AOH :: MDM4.TXT|
An introduction to modems
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 4)
Chapter 2 covered the basics of telecomputing equipment and
software, and Chapter 3 explained the technical aspects of
personal computer telecommunications. This prepared you the in-
depth look at modems in this chapter.
The major topics in this chapter are modem types, modem
features and options, and important factors to consider when
buying a modem. I've included tips on using modems, too.
If you don't yet have a modem, or are considering an upgrade
to one with more features or higher speed, read this chapter; it
takes the mystery out of modems.
Modems come in two basic physical types: acoustic and direct
connect. Most computer systems operate with either type, and
each has its advantages and disadvantages, as described below.
The simplest type of modem is the acoustic modem (sometimes
called an acoustic coupler). This kind of modem was originally
developed to circumvent now-defunct telephone company regulations
involving devices connected to telephone lines. No part of an
acoustic modem is electrically connected to a telephone or
telephone lines. Instead, the modem communicates with the
telephone via sound waves.
A typical acoustic modem configuration (as shown in Figure
4.1) consists of the modem itself (which may be a stand-alone
desktop unit or an internal card), and rubber microphone and
speaker cups that connect the modem to a standard desk telephone
handset. (NOTE: You'll find it difficult or impossible to use an
acoustic modem with "designer"-style telephones, cordless
telephones, and other telephones that have a non-standard
handset. The speaker cups fit snugly over a standard telephone
mouthpiece and earpiece, to filter out outside noise.) Another
style of acoustic modem provides an appropriately-shaped cradle
for the telephone handset.
Figure 4.1 Acoustic Modem Configuration
In operation, an acoustic modem converts a computer's
digital signals to audio tones. These tones are emitted by the
modem's speaker cup and picked up by the microphone in the
telephone's mouthpiece. The telephone then sends the signals
over telephone lines. On the receiving end, the modem's
microphone cup picks up the tones from the telephone's earpiece.
The signals are sent over a connecting to the modem, which
converts them back to digital data.
An acoustic modem is usually less expensive than a direct
connect modem. And, because an acoustic modem requires no
special plugs or outlets, it is convenient to use with hardwired
hotel or pay telephones. (These are the most practical and
popular applications for acoustic modems.)
On the negative side, almost all acoustic modems require
manual operation (i.e., you must dial the telephone number and
listen for the remote system to answer, hang up the telephone
manually, etc.). And, because they are less popular than direct
connect modems, you'll have a difficult time finding acoustic
modems with the enhancements offered by direct connect modems.
Actually, it's hard to find acoustic modems nowadays. This
is because the situations that made acoustic modems necessary
have pretty much ceased to exist. However, acoustic couplers are
available for use with direct-connect modems. (The modem must
have an interface for the acoustic coupler.) These are most
useful if you frequently take your modem and computer on the
Direct Connect Modems
A direct connect modem bypasses the telephone set altogether by
connecting directly with a telephone line via a standard RJ-11
jack. As described in Chapter 2, an RJ-11 jack is the receptacle
on a telephone into which the telephone line (equipped with an
RJ-11 plug) is plugged.
Though more expensive than acoustic modems, direct connect
modems are very popular for several reasons. Most direct connect
modems feature auto dial capability (the ability to dial a phone
number without using a telephone set) as well as other features
not usually found in acoustic modems. A direct connect modem may
also allow you to connect both the modem and the telephone set to
a telephone line simultaneously, which means you don't have to
plug and unplug the phone line to switch between modem and voice
communications. And, because data are not transmitted via sound
waves, there is less potential for garbled signals.
There are two classes of direct connect modems--internal and
Internal direct connect modem. An internal direct connect modem
consists of an IC board, or "card," mounted in a computer's
expansion slot. A typical internal modem contains the modem plus
a serial port and all required connectors--including a modular
jack for a telephone line. (Many laptop computers feature built-
in internal modems.) Figure 4.2 shows a typical internal direct-
Figure 4.2 Internal Direct-Connect Modem
An internal modem has several advantages over an external
modem, not the least of which is reducing desktop clutter.
Once an internal modem is installed, there are no cables in
the way, and the modem is less susceptible to external physical
disturbance than an external modem. If you have a "luggable" or
laptop computer, using an internal modem means one less piece of
hardware to carry around. And because an internal modem draws
power from its host computer's power supply, it doesn't require
an extra electrical outlet. (The computer's power supply should
be hefty enough to handle the modem's requirements, of course.)
Finally, an internal modem leaves a computer's serial port free
for other applications.
The major disadvantage of an internal modem is that, because
it is an integral part of a computer, it is machine-specific and
function only with the type of computer for which it is designed.
So, if you upgrade to a different kind of computer, you'll have
to buy a new modem.
Another disadvantage of an internal modem is the fact that
you cannot monitor a call's status via external indicators.
Finally, an external modem does require a slot--which leaves you
with one less slot to devote to another device.
External direct connect modem. A typical external direct connect
modem (as shown in Figures 4.3 and 4.4) is a thin rectangular box
with a power line, one or two modular jacks, and an RS-232C
connector. The RS-232C connector accommodates a cable from the
computer, and is usually mounted on the back panel. (As
explained in Chapter 3, an RS-232C connector can be a 5-, 9-, or
25-pin male or female connector. The shape of the connector is
either that of an elongated letter "D" or a circle.)
An external modem usually has a row of LEDs mounted on its
front panel. These are the modem's status lights. Status lights
let you monitor the modem's operation and the status of a call.
An external modem may also have controls of various kinds mounted
on its front panel.
Figure 4.3 External Direct-Connect Modem
Figure 4.4 External Direct-Connect Modem ("Economy" Model)
The main advantage of external modems over internal modems
is the fact that they can be used with more than one kind of
And there are other advantages. If an external modem needs
repairs, you can fix it (or take it into the shop) without
disassembling the computer. Getting at the modem's DIP switches
is easy, too. (That is, you don't have to open your computer;
you may have to open the modem, although you can access DIP
switches on some modems externally.) And, if you feel
comfortable being able to monitor the status of a call visually,
an external modem's status lights are a welcome feature.
(Note that most portable modems have neither DIP switches
nor status lights.)
Finally, external modems cost less than internal modems.
This is because they are not specially-engineered to operate with
a certain computer, and because--unlike internal modems--they do
not have built-in serial ports.
On the negative side, an external modem requires a separate
power source--more often than not, a transformer that plugs into
a wall outlet. (There are battery-powered modems, and modems
that take their power directly from a telephone line, but these
are rare.) Too, using an external modem means having to find
room for one more piece of equipment on your desk or work table.
(Note that the comments about external modems apply in
general to portable modems as well as desktop modems. Chapter 18
has more to say about probable modems.)
Several modem manufacturers (notably, AT&T and Tandy) have
experimented with combining a telephone set with a direct-connect
modem, as illustrated in Figure 4.5. This type of modem is
convenient and saves space.
Figure 4.5 A Telephone/Modem Combination
Modems are also classified by whether they communicate in
asynchronous or synchronous mode, and whether they can
communicate over common telephone lines to access online services
and BBSs or work over dedicated lines at ultra high speeds
(dialup vs. leased line). These distinctions are described
If you use dialup services, you'll probably never need a
synchronous modem. All BBSs and popular dialup services
(including those discussed in this book) provide asynchronous
service. The same is true of the major packet switching
Leased line (see below) and mainframe connections usually
require synchronous communications.
If your applications are dialup, make sure the modem you buy
is capable of asynchronous operation. If you have applications
in both areas, you'll be happy to know there are a number of
modems that operate in either asynchronous or synchronous mode.
(The differences between asynchronous and synchronous operation
are explained in Chapter 3.)
Dialup and Leased Line Modems
Dialup modems. Simply described, a dialup modem is one that
operates within a bps range (0 to 9600) that is effective for
data transmission via voice-grade telephone lines. (9600 bps is
the upper limit for reliable data communications within the
frequency bandwidth available on voice telephone lines.) A
dialup modem also uses communications parameters (parity, data
bits, number of stop bits, etc.) that are compatible with other
dialup modems and packet switching networks.
Most (but not all) dialup modems operate in asynchronous
mode. Some high-end modems, such as the Hayes Smartmodem 2400,
are switchable between asynchronous and synchronous modes. Dial
up modems are, in general, less expensive than leased line
Virtually all modems sold for use with personal computers
are asynchronous dialup modems.
Leased line modems. Leased line modems use extremely high bps
rates (9600 and up) over dedicated two- or four-wire telephone
lines, or other direct-connect lines shielded against outside
interference. (These are also called "conditioned" lines.)
Leased line modems may operate in synchronous or asynchronous
mode, or switch between modes. Leased line modems usually
transmit data in full duplex (two-way) mode. A typical leased
line modem is shown in Figure 4.6.
Figure 4.6 Leased Line Modem
Leased line modems are useful in LAN (Local Area Network)
and private network applications (such as in commercial
applications where large amounts of data pass rapidly over
conditioned or four-wire lines). These modems almost always
include the more esoteric of the modem options discussed on the
following pages, such as data encryption, automatic dialback, and
data compression. (This is not to say that dialup modems cannot
include these features.)
Unless you are setting up your own network, or intend to use
a terminal (or a personal computer as a terminal) to communicate
with a mainframe or mini-computer in a nearby location, you won't
need a leased line modem.
Analog and Digital Modems
The modem applications discussed in this book concern analog
modems (that is, modems that transmit and receive data over
communications links in analog form).
You'll be interested to know, however, that digital modems
can transmit data via special digital networks in true digital
form. They don't have to convert data to analog form as analog
modems do. Applications for digital modems involve specialized
digital telephone networks of the type described in the preceding
Special note on digital telephone systems. Many large offices
have their own internal telephone network. One incoming number
handles multiple calls to various extensions, as well as
telephone traffic from one office to another. Such a system is
called a "Private Branch Exchange," or "PBX" for short. There is
usually no problem with using a modem through a conventional PBX,
unless the system has a Call-Waiting-type feature.
However, many modern PBX systems are digital systems. If
you wish to telecompute via a digital PBX, you may have to
install an analog (standard) telephone line. (This is would be a
separate line with its own number, one that doesn't go through
the PBX.) Try your modem with the PBX system before installing a
new line, though; chances are, your modem should operate with the
An alternative to having a dedicated phone line installed is
to buy a digital-to-analog converter. Also known as analog-to-
digital converters, or "A-to-D" or "D-to-A" converters for short,
these devices handle changing a modem's analog output to a format
compatible with a digital PBX. They also convert incoming data
from a digital PBX to the analog format recognized by dialup
modems. Check with your computer dealer or phone company on the
availability of A-to-D converters.
Selecting a modem deserves at least as much thought and
consideration as you put into selecting your computer. Even
bargain-priced modems are not inexpensive, and you have to live
with your mistakes (unless, that is, your income and spending
habits can easily absorb a useless expenditure of several hundred
dollars--or more). So, take your time and choose carefully!
You may find the "do it yourself" modem comparison chart in
Table 4.2 helpful in selecting your modem. (Make a copy of the
chart if you'd rather not mark in this book.) Write the model
names of the modems you're considering in the blank row at the
top of the chart, highlight the features in the left hand column
that interest you, and check off those features as they apply to
each modem model whose name you enter at the top of the page.
MODEM FEATURES AND OPTIONS
The number of modem features and options available is
surprisingly large, and the source of more than a little
confusion (especially since communications software packages
offer many of the same features). I'll describe the more
important features here, to acquaint you with what's available
and to help you develop a "shopping list" of modem features.
You'll also find this section handy when looking over a list of
specifications for a particular modem. (Spec sheets tend to list
buzz words without explaining what those words mean.)
Not all software packages can use all the features offered
by some modems. Some features may require communications
software designed for use with the modem in question. Too, some
modem features (like built-in error-checking protocol) will not
operate unless you're calling a modem with the same features.
You should also be aware that some features are available
only with dialup modems, while others are available only with
leased line modems. Unless otherwise noted, the features
discussed in the following pages are available for dialup modems
(sometimes as extra-cost options) as well as for leased line
modems. Not every modem offers every feature discussed here.
The features aren't listed in order of importance.
Importance is subjective and depends on your application.
Finally, some of the capabilities discussed in the following
pages are also available as communications software features.
Where appropriate, I've noted when this is the case.
Notes on "Intelligent" Modems and Features
Dialup modems are frequently advertised as "intelligent." This
term usually alludes to the ability to perform certain functions
automatically, such as dialing numbers, answering the telephone,
etc. The "intelligence" consists of programming in ROM or
Advanced functions offered by intelligent modems are
normally performed in response to commands issued from the
keyboard or (more typically) by your communications software, or
in response to signals from the system you have dialed up.
As with people, modem intelligence varies; an "intelligent"
modem may offer any number of the functions discussed here. If
you see a modem labeled "intelligent" (or "full-featured"), don't
count on it having everything you want. Be sure to get a
complete list of a modem's features before buying it.
Variable Communications Parameters
Unless you plan to use a modem to dial up only one other system,
don't buy a modem with fixed communication parameters. Such a
modem will not respond to software commands to change parameters.
For maximum utility, these parameters should be variable:
* duplex (echo)
* data bits
* stop bit(s)
Automatic parameter adjustment. Some top of the line modems
(both leased line and dialup) sense and adjust to the
communications parameters in use by a remote system. This
capability is often called "automatic feature negotiation."
Autodial. One of the most popular modem features is "autodial"
(or, "auto dial")--the ability to dial a number without the use
of a telephone set. With autodial, all your communications
software has to do is send a command telling the modem to dial a
number. (The command may come from your keyboard, or from a
communication program's autologon script file.)
Once it receives the dial commands and number, the modem opens
the phone line, dials the number and--if there is an answer on
the other end--lets the answering computer know a computer is
calling. Most autodial modems can dial using either tones or
Auto answer. Auto answer capability means that the modem can
detect an incoming ring (via a variation in telephone line
voltage), "answer" the phone, keep the line open, and let your
software know there is an incoming call. (The communications
software--assuming it has the capability--takes over from there
and sends the appropriate commands to the modem.)
Selective Dialing Capability (Tone/Pulse)
This is the ability to dial by either tone or pulse. While the
majority of telephone exchanges in the U.S. handle tone dialing,
it is a good idea to have both capabilities.
Tone dialing. Tone dialing is dialing by sending DTMF signals
over the phone line. DTMF is an acronym for "Dual Tone Modulated
Frequency," which is the type of signal used by a touch tone
system. If you've a good ear, you may hear two tones when you
press a touch tone telephone button. The tones are carrier
tones, modulated with information that identifies the number
pressed. These tones go to a local telephone switching computer,
which connects you with the number dialed. A modem with tone
dialing capability can generate these tones.
Pulse dialing. Pulse dialing is used with "rotary" style phones
(the old ones with a true dial rather than push buttons). When a
number is dialed with pulses, a series of clicks is sent through
the telephone line--one click for the number 1, two clicks for
the number 2, and so on, all the way up to ten clicks for 0.
This is rather slow, but necessary with old telephone systems in
some parts of the country.
(You can try pulse dialing manually with your phone--even if
it is a Touch Tone telephone. Lift the handset, then depress the
"hangup button" rapidly for each number--once for 1, twice for 2,
three times for 3, and so on. You can actually dial a number in
this manner--which is the way a pulse dial modem does it.)
You can often use tone dialing even if there is a rotary
phone connected to the line. You'll often find locations that
use rotary phones even though tone dialing is supported; it's a
matter of economy. So, always try tone dialing before you try
pulse dialing. The presence of a rotary phone doesn't mean tone
dialing isn't supported.
If dialing a number using tones has no effect, you can
always switch your modem to dial using pulses (normally with the
Tone dialers. If you have a manual-dial modem, you can "upgrade"
it to autodialing, even if you have a rotary-dial telephone. All
you need is a device known as a "tone dialer." As illustrated in
Figure 4.7, a tone dialer is a handheld device with a speaker and
Figure 4.7 Tone Dialer
When the buttons are pressed, the tone dialer's speaker
emits the standard DTMF signals used by phone systems. These
tones are picked up by a telephone's microphone and have the same
effect as dialing with a telephone.
The better tone dialers store several numbers for later
playback. You can use this kind of tone dialer as an online
"phone directory" to speed up manual dialing; all you have to do
to dial a number is press a couple of keys. This gives you the
convenience of an autodial modem with number storage. It also
speeds up dialing and eliminates errors.
Adaptive dialing. Modems with adaptive dialing sense whether
they should use tone or pulse dialing, depending upon whether the
local telephone system is equipped to handle tone dialing. This
information is present in the dial tone sent by tone-dial
When a modem with adaptive dialing receives a dial command
without tone or pulse specification, it will try tone dialing
first; if the dial tone is still present, it switches to pulse
dialing. The U.S. Robotics Courier 2400e is one example of a
modem that implements this feature.
Default dial settings. Intelligent modems can be set to dial
using tone or pulse only. When so programmed, a dial command
sent to the modem doesn't have to include the tone or pulse
specification, unless the non-default setting is to be used.
The ability to pause during dialing is necessary if a modem is
dialing through a PBX, certain long-distance services, or any
phone system that pauses before a dial tone comes up.
For example, if a PBX system requires you to dial 9 to get
an outside line, you may have to wait one or two seconds after
you dial 9 before the dial tone is heard. A communications
program can tell a modem with pause capability to wait one second
after dialing 9 before dialing the rest of the number. This is
normally done by embedding the standard AT pause command (,) in
the number, thus: 9,7221969. This sequence, sent to a modem as
the number to be dialed, would tell the modem to dial 9, wait one
second, then dial 7221969.
A speaker is useful when you wish to monitor a dialup. With a
speaker, you can hear if there's a busy signal, and thus not
wonder why you're not getting an answer. And, if you get a voice
response ("live," or, in the case of a changed number, a
recording), you can respond appropriately. (The speaker is
mounted inside the modem.)
Volume control. A nice extra that's not available with all
speaker-equipped modems is a volume control. If you buy a modem
that doesn't have a volume control, you may wish it did after a
few online sessions. Hearing a dial tone and the sound of the
modem dialing is reassuring to some, but annoying to others.
Most volume controls are external (manual). But many modems
(among them the Hayes Smartmodem 2400 and the U.S. Robotics
Courier 2400e), let you adjust speaker volume with software
commands. Some modems offer both external and software volume
As of this writing, 2400 bps is the "standard" communications
speed for the vast majority of BBSs and online services, while a
few offer 9600-bps service. (The larger online services and
packet switching networks are moving in the direction of 9600-bps
dialup service. Look for 9600-bps service to become the standard
before the decade's out.)
Despite all this, you'll still find systems limited to 300
or 1200 bps. And you will sometimes want to use 300 or 1200 bps
on a commercial online service.
So, while you'll want a modem with a maximum speed of at
least 2400 bps, be sure it can handle 300- and 1200-bps
communication, as well.
Modem standards are discussed in detail in Chapter 3. Suffice to
say that if you use a modem in the U.S., it should conform to one
of these standards:
300 bps Bell 103 (North America)
1200 bps Bell 212A (North America)
300/1200 bps CCITT V.22 and V.32 (International)
2400 bps CCITT V.22 bis (North America/International)
Multi-speed modems are usually "downward compatible" with
reference to standards. If a modem conforms to a recommended
standard at its highest bps rate, it generally conforms to that
standard's counterpart at lower rates. For instance, a 2400-bps
modem that is compatible with the CCITT V.22 bis standard is
compatible with the Bell 212A standard at 1200 bps (and probably
the CCITT V.22 and V.32 standards, as well).
Modems that accommodate both Bell and CCITT standards are
offered by a number of manufacturers.
Any modem you use in the U.S. should also be registered with
the FCC (Federal Communications Commission) as meeting that
body's standards. You'll find evidence of such registration on
the modem's serial-number plate, and in the modem's manual.
Don't buy a modem that isn't FCC registered.
Dual Phone Jacks
External direct-connect modems with autodial capability do not,
of course, have to be connected to a telephone to function. But
you may wish to have a telephone set connected to the telephone
line along with the modem. This way, you can use either the
phone or the modem without switching the telephone line from one
to the other.
One way to do this is with a "Y-jack" (sometimes called a Y-
plug or Y-cable). This is a short cable with an RJ-11 jack, or
receptacle at one end; the other end of the cable splits into two
wires, each of which has an RJ-11 plug at its end. The telephone
line is plugged into the single RJ-11 jack, and the two RJ-11
plugs are connected to your modem and your telephone set,
respectively. You can use a "duplex jack" to do the same thing.
(A duplex jack is a plastic block with an RJ-11 plug on one end
and two RJ-11 jacks on the other, as shown in Figure 4.8.)
Figure 4.8 Duplex Jack
A more convenient approach is to buy a modem with two RJ-11
jacks. The incoming telephone line is plugged into one jack, and
a short cord with an RJ-11 plug on each end (usually called the
"telephone cable" and supplied with the modem) is plugged into
the other jack. The free plug on the short cord is then plugged
into the telephone set. The net effect is that the modem and
telephone are connected to the same phone line in parallel.
Automatic Hangup on Carrier Loss
A few autodial/auto answer modems don't have this feature--and
it's an important one to have. With automatic hangup, the modem
automatically hangs up the phone (disconnects from the phone
line) if the remote system hangs up or is accidentally
disconnected. This feature is especially useful if you use a
phone line for both voice and data; if it's not present, your
phone may stay "off hook" after a remote system disconnects and
prevent calls from coming in.
A few modems with this capability will disconnect only when
told to do so by communications software. Because of this, you
must check carefully to make sure automatic hangup means the
modem and not the software performs the operation. (There's
nothing wrong with relying on your software to do this; but the
feature should be available if you need it.)
Visual Call Status and System Monitoring
Most external desktop modems have status lights (usually LEDs--
light-emitting diodes--rather than lights) on their front panels,
as illustrated in Figure 4.9. These indicators provide a way to
visually monitor the status of a call and the telecomputing
Figure 4.9 A Modem with Front-Panel Indicators
Status lights are usually marked with two- or three-letter
abbreviations to indicate their purpose. The status indicators
provided most frequently are:
* AA "Auto Answer." This indicator is on when a modem
with auto answer capability is set to answer incoming
* CD "Carrier Detect." This indicator lights when a modem
detects a carrier tone from an answering computer during
a call. (At the same time, the modem sends a "carrier
detect" signal to its computer.)
* HS When the "HS" indicator is on, it means that the
modem is set to 1200 or 2400 baud.
* MR "Modem Ready." MR lights when a modem is ready to
receive or send data (normally on whenever there is power
to the modem).
* OH "Off Hook." This means that the telephone line
connected to a modem is currently in use, as when the
modem takes control of the line to make a call.
* RD "Receiving Data." This indicator flashes with each
bit of data your computer receives.
* SD "Sending Data." This indicator flashes with each bit
of data your computer sends. (Watch it as you type; it
flashes each time you press a key.)
* TR "Terminal Ready." This means your terminal is ready
to dial out or receive calls.
You'll find other, more specialized indicators on some
modems. The more elaborate communications software packages (or
ancillary programs) provide the same kinds of information
Call Status Monitoring
If you don't have a speaker (or even if you do), it is nice to
have a modem that detects and reports the status of a call by
sending messages to your screen similar to these:
BUSY (busy signal detected)
CONNECT (connection established)
CONNECT 300 (connection established at 300 bps)
CONNECT 1200 (connection established at 1200 bps)
CONNECT 2400 (connection established at 2400 bps)
NO ANSWER (no response after a specified number of
NO DIAL TONE (no dial tone present)
RINGING (the number is ringing)
UNSUCCESSFUL (the call was aborted or interrupted)
VOICE (a voice, rather than a modem, answered
Advanced communications programs react to such messages by
disconnecting and/or redialing as appropriate when, for example,
there's no answer or a voice answers the phone. More expensive
call-monitoring modems handle these situations without waiting
for software commands.
This feature is "switchable" in many modems. This means the
level of response can be adjusted to brief numeric codes, or
turned off entirely.
Auto Baud Rate Switching
This feature is sometimes available whether or not a modem has
call status monitoring. A modem with auto baud rate switching
senses and matches the baud rate in use by a remote system when
connection is established.
Auto baud rate is especially convenient when you're dialing
a BBS new to you; you don't have to worry about which baud rate
Modems capable of auto redial store the most recently-dialed
number in RAM, and will redial that number in response to a brief
command. This is convenient if you call a system and receive a
busy signal, and wish to try the system again in a short time.
Modems with this feature can often be set to redial a number
after a specified time if there's no carrier or a busy signal is
Many communications software packages offer this function,
Some modems with autodial capability also store numbers in non-
volatile memory or EEPROM. This means the numbers are
"remembered" even when power to the modem is off.
Numbers are usually stored in a "directory" format,
accessible by name. Given the proper command, the modem will
retrieve a specified number from its memory and call it.
Here again, many communications software packages offer this
feature, eliminating the need for modem number storage. (I've
found that using a communication program's "dialing directory"
feature is easier than using that of a modem. This feature is
most useful in a modem that you'll be using with different
Parameter and Configuration Storage
Modems capable of storing and retrieving numbers may also be able
to store communications parameters and operating configurations.
Parameter storage. Parameter storage is useful if you always use
the same communications parameters. With the communications
speed, duplex, parity, and number of data bits and stop bits
stored in your modem's memory, all you have to do is tell the
modem to dial a number. There's no need to set the parameters
from your terminal.
If you use autologon files with your communications program,
however, you probably won't need parameter storage in your modem.
Each time an autologon file is run, the communications program
resets the modem parameters to those specified in the autologon
Configuration Storage. If you change communications software
frequently, and/or use your modem with more than one computer,
you may have to change the modem's operating configuration to
accommodate the new software or computer.
The elements of a modem's operating configuration include
carrier detect override, local echo, result code display, and
more. (These elements vary from modem to modem; consult your
modem's manual for details.) The operating configuration is
normally set via a modem's DIP switches, and resetting it can
become a tedious chore if you have to do it frequently. With
configuration storage, you can use AT-type commands to reset the
modem's operating configuration from your keyboard.
(Incidentally, Touchbase Systems' "WorldPort 2400" portable
modem is a prime example of the utility of configuration storage.
Able to operate with virtually any computer that has an RS-232C
interface, it is ideal if you want a modem to use with both a
desktop and a portable computer. Connection is fast and easy,
and it does away with DIP switches entirely by using parameter
and configuration storage.)
Automatic "Fall Back"
When a poor connection exists as a result of a "noisy" telephone
line, it is sometimes necessary for a modem to fall back to a
lower transmission speed (perhaps 100 bps less). Many high speed
modems cannot fall back once connection is made, but this feature
is becoming more common--especially in modems with built-in error
Built-in Communications Software
Having communications software built into a modem may seem a good
idea. You don't have to install and load software from a disk,
and you know the software will work with the modem.
However, you'll probably find it more convenient to use
disk-based communications software. Disk-based software is
generally more flexible and easier to use than built-in software.
(Interestingly enough, every modem user I've talked with who has
a modem with built-in software uses a disk-based program, if
Modems with built-in terminal software include many of the
features for terminal software (as described in Chapter 5).
Automatic Data/Voice Switching
Lower-cost modems may make use of a telephone set's circuitry to
generate DTMF signals for dialing, or require you to dial numbers
manually. Such modems must be connected to a telephone set
(rather than directly to a telephone line), and you have to
switch to voice mode to dial out, then to data mode when the
system you are calling answers. (Switching to data mode allows
the modem to route incoming data to your computer.)
Similarly, many acoustic modems require that you dial
numbers for them, then flip from voice to data mode when the
remote system answers.
Manual voice/data switching seems to be the norm for these
modems, but it is obviously less desirable than automatic
switching, available on some modems that require a telephone set
to dial a number. So, look for automatic voice/data switching on
any modem that must be connected to a telephone set.
(NOTE: You can avoid the data/voice switching problem
entirely by buying a modem with auto dial capability!)
Telephone line- and Battery-Powered Modems
It is possible to buy a modem that draws its power from a
telephone line, but this is not an important feature unless you
have absolutely no extra electrical outlets available where you
use your computer.
The same is true for battery power. Unless you use a
portable modem with a laptop computer, battery power is not an
important feature. Note, too, that battery life is limited with
portable modems. I've got as little as two hours use from a new
alkaline battery with a portable modem.
Call Duration Reporting
1200 and 2400 bps modems with this option can display the
duration of calls in hours, minutes, and seconds on your computer
screen. This provides an audit record of online activities.
(This feature sometimes includes an optional real-time clock.)
This is a handy feature that prevents online charges from piling
up if you leave your computer and forget to sign off a commercial
online service. Modems with this feature disconnect if there is
no activity (data sent or received) after a preset length of
This feature is sometimes offered by communications
software, as well.
Built-in Error Correction
A popular trend among modem manufacturers is to include built-in
error-checking protocol as a feature with dial-up and leased line
modems. Several hardware error-checking protocols are in use,
including MNP (Microcom Network Protocol), X.25, LAP-B, AFT, and
various other proprietary protocols, but the feature is not
useful unless both modems involved in a data transfer use the
It's difficult to recommend one protocol over the other,
but, as of this writing, MNP seems to be the leader among dialup
modems, as it has the largest installed user base. X.25 and
X.400 are close behind, with X.25 being the inter-system E-mail
communication standard and X.400 emerging as the international
In some instances, the use of a hardware error-checking
protocol slows real-time communications slightly. These
protocols typically use a system of sending groups of characters
in "packets." Sometimes there is a brief delay before a
character is sent because the modem waits to be sure no
additional characters are coming before it sends a data packet.
(Data packets and related topics are addressed in later
(NOTE: This type of protocol should not be confused with
software error-checking protocols such as Xmodem, Zmodem, Kermit,
SELECTING A MODEM
By now, you should have a fairly good idea of whether you will be
better off with an acoustic modem or an internal, external, or
portable direct-connect modem. You should also know which
features and options you want (or need).
Deciding the configuration and features you want is only the
first step in selecting a modem, however. You must also consider
hardware compatibility, software compatibility, command set,
price, and other factors.
First, a modem must be compatible with your computer system. One
way to find out if a modem is compatible with your computer
system is to read the modem's specifications, found on the modem
package or in advertising material. You might also read magazine
reviews, which usually tell you what kinds of the equipment you
can use a modem with.
If you're lucky, your computer will be compatible with the
modem you're considering. In this case, you have only to make or
buy the proper cable to connect the modem to your computer.
The simplest way to ensure that a modem will work with your
computer, however, is to buy a "system-ready" modem. A system-
ready modem is typically a dial-up modem that operates with a
variety of computers, but packaged with the appropriate cable and
documentation for a specific brand/model of computer.
Or, you might look at dial-up modems designed specifically
for a computer brand/model (these are commonly internal modems,
discussed earlier in this chapter).
Some modems are labeled as compatible with any computer.
Modems so labeled probably work with just about any computer that
has a serial port. Of course, you may have to find or make the
proper cable to connect the modem to your computer (see Chapter 3
for more information on cables). To do this, you will have study
"pinout" diagrams for the modem and your computer, and perhaps
experiment a bit. (A pinout diagram is a diagram that
illustrates a connector's pin assignments.)
Remember, too, that a few modems communicate via a
computer's parallel port. So be sure you're getting a modem with
the appropriate connection--serial or parallel--for your
If you have any doubt as to whether a particular modem can
be used with your computer, contact the manufacturer or a
knowledgeable computer salesperson.
(Don't count on information from computer salespeople being
100% accurate, though. It is an unfortunate fact that all too
few computer salespeople are proficient in things technical;
after all, most are salespeople first. I'm reminded here of a
local branch of a national computer retail chain that frequently
advertises for salespeople, requesting used-car salespeople as
prime candidates ....)
As a backup to information given to you by salespeople,
check out manufacturers' brochures and specification sheets.
You might also check with acquaintances who use your brand
of computer. Chances are, you'll find someone who uses the
modem(s) you are considering with the computer you use. Or, your
friend(s) can advise you as to which modem is best for your
computer. If nothing else, you'll learn a lot about other modems
that are compatible with your computer.
If you're buying modem accessory hardware, or plan to use
modem features that require compatibility between two modems,
your safest course is to buy all equipment from the same
Another important consideration in selecting a modem is software
compatibility. That is, the modem you select should work with
the communications software you intend to use. This may seem
obvious, but not all modems respond to the same set of commands.
And unless your software issues commands to which the modem
responds, you'll have trouble using the two together. (I am, of
course, speaking of the actual commands the software sends to the
modem--not the commands you type at your keyboard.)
Fortunately, most modems use the same command set (or
portions thereof)--a de facto standard called the "AT" or "Hayes"
command set, which we'll examine in a few paragraphs.
It also happens that some software packages come with
prepared command or script files that enable them to "talk" with
a variety of modems. With this type of software, you merely
select the appropriate brand and model of modem from a menu
during installation. The program automatically uses the proper
command set with the modem. (You will have to change a DIP
switch setting or two on some modems.) Usually only the more
popular modems are so accommodated--yet another factor to keep in
mind when shopping for a modem and software.
The best way to determine modem/software compatibility is to
try the software with the modem before you buy either. If you
can't do that, consult with someone who already uses the
modem/software combination you're considering.
Bundled software. You'll find communications software included
with many modems (this is known as "bundled" software). You
don't have to worry about software compatibility when the modem
and software come as a package--unless, that is, you find you
don't like the bundled software.
Figure 4.11 A "Bundled" Modem-and-Software Package
If you don't want to use the software provided with a modem,
you'll have to look for a program better suited to your needs and
tastes. In this case, you should have a modem that responds to
the aforementioned AT command set, again because the majority of
communications programs use this command set.
Conflicting "standards" proliferate throughout the computer
industry, and telecomputing is no exception. Fortunately, the
standards for modem commands have been pretty well settled. Most
communications software publishers have adopted what is known as
the "AT command set," or the "Hayes Standard AT command set" in
recent years. This is partly because, early on, the best-selling
modems used this command set and partly because a standard was
A modem labeled "AT compatible" or "Hayes compatible" uses
(or ostensibly uses) the AT command set. Not every "Hayes
compatible" modem is 100% compatible with this command set, but a
modem labeled AT- or Hayes-compatible should be close enough in
compatibility that software issuing standard AT commands can
communicate with it.
By the way, most modem manufacturers have adopted this
command set, too. This way, their products are compatible with
popular communications software packages. (A selling strategy,
of course--the easier a product is to use, the better it sells.)
The AT command set is so named because the command used to
"wake up" (initiate) a modem is "AT," which stands for
"attention." Other commands include "D" for "dial," "T" for "use
tones when dialing," and "P" for "use pulse dialing." (Thus, the
command string "ATDT" tells a modem to wake up and dial a number
using tones.) Table 4.1 lists the basic standard AT commands.
Table 4.1 Standard AT Command Set
Note that the commands listed in Table 4.1 are those
commonly used in dialing and answering operations. Your modem
may offer additional commands for configuration, setup, and self-
diagnostics, particularly if it is a Hayes modem.
Some modem manufacturers (such as U.S. Robotics and Hayes)
use what is called an "extended AT command set." This is a set
of specialized commands--based on the AT command set--used to
access their modems' special features. The AT commands shown in
Table 4.1 also operate with such modems, of course.
The chances are good that any modem you are considering uses
the AT command set. But, again, make sure about this before you
buy it. Otherwise, you could find yourself stuck with a limited
range of choices in communications software, unless you want to
write your own!
((EXCLUDED: Several sections following provide more information
on selecting a modem.))
Selecting a modem deserves at least as much thought and
consideration as you put into selecting your computer. Even
bargain-priced modems can be costly, in terms of living with your
mistakes. (Unless, that is, your income and spending habits can
easily absorb a useless expenditure of several hundred dollars--
or more). So, take your time and choose carefully!
You may find the accompanying "do it yourself" modem
comparison chart (Table 4.2) helpful in selecting your modem.
Write the model names of the modems you're considering in the
blank row at the top of the chart, highlight the features in the
left hand column that interest you, and check off those features
as they apply to each modem model whose name you enter at the top
of the page. (Make a copy of the chart if you'd rather not mark
in this book.)
Table 4.2 Modem Comparison Chart
((OMITTED: The remainder of the chapter provides hints and tips
on modem installation and use.))
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, recommended by Jerry Pournelle in Byte, The New York
times, The Smithsonian Magazine, various computer magazines, etc.
(Excerpts from this book accompany this file.) THE MODEM
REFERENCE published by Brady Books/Simon & Schuster, and is
available at your local B. Dalton's, WaldenSoftware,
Waldenbooks, or other bookstore, either in stock or by order.
Or, phone 800-624-0023 to order direct.
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,
Prodigy, 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
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
Want the lowdown on getting more out of your word processor?
Read the only book on word processing written by writers, for
writers: WORD PROCESSING SECRETS FOR WRITERS, by Michael A. Banks
& Ansen Dibel (Writer's Digest Books). WORD PROCESSING SECRETS
FOR WRITERS is available at your local B. Dalton's, Waldenbooks,
or other bookstore, either in stock or by order. Or, phone
800-543-4644 (800-551-0884 in Ohio) to order direct.
Do you use DeskMate 3? Are you getting the most out of the
program? To find out, get a copy of GETTING THE MOST OUT OF
DESKMATE 3, by Michael A. Banks, published by
Brady Books/Simon & Schuster, and available in your local
Tandy/Radio Shack or Waldenbooks store now. Or, phone 800-624-
0023 to order direct.
Other books by Michael A. Banks
UNDERSTANDING FAX & E-MAIL (Howard W. Sams & Co.)
THE ODYSSEUS SOLUTION (w/Dean Lambe; SF novel; Baen Books)
JOE MAUSER: MERCENARY FROM TOMORROW (w/Mack Reynolds; SF novel; Baen Books)
SWEET DREAMS, SWEET PRICES (w/Mack Reynolds; SF novel; Baen Books)
COUNTDOWN: THE COMPLETE GUIDE TO MODEL ROCKETRY (TAB Books)
THE ROCKET BOOK (w/Robert Cannon; Prentice Hall Press)
SECOND STAGE: ADVANCED MODEL ROCKETRY (Kalmbach Books)
For more information, contact:
Michael A. Banks
P.O. Box 312
Milford, OH 45150
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