A BERLITZ COURSE IN HIGH SPEED DATA COMMUNICATIONS
A BERLITZ COURSE IN HIGH SPEED DATA COMMUNICATIONS
or
How to Speak Modem
by
Theodore M. Rosenberg
(301) 467-8988
2541 St. Paul St.
Baltimore, MD 21218
Prepared for:
The Columbia-Baltimore Users Group
and
Users of the PC_Tech Echo on EchoNet
Available on:
Columbia Online and
The Writers Block
Copyright 1990, Theodore M. Rosenberg, all rights reserved.
Non-commercial distribution permitted without charge as
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Rev 2.0 - July 14, 1991What is a Modem?
A modem is a Modulator - Demodulator, it is a device that allows
the transfer of digital data as an analog signal - such as over a
phone line.
Who Makes them?
About 70% of all modems available in the US are Rockwell
International chipsets or boards, with the named manufacturer
providing some firmware and the packaging. Most of the rest are
made by Motorola, with a few brave companies actually making
their own from scratch. This could change with AT&T entering the
market.
Who Sets The Standards?
CCITT: The Consultative Committee on International Telephone
and Telegraph is an international body of technical experts
responsible for developing data communications standards for the
world. This is a United Nations sponsored group, and its members
include representatives from major modem manufacturers, common
carriers (such as AT&T), and governmental bodies.
The CCITT establishes standards for modulation -- actual modem
signaling methods. It also determines standards for error
correction and data compression. It is possible (and likely)
that one modem might follow 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 that are primarily
modulation do include some of the higher layers (such as
negotiation) as well. Multi-speed modems may use several of
these standards.
CCITT standards for analog modems begin with "V.", digital
standards begin "X." pronounced "Vee dot" or "Ex Dot." Standards
also may end with a "bis" or "ter," meaning second generation or
third generation respectively. A common confusion arises out of
connector standards with similar numbers. The CCITT does NOT set
connector standards.
ANSI: The American National Standards Institute, is a US
standards development organization. The ANSI develops standards
for thousands of different areas, from architectural
specifications for the handicapped to computer programming
languages.
With regard to data communication, the term "ANSI" generally
refers to ANSI standard X.64 as implemented by IBM in ANSI.SYS.
The ANSI X.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.
ASCII: The American Standard Code for Information Interchange is
a standard that defines 128 different characters that can be
page 1used for data transmission. These include control characters,
letters of the alphabet, both upper and lower case, numbers, and
a full set of punctuation characters. Because there are only 128
ASCII characters, only 7 bits are required to form each of the
128 possibilities.
There are also versions of ASCII for less than 7 bits, more
commonly called BAUDOT, Correspondence, or "Navy."
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" characters, or "ANSI graphics".
In addition, some languages, (such as APL) also define the
extended characters for special purposes.
What do the various standards mean?
Modulation
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. For example the Bell 212A and V.22
modulation standards that both specify 1200 bps modulation, but
they work differently, and are not directly compatible.
Common Modulation Standards
V.21: a data transmission standard at 300 bps. This standard
is used primarily outside the United States. (300 bps
transmissions in the United States primarily use the Bell 103
standard). It is Full-Duplex and is Frequency Shift Keyed.
Bell 103: a 300 bps transmission standard. This standard is used
primarily inside the United States. (300 bps transmissions
outside the United States primarily use the V.21). It is Full-
Duplex and is Frequency Shift Keyed (FSK).
Bell 202: 1800 bps Asynchronous 4 wire leased line, or 1200 bps
Synchronous 2 wire leased line.
Bell 208: 4800 bps Synchronous leased line. 208A is 4 wire and
208B is switched.
Bell 212A: another 1200 bps transmission standard. This standard
is also used primarily inside the United States. (1200 bps
transmissions outside the United States primarily use the V.22
standard). It is Full-Duplex and is Phase Shift Keyed (PSK or
DPSK).
page 2V.17:a Half-duplex implementation of V.33, 14000 bps
transmission, now used by 14400 bps Group III fax.
V.22: a data transmission standard at 1200 bps. This
standard is also used primarily outside the United States. (1200
bps transmissions in the United States primarily use the Bell
212A standard). It is Full-Duplex and is PSK.
V.22bis: the international data transmission standard at 2400
bps.It is used both inside and outside the United States, and is
Full-Duplex.
QAM, Quadrature Amplitude Modulation allows modems to
increase speed from 1200 bps to 2400 bps.In QAM, each signal
represents four data bits.Both 1200 bps and 2400 bps modems
use the same 600 baud rate, but each 1200 bps signal carries
two data bits, while each 2400 bps signal carries four data
bits: 600 signals per second X 4 bits per signal = 2400 bps.
A technique known as Automatic Adaptive Equalization enables
2400 bps modems to adapt to phone line impairments call-by-
call. Essentially, if the modem is experiencing problems
with a noisy line, it looks for a "sweet spot" in the
bandwidth and attempts to avoid troublesome frequencies.
This technique makes 2400 bps modems more tolerant of line
noise than their 1200 bps counterparts that use compromise
equalization.
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 "semi-full-duplex," meaning that it
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. It is PSK.
V.26: 2400 bps Synchronous 4 wire leased line.
V.26bis: 1200 bps/2400 bps Asynchronous 2 wire leased line.
V.27bis: 4800 bps/2400 bps Synchronous 2/4 wire leased line.
V.27ter: 4800 bps/2400 bps Asynchronous 2 wire leased line. Also
used by Group III Fax 4800 bps/2400 bps
V.29: is a 9600 bps data transmission standard that 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 at 9600
bps/7200 bps. 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
(not faxes) from different manufacturers will not necessarily
communicate with one another. Prior to the development of Group
III Fax, V.29 was used primarily for 4 wire leased line, or short
haul transmission.
page 3V.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. This is generally considered
"the" standard for high-speed modems today. It is Full-Duplex
and uses Quadrature Amplitude Modulation and Trellis Coded
Modulation.
ECHO-CANCELLATION is the method V.32 uses to solves the
problem of overlapping transmit and receive channels. The
transmit and receive bands overlap almost completely, each
occupying 90 percent of the available bandwidth. Measured
by computations per second and bits of resolution, a V.32
modem is roughly 64 times more complex than a 2400 bps
modem.
V.32bis: is the new high speed standard. V.32bis operates at
14400 bps and, like V.32, will be a Full-duplex method. V.32bis
will be generally available by the end of 1991 and will rapidly
replace V.32 in general use.
V.33: A version of V.32 without echo cancellation, uses QAM
and TCM at 14400 bps, normally Synchronous 4 wire leased line,
but some two wire implementations exist.
V.35: Even though the number is higher, this is a supposedly
obsolete standard for high-speed (19200 bps) communication
originally used on leased lines. As I talk to manufacturers, I
am finding that this is still in use.
Hayes "V": Also called Hayes Ping Pong modulation. Hayes
also developed its own technology for high speed transmission.
Like the others Proprietary modems, Hayes "V" series high speed
modems only talk high speed to other Hayes modems.
HST: High Speed Technology is a proprietary method of U.S.
Robotics, it is not Full-duplex, and it does not support high
speed transmission in BOTH directions. Current HST modems send
data at 14400 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 bps, 1200
bps and 2400 bps operation.
PEP: Packetized Ensemble Protocol is a proprietary method used
by Telebit in their Trailblazer modem series. Like the HST, PEP
modems will only connect at high speed with other PEP modems.
PEP communicates at 20600 bps., the highest speed in general use.
PEP is based on a multi-carrier technique, the transmission
channel is divided into 512 independent, very narrow channels.
The main advantage is that no receiver adaptive equalizer is
needed because each channel is very narrow compared to the
overall channel bandwidth. The modulation rate in each narrow
channel can be changed somewhat independently. Trailblazer is
different from many other modems in that the decision to fall
back to lower speeds is built into the modem protocol, rather
than controlled by the user's computer port. Traditional
modulation systems would have to fall back in larger steps. But
there are three problems:
page 41. The turn-around delay is very long compared to
conventional modulation techniques because data must be sent
in large blocks. A typed character may take as much as a
half of a second to be echoed back to the system that sent
it. As a result, the system is not the best for interactive
online sessions.
2. The Trailblazer receiver cannot track carrier phase
jitter, Instead of canceling out phase jitter, PEP can only
respond by lowering throughput.
3. The ability to transmit at the maximum rate when subject
to some types of channel impairment is considerably less
than for conventional modems, HOWEVER, The multiple channel
technique offers extremely good immunity to impulse noise
(the most common) because the impulse energy is distributed
over narrow channels.
Due to the better overall performance of PEP, and the better
turnaround time of HST, US Robotics had captured a lot of the
general high speed traffic in the PC world, and Telebit captured
the majority of similar high speed traffic in the Unix world
prior to V.42bis.
Both US Robotics and Telebit both offer modems that have both the
proprietary and standard transmission options.
DIS : A relatively inexpensive, non-standard method of Full-
duplex 9600 bps communication is Dynamic Impedance Stabilization,
which improves the signal-to-noise ratio of the telephone line by
increasing the clarity and power of the signal, and automatically
compensating for impedance variations on the phone line. It has
a fallback rate of 7200 bps if too much line noise exists for
9600 bps communications. DIS is a proprietary method owned by
CompuCom, and is a companion to CSP data compression,(see below).
Error Correction
Most error correction would better be called error detection, it
refers to the ability to identify errors during a transmission,
and to automatically resend data that appears to have been
damaged in transit. If error correction is to be used, both
modems must adhere to the same error correction standard to make
it work. Most error correction is Automatic request for
retransmission (ARQ), in that the modem sends a block and a
cyclical redundancy checksum (CRC). The receiving modem
recalculates the CRC, and if it doesn't match, asks for a re-
transmit.
TCM: Trellis-Coded Modulation (with Viterbi coding) is an
optional error-correction method included in the V.32 standard.
TCM allows modems to check for transmission errors with a
redundancy bit, which results in fewer errors on noisy lines.
Trellis Encoding works WITH other error correction methods such
as V.42. Unlike other methods, TCM is true error correction, in
that for small errors, it does not force a retransmit, it
actually fixes them.
MNP - Microcom Network Protocol is a set of standards developed
by Microcom, and made available by them to other manufacturers.
page 5MNP Class 1 is referred to as Block Mode. It uses asynchronous,
byte-oriented, Half-duplex transmission. This method provides
only about 70% efficiency, and is rarely used today.
MNP Class 2 is called Stream Mode, and uses asynchronous, byte-
oriented, Full-duplex transmission. 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 and is rarely used today.
MNP Class 3 incorporates Class 2, and is more efficient. It uses
a synchronous, bit-oriented, Full-duplex method. This procedure
yields throughput about 108% of that of a modem without MNP,
delivering about 254 cps at 2400 bps.
MNP Class 4 uses Adaptive Packet Assembly and Optimized Data
Phase techniques, it improves throughput and performance by about
5%, although actual increases depend on the type of call (noisy
or clean), and can be as high as 25% to 50% on some links.
V.42: is a CCITT error-correction standard that's similar to MNP
Class 4, 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
LAPM : Link Access Procedure for Modems, 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.
LAPB : Link Access Procedure Binary, like MNP or LAPM, 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. Used with X.25.
LAPD : Link Access Procedure Direct, like MNP or LAPM & B, but
for internal or leased lines.
Data Compression
Data compression refers to the ability in some modems to compress
the data they're sending, squeezing data to a smaller size as it
is sent. This saves time and can result in considerable money
saved by long-distance modem users. Not all types of data can be
compressed by the same amount, but gains can nearly always be
realized. Raw text files will allow the highest increase, while
program files cannot be compressed as much and the increase in
transfer speed will be less, graphic files even less, and
compressed files such as ARC and ZIP, cannot be compressed at
all. Current compression methods are still improving, with
Bell's application of B-Trees and Brents application of Huffman
encoding to LZ methods. The latest methods (Fiala-Greene TRIE
data structures) are not yet in commercial form, but promise
faster and greater compression.
MNP Class 5 is a Data Compression protocol which uses a real-time
adaptive algorithm. It can provide up to 2-1 compression, On
page 6pre-compressed data MNP 5 can actually EXPAND the data and
performance can actually decrease. For this reason, MNP 5 is
often disabled on BBS systems. MNP 5 uses a form of Huffman
Coding and a dynamic dictionary algorithm. Huffman Coding
replaces ASCII with a variable length "minimum redundancy code"
with the length of the character based on the frequency of
occurrence. For example the code for the most common character
"E" is 00, and the code for the much less common character "D" is
11000.
MNP Class 7 provides Enhanced Data Compression. When combined
with Class 4, it can obtain about a 3:1 improvement in
performance. It is designed primarily for use with a 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 be seen in the future.
V.42bis: a CCITT data compression standard similar to MNP Class
5, but providing about 4-1 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.
V.42bis uses Lempel-Ziv encoding (like the PKZIP and LHARC
programs) which is a dynamic dictionary algorithm which assigns
codes to repeated strings in the actual file being compressed,
rather than theoretical forms.
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.
Most V.42bis modems also support MNP5, however it is not required
by the standard.
CSP: CompuCom Speed Protocol offers compression up to 4:1. CSP
is a proprietary method owned by CompuCom, and requires DIS.
Fractal: I am unable to locate the material at present, however a
firm has developed a unique data compression algorithm
specifically for graphics files, using fractal analysis methods.
This method is currently only available in software (like zip,
arc, etc. for data) but, when I spoke to the developers a few
months ago, they said that they were in negotiation with a modem
manufacturer to provide a hardware solution for people who need
to transmit large amounts of graphics data regularly. They were
also hoping to provide a better alternative to V.29 Group III
Fax. A Fax is just a graphics file. This is especially
appropriate, as all modern data transmission is based on Benoit
Mandelbrot's early work on the nature of telephone line noise.
His discovery that line noise could be described as a Cantor set
was one of the first steps in the development of fractal
mathematics.
Fallback Negotiation
page 7If the line noise is high enough, more data will get through with
a lower speed than with the high speed and retransmission.
V.32(bis): includes an optional fallback to 4800 bps under
conditions of extreme line noise.
MNP Class 10: is a fallback protocol developed by Microcom, which
provides for small movements down (and back up) in speed under
poor line conditions. It has been recently licensed to Rockwell
International. Since Rockwell makes at least 70% of all modem
chipsets sold in the US, it will rapidly become a de-facto
standard in all US Modems.
Connection Negotiation
Connection 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 each modem standard. It should be noted here that the
tones sent to originate a call are usually different than those
sent to answer a call. Some modems can originate a call forcing
an answer set of tones, or vice-versa.
Most negotiation standards are included in the base standard, the
only separately named one that I have seen is Annex A, the method
used by V.42 modems to fall back from V.42 to MNP if the other
modem is not a V.42, and then to non-corrected if the responding
modem is not MNP.
The fancier the modem, the longer it can take to negotiate a
connection. If you stop and think about this, it is obvious,
with more choices, the two modems will spend more time finding
the best combination.
WARNING: With most high speed modems you must "lock" your port
speed at a set speed ( as high as your system will allow). The
modems will negotiate the best speed that they actually use, and
may change it as line conditions dictate. If your communications
software is set to detect baud rates, the software will also try
to change, but the modems will ignore it and you will lose
contact with your own modem.
Duplex
Duplex 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 modems are pseudo Full-duplex. This means they cannot
transmit data at high speed in both directions at the same time
page 8because they are really operating in a fast turn-around Half-
duplex mode internally.
Bits and Parity
Data Bits: 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, 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
data. Some systems have even 5-bit and 6-bit data, and use
character sets such as BAUDOT and Selectric, but these systems
are uncommon today. For some reason, TTY's for the deaf use the
extremely obsolete BAUDOT codes, making them out of step with the
rest of the communications world.
Parity Bit: 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 the receiver.
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.
page 9In 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
involved.
Asynchronous: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 bits.
Synchronous (or bisynchronous): 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, null 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, it is used
is with high speed modems. When these modems use MNP or V.42
protocols they use synchronous communications between the modems
themselves. However, you still use asynchronous communications
between the computer and the modem.
There are software emulations of Synchronous communications, such
as Flashlink from Cardinal, or SynchUp from Motorola that will
increase thruput on lower speed modems.
Flow Control
Flow control refers to the 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.
Hardware flow control is not always required. It is generally
needed only with high speed modems. Hardware 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.
CTS/RTS: Clear To Send - Ready To Send is one form of Hardware
flow control.
DSR/DTR: Data Set Ready - Data Terminal Ready is another form of
Hardware flow control.
page 10
V.24: is the CCIIT standard for, not only flow control, but
all of the communications between the modem and the serial
interface.
X-ON/X-OFF: 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 can be used by
either the user or the application program to start and stop data
transmission by using control keys. <Ctrl>S to temporarily halt
data flow, and <Ctrl>Q at any time to restart data flow.
Software flow control has two problems;
1) Due to the time it can take for an X-off to reach the
processor, data can overrun the buffers. To prevent this,
X-off is normally sent at 80% of capacity. At high speeds,
this may not be enough.
2) It is easy to have an accidental X-off triggered, either
by a code in a binary file, or by operator error. On many
types of keyboards it is surprisingly easy to accidently hit
a <ctrl>S. Anyone who has worked on a large system will
have run into the mysterious intermittent terminal lock up
problem.
Commands
To tell the modem what you want it to do, you have to be able to
communicate to the modem itself. There are two common ways to do
this:
AT: The common US command set was developed by Hayes, and is
called the "AT" command set because commands begin with the
prefix "AT". If a modem uses this set, it is usually advertised
as "Hayes compatible". If it doesn't use this set, it probably
is not sold in the US.
V.25bis: This is approximately the European equivalent of the
Hayes set. It includes auto-dial from NVRAM.
V.54: Actually not a command set, this is the specification
for a "loop-back" test to test the modems performance and status.
X.25: This command set is a special set, used to communicate
with a "PAD" or Packet Assembler Disassembler. A PAD allows you
to communicate simultaneously with more than one system. Put
simply, it puts data going to each destination into a "packet",
tags it with an address, and ships it off. Incoming packets are
sorted and sent to each actual and "virtual" destination.
When you are on-line to any major network (such as
Timenet,Telenet, Sprint, PDN, etc) you are actually hooked
up to a PAD. If you have X.25, and the right communications
software, you can do more than one thing simultaneously,such
as download, read mail, and upload at the same time.As far
as I know, Hayes and TL Systems are the only general market
modem companies to offer X.25 at the present time, more WILL
follow in coming years.
page 11
All RBOC's, and most, if not all, long distance carriers and
independent telephone companies offer X.25 dial up service.
Short Haul
Some modems are sold as "short haul" modems, these differ from
regular modems in that they are really a type of repeater used to
boost the signal on long runs of serial cable. Short haul modems
need not use any common protocol as they are always Full-duplex,
used on dedicated cable, and in pairs. In addition, they must
provide the power to send the signal along the line to the other
end. On the other hand, they don't need error correction,
compression, connection negotiation, fallback, or any of the
other stuff I've been discussing, they just squirt the data down
the line, and demodulate what is coming back.
Point To Point / Switched
You may sometimes see modems called "point to point" or
"switched". These modems are for use on leased or dedicated
lines. A pure point-to-point modem may actually be a short-haul,
or it may be too dumb to survive a switching network. A switched
line modem is very similar to a regular dial-up modem, but is
intended for use on leased or internal lines.
More Buzzwords
PSTN : Public Switched Telephone Network - regular voice dial-up
service.
ISDN: Integrated Services Digital Network - a new switched
digital high-speed system expected to eventually replace the
current analog PSTN.
RS-469 (A or B): A specification for testing modems, adopted by
the Telecommunications Industry Association and the Electronics
Industries Association.
NVRAM: Non Volatile Random Access Memory, on board memory which
does not require power to keep data. It is used to store phone
numbers and setup information.
What's New
New products starting to arrive are:
The Pocket Modem: a very small external modem (currently only
available at 2400 bps) which takes its power off of the serial
line. It can be put in your pocket and shifted from system to
system. It is mainly used with laptops or smaller systems.
The Cellular Modem and/or FAX: a combination Cellular Telephone
and modem. These have two uses: The most obvious is for travel,
but some are being made to function as backups for WAN's, so that
even if the phone lines go down, the system stays up.
The Gateway modem: These operate directly off of LAN's, and do
not require a communications server - just plug in the ethernet
cable.
page 12What's Coming
A new high-speed protocol will probably be released in 1991
(V.32ter ??) the main disagreement now seems to be whether it
will be 19200 bps, 24000 bps, or 25600 bps. It will take at
least a year after that for any of these modems to reach the
market.
V.17, 14400 bps Group III fax is out, but few manufacturers are
offering it now, within the next year, it will replace 9600 bps
fax on most new purchases - at lower cost.
2400 bps modems are now where 1200 bps modems were two years
ago - within the next one to two years, no one will be making
them, and 14400 (or higher) will be the standard.
page 13
Litigation
As far as I can ascertain, there is only one current major suit
in the data communications area (unlike the rest of the computer
field). A few years ago, Hayes sued some modem manufacturers
over the use of one element of the Hayes AT command set, the only
part that they had not released into the public domain. That was
the programmable escape code, i.e. the ability to CHANGE the
escape codes with software. I was not able to get any
information from Hayes, but others in the industry said that the
suit seemed to be slowly dragging on. Recent news reports,
however, seem to indicate that the action on this suit is heating
up.
Licenses
The V.42bis is an unusual standard in that, while it is an
international standard, the Zemple-Lev-Welsh (LZ84) Compression
algorithm that it uses, infringes on patents held by IBM, British
Telcom, and UNISYS. To produce a legal V.42bis modem, one must
have licenses from all three.
I was unable to get details of licensing arrangements from either
Unisys or BT, but IBM is probably rather typical in their
approach. Unlike their usual closely protected licenses, IBM
will make its patent rights under patent #4814746 available to
ANY modem manufacturer for either.
1) A one-time payment of $20,000, or
2) A royalty of 1% of V.42bis sales, or
3) If you have anything IBM is interested in, or are doing
other business with them, as part of a negotiated license or
cross-license agreement.
MNP-1 to MNP-4 have been put in the public domain by Microcom, as
has most of the Hayes AT command set.
MNP-5 and MNP-10 require licenses from Microcom, and, to actually
build a saleable modem, licenses MAY be necessary from AT&T,
Motorola, and/or Rockwell International.
The result of this is to make business easier for Rockwell and
Motorola, aside from their continuing lead in technology, they
have the legal staff to keep everything straight, a rough job for
a start-up or small company.
Pricing
I remember RENTING 100 bps modems for $45/month from AT&T, later
I bought a high speed (300 bps) modem for $1,200. Now, 1200 bps
modems are dead, almost no-one makes them, and they cost MORE to
make than 2400 bps modems.
A 2400 bps internal modem should cost under $70, street price,
with MNP-5, maybe $95. A 9.6 kbs V.42bis modem can be bought for
$300-500. External modems cost a little more due to their case
and power supply.
page 14
Modem prices should continue to drop, with many observers calling
for V.32bis\V.42bis internal modems available as low as $100 by
the end of 1992. Fax capability $10-$25 extra.
This is due in part to:
1) The cost of making modems has plummeted. If you look at
an old 300 bps modem, you can see that it is MUCH larger
than a new high-speed modem, it has a LOT more chips, and
lots of circuit traces. A new modem uses LSI low power
chips.
2) There are a lot more modems sold today, and with new
services such as Prodigy, and IBM's decision to make modems
standard on the PS/1, more people are becoming aware of
modems than ever before. You can spread the costs of ramp-
up over millions of units, rather than thousands.
3) The promised entry of AT&T into the market as a
competitor to Rockwell creates more competition.
Tables:
Signal
Baud BITS/ Constell Type
Modulation Bits/Sec Rate Symbol Points
v.32bis TCM 14,400 2400 6+TCM 126
v.32bis TCM 12,000 2400 5+TCM 64
v.32 TCM 9,600 2400 4+TCM 32
v.32 QAM 9,600 2400 4 16
v.22bis QAM 2,400 600 4 16
Bell 212A QAM 1,200 600 2 4
Bell 103 FSK 300 300 1 -
page 15
Thanks to:
Dick Checket and Richard McCarty of A T & T Paradyne
Ed Prentiss of Microcom - Racal Vadic
Steve Mills of UDS - Motorola
Peter Theune of Johns Hopkins Applied Physics Lab
MICC technical support (probably Casey Garrigan)
Rich Blatt - Octocom Systems
Special Thanks to:
Mitch Baker, Senior Engineer, Rockwell International
Gary Sanderson, Chief Engineer, Cardinal Industries
John Lowe, Intellectual Property Attorney, and Commercial
Relations program manager for V.42bis, IBM CHQ
No Thanks to:
Hayes, who could only send me the regular handouts, no
useful information, and couldn't put me in touch with anyone
but untrained marketing reps.
Unisys, who after three weeks of passing me through their
system, never located anyone who knew anything about either
modems or patents.
Bibliography:
Sales materials, users manuals, technical manuals, etc. for about
50 different modems, and lots of columns and articles,
particularly in PC Week (Ziff Davis).
A Comparison of High Speed Modems, Mike Ehlert
SysOp: PACIFIC COAST MICRO BBS
Taking The "Buzz" Out of Buzz Words, Alan D. Applegate
eSoft Possibilities Newsletter, June, July, and August 1990
issues. A monthly customer support publication of eSoft, Inc.,
Aurora, Co
While I did not receive it until after Rev 1.4, recommended
reading includes:
Racal-Vadic's 1991 High Speed Dial-Up Modem Handbook
Racal-Vadic Communications Group, Milpitas California
page 16Sources
Manufacturer Toll Free Charge Call Support BBS
=================================================================
Anchor Automation 1 (818) 998-6100
American Mitec 2 (800) 648-2287 (408) 432-1160
Anderson Jacobson (408) 435-8520
ATI Technologies 2 (416) 756-0718
Best Data Prod.1 (800) 632-2378 (818) 773-9600
Black Box Corp 2 (412) 746-5500
Cardinal Technologies 2 (717) 293-3800
CMS Enhancements 1 (714) 222-6000
Codex - Motorola 3 (800) 426-1212 (508) 261-4000
Compucom 4 (800) 228-6648 (408) 732-4500 (714) 946-9337
also Educational Progr. (918) 224-0065 (918) 224-0005
Computer Perip. 2 (800) 854-7600 (805) 499-5751 (805) 499-9646
Data Systems, Inc.1 (708) 459-8881
Digicom 3 1 (800) 574-2730 (408) 262-1277
Dove Computer 1 (800) 622-7627 (919) 763-7918
Dowty Comm. 2 (800) 227-3134 (301) 317-7710
Everex Systems 1 (415) 498-1111
E-Tech Research 2 (800) 328-5538 (408) 730-1388
Farallon (415) 596-9100
Fastcomm Comm 2 (800) 521-2496 (703) 620-3900
Forval America 3 (800) FORVAL-1 (801) 561-8080
General Datacom 2 (203) 574-1118
GVC Technologies2 1 (800) 289-4821 (201) 579-2702
Hayes Micro 5 1 (800) 241-9625 (404) 441-1617 (800) 874-2937
Image Comm. 2 (800) 666-2496 (201) 935-8800
Inmac 2 1 (800) 547-5444 (408) 727-1970
Intel Corp 2 (800) 538-3373
Logicode 2 (818) 879-0533
Magic Modems (800) 622-3475 (512) 343-3421
Mastercom (213) 834-6666
Memotec DATA 3 (800) 423-6144 (508) 681-0600
MICC 2 (800) 289-6422 (408) 980-9565
Microcom Inc. 2 1 (800) 822-8224 (617) 551-1000
Micro Electronic 1 (508) 435-9057
Multi-Tech Syst. 2 (800) 328-9717 (612) 785-3500
NEC America 2 (800) 222-4632 (408) 433-1250
Octocom Systems 3 (508) 658-6050
OmniTel Inc. 3 (800) 666-4835 (415) 490-2202
Outbound Systems 1 (800) 444-4607 (303) 786-9200
Paradyne-A T & T (800) 482-3333
Patton Electron 1 (301) 975-1000
Penril DataComm 3 (301) 921-8600
Practical Perip.2 1 (800) 442-4774 (818) 706-0333
Prometheus Prod.2 1 (800) 477-3473 (503) 624-0571
Racal Data Comm.2 (800) 722-2555 (305) 846-4942
Racal-Vadic 2 (800) 482-3427 (408) 432-8008
Racal Milgo (800) 327-7909 (305) 846-1601
Shiva 6 (800) 458-3550 (617) 252-6400
Spectron Celluar 1 (214) 630-9825
Telcor Systems 2 (800) 826-2938 (508) 651-0065
Telebit Corp 7 1 (800) 835-3248 (408) 734-4333
Telenetics 1 (800) 826-6336 (714) 779-2766
Toshiba Inc. 2 (212) 682-2595
Touchbase Sys. 1 (800) 541-0345 (516) 261-0423
TL Systems 8 (508) 970-1295
UDS - Motorola 2 (800) 451-2369 (205) 430-8000
page 17
Manufacturer Toll Free Charge Call Support BBS
============ ===============================================
US Robotics 9 1 (800) DIAL-USR (708) 982-5001 (708) 982-5092
Ven-Tel 2 (800) 538-5121 (408) 463-7400
Vocal Tech. 1 (203) 356-1837
Western DataCom 2 (800) 262-3311 (216) 835-1510
1. Pocket or Cellular Modem.
2. V.32/V.42/V.42bis
3. V.32/V.32bis/V.42/V.42bis
4. CSP/DIS
5. V.32/V.42/V.42bis, X.25, and "Hayes Ping Pong"
6. V.32/V.42/V.42bus - Running under AppleTalk and
Novel IPX. Ethernet port only, not serial.
7. V.32/V.42/V.42bis and PEP.
8. V.32/V.42/V.42bis and X.25
9. V.32/V.32bis/V.42/V.42bis and HST.
Rev List.
1.1 Added leased line material
1.2 Updated Mfgrs names & numbers
1.3 Added CTS/RTS and DSR/DTR specifics. Changes in V.35, V.29,
V.27ter definition. Added Point-Point/Switched and Short
Haul, Port locking.
1.4 Added V.17, V.24.
1.5 More info on AT&T.
2.0 A lot of small corrections and additions; Baud/BPS/signal
table, more data on LZ & Huffman coding, Fiala Greene,
Brent, & Timothy Bell, additional buzzwords, more sources,
whats coming, whats new.
page 18
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