Abstract: A circuit for a central control TDM PCM telecommunications exchange with groups of terminals connected to a switching network is disclosed in which the terminal groups which are each connected to the duplicated switching network over TDM lines and which each include a subsidiary switching network and a decentral control unit are assigned to one another in pairs. In preparation for the stand-by substitution of the central switching network and associated central processor, in the two terminal groups, assigned to one another in pairs, the subscriber lines and/or connection lines connected to the first of said terminal groups can be switched-over from the subsidiary switching network of the first terminal group to the subsidiary switching network of the partner terminal group, whereupon the decentral control unit additionally undertakes all the control functions for the decentral control unit of the first terminal group.

Abstract: Line trunk groups coupled to a double central switching matrix by way of two PCM lines and including sub-switching matrix and group control units are allocated to one another in pairs. Prepared for stand-by service switch-over by the central switching matrix and the processor are subscriber and trunk lines within each line trunk group pair, which are connected to the sub-switching matrix of a first line trunk group and are switchable to that of a second line trunk group, such that their group control unit also assumes the control functions. The processor going into service with the stand-by switch-over is connected with the group control units of all first line trunk groups via its central switching matrix, while all switching service continues with those of all second line trunk groups. Connection data of all existing connections are stored in all group control units.

Abstract: A circuit arrangement, with a doubled central switching matrix to which two line trunk groups having a sub-switching matrix each and a group control unit each, are allocated to each other in pairs via two PCM-lines. The line facilities which are individually allocated to the one and the other of the two line trunk groups and which are connected to the sub-switching matrix of their own respective line trunk group during normal service, are switchable during emergency service of one of the two line trunk groups to the sub-switching matrix of the respective other line trunk group. In order that no interconnected calls are interrupted, each local control unit stores the call data of all interconnected calls during normal service.