It began as a work-around and survived to usher in the Internet
THE STORY OF THE computer modem begins in the late 1940s, when Cold War concerns caused the United States military to reconsider its defense against longrange bombers. A key component of this system was an automated collection of hundreds of radar earlywarning stations, which would detect possible intruders and send information about them to commandand-control centers. The plan was to transmit these radar images using microwave relays, but building such a network would take many years and vast amounts of money.
Fortunately, a convenient method of communication that was already in place could serve as a stopgap—the telephone system. The main problem was that radar images had to be transmitted in digital form so that they could be processed by digital computers, while the telephone network was designed to transmit analogue voice signals.
A digital signal consists of rapid alternations between a high and low voltage, which correspond to 1 and 0. The 1’s and 0’s are separated by pauses. An analogue signal, by contrast, is a continuous wave (known as the carrier) whose up-and-down oscillations are altered, or modulated, by the addition of a second signal. For air defense, the military needed something that would take digital signals, convert them to analogue for transmission, and then convert them back to digital afterward. This device was called a modem, short for modulator-demodulator.
Modems already existed for use in the Teletype, which was basically a remotecontrolled electric typewriter. But Teletype data was transmitted at top speeds of around 150 bits per second (bps), which, at 8 bits per character, worked out to about 20 characters per second. The military needed 10 times that. So in the mid1950s Bell Laboratories developed improved modems that used amplitude modulation over dedicated phone lines to transmit at speeds of 1,600 bps or more.
The commercial market was slower to develop, but in 1958 Bell Labs’ parent company, AT&T, began leasing modems to businesses. Since the amplitudemodulation method was not robust enough to stand up to the noise of ordinary phone lines, the Bell 103 used the slower technique of “frequency-shift keying,” in which the carrier signal shifts between two different frequencies. The Bell 103 had a top speed of 300 bps, but it was sturdy; technicians boasted that it would work over barbed wire. By the late 1960s AT&T was offering modems with speeds up to 1,200 bps over dialup connections and 9,600 bps over dedicated lines.
At first AT&T had the modem market all to itself, mainly because it prohibited customers from connecting third-party equipment to its telephone lines. In 1968 the Federal Communications Commission voided this prohibition, and soon AT&T had to compete with modems from such companies as General Electric and MultiTech. Unlike AT&T modems, which had to be installed by a technician, most of these used a simpler, userinstalled solution, the acoustic coupler.
An acoustic coupler consisted of a pair of rubber cups into which the user inserted an ordinary telephone handset. One cup picked up the telephone’s sound output with a microphone, which converted it into an analogue signal. This signal was then converted to digital and sent to the user’s computer (or terminal). The other cup received the computer’s (or terminal’s) output, converted it from digital to analogue, and then used a speaker to convert that signal into sound, which was picked up by the telephone’s mouthpiece. Acoustic couplers made it simpler and cheaper to get connected, but they introduced an extra step to the process—the conversion from signal to sound and back again. Although this was not a problem at rates of a few hundred bps, it had to be avoided if modems were to get any faster.
Over the 1970s and 1980s the RJ-11 connector—the now-ubiquitous modular phone plug—gradually rendered the acoustic coupler obsolete. Top speeds climbed in dizzying fashion, mostly by compressing information and transmitting on multiple frequencies, with accompanying advances in filtering noise and detecting and correcting errors. Today’s fastest modems have a nominal capacity of 56,000 bps, though in most cases they go no faster than 40,000 or so.
Joshua Bloch of Sun Microsystems, the author of Effective Java Programming Language Guide , recalls the steady progression in modem speeds: “The first modem I used was a 110 bps Anderson-Jacobson rented by my high school. A couple of years later we upgraded to an LA36 DECWriter II terminal with a 300 bps modem. It felt blazingly fast. In 1979 I bought my own first modem, a 300 bps Novation CAT. I used this for the next five years until my brother gave me a very generous Christmas gift, an Anchor Automation Signalman Mark XII, the first affordable 1,200 bps modem. Gone was the acoustic coupler; I felt as if I was on cloud nine.
“I continued to use the Signalman until 1989, when my company lent me a Telebit TrailBlazer, which cost about $700. It seemed astonishingly fast at 9,600 bps, with even higher burst rates. The TrailBlazer acquired a certain degree of fame among geeks as the workhorse of UUCP/Usenet, the predecessor of today’s Internet newsgroups. About a year later I tried to sell the old Signalman for 50 cents at a yard sale. There were no takers.
“Now, of course, you can get a 56K modem for around $20, but it seems quaint and snail-like. I can’t remember the last time I used a telephone modem. I’m currently connected to the Internet via DSL [digital subscriber line], which can handle 340 Kbps. Although it’s 3,000 times faster than my first modem and always on, I complain that it’s too slow; my friend across town gets more than a megabit per second over his DSL line.”
In fact, all-digital connections were available as far back as the early 1960s, though not to individual users. A digital network formed the backbone of ARPANET , the militaryacademic electronic community that eventually turned into the Internet. Today even telephone companies have gone mostly digital, with analogue connections surviving only between the user’s home and the central station. At the central station, signals are digitized using pulse-code modulation—sampled 8,000 times per second, with each sample yielding 7 bits for a total rate of 56,000 bps. Because all telephone signals undergo this process, the limit on modem speed has finally been reached; no dial-up modem can go any faster.
Modems of the 1960s were thought of as laboratory instruments and designed as utilitarian boxes. Today’s external modems are modest-priced consumer products and accordingly have the low, sculpted white blandness of an intercom or answering machine. While this fate is inescapable in the computer world, the story of the modem, which was originally meant as a stopgap until something better came along, serves as a remir.der of the ancient days when it actually took several decades for a piece of equipment to go from futuristic to banal.