JP2009225102A - Line switching system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To distribute risks due to failure of an extension exchange etc. while relieving economic burdens on a user side in a line switching system which switches and connects an ISDN pubic network and an extension terminal of a private branch extension network. <P>SOLUTION: The line switching system 1 switches, connects the ISDN public network 4 and an IP telephone 7 of the private branch extension network, and is provided with a media gateway 3 connected between the ISDN public network 4 and the private branch extension network, and a plurality of IP-PBXs 2a, 2b connected to a single ISDN line 5 via the media gateway 3. Channel setting information 32 indicating correspondence between telephone call channels ch1-ch23 and the respective IP-PBXs 2a, 2b is recorded beforehand in a memory 31 of the media gateway 3. The media gateway 3 reads a channel number from a SETUP message, collates the channel number with the channel setting information 32 to select the IP-PBX to which a UDP packet is transmitted. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a circuit switching system for switching and connecting an ISDN public network and an extension terminal of a local extension network, and more particularly to a circuit switching system for connecting an ISDN line and a local LAN via a media gateway.

  2. Description of the Related Art Conventionally, a private branch exchange system in which a plurality of extension telephones are accommodated in an IP-PBX (extension telephone exchange) and a local extension network is unified with a LAN cable or the like is widely used in government offices and companies. Also, in the private branch exchange system, the private branch network is connected to the ISDN (Integrated Services Digital Network) public network through a relay device such as a media gateway, and the interconnection between the internal line and the external line is also attempted (see, for example, Patent Document 1). ).

In particular, in a private branch exchange system for business use, a primary group interface line that divides and uses one communication line into 24 channels is often used because the network scale tends to be large. In a primary group interface in a domestic communication service, a call channel composed of 23 B channels and a control signal channel composed of one D channel are usually prepared.
Japanese Patent Laid-Open No. 10-243429

  By the way, in the above private branch exchange system, in order to establish a communication path between the public network and the extension network, both the exchange on the public network side and the IP-PBX on the extension network side need to function normally. is there. For this reason, even if the exchange on the public network side is functioning normally, if any failure or the like occurs in the IP-PBX on the extension network side, there is a possibility that communication between the two is completely blocked.

  In addition, when the above private branch exchange system is used in a call center or the like whose main business is telephone reception, in these facilities, incoming calls may be concentrated in a short period of time, which places a heavy load on the IP-PBX. It becomes easy. At this time, depending on the processing capability of the IP-PBX, there is a possibility of causing congestion, so it is necessary to temporarily block the used line to suppress incoming calls.

  In order to cope with these problems, it is desirable to install a spare IP-PBX and switch over as necessary. However, for the installation of the IP-PBX, one unit per line is adopted. Since it is general, in order to add IP-PBX, it is necessary to separately make a contract for one line for backup. As a result, a total of two lines of contracts are required, and there is a problem that the economic burden on the user side increases.

  Therefore, the present invention has been made in view of the above-described problems in the prior art, and reduces the economic burden on the user side when the ISDN public network and the extension terminal of the private extension network are connected for exchange. On the other hand, it is an object of the present invention to provide a circuit switching system capable of distributing the risk associated with a failure of an extension telephone exchange or the like and distributing the load when incoming calls are concentrated.

  In order to achieve the above object, the present invention provides a circuit switching system for exchanging and connecting an ISDN public network and an extension terminal of a local extension network, wherein the gateway apparatus is connected between the ISDN public network and the local extension network. And a plurality of extension telephone exchanges connected to a single ISDN line via the gateway device, and the gateway device selects a preset when receiving an incoming call from the ISDN public network. According to the setting, one of the plurality of extension telephone exchanges is selected, and the incoming call is transmitted to the selected exchange.

  According to the present invention, since a single ISDN line can be shared by a plurality of extension telephone exchanges, even if a failure or the like occurs in any of the extension telephone exchanges, the exchange on the side functioning normally Can be used to establish communication between the ISDN public network and the private extension network, and it is possible to disperse risks associated with failures and the like. In terms of line contracts, users (contractors) can install multiple extension telephone exchanges only by exchanging contracts for one line, reducing the economic burden on the user side. It becomes possible to do.

  In the circuit switching system, the gateway device includes a memory in which channel setting information indicating correspondence between each of a plurality of call channels used on the ISDN line and each of the plurality of extension telephone switches is recorded. When an incoming call from the ISDN public network is accepted, a channel number is read from the incoming call, and the extension telephone exchange that transmits the incoming call is selected by comparing the read channel number with the channel setting information. can do.

  In the circuit switching system, the plurality of extension telephone exchanges includes a memory in which channel assignment information indicating assignment of a use channel to the own apparatus among a plurality of call channels used in the ISDN line is recorded in advance, When an outgoing call from an extension terminal is accepted, one channel is selected from the used channels with reference to the channel assignment information, and the outgoing call can be transmitted to the gateway device.

  In the circuit switching system, the gateway device selects one of the plurality of extension telephone exchanges according to an incoming call reception order from the ISDN public network, and transmits the incoming call to the selected exchange. can do.

  According to the above configuration, since incoming calls are distributed to a plurality of extension telephone exchanges for each incoming call reception order, the load applied to the extension network can be distributed to a plurality of extension telephone exchanges, reducing the load on each exchange. It becomes possible to do.

  In the circuit switching system, the gateway device can alternately select the plurality of extension telephone exchanges in accordance with the reception order of incoming calls from the ISDN public network.

  As described above, according to the present invention, when the ISDN public network and the extension terminal of the private extension network are exchanged and connected, the risk associated with the failure of the extension telephone exchange is reduced while reducing the economic burden on the user side. It is possible to distribute the load or distribute the load when incoming calls are concentrated.

  Next, embodiments of the present invention will be described in detail with reference to the drawings. In the following, a case where the circuit switching system according to the present invention is applied to a system using a primary group interface line (23B + D) will be described as an example.

  FIG. 1 shows an embodiment of a circuit switching system according to the present invention. This circuit switching system 1 is roughly divided into an IP telephone 7 for extension connected to a local LAN 8 and 2 connected to a LAN 8. IP-PBXs 2a and 2b, and a media gateway 3 connected between the IP-PBXs 2a and 2b and the ISDN public network 4.

  In this configuration, communication between the ISDN public network 4 and the IP-PBXs 2 a and 2 b is performed via the media gateway 3. In this case, the communication path control between the ISDN public network 4 and the media gateway 3 is performed by circuit switching on the public station side, while the communication path control between the media gateway 3 and the IP-PBX 2a, 2b is controlled via the LAN 8. This is done by sending and receiving signals.

  The media gateway 3 is a relay device that connects between the ISDN line 5 and the LAN 8 (between different lines), and has a conversion function between the ISDN interface and the LAN interface. When the media gateway 3 accepts an incoming call transmitted through the ISDN line 5, it generates a UDP packet for IP communication, encapsulates a layer 3 message (call control signal), and outputs it to the LAN 8. The UDP packet has the format shown in FIG. 2, and the layer 3 message is stored in the application data portion of the UDP packet format.

  Further, the memory 31 of the media gateway 3 stores channel setting information 32 indicating the association between the 23 B channels (call channels) and the IP-PBXs 2a and 2b. The media gateway 3 distributes and accepts the received incoming call to the two IP-PBXs 2a and 2b according to the channel setting information 32.

  The IP-PBXs 2a and 2b are extension exchange servers that manage the correspondence between the telephone number of the IP telephone 7 and the IP address and control the communication path of each IP telephone 7. These IP-PBXs 2a and 2b both have the same configuration, a CPU 21 that performs speech path control according to a predetermined control signal, a LAN control unit 22 that controls transmission and reception of IP packets between the CPU 21 and the media gateway 3, And a memory 23 in which PBX setting information 24 is stored.

  The PBX setting information 24 in the memory 23 is set in association with the channel setting information 32 stored in the memory 31 of the media gateway 3, and indicates the allocation of the used channel to the local exchange.

  Next, switching processing using the circuit switching system 1 having the above configuration will be described with reference to FIGS. In the present system 1, since the channel setting information 32 and the PBX setting information 24 must be recorded in the memories 31 and 23 before the exchange process, various setting processes must be performed. explain.

  As shown in FIG. 3, channel setting information 32 recorded in the memory 31 of the media gateway 3 includes channel number information 32a indicating channel numbers of ch1 to ch23, and IP address information 32b recorded in association with each channel number. Have Among these, the IP address information 32b indicates the IP addresses of the IP-PBXs 2a and 2b, and the channel number information 32a and the IP address information 32b are recorded according to the predetermined allocation of the IP-PBXs 2a and 2b.

  For this reason, for example, when 10 of the 23 channels (ch1 to ch10) are assigned to the IP-PBX 2a and the remaining 13 channels (ch11 to ch23) are assigned to the IP-PBX 2b, they correspond to ch1 to ch10. The IP address of the IP-PBX 2a is recorded in the IP address information 32b, and the IP address of the IP-PBX 2b is recorded in the IP address information 32b corresponding to ch11 to ch23.

  Incidentally, CR (call number) information 32c in the figure is information for identifying individual incoming calls. For this reason, it is not recorded in advance, but is recorded when the media gateway 3 accepts an incoming call from the ISDN public network 4. The information (call number) once recorded in the CR information 32c is retained from when the incoming call is accepted until the call is terminated, and is deleted after the call is terminated.

  On the other hand, as shown in FIG. 4, the PBX setting information 24 recorded in the memory 23 of the IP-PBX 2a, 2b includes GWIP address information 24a indicating the IP address of the media gateway 3, channel number information 24b indicating the channel number, And allocation information 24c indicating whether or not each channel is allocated.

  Among them, the allocation information 24c is recorded in association with the channel setting information 32 of the media gateway 3. For example, when ch1 to ch10 are allocated as the use channels of the IP-PBX 2a, the IP-PBX 2a side In the memory 23, a value indicating “valid” is recorded in the allocation information 24c corresponding to ch1 to ch10.

  Note that CR (call number) information 24d in the figure is information for identifying individual incoming calls in the same manner as the CR information 32c in the channel setting information 32. The CR information 24d is recorded when the IP-PBX 2a, 2b receives an incoming call relayed by the media gateway 3, and is held until the call is terminated.

  Next, the exchange operation will be described. First, a case where a call is made from the subscriber telephone 6 of the ISDN public network 4 to the extension network will be described with reference to FIG.

  When the user of the subscriber telephone 6 dials the number of the ISDN line 5, a layer 3 SETUP message (call setting) is transmitted to the media gateway 3 via the ISDN public network 4 as shown in FIG.

  When the media gateway 3 receives the SETUP message, it reads the channel number from the channel identifier information in the message (step S1). Note that the channel number in the channel identifier information is designated by the public station that controls the ISDN public network 4, and here, it is assumed that ch1 is designated.

  Next, the read channel number is compared with the channel setting information 32 (see FIG. 3) in the memory 31, and the IP address information 32b corresponding to the channel number is read (step S2). For example, as described above, when the IP-PBX 2a is specified for the exchange used in ch1 to ch10, the IP address of the IP-PBX 2a is read as the IP address information 32b.

  At the same time, CR (call number) is read from the SETUP message, and the call number is recorded in the memory 31 as CR information 32c corresponding to ch1 (step S3). Thus, after identifying the incoming call, the fact that the incoming call has been accepted is registered in the media gateway 3. Here, it is assumed that CR1 is assigned as the call number and CR1 is recorded in the CR information 32c of ch1.

  Next, the received SETUP message is converted into a UDP packet format (see FIG. 2), and the SETUP message converted into a UDP packet is transmitted to the IP-PBX 2a (step S4).

  When the IP-PBX 2a receives the SETUP message from the media gateway 3, the channel number and the incoming call number are read from the message (step S5). Then, the use state of the channel (ch1) and the destination IP phone 7 is confirmed. If both are in an empty state, the PBX setting information 24 (see FIG. 4) in the memory 23 indicates the CR information of ch1. The call number (CR1) is recorded in 32c (step S6).

  Note that the above-described determination of the channel availability can be performed with reference to the CR information 32c of the PBX setting information 24. That is, since the call number recorded in the CR information 32c is held only during the duration of the call, it can be determined whether there is a vacancy depending on whether the call number is recorded. For this reason, for example, if the call number is recorded in the CR information 32c corresponding to ch1, it can be determined that ch1 is in use, and conversely, if no information is recorded, the state is empty. It can be judged.

  Next, in the IP-PBX 2a, the managed IP telephone 7 is called, and a CALLPROC (call setting reception) and an ALERT message (call) are created (step S7). These messages are converted into a UDP packet format and transmitted to the media gateway 3 via the LAN 8.

  When the media gateway 3 receives the CALPROC or ALERT message in the form of a UDP packet, it extracts the layer 3 message portion and reads the call number (CR1). Then, it is confirmed whether or not the read call number matches the call number recorded in the memory 31 in the previous step S3 (step S8). If the match is recognized, the CALLPROC and ALERT messages are sent to the ISDN public network 4. Forward.

  Next, when a response on the IP telephone 7 side is detected in the IP-PBX 2a, a CONN message (response) is created (step S9), converted into a UDP packet format, and transmitted to the media gateway 3. Next, as in the previous step S8, the media gateway 3 confirms the call number in the packet (step S10), and if it is the same number as the call number recorded in the memory 31, the CONN message is sent to the ISDN public network. Forward to 4.

  Thereafter, a communication path on the LAN interface between the IP telephone 7 under management and the media gateway 3 is established in the IP-PBX 2a, and the communication on the LAN interface between the IP gateway P2a and the media gateway 3 is also established. A path and a communication path for circuit switching with the ISDN public network 4 are established. As a result, a communication path between the subscriber telephone set 6 and the IP telephone set 7 is formed, and a call can be made.

  If the channel number read by the media gateway 3 in the previous step S1 is the number assigned to the IP-PBX 2b (for example, ch11), as shown in FIG. 6, the media gateway 3 and the IP- The processing shown in FIG. 5 is executed between the PBXs 2b, and a communication path with the IP telephone set 7 managed by the IP-PBX 2b is established.

  Next, the exchange operation when a call is made from the IP telephone 7 on the extension side to the subscriber telephone 6 will be described with reference to FIG.

  When the IP telephone 7 under the management of the IP-PBX 2a dials the number of the subscriber telephone 6 of the ISDN public network 4, it can be used by referring to the PBX setting information 24 (see FIG. 4) in the memory 23 in the IP-PBX 2a. An empty channel is searched from among the active channels (step S11). When there is an empty channel, the channel is selected as a use channel. Here, it is assumed that ch2 is selected as the use channel.

  Next, an ISDN layer 3 SETUP message is created, and the channel number of the selected channel and the call number of the outgoing call are set (step S12). Here, it is assumed that CR2 is set as the call number. At the same time, the call number (CR2) is recorded in the CR information 24d of the selected channel number (ch2) in the PBX setting information 24 of the memory 23 (step S13). Thereafter, the GWIP address 24 a is read from the memory 23, and a SETUP message converted into a UDP packet is transmitted to the media gateway 3.

  When the media gateway 3 receives the UDP packet, it extracts the SETUP message part. Then, the call number (CR2) is read from the message, and the channel number (ch2) is read from the channel number information element (step S14). Next, the read call number (CR2) is recorded in the CR information 32c corresponding to ch2 of the channel setting information 32 (see FIG. 3) (step S15), and a SETUP message is transmitted to the ISDN public network 4.

  Next, in the ISDN public network 4, the subscriber telephone 6 is derived from the number information of the SETUP message. Next, the CALLPROC message and the ALERT message are returned to the media gateway 3 through the ISDN line 5 (step S16).

  When the media gateway 3 receives these messages, the call number (CR2) in the message is read (step S17), the call number (CR2) is compared with the channel setting information 32 in the memory 31, and the corresponding channel number ( Search for ch2). Next, the IP address of the IP-PBX (IP-PBX 2a) to which the channel is assigned is read from the memory 31 (step S18), and the message is transferred as a UDP packet.

  Next, when a response on the subscriber telephone 6 side is detected in the ISDN public network 4, a CONN message (response) is created (step S19) and transmitted to the media gateway 3. Next, the media gateway 3 reads out the call number (CR2) from the message as in the previous steps S17 and S18, and converts the message into a UDP packet and transfers it to the IP-PBX 2a (steps 20 and 21).

  Thereafter, the IP-PBX 2a establishes a communication path between the IP telephone 7 and the media gateway 3, and the media gateway 3 also establishes a communication path between the IP-PBX 2a and the ISDN public network 4. Each communication path is established.

  When a call is made from the IP telephone 7 managed by the IP-PBX 2b, the process shown in FIG. 7 is executed between the IP-PBX 2b and the media gateway 3 as shown in FIG. A communication path with the telephone 6 is established.

  As described above, according to the present embodiment, the channel setting information 32 that defines the correspondence between each of the call channels used in the ISDN line 5 and the plurality of IP-PBXs 2a and 2b is stored in the memory of the media gateway 3. 31. When the media gateway 3 receives an incoming call from the ISDN public network 4, the media gateway 3 distributes the incoming call to the IP-PBXs 2a and 2b according to the channel setting information 32 in the memory 31. The ISDN line 5 can be shared by a plurality of IP-PBXs 2a and 2b.

  As a result, even if a failure or the like occurs in either of the IP-PBXs 2a and 2b, communication between the ISDN public network 4 and the private extension network can be established using the IP-PBX on the side that is functioning normally. It becomes possible to disperse the risks associated with failures. Also, in terms of line contracts, the user (contractor) can install multiple IP-PBXs only by exchanging contracts for one line, thus reducing the economic burden on the user side. It becomes possible to do.

  In the above embodiment, the first half of the 23 channels is assigned to the IP-PBX 2a and the rear half is assigned to the IP-PBX 2b. How are the two IP-PBXs 2a and 2b assigned to the call channel? For example, the IP-PBXs 2a and 2b can be alternately assigned in order from the top of the call channel.

  Next, a second embodiment of the present invention will be described. The circuit switching system according to the present embodiment is mainly intended to distribute the load applied to one ISDN line by a plurality of IP-PBXs. In particular, telephone reception such as a call center is a main business. It is suitable for the facility to do.

  The circuit switching system according to the present embodiment has substantially the same configuration as that of the circuit switching system according to the first embodiment. Therefore, the following description will be given with reference to FIG. However, since there is a difference in the allocation method of the IP-PBX 2a and 2b and the information recorded in the memories 31 and 23 of the media gateway 3 and the IP-PBX 2a and 2b, these points will be mainly described here.

  In the first embodiment, ch1 to ch23 and IP-PBXs 2a and 2b are associated in advance. However, in the circuit switching system according to the present embodiment, these associations are not performed in advance, and the ISDN public network 4 The IP-PBXs 2a and 2b are alternately distributed in the order of receiving incoming calls. This distribution process is performed by the media gateway 3, and each time an incoming call from the ISDN public network 4 is received, the media gateway 3 designates an IP-PBX to be used and transmits a control IP packet to the IP-PBX. To do.

  Therefore, in the first embodiment, the channel number information 32a and the IP address information 32b of the channel setting information 32 shown in FIG. 3 are recorded in the memory 31 of the media gateway 3 prior to the exchange process. In the embodiment, the channel setting information 32 is not recorded in the memory 31 before the exchange process is performed.

  The information as the channel setting information 32 (channel number information 32a, IP address information 32b and CR information 32c in FIG. 3) is recorded when the IP-PBX is selected by the media gateway 3, and the channel number to be used and the selected IP -It is performed by recording the IP address of the PBX in the memory 31. Therefore, for example, when the designated channel accepts an outgoing call of ch1, and when IP-PBX 2a is selected at that time, the channel number indicating ch1, IP address information indicating the IP address of IP-PBX 2a, and The CR information indicating the accepted outgoing call is recorded in association with each other.

  This also applies to the PBX setting information 24 (see FIG. 4) recorded in the memory 23 of the IP-PBX 2a, 2b. Prior to the exchange process, the channel number information 24b, the allocation information 24c, The CR information 24d is not recorded in the memory 23. However, the GWIP address information 24a indicating the IP address of the media gateway 3 is recorded in the memory 23 in advance.

  Note that the distribution of the IP-PBXs 2a and 2b is not limited to alternating, and other rules may be adopted. For example, by providing a difference in the degree of allocation according to the processing capabilities of the IP-PBXs 2a and 2b, it is possible to take measures such as increasing the use frequency of the IP-PBX on the higher processing capability side. However, since the rules for distribution need to be set in the media gateway 3 in advance, it is necessary to use an allocation method having some regularity.

  Hereinafter, the exchange operation in the case of having the above configuration will be described in detail with reference to FIG. Here, a case where the IP-PBXs 2a and 2b are alternately selected will be described as an example. In FIG. 9, the same processing as the incoming call processing sequence shown in FIG. 5 is not shown.

  When the user of the subscriber telephone 6 dials the number of the ISDN line 5, a layer 3 SETUP message (call setting) is transmitted to the media gateway 3 via the ISDN public network 4.

  When the media gateway 3 receives the SETUP message, it reads CR (call number) from the message and also reads the channel number from the channel identifier information (step S31). Here, ch1 is designated as the channel number, and CR1 is designated as the call number.

  Next, the memory 31 is accessed, the channel number (ch1) is written in the channel number information 32a (see FIG. 3), and the call number (CR1) is written in the corresponding CR information 32c (step S32). Next, the IP-PBX 2a is selected as the exchange to be used (step S33), and the IP address of the IP-PBX 2a is written in the memory 31 (step S34).

  Next, the received SETUP message is converted into a UDP packet format (see FIG. 2), and the SETUP message converted into a UDP packet is transmitted to the IP-PBX 2a.

  When the IP-PBX 2a receives the SETUP message, the channel number and the incoming call number are read from the message (step S35), and the call number (CR1) is stored in the ch1 CR information 24d and the allocation information 24c (see FIG. 4) in the memory 23. ) And valid are written (steps S36 and S37).

  Thereafter, in the same manner as steps S7 to S10 shown in FIG. 5, various messages are transmitted and received among the three parties of the IP-PBX 2a, the media gateway 3 and the ISDN public network 4, and the communication between the subscriber telephone 6 and the IP telephone 7 is performed. Establish a path.

  When the second incoming call is received on the ISDN line 5 and received by the media gateway 3, the apparatus 3 selects the IP-PBX 2b as the exchange to be used (step S38). At that time, similarly to the previous steps S31, 32 and 34, the read channel number and call number are written in the channel number information 32a and CR information 32c (FIG. 3) of the memory 31, and the IP- Write the IP address of PBX2b.

  Thereafter, in the same manner as in steps S35 to S37, various messages are transmitted and received among the three parties of the IP-PBX 2b, the media gateway 3 and the ISDN public network 4, and the communication between the subscriber telephone 6 and the other IP telephone 7 is performed. Establish a path.

  As described above, according to the present embodiment, in order to distribute incoming calls to the two IP-PBXs 2a and 2b according to the incoming call acceptance order, the load applied to the extension network is shared by the IP-PBXs 2a and 2b. It is possible to reduce the load on each exchange. As a result, the risk of congestion is reduced, so that incoming calls can be accepted without blocking used lines even when incoming calls are concentrated.

It is a block diagram which shows one Embodiment of the circuit switching system concerning this invention. It is a figure which shows a UDP packet format. It is a figure which shows the channel setting information recorded on the memory of a media gateway. It is a figure which shows the PBX setting information recorded on the memory of IP-PBX. It is a figure which shows an incoming call process sequence. It is a figure which shows an incoming call process sequence. It is a figure which shows a transmission process sequence. It is a figure which shows a transmission process sequence. It is a figure which shows the incoming call process sequence of the circuit switching system concerning other embodiment.

Explanation of symbols

1 Circuit switching system 2 (2a, 2b) IP-PBX
3 Media Gateway 4 ISDN Public Network 5 ISDN Line 6 Subscriber Telephone 7 IP Telephone 8 LAN
21 CPU
22 LAN controller 23 Memory 24 PBX setting information 24a Media gateway IP address information 24b Channel number information 24c Assignment information 24d CR information 31 Memory 32 Channel setting information 32a Channel number information 32b IP address information 32c CR information

Claims (5)

A circuit switching system for switching and connecting an ISDN public network and an extension terminal of a local extension network,
A gateway device connected between the ISDN public network and the local extension network;
A plurality of extension telephone exchanges connected to a single ISDN line via the gateway device;
When receiving an incoming call from the ISDN public network, the gateway device selects one of the plurality of extension telephone exchanges according to a preset selection setting, and sends the incoming call to the selected exchange. A circuit switching system characterized by transmitting. The gateway device is
A memory in which channel setting information indicating correspondence between each of a plurality of call channels used on the ISDN line and each of the plurality of extension telephone exchanges is recorded;
When an incoming call from the ISDN public network is received, a channel number is read from the incoming call, and the extension telephone exchange that transmits the incoming call is selected by comparing the read channel number with the channel setting information. The circuit switching system according to claim 1. The plurality of extension telephone exchanges are:
A memory in which channel assignment information indicating assignment of a use channel to the own device among a plurality of call channels used in the ISDN line is recorded;
2. When an outgoing call from the extension terminal is received, one channel is selected from the used channels with reference to the channel assignment information, and the outgoing call is transmitted to the gateway device. Or the circuit switching system of 2.   The gateway device selects one of the plurality of extension telephone exchanges according to the order of acceptance of incoming calls from the ISDN public network, and transmits the incoming call to the selected exchanges. The circuit switching system according to claim 1.   5. The circuit switching system according to claim 4, wherein the gateway device alternately selects the plurality of extension telephone exchanges according to an incoming call reception order from the ISDN public network.

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