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This is an edited and condensed excerpt from The Modem
Reference, written by Michael A. Banks and recommended by the
Associated Press, The Smithsonian Magazine, Jerry Pournelle in
Byte, et al.
The right to reproduce this article is granted on the
condition that all text, including this notice and the notice at
the end of the article, remain unchanged, and that no text is
added to the body of the article. Thanks! --MB
Copyright (c), 1988, 1989, 1990 Michael A. Banks
All Rights Reserved
(From Chapter 3)
HOW TELECOMPUTING WORKS
You don't have to understand how something works to use it,
but understanding sure makes things easier--especially when
trouble pops up.
This is true of using any complex device or system--be it an
automobile, a VCR, or a library. And it's especially true of
using a computer in any application. When you understand even a
little of what's happening "behind the scenes," as it were,
you'll find that you have a lot more control over what's going on
with your equipment and software.
Which brings us to my purpose in writing this excerpt: I
want you to have a solid picture of what's happening when your
computer communicates with another system.
We'll first examine the basic elements of telecomputing,
followed by data formats. Then, we'll segue into how data are
transmitted (focusing on hardware and software elements), and
wrap up with communications parameters and error control
protocols. (And, yes--you can understand all this "technical"
stuff; what one human creates, another human can understand, if
he or she so desires.) You won't have to pull out engineering
texts to keep up with me. I'll explain terms and concepts as
necessary, and you can supplement those explanations with the
Glossary of Terms in Section Four of THE MODEM REFERENCE.
If you're relatively new to telecomputing, read this chapter
from start to finish. Otherwise, you may skim the headings, if
you wish, to find only the information you need to fill the gaps
in your telecomputing knowledge.
TELECOMPUTING BASICS
As indicated in Chapter 2 of THE MODEM REFERENCE,
telecomputing (also called data communications or microcomputer
telecommunications) is the transfer of data of any type between
two or more computers via a transmission link.
Most telecomputing (especially telecomputing involving dial-
up systems) goes like this:
Computer A transmits binary data (also known as digital
data) to a modem, in the form of a sequence of bits. The modem
converts the bits to an analog signal which mimics the
distinction between the binary 1s and 0s. The analog signal is
then transmitted over voice-grade telephone lines.
At the receiving end, a modem connected to computer B
converts the analog signal back into a binary signal that is
basically a copy of what computer A sent to its modem. The modem
then sends the binary signal to computer B.
At this point, Computer B has in its memory a duplicate of
the data Computer A originally sent to its modem. (This process
works in both directions, of course.)
This is a greatly simplified description of data transfer
from one computer to another via modem, but it should give you a
picture of how the basic elements of telecomputing (as described
below) interact. (If any of the terms used in the description
caught you off guard, don't despair--they are fully explained in
the following pages.)
Basic Elements of Telecomputing
Successful telecomputing involves four major elements: data,
data terminal equipment, data communications equipment, and a
communications link. Various sub-elements are also involved, as
described in the text following.
Data. Computer data is defined as machine-readable
information of any kind. The information may consist of business
or personal messages, other kinds of text files (articles,
contracts, documentation, reports), spreadsheets, "lines" of a
realtime conference, graphic images, data base files, executable
programs in binary data format, data files for programs, etc.
Data are handled within a computer as binary information,
and may be entered in "real time" from a keyboard, retrieved from
a mass storage device like a disk, generated by a program's
operation, or received via an interface from an external source.
By the way, this chapter focuses more on the format and mode
of data transfer--before, during, and after transmission--than on
its content. The successful delivery of data with its content
intact is of course the primary goal of data communications but,
as you'll learn, data format in large part determines the mode--
and success--of transmission.
Data Terminal Equipment. As imposing as the phrase may
sound, Data Terminal Equipment (DTE) is nothing more than the
computers or terminals used in telecomputing--the source and
destination of data. The use of the word "terminal" in the
phrase alludes to the fact that the computers or terminals
involved are the beginning and ending points of data
transmission--which is just what a terminal is: a place where a
journey begins and ends, ala a train or bus terminal.
Communications Links. Computer-to-computer data transfer
may take place in a variety of ways. In its most basic form,
telecomputing involves linking two computers directly, using what
is called a "null-modem cable" to connect their serial ports. In
this kind of setup, the computers are only a few feet apart, and
data are transferred in true binary format. (Appendix E contains
instructions on making a null-modem cable.)
The majority of telecomputing activities, however, take
place via telephone lines--either the public voice telephone
system or what are known as "dedicated" telephone lines as the
communications link.
Although it is the least expensive and most readily
available channel of data transfer, the voice telephone system
cannot handle computer data in its native binary format. Thus, a
modem must be used to translate data from binary (or "digital")
format into analog format--a format that can be successfully
transmitted via ordinary phone lines.
(There is another type of phone system that does accommodate
direct binary data transmission. Called a digital network, it is
used in a applications where extremely speed and accuracy are
necessary--but setting up and using such a system is an extremely
expensive proposition.)
A telephone link does not, by the way, consist solely of
telephone wires. Electro-mechanical, electronic, and computer
switching equipment is involved, as well as microwave
transmitters and satellite up- and down-links. For the purposes
of this chapter, however, such elements will remain transparent.
Data Communications Equipment. Simply put, Data
Communications Equipment (DCE) consists of modems and their
associated interfaces, connectors, and cables.
Modems have a number of functions, including but not limited
to:
* establishing and maintaining a communications link
* translating data from digital to analog format, and vice-
versa
* transmitting and receiving data
The interface between a modem and its computer is typically
a serial interface (specifically, an RS-232C interface), although
some few modems use a parallel interface. For the dial-up
applications discussed in this book, you'll probably use a serial
interface.
DATA FORMATS AND DATA TRANSFER IN COMPUTERS
As noted above, computer data must be converted from digital
to analog format before it can be transmitted via voice telephone
lines.
Before you can understand how data are physically converted
and transmitted over communications links, however, you need to
understand how computers handle data internally.
This section presents important information about digital
data organization and handling in both your computer and portions
of a communications link. Basically, I'm going to show data to
you as your computer sees it.
Computer Data Format
Continuing improvements in computer hardware and software
over the years (as well as competition among various
manufacturers) have made for a veritable Tower of Babel when it
comes to mainipulating, storing, and transfering information.
The differences in how data are stored on, say, an IBM AT quad-
density disk versus a Commodore 128 disk are so great that there
is no possible comparison of the two (and there's certainly no
way to transfer data between machines using one machine's disk
and the other machine's drive). Too, how various brands and
models of computers handle data internally differs substantially.
Fortunately, all modern computers have one thing in common:
they handle data in digital format (which is why they're called
digital computers). This means that they "see" data characters
as strings of binary digits.
Note: The terms "digital" and "binary" are
sometimes used interchangeably. "Digital"
refers to discrete, uniform signals of any
type--binary or otherwise--that do not vary
in a continuous manner. Rather, such signals
are identified by specific levels or values
such as "on" or "off." Digital signals
change immediately from one state to another,
and are the antithesis of analog signals.
Because we are concentrating on binary data
and binary signals in this book, "digital"
refers to binary data and signals.
Further, almost all computers (some mainframe computers
excepted) use the same numeric code to represent each character--
numbers from the American Standard Code for Information
Interchange (ASCII).
As you'll see in the following pages, computers that share
these attributes can easily exchange data.
Binary data. Even if you're not technically oriented, I'm
sure you've at least heard rumors about data being stored and
manipulated by digital computers in something called "digital" or
"binary" data format.
Binary data format means that each character (letter,
number, symbol, or control character) a computer handles is
operated on and stored as a specific binary number. (There are
several excellent reasons for this, one of which you'll discover
presently.)
Binary numbers. A binary number is a string of binary
digits, such as "1010" or "10011." Only 0s and 1s are used in
binary notation (as opposed to the numerals 0 through 9, which
are used by the decimal system).
Unlike the decimal system, the values of the numerals
themselves (0 and 1) are not used to determine the total value of
a binary number; instead, the values of the places marked by a 1
are summed. Each place, of course, has a set value. The first
place on the right in a binary number has a value of 1, the
second place a value of 2, the third place a value of 4, and so
on, with the value doubling with each place.
Again, the value of a binary number is determined by adding
up the values of the places that contain a 1. If there is a 0 in
a place, that place's value is not counted.
Consulting Table 3.1 in THE MODEM REFERENCE, it is easy to
see that the binary number "11" is the same as the decimal number
"3" (add the values of the places: 2 + 1 = 3). Similarly, the
binary number 1010 is the same as the decimal number 10 (add the
value of the places that contain a 1: 8 + 2 = 10). Nothing to
it, right? Right! Keep this up and you'll be a math wizard in
no time.
Bits and bytes. A binary digit is called a "bit," by the
way, and it happens to be the smallest unit of computer data. A
binary number representing a computer character (sometimes called
a data "word") is called a byte.
The ASCII character set. I hope the explanation of binary
data left you wondering what the key to the numeric "code" of
binary numbers might be--which is to say, I hope it started you
thinking. There is indeed a code to those numbers, a "Rosetta
stone" you can use to translate the decimal equivalents of binary
numbers into characters. It's called ASCII.
Each and every character that a computer handles is assigned
a number in the ASCII character set, which is shown in Table 3.2.
(Note that only the first 128 ASCII characters are shown;
certain computers--such as the IBM PC and Apple's MacIntosh--use
an additional 128 ASCII characters, many of them dedicated to
graphics. Not all computers recognize such extended ASCII
character sets. Also note, for later reference, that the first
128 ASCII characters--0 through 127--can be represented by a
string of seven binary digits, with leading 0's added as
necessary.)
As you may have inferred from the foregoing, a digital
computer manipulates characters as the binary counterparts of
these ASCII numbers. For instance, the letter "A" (ASCII 65) is
represented within a computer as "1000001"--the binary
counterpart of 65.
Thus, any computer that uses binary data format and the
ASCII character set recognizes the binary digit "100001" as the
letter "A" when it is received as input--via its keyboard or from
an external source such as a modem.
Binary signals. Within a computer, each string of binary
digits representing a character is manipulated as a discreet
unit. This unit, as you may have noticed a few paragraphs back,
is called a byte. A byte is normally composed of 8 bits.
When a computer sends data to a peripheral--printer, modem,
etc.--each byte is transmitted as a binary signal. The signal is
literally a series of negative and positive voltages, as
illustrated in Figure 3.2. The negative state represents a
binary 1, while the positive state represents a binary 0.
This particular signal contains the binary number
"11000001."
And yes, the order of the binary digits is reversed; that
is, the figure does show "10000011," even though the number it
represents is "11000001."
The figure is reversed because the binary digits that make
up a character are transmitted that way (i.e., the "low-order"
bit [the bit with the lowest-value] is transmitted first). This
is done so parity can be calculated as a character is sent (more
on that later in the chapter).
(NOTE: I wanted you to see this now, so you
won't go slowly nuts trying to figure out why
the digits are reversed in the figures in this
book that illustrate binary signals. That's
pretty much what happened to me the first few
times I studied tutorials on data
communication. None of the authors bothered to
explain why the figures showed the data bit
order as the reverse of what it was in the
text.
(Whether this was out of ignorance or due
to a lack of consideration for the reader, I
don't know. And I never did find a book that
explains this situation; I eventually found out
what was going on by asking several engineers
who work in the field.
(All of which is not to plug this book as
the ultimate resource on data communications
for the layman, but to make this point: Books
and manuals are not infallible. Writers
sometimes assume too much knowledge on the part
of the reader, and are occasionally themselves
confused. Too, the computer field is notorious
for having several terms with the same meaning,
or several meanings for one term.
So, if something doesn't make sense when
you read about it in your modem or software
manual, check it out! See if this book
provides the information you need. If not, ask
someone who knows (or should know); contact the
modem manufacturer or software publisher whose
manual is the source of your confusion.)
Why binary? The binary data system is used with modern
computers primarily because it is faster and more reliable than
analog systems.
The two-state binary system is extremely simple; a signal is
either there or not there, negative or positive, etc. Therefore,
there is very little chance of information getting lost or
"scrambled." Such information can be received and handled by a
computer at extremely high rates, as cumbersome as this system is
for humans to use. Using the analog technique for information
transfer and manipulation, on the other hand, requires reading
the relative strengths of any of a large number signals, as well
as the calibration and checking of those signals. This slows
down data manipulation tremendously.
The electronic components used in digital computers
(initially transistors and later integrated circuits of various
types) exactly mimic the binary system, switching off and on
(between two states) at extremely high speed.
Data Transfer
The astute reader will have realized by now that all that's
necessary for the letter "A" to be transmitted from computer A to
computer B is for computer A to send the binary digit "1000001"
to computer B. If you've figured that out, pat yourself on the
back--it represents the very essence of telecomputing.
This all looks good on paper, but we still have to move that
"1000001" from computer A to computer B. This requires a chain
of devices and connections, the first link in which is a
computer's port.
SERIAL PORTS AND THE RS-232C STANDARD
Computers aren't telepathic, and they seem immune to magic
spells, so they must be physically connected before they can
communicate. Moreover, they must be connected with one another
or with their associated modems in a special way.
The hardware medium for such a connection, whether the
computers are linked via a null-modem cable or modems, is known
as a port.
Ports
A computer's ports provide the physical (electrical)
connections by which a computer's internal workings communicates
with peripherals such as printers and modems. (A seaport might
be used as a valid analogy for a communications port, as this is
where data are sorted, organized, and shipped.)
A port consists of a group of connectors in the form of pins
(male) or sockets (female). Not all connectors are used by every
computer, but each connector that is used is connected to a
specific part of the computer's circuitry where signals are
received and/or sent according to instructions from a program or
the computer's operating system. (It gets more complicated, but
I promised you wouldn't have to drag out the engineering texts,
so we'll leave it at that.)
There are two kinds of ports--parallel and serial. The name
of each describes how it transfers data.
Parallel ports. A parallel port sends and receives data
over at least eight wires at once. This means it simultaneously
transmits all the data bits that make up a character. Parallel
ports are rarely used for data communication; they are most
frequently used to connect computers with devices such as
printers, where extremely high data transmission rates are
necessary and the short distance involved won't cause problems.
Serial ports. A serial port is a type of port which sends
and receives data over one wire. Figure 3.3 shows typical serial
port configuration.
Note that Figure 3.3 shows two kinds of serial ports--one
with 25 connectors and one with nine connectors. The difference
in the number of connectors is a bit confusing, until you learn
that not all 25 pins are used. The average microcomputer uses
only eight of the pins in data communications applications (the
IBM PC, among others, uses nine pins.)
Serial ports and their associated cables function as an
interface between computers and modems. Or, if you like
technical jargon, as a "DTE/DCE interface."
The RS-232C Standard
This is where we un-buzz a buzz word (which should be done
whenever possible). You've undoubtedly heard that this or that
port or modem or cable conforms to something called the "RS-232C"
standard. And you may well have wondered just what RS-232C
means--and whether there's an RS-232A or B, for that matter.
Well, I hope I don't shatter any cherished illusions, but RS-232C
isn't as mystic as it may look in print. Let's look at it.
What it is. RS-232C (often called simply RS-232) is a
reference on how a serial port should physically communicate.
Functionally, it is a standard for the design of the interface
between a computer and its modem. This recommendation was
established by the Electronics Industry Association (EIA) in 1969
to provide a standard for manufacturers of data communications
equipment to follow in the design of such equipment.
Semantically speaking, the designation breaks down thus: RS
is an acronym for Recommended Standard; 232 is the ID number for
this particular standard; and C is the latest revision of this
standard. (The full name of RS-232C, by the way, is "Interface
Between Data Terminal Equipment and Data Communication Equipment
Employing Serial Binary Data Interchange.")
What it does. The RS-232C recommendation covers the
electrical and mechanical characteristics of the interface, the
function of each signal (pin), and, for certain applications,
secondary functions of signals.
The reason for designing a serial interface that follows the
RS-232C standard is to enable it to exchange data with other
serial interfaces in proper form. To this end, the standard
dictates how data are to be handled by the port, which indirectly
dictates how the port's pins are connected to the computer's or
modem's circuitry. Elements covered by the standard include
voltage levels, which pins on the port are used to send and
receive data, which pins detect various status signals, and more.
In addition to the above, the RS-232C standard dictates that
DCE (modems) use a female RS-232C connector, and DTE (computers)
use a female connector. The actual configuration of a connector
is not covered by the RS-232C standard, however.
What a computer or modem does with a signal received at an
RS-232C port is its own business; the important thing is to have
the receiving device perceive data it receives in exactly the
form it was sent, and at the appropriate location in its
circuitry.
How it works. Serial data ports that are designed in
conformance with the RS-232C standard have standardized pin
assignments. That is, each pin in the port's connector has a
designated purpose, as shown in Figure 3.4
As you can see, the pin assignments cover just about every
job that might come up in transmitting and receiving data.
Again, each pin is connected to its device's circuitry as
necessary to route signals to and from the appropriate elements.
Thus, a computer with a properly-designed RS-232C port should,
for example, send data at pin 2, and receive data at pin 3.
In transfers via telephone line, each computer's serial port
is connected to a modem via a cable that is wired in conformance
with the RS-232C standard. (Presumably, the modem's serial port
likewise conforms to the RS-232C standard.) In null-modem
transfers, the computers' serial ports are connected via a cable
that is wired to match the RS-232C standard for pin connections--
but with two connections reversed.
Figure 3.4 covers pin assignments for DTE only, by the way.
A DCE serial port (as wired for a modem) has slightly different
pin assignments (such as using pin 2 to receive data from a
computer, and pin 3 to send data).
Other RS-232C Functions. A serial port is much more than a
simple data conduit. In most telecomputing applications, the
serial port performs additional tasks, some of which may not be
covered by the RS-232C standard. These include organizing the
parallel data bits transmitted by a computer into serial form,
parity checking, adding and stripping start and stop bits, and--
on relatively rare occasions--flow control (more on those later
in this chapter).
Limitations. Because of the low voltages used to transmit a
signal via an RS-232C port, the length, or "run," of a cable is
limited to about 50 feet. If a longer cable is used, there is a
good chance that data will be lost.
However, the cable between a computer and modem is generally
only two or three feet in length. Thus the RS-232C is ideal for
this application.
The maximum transmission speed of an RS-232C transmitter is
20,000 bps, but this is far faster than most telecomputing
applications.
Connectors and Cables
The cables (and the connectors used with those cables) that
connect a computer's serial port with a modem are obviously very
important elements in the data communications chain. Like serial
ports, connectors and cables used with serial ports must conform
to the RS-232C standard.
Connectors. There are two types of RS-232 connectors in
common use--DB-9 and DB-25. Remember the diagram in Figure 3.3?
It showed a port connector with nine pins and another with 25
pins. Nine- and 25-pin connectors are known as DB-9 and DB-25
connectors, respectively. Each type has numbered connectors
(very important if you intend to make your own cables--saves a
lot of messing around with a continuity tester).
You'll sometimes see the letter "P" or "S" appended to DB-9
or DB-25. This letter indicates whether the connector is male
("P" for "plug") or female ("S" for socket).
DB connectors are essentially "mirror images" of the serial
ports on your computer and modem, and are used at each end of the
computer-to-modem connecting cable.
As noted earlier, the physical configuration, or shape, of
an RS-232C connector is not defined by the RS-232C standard. DB-
style connectors have become the de facto standard, however,
simply because almost all manufacturers use them.
Cables. A connecting cable is typically a "ribbon cable"--a
flat cable with multiple conductors, as shown in Figure 3.5.
Mixing connectors. DB-25 and DB-9 connectors can be used at
opposite ends of a cable if necessary (as when a computer's
serial port has a DB-9 connector and its modem has a DB-25
connector). Only eight of the pins are normally used, and all
that's required for the connection to be successful is that the
pins on each connector be properly wired (i.e., one connector
should be wired as DCE [usually the DB-9], and the other as DTE
[usually the DB-25]). Some modem manufacturers, such as U.S.
Robotics and Migent, provide a DB-25 to DB-9 adaptor cable with
certain models of their modems.
(See Appendix E for more information on cable/connector
hookups and making your own cables.)
Connector and cable "gender". You may recall my saying a
few paragraphs back that the RS-232 standard specifies a that
female connector should be used on modems, and a male connector
on computers. Thus, a "standard" RS-232C cable has a male
connector on one end (to connect with the modem) and a female
connector on the other end (to connect with the computer).
Unfortunately, not all manufacturers follow this standard
regarding the gender of their serial ports. So, with certain
equipment you may find what is called a "gender problem." (No
sex-change jokes please--this is serious stuff!) When this is
the case, you'll have to buy or make an appropriate cable with
both female or both male connectors. (See Appendix C for more
information on this.)
RS-232 Caveats
Warning: Human nature being what it is, a few unscrupulous
manufacturers and suppliers are rather more extravagant in their
product claims than they should be. The result is that not every
cable or device labeled "RS-232C compatible" is set up to operate
exactly as a RS-232C device should. Generally, if a reputable
manufacturer (or someone you trust) tells you that a cable,
modem, or serial port conforms to the RS-232C standard, it's a
fact . . . but watch out for Brand XYZ--especially if the
company's home office is a post office box.
You should of course refer to your computer's manual and
your modem's manual before making connections of any type.
Connecting your computer to a modem may require a cable of
modified gender. You may find that, due to the peculiarities of
your computer's design, you will need a special cable (as is the
case with the IBM PCjr, which doesn't use a DB connector with its
serial port). Or, it may happen that your computer's RS-232 port
is wired slightly different from the standard.
#
Now that you've seen how a computer handles data internally,
and how it transmits that data to the outside world, let's take a
look at the next link in the data communications chain: modems.
#
MODEMS
By now you probably have a mental picture of 0's and 1's
hanging around inside a computer in groups that represent various
characters, and, on demand, zipping through circuitry and pouring
out a serial port and along a cable in tight formation. (If you
don't have that picture, take a break, do whatever it is you do
to clear your mind, and reread the preceding portion this
chapter.)
With that picture in mind, let's look at where data goes
after it leaves a serial port, what happens to it, and why.
What Does a Modem Do With Data?
As you know, data intended for modem transmission normally
leaves a computer via its serial port. A cable, connected to the
port via a properly-wired connector, conducts the data to the
modem's serial port.
The modem, in the meantime, is busy maintaining the
communications link that it originally established, watching for
incoming data from the communications link, and watching for data
and commands from the computer to which it is connected.
When data arrives at a modem's serial port, the modem has
two jobs to do:
1. Translate the data from digital to analog format.
2. Transmit the data via the communications link to a
remote system.
Similarly, when data arrives via the communications link,
the modem must perform these tasks:
1. Translate the data from analog to digital format.
2. Send the data to its computer.
Depending on its capabilities and the form of transmission,
the modem handles flow control and performs certain kinds of
error checking. These jobs are more often handled by software,
however (and you'll learn more about them in a few pages.)
The primary functions of a modem, then, are data translation
and transmission.
Data Translation: Digital vs. Analog Signals
I've indicated more than once that data must be translated
from digital (or binary) signals to analog signals before it can
be transmitted over a voice-grade telephone line. You're already
familiar with digital signals, so let's take a closer look at
analog signals and why they must be used in data communications.
What is an analog signal? Strictly defined, an analog
signal is a signal that varies in a continuous manner, as opposed
to a digital signal, which varies in a discontinuous manner.
Figure 3.7 provides a visual comparison of analog and digital
signals.
As the diagram illustrates, analog signals vary continuously
between their minimum and maximum values, while digital signals
do not vary between values. A digital signal is always at either
a minimum or a maximum level or value (or in one state or the
other), with no in-between. Analog signals, on the other hand,
cover the entire range between maximum and minimum values.
Note that the analog signal in Figure 3.7 is in the form of
a sine wave, while a digital, or binary, signal (as illustrated
in Figure 3.2) is in the form of a square wave.
Why analog? Modems were developed because the nature of the
telephone system places several limitations on how data may be
transferred:
* Telephone lines and their associated switching equipment
are designed for voice communications, which means they
accommodate a limited range of frequencies. This
frequency range, called the bandwidth, does not include
all frequencies used by digital data.
* Various amplifiers and filtering circuits involved in
telephone line transmission sometimes "cut off" the upper
end of a square-wave signal.
* The sometimes-poor quality of telephone lines (which
you've experienced aurally during long distance and even
local calls) makes tolerance for variations in signal
quality necessary.
A computer's digital data signals must be changed to a
format compatible with these limitations. To be compatible, the
signal format must use the telephone bandwidth, and must be
flexible enough so that minor variations in the signal do not
result in lost data.
Analog signals meet all the requirements dictated by the
telephone system's limitations (the telephone system was, after
all, designed to carry analog signals). And, as the diagram in
Figure 3.8 shows, an analog signal can be made to approximate the
variations of a digital signal.
Data Transmission
Once a signal has been converted from digital to analog
format, it is a relatively simple matter to transmit the analog
version by telephone lines. The signal is placed on the
telephone line carrier wave via a process called modulation.
This takes place in much the same way that radio transmission
takes place.
Carrier wave. A carrier wave is a tone transmitted on a
telephone line. The carrier is a tone of constant frequency and
strength--a sine wave, as shown in Figure 3.9.
Carrier waves are so named because they are periodic
electromagnetic impulses (waves) that literally carry
information.
Modulation. Modulation is a process whereby a signal is
placed on a carrier wave by varying one or more of the wave's
characteristics. These characteristics include amplitude,
frequency, and phase.
1. Amplitude modulation. If the strength of a carrier wave
is varied to carry a signal, it might appear much as in
Figure 3.10
In this example, each period of maximum high and
low variation might represent a binary 1, while each
period of low variation might represent a binary 0.
Because of the prevalence of varying amplitude on a
telephone carrier (in the form of voltage spikes and
other line "noise"), straight amplitude modulation is
not commonly used.
2. Frequency modulation. If the frequency of a wave is
varied, it might look like the wave in Figure 3.11.
Each area of high frequency (i.e., when the waves are
closest together) could represent a binary 1, while low-
frequency waves could represent a binary 0.
A simple variation of frequency modulation, called
Frequency Shift Keying, is used by low-speed (300 bps)
modems. Four frequencies are used in this technique,
two of which represent the binary 1 and 0 of one modem,
and two which represent the binary 1 and 0 of the other.
The frequencies are turned on or off as necessary to
transmit the 1s and 0s of each modem. (If you listen in
on a telephone line while a modem using this kind of
modulation is connected, you can hear the frequencies
change.)
3. Phase modulation. Phase modulation is represented in
Figure 3.12. Two waves of the same frequency are
transmitted, and modulation is achieved by varying the
time lag between the two waves. A variation on phase
modulation is commonly used in higher speed modems,
because it requires the smallest bandwidth of any
modulation techniques.
In the example in Figure 3.12, each change in phase
represents a digital 1, while periods of no change
represent a digital 0. (A change in phase occurs when
the variable carrier wave is started at a new time in
relation to the other carrier wave.)
Several combinations of and variations on these modulation
techniques are used in higher speed data transmission, but we
won't concern ourselves with such here. The foregoing
explanations and examples of modulation techniques are intended
to help you visualize what happens when data are transmitted,
rather than to serve as a tutorial.
Data Reception
You've seen how data are translated and transmitted by a
modem, but what happens when a modem receives data? As you might
expect, the reverse of the modulation process takes place.
When a modem receives data, it senses the modulation of the
carrier wave and demodulates the signal to recover the data from
the carrier wave. This involves reconstructing a true digital
signal from the analog signal's approximation of a digital
signal.
Once the translation is achieved, the modem transfers the
data to the computer, sending it through its serial port to the
computer's serial port.
#
Not incidentally, the word "modem" is a contraction of the
full name of the device: "modulating/demodulating device." This
name, in turn, is derived from the modem's functions of
modulating and demodulating signals.
Modem Standards
As with serial data ports, there are recommended standards
for modem operation. Rules and conventions for the design of
modems have been established by several trade organizations and
corporations. The standards cover the modulation and
transmission techniques used by modems, as well as other elements
of their operation.
Virtually all BBSs, online services, and packet switching
networks in the U.S. and Canada use modems that are compatible
with Bell 103 (300 bps) and Bell 212 (1200 bps) standards. So,
it's a good idea to use a modem that conforms to these standards,
which were developed by Bell Laboratories and adopted by the
telecomputing industry in the U.S. 2400 bps modems generally
conform to the international CCITT V.22bis standard, described
below. If your modem does not meet the appropriate standard(s),
you won't be able to communicate with the vast majority of dial-
up systems in the U.S.; in fact, you'll probably be unable to
communicate with any system unless it uses the exact brand and
model of nonstandard modem!
(NOTE: The Bell standards have nothing to do with the RS-
232C standard. You must, of course, make sure your modem uses an
RS-232C connector at its serial port.)
If you're going to use a modem in another country--
especially in a European nation--it will probably have to conform
to a different standard, known as the CCITT standard. This
standard was developed by the Consultive Committee on
International Telegraphy and Telephony, an international
telecommunications standards committee. It is the standard for
1200 baud communications in western Europe and most of the world
outside North America. Thus it is necessary to have a modem that
operates on the CCITT standard if you wish to access systems
outside North America. (The current versions of the CCITT
standard in use are CCITT V.22 and V.32 for dial-up modems up to
1200 bps, V.22bis for 2400 bps dial-up modems, and V.29 for
leased line modems.)
1200 bps modems that meet the CCITT standard are
manufactured by several U.S. modem manufacturers, by the way, and
it is becoming more and more common to find modems manufactured
in the U.S. that recognize both Bell and CCITT standards. You'll
even find some modems, such as the Hayes Smartmodem 2400, that
are compatible with CCITT V.22 bis at 2400 bps, CCITT V.22 and
Bell 212A at 1200 bps, and Bell 103 at 0 to 300 bps. (With a
modem like this, you'll have no problem communicating with any
system in the U.S. or Europe!)
FCC Registration. It is worth noting here that any device
sold in the U.S. for connection to a telephone must be registered
with the Federal Communications Commission. You'll find evidence
of such registration on a plate attached to the modem, and
probably in the modem's manual, as well. Don't not buy a modem
that isn't FCC registered.
#
If you found this excerpt useful, you may want to pick up a
copy of the book from which it was excerpted:
THE MODEM REFERENCE
by Michael A. Banks
Published by Brady Books/Simon & Schuster
ISBN # 0-13-586646-4 $21.95
In addition to explaining the technical aspects of modem
operation, communications software, data links, and other
elements of computer communications, the book provides detailed,
illustrated "tours" of major online services such as UNISON,
CompuServe, DELPHI, BIX, Dow Jones News/Retrieval, MCI Mail, and
others. It contains information on using packet switching
networks and BBSs, as well as dial-up numbers for various
networks and BBSs, and the illustrations alluded to in this
excerpt.
You'll also find hands-on guides to buying, setting up,
using, and troubleshooting computer communications hardware and
software. (And the book "supports" all major microcomputer
brands.)
For more information, contact:
Michael A. Banks
P.O. Box 312
Milford, OH 45150