1. Field of the Invention
The present invention relates generally to communications between private branch exchanges (PBXs), and more specifically to a technique for establishing an alternate route between the PBXs.
2. Description of the Related Art
It is known to use the public switched ISDN (integrated services digital network) as a backup facility for carrying traffic between two PBXs which are normally interconnected by a dedicated tie trunk. As disclosed in Japanese Provisional Patent Publication 3-106234, the traffic from each of the interconnected PBXs is multiplexed by a time-division multiplexer/demultiplexer and converted to a high bit-rate multiplex signal and transmitted over a tie trunk. At the other end of the trunk, the transmitted signal is demultiplexed for coupling to the other PBX. Each multiplexer/demultiplexer includes an ISDN interface for accommodating an ISDN line connected to the public switched ISDN. The tie trunk is constantly monitored at each end of the trunk. Should the tie trunk fail, a backup route is established between the PBXs through the ISDN interface. A protection switching circuit is provided in each time-division multiplexer for switching the traffic from the tie trunk to the backup route.
However, it is not economical to use the ISDN interface only as a backup facility. Additionally, the provision of the protection switching circuit at each PBX increases the total cost of the interconnected PBXs.