Abstract:
To detect a failure in a link or a failure in a network due to a frame loss between apparatuses, and degenerate a band in communication in which a frame is divided and divided frames are transmitted. A communication apparatus ( 100 ) includes a transmitter-receiver ( 101 ) configured to receive divided frames transmitted through a plurality of paths, the divided frame being obtained by dividing a frame, and a coupler ( 103 ) configured to couple the received divided frames, in which when the coupling of the received divided frames has failed in the coupler, the transmitter-receiver ( 101 ) transmits a signal-faulty signal indicating a transmission fault. When the communication apparatus ( 100 ) has failed in coupling frames, it can detect a failure in a link or a failure in a network due to a frame loss between apparatuses by transmitting a signal-faulty signal indicating a transmission fault.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a communication apparatus, a communication system, and a communication method. In particular, the present disclosure relates to a communication apparatus, a communication system, and a communication method for performing communication by using a link aggregation function. 
       BACKGROUND ART 
       [0002]    In recent years, as a technique for expanding a communication band, there is a link aggregation in which a plurality of physical links are bundled and used as one logical link. 
         [0003]    In the link aggregation, since the band width of the logical link is equal to the total of the band widths of individual links, the band width can be expanded. Further, in the link aggregation, even if a failure occurs in one of the links, communication is continued by using the remaining links, i.e., a degeneracy process is performed. Therefore, redundancy is ensured. 
         [0004]    For example, Patent Literatures 1 to 3 disclose a technique for improving redundancy in a link aggregation or a technique for coping with a failure. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2008-160227 
         [0006]    Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2005-347943 
         [0007]    Patent Literature 3: Japanese Unexamined Patent Application 
         [0008]    Publication No. 2006-245849 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    However, a related-art communication apparatus detects a failure between apparatuses on both sides of a port for which a link aggregation is implemented. Therefore, there is a problem that when a link aggregation is implemented by dividing a frame and transmitting divided frame in communication that is performed through a plurality of links, it is impossible to detect a failure that occurs in one of links that are adjacent to neither the communication source nor the communication entity on the other side. 
       Solution to Problem 
       [0010]    A communication apparatus according to the present disclosure includes: a transmitter-receiver configured to receive divided frames transmitted through a plurality of paths, the divided frame being obtained by dividing a frame; and a coupler configured to couple the divided frames, in which when the coupling of the divided frames has failed in the coupler, the transmitter-receiver transmits a signal-faulty signal indicating a transmission fault. 
         [0011]    A communication system according to the present disclosure is a communication system configured to perform communication through a plurality of paths, the communication system including: a first communication apparatus configured to divide a frame and transmit divided frames through the plurality of paths; and a second communication apparatus configured to receive the divided frames through the plurality of paths and couple the received frames, in which when the coupling of the frames has failed, the second communication apparatus transmits a signal-faulty signal indicating a transmission fault to the first communication apparatus. 
         [0012]    A communication method according to the present disclosure is a communication method in which first and second communication apparatuses perform communication through a plurality of paths, the communication method including: a divided frame transmission step of, in the first communication apparatus, dividing a frame and transmitting divided frames through respective paths; a coupling step of, in the second communication apparatus, coupling frames received through the plurality of paths; and an SF signal transmission step of, in the second communication apparatus, transmitting a signal-faulty signal indicating a transmission fault to the first communication apparatus when the coupling of the frames has failed. 
       Advantageous Effects of Invention 
       [0013]    A communication apparatus according to the present disclosure can detect a failure in a link or a failure in a network due to a frame loss between apparatuses, and degenerate a band in communication in which a frame is divided and divided frames are transmitted. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a block diagram showing a configuration of a communication apparatus according to an embodiment; 
           [0015]      FIG. 2  is a flowchart sowing an operation of the communication apparatus according to the embodiment; 
           [0016]      FIG. 3  is a flowchart sowing an operation of the communication apparatus according to the embodiment; 
           [0017]      FIG. 4  is a block diagram sowing a configuration of the communication apparatus according to the embodiment; 
           [0018]      FIG. 5  is a flowchart sowing an operation of the communication apparatus according to the embodiment; 
           [0019]      FIG. 6  shows an outline of a communication system using communication apparatuses according to the embodiment; 
           [0020]      FIG. 7  shows an outline of a communication system using the communication apparatus according to the embodiment; 
           [0021]      FIG. 8  is a schematic diagram showing an example of transmission and reception of a signal in the communication system according to the embodiment; 
           [0022]      FIG. 9  is a schematic diagram showing an example of transmission and reception of a signal in the communication system according to the embodiment; 
           [0023]      FIG. 10  is a schematic diagram showing an example of transmission and reception of a signal in the communication system according to the embodiment; 
           [0024]      FIG. 11  is a schematic diagram showing an example of transmission and reception of a signal in the communication system according to the embodiment; 
           [0025]      FIG. 12  is a schematic diagram showing an example of transmission and reception of a signal in the communication system according to the embodiment; 
           [0026]      FIG. 13  is a schematic diagram showing an example of transmission and reception of a signal in the communication system according to the embodiment; and 
           [0027]      FIG. 14  is a block diagram showing a configuration of a communication apparatus according to the embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments 
       [0028]    Embodiments according to the present disclosure are explained hereinafter with reference to the drawings.  FIG. 1  is a block diagram showing a configuration of a communication apparatus according to this embodiment. In  FIG. 1 , a communication apparatus  100  includes an interface  101 , a CH information checker  102 , a coupler  103 , a communication device  104 , an interface  105 , a divider  106 , a buffer  107 , a controller  108 , and a control header generator  109 . 
         [0029]    Further, the interface  101  includes interfaces  101 - 1  to  101 - 3  which perform communication on a port-by-port basis. The following explanation is given on the assumption that: the interface  101 - 1  corresponds to a port A; the interface  101 - 2  corresponds to a port B; and the interface  101 - 3  corresponds to a port C. The above-described configuration is preferably implemented by hardware such as a logic circuit. 
         [0030]    The interfaces  101 - 1  to  101 - 3  receive divided frames transmitted from a counterpart communication apparatus and output them to the CH information checker  102 . These divided frames are obtained by dividing a frame. Further, the interfaces  101 - 1  to  101 - 3  transmit divided frames to the counterpart communication apparatus. 
         [0031]    For example, the interfaces  101 - 1  to  101 - 3  are interfaces in a layer 1. Specifically, for the interfaces  101 - 1  to  101 - 3 , a wired communication apparatus such as an Ethernet (Registered Trademark) or a wireless communication apparatus using FDD (Frequency Division Duplex) or the like is preferred. 
         [0032]    The CH information checker  102  checks a content of CH information (Control Header Information) of the received divided frame and detects whether or not control information is included in the CH information of the received divided frame. Then, the CH information checker  102  outputs information on a port corresponding to the received divided frame and the control information to the controller  108 . Specifically, the control information preferably includes information as to whether frames to be transmitted should be coupled or should not be coupled. 
         [0033]    Further, the CH information checker  102  extracts a sequence number included in the CH information of the received divided frame and outputs the extracted sequence number together with the divided frame to the coupler  103 . This sequence number indicates an order according to which the divided frames are coupled. 
         [0034]    The coupler  103  couples the divided frames according to the order of the sequence number and outputs the coupled frame to the communication device  104 . Further, when the coupler  103  has failed in coupling the divided frames, it outputs information that the coupling has failed to the controller  108 . For example, when some number is skipped in the order of the sequence number, the coupler  103  outputs information that the coupling of frames has failed and the skipped sequence number (or information on a corresponding port) to the controller  108 . 
         [0035]    The communication device  104  performs a predetermined communication process for the coupled frame and outputs the processed frame to the interface  105 . Further, the communication device  104  performs a predetermined communication process for a frame output from the interface  105  and outputs the processed frame to the divider  106 . For example, it is preferably a communication process in a data link layer (Layer 2) or a higher layer, i.e., a layer higher than the physical layer. 
         [0036]    The interface  105  transmits the processed frame to another communication entity with which the communication apparatus  100  communicates through a wire or wirelessly. Further, the interface  105  performs a process for a frame that is transmitted from another communication entity with which the communication apparatus  100  communicates through a wire or wirelessly, and outputs the processed frame to the communication device  104 . 
         [0037]    The divider  106  divides a frame according to an instruction from the controller  108  and outputs the divided frames to the buffer  107 . For example, when the divider  106  is instructed to divide a frame for three ports, i.e., the ports A to C, the divider  106  divides the frame into three divided frames, sorting out the divided frames on a port-by-port basis, and outputs the divided frames to the buffer  107 . 
         [0038]    The buffer  107  sorts out the divided frames on a port-by-port basis, temporarily stores the sorted-out divided frames, and outputs them to the interfaces  101 - 1  to  101 - 3 . For example, the buffer  107  temporarily stores a divided frame that should be transmitted from the port A and outputs that divided frame to the interface  101 - 1 . Similarly, the buffer  107  temporarily stores a divided frame that should be transmitted from the port B and outputs that divided frame to the interface  101 - 2 . Similarly, the buffer  107  temporarily stores a divided frame that should be transmitted from the port C and outputs that divided frame to the interface  101 - 3 . 
         [0039]    Further, when there is no frame that should be output to the interfaces  101 - 1  to  101 - 3  in the buffer  107 , it informs the controller  108  that the buffer  107  is empty. Further, the buffer  107  inserts a header output from the control header generator  109  into the divided frame and outputs the divided frame to the interfaces  101 - 1  to  101 - 3 . 
         [0040]    The control header generator  109  generates a control header according to an instruction from the controller  108  and outputs the generated control header to the buffer  107 . 
         [0041]    When the controller  108  receives network failure information, it degenerates the band. Further, when the controller  108  receives information that the network failure has been solved, it cancels the degeneracy of the band. 
         [0042]    Specifically, when the interfaces  101 - 1  to  101 - 3  cannot perform communication with a counterpart communication apparatus, they inform the controller  108  that a failure has occurred in the link. Then, when the controller  108  is informed that a failure has occurred in the link from one of the interfaces  101 - 1  to  101 - 3 , the controller  108  determines that a failure has occurred in the link in communication by using the corresponding port. 
         [0043]    Further, when the coupler  103  has failed in coupling divided frames, it outputs information that the coupling has failed to the controller  108 . For example, when some number is skipped in the order of the sequence number, the coupler  103  outputs information that the coupling of frames has failed and the skipped sequence number (or a corresponding port) to the controller  108 . Then, the controller  108  determines that a failure has occurred in the link in communication using the port corresponding to the skipped sequence number. 
         [0044]    By the above-described determination, the controller  108  identifies the port of the communication path in which the failure has occurred in the link and transmits divided frames to the entity with which the communication apparatus  100  communicates without using the port in which the failure has occurred. For example, when the port of the communication path in which the failure has occurred in the link is the port A, the controller  108  instructs the divider  106  to divide a frame into two frames for the ports B and C and transmit the divided frames from the ports B and C. Then, the divided frames are transmitted from the interface  101 - 2  corresponding to the port B and the interface  101 - 3  corresponding to the port C. 
         [0045]    Meanwhile, a determination for cancelling degeneracy is performed by reception of divided frames in which control information is inserted on a port-by-port basis. That is, when control information is included in CH information of a received divided frame, it can be determined that communication has been successfully performed for the port through which that divided frame has been received. When the controller  108  has received a control signal through a port in which a failure has occurred in the link and which has been excluded based on the content of CH information from the CH information checker  102 , the controller  108  determines that that port has been restored to a state in which communication can be performed and adds that port as a port through which a divided frame is transmitted. 
         [0046]    For example, in a degenerated state in which a frame is divided into two frames for the ports B and C and transmitted therethrough and the port A is not used, when the controller  108  receives a control signal through the port A, it instructs the divider  106  to divide a frame into three frames for the ports A, B and C and transmitted them therethrough. The divided frames are transmitted from the interface  101 - 1  corresponding to the port A, the interface  101 - 2  corresponding to the port B, and the interface  101 - 3  corresponding to the port C. 
         [0047]    By the above-described configuration, the communication apparatus  100  carries out detection of a failure in the link or a failure in the network due to a frame loss between apparatuses, degeneracy of the band, and cancellation of the degeneracy. Next, an operation of the communication apparatus  100  is explained.  FIG. 2  is a flowchart showing an operation of a communication apparatus according to this embodiment. The flowchart shown in  FIG. 2  includes steps A 01  to A 07 . 
         [0048]    Firstly, in the step A 01 , the interfaces  101 - 1  to  101 - 3  determine whether or not they have been able to receive divided frames from the counterpart communication apparatus within a specific period. Then, when they have been able to receive divided frames in the specific period, the process proceeds to the step A 02 , whereas when they have not been able to receive divided frames in the specific period, the process proceeds to the step A 07 . In the step A 02 , the CH information checker  102  extracts CH information from the divided frame and outputs the extracted CH information to the controller  108 . Then, the process proceeds to the step A 03 . 
         [0049]    In the step A 03 , the coupler  103  couples the divided frames in the order of their sequence numbers and the process proceeds to the step A 04 . 
         [0050]    In the step A 04 , the coupler  103  determines whether the coupling of the divided frames has succeeded or failed. When the coupling of the divided frames has succeeded, the process proceeds to the step A 05 , whereas when the coupling of the divided frames has failed, the process proceeds to the step A 07 . 
         [0051]    In the step A 05 , the coupled frame is processed in the communication device  104  and the interface  105  and the process proceeds to the step A 06 . 
         [0052]    In the step A 06 , when the communication apparatus  100  continues the communication, the process returns to the step A 01 , whereas when the communication apparatus  100  finishes the communication, the process is finished. 
         [0053]    In the step A 07 , the controller  108  excludes the port corresponding to the link in which the failure has occurred in the step A 01 , A 04  or A 04  from the communication, and thereby degenerates the band. Then, the process returns to the step A 01 . 
         [0054]    Through the above-described operation, the communication apparatus  100  degenerates the band for which the link aggregation has been implemented. Next, an operation for cancelling degeneracy is explained.  FIG. 3  is a flowchart showing an operation of a communication apparatus according to this embodiment. The flowchart shown in  FIG. 3  includes steps B 01  to B 06 . 
         [0055]    Firstly, in the step B 01 , the controller  108  determines whether or not the band for which the link aggregation is implemented has been degenerated. Then, when the band has been degenerated, the process proceeds to the step B 02 , whereas when the band has not been degenerated, the process proceeds to the step B 04 . 
         [0056]    In the step B 02 , the CH information checker  102  determines whether or not a frame including control information has been received through a port that has been excluded from the link aggregation due to the degeneracy of the band. Then, when a frame including control information has been received, the process proceeds to the step B 03 , whereas when a frame including control information has not been received, the process proceeds to the step B 04 . 
         [0057]    In the step B 03 , the controller  108  cancels the degeneracy and the process proceeds to the step B 04 . In the step B 04 , the communication apparatus  100  receives divided frames and performs a coupling process. Then, the process proceeds to the step B 05 . 
         [0058]    In the step B 05 , the coupled frame is processed in the communication device  104  and the interface  105  and the process proceeds to the step B 06 . 
         [0059]    In the step B 06 , when the communication apparatus  100  continues the communication, the process returns to the step B 01 , whereas when the communication apparatus  100  finishes the communication, the process is finished. 
         [0060]    By the above-described configuration, the communication apparatus  100  detects a failure in the link or a failure in the network due to a frame loss between apparatuses, degenerates the band for which the link aggregation is implemented, and cancels the degeneracy. 
         [0061]    Next, a communication apparatus that communicates with the communication apparatus  100  is explained.  FIG. 4  is a block diagram showing a configuration of a communication apparatus according to this embodiment. In  FIG. 4 , the same numbers as those in  FIG. 1  are assigned to the same structures as those in  FIG. 1 , and their explanations are omitted. In  FIG. 4 , a communication apparatus  200  includes an interface  101 , a CH information checker  201 , a coupler  103 , a communication device  104 , an interface  105 , a divider  106 , a buffer  107 , a controller  108 , and a control header generator  109 . 
         [0062]    The interfaces  101 - 1  to  101 - 3  receive divided frames transmitted from a counterpart communication apparatus and output them to the CH information checker  201 . 
         [0063]    The CH information checker  201  checks a content of CH information of the received divided frame and detects the presence/absence of information indicating signal failure (SF). Then, the CH information checker  201  outputs information on a port corresponding to the received divided frame and the SF to the controller  108 . 
         [0064]    Similarly, the CH information checker  201  detects whether or not control information is included in the CH information of the received divided frame. Then, the CH information checker  201  outputs information on a port corresponding to the received divided frame and the control information to the controller  108 . Specifically, the control information preferably includes information as to whether frames to be transmitted should be coupled or should not be coupled. 
         [0065]    Further, the CH information checker  201  extracts a sequence number included in the CH information of the received divided frame and outputs the extracted sequence number together with the divided frame to the coupler  103 . This sequence number indicates an order according to which the divided frames are coupled. 
         [0066]    The coupler  103  couples the divided frames according to the order of the sequence number and outputs the coupled frame to the communication device  104 . 
         [0067]    When the controller  108  receives network failure information, it degenerates the band. Further, when the controller  108  receives information that the network failure has been solved, it cancels the degeneracy of the band. 
         [0068]    Specifically, the CH information checker  201  associates an SF included in a header of the divided frame with a port through which that divided frame has been received, and informs the controller  108  of them in the associated state. The controller  108  determines that a failure has occurred in the link in the port through which a divided frame including an SF has not been received. 
         [0069]    By the above-described determination, the controller  108  identifies the port of the communication path in which the failure has occurred in the link and transmits divided frames to the entity with which the communication apparatus  200  communicates without using the port in which the failure has occurred. For example, when the port of the communication path in which the failure has occurred in the link is the port A, the controller  108  instructs the divider  106  to divide a frame into two frames for the ports B and C and transmit the divided frames from the ports B and C. Then, the divided frames are transmitted from the interface  101 - 2  corresponding to the port B and the interface  101 - 3  corresponding to the port C. 
         [0070]    Meanwhile, a determination for cancelling degeneracy is performed by reception of divided frames in which control information is inserted on a port-by-port basis. That is, when control information is included in CH information of a received divided frame, it can be determined that communication has been successfully performed for the port through which that divided frame has been received. When the controller  108  has received a control signal through a port in which a failure has occurred in the link and which has been excluded based on the content of CH information from the CH information checker  201 , the controller  108  determines that that port has been restored to a state in which communication can be performed and adds that port as a port through which a divided frame is transmitted. 
         [0071]    For example, in a degenerated state in which a frame is divided into two frames for the ports B and C and transmitted therethrough and the port A is not used, when the controller  108  receives a control signal through the port A, it instructs the divider  106  to divide a frame into three frames for the ports A, B and C and transmitted them therethrough. The divided frames are transmitted from the interface  101 - 1  corresponding to the port A, the interface  101 - 2  corresponding to the port B, and the interface  101 - 3  corresponding to the port C. 
         [0072]    By the above-described configuration, the communication apparatus  200  carries out degeneracy of the band and cancellation of the degeneracy. Next, an operation of the communication apparatus  200  is explained.  FIG. 5  is a flowchart showing an operation of a communication apparatus according to this embodiment. The flowchart shown in  FIG. 5  includes steps C 01  to C 06 . 
         [0073]    Firstly, in the step C 01 , the CH information checker  201  extracts CH information from the divided frame and outputs the extracted CH information to the controller  108 . Then, the process proceeds to the step C 02 . 
         [0074]    In the step C 02 , the controller  108  determines whether or not there is a port through which a divided frame including an SF has not been received. Then, when there is a port through which a divided frame including an SF has not been received, the process proceeds to the step C 06 , whereas when there is no port through which a divided frame including an SF has not been received, the process proceeds to the step C 03 . Note that the step C 02  is an operation that corresponds to a case where a divided frame including an SF is transmitted from the communication entity with which the communication apparatus  200  communicates as a result of a link failure. 
         [0075]    In the step C 03 , the coupler  103  couples the divided frames in the order of their sequence numbers and the process proceeds to the step C 04 . 
         [0076]    In the step C 04 , the coupled frame is processed in the communication device  104  and the interface  105  and the process proceeds to the step C 05 . 
         [0077]    In the step C 05 , when the communication apparatus  200  continues the communication, the process returns to the step C 01 , whereas when the communication apparatus  200  finishes the communication, the process is finished. 
         [0078]    In the step C 06 , the controller  108  excludes the port corresponding to the link in which the failure has occurred in the step C 01 , C 02  or C 05  from the communication, and thereby degenerates the band. Then, the process returns to the step C 01 . 
         [0079]    Through the above-described operation, the communication apparatus  200  degenerates the band for which the link aggregation has been implemented. Note that the operation for cancelling the degeneracy is similar to the operation performed by the communication apparatus  100  shown in  FIG. 3 . 
         [0080]    Next, a communication system using communication apparatuses  100  and  200 , and degeneracy of a communication band and cancellation of the degeneracy are explained. 
         [0081]      FIG. 6  shows an outline of a communication system using communication apparatuses according to this embodiment. In  FIG. 6 , a communication apparatus  100  includes the configuration of the communication apparatus  100  shown in  FIG. 1  and a communication apparatus  200  includes the configuration of the communication apparatus  200  shown in  FIG. 4 . Further, the communication apparatuses  100  and  200  form a link through AORs  110 A to  140 B and perform communication. Each of the AORs  110 A to  140 B is an outdoor integral-type communication system (i.e., an AOR: All Outdoor Radio) and relays communication through a wire or wirelessly. 
         [0082]    Each of the communication apparatuses  100  and  200 , and the AORs  110 A to  140 B communicates with an apparatus that is adjacent to and opposed to that apparatus as shown in  FIG. 6 . 
         [0083]    Further, communication through the port A is performed through a path of the communication apparatus  100 , the AORs  110 A,  120 A,  130 A and  140 A, and the communication apparatus  200 . Similarly, communication through the ports B and C is performed through a path of the communication apparatus  100 , the AORs  110 B,  120 B,  130 B and  140 B, and the communication apparatus  200 . 
         [0084]    Here, when a communication failure has occurred in a part of the link, the state of the communication system becomes a state shown in  FIG. 7 .  FIG. 7  shows an outline of a communication system using communication apparatuses according to this embodiment. In  FIG. 7 , the same numbers as those in  FIG. 6  are assigned to the same structures as those in  FIG. 6 , and their explanations are omitted. 
         [0085]    As shown in  FIG. 7 , when a failure occurs in a link between the AORs  120 A and  130 A, the AORs  120 A and  130 A can detect the occurrence of the failure. However, since the communication between the AORs  120 A and  130 A is for the opposed communication apparatuses, the communication apparatuses  100  and  200  (and the AORs  110 A and  140 A) requires a configuration for detecting the failure between the AORs  120 A and  130 A. 
         [0086]    In this embodiment, by the configuration of the communication apparatus  100  shown in  FIG. 1 , the communication apparatuses  100  and  200  detect a failure in the link between the AORs  120 A and  130 A, which are not adjacent to the communication apparatuses  100  and  200 , or a failure in the network due to a frame loss between the apparatuses, and degenerates the band. Transmission and reception of signals between the communication apparatuses  100  and  200  are explained hereinafter.  FIGS. 8 to 12  are schematic diagrams showing examples of transmission and reception of signals in the communication system according to this embodiment.  FIGS. 8 to 12  correspond to  FIGS. 6 and 7 , and the AORs  110 A to  140 B are omitted in the figures. 
         [0087]    When a failure occurs in the path of the port A as shown in  FIG. 8 , a divided frame that passes through the port A cannot reach the communication apparatus  100  or  200 . That is, only divided frames that pass through the ports B or C reach the communication apparatus  100  or  200 . 
         [0088]    Since the communication apparatus  100  cannot receive the divided frame through the port A, it fails in coupling the divided frames of the ports A to C. 
         [0089]    Therefore, as shown in  FIG. 9 , the communication apparatus  100  transmits a signal with an SF assigned thereto to the communication apparatus  200  through the ports A to C. After that, the communication apparatus  100  degenerates the ports A to C into the ports B and C. Although the communication apparatus  200  can receive the signal with the SF assigned thereto through the ports B and C, it cannot receive the signal with the SF assigned thereto through the port A due to the failure. Therefore, the communication apparatus  200  degenerates the band into the ports B and C through which the signal with the SF assigned thereto has been able to be received. 
         [0090]    Since both of the communication apparatuses  100  and  200  degenerate the band into the ports B and C, they perform communication through the ports B and C as shown in  FIG. 10 . 
         [0091]    As described above, when the communication system according to this embodiment detect an occurrence of a failure, it transmits a signal including an SF from each of the ports and the communication entity on the other side degenerates the band into the ports through which the signal including the SF has been able to be received. 
         [0092]    Next, cancellation of degeneracy is explained. For example, when a specific period has elapsed from the degenerated state shown in  FIG. 10 , an operation for checking whether or not the degeneracy can be cancelled is performed. 
         [0093]    As shown in  FIG. 11 , the communication apparatuses  100  and  200  transmit a signal including a control signal through the port A, which has been stopped performing communication due to the degeneracy. Since  FIG. 11  shows a state in which the path of the port A has not been restored from the failure, the signal including the control signal does not reach the communication entity on the other side. 
         [0094]    As shown in  FIG. 12 , when the path of the port A has been restored from the failure, each of the communication apparatuses  100  and  200  can receive a signal including a control signal from the communication entity on the other side. As a result, since the communication apparatuses  100  and  200  can determine that the path of the port A has been restored from the failure, the communication apparatuses  100  and  200  cancel the degeneracy and perform communication through the ports A to C as shown in  FIG. 13 . 
         [0095]    As described above, when the communication system according to this embodiment has failed in coupling frames, it transmits a signal including an SF from each of the ports. By doing so, the communication system can detect a failure in the link or a failure in the network due to a frame loss between apparatuses in communication in which a flame is divided and divided frames are transmitted. 
         [0096]    Further, the communication system according to this embodiment can degenerate the band in communication in which a frame is divided and divided frames are transmitted by degenerating the band into ports through which a signal including an SF has been able to be received. 
         [0097]    Note that the present disclosure is not limited to the above-described embodiments and can be modified as appropriate without departing from the spirit and scope of the present disclosure. 
         [0098]    For example, an embodiment may include both the communication apparatus  100  shown in  FIG. 1  and the communication apparatus  200  shown in  FIG. 4 .  FIG. 14  is a block diagram showing a configuration of a communication apparatus according to this embodiment. In  FIG. 14 , the same numbers as those in  FIG. 1  are assigned to the same structures as those in  FIG. 1 , and their explanations are omitted. In  FIG. 14 , a communication apparatus  300  includes an interface  101 , a CH information checker  301 , a coupler  103 , a communication device  104 , an interface  105 , a divider  106 , a buffer  107 , a controller  108 , and a control header generator  109 . 
         [0099]    Further, the interface  101  includes interfaces  101 - 1  to  101 - 3  which perform communication on a port-by-port basis. 
         [0100]    The CH information checker  301  checks a content of CH information of the received divided frame and detects the presence/absence of information indicating signal failure (SF). Then, the CH information checker  301  outputs information on a port corresponding to the received divided frame and the SF to the controller  108 . 
         [0101]    Similarly, the CH information checker  301  detects whether or not control information is included in the CH information of the received divided frame. Then, the CH information checker  301  outputs information on a port corresponding to the received divided frame and the control information to the controller  108 . Specifically, the control information preferably includes information as to whether frames to be transmitted should be coupled or should not be coupled. 
         [0102]    Further, the CH information checker  301  extracts a sequence number included in the CH information of the received divided frame and outputs the extracted sequence number together with the divided frame to the coupler  103 . This sequence number indicates an order according to which the divided frames are coupled. 
         [0103]    The coupler  103  couples the divided frames according to the order of the sequence number and outputs the coupled frame to the communication device  104 . Further, when the coupler  103  has failed in coupling divided frames, it outputs information that the coupling has failed to the controller  108 . For example, when some number is skipped in the order of the sequence number, the coupler  103  outputs information that the coupling of frames has failed and the skipped sequence number (or information on a corresponding port) to the controller  108 . 
         [0104]    When the controller  108  receives network failure information, it degenerates the band. Further, when the controller  108  receives information that the network failure has been solved, it cancels the degeneracy of the band. 
         [0105]    Specifically, when the interfaces  101 - 1  to  101 - 3  cannot perform communication with a counterpart communication apparatus, they inform the controller  108  that a failure has occurred in the link. Then, when the controller  108  is informed that a failure has occurred in the link from one of the interfaces  101 - 1  to  101 - 3 , the controller  108  determines that a failure has occurred in the link in communication by using the corresponding port. 
         [0106]    Further, when the coupler  103  has failed in coupling divided frames, it outputs information that the coupling has failed to the controller  108 . For example, when some number is skipped in the order of the sequence number, the coupler  103  outputs information that the coupling of frames has failed and the skipped sequence number (or a corresponding port) to the controller  108 . Then, the controller  108  determines that a failure has occurred in the link in communication using the port corresponding to the skipped sequence number. 
         [0107]    Furthermore, the CH information checker  301  associates an SF included in a header of the divided frame with a port through which that divided frame has been received, and informs the controller  108  of them in the associated state. The controller  108  determines that a failure has occurred in the link in the port through which a divided frame including an SF has not been received. 
         [0108]    By the above-described determination, the controller  108  identifies the port of the communication path in which the failure has occurred in the link and transmits divided frames to the entity with which the communication apparatus  300  communicates without using the port in which the failure has occurred. 
         [0109]    Meanwhile, a determination for cancelling degeneracy is performed by reception of divided frames in which control information is inserted on a port-by-port basis. That is, when control information is included in CH information of a received divided frame, it can be determined that communication has been successfully performed for the port through which that divided frame has been received. When the controller  108  has received a control signal through a port in which a failure has occurred in the link and which has been excluded based on the content of CH information from the CH information checker  301 , the controller  108  determines that that port has been restored to a state in which communication can be performed and adds that port as a port through which a divided frame is transmitted. 
         [0110]    By the above-described configuration, the communication apparatus  300  carries out detection of a failure in the link or a failure in the network due to a frame loss between apparatuses, degeneracy of the band, and cancellation of the degeneracy. 
         [0111]    Further, the above-described configuration is preferably formed by a logic circuit such as an ASIC (Application Specific Integrated Circuit). However, the configuration may be implemented by a circuit such as a CPU (Central Processing Unit). 
         [0112]    Although the present disclosure is explained above with reference to embodiments, the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure. 
         [0113]    This application is based upon and claims the benefit of priority from Japanese patent applications No. 2015-4661, filed on Jan. 14, 2015, the disclosure of which is incorporated herein in its entirety by reference. 
       Industrial Applicability 
       [0114]    The present disclosure can be used for a communication apparatus or a communication system. 
       Reference Signs List 
       [0115]      100 ,  200 ,  300  COMMUNICATION APPARATUS 
         [0116]      101  INTERFACE 
         [0117]      101 - 1  TO  101 - 3 ,  104  INTERFACE 
         [0118]      102 ,  201 ,  301  CH INFORMATION CHECKER 
         [0119]      103  COUPLER 
         [0120]      105  INTERFACE 
         [0121]      106  DIVIDER 
         [0122]      107  BUFFER 
         [0123]      108  CONTROLLER 
         [0124]      109  CONTROL HEADER GENERATOR