JPH09266509A - Virtual private line connection system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a virtual private line. SOLUTION: Instead of providing a private line all the times, but a virtual private line is formed only when necessary by making use of an ISDN line 10. To form this virtual private line, private branch exchanges PBX-A and PBX-B to which extension telephone terminals are connected are connected by the ISDN line 10. Through the connected ISDN line 10, the extension telephone terminals STA-A and STA-B connected to the private branch exchanges PBX-A and PBX-B are connected. Further, a communication is made between those connected extension telephone terminals. Furthermore, the telephone charge is reducible by carrying on communication utilizing speech compression and expansion provided by speech compression trunks VCT-A and VCT-B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a virtual private line connection system, and in particular to PBs connected to private branch exchanges, respectively.
X (Private Branch Exchange)
e) A virtual private line connection system for connecting terminals to each other.

[0002]

2. Description of the Related Art Conventionally, a dedicated line has been used to connect PBX terminals housed in different private branch exchanges, respectively. This leased line is connected after making a contract with a telecommunications carrier. The dedicated line includes an analog dedicated line and a digital dedicated line.

Here, Japanese Patent Laid-Open No. 4-51647 describes a technique for connecting a private branch exchange with a switching network interface device to make a dedicated line connection. That is,
As shown in FIG. 10, an exchange network interface device is externally attached to the private branch exchange, and the private branch exchange and the public digital network are connected via this interface device.

In such a case, direct access to the public digital network is performed by the switching network interface device,
In the private branch exchange, the processing is performed as if the leased line connection is normally performed. Therefore, functionally, a private line is constructed by adding an adapter to the private branch exchange.

In the figure, the exchange control device 12 detects an activation request for the leased line 41 from the extension terminal of the private branch exchange.
Activates the leased line setting (virtual leased line) using the control line 43 provided between the switching network interface device 3 and the switching network interface device 3. In the exchange network interface device 3, the transmission control interface 33, the interface control unit 34, the transmission control unit 35, the line setting confirmation unit 36, and the network interface 32 set the line of the public digital network.
By this activation, the exchange switch 11 of the private branch exchange 1 and the leased line interface 31 of the exchange network interface device 3
And the public digital network 2 via the network interface 32
For this reason, the setting of the dedicated line is realized.

[0006]

In the above-mentioned prior art, since the private network interface of the private branch exchange is provided with the switching network interface device in front of it, and the public digital network is further provided in front of it, the public digital network is provided. There is a drawback in that the network line must be dedicated to the dedicated line setting (virtual dedicated line).

Further, since the private line is set by the dedicated control line, the system configuration is separated from the private branch exchange, and the services close to those of the private branch exchange (for example, exclusive operation by operation from a telephone or the like). There is a drawback that line setting) cannot be performed.

The present invention has been made to solve the above-mentioned drawbacks of the prior art, and its purpose is to automatically bypass a private line of a private branch exchange via a public network and to provide services in the private branch exchange. It is to provide a virtual private line connection system that can provide close services.

[0009]

A virtual private line connection system according to the present invention is a virtual private line connection system for connecting PBX terminals respectively connected to a first private branch exchange and a second private branch exchange. In response to the ISDN connection means for connecting the private branch exchanges to each other by an ISDN line, and a PB connecting the PBX terminals respectively connected to the private branch exchanges via the connected ISDN line.
It is characterized in that it includes an X connection means, and that communication is performed between these PBX terminals connected.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation of the present invention is as follows.

In the present invention, the dedicated line is not always provided, but only when necessary. Moreover,
Instead of using a leased line network, the well-known ISDN (In
integrated Services Digital
The network is used to form a virtual private line. When forming this virtual private line, first, the private branch exchanges to which the PBX terminals are connected are
Connect by SDN line. Then this connected I
Ps connected to private branch exchanges via SDN lines
Connect BX terminals together. By doing so, communication can be performed between the PBX terminals connected to each other. Furthermore, if voice communication is performed after voice compression, the call charge will be reduced.

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of a virtual private line connection system according to the present invention. In the figure, a virtual private line connection system according to an embodiment of the present invention shows an extension telephone terminal STA connected to a private branch exchange PBX-A via a virtual private line by an ISDN line 10.
(Station) -A communicates with the extension telephone terminal STA-B connected to the private branch exchange PBX-B.

First, the private branch exchange PBX-A on the calling side
Switch SW-A for performing exchange and exchange control device CC (call control) for performing exchange control
-ISDN line trunk PRT (primary) for realizing the interface between A and the ISDN line 10
trunk-A and a multiplexed trunk HMT (heterogeneous multi) for multiplexing the line.
Plex trunk) -A and extension telephone terminal STA-
A voice compression trunk VCT (voi) for compressing the voice from A and connecting to the individual channel of the multiplexing trunk HMT-A.
ce compression trunk) -A. And multiplex trunk HMT
-A is a private branch exchange PBX-A and a private branch exchange PBX-
When a virtual leased line is set up with B, the inter-PBX controller LP (li) that controls the control channel between both exchanges.
nk protocol) -A is provided.

On the other hand, the private branch exchange PBX-B on the receiving side is
A switch SW-B, a switching control device CC-B for performing switching control, an ISDN line trunk PRT-B for realizing an interface with the ISDN line 10, and a multiplexing trunk HMT-for performing line multiplexing. B and a voice compression trunk VCT-B which compresses voice from the extension telephone terminal STA-B and connects it to an individual channel of the multiplexing trunk HMT-B. Then, in the multiplex trunk HMT-A, when a virtual leased line is set up between the private branch exchange PBX-A and the private branch exchange PBX-A, the inter-PBX control device LP for controlling the control channel between the two private branch exchanges. -B is provided.

In order to realize an interface with an analog or digital leased line, the leased line trunk TI is used.
E (tie line trunk) -A is provided in the private branch exchange PBX-A, and private line trunk TIE-B is provided in the private branch exchange PBX-B.

Voice compression trunk VCT-A (VCT-
B) is a 64 [kbps] side channel connected to the extension telephone terminal STA-A (STA-B) and a multiple side channel connecting the compressed channel of the multiplexing trunk HMT-A (HMT-B). have. Then, multiple lines are used to improve line efficiency.

Both exchange control devices CC-A and CC-
It is assumed that each B has a table for associating an extension number with a called subscriber number of a private branch exchange to which a telephone terminal having the extension number is accommodated and connected.

In such a configuration, the extension telephone terminal STA
-When the extension number of the extension telephone terminal STA-B is input in -A, the called subscriber number of the private branch exchange (in this example, the called subscriber number of the private branch exchange PBX-B) corresponding to the extension number is the switching control device. It is read from the table in CC-A. Then, a call to the private branch exchange PBX-B is made by using the read called subscriber number.
From the ISDN line 10 according to the procedure described later.
Private branch exchange PBX-A and private branch exchange PBX-B via
Are connected. Furthermore, by using this ISDN line 10, the extension telephone terminal STA-A and the extension telephone terminal STA-B
And are connected. As a result, even if the private branch exchanges accommodated are different, it is possible to communicate as an extension telephone as if they were accommodated in the same private branch exchange. In other words, from the extension telephone terminal, it is only necessary to input the extension number of the communication partner and not the number of the outside line.

By doing so, the private line is not always formed, but the virtual private line is formed only when necessary, so that the call charge can be reduced. In short, by forming a virtual private line by the procedure described later, the inter-PBX controller LP-A and the receiving side private branch exchange PBX provided in the originating private branch exchange PBX-A.
The link between the private branch exchanges is formed by the communication state with the inter-PBX control device LP-B provided in the -B. Then, when communication is performed between extension telephone terminals housed and connected to both private branch exchanges, the voice is compressed.

As a result, the leased line of the private branch exchange is virtually formed via the public line, and the voice compression is performed to improve the line efficiency, and the link with the service originally possessed by the private branch exchange is realized. You can do it.

In addition to forming a virtual private line in response to a call from one of the extension telephone terminals to the other, activation by a so-called maintenance call (local or remote) from the maintenance terminal, operation of an external switch (not shown), and the like. The internal clock may be activated at regular intervals.

It is also possible to always form a dedicated line and additionally form a virtual dedicated line only when the traffic on this dedicated line becomes large.

Hereinafter, from the extension telephone terminal STA-A accommodated in the private branch exchange PBX-A to the private branch exchange PBX-B.
The operation until a telephone call is made by automatically setting a virtual private line by performing a dial operation for calling the extension telephone terminal STA-B accommodated in the above will be described.

FIG. 2 is a flow chart showing the setting operation of the virtual private line by starting from the extension. First, an extension telephone, which is a PBX terminal accommodated in the private branch exchange PBX-A, is connected to the private branch exchange PBX-B at the terminal STA-A.
Extension telephone terminal STA which is a PBX terminal accommodated in
-Receive an access code and a called subscriber number for calling A (step 101).

Next, it is judged whether the private branch exchange PBX-A accommodates an analog or digital leased line trunk TIE-A (step 202). When the leased line trunk TIE-A is accommodated, the line is selected for the analog or digital leased line (step 103). When the line selection result can be captured, the analog or digital leased line is connected. Do (Step 1
04 → 105).

If the line selection result cannot be captured in step 104, or in step 102, the analog or digital leased line trunk TIE is used.
-If A is not accommodated, private line activation information from the extension is stored in the corresponding memory (step 10).
6).

Next, the voice compression trunk VCT-A for performing voice compression is captured (step 107), and the multiplex trunk HMT-A for multiplexing the band is captured (step 108). . In addition, ISDN, which is a public network
In order to interface with the line, the ISDN line trunk PRT-A is captured (step 109), and a line start signal for starting the ISDN line, which is a public network, is sent (step 110). The incoming call is made on the opposite side via the public network (ISDN line).

When the private branch exchange PBX-B, which is the opposite station, responds, the private branch exchange PBX-B causes the ISDN line 1 of the public network.
By sending a response signal to 0, ISD
The local station receives the response signal via the N line 10 (step 111). As a result, switch connection of the multiplex side channel between the ISDN line trunk PRT-A and the multiplex trunk HMT-A is performed (step 11).
2).

As described above, the multiplexed trunk HMT-A of the private branch exchange PBX-A is connected to the ISDN line trunk PRT.
-A, the ISDN line 10 which is a public network, and the PBX of the private branch exchange PBX-A by being connected to the multiplexing trunk HMT-B via the ISDN line trunk PRT-B of the private branch exchange PBX-B. Controller LP-A
And the inter-PBX controller LP-B of the private branch exchange PBX-B are connected to form a control link between the PBXs (step 113).

In step 114, the private line activation information from the extension stored in step 106 is read from the corresponding memory, and in step 115, the PBX is read.
By transmitting the extension selection signal of the called party to the inter-station control apparatus LP-A, it is sent to the inter-PBX control apparatus LP-B of the private branch exchange PBX-B, and the extension telephone terminal ST which is the called subscriber.
AB is called.

After that, the extension telephone terminal STA-B responds, so that the response signal is the PB of the private branch exchange PBX-B.
It is transmitted in the inter-X control device LP-B and is transmitted to the private branch exchange P.
The response is received by the inter-PBX controller LP-A of BX-A (step 116). Then multiplex trunk HMT
A switch connection is made between the individual channel of -A and the voice compression trunk VCT-A (step 117). Further, switch connection is established between the 64 [kbps] side channel of the voice compression trunk VCT-A and the extension telephone terminal STA-A (step 118).

After the above connection is completed, the extension telephone terminal STA-A accommodated in the private branch exchange PBX-A and the extension telephone terminal STA-B accommodated in the private branch exchange PBX-B are connected by the virtual leased line. Is communicated with (step 119).

FIG. 3 is a flow chart showing the setting operation of the virtual private line by the game activation according to the present invention.

First, the incoming signal from the private branch exchange PBX-A via the ISDN line of the public network, which is activated by the virtual private line, is received by the ISDN accommodated in the private branch exchange PBX-B.
It is received via the line trunk PRT-B (step 201). Next, the multiplex trunk HMT-B for multiplexing the band is captured (step 202). Then, a response signal for responding to the ISDN line,
The data is transmitted via the ISDN line trunk PRT-B (step 203). As a result, switch connection is established between the ISDN line trunk PRT-B and the multiplex side channel of the multiplex trunk HMT-B (step 204).

The multiplexing trunk H of the private branch exchange PBX-B
The MT-B has an ISDN line trunk PRT-B, an ISDN line 10 which is a public network, and an IS of a private branch exchange PBX-A.
The private branch exchange PBX is connected by being connected to the multiplexing trunk HMT-A via the DN line trunk PRT-A.
-A-to-PBX controller LP-A and private branch exchange PBX-
The inter-PBX controller LP-B of B is connected to form a control link between the PBXs (step 205).

Since the control link between the PBXs is formed, the private branch exchange PBX-A is controlled by the inter-PBX control unit L.
The selection signal of the called party is sent to PA, and the inter-PBX controller LP-B of the private branch exchange PBX-B receives the selection signal of the called party (step 206).

Next, the voice compression trunk VCT-B for performing voice compression is captured (step 207). Then, a switch connection is established between the individual side channel of the multiplexed trunk HMT-B and the voice compression trunk VCT-B (step 208). In addition, voice compression trunk VCT
-64 [kpbs] side channel of B and extension telephone terminal ST
Switch connection with AB is performed (step 209).

Then, the extension telephone terminal STA-B corresponding to the selection signal of the called party is called (step 21).
0). Since the connection is completed by the response from the extension telephone terminal STA-B, the private branch exchange PBX-A using the virtual private line
The extension telephone terminal STA-A accommodated in the private branch exchange and the extension telephone terminal STA-B accommodated in the private branch exchange PBX-B communicate with each other (step 211). Then, in VCT-A and VCT-B of the voice compression trunk, voice compression and expansion are performed.

FIG. 4 is a flow chart showing the setting operation of the virtual private line according to the service starting condition relating to this system.

In this embodiment, the start condition is detected from the maintenance console (maintenance console) (step 3
01), detection of the start condition from the external key (step 30)
2), detection of start condition from internal clock (step 30)
3), detection of activation condition from extension telephone terminal (step 3)
04), in response to any of the detection of the activation condition from other services (step 305), the multiplex trunk HMT-A for multiplexing the band is captured (step 306). Next, in order to interface with the ISDN line 10 which is the public network, the ISDN line trunk PRT-A is captured (step 307), and the line is activated to activate the ISDN line 10 which is the public network. Send a signal. As a result, an incoming call is made at the opposite station via the public network (ISDN line) (step 308).

When the private branch exchange PBX-B, which is the opposite station, responds, the private branch exchange PBX-B causes the ISDN line 1 of the public network.
By sending a response signal to 0, ISD
The response signal is received at the local station via the N line 10 (step 309). As a result, switch connection of the multiplex side channel between the ISDN line trunk PRT-A and the multiplex trunk HMT-A is performed (step 31).
0).

In this way, the multiplexed trunk HMT-A of the private branch exchange PBX-A is connected to the ISDN line trunk PRT.
-A and the PBX of the private branch exchange PBX-A because the multiplex trunk HMT-B is connected via the ISDN line 10 which is the public network and the ISDN line trunk PRT-B of the private branch exchange PBX-B. Controller LP-A
And the inter-PBX control device LP-B of the private branch exchange PBX-B are connected to form a control link between the PBXs (step 311).

FIG. 5 is a flow chart showing the operation of releasing the virtual private line.

First, periodic monitoring is executed at a preset start time to check the state of the ISDN line trunk (step 401), and it is determined whether the virtual private line is being set (step 402). If no virtual leased line is set, wait until the next time (step 402 →
401). On the other hand, if a virtual leased line is set,
It is determined whether or not there is a call in progress by performing voice compression or the like on the B channel (64 [kbps]) in the virtual private line (step 402 → 403). If there is a line in use, wait until the next monitoring time (step 403 → 4)
01). If all of the B channels are unused,
The process of releasing the virtual private line is performed as follows.

First, a release instruction is issued to the ISDN line which is a public network (step 404). Next, the ISDN line trunk PRT-A is released (step 40).
5). Further, the virtual private line is released (step 407) by releasing the multiplexed trunk HMT-A (step 406).

In short, this system includes a digital leased line trunk for interfacing with a leased line accommodated in a private branch exchange, an ISDN line trunk for interfacing with a public digital line, and a multiplexing trunk for multiplexing voice. , And a voice compression trunk for compressing voice. At the same time, the following functions are newly added to the exchange control device, which is a control unit in the exchange, in addition to the general call originating / receiving function.

When a virtual private line connection request is made from a terminal or a trunk accommodated in the private branch exchange, the public digital line for setting the virtual private line is activated, connected and set, and the virtual private line connection is requested. A function to wait until a virtual leased line is set up for a call to a terminal or a trunk, and a function to connect the leased line using the virtual leased line by the leased line control signal channel after the leased line is set And are added.

When the virtual private line is activated from the game station, the incoming call request is accepted via the public digital channel, and the exchange control device performs matching with the game station.
When matching is completed, a function for connecting and setting a virtual leased line and waiting for an incoming call on the analog or digital leased line is added.

Furthermore, when an analog leased line or a digital leased line is set in advance, when all of these line groups are busy or cannot be used due to temporary restrictions, etc., the virtual leased line is set by the above procedure. Is possible.

By using the voice multiplexing trunk and the voice compression trunk in the above, a function for performing voice compression in the line in which the virtual private line is set and performing communication is provided.

Further, as conditions for starting, connecting, and setting the virtual private line, in addition to the above, operation from the maintenance console (maintenance terminal), setting with an external key, setting with an internal clock, setting from the accommodated terminal Etc. are possible.

As a function of releasing the set virtual private line, if there is no call using the line that is the target of the virtual private line, the virtual private line is automatically set by time monitoring or the like. It has the function of releasing. In addition, the above release is also possible.

Here, the tables provided in the switching control devices CC-A and CC-B in FIG. 1, that is, the extension number and the called terminal of the private branch exchange to which the telephone terminals of the extension numbers are accommodated and connected are called. A table for associating the person numbers with each other will be described.

FIG. 6 is a diagram showing an example of a table in which an extension number and a called subscriber number of a private branch exchange to which a telephone terminal having the extension number is accommodated and connected are associated with each other. First of all, FIG. 1A shows a table showing the relationship between the numbers of the multiplex trunks (HMT) and the number information of the multiplex trunks of the game. This table is referred to in the case of activation by the above-mentioned external key (step 302 in FIG. 4) or activation from the internal clock (step 303 in FIG. 4). The number information corresponding to the trunk number of the other party, that is, the multiplex trunk of the opposite station is read from this table, and by using this number, a virtual private line by the ISDN line is formed. For example, when forming a virtual leased line with the multiplexed trunk with the number "0", the game number information "0471
By using "857712", private branch exchanges are connected to each other by ISDN lines.

Further, FIG. 6B shows a table showing the relationship between the partner number and the number information of the multiplexed trunk of the opposite game. This table is referred to in the case of activation by a call from the above-mentioned extension telephone terminal (step 304 in FIG. 4). The virtual private line by the ISDN line is formed by reading the number information of the multiplexed trunk of the game corresponding to the extension telephone terminal of the communication partner from this table and using it. For example, if the extension number “26200” is entered in the extension telephone terminal, the private branch exchanges can be exchanged by using the number information “0471885712” of the private branch exchange accommodating the extension telephone terminal to which the extension number is assigned. Are connected by the ISDN line to form a virtual private line. This allows
Extension telephone terminals accommodated in different private branch exchanges can communicate with each other in the same manner as when they are accommodated in the same private branch exchange.

Note that these tables are held in memories inside the switching control devices CC-A and CC-B, respectively. Then, the contents of these tables are registered by the maintenance command from the maintenance console.

Next, the multiplexed trunks HMT-A and HM
An example of the internal configuration of TB will be described. FIG. 7 is a block diagram showing the internal structure of the multiplex trunk shown in FIG.
In the figure, the multiplexed trunk is a communication path control unit 71 and a D
The channel control unit 72, the multiplexing control unit 73, the IO control unit 74, and the CPU (Central Process)
ng Unit) 75 and ROM (Read Only)
Memory 76 and RAM (Random Ac)
cess Memory) 77.

The communication path control unit 71 is a part that interfaces with a switch in the PBX. The D channel control unit 72 is a unit that performs link control of the control channel in the PBX. The multiplexing control unit 73 is a unit that performs multiplexing control in units of subrates. The IO control unit 74 is a unit that performs I / O (input / output) control. CP
U75 is a processor for controlling in the multiplexed trunk. The ROM 76 is a ROM for controlling the processor in the multiplexed trunk. The RAM 77 is a RAM for controlling the processor in the multiplexed trunk.

Further, an example of the internal structure of the voice compression trunks VCT-A and VCT-B will be described. FIG. 8 is a block diagram showing an internal configuration of the voice compression trunk in FIG. In the figure, the voice compression trunk is configured to include a speech path control unit 81, an LD-CELP control unit 82, a PAD control unit 83, and an EC control unit 84.

The communication path control section 81 is a section for interfacing with a switch in the PBX. The LD-CELP control unit 82 is a control unit for compressing voice. PAD
The control unit 83 is a unit that controls the adjustment of the audio level.
The EC control unit 84 is a part that performs control related to echo.

The state of the formed virtual private line will be described with reference to FIG. In the figure, the line connection state of the private branch exchange PBX-A in FIG. 1 is shown.

Extension telephone terminal STA (not shown)
-The line L1 from A is a line of 64 [kbps], which is 16 in the voice compression trunk VCT-A.
The line L2 is compressed into the line of [kbps]. This compressed line L2 is input to the multiplexing trunk HMT-A, and the other line L3-2,
The line L3-3 and the line L3-4 are multiplexed. The multiplexed output of this multiplexed trunk HMT-A is the private branch exchange PB.
Input to X-A, 16 [kbps] × 4, that is, 6
ISDN as line L4 which is a 4 [kbps] line
It is connected to the ISDN line via the line trunk PRT-A.

Here, the line L3-4 in the multiplex trunk HMT-A is used as a control line by the inter-PBX controller LP-A. Further, with respect to the line L3-2 and the line L3-3, an extension telephone terminal can be further added by connecting an additional voice compression trunk.

As described above, according to this system, ISD
A virtual leased line using N lines can be formed only when necessary. Further, the virtual private line can be formed as a detour only when the traffic of the private line already provided increases. As a result, the communication cost will be lower than when a regular leased line is always provided. In addition, by performing voice compression, it is possible to efficiently use the leased line virtually formed and reduce the call charge.

[0066]

As described above, the present invention has the effect of reducing the communication cost by virtually forming a dedicated line using an ISDN line when necessary. In addition, by performing voice compression, it is possible to efficiently use the virtually formed leased line and to reduce the call charge.

[Brief description of drawings]

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

FIG. 2 is a flowchart showing an operation of forming a virtual leased line with an extension telephone terminal of a communication partner in response to a call originated from the extension telephone terminal.

FIG. 3 is a flowchart showing an operation in a private branch exchange on the called side.

FIG. 4 is a flowchart showing an operation of forming a virtual private line by various services.

FIG. 5 is a flowchart showing an operation of releasing a formed virtual private line.

FIG. 6A is a table showing the relationship between the numbers of multiplexed trunks and the number information of the multiplexed trunks of the opposite station, and FIG. 6B shows the relationship between the numbers of the other party and the number information of the multiplexed trunks of the opposite station. It is a figure which shows the table shown.

FIG. 7 is a block diagram showing an internal configuration of a multiplexing trunk in FIG.

8 is a block diagram showing an internal configuration of a voice compression trunk in FIG.

9 is a connection diagram showing a line connection state of the private branch exchange PBX-A in FIG. 1 in a state in which a virtual private line is formed.

FIG. 10 is a block diagram showing a configuration of a conventional leased line connection system.

[Explanation of symbols]

 10 ISDN line CC-A, CC-B Switching control equipment HMT-A, HMT-B Multiplexing trunk LP-A, LP-B Inter-PBX control equipment PBX-A, PBX-B Private branch exchange PRT-A, PRT-B ISDN line trunk STA-A, STA-B Extension telephone terminal SW-A, SW-B switch TIE-A, TIE-B Private line trunk VCT-A, VCT-B Voice compression trunk

Claims (9)

[Claims] 1. A virtual private line connection system for connecting PBX terminals respectively connected to a first private branch exchange and a second private branch exchange, wherein the private branch exchanges are connected by an ISDN line in response to an external command. ISDN connection means,
PBX connecting means for connecting PBX terminals respectively connected to the private branch exchange via the connected ISDN line, and communication is carried out between these connected PBX terminals. Virtual private line connection system. 2. The ISDN connecting means connects the private branch exchanges to each other by transmitting the station number information of the other private branch exchange to the other in response to the external command. The virtual private line connection system according to claim 1. 3. A compression unit provided on the transmitting side of the connected PBX terminals for compressing information to be transmitted, and a decompressing unit provided on the receiving side of the connected PBX terminals for decompressing the information. The virtual private line connection system according to claim 1 or 2, further comprising: 4. The virtual private line connection system according to claim 1, wherein the external command is a call from one of the PBX terminals to the other. 5. The virtual private line connection system according to claim 1, wherein the external command is transmitted from a maintenance terminal. 6. The virtual private line connection system according to claim 1, wherein the external command is transmitted in response to an operation of an external switch. 7. The virtual private line connection system according to claim 1, wherein the external command is transmitted at a specific time. 8. The P connected by the PBX connecting means
8. The virtual private line connection system according to claim 1, further comprising connection releasing means for releasing the connection only when communication is not performed between the BX terminals. 9. The P connected by the PBX connecting means
Monitoring means for periodically monitoring whether or not communication is performed between BX terminals, and connection releasing means for releasing the connection between the PBX terminals when it is detected from the monitoring result that communication is not being performed. The virtual private line connection system according to claim 1, further comprising: