Abstract: A concentrator equipped with two processors is connected via two corresponding channel systems to a higher ranking exchange. Both processors operate independently of each other and poll all subscriber line circuits for switching codes. In the event that a channel system malfunctions, but the concentrator processor is operational, one processor transmits request signals, as well as an emergency operating state signal, to the subscriber line circuits. If the processor itself malfunctions, it transmits request signals and, in addition, an out-of-order state signal. The emergency operating signal and out-of-order signal are stored by each of the subscriber line circuits. Switching codes are transmitted to each of the two processors when both of these processors are in standard operating mode or in emergency mode.