Modem

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Acoustic coupler modem

A modem (modulator-demodulator) is a network hardware device that modulates one or more carrier wave signals to encode digital information for transmission and demodulates signals to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data.

Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (symbol bit/s, sometimes abbreviated "bps"), or bytes per second (symbol B/s). Modems can also be classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency shift keying with two possible frequencies, corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which could transmit and receive four distinct symbols (two bits per symbol), transmitted 1,200 bits by sending 600 symbols per second (600 baud) using phase shift keying.

Dialup modem[edit]

History[edit]

TeleGuide terminal

News wire services in the 1920s used multiplex devices that satisfied the definition of a modem. However, the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems. Modems grew out of the need to connect teleprinters over ordinary phone lines instead of the more expensive leased lines which had previously been used for current loop–based teleprinters and automated telegraphs.

Mass-produced modems in the United States began as part of the SAGE air-defense system in 1958 (the year the word modem was first used^[1]), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. and Canada. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems.

The 201A and 201B Data-Phones were synchronous modems using two-bit-per-baud phase-shift keying (PSK). The 201A operated half-duplex at 2,000 bit/s over normal phone lines, while the 201B provided full duplex 2,400 bit/s service on four-wire leased lines, the send and receive channels each running on their own set of two wires.

The famous Bell 103A dataset standard was also introduced by AT&T in 1962. It provided full-duplex service at 300 bit/s over normal phone lines. Frequency-shift keying was used, with the call originator transmitting at 1,070 or 1,270 Hz and the answering modem transmitting at 2,025 or 2,225 Hz. The readily available 103A2 gave an important boost to the use of remote low-speed terminals such as the Teletype Model 33 ASR and KSR, and the IBM 2741. AT&T reduced modem costs by introducing the originate-only 113D and the answer-only 113B/C modems.

Acoustic couplers[edit]

Carterfone and direct connection[edit]

The Hush-a-Phone decision applied only to mechanical connections, but the Carterfone decision of 1968, led to the FCC introducing a rule setting stringent AT&T-designed tests for electronically coupling a device to the phone lines. This opened the door to direct-connect modems that plugged directly into the phone line rather than via a handset. However, the cost of passing the tests was considerable, and acoustically coupled modems remained common into the early 1980s.

The rapidly falling prices of electronics in the late 1970s led to an increasing number of direct-connect models around 1980. In spite of being directly connected, these modems were generally operated like their earlier acoustic versions – dialling and other phone-control operations were completed by hand, using an attached handset. A small number of modems added the ability to automatically answer incoming calls, or automatically place an outgoing call to a single number, but even these limited features were relatively rare or limited to special models in a lineup. When more flexible solutions were needed, 3rd party "dialers" were used to automate calling, normally using a separate serial port to communicate with the dialler, which would then control the modem through a private electrical connection.

The introduction of microcomputer systems with internal expansion slots made the first software-controllable modems common. Slot connections gave the computer complete access to the modem's memory or I/O channels, which allowed software to send commands to the modem, not just data. This led to a series of popular modems for the S-100 bus and Apple II computers that could directly dial the phone, answer incoming calls, and hang up the phone, the basic requirements of a bulletin board system (BBS). The seminal CBBS was created on an S-100 machine with a Hayes internal modem, and a number of similar systems followed.

The Smartmodem and the rise of BBSs[edit]

The original model 300-baud Hayes Smartmodem

In later years, US Robotics Courier modems were common at many BBS's because they had better compatibility with more brands of modems

The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300-bit/s direct-connect modem, but it was attached to a small microcontroller that watched the data stream for certain character strings representing commands. This allowed both data and commands to be sent through a single serial port. The now-standard Hayes command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls, among others. This was similar to the commands offered by the internal modems, but unlike them, the Smartmodem could be connected to any computer with an RS-232 port, which was practically every microcomputer built.

The introduction of the Smartmodem made communications much simpler and more easily accessed. This provided a growing market for other vendors, who licensed the Hayes patents and competed on price or by adding features. Through the 1980s, a number of new higher-speed modems, first 1,200 and then 2,400 bit/s, greatly improved the responsiveness of the online systems, and made file transfer practical. This led to rapid growth of online services with their large file libraries, which in turn gave more reason to own a modem. The rapid update of modems led to a similar rapid increase in BBS use, which was helped by the fact that BBSs could control the modem simply by sending strings, rather than talking to a device driver that was different for every direct-connect modem.

1200 and 2400 bit/s[edit]

The 300 bit/s modems used audio frequency-shift keying to send data. In this system the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines. In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz. These frequencies were chosen carefully; they are in the range that suffers minimum distortion on the phone system and are not harmonics of each other.

In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase. Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid-1980s. A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe. The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still only 600 baud (symbols per second). The bit rate increases were achieved by defining four or eight distinct symbols, which allowed the encoding of two or three bits per symbol instead of only 1. The use of smaller shifts had the drawback of making each symbol more vulnerable to interference, but improvements in phone line quality at the same time helped compensate for this. By the late 1980s, most modems could support all of these standards and 2,400-bit/s operation was becoming common.

Proprietary standards[edit]

Many other standards were also introduced for special purposes, commonly using a high-speed channel for receiving, and a lower-speed channel for sending. One typical example was used in the French Minitel system, in which the user's terminals spent the majority of their time receiving information. The modem in the Minitel terminal thus operated at 1,200 bit/s for reception, and 75 bit/s for sending commands back to the servers.

Three U.S. companies became famous for high-speed versions of the same concept. Telebit introduced its Trailblazer modem in 1984, which used a large number of 36 bit/s channels to send data one-way at rates up to 18,432 bit/s. A single additional channel in the reverse direction allowed the two modems to communicate how much data was waiting at either end of the link, and the modems could change direction on the fly. The Trailblazer modems also supported a feature that allowed them to spoof the UUCP g protocol, commonly used on Unix systems to send e-mail, and thereby speed UUCP up by a tremendous amount. Trailblazers thus became extremely common on Unix systems, and maintained their dominance in this market well into the 1990s.

USRobotics (USR) introduced a similar system, known as HST, although this supplied only 9,600 bit/s (in early versions at least) and provided for a larger backchannel. Rather than offer spoofing, USR instead created a large market among Fidonet users by offering its modems to BBS sysops at a much lower price, resulting in sales to end users who wanted faster file transfers. Hayes was forced to compete, and introduced its own 9,600-bit/s standard, Express 96 (also known as Ping-Pong), which was generally similar to Telebit's PEP. Hayes, however, offered neither protocol spoofing nor sysop discounts, and its high-speed modems remained rare.

A common feature of these high-speed modems was the concept of fallback, or speed hunting, allowing them to communicate with less-capable modems. During the call initiation, the modem would transmit a series of signals and wait for the remote modem to respond. They would start at high speeds and get progressively slower until there was a response. Thus, two USR modems would be able to connect at 9,600 bit/s, but, when a user with a 2,400 bit/s modem called in, the USR would fall back to the common 2,400 bit/s speed. This would also happen if a V.32 modem and a HST modem were connected. Because they used a different standard at 9,600 bit/s, they would fall back to their highest commonly supported standard at 2,400 bit/s. The same applies to V.32bis and 14,400 bit/s HST modem, which would still be able to communicate with each other at 2,400 bit/s.

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