(EN) For example, in order to simplify the backup system of an on-line service system, according to the invention, a host computer, a communication manager, and a number of ISDN compound switching units (I-SW) are installed on the part of a central controller unit. In addition, a terminal control unit and an I-SW remote manager are installed at the central controller unit or each of specific business offices. A plurality of ISDN compound switching units, a network terminal equipment, a digital service unit (DSU), a modem M, a terminal control unit, a plurality of terminal equipment, and automatic equipment, are installed in each business office. Using private lines, the communication manager unit is connected to the ISDN com-pound switching units of respective business offices via the DSU and modem (M). The communication manager unit (CCU) is connected to the ISDN network via the ISDN compound switching units and the network terminal equipment. The ISDN compound switching units in respective business offices can be connected to the ISDN network via the ISDN lines. In the event that any failure arises and the backup function is needed, the ISDN compound switching units are switched over to the connection with the ISDN line under the control of the I-SW remote manager on the part of the central controller unit or the business office demanding the backup operation.
