Abstract:
A regenerative relay method includes the steps of: i) calculating an error rate of a transmission path between the first half apparatus and a main apparatus; ii) calculating an error rate of a transmission path between the main apparatus and the latter apparatus; iii) adding the error rates; iv) selecting the error correction code and data before the error is corrected in the main apparatus so as to be supplied to the latter apparatus if the added error rates are lower than a designated error correction threshold; and v) selecting data after the error is corrected in the main apparatus and the other error correction code generated from the data so as to be supplied to the latter apparatus if the added error rates are higher than the designated error correction threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of application Ser. No. 11/284,210, filed Nov. 21, 2005, which claims the benefit under 35 USC 120 and 365(c) of PCT application JP2003/009928, filed Aug. 5, 2003. The foregoing applications are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to regenerative relay methods and regenerative relay apparatuses, and more specifically, to a regenerative relay method and regenerative relay apparatus whereby, for example, a regenerative relay of a signal is implemented in an optical transmission system such as a Dense Wavelength Division Multiplexer (DWDM) system. 
         [0004]    2. Description of the Related Art 
         [0005]    Recently and continuing, due to the increase of communications capability, a large amount of data can be transmitted by a transmission apparatus and long distance transmission of data can be done. In this state, an efficient optical transmission system using Wavelength Division Multiplexing is utilized. In this Wavelength Division Multiplex transmission, deterioration of light often happens due to mixing of noise by the optical amplifier, light dispersion or polarization by a long distance transmission, or the like. Because of this, instead of use of an expensive compensator or low noise amplifier, an error correction function whereby a signal is encoded by adding a redundancy bit and the signal is decoded at a receiving side, has been used. 
         [0006]    An S/N ratio improved by the error correction is generally called a coding gain. Various kinds of error correction have been developed depending on their coding rules or the number of the redundancy bits. Generally, as the coding gain is larger, that is, the error correction ability is higher, the number of the redundancy bits is bigger, circuit size is bigger, delay time is longer, and consumption of electric power is larger. 
         [0007]    Transmission capacity has been recently extremely improved so that a transmission capacity of 10 Gbit/s or 40 Gbit/s has been utilized. Therefore, an error correction method wherein the number of the redundancy bits, the circuit size, the delay time, and the consumption of electric power are respectively small but the coding gain is large in demanded. 
         [0008]    In addition, an INVERSE-MUX method has been used. According to this method, a signal having a larger capacity is divided into plural routes and transmitted so that the signal is multiplexed at a transmitted side and the original signal is regenerated. In this method, for example, in order to divide and transmit a signal having a capacity of 40 Gbit/s into four routes of 10 Gbit/s each and then multiplex to generate the signal having a capacity of 40 Gbit/s again, it is necessary to minimize the differences of the delay times of the transmitted four routes and therefore the delay time is increased due to the number of the relay apparatuses of the respective routes. 
         [0009]      FIG. 1  is a block diagram of an example of a related art transmission system. Referring to  FIG. 1 , in a terminal apparatus  10 , a redundancy bit is added to a client signal by using an encoding part  12  so that the client signal is encoded. The coded signal is converted to an optical signal by an electric/optical conversion part  14 . The converted optical signal is multiplexed by an optical wavelength multiplexer  16  so as to be sent out to a transmission path at a network side. 
         [0010]    The optical multiplex signal that is sent out is optical-level generated by an optical amplifier (ILA)  17   1 ,  17   2 , and  17   3  arranged on the transmission path of the network so as to be transmitted. In addition, the signals are divided into individual optical signals by an optical wavelength divider  18  so as to be transmitted to a regenerative relay apparatus  20 . 
         [0011]    In the regenerative relay apparatus  20 , the signal converted to the electric signal by the optical/electric conversion part  22  is decoded so that the error correction is made and the signal is regenerated by the signal generation part  26 . After that, a redundancy bit is added again by an encoding part  28  so that the signal is encoded. The signal is converted to an optical signal by an electric/optical conversion part  30 , and then the signal is multiplexed by an optical wavelength multiplexer  32  so as to be sent out to the transmission path at the network side. The signal is transmitted to a terminal apparatus  34  situated in a remote position by repeating this process. In the terminal apparatus receiving the signal, after the error correction is made, the signal is used as a transmission signal to the client. 
         [0012]    In the meantime, Japan Laid-Open Patent Application Publication No. 2000-341344 discloses that a signal converted to an electric signal by a light receiving element is binarized and demultiplexed into n-channels by a comparator, errors of the binarized signals of respective channels are corrected by an error correction circuit, a total error number obtained by counting the number of the errors of the channels is supplied to a threshold value control circuit, the threshold control circuit allows a threshold generation circuit to generate plural thresholds used by the comparator, and an optimum threshold is determined on the basis of the total error number. 
         [0013]    In the related art transmission system, decoders having constant error correction abilities regardless of the transmission path qualities are provided in the regenerative relay apparatus and the terminal apparatus. Therefore, the signal passes through the decoder even at a section where the number of errors is small so that signal delay by the decoder and the encoder is increased. 
       SUMMARY OF THE INVENTION 
       [0014]    Accordingly, it is a general object of the present invention to provide a novel and useful regenerative relay method and regenerative relay apparatus. 
         [0015]    Another and more specific object of the present invention is to provide a regenerative relay method and regenerative relay apparatus whereby a necessary error correction is made without increasing delay time and good transmission quality can be secured. 
         [0016]    The above object of the present invention is achieved by a regenerative relay method wherein data to which an error correction code is added are transmitted from a first half apparatus so that an error correction is made, and another error correction code is generated from the data after the error is corrected so as to be transferred to a latter apparatus, the method including the steps of: 
         [0017]    calculating an error rate of a transmission path between the first half apparatus and a main apparatus; 
         [0018]    calculating an error rate of a transmission path between the main apparatus and the latter apparatus; 
         [0019]    adding the error rates; 
         [0020]    selecting the error correction code and data before the error is corrected in the main apparatus so as to be supplied to the latter apparatus if the added error rates are lower than a designated error correction threshold; and 
         [0021]    selecting data after the error is corrected in the main apparatus and the other error correction code generated from the data so as to be supplied to the latter apparatus if the added error rates are higher than the designated error correction threshold. 
         [0022]    According to the present invention, it is possible to make a necessary error correction without increasing delay time and secure good transmission quality. 
         [0023]    Other objects, features, and, advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a block diagram of an example of a related art transmission system; 
           [0025]      FIG. 2  is a structural view of a first embodiment of a relay method of the present invention; 
           [0026]      FIG. 3  is a view showing an example of a frame format of a transmission signal; 
           [0027]      FIG. 4  is a view for explaining generation and storage of an error detecting code BIP 8 ; 
           [0028]      FIG. 5  is a structural view of a second embodiment of the relay method of the present invention; 
           [0029]      FIG. 6  is a structural view of a third embodiment of the relay method of the present invention; 
           [0030]      FIG. 7  is a structural view of a fourth embodiment of the relay method of the present invention; 
           [0031]      FIG. 8  is a structural view of a fifth embodiment of the relay method of the present invention; 
           [0032]      FIG. 9  is a structural view of a sixth embodiment of the relay method of the present invention; 
           [0033]      FIG. 10  is a structural view of a seventh embodiment of the relay method of the present invention; 
           [0034]      FIG. 11  is a structural view of an eighth embodiment of the relay method of the present invention; 
           [0035]      FIG. 12  is a structural view of an example of an optical transmission system; and 
           [0036]      FIG. 13  is a flowchart of an example of a determination process implemented by a monitor apparatus  810 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    A description is given below, with reference to the  FIG. 2  through  FIG. 13  of embodiments of the present invention. 
       First Embodiment of the Present Invention 
       [0038]      FIG. 2  is a structural view of a first embodiment of a relay method of the present invention. Referring to  FIG. 2 , an error correction code that is a redundancy bit for error correction is added to a signal in an encoding part  41  of a terminal apparatus  40 . Overhead is added to the signal by an OH transmission part  42  so as to be transmitted to a regenerative relay apparatus  44  via a going-up (upstream) circuit. 
         [0039]      FIG. 3  is a view showing an example of a frame format of a transmission signal. This shows the frame structure of a digital wrapper recommended by ITU-T G709. The transmission signal has a 4080×4 byte structure. This frame structure includes the overhead OH of 56 (=14×4) bytes, a data area OPUk of 15296 (=3824×4) bytes, and an error correction code FEC (Forward Error Correction) of 1024 byte. 
         [0040]    An error detection code BIP  8  (Bit Interleaved Parity-level  8 ) is stored in the ninth column of the first line of the overhead. Backward error information BEI (Backward Error Indication) and forward error information (Forward Error Indication) are stored in the tenth column. 
         [0041]    Backward correction information BCI (Backward Correction Indication) and forward correction information FCI (Forward Correction Information) are stored in the thirteenth column. Control information CMD is stored in the fourteenth column. 
         [0042]    As shown in  FIG. 4 , the error detection code BIP  8  stores a parity value calculated from a data area (OPUk) of a frame i in the overhead of a frame i+2 which is two frames after the frame i. In addition, an error correction code (FEC) is, for example, a BCH (Bose Chaudhuri Hocquengham) code or a Reed Solomon code calculated from data of a data area OPUk of the frame i, and is stored in the frame i. 
         [0043]    In an OH receiving part  45  of the regenerative relay apparatus  44  shown in  FIG. 2 , the overhead of a receiving frame is terminated, and error detection is done by using the error detection code BIP  8  in the overhead and data of the data area OPUk of the frame preceding by two frames so as to be supplied to an error count part  46 . Furthermore, data in the data area OPUk and the error correction code FEC are supplied to a decoding part  47  and a selection part  49 . 
         [0044]    The decoding part  47  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. The data (corresponding to the data area OPUk) for which the error correction is implemented are encoded again by the encoding part  48  so that the error correction code FEC is generated, and are then supplied to the selection part  49 . 
         [0045]    The error count part  46  calculates an error rate of a transmission path between the terminal apparatus  40  and the regenerative relay apparatus  44  from an error detection result supplied from the OH receiving part  45  and supplies the error rate to an adder  51 . The OH receiving part  52  terminates the overhead of the receiving frame of a going-down (downstream) circuit so as to take out the backward error information BEI in the overhead. The backward error information BEI is an error rate between the regenerative relay apparatus  44  and the terminal apparatus  54  and supplied to the adder  51 . 
         [0046]    The adder  51  supplies the total of an error rate of the transmission path between the terminal apparatus  40  and the regenerative relay apparatus  44  and the error rate of the transmission path between the regenerative relay apparatus  44  and the terminal apparatus  54  to a threshold determination part  53 . 
         [0047]    The threshold determination part  53  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  57  of the terminal apparatus  54  and the total error rate supplied from the adder  51 . In a case where the total error rate does not exceed the threshold, a signal route is selected by the selection part  49  so that the signal goes to the selection part  49  without going through the decoding part  47  and the encoding part  48 . On the other hand, in a case where the total error rate exceeds the threshold, a signal route is selected by the selection part  49  so that the signal goes to the selection part  49  via the decoding part  47  and the encoding part  48 . 
         [0048]    The overhead is added to the error correction code FEC and the data in the data area OPUk being output by the selection part  49  by an OH transmission part  50 . Then, the data in the data area OPUk and the error correction code FEC are transmitted to the terminal apparatus  54  by the going-up (upstream) circuit. 
         [0049]    The overhead of the receiving frame is terminated at the OH receiving part  55  of the terminal apparatus  54 . Error detection is done by using the error detection code BIP  8  in the overhead and data of the data area OPUk of the frame preceding by two frames so as to be supplied to an error count part  56 . Furthermore, data in the data area OPUk and the error correction code FEC are supplied to a decoding part  57 . 
         [0050]    The decoding part  57  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. 
         [0051]    The error count part  56  calculates an error rate of a transmission path between the terminal apparatus  50  and the regenerative relay apparatus  44  from an error detection result supplied from the OH receiving part  55  and supplies the error rate to an OH transmission part  58 . The OH transmission part  58  stores the error rate supplied from the error count part  56  in the backward error information BEI in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the error rate to the regenerative relay apparatus  44  via a going-down (downstream) circuit. 
         [0052]    Thus, by the threshold determination part  53 , the total of the error rate of the transmission path between the terminal apparatus  40  and the regenerative relay apparatus  44  and the error rate of the transmission path between the regenerative relay apparatus  44  and the terminal apparatus  54  is compared with error correction threshold determined by the error correction ability set in advance at the decoding part  57 . In the case where the total error rate does not exceed the error correction threshold, the signal route is selected by the selection part  49  so that the signal goes to the selection part  49  without going through the decoding part  47  and the encoding part  48 . As a result of this, the error in a transmission section from the terminal apparatus  40  and the terminal apparatus  54  is corrected by the error correction function of the terminal apparatuses  40  and  54  so that the signal does not go through the decoding part  47  and the coding part  48  of the regenerative relay apparatus  44 . Hence, it is possible to minimize the signal delay. 
         [0053]    In the above-discussed first embodiment, the regenerative relay apparatus  44  is connected to the terminal apparatuses  40  and  54 . However, the regenerative relay apparatus  44  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  40  and  54 . 
       Second Embodiment of the Present Invention 
       [0054]      FIG. 5  is a structural view of a second embodiment of the relay method of the present invention. Referring to  FIG. 5 , an error correction code that is a redundancy bit for error correction is added to a signal in an encoding part  141  of a terminal apparatus  140 . Overhead is added to the signal by an OH transmission part  142  so as to be transmitted to a regenerative relay apparatus  144  via a going-up (upstream) circuit. 
         [0055]    In an OH receiving part  145  of the regenerative relay apparatus  144 , the overhead of a receiving frame is terminated. Furthermore, data in the data area OPUk among the transmission signals of the frame format shown in  FIG. 3  and the error correction code FEC are supplied to a decoding part  147  and a selection part  149 . 
         [0056]    The decoding part  147  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. At this time, an error corrected part is supplied to an error correction amount count part  146 . The data (corresponding to the data area OPUk) for which the error correction is implemented are encoded again by the encoding part  148  so that the error correction code AFEC is generated, and are then supplied to the selection part  149 . 
         [0057]    The error count part  146  calculates the number of bits of the error corrected part supplied from the decoding part  147  and supplies the number of error correction bit generated at the transmission path between the terminal apparatus  140  and the regenerative relay apparatus  144  to an adder  151 . The OH receiving part  152  terminates the overhead of the receiving frame of a going-down (downstream) circuit so as to take out the backward error information BCI in the overhead. The backward error information BCI is an error rate between the regenerative relay apparatus  144  and the terminal apparatus  154  and supplied to the adder  151 . 
         [0058]    The adder  151  supplies the total of the number of error correction bits of the transmission path between the terminal apparatus  140  and the regenerative relay apparatus  144  and the number of error correction bits of the transmission path between the regenerative relay apparatus  144  and the terminal apparatus  154  to a threshold determination part  153 . 
         [0059]    The threshold determination part  153  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  157  of the terminal apparatus  154  and the total number of the error correction bits supplied from the adder  151 . In a case where the total error rate does not exceed the threshold, a signal route is selected by the selection part  49  so that the signal goes to the selection part  149  without going through the decoding part  147  and the encoding part  148 . On the other hand, in a case where the total error rate exceeds the threshold, a signal route is selected by the selection part  149  so that the signal goes to the selection part  149  via the decoding part  147  and the encoding part  148 . 
         [0060]    The overhead is added to the error correction code FEC and the data in the data area OPUk being output by the selection part  149  by an OH transmission part  150 . Then, the data in the data area OPUk and the error correction code FEC are transmitted to the terminal apparatus  154  by the going-up (upstream) circuit. 
         [0061]    The overhead of the receiving frame is terminated at the OH receiving part  155  of the terminal apparatus  154 . Data in the data area OPUk and the error correction code FEC among transmission signals of the frame format shown in  FIG. 3  are supplied to a decoding part  157 . 
         [0062]    The decoding part  157  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. At this time, the error corrected part is supplied to an error correction amount count part  156 . 
         [0063]    The error count part  156  calculates the number of the bits of the error corrected part supplied from the decoding part  157  and supplies the number of the error correction bits generated at the transmission path between the regenerative relay apparatus  144  and the terminal apparatus  154  to an OH transmission part  158 . The OH transmission part  158  stores the number of the error correction bits supplied from the error correction amount count part  156  in the backward error information BCI in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the number of the bits to the terminal apparatus regenerative relay apparatus  144  via a going-down (downstream) circuit. 
         [0064]    Thus, by the threshold determination part  153 , the total of the error correction amount of the transmission path between the terminal apparatus  140  and the regenerative relay apparatus  144  and the error correction amount of the transmission path between the regenerative relay apparatus  144  and the terminal apparatus  154  is compared with the error correction threshold determined by the error correction ability set in advance at the decoding part  157 . In the case where the total error correction amount does not exceed the error correction threshold, the signal route is selected by the selection part  149  so that the signal goes to the selection part  149  without going through the decoding part  147  and the encoding part  148 . As a result of this, the error in a transmission section from the terminal apparatus  140  and the terminal apparatus  154  is corrected by the error correction function of the terminal apparatuses  140  and  154  so that the signal does not go through the decoding part  147  and the encoding part  148  of the regenerative relay apparatus  144 . Hence, it is possible to minimize the signal delay. 
         [0065]    In the above-discussed second embodiment, the regenerative relay apparatus  144  is connected to the terminal apparatuses  140  and  154 . However, the regenerative relay apparatus  144  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  140  and  154 . 
       Third Embodiment 
       [0066]      FIG. 6  is a structural view of a third embodiment of the relay method of the present invention. The difference between the first embodiment and the third embodiment is that, in the third embodiment, a threshold determination part is provided at a latter part terminal apparatus and a result of determination by the threshold determination part goes back to the regenerative relay apparatus so that control of the selection part is implemented. 
         [0067]    Referring to  FIG. 6 , an error correction code that is a redundancy bit for error correction is added to a signal in an encoding part  241  of a terminal apparatus  240 . Overhead is added to the signal by an OH transmission part  242  so as to be transmitted to a regenerative relay apparatus  244  via a going-up (upstream) circuit. 
         [0068]    In an OH receiving part  245  of the regenerative relay apparatus  244 , the overhead of a receiving frame shown in  FIG. 3  is terminated, and error detection is done by using the error detection code BIP  8  in the overhead and data of the data area OPUk of the frame preceding by two frames so as to be supplied to an error count part  246 . Furthermore, data in the data area OPUk and the error correction code FEC are supplied to a decoding part  247  and a selection part  249 . 
         [0069]    The decoding part  247  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. The data (corresponding to the data area OPUk) for which the error correction is implemented are encoded again by the encoding part  248  so that the error correction code FEC is generated, and are then supplied to the selection part  249 . 
         [0070]    The error count part  246  calculates an error rate of a transmission path between the terminal apparatus  240  and the regenerative relay apparatus  244  from an error detection result supplied from the OH receiving part  245  and supplies the error rate to an OH transmission part  250 . 
         [0071]    In the OH receiving part  252 , the overhead of the receiving frame of the going-down (downstream) circuit is terminated and control information CMD in the overhead is taken out. The control information CMD is an order for making the selection part  249  select a signal route without going through the decoding part  247  and a encoding part  248  or a signal route going through the decoding part  247  and the encoding part  248 . The selection part  249  follows this order so as to select either one of the signal routes and supplies the signal to the OH receiving part  250 . 
         [0072]    The data in the data area OPUk output from the selection part  249  and the error correction code FEC are supplied to the OH receiving part  250 . In the OH receiving part  250 , the error rate supplied from the error count part  246  is stored in forward error information FEI in the overhead so that the going-up (upstream) circuit is transmitted to the terminal apparatus  254 . 
         [0073]    In an OH receiving part  255  of the regenerative relay apparatus  254 , the overhead of a receiving frame is terminated, and error detection is done by using the error detection code BIP  8  in the overhead and data of the data area OPUk of the frame preceding by two frames so as to be supplied to an error count part  256 . In addition, data in the data area OPUk and the error correction code FEC are supplied to a decoding part  257 . Furthermore, forward error information FEI (an error rate of the transmission path between the terminal apparatus  240  and the regenerative relay apparatus  244 ) in the overhead is taken out and supplied to an adder  259 . 
         [0074]    The decoding part  257  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. 
         [0075]    The error count part  256  calculates the error rate of a transmission path between the terminal apparatus  254  and the regenerative relay apparatus  244  from the error detection result supplied from the OH receiving part  255  and supplies the error rate to an adder  259 . 
         [0076]    The adder  259  supplies the total of an error rate of the transmission path between the terminal apparatus  240  and the regenerative relay apparatus  244  and an error rate of the transmission path between the terminal apparatus  254  and the regenerative relay apparatus  244 , to the threshold determination part  260 . 
         [0077]    The threshold determination part  260  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  257  of the terminal apparatus  254  and the total error rate supplied from the adder  259 . In a case where the total error rate does not exceed the threshold, a signal route is selected by the selection part  249  so that the signal goes to the selection part  249  without going through the decoding part  247  and the encoding part  248 . On the other hand, in a case where the total error rate exceeds the threshold, an order is supplied to the OH receiving part  261 , so that a signal route is selected by the selection part  249  and thereby the signal goes to the selection part  249  via the decoding part  247  and the encoding part  248 . 
         [0078]    The OH transmission part  261  stores the order supplied from the threshold determination part  260  in the control information CMD in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the order to the regenerative relay apparatus  44  via a going-down (downstream) circuit. 
         [0079]    Thus, in a case where the total of the error rate of the transmission path between the terminal apparatus  240  and the regenerative relay apparatus  244  and the error rate of the transmission path between the regenerative relay apparatus  244  and the terminal apparatus  254  does not exceed the error correction threshold, the signal route is selected by the selection part  249  so that the signal goes to the selection part  249  without going through the decoding part  247  and the encoding part  248 . As a result of this, the error in a transmission section from the terminal apparatus  240  to the terminal apparatus  254  is corrected by the error correction function of the terminal apparatuses  240  and  254  so that the signal does not go through the decoding part  247  and the coding part  248  of the regenerative relay apparatus  244 . Hence, it is possible to minimize the signal delay. 
         [0080]    In the above-discussed third embodiment, the regenerative relay apparatus  244  is connected to the terminal apparatuses  240  and  254 . However, the regenerative relay apparatus  244  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  240  and  254 . 
       Fourth Embodiment 
       [0081]      FIG. 7  is a structural view of a fourth embodiment of the relay method of the present invention. The difference between the second embodiment and the fourth embodiment is that, in the fourth embodiment, a threshold determination part is provided in a latter part terminal apparatus and a result of determination by the threshold determination part goes back to the regenerative relay apparatus so that control of the selection part is implemented. 
         [0082]    Referring to  FIG. 7 , an error correction code that is a redundancy bit for error correction is added to a signal in an encoding part  341  of a terminal apparatus  340 . Overhead is added to the signal by an OH transmission part  342  so as to be transmitted to a regenerative relay apparatus  344  via a going-up (upstream) circuit. 
         [0083]    In an OH receiving part  345  of the regenerative relay apparatus  344 , the overhead of a receiving frame is terminated. Furthermore, data in the data area OPUk among the transmission signals of the frame format shown in  FIG. 3  and the error correction code FEC are supplied to a decoding part  347  and a selection part  349 . 
         [0084]    The decoding part  347  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. At this time, an error corrected part is supplied to an error correction amount count part  346 . The data (corresponding to the data area OPUk) for which the error correction is implemented is encoded again by the encoding part  348  so that the error correction code FEC is generated, and are then supplied to the selection part  349 . 
         [0085]    The error count part  346  calculates the number of bits of the error corrected part supplied from the decoding part  347  and supplies the number of error correction bit generated at the transmission path between the terminal apparatus  340  and the regenerative relay apparatus  344  to an OH transmission part  350 . 
         [0086]    In the OH receiving part  352 , the overhead of the receiving frame of the going-down (downstream) circuit is terminated and control information CMD in the overhead is taken out. The control information CMD is an order for making the selection part  349  select a signal route without going through a decoding part  347  and a encoding part  348  and or a signal route going through the decoding part  347  and the encoding part  348 . The selection part  349  follows this order so as to select either one of the signal routes and supplies the signal to the OH receiving part  350 . 
         [0087]    The data in the data area OPUk output from the selection part  349  and the error correction code FEC are supplied to the OH receiving part  350 . In the OH receiving part  350 , the error correction bit number supplied from the error correction number count part  346  is stored in forward error correction information FEC in the overhead so that the FEC is transmitted to the terminal apparatus  354  via the going-up (upstream) circuit. 
         [0088]    The overhead of the receiving frame is terminated at the OH receiving part  355  of the terminal apparatus  354 . Data in the data area OPUk and the error correction code FEC among transmission signals of the frame format shown in  FIG. 3  are supplied to a decoding part  357 . In addition, forward error correction information FCI (the number of correction bits of the transmission path between the terminal apparatus  340  and the regenerative relay apparatus  344 ) in the overhead is taken out and supplied to an adder  359 . 
         [0089]    The decoding part  357  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others. At this time, the error corrected part is supplied to an error correction amount count part  356 . 
         [0090]    The error count part  356  calculates the number of the bits of the error corrected part at the transmission path between the regenerative relay apparatus  344  and the terminal apparatus  354  from the error corrected part supplied from the OH receiving part  355  and supplies it to the adder  359 . 
         [0091]    The adder  359  supplies the total of the number of error correction bits generated at the transmission path between the terminal apparatus  340  and the regenerative relay apparatus  344  and the number of error correction bits generated at the transmission path between the terminal apparatus  354  and the regenerative relay apparatus  344 , to the threshold determination part  360 . 
         [0092]    The threshold determination part  360  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  357  of the terminal apparatus  354  and the total error rate supplied from the adder  359 . In a case where the total error rate does not exceed the threshold, a signal route is selected by the selection part  349  so that the signal goes to the selection part  349  without going through the decoding part  347  and the encoding part  348 . On the other hand, in a case where the total error rate exceeds the threshold, an order is supplied to the OH receiving part  361 , so that a signal route is selected by the selection part  349  and thereby the signal goes to the selection part  349  via the decoding part  347  and the encoding part  348 . 
         [0093]    The OH transmission part  361  stores the order supplied from the threshold determination part  360  in the control information CMD in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the order to a terminal apparatus regenerative relay apparatus  344  via a going-down (downstream) circuit. 
         [0094]    Thus, in a case where the total of the error correction amount of the transmission path between the terminal apparatus  340  and the regenerative relay apparatus  344  and the error correction amount of the transmission path between the regenerative relay apparatus  344  and the terminal apparatus  354  does not exceed the error correction threshold, the signal route is selected by the selection part  349  so that the signal goes to the selection part  349  without going through the decoding part  347  and the encoding part  348 . As a result of this, the error in a transmission section from the terminal apparatus  340  and the terminal apparatus  354  is corrected by the error correction function of the terminal apparatuses  340  and  354  so that the signal does not go through the decoding part  347  and the coding part  348  of the regenerative relay apparatus  344 . Hence, it is possible to minimize the signal delay. 
         [0095]    In the above-discussed first embodiment, the regenerative relay apparatus  344  is connected to the terminal apparatuses  340  and  354 . However, the regenerative relay apparatus  344  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  340  and  354 . 
       Fifth Embodiment 
       [0096]      FIG. 8  is a structural view of a fifth embodiment of the relay method of the present invention. Referring to  FIG. 8 , an error correction code that is a redundancy bit for error correction is added to signals in encoding parts  441  and  442  of a terminal apparatus  440 . Either one of the signals is selected by a selection part  443  and overhead is added to the selected signal by an OH transmission part  444  so as to be transmitted to a regenerative relay apparatus  450  via a going-up (upstream) circuit. 
         [0097]    Here, the encoding parts  441  and  442  implement encoding wherein each has an error correction ability that is different such that a redundancy bit rate is 3% or 7% (or, 12% or 25%). The error correction ability of the encoding part  442  is greater than the error correction ability of the encoding part  441 . A delay time at the time of decoding in the encoding part  442  is greater than a delay time at the time of decoding in the encoding part  441 . In the encoding parts  441  and  442 , encoding with different methods such as a Reed Solomon code or a BHC code may be implemented. 
         [0098]    In the OH receiving part  445 , the overhead of the receiving frame of the going-down (downstream) circuit is terminated and control information CMD in the overhead is taken out. The control information CMD is an order for the selection part  443  to select the encoding part  441  or  442 . The selection part  443  follows this order so as to select either one of the output signal and supplies the selected signal to the OH transmission part  444 . 
         [0099]    In an OH receiving part  445  of the regenerative relay apparatus  450 , the overhead of a receiving frame shown in  FIG. 3  is terminated, and error detection is done by using the error detection code BIP  8  in the overhead and data of the data area OPUk of the frame preceding by two frames so as to be supplied to an error count part  452 . Furthermore, data in the data area OPUk and the error correction code FEC are supplied to decoding parts  453  and  454 . 
         [0100]    The decoding parts  453  and  454  implement decoding corresponding to the encoding parts  441  and  442 . The decoding parts  453  and  454  decode data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implement timing regeneration, waveform shaping, and others. 
         [0101]    The data (corresponding to the data area OPUk) for which the error correction is implemented in the decoding parts  453  and  454  are supplied to the selection part  455 . Either one of the data sets is selected and encoded again by the encoding part  458  so that the error correction code FEC is generated, the overhead is added by the OH transmission part  459 , and the frame is transmitted to the terminal apparatus  460 . 
         [0102]    The error count part  452  calculates an error rate of a transmission path between the terminal apparatus  440  and the regenerative relay apparatus  450  from an error detection result supplied from the OH receiving part  451  and supplies the error rate to a threshold determination part  456 . 
         [0103]    The threshold determination part  456  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  453  and the error of the error count part  452 . In a case where the error rate does not exceed the threshold, the decoding part  453  is selected. In a case where the error rate exceeds the threshold, an order is supplied to the selection part  455  and the OH transmission part  457 , so that the decoding part  454  is selected. 
         [0104]    The OH transmission part  457  stores the order supplied from the threshold determination part  456  in the control information CMD in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the order to the terminal apparatus  440  via a going-down (downstream) circuit. 
         [0105]    The overhead of the receiving frame is terminated in the OH receiving part  461  of the terminal apparatus  460  and the data in the data area OPUk and the error correction code FEC are supplied to the decoding part  462 . The decoding part  462  decodes the data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implement timing regeneration, waveform shaping, and others again. Thus, in a case where the error rate of the transmission path between the terminal apparatus  440  and the regenerative relay apparatus  450  does not exceed the error correction threshold, the decoding part  453  and the encoding part  441  whose delay time is small are selected by the selecting parts  443  and  455 . In a case where error rate of the transmission path between the terminal apparatus  440  and the regenerative relay apparatus  450  exceeds the error correction threshold, the decoding part  454  and the encoding part  442  whose delay time is greater are selected so that it is possible to minimize the signal delay. 
         [0106]    In the above-discussed embodiment, the regenerative relay apparatus  450  is connected to the terminal apparatuses  440  and  460 . However, the regenerative relay apparatus  450  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  440  and  460 . 
       Sixth Embodiment 
       [0107]      FIG. 9  is a structural view of a sixth embodiment of the relay method of the present invention. Referring to  FIG. 9 , an error correction code that is a redundancy bit for error correction is added to signals in encoding parts  541  and  542  of a terminal apparatus  540 . Either one of the signals is selected by a selection part  543  and overhead is added to the selected signal by an OH transmission part  544  so as to be transmitted to a regenerative relay apparatus  550  via a going-up (upstream) circuit. 
         [0108]    Here, the encoding parts  541  and  542  implement encoding wherein each has an error correction ability that is different such that a redundancy bit rate is 3% or 7% (or, 12% or 25%). 
         [0109]    The error correction ability of the encoding part  542  is greater than the error correction ability of the encoding part  541 . A delay time at the time of decoding in the encoding part  542  is greater than a delay time at the time of decoding in the encoding part  541 . In the encoding parts  541  and  542 , encoding with different methods such as a Reed Solomon code or a BHC code may be implemented. 
         [0110]    In the OH receiving part  545 , the overhead of the receiving frame of the going-down (downstream) circuit is terminated and control information CMD in the overhead is taken out. The control information CMD is an order for the selection part  543  to select the encoding part  541  or  542 . The selection part  543  follows this order so as to select either one of the output signal and supplies the selected signal to the OH transmission part  544 . 
         [0111]    In an OH receiving part  545  of the regenerative relay apparatus  550 , the overhead of a receiving frame shown in  FIG. 3  is terminated, and data in the data area OPUk and the error correction code FEC are supplied to decoding parts  553  and  554 . 
         [0112]    The decoding parts  553  and  554  implements decoding corresponding to the encoding parts  541  and  542 . The decoding parts  553  and  554  decode data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implement timing regeneration, waveform shaping, and others. The error corrected parts of the decoding parts  553  and  554  are supplied to the selection part  556 . Either one of the error corrected parts is selected and supplied to the error correction amount count part  557 . 
         [0113]    The data (corresponding to the data area OPUk) for which the error correction is implemented in the decoding parts  553  and  554  are supplied to the selection part  555 . Either one of the data sets is selected and encoded again by the encoding part  560  so that the error correction code FEC is generated, the overhead is added by the OH transmission part  561 , and the frame is transmitted to the terminal apparatus  570 . 
         [0114]    The error correction amount count part  557  calculates the number of bits of the error corrected part supplied from the selection part  556 , and supplies the number of error correction bits generated at the transmission path between the terminal apparatus  540  and the regenerative relay apparatus  550  to a threshold determination part  558 . 
         [0115]    The threshold determination part  558  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  553  and the error of the error correction amount count part  557 . In a case where the error rate does not exceed the threshold, the decoding part  553  is selected. In a case where the error rate exceeds the threshold, an order is supplied to the selection parts  555  and  556  and the OH transmission part  559 , so that the decoding part  554  is selected. 
         [0116]    The OH transmission part  559  stores the order supplied from the threshold determination part  558  in the control information CMD in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the order to the terminal apparatus  540  via a going-down (downstream) circuit. 
         [0117]    The overhead of the receiving frame is terminated in the OH receiving part  571  of the terminal apparatus  570  and the data in the data area OPUk and the error correction code FEC are supplied to the decoding part  572 . The decoding part  572  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others again. 
         [0118]    Thus, in a case where the error rate of the transmission path between the terminal apparatus  540  and the regenerative relay apparatus  550  does not exceed the error correction threshold, the decoding part  553  and the encoding part  541  whose delay time is small are selected by the selecting parts  543 ,  555  and  556 . In a case where error rate of the transmission path between the terminal apparatus  540  and the regenerative relay apparatus  550  exceeds the error correction threshold, the decoding part  554  and the encoding part  542  whose delay time is greater are selected so that it is possible to minimize the signal delay. 
         [0119]    In the above-discussed embodiment, the regenerative relay apparatus  550  is connected to the terminal apparatuses  540  and  570 . However, the regenerative relay apparatus  550  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  540  and  570 . 
       Seventh Embodiment 
       [0120]      FIG. 10  is a structural view of a seventh embodiment of the relay method of the present invention. The difference between the fifth and seventh embodiments is that, in the seventh embodiment, the threshold determination part is provided in a terminal apparatus, too. 
         [0121]    Referring to  FIG. 10 , an error correction code that is a redundancy bit for error correction is added to signals in encoding parts  641  and  642  of a terminal apparatus  640 . Either one of the signals is selected by a selection part  643  and an overhead is added to the selected signal by an OH transmission part  644  so as to be transmitted to a regenerative relay apparatus  650  via a going-up (upstream) circuit. 
         [0122]    Here, the encoding parts  641  and  642  implement encoding wherein an error correction capability is different for each such that a redundancy bit rate is 3% or 7% (or, 12% or 25%). The error correction ability of the encoding part  642  is greater than the error correction ability of the encoding part  641 . A delay time at the time of decoding in the encoding part.  642  is greater than a delay time at the time of decoding in the encoding part  641 . In the encoding parts  641  and  642 , encoding system whose methods are different such as a Reed Solomon code or a BHC code may be implemented. 
         [0123]    The OH receiving part  645  terminates the overhead of the receiving frame of a going-down (downstream) circuit so as to take out the backward error information BEI in the overhead and supply the BEI to the threshold determination part  646 . The backward error information BEI is an error rate between the regenerative relay apparatus  650  and the terminal apparatus  640 . 
         [0124]    The threshold determination part  646  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  653  and the error rate. In a case where the error-rate does not exceed the threshold, the encoding part  641  is selected by the selection part  643 . On the other hand, in a case where the error rate exceeds the threshold, the encoding part  642  is selected by the selection part  643 . 
         [0125]    In an OH receiving part  651  of the regenerative relay apparatus  650 , the overhead of a receiving frame shown in  FIG. 3  is terminated, and error detection is done by using the error detection code BIP  8  in the overhead and data of the data area OPUk of the frame preceding by two frames so as to be supplied to an error count part  652 . Furthermore, data in the data area OPUk and the error correction code FEC are supplied to decoding parts  653  and  654 . 
         [0126]    The decoding parts  653  and  654  implement decoding corresponding to the encoding parts  651  and  642 . The decoding parts  653  and  654  decode data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implement timing regeneration, waveform shaping, and others. The data (corresponding to the data area OPUk) for which the error correction is implemented in the decoding parts  653  and  654  are supplied to the selection part  655 . Either one of the data sets is selected and encoded again by the encoding part  658  so that the error correction code FEC is generated, the overhead is added by the OH transmission part  659 , and the frame is transmitted to the terminal apparatus  660 . 
         [0127]    The error count part  652  calculates an error rate of a transmission path between the terminal apparatus  640  and the regenerative relay apparatus  650  from an error detection result supplied from the OH receiving part  651  and supplies the error rate to a threshold determination part  656  and the OH receiving part  657 . 
         [0128]    The threshold determination part  656  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  653  and the error of the error count part  652 . In a case where the error rate does not exceed the threshold, the decoding part  653  is selected. In a case where the error rate exceeds the threshold, an order is supplied to the selection part  655 , so that the decoding part  654  is selected. 
         [0129]    The OH transmission part  657  stores an error rate supplied from the threshold determination part  656  in the backward error information BEI in the overhead of the transmission frame of the going-down (downstream) circuit and transmits the error rate to a terminal apparatus regenerative relay apparatus  640  via a going-down (downstream) circuit. 
         [0130]    The overhead of the receiving frame is terminated in the OH receiving part  661  of the terminal apparatus  660  and the data in the data area OPUk and the error correction code FEC are supplied to the decoding part  662 . The decoding part  662  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others again. 
         [0131]    Thus, in a case where the error rate of the transmission path between the terminal apparatus  640  and the regenerative relay apparatus  650  does not exceed the error correction threshold, the decoding part  653  and the encoding part  641  whose delay time is small are selected by the selecting parts  643  and  655 . In a case where error rate of the transmission path between the terminal apparatus  640  and the regenerative relay apparatus  650  exceeds the error correction threshold, the decoding part  654  and the encoding part  642  whose delay time is greater are selected so that it is possible to minimize the signal delay. 
         [0132]    In the above-discussed embodiment, the regenerative relay apparatus  650  is connected to the terminal apparatuses  640  and  660 . However, the regenerative relay apparatus  650  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  640  and  660 . 
       Eighth Embodiment 
       [0133]      FIG. 11  is a structural view of an eighth embodiment of the relay method of the present invention. The difference between the sixth and eighth embodiments is that, in the eighth embodiment, the threshold determination part is provided in a terminal apparatus, too. Referring to  FIG. 11 , an error correction code that is a redundancy bit for error correction is added to signals in encoding parts  741  and  742  of a terminal apparatus  740 . Either one of the signals is selected by a selection part  743  and overhead is added to the selected signal by an OH transmission part  744  so as to be transmitted to a regenerative relay apparatus  750  via a going-up (upstream) circuit. 
         [0134]    Here, the encoding parts  741  and  742  implement encoding wherein an error correction capability is different for each such that a redundancy bit rate is 3% or 7% (or, 12% or 25%). The error correction ability of the encoding part  742  is greater than the error correction ability of the encoding part  741 . A delay time at the time of decoding in the encoding part  742  is greater than a delay time at the time of decoding in the encoding part  741 . In the encoding parts  741  and  742 , encoding system with different methods such as a Reed Solomon code or a BHC code may be implemented. 
         [0135]    In the OH receiving part  745 , the overhead of the receiving frame of the going-down (downstream) circuit is terminated and the backward error correction information BCI in the overhead is taken out. The backward error correction information BCI is the number of the error correction bits at the transmission path between the regenerative relay apparatus  750  and the terminal apparatus  740 . The threshold determination part  746  compares an error correction threshold determined by an error correction ability set in advance at the decoding part  753  and the number of the error correction bits. In a case where the number of the error correction bits does not exceed the threshold, the encoding part  741  is selected by the selection part  743 . On the other hand, in a case where the number of the error correction bits exceeds the threshold, the coding part  742  is selected by the selection part  743 . 
         [0136]    In an OH receiving part  751  of the regenerative relay apparatus  750 , the overhead of a receiving frame shown in  FIG. 3  is terminated, and data in the data area OPUk and the error correction code FEC are supplied to decoding parts  753  and  754 . 
         [0137]    The decoding parts  753  and  754  implement decoding corresponding to the encoding parts  741  and  742 . The decoding parts  753  and  754  decode data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implement timing regeneration, waveform shaping, and others. The error corrected parts of the decoding parts  753  and  754  are supplied to the selection part  756 . Either one of the error corrected parts is selected and supplied to the error correction amount count part  757 . 
         [0138]    The data (corresponding to the data area OPUk) for which the error correction is implemented in the decoding parts  753  and  754  are supplied to the selection part  755 . Either one of the data sets is selected and encoded again by the encoding part  760  so that the error correction code FEC is generated, the overhead is added by the OH transmission part  761 , and the frame is transmitted to the terminal apparatus  770 . 
         [0139]    The error correction amount count part  757  calculates the number of bits of the error corrected part supplied from the selection part  756 , and supplies the number of error correction bit generated at the transmission path between the terminal apparatus  740  and the regenerative relay apparatus  750  to the threshold determination part  758 . 
         [0140]    The threshold determination part  758  compares an error correction threshold determined by an error correction capability set in advance at the decoding part  753  and the error of the error correction amount count part  757 . In a case where the error rate does not exceed the threshold, the decoding part  753  is selected. In a case where the error rate exceeds the threshold, an order is supplied to the selection parts  755  and  756  and the OH transmission part  759 , so that the decoding part  754  is selected. 
         [0141]    In the OH transmission part  759 , the number of the error correction bits supplied from the error correction amount count part  757  is stored in the backward error correction information BCI in the overhead of the transmission frame of the going-down (downstream) circuit so as to be transmitted to the terminal apparatus  740  via a going-down (downstream) circuit. 
         [0142]    The overhead of the receiving frame is terminated in the OH receiving part  771  of the terminal apparatus  770  and the data in the data area OPUk and the error correction code FEC are supplied to the decoding part  772 . The decoding part  772  decodes data by implementing the error correction of the data of the data area OPUk by using the error correction code FEC and implements timing regeneration, waveform shaping, and others again. 
         [0143]    Thus, in a case where the error rate of the transmission path between the terminal apparatus  740  and the regenerative relay apparatus  750  does not exceed the error correction threshold, the decoding part  753  and the encoding part  741  whose delay time is small are selected by the selecting parts  743 ,  755  and  756 . In a case where error rate of the transmission path between the terminal apparatus  740  and the regenerative relay apparatus  750  exceeds the error correction threshold, the decoding part  754  and the coding part  742  whose delay time is great are selected so that it is possible to minimize the signal delay. 
         [0144]    In the above-discussed embodiment, the regenerative relay apparatus  750  is connected to the terminal apparatuses  740  and  770 . However, the regenerative relay apparatus  750  may be connected to another regenerative relay apparatus, instead of the terminal apparatuses  740  and  770 . 
         [0145]    Here, as shown in  FIG. 12 , in order to reduce the delay time in the optical transmission system where plural regenerative relay apparatuses  802 ,  803 ,  804 , and  805  are made in a cascade connection between the terminal apparatuses  800  and  801 , it is necessary to not only apply the first through fourth embodiments but also determine which section the signal does not pass through by using a monitor apparatus  810 . 
         [0146]    In a case where encoding and decoding are implemented in all of the regenerative relay apparatuses  802 ,  803 ,  804  and  805 , the delay time of the section A between the terminal apparatus  800  and the regenerative relay apparatus  802  is regarded as τ; the error rate of the section B between the regenerative apparatuses  802  and  803  is regarded as α; the error rate of the section C between the regenerative apparatuses  803  and  804  is regarded as 2α; the error rate of the section D between the regenerative apparatuses  804  and  805  is regarded as 2α; the error rate of the section E between the regenerative apparatus  805  and the terminal apparatus  801  is regarded as α; and an error correction threshold determined by the monitor apparatus  810  is regarded as 3α. 
         [0147]      FIG. 13  is a flowchart of an example of a determination process implemented by the monitor apparatus  810 . In  FIG. 13 , error rates at all of the sections of the route are measured in step S 10 . 
         [0148]    Next, in step S 12 , a section I where the error rate is highest is determined. In a case where the error rates are the same, a section near the terminal apparatus  800  is selected, for example. In step S 14 , a section J is determined. The error rate of the section J is highest in two sections neighboring to the section I whose error rate is determined to be highest in step S 14 . In step S 16 , the error rates of the section I and Section J are added. In step S 18 , whether a sum of the error rates exceeds the error correction threshold (3α) is determined. 
         [0149]    In a case where the sum of the error rates is lower than the error correction threshold, in step S 20 , the section I and section J (or the section K) are made to be a single section so that the signal does not pass through the section I and section J of the regenerative relay apparatus and the process goes to step S 14 . 
         [0150]    On the other hand, if the sum of the error rates exceeds the error correction threshold, the process goes to step S 22  so that whether the sum of the error rates exceeds the threshold twice consecutively according to the determination in step S 18  is determined. If the sum of the error rates exceed the threshold only one time, the process goes to step S 24  so that a section K is determined. The error rate of the section K is low in the two sections neighboring to the section I. In step S 26 , the error rates of the section I and the section K are added and the process returns to step S 18  so that whether the sum of the error rates exceed the error correction threshold (3α) is determined. 
         [0151]    Whether the sum of the error rates exceeds the threshold twice consecutively is determined in step S 22 , the section J and the section K neighboring to the section I cannot be made into a single section. Hence, in step S 28  the route is divided into two routes in a state where the section I is the boundary and the process returns to step S 10  so that the process after step  10  is repeated for the divided routes. 
         [0152]    If the determination process shown in  FIG. 13  is not implemented such that the error correction threshold is determined from a side of the terminal apparatus  800  in turn, the section A and section B are made single so as to be a new section (the error rate of the new section is 2α) so that encoding and decoding in the regenerative relay apparatus  802  are stopped. Hence, four error correction sections (A+B, C, D, and E) are required. 
         [0153]    On the other hand, if the determination process shown in  FIG. 13  is implemented, the section B and section C are made single and the section D and section E are made single. Only three error correction sections (A, B+C and D+E) are required so that the delay time can be efficiently reduced. 
         [0154]    According to the above-discussed embodiment of the present invention, it is possible to form a network wherein transmission delay is small so that the error correction ability based on compositing and decoding can be used to the maximum. In addition, by reducing the difference of the delay time between the routes, it is possible to reduce the amount of memory necessary for eliminating a phase difference between the routes in the INVERSE-MUX method wherein signals having large capacities are divided into and transmitted by plural routes and at a part after the signals are transmitted, the signals are multiplexed so that the original signal is regenerated. 
         [0155]    The present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention. 
         [0156]    For example, in the above-discussed embodiments, the error rate is compared with the error correction threshold. Instead of the error rate, the error correction amount may be compared with the error correction threshold. 
         [0157]    Either of the first through fourth embodiments may be combined with the either of the fifth through eighth embodiments. The present invention is not limited to this. The decoding part  47 ,  147 ,  257 ,  357 ,  453 ,  454 ,  553 ,  554 ,  653 ,  654 ,  753 , or  754  corresponds to a decoding part mentioned in the following claims. The encoding part  48 ,  148 ,  641  or  642  corresponds to an encoding part mentioned in the following claims. The error count part  46 ,  256 ,  452  or  652  corresponds to an error count part mentioned in the following claims. The OH receiving part  52 ,  152 ,  255 , or  355  corresponds to a transfer part mentioned in the following claims. The adder  51 ,  151 ,  259  or  359  corresponds to an adding part mentioned in the following claims. The threshold determination part  53 ,  153  or  260  or the section part  49 ,  149 ,  455 , or  555  corresponds to a selection part mentioned in the followings claims. The error correction amount count part  146 ,  356 ,  557  or  757  corresponds to an error correction amount count part mentioned in the following claims. The threshold determination part  260 ,  360 ,  456 , or  558  or the OH transmitting part  261 ,  361 ,  457 , or  559  corresponds to a selection order part mentioned in the following claims. The threshold determination part  656  or  758  or the selection part  655 ,  755  or  756  corresponds to a comparison selection part mentioned in the following claims. The OH transmitting part  657  or  759  corresponds to a notification part.