Unit and method for duplex call control for SVC connection

[0002] The present invention relates to a unit and a method for duplex call control for SVC (Switched Virtual Channel) connection, which is implemented as an ATM switch, an ATM subscriber line concentrator, etc. having duplex composition composed of an operating (working) system and a standby (protection) system.

[0003] In ATM (Asynchronous Transfer Mode) networks, three protocols are used as control protocols for establishing/releasing SVC (Switched Virtual Channel) connection. The first is a protocol according to ITU-T Q.2110 (hereafter, referred to as “SSCOP (Service Specific Connection Oriented Protocol)”). The second is a protocol according to ITU-T Q.2130 (hereafter, referred to as “SSCF (Service Specific Coordination Function)”). The third is a protocol for the signaling layer, according to ITU-T Q.2931, ITU-T Q.2971, ATM Forum Technical Committee UNI (User-Network Interface) Specification Version 3.1 (hereafter, referred to as “UNI3.1”), or ATM Forum Technical Committee ATM UNI Signaling Specification Version 4.0 (hereafter, referred to as “UNI4.0”). Each of the four standard specifications (ITU-T Q.2931, ITU-T Q.2971, UNI3.1, UNI4.0) for the signaling layer is hereafter referred to as “UNI signaling”.

[0004] FIG. 1 is a block diagram showing an example of the composition of a conventional duplex call control unit. The conventional duplex call control unit shown in FIG. 1 is composed of an operating (working) system (including an ATM switch section 100aand a call control section 101a), a standby (protection) system (including an ATM switch section 100band a call control section 101b), and line interface sections 102a, 102b, . . . , 102n. The operating system ATM switch section 100aand the standby system ATM switch section 100bare connected to the line interface sections 102a, 102b, . . . , 102nvia buses 151a, 151b, 150, 154a, 154b, . . . , 154n. The standby system ATM switch section 100bcan not receive ATM cells from the line interface sections 102a, 102b, . . . , 102nvia the buses 151b, 150, 154a, 154b, . . . , 154nuntil a switch-over from the operating system ATM switch section 100aoccurs. The operating system ATM switch section 100ais connected to the operating system call control section 101avia a bus 152a, and the standby system ATM switch section 100bis connected to the standby system call control section 101bvia a bus 152b. The operating system call control section 101aand the standby system call control section 101bare connected together via a bus 153. The line interface sections 102a, 102b, . . . , 102ntransmit and receive ATM cells to/from ATM networks or subscriber terminals via ATM lines 155a, 155b, . . . , 155n, respectively.

[0005] The operating system ATM switch section 100areceives ATM cells via the bus 151a, extracts call control cells for SVC connection (VPI=0, VCI=5) from the ATM cells, and sends the extracted call control cells to the operating system call control section 101avia the bus 152a. The operating system call control section 101awhich received the call control cells executes call control according to the call control cells. In the call control, the operating system call control section 101acommunicates UNI signaling messages of the signaling layer by use of the call control cells. The operating system call control section 101afirst receives a UNI signaling message “SETUP” and transmits the UNI signaling message “SETUP” to the destination of the SVC connection. When a UNI signaling message “CONNECT” from the destination arrived, the operating system call control section 101asets SVC connection information (as the result of the call control) to the operating system ATM switch section 100a, and thereby establishes the SVC connection. The operating system call control section 101asends the SVC connection information of the established SVC connection only, to the standby system call control section 101bvia the bus 153. The standby system call control section 101bwhich received the SVC connection information sets the same SVC connection information to the standby system ATM switch section 100band thereby establishes the same SVC connection (as the SVC connection which has been set to the operating system ATM switch section 100a) to the standby system ATM switch section 100b. When failure or maintenance occurred to the operating system ATM switch section 100aor the operating system call control section 101a, the bus 151afor connecting the operating system ATM switch section 100aand the bus 150 is disabled and the bus 151bfor connecting the standby system ATM switch section 100band the bus 150 is enabled, thereby the reception and transmission of ATM cells are thereafter executed by the standby system ATM switch section 100b. Thereafter, the standby system call control section 101bexecutes the call control.

[0006] However, in the conventional duplex call control unit which has been explained above, SSCOP link connections are necessitated to be disconnected when the switch-over from the operating system to the standby system is executed. Information concerning the SSCOP layer which is being operated is not communicated between the operating system call control section 101aand the standby system call control section 101b, therefore, mismatch of the SSCOP layer status occurs between the operating system call control section 101aand the standby system call control section 101b. Therefore, the SSCOP layer status of the standby system call control section 101bon the switch-over becomes different from peer protocol, that is, mismatch of the SSCOP layer status occurs between the standby system call control section 101band an adjacent ATM switch or a subscriber terminal which has been linked with the operating system call control section 101a, and thus the SSCOP link connections are necessitated to be disconnected on the switch-over.

[0007] Due to the disconnection of the SSCOP link connections, some of signaling messages which should be communicated via the SSCOP link connections are missed or lost, and thereby a call during establishment/release of the SVC connection is necessitated to be interrupted.

[0008] It is therefore the primary object of the present invention to provide a duplex call control unit and a duplex call control method for SVC connection by which the disconnection of the SSCOP link connections on the switch-over from the operating system to the standby system is eliminated, information concerning established SVC connection can be held on the switch-over, and call control for a call during SVC connection establishment/release can also be continued and executed normally on the switch-over.

[0009] In accordance with a first aspect of the present invention, there is provided a duplex call control unit comprising a first ATM switch section, a second ATM switch section, a first call control section, a second call control section, a call control parameter database section, a control section, a first switch-over section and a second switch-over section. The first ATM switch section for switching ATM cells extracts call control cells for establishment/release of SVC connection from ATM cells which are supplied via ATM lines, and when call control cells as response are supplied thereto, inserts the call control cells in proper ATM cells and outputs the ATM cells including the call control cells so as to be transmitted to proper ATM lines. The second ATM switch section also executes the above operations of the ATM switch section similarly to the first ATM switch section. The first call control section receives call control cells which has been extracted by the first ATM switch section or the second ATM switch section, outputs call control cells as response to the first ATM switch section or the second ATM switch section, and when call control for an SVC connection is completed, sets SVC connection information to the first ATM switch section or the second ATM switch section. The second call control section also executes the above operations of the call control section similarly to the first call control section. The call control parameter database section stores call control parameters with respect to SVC connection information with regard to SVC connection which has already been established and call control parameters with respect to the statuses of call control protocols for SVC connection which is being established or being released. The call control parameters stored in the call control parameter database section are referred to by the first call control section or the second call control section that is operating as a standby system call control section or that is restarting working, for adjusting its call control parameters. The control section monitors operating statuses of the first ATM switch section, the second ATM switch section, the first call control section and the second call control section, and outputs instructions concerning bus connection depending on the monitored operating statuses. The first switch-over section switches bus connection between the ATM lines and the ATM switch sections according to the instructions supplied from the control section so that the aforementioned operations of the ATM switch section can be executed at least by the first ATM switch section or the second ATM switch section that is working. The second switch-over section switches bus connection between the ATM switch sections and the call control sections according to the instructions supplied from the control section so that the aforementioned operations of the call control section can be executed at least by the first call control section or the second call control section that is working.

[0010] In accordance with a second aspect of the present invention, in the first aspect, the call control parameters with respect to SVC connection information include “VPI (Virtual Path Identifier)”, “VCI (Virtual Channel Identifier)”, “traffic information”, “band width”, and “call reference”.

[0011] In accordance with a third aspect of the present invention, in the second aspect, the call control parameter “traffic information” includes “priority” and “QoS (Quality of Service)”.

[0012] In accordance with a fourth aspect of the present invention, in the first aspect, the call control parameters with respect to call control protocol statuses include call control parameters for the SSCOP layer, SSCF layer and call control parameters for the signaling layer.

[0013] In accordance with a fifth aspect of the present invention, in the fourth aspect, the call control parameters for the SSCOP layer include “SSCOP state variables”, “SSCOP timer value”, “SSCOP parameters”, “maximum SSCOP SDU size”, and “maximum SSCOP UU size(j)” for each SSCOP link connections.

[0014] In accordance with a sixth aspect of the present invention, in the fourth aspect, the call control parameters for the SSCF layer include “SSCF state variables” for each SSCOP link connections.

[0015] In accordance with a seventh aspect of the present invention, in the fourth aspect, the call control parameters for the signaling layer include “call reference” and “status for call reference” for each calls.

[0016] In accordance with an eighth aspect of the present invention, in the first aspect, in the case where the control section detected that all of the four monitored components are working, the control section instructs the first switch-over section to execute bus connection for transferring the ATM cells supplied via the ATM lines to both the first ATM switch section and the second ATM switch section and transmitting ATM cells supplied from the first ATM switch section to the ATM lines, and instructs the second switch-over section to execute bus connection for transferring the call control cells extracted by the first ATM switch section to both the first call control section and the second call control section, transferring the call control cells as response outputted by the first call control section to the first ATM switch section, transferring SVC connection information outputted by the first call control section as the result of the call control to the first ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to the second ATM switch section. In the case where the control section detected that the first call control section out of the four monitored components is not working, the control section instructs the first switch-over section to execute bus connection for transferring the ATM cells supplied via the ATM lines to both the first ATM switch section and the second ATM switch section and transmitting ATM cells supplied from the first ATM switch section to the ATM lines, and instructs the second switch-over section to execute bus connection for transferring the call control cells extracted by the first ATM switch section to the second call control section, transferring the call control cells as response outputted by the second call control section to the first ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to both the first ATM switch section and the second ATM switch section. In the case where the control section detected that the first ATM switch section out of the four monitored components is not working, the control section instructs the first switch-over section to execute bus connection for transferring the ATM cells supplied via the ATM lines to the second ATM switch section and transmitting ATM cells supplied from the second ATM switch section to the ATM lines, and instructs the second switch-over section to execute bus connection for transferring the call control cells extracted by the second ATM switch section to both the first call control section and the second call control section, transferring the call control cells as response outputted by the second call control section to the second ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to the second ATM switch section. In the case where the control section detected that the first call control section and the first ATM switch section out of the four monitored components are not working, the control section instructs the first switch-over section to execute bus connection for transferring the ATM cells supplied via the ATM lines to the second ATM switch section and transmitting ATM cells supplied from the second ATM switch section to the ATM lines, and instructs the second switch-over section to execute bus connection for transferring the call control cells extracted by the second ATM switch section to the second call control section, transferring the call control cells as response outputted by the second call control section to the second ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to the second ATM switch section.

[0017] In accordance with a ninth aspect of the present invention, there is provided a duplex call control method for controlling a duplex call control unit which is provided with a first ATM switch section, a second ATM switch section, a first call control section and a second call control section. The first ATM switch section for switching ATM cells extracts call control cells for establishment/release of SVC connection from ATM cells which are supplied via ATM lines, and when call control cells as response are supplied thereto, inserts the call control cells in proper ATM cells and outputs the ATM cells including the call control cells so as to be transmitted to proper ATM lines. The second ATM switch section also executes the above operations of the ATM switch section similarly to the first ATM switch section. The first call control section receives call control cells which has been extracted by the first ATM switch section or the second ATM switch section, outputs call control cells as response to the first ATM switch section or the second ATM switch section, and when call control for an SVC connection is completed, sets SVC connection information to the first ATM switch section or the second ATM switch section. The second call control section also executes the above operations of the call control section similarly to the first call control section. The duplex call control method comprises a call control parameter storage step, a call control parameter reference step and a bus connection control step. In the call control parameter storage step, call control parameters with respect to SVC connection information with regard to SVC connection which has already been established and call control parameters with respect to the statuses of call control protocols for SVC connection which is being established or being released are stored in a call control parameter database section to be referred to by the first call control section or the second call control section for adjusting its call control parameters. In the call control parameter reference step, the call control parameters stored in the call control parameter database section is referred to by the first call control section or the second call control section that is operating as a standby system call control section or that is restarting working, and thereby call control parameters of the call control section is adjusted. In the bus connection control step, operating statuses of the first ATM switch section, the second ATM switch section, the first call control section and the second call control section are monitored, and instructions concerning bus connection are generated and outputted depending on the monitored operating statuses so as to control bus connection of a first switch-over section which is provided between the ATM lines and the ATM switch sections and bus connection of a second switch-over section which is provided between the ATM switch sections and the call control sections so that the aforementioned operations of the ATM switch section can be executed at least by the first ATM switch section or the second ATM switch section that is working and so that the aforementioned operations of the call control section can be executed at least by the first call control section or the second call control section that is working.

[0018] In accordance with a tenth aspect of the present invention, in the ninth aspect, the call control parameters with respect to SVC connection information include “VPI (Virtual Path Identifier)”, “VCI (Virtual Channel Identifier)”, “traffic information”, “band width”, and “call reference”.

[0019] In accordance with an eleventh aspect of the present invention, in the tenth aspect, the call control parameter “traffic information” includes “priority” and “QoS (Quality of Service)”.

[0020] In accordance with an twelfth aspect of the present invention, in the ninth aspect, the call control parameters with respect to call control protocol statuses include call control parameters for the SSCOP layer, SSCF layer and call control parameters for the signaling layer.

[0021] In accordance with a thirteenth aspect of the present invention, in the twelfth aspect, the call control parameters for the SSCOP layer include “SSCOP state variables”, “SSCOP timer value”, “SSCOP parameters”, “maximum SSCOP SDU size”, and “maximum SSCOP UU size(j)” for each SSCOP link connections.

[0022] In accordance with a fourteenth aspect of the present invention, in the twelfth aspect, the call control parameters for the SSCF layer include “SSCF state variables” for each SSCOP link connections.

[0023] In accordance with a fifteenth aspect of the present invention, in the twelfth aspect, the call control parameters for the signaling layer include “call reference” and “status for call reference” for each calls.

[0024] In accordance with a sixteenth aspect of the present invention, in the bus connection control step of the ninth aspect, in the case where all of the four monitored components are detected to be working, the first switch-over section is instructed to execute bus connection for transferring the ATM cells supplied via the ATM lines to both the first ATM switch section and the second ATM switch section and transmitting ATM cells supplied from the first ATM switch section to the ATM lines, and the second switch-over section is instructed to execute bus connection for transferring the call control cells extracted by the first ATM switch section to both the first call control section and the second call control section, transferring the call control cells as response outputted by the first call control section to the first ATM switch section, transferring SVC connection information outputted by the first call control section as the result of the call control to the first ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to the second ATM switch section. In the case where the first call control section out of the four monitored components is detected to be not working, the first switch-over section is instructed to execute bus connection for transferring the ATM cells supplied via the ATM lines to both the first ATM switch section and the second ATM switch section and transmitting ATM cells supplied from the first ATM switch section to the ATM lines, and the second switch-over section is instructed to execute bus connection for transferring the call control cells extracted by the first ATM switch section to the second call control section, transferring the call control cells as response outputted by the second call control section to the first ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to both the first ATM switch section and the second ATM switch section. In the case where the first ATM switch section out of the four monitored components is detected to be not working, the first switch-over section is instructed to execute bus connection for transferring the ATM cells supplied via the ATM lines to the second ATM switch section and transmitting ATM cells supplied from the second ATM switch section to the ATM lines, and the second switch-over section is instructed to execute bus connection for transferring the call control cells extracted by the second ATM switch section to both the first call control section and the second call control section, transferring the call control cells as response outputted by the second call control section to the second ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to the second ATM switch section. In the case where the first call control section and the first ATM switch section out of the four monitored components are detected to be not working, the first switch-over section is instructed to execute bus connection for transferring the ATM cells supplied via the ATM lines to the second ATM switch section and transmitting ATM cells supplied from the second ATM switch section to the ATM lines, and the second switch-over section is instructed to execute bus connection for transferring the call control cells extracted by the second ATM switch section to the second call control section, transferring the call control cells as response outputted by the second call control section to the second ATM switch section, and transferring SVC connection information outputted by the second call control section as the result of the call control to the second ATM switch section.

[0025] In accordance with seventeenth through twenty-second aspects of the present invention, there are provided machine-readable record mediums storing programs for instructing a microprocessor unit etc. to execute duplex call control processes according to the duplex call control methods of the eighth, ninth, eleventh, twelfth, thirteenth and fourteenth aspects of the present invention.

[0026] The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings, in which:

[0027] FIG. 1 is a block diagram showing an example of the composition of a conventional duplex call control unit;

[0028] FIG. 2 is a block diagram showing the composition of a duplex call control unit in accordance with an embodiment of the present invention;

[0029] FIG. 3A is a schematic diagram showing bus connection of an ATM switch section switch-over section of the duplex call control unit of FIG. 2 for distributing ATM cells supplied via ATM lines to an operating system ATM switch section and a standby system ATM switch section of the duplex call control unit of FIG. 2;

[0030] FIG. 3B is a schematic diagram showing bus connection of the ATM switch section switch-over section for transmitting ATM cells supplied from the ATM switch sections to the ATM lines;

[0031] FIG. 4A is a schematic diagram showing bus connection between the ATM switch section switch-over section and a call control section switch-over section of the duplex call control unit of FIG. 2 for communicating ATM cells and call control cells when both the operating system ATM switch section and the standby system ATM switch section are working;

[0032] FIG. 4B is a schematic diagram showing bus connection between the ATM switch section switch-over section and the call control section switch-over section for communicating ATM cells and call control cells when only the standby system ATM switch section is working due to failure, maintenance, etc. of the operating system ATM switch section;

[0033] FIG. 5A is a schematic diagram showing bus connection which is executed by the call control section switch-over section and selectors of the duplex call control unit of FIG. 2 when a normal operation instruction is supplied;

[0034] FIG. 5B is a schematic diagram showing bus connection which is executed by the call control section switch-over section and the selectors when an operating system call control section cut-off instruction is supplied;

[0035] FIG. 5C is a schematic diagram showing bus connection which is executed by the call control section switch-over section and the selectors when an operating system ATM switch section cut-off instruction is supplied;

[0036] FIG. 5D is a schematic diagram showing bus connection which is executed by the call control section switch-over section and the selectors when an operating system cut-off instruction is supplied;

[0037] FIG. 6 is a block diagram showing a first example of implementation of the duplex call control unit according to the embodiment of the present invention;

[0038] FIG. 7 is a block diagram showing a second example of implementation of the duplex call control unit according to the embodiment of the present invention; and

[0039] FIG. 8 is a block diagram showing a third example of implementation of the duplex call control unit according to the embodiment of the present invention.

[0040] Referring now to the drawings, a description will be given in detail of preferred embodiments in accordance with the present invention.

[0041] FIG. 2 is a block diagram showing the composition of a duplex call control unit in accordance with an embodiment of the present invention. The duplex call control unit of FIG. 2 comprises a control section 10, an ATM switch section 13aof the operating system, an ATM switch section 13bof the standby system, a call control section 14aof the operating system, a call control section 14bof the standby system, an ATM switch section switch-over section 11, a call control section switch-over section 12, call control parameter database section 15, and selectors 16aand 16b.

[0042] The control section 10 is implemented by, for example, a microprocessor unit which is composed of a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc., and appropriate software. Such software for realizing the operations of the control section 10 is stored in one or more record mediums. The ATM switch section switch-over section 11 is implemented by, for example, a bus switching LSI. The call control section switch-over section 12 is also implemented by, for example, a bus switching LSI. Each of the ATM switch sections 13aand 13bis implemented by, for example, an ATM switch module LSI. Each of the call control sections 14aand 14bis implemented by, for example, a microprocessor unit which is composed of a CPU, ROM, RAM, etc., and appropriate software. Such software for realizing the operations of each call control section (14a, 14b) is stored in one or more record mediums. The call control parameter database section 15 is implemented by, for example, flash RAM.

[0043] ATM lines 56a, 56b, . . . , 56nfor inputting/outputting ATM cells from/to ATM networks, subscriber terminals, etc. are connected to the ATM switch section switch-over section 11. The control section 10 is connected to the ATM switch section switch-over section 11 and the call control section switch-over section 12 by buses 50 and 51, respectively. The control section 10 is also connected to other components of the duplex call control unit by buses 49 etc. The ATM switch section switch-over section 11 is connected to the operating system ATM switch section 13aand the standby system ATM switch section 13bby buses 53aand 53b, respectively.

[0044] The control section 10 monitors the statuses of the components of the duplex call control unit by means of polling etc. and outputs signals (instructions) for controlling the switch-over between the operating system and the standby system depending on the statuses of the components.

[0045] The ATM switch section switch-over section 11 controls the connection between the ATM lines (56a, 56b, . . . , 56n) and the buses 53aand 53band thereby controls switch-over of the operations of the operating system ATM switch section 13aand the standby system ATM switch section 13b.

[0046] The call control section switch-over section 12 controls the connection between the ATM switch sections (13a, 13b) and the call control sections (14a, 14b), and thereby controls switch-over of the operations of the operating system call control section 14aand the standby system call control section 14b.

[0047] The call control parameter database section 15 is provided to the duplex call control unit for storing call control parameters with respect to SVC connection information and call control protocol statuses. The SVC connection information is information with regard to SVC connection which has already been established. The call control protocol statuses are statuses of call control protocols for SVC connection which is being established or being released. The call control parameters with respect to SVC connection information which are stored in the call control parameter database section 15 include: VPI (Virtual Path Identifier), VCI (Virtual Channel Identifier), traffic information (priority, QoS (Quality of Service), etc.), band width, and call reference, for example. The call control parameters with respect to call control protocol statuses which are stored in the call control parameter database section 15 include call control parameters for the SSCOP layer, SSCF layer and call control parameters for the signaling layer (UNI signaling). The call control parameters for the SSCOP layer include: SSCOP state variables (ITU-T Q.2110 pages 17, 18 (July 1994)), SSCOP timer value (ITU-T Q.2110 pages 19, 20), SSCOP parameters (ITU-T Q.2110 pages 20, 21), maximum SSCOP SDU (Service Data Unit) size (ITU-T Q.2130 page 14 (July 1994)), and maximum SSCOP UU size(j) (ITU-T Q.2130 page 14 (July 1994)), for example, for each SSCOP link connections. The call control parameters for the SSCF layer include: SSCF state information(ITU-T Q.2130 page 7 (July 1994), for example, for each SSCOP link connections. The call control parameters for the signaling layer include: call reference and status for call reference, for example, for each calls. The call control parameters stored in the call control parameter database section 15 are updated and referred to by the operating system call control section 14aand the standby system call control section 14b.

[0048] The switching of the selectors 16aand 16bare controlled according to signals (instructions) which are supplied from the control section 10. The selectors 16aand 16bare used for connecting buses when SVC connection information is set to each of the operating system ATM switch section 13aand the standby system ATM switch section 13bby the operating system call control section 14aor the standby system call control section 14b.

[0049] Data transmission from the operating system ATM switch section 13ato the operating system call control section 14ais executed via a (unidirectional) bus 54a, and data transmission from the operating system call control section 14ato the operating system ATM switch section 13ais executed via a (unidirectional) bus 55a. Data transmission from the standby system ATM switch section 13bto the standby system call control section 14bis executed via a (unidirectional) bus 54b, and data transmission from the standby system call control section 14bto the standby system ATM switch section 13bis executed via a (unidirectional) bus 55b.

[0050] Under normal operating conditions, the operating system call control section 14aexecutes establishment/release of SVC connection (setting of SVC connection information) to the operating system ATM switch section 13avia buses 52aand 61a, and the standby system call control section 14bexecutes establishment/release of the same SVC connection (setting of SVC connection information) to the standby system ATM switch section 13bvia buses 52band 61b. The operating system call control section 14ais also connected to the selector 16bvia a bus 57band the standby system call control section 14bis also connected to the selector 16avia a bus 57aso that the establishment/release of SVC connection to both ATM switch sections 13aand 13bcan be executed by one call control section 14aor 14bin cases where the operating system call control section 14aor the standby system call control section 14bcan not work (in cases where failure, maintenance etc. occurred to one of the call control sections 14aand 14b).

[0051] In the following, the operation of the duplex call control unit of FIG. 2 will be described in detail.

[0052] ATM cells sent from ATM networks, ATM subscriber terminals, etc. are supplied to the ATM switch section switch-over section 11 via the ATM lines 56a, 56b, . . . , 56n. The ATM switch section switch-over section 11 distributes the received ATM cells to the operating system ATM switch section 13aand the standby system ATM switch section 13b, according to a signal (instruction) supplied from the control section 10 via the bus 50.

[0053] The control section 10 monitors the components of the duplex call control unit by polling etc. as mentioned before, and outputs a normal operation instruction when the ATM switch sections 13aand 13band the call control sections 14aand 14bare working correctly without failure, maintenance, etc. The control section 10 outputs an operating system ATM switch section cut-off instruction when the operating system ATM switch section 13ais not working correctly due to failure, maintenance, etc. The control section 10 outputs an operating system call control section cut-off instruction when the operating system call control section 14ais not working correctly due to failure, maintenance, etc. The control section 10 outputs an operating system cut-off instruction when both the operating system ATM switch section 13aand the operating system call control section 14aare not working correctly due to failure, maintenance, etc.

[0054] FIG. 3A is a schematic diagram showing bus connection of the ATM switch section switch-over section 11 for distributing ATM cells supplied via the ATM lines 56a, 56b, . . . , 56nto the ATM switch sections 13aand 13b. When the normal operation instruction or the operating system call control section cut-off instruction is supplied from the control section 10 via the bus 50 (i.e. when both the ATM switch sections 13aand 13bare working without failure, maintenance, etc.), the ATM switch section switch-over section 11 duplicates all the ATM cells received via the ATM lines 56a, 56b, . . . , 56n, and sends the ATM cells to both the operating system ATM switch section 13aand the standby system ATM switch section 13bvia the buses 53aand 53brespectively.

[0055] When the operating system ATM switch section cut-off instruction or the operating system cut-off instruction is supplied from the control section 10 via the bus 50 (i.e. when the operating system ATM switch section 13ais not working due to failure, maintenance, etc.), the ATM switch section switch-over section 11 disables the bus 53awhich is connected to the operating system ATM switch section 13awhich is not working, and thereby stops the ATM cell duplication. Thereby the ATM cells received via the ATM lines 56a, 56b, . . . , 56nare supplied to the working standby system ATM switch section 13bonly.

[0056] FIG. 3B is a schematic diagram showing bus connection of the ATM switch section switch-over section 11 for transmitting ATM cells supplied from the ATM switch sections 13aand 13bto the ATM lines 56a, 56b, . . . , 56n. In the case where the normal operation instruction or the operating system call control section cut-off instruction is supplied from the control section 10 via the bus 50 (i.e. in the case where both the ATM switch sections 13aand 13bare working without failure, maintenance, etc.), the ATM switch section switch-over section 11 enables the bus 53awhich is connected to the operating system ATM switch section 13aand disables the bus 53bwhich is connected to the standby system ATM switch section 13b, and thereby transmits ATM cells supplied from the operating system ATM switch section 13ato the ATM lines 56a, 56b, . . . , 56n. In this case, ATM cells supplied from the standby system ATM switch section 13bare abandoned in the ATM switch section switch-over section 11.

[0057] In the case where the operating system ATM switch section cut-off instruction or the operating system cut-off instruction is supplied from the control section 10 via the bus 50 (i.e. when the operating system ATM switch section 13ais not working due to failure, maintenance, etc.), the ATM switch section switch-over section 11 enables the bus 53bwhich is connected to the standby system ATM switch section 13band disables the bus 53awhich is connected to the operating system ATM switch section 13a, and thereby transmits ATM cells supplied from the standby system ATM switch section 13bto the ATM lines 56a, 56b, , 56n.

[0058] By the above operation of the ATM switch section switch-over section 11, even if the status of the duplex call control unit changed from the status where both ATM switch sections 13aand 13bare working to the status where the operating system ATM switch section 13ais not working due to failure, maintenance, etc., the missing or loss of the ATM cells which are being communicated is prevented. Further, under normal operating conditions, the same (duplicated) ATM cells are supplied to the operating system ATM switch section 13aand the standby system ATM switch section 13b, and thus the same call control cells are extracted by the ATM switch sections 13aand 13band supplied to the call control section switch-over section 12. Therefore, even if one of the ATM switch sections (13aor 13b) stopped working, the extraction of the call control cells from the received ATM cells can be continued by the operating system ATM switch section 13aor the standby system ATM switch section 13b, and the extracted call control cells can be supplied to the call control section switch-over section 12.

[0059] Each ATM switch section (13a, 13b) which received the ATM cells from the ATM switch section switch-over section 11 extracts particular call control cells (in which specific VPI/VCI for call control (VPI=0, VCI=5) have been set) from headers of the ATM cells, and sends the call control cells to the call control section switch-over section 12 via the bus (54a, 54b).

[0060] FIG. 4A is a schematic diagram showing bus connection between the ATM switch section switch-over section 11 and the call control section switch-over section 12 for communicating ATM cells and call control cells in the case where both the operating system ATM switch section 13aand the standby system ATM switch section 13bare working (that is, in the case where the normal operation instruction or the operating system call control section cut-off instruction is supplied from the control section 10). Referring to FIG. 4A, each ATM switch section (13a, 13b) extracts the call control cells from the ATM cells supplied from the ATM switch section switch-over section 11, and sends the extracted call control cells to the call control section switch-over section 12 via the bus (54a, 54b). In the case where the normal operation instruction is outputted by the control section 10, the operating system ATM switch section 13aalso receives call control cells (which have been generated by the operating system call control section 14aas response) from the call control section switch-over section 12 via the bus 55a, inserts the call control cells in suitable ATM cells, and sends the ATM cells to the ATM switch section switch-over section 11. In the case where the operating system call control section cut-off instruction is outputted by the control section 10, the operating system ATM switch section 13aalso receives call control cells (which have been generated by the standby system call control section 14bas response) from the call control section switch-over section 12 via the bus 55a, inserts the call control cells in suitable ATM cells, and sends the ATM cells to the ATM switch section switch-over section 11.

[0061] FIG. 4B is a schematic diagram showing bus connection between the ATM switch section switch-over section 11 and the call control section switch-over section 12 for communicating ATM cells and call control cells in the case where only the standby system ATM switch section 13bis working due to failure, maintenance, etc. of the operating system ATM switch section 13a(that is, in the case where the operating system ATM switch section cut-off instruction or the operating system cut-off instruction is supplied from the control section 10). Referring to FIG. 4B, the ATM switch section switch-over section 11 does not execute the ATM cell duplication and enables the bus 53bonly, and thereby distributes the ATM cells supplied via the ATM lines 56a, 56b, . . . , 56nto the standby system ATM switch section 13bonly. The standby system ATM switch section 13bextracts the call control cells from the ATM cells and sends the extracted call control cells to the call control section switch-over section 12 via the bus 54b. In this case where the operating system ATM switch section cut-off instruction or the operating system cut-off instruction is outputted by the control section 10, the standby system ATM switch section 13balso receives call control cells (which have been generated by the standby system call control section 14bas response) from the call control section switch-over section 12 via the bus 55a, inserts the call control cells in suitable ATM cells, and sends the ATM cells to the ATM switch section switch-over section 11.

[0062] When the operating system ATM switch section 13ais restored and restarts working from the state shown in FIG. 4B (where the operating system ATM switch section cut-off instruction or the operating system cut-off instruction is outputted by the control section 10), the control section 10 outputs the normal operation instruction or the operating system call control section cut-off instruction. The ATM switch section switch-over section 11 which received the normal operation instruction or the operating system call control section cut-off instruction restores the bus connection to the bus connection shown in FIG. 4A.

[0063] The call control section switch-over section 12 receives the call control cells from both ATM switch sections 13aand 13b, or from the standby system ATM switch section 13bonly, depending on the bus connections which have been explained above. The call control section switch-over section 12 controls the distribution of the received call control cells to the operating system call control section 14aand the standby system call control section 14bas follows, depending on the instructions supplied from the control section 10.

[0064] FIG. 5A is a schematic diagram showing bus connection which is executed by the call control section switch-over section 12 and the selectors 16aand 16bin the case where the normal operation instruction is supplied from the control section 10 via the bus 51 (that is, in the case where both ATM switch sections 13aand 13band both call control sections 14aand 14bare working without failure, maintenance, etc.).

[0065] In the case of FIG. 5A, the call control section switch-over section 12 duplicates call control cells supplied from the operating system ATM switch section 13a, and sends the same (duplicated) call control cells to the operating system call control section 14a(via the bus 54a) and the standby system call control section 14b(via a bus 59 and the bus 54b). Call control cells supplied from the standby system ATM switch section 13bare abandoned in the call control section switch-over section 12. The call control section switch-over section 12 also receives call control cells from the operating system call control section 14aand sends the call control cells to the operating system ATM switch section 13avia the bus 55a. Call control cells supplied from the standby system call control section 14bvia the bus 55bare abandoned in the call control section switch-over section 12.

[0066] Since the normal operation instruction is supplied from the control section 10, the selector 16aselects the bus 52a(which is connected to the operating system call control section 14a) as its input and the selector 16bselects the bus 52b(which is connected to the standby system call control section 14b) as its input, thereby the operating system call control section 14asets SVC connection information (as the result of the call control) to the operating system ATM switch section 13avia the buses 52aand 61a, and the standby system call control section 14bsets the SVC connection information (as the result of the call control) to the standby system ATM switch section 13bvia buses 52band 61b. By such bus connection and operation shown in FIG. 5A, the duplex composition of the ATM switch sections (13aand 13b) and the duplex composition of the call control sections (14aand 14b) are maintained.

[0067] FIG. 5B is a schematic diagram showing bus connection which is executed by the call control section switch-over section 12 and the selectors 16aand 16bin the case where the operating system call control section cut-off instruction is supplied from the control section 10 via the bus 51 (that is, in the case where the operating system call control section 14ais not working due to failure, maintenance, etc. (the standby system call control section 14band both ATM switch sections 13aand 13bare working without failure, maintenance, etc.)).

[0068] In the case of FIG. 5B, the call control section switch-over section 12 disables rear part of the bus 54a, thereby stops the duplication of the call control cells, and thereby sends the call control cells supplied from the operating system ATM switch section 13ato the standby system call control section 14bonly via the bus 59 and the bus 54b. The call control section switch-over section 12 disables front part of the bus 55aand enables a bus 60 which connects front part of the bus 55band rear part of the bus 55aso that call control cells can be transferred from the standby system call control section 14bto the operating system ATM switch section 13avia the buses 55b, 60 and 55a.

[0069] Since the operating system call control section cut-off instruction is supplied from the control section 10, the selector 16aselects the bus 57a(which is connected to the standby system call control section 14b) as its input and the selector 16bselects the bus 52b(which is connected to the standby system call control section 14b) as its input, thereby the standby system call control section 14bsets SVC connection information (as the result of the call control) to the operating system ATM switch section 13avia the buses 57aand 61a, and to the standby system ATM switch section 13bvia the buses 52band 61b. By such bus connection and operation shown in FIG. 5B, the duplex composition of the ATM switch sections (13aand 13b) is maintained.

[0070] FIG. 5C is a schematic diagram showing bus connection which is executed by the call control section switch-over section 12 and the selectors 16aand 16bin the case where the operating system ATM switch section cut-off instruction is supplied from the control section 10 via the bus 51 (that is, in the case where the operating system ATM switch section 13ais not working due to failure, maintenance, etc. (the standby system ATM switch section 13band both call control sections 14aand 14bare working without failure, maintenance, etc.)). In this case, the ATM switch section switch-over section 11 is also supplied with the operating system ATM switch section cut-off instruction from the control section 10, and thus bus connection in the ATM switch section switch-over section 11 is set as shown in FIG. 4B.

[0071] In the case of FIG. 5C, the call control section switch-over section 12 disables front part of the bus 54aand enables front part of the bus 54b(in comparison with the bus connection shown in FIG. 5A) in order to stop call control cell reception from the operating system ATM switch section 13aand start call control cell reception from the standby system ATM switch section 13b. Thereafter, the call control section switch-over section 12 receives call control cells from the standby system ATM switch section 13band sends the call control cells to the operating system call control section 14a(via the buses 59 and 54a) and the standby system call control section 14b(via the bus 54b). The call control section switch-over section 12 also receives call control cells from the standby system call control section 14band sends the call control cells to the standby system ATM switch section 13bvia the bus 55b. Call control cells supplied from the operating system call control section 14avia the bus 55aare abandoned in the call control section switch-over section 12.

[0072] Since the operating system ATM switch section cut-off instruction is supplied from the control section 10 (since the operating system ATM switch section 13ais not working), it is impossible for the operating system call control section 14ato set SVC connection information (as the result of the call control) to the operating system ATM switch section 13avia the buses 52aand 61a. In this case, the selector 16awhich is supplied with the operating system ATM switch section cut-off instruction disables its output and does not output call control cells to the operating system ATM switch section 13a. The selector 16bwhich is supplied with the operating system ATM switch section cut-off instruction selects the bus 52b(which is connected to the standby system call control section 14b) as its input, thereby the standby system call control section 14bsets SVC connection information (as the result of the call control) to the standby system ATM switch section 13bvia the buses 52band 61b. By such bus connection and operation shown in FIG. 5C, the duplex composition of the call control sections (14aand 14b) is maintained.

[0073] FIG. 5D is a schematic diagram showing bus connection which is executed by the call control section switch-over section 12 and the selectors 16aand 16bin the case where the operating system cut-off instruction is supplied from the control section 10 via the bus 51 (that is, in the case where the operating system ATM switch section 13aand the operating system call control section 14aare not working due to failure, maintenance, etc. (the standby system ATM switch section 13band the standby system call control section 14bare working without failure, maintenance, etc.)). In this case, the ATM switch section switch-over section 11 is also supplied with the operating system cut-off instruction from the control section 10, and thus bus connection in the ATM switch section switch-over section 11 is set as shown in FIG. 4B.

[0074] In the case of FIG. 5D, the call control section switch-over section 12 enables the buses 54band 55bonly so as to connect the standby system ATM switch section 13band the standby system call control section 14b. The call control section switch-over section 12 receives call control cells from the standby system ATM switch section 13band sends the call control cells to the standby system call control section 14bvia the bus 54b. The call control section switch-over section 12 also receives call control cells from the standby system call control section 14band sends the call control cells to the standby system ATM switch section 13bvia the bus 55b.

[0075] Since the operating system cut-off instruction is supplied from the control section 10 (since the operating system ATM switch section 13aand the operating system call control section 14aare not working), it is impossible for the operating system call control section 14ato set SVC connection information (as the result of the call control) to the operating system ATM switch section 13avia the buses 52aand 61a. In this case, the selector 16awhich is supplied with the operating system cut-off instruction disables its output and does not output call control cells to the operating system ATM switch section 13a. The selector 16bwhich is supplied with the operating system cut-off instruction selects the bus 52b(which is connected to the standby system call control section 14b) as its input, thereby the standby system call control section 14bsets SVC connection information (as the result of the call control) to the standby system ATM switch section 13bvia the buses 52band 61b.

[0076] Incidentally, while the ATM switch section 13ahas been referred to as an “operating system ATM switch section 13a” and the ATM switch section 13bhas been referred to as an “standby system ATM switch section 13b” (that is, the functions of the ATM switch section 13ahas been fixed as an ATM switch section for the “operating system” and the functions of the ATM switch section 13bhas been fixed as an ATM switch section for the “standby system”) in the above explanation, it is also possible to let the ATM switch sections 13aand 13boperate relatively as the “operating system ATM switch section” or the “standby system ATM switch section” depending on cases. Concretely, when the ATM switch section 13awhich has been operating as the “operating system ATM switch section” stopped working due to failure, maintenance, etc., it is also possible to let the ATM switch section 13bthereafter operate as the “operating system ATM switch section” and let the ATM switch section 13aoperate as the “standby system ATM switch section”. In such relative operation, when the ATM switch section 13a(“standby system ATM switch section”) is restored and restarts working, the ATM switch section 13aremains as the “standby system ATM switch section” and the ATM switch section 13bremains as the “operating system ATM switch section”.

[0077] In the same way, while the call control section 14ahas been referred to as an “operating system call control section 14a” and the call control section 14bhas been referred to as an “standby system call control section 14b” (that is, the functions of the call control section 14ahas been fixed as a call control section for the “operating system” and the functions of the call control section 14bhas been fixed as a call control section for the “standby system”) in the above explanation, it is also possible to let the call control sections 14aand 14boperate relatively as the “operating system call control section” or the “standby system call control section” depending on cases. Concretely, when the call control section 14awhich has been operating as the “operating system call control section” stopped working due to failure, maintenance, etc., it is also possible to let the call control section 14bthereafter operate as the “operating system call control section” and let the call control section 14aoperate as the “standby system call control section”. In such relative operation, when the call control section 14a(“standby system call control section”) is restored and restarts working, the call control section 14aremains as the “standby system call control section” and the call control section 14bremains as the “operating system call control section”.

[0078] Such operating/standby relative operation is implemented by bus connection of the ATM switch section switch-over section 11, the call control section switch-over section 12 and the selectors 16aand 16bdepending on instructions which are supplied from the control section 10, similarly to the operations which have been explained referring to FIGS. 3A through 5D. The control section 10 always grasps which of the ATM switch sections (13aor 13b) is the “operating system ATM switch section” now and which of the call control sections (14aor 14b) is the “operating system call control section” now, based on the result of the monitoring (by means of polling etc.), instructions it outputted. etc. The control section 10 instructs the ATM switch section switch-over section 11, call control section switch-over section 12 and the selectors 16aand 16bexecute bus connection similarly to the above explanation, depending on which ATM switch section (13aor 13b) is the “operating system ATM switch section” now and which call control section (14aor 14b) is the “operating system call control section” now.

[0079] The operating system call control section 14aand the standby system call control section 14bare designed to receive the same call control cells under normal operating conditions. However, mismatch of statuses of call control parameters can occur between the operating system call control section 14aand the standby system call control section 14bfor some reasons. Further, when the status of the duplex call control unit changes from the status shown in FIG. 5B to the status shown in FIG. 5A, when the status changes from the status shown in FIG. 5D to the status shown in FIG. 5C, etc., a call control section (14aor 14b) which has not been working has to obtain SVC connection information and call control protocol statuses which has been operated and held by the other call control section (14bor 14a). Therefore, the call control sections 14aand 14buse the call control parameter database section 15 for checking the SVC connection information and the call control protocol statuses which are now being operated and for setting (updating) the SVC connection information and the call control protocol statuses of itself.

[0080] The operating system call control section 14amakes access to the call control parameter database section 15 via a bus 58a. The standby system call control section 14bmakes access to the call control parameter database section 15 via a bus 58b. For example, in the case of FIG. 5B where the operating system call control section 14ais not working, the standby system call control section 14bexecutes update of the call control parameters which are stored in the call control parameter database section 15. When the operating system call control section 14ais restored and the duplex operation is restarted, the operating system call control section 14arefers to the call control parameter database section 15 and thereby acquires the call control parameters with respect to SVC connection information and call control protocol statuses.

[0081] As mentioned before, the call control parameters stored in the call control parameter database section 15 include call control parameters with respect to SVC connection information and call control parameters with respect to call control protocol statuses. The SVC connection information is information with regard to SVC connection which has already been established. The call control protocol statuses are statuses of call control protocols for SVC connection which is being established or being released. The call control parameters with respect to SVC connection information include: VPI, VCI, traffic information (priority, QoS (Quality of Service), etc.), band width, and call reference, for example. The call control parameters with respect to call control protocol statuses include call control parameters for the SSCOP layer, SSCF layer and call control parameters for the signaling layer (UNI signaling). The call control parameters for the SSCOP layer include: SSCOP state variables, SSCOP timer value, SSCOP parameters, maximum SSCOP SDU size, and maximum SSCOP UU sizes), for example, for each SSCOP link connections. The call control parameters for the SSCF layer include: SSCF state information, for example, for each SSCOP link connections. The call control parameters for the signaling layer include: call reference and status for call reference, for example, for each calls, as mentioned before.

[0082] When both the operating system call control section 14aand the standby system call control section 14bare working as shown in FIG. 5A or FIG. 5C (that is, when the normal operation instruction or the operating system ATM switch section cut-off instruction is outputted by the control section 10), the operating system call control section 14aupdates the call control parameters stored in the call control parameter database section 15 during processing call control cells. The standby system call control section 14brefers to the call control parameters stored in the call control parameter database section 15. If call control parameters held by the standby system call control section 14bdid not match the call control parameters of the call control parameter database section 15, the standby system call control section 14bforcibly replaces its call control parameters by the call control parameters of the call control parameter database section 15, and adjust its call control protocol statuses and SVC connection information according to the new call control parameters.

[0083] When the operating system call control section 14ais not working as shown in FIG. 5B or FIG. 5D (that is, when the operating system call control section cut-off instruction or the operating system cut-off instruction is outputted by the control section 10), the standby system call control section 14bupdates the call control parameters stored in the call control parameter database section 15 during processing call control cells.

[0084] When the operating system call control section 14awhich has not been working restarts working from its initial state, the operating system call control section 14aobtains call control parameters stored in the call control parameter database section 15, forcibly sets its call control parameters as those of the call control parameter database section 15, adjusts its call control protocol statuses and SVC connection information according to the new call control parameters, and thereafter sends an operating system call control section restart signal to the control section 10. The control section 10 which received the operating system call control section restart signal outputs the normal operation instruction or the operating system ATM switch section cut-off instruction, and thereby the duplex call control operation is restarted.

[0085] In the case where the aforementioned “operating/standby relative operation” is employed, the call control parameters stored in the call control parameter database section 15 are updated by the call control section 14aor 14bthat is now regarded as the “operating system ATM switch section”, and are referred to by the call control section 14bor 14athat is now regarded as the “standby system ATM switch section”. The call control sections 14aand 14bare informed by the control section 10 about which (14aor 14b) is the “operating system ATM switch section” and which (14bor 14a) is the “standby system ATM switch section”, and execute the update and reference of the call control parameter database section 15 according to the information concerning “operation system” and “standby system”.

[0086] When one of the call control sections (14aor 14b) is not working, the other call control section (14bor 14a) updates the call control parameters stored in the call control parameter database section 15 during processing call control cells.

[0087] When the call control section (14aor 14b) which has not been working restarts working from its initial state, the call control section (14aor 14b) acquires call control parameters stored in the call control parameter database section 15, forcibly sets its call control parameters as those of the call control parameter database section 15, adjusts its call control protocol statuses and SVC connection information according to the new call control parameters, and thereafter sends a call control section restart signal (of the call control section 14aor the call control section 14b) to the control section 10. The control section 10 which received the call control section restart signal outputs an instruction (which is similar to the normal operation instruction or the operating system ATM switch section cut-off instruction), and thereby the duplex call control operation is restarted.

[0088] In the following, some examples of implementation of the duplex call control unit according to the embodiment of the present invention will be explained.

[0089] FIG. 6 is a block diagram showing a first example of implementation of the duplex call control unit according to the embodiment of the present invention. Referring to FIG. 6, the control section 10, the ATM switch section switch-over section 11, the call control section switch-over section 12 and the call control parameter database section 15 are mounted on a control board 300. The operating system ATM switch section 13aand the standby system ATM switch section 13bare separately mounted on an operating system ATM switch board 301aand a standby system ATM switch board 301b, respectively. The operating system call control section 14aand the standby system call control section 14bare separately mounted on an operating system call control board 302aand a standby system call control board 302b, respectively. By developing an ATM switch or an ATM subscriber line concentrator (the duplex call control unit) in such board combination, duplex composition of the ATM switch boards (301aand 301b) and duplex composition of the call control boards (302aand 302b) are realized. By such board combination, the duplex composition of the ATM switch boards (301aand 301b) and the duplex composition of the call control boards (302aand 302b) can be maintained independently by the operations of the control section 10, the ATM switch section switch-over section 11, the call control section switch-over section 12 and the selectors 16aand 16bwhich have been explained referring to FIGS. 4A through 5D.

[0090] FIG. 7 is a block diagram showing a second example of implementation of the duplex call control unit according to the embodiment of the present invention. Referring to FIG. 7, the control section 10, the ATM switch section switch-over section 11, the call control section switch-over section 12 and the call control parameter database section 15 are mounted on a control board 300. The operating system ATM switch section 13aand the operating system call control section 14aare mounted on an operating system ATM switch call control board 303a. The standby system ATM switch section 13band the standby system call control section 14bare mounted on a standby system ATM switch call control board 303b. By developing an ATM switch or an ATM subscriber line concentrator (the duplex call control unit) in such board combination, duplex composition of the duplex call control unit is made similar to that of the conventional duplex call control unit of FIG. 1 from the viewpoint of hardware composition (board combination). However, the control board 300 is provided with the ATM switch section switch-over section 11 and the call control parameter database section 15, and thus the missing (loss) of ATM cells on the operating/standby switch-over is prevented and a call during establishment/release can be relieved from being interrupted. Therefore, duplex composition of higher reliability in comparison with the conventional duplex call control unit can be realized by the second example.

[0091] FIG. 8 is a block diagram showing a third example of implementation of the duplex call control unit according to the embodiment of the present invention. Referring to FIG. 8, the control section 10, the ATM switch section switch-over section 11 and the call control section switch-over section 12 are mounted on a control board 300. The operating system ATM switch section 13aand the standby system ATM switch section 13bare separately mounted on an operating system ATM switch board 301aand a standby system ATM switch board 301b, respectively. The operating system call control section 14aand a call control parameter database section 15afor the operating system are mounted on an operating system call control board 302a. The standby system call control section 14band a call control parameter database section 15bfor the standby system are mounted on a standby system call control board 302b. The call control parameter database section 15aof the operating system call control board 302aand the call control parameter database section 15bof the standby system call control board 302bare connected together by a bus 62. The call control parameter database sections 15aand 15bare provided with functions for checking whether or not the call control parameters held by them are identical to each other. By developing an ATM switch or an ATM subscriber line concentrator (the duplex call control unit) in such board combination, in comparison with the first example of FIG. 6, loss of call control parameters can be prevented even when failure occurred to the control board 300. Even when abnormality occurred in call control parameters stored in one of the call control parameter database sections (15aor 15b), call control parameters stored in the other call control parameter database section (15bor 15a) can be used. Therefore, reliability of the duplex composition can be improved by the board combination of FIG. 8.

[0092] As described above, in the duplex call control unit according to the embodiment of the present invention, the control section 10 monitors the operating statuses of the two ATM switch sections 13aand 13band the two call control sections 14aand 14bof the duplex call control unit, and outputs instructions concerning bus connection to the ATM switch section switch-over section 11, the call control section switch-over section 12 and the selectors 16aand 16bdepending on the monitored operating statuses so that the operations of the ATM switch section can be executed at least by the first ATM switch section or the second ATM switch section that is working and so that the operations of the call control section can be executed at least by the first call control section or the second call control section that is working. In the call control parameter database section 15, call control parameters with respect to SVC connection information with regard to SVC connection which has already been established and call control parameters with respect to the statuses of call control protocols for SVC connection which is being established or being released are stored (updated by one of the call control section (14aor 14b) which is actually executing call control). The call control parameters stored in the call control parameter database section 15 are referred to by the other call control section that is operating as a standby system call control section or that is restarting working for adjusting its call control parameters.

[0093] Therefore, the operations of the ATM switch section and the call control section, that is, the extraction of call control cells for SVC connection from the ATM cells supplied via the ATM lines 56a, 56b, . . . , 56n, the reception of the call control cells, the outputting of call control cells as response, the setting of SVC connection information (as the result of the call control for a SVC connection) to a working ATM switch section, are executed by at least one ATM switch section (13aor 13b) that is working and one call control section (14aor 14b) that is working even if one of the ATM switch sections and/or one of the call control sections stopped working due to failure, maintenance, etc.

[0094] Therefore, loss of ATM cells for SVC connection on the switch-over from the operating system to the standby system is eliminated, and the disconnection of the SSCOP link connections on the switch-over from the operating system to the standby system is also eliminated. By the elimination of the SSCOP link disconnection, the SVC connection is saved from being released by another ATM switch or ATM subscriber terminal.

[0095] Signaling messages are communicated in the PDU (Protocol Data Unit) of the SSCOP layer. The disconnection of the SSCOP link connections on the switch-over from the operating system to the standby system is avoided as mentioned above. Therefore, loss of the signaling messages on the switch-over from the operating system to the standby system is also avoided. Consequently, duplicate control can be realized and executed for calls during call control, that is, for calls during establishment/release of SVC connection, as well as for calls whose SVC connection have already been established.

[0096] During call control, the call control parameters stored in the call control parameter database section 15 are updated by a call control section (14aor 14b) which is operating as the “operating system call control section”, and are referred to by the other call control section (14bor 14a). When mismatch occurred in the call control parameters (with respect to the SVC connection information and the call control protocol statuses), the call control section (14bor 14a) which is referring to the call control parameter database section 15 forcibly sets its call control parameters as those of the call control parameter database section 15 (that is, as those of the “operating system call control section” (14aor 14b) which is updating the call control parameter database section 15). Therefore, the statuses of SVC connection setting to the ATM switch section (13a, 13b) and the statuses of the call control protocols in the SSCOP layer and the signaling layer can be made exactly the same between the two call control sections 14aand 14bof the operating system and the standby system.

[0097] The duplex ATM switch sections (13aand 13b) and the duplex call control sections (14aand 14b) are provided to the duplex call control unit as two independent duplex components, and the duplex composition of the call control sections 14aand 14bcan be maintained even if one of the ATM switch sections 13aand 13bstopped working, and the duplex composition of the ATM switch sections 13aand 13bcan be maintained even if one of the call control sections 14aand 14bstopped working. Therefore, a duplex call control unit of improved reliability can be provided.

[0098] Incidentally, as mentioned before, while the above explanation has been mainly given on the assumption that the functions of the ATM switch section 13aand the call control section 14aare fixed as the “operating system ATM switch section 13a” and the “operating system call control section 14a” and the functions of the ATM switch section 13band the call control section 14bare fixed as the “standby system ATM switch section 13b” and the “standby system call control section 14b”, the aforementioned “operating/standby relative operation” can also be employed. In such cases, the control section 10 grasps which of the ATM switch sections (13aor 13b) is operating as the “operating system ATM switch section” now and which of the call control sections (14aor 14b) is operating as the “operating system call control section” now, and outputs instructions to the ATM switch section switch-over section 11, the call control section switch-over section 12 and the selectors 16aand 16bto execute proper bus connection similarly to the explanation which has been given referring to FIGS. 3A through 5D.

[0099] While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by those embodiments but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention.