Abstract: A circuit for performing a switching of a protection line between a common section and a channel section includes a common section and a channel section connected by signal lines. The common section has a working interface circuit and a protecting interface circuit. The channel section only has a working interface circuit. The working interface circuit of the common section is operably connected to the working interface circuit of the channel section by a working cable. The protecting interface circuit of the common section is connected to the working interface circuit of the channel section by a protecting cable. When a disconnection or a slip of the working cable occurs, a signal from the working interface circuit of the common section is transmitted and inputted into the protecting interface circuit via the protecting cable.

Abstract: A system for monitoring the status of a plurality of repeaters located along a transmission line connecting two terminal stations, wherein data is transmitted between the two terminal stations through the plurality of repeaters, in the form of a multiframe consisting of a group of consecutive frames, each of the consecutive frames in the multiframe transmitting an address of the corresponding repeater, respectively, and the status of each of the repeaters being written in the corresponding frame when each of the repeaters has received the corresponding one of the consecutive frames.