Abstract:
There is provided an optical transmission system including: an optical transmitting apparatus including: a first processing circuit configured to process a transmission signal to be transmitted, a second processing circuit configured to process overhead data, the processed overhead data being multiplexed to the transmission signal, a retaining circuit configured to retain the overhead data, the retained overhead data being multiplexed to the transmission signal; and an insertion circuit configured to generate an identifier to be inserted into the retained overhead data; and an optical receiving apparatus including a detecting circuit configured to receive the transmission signal transmitted from the optical transmitting apparatus, and detect the identifier, wherein, when the overhead data is a predetermined state, the first processing circuit multiplexes the retained overhead data into which the identifier is inserted and the detecting circuit detects the identifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-059857, filed on Mar. 17, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to an optical transmission system. 
     BACKGROUND 
     An optical transport network (OTN) is one of optical transmission technologies that have been used. OTN is being standardized by the international telecommunication union-telecommunication sector (ITU T) and the institute of electrical and electronic engineers (IEEE). 
     The standardization of OTN has not been completed yet. Therefore, some optical transmission apparatuses using OTN use programmable large scale integrations (LSIs) such as field programmable gate arrays (FPGAs) so as to be applicable to OTN before and after the completion of the standardization. 
     In some cases, the scale of a circuit used to process OTN functions is larger than the circuit scale of available programmable LSIs. In such a case, the circuit processing the OTN functions is divided into a main signal system LSI performing main signal processing and a control system LSI performing processing excluding the main signal processing, for example. Examples of the processing excluding the main signal processing include overhead processing, an interface function of a central processing unit (CPU), and reset processing. In OTN, overheads (OHs) for monitoring a network are defined. Some of the overheads are dynamically changed by being processed by the CPU. 
     In an optical transmission system including an optical transmission apparatus provided with such a programmable LSI, there has been a technique of updating firmware of the programmable LSI included in the optical transmission apparatus without signal interruption in the main signal system. For example, there has been a technique of downloading new firmware of the control system LSI without stopping operation of the main signal system LSI, which is a so-called uninterrupted (hitless) firmware download, and updating the firmware of the control system LSI. 
     As such a related art technique, there has been a technique in which the main signal system LSI retains the overhead data processed by the control system LSI before the control system LSI starts the uninterrupted firmware download and the firmware of the control system LSI is updated while the main signal system LSI retains the overhead data. In OTN, a general communication channel (GCC) and an automatic protection switching (APS) are defined as the overheads, for example. The GCC and the APS are passed from the control system LSI to the CPU and processed by the CPU when the optical transmission apparatus is operating normally. Accordingly, they change dynamically. 
     As an example of the related art technique, there has been a system in which an identifier is added to an original signal on a transmitting side and the resulting signal is transmitted from the transmitting side to a receiving side through two different paths while one of the signals received on the receiving side through the two paths is restored on the receiving side, so that signal paths are switched in an uninterruptible manner. There has been a system in which a GCC mode switching dedicated frame is inserted into the GCC overhead on the transmitting side and sent to the receiving side while the GCC modes are switched between by determining whether the GCC mode switching dedicated frame is inserted into the GCC overhead on the receiving side. 
     Japanese Laid-open Patent Publication No. 2004-266480, Japanese Laid-open Patent Application No. 2010-166254 and “ Interfaces for the optical transport network  ( OTN )”, ITU-T G.709/Y.1331 (December 2009) are examples of the related art. 
     SUMMARY 
     According to an aspect of the embodiment, there is provided an optical transmission system including: an optical transmitting apparatus including: a first processing circuit configured to process a transmission signal to be transmitted, a second processing circuit configured to process overhead data, the processed overhead data being multiplexed to the transmission signal, a retaining circuit configured to retain the overhead data, the retained overhead data being multiplexed to the transmission signal, and an insertion circuit configured to generate an identifier to be inserted into the retained overhead data; and an optical receiving apparatus including a detecting circuit configured to receive the transmission signal transmitted from the optical transmitting apparatus, and detect the identifier, wherein, when the overhead data is a predetermined state, the first processing circuit multiplexes the retained overhead data into which the identifier is inserted and the detecting circuit detects the identifier. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an optical transmission system according to a first embodiment; 
         FIG. 2  is a block diagram illustrating a signal flow in the optical transmission system according to the first embodiment; 
         FIG. 3  is a flowchart illustrating an optical transmission method according to the first embodiment; 
         FIG. 4  is a block diagram illustrating an optical transmission apparatus according to a second embodiment; 
         FIG. 5  is a block diagram illustrating a signal flow on a transmitting side when the optical transmission apparatus according to the second embodiment is operating normally; 
         FIG. 6  is a block diagram illustrating a signal flow on the transmitting side when the optical transmission apparatus according to the second embodiment is performing uninterrupted firmware download operation; 
         FIG. 7  is a block diagram illustrating a signal flow on a receiving side when the optical transmission apparatus according to the second embodiment is performing the uninterrupted firmware download operation; 
         FIG. 8  is a schematic illustrating a frame format of an optical transfer network (OTN); 
         FIG. 9  is a schematic illustrating a fault type &amp; fault location reporting channel (FTFL) message structure; and 
         FIG. 10  is a table illustrating an example of definitions of fault indication codes. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the related-art optical transmission apparatus, the overhead data retained by the main signal system LSI changes to an incorrect data because the overhead data, which was originally dynamically changing, is retained by the main signal system LSI when the uninterrupted firmware download of the control system LSI is executed. As a result, the optical transmission apparatus on the transmitting side transmits a signal having incorrect overhead data. Upon receiving the incorrect overhead data, the optical transmission apparatus on the receiving side sends an unnecessary alarm or switches a line in current use to another line although the line is actually normal, for example, because no information indicating that the optical transmission apparatus on the transmitting side is performing the uninterrupted firmware download operation is available. 
     Some optical transmission apparatuses transmit maintenance signals during execution of the uninterrupted firmware download. In an example of such optical transmission apparatuses, upon receiving the maintenance signals, the optical transmission apparatus on the receiving side sends an unnecessary alarm or switches a line in current use to another line, for example, in the same manner as in the case of receipt of the incorrect overhead data, because no information is available indicating that the uninterrupted firmware download operation is in progress in the optical transmission apparatus on the transmitting side. 
     The embodiments discussed herein aim to provide an optical transmission system that can avoid sending of an unnecessary alarm or unnecessary switching of lines. 
     Embodiments of an optical transmission system, an optical transmission apparatus, and an optical transmission method are described in detail below with reference to the accompanying drawings. In the optical transmission system, the optical transmission apparatus, and the optical transmission method, the apparatus on a transmitting side retains overhead data, and inserts an identifier relating to the retained overhead data into the overhead data while the apparatus on a receiving side detects the identifier. In the following embodiments, the same elements are labeled with the same reference numerals and repeated description thereof is omitted. 
     First Embodiment 
       FIG. 1  is a block diagram illustrating an optical transmission system according to a first embodiment. As illustrated in  FIG. 1 , the optical transmission system includes an optical transmission apparatus  1  on the transmitting side (also referred to as an optical transmitting apparatus) and an optical transmission apparatus  2  on the receiving side (also referred to as an optical receiving apparatus). The optical transmission apparatus  1  on the transmitting side includes a first processing circuit  3 , a second processing circuit  4 , a retaining circuit  5 , and an insertion circuit  6 . 
     The first processing circuit  3  is connected to optical transmission paths  8  and  9  such as optical fibers. The first processing circuit  3  processes a signal to be transmitted to the optical transmission apparatus  2  on the receiving side. The second processing circuit  4  is coupled to the first processing circuit  3 . The second processing circuit  4  processes the overhead data to be added to the signal to be transmitted to the optical transmission apparatus  2  on the receiving side. The retaining circuit  5  is coupled to the first processing circuit  3 . The retaining circuit  5  retains the overhead data. The insertion circuit  6  is coupled to the first processing circuit  3 . The insertion circuit  6  inserts the identifier relating to the overhead data into the overhead data. 
     The optical transmission apparatus  2  on the receiving side includes a detecting circuit  7 . The detecting circuit  7  is connected to the optical transmission path  9 . The detecting circuit  7  detects the identifier included in the overhead data of a received signal. 
       FIG. 2  is a block diagram illustrating a signal flow in the optical transmission system according to the first embodiment.  FIG. 3  is a flowchart illustrating an optical transmission method according to the first embodiment. As illustrated in  FIGS. 2 and 3 , the first processing circuit  3  receives a signal from another optical transmission apparatus (not illustrated) through the optical transmission path  8  in the optical transmission apparatus  1  on the transmitting side. The first processing circuit  3  extracts the overhead data included in the received signal and passes the overhead data to the second processing circuit  4  or the retaining circuit  5 . The second processing circuit  4  processes the overhead data to be added to the signal to be transmitted to the optical transmission apparatus  2  on the receiving side. The retaining circuit  5  retains the overhead data (operation S 1  in  FIG. 3 ). 
     The first processing circuit  3  processes a signal to be transmitted (also referred to as a transmission signal) to the optical transmission apparatus  2  on the receiving side. When processing the transmission signal, the first processing circuit  3  adds the overhead data passed from the second processing circuit  4  or the retaining circuit  5  to the transmission signal. The insertion circuit  6  inserts the identifier relating to the overhead data into the overhead data (operation S 2  in  FIG. 3 ). The first processing circuit  3  transmits the signal to which the overhead data has been added to the optical transmission apparatus  2  on the receiving side through the optical transmission path  9  (operation S 3  in  FIG. 3 ). 
     In the optical transmission apparatus  2  on the receiving side, the detecting circuit  7  receives the signal from the optical transmission apparatus  1  on the transmitting side through the optical transmission path  9  (operation S 4  in  FIG. 3 ). The detecting circuit  7  extracts the overhead data included in the received signal and detects the identifier included in the overhead data (operation S 5  in  FIG. 3 ). 
     According to the first embodiment, the optical transmission apparatus  2  on the receiving side can identify a state of the received overhead data by detecting the identifier included in the received overhead data. The optical transmission apparatus  2  on the receiving side can avoid a situation in which the optical transmission apparatus  2  sends an unnecessary alarm or unnecessarily switching lines by identifying the state of the received overhead data. 
     Second Embodiment 
     In a second embodiment, the optical transmission system and the optical transmission apparatus according to the first embodiment are applied to an optical transfer network (OTN). The optical transmission system and the optical transmission apparatus according to the first embodiment can be applied to any system besides OTN. In the second embodiment, a field programmable gate array (FPGA) is used as an example of a programmable large scale integration (LSI). The programmable LSI is not limited to an FPGA. 
       FIG. 4  is a block diagram illustrating an optical transmission apparatus according to the second embodiment. As illustrated in  FIG. 4 , an optical transmission apparatus  11  includes a main signal system FPGA  12  as a first processing circuit and a control system FPGA  13  as a second processing circuit. The main signal system FPGA  12  and the control system FPGA  13  can be updated by individually downloading firmware. The main signal system FPGA  12  and the control system FPGA  13  may be provided in different chips or in one chip. 
     The main signal system FPGA  12  includes a main signal processing module  14 , an identifier insertion module  15  as an example of an insertion circuit, an overhead processing selection module  16  as an example of a selection circuit, and a front-end retaining module  17  as an example of a retaining circuit. 
     The main signal processing module  14  is connected to optical transmission paths  21  and  22  such as the optical fibers. The main signal processing module  14  processes a signal to be transmitted to the optical transmission apparatus on the receiving side (not illustrated). 
     The front-end retaining module  17  is coupled to the overhead processing selection module  16 . The front-end retaining module  17  retains the overhead data. 
     The overhead processing selection module  16  is coupled to the main signal processing module  14 . The overhead processing selection module  16  exclusively selects either the overhead data retained by the front-end retaining module  17  or the overhead data processed by the control system FPGA  13 . 
     The identifier insertion module  15  is coupled to the main signal processing module  14 . The identifier insertion module  15  inserts the identifier relating to the overhead data into the overhead data. An example of the identifier relating to the overhead data is an identifier indicating that it is unclear whether the overhead data is correct. 
     The control system FPGA  13  is coupled to the main signal processing module  14 . The control system FPGA  13  processes the overhead data to be added to the signal to be transmitted to the optical transmission apparatus on the receiving side. The control system FPGA  13  includes an identifier detecting module  18  as an example of a detecting circuit. The identifier detecting module  18  detects the identifier included in the overhead data of a received signal. 
     The optical transmission apparatus  11  includes a central processing unit (CPU) processing module  19  as an example of a processing circuit. The CPU processing module  19  is coupled to the identifier detecting module  18 , the control system FPGA  13 , the identifier insertion module  15 , and the overhead processing selection module  16 . The CPU processing module  19  controls the overhead processing selection module  16  so that the overhead processing selection module  16  selects the overhead data and the identifier insertion module  15  so that the identifier insertion module  15  inserts the identifier based on the identifier detected by the identifier detecting module  18 . The CPU processing module  19  masks sending of an alarm or switching of lines controlled by the control system FPGA  13  based on the identifier detected by the identifier detecting module  18 . 
       FIG. 5  is a block diagram illustrating a signal flow on the transmitting side when the optical transmission apparatus according to the second embodiment operates normally. As illustrated in  FIG. 5 , the main signal processing module  14  receives a signal from another optical transmission apparatus (not illustrated) through the optical transmission path  21  when the optical transmission apparatus  11  on the transmitting side operates normally. The main signal processing module  14  extracts the overhead data included in the received signal and passes the overhead data to the control system FPGA  13 . 
     The control system FPGA  13  processes the overhead data to be added to a signal to be transmitted to the optical transmission apparatus  2  on the receiving side. The control system FPGA  13  passes the overhead data that dynamically changes such as a general communication channel (GCC) and automatic protection switching (APS) to the CPU processing module  19 . The CPU processing module  19  processes the overhead data that dynamically changes such as the GCC and the APS and returns the processed overhead data to the control system FPGA  13 . The control system FPGA  13  outputs the overhead data processed by the CPU processing module  19  and the overhead data processed by the control system FPGA  13  to the overhead processing selection module  16 . 
     The overhead processing selection module  16  outputs the overhead data processed by the CPU processing module  19  and the overhead data processed by the control system FPGA  13  to the main signal processing module  14  under control of the CPU processing module  19 . The main signal processing module  14  adds the overhead data passed from the overhead processing selection module  16  to a signal to be transmitted and transmits the signal to the optical transmission apparatus on the receiving side (not illustrated) through the optical transmission path  22 . 
       FIG. 6  is a block diagram illustrating a signal flow on the transmitting side when the optical transmission apparatus according to the second embodiment is performing uninterrupted firmware download operation. As illustrated in  FIG. 6 , upon receiving an uninterrupted firmware download request from a network management system (not illustrated), the CPU processing module  19  outputs control signals to the identifier insertion module  15  and the overhead processing selection module  16  in the optical transmission apparatus  11  on the transmitting side ready for uninterrupted firmware download operation. 
     The identifier insertion module  15  produces the identifier to be inserted into the overhead data and outputs the identifier to the main signal processing module  14  under control of the CPU processing module  19 . The overhead processing selection module  16  selects the front-end retaining module  17  as a supply source of the overhead data provided to the main signal processing module  14 . The front-end retaining module  17  retains the overhead data included in a signal received by the main signal processing module  14  from another optical transmission apparatus (not illustrated) through the optical transmission path  21 . 
     When the front-end retaining module  17  retains the overhead data, the control system FPGA  13  starts uninterrupted firmware download. While the uninterrupted firmware download is being executed, the overhead data retained by the front-end retaining module  17  and the identifier to be inserted by the identifier insertion module  15  are provided to the main signal processing module  14 . The main signal processing module  14  inserts the identifier into the overhead data passed from the overhead processing selection module  16 . The main signal processing module  14  adds the overhead data into which the identifier has been inserted to a signal to be transmitted and transmits the resulting signal to the optical transmission apparatus on the receiving side (not illustrated) through the optical transmission path  22 . 
       FIG. 7  is a block diagram illustrating a signal flow on the receiving side when the optical transmission apparatus according to the second embodiment is performing the uninterrupted firmware download operation. As illustrated in  FIG. 7 , the main signal processing module  14  receives a signal from another optical transmission apparatus, which is not illustrated, (the optical transmission apparatus  11  on the transmitting side in  FIG. 6 ) through the optical transmission path  21  when the optical transmission apparatus  11  on the receiving side is performing the uninterrupted firmware download operation. The main signal processing module  14  extracts the overhead data included in the received signal and passes the overhead data to the control system FPGA  13 . 
     The identifier detecting module  18  of the control system FPGA  13  detects the identifier included in the overhead data passed from the main signal processing module  14 . The CPU processing module  19  polls the identifier detecting module  18  and acquires the identifier detected by the identifier detecting module  18 . Upon acquiring the identifier relating to the overhead data, e.g., the identifier indicating that it is unclear whether the overhead data is correct, the CPU processing module  19  outputs a control signal to the control system FPGA  13  so as to mask sending of an alarm or switching of lines. Upon receiving the control signal to mask the sending of the alarm or the switching of the lines, the control system FPGA  13  controls the main signal processing module  14  so that the main signal processing module  14  does not send the alarm or switch the lines. 
       FIG. 8  is a schematic illustrating a frame format of OTN. As illustrated in  FIG. 8 , a frame structure  31  of OTN is structured as follows: an OPUk overhead is added to an OPUk payload that is a client signal, an ODUk overhead is added to the OPUk overhead, and an FA overhead and an OTUk overhead are further added to the ODUk overhead. In an OTUK frame, an FEC for error correction is further added to the end of the frame structure  31  of OTN, for example. 
     OPUk, ODUk, and OTUk stand for optical channel payload unit-k, optical data unit-k, and optical channel transport unit-k, respectively. FA stands for frame alignment. FEC stands for forward error correction. 
     An overhead  32  includes the OTUk overhead, the ODUk overhead, and the OPUk overhead and has an FTFL  33  of 1 byte, and an RES 34  and an RES  35  each of 10 bytes, for example. In  FIG. 8 , one column is one byte. FTFL stands for fault type &amp; fault location reporting channel. The FTFL is a region used for forwarding fault information of the lines. RES stands for reserved for future international standardization. The RES is a region reserved for future use. 
       FIG. 9  is a schematic illustrating an FTFL message structure. As illustrated in  FIG. 9 , an FTFL message  41  is composed of 256 frames. A forward field  42  to be sent in a forward direction is allocated to bytes  0  through  127 , i.e., 128 bytes, of the FTFL message  41 . A backward field  43  to be sent in a backward direction is allocated to bytes  128  through  255 , i.e., 128 bytes, of the FTFL message  41 . 
     When the identifier is inserted into the FTFL message, the identifier may be inserted into a fault indication field  44  in the forward field  42  of the FTFL message  41 . A fault indication code representing a failure type is set into the fault indication field  44 . The length of the fault indication code is one byte, for example. 
       FIG. 10  is a table illustrating an example of definitions of the fault indication codes. As illustrated in a definition list  46  of  FIG. 10 , the fault indication codes from “00000011” to “11111111” are reserved for future use as a reserved region  47 . The identifier may be inserted into the FTFL message by using the fault indication codes in the reserved region  47 . In this case, the identifier insertion module  15  includes an insertion circuit that inserts 256 frames (256 frames form a multi-frame structure, and are also referred to as 256 multiframes). 
     In the fault indication codes of the reserved region  47 , the most significant bit of the fault indication code may be defined as a restart status code, for example. For example, the restart status code may be set to “1” by the identifier insertion module  15  when the CPU processing module  19  of the optical transmission apparatus  11  receives the uninterrupted firmware download request. The restart status code of “1” indicates that it is unclear whether the overhead data is correct, for example. For example, the restart status code may be set to “0” by the identifier insertion module  15  when the CPU processing module  19  receives a request to complete uninterrupted firmware download. In this case, the identifier insertion module  15  includes a circuit that inserts “0” or “1” into the most significant bit of the fault indication code in the reserved region  47 . 
     In the fault indication codes of the reserved region  47 , the second bit from the most significant bit of the fault indication code may be defined as an overhead status code. For example, the overhead status code may be set to “1” by the identifier insertion module  15  when the CPU processing module  19  of the optical transmission apparatus  11  receives the uninterrupted firmware download request. For example, the overhead status code may be set to “0” by the identifier insertion module  15  when the CPU processing module  19  receives the request to complete uninterrupted firmware download. In this case, the identifier insertion module  15  includes a circuit that inserts “0” or “1” into the second bit from the most significant bit of the fault indication code in the reserved region  47 . 
     In the fault indication codes of the reserved region  47 , the lower six bits (“000000” to “111111”) of each fault indication code may be used as a number representing each byte of the overhead  32  of 64 bytes illustrated in  FIG. 8 , for example. For example, in the overhead  32  illustrated in  FIG. 8 , the first column at the first row (the head of the FAS) is given a value of “000000” and the sixteenth column at the fourth row (the end of the OPUk overhead) is given a value of “111111”. The GCC, which is one of the pieces of overhead data that dynamically change, is given a value in a range from “110001” to “110100”. The APS is given a value in a range from “110101” to “111000”. In this case, the identifier insertion module  15  includes a circuit that inserts the bits ranging from “000000” to “111111” into the lower six bits of the fault indication code in the reserved region  47 . 
     The overhead status code of “1” indicates that it is unclear whether the overhead data identified with the lower six bits of the fault indication code in the reserved region  47  is correct, for example. When the fault indication code in the reserved region  47  is used, information of whether the overhead data of 1 byte is correct can be sent with the FTFL message of 256 frames, for example. Therefore, when information indicating whether the overhead data is correct is sent for all of the bytes of the overhead  32  of 64 bytes illustrated in  FIG. 8 , the identifier insertion module  15  includes a circuit that inserts 64 multiframes each composed of the FTFL of 256 multiframes. 
     When the identifier is inserted into the RES of the overhead  32  of 64 bytes illustrated in  FIG. 8 , the identifier may be inserted into any 1 byte out of 10 bytes of the RES. In 8 bits, i.e., 1 byte, of the RES into which the identifier is inserted, the most significant bit may be defined as the restart status code, for example, in the same manner that the identifier is inserted into the FTFL message as described above. Likewise, the second bit from the most significant bit may be defined as the overhead status code, for example. In addition, the low six bits may be used for a number representing each byte of the overhead  32  of 64 bytes illustrated in  FIG. 8 , for example. 
     In this case, the identifier insertion module  15  includes a circuit that inserts “0” or “1” into the most significant bit or the second bit from the most significant bit of the RES. The identifier insertion module  15  includes a circuit that inserts bits ranging from “000000” to “111111” into the lower six bits of the RES. Information indicating whether the overhead data of one byte is correct can be sent with the RES of one byte. Therefore, when information indicating whether the overhead data is correct is sent for all of the bytes of the overhead  32  of 64 bytes illustrated in  FIG. 8 , the identifier insertion module  15  includes a circuit that inserts 64 multiframes. 
     The overhead data into which the identifier is inserted is not limited to the FTFL message and the RES. Any overhead data can be used as long as the overhead data is forwarded. Instead of sending the information indicating whether the overhead data is correct for all of the bytes of the overhead  32  of 64 bytes illustrated in  FIG. 8 , only the overhead data for which it is unclear whether the overhead data is correct may be sent. The CPU processing module  19  may include a program, a memory storing data, and a processor executing the program and realize some of the functions of the above-described optical transmission apparatus with software. 
     According to the second embodiment, the same effect as the first embodiment can be obtained. The overhead data for which it is unclear whether the overhead data is correct can be identified, for example, by providing numbers to the overhead  32  with codes of 6 bits, for example. As a result, the optical transmission apparatus on the receiving side can send an alarm or not or switch the lines or not depending on the overhead data for which it is unclear whether the overhead data is correct. 
     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.