Abstract:
A transmission device for receiving a user signal and transmitting to another device a transmission signal, the transmission device comprising: a plurality of interface units each capable of accommodating a communication line; and a cross-connect unit for branching the user signal and coupling the communication line accommodated by each of the plurality of interface units to a client device, wherein the transmission device is configured to provide redundancy to the communication line by using the plurality of interface units, wherein the each of the plurality of interface units having a switching control module for performing switching control of an interface unit that accommodates a communication line for transmitting a main signal to the client device.

Description:
CLAIM OF PRIORITY 
       [0001]    This U.S. non-provisional patent application is a continuation of U.S. patent application Ser. No. 13/682,094 filed on Nov. 20, 2012 which claims priority to Japanese Patent Applications No. 2011-263830 filed on Dec. 1, 2011, the entire contents of each application being hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to a transmission device and an interface device for realizing switching control in packet transport such as the MPLS-TP. 
         [0003]    In recent years, broadband lines represented by the Internet have spread into houses, which results in an increased need for the lines mainly by IP traffics and a rapid increase in various services provisioned therein. 
         [0004]    Under such circumstances, as a cross-connect method in a layer 1 transmission device, the mainstream has shifted from the conventional “line switching” represented by the SDH to “packet switching” having high affinity to IP networks represented by routers. 
         [0005]    A representative cross-connect method of the packet switching is Multi Protocol Label Switch (MPLS) (see, for example, IETF RFC 3031 “Multiprotocol Label Switching Architecture”). In the MPLS, generally, a route is determined by a predetermined algorithm, and information is exchanged between nodes coupled via the determined route to determine a label. Each node holds, as route information, a table showing correspondence between an input label and an output port, and transfers data to the output port specified based on the table. 
         [0006]    In a packet transmission network represented by the MPLS, the concept of “path”, which is a physical transfer path in the SDH, does not exist. As a method of managing the route, a distributed management method involving determining a transfer destination for each node based on a predetermined policy such as the resources of the network and the number of hops is used as a basic method. 
         [0007]    In the packet transmission network described above, data is transmitted on a packet-by-packet basis without securing a fixed time slot, and hence there is an advantage in that the traffic may be accommodated efficiently. However, each device determines the route autonomously based on a route selection algorithm, and hence an explicit route is not known. Further, when a failure occurs, a failure point and an affected range are difficult to identify, which is a big problem in maintainability and in terms of management. 
         [0008]    In order to overcome the above-mentioned problem in maintainability and in terms of management, standardization of a technology called Multi Protocol Label Switch Transport Profile (MPLS-TP) is currently underway in the IETF (see, for example, IETF RFC 5654 “Requirements of an MPLS Transport Profile”). 
         [0009]    The MPLS-TP is a technology which adopts the transmission method of transmitting packets in the MPLS and which combines the reliability and high operability in terms of maintenance and operation in the conventional legacy line such as the SDH. Therefore, in the MPLS-TP, characteristic functions (ideas) of “expanding the operation, administration, and maintenance (OAM) function”, “explicit route management”, and “separation of the control plane and the data plane” are introduced to the conventional MPLS. 
         [0010]    The “OAM function” is a collective name of various maintenance and operation functions for supporting stable data transfer at high quality, and is constituted of “failure detecting function”, “failure point identifying function”, “failure notifying function”, “performance monitoring function”, and “protecting function”. In order to realize the above-mentioned functions, separately from the packet for transferring the service, normalization of the packet dedicated to the OAM is underway (see, for example, IETF RFC 5860 “Requirements for Operations, Administration, and Maintenance (OAM) in MPLS Transport Networks”). 
         [0011]    The “explicit route managing function” allows, as opposed to autonomic distributed route determination method which has been a mainstream in the conventional packet transmission network, a person in charge of maintenance to specify a start point, an end point, and a via point as the route through which the traffic passes, to thereby set the route statically. The route which is specified once is not updated automatically by factors other than a setting instruction from the person in charge of maintenance. This allows the person in charge of maintenance to clearly know where the service line is actually transmitted through. 
         [0012]    The “separation of the control plane and the data plane” means that the functional module for controlling the modules in the node and the functional module for transferring data are separated, which is an idea for preventing, even when a failure occurs in the functional module for controlling the modules in the node, for example, the functional module for transferring data from being affected. 
         [0013]    The MPLS-TP follows a transmission method similar to the MPLS as described above. Therefore, the MPLS-TP has features of allowing multiprotocol accommodating, which accommodates user signals irrespective of the types of the accommodated signals by using the MPLS label, and of being capable of separating clear service classes among the user signals by using QoS control. 
         [0014]    The MPLS-TP is a growing technical field as a technology of combining the above-mentioned technology and the line emulation technology to accommodate the rapidly increasing IP traffics and the conventional SDH line such as the SDH in the same platform and to manage a layer 1 network and a layer 2 network in combination. Related art documents relating to the switching control method of the transmission device include, for example, Japanese Patent Application Laid-open Nos. 2004-207849, 06-311131, and 2001-160793. 
       SUMMARY OF THE INVENTION 
       [0015]    Those characteristic functions and ideas of the MPLS-TP, that is, “expanding the operation, administration, and maintenance (OAM) function”, “explicit route management”, and “separation of the control plane and the data plane” are necessitated as a result of pursuing high maintainability, reliability, and fault tolerance of the MPLS-TP. Among others, the separation of the control plane for monitoring and controlling the line and the data plane for transmitting and receiving the user signals is an important function in terms of the maintainability and the fault tolerance. 
         [0016]    The conventional transmission device generally includes, as described in Japanese Patent Application Laid-open Nos. 2004-207849 and 06-311131, an interface unit for executing main signal processing such as signal termination processing, and a monitoring and control module for monitoring and controlling the device. 
         [0017]    The transmission device has a configuration in which, in order to increase the reliability and the fault tolerance of a main signal transfer path, the main signal transfer path is duplicated as a first route and a second route. When a failure occurs in the first route, the transmission device executes a failure relief processing by switching to the second route so that a disconnected state of a main signal to be transferred between a device as a transfer destination of the main signal and the transmission device does not continue. The route switching control described above is another function of the monitoring and control module. 
         [0018]    In the conventional transmission device, in order to increase the fault tolerance of the main signal, an interface card for accommodating the first route and an interface card for accommodating the second route are different cards. Further, the conventional transmission device needs to have a configuration capable of monitoring the interface cards, and hence the monitoring and control module for performing switching control between the interface cards and executing processing of transmitting switching information to an opposing device is mounted in a card different from the interface cards. 
         [0019]    However, the above-mentioned monitoring and control module may temporarily become out of service during a series of maintenance operations, such as a file update operation of software for realizing the monitoring and control module, and in a case where the person in charge of maintenance resets the monitoring and control module intentionally. Therefore, there has been a problem in that, during the temporal out-of-service period, the transmission device cannot switch the transmission path, and hence in case where a failure that affects the main signal occurs, the main signal remains in a blocked state and is not relieved. 
         [0020]    There is also known means for continuing, in a case where the monitoring and control module is reset during the switching control and the switching information is initialized, the switching control immediately after the initialization without any contradiction from the state before the resetting (see, for example, Japanese Patent Application Laid-open No. 2001-160793). 
         [0021]    However, with the means described in Japanese Patent Application Laid-open No. 2001-160793, the switching control stops from the reset to the restart of the monitoring and control module. Therefore, means for ensuring the switching control from the reset to the restart of the monitoring and control module is not provided. 
         [0022]    On the other hand, there may be contemplated a method involving duplicating the monitoring and control module per se so that, even if one monitoring and control module enters the out-of-service state, the other monitoring and control module continues the switching control, to thereby improve the reliability of the switching control. 
         [0023]    However, in a configuration in which many lines are accommodated under the monitoring and control module, in a case where line failures occur simultaneously and many main signals are to be switched at once, it takes time from the occurrence of the line failure to the completion of the switching control, the time corresponding to the number of provisioned main signals. 
         [0024]    For example, there is a problem in that the time may exceed 50 ms, which is specified for the switching time of the legacy line such as the SDH, and even when the monitoring and control module is duplicated, a restriction is imposed on the scalability of the number of interface cards and the number of accommodated lines. Even when the switching time does not exceed 50 ms, the time required until the completion of the switching processing is lengthened proportionally to the number of accommodated lines. 
         [0025]    The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: a transmission device for receiving a user signal and transmitting to another device a transmission signal obtained by reflecting an APS transmission value on the received user signal. The transmission device comprises: a plurality of interface units each capable of accommodating a communication line; and a cross-connect unit for branching the user signal, which is transmitted from a client device, to be transmitted to each of the plurality of interface units, and coupling the communication line accommodated by each of the plurality of interface units to the client device. The transmission device is configured to provide redundancy to the communication line by using the plurality of interface units. The each of the plurality of interface units has a switching control module for performing switching control of an interface unit that accommodates a communication line for transmitting a main signal to the client device. The switching control module is configured to: select the interface unit for transmitting the main signal from among the plurality of interface units providing the redundancy; and switch to any one of the plurality of interface units providing the redundancy to continue the transmission of the main signal, in a case where a failure occurs in the selected interface unit. 
         [0026]    With each interface card including the switching control module for performing switching control of the communication line through which the main signal is transmitted, the single point of failure due to the failure of the switching control module may be avoided. In addition, the failure of the switching control module may be addressed quickly. Further, necessary processing is executed as processing in the interface card, and hence the switching of the communication line may be performed at high speed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
           [0028]      FIG. 1  is a block diagram illustrating a configuration example of a transmission device according to the embodiment of this invention;  FIGS. 2A and 2B  are block diagrams illustrating detailed configurations of a system  0  interface car, a system  1  interface card, and a cross-connect card according to the embodiment of this invention; 
           [0029]      FIG. 3  is an explanatory diagram illustrating a concept of a transition table according to the embodiment of this invention; 
           [0030]      FIG. 4  is an explanatory diagram illustrating a specific example of the transition table according to the embodiment of this invention; 
           [0031]      FIGS. 5A and 5B  are explanatory diagrams illustrating a switching control of a line according to the embodiment of this invention; 
           [0032]      FIGS. 6A and 6B  are explanatory diagrams illustrating the switching control of a line according to the embodiment of this invention; 
           [0033]      FIGS. 7 and 8  are sequence diagrams illustrating a flow of the processing of switching between a master and slave according to the embodiment of this invention; and 
           [0034]      FIG. 9  is an explanatory diagram illustrating the conventional transmission device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    Hereinafter, an embodiment of this invention is described with reference to the accompanying drawings. 
         [0036]    First, a configuration example of a conventional transmission device is described. 
         [0037]      FIG. 9  is an explanatory diagram illustrating the conventional transmission device. 
         [0038]    A conventional transmission device  100  includes a monitoring and control module  113 , a section termination module  101 , a signal transmission module  105 , a switching factor reception module  107 , a switching information reception module  108 , a switching information transmission module  109 , a section termination module  102 , a signal transmission module  106 , a switching factor reception module  110 , a switching information reception module  111 , a switching information transmission module  112 , a selector  103 , and a branch module  104 . 
         [0039]    The monitoring and control module  113  controls switching of a line (transmission path). The section termination module  101  receives signals from a system  0  transmission path  1102 , and the signal transmission module  105  transmits signals to the system  0  transmission path  1102 . 
         [0040]    The switching factor reception module  107  receives a switching factor  1108  from the section termination module  101  and transfers the switching factor  1108  to the monitoring and control module  113 . The switching information reception module  108  receives received switching information  1110  from the section termination module  101  and transfers the received switching information  1110  to the monitoring and control module  113 . The switching information transmission module  109  receives transmission switching information  1111  from the monitoring and control module  113  and transfers the transmission switching information  1111  to the signal transmission module  105 . 
         [0041]    The section termination module  102  receives signals from a system  1  transmission path  1103 , and the signal transmission module  106  transmits signals to the system  1  transmission path  1103 . 
         [0042]    The switching factor reception module  110  receives a switching factor  1112  from the section termination module  102  and transfers the switching factor  1112  to the monitoring and control module  113 . The switching information reception module  111  receives received switching information  1113  from the section termination module  102  and transfers the received switching information  1113  to the monitoring and control module  113 . The switching information transmission module  112  receives transmission switching information  1114  from the monitoring and control module  113  and transfers the transmission switching information  1114  to the signal transmission module  106 . 
         [0043]    The selector  103  selects one of a system  0  received signal  1104  and a system  1  received signal  1105  based on an instruction from the monitoring and control module  113 . The branch module  104  branches a main signal  1101  into a system  0  transmission signal  1106  to be transmitted to the system  0  transmission path  1102  and a system  1  transmission signal  1107  to be transmitted to the system  1  transmission path  1103 . 
         [0044]    In other words, in a transmission direction (bottom to top direction of  FIG. 9 ) of the transmission path, the branch module  104  branches the main signal  1101  into the system  0  transmission signal  1106  and the system  1  transmission signal  1107 , and in a reception direction (top to bottom direction of  FIG. 9 ) of the transmission path, the selector  103  selects one of the system  0  received signal  1104  and the system  1  received signal  1105 . 
         [0045]    As a result, a configuration in which the transmission path is redundant is provided. 
         [0046]    The monitoring and control module  113  receives the switching factor  1108  and the received switching information  1110  from the switching factor reception module  107  and the switching information reception module  108  on the system  0  side, respectively, and receives the switching factor  1112  and the received switching information  1113  from the switching factor reception module  110  and the switching information reception module  111  on the system  1  side, respectively. 
         [0047]    The monitoring and control module  113  determines, based on the received information, which of the system  0  received signal  1104  and the system  1  received signal  1105  is to be set as the main signal, and transmits a switching instruction  1115  including the determination result to the selector  103 . Further, in order to notify a coupling destination device of each of the system  0  transmission path  1102  and the system  1  transmission path  1103  of contents of the switching instruction  1115 , the monitoring and control module  113  transmits the transmission switching information  1111  to the signal transmission module  105  of the system  0  or transmits the transmission switching information  1114  to the signal transmission module  106  of the system  1 . The transmission switching information  1111  is transmitted to the coupling destination device via the system  0  transmission path  1102 , and the transmission switching information  1114  is transmitted to the coupling destination device via the system  1  transmission path  1103 . 
         [0048]    The selector  103  selects one of the system  0  received signal  1104  and the system  1  received signal  1105  based on the switching instruction  1115  received from the monitoring and control module  113 . 
         [0049]    As described above, the transmission device  100  generates the switching instruction based on information such as the switching factor, and transmits its own switching information to the coupling destination device so that the switching information is exchanged from/to the coupling destination device to implement the line switching control. 
         [0050]    The series of processing is performed by the monitoring and control module  113 . Therefore, as illustrated in  FIG. 9 , with the configuration of the conventional transmission device  100 , in a case where the monitoring and control module  113  is out of service, a state in which the line switching control cannot be performed occurs. 
         [0051]      FIG. 1  is a block diagram illustrating a configuration example of a transmission device according to the embodiment of this invention. 
         [0052]    As illustrated in  FIG. 1 , a transmission device  200  according to this embodiment includes a system  0  interface card  251 , a system  1  interface card  252 , and a cross-connect card  253 . 
         [0053]    The system  0  interface card  251  and the system  1  interface card  252  transmit signals received from an opposing device such as another transmission device to the cross-connect card  253 , and the cross-connect card  253  transmits a main signal to a client device or the like. 
         [0054]    The cross-connect card  253  also branches the main signal received from the client device or the like, and transmits the main signal branched to the system  0  interface card  251  and the system  1  interface card  252 . Each of the system  0  interface card  251  and the system  1  interface card  252  transmits the received main signal to the opposing device. 
         [0055]    In the example illustrated in  FIG. 1 , the transmission device  200  includes only two interface cards. However, this invention is not limited thereto, and the transmission device  200  may include three or more interface cards. The transmission device  200  may also include another component (not shown). 
         [0056]    The system  0  interface card  251 , the system  1  interface card  252 , and the cross-connect card  253  are communicably connected to one another. Each of the system  0  interface card  251 , the system  1  interface card  252 , and the cross-connect card  253  includes at least one processor (not shown), at least one memory (not shown), and the like as hardware components. 
         [0057]    Next, details of the system  0  interface card  251 , the system  1  interface card  252 , and the cross-connect card  253  are described. 
         [0058]      FIGS. 2A and 2B  are block diagrams illustrating detailed configurations of the system  0  interface card  251 , the system  1  interface card  252 , and the cross-connect card  253  according to the embodiment of this invention. 
         [0059]    As illustrated in  FIGS. 2A and 2B , the system  0  interface card  251  accommodates a system  0  transmission path  2101  and the system  1  interface card  252  accommodates a system  1  transmission path  2121  so that the line is made redundant by the system  0  transmission path  2101  and the system  1  transmission path  2121 . 
         [0060]    It should be noted that, in a case where the transmission device  200  includes three or more interface cards, the line may be made redundant by the three or more interface cards. 
         [0061]    The system  0  interface card  251  includes a signal termination module  201 , an own system failure detection module  203 , an own system APS reception module  205 , a packet transmission control module  206 , a switching information transmission module  204 , a signal transmission module  202 , an other system failure notification module  210 , an other system APS notification module  209 , a switching control module  208 , a transition information management module  207 , an other system monitoring module  212 , an M/S determination module  211 , and an inter-adjacent-card IF  213 . 
         [0062]    The signal termination module  201  executes termination processing on a system  0  transmission path received signal  2102 . 
         [0063]    The own system failure detection module  203  detects a failure state of the system  0  interface card  251 . The own system failure detection module  203  also transmits own system failure information  2112  including the detected failure state to the switching control module  208  and the inter-adjacent-card IF  213 . The own system APS reception module  205  extracts a received APS byte from the system  0  transmission path received signal  2102 . The own system APS reception module  205  also transmits own system APS information  2111  including the extracted received APS byte to the switching control module  208  and the inter-adjacent-card IF  213 . 
         [0064]    The packet transmission control module  206  performs transmission control and stop control on a packet signal from the system  0  interface card  251  to the cross-connect card  253 . 
         [0065]    The switching information transmission module  204  reflects switching information  2105  including a switching state of the system  0  interface card  251  itself on the APS byte. The signal transmission module  202  generates a system  0  transmission path transmission signal  2103  by multiplexing the main signal received from the cross-connect card  253  with the APS byte received from the switching information transmission module  204 . 
         [0066]    The other system failure notification module  210  receives, from the system  1  interface card  252 , failure information (own system failure information  2132 ) of a system  1  transmission path received signal  2122  received by the system  1  interface card  252 . The other system APS notification module  209  receives, from the system  1  interface card  252 , APS information (own system APS information  2131 ) of the system  1  transmission path received signal  2122  received by the system  1  interface card  252 . 
         [0067]    The switching control module  208  performs switching control of the transmission path (line) between the system  0  transmission path  2101  and the system  1  transmission path  2121 . Specifically, in the switching control, which of the system  0  transmission path received signal  2102  and the system  1  transmission path received signal  2122  is to be transmitted as the main signal from the cross-connect card  253  is determined. In this embodiment, the switching control module  208  switches the transmission path by transmitting an own system shutdown instruction  2104  to the packet transmission control module  206  or transmitting an other system shutdown instruction  2126  to a packet transmission control module  226 . 
         [0068]    The transition information management module  207  holds a transition table  300  (see  FIGS. 3 and 4 ), and refers to the transition table  300  (see  FIGS. 3 and 4 ) based on the received failure information and the received APS information to read information necessary for the switching control. Further, the transition information management module  207  notifies the switching control module  208  of the read information. It should be noted that details of the transition table  300  are described below with reference to  FIGS. 3 and 4 . 
         [0069]    The other system monitoring module  212  checks an operating state of a switching control module  228  of the system  1  interface card  252 , which is the other system. Specifically, the other system monitoring module  212  checks the operating state of the switching control module  228  by transmitting a monitoring signal  2113  to an other system monitoring module  232 . The other system monitoring module  212  also transmits a monitoring result  2110  to the M/S determination module  211 . 
         [0070]    The M/S determination module  211  determines, based on the monitoring result of the other system monitoring module  212 , which of the switching control module  208  of the system  0  interface card  251  and the switching control module  228  of the system  1  interface card  252  serves as a master. The M/S determination module  211  also transmits M/S information  2109  including the determination result to the switching control module  208 . 
         [0071]    It should be noted that in this embodiment, the switching control module  208  or the switching control module  228  serving as the master performs the switching control initiatively. This indicates that the switching control module  208  or the switching control module  228  serving as the master is a switching control module of an active system. On the other hand, the switching control module  208  or the switching control module  228  not serving as the master serves as a slave (standby system). 
         [0072]    The inter-adjacent-card IF  213  is an interface for transmitting and receiving various kinds of information to/from the system  1  interface card  252 . 
         [0073]    The system  1  interface card  252  includes, similarly to the system  0  interface card  251 , a signal termination module  221 , an own system failure detection module  223 , an own system APS reception module  225 , the packet transmission control module  226 , a switching information transmission module  224 , a signal transmission module  222 , an other system failure notification module  230 , an other system APS notification module  229 , the switching control module  228 , a transition information management module  227 , the other system monitoring module  232 , an M/S determination module  231 , and an inter-adjacent-card IF  233 . 
         [0074]    The signal termination module  221  executes termination processing on the system  1  transmission path received signal  2122 . 
         [0075]    The own system failure detection module  223  detects a failure state of the system  1  interface card  252 . The own system failure detection module  223  also transmits the own system failure information  2132  including the detected failure state to the switching control module  228  and the inter-adjacent-card IF  233 . The own system APS reception module  225  extracts a received APS byte from the system  1  transmission path received signal  2122 . The own system APS reception module  225  also transmits the own system APS information  2131  including the extracted received APS byte to the switching control module  228  and the inter-adjacent-card IF  233 . 
         [0076]    The packet transmission control module  226  performs transmission control and stop control on a packet signal from the system  1  interface card  252  to the cross-connect card  253 . 
         [0077]    The switching information transmission module  224  reflects switching information  2125  including a switching state of the system  1  interface card  252  itself on the APS byte. The signal transmission module  222  generates a system  1  transmission path transmission signal  2123  by multiplexing the main signal received from the cross-connect card  253  with the APS byte received from the switching information transmission module  224 . 
         [0078]    The other system failure notification module  230  receives, from the system  0  interface card  251 , failure information (the own system failure information  2112 ) of the system  0  transmission path received signal  2102  received by the system  0  interface card  251 . The other system APS notification module  229  receives, from the system  0  interface card  251 , APS information (the own system APS information  2111 ) of the system  0  transmission path received signal  2102  received by the system  0  interface card  251 . 
         [0079]    The switching control module  228  performs switching control of the transmission path (line) between the system  0  transmission path  2101  and the system  1  transmission path  2121 . Specifically, in the switching control, which of the system  0  transmission path received signal  2102  and the system  1  transmission path received signal  2122  is to be transmitted as the main signal from the cross-connect card  253  is determined. In this embodiment, the switching control module  228  switches the transmission path by transmitting an own system shutdown instruction  2124  to the packet transmission control module  226  or transmitting an other system shutdown instruction  2106  to the packet transmission control module  206 . 
         [0080]    The transition information management module  227  holds the transition table  300  (see  FIGS. 3 and 4 ), and refers to the transition table  300  (see  FIGS. 3 and 4 ) based on the received failure information and the received APS information to read information necessary for the switching control. Further, the transition information management module  227  notifies the switching control module  228  of the read information. It should be noted that details of the transition table  300  are described below with reference to  FIGS. 3 and 4 . 
         [0081]    The other system monitoring module  232  checks an operating state of the switching control module  208  of the system  0  interface card  251 , which is the other system. Specifically, the other system monitoring module  232  checks the operating state of the switching control module  208  by transmitting a monitoring signal  2133  to an other system monitoring module  212 . The other system monitoring module  232  also transmits a monitoring result  2130  to the M/S determination module  231 . 
         [0082]    The M/S determination module  231  determines, based on the monitoring result of the other system monitoring module  232 , which of the switching control module  208  of the system  0  interface card  251  and the switching control module  228  of the system  1  interface card  252  serves as a master. The M/S determination module  231  also transmits M/S information  2129  including the determination result to the switching control module  228 . In this embodiment, the switching control module  208  or the switching control module  228  serving as the master performs the switching control initiatively. 
         [0083]    The inter-adjacent-card IF  233  is an interface for transmitting and receiving various kinds of information detected by the system  0  interface card  251 . 
         [0084]    The system  0  interface card  251  and the system  1  interface card  252  according to this embodiment include the switching control module  208  and the switching control module  228  for performing the line switching control, respectively. It should be noted, however, that in a case where the switching control module  208  and the switching control module  228  operate simultaneously, inconsistency arises in the switching control, and hence the master for performing the switching control initiatively is set. 
         [0085]    In this embodiment, constituent modules of the system  0  interface card  251  and the system  1  interface card  252  are implemented by using special hardware. For example, a special processor and a special memory may be provided separately from the processor and the memory used for transmitting and receiving the signals, and the special processor may execute programs stored in the specific memory to implement the constituent modules. This invention is not limited thereto, and alternatively the processor may execute programs stored in the memory to implement equivalent functions. 
         [0086]    The system  0  interface card  251  and the system  1  interface card  252  are connected to each other by a plurality of signal lines via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . 
         [0087]    The first signal line is a signal line for notifying each other of the failure information. 
         [0088]    Specifically, in the signal line, the own system failure information  2112  including the failure state detected by the own system failure detection module  203  is transmitted to the other system failure notification module  230  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . Further, the own system failure information  2112  is transmitted from the other system failure notification module  230  to the switching control module  228 . Similarly, in the signal line, the own system failure information  2132  including failure information detected by the own system failure detection module  223  is transmitted to the other system failure notification module  210  via the inter-adjacent-card IF  233  and the inter-adjacent-card IF  213 . Further, the own system failure information  2132  is transmitted from the other system failure notification module  210  to the switching control module  208 . 
         [0089]    The second signal line is a signal line for notifying each other of the APS information. 
         [0090]    Specifically, in the signal line, the own system APS information  2111  including the received APS byte detected by the own system APS reception module  205  is transmitted to the other system APS notification module  229  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . Further, the own system APS information  2111  is transmitted from the other system APS notification module  229  to the switching control module  228 . Similarly, in the signal line, the own system APS information  2131  including received APS byte detected by the own system APS reception module  225  is transmitted to the other system APS notification module  209  via the inter-adjacent-card IF  233  and the inter-adjacent-card IF  213 . Further, the own system APS information  2131  is transmitted from the other system APS notification module  209  to the switching control module  208 . 
         [0091]    The third signal line is a signal line for notifying each other of the shutdown instruction. 
         [0092]    Specifically, in the signal line, the other system shutdown instruction  2126  transmitted from the switching control module  208  is transmitted to the packet transmission control module  226  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . Similarly, in the signal line, the other system shutdown instruction  2106  transmitted from the switching control module  228  is transmitted to the packet transmission control module  206  via the inter-adjacent-card IF  233  and the inter-adjacent-card IF  213 . 
         [0093]    The fourth signal line is a signal line for transmitting monitoring signals of the switching control module  208  and the switching control module  228 . 
         [0094]    Specifically, in the signal line, the monitoring signal  2113  transmitted from the other system monitoring module  212  is transmitted to the other system monitoring module  232  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . Similarly, in the signal line, the monitoring signal  2133  transmitted from the other system monitoring module  232  is transmitted to the other system monitoring module  212  via the inter-adjacent-card IF  233  and the inter-adjacent-card IF  213 . 
         [0095]    The fifth signal line is a signal line for notifying each other of the switching information. 
         [0096]    Specifically, in the signal line, the switching information  2105  transmitted from the switching control module  208  is transmitted to the switching information transmission module  224  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . Similarly, in the signal line, the switching information  2125  transmitted from the switching control module  228  is transmitted to the switching information transmission module  204  via the inter-adjacent-card IF  233  and the inter-adjacent-card IF  213 . 
         [0097]    The inter-adjacent-card IF  213  and the inter-adjacent-card IF  233  are connected to each other symmetrically via the above-mentioned five signal lines. In this embodiment, the system  0  interface card  251  and the system  1  interface card  252  are desirably connected by hardware without any intervening software. In other words, a configuration in which the above-mentioned pieces of information may be transferred by using hardware is desired. 
         [0098]    This is because, in a case where the above-mentioned information and control signals are transferred by intervening software, when a failure occurs in the software running on one of the system  0  interface card  251  and the system  1  interface card  252 , necessary information cannot be transmitted or received, with a result that the switching control cannot be performed. 
         [0099]    Assuming the configuration as described above, the specific switching control according to this invention is described. It should be noted that in the following, a case where the switching control module  208  of the system  0  interface card  251  operates as the master and a case where the switching control module  228  of the system  1  interface card  252  operates as the master are described separately. 
         [0100]    First, a configuration example of the transition table  300  held in each of the transition information management module  207  and the transition information management module  227  is described. 
         [0101]      FIG. 3  is an explanatory diagram illustrating a concept of the transition table  300  according to the embodiment of this invention.  FIG. 4  is an explanatory diagram illustrating a specific example of the transition table  300  according to the embodiment of this invention. 
         [0102]    As illustrated in  FIG. 3 , the transition table  300  forms a matrix having a state  301  in the horizontal direction and a state transition occasion  302  in the vertical direction. Further, at intersecting portions of rows and columns, transition information necessary for the switching control is defined. Specifically, a “state”, “control”, and “APS” are defined as the transition information. 
         [0103]    The state  301  indicates states of the switching control. In this embodiment, the content of the switching control is defined depending on the states. The state transition occasion  302  indicates occasions for state transition. 
         [0104]    The “state” indicates a state to which the switching control transitions. The “control” indicates a specific content of the switching control. The “APS” indicates an APS transmission value to be reflected on the signal transmitted from a branch module  242 . 
         [0105]    For example, in the example illustrated in  FIG. 3 , in a state of “State 01”, in a case where “Occasion 03” arises, the switching control is performed based on the transition information defined in the intersecting portion (hatched portion) of State 01and Occasion 03. In this case, the switching control module  208  or the switching control module  228  performs “control: B”, transmits “APS: b” as the APS transmission value, transitions to a state of “State 04”, and waits for the next occasion for state transition. 
         [0106]    Each time an occasion corresponding to the state transition occasion  302  arises, the transition information management module  207  and the transition information management module  227  determine the transition information (state, control, and APS) corresponding to the occasion that has arisen. 
         [0107]    The specific transition table  300  illustrated in  FIG. 4  forms a matrix similarly to the transition table  300  illustrated in  FIG. 3 . It should be noted that the state  301  is defined by a combination of a switching state  401  and a selected system  402 . As the state transition occasion  302 , occurrence of system  0  SF, recovery from system  0  SF, occurrence of system  1  SF, recovery from system  1  SF, reception of RR, and the like are defined by all pieces of information detected by the own system APS reception module  205 , the own system failure detection module  203 , the own system APS reception module  225 , and the own system failure detection module  223  in  FIGS. 2A and 2B . 
         [0108]    At intersecting portions of rows and columns, similarly to the transition table  300  illustrated in  FIG. 3 , the transition information including the “state”, “control”, and “APS” is defined. 
         [0109]    In this embodiment, it is assumed that the transition table  300  is provided for each of protocols such as ITU-T G.841, ITU-T G.841 Annex B, ITU-T G.8031, and Telcordia GR-253-CORE. 
         [0110]    Next, details of the switching control are described by taking the transition table  300  illustrated in  FIG. 4  as an example. 
         [0111]    (1) Case where the system  0  interface card  251  operates as the master, and a failure occurs in the system  0  transmission path  2101  as a accommodated line. 
         [0112]      FIGS. 5A and 5B  are explanatory diagrams illustrating the switching control of a line according to the embodiment of this invention.  FIGS. 5A and 5B  illustrate the switching control performed by the system  0  interface card  251  operating as the master. 
         [0113]    In this case, it is assumed that the system  0  interface card  251  operates as the master and that the system  0  transmission path  2101  is a accommodated line. The switching control performed in a case where a state in which the transmission paths of the system  0  and the system  1  are both in a normal state is changed to a state in which a failure occurs in the system  0  transmission path is described below. It should be noted that in this embodiment, at the time of initial setting of the transmission device  200 , the state  301  is set to “State 01”. 
         [0114]    In the case where the system  0  interface card  251  operates as the master, the switching control module  228  and the transition information management module  227  of the system  1  interface card  252  operating as the slave are set to a stopped state. 
         [0115]    Therefore, the switching control module  228  does not transmit the own system shutdown instruction  2124  to the packet transmission control module  226 , and does not transmit the other system shutdown instruction  2106  to the packet transmission control module  206 , either. 
         [0116]    On the other hand, the switching control module  208  and the transition information management module  207  of the system  0  interface card  251  operating as the master are set to a state of being able to operate. Therefore, the switching control module  208  and the transition information management module  207  monitor a failure state and the APS information between the system  0  transmission path  2101  and the system  1  transmission path  2121 , and perform the switching control. 
         [0117]    The switching control module  208  always monitors a failure state of each of the system  0  transmission path received signal  2102  and the system  1  transmission path received signal  2122  which have been received, and the received APS information. Specifically, each of the constituent modules transmits a signal or information as follows. 
         [0118]    The signal termination module  201  receives the system  0  transmission path received signal  2102 , and executes termination processing on the received signal. Further, the signal termination module  201  transmits the system  0  transmission path received signal  2102  on which the termination processing has been executed to the own system failure detection module  203  and the own system APS reception module  205 . 
         [0119]    The own system failure detection module  203  monitors the failure state of the system  0  transmission path received signal  2102 , and transmits to the switching control module  208  the own system failure information  2112  including a result of the monitoring. The own system APS reception module  205  monitors the received APS information of the system  0  transmission path received signal  2102 , and transmits to the switching control module  208  the own system APS information  2111  including a result of the monitoring. 
         [0120]    On the other hand, the signal termination module  221  receives the system  1  transmission path received signal  2122 , and executes termination processing on the received signal. Further, the signal termination module  221  transmits the system  1  transmission path received signal  2122  on which the termination processing has been executed to the own system failure detection module  223  and the own system APS reception module  225 . 
         [0121]    The own system failure detection module  223  monitors the failure state of the system  1  transmission path received signal  2122 , and transmits to the inter-adjacent-card IF  233  the own system failure information  2132  including a result of the monitoring. The own system failure information  2132  is transmitted to the switching control module  208  via the inter-adjacent-card IF  213  and the other system failure notification module  210 . 
         [0122]    Further, the own system APS reception module  225  monitors the received APS information of the system  1  transmission path received signal  2122 , and transmits to the inter-adjacent-card IF  233  the own system APS information  2131  including a result of the monitoring. The own system APS information  2131  is transmitted to the switching control module  208  via the inter-adjacent-card IF  213  and the other system APS notification module  209 . 
         [0123]    In a case where a signal fail (SF) occurs in the system  0  transmission path  2101 , the own system failure detection module  203  detects “system  0  SF”, and transmits to the switching control module  208  the own system failure information  2112  notifying of the detected “system  0  SF”. 
         [0124]    On the other hand, the system  1  transmission path  2121  is normal, and hence the own system failure detection module  223  transmits to the switching control module  208 , via the inter-adjacent-card IF  233 , the inter-adjacent-card IF  213 , and the other system failure notification module  210 , the own system failure information  2132  notifying of no failure in system  1 ″. 
         [0125]    Further, the own system APS reception module  225  extracts the APS information from the system  1  transmission path received signal  2122 . The own system APS reception module  225  transmits to the switching control module  208 , via the inter-adjacent-card IF  233 , the inter-adjacent-card IF  213 , and the other system APS notification module  209 , the own system APS information  2131  notifying of “received APS byte: no switching to select system  0 ”. 
         [0126]    As used herein, the “APS information” refers to information indicating a switching state of the opposing device coupled via the system  0  transmission path  2101  or the system  1  transmission path  2121 . Each of the interface cards and its corresponding device exchange the “APS information” with one another, and by confirming the arrival of the “APS information”, the switching control between the system  0  transmission path  2101  and the system  1  transmission path  2121  is performed between the interface card and the opposing device. 
         [0127]    The switching control module  208  transmits to the transition information management module  207  the received information  2107  including information received from the respective constituent modules, and makes an inquiry about a transition destination in the switching state. In this case, the received information  2107  includes “System  0  SF” received from the own system failure detection module  203 , no failure in system  1 ″ received from the other system failure notification module  210 , and “received APS byte: no switching to select system  0 ” received from the other system APS notification module  209 . 
         [0128]    The transition information management module  207  refers to the transition table  300  based on the received information  2107  which has been received so as to determine the transition information, and transmits to the switching control module  208  the transition information  2108  including the “state”, “control”, and “APS” as a response. 
         [0129]    Before the failure (SF) occurs in the system  0  transmission path  2101 , the system  0  transmission path  2101  is selected and the transmission paths of both systems are normal, and hence the switching control module  208  remains in State 01 without making a transition of the state. 
         [0130]    In a case where the failure occurs in the system  0  transmission path  2101  (occurrence of system  0  SF), such a state transition as indicated by a route  4001  of  FIG. 4  is performed. Specifically, the transition information management module  207  determines the transition information, and transmits the corresponding transition information  2108  to the switching control module  208 . 
         [0131]    Specifically, in a case where the state  301  is “State 01” and the state transition occasion  302  is “occurrence of system  0  SF”, in the transition table  300 , “state: 09, control: wait for RR response, APS: SFW” are defined as the transition information. Therefore, the system  0  interface card  251  makes a transition to State 09, transmits a code corresponding to “working signal fail (SFW)”, and makes a transition to a state of waiting for a response of RR from the opposing device. 
         [0132]    Then, in State 09, in a case where the system  0  interface card  251  receives the response of RR from the opposing device, such a state transition as indicated by a route  4002  of  FIG. 4  is performed. Specifically, the transition information management module  207  determines the transition information, and transmits the corresponding transition information  2108  to the switching control module  208 . 
         [0133]    Specifically, in a case where the state  301  is “State 09” and the state transition occasion  302  is “reception of RR”, in the transition table  300 , “state:  10 , control: switch to system  1 , APS: SFW” are defined as the transition information. Therefore, the system  0  interface card  251  makes a transition to State  10 , continues transmitting the code corresponding to the SFW, switches a transmission path (line) to the system  1  transmission path  2121 , and waits for the next state transition occasion. 
         [0134]    An example of a general APS switching has been described above, but there exist various standards for APS switching, and a switching protocol, a sequence, and a code for the APS value vary for each standard. Examples of the standards for APS switching include ITU-T G.841, ITU-T G.841 Annex B, ITU-T G.8031, and Telcordia GR-253-CORE. 
         [0135]    As described above, the switching control module  208  receives from the transition information management module  207  the transition information  2108  including the “state”, “control”, and “APS”, performs the switching control based on the received transition information, and further, transmits the APS transmission value. 
         [0136]    First, a method of transmitting the APS value is described. 
         [0137]    The switching control module  208  extracts the APS transmission value included in the transition information  2108 , and transmits to the switching information transmission module  204  the switching information  2105  including the extracted APS transmission value. The switching information  2105  is further transmitted to the signal transmission module  202 . 
         [0138]    The signal transmission module  202  multiplexes the main signal branched by the branch module  242  of the cross-connect card  253  and the switching information  2105  to generate the system  0  transmission path transmission signal  2103 . 
         [0139]    It should be noted that whether the APS transmission value is transmitted by using the system  0  transmission path transmission signal  2103  or the APS transmission value is transmitted by using the system  1  transmission path transmission signal  2123  differs for each standard. 
         [0140]    For example, in a case where the APS transmission value is transmitted by using the system  1  transmission path transmission signal  2123 , the switching control module  208  may transmit to the switching information transmission module  224 , via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 , the switching information  2105  including the APS transmission value. Then, the signal transmission module  222  multiplexes the main signal branched by the branch module  242  of the cross-connect card  253  and the switching information  2105  to generate the system  1  transmission path transmission signal  2123 . 
         [0141]    Next, a method of switching the transmission path is described. 
         [0142]    The switching of the transmission path refers to processing of selecting, by the cross-connect card  253 , one of the system  0  transmission path received signal  2102  and the system  1  transmission path received signal  2122 . It should be noted that the above-mentioned processing of this embodiment is different from a method of selecting a signal by the conventional selector  103 . 
         [0143]    Specifically, the packet transmission control module  206  and the packet transmission control module  226  stop transmission of a packet signal of one of the system  0  transmission path and the system  1  transmission path, of the packet signals to be transmitted from the own system APS reception module  205  and the own system APS reception module  225  to the cross-connect card  253 , and a merge module  241  of the cross-connect card  253  merges the signal. 
         [0144]    For example, in a case where the system  0  transmission path received signal  2102  is selected, the packet transmission control module  206  sets the packet signal to be transmitted to the merge module  241  of the cross-connect card  253  to “transmission”, and the packet transmission control module  226  sets the packet signal to be transmitted to the merge module  241  of the cross-connect card  253  to “stop”. As a result, only the system  0  transmission path received signal  2102  is transmitted from the merge module  241 . Therefore, logically, a state in which the system  0  transmission path received signal  2102  is selected is obtained. 
         [0145]    In a case where the configuration in which the transmission path is redundant is provided as in this embodiment, the selector  103  for selecting a signal is normally mounted on a card different from the system  0  interface card  251  and the system  1  interface card  252 . On the other hand, in the case of the packet signal such as that in the MPLS-TP as described above, it is not necessary to directly control the selector  103 , and the line switching control can be performed in the system  0  interface card  251  and the system  1  interface card  252 . 
         [0146]    As described above, in the route  4002  from State 09 to State 10, the transmission path is switched from the system  0  transmission path  2101  to the system  1  transmission path  2121 . In this case, the switching control module  208  transmits the own system shutdown instruction  2104  to the packet transmission control module  206 , to thereby stop the transmission of the system  0  transmission path received signal  2102 . Meanwhile, the switching control module  208  transmits the other system shutdown instruction  2126  to the packet transmission control module  226 , to thereby start the transmission of the system  1  transmission path received signal  2122 . The system  1  transmission path received signal  2122  is transmitted to the merge module  241 , and the system  1  transmission path received signal  2122  is transmitted from the merge module  241 . 
         [0147]    In this manner, the switching is performed as follows. One of the packet transmission control modules is controlled to be a stop (invalid) state, and the other of the packet transmission control modules is controlled to be a transmission (valid) state, and one of the system  0  transmission path received signal  2102  and the system  1  transmission path received signal  2122  is transmitted to the cross-connect card  253 . 
         [0148]    To summarize the above description, in the case where the system  0  interface card  251  operates as the master, a monitoring and control route regarding the switching control is a route indicated by the thick lines of  FIGS. 5A and 5B . Specifically, the monitoring and control route is as follows. 
         [0149]    First, the switching control module  208  monitors the own system APS reception module  205  and the own system failure detection module  203 , and further, monitors the own system APS reception module  225  and the own system failure detection module  223  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 . The switching control module  208  transmits to the transition information management module  207  the received information  2107  including information received from the respective constituent modules. 
         [0150]    The transition information management module  207  refers to the transition table  300  based on the received information  2107  to determine the transition information (state, control, and APS), and transmits the corresponding transition information  2108  to the switching control module  208 . 
         [0151]    The switching control module  208  controls the packet transmission control module  206  and the packet transmission control module  226  based on the received transition information  2108 . Further, the switching control module  208  transmits to the signal transmission module  222 , via the inter-adjacent-card IF  213 , the inter-adjacent-card IF  233 , and the switching information transmission module  224 , the switching information  2105  including the APS transmission value. In this manner, the system  1  transmission path transmission signal  2123  including the APS transmission value is transmitted. 
         [0152]    In short, the switching control module  208  of the system  0  interface card  251  operating as the master monitors the system  0  interface card  251  and the system  1  interface card  252 , and further, performs the switching control. 
         [0153]    (2) Case where the system  1  interface card  252  operates as the master, and a failure occurs in the system  1  transmission path  2121  as a accommodated line. 
         [0154]      FIGS. 6A and 6B  are explanatory diagrams illustrating the switching control of a line according to the embodiment of this invention.  FIGS. 6A and 6B  illustrate the switching control performed by the system  1  interface card  252  operating as the master. 
         [0155]    In a case where the system  1  interface card  252  operates as the master, a monitoring and control route regarding the switching control is a route indicated by the thick lines of  FIGS. 6A and 6B . Specifically, the monitoring and control route is as follows. 
         [0156]    First, the switching control module  228  monitors the own system APS reception module  225  and the own system failure detection module  223 , and further, monitors the own system APS reception module  205  and the own system failure detection module  203  via the inter-adjacent-card IF  233  and the inter-adjacent-card IF  213 . The switching control module  228  transmits to the transition information management module  227  the received information  2127  including information received from the respective constituent modules. 
         [0157]    The transition information management module  227  refers to the transition table  300  based on the received information  2127  to determine the transition information (state, control, and APS), and transmits corresponding transition information  2128  to the switching control module  228 . 
         [0158]    The switching control module  228  controls the packet transmission control module  206  and the packet transmission control module  226  based on the received transition information  2128 . Further, the switching control module  228  transmits to the signal transmission module  222 , via the switching information transmission module  224 , the switching information  2125  including the APS transmission value. In this manner, the system  1  transmission path transmission signal  2123  including the APS transmission value is transmitted. 
         [0159]    In short, the switching control module  228  of the system  1  interface card  252  operating as the master monitors the system  0  interface card  251  and the system  1  interface card  252 , and further, performs the switching control. 
         [0160]    According to this invention, based on the operating states of the switching control module  208  of the system  0  interface card  251  and the switching control module  228  of the system  1  interface card  252 , it is determined whether each of the interface cards is to be the master or slave. It should be noted that the switching control between the master and slave is performed based only on the operating state of the switching control module  208  (or the switching control module  228 ), and the state of the line (transmission path) is not considered. 
         [0161]    In other words, the switching control of the transmission path and the switching control between the master and slave are independently performed. Therefore, there may be a case where the system  0  interface card  251  operates as the master and the accommodated line is the system  1  transmission path  2121 , or conversely, there may be a case where the system  1  interface card  252  operates as the master and the accommodated line is the system  0  transmission path  2101 . 
         [0162]    The switching control of the line to be accommodated (transmission path) and the switching control between the master and slave are independently performed for the following two reasons. 
         [0163]    (Reason 1) For example, under the standard such as ITU-T G.841, ITU-T G.8031, and Telcordia GR-253-CORE, even in a case where the system  0  interface card  251  is not mounted on the transmission device  200 , the system  0  transmission path  2101  may be selected as the line to be provisioned. Therefore, if the switching control between the master and slave is synchronized with the switching control of the provisioned line, there is a fear that an interface card that is not mounted may be selected as the master. In this case, the transmission device  200  becomes a deadlock state in which the monitoring of the failure state and the switching control cannot be performed. 
         [0164]    (Reason 2) A recovery time of a service is desirably as early as possible, but if the master and slave are switched every time the line is switched, the recovery time of the service is delayed by the period of time required for the processing of switching between the master and slave. 
         [0165]    In particular, (Reason 1) is a critical failure, and hence the switching control between the master and slave needs to be performed independently of the line switching control. 
         [0166]    According to this invention, the other system monitoring module  212  of the system  0  interface card  251  and the other system monitoring module  232  of the system  1  interface card  252  check the operating states of the switching control module  208  and the switching control module  228  via the inter-adjacent-card IF  213  and the inter-adjacent-card IF  233 , and transmit/receive a message to/from each other, to thereby switch between the master and slave. Specifically, the following processing is executed. 
         [0167]      FIGS. 7 and 8  are sequence diagrams illustrating a flow of the processing of switching between the master and slave according to the embodiment of this invention. 
         [0168]    In a case where the system  0  interface card  251  and the system  1  interface card  252  both start as a slave, the other system monitoring module  212  and the other system monitoring module  232  transmit slave notifications  703  notifying each other that the own interface card is the slave. 
         [0169]    The other system monitoring module  212  which has received the slave notification  703  switches the own interface card to a master, and transmits to the other system monitoring module  232  a master notification  704  notifying the other system monitoring module  232  that the own interface card has become the master. It should be noted that an initial setting of the master may be made in advance, or may be instructed by an administrator of the transmission device  200 . In this case, it is assumed that the initial setting is made in advance. 
         [0170]    In a case of receiving the master notification  704 , the other system monitoring module  232  transmits an ACK  705  as a response to the other system monitoring module  212 , and thereafter, transmits a normality check  706  to the other system monitoring module  212  of the system  0  interface card  251  operating as the master. The other system monitoring module  232  determines, based on a response to the normality check  706 , whether or not the operating state of the switching control module  208  of the system  0  interface card  251  is normal. 
         [0171]    The other system monitoring module  232  transmits normality checks  801  and  802  to the other system monitoring module  212  to monitor the operating state of the switching control module  208  of the system  0  interface card  251  operating as the master. 
         [0172]    In a case where a failure such as down occurs in the switching control module  208 , there is no response to the normality check  802  from the other system monitoring module  212 , and hence the other system monitoring module  232  detects the occurrence of the failure. Accordingly, the other system monitoring module  232  transmits an instruction to transition to slave  803  to the other system monitoring module  212 . 
         [0173]    In a case of receiving the instruction to transition to slave  803 , the other system monitoring module  212  switches the own interface card to the slave, and transmits to the other system monitoring module  232  an ACK  804  as a response. In a case of receiving the ACK  804 , the other system monitoring module  232  switches the own interface card to the master. 
         [0174]    Thereafter, the other system monitoring module  212  switched to the slave transmits a normality check  805  to the other system monitoring module  232 . 
         [0175]    According to this invention, each of the interface cards includes the switching control module for performing the line switching control, and hence even in a case where an abnormality occurs in the switching control module of one of the interface cards, the line switching control can be performed. As a result, the separation of the control plane and the data plane is implemented. 
         [0176]    Further, the switching control module is disposed for each of the interface cards in a distributed manner, and hence the switching control is performed on a card-by-card basis. Therefore, even in a case where the number of interface cards in the entire device and the number of accommodated lines increase, the line switching can be implemented without exceeding 50 ms, which is specified for the switching time of the legacy line such as the SDH. As a result, no restriction is imposed on the scalability of the number of accommodated lines and the number of interface cards. 
         [0177]    Still further, the switching control module is disposed for each of the interface cards in a distributed manner, and hence the number of lines under the control of the switching control module can be made smaller. As a result, a period of time in which a service provisioned in the lines is disconnected is shortened as compared with a conventional case. 
         [0178]    While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.