The 202 Data-Phone was a half-duplex asynchronous service that was marketed extensively in late 1960. In summer 1960, the name Data-Phone was introduced to replace the earlier term digital subset. While they ran on dedicated telephone lines, the devices at each end were no different from commercial acoustically coupled Bell 101, 110 baud modems. SAGE modems were described by AT&T's Bell Labs as conforming to their newly published Bell 101 dataset standard. 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), connecting terminals at various airbases, radar sites, and command-and-control centers to the SAGE director centers scattered around the U.S. In 1942, IBM adapted this technology to their unit record equipment and were able to transmit punched cards at 25 bits/second. 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. However the modem function was incidental to the multiplexing function, so they are not commonly included in the history of modems. News wire services in the 1920s used multiplex devices that satisfied the definition of a modem. 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. 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. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. Modems can also be classified by their symbol rate, measured in baud. Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s or bps), or bytes per second (B/s). These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data. The most familiar type is a voice band modem that turns the digital data of a computer into modulated electrical signals in the voice frequency range of a telephone channel. Modems can be used with any means of transmitting analog signals, from light emitting diodes to radio. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. A modem ( modulator- demodulator) is a device that modulates an analog carrier signal to encode digital information and demodulates the signal to decode the transmitted information.