Abstract: The interfaces between line terminator groups of a pair of redundant line terminator groups each comprise an interface circuit which makes it possible, by being equipped with a write-read memory, a switch-over device and a control device that controls these components, to test the interface parts belonging to an active line terminator group in a spot-check fashion during a time channel reserved for this purpose and to test the interface parts belonging to the passive line terminator group during all time channels on the basis of a respective check word mirroring.

Abstract: In a method for processing incoming code words that are accompanied by parity bits and that, in the course of being transmitted, undergo a digital attenuation and/or a code conversion for generating further code words that are to be transmitted, the code words and the parity bits are subjected to parity tests for indicating whether parity errors exist prior to the performance of the digital attenuation and/or code conversion. Then, for each of the further code words a further parity bit is generated that is also to be transmitted, but that is inverted, if the parity test of a corresponding code word has shown that a parity error exists.

Abstract: Terminal groups are connected to a central switching matrix network via time-division multiplex lines and each comprises a switching matrix sub-network and a group controller which are assigned to one another in pairs. In addition to the connection equipment that are individually assigned to the one and to the other of the two terminal groups and that are respectively connected to the switching matrix sub-network of its own terminal group via internal lines during normal operation and that are switchable to the switching matrix sub-network of the respective other terminal group, the time-division multiplex lines individually extending from the central switching matrix network to each of the terminal groups are also switchable from their own terminal group to the respective partner terminal group.