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Bell 101 - Wikipedia

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Commercial modem

The back of a Teletype Model 33 teleprinter with a Bell 101C Data Set in the pedestal, 1963.

The Bell 101 Data Set was the first commercial modem for computers, released by AT&T Corporation in 1958 for use by SAGE, and made commercially available in 1959, shortly after AT&T's Bell Labs announced their 110 baud modulation frequencies.^[1]^[2] The Bell 101 allowed digital data to be transmitted over regular unconditioned telephone lines at a speed of 110 bits per second.

The Bell 101 modem used audio frequency-shift keying to encode data. Different pairs of audio frequencies were used by each station:^[3]

The originating station used a mark tone of 1,270 Hz and a space tone of 1,070 Hz.
The answering station used a mark tone of 2,225 Hz and a space tone of 2,025 Hz.

Bell 101 modems are no longer in use and were quickly replaced by its successor the Bell 103 modem. SAGE modems were described by AT&T's Bell Labs as conforming to the Bell 101 data set standard.

Weber, Laurance A. (1959). "A frequency-modulation digital subset for data transmission over telephone lines". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 77 (6): 867–872. doi:10.1109/TCE.1959.6372907. S2CID 51673551.
Soffel, R. O.; Spack, E. G. (1959). "SAGE data terminals". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 77 (6): 872–879. doi:10.1109/TCE.1959.6372908. ISSN 0097-2452. S2CID 51676153.
Data Set 101C: Identification and Operation (PDF). Bell System Practices. AT&T Co. March 1963.
Kline, Ronald R. (2019). "The Modem that Still Connects Us". Historical Studies in Computing, Information, and Society. History of Computing. pp. 29–50. doi:10.1007/978-3-030-18955-6_3. ISBN 978-3-030-18954-9. S2CID 213434332.

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