with another computer, parity
is set to "even," "odd," "mark," "space" or "none." These are
terms for the manner in which the parity bit is interpreted by
Parity is a primitive form of error-checking. The state of the
parity bit, when set to be even or odd, is based on a simple
mathematical formula. Depending on the data bits, the parity bit
will either be on or off. Normally, the limited error checking
capabilities are not utilized. This explains why the setting of
parity to "none" is so common in communications today. This
allows the parity bit to be used as a normal data bit instead.
Start and Stop Bits
Start and stop bits allow each character sent to be set in a
"frame." The beginning of the character, the first part sent, is
the start bit, and the end of the character, the last part sent,
is the stop bit. Each character sent is thus framed with a
distinct beginning and ending bit and this allows the receiving
system to know when each complete character has been sent.
There is always just one start bit. However, there may be one,
one and a half or two stop bits.
Stop bit length used to be critical when serial communication was
primarily handled with electromechanical equipment, such as an
old-fashioned Teletype machine. The print head in this type of
equipment took a fixed amount of time to return to its "home"
position, and this was accomplished during the sending of the
stop bits. A longer stop bit length gave the print head more
time to return to its home position.
In modern all-electronic serial communication, the stop bit is
still necessary, but only to mark the end of a character. A
delay isn't necessary as there isn't usually anything mechanical
Framing the character with start and stop bits forms the basis
for "asynchronous" communications. In asynchronous transmission,
characters do not have to flow constantly - there can be "gaps,"
or spaces, between each character. The receiver knows when a
character is sent by the framed nature of asynchronous
transmission - the start and stop points can easily be
An alternate serial transmission method exists known as
synchronous communications. It occurs when there are no start or
stop bits, and is possible only if data characters flow
constantly at a fixed bit rate with no interruptions. When there
is no data to send, idle or padding characters are sent at the
fixed rate (to keep data bits flowing constantly), but they are
discarded by the receiver.
Because there are no start or stop bits, it is possible to remove
2 of every 10 bits used in Asynchronous communications. This
results in a 20% faster data speed with the same serial bit rate.
However, because of the requirement for constant data flow,
Synchronous transmission requires additional protocol and is
primarily used in mainframe computer or specialized applications.
One place it is used with TBBS is hidden inside of high speed
modems. When these modems use MNP or V.42 protocols, they have
the needed protocol to use synchronous communications between the
modems themselves. However, you still use asynchronous
communications between the computer and the modem so this
instance of hidden synchronous communications is primarily of
interest as trivia.
"Duplex" is a term which refers to whether a data communications
path is one- way or two-way. "Full duplex" means that data can
flow in both directions at the same time. "Half duplex" means
that data can flow in only one direction at one time. Most
modems are full duplex, but communications software can most
often still be set to take advantage of half duplex connections.
Some less expensive high speed (9600+ bps) modems are pseudo-
full-duplex. This means they cannot transmit data at high speed
in both directions at the same time because they are really
operating in a fast turn-around half duplex mode internally.
The term "flow control" refers to a method of controlling the
flow of transmitted data, so it doesn't "overrun" the data
receiver's ability to receive the incoming signals. Flow control
allows the receiver to signal the transmitter to pause, while
recently received data is properly assimilated, then signal it to
restart the data flow when it's ready to receive more.
There are generally two forms of flow control - software and
Hardware flow control is not always required. It is generally
needed only with modems that are capable of "buffering" out-going
data, or with high speed modems. Hardware flow control, called
RTS/CTS flow control, uses two of the RS-232 (serial) pins to
start and stop the data flow. Its advantage is that it is data
independent and thus can be used for reliable flow control with
any type of data stream.
Software flow control, called XON/XOFF flow control, starts and
stops the data flow based on the reception of certain control
characters. Although this type of flow control can be used by
hardware devices, software flow control is usually used with
TBBS, to allow the TBBS user to start and stop data transmission
by using control keys. This allows the user to press Ctrl-S at
any time to temporarily halt data flow, and then press Ctrl-Q at
any time to restart data flow.
Even when hardware flow control is in use, TBBS will honor
software flow control codes to start and stop the flow of text
What is ANSI?
"ANSI" is a common term in the bulletin board community today,
but it's also a term that's usually misused.
ANSI stands for the American National Standards Institute, a
standards development organization (sort of like the CCITT, which
I discussed in my last column). ANSI develops and documents
standards for thousands of different areas, from architectural
specifications for the handicapped to computer programming
Within the bulletin board community, the term "ANSI" generally
refers to an ANSI standard called X3.64 as implemented by IBM in
ANSI.SYS. The ANSI X3.64 standard specifies a series of codes
that a host system can send to a remote data terminal to control
color attributes, cursor positioning, inverse video and screen
clearing on the terminal display.
"ANSI Graphics" is a term that is often used in the bulletin
board community, but this actually refers to two separate
elements. "ANSI" controls color and cursor positioning, while
"Graphics" usually refers to characters in the IBM PC extended
character set, such single- and double-line boxes, shading
characters, and so on. "ANSI Graphics" is a common term, since
normally only an IBM PC is capable of handling both ANSI and
Graphics. In reality, many data terminals and software packages
for various computers are capable of handling ANSI codes,
although they may not always handle the IBM extended characters.
Actually, "ANSI Graphics" does NOT refer to a standard for
displaying pictures or graphic images on the remote terminal.
The VT-100 terminal (a data terminal from Digital Electronics
Corporation) and software that emulates a VT-100 terminal can
also be used with ANSI escape codes, since the codes for both
ANSI and VT-100 are very similar.
ANSI works by sending a series of characters to the remote
terminal. The codes all begin with an escape character and a
left bracket, and are followed by a variable quantity of numbers
and letters. The terminal understands the meaning of these
codes, and acts accordingly by setting screen colors or moving
Graphics, as I mentioned previously, are the characters in the
IBM PC extended character set. They are characters beyond the
original 127 possible ASCII characters as defined by IBM in all
of their display adapters. These include single- and double-line
boxes, shading characters, international characters and
IBM Graphics characters have become enough of a de-facto
standard, that many other computers now emulate them. Many
terminal programs on the Apple Macintosh computer will allow
proper display of the IBM graphics character set, as will many of
the true display terminals on the market today.
That pretty well covers most of the common modem and
telecommunications program terms and standards in use today. I
hope this series of articles has made you better able to
understand the seemingly endless number of buzz words you find in
microcomputer communications. You should now be able to
understand better why terminal programs must be configured to
operate correctly. You also should be able (with information
from the first two parts of this series) to better choose the
type of modem you need to meet your applications. I hope you'll
let us know if you have any questions or need further help
understanding anything that I've already discussed. It's been
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