the facility shown in the drawing is part of a digital concentrator 34 digital line unit dlu 34 the transmitting channel c is a central character channel common to several channels over which all information serving to establish connection to maintain connection and to dissolve connection are transmitted in the journal 34 telcom report 34 no 4 of 1982 english edition the principle generally of a digital concentrator are shown and described it can have several concentration steps it is evident from the illustration that devices a1 to am belong to a first concentration step while device b is part of a second concentration such devices are described in the journal 34 telecom report 34 no 4 1982 page 262 pp of the english edition the devices a1 to am can be multiple subscriber line circuits slm with one module processor slmcp while the device b represents a concentrator processor dicc dic such as described in detail in german pat no 35 12 846 more than two concentration steps may be used the concentration includes gate circuits which fulfil the logic or condition these are or circuits which are able to transmit each of the two possible binary signals relatively low ohmicly transmitters k1 to kn are in addition to the common transmitter channel c connected with a common test channel z they lead their binary coded information simultaneously to the common transmitting channel c and to the common test channel z over the concentration circuits x1 to xm and y which with respect to their outer connections correspond to the concentration steps a1 am and b the transmitters always receive back the information transmitted on and over the test channel and carry out with one.