PURPOSE:To eliminate system-down due to a failure from a frame generating section, by confirming the folding of a frame of a main logical section, when a monitor section detects out-of-synchronism of a system in use. CONSTITUTION:Each remote terminal station is under loop transmission by an N system transmission line, an N system repeater, an N major logical section 6, and a channel section 7 normally. In case of a failure of the N system, the failure is detected at the monitor section and the system N and E are changed over. Especially, in case of out-of-synchronism, it is detected and an electric folding section 9 of the system in use is operated for the performance of the frame test, and if the failure is a failure of a frame generating section of the N major logical section, the logical section is instantly switched to the E major logical section 6'.
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