Guide to Modem Communication Standards V1.2
September 5, 1991
by Paul Gani
Several years ago, you probably purchased a 1200 or 2400 baud
modem. Today, you may have noticed that many of the BBSs you call
now use some sort of 9600 baud (or faster) modem. You savor the
claims of 1600+ cps and error free communication, but you don't
understand all this lingo about V.32, MNP, V.42bis, HST, etc...
This small guide is written to explain all relevant information
about those confusing terms, and to help you choose the right modem
for today and the future.
Please note that this article will only discuss dial up modems used
by BBSs & information services such as Compuserve. There are super
high speed modems available for mainframes using special high
bandwidth (T1) phone lines, but they are of no use to most of us.
There are/were two committees who set standards for modem
protocols, Bell Labs & the CCITT (a division of the United Nations,
English translation: International Telegraph and Telephone
Consultive Committee.) Today, Bell Labs no longer sets standards
for modems, although several of its protocols are still used. Thus
most modems today conform to some CCITT standard.
In addition, almost all modems today claim to be "Hayes
Compatible". This does not refer to any communication protocol,
but instead to the commands required to operate the modem. Since
it is virtually impossible to find a modem today that DOESN'T use
the Hayes command set, this should not be a concern no matter what
modem you buy.
First, we must take all of those Bell and CCITT numbers & separate
them into three categories:
1) Modulation Standards: Bell 103, Bell 212A, CCITT V.22bis,
V.29, V.32, V.32bis, HST, PEP, DIS
2) Error Correction Standards: MNP 1-4, V.42, Hayes V-series
3) Data Compression Standards: MNP 5, V.42bis, CSP
The first category, modulation standards, represents the actual
protocols which convert data to sound and visa versa. They are
implemented by hardware chips and cannot be emulated by software.
The second category, error correction standards, works ON TOP of
the modulation standards. Let's pretend that two PCs with modems
are talking, and instead of the PCs sending data directly to the
modem to be transferred, all data is sent to this intermediate
program (running on both PCs). This intermediate program transfers
the information surrounded by various "checksum" bytes. If
information is sent, and line noise corrupts it, the two programs
are smart enough to resend the data error free. If this sounds
suspiciously like XMODEM or KERMIT or ZMODEM, you're right! They
all work using similar methods. And just like XMODEM, etc... these
protocols are independent of whatever modulation you are using
(i.e. they will work with any modulation speed from 1200 baud+).
All of these standards can be implemented by PC software, but many
good modems implement them in their modem hardware.
The third category, data compression standards, is very similar to
the error correction standards. In fact - they work ON TOP of the
error correction standards. Before the error correction programs
send the data to the other modem, they are first compressed.
Again, these can be implemented either with software or built into
a modem's hardware.
Let's now describe each standard in detail:
Bell 103 - the U.S. & Canadian 300 baud modulation standard,
utilizing Frequency Shift Keying (FSK). The international
equivalent (though not compatible) is CCITT V.21.
Bell 212A - the U.S. & Canadian 1200 baud modulation standard,
utilizing Differential Phase Shift Keying (DPSK). The
international equivalent (though not compatible) is CCITT V.22.
CCITT V.22bis - the U.S. and international 2400 baud modulation
standard, utilizing Quadrature Amplitude Modulation (QAM).
CCITT V.29 - a 9600 baud half duplex modulation protocol, most
commonly implemented in Fax machines & Hayes 9600 baud modems
(ping-pong). V.29 modems of different brands seldom communicate
with each other (with the exception of Fax machines & boards).
CCITT V.32 - a 9600 baud full duplex modulation protocol, now
widely implemented by many modem manufacturers. Inter-brand
communication is common. Since a one way 9600 baud stream uses
virtually the entire bandwidth of the phone line, V.32 modems
implement "echo cancellation", which means that they cancel out the
overlapping signal that their own modems transmit, and just listen
to the other modem's signal. This procedure is complicated and
CCITT V.32bis - a 14400 baud full duplex modulation protocol, with
fallback to V.32. Not as common as V.32, because of price, but
also destined to become a popular standard.
HST - a 14400 & 9600 baud modified half duplex proprietary
modulation protocol used by U.S. Robotics. Very common in the BBS
world, but probably destined for extinction within the next few
years, as V.32 modems become more competitive in price. HST modems
run at 9600 baud or 14400 baud in one direction, and 300 or 450
baud in the other direction. This is an ideal protocol for
interactive sessions. Because echo cancellation circuitry is not
required, costs are lower.
PEP - a high speed modified half duplex proprietary modulation
protocol used by Telebit. Uses a multicarrier system which is
supposed to be superior at rejecting certain types of line noise.
Common in the Unix world, but probably destined to fade away.
DIS - a 9600 baud proprietary modulation protocol by CompuCom.
Utilizes Dynamic Impedance Stabilization (DIS), with claimed
superiority in noise rejection over V.32. Implementation appears
to be very inexpensive, but like HST, only one company makes modems
with the DIS standard.
MNP levels 1-4 - the Microcom Networking Protocol (MNP), an error
correction protocol developed by Microcom (since released to the
public domain.) At level 4, can increase throughput by 20% by
stripping start & stop bits. Implemented by hardware or software.
CCITT V.42 - an error correction protocol, with fallback to MNP
1-4. Slightly better; also can increase throughput by 20%.
Hayes V-series - a proprietary error correction protocol by Hayes.
MNP 5 - a data compression protocol which can sometimes double
throughput (i.e. at 2400 baud will Xfer up to 4800 bps).
Implemented by hardware or software.
V.42bis - a data compression protocol, with fallback to MNP 5. Can
sometimes quadruple throughput. A 2400 baud V.42bis modem might
advertise "9600 bps", but be forewarned - that is only possible
with highly uncompressed data. Likewise, many 9600 baud V.42bis
makers advertise "up to 38.4K bps". V.42bis is superior to MNP 5
because it analyzes the data first, and then determines whether
compression would be useful or not. MNP 5 always attempts to
compress the data, which slows down throughput on previously
compressed files (i.e. ZIP files.)
CSP - CompuCom Speed Protocol, a error correction/data compression
protocol available on CompuCom DIS modems.
There is already talk of a 19200 baud modulation standard -
tentatively dubbed V.high. But it is at least two years away.
Today, many 2400 baud modems come with MNP or V.42bis. They can
connect to 9600 baud or faster modems with MNP or V.42bis, but the
modulation speed will still be at 2400. Likewise, a HST V.42bis
can connect to a V.32/42bis, but only at their highest common speed
of 2400 baud. Of course, since they both have V.42bis, UP TO 9600
bps throughput is possible.
The BEST modem to buy to connect to as many BBSs as possible is the
U.S. Robotics Dual Standard (V.32bis/V.42bis/HST). But it costs
$720+ street price, whereas new V.32/42bis modems can be had for
$299+ street price.
It is probable that like 2400 baud modems, 9600 baud modems will
eventually drop to a couple hundred dollars or less. Several
"generic" V.32 modems can be seen in Computer Shopper for $299. So
far, all of them seem to use the same Rockwell daughterboard that
U.S. Robotics uses in their modems. Until we see some Taiwanese
clone V.32 chipsets, prices are unlikely to drop much further.
CompuCom 9600 baud DIS modems can be purchased for $149 though the
factory ($299 & up for modems with additional V.32/V.32bis
capability). The price for the base model is competitive even
against 2400 baud modems. Many large multi-line BBSs have
CompuComs on a few lines. However, its success has yet to be
Many modem manufacturers offer SYSOPs big discounts on their
modems. For example, U.S. Robotics offers their Dual Standard
(V.32bis/V.42bis/HST) for around $500 (normal list $1,295). Hayes
offers their Ultra (V.29/V.32/V.42bis) also for around $500 ($999
list), and Practical Peripherals offers their V.32/V.42bis modem
for $339 (list $699). Recently, CompuCom began offering V.32 and
V.32bis modems (with DIS capability) to sysops for $199 and $299
respectively! This appears to be an attempt at mimicking USR's
successful SYSOP program, hopefully making their DIS standard as
popular as the HST standard. These prices are available only to
established SYSOPs of BBSs. Normal street prices for these modems
are significantly higher.
Because of the various SYSOP programs, 9600+ baud BBSs are very
common today. Most have HST and/or V.32 capability, with an
increasing number supporting the CompuCom DIS standard. The
factory direct price for a CompuCom V.32/DIS modem is $299 - a very
attractive price compared to the $700+ cost of a USR Dual Standard.
There is big debate on whether one should buy a plain HST modem
from U.S. Robotics. They are very common in the BBS world
(currently more common than V.32), and they can be often purchased
used at a low price. Since the U.S. Robotics Dual Standard (with
V.32 and HST) appears to be the defacto standard among serious
BBSs, this implies that the HST protocol will still be around for
many more years.
Oh well. The End.
Any questions, comments, corrections (I KNOW I've made some
misstatements!), please contact me at:
Prog Corner: (301)-995-1508 8/N/1 V.32/MNP-5