Abstract: In an apparatus and a method for interfacing a private exchange to an ISDN, the apparatus controls intralayer and interlayer entity communications to interface the private exchange to the ISDN. The apparatus is disposed in a universal card slot of a subscriber shelf of the private exchange in the form of a card to accommodate an ISDN terminal in the private exchange or connect the private exchange to an ISDN public network, thereby enabling voice and data communications.

Abstract: A technique for realizing a fault tolerance of an integrated services digital network private branch exchange (ISDN PBX), includes: dualizing main control devices of the ISDN PBX into an operating section and a waiting section so as to sustain a subscriber service in progress even when the ISDN PBX has an unexpected problem; allowing the operating section to transfer to the waiting section various information required for performing a section switching to sustain an identical status between the operating section and waiting section, while providing a service requested by a user; and allowing the waiting section to perform the section switching so as to sustain the service as a new operating section when the operating section has an error.