Abstract: In a channel bank at one of TDM hierarchical levels, a TDM demultiplexer includes a plurality of high-speed interfaces connected respectively to high-speed digital transmission lines. Each interface receives detects a line fault and appends an alarm signal of a time-slot duration to time slots of digital signals. The outputs of the high-speed interfaces are multiplexed into a TDM frame sequence and the time slots of data signals in that TDM frame sequence are interchanged according to a prescribed rule. The slot-interchange frame sequence is demultiplexed into plurality of TDM signals greater in number than the number of the high-speed interfaces. Low-speed interfaces respectively receive the demultiplexed TDM signals to detect the time slot in which the alarm signal is contained and transmits a line fault signals to a respective one of low-speed transmission lines.