48 this system is set up for duplex traffic in addition to the digital signal source 11 and the switching matrix network a in the upper half a switching matrix network aa and a digital signal sink 11a are arranged in a lower half these therefore act like the switching matrix network b and the digital signal sink 12 in the digital signal source 11 the digital signal source sink device 34 contains an input 35 a regenerator 36 an interface code decoder 37 a multiplexer 38 an interface code encoder 39 an amplifier 40 a source identification transmitter 13a and a source identification marker 13b the digital signal sink 11a contains a source identification receiver 41 a regenerator 42 an interface code decoder 43 a demultiplexer 44 an interface code encoder 45 an amplifier 46 and an output 47 the source identification transmitter 13a and the source identification receiver 41 are connected to one another and to the control device 23 by way of buses 18 18a and 18 18a and 18b in the region of the transmission from the input 1 to the output 2 the switch matrix 48 contains a regenerator 49 an interface code decoder 50 the switching matrix network a a switch over device 51 an interface code encoder 52 an amplifier 53 a frame identification device 54 and 55 source identification receivers 15a and 15b in an address memory 15 see fig4 as well as an alternate circuit identification transmitter 56 in the region between an input 67 and an output 68 the switching matrix 48 contains an alternate circuit identification transmitter.