PURPOSE: To manage definitely a fault of layer 1 or 2 by demultiplexing a call control packet and a data packet from a D-channel packet, converting them into the call control packet and a message of the same form as the signal message of D-channel and supplying them to the ISDN exchange.

CONSTITUTION: When a D-channel packet is sent from a terminal equipment of a network termination NTi of i-th ISDN subscriber line 15i, it is received by a termination circuit 16i of the i-th layer 1 and a termination circuit 17i of the i-th layer 2. A packet demultiplex circuit 26 checks a call identifier in the layer 3 of the received data to discriminate the presence of the call control. Since the call identifier is a call packet in the signal processing section 31i, the packet is converted into a D channel message type (SETUP) corresponding to the call packet and converted into a message form represented by the CCITT Q.931. Thus, since the D channel packet is processed similarly to the line exchange call, the fault on a subscriber line or the like is processed at a batch.