Abstract: Line groups with doubled central switching network and connected via correspondingly two time division multiplex lines and having inter alia one sub-switching network and one group control system each are assigned to each other in pairs. The connection devices, which are individually assigned to the one and the other of two line groups, and which are connected via internal lines in normal operation to the sub-switching network in each instance of their own line group, can be changed over to the sub-switching network of the particular other line group.