Local communication system among ISDN terminal equipments

A local communication system among ISDN terminal equipment comprising:

• a plurality of terminal equipment, each of said terminal equipment including a requesting means;
• a network termination unit terminating a subscriber line from a network, said network termination unit including a switching means;
• a passive bus connecting the plurality of terminal equipment to the network termination unit, the passive bus having a two-way communication function containing two two-way circuit-switched communication channels,
• signals on the passive bus being transferred to said network termination unit and said terminal equipment in two directions of transmission according to a predetermined frame structure including bits for the two two-way circuit-switched communication channels,
• a multiframe being configured on the passive bus by combining a predetermined number of frames together by utilizing an auxiliary framing bit in each frame,
• said switching means for coupling two-way circuit-switched communication channels on the passive bus to establish a loop-backed connection for a local communication among the terminal equipment through the network termination unit and for coupling the passive bus with the subscriber line to establish a connection for an external communication,
• said requesting means for requesting the loop-backed connection for the local communication and the connection for the external communication selectively, by controlling the auxiliary framing bits of the frames in the multiframe, wherein
• when the loop-backed connection is requested by one of the terminal equipment, the switching means in the network termination unit changes the connection from the external communication into the loop-backed connection.