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.
Contents
- 1 Dialup modem
Dialup modem
History
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). [see MODULSTION as in AM; does not seem to be realted to freq hop]
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
See also: Acoustic coupler
The Novation CAT acoustically coupled modem
For many years, the Bell System (AT&T) maintained a monopoly on the use of its phone lines and what devices could be connected to them. However, the seminal Hush-a-Phone v. FCC case of 1956 concluded it was within the FCC's jurisdiction to regulate the operation of the Bell System. The FCC found that as long as a device was not electronically attached to the system, it would not threaten its integrity. This led to a number of devices that mechanically connected to the phone through a standard handset. Since most handsets were supplied by Western Electric and thus of a standard design, acoustic couplers were relatively easy to build. This type of connection was used for many devices, such as answering machines.
Acoustically coupled Bell 103A-compatible 300 bit/s modems were common during the 1970s. Well-known models included the Novation CAT and the Anderson-Jacobson, the latter spun off from an in-house project at Stanford Research Institute (now SRI International). An even lower-cost option was the Pennywhistle modem, designed to be built using parts from electronics scrap and surplus stores.
In December 1972, Vadic introduced the VA3400, notable for full-duplex operation at 1,200 bit/s over the phone network. Like the 103A, it used different frequency bands for transmit and receive. In November 1976, AT&T introduced the 212A modem to compete with Vadic. It was similar in design, but used the lower frequency set for transmission. One could also use the 212A with a 103A modem at 300 bit/s. According to Vadic, the change in frequency assignments made the 212 intentionally incompatible with acoustic coupling, thereby locking out many potential modem manufacturers. In 1977, Vadic responded with the VA3467 triple modem, an answer-only modem sold to computer center operators that supported Vadic's 1,200-bit/s mode, AT&T's 212A mode, and 103A operation.
Carterfone and direct connection
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
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....