Contents of the V42.TXT file
A close look at the Bell and CCITT standards
for modem communications
Sending and receiving files over modem connections is a routine
procedure for most persona computer users. It's not unusual however to
find modems that can't communicate effectively because of
compatibility problems--they don't all follow the same standards. For
users, just understanding modem standards can be problem. The maze of
modem standards grows constantly Look at modem advertisements and
you'll see a long list--Bell 103J, Bell 212A. V.22, V.22bis, V.32--not
to mention proprietary technology and protocols that are licensed by
individual companies .
These standards cover a variety of transmission speeds and such
features as error correction and data compression. The modem standards
in use today come primarily from three sources: Bell Standards, CCITT
Recommendations, or EIA/ TIA Standards. (For definitions and an
explanation of how modem standards are established, see the text box
"Where Modem Standards Come From" on page 354.) Table I shows the most
common modem standards for data rates of from 300 bps to 14,400 bps,
over leased-line and dial-up telephone lines.
The most common low-speed standards in use are the Bell 103J standard
for 300-bps transmission and the Bell 212A standard for 1200-bps
transmission. Almost every modem sold in the U.S. supports these
standards, either as the primary rate or as secondary fallback rates.
Fallback rates are used when the modem is unable to connect at higher
rates, usually because the telephone channel is too noisy to provide
error-free communication at that rate. For example, if a modem
attempts to connect at 2400 bps but determines that the line will not
support that rate, the modem may try to connect at 1200 bps or 300 bps
The Bell 103J and 212A standards are two-wire, full-duplex standards.
This means that modems that support those standards use ordinary
telephone lines, and they transmit and receive data in both directions
simultaneously. Even at 1200 bps (212A), the data rate is low enough
that the data channel for both directions of transmission can fit
comfortably within the 3000-Hz-wide voiceband telephone channel.
Because the CCITT was developing international standards during the
1960s (while Bell was defining U.S. standards), most 1 200-bps modems
in the rest of the world operator using a standard known as V .22 .
This is similar to the Bell 21 2A standard, but the carrier
frequencies at which the data channels are modulated are different.
Thus, V.22 modems and 212A modems are not compatible, unless special
design changes are in-corporated.
For 2400-bps transmission, most personal computer modems in use today
implement V.22bis. The Bell Standard for 2400-bps data was never
completely accepted because at the time the telephone company's
monopoly was dissolved, 2400-bps transmission wasn't yet perfected. As
a result, there is almost universal compatibility among 2400-bps
modems based on V.22bis.
Like the lower-speed standards, V.22bis is a two-wire (dial-up line),
full-duplex standard. To fit two 2400-bps data chan-nels in the 3000-
Hz-wide voiceband telephone channel, the data bits are encoded into 4-
bit bytes before transmission. Each data signal is then transmitted at
600 baud, and the two modem channels can again fit comfortably within
the telephone-line channel .
High-Speed Standards Grow
Prior to 1984, modem transmission at speeds above 2400 bps was
possible only by transferring the data over expensive fourwire
(leased) telephone lines. Special standards, such as Bell 208 for 4800
bps, V.29 for 9600 bps, and V.33 for 14,400 bps, were available for
use with these leased lines. However, only users needing to transfer
very large amounts of data could justify the cost of leasing the
telephone lines and buying the more expensive modems.
In 1984, the CCITT approved V.32 for use with standard dialup
telephone lines. V.32 leapfrogged from 2400 bps to 9600 bps,
representing a 4to1 increase in throughput over modems using V.22bis.
Using advanced technology to provide 9600bps transmission over
ordinary telephone lines, V.32 put the everyday personal computer user
in the highspeed data business for the first time by opening new doors
to sharing files and programs rapidly over modem connections.
The technology required to implement V.32 modems did no come easily.
The level of technical expertise needed in developing V.32 modems has
been conservatively estimated to b 100 times greater than for V.22bis
modems. As a result full, functional V.32 modems did not become widely
available until late 1986--two years after V.32 was adopted.
To send 9600 bps data, V.32 modems group the data into 4-bit bytes and
transmit them at 2400 baud. Since there is room for only one 2400 baud
data channel within the 3000-Hz-wide telephone channel, V.32 calls for
both modems to transmit over the same channel at the same time. Each
modem must then sort out its own transmitted signal from the signal it
is receiving from the other modem. To do this V.32 modems use echo
cancelers. Figure I shows a typical modem connection, with the echo
cancelers included in the modems at each end.
C I c >Transmitter Transmitter< I c C
O n i n i O
M t r t r M
T P e c e c T P
O U<->r u >> r u<->O U
T f i v v 4 c c v v f i T
E a t a i a t E
R c r Echo w r r Echo c r R
e y Canceler Hybridei r c Hybride Canceler e y
^r i u ^
^ e e i ^
Receiver - + + - Receiver
v v 2 wire v v
^ local ^
Local ++ loop ++ Remote
Hybrid circuits inside all modems are designed to match the
characteristics of the modem to the telephone line. Since the nature
of the telephone network changes constantly, this match is never
ideal. This results in part of a modem's transmitted signal being
reflected through the hybrid and back into the modem's receiver.
In addition, echoes of the transmitted signal from the hybrid circuits
out in the telephone network bounce back into the modem's receiver. To
get a good strong received signal, these reflected echoes must be
removed before the modem receiver processes its input.
The echo canceler, which is driven by the known transmitted signal,
models the echoes produced by hybrid circuits in the modem and the
network. The output of the echo canceler is subtracted from the
received signal before it goes into the modem receiver for processing,
thus eliminating the effects of the echoes. This is not a simple
task. The precision that is required in the echo canceler to remove
the echoes is substantial. Since the transmit signal is constantly
fluctuating with changes In the
data, the echo canceler must continuously adapt to those
changes as it mimics the transmitted signal's echo.
Since at any given moment a V.32 modem is transmitting more data than
a lower-speed modem, the individual V.32 signals are much weaker and
harder to detect. For this reason V.32 incorporates advanced coding
techniques such as trellis encoding. Trellis encoding allows the modem
to examine several consecutive received signals and look for known
patterns before deciding the value of the signal.
This memory effect can produce dramatic reductions in error rate. The
end result is that well-made V.32 modems produce very low error rates
and provide reliable, highspeed data transfer between modems. This
allows personal computer users to trade programs and download files at
rates unimagined in the early 1980s.
In an attempt to push technology barriers even further, the CCITT
began, in 1989 to study the idea of extending V.32 up to a 14,400-bps
rate. This standard was named V.32bis, since it represented an
outgrowth of V.32 rather than a new idea. V.32bis requires even better
echo cancelers than does V.32 also requires an overall improvement in
receiver quality. Testing has shown, however that 14,400bps
transmission over standard telephone lines is quite feasible with
V.32bis is expected to be formally approved by the CCITT by mid 1991.
Once adopted, V.32bis will open the door even wider for very fast data
transfer between personal computer A summary of new and evolving modem
standards and their status is detailed in table 2.
Data Manipulation Standards
With the basic modulation rates approaching the theoretical limits of
telephone line channels, modem makers and the CCITT have turned to new
ways of improving performance and increasing the data rates. The two
most important steps in this direction are V.42 for error correction,
and its companion, V.42bis for data compression .
The error correction and data compression functions are ap
plied to the data before modulation and stripped off before the modem
receiver decodes the data at the other end. An expanded view of these
functions inside the modem is depicted in
At high speeds, modems are prone to making more errors,
not only because of the reduced power in highspeed modem signals, but
also because they use the edges of the bandwidth (which tend to be
noisier) to carry data. V.42, formally approved in 1988, provides
error correction using the automatic repeat request (ARQ) principle.
Under ARQ, data is grouped into blocks at the transmitter and an
advanced cyclic redundancy check is applied across each block. This is
the same CRC concept already used to ensure the integrity of file
transfers in techniques such as XMODEM. The main difference is that
V.42 provides error corrected operation for all information exchanges,
not just file transfers using specific computer software programs.
Since the technique for checking the received data and re-transmitting
flawed blocks is contained directly in the modem itself, it is
completely transparent to the user and speeds up the transfer process.
The main drawback of V.42, as with any error correction technique, is
that when numerous errors are detected, the throughput rate suffers as
blocks of data are re-transmitted. However, this only comes into play
when errors are actually present, and even then the slowdown in the
transfer rate is a small price to pay for the capability to identify
and correct those errors.
Modems equipped with V.42 were originally introduced i late 1988 in
V.22bis products. It is now widely available b V.32 modems as well.
Data Compression with V.42bis
Approved in late 1989, V.42bis provides the first "official method for
compressing and decompressing data in modem (Several proprietary
compression techniques have been available for some time, the most
notable being Microcom's MNP level 5 technique.)
As with V.42, the CCITT adopted a technique similar those already in
use in the computer industry when it selected method for V.42bis. This
method is a variant of the Lempel-Ziv compression algorithm, the same
type of compression used in the familiar .ARC and .ZIP techniques.
However, instead of applying only to files compressed in advance,
V.42bis performs automatic, real-time compression ar decompression on
all the data flowing between the modem This can bring about dramatic
reductions in the amount of time needed to send and receive data. For
example, it is possible to achieve up to 4-to-1 compression ratios
with V.42bis. That could mean effective rates of up to 38,400 bps with
a V.32 modem or rates even greater than the 56,000 bps offered by
digital leased line service when used with a V.32bis modem. The
advantages of reducing the time required to transmit files across a
modem connection by a factor of four are obvious, especially if the
telephone call is long distance.
The amount of compression that V.42bis can actually provide depends on
the type of data being transmitted. Compression algorithms work by
recognizing repeated patterns in data and substituting shorter symbols
for them. This reduces the number of characters needed to represent a
given set of information. The more repetition a data file has, the
greater the compression. On the other hand, purely random data
contains no patterns at all, and it is non-compressible.
Figure 3 provides a comparison of how well V.42bis works on various
types of data. Assembly language and computer source code contain many
short, repeated commands, since the language has a limited command
set. As a result, data compression ratios on these types of files are
generally quite high. Conversely, pre-compressed files such as .ARC or
them through V.42bis usually does not provide much more improvement.
Data files that have been encrypted through a randomization process
will also show little reduction in file size and transmit time,
because the data has been pre-processed to remove identifiable
patterns. For the average personal computer user, however, V.42bis
should reduce modem signaling time and expense considerably.
V.42bis began appearing in modem products this summer, first in
V.22bis modems and later in V.32 modems. Many of the first V.32bis
modems will have V.42bis compression capability as soon as they hit
V.42bis relies on V.42 for its modem protocol and control
functions. Because of this, only those modems that have V.42 will
contain V.42bis. Fortunately, since V.42bis is a software
intensive technique, it doesn't require extensive modem re-design, and
most modem makers are offering it in their products at a minimal
increase in cost.
Standards to Watch For
The CCITT is continuing to develop new modem standards, pushing the
technology envelope a little further each time. A new effort is under
way tn standardize a 19,200bps modem. Another CCITT standard currently
under development will provide a uniform interworking procedure to
ensure that modems implementing a number of different standards can
For example, if a V.32 modem calls a V.22bis modem, the new
interworking protocol provides a way for the V.32 modem
to identify the receiving modem's standard and fall back to V.22bis
mode to match it. While many Modems are already
capable of this, there is no standardized format to ensure that all
modems do it in the same way. The new interworking standard should
improve compatibility by increasing conformity. Expect the new
interworking scheme to begin appearing in modems by 1991.
Another important standards issue that the CCITT expects to take up
soon involves interworking between cellular modems and regular
telephone line modems. There is currently no accepted way to guarantee
that these modems can communicate but with the explosive growth of
cellular technology and the increased mobility of laptop computers,
this will become a major issue in a few years. Hopefully, the CCITT
will finalize a standard to solve this problem soon.
Steven E. Turner is manager of technical staff research at UDS
Motorola (Huntsville, AL) which manufacturers modems. He participates
in several TIA and CCITT committees that develop modem standards. He
can be reached on BIX c/o "editors".
(c) Copyright 1990, Byte Magazine.