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An answer-only modem is a network device that is capable of receiving messages, but cannot send messages. Answer-only modems were usually the most inexpensive modems available.
Answer-only modems were first manufactured by Bell Systems around 1962. They used frequency-shift keying (an early frequency modulation scheme) and received messages at a rate of 2,025 Hz. AT&T later came out with both an originate-only modem, model 113D, and an answer-only modem, model 113B/C. In 1977, Vadic came out with the VA3467 triple modem, an answer-only modem; it was sold to computer center operators and had a 1,200-bps mode.
In 1981, Hayes introduced its Smartmodem and later produced an answer-only modem model, which operated on the server side but was much more expensive than the acoustically coupled models (picked up audio signals from a telephone handset) that operated on the client side. However, this answer-only modem was a low-cost, server-side modem that helped make the bulletin board systems (BBSs) flourish.