ISDN network termination unit enabling to establish data link between one terminal equipment and other terminal equipments connected to the unit without use of subscriber line

A network termination unit for use in an integrated services digital network (ISDN) system, said network termination unit comprising a first port to which a plurality of terminal equipments are connected in parallel with one another, a second port to which a digital subscriber line is connected, a demultiplexer coupled to the first port for demultiplexing a first TDM (Time Division Multiplexed) signal incoming through the first port from one of the terminal equipments to extract a first control channel signal for establishing a data link, the first control channel signal having an identifier code, a digital subscriber line interface circuit coupled to the second port for transmitting the first control channel signal as a second control channel signal in a second TDM signal to the second port, said digital subscriber line interface circuit receiving a third TDM signal incoming through the second port to extract a third control channel signal, and a multiplexer coupled to the first port for multiplexing the third control channel signal as a fourth control channel signal in a fourth TDM signal to deliver the fourth TDM signal to the first port, wherein the improvement comprises:

• code holding means for holding a predetermined code;
• code deciding means coupled with said demultiplexer and said code holding means for deciding whether or not said identifier code in said first control signal is coincident with said predetermined identifier code, said code deciding means producing a return control signal when said identifier code is coincident with said predetermined code;
• and
• returning means coupled with said code deciding means and responsive to said return control signal for returning said first control channel signal as a return signal to said multiplexer, said multiplexer receiving said return signal as the third control channel signal to thereby return said first control channel signal to said first port as the fourth control channel signal in the fourth TDM signal without delivering the second TDM signal including the first control channel signal to said second port.