ITU V.91:1999 A Digital Modem Operating At Data Signalling Rates Of Up To 64000 Bit/s For Use On A 4-wire Circuit Switched Connection And On Leased Point-to-point 4-wire Digital Circuits Defines the operating of a duplex digital modem for a 4-wire circuit switched connection and on leased point-to-point 4-wire digital circuits at data signalling rates not in excess of 64 000 bit/s.