Private network with means for establishing virtual tie trunks between PBXS through ISDN public network

A private switched communications system having first and second PBX (private branch exchange) systems connected through a respective primary rate interface to an ISDN (Integrated Services Digital Network) public network, said first PBX system comprising:

• memory means for storing routing data identifying a virtual signaling tie trunk and a virtual speech tie trunk to be established by speech channels of the primary rate interface, first channel status data indicating set-up/clear-down command status of the virtual signaling and speech tie trunks, and second channel status data indicating working/nonworking status of said virtual tie trunks, said first channel status data being variable as a function of time and as a function of calendar day;
• and
• control means for accessing said first channel status data at periodic intervals, transmitting a call setup message or a call clearing message to said ISDN public network through a signaling channel of the primary rate interface to set-up or clear-down said virtual signaling and speech tie trunks between the first and second PBX systems according to contents of said channel status data, receiving a response message from the distant PBX system, causing the working/nonworking status of said virtual signaling and speech tie trunks in said memory means to change according to said first channel status data following reception of said response message, causing a path to be established in the first PBX system from one of the user stations to one end of said virtual speech tie trunk in response to a call request therefrom when the status of said virtual speech tie trunk is indicative of the working state, and transmitting a call setup message to said second PBX system through said virtual signaling tie trunk to establish a path from the other end of the virtual speech tie trunk to a destination user station.

A method for establishing a virtual tie trunk between first and second PBX (private branch exchange) systems each being connected through a respective primary rate interface to an ISDN (Integrated Services Digital Network) public network, the method comprising the steps of:

• a) storing routing data identifying a virtual tie trunk to be established by one of the speech channels of said primary rate interfaces;
• b) periodically generating channel status data indicating set-up or clear-down state of the virtual tie trunk, said channel status data being variable as a function of time and as a function of calendar day;
• c) transmitting a call setup message or a call clearing message through a signaling channel of the primary rate interface of the first PBX system to said ISDN public network to set-up or clear-down said virtual tie trunk according to contents of the first channel status data;
• d) receiving a response message from said second PBX system;
• e) indicating working/nonworking status of said virtual tie trunk in said memory means according to said channel status data upon receipt of the response message;
• and
• f) establishing a path from one of said user stations to said virtual tie trunk in response to a request therefrom when said status of said virtual tie trunk is indicative of the working state.

A method for establishing virtual tie trunks between first and second PBX (private branch exchange) systems each being connected through a respective primary rate interface to an ISDN (Integrated Services Digital Network) public network, the method comprising the steps of:

• a) storing routing data identifying a virtual signaling tie trunk and a virtual speech tie trunk to be established by ones of the speech channels of said primary rate interfaces;
• b) periodically generating channel status data indicating set-up or clear-down state of the virtual signaling and speech tie trunks, said channel status data being variable as a function of time and as a function of calendar day;
• c) transmitting a call setup message or a call clearing message to said ISDN public network to set-up or clear-down said virtual signaling and speech tie trunks through a signaling channel of the interface of the first PBX system according to contents of the channel status data;
• d) receiving a response message from said second PBX system;
• e) indicating working/nonworking status of said virtual tie trunk in said memory means according to said channel status data upon receipt of the response message;
• f) establishing a path from one of said user stations of the first PBX system to one end of said virtual speech tie trunk in response to a request therefrom when said working/nonworking status of said virtual speech tie trunk is indicative of the working state;
• and
• g) transmitting a call setup message to said second PBX system through said virtual signaling tie trunk to cause a path to be established from the other end of the virtual speech tie trunk to a destination user station of the second PBX system.