Abstract:
A transmission system includes: a first transmission device configured to transmit an input signal to a second transmission device via a transmission section, the first transmission device preforms operations of: saving overhead information included in a first frame of the input signal from a region in use to an unused region of the first frame, the overhead information corresponding to an object of termination in the transmission section; and transmitting, to the transmission section, a first signal including a second frame in which the overhead information is saved.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-233869, filed on Nov. 18, 2014, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a transmission system, a transmission device, and a transmission method. 
       BACKGROUND 
       [0003]    In an optical transport network (OTN) transmission system illustrated in an international telecommunication union (ITU)-T G.709 standard, a client signal flowing into an optical network is transmitted as an optical channel transport unit (OTU). The OTU stores a payload storing the client signal as well as overhead (OH) information of an optical channel payload unit (OPU) and OH information of an optical channel data unit (ODU). 
         [0004]    A related technology is disclosed in Japanese Laid-open Patent Publication No. 2004-266480. 
       SUMMARY 
       [0005]    According to an aspect of the embodiments, a transmission system includes: a first transmission device configured to transmit an input signal to a second transmission device via a transmission section, the first transmission device preforms operations of: saving overhead information included in a first frame of the input signal from a region in use to an unused region of the first frame, the overhead information corresponding to an object of termination in the transmission section; and transmitting, to the transmission section, a first signal including a second frame in which the overhead information is saved. 
         [0006]    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. 
         [0007]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  illustrates an example of OTN transmission; 
           [0009]      FIG. 2  illustrates an example of OTN transmission; 
           [0010]      FIG. 3  illustrates an example of a transmission system; 
           [0011]      FIG. 4  illustrates an example of hardware configuration of a transmission system; 
           [0012]      FIG. 5A  illustrates an example of processing on a transmitting side; 
           [0013]      FIG. 5B  illustrates an example of processing on a receiving side; 
           [0014]      FIG. 6  illustrates an example of overhead information; 
           [0015]      FIG. 7  illustrates an example of overhead information; 
           [0016]      FIG. 8  illustrates an example of an OTU4 frame; 
           [0017]      FIG. 9  illustrates an example of frames as saving destinations; 
           [0018]      FIG. 10  illustrates an example of a transmission system; 
           [0019]      FIG. 11A  illustrates an example of processing on a transmitting side; 
           [0020]      FIG. 11B  illustrates an example of processing on a receiving side; 
           [0021]      FIG. 12  illustrates an example of processing of a transmission system; 
           [0022]      FIG. 13A  illustrates an example of processing on a master (transmitting) side; and 
           [0023]      FIG. 13B  illustrates an example of processing on a slave (receiving) side. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    In OTN transmission, OH information in a client signal may not be transmitted transparently in a given transmission section.  FIG. 1  and  FIG. 2  illustrate an example of OTN transmission. In transmission systems  200  and  300  illustrated in  FIG. 1  and  FIG. 2 , a client signal D 1  is input from network equipment A, passed through network equipment B, and then output from network equipment C. 
         [0025]    As illustrated in  FIG. 1 , the client signal D 1  in the transmission system  200  may be a synchronous digital hierarchy (SDH)/Ethernet (registered trademark) signal or an OTUk signal in a low-level (lower) layer. In an OTUk section of the pieces of network equipment A and B and an OTUk section of the pieces of network equipment B and C in an ODUk path between the pieces of network equipment A and C, OTUk OH information is terminated. 
         [0026]    The client signal D 1  input to a transmission device  210 A of the network equipment A is mapped into a payload of an OTUk signal in a high-level (upper) layer, and is then transmitted to a transmission device  210 B of the network equipment B. The client signal D 1  is transmitted through the transmission device  210 B to a transmission device  210 C of the network equipment C, extracted by the transmission device  210 C from the payload of the OTUk signal in the high-level (upper) layer, and then output. The client signal D 1  input to the network equipment A and output from the network equipment C is mapped into the payload of the OTUk signal in the high-level (upper) layer, and then transmitted. Therefore, the OH information in the client signal D 1  is transparently transmitted without being terminated. 
         [0027]    As illustrated in  FIG. 2 , in the transmission system  300 , a multiplex (OTUk) layer on the side of a client signal input to network equipment A and a multiplex (OTUk) layer on the side of a transmission line (between the pieces of network equipment A, B, and C) are substantially identical with each other, and, for example, an OTU4 is used on both of the client signal side and the transmission line side. In this case, in the standard, OTUk OH information is terminated on the input side, and the values are rewritten on the output side. 
         [0028]    For example, a transmission device  310 A of the network equipment A terminates and rewrites the OTUk OH information of the client signal D 1 , and transmits the OTUk OH information to a transmission device  310 B of the network equipment B. The transmission device  310 B terminates and rewrites the OTUk OH information transmitted from the transmission device  310 A, and transmits the OTUk OH information to a transmission device  310 C of the network equipment C. The transmission device  310 C terminates and rewrites the OTUk OH information transmitted from the transmission device  310 B, and outputs the OTUk OH information. The OH information in the client signal D 1  input to the network equipment A and output from the network equipment C is rewritten to different values, and therefore may not be transmitted transparently. 
         [0029]    Constitutions having substantially the same or similar functions may be identified by the same reference symbols, and repeated description thereof may be omitted or reduced. 
         [0030]      FIG. 3  illustrates an example of a transmission system. In a transmission system  1  of  FIG. 3 , a client signal may be input from network equipment A, passed through network equipment B, and then output from network equipment C. A similar description may be applied to a client signal in an opposite direction from that of  FIG. 3 . 
         [0031]    As illustrated in  FIG. 3 , in the transmission system  1 , a multiplex (OTUk) layer on the side of the client signal input to the network equipment A and a multiplex (OTUk) layer on the side of a transmission line (between the pieces of network equipment A, B, and C) may be substantially identical with each other. For example, an OTU4 may be used on both of the client signal side and the transmission line side. OTUk OH information is terminated in an OTUk section (between A and B) and an OTUk section (between B and C) in an ODUk path between the pieces of network equipment A and C. 
         [0032]    Transmission devices  10 A,  10 B, and  10 C that transmit the client signal by an OTN transmission system are provided to the pieces of network equipment A, B, and C, respectively.  FIG. 4  illustrates an example of hardware configuration of a transmission system. The transmission system illustrated in  FIG. 4  may be the transmission system  1  illustrated in  FIG. 3 . 
         [0033]    As illustrated in  FIG. 4 , the transmission device  10 A includes an optical/electricity (O/E) converting unit  11 A, a frame processing unit  12 A, and an electricity/optical (E/O) converting unit  13 A. The frame processing unit  12 A includes a forward error correction (FEC) processing unit  121 A, an OTUk OH processing unit  122 A, an ODUk frame generating unit  123 A, and an FEC inserting unit  124 A. 
         [0034]    Similarly, the transmission device  10 B includes an O/E converting unit  11 B, a frame processing unit  12 B, and an E/O converting unit  13 B. The frame processing unit  12 B includes an FEC processing unit  121 B, an OTUk OH processing unit  122 B, an ODUk frame generating unit  123 B, and an FEC inserting unit  124 B. Similarly, the transmission device  10 C includes an O/E converting unit  11 C, a frame processing unit  12 C, and an E/O converting unit  13 C. The frame processing unit  12 C includes an FEC processing unit  121 C, an OTUk OH processing unit  122 C, an ODUk frame generating unit  123 C, and an FEC inserting unit  124 C. For convenience, the configuration and functions of the transmission device  10 A will be described in the following. However, the configuration and functions of the other transmission devices may be substantially the same or similar. 
         [0035]    The O/E converting unit  11 A converts the client signal, for example, an optical signal into an electric signal, and then outputs the electric signal after the conversion to the frame processing unit  12 A. The frame processing unit  12 A subjects an OTN frame included in the electric signal to given frame processing, and outputs the electric signal after the processing to the E/O converting unit  13 A. The E/O converting unit  13 A converts the electric signal processed by the frame processing unit  12 A into an optical signal, and then transmits the optical signal. The optical signal converted by the E/O converting unit  13 A is transmitted to the transmission device  10 B via the OTUk section (between A and B). 
         [0036]    The FEC processing unit  121 A subjects payload data of the OTN frame to error correction code processing, and then outputs the frame after the processing to the OTUk OH processing unit  122 A and the ODUk frame generating unit  123 A. The OTUk OH processing unit  122 A stores various kinds of operation information in an OTUk overhead region within the frame to be processed by the ODUk frame generating unit  123 A. The ODUk frame generating unit  123 A subjects the frame (ODUk frame) to given processing. The FEC inserting unit  124 A inserts an FEC code into the frame generated by the ODUk frame generating unit  123 A, and then outputs the frame to the E/O converting unit  13 A. 
         [0037]    The frame processing unit  12 A saves overhead information (OTUk OH information) that is included in the input signal and is an object of termination in the ODUk path, to a given unused region within the frame based on a user setting. For example, when a setting for saving the OTUk OH information is made by a user, the OTUk OH processing unit  122 A saves the OTUk OH information of the input client signal to the given unused region within the frame to be processed by the ODUk frame generating unit  123 A. 
         [0038]    The frame processing unit  12 A writes back and restores the overhead information (OTUk OH information) included in the input signal and saved to the unused region of the frame to the original position within the frame based on a user setting. For example, when a setting for restoring the OTUk OH information is made by the user, the OTUk OH processing unit  122 A writes back and restores, to the original OH region, the OTUk OH information saved to the unused region within the frame to be processed by the ODUk frame generating unit  123 A. 
         [0039]    The user setting for saving or restoring the OTUk OH information may be made by console operation on the transmission device itself, or may be made via an external device such as a network monitoring device that monitors the pieces of network equipment A, B, and C or the like. For example, the transmission device  10 A of the network equipment A may be set to save the OTUk OH information, and the transmission device  10 C of the network equipment C may be set to restore the OTUk OH information. 
         [0040]    As illustrated in  FIG. 3 , a client signal is input to the transmission device  10 A (S 1 ). OH information  20  of the client signal includes a frame alignment signal (FAS)  21 , OTUk OH information  22 , and ODUk OH information  23 . The transmission device  10 A is set to save the OTUk OH information  22  as an object of termination. Therefore, the frame processing unit  12 A of the transmission device  10 A saves the OTUk OH information  22  stored in a region R 1  to a region R 2  storing an unused RES (Reserved Byte) in the ODUk OH information  23  within the frame (S 2 ). The frame in which the OTUk OH information  22  included in the client signal is saved to the unused region R 2  is thus transmitted in the ODUk path. 
         [0041]    In the OTUk section (between A and B), the OTUk OH information in the region R 1  is overwritten and terminated (S 3 ). In the OTUk section (between B and C), the OTUk OH information in the region R 1  is overwritten and terminated (S 4 ). 
         [0042]    The transmission device  10 C is set to restore the OTUk OH information  22  as an object of termination. Therefore, the frame processing unit  12 C of the transmission device  10 C checks for the presence or absence of a transmission line alarm and/or the normality of the OTUk OH information  22  saved in the unused region R 2  within the frame (S 5 ). When there is no transmission line alarm, and the normality of the OTUk OH information  22  is confirmed, the frame processing unit  12 C writes back and restores, to the original region R 1 , the OTUk OH information  22  saved in the unused region R 2  within the frame (S 6 ). The OTUk OH information  22  of the client signal input to the transmission device  10 A is restored, and then output from the transmission device  10 C (S 7 ). 
         [0043]      FIG. 5A  illustrates an example of processing on a transmitting side.  FIG. 5B  illustrates an example of processing on a receiving side. 
         [0044]    As illustrated in  FIG. 5A , the frame processing units  12 A,  12 B, and  12 C determine whether or not to save the OTUk OH information  22  based on a user setting (S 10 ). The transmission device  10 A is set to save the OTUk OH information  22 . Thus, the frame processing unit  12 A determines that the frame processing unit  12 A is to perform saving (S 10 : YES). The frame processing units  12 B and  12 C are not set to save the OTUk OH information  22 . Thus, the frame processing units  12 B and  12 C determine that the frame processing units  12 B and  12 C are not to perform saving (S 10 : NO). 
         [0045]    Because the frame processing unit  12 A is to perform saving, the frame processing unit  12 A performs saving processing that saves the OTUk OH information  22  stored in the region R 1  to the unused region R 2  within the frame (S 11 ). 
         [0046]      FIG. 6  and  FIG. 7  illustrate an example of overhead information.  FIG. 6  illustrates the position of the region R 1 .  FIG. 7  illustrates the position of the region R 2 . As illustrated in  FIG. 6 , information such as a section monitoring (SM), a general communication channel 0 (GCC0), and an RES or the like stored in the region R 1  is saved. Information to be saved among the SM, the GCC0, and the RES may be arbitrarily selected by a user setting, for example. 
         [0047]    As illustrated in  FIG. 7 , the unused region R 2  in the OH information  20  includes an RES (row 2, 3 bytes), an EXP (row 3, 2 bytes), and an RES (row 4, 6 bytes), or the like. A region as a saving destination in the region R 2  may be a single region or an arbitrary combination of a plurality of regions based on a user setting. 
         [0048]    The region as a saving destination in the OTUk OH information  22  is not limited to a region within the OH information  20 , but may be within a payload as long as the region is an unused region within the frame.  FIG. 8  illustrates an example of an OTU4 frame. As illustrated in  FIG. 8 , the OTUk OH information  22  may be saved to arbitrary bytes in a region R 3  of Fixed stuff bytes (columns 3817 to 3824, 32 bytes) within the payload. 
         [0049]    The region as a saving destination in the OTUk OH information  22  may not be only within the identical frame that includes the OTUk OH information  22 , but may be an unused region in one of a previous frame and a subsequent frame based on a user setting. 
         [0050]      FIG. 9  illustrates an example of frames as saving destinations. As illustrated in a case C 1  of  FIG. 9 , the frame processing unit  12 A may save the OTUk OH information  22  stored in the region R 1  of a frame (n) to the region R 2  of the identical frame. As illustrated in a case C 2 , the frame processing unit  12 A may save the OTUk OH information  22  stored in the region R 1  of the frame (n) to the region R 2  of a previous frame (n−1). As illustrated in a case C 3 , the frame processing unit  12 A may save the OTUk OH information  22  stored in the region R 1  of the frame (n) to the region R 2  of a subsequent frame (n+1). 
         [0051]    In saving and restoring the OTUk OH information  22  to and from the unused region of one of the previous frame and the subsequent frame, data for restoration, for example, frame information, and/or the OTUk OH information  22  to be saved and restored are/is temporarily stored in a memory. The temporarily stored data may be processed in accordance with frames processed sequentially. 
         [0052]    As illustrated in  FIG. 5B , the frame processing units  12 A,  12 B, and  12 C determine whether or not to write back the OTUk OH information  22  saved to the region R 2  or R 3  based on a user setting (S 20 ). The transmission device  10 C is set to restore the OTUk OH information  22 , for example, and therefore the frame processing unit  12 C determines that the frame processing unit  12 C is to perform writing back (S 20 : YES). The frame processing units  12 A and  12 B are not set to restore the OTUk OH information  22 , and therefore determine that the frame processing units  12 A and  12 B are not to perform writing back (S 20 : NO). When writing back is not to be performed, the processing is ended. 
         [0053]    Because the frame processing unit  12 C is to perform writing back, the frame processing unit  12 C determines whether or not a transmission line alarm is absent and whether or not the OTUk OH information  22  is normal (S 21 ). For example, the frame processing unit  12 C determines whether or not the input signal includes signal disappearance (loss of signal)/frame synchronization loss (loss of frame) information (LOS/LOF information), a transmission line alarm such as an alarm indication signal (AIS) alarm or the like. The frame processing unit  12 C may determine the normality of the saved OTUk OH information  22  by checking the saved OTUk OH information  22 . For example, in the SM in the OTUk OH information  22 , the normality may be determined based on source access point identifier/destination access point identifier information (SAPI/DAPI information) and/or bit interleaved parity (BIP) 8 information. 
         [0054]    When the transmission line alarm is absent, and the normality of the OTUk OH information  22  is confirmed (S 21 : YES), the frame processing unit  12 C performs writing-back processing that writes back and restores the OTUk OH information  22  saved in the unused region R 2  or R 3  within the frame to the original region R 1  (S 22 ). 
         [0055]    The writing-back processing is performed after it is confirmed that there is no transmission line alarm and the normality of the OTUk OH information  22  is confirmed. Therefore, even when the saved OTUk OH information  22  is lost due to an abnormality in the ODUk path, the restoration using wrong information, for example, the fixed pattern of the AIS signal may be reduced. 
         [0056]    In the above-described writing-back processing, based on a user setting, the frame processing unit  12 C reads the OTUk OH information  22  saved to the arbitrarily selected region R 2  or R 3 , and restores the OTUk OH information  22  to the original region R 1 . Also in the case where the OTUk OH information  22  is saved to the unused region of one of the previous frame and the subsequent frame, the frame processing unit  12 C similarly restores the OTUk OH information  22  by temporarily storing appropriate data in a memory in advance and processing the temporarily stored data in accordance with frames processed sequentially. 
         [0057]    In the transmission system  1 , the transmission device  10 A saves the OTUk OH information  22 , which is included in a frame of the client signal and is an object of termination in the ODUk path, to the unused region R 2  within the frame, and transmits an optical signal including the frame in which the OTUk OH information  22  is saved to the unused region R 2 . After the transmission device  10 C restores the saved OTUk OH information  22  by writing back the OTUk OH information  22  saved to the region R 2  in the frame included in the received optical signal to the original position, the transmission device  10 C outputs the signal including the restored frame. Therefore, in the transmission system  1 , the OTUk OH information  22  of the input signal is transmitted transparently in the transmission section in which the OTUk OH information  22  is terminated. 
         [0058]    Transparently transmitting the OTUk OH information  22  in the section from the network equipment A to the network equipment C, for example, the sections of the network equipment B provides a great advantage also in network management. 
         [0059]    For example, when the service area of the transmission system  1  becomes larger, realizing service in the entire area with only the network equipment of one communication carrier may result in poor cost effectiveness. As a measure, operation may be performed in which the network equipment of another communication carrier is rented. However, in the case of the operation in which the network equipment of another communication carrier is rented, the monitoring of the entire network may be affected. 
         [0060]    For example, when the network equipment B illustrated in  FIG. 3  is equipment rented from another communication carrier, the monitoring of the sections of the network equipment B may be difficult due to differences in specifications related to network management or the like. For example, when the OTUk OH information  22  is transmitted transparently in the sections of the network equipment B, the sections of the network equipment B may be treated as equivalent to a simple light transmission line. The network management may therefore be performed without awareness of the sections of the network equipment B. 
         [0061]      FIG. 10  illustrates an example of a transmission system. As illustrated in  FIG. 10 , in a transmission system  1   a,  when a frame processing unit  12 A of a transmission device  10 A saves OTUk OH information  22 , the frame processing unit  12 A inserts (stores), into a region R 2 , saving information indicating that the OTUk OH information  22  is saved (S 2   a ). A frame processing unit  12 C of a transmission device  10 C detects the saving information stored in the region R 2  (S 5   a ). When the saving information is stored, the frame processing unit  12 C writes back the OTUk OH information  22  saved to the region R 2 . 
         [0062]      FIG. 11A  illustrates an example of processing on a transmitting side. The processing illustrated in  FIG. 11A  may be performed in the transmission system la illustrated in  FIG. 10 . 
         [0063]    As illustrated in  FIG. 11A , frame processing units  12 A,  12 B, and  12 C determine whether or not to save the OTUk OH information  22  based on a user setting (S 10 ). The transmission device  10 A is set to save the OTUk OH information  22 , for example. Thus, the frame processing unit  12 A determines that the frame processing unit  12 A is to perform saving (S 10 : YES). The frame processing units  12 B and  12 C are not set to save the OTUk OH information  22 . The frame processing units  12 B and  12 C therefore determine that the frame processing units  12 B and  12 C are not to perform saving (S 10 : NO). 
         [0064]    The frame processing unit  12 A is to perform saving. The frame processing unit  12 A therefore performs saving processing that sets (stores) saving information indicating saving in the unused region R 2  within the frame (S 10   a ) and that saves the OTUk OH information  22  in the region R 2  (S 11 ). 
         [0065]    As illustrated in  FIG. 11B , the frame processing units  12 A,  12 B, and  12 C determine whether or not to write back the OTUk OH information  22  saved to the region R 2  or a region R 3  based on a user setting (S 20 ). The transmission device  10 C is set to restore the OTUk OH information  22 , for example. Thus, the frame processing unit  12 C determines that the frame processing unit  12 C is to perform writing back (S 20 : YES). The frame processing units  12 A and  12 B are not set to restore the OTUk OH information  22 . The frame processing units  12 A and  12 B therefore determine that the frame processing units  12 A and  12 B are not to perform writing back (S 20 : NO). When writing back is not to be performed, the processing is ended. 
         [0066]    The frame processing unit  12 C is to perform writing back. Thus, the frame processing unit  12 C refers to the region R 2 , and determines whether or not the saving information is detected (S 21   a ). When the saving information is not detected (S 21   a : NO), the OTUk OH information  22  is not saved in the region R 2 , and therefore the frame processing unit  12 C ends the processing without performing the writing-back processing. When the saving information is detected (S 21   a : YES), the OTUk OH information  22  is saved to the region R 2 , and therefore the frame processing unit  12 C performs the writing-back processing (S 22 ). 
         [0067]    When whether or not to perform the writing-back processing is determined based on a transmission line alarm and the like, a plurality of alarms may be integrated. When the saving information is used, for example, only the monitoring of the region R 2  is performed, so that a processing configuration may be simplified. In a case where normality is determined based on SAPI/DAPI information in an SM and the writing-back processing is performed, the SAPI/DAPI information is transferred in multiple frames. Therefore the determination may be made after reception of a plurality of frames. When the saving information is used, for example, only the monitoring of the region R 2  within one frame is performed, so that an instant determination may be made. Even when the monitoring of the region R 2  is performed, another method such as the transmission line alarm and the like may be used for the determination, for example. 
         [0068]    When normality is determined based on BIP8 information in the SM, and the writing-back processing is performed, an instant determination may be made because the BIP8 information is information that precedes by two frames. For example, error information of the BIP8 information does not include the OH region. Thus, when saving is performed to an OH region, the determination of exact normality may be difficult. When the saving information is used, the saving information is stored in the same region as the saved OTUk OH information  22 . Therefore a more accurate determination may be made. 
         [0069]    When normality is determined based on GCC0 information, and the writing-back processing is performed, there is a possibility of the GCC0 information being a random signal, so that the determination of normality may be difficult. In the case where the saving information is used, for example, even when a signal in a format that makes the determination of normality difficult such as the GCC0 information or the like is saved, the determination of normality may be made. 
         [0070]      FIG. 12  illustrates an example of processing of a transmission system.  FIG. 12  represents a ladder chart of operation of a transmission system  1   b.  A configuration of the transmission system  1   b  may be substantially the same as or similar to those of the transmission systems  1  and  1   a,  and therefore description thereof may be omitted and reduced. 
         [0071]    As illustrated in  FIG. 12 , in the transmission system  1   b,  a master side that saves OTUk OH information  22  and transmits the OTUk OH information  22  (transmission device  10 A) makes a test transmission for all of unused regions R 2  within a frame to a slave side that restores the OTUk OH information  22  (transmission device  10 C). The transmission device  10 A determines a region R 2  in which there is a response to the test transmission from the transmission device  10 C as a candidate destination to which the OTUk OH information  22  is saved. 
         [0072]    For example, the transmission device  10 A adds a given test flag to all of unused regions R 2  (all of saving candidate destinations) within a frame, and makes a transmission (S 30 ). 
         [0073]    When a usually unused region R 2  is used between the transmission devices  10 A and  10 C, the added test flag is rewritten into other data. Therefore, when the transmission device  10 C confirms the given test flag, it may be confirmed that the region R 2  is unused between the transmission devices  10 A and  10 C. 
         [0074]    The transmission device  10 C receives the frame in which the test flags are added (S 31 ), adds a response flag to the regions R 2  as candidate destinations in which the given test flags are received, and then returns the frame (S 32 ). 
         [0075]    The transmission device  10 A receives the frame in which the response flags are added (S 33 ), and determines the regions R 2  to which the response flags are added as candidate destinations to which the OTUk OH information  22  is saved (S 34 ). Therefore, in the transmission system  1   b,  the OTUk OH information  22  is saved to the regions R 2  not used between the transmission devices  10 A and  10 C, for example, by the network equipment B. 
         [0076]      FIG. 13A  illustrates an example of processing on a master (transmitting) side. As illustrated in  FIG. 13A , the frame processing unit  12 A determines whether or not to save the OTUk OH information  22  based on a user setting (S 40 ). When the frame processing unit  12 A is not to perform saving (S 40 : NO), the frame processing unit  12 A ends the processing. 
         [0077]    When the frame processing unit  12 A is to perform saving (S 40 : YES), the frame processing unit  12 A sets a given test flag in a plurality of regions R 2  unused within a frame (S 41 ). An RES (row 2, 3 bytes), an EXP (row 3, 2 bytes), an RES (row 4, 6 bytes), Fixed stuff bytes within a payload, and the like may be used as the regions R 2  in which the test flags are set. 
         [0078]    The frame processing unit  12 A transmits the frame in which the test flags are set to the slave side via an E/O converting unit  13 A (S 42 ). The frame processing unit  12 A determines whether or not there is a response flag from the slave side (S 43 ). When there is no response flag from the slave side (S 43 : NO), the frame processing unit  12 A determines whether or not a time-out has occurred (S 44 ). When the time-out has not occurred (S 44 : NO), the frame processing unit  12 A sets the processing in a waiting state. When the time-out has occurred (S 44 : YES), the frame processing unit  12 A makes an abnormal end because there is no region R 2  as a candidate for a saving destination and thus saving is difficult (S 45 ). In the case of the abnormal end, the user may be notified by outputting an alert, for example, that it is difficult to save the OTUk OH information  22 . 
         [0079]    When there is a response flag from the slave side (S 43 : YES), the frame processing unit  12 A determines whether or not there is OTUk OH information  22  that can be used in (that can be saved to) the region R 2  to which the response flag is added (S 46 ). For example, when various kinds of information of the OTUk OH information  22  can be stored in the region R 2  to which the response flag is added, it may be determined that the information can be saved. The frame processing unit  12 A sets a saving location of the OTUk OH information  22 , which is determined to be able to be saved, in the region R 2  to which the response flag is added (S 47 ). 
         [0080]      FIG. 13B  illustrates an example of processing on the slave (receiving) side. As illustrated in  FIG. 13B , the frame processing unit  12 C determines whether or not to save the OTUk OH information  22  based on a user setting (S 50 ). When the frame processing unit  12 C is not to perform saving (S 50 : NO), the frame processing unit  12 C ends the processing. 
         [0081]    When the frame processing unit  12 C is to perform saving (S 50 : YES), the frame processing unit  12 C extracts information from the plurality of regions R 2  unused within the frame (S 51 ), and determines whether or not there is a test flag from the master side (S 52 ). When there is no test flag (S 52 : NO), the frame processing unit  12 C determines whether or not a time-out has occurred (S 53 ). When the time-out has not occurred (S 53 : NO), the frame processing unit  12 C sets the processing in a waiting state. When the time-out has occurred (S 53 : YES), the frame processing unit  12 C ends the processing. 
         [0082]    When there is a test flag (S 52 : YES), the frame processing unit  12 C sets a response flag in the region R 2  in which the test flag is detected (S 54 ), and transmits the frame in which the response flag is set to the master side via an E/O converting unit  13 C in an opposite direction from  FIG. 4  (S 55 ). 
         [0083]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present 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.