PURPOSE: To attain mutual communication between TEs (subscriber terminals), which are received on a same line, without giving an influence to a switchboard by abolishing a specified frame or a message on a D-channel not to defined with the switchboard in an NT(network termination device).

CONSTITUTION: The respective TEs have a D-channel transmission circuit 21, a D-channel reception circuit 22, an echo channel reception circuit 23, a terminal processing circuit 24, which includes the software for application of the TE, and a collison detecting circuit (CD) 25. An NT1 has a frame abolishing circuit 11 and this frame abolishing circuit detects the specified frame on the D-channel and abolishes it. A signal string on a Du is always returned to an echo channel by the NT and the Du is always monitored in the frame abolishing circuit 11. Then, when a specified input pattern is detected, an output to a switchboard side is prohibited for the signal string, which follows the pattern, until a next closing flag arrives. The respective TEs not only uses an E-bit for normal collison detection but also receives the E-bit always and when a frame for extension mutual communication is obtained, the contents of the frame are sent to the terminal processing circuit 24 and a necessary processing is executed.