Standard connections in the ISDN network rely upon the ISDN Basic Rate Interface (BRI). This comprises the so-called S0-bus with S0-interface (four-wire interface) for up to eight end devices on the subscriber side; two end devices can be operated simultaneously. The bus has two service channels operating at 64 kbit/s (B channels) as well as a 16 kbit/s control channel (Channel D).

Analogue end devices need an external ISDN terminal adapter or an ISDN controller with an integrated a/b converter for access to ISDN services. Furthermore, analogue end devices such as facsimile or modems can continue to be used with an ISDN interface. Serial devices can be connected directly to the ISDN network via a TA (Terminal Adapter, also incorrectly called an ISDN Modem).

In addition to the ISDN basic rate interference, the primary rate interface (PRI) is available, also known as PMx standing for Primary Multiplex access. The PRI has 30 B-channels each operating at 64 kbit/s and one D-channel also operating at 64 kbit/s. On the subscriber side it is equipped with the four-wire S2M interface. For cost reasons in particular, the PRI is a good choice for the connection of large telecommunication systems, data servers or for inter-linking multiple local networks.