Abstract: A packet data communication system with a ring-type transmission line circulates a predetermined number of slots in which packets to be transmitted are loaded. The system includes interface nodes, provided on the ring-type transmission line, for loading packets on slots and receiving packets from slots for terminals connected thereto. In each interface node, there are a predetermined number of sub-transmission lines, for passing each of slots in parallel through the interface node, terminal interfaces each having a plurality of sending buffers for storing packets and a plurality of receiving buffers for storing packets, and a switch for connecting each of the sending buffers and receiving buffers to any one of the sub-transmission lines.

Abstract: A loop transmission system with frame synchronization control includes a loop (1) through which information in the form of a sequence of frames is transmitted, a plurality of node units (51, 52, . . . , 5n; 3, 4) inserted at predetermined positions in the loop (1) for receiving and delivering information, and a supervisory device (2) inserted at a predetermined position in the loop (1) for supervising the transmission of information through the loop (1) in association with the node units (51, 52, . . . , 5n; 3, 4). The supervisory device (2) is operable to deliver a frame synchronization cancellation signal. Each of the node units (51, 52, . . . , 5n; 3, 4) is adapted to detect the frame synchronization cancellation signal and cancel the synchronized operation state in bypassing or taking-in of a node and to restore synchronization in the frame next to the synchronization cancelled frame. Therefore, quick restoration of synchronization after bypassing or taking-in of a node is ensured.

Abstract: A loop transmission system and a method of controlling the loop-back condition thereof. The loop transmission system comprises a plurality of node stations, a supervisory station, and two duplicate loop transmission lines which transmit signals in opposite directions. In the loop transmission system, the supervisory station first sends out loop-back commands via both of the duplicate loop transmission lines when faults are detected on both of the duplicate loop transmission lines at the same time. Each of the node stations establishes a loop-back path while retaining a connection path to a succeeding node station upon the receipt of a loop-back command. The loop-back path is released only in the node stations which receive signals normally from both of the duplicate loop transmission lines when the supervisory station sends release commands to the node stations after sending the loopback commands.