This Recommendation specifies the operation of a duplex digital modem for use on a 4-wire circuit switched connection and on leased point-to-point 4-wire digital circuits at data signalling rates of up to 64 000 bit/s. The modem is specified in terms of coding, start-up sequences, operating procedures and DTE-DCE interface functionalities. The modem includes an optional control channel and support for transparent mode on unrestricted 64 000 bit/s channels. The network interface of the modem and the signalling rate that is used to connect the modem locally to a 4-wire connection are considered to be national matters and are hence not specified.