Patent Publication Number: US-2023163873-A1

Title: Station-side device, optical communication system, and search method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2021-191278, filed on Nov. 25, 2021, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosure relates to a station-side device, an optical communication system, and a search method. 
     2. Description of the Related Art 
     As a method for providing optical access services, such as fiber-to-the-home (FTTH), to users, there is a passive optical network (PON) technique. 
     In a PON system using the PON technique, multiple optical network units (ONUs) or subscriber-side devices are connected to an optical line terminal (OLT) or a station-side device. PON systems are capable of providing a high-speed access service at a low cost by using optical fibers as transmission lines. Today, ONUs are widely used in homes. 
     In a PON system, an OLT and multiple ONUs are connected by optical fibers, and in order for the OLT to correctly reproduce a signal, it is necessary to manage the optical transmission timings of the ONUs so that multiple ONUs do not simultaneously emit light. Thus, the OLT instructs a target ONU of the light emission timing to control the target ONU so as to emit light only at an instructed time. 
     In WO 2013/140454, Hamaoka discloses an OLT capable of specifying ONUs that are constantly emitting light. According to Hamaoka, when all ONUs other than a single ONU enter an unregistered state within a predetermined time, the OLT specifies the single ONU as an ONU that constantly emits light. 
     Searching for an ONU with a conventional technique may result in erroneously determining a normal ONU as an ONU in an abnormal state if the ONU is searched for, for example, during temporary resolution of abnormal light emission that is occurring irregularly. Such erroneous determination may result in normal ONUs being shut down one after another. 
     Accordingly, it is an object of one or more aspects of the disclosure to more accurately specifies ONUs that are the source of abnormal light emission. 
     SUMMARY OF THE INVENTION 
     A station-side device according to an aspect of the disclosure includes an optical transmission/reception unit configured to transmit and receive optical signals by time division to and from a plurality of subscriber-side devices; an abnormal-light-emission detection unit configured to monitor the optical signals received by the optical transmission/reception unit, and detect a state in which the optical signals are received for a predetermined period or longer as abnormal light emission in which the optical signals from one or more of the subscriber-side devices are not received; and an optical-communication control unit configured to sequentially select a subscriber-side device one at a time from the plurality of subscriber-side devices other than the one or more subscriber-side devices as a target subscriber-side device, test whether or not the abnormal light emission is resolved by stopping transmission of the optical signal from each target subscriber-side device via the optical transmission/reception unit, and, if the abnormal light emission is resolved, specifies the target subscriber-side device being tested as a causative subscriber-side device, the causative subscriber-side device being a subscriber-side device that is a source of the abnormal light emission. 
     An optical communication system according to another aspect of the disclosure includes: a plurality of subscriber-side devices; and a station-side device configured to transmit and receive optical signals to and from the plurality of subscriber-side devices, wherein, the station-side device comprises: an optical transmission/reception unit configured to transmit and receive an optical signal by time division to and from a plurality of subscriber-side devices; an abnormal-light-emission detection unit configured to monitor the optical signals received by the optical transmission/reception unit, and detect a state in which the optical signals are received for a predetermined period or longer as abnormal light emission in which the optical signals from one or more of the subscriber-side devices are not received; and an optical-communication control unit configured to sequentially select a subscriber-side device one at a time from the plurality of subscriber-side devices other than the one or more subscriber-side devices as a target subscriber-side device, test whether or not the abnormal light emission is resolved by stopping transmission of the optical signal from each target subscriber-side device via the optical transmission/reception unit, and, if the abnormal light emission is resolved, specifies the target subscriber-side device being tested as a causative subscriber-side device, the causative subscriber-side device being a subscriber-side device that is a source of the abnormal light emission. 
     A searching method according to another aspect of the disclosure includes the steps of: receiving optical signals by time division from a plurality of subscriber-side devices; monitoring the received optical signals, and detecting a state in which the optical signals are received for a predetermined period or longer as abnormal light emission in which the optical signals from one or more of the subscriber-side devices are not received; sequentially selecting a subscriber-side device one at a time from the plurality of subscriber-side devices other than the one or more subscriber-side devices as a target subscriber-side device, and testing whether or not the abnormal light emission is resolved by stopping transmission of the optical signal from each target subscriber-side device; and if the abnormal light emission is resolved, specifying the target subscriber-side device being tested as a causative subscriber-side device, the causative subscriber-side device being a subscriber-side device that is a source of the abnormal light emission. 
     According to one or more aspects of the disclosure, ONUs that are the source of abnormal light emission can be more accurately specified. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram schematically illustrating the configuration of a PON system according to first and second embodiments. 
         FIG.  2    is a schematic diagram illustrating a first example of link-state management information according to the first embodiment. 
         FIGS.  3 A and  3 B  are block diagrams illustrating hardware configuration examples. 
         FIG.  4    is a flowchart illustrating the operation of an OLT searching for an ONU that is the source of abnormal light emission in the first embodiment. 
         FIG.  5    is a schematic diagram illustrating a second example of link-state management information according to the first embodiment. 
         FIG.  6    is a schematic diagram illustrating a third example of link-state management information according to the first embodiment. 
         FIG.  7    is a schematic diagram illustrating a fourth example of link-state management information according to the first embodiment. 
         FIG.  8    is a schematic diagram illustrating a fifth example of link-state management information according to the first embodiment. 
         FIG.  9    is a schematic diagram illustrating a sixth example of link-state management information according to the first embodiment. 
         FIG.  10    is a schematic diagram illustrating a seventh example of link-state management information according to the first embodiment. 
         FIG.  11    is a first schematic diagram for explaining the outline of the operation of a PON system according to the first embodiment. 
         FIG.  12    is a second schematic diagram for explaining the outline of the operation of a PON system according to the first embodiment. 
         FIG.  13    is a third schematic diagram for explaining the outline of the operation of a PON system according to the first embodiment. 
         FIG.  14    is a schematic diagram illustrating a first example of link-state management information according to the second embodiment. 
         FIG.  15    is a first flowchart illustrating the operation of an OLT searching for an ONU that is the source of abnormal light emission in the second embodiment. 
         FIG.  16    is a second flowchart illustrating the operation of an OLT searching for an ONU that is the source of abnormal light emission in the second embodiment. 
         FIG.  17    is a third flowchart illustrating the operation of an OLT searching for an ONU that is the source of abnormal light emission in the second embodiment. 
         FIG.  18    is a schematic diagram illustrating a second example of link-state management information according to the second embodiment. 
         FIG.  19    is a schematic diagram illustrating a third example of link-state management information according to the second embodiment. 
         FIG.  20    is a schematic diagram illustrating a fourth example of link-state management information according to the second embodiment. 
         FIG.  21    is a schematic diagram illustrating a fifth example of link-state management information according to the second embodiment. 
         FIG.  22    is a schematic diagram illustrating a sixth example of link-state management information according to the second embodiment. 
         FIG.  23    is a schematic diagram illustrating a seventh example of link-state management information according to the second embodiment. 
         FIG.  24    is a schematic diagram illustrating an eighth example of link-state management information according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
       FIG.  1    is a block diagram schematically illustrating the configuration of a PON system  100  serving as an optical communication system according to the first embodiment. 
     The PON system  100  includes an OLT  110  or a station-side terminal device, and multiple ONUs  130 . 
     In the PON system  100 , the OLT  110  and the ONUs  130  are connected via an optical fiber  101  and an optical splitter  102 . 
     The ONUs  130  are assigned identifications (IDs) serving as ONU identification information for identifying the respective ONUs  130 . Here, positive integers are assigned as IDs in order from “1.” In the following description, an ONU  130  having an ID=i may be represented as ONU #i. For example, the ONU  130  having the ID=1 may be represented as ONU # 1 . As illustrated in  FIG.  1   , i is a positive integer satisfying 1≤i≤n (where n is a positive integer greater than or equal to two). 
     The OLT  110  includes an optical transmission/reception unit  111 , an abnormal-light-emission detection unit  112 , a link-state storage unit  113 , an optical-communication control unit  114 , and a communication unit  118 . 
     The optical transmission/reception unit  111  transmits and receives optical signals to and from the ONUs  130  by time division. 
     For example, the optical transmission/reception unit  111  receives an uplink signal from an ONU  130  and provides this signal to a PON control unit  117 . The optical transmission/reception unit  111  transmits a downlink signal to an ONU  130 . The optical transmission/reception unit  111  has a wavelength division multiplexing (WDM) function for multiplexing an uplink signal and a downlink signal. 
     The abnormal-light-emission detection unit  112  monitors the optical signals received by the optical transmission/reception unit  111 , and detects a state in which an optical signal is being received for a predetermined period or longer as abnormal light emission, which is a state in which reception of optical signals from one or more of the ONUs  130  is disrupted. 
     For example, the abnormal-light-emission detection unit  112  monitors the uplink signals transmitted from an ONU  130  to the optical transmission/reception unit  111 , detects abnormal light emission of the ONU  130  when a light emitting state continues for a predetermined period, and sends an abnormal light emission notification to a link-state management unit  115  to notify the link-state management unit  115  of the abnormal light emission of the ONU  130 . If the link between the OLT  110  and each of the ONUs  130  is normal, each ONU  130  maintains the light emitting state for shorter than the predetermined period. Thus, if the light emitting state extends longer than or equal to the predetermined period, one of the ONUs  130  can be detected to be the source of abnormal light emission. If the light emitting state extending for the predetermined period is resolved, the abnormal-light-emission detection unit  112  provides an abnormal-light-emission resolution notification to the link-state management unit  115  to notify the link-state management unit  115  that the abnormal light emission of the ONU  130  has been resolved. 
     The link-state storage unit  113  stores link-state management information for managing the link state of the ONUs  130 . 
       FIG.  2    is a schematic diagram illustrating an example of link-state management information according to the first embodiment. 
     As illustrated in  FIG.  2   , a link-state management table  113   a , which is an example of link-state management information, is data in a table format containing an ID column  113   b , an ONU state column  113   c , an abnormal-light-emission detection state column  113   d , and a classification column  113   e.    
     The ID column  113   b  stores the IDs assigned to the ONUs  130 . 
     The ONU state column  113   c  stores the ONU state, which is the state of each ONU  130 . When an ONU  130  is linked with the OLT  110 , the value “Register” is stored to indicate a registered state; when an ONU is not linked with the OLT  110 , the value “deregister” is stored to indicate a deregistered state; and when the power of an ONU is turned off, the value “Power off” is stored to indicate a power-off state. 
     The abnormal-light-emission detection state column  113   d  stores an abnormal-light emission detection state value indicating whether or not abnormal light emission of an ONU  130  connected to the OLT  110  is detected. If abnormal light emission is detected in none of the ONUs  130  connected to the OLT  110 , the value “Normal” indicating a normal light emitting state is stored, and if abnormal light emission is detected in any of the ONUs  130  connected to the OLT  110 , the value “Abnormal” indicating an abnormal light emitting state is stored. 
     In the OLT  110 , the classification column  113   e  stores the test state classification of the test conducted to check the light emitting state of the ONUs  130 . 
     For example, if abnormal light emission is detected in none of the ONUs  130  connected to the OLT  110 , that is, if the abnormal-light-emission detection state column  113   d  stores the value “Normal,” the value “normal” indicating a normal state is stored in the classification column  113   e  if the ONU state column  113   c  stores the value “Register,” and the same value as that in the ONU state column  113   c  is stored in the classification column  113   e  if the ONU state column  113   c  stores a value other than “Register.” 
     On the other hand, if abnormal light emission is detected in an ONU  130  connected to the OLT  110 , that is, if the value “Abnormal” is stored in the abnormal-light-emission detection state column  113   d , the value stored in the classification column  113   e  is as follows: if the corresponding ONU  130  has been deregistered in response to the detection of abnormal light emission, stored is the value “damaged” indicating that the corresponding ONU  130  is not to be tested; if the corresponding ONU  130  is being tested, stored is the value “in test” indicating that the corresponding ONU  130  is being tested; if the test of the corresponding ONU  130  is completed, and it cannot be confirmed that there is not abnormal light emission, stored is the value “tested” indicating the completion of the test; and if there is a possibility of abnormal light emission of the corresponding ONU  130 , stored is the value “suspected.” 
     Referring back to  FIG.  1   , the optical-communication control unit  114  controls the communication of the OLT  110 . 
     For example, when abnormal light emission that prevents reception of optical signals from one or more of the ONUs  130  is detected, the optical-communication control unit  114  selects an ONU  130  as a target ONU  130  one at a time from the ONUs  130  other than the one or more ONUs  130 , tests whether or not the abnormal light emission can be resolved by stopping the transmission of optical signals from the target ONU  130  via the optical transmission/reception unit  111 ; and if the abnormal light emission is resolved, specifies the target ONU  130  at that time as a causative ONU  130  that is the source of the abnormal light emission. 
     The optical-communication control unit  114  includes a link-state management unit  115 , an abnormal-light-emission management unit  116 , and a PON control unit  117 . 
     The link-state management unit  115  uses the link-state management information stored in the link-state storage unit  113  to manage the link state of each ONU  130  and the abnormal light emitting state of each ONU  130 . 
     The link-state management unit  115  receives, from the PON control unit  117 , notification of a change in or a request for the link state of each ONU  130 , updates or confirms the link state of each ONU  130  in the link-state management information, and notifies the PON control unit  117  of the content of the update or confirmation. 
     The link-state management unit  115  receives an abnormal light emission notification or an abnormal-light-emission resolution notification from the abnormal-light-emission detection unit  112 , and updates the abnormal-light-emission detection state value in the link-state management information to manage the abnormal-light-emission detection state of the ONUs  130 . 
     The link-state management unit  115  also manages the test state classification of the test conducted to check the light emitting state in the link-state management information on the basis of the abnormal light emission notification or the abnormal-light-emission resolution notification from the abnormal-light-emission detection unit  112  and the link state of each ONU  130 . On the basis of the link state of each ONU  130  and the test state classification of the test conducted to check the light emitting state, the link-state management unit  115  notifies the abnormal-light-emission management unit  116  of the ID of the ONU  130  to be the subject of ONU optical shutdown or ONU optical shutdown cancellation and an ONU optical shutdown flag or an ONU optical shutdown cancellation flag. 
     On the basis of the ID sent from the link-state management unit  115  and the ONU optical shutdown flag or the ONU optical shutdown cancellation flag, the abnormal-light-emission management unit  116  provides the PON control unit  117  with an ONU optical shutdown notification indicating that optical shutdown is to be performed on the ONU  130  corresponding to the ID or an ONU optical shutdown cancellation notification indicating that optical shutdown is to be canceled for the ONU  130  corresponding to the ID. 
     The PON control unit  117  comprehensively controls the processing by the OLT  110 . 
     For example, the PON control unit  117  performs PON interface control. 
     Specifically, when the PON control unit  117  receives the ONU optical shutdown notification or the ONU optical shutdown cancellation notification from the abnormal-light-emission management unit  116 , the PON control unit  117  transfers the ONU optical shutdown notification or the ONU optical shutdown cancellation notification to the ONU  130  via the optical transmission/reception unit  111 . 
     The PON control unit  117  transmits warning information sent from an ONU  130  to an external monitoring system  150  via the communication unit  118 , and provides the warning information to the link-state management unit  115 . 
     The PON control unit  117  receives a setting or a request notification regarding the PON terminals between the OLT  110  and each ONU  130  from the external monitoring system  150  via the communication unit  118 . The PON control unit  117  receives information on the link state of each ONU  130  from the external monitoring system  150  via the communication unit  118 , provides the information to the link-state management unit  115 , receives a response from the link-state management unit  115 , and sends the response to the external monitoring system  150  via the communication unit  118 . 
     The communication unit  118  communicates with the external monitoring system  150 . 
     A portion or the entirety of the abnormal-light-emission detection unit  112  and the optical-communication control unit  114  described above can be implemented by, for example, a memory  10  and a processor  11 , such as a central processing unit (CPU), that executes the programs stored in the memory  10 , as illustrated in  FIG.  3 A . Such programs may be provided via a network or may be recorded and provided on a recording medium. That is, such programs may be provided as, for example, program products. 
     A portion or the entirety of the abnormal-light-emission detection unit  112  and the optical-communication control unit  114  can be implemented by, for example, a processing circuit  12 , such as a single circuit, a composite circuit, a processor running on a program, a parallel processor running on a program, an application-specific integrated circuit (ASIC), or a field programmable gate array (FPGA), as illustrated in  FIG.  3 B . 
     As described above, the optical-communication control unit  114  can be implemented by processing circuitry. 
     The optical transmission/reception unit  111  can be implemented by an optical communication interface for performing optical communication via a connected optical fiber  101 . 
     The communication unit  118  can be implemented by a communication interface, such as a network interface card (NIC) for wired or wireless communication. 
     The link-state storage unit  113  can be implemented by a volatile or non-volatile memory. 
     Referring back to  FIG.  1   , each ONU  130  includes an optical transmission/reception unit  131 , a PON control unit  132 , and an optical output control unit  133 . 
       FIG.  1    illustrates the internal configuration of only the uppermost ONU  130 , but the other ONUs  130  each have the same configuration. 
     The optical transmission/reception unit  131  receives a signal that is transmitted from the optical transmission/reception unit  111  of the OLT  110  through the optical fiber  101  and the optical splitter  102 . The optical transmission/reception unit  131  feeds this signal to the PON control unit  132 . 
     The optical transmission/reception unit  131  transmits signals fed from the PON control unit  132  to the OLT  110 . 
     The optical transmission/reception unit  131  stops the emission of transmission light or cancels the stop of transmission light on the basis of notification for stopping the transmission light or notification for cancelling the stop of the transmission light issued by the optical output control unit  133 . 
     The PON control unit  132  controls the processing by the ONU  130 . 
     For example, the PON control unit  132  performs PON interface control. 
     Specifically, the PON control unit  132  performs PON termination processing on the ONU  130  side for the signal transmitted from the OLT  110  through the optical transmission/reception unit  111 . 
     The PON control unit  132  notifies the OLT  110  of the warning information of the ONU  130  and the current status of the ONU  130  via the optical transmission/reception unit  131 . 
     The PON control unit  132  transfers a downlink signal received from the optical transmission/reception unit  131  to an external terminal (not illustrated). 
     The PON control unit  132  provides the ONU optical shutdown notification or the ONU optical shutdown cancellation notification transmitted from the OLT  110  to the optical output control unit  133 . 
     The optical output control unit  133  instructs the optical transmission/reception unit  131  to stop the transmission light emitted from the optical transmission/reception unit  131  or to cancel the stop of the transmission light on the basis of the ONU optical shutdown notification or the ONU optical shutdown cancellation notification fed from the PON control unit  132 . 
     A portion or the entirety of the PON control unit  132  and the optical output control unit  133  described above can be composed of, for example, a memory  10  and a processor  11 , such as a CPU, that executes the programs stored in the memory  10 , as illustrated in  FIG.  3 A . Such programs may be provided via a network or may be recorded and provided on a recording medium. That is, such programs may be provided as, for example, program products. 
     Alternatively, a portion or the entirety of the PON control unit  132  and the optical-communication control unit  133  can be implemented by, for example, a processing circuit  12 , such as a single circuit, a composite circuit, a processor running on a program, a parallel processor running on a program, an ASIC, or an FPGA, as illustrated in  FIG.  3 B . 
     As described above, the PON control unit  132  and the optical output control unit  133  can be implemented by processing circuitry. 
     The optical transmission/reception unit  111  can be implemented by an optical communication interface for performing optical communication via a connected optical fiber  101 . 
     The operation of the OLT  110  for managing the link state of an ONU  130  will now be explained. 
       FIG.  4    is a flowchart illustrating the operation of the OLT  110  searching for an ONU  130  that is the source of abnormal light emission. 
     When the ONU  130  that is the source of abnormal light emission is not connected to the OLT  110 , the abnormal-light-emission detection state column  113   d  stores the value “Normal,” as in the link-state management table  113   a  illustrated in  FIG.  2   . 
     First, the abnormal-light-emission detection unit  112  monitors the uplink signals transmitted from the ONU  130  to the optical transmission/reception unit  111  and determines whether or not abnormal light emission is detected (step S 10 ). Here, the abnormal-light-emission detection unit  112  detects abnormal light emission when the light emitting state continues for a predetermined period. If abnormal light emission is detected (Yes in step S 10 ), the abnormal-light-emission detection unit  112  provides an abnormal light emission notification of the corresponding ONU  130  to the link-state management unit  115 , and the process proceeds to step S 11 . 
     When the link-state management unit  115  receives the abnormal light emission notification, the link-state management unit  115  updates the abnormal light emission detection state in the link-state management information stored in the link-state storage unit  113  so as to indicate the abnormal light emitting state, and checks the ONU state and the test state classification (step S 11 ). For example, as in the link-state management table  113   a  # 1  illustrated in  FIG.  5   , the link-state management unit  115  updates the value in the abnormal-light-emission detection state column  113   d  to “Abnormal” indicating an abnormal light emitting state, and checks the values in the ONU state column  113   c  and the classification column  113   e.    
     The link-state management unit  115  then checks the ONU state and the test state classification of the link-state management information stored in the link-state storage unit  113 , to determine whether or not there is an ONU  130  that had entered a deregistered state when the abnormal light emission was detected (step S 12 ). When abnormal light emission is detected, the light emitting state of the relevant ONU  130  continues for a predetermined period; as a result, the optical signal transmitted from other ONUs  130  that are to transmit optical signals cannot be received during this period. Thus, as illustrated in the link-state management table  113   a  # 2  illustrated in  FIG.  6   , there are ONUs  130  of which the value “normal” is stored in the classification column  113   e  and the value “Deregister” is stored in the ONU state column  113   c . In the example illustrated in  FIG.  6   , ONU # 2  and ONU # 3  are the ONUs  130  in a deregistered state. If there is an ONU  130  in the deregistered state (Yes in step S 12 ), the process proceeds to step  313 , and if there is no ONU  130  in the deregistered state (No in step S 12 ), the process proceeds to step  314 . 
     In step S 13 , the link-state management unit  115  updates the test state classification of the test conducted for the ONU  130  that entered the deregistered state when the abnormal light emission was detected to be ineligible. For example, if ONU # 2  and ONU # 3  enter a deregistered state, as illustrated in  FIG.  6   , the link-state management unit  115  updates the values of ONU # 2  and ONU # 3  in the classification columns  113   e  to “damaged,” as in the link-state management table  113   a  # 3  illustrated in  FIG.  7   . The processing then proceeds to step S 14 . 
     In step  314 , the link-state management unit  115  specifies one ONU  130  of the ONUs  130  whose test state classification is a normal state, and notifies the abnormal-light-emission management unit  116  of the ID and the optical shutdown flag of the specified ONU  130  so as to optically shut down the specified ONU  130 . Here, the link-state management unit  115  specifies an ONU  130  one at a time in ascending order of the ID number. The ONU  130  specified here is also referred to as a target ONU  130 . The abnormal-light-emission management unit  116  that has been notified of the ID and the optical shutdown flag generates an ONU optical shutdown notification indicating that the ONU  130  corresponding to the ID is to be optically shut down, and provides the ONU optical shutdown notification to the PON control unit  117 . When the PON control unit  117  receives the ONU optical shutdown notification from the abnormal-light-emission management unit  116 , the PON control unit  117  transfers the ONU optical shutdown notification to the corresponding ONU  130  via the optical transmission/reception unit  111 . 
     The PON control unit  132  of the ONU  130  that has received the ONU optical shutdown notification provides the ONU optical shutdown notification to the optical output control unit  133 , and the optical output control unit  133  causes the optical transmission/reception unit  131  to stop the emission of the transmission light and to transmit a response to the optical shutdown notification to the OLT  110 . The OLT  110  that received such a response sends the response to the link-state management unit  115  via the PON control unit  117 . The link-state management unit  115  updates the test state classification of the ONU  130  that has transmitted the response to the value “in test” (step S 15 ). For example, as illustrated in  FIG.  8   , when a response is received from ONU # 1 , the value of ONU # 1  in the classification column  113   e  in the link-state management table  113   a  # 4  is updated to the value “in test.” Note that when there is an ONU  130  in the “in test” state, the link-state management unit  115  does not allow an optical shutdown notification to be sent to other ONUs  130 . 
     The abnormal-light-emission detection unit  112  then monitors the uplink signals transmitted from the ONU  130  to the optical transmission/reception unit  111  and determines whether or not the abnormal light emission is resolved after the execution of the ONU optical shutdown (step S 16 ). Here, the abnormal-light-emission detection unit  112  monitors the uplink signals from the ONU  130  for a predetermined period and if the light emitting state is canceled within this period, determines that the abnormal light emission has been resolved. If the abnormal light emission is resolved (Yes in step S 16 ), the abnormal-light-emission detection unit  112  provides an abnormal-light-emission resolution notification of the ONU  130  to the link-state management unit  115 , and the processing proceeds to step S 17 . If the abnormal light emission is not resolved (No in step S 16 ), the processing proceeds to step S 18 . 
     In step S 17 , since the abnormal light emission is resolved by shutting down the optical signal of the target ONU  130 , the link-state management unit  115  specifies the target ONU  130  as the causative ONU  130  that is a source of the abnormal light emission. For example, if ONU # 1  is the target ONU  130 , the value in the classification column  113   e  of ONU # 1  is updated to “suspected,” as in the link-state management table  113   a  illustrated in  FIG.  9   . Note that in such a case, the ONU state of the ONU  130  that was a deregistered state returns to a registered state. Thus, for example, if ONU # 2  and ONU # 3  are in a deregistered state, as illustrated in  FIG.  8   , the values in the ONU state column  113   c  corresponding to ONU # 2  and ONU # 3  are updated to “Register,” and the values in the classification column  113   e  corresponding to ONU # 2  and ONU # 3  are updated to “normal.” Moreover, the value in the abnormal-light-emission detection state column  113   d  is updated to “Normal.” 
     In step S 18 , since the abnormal light emission is not resolved even after the optical signal of the target ONU  130  has been shut down, the target ONU  130  is not the causative ONU  130 . Thus, the link-state management unit  115  notifies the abnormal-light-emission management unit  116  of the ID of the target ONU  130  and the optical shutdown cancellation flag (step S 18 ). The abnormal-light-emission management unit  116  that has been notified of the ID and the optical shutdown cancellation flag generates an ONU optical shutdown cancellation notification indicating that optical shutdown of the ONU  130  corresponding to the ID is to be canceled, and provides the ONU optical shutdown cancellation notification to the PON control unit  117 . When the PON control unit  117  receives the ONU optical shutdown cancellation notification from the abnormal-light-emission management unit  116 , the PON control unit  117  transfers the ONU optical shutdown cancellation notification to the corresponding ONU  130  via the optical transmission/reception unit  111 . 
     The PON control unit  132  of the ONU  130  that has received the ONU optical shutdown cancellation notification provides the ONU optical shutdown cancellation notification to the optical output control unit  133 , and the optical output control unit  133  causes the optical transmission/reception unit  131  to resume the emission of the transmission light and to transmit a response to the optical shutdown cancellation notification to the OLT  110 . The OLT  110  that received such a response sends the response to the link-state management unit  115  via the PON control unit  117 . The link-state management unit  115  updates the test state classification value of the target ONUs  130  to “tested” (step S 19 ). For example, when a response is received from ONU # 1 , the value in the classification column  113   e  of ONU # 1  is updated to “tested,” as in the link-state management table  113   a  # 6  illustrated in  FIG.  10   . The link between the target ONU  130  and the OLT  110  is restored by canceling the stop of the emission of the transmission light, and the PON controller  117  sends an ONU linkup notification to the link-state management unit  115  to update the ONU state of the target ONU  130  to a registered state. For example, when the target ONU  130  is ONU # 1 , the value in the ONU state column  113   c  of ONU # 1  in the link-state management table  113   a  # 6  is updated to “Register,” as illustrated in  FIG.  10   . 
     The link-state management unit  115  then refers to the link-state management information stored in the link-state storage unit  113  to determine whether or not there are remaining ONUs  130  that are classified as being in a normal state (step S 20 ). 
     If ONUs  130  classified as being in a normal state still remain (Yes in step S 20 ), the processing returns o step S 14 , and the link-state management unit  115  specifies one of the ONUs  130  in the normal state as the target ONU. For example, in the link-state management table  113   a  # 6  illustrated in  FIG.  10   , ONU # 3  that has the smallest ID number out of the ONUs  130  of which the value in the classification column  113   e  is “normal” is specified as the next target ONU. 
     If there are no ONUs  130  of which the test state classification is “normal” (No in step S 20 ), the processing proceeds to step S 21 . 
     In step S 21 , the link-state management unit  115  determines that none of the ONUs  130  performing uplink is in an abnormal light emitting state because the test state classification of all ONUs  130  is a state other than “normal.” Thus, the link-state management unit  115  warn the external monitoring system  150  that the ONU that is the source of the abnormal light emitting state is unspecified through the PON control unit  117 . This ends the search for the ONU  130  in an abnormal light emitting state. 
     The above operation in the flowchart will now be explained in detail with reference to  FIGS.  11  to  13   . 
       FIGS.  11  to  13    are schematic diagrams for explaining the outline of the operation of the PON system  100  according to the first embodiment. 
     In  FIGS.  11  to  13   , it is assumed that four ONUs  130  are connected to one OLT  110 . 
       FIG.  11    illustrates the outline of the operation of each ONU  130  in a normal state. 
     Uplink frames  1  to  4  input to the respective ONUs  130  from a low-order terminal (not illustrated) are transmitted to the OLT  110  at respective timings under time division control. 
     Since the frames received by the OLT  110  are time-division multiplexed and transferred in a normal state and do not collide, the ONU state of each ONU  130  is indicated in the OLT  110  by the value “Register” indicating a registered state. 
       FIG.  12    illustrates the outline of the operation when a failure occurs in ONU # 1 , and abnormal uplink signals are generated irregularly. 
     When ONU # 1  irregularly enters an abnormal light emitting state, the signals received by the OLT  110  are frame  1  transmitted from ONU # 1 , a signal resulting from a collision of frame  2  transmitted from ONU # 2  and frame  3  transmitted from ONU # 3 , and frame  4  transmitted from ONU # 4 . In such a case, the OLT  110  cannot receive frames  2  and  3  correctly. Thus, in the OLT  110 , the ONU state of ONU # 1  and ONU # 4  is set to the value “Register” indicating a registered state, and the ONU state of ONU # 2  and ONU # 3  is set to the value “Deregister” indicating a deregistered state. 
       FIG.  13    illustrates the outline of the operation when ONU # 1  is detected as the source of abnormal light emission. 
     When the OLT  110  detects abnormal light emission of an ONU  130 , the OLT  110  sequentially issues optical shutdown instructions to the ONUs  130  set to the value “Register” indicating a normal state. Here, by issuing an optical shutdown instruction to ONU # 1 , frame  1  colliding with frames  2  and  3  disappears, and the OLT  110  can receive frames  2  and  3 . This causes ONU # 2  and ONU # 3  to return to the registered state, “Register.” Thus, the OLT  110  can specify ONU # 1  as the causative ONU  130 . 
     According to the first embodiment described above, it is possible to automatically search for and specify an ONU  130  that is irregularly emitting light during linkup without dedicated detection circuits provided in the OLT  110  and the ONUs  130 . This allows the operator to take action without having to deal with the problem manually. 
     Second Embodiment 
     While the first embodiment describes a configuration for automatically searching for a device that is the source of irregular abnormal light emission, the second embodiment will describe a configuration for preventing erroneously specifying a normal device as the source of abnormal light emission while normal devices are being searched for during temporary resolution of the irregular abnormal light emission. 
     As illustrated in  FIG.  1   , a PON system  200  according to the second embodiment includes an OLT  210  or a station-side terminal device, and multiple ONUs  130 . 
     The ONUs  130  in the PON system  200  according to the second embodiment are the same as the ONUs  130  in the PON system  100  according to the first embodiment. 
     The OLT  210  includes an optical transmission/reception unit  111 , an abnormal-light-emission detection unit  112 , a link-state storage unit  213 , an optical-communication control unit  214 , and a communication unit  118 . 
     The optical transmission/reception unit  111 , the abnormal-light-emission detection unit  112 , and the communication unit  118  of the OLT  210  according to the second embodiment are respectively the same as the optical transmission/reception unit  111 , the abnormal-light-emission detection unit  112 , and the communication unit  118  of the OLT  110  according to the first embodiment. 
     The optical-communication control unit  214  controls the communication of the OLT  210 . 
     The optical-communication control unit  214  of the second embodiment performs substantially the same processing as the optical-communication control unit  114  of the first embodiment, but in the second embodiment, the optical-communication control unit  214  stops the transmission of optical signals from target ONUs  130 , counts the number of times that abnormal light emission has been resolved for each target ONU  130 , and specifies a target ONU  130  of which this number exceeds a predetermined threshold as a causative ONU  130  that is the source of the abnormal light emission. 
     The link-state storage unit  213  stores link-state management information for managing the link state of the ONUs  130 . 
       FIG.  14    is a schematic diagram illustrating an example of link-state management information according to the second embodiment. 
     As illustrated in  FIG.  14   , a link-state management table  213   a , which is an example of link-state management information, is data in a table format having an ID column  113   b , an ONU state column  113   c , an abnormal-light-emission detection state column  113   d , a classification column  113   e , and a test number column  213   f.    
     The ID column  113   b , the ONU state column  113   c , the abnormal-light-emission detection state column  113   d , and the classification column  113   e  of the link-state management table  213   a  according to the second embodiment are respectively the same as the ID column  113   b , the ONU state column  113   c , the abnormal-light-emission detection state column  113   d , and the classification column  113   e  of the link-state management table  113   a  according to the first embodiment. 
     The test number column  213   f  stores the number of times the corresponding ONU  130  is tested. 
     Here, stored is the number of times the ONU  130  is optically shut down and tested for whether the ONU  130  is the source of abnormal light emission. 
     The optical-communication control unit  214  includes a link-state management unit  215 , an abnormal-light-emission management unit  116 , and a PON control unit  117 . 
     The abnormal-light-emission management unit  116  and the PON control unit  117  of the optical-communication control unit  214  according to the second embodiment are respectively the same as the abnormal-light-emission management unit  116  and the PON control unit  117  of the optical-communication control unit  114  according to the first embodiment. 
     The link-state management unit  215  uses the link-state management information stored in the link-state storage unit  213  to manage the link state and the abnormal light emitting state of each ONU  130 . 
     The link-state management unit  215  according to the second embodiment performs substantially the same processing as the link-state management unit  115  according to the first embodiment, but if an ONU  130  is suspected multiple times to be the source of abnormal light emission while an ONU  130  that is the source of the abnormal light emission is being searched for, the suspected ONU  130  is specified as the source of the abnormal light emission. 
     The operation of the OLT  210  for managing the link state of an ONU  130  will now be explained. 
       FIGS.  15  to  17    are flowcharts illustrating the operation of the OLT  210  searching for an ONU  130  that is the source of abnormal light emission. 
     If an ONU  130  that is the source of abnormal light emission is not connected to the OLT  210 , the abnormal-light-emission detection state column  113   d  stores the value “Normal,” and the test number column  213   f  stores the value “0” for all ONUs  130 , as in the link-state management table  213   a  illustrated in  FIG.  14   . 
     Among the steps in the processing of the flowcharts illustrated in  FIGS.  15  to  17   , the steps that are the same as those in the flowchart illustrated in  FIG.  4    of the first embodiment are denoted by the same reference numerals as those in  FIG.  4   . 
     The processing of steps S 10  to S 13  in the flowchart illustrated in  FIG.  15    are respectively the same as the processing of steps S 10  to S 13  in the flowchart illustrated in  FIG.  4   . However, in  FIG.  15   , the processing proceeds to step S 30  after the processing of step S 13 . 
     In step S 30 , the link-state management unit  215  specifies an ONU  130  of the ONUs  130  of which the test state classification value is “normal,” and notifies the abnormal-light-emission management unit  116  of the ID and the optical shutdown flag of the specified ONU  130  so as to optically shut down the specified ONU  130 . Here, the link-state management unit  215  specifies an ONU  130  one at a time in ascending order of the ID number. The ONU  130  specified here is also referred to as a target ONU  130 . The abnormal-light-emission management unit  116  that has been notified of the ID and the optical shutdown flag generates an ONU optical shutdown notification indicating that the ONU  130  corresponding to the ID is to be optically shut down, and provides the ONU optical shutdown notification to the PON control unit  117 . When the PON control unit  117  receives the ONU optical shutdown notification from the abnormal-light-emission management unit  116 , the PON control unit  117  transfers the ONU optical shutdown notification to the corresponding ONU  130  via the optical transmission/reception unit  111 . 
     Also, the link-state management unit  215  counts up by one the number of times the target ONU is tested in the specifies link-state management information stored in the link-state storage unit  213 . For example, when the target ONU is ONU # 1  and is tested for the first time, the value in the test number column  213   f  corresponding to ONU # 1  is updated to “1,” as in the link-state management table  213   a  # 1  illustrated in  FIG.  18   . The processing then proceeds to step S 15 . 
     The processing of steps S 15  and S 16  in the flowchart illustrated in  FIG.  15    are respectively the same as the processing of steps S 15  and S 16  in the flowchart illustrated in  FIG.  4   . However, in  FIG.  15   , if the abnormal light emission is resolved in step S 16  (Yes in step S 16 ), the abnormal-light-emission detection unit  112  provides an abnormal-light-emission resolution notification of the ONU  130  to the link-state management unit  215 , and the processing proceeds to step S 31 ; if the abnormal light emission is not resolved in step S 16  (No in step S 16 ), the processing proceeds to step S 18  in  FIG.  16   . 
     For example, in step S 14 , the link-state management information is that in the link-state management table  213   a  # 1  illustrated in  FIG.  18   , and if it is the first test, steps S 15  and S 16  in  FIG.  15    are performed to update the link-state management information to that in the link-state management table  213   a  # 2  illustrated in  FIG.  19   . 
     In step S 31 , since the abnormal light emission is resolved by the shutdown of the optical signal of the target ONU  130 , the link-state management unit  215  suspects that the target ONU  130  is the source of the abnormal light emission and specifies the target ONU  130  as an abnormal-light-emission suspect ONU  130 . For example, if ONU # 1  is the target ONU, and the link-state management information after the determination in step S 16  is that in the link-state management table  213   a  # 2  illustrated in  FIG.  19   , the value in the classification column  113   e  of ONU # 1  is updated to “suspected,” as in the link-state management table  213   a  # 3  illustrated in  FIG.  20   . In such a case, the ONU state of the ONUs  130  in the deregistered state returns to a registered state. Thus, for example, if ONU # 2  and ONU # 3  are in a deregistered state, the values corresponding to ONU # 2  and ONU # 3  in the ONU state column  113   c  are updated to “Register,” and the values corresponding to ONU # 2  and ONU # 3  in the classification column  113   e  are updated to “normal.” Moreover, the value in the abnormal-light-emission detection state column  113   d  is updated to “Normal.” 
     The link-state management unit  215  then determines whether or not the number M of tests conducted on the target ONU  130  is larger than a predetermined threshold L (step S 32 ). Here, the threshold L is an integer larger than or equal to one. If the number M of tests is larger than the threshold L (Yes in step S 32 ), the processing proceeds to step  333 ; if the number M of tests is smaller than or equal to the threshold L (No in step S 32 ), the processing proceeds to step S 34  in  FIG.  17   . 
     In step S 33 , since the abnormal light emission is resolved by the shutdown of the optical signal of the target ONU  130  and the number of tests exceeds the threshold, the link-state management unit  215  specifies the target ONU  130  at this time as the causative ONU  130  that is the source of the abnormal light emission. 
     The processing of steps S 18  to S 21  in the flowchart illustrated in  FIG.  16    are respectively the same as the processing of steps S 18  to S 21  in the flowchart illustrated in  FIG.  4   . 
     In step S 34  of  FIG.  17   , the link-state management unit  215  cancels the optical shutdown of the abnormal-light-emission suspect ONU  130 , sets the test state classification to “in test,” and counts up the number of tests by one. 
     For example, the link-state management unit  215  notifies the abnormal-light-emission management unit  116  of the ID and the optical shutdown flag so as to optically shut down the abnormal-light-emission suspect ONU  130 . The abnormal-light-emission management unit  116  that has been notified of the ID and the optical shutdown flag generates an ONU optical shutdown notification indicating that the ONU  130  corresponding to the ID is to be optically shut down, and provides the ONU optical shutdown notification to the PON control unit  117 . When the PON control unit  117  receives the ONU optical shutdown notification from the abnormal-light-emission management unit  116 , the PON control unit  117  transfers the ONU optical shutdown notification to the corresponding ONU  130  via the optical transmission/reception unit  111 . 
     The link-state management unit  215  sets, in the link-state management information stored in the link-state storage unit  213 , the value in the ONU state column  113   c  of the abnormal-light-emission suspect ONU  130  to “Register” indicating a registered state, and counts up by one the value in the test number column  213   f  of the abnormal-light-emission suspect ONU  130 . 
     For example, if the link-state management information before the processing of step S 34  is that in the link-state management table  213   a  # 3  illustrated in  FIG.  20   , the value in the ONU state column  113   c  of ONU # 1  is updated to “Register,” the value in the classification column  113   e  of ONU # 1  is updated to “in test,” and the value in the test number column  213   f  of ONU # 1  is updated to “2,” as in the link-state management table  213   a  # 4  illustrated in  FIG.  21   . 
     The abnormal-light-emission detection unit  112  then monitors the uplink signals transmitted from the ONU  130  to the optical transmission/reception unit  111  and determines whether or not abnormal light emission is detected (step S 35 ). Here, the abnormal-light-emission detection unit  112  monitors the uplink signals from the ONU  130  for a certain period, and detects whether or not the light emitting state continues for a predetermined period. If abnormal light emission is detected (Yes in step S 35 ), the processing proceeds to step S 36 ; if no abnormal light emission is detected (No in step S 35 ), the processing proceeds to step S 39 . 
     In step S 36 , the link-state management unit  215  receives an abnormal light emission notification from the abnormal-light-emission detection unit  112 , and updates the abnormal light emission detection state in the link-state management information stored in the link-state storage unit  213  to an abnormal light emitting state. 
     For example, if the link-state management information before the processing of step S 36  is as that in the link-state management table  213   a  # 4  illustrated in  FIG.  21   , the link-state management unit  215  updates the value of the abnormal-light-emission detection state column  113   d  to “Abnormal” indicating an abnormal light emitting state, as in the link-state management table  213   a  # 5  illustrated in  FIG.  22   . 
     The link-state management unit  115  then checks the ONU state and the test state classification in the link-state management information to determine whether or not there is an ONU  130  that had entered a deregistered state when the abnormal light emission was detected (step S 37 ). When abnormal light emission is detected, the light emitting state of the relevant ONU  130  continues for a predetermined period; as a result, the optical signal transmitted from other ONUs  130  that are to transmit optical signals cannot be received during this period. Thus, for example, there are ONUs  130  of which the value “normal” is stored in the classification column  113   e  and the value “Deregister” is stored in the ONU state column  113   c , as in the link-state management table  213   a  # 5  illustrated in  FIG.  22   . In the example illustrated in  FIG.  22   , ONU # 2  and ONU # 3  are ONUs  130  in a deregistered state. If there is an ONU  130  in a deregistered state (Yes in step S 37 ), the process proceeds to step S 38 ; if there is no ONU  130  in a deregistered state (No in step S 37 ), the process proceeds to step S 39 . 
     In step S 38 , the link-state management unit  215  updates the test state classification of the test conducted for the ONU  130  that entered the deregistered state when the abnormal light emission was detected to be ineligible. For example, when ONU # 2  and ONU # 3  are in a deregistered state, as illustrated in  FIG.  22   , the link-state management unit  215  updates the values of the classification columns  113   e  of ONU # 2  and ONU # 3  to “damaged,” as in the link-state management table  213   a  # 6  illustrated in  FIG.  23   . The process then proceeds to step S 39 . 
     In step S 39 , the link-state management unit  215  notifies the abnormal-light-emission management unit  116  of the ID and the optical shutdown flag to optically shut down the target ONU  130 . The abnormal-light-emission management unit  116  that has been notified of the ID and the optical shutdown flag generates an ONU optical shutdown notification indicating that the ONU  130  corresponding to the ID is to be optically shut down, and provides the ONU optical shutdown notification to the PON control unit  117 . When the PON control unit  117  receives the ONU optical shutdown notification from the abnormal-light-emission management unit  116 , the PON control unit  117  transfers the ONU optical shutdown notification to the corresponding ONU  130  via the optical transmission/reception unit  111 . 
     The PON control unit  132  of the ONU  130  that has received the ONU optical shutdown notification provides the ONU optical shutdown notification to the optical output control unit  133 , and the optical output control unit  133  causes the optical transmission/reception unit  131  to stop the emission of the transmission light and to transmit a response to the optical shutdown notification to the OLT  110 . The process then returns to step S 16  in  FIG.  15   . 
     Since the abnormal light emission is not detected even when the optical signal of the target ONU  130  is shut down, the target ONU  130  is not the causative ONU  130  in step S 40 . Thus, the link-state management unit  215  updates the test state classification of the test conducted for the target ONU  130  in the link-state management information stored in the link-state storage unit  213  to “tested.” For example, if the link-state management information before the processing of step S 40  is as that in the link-state management table  213   a  # 4  illustrated in  FIG.  21   , the link-state management unit  215  updates the value of the classification column  113   e  of ONU # 1  to “tested,” as in the link-state management table  213   a  # 7  illustrated in  FIG.  24   . The process then returns to step S 20  in  FIG.  16   . 
     According to the second embodiment as described above, when a normal ONU  130  is being tested at a timing at which irregular abnormal light emission is temporarily resolved, erroneously determining the normal ONU  130  as the ONU  130  that is source of the abnormal light emission can be avoided by testing the ONU  130  multiple times. 
     In the first and second embodiments, the ONUs  130  are optically shut down one at a time while searching for the ONU  130  that is the source of abnormal light emission, but the first or second embodiment is not limited to such an example. For example, multiple ONUs  130  may be optically shut down at once to reduce the number of searches performed to find one ONU  130  that is the source of abnormal light emission. Specifically, if the abnormal light emission is resolved when multiple ONUs  130  are optically shut down, these ONUs  130  may be optically shut down one at a time to reduce the number of searches. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       100 ,  200  PON system;  110 ,  210  OLT;  111  optical transmission/reception unit;  112  abnormal-light-emission detection unit;  113 ,  213  link-state storage unit;  114 ,  214  optical-communication control unit;  115 ,  215  link-state management unit;  116  abnormal-light-emission management unit;  117  PON control unit;  118  communication unit;  130  ONU;  131  optical transmission/reception unit;  132  PON control unit;  133  optical output control unit.