Abstract: ISDN basic interface S.sub.o of an ISDN communication system that can be connected to an S.sub.o interface of an ISDN communication terminal equipment or of an ISDN terminal of a public communication network (ISDN-PSTN) is provided. For automatic recognition and commissioning of such an ISDN terminal, this is first initialized as one of the possible connection modes and the processes of layer 1 and layer 2 are activated in interaction with the administration and maintenance and dependability processes. When it occurs from the message traffic of the individual components that a commissioning is not possible, then the appertaining S.sub.o terminal is initiated with the next connection mode and the commissioning is restarted. The controller previously made the corresponding, new configuring data available. After a successful commissioning, the S.sub.