Abstract:
A line accommodating device for accommodating lines of a first communication network and a second communication network, the line accommodating device includes an interface connecting to the first communication network; a connecting block for connecting to the interface; and a plurality of relay modules connecting to the connecting block and the second communication network, respectively, each of the relay module including a reporting unit for notifying the connected relay modules via the connecting block of identification information of the relay module&#39;s own, an obtaining unit for obtaining identification information of the connected relay modules via the connecting block, and a controller including processes of determining at least one connection target of the relay module in the properly obtained identification information of the relay module by the obtaining unit, and controlling for switching the connection target of the relay module in accordance with a determined result of the determining.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2008-090933, filed on Mar. 31, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a line accommodating device in communication network. 
     BACKGROUND 
     A resilient packet ring (RPR) is a scheme, defined in IEEE 802.17, for providing relief against a network failure. The RPR is a system in which multiple RPR relay devices are connected by two ring-type transmission paths for clockwise and anticlockwise transmissions and the two transmission paths are switched so as to bypass a transmission path and/or a station where a failure occurred, to thereby provide relief against a network failure. For example, refer to Japanese Laid-open Patent Publication No. 2006-279891 and International Publication Pamphlet No. WO 2005/015851. 
     As the communications media for the RPR, for example, SONET/SDH transmission paths are used. In this case, through the use of Generic Framing Procedure (GFP) and Virtual Concatenate (VCAT), an RPR frame is mapped on a SONET frame for transmission on the SONET/SDH transmission paths. 
     Also, in general, a line accommodating device that can provide large-capacity line services employs a shelf structure. An RPR relay device is realized in the form of a card, such as a station, and can be freely attached to and detached from the shelf of the line accommodating device. Thus, when the number of lines is increased, multiple stations may be accommodated in one shelf or stations accommodated in one shelf may belong to multiple RPR networks. 
     With such a configuration, when a failure occurs in one of the stations, which are RPR relay devices, an RPR failure recovery function works but a failure recovery function for yet another failure is lost. As opposed to it, a technology is disclosed in which a management unit detects a failure in each station and a connection path of stations where no failure is occurring is formed so as to bypass the station where the failure occurred. For example, refer to Japanese Laid-open Patent Publication No. 2006-279891. 
     However, in the related technologies noted above, when a failure occurs in a station during occurrence of a failure in the management unit, there is a problem in that switching to a connection path that bypasses the station where the failure occurred cannot be performed. Thus, when the network does not have a failure recovery function, such as an RPR, there is a problem in that the portion of the network which is in an interrupted state due to the station failure cannot be recovered from the interrupted state. 
     Also, when the network is an RPR network and a station where a failure occurred remains on the network, the RPR-based failure recovery function works, but the redundancy function is lost. Thus, there is a problem in that relief cannot be provided during occurrence of another failure. 
     Also, the management unit also performs overall line accommodating device control, such as control of each station for switching between the signal transmission paths on the network. Thus, when the management unit performs processing for detecting a failure through constant monitoring of the state of each station and/or processing such as re-formation of the connection path of the stations when a failure is detected, there is a problem in that a load on the management unit increases. As a result, it is conceivable that the functions of the management unit which include detection of a failure in each station and re-formation of a connection path cannot be fully exercised. 
     SUMMARY 
     According to an aspect of the invention, a line accommodating device for accommodating lines of a first communication network and a second communication network, the line accommodating device includes an interface connecting to the first communication network; a connecting block for connecting to the interface; and a plurality of relay modules connecting to the connecting block and the second communication network, respectively, each of the relay module including a reporting unit for notifying the connected relay modules via the connecting block of identification information of the relay module&#39;s own, an obtaining unit for obtaining identification information of the connected relay modules via the connecting block, and a controller including processes of determining at least one connection target of the relay module in the properly obtained identification information of the relay module by the obtaining unit, and controlling for switching the connection target of the relay module in accordance with a determined result of the determining. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram part  1  illustrating an overview of this line accommodating device. 
         FIG. 2  is a diagram part  2  illustrating an overview of this line accommodating device. 
         FIG. 3  is a diagram part  3  illustrating an overview of this line accommodating device. 
         FIG. 4  is a block diagram illustrating the configuration of a network according to a first embodiment. 
         FIG. 5  is a block diagram illustrating the configuration of a line accommodating device according to the first embodiment. 
         FIG. 6  is a block diagram illustrating a specific example of a station illustrated in  FIG. 5 . 
         FIG. 7  is a flowchart illustrating one example of the operation of the station illustrated in  FIG. 5 . 
         FIG. 8  is a flowchart illustrating a specific example of step S 705  illustrated in  FIG. 7 . 
         FIG. 9  is a flowchart illustrating another example of the operation of the station illustrated in  FIG. 5 . 
         FIG. 10  is a drawing part  1  illustrating a table created by a control unit. 
         FIG. 11  is a drawing part  2  illustrating a table created by the control unit. 
         FIG. 12  is a diagram part  1  illustrating a switching operation of each station when a failure occurs. 
         FIG. 13  is a drawing part  3  illustrating a table created by the control unit. 
         FIG. 14  is a diagram part  2  illustrating a switching operation of each station when a failure occurs. 
         FIG. 15  is a block diagram illustrating the configuration of a line accommodating device according to a second embodiment. 
         FIG. 16  is a block diagram illustrating a specific example of a station illustrated in  FIG. 15 . 
         FIG. 17  is a diagram part  4  illustrating an overview of this line accommodating device. 
         FIG. 18  is a diagram part  5  illustrating an overview of this line accommodating device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Preferred embodiments of a line accommodating device and a control method will be described below in detail with reference to the accompanying drawings. 
     Overview of Line Accommodating Device 
       FIG. 1  is a diagram (part  1 ) illustrating an overview of this line accommodating device. A network  100  illustrated in  FIG. 1  is a ring-type network in which nodes #A to #D are connected in a ring shape. The node #A is provided with a line accommodating device  110 . The line accommodating device  110  accommodates relay modules  111  to  113  (Relay MODs). The relay modules  111  to  113  are connected in series so as to form a portion  101  of the network  100 . 
     Also, the individual relay modules  111  to  113  are connected to a network  120  that is different from the network  100 , as indicated by up-down solid-line arrows. The network  120  to which the relay modules  111  to  113  are connected may be a network that is different for each of the relay modules  111  to  113  or may be the identical network. 
     A portion surrounded by a dotted line schematically indicates the relationships of connections of the relay modules  111  to  113  in the line accommodating device  110 . A first interface  115  is provided at an end of the line accommodating device  110 , the end being adjacent to the node #B, and performs communication with the node #B. A second interface  116  is provided at an end of the line accommodating device  110 , the end being adjacent to the node #D, and performs communication with the node #D. 
     The relay modules  111  to  113  are connected in series between the first interface  115  and the second interface  116 . That is, each of the relay modules  111  to  113  has at least two connection targets (hereinafter referred to an “east-side connection target” and a “west-side connection target”) that are each connected to the first interface  115 , the second interface  116 , or another relay module. 
     The relay module  111  is connected to the first interface  115  and the relay module  112 . The relay module  112  is connected to the relay module  111  and the relay module  113 . The relay module  113  is connected to the relay module  112  and the second interface  116 . 
     In this manner, the relay modules  111  to  113  are connected in series between the first interface  115  and the second interface  116  to thereby form, in the network  100 , the portion  101  of a path that heads counterclockwise from the node #B to the node #D (or a path that heads clockwise from the node #D to the node #B). 
     Each of the relay modules  111  to  113  repeatedly reports identification information (dotted-line arrows) for identifying the self module to, of the relay modules  111  to  113 , the relay modules that are different from the self module. Also, each of the relay modules  111  to  113  obtains the identification information repeatedly reported from the other relay modules. The repeated reporting means periodical or irregular, continuous reporting (e.g., reporting at a period of several milliseconds). 
     For example, the relay module  111  repeatedly reports identification information # 1  for identifying the relay module  111  to the relay module  112  and the relay module  113 . Also, the relay module  111  obtains identification information # 2  for identifying the relay module  112 , the identification information # 2  being repeatedly reported from the relay module  112 , and identification information # 3  for identifying the relay module  113 , the identification information # 3  being repeatedly reported from the relay module  113 . 
     The reporting of the identification information is performed using a control-signal line (not illustrated) that interconnects the relay modules  111  to  113 . The control-signal line is always reserved regardless of the state of a main-signal line for forming the portion  101  of the network  100 . The control-signal line may be provided separately from the main-signal line or may be provided utilizing part of the main-signal line. 
       FIG. 2  is a diagram (part  2 ) illustrating an overview of this line accommodating device. In  FIG. 2 , the same portions as those illustrated in  FIG. 1  are denoted by the same reference numerals, and descriptions thereof are omitted. A case in which a failure occurs in the relay module  112 , as illustrated in  FIG. 2 , is described. When a failure occurs in the relay module  112 , an interrupted state occurs in the portion  101  of the network  100 , the portion  101  being formed by the relay modules  111  to  113 , and thus the network  100  fails. 
     Meanwhile, when a failure occurs in the relay module  112 , the relay module  112  may not report the identification information # 2 , or may not report it in a normal state even if possible. Thus, the relay module  111  and the relay module  113  may not properly detect the identification information # 2 . Therefore, the relay module  111  and the relay module  113  can detect the occurrence of the failure in the relay module  112 . 
     In contrast, the relay module  111  and the relay module  113  switch the connection targets to recover the portion  101  of the network  100  from the interrupted state. Specifically, the relay module  111  and the relay module  113  determine the connection-target relay module(s) of the relay module(s) indicated by identification information that has been properly obtained at the time. 
     When multiple pieces of identification information which have been properly obtained exist, the relay modules indicated by identification information having the next smaller value than the value of the identification information of the self module and identification information having the next larger value than the identification information of the self module, of the multiple pieces of identification information, are determined as the connection-target relay modules. The value of the identification information is a value that is determined by the identification information and that differs for each piece of identification information. 
     For example, when the identification information is information indicating a numeric value or a number, the value of the identification information is the numeric value or number itself. Also, when the identification information is not information indicating a numeric value or a number, the value of the identification information is a value obtained by converting the identification information into a numeric value in accordance with a rule that is common to the relay modules. This rule is a rule that can convert the identification information into a numeric value that is different for each piece of identification information. 
     For example, when the identification information is information indicating the name of the relay module, the value of the identification information is a value obtained by converting the character string of the name into a character code. The use of values different for the respective pieces of the identification information and the interconnection of the relay modules having the values that are the closest to each other, as described above, makes it possible to form a connection path that directly connects all relay modules where no failure is occurring. 
     Also, when each relay module does not properly obtain identification information having a value smaller than the value of the identification information of the self module, the relay module determines, as the connection targets, the first interface  115  and the relay module indicated by identification information having the next larger value than the identification information of the self module. Also, when each relay module does not properly obtain identification information having a value larger than the value of the identification information of the self module, the relay module determines, as the connection targets, the relay module indicated by identification information having the next smaller value than the value of the identification information of the self module and the second interface  116 . 
     In this case, the relay module  111  has not obtained identification information having a value smaller than the identification information # 1  of the self module. Thus, the relay module  111  determines that the west-side connection target is the first interface  115 . Also, the relay module  111  has properly obtained the identification information # 3  reported from the relay module  113 . Thus, the relay module  111  determines that the east-side connection target is the relay module  113 . 
     Also, the relay module  113  has properly obtained the identification information # 1  reported from the relay module  111 . Thus, the relay module  113  determines that the west-side connection target is the relay module  111 . Also, the relay module  113  has not obtained identification information having a value larger than the identification information # 3  of the self module. Thus, the relay module  113  determines that the east-side connection target is the second interface  116 . 
     Also, in this case, since the relay module  111  and the relay module  113  have only one piece of identification information that has been property obtained, the connection targets can be uniquely determined without performing comparison of the values of the identification information. Also, although a case in which a failure occurs in the relay module  112  has been described above, the above-described determination method can also be used to form a connection path even in a case in which a failure occurs in the relay module  111  or the relay module  113 . 
       FIG. 3  is a diagram (part  3 ) illustrating an overview of this line accommodating device. In  FIG. 3 , the same portions as those illustrated in  FIG. 2  are denoted by the same reference numerals, and descriptions thereof are omitted. As illustrated in  FIG. 3 , the relay module  111  switches the east-side connection target from the relay module  112  to the relay module  113 , on the basis of the result of the determination (the determination result) described in  FIG. 2 . 
     The first interface  115  is maintained as the west-side connection target of the relay module  111 . Also, the relay module  113  switches the west-side connection target from the relay module  112  to the relay module  111 , on the basis of the determination described in  FIG. 2 . The second interface  116  is maintained as the east-side connection target of the relay module  113 . 
     In this manner, the relay module  111  and the relay module  113  can automatically switch the connection targets when a failure occurs in the relay module  112 . Also, the connection path formed by the connection-target switching performed by the relay module  111  and the relay module  113  serves as a path that bypasses the relay module  112  where the failure occurred. Thus, it is possible to recover the portion  101  of the network  100  from the interrupted state. 
     The relay modules report the identification information at the same reporting timing. In this case, the relay modules create information illustrating a list of the pieces of identification information simultaneously obtained at the reporting timing. The relay modules then determine that identification information contained in the information illustrating the created list is identification information that has been properly obtained. Also, the relay modules determine that identification information that is not contained in the information illustrating the created list is identification information that has not been properly obtained. 
     Alternatively, the relay modules may report the identification information at timings that are different from one another. In this case, every time each relay module obtains new identification information, it stores the identification information in a table. For each piece of identification information stored in the table, the relay module constantly measures elapsed time from when the identification information was obtained last time. Then, when the elapsed time exceeds a predetermined time, the relay module deletes the identification information from the table. Each relay module determines, as properly obtained identification information, the identification information stored in the table. 
     Although a case in which the relay module  111  and the relay module  113  detect the occurrence of a failure in the relay module  112  has been described above, the arrangement may be such that the operation for detecting the occurrence of the failure is not performed. In this case, the relay modules  111  to  113  determine connection targets each time they obtain the identification information repeatedly reported from the other relay modules. Then, when the determined connection targets are different from the current connection targets, the relay modules  111  to  113  switch the connection targets. 
     Specifically, in the state illustrated in  FIG. 1 , the relay module  111  has not obtained identification information having a value smaller than the identification information # 1  of the self module. Thus, the relay module  111  determines that the west-side connection target is the first interface  115 . Also, the relay module  111  has properly obtained the identification information # 2  and the identification information # 3 . Thus, the relay module  111  determines, as the east-side connection target, the relay module  112  indicated by the identification information # 2  having the next larger value than the identification information # 1  of the self module. 
     Also, the relay module  112  has properly obtained the identification information # 1  and the identification information # 3 . Thus, the relay module  112  determines, as the west-side connection target, the relay module  111  indicated by the identification information # 1  having the next smaller value than the identification information # 2  of the self module. Also, the relay module  112  determines, as the east-side connection target, the relay module  113  indicated by the identification information # 3  having the next larger value than the identification information # 2  of the self module. 
     Also, the relay module  113  has properly obtained the identification information # 1  and the identification information # 2 . Thus, the relay module  113  determines, as the west-side connection target, the relay module  112  indicated by the identification information # 2  having the next smaller value than the identification information # 3  of the self module. Also, since the relay module  113  has not obtained identification information having a value larger than the identification information # 3  of the self module, the relay module  113  determines that the east-side connection target is the second interface  116 . 
     Even in such a case in which each relay module determines connection targets every time it obtains identification information repeatedly reported from the other relay modules, the connection path of the relay modules  111  to  113  is maintained before the occurrence of a failure. Then, when a failure occurs in the relay module  112 , as illustrated in  FIG. 2 , the relay module  111  and the relay module  113  can automatically switch the connection targets, as described in  FIGS. 2 and 3 . 
     FIRST EMBODIMENT 
       FIG. 4  is a block diagram illustrating the configuration of a network according to a first embodiment. As illustrated in  FIG. 4 , a network  400  is a ring-type network in which nodes #A to #D are connected in a ring shape. LANs (local area networks), such as user networks  410 A to  410 D, are connected to the nodes #A to #D, respectively. 
     The network  400  is an optical network for transmitting optical frames according to a standard of a SONET/SDH or the like. Also, the network  400  has a clockwise transmission path  421  and a counterclockwise transmission path  422 . The network  400  is an RPR network standardized by IEEE 802.17. 
     For example, it is assumed that data received from the user network  410 A is relayed by the node #A, the node #B, and the node #C through the use of the transmission path  421  and is transmitted to the user network  410 C, as denoted by reference numeral  431 . A description is given of a case in which, in this case, a failure occurs in the transmission path  421  between the node #A and the node #B, as denoted by reference numeral  423 . 
     The node #A switches the transmission path for transmitting the data, received from the user network  410 A, from the transmission path  421  to the transmission path  422 . Thus, the data transmitted from the node #A is relayed by the node #D and the node #C and is transmitted to the user network  410 C, as denoted by reference numeral  432 . In this manner, switching between the transmission paths so as to bypass a portion where a failure occurred makes it possible to improve the tolerance of the network  400  against a failure. 
       FIG. 5  is a block diagram illustrating the configuration of a line accommodating device according to the first embodiment. In  FIG. 5 , solid lines indicate flows of data and dotted lines indicate flows of control signals (the same applies to subsequent block diagrams). The line accommodating device according to the first embodiment is a line accommodating device provided at, for example, the node #A illustrated in  FIG. 4 . As illustrated in  FIG. 5 , a line accommodating device  500  according to the first embodiment has a first shelf  510  and a second shelf  520 . 
     The first shelf  510  is provided with a monitoring unit  511 , a first high-speed I/F  512 , a second high-speed I/F  513 , and a connection changing switch  514 . The monitoring unit  511  is responsible for overall control of the line accommodating device  500 . For example, the monitoring unit  511  controls communications performed by the first high-speed I/F  512  and the second high-speed I/F  513  with other nodes, performs overall control of RPR processing, and so on. The connection changing switch  514  is a connecting block for connecting the first high-speed I/F  512 , the second high-speed I/F  513  and relay modules. 
     The first high-speed I/F  512  performs communication with the node #B in accordance with control of the monitoring unit  511 . The first high-speed I/F  512  receives an optical frame transmitted from the node #B through the transmission path  422 , converts the received optical frame into an electrical signal frame, and outputs the resulting frame to the connection changing switch  514 . Also, the first high-speed I/F  512  converts a frame, output from the connection changing switch  514 , into an optical frame, and transmits the optical frame to the node #B through the transmission path  421 . 
     The second high-speed I/F  513  performs communication with the node #D in accordance with control of the monitoring unit  511 . The second high-speed I/F  513  receives an optical frame transmitted from the node #D through the transmission path  421 , converts the received optical frame into an electrical signal frame, and outputs the resulting frame to the connection changing switch  514 . Also, the second high-speed I/F  513  converts a frame, output from the connection changing switch  514 , into an optical frame, and transmits the optical frame to the node #D through the transmission path  422 . 
     The second shelf  520  has slots  521  to  526 , which are arranged. Slot numbers Slot 1  to Slot 6  indicating the arrangement sequence of the slots are attached to the slots  521  to  526 , respectively. A station can be freely attached to and detached from each of the slots  521  to  526 . The station is a relay module implemented in the form of a card. 
     Also, the second shelf  520  is provided with a control-signal line  540  for interconnecting the slots  521  to  526 . The stations accommodated in the slots  521  to  526  can report control signals each other through the use of the control-signal line  540 . The control signal is, for example, the slot number of the slot that accommodates the self station. 
     In this case, a station  531  is accommodated in the slot  521 , a station  532  is accommodated in the slot  523 , a station  533  is accommodated in the slot  524 , and a station  534  is accommodated in the slot  526 . No stations are accommodated in the slots  522  and the slot  525 . The stations  531  to  534  have configurations corresponding to the relay modules  111  to  113  illustrated in  FIG. 1 . 
     Each of the stations  531  to  534  is provided with two connection portions that are connected to the connection changing switch  514  and a connection portion connected to the user network  410 A. Hereinafter, of the two connection portions connected to the connection changing switch  514 , the connection portion at the left side in the figure is referred to as a “west-side connection portion” and the connection portion at the right side in the figure is referred to as an “east-side connection portion”. For example, the station  531  is provided with a west-side connection portion  531   a  and an east-side connection portion  531   b  which are connected to the connection changing switch  514 . 
     The connection changing switch  514  provided at the first shelf  510  switches connections between the stations accommodated in the slots  521  to  526 . The connection changing switch  514  also switches connections between the first high-speed I/F  512  and the stations and between the second high-speed I/F  513  and the stations. The connection changing switch  514  performs switching in accordance with control of each station. 
     In this case, the connection changing switch  514  connects the first high-speed I/F  512  and the west-side connection portion  531   a  of the station  531 . The connection changing switch  514  also connects the east-side connection portion  531   b  of the station  531  and the west-side connection portion  532   a  of the station  532 . The connection changing switch  514  also connects the east-side connection portion  532   b  of the station  532  and the west-side connection portion  533   a  of the station  533 . 
     Also, the connection changing switch  514  connects the east-side connection portion  533   b  of the station  533  and the west-side connection portion  534   a  of the station  534 . The connection changing switch  514  also connects the east-side connection portion  534   b  of the station  534  and the second high-speed I/F  513 . In this manner, the connection path in which the stations  531  to  534  are connected in series between the first high-speed I/F  512  and the second high-speed I/F  513  is formed. 
     The stations  531  to  534  connected in series form a portion of the network  400 . This portion of the network  400  is a portion corresponding to the portion  101  of the network  100  illustrated in  FIG. 1 . The user network  410 A to which the stations  531  to  534  are connected may be a network that is different for each of the stations  531  to  534  or may be the identical network. 
       FIG. 6  is a block diagram illustrating a specific example of the station illustrated in  FIG. 5 . In  FIG. 6 , the same configurations as those illustrated in  FIG. 5  are denoted by the same reference numerals, and descriptions thereof are omitted. The station illustrated in  FIG. 6  is the station  531  illustrated in  FIG. 5 . As illustrated in  FIG. 6 , the station  531  has a data transmission/reception unit  610 , a first RPR processing unit  621 , a second RPR processing unit  622 , a first framer  631 , a second framer  632 , a control unit  640 , a reporting unit  650 , an obtaining unit  660 , and a memory  670 . 
     The data transmission/reception unit  610  transmits/receives data to/from the user network  410 A. Specifically, the data transmission/reception unit  610  receives data transmitted from the user network  410 A. The data transmission/reception unit  610  outputs the received data to the first RPR processing unit  621  and the second RPR processing unit  622 . Also, the data transmission/reception unit  610  transmits data, output from the first RPR processing unit  621  or the second RPR processing unit  622 , to the user network  410 A. 
     In accordance with control of the control unit  640 , the first RPR processing unit  621  outputs the data, output from the data transmission/reception unit  610 , to the first framer  631 . Also, in accordance with control of the control unit  640 , the first RPR processing unit  621  outputs data, output from the first framer  631 , to the data transmission/reception unit  610  or the second RPR processing unit  622 . 
     In accordance with control of the control unit  640 , the second RPR processing unit  622  outputs the data, output from the data transmission/reception unit  610 , to the second framer  632 . Also, in accordance with control of the control unit  640 , the second RPR processing unit  622  outputs data, output from the second framer  632 , to the data transmission/reception unit  610  or the first RPR processing unit  621 . 
     The first framer  631  maps the data, output from the first RPR processing unit  621 , on a SONET (e.g., GFP/VCAT) frame. The first framer  631  outputs the data-mapped frame via the west-side connection portion  531   a . The frame output via the connection portion  531   a  is output to the connection target (the first high-speed I/F  512  in  FIG. 5 ) connected to the connection portion  531   a  by the connection changing switch  514 . 
     The second framer  632  maps the data, output from the second RPR processing unit  622 , on a SONET (e.g., GFP/VCAT) frame. The second framer  632  outputs the data-mapped frame via the east-side connection portion  531   b . The frame output via the connection portion  531   b  is output to the connection target (the connection portion  532   a  of the station  532  in  FIG. 5 ) connected to the connection portion  531   b  by the connection changing switch  514 . 
     Also, the first framer  631  extracts data from a frame input via the connection portion  531   a . The first framer  631  outputs the extracted data to the first RPR processing unit  621 . The second framer  632  extracts data from a frame input via the connection portion  531   b . The second framer  632  outputs the extracted data to the second RPR processing unit  622 . 
     The control unit  640  is responsible for overall control of the station  531 . When the station  531  is installed in the slot of the shelf, the control unit  640  accesses the monitoring unit  511  to obtain the slot number of the station  531 . Also, the control unit  640  controls the first RPR processing unit  621  and the second RPR processing unit  622  to perform RPR protocol control. 
     Also, the control unit  640  outputs the slot number Slot 1  of the station  531 , the slot number being obtained from the monitoring unit  511 , to the reporting unit  650  as identification information for identifying the station  531 . Also, the control unit  640  creates a table illustrating a list of the slot numbers Slot 3 , Slot 4 , and Slot 6  of the stations  532  to  534 , the slot numbers being output from the obtaining unit  660 , and the slot number Slot 1  of the station  531 . The control unit  640  causes the memory  670  to store the table. 
     Also, on the basis of the table stored in the memory  670 , the control unit  640  determines the connection target of the connection portion  531   a  of the station  531  and the connection target of the connection portion  531   b . Also, the control unit  640  switches the connection targets of the connection portion  531   a  and the connection portion  531   b  to the connection targets determined on the basis of the table. Specifically, the control unit  640  outputs a switching instruction based on the determined connection targets to the connection changing switch  514 , to thereby switch the connection targets. 
     The reporting unit  650  repeatedly reports the slot number Slot 1 , output from the control unit  640 , to the stations  532  to  534 . The obtaining unit  660  obtains the slot numbers Slot 3 , Slot 4 , and Slot 6  repeatedly reported from the stations  532  to  534  and outputs the slot numbers Slot 3 , Slot 4 , and Slot 6  to the control unit  640 . The slot number reporting and obtaining performed by the reporting unit  650  and the obtaining unit  660  are performed through the control-signal line  540  illustrated in  FIG. 5 . 
     The control unit  640  is, for example, a CPU (central processing unit). The first RPR processing unit  621 , the second RPR processing unit  622 , the first framer  631 , and the second framer  632  are, for example, dedicated chips having those functions. Although a specific example of the configuration of the station  531  has been described above, specific examples of the configurations of the stations  532  to  534  are also analogous thereto. 
       FIG. 7  is a flowchart illustrating one example of the operation of the station illustrated in  FIG. 5 . As illustrated in  FIG. 7 , first, the control unit  640  obtains the slot number of the slot  521  that accommodates the self station (the station  531 ) from the monitoring unit  511  (step S 701 ). Next, the reporting unit  650  reports the slot number, obtained in step S 701 , to the other stations (the stations  532  to  534 ) (step S 702 ). 
     Next, the obtaining unit  660  obtains the slot numbers reported from the other stations (step S 703 ). Next, the control unit  640  creates a table illustrating a list of the slot number obtained in step S 701  and the slot numbers obtained in step S 703  (step S 704 ). Next, on the basis of the table created in step S 704 , the control unit  640  determines connection targets of the self station (step S 705 ). 
     Next, the control unit  640  switches the connection targets of the self station to the connection targets determined in step S 705  (step S 706 ). Next, the control unit  640  determines whether or not an operation end condition is satisfied (e.g., whether or not an end instruction given by a user is received) (step S 707 ). When the end condition is not satisfied (step S 707 : No), the process returns to step S 702  to continue the processing. 
     When the end condition is satisfied (step S 707 : Yes), the series of processing ends. A step that is analogous to each step described above is simultaneously executed on the stations  532  to  534 , so that, when a failure occurs in any of the stations  531  to  534 , a connection path that bypasses the station where the failure occurred can be automatically formed. 
     Specifically, each of the stations  531  to  534  executes step S 704  in  FIG. 7 , so that the stations  531  to  534  create the identical table. Then, on the basis of the created table, each of the stations  531  to  534  determines connection targets of the self station by the same method (see  FIG. 8 ) and switches the connection targets. 
       FIG. 8  is a flowchart illustrating a specific example of step S 705  illustrated in  FIG. 7 . As illustrated in  FIG. 8 , first, a determination is made as to whether or not the slot number of the self station is the smallest of the slot numbers in the table created in step S 704  in  FIG. 7  (step S 801 ). When the slot number of the self station is the smallest of the slot numbers (step S 801 : Yes), it is determined that the west-side connection target of the self station is the first high-speed I/F  512  (step S 802 ), and the process proceeds to step S 804  to continue the processing. 
     When the slot number of the self station is not the smallest of the slot numbers in step S 801  (step S 801 : No), it is determined that the west-side connection target of the self station is the east-side connection portion of the station having the next smaller slot number than the slot number of the self station (step S 803 ), and the process proceeds to step S 804  to continue the processing. 
     Next, a determination is made as to whether or not the slot number of the self station is the largest of the slot numbers in the table created in step S 704  in  FIG. 7  (step S 804 ). When the slot number of the self station is the largest of the slot numbers (step S 804 : Yes), it is determined that the east-side connection target of the self station is the second high-speed I/F  513  (step S 805 ), and the series of processing ends. 
     When the slot number of the self station is not the largest of the slot numbers in step S 804  (step S 804 : No), it is determined that the east-side connection target of the self station is the west-side connection portion of the station having the next larger slot number than the slot number of the self station (step S 806 ), and the series of processing ends. 
       FIG. 9  is a flowchart illustrating another example of the operation of the station illustrated in  FIG. 5 . In  FIG. 9 , steps S 901  to S 904  are analogous to steps S 701  to S 704  illustrated in  FIG. 7 , and thus, descriptions thereof are omitted. After step S 904 , the control unit  640  determines whether or not the latest table crated in step S 904  has changed from the table created in step S 904  of the last time (step S 905 ). 
     When the table has not changed in step S 905  (step S 905 : No), the process returns to step S 902  to continue the processing. When the table has changed (step S 905 : Yes), the control unit  640  determines the connection target of the self station (step S 906 ) on the basis of the table created in step S 904 . 
     Since steps S 907  and S 908  are analogous to steps S 706  and S 707  illustrated in  FIG. 7 , descriptions thereof are omitted. In this manner, only when the created table has changed, the process proceeds to the determination process in step S 906 . Thus, it is possible to prevent the determination process illustrated in  FIG. 8  from being continuously executed also when no failure has occurred in another station. 
     Thus, it is possible to reduce the period of the loop of steps S 901  to S 905  without increasing the amount of processing performed by the control unit  640 . A reduction in the period of the loop of steps S 901  to S 905  makes it possible to immediately detect a failure that occurs in another station. The specific example of the connection-target determination process illustrated in  FIG. 8  can also be used in step S 906  in  FIG. 9 . Next, a specific example of the operation described in  FIGS. 7 to 9  is descried. 
       FIG. 10  is a drawing (part  1 ) illustrating a table created by the control unit. A table  1000  illustrated in  FIG. 10  is a table mutually created by the stations  531  to  534  when the line accommodating device  500  is in the state illustrated in  FIG. 5 . In the table  1000 , the presence/absence of reporting of a slot number is stored for each of all the slot numbers (Slot 1  to Slot 6 ). In the table  1000 , a slot number that has been properly obtained is expressed by “◯” and a slot number that has not been properly obtained is expressed by “x”. 
     When the line accommodating device  500  is in the state illustrated in  FIG. 5 , the stations  531  to  534  are installed in the corresponding slots  521 ,  523 ,  524 , and  526  and no failure is occurring in the stations  531  to  534 . Thus, each of the stations  531  to  534  can properly obtain all the slot numbers of the other stations. 
     Thus, the presence/absence of the reporting of the slot numbers Slot 1 , Slot 3 , Slot 4 , and Slot 6  is expressed by “◯”. Also, no stations are installed in the slot  522  and the slot  525 . Thus, the presence/absence of the reporting of the slot numbers Slot 2  and Slot 5  is expressed by “x”. 
     Connection-target determination performed by the station  531  in this case is described. The slot number Slot 1  of the station  531  is the smallest of the slot numbers having “◯” in the table  1000 . Thus, the station  531  determines that the connection target of the west-side connection portion  531   a  of the self station is the first high-speed I/F  512 . 
     Also, the next smaller slot number than the slot number Slot 1  of the station  531  is Slot 3  of the slot numbers having “◯” in the table  1000 . Thus, the station  531  determines that the connection target of the connection portion  531   b  of the self station is the west-side connection portion  532   a  of the station  532  accommodated in the slot  523 . 
     Connection-target determination performed by the station  532  is described next. The next smaller slot number than the slot number Slot 3  of the station  532  is Slot 1  of the slot numbers having “◯” in the table  1000 . Thus, the station  532  determines that the connection target of the west-side connection portion  532   a  of the self station is the east-side connection portion  531   b  of the station  531  accommodated in the slot  521 . 
     Also, the next larger slot number than the slot number Slot 3  of the station  532  is Slot 4  of the slot numbers having “◯” in the table  1000 . Thus, the station  532  determines that the connection target of the east-side connection portion  532   b  is the west-side connection portion  533   a  of the station  533  accommodated in the slot  524 . 
     Connection-target determination performed by the station  533  is described next. The next smaller slot number than the slot number Slot 4  of the station  533  is Slot 3  of the slot numbers having “◯” in the table  1000 . Thus, the station  533  determines that the connection target of the west-side connection portion  533   a  of the self station is the east-side connection portion  532   b  of the station  532  accommodated in the slot  523 . 
     Also, the next larger slot number than the slot number Slot 4  of the station  533  is Slot 6  of the slot numbers having “◯” in the table  1000 . Thus, the station  533  determines that the connection target of the east-side connection portion  533   b  is the west-side connection portion  534   a  of the station  534  accommodated in the slot  526 . 
     Connection-target determination performed by the station  534  is described next. The next smaller slot number than the slot number Slot 6  of the station  534  is Slot 4  of the slot numbers having “◯” in the table  1000 . Thus, the station  534  determines that the connection target of the west-side connection portion  534   a  of the self station is the east-side connection portion  533   b  of the station  533  accommodated in the slot  524 . 
     Also, the slot number Slot 6  of the station  534  is the largest of the slot numbers having “◯” in the table  1000 . Thus, the station  534  determines that the connection target of the west-side connection portion  534   b  of the self station is the second high-speed I/F  513 . Consequently, a connection path in which the stations  531  to  534  are connected in series between the first high-speed I/F  512  and the second high-speed I/F  513  is formed as illustrated in  FIG. 5 . 
       FIG. 11  is a drawing (part  2 ) illustrating a table created by the control unit. A table  1100  illustrated in  FIG. 11  is a table created by each of the station  531 , the station  533 , and the station  534  when the line accommodating device  500  is in the state illustrated in  FIG. 5  and a failure occurs in the station  532 . The contents of the tables  1100  created by the station  531 , the station  533 , and the station  534  are the same. 
     When a failure occurs in the station  532 , the station  532  may not report the slot number Slot 3 . Therefore, the station  531 , the station  533 , and the station  534  may not properly obtain the slot number Slot 3 . Thus, the presence/absence of the reporting is changed from “◯” to “x” with respect to the slot number Slot 3 . 
       FIG. 12  is a diagram (part  1 ) illustrating a switching operation of each station when a failure occurs.  FIG. 12  illustrates a switching operation of each station when a failure occurs in the station  532 , as described in  FIG. 11 . The station  531 , the station  533 , and the station  534  determine the connection targets on the basis of the table  1100  illustrated in  FIG. 11 . 
     Connection-target determination performed by the station  531  in this case is described. The slot number Slot 1  of the station  531  is the smallest of the slot numbers having “◯” in the table  1100 . Thus, the station  531  maintains the first high-speed I/F  512  as the connection target of the west-side connection portion  531   a  of the self station. 
     Also, the next smaller slot number than the slot number Slot 1  of the station  531  is Slot 4  of the slot numbers having “◯” in the table  1100 . Thus, the station  531  switches the connection target of the east-side connection portion  531   b  to the west-side connection portion  533   a  of the station  533  accommodated in the slot  524 . 
     Connection-target determination performed by the station  533  is described next. The next smaller slot number than the slot number Slot 4  of the station  533  is Slot 1  of the slot numbers having “◯” in the table  1100 . Thus, the station  533  switches the connection target of the west-side connection portion  533   a  of the self station to the east-side connection portion  531   b  of the station  531  accommodated in the slot  521 . 
     Also, the next larger slot number than the slot number Slot 4  of the station  533  is Slot 6  of the slot numbers having “◯” in the table  1100 . Thus, the station  533  maintains the west-side connection portion  534   a  of the station  534 , accommodated in the slot  526 , as the connection target of the east-side connection portion  533   b.    
     Connection-target determination performed by the station  534  is described next. The next smaller slot number than the slot number Slot 6  of the station  534  is Slot 4  of the slot numbers having “◯” in the table  1100 . Thus, the station  534  maintains the east-side connection portion  533   b  of the station  533 , accommodated in the slot  524 , as the connection target of the west-side connection portion  534   a  of the self station. 
     Also, the slot number Slot 6  of the station  534  is the largest of the slot numbers having “◯” in the table  1100 . Thus, the station  534  maintains the second high-speed I/F  513  as the connection target of the east-side connection portion  534   b . Consequently, a connection path that bypasses the station  532  where the failure occurred is automatically formed. 
       FIG. 13  is a drawing (part  3 ) illustrating a table created by the control unit. A table  1300  illustrated in  FIG. 13  is a table created by each of the stations  531  and the station  533  when the line accommodating device  500  is in the state illustrated in  FIG. 12  and a failure further occurs in the station  534 . The contents of the tables  1300  created by the station  531  and the station  533  are the same. 
     When a failure occurs in the station  534 , the station  534  may not report the slot number Slot 6 . Therefore, the station  531  and the station  533  may not properly obtain the slot number Slot 6 . Thus, the presence/absence of the reporting is changed from “◯” to “x” with respect to the slot number Slot 6 . 
       FIG. 14  is a diagram (part  2 ) illustrating a switching operation of each station when a failure occurs.  FIG. 14  illustrates a switching operation of each station when a failure occurs in the station  534 , as described in  FIG. 13 . The station  531  and the station  533  determine the connection targets on the basis of the table  1300  illustrated in  FIG. 13 . 
     Connection-target determination performed by the station  531  in this case is described. The slot number Slot 1  of the station  531  is the smallest of the slot numbers having “◯” in the table  1300 . Thus, the station  531  maintains the first high-speed I/F  512  as the connection target of the west-side connection portion  531   a  of the self station. 
     Also, the next larger slot number than the slot number Slot 1  of the station  531  is Slot 4  of the slot numbers having “◯” in the table  1300 . Thus, the station  531  maintains the west-side connection portion  533   a  of the station  533  as the connection target of the east-side connection portion  531   b  of the self station. 
     Connection-target determination performed by the station  533  is described next. The next smaller slot number than the slot number Slot 4  of the station  533  is Slot 1  of the slot numbers having “◯” in the table  1300 . Thus, the station  533  maintains the east-side connection portion  531   b  of the station  531  as the connection target of the west-side connection portion  533   a  of the self station. 
     Also, the slot number Slot 4  of the station  533  is the largest of the slot numbers having “◯” in the table  1300 . Thus, the station  533  switches the connection target of the east-side connection portion  533   b  of the self station from the west side of the station  534  to the second high-speed I/F  513 . Consequently, a connection path that bypasses the station  534  where the failure occurred can be automatically formed. 
     As described above, according to the line accommodating device  500  according to the first embodiment, when a failure occurs in one of the stations, the stations in which no failure is occurring can connect each other by independent operations. Thus, even when a failure occurs in one of the stations during occurrence of a failure in the monitoring unit  511 , a connection path that bypasses the station where the failure occurred can be automatically formed. 
     Thus, it is possible to improve the tolerance of the network  400  against a failure in the stations. Also, since the failure detection and the recovery operation can be performed without dependence on the monitoring unit  511 , it is possible to reduce the load on the monitoring unit  511 . 
     According to the line accommodating device  500 , each station determines, as the connection targets, the station indicated by the next smaller slot number than the slot number of the self station and the station indicated by the next larger slot number than the slot number of the self station. Thus, it is possible to automatically form a connection path in which all of the stations where no failure is occurring are connected in series. 
     Thus, it is possible to prevent the stations where no failure is occurring and the user networks connected to the stations from being disconnected from the network  400 . In addition, the stations  531  to  534  are accommodated in the slots  521  to  526  arranged in the line accommodating device  500 . Also, since the slot numbers Slot 1  to Slot 6  used as the identification information are attached in order of the arrangement of the slots  521  to  526 , a connection path that is automatically formed can be formed to be a shortest path that connects the stations where no failure is occurring. 
     Also, according to the line accommodating device  500 , when the next smaller slot number than the slot number of the self station does not exist, each station can determine the first high-speed I/F  512  as the west-side connection target, and when the next larger slot number than the slot number of the self station does not exist, each station can determine that second high-speed I/F  513  as the east-side connection target. 
     Thus, even when a failure occurs in the station connected to the first high-speed I/F  512  or the second high-speed I/F  513 , the first high-speed I/F  512  and the second high-speed I/F  513  can be automatically connected to the stations where no failure is occurring. Thus, it is possible to further improve the tolerance of the network  400  against a failure in the stations. 
     Also, since the network  400  is a ring-type network, the entire network  400  is affected if one portion of the network  400  is disconnected. In contrast, when the line accommodating device  500  is used for at least any of the nodes #A to #D, even if a station failure occurs during occurrence of a failure in the monitoring unit  511  and a portion of the network  400  is put into an interrupted state, the interrupted state can be automatically recovered. Thus, it is possible to improve the tolerance of the entire network  400  against a failure in the stations. 
     Also, the network  400  is an RPR-system network. Thus, an RPR failure relief function works when a failure occurs in one of the stations, but the redundancy function is lost against a further failure. In contrast, when the line accommodating device  500  is used for at least any of the nodes #A to #D, even if a station failure occurs during occurrence of a failure in the monitoring unit  511 , the station where the failure occurred can be automatically separated from the network  400 . Thus, it is possible to maintain the RPR function. Therefore, it is possible to improve the tolerance of the network  400  against a failure in the stations. 
     A data transmission operation after the ring network is constructed using the above-described procedure is performed as follows. A case in which data transmitted from the user network  410 A is relayed by the node #A, the node #B, and the node #C through the use of the transmission path  421  and is transmitted to the user network  410 C, as denoted by reference numeral  431  in  FIG. 4 , is described by way of example. It is assumed in this case that the data from the user network  410 A is received by the station  531 . It is also assumed that the stations  531 ,  532 ,  533 , and  534  are installed in the node #A, as illustrated in  FIG. 5 . 
     The station  531  outputs the data, received from the user network  410 A, via the west-side connection portion  531   a  in accordance with an RPR protocol. The data output from the connection portion  531   a  of the station  531  is input to the first high-speed I/F  512 . The first high-speed I/F  512  transmits the signal, output from the station  531 , to the node #B. Consequently, the data from the user network  410 A is transmitted to the node #B. 
     A case in which the transmission path for transmitting the data, transmitted from the user network  410 A, is switched from the transmission path  421  to the transmission path  422 , as denoted by reference numeral  432  illustrated in  FIG. 4 , is described as a second example of the transmission operation of the line accommodating device  500 . It is also assumed in this case that the data from the user network  410 A is received by the station  531 . The station  531  outputs the data, received from the user network  410 A, via the east-side connection portion  531   b  in accordance with the RPR protocol. 
     The data output via the connection portion  531   b  is input to the west-side connection portion  532   a  of the station  532 . The station  532  outputs the data, input from the connection portion  532   a , via the connection portion  532   b  in accordance with the RPR protocol. The data output from the east-side connection portion  532   b  of the station  532  is input to the west-side connection portion  533   a  of the station  533 . The station  533  outputs the data, input from the connection portion  533   a , via the connection portion  533   b  in accordance with the RPR protocol. 
     The signal output from the connection portion  533   b  of the station  533  is input to the west-side connection portion  534   a  of the station  534 . The station  534  outputs the signal, input from the connection portion  534   a , via the connection portion  534   b  in accordance with the RPR protocol. The data output from the east-side connection portion  534   b  of the station  534  is input to the second high-speed I/F  513 . The second high-speed I/F  513  transmits the data, input from the station  534 , to the node #D. Consequently, the data from the user network  410 A is transmitted to the node #D. 
     A case, illustrated in  FIG. 4 , in which data transmitted from the user network  410 B is relayed by the node #B, the node #A, and the node #D through the use of the transmission path  422  and is transmitted to the user network  410 D is described as a third example of the transmission operation of the line accommodating device  500 . It is also assumed in this case that the line accommodating device  500  relays the data transmitted from the node #B and transmits the data to the node #D. 
     The first high-speed I/F  512  receives the data transmitted from the node #B. The first high-speed I/F  512  outputs the received data to the west-side connection portion  531   a  of the station  531 . The station  531  outputs the data, output from the first high-speed I/F  512 , via the east-side connection portion  531   b  in accordance with the RPR protocol. 
     The data output from the connection portion  531   b  is input to the west-side connection portion  532   a  of the station  532 . The station  532  outputs the data, input from the connection portion  532   a , via the connection portion  532   b  in accordance with the RPR protocol. The data output via the east-side connection portion  532   b  of the station  532  is input to the west-side connection portion  533   a  of the station  533 . The station  533  outputs the data, input from the connection portion  533   a , via the connection portion  533   b  in accordance with the RPR protocol. 
     The data output via the connection portion  533   b  of the station  533  is input to the west-side connection portion  534   a  of the station  534 . The station  534  outputs the data, input from the connection portion  534   a , via the connection portion  534   b  in accordance with the RPR protocol. The data output via the east-side connection portion  534   b  of the station  534  is input to the second high-speed I/F  513 . The second high-speed I/F  513  transmits the data, input from the station  534 , to the node #D. Consequently, the data transmitted from the node #B is transmitted to the node #D. 
     A case, illustrated in  FIG. 4 , in which data transmitted from the user network  410 B is relayed by the node #B and the node #A through the use of the transmission path  422  and is transmitted to the user network  410 A is described as a fourth example of the transmission operation of the line accommodating device  500 . It is assumed in this case that the line accommodating device  500  relays the data transmitted from the node #B and transmits the data to the user network  410 A. 
     It is also assumed that the data is transmitted to, of the user network  410 A, particularly, the network connected to the station  533 . The first high-speed I/F  512  receives the data transmitted from the node #B and outputs the received data to the west-side connection portion  531   a  of the station  531 . The station  531  outputs the data, output from the first high-speed I/F  512 , via the east-side connection portion  531   b  in accordance with the RPR protocol. 
     The data output via the connection portion  531   b  is input to the west-side connection portion  532   b  of the station  532 . The station  532  outputs the data, input from the connection portion  532   a , via the connection portion  532   b  in accordance with the RPR protocol. The data from the connection portion  532   b  of the station  532  is input to the connection portion  533   a  of the station  533 . 
     The station  533  transmits the data, input from the connection portion  533   a , to the user network  410 A in accordance with the RPR protocol. Consequently, the data transmitted from the node #B is transmitted to, of the user network  410 A, the network connected to the station  533 . In this manner, each station performs an operation according to the RPR protocol to thereby change the transmission direction between the node #B, the node # 3 , and the user network  410 A. 
     SECOND EMBODIMENT 
       FIG. 15  is a block diagram illustrating the configuration of a line accommodating device according to a second embodiment. In  FIG. 15 , the same configurations as those illustrated in  FIG. 5  are denoted by the same reference numerals, and descriptions thereof are omitted. As illustrated in  FIG. 15 , a line accommodating device  500  according to the second embodiment has a mesh network  1510  instead of the connection changing switch  514  in the configuration illustrated in  FIG. 5 . 
     The mesh network  1510  is a connecting block that connects, in a meshed manner, the first high-speed I/F  512 , the second high-speed I/F  513 , and the west-side and the east-side connection portions of the stations installed in the slots  521  to  526 . Each of the stations  531  to  534  has a group of connection paths  1520  for connection with the first high-speed I/F  512 , the second high-speed I/F  513 , and another station via the mesh network  1510 . 
     Each of the stations  531  to  534  selects the west-side and east-side connection portions of the self station from the group of connection paths  1520  to thereby switch the connection targets of the self station. Thus, the stations  531  to  534  can switch the connection targets of the self stations by respective independent operations. 
     In this case, the connection changing switch  514  is not utilized. Thus, it is possible to prevent an event in which the network  400  may not recover from an interrupted state when a failure occurs in one of the relay modules during occurrence of a failure in the connection changing switch  514 . 
       FIG. 16  is a block diagram illustrating a specific example of the station illustrated in  FIG. 15 . In  FIG. 16 , the same configurations as those illustrated in  FIG. 6  or  FIG. 15  are denoted by the same reference numerals, and descriptions thereof are omitted. As illustrated in  FIG. 16 , the station  531  has a connection changing switch  1610  connected to a group of connection paths  1620 , in addition to the configuration illustrated in  FIG. 6 . 
     The first framer  631  outputs a mapped signal to the connection changing switch  1610 . Also, the first framer  631  extracts data from a signal input from the connection changing switch  1610 . The second framer  632  outputs a mapped signal to the connection changing switch  1610 . Also, the second framer  632  extracts data from a signal input from the connection changing switch  1610 . 
     In accordance with control of the control unit  640 , the connection changing switch  1610  selects, from the group of connection paths  1620 , a connection target for performing signal input/output with the first framer  631 . In accordance with control of the control unit  640 , the connection changing switch  1610  selects, from the group of connection paths  1620 , a connection target for performing signal/output with the second framer  632 . The control unit  640  outputs a switching instruction based on the determined connection targets to the connection changing switch  1610  to thereby perform switching similar to the connection-target switching described in  FIG. 6 . 
     As described above, the line accommodating device  500  according to the second embodiment provides advantages of the line accommodating device  500  according to the second embodiment and also can switch the connection targets of the stations by independent operations of the stations  531  to  534 . Thus, it is possible to further improve the tolerance of the network  400  against a failure in the relay modules, compared to a case using the connection changing switch that switches the connections of the stations in an integrated manner. 
     Overview of Operation of Line Accommodating Device During Recovery of Relay Module 
       FIG. 17  is a diagram (part  4 ) illustrating an overview of this line accommodating device. In  FIG. 17 , the same portions as those illustrated in  FIG. 3  are denoted by the same reference numerals, and description thereof are omitted. A description is given of a case in which, after the state illustrated in  FIG. 3 , the relay module  112  recovers from the failure. As illustrated in  FIG. 17 , the relay module  112  that has recovered from the failure resumes the reporting of the identification information # 2 . 
     When the reporting of the identification information # 2  from the relay module  112  is resumed, the relay module  111  and the relay module  113  can detect that the relay module  112  has recovered from the failure. In this state, the connection-target determination operation performed by each of the relay modules  111  to  113  is the same as the operation described in  FIGS. 1 to 3 , and thus, a description thereof is omitted here. 
       FIG. 18  is a diagram (part  5 ) illustrating an overview of this line accommodating device. In  FIG. 18 , the same portions as those illustrated in  FIG. 3  are denoted by the same reference numerals, and description thereof are omitted. After the relay module  112  recovers from the failure (see  FIG. 17 ), the relay module  111  switches one of the connection targets from the relay module  113  to the relay module  112 . As the other connection target of the relay module  111 , the first interface  115  is maintained. 
     Also, the relay module  112  has properly obtained the identification information # 1  and the identification information # 3 . Thus, the relay module  112  connects to the relay module  111  indicated by the identification information # 1  having the next smaller value than the identification information # 2  of the self module and also connects to the relay module  113  indicated by the identification information # 3  having the next larger value than the identification information # 2 . 
     Also, the relay module  113  switches one of the connection targets from the relay module  111  to the relay module  112 . As the other connection target of the relay module  113 , the second interface  116  is maintained. In this manner, when the relay module  112  recovers from a failure, a connection path that passes through the recovered relay module  112  is automatically formed. 
     As described above, according to the disclosed line accommodating device and the control method, it is possible to improve the tolerance of the network against a failure in the relay modules. Although a case in which the network  400  is a ring-type network that supports an RPR has been described in each embodiment described above, the present invention is not necessarily limited to a case in which the network  400  supports an RPR, and the network  400  may be a network in which the nodes #A to #D are connected on a straight line. In this case, provision of an intermediate node, which is a non-terminal node, of the nodes #A to #D with the line accommodating device  500  makes it possible to improve the tolerance of the network. 
     Although a configuration in which signals to be transmitted between the nodes are relayed after being temporarily converted by the line accommodating device  500  into electrical signals has been described in each embodiment described above, a configuration in which signals to be transmitted between the nodes are relayed as optical signals may also be used. In this case, a connection path formed by the stations is also an optical path. Also, although the network  400  has been described as being an optical network, the line accommodating device  500  is also applicable to an electrical-line network. 
     Also, a case in which all of the stations  531  to  534  accommodated by the line accommodating device  500  are stations that belong to the ring-type network  400  has been described in each embodiment described above. As opposed to it, one of the stations  531  to  534  may be a station that belongs to a network that is different from the network  400 . 
     In this case, the stations  531  to  534  attach the number of the network to which the self stations belong to the slot numbers that serve as identification information of the self stations, and report the resulting numbers to the other stations. The stations  531  to  534  then each create a table illustrating a slot-number list in which, of the slots numbers obtained from the other stations, the attached network numbers are the same as the network number of the network to which the self station belongs. 
     Thus, of the stations  531  to  534 , the stations that belong to the network  400  can form a connection path that bypasses, of the stations  531  to  534 , the stations that belongs to another network. Consequently, the line accommodating device  500  can also accommodate, in a mixed manner, a station or stations that belong to a network that is different from the network  400 . With respect to the above-described embodiments, the following appendices are further disclosed. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.