
        
        MODEM 101: An Introductory Lesson in High-Speed Modems
        
                          by Nancy Hattaway
                    Sempervirens BBS, Concord, CA
                             415-685-0644

Jim, an avid bulletin board caller, wanted to upgrade from his 2400
baud modem to a high-speed modem.  He went to his computer dealer and
asked for the best 9600 baud modem, and purchased a Hayes V-series
Ultra Smartmodem 96, which is indeed one of the best on the market.
Jim felt that getting the best justified spending $800 on the modem.
However, when Jim got his new modem home and set it up, he found that
all of his connections were still at 2400 baud.  Many of the sysops
who ran the bulletin boards Jim called were as confused as he, but one
of them targeted the problem.  While Jim's modem was an excellent V.32
modem, the bulletin boards were using HST modems.  Jim wasn't quite
sure what this meant, but returned to his computer dealer and
explained the problem.  His vendor made him a fair compromise offer:
for returning the Hayes and $100, he sold Jim a U.S. Robotics Dual
Standard, which, although more expensive than the Hayes Ultra, can
connect to both V.32 and HST modems at high speed. Jim was very happy
with his new modem. However, he was not so happy when he saw a V.42bis
modem priced at $179, much less than he had paid for his V.32.  Jim
called the sysop who had understood his first problem and was told
that this time he had no problem, because a V.42bis modem with no
other designation was a 2400 baud modem, no faster than Jim's older
2400 baud.

Although the name was changed, Jim's story is true, and illustrates
one of the most common areas of confusion in today's telecommunication
field.  Jim faced several sources of confusion.  The designations
V.32, V.42bis and HST indicate the modem's abilities, but do not
describe them to the uninitiated.  Jim was also using the word "baud"
imprecisely without knowing it.  If a modem buyer doesn't understand
the jargon of modem descriptions, it is easy to spend a large amount
of money and purchase a modem that doesn't function with any more
efficiency than a $79 modem from a warehouse store.


                  
                  What makes two modems compatible?
                  

Modems speak to other modems.  In order to do so, the two modems in
question must speak the same "language", otherwise they cannot
communicate.  In the early days of modems, the "language" used by the
Hayes Microcomputer Products company was taken as a standard by most
modem manufacturers.  For medium speed modems (1200 and 2400 bits per
second, or bps), this is still the case.  Most of these modems use the
Hayes "AT" command set and speak freely to one another.

When manufacturers first began making high speed modems (9600 bps and
above), no clear standard evolved.  Several manufacturers developed
proprietary protocols such as HST and PEP (see below).  Proprietary
protocols are owned by the company which developed them, and thus none
of these modems could communicate with any of the other types at 9600
bps, because none of them spoke the same "language."

The United Nations, through the Comit Consultatif International de
Telegraphie et Telephonie (known as the CCITT), is charged with
establishing a recognized standard for high speed telecommunication.
The CCITT, based in Geneva, has defined many telecommunication
standards, some relating to modems, others to fascimile transmission,
and still others to packet-switching and other telecommunications.
All of the CCITT standards pertinent to modems are recognizable by
their "V.nn" designation.  One thing to keep in mind as you are
reading is that the "V-dot" protocols such as V.32, V.42, and V.42bis
are totally different standards, although they are commonly confused.
The -bis suffix also causes confusion.  It is easiest to think of -bis
as meaning "another protocol".  We'll describe each CCITT standard in
turn, then summarize the differences.

Before beginning, you should know that "baud" does not technically
refer to the speed of a modem, but to an aspect of how the
transmission occurs (more precisely, the number of changes of state in
the communication line per second).  The smallest unit of binary data
(0 or 1) is called a "bit", short for BInary digiT.  A 300 baud modem
utilizing a method known as frequency shift keying sends one bit per
baud and is therefore also a 300 bps modem.  A 1200 bps modem is
usually a 300 baud modem using a different method in order to transmit
four bits per baud.  Confused?  Just forget you ever heard the word
"baud" and use the initials "bps".

Another term used in modem descriptions is "duplex".  This term, when
used in reference to a modem, indicates whether or not data is
transmitted out and received in simultaneously at the designated
speed.  Modems using full duplex protocols can transfer data both
directions simultaneously at their rated speed.  Half duplex protocols
allow data to be sent in only one direction at a time.  A signal on
the end of the information tells the receiving modem that it is now
free to transmit.  "Asymmetrical" duplex indicates that information
flows in both directions simultaneously, but at different speeds.
"Adaptive" duplex means that the modems may transmit anything from
full to half duplex, depending on the situation.


            
            CCITT protocols: data transmission or "speed"
            

V.22:  You may rarely see a reference to the V.22 protocol.  Modems
using V.22 are almost universally called "1200 baud" modems rather
than V.22 modems.  This is a 1200 bps data transmission protocol.  A
data transmission protocol specifies the "modulation technique", or
method used to transfer the data.  It therefore dictates the fastest
speed at which information can be transferred.  Thus, data
transmission protocols are often called "speed" protocols.  In
addition to the rate of the character stream, data transmission
protocols define such things as methods used to limit the effect of
telephone line noise, so they are not technically just "speed"
protocols.

V.22bis: V.22bis is the data transmission protocol recommended by the
CCITT for 2400 bps modems.  The modulation technique used by V.22bis
modems transmits four bits per baud, and these modems typically
are 600 baud modems.  Four bits per baud at 600 baud is the same
as 2400 bps.  These modems are usually called 2400 baud modems,
which is technically incorrect.  From the consumer's standpoint, it
doesn't matter if your modem is a 600 baud modem transmitting four
bits per baud, or a 2400 baud modem transmitting one bit per baud.
Both have a "speed" of 2400 bps.  V.22bis is a full duplex protocol.

V.32: V.32 is also a data transmission protocol.  It is a 4800 bps and
9600 bps standard employing a method called trellis coded quadrature
amplitude modulation (TCQAM) at 2400 baud.  TCQAM encodes 2 or 4 bits per
baud and is a full duplex protocol.  Until the advent of V.32bis, V.32
was considered to be the standard for high-speed modems.  However, it
was introduced only after certain proprietary transmission protocols
had become well established, and thus has shared, but not dominated
the high speed data transmission market.

V.32bis: V.32bis is the newest data transmission protocol from the
CCITT, with final approval of the standard expected in spring or
summer of 1991.  V.32bis is a 14400 bps full duplex protocol, encoding
6 bits per baud at 2400 baud.  There are already several modems being
marketed which adhere to the proposed V.32bis standard.  It is
considered unlikely that major changes will occur prior to final
approval that would cause these "pre-approval" V.32bis modems to be
non-standard.

Thus, the "V-dot" protocols that determine data transmission or
"speed" are, in order from slowest to fastest, V.22 (1200 bps),
V.22bis (2400 bps), V.32 (4800 bps or 9600 bps), and V.32bis (14400
bps).  All are full duplex protocols.  The remaining "V-dot" protocols
do not determine "speed", but are concerned with error correction
(ensuring that the data received is an exact copy of the data sent)
and data compression (coding the data into a smaller form so that it
takes less time to send at the rated speed).


        
        CCITT protocols: error correction and data compression
        

V.42: V.42 is an error correction protocol.  V.42 uses a method known
as link access protocol for modems, or LAP-M.  It helps to ensure that
transmission of data is done without error.  Unlike the protocols
discussed above, V.42 does not relate to the speed of data
transmission, only to its correctness.  However, V.42 can decrease the
actual time for transmission at a given transmission speed.

This requires a little more explanation.  One way of measuring the
speed of data transmission is by characters per second, or cps.  Each
character consists of 10 bits, thus a 2400 bps modem has a theoretical
character rate of 240 cps (the number of bits per second divided by
the number of bits in a character).  In reality, this figure is closer
to 235 cps, because no transmission is totally efficient.  This rate is
commonly referred to as the "throughput" because it indicates how many
characters the modem can "put through" in a given time.  The 10 bits
in each character include 8 bits of data plus a "start" bit and a
"stop" bit.  The V.42 error correction protocol strips off the excess
start and stop bits and thus reduces the data load by 20%.  The actual
throughput increase is less, due to protocol overhead, but is about
15% (270 cps at 2400 bps with excess bits stripped off compared to 235
cps at 2400 bps without).

V.42bis:  V.42bis is a data compression standard.  Data compression is
commonly used to reduce the size of a file for storage or
transmission.  Smaller files naturally take less time to transmit.
V.42bis uses a method of compression called Limpel-Ziv encoding, which
typically can achieve a 4:1 compression ratio on an uncompressed
ASCII text file, meaning four times as much data can be sent in a
given time at a given transmission rate.  With V.42bis compression, a
V.32 (9600 bps) modem can achieve an effective transfer rate of 19200
bps.

However, many files have been compressed before they are transmitted,
usually so that they can be stored in less space on a hard disk or
floppy disk.  This process, commonly called "archiving", uses one of a
variety of proprietary or public domain compression techniques.  DOS
files that have been compressed usually have extensions such as .ZIP,
.LZH, .ARC or .GIF which identify the compression technique used.
If a modem tries to further compress a file that has already been
compressed, it actually increases the time needed for transmission.
Therefore, the V.42bis standard includes the ability to "sense"
pre-compressed files and disable the V.42bis compression for such
files.

V.42 and V.42bis are commonly confused with the other V-dot protocols.
Unlike V.22, V.22bis, V.32 and V.32bis, all of which define a data
transmission speed, V.42 and V.42bis have no effect on speed, but are,
respectively, error correction and data compression protocols.  The
modem Jim saw advertised for $179 was a 2400 bps modem with V.42bis
data compression ability.


               
               Proprietary data transmission protocols
               

The CCITT standards are, by definition, world-wide and non-proprietary.
However, other protocols exist that define data transmission, error
correction and data compression.  Proprietary data transmission
protocols in use in modems being sold today include the HST, DIS and
PEP protocols.  Proprietary error correction and data compression
protocols in use today include MNP level 6 and higher and CSP.

Proprietary high speed data transmission protocols are owned by the
companies which developed them and cannot be used by anyone else
without a license.  The cardinal rule to remember about data
transmission protocols is that two modems with different high speed
protocols will not be able to communicate with each other at high
speed.  However, since all these modems have a standard 2400 bps
protocol as a "fall-back" protocol, these modems will be able to
connect in most cases and communicate at 2400 bps.

HST:  The HST (High Speed Transmission) modulation is similar to
V.32 in the fact it uses a trellis coded quadrature amplitude
modulation (TCQAM) technique. Unlike the V.32, it is not full
duplex, but instead sends data at a maximum of 14400 bps in one
direction with either a 300 or 450 bps frequency shift keyed reverse
channel.  This makes it an "asymetrical duplex" modem. The main TCQAM
channel encodes up to 7 bits per baud, at 2400 baud, with one bit used
as parity, for the theoretical maximum of 14400 bps in one direction.
The modem will switch channels as the data demands.  The HST protocol
is proprietary to U.S. Robotics (USR).

The meanings of the initials USR and HST are often confused.  All HST
modems are made by USR, but USR makes other modems which do not use
the company's proprietary protocol.  For instance, USR makes a modem
which uses only their proprietary HST protocol, the USR Courier HST.
However, they also make a high speed modem that conforms to the CCITT
standard only, the USR Courier V.32.  A third, and very popular, modem
marketed by USR includes both the CCITT V.32 protocol and the proprietary
HST protocol.  This modem, the USR HST Dual Standard HST V.32, will
connect at high speed to an HST only modem or to a modem that has V.32
only, thus the name Dual Standard.  The logic for both protocols
actually exists side-by-side in the Dual Standard modems.  Recently,
USR has begun producing modems that utilize the new V.32bis protocol,
both as a single protocol modem and as a Dual Standard modem.

DIS:  The DIS protocol, like the V.32, V.32bis and HST protocols, uses
quadrature amplitude modulation to acheive a data transmission rate of
9600 bps.  The major difference between DIS modems and others,
however, is the method used to control "noise", or unwanted signals on
the telephone line.  A "noisy" telephone line can cause errors in data
transmission or even cause loss of carrier before the transmission is
finished.  The CCITT standards call for use of echo cancellation to
filter out unwanted line noise.  Echo cancellation  requires a digital
signal processor, which greatly increases the cost of the modem.  DIS
uses a method of improving the signal-to-noise ratio which does not
require the processor.  Thus, modems using the DIS protocol are
significantly less expensive than V.32, V.32bis, or HST modems. DIS is
a proprietary product of CompuCom Corporation.

PEP: The PEP (Packetized Ensemble Protocol) modulation technique,
which is a proprietary product of the Telebit Corporation, is totally
dissimilar to the protocols described above.  It uses a method called
dynamic adaptive quadrature amplitude modulation (DAQAM). Effectively,
it splits the phone line into 511 sections and puts a 34 baud carrier
on each section.  By encoding up to 4 bits per baud, PEP achieves
maximum speeds of 18000 bps (uncompressed).  Each channel can be going
only one direction, so full duplex operation can be had at 9000 bps.
PEP also utilizes adaptive duplex, which means that the speed over the
various channels will be determined by the data being sent. If data is
being sent in only one direction, it will transmit at 18000 bps in
that direction.  However, if full duplex is needed, data will be sent
at 9000 bps in both directions.  If traffic is heavier in one
direction than the other, the PEP protocol adjusts to ensure a maximum
data transmission rate. Modems using the PEP protocol are common in
systems using the UNIX operating system.


        
        Other error correction and data compression protocols
        

The most common non-CCITT error correction and data compression
protocols are those developed by Microcom, Inc.  These are all
indicated by the letters MNP (Microcom Network Protocol) and a number
which differentiates between the various MNP protocols.  MNP levels
2-4 (error correction) and MNP level 5 (data compression) are in
widespread use and are found on most high speed modems, including
those that meet CCITT standards.

The reason for this is that both the V.42 standard and the V.42bis
standard include an annex which requires that MNP levels 2-4 (for
V.42) or MNP level 5 (for V.42bis) be available as "fall-back"
protocols.  In the case of error correction, this ensures that if both
modems are not V.42 compliant, at least MNP level 4 error correction
will be used. MNP level 4 is similar to V.42 in that it strips the
start and stop bits from each 10-bit character, thus increasing
throughput by about 15%.  The MNP level 4 protocol includes MNP levels
2 and 3, which are also error correction protocols.

Note that some 2400 bps modems do not have any error correction or
data compression ability.  These are often referred to as "non-MNP" or
"non-error correcting" modems.  When such modems are used for data
transmission, software (such as the Zmodem file transfer protocol) is
used to provide error correction.  Modems which have at least MNP
level 4 error correction or V.42 error correction (which includes MNP
level 4 as a fall-back protocol) can instead use a file transfer
protocol such as Ymodem-G, which is faster because it sends the data
in a stream with no software-controlled error correction.
                                                         
MNP level 5 is the most commonly seen non-CCITT data compression
protocol, and is included in the CCITT V.42bis standard as a fall-back
protocol.  MNP level 5 differs from V.42bis in two important ways.
First, while V.42bis uses a 4:1 data compression protocol, MNP level 5
uses a method called run length encoding, which is only a 2:1 data
compression protocol.  Microcom did develop a 4:1 compression protocol
(MNP level 9) but that did not provide much of a challenge to V.42bis.

The second difference between V.42bis and MNP level 5 is that, while
V.42bis can "sense" previously compressed files and disable V.42bis
compression, MNP level 5 does not have this capability.  Attempting to
further compress an already compressed file slows transmission.
Therefore, if a modem with MNP level 5 (but not V.42bis) capabilities
is used mostly for previously compressed files, as is the case with
most bulletin board file transfers, MNP level 5 should be disabled on
that modem, usually by toggling a DIP switch or changing a software
setting.  If the modem is used for uncompressed text file transfers,
MNP level 5 should remain enabled.

MNP level 5 is often erroneously referred to as an error correction
protocol.  This is partially because modems tend to be referred to by
their most sophisticated feature.  Thus, an "MNP 5 modem" is
considered better than an "MNP 4 modem".  In fact, this is true,
because the MNP level 5 modems always include MNP levels 2-4.
However, it is imprecise to refer to a modem as an "MNP 5 error
correcting modem".  In such a modem, only MNP levels 2-4 have anything
to do with error correction, while MNP level 5 is strictly a data
compression protocol.  It is more precise to describe such a modem as
an "error correcting modem with MNP level 5 compression".

A non-CCITT, non-Microcom error correction and data compression
protocol called CSP (Compucom Speed Protocol) is used with modems
employing the DIS data transmission protocol.  These same modems offer
MNP levels 2-5 as fall-back routines for times when the DIS modem
connects to a non-DIS modem (at 2400 bps, as DIS is a proprietary
protocol).  CSP offers compression of up to 4:1 on noncompressed files
without apparent degradation of file transfers on precompressed files.
It is a proprietary technology of CompuCom Corporation.


                            
                            Summing it up
                            

Modem protocols can dictate the data transmission characteristics such
as speed and telephone line noise reduction.  Examples of this type of
protocol include V.22, V.22bis, V.32, V.32bis, HST, DIS and PEP.

Modem protocols can also provide error correction, at the same time
increasing throughput by about 15%.  Examples of this type of protocol
include V.42 and MNP levels 2-4.

Finally, modem protocols can compress data that has not been
previously compressed, which shortens transmission time by decreasing
file size.  Examples of this type of protocol are V.42bis and MNP
level 5.  The CSP protocol handles both error correction and data
compression for modems using the DIS data transfer protocol.


                        
                        Trademark Information
                        

"Dynamic Impedance Stabilization", "DIS", "CompuCom Speed Protocol",
and "CSP" are trademarks of CompuCom Corporation.

"Hayes V-series" is a registered trademark of Hayes Microcomputer
Products, Inc.
"ULTRA Smartmodem 9600", "Ultra" and "Smartmodem" are trademarks of
Hayes Microcomputer Products, Inc.

"Microcom Networking Protocol" and "MNP" are registered trademarks of
Microcom, Inc.

"MS-DOS" is a registered trademark of Microsoft Corporation.

"PEP" and "Packetized Ensemble Protocol" are trademarks of Telebit
Corporation.

"UNIX" is a registered trademark of American Telephone & Telegraph.

"USRobotics" is a registered trademark of U.S. Robotics, Inc.
"Courier", "HST", and "Dual Standard" are trademarks of U.S. Robotics,
Inc.
