Abstract:
A virtual private line connection method allowing efficient and economical virtual private line service in the public ISDN is disclosed. When a SS 7  (common channel signalling system No.  7 ) message is generated in response to a call originated by a terminal accommodated in a PBX, the SS 7  message is encapsulated in an ISDN message and the ISDN message is sent to the destination PBX on a D channel. When the destination PBX receives the ISDN message, the SS 7  message is read from the received ISDN message to establish the virtual private line between the PBXs.

Description:
BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED 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.