Classic Computer Magazine Archive CREATIVE COMPUTING VOL. 11, NO. 5 / MAY 1985 / PAGE 14

Modem magic; more baud for the buck: a buyer's guide to understanding, choosing, and using modems. Russ Lockwood.

In the squat ivory tower that houses our editorial offices, telecommunications is one of the hottest topics around. Our survey in the February issue indicates that it is popular among readers as well. The increased sales of hardware and software products, coupled with a proliferation of on-line services, reflects this trend. All across the market, from the low end to the upscale, people are discovering the ease, speed, and convenience of telecommunications.

The key to unlocking this long distance power is a modem, a device that allows computers to communicate with one another over common telephone lines. It translates the digital signals of your computer into the analog signals recognized by the telephone system. A modem at the other end of the line converts the analog signals back into digital form.

This process is called modulation-demodulation, and the modem derives its name from what it is: modulator-demodulator.

Modems generally use either synchronous or asynchronous communications. Synchronous modems organize data into blocks and transmit all the blocks at one time. Asynchronous modems send data in a long stream, one character at a time. The vast majority of modems for microcomputers are asynchronous.

Long Distance Chaos

A computer creates digital signals by using bursts of voltage, representing on and off (1 and 0). These binary electrical signals must be precise; any deviation causes errors in the data.

On the other hand, the telephone system transmits analog signals, which carry information over the lines using fluctuating electrical voltage that reacts to changes in sound pressure. The louder you talk into a telephone, the higher the voltage in the line.

Both types of signal run into problems during long distance transmission. The longer the distance between communication points, the weaker and more distorted the signal. Eventually, both digital and analog signals fade into unrecognizable static.

The telephone company solves the problem by using special analog filters and amplifiers that select and boost a limited bandwidth of comprehensible voice communication frequencies. Signals outside this range of 300 Hz to 3300 Hz are discarded.

Unmodulated digital signals receive a hostile welcome from the telephone lines. The filtering and amplifying blur the precise signals. General line noise mimics digital pulses and produces echoes. If a signal falls outside the 300 to 3300 Hz range, it is discarded. All this adds up to distorted transmissions.

To avoid these problems, a modem changes digital signals into audio tones and transmits these tones over specific frequencies. And just as different voltage bursts in a computer represent on and off (1 and 0), so the modem uses different audio frequencies to duplicate on and off patterns.

Since telecommunication is usually two-way, a modem uses two sets of tones to distinguish the data it transmits from the data it receives. Furthermore, to coordinate the tone frequencies, one modem acts as a signal originator and the other acts as a signal answerer.

Figure 1 shows the frequencies used by the most common type of modem (AT&T Bell 103 standard). The technical term for this modulation-demodulation technique is Frequency Shift Keying. The signal originator transmits at one pair of frequencies, which is the exact pair of frequencies the signal answerer expects to receive. Notice that when the signal answerer sends data to the originator, the two sets of transmitting and receiving frequencies also match.

The proper term for simultaneous, two-way data transmission is full-duplex. As you may have guessed, one-way data transmission is called half-duplex. Just about all Bell 103 standard modems are full-duplex.

Greased Lightning

The data you send over a telephone line uses a code called ASCII, short for American Standards Code for Information Interchange. Each transmitted character (letter, number, punctuation, or symbol) is made up of seven bits, has two more framing bits to tell each computer when the bit pattern for a character starts and stops, and may include one or two parity bits for error checking. All told, a single character can be up to 11 bits long.

The speed of a modem is measured in bits per second, more commonly referred to as baud. Actually, baud is something of a misnomer, referring to the number of times a modem signals another modem. At first, bits per second and baud could be used interchangeably, but with advances in technology, differences have appeared. Nevertheless, baud remains a popular, if slightly improper, term for transmission speed.

Back in the days of the Teletype machine, transmission speed was a hair-raising 110 baud, roughly 10 or 11 characters per second. The Bell 103 standard mentioned above transmits at a maximum of 300 baud, or roughly 30 characters per second.

Transmission speeds faster than 300 baud are possible using the Frequency Shift Keying modulation technique. However, full two-way communication places some of the rapidly shifting frequencies outside the limited bandwidth of 300 Hz to 3300 Hz. As mentioned above, signals outside this range are discarded.

To meet the need for higher speed, AT&T introduced the Bell 202 standard. While technically a full-duplex modem, it allows 1200 baud transmissions in only one direction. The return channel is only five baud, which is practically useless for transferring long files. Although cumbersome, a Bell 202 standard modem can to used for half-duplex telephone communication. In this case, the parties must take turns sending data at 1200 baud.

Modem Wars

The breakthrough came in late 1972, when Racal-Vadic introduced the VA3400, a full-duplex 1200 baud modem. The VA3400 uses Quadrature Amplitude Modulation, one variation of a technique called Phase Shift Keying, to make the digital-analog signal conversion. As before, one modem functions as the signal originator while the other is the signal answerer.

The signal originator transmits a constant signal at 2250 Hz, and the signal answerer transmits a constant signal at 1150 Hz. Instead of changing frequencies as in Frequency Shift Keying, Quadrature Amplitude Modulation keeps a constant frequency but changes the starting point of the analog sound waves. The modem sends out an electronic marker and regularly checks to see where that marker is in relation to the incoming waves from the other modem. The "Quad" refers to the four positions at which the modem detects the marker--the top of the wave, the bottom of the wave, midway between the top and bottom on the "up" side, and midway between the top adn bottom of the "down" side. Each position actually represents two bits (called dibits) of information.

AT&T Strikes Back

In 1976, AT&T introduced its own full-duplex 1200 baud modem, the Bell 212A standard. This modem uses a variation of Phase Shift Keying technique, called Four Phase Differential Phase Shift Keying, which is completely incompatible with Quadrature amplitude Modulation. The signal originator transmits at 1200 Hz, and the signal answerer transmits at 2400Hz, roughly the opposite of the VA3400 from Racal-Vadie.

Most 1200 band modems use the Bell 212A standard, just as most 300 baud modems use the Bell 202 protocols are not extinct. Indeed, many information services offer VA3400 compatibility. However, Bell 212A is significantly more popular than either of them.

The rest of the world does not recognize the Bell 212A standard. Instead, by treaty, other countries have settled on an incompatible standard called V.22 put forth by the Consultive Committee on International Telephone and Telegraph (CCITT). It uses a different modulation technique operating on different frequencies.

Return of the Innovator

At this time, 11 companies--Racal-Vadic, US Robotics, Hayes Microcomputer Products, Multi-Tech Systems, Novation, Micom, Penril DataComm, Case Rixon, GandalfData, Concord Data Systems, and Cermetek Microelectronics--have introduced 2400 baud modems, and others are soon to follow. AT&T also introduced a 2400 baud modem, but the Bell 2224 protocol has yet to catch the fancy of the industry.

Predicting which protocol will succeed as the 2400-baud industry standard is difficult to do. Essentially, manufacturers are cramming 2400 bits where 1200 bits used to be using a variation of Phase Shift Keying. There seems to be some consensus in using adaptive equalizing techinques to preserve signal quality and prevent errors, but that is not a transmission technique.

CCITT already has specifications for 2400 baud modems: V.22bis. Only time will tell whether it becomes a North American standard, but most of the manufacturers are offering CCITT compatibility with their 2400 baud modems.

Several manufacturers offer modems with speeds of 9600 baud and beyond. Some of them use Phase Amplitude Modulation, another version of Phase Shift Keying. Note that these high-speed modems are usually reserved for mainframe communications. In any event, one of these superfast modems can cost more than your entire computer system.

A Port of Call

Back in 1969, the Electronics Industry Association adopted RS-232 as a standard for serial communication between data terminal equipment (your computer) and data communication equipment (the modem). RS-232C stands for Recommended Standard 232 Version C and is indeed the industry standard port for telecommunications.

Among other things, RS-232C calls for a 25-pin connection between modem and computer and specifies which signal each pin carries. Thus, pin 1 is a ground, pin 2 transmits data, pin 3 receives data, and so on.

The more-or-less standard connector to the RS-232C port is called DB-25. It derives its name, in part, because it is shaped liked the letter D. Most, but not all, computers and modems use a female DB-25 connector, which means the intervening cable requires male DB-25 connectors at each end. However, the IBM PC communications card, for example, has a male DB-25 connector, which means you need a female connector on one end of the modem cable and a male connector at the other.

Note that many modems do not send signals through all 25 pins. Ignoring a few pins is perfectly acceptable. A manufacturer is said to support RS-232C as long as the signals do not directly violate it. Many modems will function using only pin 2 (transmit data), pin 3 (receive data), and pin 7 (signal ground).

Finally, the modem must be connected to the telephone system--a task often accomplished by plugging the line from the modular phone jack on the wall into the modem. Although this should go without saying, in more than a few cases, novice telecommunicators have neglected to perform this step, producing far from satisfactory results.

A Fist Full of Features

Now that you understand the fundamentals of how modems operate, what features should you look for? Choosing a modem takes as much time and effort as choosing a printer or monitor--certainly the costs are roughly the same. As always, your decision should reflect your needs. It should meet your current requirements and allow for future growth. Contrary to marketing claims, many people just do not need the fastest, fanciest, and costliest modem around.


Basically, you can choose from 300 baud, 1200 baud, and 2400 baud modems. Quite often, the higher speed modem also includes the electronic circuitry for the lower speeds. Thus, one of the most popular combinations around is a 1200/300 baud modem.

A 1200 baud modem is four times as fast as a 300 baud modem. Likewise, a 2400 baud modem is twice as fast as a 1200 baud modem. To give you an idea of the difference in transmission time, a 10,000-word data file should take approximately three minutes to send at 2400 baud, a little over six minutes at 1200 baud, and an agonizing 24.5 minutes at 300 baud. Clearly, if you routinely transmit and receive long files, and the business axiom "time is money" applies, the higher speed is desirable.

On the other hand, a 2400 baud modem for accessing a consumer telecommunications service represents overkill. If you are using the Source to "chat" or CompuServe to play games, a 300 baud modem works just fine. Most people do not type 30 characters a second, and waiting around for the mainframe to execute your command does not take advantage of that super speed.

Note that most telecommunications services charge extra for using faster speeds, and many do not support 2400 baud transmission. Also not that many networks (Tymnet, Uninet, MCI, et al.) are not capable of 2400 baud communication. This is probably a short-term problem though.

Also remember that you need modems of equal speed at both ends of the line. A 1200 baud modem cannot directly connect with a 300 baud modem at 1200 baud--it needs to go down to 300 baud. Likewise, you need to compatible modems at each end. Buying a VA3400 modem will not let you directly connect with a Bell 212A standard modem.


Modems come in two types: acoustic and direct connect. A coustic modems were first on the market, in part because AT&T held a monopoly on direct connection into the telephone system. The handset of the phone fits snugly in the two rubber cups of an acoustic modem: one holds a speaker to generate tones and the other holds a microphone to receive tones sent by the modem at the other end of the line.

Direct connect modems bypass the telephone entirely. You plug the line from the modular jack on the wall directly into the modem.

Direct connect modems are by far the more reliable. Acoustic modems are notorious for picking up external noise that results in transmission errors. Many of the handsets on newer telephones do not fit in the cups, which increases the risk of errors. While direct connect modems are generally more expensive than acoustic modems, the improved signal quality is worth the extra cost.

However, older houses may not have the modular plugs needed by direct-connect modems, so unless you want the phone company to install a new jack (or you do it yourself), an acoustic modem is your only choice.

Many direct-connect modems provide a second jack on the modem itself to hook up the telephone. You can use either the modem or the telephone; this handy arrangement relieves you of the chore of plugging and unplugging the telephone every time you use the modem.

Internal Versus External

This is one debate that has no clear-cut answer. Both types have numerous advantages and disadvantages. The choice is really a personal one.

An internal modem (an expansion board) fits into an expansion slot inside your computer. An external modem resides outside, connected via an RS-232C port and a cable.

Internal modems tuck neatly away inside the system unit, are automatically transported with the computer, and require no serial port, modem cable, or external power supply. On the other hand, they occupy valuable expansion slots, increase internal heat, show to external lights, can be reset only by turning the computer off and on, and cannot be transferred to an incompatible computer (you cannot use an Apple internal modem inside an IBM PC).

External modems are easily ported to other computers, have lots of indicator lights, have power on/off switches for instant reset, and include DIP switches for special use. On the negative side, they require desk space, a power adapter, a cable between computer and modem, and a special serial communications card (or the capability on a multifunction card).

You may want to give additional consideration to an external modem only because of portability. If you upgrade from one computer to another, you can take the modem along with you. However, an external modem costs more than an internal one--somewhere in the neighborhood of $100 more--even if you buy through a mail order house.

Voice/Data Jacks

As we mentioned before, many direct connect modems (internal and external) include two modular phone jacks--one for data transmissions and one for a voice line. The smarter modems automatically switch between the two, although most change with the flick of a switch.

Self Test

Many modems perform a self test. A pattern generator and error checking circuitry verify the ability to send and receive data. The three tests are analog loop back, digital loop back, and remote digital loop back.

In the analog loop back test, data are sent from the pattern generator through the transmitter. The receiver routes the data into the error checking circuitry, which compares them with the original data and flags any errors.

The digital loop back test allows you to test another modem. Data are sent from your modem to the receiver of the other modem, through the transmitter, and back to the error checking circuitry in your modem. Note that the digital loop back test requires the help of a person to tend the other modem. The remote digital loop back test is the same as the digital loop back test, but needs no additional person at the other end.


Half duplex, or one-way communication, has generally been superceded by full duplex, or two-way communication. Unless you you need a modem for specialized use that requires only half duplex operation, stick to full duplex.

Automatic Response

Auto dial/auto answer are two of the more popular modem features. With the old acoustic modems, you had to dial the number by hand. Newer, smarter modems allow you to dial numbers through the software from the keyboard. The modem mimics the tones of the phone to call out. Likewise, the modem can automatically answer the phone, recognizing the tones and beginning appropriate procedures.


Many modem manufacturers bundle software with their modems. For an overview of packages available separately, see the February 1985 issue in which we reviewed 17 telecommunications software packages. Unfortunately, three of the real heavyweights were absent--Hayes Smartcom II, US Robotics Telpac, and IBM Personal Communications Manager.

Smartcon II is the choice of many Hayes Smartmodem and Smartmodem-compatible users. It allows for easy changes in baud rate, bit patterns, and most other modem functions. It provides a directory for storing numbers and log-in procedures that are available at the touch of the button. It requires 96K to run, and retails for $119. Versions are available for many popular computers.

Telpac is compatible with many popular computers, including the IBM PC and compatibles, Apple II, and computers running CP/M. It supports many modems, including Multi-Tech, Novation, Anchor, Prentice, Hayes, Ven-Tel, Prometheus, and of course, US Robotics. It includes terminal emulation (VT 52/100, Televideo 910, and TTY ASCII), and the latest version includes windowing and instant access to menus. It requires 192K (256K suggested) and retails for $99.

Personal Communications Manager is the official IBM communications package for the PCjr. It stores telephone numbers and log-on procedures, transmits and receives files, and keeps track of how long you have been on line. It also includes a handy electronic mail function and the ability to program function keys. Personal Communications Manager requires 128K and sells for $100.

If software is not bundled with the modem you buy, you must purchase it. You absolutely need telecommunications software to run a modem. IBM PC owners can probably find a copy of PC Talk III at a local user's group. PC Talk III is called freeware, which means you are encouraged to copy and pass out the software. Not only does it carry an attractive "price," but it is a good telecommunications program.

The two types of programs generally available are menu-driven and command-driven. Menu-driven software (like Hayes Smartcom II) is easier to learn, but takes longer to use. Command-driven software (like Microstuf Crosstalk XVI) is harder to learn, but faster to use once learned. Features to look for include selectable baud rate, selectable bit patterns, directory, automatic log-on procedures, file transfer prompts, automatic redial, and help menus.

The Mainframe Connection

Right now, 2400 baud seems to be the "state-of-the-art" speed for microcomputer modems, with 4800 baud and 9600 baud looming on the horizon. Yet telecommunications within networks and between mainframes exceeded that speed long ago. These sophisticated (and expensive) modems shoot between 9600 and 19,200 bits per second down the line--using the same RS-232C port found on your computer.

You can gain the speed benefits of these sophisticated modems if you are willing to bear the cost of the modem and the cost of leasing a dedicated telephone line. The basic level of quality you can expect is termed 3002. Better quality lines are rated C-1, C-2, and so on. Higher speeds increase the probability of transmission errors, so anything less than a leased line is data suicide.

Telecommunication Tolls

Hardware and software prices fluctuate dramatically. Usually, the trend is downward. A quick look through the advertisements of mail order houses confirms that many 300 baud modems (which do an admirable job of accessing telecommunications services) cost under $100. Add the cost of a cable (roughly $25) and you are ready to access the world.

On the other hand, a feature-packed top-of-the-line modem can cost you several hundred dollars. You must decide what will best meet your needs. For the home, where telecommunication is more a luxury than a neccesity, 300 baud is a good starting point-inexpensive and fast enough for occasional use. Businesses and professionals probably need more speed--a 1200/300 baud modem should do the trick. To steal a phrase, they give you the most baud for the buck.

The new 2400 baud modems are lightning fast, but at this time, the scarcity of compatible hardware and acceptable high speed links make this a longer term investment. However, just about all include 1200 and 300 baud capabilities, so if you buy now, you will not have to upgrade in the future. Falling prices, increased sales, and wider service will make 2400 baud modems an attractive investment over the next year.

All this assumes you already own a serial card with an RS-232C serial port. Most multifunction cards and some computers include one as standard equipment. If not, you must purchase and install an expansion card with an RS-232C serial port before you can use a modem.

To use a modem, you must use a telephone line, and that means paying telephone charges just as if you were talking to someone. The usual rules apply: calling during business hours incurs the most expensive tolls, while calling after 11:00 p.m. local time gets you the cheapest rate. Yes, you can put a big dent in your pocketbook by calling all the marvelous bulletin boards and services across the country;

Note that you can use alternate long distance services like MCI and Sprint, but your software and hardware should pause and recognize the second dial tone that sounds when you connect with the network. Otherwise, themodem will begin to transmit the phone number, log-on sequence, and other data before you actually enter the network, resulting in a telecommunications tangle.

Fortunately, to limit long distance charges and help prevent transmission errors, most telecommunications services allow access through dedicated telecommunications networks that operate local trunk lines. CompuServe, for example, operates its own network. Tymnet and Telenet, two network giants, serve most of the major services, including CompuServe and The Source.

Unfortunately, these local trunk lines are sometimes not so local. Quite often, especially in remote areas, you still incur charges because the phone number lies outside your local calling range. Also, some services charge you a premium for accessing them through other telecommunications networks. However, all things considered, going through a network is generally less expensive than calling the service direct.

And lest you forget, telecommunications services cost money. They usually charge by the hour at rates from $2 to $300. Before you sign up, ask for information. Most send a packet that extolls the virtues of their service and tells you the rates. For a list of the more popular services, see the sidebar.

The End of the Line

What does the future hold for telecommunications? It is difficult to say, but in "Telecommunications: The Ideal and the Reality" (February 1985 issue), Barry Keating provides a good idea of how the business professional can use a modem to improve work flow.

The possibilities for students are equally impressive. Telecommunications can replace the limitations of the local library with the virtually boundless resources of huge databases. Chances are, if you want to research a particular topic, exhaustive information is already available from one of the many services.

A growing number of people use telecommunications for leisure. You may not be able to snare a local opponent for chess at 10:00 p.m., but you can find someone across the country. Many people find new friends with the same interests through telecommunications. The CB on CompuServe and Chat on The Source are two of the most popular meeting places.

The comparison chart will help you decide which modem is right for you. Armed with the appropriate hardware and software, you too can take advantage of the speed, ease, and convenience of telecommunication to improve productivity, research topics, and just plain relax. And the best part--it is only a phone call away.