Abstract: Multiplexer/de-multiplexer units 201 to 296 directly multiplexes or de-multiplexes data signals from terminals. A leased line allotting unit 20 normally allotting all time slots on a multiplexed signal bus 25 to the side of a high rate 6 Mbps digital leased line 3. Switched line allotting units 21 to 24 execute allotting, in the event of a trouble in the leased line, the multiplexed signal time slots on the multiplexed signal bus in four divisions to the side of respective 1.5 Mbps switched lines 41 to 44 through transmission rate conversion. A control unit executes a time slot allotting change.

Abstract: A transmission path back-up system according to the invention has a multiplexer for dividing a set of time slots on a digital private line into a first set of time slots to accommodate the group of signals to be backed up and a second set of time slots to accommodate the group of signals not to be backed up. A selector sends out these multiplexed signals on the digital private line as transmission path when the digital private line is in a normal state, or selectively extracts the group of signals to be backed up and sends them out to an exchange line when the digital private line is in trouble. The back-up system makes it possible to realize an economical and yet reliable network which can extract and send out to the back-up ISDN line only the set of time slots for the group of signals to be backed up when the private line is in trouble.