This Recommendation defines a modem for use in telegraph TDM systems. The principal characteristics of this Recommendation are for use on 4-wire leased telephone-type circuits. Other applications, such on the GSTN are for further study.
ITU-T R.21 SPANISH-1996 Referenced Document
ITU-T H.22-1984 Transmission requirements of international voice-frequency telegraph links (at 50@ 100 and 200 bauds)
ITU-T M.1020-1993 Characteristics of Special Quality International Leased Circuits with Special Bandwidth Conditioning - Maintenance: International Leased Circuits (Study Group IV) 7 pp
ITU-T M.1025-1993 Characteristics of Special Quality International Leased Circuits with Basic Bandwidth Conditioning - Maintenance: International Leased Circuits (Study Group IV) 11 pp
ITU-T V.10-1993 Electrical Characteristics for Unbalanced Double-Current Interchange Circuits Operating at Data Signalling Rates Nominally up to 100 kbit/s - Data Communication over the Telephone Network (Study Group XVII) 21 pp [Superseded: ITU-T X.26]
ITU-T V.28-1993 Electrical Characteristics for Unbalanced Double-Current Interchange Circuits (Study Group XVII)