The family of modems covered by this Recommendation should be applicable to the following uses: a) transmission of data between customers on leased circuits; b) transmission of a multiplex aggregate bit stream for public data networks; c) extension of a PCM channel at 64 kbit/s over analogue facilities; d) transmission of a common channel signalling system for telephony and/or public data networks; e) extension of Single-Channel-Per-Carrier (SCPC) circuit from a satellite earth station; f) transmission of a multiplex aggregate bit stream for telegraph and data signals. Principal recommended characteristics to be used for simultaneous both-way synchronous operation.