This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-167099, filed on Jun. 26, 2007, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and device for switching interfaces in a communication device included in a redundant system. 
     2. Description of the Related Art 
     SDH (Synchronous Digital Hierarchy)/SONET (Synchronous Optical NETwork) is widely used as global-standard network interfaces particularly in wide area networks. In recent years, for the purpose of further bringing down costs, there is a move afoot to apply Ethernet™ (hereinafter, referred to as “Ethernet”) to wide area networks. In this case, since there are many occasions when a single communication device must accommodate both SDH/SONET and Ethernet interfaces, a device has also been developed in which a SDH/SONET interface card and an Ethernet interface card can be freely replaced with one another in a single slot. 
     To apply Ethernet to wide area networks, it is necessary to enhance the functions of monitoring and controlling a network, and work for the standardization thereof is now in progress in ITU-T. Specifically, a frame for monitoring and controlling (Ethernet OAM frame) is defined in an Ethernet frame, causing a wide area network to be monitored and controlled. In this standardization work, in order to improve the reliability of Ethernet, it has been standardized to implement a redundant system of SDH/SONET on Ethernet (see ITU-T Standard G.8031/Y.1342, pp. 15-22). Specifically, a frame for switching control, called ETH-APS, is defined, and control information is exchanged between link-connected opposite devices, whereby bidirectional switching is accomplished. 
     However, although information carried in the above-mentioned ETH-APS frame is basically and approximately the same as the automatic protection switching (APS) bytes K 1  and K 2  of SDH/SONET, there is an essential difference between them: a main-signal interface for one of them is of a synchronization type, and that for the other is of a non-synchronization type. Additionally, the APS bytes and ETH-APS frame are also different in bit assignment. 
     Accordingly, to enable a single device to accommodate both interfaces, a switch control section for SDH/SONET and a switch control section for Ethernet may be provided independently and switched depending on the interface type. In other words, it is necessary to mount both the SDH/SONET switching control section and Ethernet switching control section on a control section, resulting in the device being complicated and expensive. 
     Moreover, the above-described method only assumes that the network interface sections support the same type of network (i.e., any one of SDH/SONET and Ethernet), and no consideration is given to a redundant system in a case of network interfaces for different types of networks. Accordingly, in the above-described example, a SDH/SONET interface section and an Ethernet interface section, for example, cannot be mounted at the same time. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an interface switching method and device that makes it possible to switch between different types of interfaces, without complicating the configuration. 
     According to the present invention, an interface switching device in a redundant system including a plurality of network interfaces which can be switched depending on automatic switch information, includes: a transformer for transforming automatic switch information between a predetermined type used for a predetermined network interface and each of other types used for network interfaces other than the predetermined network interface; a switch controller for performing switch control for automatic switch information of the predetermined type; and a control interface for connecting a first network interface to the switch controller via the transformer when first automatic switch information received from the first network interface is not of the predetermined type. 
     By virtue of the interface switching device according to the present invention, it is possible to switch between different types of interfaces, without complicating the configuration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an interface switching device according to an exemplary embodiment of the present invention. 
         FIG. 2  is a more detailed block diagram of the interface switching device shown in  FIG. 1 . 
         FIG. 3  is a format diagram showing an example of correspondences between Ethernet APS information and SDH/SONET APS information, to describe APS information transformation rules stored in an APS information transformation section. 
         FIG. 4  is a diagram of correspondences, showing an example of APS information transformation rules between SDH/SONET APS information and Ethernet APS information. 
         FIG. 5  is a diagram showing part of the example of APS information transformation rules between Ethernet APS information and SDH/SONET APS information as shown in  FIG. 4 . 
         FIG. 6  is a diagram showing other part of the example of the APS information transformation rules as shown in  FIG. 4 . 
         FIG. 7  is a block diagram of the interface switching device according to the present exemplary embodiment on which SDH/SONET interfaces are mounted for both of 0- and 1-systems. 
         FIG. 8  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 7 . 
         FIG. 9  is a block diagram of the interface switching device according to the present exemplary embodiment on which Ethernet interfaces are mounted for both of the 0- and 1-systems. 
         FIG. 10  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 9 . 
         FIG. 11  is a block diagram of the interface switching device according to the present exemplary embodiment in which a 0-system interface section serves as a SDH/SONET interface and a 1-system interface section serves as an Ethernet interface. 
         FIG. 12  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 11 . 
         FIG. 13  is a block diagram of the interface switching device according to the present exemplary embodiment in which the 0-system interface section serves as a SDH/SONET interface and the 1-system interface section serves as an Ethernet interface. 
         FIG. 14  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 13 . 
         FIG. 15  is a diagram showing a sequence of APS information communication in a redundant system in which nodes, each having the interface switching device according to the present exemplary embodiment, are link-connected. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An interface switching device according to the present invention makes it possible to control the switching between different types of network interfaces through switching control for a single type of network interface, by using a function of transforming APS information between the different types of network interfaces. The present invention particularly provides a method for integrationally processing the switching between a digital synchronous network interface and an Ethernet interface, thereby achieving the simplification and lower price of a device. Moreover, the present invention makes it possible to build a redundant architecture in which different types of interface cards are used as a set, extending the flexibility of the network architecture, also achieving enhanced maintainability. 
     Hereinafter, an exemplary embodiment of the present invention will be described in detail by showing a digital synchronous network interface (here, SDH/SONET interface) and an Ethernet interface as an example of the different types of interfaces. 
       FIG. 1  is a schematic block diagram of an interface switching device according to an exemplary embodiment of the present invention. Any combination of SDH/SONET and Ethernet can apply to a network interface section  10 . 0  for a 0-system and a network interface section  10 . 1  for a 1-system, which will be described later. The network interface sections  10 . 0  and  10 . 1  are switched by an interface switch section  20 . Normally, the 0-system network interface section  10 . 0  is selected for working. Accordingly, of a main signal for the 0-system received by the network interface section  10 . 0  and a main signal from the 1-system received by the network interface section  10 . 1 , the main signal received by the network interface section  10 . 0  is selected and transferred into the device. A main signal to be transmitted from the device is branched at the interface switch section  20  into two signals, which are then sent out through the network interface sections  10 . 0  and  10 . 1  respectively. The details thereof will be described later. 
     Switching control of the interface switch section  20  is performed by an intra-device control section  50 . A control interface section  501  of the intra-device control section  50 , which is connected to the network interface section  10 . 0  and/or network interface section  10 . 1  through an intra-device control bus  40 , receives APS information, failure monitoring result information and the like and transmits control information, which will be described later. The intra-device control section  50  is further provided with an APS information transformation section  502  and a switching control section  503 . 
     The APS information transformation section  502  is provided with a table for transformation between Ethernet APS information (ETH_APS) and SDH/SONET APS information (K 1 , K 2 ), which will be described later. The switching control section  503  is composed of a finite state machine that performs switching control on a network interface of one of the types. 
     In the present exemplary embodiment, the switching control section  503  performs switching control on a SDH/SONET interface. Accordingly, when Ethernet APS information (ETH_APS) is input from the control bus  40 , the control interface section  501  transfers the information to the APS information transformation section  502 , where the Ethernet APS information is transformed into SDH/SONET APS information (K 1 , K 2 ), which is then output to the switching control section  503 . Additionally, when SDH/SONET APS information (K 1 , K 2 ) is input from the control bus  40 , the control interface section  501  transfers the SDH/SONET APS information directly to the switching control section  503 , without transferring the information to the APS information transformation section  502 . 
     Reversely, if the destination of APS information (K 1 , K 2 ) output from the switching control section  503  is an Ethernet interface, the control interface section  501  transfers the information to the APS information transformation section  502 , where the information is transformed into Ethernet APS information (ETH_APS), which is then transmitted to the destination Ethernet interface through the control bus  40 . When the destination of APS information (K 1 , K 2 ) output from the switching control section  503  is a SDH/SONET interface, the control interface section  501  transmits the information directly to the destination SDH/SONET interface through the control bus  40 , without transferring the information to the APS information transformation section  502 . 
     Moreover, the control interface section  501  transmits control information output from the switching control section  503  to the interface switch section  20  through the control bus  40 . When the received control information instructs to switch interfaces, the control interface section  501  carries out switching between the network interface sections  10 . 0  and  10 . 1 . 
     Hereinafter, a specific configuration and operations of the present exemplary embodiment will be described in more detail, by taking a 1+1 bidirectional system as an example. 
     1. Device Circuit Structure 
       FIG. 2  is a more detailed block diagram of the interface switching device shown in  FIG. 1 . Note that, in  FIGS. 1 and 2 , those blocks having the same functions are denoted by the same reference numerals. As described above, here, the 0-system is selected for working, and the 1-system is for protection. Since bidirectional switching is performed in APS switching, a link-connected opposite device on the other end of the link also similarly selects the 0-system as a working system. 
     The network interface sections  10 . 0  and  10 . 1  have similar configurations and include APS information termination/insertion (T/I) sections  101 . 0  and  101 . 1 , line/intra-device failure monitor sections  102 . 0  and  102 . 1 , and intra-device control interface sections  103 . 0  and  103 . 1 , respectively. In  FIG. 2 , “termination/insertion” and “information” are abbreviated to “T/I” and “INFO”, respectively. 
     The APS information termination/insertion section  101 . 0  terminates and inputs APS information from a 0-system link reception signal, and also performs processing for inserting APS information into a 0-system link transmission signal. The line/intra-device failure monitor section  102 . 0  has a function of monitoring 0-system link reception and transmission signals to detect an abnormality in a reception signal and a failure inside the interface. The control interface section  103 . 0  transmits/receives information to/from the intra-device control section  50  through the intra-device control bus  40 . Incidentally, it is defined by recommendations that APS information extracted by the APS information termination/insertion section  101 . 0  is notified to the intra-device control section  50  when the network interface section  10 . 0  is for protection (see ITU-T Standard G.8031/Y.1342, pp. 15-22, and ITU-T Standard G.841, pp. 28-32). 
     The APS information termination/insertion section  101 . 1 , line/intra-device failure monitor section  102 . 1 , and intra-device control interface section  103 . 1  of the network interface section  10 . 1  also perform similar operations, and therefore description thereof will be omitted. 
     The interface switch section  20  includes a selector switch  201  and an intra-device control interface section  202 . When the interface switch section  20  has received a control signal from the intra-device control section  50  through the intra-device control interface section  202 , the selector switch  201  selects a 0-system or 1-system reception signal in accordance with control information in the control signal. A transmission signal is branched into two signals, which are output to the 0-system and 1-system network interface sections  10 . 0  and  10 . 1  respectively. 
     The APS information transformation section  502  of the intra-device control section  50  is provided with a table storing transformation rules for transformation between the Ethernet APS information (ETH_APS) and SDH/SONET APS information (K 1 , K 2 ). The switching control section  503  is a finite state machine that performs switching control, here, on a SDH/SONET network interface. The switching control section  503  receives, as event inputs, APS information extracted by the APS information termination/insertion section of the protection-system network interface section, and results of monitoring from the line/intra-device failure monitor sections  102 . 0  and  102 . 1 . As status outputs responding to the inputs, the switching control section  503  outputs APS information and switching control information. 
     Note that although the intra-device control section  50  can be implemented as hardware, it can also be implemented as software by executing an interface switching control program on a program-controlled processor such as a CPU. 
     2. APS Information Transformation 
       FIG. 3  is a format diagram showing an example of correspondences between the Ethernet APS information and SDH/SONET APS information, to describe the APS information transformation rules used by the APS information transformation section  502 . Here, as an example, shown are correspondences between Ethernet APS information (shown in Table 11-1, ITU-T Standard G.8031/Y.1342, pp. 17-18) and K 1 /K 2  APS information (shown in Table 7-1, ITU-T Standard G.841, pp. 37-38). Specifically, “Request/State,” “Requested Signal,” and “Bridged Signal” of the Ethernet APS information correspond to “Type of Request,” “Requesting Signal,” and “Bridged Signal” of the SDH/SONET K 1 , K 2  bytes, respectively. 
       FIG. 4  is a diagram of correspondences showing an example of the APS information transformation rules for transformation between the SDH/SONET APS information and Ethernet APS information. Here, shown are correspondences between the “Request/State” of the Ethernet APS information and “Types of Request” of the SDH/SONET K 1  byte. They basically correspond to each other but are slightly different in bit assignment. For example, “Forced Switch (FS)” of the Ethernet APS information is represented by “1101,” but “Forced switch” of the SDH/SONET K 1  byte is represented by “1110.” Accordingly, similar functions between the Ethernet APS information and SDH/SONET APS information are associated with each other in advance and stored as transformation rules in the APS information transformation section  502 . An example of the transformation rules will be presented next. 
       FIGS. 5 and 6  are diagrams showing an example of the transformation rules between Ethernet APS information and SDH/SONET APS information. Here, a 1+1 bidirectional non-revertive mode is shown as an example. Note that since “Reverse Request (RR)” has not defined yet in Ethernet APS information, “Reverse request (RR)” of the SDH/SONET K 1  byte, which is “0010,” is transformed to “No Request (NR)” with “Requested Signal” in Ethernet APS information. 
     By using such transformation rules in the APS information transformation section  502 , it is possible to perform integrational interface switching control as described hereinafter. 
     3. SDH/SONET+SDH/SONET 
       FIG. 7  is a block diagram of the interface switching device according to the present exemplary embodiment on which SDH/SONET interfaces are mounted for both of the 0- and 1-systems.  FIG. 8  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 7 . 
     Referring to  FIG. 7 , since the selector switch  201  selects the 0-system as a working system, K 1  and K 2  bytes, which are SDH/SONET APS information, are extracted by the 1-system interface section  10 . 1  (Step S 601 ) and transmitted to the intra-device control section  50  along with a result of monitoring by the line/intra-device failure monitor section  102 . 1  (Step S 602 ). The line/intra-device failure monitor section  102 . 0  of the working-system interface section  10 . 0  transmits its own monitoring result to the intra-device control section  50  (Step S 603 ). 
     The control interface section  501  of the intra-device control section  50  transfers the result of line/intra-device monitoring received from the SDH/SONET interface section  10 . 0 , and the APS information (K 1  and K 2  bytes) and result of line/intra-device monitoring received from the SDH/SONET interface section  10 . 1 , as they are, to the switching control section  503  as event information (Step S 604 ). This is because, since the switching control section  503  has been configured for SDH/SONET in the present exemplary embodiment, there is no need to transform the APS information. 
     When the control interface section  501  receives APS information (K 1  and K 2  bytes) and control information from the switching control section  503  (Step S 605 ), the control interface section  501  transmits the K 1  and K 2  bytes, as they are, to the SDH/SONET interface section  10 . 1  (Step S 606 ) and transmits the control information to the interface switch section  20  (Step S 607 ). The APS information termination/insertion section  101 . 1  of the SDH/SONET interface section  10 . 1  inserts the K 1  and K 2  bytes received from the intra-device control section  50  into a transmission main signal. Moreover, if the received control information is switching control information generated due to a degradation of the reception signal on the working 0-system, a failure inside the 0-system device or the like, the interface switch section  20 , in accordance with the switching control information, allows the selector switch  20  to switch from the 0-system to the 1-system, thereby selecting the 1-system as a working system. In this case, as described already, since the K 1  and K 2  bytes inserted into the transmission main signal also instructs to switch, similar switching of working system from the 0-system to the 1-system is also performed on the opposite device side. 
     4. Ethernet+Ethernet 
       FIG. 9  is a block diagram of the interface switching device according to the present exemplary embodiment on which Ethernet interfaces are mounted for both of the 0- and 1-systems.  FIG. 10  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 9 . 
     Referring to  FIG. 9 , since the selector switch  201  selects the 0-system as a working system, Ethernet APS information ETH_APS is extracted by the 1-system interface section  10 . 1  (Step S 701 ) and transmitted to the intra-device control section  50  along with a result of monitoring by the line/intra-device failure monitor section  102 . 1  (Step S 702 ). The line/intra-device failure monitoring section  102 . 0  of the working-system interface section  10 . 0  transmits its own monitoring result to the intra-device control section  50  (Step S 703 ). 
     The control interface section  501  of the intra-device control section  50  transfers the APS information ETH_APS received from the Ethernet interface section  10 . 1  first to the APS information transformation section  502  (Step S 704 ). The APS information transformation section  502  transforms the Ethernet APS information ETH_APS into SDH/SONET APS information, K 1  and K 2  bytes, in accordance with the transformation rules shown in  FIGS. 6 and 7  and returns the obtained APS information (K 1  and K 2  bytes) to the control interface section  501  (Step S 705 ). The control interface section  501  transfers the result of line/intra-device monitoring received from the Ethernet interface section  10 . 1 , the APS information (K 1  and K 2  bytes) input from the APS information transformation section  502 , and the result of line/intra-device monitoring received from the Ethernet interface section  10 . 0  to the switching control section  503 , as event information (Step S 706 ). 
     When the control interface section  501  receives APS information (K 1  and K 2  bytes) and control information from the switching control section  503  (Step S 707 ), the control interface section  501  transfers the K 1  and K 2  bytes to the APS information transformation section  502  (Step S 708 ). The APS information transformation section  502  transforms the received APS information (K 1  and K 2  bytes) into Ethernet APS information ETH_APS in accordance with the transformation rules shown in  FIGS. 6 and 7  and returns the obtained APS information ETH_APS to the control interface section  501  (Step S 709 ). 
     The control interface section  501  transmits the APS information ETH_APS to the Ethernet interface section  10 . 1  (Step S 710 ) and transmits the control information to the interface switch section  20  (Step S 711 ). The APS information termination/insertion section  101 . 1  of the Ethernet interface section  10 . 1  inserts the APS information ETH_APS received from the intra-device control section  50  into a transmission main signal. Additionally, if the received control information is switching control information instructing to switch because of a degradation of the reception signal on the working 0-system, a failure inside the 0-system device or the like, the interface switch section  20 , based on this switching control information, allows the selector switch  201  to switch from the 0-system to the 1-system, thereby selecting the 1-system as a working system. In this case, as described already, since the APS information ETH_APS inserted into the transmission main signal also instructs to switch, similar switching of working system from the 0-system to the 1-system is also performed on the opposite device side. 
     5. SDH/SONET (Working)+Ethernet 
       FIG. 11  is a block diagram of the interface switching device according to the present exemplary embodiment in which the 0-system interface section functions as a SDH/SONET interface and the 1-system interface section functions as an Ethernet interface.  FIG. 12  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 11 . 
     Referring to  FIG. 11 , since the selector switch  201  selects the 0-system as a working system, APS information is obtained at the 1-system Ethernet interface section  10 . 1 . That is, Ethernet APS information ETH_APS is extracted by the Ethernet interface section  10 . 1  (Step S 801 ) and transmitted to the intra-device control section  50  along with a result of monitoring by the line/intra-device failure monitor section  102 . 1  (Step S 802 ). The line/intra-device failure monitor section  102 . 0  of the working-system SDH/SONET interface section  10 . 0  transmits its own monitoring result to the intra-device control section  50  (Step S 803 ). 
     The control interface section  501  of the intra-device control section  50  transfers the APS information ETH_APS received from the Ethernet interface section  10 . 1  first to the APS information transformation section  502  (Step S 804 ). The APS information transformation section  502  transforms the Ethernet APS information ETH_APS into SDH/SONET APS information, K 1  and K 2  bytes, in accordance with the transformation rules shown in  FIGS. 6 and 7  and returns the obtained APS information (K 1  and K 2  bytes) to the control interface section  501  (Step S 805 ). The control interface section  501  transfers the result of line/intra-device monitoring received from the Ethernet interface section  10 . 1 , the APS information (K 1  and K 2  bytes) input from the APS information transformation section  502 , and the result of line/intra-device monitoring received from the SDH/SONET interface section  10 . 0  to the switching control section  503 , as event information (Step S 806 ). 
     When the control interface section  501  receives APS information (K 1  and K 2  bytes) and control information from the switching control section  503  (Step S 807 ), the control interface section  501  transfers the K 1  and K 2  bytes to the APS information transformation section  502  (Step S 808 ). The APS information transformation section  502  transforms the received APS information (K 1  and K 2  bytes) into Ethernet APS information ETH_APS in accordance with the transformation rules shown in  FIGS. 6 and 7  and returns the obtained APS information ETH_APS to the control interface section  501  (Step S 809 ). 
     The control interface section  501  transmits the APS information ETH_APS to the Ethernet interface section  10 . 1  (Step S 810 ) and transmits the control information to the interface switch section  20  (Step S 811 ). The APS information termination/insertion section  101 . 1  of the Ethernet interface section  10 . 1  inserts the APS information ETH_APS received from the intra-device control section  50  into a transmission main signal. Additionally, if the received control information is switching control information instructing to switch because of a degradation of the reception signal on the working 0-system, a failure inside the 0-system device or the like, the interface switch section  20 , based on this switching control information, allows the selector switch  201  to switch from the 0-system to the 1-system, thereby selecting the 1-system as a working system. In this case, as described already, since the APS information ETH_APS inserted into the transmission main signal also instructs to switch, similar switching of working system from the 0-system to the 1-system is also performed on the opposite device side. 
     6. SDH/SONET+Ethernet (Working) 
       FIG. 13  is a block diagram of the interface switching device according to the present exemplary embodiment in which the 0-system interface section functions as a SDH/SONET interface and the 1-system interface section functions as an Ethernet interface.  FIG. 14  is a sequence diagram for describing the operations of the interface switching device shown in  FIG. 13 . 
     Referring to  FIG. 13 , since the selector switch  201  selects the 1-system as a working system, APS information is obtained at the 0-system SDH/SONET interface section  10 . 0 . That is, K 1  and K 2  bytes, which are SDH/SONET APS information, are extracted by the 0-system interface section  10 . 0  (Step S 901 ) and transmitted to the intra-device control section  50  along with a result of monitoring by the line/intra-device failure monitor section  102 . 0  (Step S 902 ). The line/intra-device failure monitor section  102 . 1  of the working-system Ethernet interface section  10 . 1  transmits its own monitoring result to the intra-device control section  50  (Step S 903 ). 
     The control interface section  501  of the intra-device control section  50  transfers the APS information (K 1  and K 2  bytes) and the result of line/intra-device monitoring received from the SDH/SONET interface section  10 . 0  and the result of line/intra-device monitoring received from the Ethernet interface section  10 . 1 , as they are, to the switching control section  503 , as event information (Step S 904 ). This is because, since the switching control section  503  has been configured for SDH/SONET in the present exemplary embodiment, there is no need to transform the APS information. 
     When the control interface section  501  receives APS information (K 1  and K 2  bytes) and control information from the switching control section  503  (Step S 905 ), the control interface section  501  transmits the K 1  and K 2  bytes, as they are, to the SDH/SONET interface section  10 . 0  (Step S 906 ) and transmits the control information to the interface switch section  20  (Step S 907 ). The APS information termination/insertion section  101 . 0  of the SDH/SONET interface section  10 . 0  inserts the K 1  and K 2  bytes received from the intra-device control section  50  into a transmission main signal. Additionally, if the received control information is switching control information instructing to switch because of a degradation of the reception signal on the working 1-system, a failure inside the 1-system device or the like, the interface switch section  20 , based on this switching control information, allows the selector switch  201  to switch from the 1-system to the 0-system, thereby selecting the 0-system as a working system. In this case, as described already, since the K 1  and K 2  bytes inserted into the transmission main signal also instructs to switch, similar switching of working system from the 1-system to the 0-system is also performed on the opposite device side. 
     7. System Example 
       FIG. 15  is a sequence diagram showing APS information communication in a redundant system in which nodes are link-connected, each node having the interface switching device according to the present exemplary embodiment. Here, it is assumed that each of the west and east nodes is provided with the interface switching device shown in  FIG. 9  on which Ethernet interfaces are mounted for both of the 0- and 1-systems. 
     Assuming that the 0-system is selected as a working system, Ethernet APS information is extracted by the 1-system interface section  10 . 1  and transmitted to the intra-device control section  50 , where, by referring to the APS information transformation section  502 , the Ethernet APS information is transformed to SDH/SONET APS information, K 1  and K 2  bytes, which are then transferred to the switching control section  503 . For example, Ethernet APS information including “No Request (NR) (r/b=null)” is transmitted from each of the west and east nodes to the other and, on each side, transformed to SDH/SONET APS information including “No request,” which is then transferred to the switching control section  503 . 
     When it is detected that some failure has occurred in the working 0-system from the west node to the east node, the switching control section  503  in the east node, in response to this detection, outputs to the control interface section  501  K 1  and K 2  bytes including as the type of request, for example, “1101,” which represents “Signal fail high priority” in SDH/SONET. The SDH/SONET K 1  and K 2  bytes including “1101” for “Signal fail high priority” are transformed by the APS information transformation section  502  to Ethernet APS information ETH_APS including “1011,” which represents “Signal Fail for Working (SF)” in Ethernet. The Ethernet APS information ETH_APS is then inserted into a transmission main signal by the APS information termination/insertion section  101 . 1  of the Ethernet interface section  10 . 1 . Thus, the Ethernet APS information ETH_APS including “1011” for “Signal Fail for Working (SF)” is transmitted to the west node through the 1-system link. 
     In the west node, the Ethernet APS information ETH_APS including “1011” for “Signal Fail for Working (SF)” is extracted from the 1-system reception main signal by the APS information termination/insertion section  101 . 1  of the interface section  10 . 1  and transmitted to the intra-device control section  50 . The control interface section  501  of the intra-device control section  50  allows the APS information transformation section  502  to transform the Ethernet APS information ETH_APS including “1011” for “Signal Fail for Working (SF)” to K 1  and K 2  bytes including “1101” for “Signal fail high priority” and outputs the obtained K 1  and K 2  bytes to the switching control section  503  (see  FIG. 5 ). 
     Since the working system is in a failed state as indicated by “Signal fail high priority,” the switching control section  503  in the west node determines to switch interfaces. The switching control section  503  transmits switching control information to the interface switch section  20 , whereby the selector switch  201  switches working system from the 0-system to the 1-system. The switching control section  503 , in parallel with this switching control, outputs to the control interface section  501  K 1  and K 2  bytes including “0010,” which represents “Reverse request,” to respond to the east node with an acknowledgement of link switching. The control interface section  501  allows the APS information transformation section  502  to transform the K 1  and K 2  bytes including “0010” for “Reverse request” to Ethernet APS information ETH_APS including “0000,” which represents “No Request (NR) (r/b=normal traffic)” (see  FIG. 6 ). The Ethernet APS information ETH_APS is then inserted into a transmission main signal by the APS information termination/insertion section  101 . 1  of the Ethernet interface section  10 . 1  and thereby transmitted to the east node through the 1-system link. 
     In the east node, the APS information termination/insertion section  101 . 1  of the Ethernet interface section  10 . 1  extracts from the reception main signal the Ethernet APS information ETH_APS including “0000” for “No Request (NR) (r/b=normal traffic),” which is then transformed by the APS information transformation section  502  of the intra-device control section  50  to K 1  and K 2  bytes including “0010” for “Reserve request,” which are then output to the switching control section  503 . In this manner, upon the occurrence of a signal fail in the 0-system link, the switching of working system from the 0-system to the 1-system is performed in both of the east and west nodes, whereby communication is continued through the 1-system link without interruption. 
     When the 0-system link from the west node to the east node is recovered and the line/intra-device failure monitor section  102 . 0  of the interface section  10 . 0  in the east node detects the recovery, then, in response to this detection, the switching control section  503  in the east node is assumed to output K 1  and K 2  bytes including “Do not revert,” for example. These K 1  and K 2  bytes including “Do not revert” are transformed by the APS information transformation section  502  to Ethernet APS information ETH_APS including “Do Not Revert (DNR),” which is then inserted into a transmission main signal by the APS information termination/insertion section  101 . 0  of the Ethernet interface section  10 . 0 . Thus, the Ethernet APS information ETH_APS including “Do Not Revert (DNR)” is transmitted to the west node through the 0-system link. 
     In the west node, the Ethernet APS information ETH_APS including “Do Not Revert (DNR)” is extracted from the 0-system reception main signal by the APS information termination/insertion section  101 . 0  of the interface section  10 . 0  and transmitted to the intra-device control section  50 . The control interface section  501  of the intra-device control section  50  allows the APS information transformation section  502  to transform the Ethernet APS information ETH_APS including “Do Not Revert (DNR)” to K 1  and K 2  bytes including “Do not revert,” which are then output to the switching control section  503  (see  FIG. 6 ). 
     Based on the K 1  and K 2  bytes including “Do not revert,” the switching control section  503  in the west node determines not to perform interface switching from the working 1-system and performs no switching control on the interface switch section  20 . Accordingly, the selector switch  201  remains selecting the 1-system as a working system. In parallel with this control, the switching control section  503  outputs to the control interface section  501  K 1  and K 2  bytes including “Reverse request” to respond to the east node. The control interface section  501  allows the APS information transformation section  502  to transform the K 1  and K 2  bytes including “Reverse request” into Ethernet APS information ETH_APS including “No Request (NR) (r/b=normal traffic)” (see  FIG. 6 ), which is then inserted into a transmission main signal by the APS information termination/insertion section  101 . 0  of the Ethernet interface section  10 . 0  and thereby transmitted to the east node through the 0-system link. 
     In the east node, the APS information termination/insertion section  101 . 0  of the Ethernet interface section  10 . 0  extracts from the reception main signal the Ethernet APS information ETH_APS including “No Request (NR) (r/b=normal traffic),” which is then transformed by the APS information transformation section  502  of the intra-device control section  50  into K 1  and K 2  bytes including “Reverse request,” which are then output to the switching control section  503 . In this manner, after the 0-system link has been recovered, the 1-system is remained selected as a working system in both of the west and east nodes, and communication is continued. 
     As described above, according to the present exemplary embodiment, the single switching control section  503  of the intra-device control section  50  can be used for SDH/SONET and for Ethernet. Accordingly, it is possible to simplify the device configuration and thus to economize the device. Since a redundant system can be constructed by arbitrarily combining SDH/SONET and Ethernet interfaces, it is possible to build a more flexible network architecture. 
     In addition, the types of interfaces can be changed at an instantaneous-decision level by utilizing redundant switching. Specifically, switching is first performed on a protection-system interface section, and after this switching is complete, switching is performed on the other interface section. 
     8. Other Exemplary Embodiments 
     In the above-described exemplary embodiment, description has been given of the case where the switching control section  503  of the intra-device control section  50  is configured for SDH/SONET. However, similar effects can also be obtained in a case where, with the switching control section  503  configured for Ethernet, the directions of transformation by the APS information transformation section  502  in the above-described exemplary embodiment are reversed. For example, in the case of SDH/SONET+SDH/SONET, it is necessary to perform ASP information transformation from ASP information for SDH/SONET to APS information for Ethernet, by using the ASP information transformation section  502 . In the case of Ethernet+Ethernet, transactions can be performed directly with the switching control section  503 , without using the APS information transformation section  502 . Moreover, in the case of SDH/SONET (working)+Ethernet, transactions can be performed directly with the switching control section  503 , without using the APS information transformation section  502 , but in the case of SDH/SONET+Ethernet (working), transformation is needed, using the APS information transformation section  502 . 
     Additionally, in the above-described exemplary embodiment, shown is the exemplary case where two types of network interfaces, SDH/SONET and Ethernet, are accommodated. However, similar effects can be obtained by applying the present invention to any communications system as long as the communications system is a redundant system based on a protocol using APS information and accommodating three or more different types of network interfaces. Similarly, the present invention is not limited to 1+1 bidirectional systems but can also be applied to 1:1 or 1:n architectures. 
     The present invention is applicable to redundant communications systems in general and particularly to redundant systems accommodating different types of network interfaces. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The above-described exemplary embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
In a redundant system where multiple network interfaces can be switched, an interface switching device includes: a transformer for transforming automatic switch information between a predetermined type used for a predetermined network interface and each of other types used for network interfaces other than the predetermined network interface; a switch controller for performing switch control for automatic switch information of the predetermined type; and a control interface for connecting a first network interface to the switch controller via the transformer when first automatic switch information received from the first network interface is not of the predetermined type.