Abstract: In the ISDN system complex line adapting circuits are provided on the two sides of the transmission line and they handle an extensive protocol. These circuits are divided into two sub-circuits connected to one another over fixed interfaces. The sub-circuit which is located nearest to the transmission line, however, is to have a considerably varying structure depending on the transmitting medium. For a modular structure it is now proposed to subdivide this sub-circuit into two partial circuits interconnected over a newly created simple binary interface. Thus, only the partial circuit which is located nearest to the transmission line needs to be specifically designed for the transmitting medium, for example, glass fibre lines, whereas the protocol management is performed in the other partial circuit, which always has the same structure irrespective of the transmitting medium.