Abstract:
A ranging signal R1 generated by a signal generator and reaches to a signal checker via a working system transmission line, a loop circuit, and an auxiliary system transmission line. The signal checker measures a delay time from the generation to the arrival of the signal R1. A ranging signal R2 generated by the signal generator and reaches to a signal checker via an auxiliary system transmission line, a loop circuit, and an auxiliary system transmission line. The signal checker measures a delay time from the generation to the arrival of the signal R12. A delay time of the working system transmission line is calculated from the delay times of the signals R1 and R2. Disruption of the services provided by the other ONUs can be prevented since the working system transmission line is not used for upstream communication of the ranging signals R1 and R2.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 USC 119 from Japanese Patent Application No. 2007-119582, the disclosure of which is incorporated by reference herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an improvement of a ranging function provided in a redundant optical access system. The invention is applicable to a redundant optical access system for constructing an access system such as a PON (Passive Optical Network) and the like. 
         [0004]    2. Description of the Related Art 
         [0005]    Hitherto, FTTx (Fiber To The x) has been known as an optical access network for providing communication services such as Internet, IP (Internet Protocol) telephone, distribution of video and the like. As the FTTx, there have been known FTTH (Fiber To The Home), FTTC (Fiber To The Curb), FTTN (Fiber To The Node), FTTP (Fiber To The Premises) and the like for example. The PON is also known as a subscriber access technology for realizing the FTTx at low cost. As the PON, there have been known ATM-PON (Asynchronous Transfer Mode-PON: technology standardized by ITU-T G.983.1 and G983.2), B-PON (Broadband-PON: technology standardized by ITU-T G.983.3), G-PON (Gigabit-PON: technology standardized by ITU-T G.984) and GE-PON (Gigabit Ethernet (Registered Mark)-PON: technology standardized by IEEE802.3ah). 
         [0006]      FIG. 3  is a conceptual diagram showing a topology of the PON. As shown in  FIG. 3 , an OLT (Optical Line Terminal: subscriber station unit)  301  accommodates n ONUs (Optical Network Unit: optical line terminal unit)  305 - 1 , . . . ,  305 -n via a splitter  302  and optical fibers  303  and  304 - 1 , . . . ,  304 -n. The OLT  301  and the ONUs  305 - 1 , . . . ,  305 -n use different frequencies for downstream and upstream communications. Therefore, it is possible to carry out the downstream and upstream communications in parallel. 
         [0007]    In the downstream communication, the OLT  301  receives IP packets addressed to the ONUs  305 - 1 , . . . ,  305 -n from a host network  311 . Then, the OLT  301  generates time division multiplexed downstream signals from these IP packets. The downstream signal may contain communication data D 1 , . . . , Dn addressed to each of the ONUs  305 - 1 , . . . ,  305 -n. This downstream signal is outputted from the OLT  301  and arrives at the splitter  302  via the optical fiber  303 . The splitter  302  outputs the same downstream signal to each of the optical fibers  304 - 1 , . . . ,  304 -n. Receiving the downstream signal from the corresponding optical fibers  304 - 1 ,  . . . ,  304 -n, the ONUs  305 - 1 , . . . ,  305 -n extract the IP packets D 1 , . . . , Dn addressed to own, to convert into communication data and sends to corresponding communication terminals  312  (e.g., a personal computer, an IP telephone and the like). It is noted that the OLT  301  transmits the communication data D 1 , . . . , Dn by encrypting them in order to assure confidentiality of the communication (i.e., so that the ONUs other than the addressed ONU cannot decode the communication data D 1 , . . . , Dn). 
         [0008]    On the other hand, during the upstream communication, the ONUs  305 - 1 , . . . ,  305 -n receive communication data U 1 , . . . , Un from the corresponding communication terminals  312 . The communication data U 1 , . . . , Un are outputted at timing set per each ONU  305 - 1 , . . . ,  305 -N. The communication data U 1 , . . . , Un arrive at the splitter  302  via the optical fibers  304 - 1 , . . . ,  304 -n. The splitter  302  superimposes the communication data U 1 , . . . , Un as they are. At this time, a multiplexed upstream signal may be generated by adequately setting the timing for outputting the communication data U 1 , . . . , Un from each of the ONUs  305 - 1 , . . . ,  305 -n, because the splitter  302  superimposes the communication data. The upstream signal is outputted from the splitter  302  and arrives at the OLT  301  via the optical fiber  303 . The OLT  301  multiplies and separates the upstream signal to generate IP packets and sends them to the host network  311 . 
         [0009]    In order for the splitter  302  to multiply the upstream signal in the time-division manner as described above, it is necessary to coordinate the output timing of the communication data U 1 ,  . . . , Un per each of the ONUs  305 - 1 , . . . ,  305 -n. Here, a distance from the ONUs  305 - 1 , . . . ,  305 -n to the splitter  302  differ per each of the ONUs  305 - 1 , . . . ,  305 -n. Therefore, a delay time (signal propagating time) from the ONUs  305 - 1 , . . . ,  305 -n to the splitter  302  also differs from each other. Therefore, it is necessary to take the difference of the delay times into account in coordinating the signal output timing of each of the ONUs  305 - 1 , . . . ,  305 -n in order to carry out the time-division multiplication by the splitter  302 . For such reason, it is necessary to precisely measure the delay time from each of the ONUs  305 - 1 , . . . ,  305 -n to the splitter  302  in the PON. 
         [0010]    As a method for measuring such a delay time, there has been known a method called as ranging. As a ranging system, there is a system stipulated in ITU-T Recommendation G.983.1 for example (see “ITU-T Recommendation” issued by International Telecommunications Union, January 2005, p. 72, FIG. 25/G983.1-Configuration of the specification points).  FIG. 4  is a conceptual diagram for explaining this method and is substantially the same diagram with  FIG. 25  in ITU-T Recommendation G.983.1. 
         [0011]    In the technology shown in  FIG. 4 , a processing circuit  402  of the OLT  401  generates and outputs an electrical signal for measuring the delay (referred to as a “ranging signal” hereinafter) at first. The ranging signal is converted into an optical signal by an electrical/optical converter  403  and is sent to an ONU  405  via an optical communication line  404 . The inputted ranging signal is converted into an electrical signal by an optical/electrical converter  406  and is inputted to a processing circuit  407 . The processing circuit  407  transfers this ranging signal to an electrical/optical converter  408 . This ranging signal is then converted again into an optical signal by the electrical/optical converter  408  and is returned to the OLT  401 . It is converted into an electrical signal again by an optical/electrical converter  409  and is inputted to the processing circuit  402 . The processing circuit  402  measures an elapsed time Tconst from the output to the input of the ranging signal by using a built-in timer not shown. 
         [0012]    Here, a delay time when the ranging signal passes through the optical communication line  404  in the downstream direction is the same with that in the upstream direction, such delay time will be defined as Tpd, respectively. Delay times in passing through the converters  403 ,  406 ,  408  and  409  will be defined as TiS 1 , TiO 1 , TiO 2  and TiS 2 , a delay time when the processing circuit  407  transfers the ranging signal from the optical/electrical converter  406  to the electrical/optical converter  408  will be defined as Ts and an equalized delay time of the processing circuit  407  (i.e., a transmission delay time between the OLT through the ONU) will be defined as Td. Here, TiS 1  and TiS 2  may be measured or set independently. A sum Tresponce of TiO 1 , Ts, Td and TiO 2  is also measurable. Accordingly, it is possible to obtain the delay time Tpd from Tconst by the following expressions ( 1   a ) and ( 1   b ): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Tconst 
                         = 
                         
                           
                             TiS 
                              
                             
                                 
                             
                              
                             1 
                           
                           + 
                           Tpd 
                           + 
                           
                             TiO 
                              
                             
                                 
                             
                              
                             1 
                           
                           + 
                           Ts 
                           + 
                           Td 
                           + 
                           
                             TiO 
                              
                             
                                 
                             
                              
                             2 
                           
                           + 
                           Tpd 
                           + 
                           
                             TiS 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             2 
                             × 
                             Tpd 
                           
                           + 
                           Tresponse 
                           + 
                           
                             TiS 
                              
                             
                                 
                             
                              
                             1 
                           
                           + 
                           
                             TiS 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     1 
                      
                     a 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     Tresponse 
                     = 
                     
                       
                         TiO 
                          
                         
                             
                         
                          
                         1 
                       
                       + 
                       Ts 
                       + 
                       Td 
                       + 
                       
                         TiO 
                          
                         
                             
                         
                          
                         2 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     ( 
                     
                       
                         
                           when 
                            
                           
                               
                           
                            
                           Td 
                         
                         = 
                         0 
                       
                       , 
                       
                         Tresponse 
                         = 
                         
                           
                             TiO 
                              
                             
                                 
                             
                              
                             1 
                           
                           + 
                           Ts 
                           + 
                           
                             TiO 
                              
                             
                                 
                             
                              
                             2 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   
                     1 
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    The splitter  302  can generate the time-division multiplexed upstream signal by finding the delay time Tpd of each of the ONUs  305 - 1 , . . . ,  305 -n (see  FIG. 3 ) through the procedure described above and by coordinating the output timing of the ONUs  305 - 1 , . . . ,  305 -n. 
         [0013]    As shown in  FIG. 3 , one OLT  301  can accommodate plural numbers of ONUs  305 - 1 , . . . ,  305 n in the PON and can additionally accommodate ONUs after initiating the operation of the PON. It is necessary to carry out the ranging as described above for the ONU newly accommodated, when adding the ONU. Beside the case of accommodating the new ONU, there is a case when the ranging needs to be carried out while the PON is in-service. When the ranging of either one ONU is being carried out, the other ONUs are required to stop the upstream communication. It is because reliability of the upstream signal of the other ONUs cannot be guaranteed since the upstream communication of the ranging signal is carried out even though its delay time (see  FIG. 4 ) is not specified, and there is a possibility that the ranging signal collides with the upstream signal of the other ONUs. Therefore, ITU-T Recommendation G.983 and G984 stipulate that the other ONUs should not transmit upstream signals during a ranging period (i.e., a ranging window). 
         [0014]      FIGS. 5A and 5B  are conceptual diagrams for explaining the ranging operation in the PON.  FIG. 5A  is a conceptual diagram showing a configuration of the PON and  FIG. 5B  is a conceptual diagram showing the operation. The splitter and others are omitted in  FIG. 5A .  FIGS. 5A and 5B  show a case when the ranging of the ONU # 1  is carried out during when a communication is made between the OLT and the ONU # 2  via an optical communication line  501 . In this case, it is unable to guarantee the reliability of the upstream communication of the ONU # 2  as described above. Therefore, the upstream communication of the ONU # 2  is forbidden from the beginning to the end of ranging of the ONU # 1  (i.e., from when the OLT transmitted the ranging signal until when it receives the ranging signal). The communication between the OLT and the ONU # 2  is started again when the ranging ends (see a, b and c in  FIG. 5B ). 
         [0015]    However, it is not desirable to disrupt the service of the ONU for the ranging from a point of view of assuring quality and reliability of the service. Therefore, it has been desired to provide a countermeasure for carrying out the ranging without disrupting the service to the other ONU. 
         [0016]    As a countermeasure for that, there has been a method of providing a buffer on the side of the ONU to temporarily accumulate upstream signals.  FIG. 6  is a conceptual diagram showing a case when the buffer is provided in the ONU. As shown in  FIG. 6 , signals outputted out of a communication terminal are accumulated once in the buffer  604  within the buffer  604 . The signals accumulated in the buffer  604  are outputted with timing specified by a read control circuit  605  and arrive at the OLT  601  via an optical communication cable  603 . 
         [0017]    When no buffer is provided in the ONU, the object ONU of ranging operates as shown in  FIG. 7A . That is, this ONU cannot transmit upstream signals inputted from the communication terminal to the OLT during a period corresponding to the ranging window W. Therefore, the upstream signals fall out. 
         [0018]    When the buffer is provided in the ONU, the ONU that is not the object of ranging operates as shown in  FIG. 7B . The upstream signals inputted from the communication terminal are accumulated once in the buffer and are outputted sequentially with the timing specified by the read control circuit (see  FIG. 6 ). Accordingly, the timing for outputting each upstream signal of the ONU becomes late as compared to the case of  FIG. 7A  even before the ranging window W is initiated. When the ranging window W is initiated, the read control circuit stops to read the accumulated upstream signals. Therefore, a number of upstream signals accumulated in the buffer increases. After that, when the ranging window W ends, the read control circuit quickly and sequentially reads the upstream signals accumulated in the buffer to output from the ONU. 
         [0019]    However, the method of using the buffer leads to an increase of cost of the PON since the required buffer capacity is large. The larger the number of ONUs accommodated in one OLT, the larger the required buffer capacity becomes. Further, the larger the number of splits of the optical communication line (i.e., a number of the splitters) and the larger the transmission band, the larger the buffer capacity becomes. 
         [0020]    In addition to that, because the upstream signals need to be quickly read and transmitted when the buffer is provided on the side of the ONU, a high precision and complex control is required. 
       SUMMARY OF THE INVENTION 
       [0021]    The present invention provides a low-cost optical access system that can continue services during the ranging window with simple controls and at a low cost. 
         [0022]    According to a first aspect of the invention, an optical access system including, a station unit having a working system station interface and an auxiliary system station interface, an optical communication cable having a working system transmission line connected to the working system station interface and an auxiliary system transmission line connected to the auxiliary system station interface, a terminal access unit having a working system terminal interface connected to the working system transmission line and an auxiliary system terminal interface connected to the auxiliary system transmission line, a first ranging signal generator that generates a first ranging signal that outputs from the working system station interface to the working system transmission line, a first loop circuit that transfers the first ranging signal received by the working system terminal interface from the working system transmission line to the auxiliary system terminal interface, a second loop circuit that transfers the first ranging signal, which is received by the auxiliary system station interface from the auxiliary system terminal interface via the auxiliary system transmission line, to the working system station interface, a first checker that checks a time from when the working system station interface transmits the first ranging signal until the working system station interface receives the first ranging signal, a second ranging signal generator that generates a second ranging signal that outputs from the auxiliary system station interface to the auxiliary system transmission line, a third loop circuit that outputs the second ranging signal, which is received by the auxiliary system terminal interface from the auxiliary system transmission line, to the auxiliary system transmission line from the auxiliary system terminal interface, and a second checker that checks a time from when the auxiliary system station interface transmits the second ranging signal until the auxiliary system station interface receives the second ranging signal. 
         [0023]    In the aspect described above, the station unit may include a working system common section and an auxiliary system common section that respectively mediate communication between the working system station interface and a host network, and the auxiliary system station interface and the host network, a first selector that selectively supplies a signal outputted from either one of the working system station interface or the auxiliary system station interface, to the working system common section or the auxiliary system common section, and a second selector that selectively supplies a signal outputted from either one of the working system common section or the auxiliary system common section, to the working system station interface or the auxiliary system station interface. 
         [0024]    According to a second aspect of the invention, a ranging method of the optical access system including a station unit having a working system station interface and an auxiliary system station interface, an optical communication cable having a working system transmission line connected to the working system station interface and an auxiliary system transmission line connected to the auxiliary system station interface, and a terminal access unit having a working system terminal interface connected to the working system transmission line and an auxiliary system terminal interface connected to the auxiliary system transmission line, the ranging method including, a first step of generating a first ranging signal and outputting the first ranging signal from the working system station interface to the working system transmission line, a second step of transferring the first ranging signal received by the working system terminal interface from the working system transmission line, to the auxiliary system terminal interface, a third step of transferring the first ranging signal to the working system station interface via the auxiliary system terminal interface, the auxiliary system transmission line and the auxiliary system station interface, a fourth step of generating a second ranging signal and outputting the second ranging signal from the auxiliary system station interface to the auxiliary system transmission line, a fifth step of transferring the second ranging signal, which is received by the auxiliary system terminal interface via the auxiliary system transmission line, back to the auxiliary system station interface from the auxiliary system terminal interface, via the auxiliary system transmission line, and a sixth step of computing a delay time of the working system transmission line based on a time from when the working system station interface transmits the first ranging signal until the working system station interface receives the first ranging signal, and a time from when the auxiliary system station interface transmits the second ranging signal until the auxiliary system station interface receives the second ranging signal. 
         [0025]    According to the above-mentioned aspects, the working (actually-used) system transmission line is used for the downstream communication (communication from the station side to the terminal interface side) and the auxiliary system transmission line is used for the upstream communication (communication from the terminal interface side to the station side) during ranging. Here, because the station unit performs time-divided multiplexing for the downstream communication, there is no possibility of causing collision of signals. Further, the auxiliary system transmission line is used for the ranging signals, i.e., the first and second ranging signals, there is also no possibility that they collide with signals transmitted from other access terminals. Therefore, the invention can prevent the disruption of services during a ranging window at low cost without involving complicated control. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0027]      FIG. 1  is block diagram schematically showing a redundant configuration of a PON system of an embodiment; 
           [0028]      FIG. 2  is a block diagram schematically showing a functional configuration of the PON system of the embodiment; 
           [0029]      FIG. 3  is a conceptual diagram showing a topology of the PON; 
           [0030]      FIG. 4  is a conceptual diagram for explaining a ranging method stipulated by ITU-T Recommendation G.983.1; 
           [0031]      FIG. 5A  is a conceptual diagram for explaining a ranging operation in the PON; 
           [0032]      FIG. 5B  is a conceptual diagram for explaining the ranging operation in the PON; 
           [0033]      FIG. 6  is a conceptual diagram showing an exemplary configuration of a conventional art PON system; 
           [0034]      FIG. 7A  is a conceptual diagram showing an exemplary operation of the conventional art PON system; and 
           [0035]      FIG. 7B  is a conceptual diagram showing an exemplary operation of the conventional art PON system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    An embodiment of an optical access system of the invention will be explained below by exemplifying a case when the invention is applied to a PON system and by using  FIGS. 1 and 2 . It is noted that size, shape and disposition of each component are schematically shown only to a degree of helping to understand the invention and numerical conditions explained below are merely examples. 
         [0037]      FIG. 1  is block diagram schematically showing a redundant configuration of a PON system of the embodiment. As shown in  FIG. 1 , the PON system  100  of the embodiment includes an OLT  110 , an ONU  120  and an optical communication cable  130 . 
         [0038]    The OLT  110  includes a working system interface  111  and an auxiliary system interface  112 , a working system common section  113  and an auxiliary system common section  114 . 
         [0039]    The working system interface  111  communicates with a working system interface  121  of the ONU  120  via a working system transmission line  131  (described later). The working system interface  111  includes a PON end layer processing section  111   a , an optical/electrical converter  111   b , an electrical/optical converter  111   c  and a selector  111   d.    
         [0040]    The PON end layer processing section  111   a  performs signal processing for making PON communication in the same manner with the conventional art and processing of the ranging of the embodiment. An internal structure and operation of the PON end layer processing section  111   a  will be described later. 
         [0041]    The optical/electrical converter  111   b  converts an optical signal inputted from the working system transmission line  131  into an electrical signal to output to the PON end layer processing section  111   a.    
         [0042]    The electrical/optical converter  111   c  converts the electrical signal inputted from the PON end layer processing section  111   a  into an optical signal to output to the transmission line  131 . 
         [0043]    The selector  111   d  selects either one of the working system common section  113  or the auxiliary system common section  114  and sends an output signal of the selected common section to the PON end layer processing section  111   a.    
         [0044]    The auxiliary system interface  112  communicates with an auxiliary system interface  122  of the ONU  120  via an auxiliary system transmission line  132  (described later). The auxiliary system interface  112  includes a PON end layer processing section  112   a , an optical/electrical converter  112   b , an electrical/optical converter  112   c  and a selector  112   d.    
         [0045]    The PON end layer processing section  112   a  performs processing related to the ranging of the present embodiment. An internal structure and operation of the PON end layer processing section  112   a  will be described later. 
         [0046]    The optical/electrical converter  112   b  converts an optical signal inputted from the auxiliary system transmission line  132  into an electrical signal to output to the PON end layer processing section  112   a.    
         [0047]    The electrical/optical converter  112   c  converts the electrical signal inputted from the PON end layer processing section  111   a  into an optical signal to output to the transmission line  132 . 
         [0048]    The selector  112   d  selectively sends an output signal of the common section  113  or  114  to the PON end layer processing section  112   a.    
         [0049]    The working system common section  113  intermediates communication of the interfaces  111  and  112  with the host network. The working system common section  113  includes a selector  113   a . The selector  113   a  selectively sends the output signal of the interfaces  111  and  112  to an outside network. 
         [0050]    The auxiliary system common section  114  intermediates communication of the interfaces  111  and  112  with the host network. The auxiliary system common section  114  includes a selector  114   a . The selector  114   a  selectively sends the output signals of the interfaces  111  and  112  to the outside network. 
         [0051]    The ONU  120  includes a working system interface  121 , an auxiliary system interface  122  and a common section  123 . 
         [0052]    The working system interface  121 communicates with the working system interface  111  of the OLT  110  via the working system transmission line  131 . The working system interface  121  includes a PON end layer processing section  121   a , an electrical/optical converter  121   b  and an optical/electrical converter  121   c.    
         [0053]    The PON end layer processing section  121   a  performs signal processing for PON communications in the same manner with the conventional art and processing related to ranging of the embodiment. An internal structure and operation of the PON end layer processing section  121   a  will be described later. 
         [0054]    The electrical/optical converter  121   b  converts an electrical signal inputted from the PON end layer processing section  121   a  into an optical signal to output to the working system transmission line  131 . 
         [0055]    The optical/electrical converter  121   c  converts an optical signal inputted from the working system transmission line  131  into an electrical signal to output to the PON end layer processing section  121   a.    
         [0056]    The auxiliary system interface  122  communicates with the auxiliary system interface  112  of the OLT  110  via the auxiliary system transmission line  132 . The auxiliary system interface  122  includes a PON end layer processing section  122   a , an electrical/optical converter  122   b  and an optical/electrical converter  122   c.    
         [0057]    The PON end layer processing section  122   a  performs processing related to ranging of the embodiment. An internal structure and operation of the PON end layer processing section  122   a  will be described later. 
         [0058]    The electrical/optical converter  122   b  converts an electrical signal inputted from the PON end layer processing section  122   a  into an optical signal to output to the auxiliary system transmission line  132 . 
         [0059]    The optical/electrical converter  122   c  converts an optical signal inputted from the auxiliary system transmission line  132  into an electrical signal to output to the PON end layer processing section  122   a.    
         [0060]    The common section  123  intermediates communication of the interfaces  121  and  122  with the communication terminal. The common section  123  is connected with the communication terminal via UNI (User Network Interface). The common section  123  includes a copying section  123   a  and a selector  123   b . The copying section  123   a  copies a signal inputted from the communication terminal and sends a totally same signal to the PON end layer processing sections  121   a  and  122   a  of the respective interfaces  121  and  122 . The selector  123   b  selectively sends an output signal of the PON end layer processing sections  121   a  and  122   a.    
         [0061]    The optical communication cable  130  has the working system transmission line  131  and the auxiliary system transmission line  132 . The working system transmission line  131 , as described above, connects the communication of the working system interface  111  of the OLT  110  with the working system interface  121  of the ONU  120 . Further, the auxiliary system transmission line  132  connects the communication of the auxiliary system interface  112  of the OLT  110  with the auxiliary system interface  122  of the ONU  120 . 
         [0062]    Although loop circuits are formed respectively within the OLT  110  and the ONU  120  in the present embodiment, as described later, they are omitted in  FIG. 1  (see  FIG. 2 ). 
         [0063]      FIG. 2  is a block diagram schematically showing a functional configuration of the PON system of the embodiment. Each of the constituent elements of  FIG. 2  with the same reference numbers as  FIG. 1  indicate the same features as those of  FIG. 1 . 
         [0064]    As shown in  FIG. 2 , a signal generator  211 , a signal checker  212  and a timer  213  are functionally configured in the PON end layer processing section  111   a . The signal generator  211  generates a ranging signal R 1  and sends the ranging signal R 1  to the electrical/optical converter  111   c . The signal checker  212  inputs the ranging signal R 1  from the loop circuit  253  (described later). The signal checker  212  also checks a time required from when the ranging signal R 1  was transmitted by the signal generator  211  until when the ranging signal R 1  arrives at the signal checker  212  by using the timer  213 . The timer  213  counts the time under the control of the signal checker  212 . 
         [0065]    In the PON end layer processing section  112   a , a signal generator  221 , a signal checker  222  and a timer  223  are functionally configured. The signal generator  221  generates a ranging signal R 2  and sends the ranging signal R 2  to the electrical/optical converter  112   c . The signal checker  222  inputs the ranging signal R 2  from the optical/electrical converter  112   b . The signal checker  222  also checks a time required from when the ranging signal R 2  was transmitted by the signal generator  221  until when the ranging signal R 2  arrives at the signal checker  222  by using the timer  223 . The timer  223  counts the time under the control of the signal checker  222 . When the ranging signal R 1  is inputted from the optical/electrical converter  112   b , the PON end layer processing section  112   a  transfers this ranging signal R 1  to the loop circuit  253 . 
         [0066]    The PON end layer processing section  121   a  transfers the ranging signal R 1  inputted from the optical/electrical converter  121   c  to the loop circuit  251 . 
         [0067]    A selector  231  is functionally configured in the PON end layer processing section  122   a . The selector  231  sends the ranging signals R 1  and R 2  inputted from the loop circuits  251  and  252  to the electrical/optical converter  122   b . The PON end layer processing section  122   a  sends the ranging signal R 2  inputted from the optical/electrical converter  122   c  to the loop circuit  252 . 
         [0068]    The loop circuit  251  is a circuit for transferring the ranging signal R 1  outputted out of the optical/electrical converter  121   c  to the electrical/optical converter  122   b.    
         [0069]    The loop circuit  252  is a circuit for transferring the ranging signal R 2  outputted out of the optical/electrical converter  122   c  to the electrical/optical converter  122   b.    
         [0070]    The loop circuit  253  is a circuit for transferring the ranging signal R 1  outputted out of the optical/electrical converter  112   b  to the signal checker  212 . 
         [0071]    Next, the operation of the PON system of the present embodiment will be explained. 
         [0072]    In the beginning, the PON system measures the signal delay time Tloop by using the ranging signal R 1  as follows. 
         [0073]    At first, the PON end layer processing section  111   a , i.e., the PON end layer processing section  111   a  provided in the working system interface  111  of the OLT  110 , generates the ranging signal R 1 . The signal generator  211  generates the ranging signal R 1  as described above. When the ranging signal R 1  is outputted, the signal checker  212  memorizes that time indicated by the timer  213 . 
         [0074]    The ranging signal R 1  is converted into an optical signal by the electrical/optical converter  111   c  and is outputted to the working system transmission line  131 . As described above, the optical signal transmitted from the OLT  110  to the ONU  120  is multiplexed in the time-division manner within the OLT  110  so that the ranging signal R 1  will not collide with optical signals directed to another ONU (not shown). 
         [0075]    The optical signals outputted to the working system transmission line  131  (i.e., the time-divided multiplexed signals including the ranging signal R 1 ) passes through the splitter (not shown in  FIGS. 1 and 2 . See  FIG. 3 ) and reaches to the ONU  120 . The optical/electrical converter  121   c  of the ONU  120  converts the optical signals into electrical signals and sends them to the PON end layer processing section  121   a . The PON end layer processing section  121   a  extracts the ranging signal R 1  from the electrical signals and outputs the ranging signal R 1  to the loop circuit  251 . 
         [0076]    The ranging signal R 1  passes through the loop circuit  251  and the selector  231  and reaches to the electrical/optical converter  122   b . The electrical/optical converter  122   b  converts the ranging signal R 1  into an optical signal and outputs the ranging signal R 1  to the auxiliary system transmission line  132 . Because the auxiliary system transmission line  132  is not used for upstream communications of the other ONUs, the ranging signal R 1  will cause no collision nor destruction. 
         [0077]    The ranging signal R 1  passes through the auxiliary system transmission line  132  and reaches to the optical/electrical converter  112   b . The ranging signal R 1  is converted into an electrical signal by the optical/electrical converter  112   b  and is received by the signal checker  212  within the PON end layer processing section  111   a  via the PON end layer processing section  112   a  and the loop circuit  253 . 
         [0078]    The signal checker  212  memorizes the ranging signal R 1  receiving time by reading from the timer  213 . Then, the signal checker  212  compares the ranging signal R 1  receiving time with the time when the signal generator  211  outputted the ranging signal R 1  to calculate a whole signal delay time Tloop. Tloop may be expressed by the following expression (2). In the expression (2), TiS 1 _w is a delay time from the signal generator  211  to an input end of the working system transmission line  131 , Tpd_w is a delay time of the working system transmission line  131 , TiO 1 _w is a delay time from an output end of the working system transmission line  131  to an input end of the loop circuit  251 , Tsd_wp is a delay time of the loop circuit  251 , TiO 2 _p is a delay time from the selector  231  to an input end of the auxiliary system transmission line  132 , Tpd_p is a delay time of the auxiliary system transmission line  132 , TiS 2 _p is a delay time from an output end of the auxiliary system transmission line  132  to the signal checker  222  and Tsd_pw is a delay time of the loop circuit  253 . 
         [0000]      Tloop= TiS 1 —   w+Tpd   —   w+TiO 1 —   w+Tsd   —   wp+TiO 2 —   p+Tpd   —   p+TiS 2 —   p+Tsd   —   pw    (2) 
         [0079]    Next, the PON system measures a signal delay time Tres_p by using the ranging signal R 2  as follows. 
         [0080]    At first, the PON end layer processing section  112   a , i.e., the PON end layer processing section  112   a  provided in the auxiliary system interface  112  of the OLT  110 , generates the ranging signal R 2 . The signal generator  221  generates the ranging signal R 2  as described above. When the ranging signal R 2  is outputted, the signal checker  222  memorizes that time indicated by the timer  223 . 
         [0081]    The ranging signal R 2  is converted into an optical signal by the electrical/optical converter  112   c  and is outputted to the auxiliary system transmission line  132 . Because the auxiliary system transmission line  132  is not used by the other ONUs as described above, the ranging signal R 2  will not collide with other signals. 
         [0082]    The ranging signal R 2  outputted to the auxiliary system transmission line  132  passes through the splitter (not shown) and reaches to the ONU  120 . The optical/electrical converter  122   c  of the ONU  120  converts the ranging signal R 2  into an electrical signal and sends the ranging signal R 2  to the PON end layer processing section  122   a . The PON end layer processing section  122   a  outputs the ranging signal R 2  to the loop circuit  252 . 
         [0083]    The ranging signal R 2  passes through the loop circuit  252  and the selector  231  and reaches to the electrical/optical converter  122   b . The electrical/optical converter  122   b  converts the ranging signal R 2  into an optical signal and outputs the ranging signal R 2  to the auxiliary system transmission line  132 . At this time too, the ranging signal R 2  will not cause any collision or destruction. 
         [0084]    The ranging signal R 2  passes through the auxiliary system transmission line  132  and reaches to the optical/electrical converter  112   b . The ranging signal R 2  is converted into an electrical signal by the optical/electrical converter  112   b  and is received by the signal checker  222 . 
         [0085]    The signal checker  222  memorizes the ranging signal R 2  receiving time by reading from the timer  223 . Then, the signal checker  222  compares the ranging signal R 2  receiving time with the time when the signal generator  221  outputted the ranging signal R 2  to calculate a whole signal delay time Tres_p. Tres_p may be expressed by the following expression (3). In the expression (3), TiS 1 _w is a delay time from the signal generator  221  to an input end of the auxiliary system transmission line  132 , Tpd_p is a delay time of the auxiliary system transmission line  132 , TiO 1 _p is a delay time from an output end of the auxiliary system transmission line  132  to an input end of the loop circuit  252 , Ts_P+Td_p is a sum of a delay time of the loop circuit  252  and an equalized delay time, TiO 2 _p is a delay time from the selector  231  to the input end of the auxiliary system transmission line  132 , Tpd_p is a delay time of the auxiliary system transmission line  132  and TiS 2 _p is a delay time from the output end of the auxiliary system transmission line  132  to the signal checker  222 . 
         [0000]      Tres —   p=TiS 1 —   p+Tpd   —   p+TiO 1 13    p+Ts   —   p+Td   —   p+TiO 2 —   p+Tpd   —   p+TiS 2 —   p    (3) 
         [0086]    The Tpd_w of the working system transmission line  131  may be expressed from the expressions (2) and (3) by the following expression (4): 
         [0000]        Tpd   —   w =Tloop−Tres —   p+Tpd   —   p− ( TiS 1 —   w+TiO 1 —   w )−( Tsd   —   wp+Tsd   —   pw ) + TiS 1 —   p+TiO 1 —   p+Ts   —   p+Td   —   p    (4) 
         [0087]    Here, the delay time Tpd_p is a delay time of the auxiliary system transmission line. Therefore, the delay time Tpd_p can be measured by using the same method with the conventional art without disrupting the operations of the working system interfaces  111  and  121 . 
         [0088]    The delay times TiSi_w and TiO 1 _w can be calculated while designing the device. Therefore, the delay times TiSi_w and TiO 1 _w can be handled as known values. 
         [0089]    The delay times Tsd_wp and Tsd_pw can be calculated while designing the device or may be measured without disrupting the operations of the working system interfaces  111  and  121 . Therefore, the delay times Tsd_wp and Tsd_pw can be handled as known values. 
         [0090]    Because the delay times TiS 1 _p, TiO 1 _p, Ts_p and Td_p can be calculated while designing the unit, the delay times TiS 1 _p, TiO 1 _p, Ts_p and Td_p can be handled as known values. 
         [0091]    Accordingly, it is possible to calculate the delay time Tpd_w by measuring the delay times Tloop and Tres_p. 
         [0092]    As described above, according to the present embodiment, the auxiliary system transmission line  132  is used for the upstream communication in ranging (i.e., the communication of sending the ranging signal R 1  from the ONU  120  to the OLT  110 ), so that the ranging signal R 1  will not collide with upstream communication signals of the other ONUs. Therefore, according to the present embodiment, it is not necessary to disrupt the service of the other ONUs during ranging. 
         [0093]    Further, according to the present embodiment, a buffer to prevent disruption of the service of the other ONUs is not necessary, and the increase of the scale of the circuit can be suppressed. Therefore, increases in the cost of the system can be suppressed. Further, a traffic control may be simplified by not using the buffer. 
         [0094]    Thus, the ranging of the ONU  120  may be performed even when another ONUs are in-service by using the interfaces  112  and  122  and the transmission line  132 , when each component (the interfaces  111  and  121  and the transmission line  131 ) of the working system is totally the same with each component (the interfaces  112  and  122  and the transmission line  132 ) of the auxiliary system.