Virtual private line connection system and method

1. Field of the Invention

The present invention relates to a technique for providing a connection service to terminals each accommodated in private branch exchanges (PBXs) through a virtual private line in Integrated Services Digital Network (ISDN).

2. Description of the Related Art

There has been proposed a technique of establishing a virtual private line as necessary in ISDN (see Japanese Patent Application Unexamined Publication No. 9-266509). More specifically, PBXs are connected to each other through an analog/digital private line and, if necessary, a virtual private line can be established between the PBXs using an IDSN line. Therefore, telephone terminals accommodated in respective PBXs are capable of communicating with each other using the virtual private line as an alternative route.

Another virtual private line connection technique using ISDN has been disclosed in Japanese Patent Application Unexamined Publication No. 4-165892. In this conventional system, each PBX is connected to the ISDN through a primary rate interface (23B+D) trunk circuit. When a virtual private line is established, one or more B channel is used for voice and another B channel is used for signalling. A D-channel controller is connected to the primary rate interface trunk circuit to control the D channel of the primary rate interface. A D-channel controller is also connected to the primary rate interface trunk circuit to control the signalling B channel of the virtual private line. More specifically, SS 7 (common channel signalling system No. 7 ) messages are controlled by the D-channel controller to be transferred on the signalling a channel of the virtual private line. Therefore, after having established the voice and signalling B channels, the existing ISDN signal procedure can be performed with no problems by conforming to the SS 7 procedures.

However, it is necessary for each PBX to be provided with the D-channel controller, resulting in great difficulty in cost reduction and the increased amount of hardware in the PBX.

Further, at least two B channels are needed to establish the virtual private line. Therefore, the efficiency of use of the network is reduced and the communication cost is increased because of a charge for at least two B channels.

An object of the present invention is to provide a virtual private line connection system and method allowing efficient and economical virtual private line service in the ISDN.

According to the present invention, a method for establishing a virtual private line between a first private branch exchange (PBX) and a second PBX through an integrated Services Digital Network (ISDN) to connect a first terminal accommodated in the first PBX and a second terminal accommodated in the second PBX, includes the steps of: at the first PBX having a first digital interface trunk circuit connected to the ISDN, a) generating an SS 7 (common channel signalling system No. 7 ) message in response to a call originated by the first terminal; b) generating an ISDN message having the SS 7 message included therein; and c) sending the ISDN message to the second PBX on a D channel of the first digital interface trunk circuit, and at the second PBX having a second digital interface trunk circuit connected to the ISDN, d) receiving the ISDN message from the first PBX on a D channel of the second digital interface trunk circuit; and e) reading the SS 7 message from the ISDN message received to establish the virtual private line between the first and second PBXs, allowing the first and second terminals to communicate with each other through the virtual private line.

The step (b) preferably includes the steps of: b.1) generating an ISDN message for the SS 7 message; b.2) setting a virtual private line flag indicating that the SS 7 message is used for virtual private line connection; b.3) storing the virtual private line flag and the SS 7 message in a facility information element of the ISDN message.

The step (e) preferably includes the steps of: e.1) reading the facility information element from the ISDN message received from the first PBX; e.2) determining whether the virtual private line flag included in the facility information element is set; e.3) when the virtual private line flag is set, reading the SS 7 message from the facility information element to establish the virtual private line.

As described above, the SS 7 message is included in the ISDN message and is sent to the other PBX on the D channel to establish the virtual private line in the ISDN. Therefore, the need for a signalling B channel is eliminated. Since the ISDN message having the SS 7 message encapsulated therein is sent on the D channel, the virtual private line can be established conforming to specifications for inter-PBX signalling protocol defined in the international standard ISO/IEC11582, ETS 300 239, JS-11582 Private Integrated Services Network (Generic Functional Protocol for the support of supplementary services). Therefore, the transparency and versatility of the public ISDN can be ensured.

FIG. 1 is a block diagram showing a virtual private line connection system according to an embodiment of the present invention;

FIG. 2 is a diagram showing a channel configuration of the embodiment as shown in FIG. 1;

FIG. 3 is a diagram showing a flow of message formation iii the embodiment as shown in FIG. 1;

FIG. 4 is a flowchart showing an operation of a sending PBX in the embodiment; and

FIG. 5 is a flowchart showing an operation of a receiving PBX in the embodiment.

Hereafter, a virtual private line connection system according to an embodiment of the present invention will be described.

As shown in FIG. 1, the virtual private line connection system is composed of PBX 100 , public ISDN 120 , PBX 130 , and terminals 110 and 140 that are accommodated in the PBXs 100 and 130 , respectively. Although each of the PBXs 100 and 130 is capable of accommodating a plurality of terminals, for simplicity only one terminal for each PBX is shown in FIG. 1 .

The PBX 100 includes a call control processor 101 , a SS 7 controller 102 , a virtual private line controller 103 , an ISDN controller 104 , a D-channel controller 105 , and a digital interface trunk circuit 106 . The call control processor 101 performs processing of a call originating from or destined to the terminal 110 depending on a SS 7 control signal. The SS 7 controller 102 performs processing of a SS 7 message according to the signalling system No. 7 .

The virtual private line controller 103 includes a registration number memory that stores registered numbers, which are used to determine whether a dialed number originating from the terminal 110 or a source number of an incoming call from the terminal 140 is a registered number. In other words, the virtual private line controller 103 has a function of determining whether an originating terminal has been permitted to use the virtual private line.

In addition, the virtual private line controller 103 performs communication procedures conforming to specifications for inter-PBX signalling protocol defined in the international standard ISO/IEC11582, ETS 300 239, JS-11562 Private Integrated Services Network (Generic Functional Protocol for the support of supplementary services).

The ISDN controller 104 controls the D-channel controller 105 and the digital interface trunk circuit 106 . The PBX 130 has the same circuit configuration as the PBX 100 . The operations of the SS 7 controller 102 , the virtual private line controller 103 , and the ISDN controller 104 will be described later.

Referring to FIG. 2, the PBX 100 is connected to the PBX 130 through a virtual private line 201 . The virtual private line 201 is composed of one ISDN D channel 202 as SS 7 signalling channel and one ISDN B channel 203 for SS 7 voice channel controlled by a SS 7 message that is encapsulated in an ISDN message transferred through the D channel 202 . In this way, the terminal 110 can communicate with the terminal 140 through the B channel 203 .

Referring to FIG. 3, when dialing a destination number at the terminal 110 , the SS 7 controller 102 generates a SS 7 message 301 and outputs it to the virtual private line controller 103 . In the virtual private line controller 103 , the SS 7 message 301 is encapsulated in on ISDN message 311 with a virtual private line flag 312 added thereto. The ISDN message 311 is output to the ISDN controller 104 and is transferred through the D channel 202 by the digital interface trunk circuit 106 under control of the D-channel controller 105 .

On the other hand, when receiving an ISDN message having a SS 7 message encapsulated therein with a virtual private line flag from the PBX 130 , the virtual private line controller 103 reads the SS 7 message from the received ISDN message and outputs the SS 7 message to the SS 7 controller 102 .

The details of the operation of the virtual private line controller 103 will be described hereafter.

Referring to FIG. 4, the virtual private line controller 103 monitors occurrence of a SS 7 message at all times. When receiving the SS 7 message 301 from the SS 7 controller 102 , the virtual private line controller 103 reads a destination number dialed by the terminal 110 from the SS 7 message 301 (step S 401 ). Then the virtual private line controller 103 searches the registration number memory for the destination number to determine whether the destination number requires the virtual private line service (step S 402 ).

When the destination number needs the virtual private line service (YES in Step S 402 ), the virtual private line controller 103 creates an ISDN message corresponding to the SS 7 message (step S 403 ). When such an ISDN message is created, a virtual private line flag is set to indicate that the ISDN message is of the virtual private line service. Then, the SS 7 message accompanied by the set virtual private line flag is encapsulated in the Facility information element of the ISDN message (step S 404 ). At this stage, the SS 7 message is “IAI (Initial Address Information)” and the corresponding ISDN message is “FACILITY”. Thereafter, the ISDN message having the SS 7 message and the virtual private line flag encapsulated therein is transmitted to the PBX 130 through the digital interface trunk circuit 106 .

The PBX 130 receives the ISDN message having the SS 7 message and the virtual private line flag encapsulated therein from the PBX 100 via the D channel 202 .

Referring to FIG. 5, when receiving the ISDN message having the SS 7 message and the virtual private line flag encapsulated therein, the virtual private line controller 103 reads the Facility information element from the received ISDN message (step S 501 ) Then, the virtual private line controller 103 determines whether the virtual private line flag of the Facility information element is set (step S 502 ). When the virtual private line flag is set (YES in step S 502 ), the virtual private line controller 103 reads the SS 7 message from the Facility information element (step S 503 ) and then outputs it to the SS 7 controller 102 .

In this way, by sending the ISDN message having the SS 7 message encapsulated therein, the virtual private line 201 is established between the PBXs 100 and 130 to allow the terminals 110 and 140 to communicate with each other.

As described above, a virtual private line is established conforming to specifications for inter-PBX signalling protocol defined in the international standard ISO/IEC11582, ETS 300 239, JS-11582 Private Integrated Services Network (Generic Functional Protocol for the support of supplementary services).

In creation of an ISDN message corresponding to SS 7 message, when the SS 7 message is not associated with the communication channel 203 , a SETUP message is used to implement communication conforming to the procedure of “connection-oriented APDU transfer independently of a call”. When the SS 7 message is associated with the communication channel 203 , a FACILITY message is used to implement communication conforming to the procedure of “APDU transfer associated with a call”.

A virtual private line may be established conforming to a protocol other that the inter-PBX signalling protocol defined in the international standard ISO/IEC11582, ETS 300 239, JS-11582 Private Integrated Services Network (Generic Functional Protocol for the support of supplementary services). Further, a virtual private line may be established by sending the ISDN message according to the packet communication protocol.