The present invention is directed toward, a data sink/data source data transmission device and data terminal device for a circuit-switched and packet-switched network, the ability to eliminate the logical separation between applications, which are based on the circuit-switched network (e.g., PSTN, ISDN), and applications, which are based on the packet-switched network, (e.g., Internet). To this end, a data transmission device for transmitting and receiving data into/from the circuit-switched network includes controllable switchover parts. This data transmission device is or can be assigned to a universally useable unit for automatically processing data and for transmitting and receiving data to/from the packet-switched network and is assigned or can be assigned to the at least one data terminal device for transmitting and receiving data into/from the circuit-switched network. The switch-over parts can be controlled in such a manner that the data terminal device which, in a first operating mode is connected to the circuit-switched device, can be switched from the first operating mode into a second operating mode, during which the data terminal device is connected to the packet-switched network via the data transmission device and the data processing device, and from the second operating mode into the first operating mode.