This I-ETS specifies the technical characteristics (physical, electrical, logical and functional) of a family of interfaces for the connection Additional Equipment to ISDN telephony terminals. These interfaces support the facilities and services applicable mainly to ISDN telephony teleservices appropriated to those functions. Two interfaces allow bidirectional transparent analogue transmission of B-channel information between the telephony terminal and the Additional Equipment with a band width of 3,1 kHz or 7 kHz. Both a ""2 wire"" and a ""4-wire"" interface.