Patent Publication Number: US-11381525-B2

Title: Communication apparatus, communication system, and communication control method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2018-240286, filed on Dec. 21, 2018, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to a communication apparatus, a communication system, and a communication control method. 
     BACKGROUND 
     There is a communication system in which an access line for accessing a ring network is made redundant by using a multi-chassis link aggregation (MC-LAG) technology. In a case where a fault occurs in the communication between communication apparatuses coupled to a redundant access line, a main signal path is switched to a bypass for a location of a fault by using, for example, an Ethernet (registered trademark; the same applies hereinafter) ring protection (ERP) function. 
     In the above instance, the monitoring control function of MC-LAG is additionally used so that a control signal path is also switched to a bypass for redundant system switchover for the communication apparatuses. Therefore, the time required for switching a main signal and a control signal becomes longer than in a case where only the main signal path is switched. Additionally, a bandwidth of the control signal is obtained for an increased number of links in accordance with a path length increase from a pre-switch state. This reduces the bandwidth of the main signal. 
     For example, Japanese Laid-open Patent Publication No. 2015-211402 is disclosed as a related art. 
     SUMMARY 
     According to an aspect of the embodiment, a communication apparatus includes a first port that is coupled to a first communication apparatus through a ring line, a second port that is coupled to a second communication apparatus through a second access line, the second communication apparatus being disposed external to the ring line, the second access line being configured redundantly with a first access line between the first communication apparatus and the second communication apparatus, a third port that is coupled to the first communication apparatus through a control line for transmitting and receiving a control signal concerning the first access line, and circuitry that detects a fault in the control line, wherein the circuitry detects a fault in the second access line, switches the second access line from an active line to a standby line in accordance with the detection of the fault in the second access line, and while the fault in the control line is detected, shuts down the first port in accordance with the detection of the fault in the second access line. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram illustrating an example of a communication system; 
         FIG. 2  is a configuration diagram illustrating an example of a communication apparatus; 
         FIG. 3  is a diagram illustrating an exemplary operation that is performed by a communication system when a fault occurs in a control line; 
         FIG. 4  is a diagram illustrating an exemplary operation that is performed by a communication system when a fault occurs in a ring line; 
         FIG. 5  is a diagram illustrating an exemplary operation that is performed by a communication system when a fault occurs in an access line; 
         FIG. 6  is a diagram illustrating an exemplary operation that is performed by a communication system when each fault occurs in an access line and in a control line; 
         FIG. 7  is a diagram illustrating an exemplary operation that is performed by a communication system in an event of a fault in a control line and a defect in a ring apparatus; 
         FIG. 8  is a state transition diagram illustrating exemplary states of a ring apparatus managed by a state control section; 
         FIG. 9  is a flowchart illustrating an example of a state control process performed on a ring apparatus; 
         FIG. 10  is a flowchart illustrating an example of a process that is performed in accordance with a state of a ring apparatus of an active system; 
         FIG. 11  is a flowchart illustrating an example of a process that is performed in accordance with a state of a ring apparatus of a standby system; 
         FIG. 12  is a sequence diagram illustrating exemplary operations that are performed by redundantly paired ring apparatuses when a fault in an access line is detected while a fault in a control line is detected; 
         FIG. 13  is a sequence diagram illustrating exemplary operations that are performed by redundantly paired ring apparatuses when a defect is detected while a fault in a control line is detected; 
         FIG. 14  is a configuration diagram illustrating an example of a communication system in which a ring line between ring apparatuses is duplexed; 
         FIG. 15  is a diagram illustrating an exemplary operation that is performed by a communication system when a fault occurs in one ring line; 
         FIG. 16  is a configuration diagram illustrating another example of a ring apparatus; 
         FIG. 17  is a flowchart illustrating an example of a control process performed on a main signal and a control signal; and 
         FIG. 18  is a state transition diagram illustrating exemplary states of a ring apparatus managed by a state control section. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Meanwhile, in a case where a communication line between communication apparatuses is duplexed to use one communication line as an intra-portal link (IPL) for a control signal, even if a failure occurs in the IPL communication line or a failure occurs in the main signal, it may be controlled so as not to affect the main signal. Only a main signal path needs to be switched even if a fault occurs in a main signal communication line. This suppresses an increase in the time required for switchover and an increase in the bandwidth of the control signal. 
     However, when a fault occurs in the communication line for the IPL, it is difficult for the communication apparatuses to transmit and receive the control signal to and from each other. Therefore, a redundant system may not be able to achieve switchover. Consequently, if a fault further occurs, for example, in an access line of an active system, it might be practically difficult to maintain main signal communication. 
     In view of the above circumstances, it is desirable to provide a communication apparatus, a communication system, and a communication control method that are capable of maintaining main signal communication even if a fault occurs in a situation where the transmission of a control signal to a redundant partner communication apparatus is unachievable. 
     According to an aspect of an embodiment, it is possible to maintain communication even if a fault occurs in a situation where the transmission of a control signal to a redundant partner communication apparatus is unachievable. 
       FIG. 1  is a configuration diagram illustrating an example of a communication system. The communication system includes terminal apparatuses  90  and  91  such as personal computers and smartphones, access apparatuses  92  and  93 , and ring apparatuses  1   a  to  1   d  forming a ring network NW. For example, a layer  2  switch may be used as the access apparatuses  92  and  93  and the ring apparatuses  1   a  to  1   d . However, the access apparatuses  92  and  93  and the ring apparatuses  1   a  to  1   d  are not limited to the layer  2  switch. A router or other relay apparatus may be used as access apparatuses  92  and  93  and the ring apparatuses  1   a  to  1   d . The ring apparatuses  1   a  to  1   d  are examples of a communication apparatus. In the following example, however, operations of the communication apparatus  1   a  are described. 
     The terminal apparatuses  90  and  91  are respectively coupled to the access apparatuses  92  and  93  through, for example, a wired local area network (LAN) or a wireless LAN. The access apparatus  92  disposed toward the terminal apparatus  90  is coupled to the ring apparatuses  1   a  and  1   b  through redundantly paired access lines La and Lb, respectively. The access apparatus  93  disposed toward the terminal apparatus  91  is coupled to the ring apparatuses  1   c  and  1   d  through redundantly paired access lines Lc and Ld, respectively. 
     The ring apparatuses  1   a  to  1   d  are coupled to a ring line Lr by using, for example, the ring network NW. The ring line Lr has an ERP function. The ring apparatuses  1   a  to  1   d  respectively include ports A 1  to A 4 , ports B 1  to B 4 , ports C 1  to C 4 , and ports D 1  to D 4 . The ports A 1  to A 4 , ports B 1  to B 4 , ports C 1  to C 4 , and ports D 1  to D 4  each include, for example, a transmitter for transmitting an optical signal and a receiver for receiving an optical signal. 
     The port A 2  of the ring apparatus  1   a  is coupled to the access line La, and the port B 2  of the ring apparatus  1   b  is coupled to the access line Lb. The ports A 2  and B 2  are set so that the access lines La and Lb configure an MC-LAG for the access apparatus  92 . In an initial state, the access line La is set as an active line, and the access line Lb is set as a standby line. 
     The port A 3  of the ring apparatus  1   a  is coupled to oppose the port B 3  of the ring apparatus  1   b  through a control line Lf. The ports A 3  and B 3  transmit and receive a control signal Sc concerning the access lines La and Lb through the control line Lf. The control signal Sc includes, for example, a fault notification for switching the access lines La and Lb from a standby line to an active line. 
     The ports A 1  and A 4  of the ring apparatus  1   a  and the ports B 1  and B 4  of the ring apparatus  1   b  are coupled to the ring line Lr. The port A 1  and the port B 1  are intercoupled so as to oppose each other through the ring line Lr. The ports A 1 , A 4 , B 1 , and B 4  transmit and receive, through the ring line Lr, a main signal Sg including data transmitted by the terminal apparatuses  90  and  91 . 
     The port C 2  of the ring apparatus  1   c  is coupled to the access line Lc, and the port D 2  of the ring apparatus  1   d  is coupled to the access line Ld. The ports C 2  and D 2  are set so that the access lines Lc and Ld configure the MC-LAG for the access apparatus  93 . In the initial state, the access line Lc is set as an active line, and the access line Ld is set as a standby line. The pair of ring apparatuses  1   a  and  1   b  and the pair of ring apparatuses  1   c  and  1   d  are examples of the communication system. In the following example, however, operations of the pair of ring apparatuses  1   a  and  1   b  are described. 
     The port C 3  of the ring apparatus  1   c  is coupled to oppose the port D 3  of the ring apparatus  1   d  through a control line Le. As is the case with the ports A 3  and B 3  and the ports C 3  and D 3  transmit and receive a control signal concerning the access lines Lc and Ld through the control line Le. 
     The ports C 1  and C 4  of the ring apparatus  1   c  and the ports D 1  and D 4  of the ring apparatus  1   d  are coupled to the ring line Lr. The port C 4  is coupled to oppose the port B 4  through the ring line Lr, and the port D 4  is coupled to oppose the port A 4  through the ring line Lr. The port C 1  and the port D 1  are intercoupled so as to oppose each other through the ring line Lr. The ports C 1 , C 4 , D 1 , and D 4  transmit and receive, through the ring line Lr, the main signal including data transmitted by the terminal apparatuses  90  and  91 . 
     The terminal apparatuses  90  and  91  communicate with each other, for example, through a path R. The path R runs through the access lines La and Lc and through the ring line Lr between the ring apparatuses  1   a ,  1   b , and  1   c . A path between the port D 1  of the ring apparatus  1   d  and the port C 1  of the ring apparatus  1   c  is set as a blocking point BP in order to suppress the main signal from continuously looping through the ring line Lr. Therefore, the ports C 1  and D 1  may not be able to transmit and receive the main signal. 
     Configurations of the communication apparatuses  1   a  to  1   d  will now be described. The following example deals with the communication apparatus  1   a . However, the other communication apparatuses  1   b  to  1   d  have configuration similar to that of the communication apparatus  1   a.    
       FIG. 2  is a configuration diagram illustrating an example of the communication apparatus  1   a . The communication apparatus  1   a  includes a central processing unit (CPU)  10 , a read only memory (ROM)  11 , a random access memory (RAM)  12 , a hardware interface section (HW-IF)  13 , a switch (SW) device  14 , a switch memory  15 , and ports A 1  to A 4 . The CPU  10  is coupled to the ROM  11 , the RAM  12 , and the HW-IF  13  through a bus  19  so that signals may be inputted and outputted between them. 
     The ROM  11  stores programs that drive the CPU  10 . The programs include, for example, software for executing a communication control method according to the embodiment. The RAM  12  functions as a working memory for the CPU  10 . 
     The HW-IF  13  processes the communication between the CPU  10 , the switch device  14 , and the ports A 1  to A 4 . The HW-IF  13  is a circuit including hardware such as a field programmable gate array (FPGA) or an application specified integrated circuit (ASIC). 
     The ports A 1  to A 4  are, for example, LAN ports, and each include an optical transceiver (not illustrated) for transmitting and receiving an Ethernet signal. The Ethernet signal includes, in addition to the main signal Sg, monitoring control data concerning the path switchover of the main signal Sg for ERP. 
     The port A 1 , which is an example of a first port, is coupled to the port B 1  of the communication apparatus  1   b  through the ring line Lr. The communication apparatus  1   b  is an example of a first communication apparatus. 
     The port A 2 , which is an example of a second port, is coupled to the access apparatus  92  through the access line La. As mentioned earlier, the access line La is configured redundantly with the access line Lb between the access apparatus  92  and the ring apparatus  1   b . The access apparatus  92  is an example of a second communication apparatus disposed external to the ring line Lr, and the access lines La and Lb are an example of a second access line and an example of a first access line, respectively. The port B 2  of the ring apparatus  1   b  is an example of an access port that is coupled to the access apparatus  92  through the access line Lb. 
     The port A 3 , which is an example of a third port, is coupled to the port B 3  of the ring apparatus  1   b  through the control line Lf. The control line Lf is configured, for example, as an IPL. Therefore, the main signal Sg does not flow through the control line Lf, and only the control signal Sc flows through the control line Lf. 
     The port A 4 , which is an example of a fourth port, is coupled to the port D 4  of the ring apparatus  1   d  through the ring line Lr. When the path R of the main signal Sg runs through the ports A 4  and D 4  and ports A 4  and D 4  transmit and receive the main signal Sg through the ring line Lr. The communication apparatus  1   d  is an example of a third communication apparatus. 
     The switch device  14  is coupled to the ports A 1  to A 4 . The switch device  14  includes monitoring circuits  141  to  144  that monitor the communication state of each of the ports A 1  to A 4 . For example, the monitoring circuits  141  to  144  output an alarm upon detection of a bit error rate of the communication along the access line La and the ring line Lr or upon detection of an interrupted optical input or output. 
     The switch device  14  also mediates the exchange of main signal Sg between the ports A 1 , A 2 , and A 4 . When the main signal Sg is transferred along the route R illustrated in  FIG. 1 , the switch device  14  transfers the main signal Sg between the ports A 1  and A 2 . The switch device  14  is a circuit including hardware such as an FPGA or an ASIC. 
     The switch device  14  is coupled to the switch memory  15  that stores an address learning table (TBL)  151 . The switch device  14  learns the address of an Ethernet signal from the main signal Sg and stores the learned address in the address learning table  151 . For example, the destination address of the main signal Sg transferred from the ports A 1 , A 2 , and A 4  is registered in the address learning table  151 . 
     The switch device  14  transfers the main signal Sg between the ports A 1 , A 2 , and A 4  in accordance with the address learning table  151 . When the CPU  10  uses an ERP function to issue an instruction for switching the path R, the switch device  14  initializes the contents of the address learning table  151  and relearns addresses. 
     The switch device  14  transmits and receives the control signal Sc through the port A 3 . The switch device  14  outputs the received control signal Sc to the CPU  10  through the HW-IF  13 . Meanwhile, the control signal Sc to be transmitted is inputted from the CPU  10  to the switch device  14  through the HW-IF  13 . 
     Upon reading a program from the ROM  11 , the CPU  10  forms, as functions, a state control section  100 , a defect detection section  101 , an MCLAG control section  102 , an ERP control section  103 , and fault detection sections  104  to  107 . 
     The fault detection section  104 , which is an example of a first detection section, detects a fault in the control line Lf by collecting alarms from the monitoring circuit  143 . The fault detection section  104  detects a fault in the control line Lf when, for example, the control signal Sc is asynchronous with respect to the redundant partner ring apparatus  1   b . The fault in the control line Lf may be, for example, a break in the optical fiber of the control line Lr or a defect of the port A 3  or B 3 . 
     The fault detection section  105 , which is an example of a second detection section, detects a fault in the access line La by collecting alarms from the monitoring circuit  142 . The fault in the access line La may be, for example, a break in the optical fiber of the access line La or a defect of the port A 4 . The fault detection section  105  of the ring apparatus  1   b  is an example of a third detection section that detects a fault in the access line Lb. 
     The defect detection section  101  detects a defect in the ring apparatus  1   a . The defect detection section  101  detects a defect when, for example, a periodic inspection signal from a monitoring target circuit is discontinued. 
     The fault detection section  106 , which is an example of a ring fault detection section, detects a fault in the ring line Lr toward the port A 1  by collecting alarms from the monitoring circuit  141 . The fault in the ring line Lr toward the port A 1  may be, for example, a break in the optical fiber between the ports A 1  and B 1  or a defect of the port A 1  etc. 
     The fault detection section  107  detects a fault in the ring line Lr toward the port A 4  by collecting alarms from the monitoring circuit  144 . The fault in the ring line Lr toward the port A 4  may be, for example, a break in the optical fiber between the ports A 4  and D 4  or a defect of the port A 4  etc. 
     The state control section  100  coordinates with the MCLAG control section  102  and the ERP control section  103  to control the state of the ring apparatus  1   a  in accordance with given conditions. The state control section  100  controls the ports A 1  to A 4  in accordance with the state of the ring apparatus  1   a.    
     Based on the results of detection by the fault detection sections  104  and  105  and the defect detection section  101 , the MCLAG control section  102  provides redundant system switching control of the access lines La and Lb redundantly configured by MC-LAG. When no fault is detected in the control line Lf by the fault detection section  104 , the MCLAG control section  102  communicates with the ring apparatus  1   b  through the control line Lf. 
     For example, when no fault is detected in the control line Lf, the MCLAG control section  102  generates, depending on the detection of a fault in the access line La by the fault detection section  105 , the control signal Sc including a fault notification concerning the fault in the access line La, and outputs the generated control signal Sc to the switch device  14  through the HW-IF  13 . Consequently, the fault notification concerning the fault in the access line La is transmitted from the port A 3  to the redundant partner ring apparatus  1   b  through the control line Lf. 
     The port B 3  of the ring apparatus  1   b  receives signals on the control line Lf, including a fault notification. The switch device  14  outputs a fault notification inputted from the port B 3  to the CPU  10  through the HW-IF  13 . Based on the fault notification, the MCLAG control section  102  of the ring apparatus  1   b  switches the access line Lb from a standby line to an active line. In this instance, the MCLAG control section  102  exercises control to switch the port B 2  of the access line Lb from a non-communicative state to a communicative state. The MCLAG control section  102  of the ring apparatus  1   b  is an example of a second switching section. 
     As described above, the MCLAG control section  102  of the ring apparatus  1   a  transmits, to the ring apparatus  1   b  at a neighboring node through the control line Lf, the control signal Sc for switching the access line Lb from a standby line to an active line. Therefore, in the path R of the main signal Sg, the faulty access line La is replaced by the nonfaulty access line Lb. Consequently, the main signal Sg is communicated continuously even after the occurrence of a fault. The MCLAG control section  102  is an example of a switching section and of a first switching section. 
     Without regard to the presence of a fault in the control line Lf, the MCLAG control section  102  of the ring apparatus  1   a  switches the access line La from an active line to a standby line in accordance with the detection of a fault in the access line La by the fault detection section  105 . In this instance, the MCLAG control section  102  exercises control to switch the port A 2  of the access line La from a communicative state to a non-communicative state. 
     As described above, when a fault occurs in the access line La, the ring apparatus  1   a  switches from an active system to a standby system. 
     However, if a fault occurs in the access line La while the control line Lf is faulty, the ring apparatus  1   a  may not be able to transmit a fault notification to the redundant partner ring apparatus  1   b  through the control line Lf. 
     Accordingly, while a fault in the control line Lf is detected, the state control section  100  shuts down the port A 1  in accordance with the detection of a fault in the access line La. The state control section  100  exercises control to switch the port A 1  from a communicative state to a non-communicative state, for example, through the HW-IF  13 . This causes the port A 1  to stop the transmission and reception of the main signal Sg. 
     In the redundant partner ring apparatus  1   b , the monitoring circuit  141  detects the shutdown of the port A 1  when, for example, an optical input from the port A 1  is discontinued. The fault detection section  106  acquires an alarm indicative of the shutdown from the monitoring circuit  141 , and notifies the ERP control section  103  of the shutdown. The monitoring circuit  141  is an example of a fourth detection section that detects a shutdown. 
     When no fault is detected in the access line Lb by the fault detection section  105  while a fault in the control line Lf is detected, the state control section  100  of the ring apparatus  1   b  coordinates with the ERP control section  103  to switch the access line Lb from a standby line to an active line in accordance with the detection of a shutdown of the port A 1 . This enables the access apparatus  92  to communicate with the ring apparatus  1   b  through the access line Lb instead of the faulty access line La. 
     Consequently, even if a fault occurs in a case where the transmission of the control signal Sc to the redundant partner ring apparatus  1   b  is unachievable, the ring apparatus  1   a  is able to maintain communication. The state control section  100  is an example of a control section that shuts down the port A 1 . 
     The MCLAG control section  102  switches the access line La from an active line to a standby line even in a case where a defect is detected by the defect detection section  101 . In this instance, the state control section  100  shuts down all the ports A 1  to A 4  so that the transmission of the main signal Sg and control signal Sc in a defect state does not allow the defect to affect the other ring apparatuses  1   a  to  1   d  and access apparatus  92 . The MCLAG control section  102  of the ring apparatus  1   b  switches the access line Lb from a standby line to an active line in accordance with the detection of a shutdown of the port A 1 . 
     Consequently, even when a defect occurs in one ring apparatus  1   a , the other ring apparatus  1   b  is able to maintain communication. However, if a defect occurs while the control line Lf is faulty, the ring apparatus  1   a  may not be able to transmit the control signal Sc to the redundant partner ring apparatus  1   b.    
     Accordingly, the state control section  100  of the ring apparatus  1   a  shuts down all the ports A 1  to A 4  in accordance with the detection of a defect by the defect detection section  101 . In this instance, the state control section  100  exercises control to switch the ports A 1  to A 4  from a communicative state to a non-communicative state through the HW-IF  13 . 
     Consequently, the MCLAG control section  102  of the partner ring apparatus  1   b  switches the access line Lb from a standby line to an active line in accordance with the detection of a shutdown of the port A 1 . Therefore, even when a defect occurs, one redundant ring apparatus  1   a  is able to let the other ring apparatus  1   b  maintain communication. 
     The ring apparatus  1   a  shuts down all the ports A 1  to A 4 . This suppresses a defect from affecting the access apparatus  92  and the other ring apparatuses  1   b  to  1   d  when, for example, an abnormal main signal Sg or control signal Sc generated due to the defect is transmitted. 
     The ERP control section  103  controls the communication of the main signal Sg through the ring line Lr in accordance with the results of detection by the fault detection sections  106  and  107  and the defect detection section  101 . The ERP control section  103  switches the path R of the main signal Sg in accordance with the detection of a fault in the ring line Lr by the fault detection sections  106  and  107 . The ERP control section  103  sets the blocking point BP in accordance with the switched path R. Therefore, the communication within the ring line Lr is maintained even if a fault occurs in the ring line Lr between the ring apparatuses  1   a  and  1   b  or between the ring apparatuses  1   a  and  1   c.    
     If there is no fault in at least either one of the ring line Lr between the ring apparatus  1   a  and the ring apparatus  1   b  and the ring line Lr between the ring apparatus  1   a  and the communication apparatus  1   d  in a case where neither a fault in the access line La nor a defect in the ring apparatus  1   a  is detected, the MCLAG control section  102  maintains the access line Lc as an active line. 
     The main signal Sg does not flow through the control line Lf. Therefore, even while the control line Lf is faulty, the ring apparatus  1   a  is able to maintain the communication of the main signal Sg from at least either one of the ports A 1  and A 4  through the ring line Lr as far as no fault occurs in the access line La and no defect occurs in the ring apparatus  1   a . Consequently, the MCLAG control section  102  does not switch the access line La from an active line to a standby line, and thus saves the time required for such switching. 
     Exemplary operations performed in the event of a fault or a defect in the ring apparatus  1   a  will now be described. The following exemplary operations are described on the assumption that the initial state of the communication system is as illustrated in  FIG. 1 . 
     (In the Event of a Fault in the Control Line Lf) 
       FIG. 3  is a diagram illustrating an exemplary operation that is performed by the communication system when a fault occurs in the control line Lf. Elements common to  FIGS. 1 and 3  are designated by the same reference symbols and will not be redundantly described. 
     A cross mark (x) indicates an exemplary location of a fault. The same applies to the subsequent exemplary operations. In the present example, a fault has occurred in the control line Lf, but no fault has occurred in the access line La and in the ring line Lr, and no defect has occurred in the ring apparatus  1   a.    
     Accordingly, the MCLAG control section  102  of the ring apparatus  1   a  maintains the access line La as an active line. This saves the time required for switching the access line La to a standby line. Even when a fault occurs in the control line Lf, the ring apparatuses  1   a  and  1   b  stop the transmission and reception of the control signal Sc without setting a path of the control signal Sc for the ring line Lr in such a manner as to bypass the fault. This suppresses the fault in the control line Lf from affecting the main signal Sg in the ring line Lr. 
     (In the Event of a Fault in the Ring Line Lr) 
       FIG. 4  is a diagram illustrating an exemplary operation that is performed by the communication system when a fault occurs in the ring line Lr. Elements common to  FIGS. 1 and 4  are designated by the same reference symbols and will not be redundantly described. 
     In the present example, a fault has occurred in the ring line Lr between the ports A 1  and B 1 , but no fault has occurred in the access line La and in the ring line Lr between the ports A 4  and D 4 , and no defect has occurred in the ring apparatus  1   a . Accordingly, the MCLAG control section  102  of the ring apparatus  1   a  maintains the access line La as an active line. 
     When the fault detection section  106  detects a fault in the above-mentioned section of the ring line Lr, the ERP control section  103  of the ring apparatuses  1   a  and  1   b  notifies each of the other ring apparatuses  1   d  and  1   c  of the detected fault through the ports A 4  and B 4 . The ERP control section  103  of the respective ring apparatuses  1   a  to  1   d  switches the path R so as to bypass the fault. In this instance, the ERP control section  103  of the respective ring apparatuses  1   a  to  1   d  sets the blocking point BP In a section where the fault of the ring line Lr has occurred. Therefore, the switched path R runs through the access apparatuses  92  and  93  and the ring apparatuses  1   a  and  1   c.    
     As described above, since no fault has occurred in the ring line Lr between the ports A 4  and D 4 , the ring apparatus  1   a  is able to continuously communicate the main signal Sg by setting the path R running through such a nonfaulty section of the ring line Lr without switching the access line La to a standby line. This suppresses the fault in the ring line Lr from affecting the control line Lf. 
     (In the Event of a Fault in the Access Line La) 
       FIG. 5  is a diagram illustrating an exemplary operation that is performed by the communication system when a fault occurs in the access line La. Elements common to  FIGS. 1 and 5  are designated by the same reference symbols and will not be redundantly described. 
     In the present example, a fault has occurred in the access line La toward the ring apparatus  1   a , but no fault has occurred in the control line Lf, and no defect has occurred in the ring apparatus  1   a.    
     Accordingly, the MCLAG control section  102  of the ring apparatus  1   a  switches the access line La from an active line to a standby line, and transmits a fault notification concerning the fault in the access line La to the redundant partner ring apparatus  1   b  through the control line Lf. In response to the fault notification, the MCLAG control section  102  of the ring apparatus  1   b  switches the access line Lb from a standby line to an active line. 
     The ERP control section  103  of the ring apparatuses  1   a  and  1   b  coordinates with the MCLAG control section  102  to switch the path R of the main signal Sg in accordance with the redundant system switching of the access lines La and Lb. Consequently, the communication of the main signal Sg is maintained even after the occurrence of the fault in the access line La. 
     (In the Event of a Fault in the Access Line La and in the Control Line Lf) 
       FIG. 6  is a diagram illustrating an exemplary operation that is performed by the communication system when a fault occurs in the access line La and in the control line Lf. Elements common to  FIGS. 1 and 6  are designated by the same reference symbols and will not be redundantly described. 
     The present example deals with a case where a fault has occurred in the access line La in a state Illustrated in  FIG. 3 . In accordance with the detection of a fault in the access line La, the MCLAG control section  102  of the ring apparatus  1   a  switches the access line La from an active line to a standby line. However, due to a fault in the control line Lf, the ring apparatus  1   a  may not be able to transmit, to the redundant partner ring apparatus  1   b , the control signal Sc including a fault notification concerning the fault in the access line La. 
     Accordingly, while the fault in the control line Lf is detected, the state control section  100  of the ring apparatus  1   a  shuts down the port A 1  in accordance with the detection of the fault in the access line La. 
     When no fault is detected in the access line Lb by the fault detection section  105  while the fault in the control line Lf is detected, the MCLAG control section  102  of the redundant partner ring apparatus  1   b  switches the access line Lb from a standby line to an active line in accordance with the detection of the shutdown of the port A 1 . For example, the MCLAG control section  102  switches the access line Lb from a standby line to an active line when the ring apparatus  1   b  is not the cause of the shutdown. 
     Accordingly, even when the transmission of the control signal Sc to the redundant partner ring apparatus  1   b  is unachievable, the ring apparatus  1   a  is able to issue a fault notification due to the shutdown of the port A 1 . This enables the ring apparatus  1   b  to maintain communication. In the present example, the ERP control section  103  of each of the ring apparatuses  1   a  and  1   b  switches the blocking point BP and the path R of the main signal Sg, as is the case with the exemplary operation illustrated in  FIG. 5 . 
     As regards the ring apparatuses  1   a  and  1   b , the active system and the standby system interchange due to the above operation. This suppresses both of the ring apparatuses  1   a  and  1   b  from becoming active. Consequently, this suppresses the occurrence of a loop in the main signal Sg transmitted through the access apparatus  92  and the ring apparatuses  1   a  and  1   b.    
     (In the Event of a Fault in the Control Line Lf and a Defect in the Ring Apparatus  1   a ) 
       FIG. 7  is a diagram illustrating an exemplary operation that is performed by the communication system when a fault occurs in the control line Lf and a defect occurs in the ring apparatus  1   a . Elements common to  FIGS. 1 and 7  are designated by the same reference symbols and will not be redundantly described. 
     The present example deals with a case where a defect has occurred in the ring apparatus  1   a  in a state Illustrated in  FIG. 3 . Due to a local defect in addition to a fault in the control line Lf, the ring apparatus  1   a  may not be able to notify the redundant partner ring apparatus  1   b  of the defect. 
     The state control section  100  of the ring apparatus  1   a  shuts down the ports A 1  to A 4  in accordance with the detection of the defect. The redundant partner ring apparatus  1   b  switches the access line Lb from a standby line to an active line in accordance with the detection of the shutdown of the port A 1 , as is the case with the foregoing exemplary operation, and is thus able to maintain communication. 
     The ring apparatus  1   a  shuts down all the ports A 1  to A 4 . Therefore, the transmission of an abnormal main signal Sg and control signal Sc, which may be generated due to a defect, is suppressed. This suppresses a defect in the ring apparatus  1   a  from affecting the other ring apparatuses  1   b  to  1   d.    
     (Processing in the CPU  10 ) 
     The processing performed by the CPU  10  will now be described. 
       FIG. 8  is a state transition diagram illustrating exemplary states of the ring apparatuses  1   a  to  1   d  managed by the state control section  100 . Defined states of the ring apparatuses  1   a  to  1   d  are, for example, a normal state, an asynchronous state, an access line fault state, and a defect state. Arrows connecting the individual states are marked with events indicative of transition conditions. 
     The normal state is a regular state where neither a fault nor a defect has occurred in the ring apparatuses  1   a  to  1   d . The asynchronous state is a state where the control signal Sc is not synchronized between the ring apparatuses  1   a  and  1   b  or between the ring apparatuses  1   c  and  1   d  due to a fault in the control lines Lf and Le. 
     The access line fault state is a state where the access lines La to Ld are faulty. The defect state is a state where the ring apparatuses  1   a  to  1   d  are defective. 
     In the normal state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into the asynchronous state in accordance with the occurrence of a fault in the control lines Lf and Le (see “CONTROL LINE FAULT OCCURRENCE”). Meanwhile, in the asynchronous state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into the regular state in accordance with the fault recovery of the control lines Lf and Le (see “CONTROL LINE FAULT RECOVERY”). 
     In the normal state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into the access line fault state in accordance with the occurrence of a fault in the access lines La to Ld (see “ACCESS LINE FAULT OCCURRENCE”). Meanwhile, in the access line fault state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into the regular state in accordance with the fault recovery of the access lines La to Ld (see “ACCESS LINE FAULT RECOVERY”). 
     In the asynchronous state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into the access line fault state in accordance with the occurrence of a fault in the access lines La to Ld (see “ACCESS LINE FAULT OCCURRENCE”). Meanwhile, in the access line fault state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into the asynchronous state in accordance with the occurrence of a fault in the control lines Lf and Le (see “CONTROL LINE FAULT OCCURRENCE”) and with the fault recovery of the access lines La to Ld (see “ACCESS LINE FAULT RECOVERY”). 
     In the normal state, in the access line fault state, and in the asynchronous state, the state control section  100  transitions the ring apparatuses  1   a  to  1   d  into an apparatus defect state in accordance with the detection of a defect in the ring apparatuses  1   a  to  1   d . The ring apparatuses  1   a  to  1   d  do not achieve recovery from a defect. Therefore, when the ring apparatuses  1   a  to  1   d  transition into the apparatus defect state, the state control section  100  stops controlling state transitions. In this instance, defective ring apparatuses  1   a  to  1   d  are replaced by new ring apparatuses. 
       FIG. 9  is a flowchart illustrating an example of a state control process performed on the ring apparatuses  1   a  to  1   d . The ERP control section  103  and the MCLAG control section  102  start conducting the main signal Sg and the control signal Sc, respectively (step St 1 ). In this instance, the ERP control section  103  and the MCLAG control section  102  perform various setups, for example, on the ports A 1  to A 4  and the switch device  14 . 
     Next, the ERP control section  103  causes the fault detection sections  106  and  107  to determine whether or not a fault is detected in the ring line Lr (step St 2 ). If no fault is detected in the ring line Lr (“NO” at step St 2 ), later-described steps St 5  and beyond are performed. 
     If a fault is detected in the ring line Lr (“YES” at step St 2 ), the ERP control section  103  changes the blocking point BP in the ring line Lr (step St 3 ). In this instance, the ERP control section  103  exercises control to place the ports A 1  to A 4 , B 1  to B 4 , C 1  to C 4 , and D 1  to D 4  corresponding to the blocking point BP in a non-communicative state. 
     Next, the ERP control section  103  switches the path R of the main signal Sg in the ring line Lr (step St 4 ). In this instance, the ERP control section  103  initializes the address learning table  151  and then causes the switch device  14  to learn addresses. 
     Next, the state control section  100  checks the results of fault and defect detection by the fault detection sections  104  and  105  and the defect detection section  101  in order to determine whether or not an event indicative of a state transition condition has occurred (step St 5 ). If the event has not occurred (“NO” at step St 5 ), steps St 2  and beyond are performed again. 
     If the event has occurred (“YES” at step St 5 ), the state control section  100  transitions the states of the ring apparatuses  1   a  to  1   d  in accordance with the event as illustrated in  FIG. 8  (step St 6 ). Next, the state control section  100  determines whether or not the ring apparatuses  1   a  to  1   d  are in the defect state (step St 7 ). 
     If the ring apparatuses  1   a  to  1   d  are not in the defect state (“NO” at step St 7 ), steps St 2  and beyond are performed again. If the ring apparatuses  1   a  to  1   d  are in the defect state (“YES” at step St 7 ), the state control process terminates. The state control process is performed in the above-described manner. 
       FIG. 10  is a flowchart illustrating an example of a process that is performed in accordance with the states of the ring apparatuses  1   a  and  1   c  of the active system. The process is repeatedly performed in parallel with the state control process performed on the ring apparatuses  1   a  and  1   c  of the active system. 
     The state control section  100  determines whether or not the ring apparatuses  1   a  and  1   c  are in the defect state (step St 11 ). If the ring apparatuses  1   a  and  1   c  are in the defect state (“YES” at step St 11 ), the MCLAG control section  102  switches the access lines La and Lc from an active line to a standby line in accordance with an Instruction from the state control section  100  (step St 12 ). Next, the state control section  100  shuts down all the ports A 1  to A 4  and C 1  to C 4  (step St 13 ). 
     As described above, the MCLAG control section  102  switches the access lines La and Lc from an active line to a standby line in accordance with the detection of a defect, and the state control section  100  shuts down the ports A 1  to A 4  and C 1  to C 4  in accordance with the detection of a defect. This permits the ring apparatuses  1   a  and  1   b  to switch the access lines Lb and Ld of the redundant partner ring apparatuses  1   b  and  1   d  from a standby line to an active line, and suppresses the defect from affecting the other ring apparatuses  1   a  to  1   d  and access apparatuses  92  and  93 . 
     Meanwhile, if the ring apparatuses  1   a  and  1   c  are not in the defect state (“NO” at step St 11 ), the state control section  100  determines whether or not the ring apparatuses  1   a  and  1   c  are in the asynchronous state (step St 14 ). If the ring apparatuses  1   a  and  1   c  are in the asynchronous state (“YES” at step St 14 ), the state control section  100  causes the fault detection section  105  to determine whether or not a fault is detected in the access lines La and Lc (step St 15 ). 
     If a fault is detected in the access lines La and Lc (“YES” at step St 15 ), the MCLAG control section  102  switches the access lines La and Lc from an active line to a standby line in accordance with an instruction from the state control section  100  (step St 16 ). Next, the state control section  100  shuts down the ports A 1  and C 1  of the ring line Lr (step St 17 ). Meanwhile, if no fault is detected in the access lines La and Lc (“NO” at step St 15 ), the process terminates. 
     As described above, the MCLAG control section  102  switches the access lines La and Lc from an active line to a standby line in accordance with the detection of a fault in the access lines La and Lc, and the state control section  100  shuts down the port A 1  in accordance with the detection of a fault in the access lines La and Lc while a fault in the control line Lf is detected. This permits the ring apparatuses  1   a  and  1   b  to switch the access lines Lb and Ld of the redundant partner ring apparatuses  1   b  and  1   d  from a standby line to an active line. 
     Meanwhile, if the ring apparatuses  1   a  and  1   c  are not in the asynchronous state (“NO” at step St 14 ), the state control section  100  determines whether or not the ring apparatuses  1   a  and  1   c  are in the regular state (step St 18 ). If the ring apparatuses  1   a  and  1   c  are not in the regular state (“NO” at step St 18 ), the process terminates. If the ring apparatuses  1   a  and  1   c  are in the regular state (“YES” at step St 18 ), the state control section  100  causes the fault detection section  105  to determine whether or not a fault is detected in the access lines La and Lc (step St 19 ). 
     If a fault is detected in the access lines La and Lc (“YES” at step St 19 ), the MCLAG control section  102  switches the access lines La and Lc from an active line to a standby line in accordance with an instruction from the state control section  100  (step St 20 ). Next, the state control section  100  transmits a fault notification concerning a fault in the access lines La and Lc to the redundant partner ring apparatuses  1   b  and  1   d  (step St 21 ). Meanwhile, if no fault is detected in the access lines La and Lc (“NO” at step St 19 ), the process terminates. 
     As described above, when no fault is detected in the control lines Lf and Le, the MCLAG control section  102  transmits a fault notification to the ring apparatuses  1   b  and  1   d  of the standby system through the control lines Lf and Le in accordance with the detection of a fault in the access lines La and Lc. This permits the ring apparatuses  1   a  and  1   b  to switch the access lines Lb and Ld of the redundant partner ring apparatuses  1   b  and  1   d  from a standby line to an active line. In the above-described manner, the ring apparatuses  1   a  and  1   c  of the active system perform the process in accordance with the current state. 
       FIG. 11  is a flowchart illustrating an example of a process that is performed in accordance with the state of the ring apparatuses  1   b  and  1   d  of the standby system. The process is repeatedly performed in parallel with the state control process performed on the ring apparatuses  1   b  and  1   d  of the standby system. 
     The state control section  100  determines whether or not the ring apparatuses  1   b  and  1   d  are in the asynchronous state (step St 31 ). If the ring apparatuses  1   b  and  1   d  are in the asynchronous state (“YES” at step St 31 ), the state control section  100  determines whether or not the shutdown of the ports A 1  and C 1  of the redundant partner ring apparatuses  1   a  and  1   c  is detected by the monitoring circuit  141  (step St 32 ). The monitoring circuit  141  detects the shutdown when, for example, an optical input from the ports A 1  and C 1  is discontinued. 
     If the shutdown is not detected (“NO” at step St 32 ), the process terminates. Meanwhile, if the shutdown is detected (“YES” at step St 32 ), the state control section  100  determines whether or not the monitoring circuits  141  to  144  have outputted an alarm that may cause the shutdown (step St 33 ). If the alarm is outputted (“YES” at step St 33 ), the process terminates. 
     If the alarm that may cause the shutdown is not outputted (“NO” at step St 33 ), the MCLAG control section  102  determines whether or not a fault is detected in the access lines Lb and Ld by the fault detection section  105  (step St 34 ). If a fault is detected in the access lines Lb and Ld (“YES” at step St 34 ), the process terminates. 
     If no fault is detected in the access lines Lb and Ld (“NO” at step St 34 ), the MCLAG control section  102  switches the access lines Lb and Ld from a standby line to an active line (step St 35 ). Next, the ERP control section  103  switches the path R of the main signal Sg so that the path R runs through the access lines Lb and Ld (step St 36 ). Upon completion of step St 36 , the process terminates. 
     As described above, when no fault is detected in the access lines La and Lc while a fault in the control lines Lf and Le is detected, the MCLAG control section  102  switches the access lines Lb and Ld from a standby line to an active line in accordance with the detection of the shutdown of the ports A 1  and C 1  of the redundant partner ring apparatuses  1   a  and  1   c . Therefore, the ring apparatuses  1   b  and  1   d  are able to maintain the communication of the main signal Sg. 
     Meanwhile, if the ring apparatuses  1   b  and  1   d  are not in the asynchronous state (“NO” at step St 31 ), the state control section  100  determines whether or not the ring apparatuses  1   b  and  1   d  are in the regular state (step St 37 ). If the ring apparatuses  1   b  and  1   d  are not in the regular state (“NO” at step St 37 ), the process terminates. 
     If the ring apparatuses  1   b  and  1   d  are in the regular state (“YES” at step St 37 ), the MCLAG control section  102  determines whether or not a fault notification concerning a fault in the access lines La and Lc is received from the redundant partner ring apparatuses  1   a  and  1   c  (step St 38 ). If the fault notification is not received (“NO” at step St 38 ), the process terminates. Meanwhile, if the fault notification is received (“YES” at step St 38 ), steps St 35  and St 36  are performed. Upon completion of steps St 35  and St 36 , the process terminates. In the above-described manner, the ring apparatuses  1   b  and  1   d  of the standby system perform the process in accordance with the current state. 
       FIG. 12  is a sequence diagram Illustrating exemplary operations that are performed by redundantly paired ring apparatuses  1   a  and  1   b  when a fault in the access line La is detected while a fault in the control line Lf is detected. The present example deals with the operations performed by the pair of ring apparatuses  1   a  and  1   b . However, the other pair of ring apparatuses  1   c  and  1   d  performs similar operations as the pair of ring apparatuses  1   a  and  1   b . At the beginning of a sequence illustrated in  FIG. 12 , the ring apparatuses  1   a  and  1   b  are both in the normal state, the ring apparatus  1   a  operates as an active system, and the ring apparatus  1   b  operates as a standby system. 
     Upon detecting a fault in the control line Lf (symbol SQ 1   a ), the ring apparatus  1   a  of the active system switches from the normal state to the asynchronous state. Upon detecting a fault in the control line Lf (symbol SQ 1   b ), the ring apparatus  1   b  of the standby system switches from the normal state to the asynchronous state. 
     Subsequently, upon detecting a fault in the access line La in the asynchronous state (symbol SQ 2 ), the ring apparatus  1   a  of the active system shuts down the port A 1  (symbol SQ 3 ). The ring apparatus  1   a  then switches from the active system to the standby system, and transitions into the access line fault state. 
     Upon detecting the shutdown of the port A 1  in the asynchronous state (symbol SQ 4 ), the ring apparatus  1   b  of the standby system switches from the standby system to the active system. As described above, when the port A 1  shuts down in a case where a fault in the access line La is detected while a fault in the control line Lf is detected, the ring apparatuses  1   a  and  1   b  switch between the active system and the standby system. 
       FIG. 13  is a sequence diagram illustrating exemplary operations that are performed by redundantly paired ring apparatuses  1   a  and  1   b  when a defect is detected while a fault in the control line Lf is detected. Operations common to  FIGS. 12 and 13  are designated by the same reference symbols and will not be redundantly described. 
     The present example deals with the operations performed by the pair of ring apparatuses  1   a  and  1   b . However, the other pair of ring apparatuses  1   c  and  1   d  performs similar operations as the pair of ring apparatuses  1   a  and  1   b . At the beginning of a sequence Illustrated in  FIG. 13 , the ring apparatuses  1   a  and  1   b  are both in the normal state, the ring apparatus  1   a  operates as an active system, and the ring apparatus  1   b  operates as a standby system. 
     Upon detecting a defect (symbol SQ 5 ), the ring apparatus  1   a  of the active system switches to the ring apparatus of the standby system and transitions into the defect state. The ring apparatus  1   a  then shuts down the ports A 1  to A 4  (symbol SQ 6 ). 
     Upon detecting the shutdown of the port A 1  in the asynchronous state (symbol SQ 7 ), the ring apparatus  1   b  of the standby system switches from the standby system to the active system. As described above, when the port A 1  shuts down in a case where a fault is detected while a fault in the control line Lf is detected, the ring apparatuses  1   a  and  1   b  switch between the active system and the standby system. 
     (Communication System in which the Ring Line Lr Between the Ring Apparatuses  1   a  and  1   b  is Duplexed) 
     In the above-described communication system, the ports A 1  and B 1  of the ring apparatuses  1   a  and  1   b  are coupled by the control line Lf through which only the control signal Sc flows. However, the communication system is not limited to such a configuration. For example, the ports A 1  and B 1  of the ring apparatuses  1   a  and  1   b  may be coupled by two ring lines through which both the main signal Sg and the control signal Sc flow. 
       FIG. 14  is a configuration diagram illustrating an example of the communication system in which the ring line Lr between the ring apparatuses  1   a  and  1   b  is duplexed. Elements common to  FIGS. 1 and 14  are designated by the same reference symbols and will not be redundantly described. The present example deals with a configuration where only ring lines Lf 1  and Lf 2  between one pair of ring apparatuses  1   a  and  1   b  are duplexed. However, the configuration is not limited to such duplexing. The other pair of ring apparatuses  1   c  and  1   d  may also be duplexed in similar manner. 
     The ring apparatus  1   a  includes ports A 1   x , A 2 , A 3   x , and A 4 , and the ring apparatus  1   b  includes ports B 1   x , B 2 , B 3   x , and B 4 . The ports A 3   x  and B 3   x  are intercoupled so as to oppose each other through the ring line Lf 1 . The ports A 1   x  and B 1   x  are intercoupled so as to oppose each other through the ring line Lf 2 . The ring line Lf 1  is an example of a control line. 
     The main signal Sg and the control signal Sc both flow through the ring lines Lf 1  and Lf 2  while they are not faulty. The ports A 3   x  and B 3   x  mutually transmit and receive the main signal Sg and the control signal Sc through the ring line Lf 1 . The ports A 1   x  and B 1   x  mutually transmit and receive the main signal Sg and the control signal Sc through the ring line Lf 2 . 
     Consequently, the path R of the main signal Sg between the ring apparatuses  1   a  and  1   b  runs through the two ring lines Lf 1  and Lf 2 . The ports A 3   x  and B 3   x  and the ports A 1   x  and B 1   x  are synchronized respectively by the control signal Sc transmitted and received through the ring lines Lf 1  and Lf 2 . 
       FIG. 15  is a diagram illustrating an exemplary operation that is performed by the communication system when a fault occurs in one ring line Lf 1 . Elements common to  FIGS. 14 and 15  are designated by the same reference symbols and will not be redundantly described. 
     Upon detecting a fault in one ring line Lf 1 , the ring apparatuses  1   a  and  1   b  stops the transmission and reception of the control signal Sc through the other ring line Lf 2 . Therefore, the bandwidth of the main signal Sg flowing through the ring line Lf 2  may be increased by an amount equivalent to the bandwidth of the control signal Sc. 
     Although not illustrated, when a fault in the ring line Lf 2  is detected conversely to the present example, the transmission and reception of the control signal Sc through the ring line Lf 1  comes to a stop. Therefore, the bandwidth of the main signal Sg flowing through the ring line Lf 1  may be increased by an amount equivalent to the bandwidth of the control signal Sc. 
     When a fault is detected in only one of the ring lines Lf 1  and Lf 2 , the ring apparatuses  1   a  and  1   b  maintain the access lines La and Lb as an active line and a standby line, respectively. This eliminates the necessity of performing a switching process on the access lines La and Lb. 
       FIG. 16  is a configuration diagram illustrating another example of the ring apparatus  1   a . Elements common to  FIGS. 2 and 16  are designated by the same reference symbols and will not be redundantly described. The ring apparatus  1   b  includes the ports B 1   x , B 2 , B 3   x , and B 4  instead of the ports A 1   x , A 2 , A 3   x , and A 4 . However, the other elements of the ring apparatus  1   b  are similar to those of the ring apparatus  1   a.    
     The port A 1   x  transmits and receives the main signal Sg and the control signal Sc to and from the ring apparatus  1   b  through the ring line Lf 2 . The port A 3   x  transmits and receives the main signal Sg and the control signal Sc to and from the ring apparatus  1   b  through the ring line Lf 1 . 
     Upon reading a program from the ROM  11 , the CPU  10  forms, as functions, fault detection sections  105   a  and  106   a  in place of the fault detection sections  105  and  106 , and a state control section  100   a , an MCLAG control section  102   a , and an ERP control section  103   a  in place of the state control section  100 , the MCLAG control section  102 , and the ERP control section  103 , respectively. The fault detection section  105   a  detects a fault in the ring line Lf 1 , and the fault detection section  106   a  detects a fault in the ring line Lf 2 . 
     The ERP control section  103   a  controls the communication of the main signal Sg through the ring lines Lf 1  and Lf 2  in accordance with the results of detection by the fault detection sections  105   a  and  106   a  and the defect detection section  101 . The ERP control section  103   a  outputs the results of detection by the fault detection sections  105   a  and  106   a  to the MCLAG control section  102   a.    
     The MCLAG control section  102   a  controls the communication of the control signal Sc in accordance with the results of detection by the fault detection sections  104 ,  105   a , and  106   a . As is the case with the MCLAG control section  102  in the foregoing example, the MCLAG control section  102   a  switches the access lines La and Lb to an active line or a standby line. 
     The state control section  100   a  is an example of the control section. In addition to the function of the state control section  100  in the foregoing example, the state control section  100   a  coordinates with the MCLAG control section  102   a  and the ERP control section  103   a  to control the bandwidths of the main signal Sg and control signal Sc, which are to be transmitted and received through the ports A 1   x  and A 3   x . The state control section  100   a  causes the port A 3   x  to stop the transmission and reception of the control signal Sc through the ring line Lf 1  in accordance with the detection of a fault in the ring line Lf 2  by the fault detection section  106   a , and causes the port A 1   x  to stop the transmission and reception of the control signal Sc through the ring line Lf 2  in accordance with the detection of a fault in the ring line Lf 1  by the fault detection section  105   a . In this instance, the state control section  100   a  instructs, for example, the switch device  14  and the MCLAG control section  102   a  to stop the transfer and generation of the control signal Sc. 
     In accordance with the detection of a fault in the ring line Lf 2  by the fault detection section  106   a , the state control section  100   a  increases the bandwidth of the main signal Sg to be transmitted and received by the port A 3   x  through the ring line Lf 1 . In accordance with the detection of a fault in the ring line Lf 1  by the fault detection section  105   a , the state control section  100   a  increases the bandwidth of the main signal Sg to be transmitted and received by the port A 3   x  through the ring line Lf 1 . The state control section  100   a  instructs, for example, the switch device  14  and the ERP control section  103   a  to increase the transfer bandwidth of the main signal Sg. 
     As described above, when a fault is detected in one of the ring lines Lf 1  and Lf 2 , the state control section  100   a  is able to increase the bandwidth allocatable to the main signal Sg by stopping the transmission and reception of the control signal Sc through the other ring line Lf 1  or Lf 2 . This suppresses a decrease in the main signal Sg between the ring apparatuses  1   a  and  1   b.    
       FIG. 17  is a flowchart illustrating an example of a control process performed on the main signal Sg and the control signal Sc. The state control section  100   a  repeatedly performs the control process in parallel with the processes illustrated in  FIGS. 9 to 11 . 
     The state control section  100   a  determines whether or not a fault is detected in either the ring line Lf 1  or the ring line Lf 2  (step St 41 ). If neither the ring line Lf 1  nor the ring line Lf 2  is detected to be faulty (“NO” at step St 41 ), the state control section  100   a  terminates the control process. 
     Meanwhile, if either the ring line Lf 1  or the ring line Lf 2  is detected to be faulty (“YES” at step St 41 ), the state control section  100   a  causes the associated port A 1   x  or A 3   x  to stop the transmission and reception of the control signal Sc through the ring line Lf 1  or Lf 2  that is not detected to be faulty (step St 42 ). Next, the state control section  100   a  increases the bandwidth of the main signal Sg transmitted and received through the ring line Lf 1  or Lf 2  that is not detected to be faulty (step St 43 ), and then terminates the control process. In the above-described manner, the control process is performed on the main signal Sg and the control signal Sc. 
       FIG. 18  is a state transition diagram illustrating exemplary states of the ring apparatuses  1   a  and  1   b  managed by the state control section  100   a . State transitions common to  FIGS. 8 and 18  will not be redundantly described. 
     In contrast to the state control section  100  in the foregoing example, the state control section  100   a  transitions the ring lines Lf 1  and Lf 2  from the normal state to the asynchronous state in accordance with the occurrence of a fault in either the ring line Lf 1  or the ring line Lf 2  (see “RING LINE FAULT OCCURRENCE”). In accordance with the fault recovery of either the ring line Lf 1  or the ring line Lf 2  (see “RING LINE FAULT RECOVERY”), the state control section  100   a  transitions the ring lines Lf 1  and Lf 2  from the asynchronous state to the normal state. 
     In the access line fault state, the state control section  100   a  transitions the ring apparatuses  1   a  and  1   b  into the asynchronous state in accordance with the occurrence of a fault in either the ring line Lf 1  or the ring line Lf 2  (see “RING LINE FAULT OCCURRENCE”) and with the fault recovery of the access lines La to Ld (see “ACCESS LINE FAULT RECOVERY”). 
     As described above, the state control section  100   a  differs from the state control section  100  in the foregoing example in conditions for transitioning from the normal state or access line fault state to the asynchronous state and in conditions for transitioning from the asynchronous state to the normal state. However, the state control section  100   a  is similar to the state control section  100  in the control process performed in each state. 
     For example, in the asynchronous state, the state control section  100   a  shuts down the port A 1   x  or the port A 3   x , whichever is not detected to be faulty, in accordance with the detection of a fault in the access line La, as is the case with the operation illustrated in  FIG. 6 . Therefore, as is the case with the foregoing example, the ring apparatus  1   a  is able to permit the redundant partner ring apparatus  1   b  to maintain communication. 
     In the present example, the duplexed ring lines Lf 1  and Lf 2  are provided between the ring apparatuses  1   a  and  1   b . However, the configuration is not limited to such duplexing. For example, a part of the bandwidth of the control line Lf in the foregoing example may be used to conduct the main signal Sg. In such an instance, the MCLAG control section  102  may coordinate with the ERP control section  103  to allocate the bandwidth of the main signal Sg. 
     The above-described embodiment is a preferred embodiment of the present technology. However, the present technology is not limited to the above-described embodiment. It may be understood that various modifications may be made without departing from the spirit of the present technology. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations 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 one or more embodiments of the present invention 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.