COMMUNICATION BACKUP METHOD, TIME DIVISION MULTIPLEXER AND COMMUNICATION SYSTEM

PURPOSE: To realize communication backup with respect to each node equipment being opposite destinations of plural communication blocks at a low cost through the use of an ISDN public line. CONSTITUTION: A high speed digital line interface multiplexer/demultiplexer section 14 receives data outputted from a terminal equipment 2 through a bus control unit 12, the data are subject to time division multiplex and the result is outputted to an SD line 4 and sent to an equipment of an opposite destination. When the SD line 4 is open, a common control section 18 selects a bus changeover switch 141 to connect the multiplexer/demultiplexer section 14 to an ISDN interface unit 15 via a common bus 17. Then a unit 15 makes a call to an opposite destination to secure a transmission channel, then the multiplexer/ demultiplexer section 14 sends multiplexed data to the opposite destination through the ISDN public line 5. The unit 15 multiplexes the data further in the case of data input from the plural multiplexer/demultiplexer sections 14 and sends the data to each opposite destination.