Line-oriented communication devices are coupled by a data network interface to a packet-oriented data network. The data network interface has a data network connection for coupling a data network cable to the data network, as well as a communication network connection for coupling a communication device for setting up line-oriented communication links. The data network interface couples the communication network connection and the data network connection to transmit communication signals on the communication links between the communication network connection and at least one supply conductor, which is provided for supplying voltage to data communication devices in the data network, in the data network cable.