Patent Publication Number: US-2012033963-A1

Title: Optical subscriber terminating device, pon system, and abnormality detecting method

Description:
FIELD 
     The present invention relates to a Passive Optical Network (PON), which is a medium-shared communication system in which data is transferred while a plurality of residence-side devices share a medium. The present invention specifically relates to an optical subscriber terminating device, a PON system, and an abnormality detecting method with which it is possible to detect a failure occurring in the optical subscriber terminating device used in an Ethernet (a registered trademark) PON (EPON) where data is transferred while being in Ethernet (a registered trademark) frames. 
     BACKGROUND 
     In recent years, the Internet is popularly used, and users are able to access and obtain various information provided at sites that are run in various places of the world. Along with this trend, broadband accesses such as ones through Asymmetric Digital Subscriber Lines (ADSLs) and Fiber To The Home (FTTH) including the PON are also getting popular. In particular, with regard to FTTH, the demand for Gigabit Ethernet (a registered trademark)-PON (GE-PON) is rapidly growing, and the communication speed thereof is expected to be even higher in the future. High-speed PON systems such as 10GE-PON are getting more and more attention. 
     A conventional PON system includes, for example, an Optical Line Terminal (OLT) that is mainly installed in a telephone station or the like; a plurality of Optical Network Units (ONUs) that are mainly installed at residences; an optical coupler that branches an optical signal transmitted from the OLT and sends the branched signals to the ONUs, and also, converges optical signals transmitted from the ONUs and sends the converged optical signal to the OLT; and user terminals each of which is connected to a different one of the ONUs. Link-up processes realized by handshakes and bandwidth distributing and allocating processes are performed between the OLT and the ONUs. 
     In the PON system configured as described above, if, for example, a failure has occurred in the circuit of one of the ONUs so that the ONU goes into a constant light-emitting state (hereinafter, a “constant emission state”), timing control of the upstream communication is not properly exercised. As a result, all the ONUs become unable to perform communication, and ONU links are cut off. In that situation, it is necessary to identify the ONU having the failure and to isolate the identified ONU from the PON system, so as to secure a communication path in the upstream direction and to recover the communication. Examples of techniques for realizing this solution are disclosed in, for example, Patent Literature 1 and Patent Literature 2 listed below. 
     By using the technique disclosed in Patent Literature 1, it is possible to detect, not only slave stations that have gone into a constant emission state due to a failure in the circuit, but also slave stations having an abnormality in the light emission period thereof. Also, Patent Literature 2 discloses a circuit for detecting an abnormality that accidentally occurred in an upstream frame. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2007-318524 
     Patent Literature 2: Japanese Patent Application Laid-open No. 2007-158943 
     SUMMARY 
     Technical Problem 
     According to the technique described in Patent Literature 1 listed above, however, it is necessary to additionally attach a photodiode to each of the ONUs, to be able to detect the light emitting state. For this reason, a problem arises where it is difficult to apply this technique to optical subscriber terminating devices, which are installed at residences and require low costs. 
     Further, the technique described in Patent Literature 2 is not applicable to the situation where the upstream communication is disabled by a continuous light-emission state of an ONU. Thus, a problem remains where it is not possible to apply the technique to continuous light-emission abnormalities. 
     In view of the circumstances described above, an object of the present invention is to obtain an optical subscriber terminating device, a PON system, and an abnormality detecting method with which it is possible to detect continuous light-emission abnormalities, while keeping the circuit to be added to general-purpose ONUs minimum. 
     Solution To Problem 
     In order to solve the above problem and in order to attain the above object, in an optical subscriber terminating device that receives a predetermined control signal transmitted by an optical subscriber terminal station device in a predetermined cycle, returns a response signal in response to the predetermined control signal, and receives a unicast frame transmitted to the optical subscriber terminating device by the optical subscriber terminal station device having received the response signal, the optical subscriber terminating device of the present invention, include: a received frame detecting unit that detects a type of signal received from the optical subscriber terminal station device; and an abnormal emission detecting unit that detects an abnormal emission state, based on a result detected by the received frame detecting unit. 
     Advantageous Effects of Invention 
     The optical subscriber terminating device, the PON system, and the abnormality detecting method according to an aspect of the present invention are obtained by adding the abnormal emission detecting unit to a general-purpose ONU, and the abnormal emission detecting unit is configured so as to determine that the ONU is in a continuous light-emission abnormal state in the situation where discovery gates have regularly been received, but no unicast frame has been received within the predetermined time period since the reception of a discovery gate. Thus, an advantageous effect is achieved where it is possible to detect continuous light-emission abnormalities, while keeping the circuit to be added to the general-purpose ONU minimum. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram of an exemplary functional configuration of an optical subscriber terminating device according to an aspect of the present invention. 
         FIG. 2  is a diagram of an exemplary configuration of a PON system. 
         FIG. 3  is a sequence chart of examples of link-up processes realized by handshakes of ONUs and a bandwidth distributing and allocating process. 
         FIG. 4  is a chart of an example of an operation sequence used when ONU link-up processes are performed again. 
         FIG. 5  is a sequence chart of an example of a method for controlling emission force-quit processes. 
         FIG. 6-1  is a flowchart of an example of a process to identify an ONU that is in a continuous light-emission abnormal state and an operation to eliminate a communication abnormality. 
         FIG. 6-2  is another flowchart of the example of the process to identify the ONU that is in the continuous light-emission abnormal state and the operation to eliminate the communication abnormality. 
     
    
    
     REFERENCE SIGNS LIST 
       1 ,  1 - 1  to  1 - 5  ONU 
       2  OPTICAL TRANSMITTING AND RECEIVING UNIT 
       3  OPTICAL INPUT DETECTING UNIT 
       4  RECEIVED FRAME DETECTING UNIT 
       5  ERRONEOUS EMISSION DETECTING UNIT 
       6  EMISSION FORCE-QUIT CONTROLLING UNIT 
       7  EMISSION FORCE-QUIT UNIT 
       8  LED CONTROLLING UNIT 
       10  DISCOVERY GATE 
       11  to  13  UNICAST FRAME 
       14  OPTICAL FIBER 
       20  OLT 
       21  OPTICAL COUPLER 
       22 - 1  to  22 - 5  USER TERMINAL 
     DESCRIPTION OF EMBODIMENTS 
     In the following sections, exemplary embodiments of an optical subscriber terminating device, a PON system, and an abnormality detecting method according to the present invention will be explained in detail, with reference to the accompanying drawings. The present invention is not limited by the exemplary embodiments. 
     Exemplary Embodiments 
       FIG. 1  is a diagram of an exemplary functional configuration of an optical subscriber terminating device (hereinafter, an “Optical Network Unit (ONU)”) according to an aspect of the present invention. In  FIG. 1 , constituent elements relevant to the present invention are shown. An ONU  1  shown in  FIG. 1  is based on a premise that the ONU  1  has principal functions of ONUs defined in the Institute of Electrical and Electronic Engineers (IEEE) Std 802.3-2005 or the IEEE 802.3av, which is in a process of standardization. 
     As shown in  FIG. 1 , the ONU  1  according to the present embodiment includes: an optical transmitting and receiving unit  2  that converts a received optical signal into an electric signal and converts an electric signal to be transmitted to an optical signal; an optical input detecting unit  3  that detects if optical signals are being received, based on whether each of the optical signals detected by the optical transmitting and receiving unit  2  has an output equal to or higher than a predetermined threshold; a received frame detecting unit  4  that detects a received frame; an erroneous light-emission (abnormal light-emission) detecting unit (hereinafter, “erroneous emission detecting unit”)  5  that detects if the ONU is in an abnormal light-emission state (hereinafter, “abnormal emission state”); an emission force-quit controlling unit  6  that controls an emission force-quit process; an emission force-quit unit  7  that implements an emission force-quit instruction; and a light emitting diode (LED) controlling unit  8  that controls an LED. Further, the ONU  1  is connected to an optical fiber  14  and receives, for example, a discovery gate (DG)  10  and unicast frames (UC)  11 - 13  via the optical fiber  14 . 
       FIG. 2  is a diagram of an exemplary configuration of a PON system according to the present embodiment. As shown in  FIG. 2 , the PON system according to the present embodiment includes: an OLT  20  that is mainly installed in a telephone station or the like; ONUs  1 - 1  to  1 - 5  that are mainly installed at residences; an optical coupler  21  that branches an optical signal transmitted from the OLT  20  and sends the branched signals to the ONUs  1 - 1  to  1 - 5 , and also, converges optical signals transmitted from the ONUs  1 - 1  to  1 - 5  and sends the converged optical signal to the OLT  20 ; and user terminals  22 - 1  to  22 - 5  that are connected to the ONUs  1 - 1  to  1 - 5 , respectively. Each of the ONUs  1 - 1  to  1 - 5  has the same configuration as that of the ONU  1  shown in  FIG. 1 . Although the number of ONUs is five in the example shown in  FIG. 2 , the number of ONUs is not limited to this example. 
     THE OLT  20  is connected to the optical coupler  21  via an optical fiber. Each of the ONUs  1 - 1  to  1 - 5  is connected to the optical coupler  21  via an optical fiber. Further, each of the ONUs  1 - 1  to  1 - 5  is connected to a corresponding one of the user terminals  22 - 1  to  22 - 5  via a cable. 
       FIG. 3  is a sequence chart of examples of link-up processes realized by handshakes of the ONUs (hereinafter, the “ONU link-up processes”) and a bandwidth distributing and allocating process that are performed in the PON system shown in  FIG. 2 . In  FIG. 3 , the number of ONUs connected to the OLT is three (i.e., the ONU- 1  to ONU- 3  shown in  FIG. 3 ). The upper section of the drawing illustrates a discovery process in which the ONUs perform the link-up processes as defined in the IEEE Std 802.3-2005 and the IEEE 802.3ay. The lower section of the drawing illustrates giving and receiving of grants and reports between the OLT and the ONUs for the purpose of allocating the bandwidth after the ONU link-up processes are completed. 
     The ONU link-up processes and the bandwidth distributing and allocating process will be explained, with reference to  FIG. 3 . First, the OLT  20  transmits, in the manner of a broadcast, a discovery gate (DG) to the ONUs  1 - 1  to  1 - 3 , for the purpose of finding ONUs that have not completed a link-up process (step S 11 ). In the present example, let us assume that the ONUs  1 - 1  to  1 - 3  have not completed the link-up processes. When having received the discovery gate from the OLT  20 , each of the ONUs  1 - 1  to  1 - 3  transmits a Register Request (RR) for the purpose of requesting a link-up process (step S 12 ). 
     When having received the register requests from the ONUs  1 - 1  to  1 - 3 , the OLT  20  transmits a register (RG) including information required to subsequently perform the ONU link-up process, to the ONU  1 - 3  (step S 13 ). After that, the OLT  20  transmits a grant (G) that defines transmission timing of frames to be transmitted by the ONU  1 - 3  (step S 14 ). 
     After confirming that the frames of the register and the grant received from the OLT  20  have properly been received, the ONU  1 - 3  transmits a Register Acknowledge (RA) for the purpose of notifying that the register has properly been received, while using the transmission timing specified in the received grant (step S 15 ). The link-up process for the ONU  1 - 3  has thus completed. 
     Subsequently, the processes at steps S 16  through S 18  and the processes at steps S 19  through S 21  are performed for the ONUs  1 - 1  and  1 - 2 , respectively, in the same manner as at steps S 13  through S 15  for the ONU  1 - 3 . The link-up processes for the ONUs  1 - 1  and  1 - 2  are thus completed. Because the OLT  20  performs the link-up process for each of the ONUs  1 - 1  to  1 - 3  independently, the transmission timing specified for each of the ONUs  1 - 1  to  1 - 3  is determined depending on a processing status of the OLT  20 . 
     The OLT  20  starts the bandwidth allocating process and data transfer for the ONUs that have completed the ONU link-up processes, according to a Multi Point Control Protocol (MPCP). First, the OLT  20  calculates a transmission starting time of a report to be transmitted by each of the ONUs, generates a grant including the calculated transmission starting time for each of the ONUs, and sequentially transmits the generated grants to the corresponding one of the ONUs  1 - 3 ,  1 - 2 , and  1 - 1  (step S 22 ). 
     Each of the ONUs  1 - 1  to  1 - 3  transmits a report including a data transmission request amount to the OLT  20 , according to the transmission starting time included in the received grant (step S 23 ). When having received the reports from the ONUs  1 - 1  to  1 - 3 , the OLT  20  calculates a data transmission starting time and a transmission permitted amount for the ONU  1 - 3 , which is the first one to be permitted to transmit data, and the OLT  20  transmits a grant storing therein the data transmission starting time and the transmission permitted amount that are calculated, to the ONU  1 - 3  (step S 24 ). 
     When having received the grant transmitted at step S 24 , the ONU  1 - 3  generates upstream data (D) based on the transmission permitted amount included in the received grant and transmits the generated upstream data (D) together with a report storing therein a transmission request amount for the next transmission, to the OLT  20  (step S 25 ). 
     The OLT  20  receives the data from the ONU  1 - 3  and calculates, in parallel therewith, a data transmission starting time and a transmission permitted amount for the ONU  1 - 2 , and transmits a grant storing therein the calculated result to the ONU  1 - 2  (step S 26 ). When having received the grant transmitted at step S 26 , the ONU  1 - 2  generates upstream data (D) based on the transmission permitted amount included in the received grant and transmits the generated upstream data (D) together with a report storing therein a transmission request amount for the next transmission, to the OLT  20  (step S 27 ). 
     The OLT  20  receives the data from the ONU  1 - 2  and calculates, in parallel therewith, a data transmission starting time and a transmission permitted amount for the ONU  1 - 1 , and transmits a grant storing therein the calculated result to the ONU  1 - 1  (step S 28 ). After transmitting the grant to the ONU  1 - 1 , the OLT  20  calculates a transmission starting time and a transmission permitted amount for the second transmission of the ONU  1 - 3  and transmits a grant storing therein the calculation result, to the ONU  1 - 3  (step S 29 ). 
     When having received the grant transmitted at step S 28 , the ONU  1 - 1  generates upstream data (D) based on the transmission permitted amount included in the received grant and transmits the generated upstream data (D) together with a report storing therein a transmission request amount for the next transmission, to the OLT  20  (step S 30 ). Also, when having received the grant transmitted at step S 29 , the ONU  1 - 3  generates upstream data, based on the transmission permitted amount included in the received grant and transmits the generated upstream data together with a report storing therein a transmission request amount for the next transmission, to the OLT  20  (step S 31 ). 
     After that, according to the data amount, a transmission of a grant and a transmission of a report and upstream data are performed in the same manner (steps S 32  and S 33 ). As explained above, the OLT  20  sequentially allocates the bandwidths to the ONUs  1 - 1  to  1 - 3  and receives the data from the ONUs. 
     If an abnormality has occurred in one or more of the ONUs  1 - 1  to  1 - 3  so that those ONUs go into a continuous light-emission state (hereinafter, “continuous emission state”), the communications from the ONUs  1 - 1  to  1 - 3  toward the OLT  20  (i.e., the communications in the upstream direction) all become impossible due to an interference among the optical signals. As a result, it becomes impossible to perform the ONU link-up processes realized by the handshakes and the bandwidth distributing and allocating process shown in  FIG. 3 . When the communications in the upstream direction became impossible in this manner, the link-up state of all the ONUs is cancelled, and the OLT  20  performs link-up processes again. 
       FIG. 4  is a chart of an example of an operation sequence used when the ONU link-up processes are performed again. When the communications in the upstream direction became impossible, the OLT  20  first transmits, as shown in  FIG. 4 , a discovery gate (DG) to the ONUs  1 - 1  to  1 - 3 , for the purpose of performing the ONU link-up processes again (step S 41 ). 
     When having received the discovery gate, each of the ONUs  1 - 1  to  1 - 3  transmits a register request to request a link-up process; however, when the one or more of the ONUs  1 - 1  to  1 - 3  are in an abnormal state and have gone into a continuous emission state where the communications are impossible, the OLT  20  is not able to receive the register requests transmitted from the ONUs  1 - 1  to  1 - 3  (step S 42 ). To cope with this situation, the OLT  20  transmits a discovery gate again, based on a discovery cycle that is set in advance (step S 43 ); however, the OLT  20  is still not able to receive the register requests transmitted from the ONUs  1 - 1  to  1 - 3  (step S 44 ). After that, the OLT  20  keeps transmitting a discovery gate at a regular interval. 
     To avoid the situation where, as shown in  FIG. 4 , the OLT  20  keeps transmitting a discovery gate, it is important to identify the ONUs that have gone into the continuous emission state and are causing hindrances, as quickly as possible, to perform an emission force-quit process on those ONUs, and to ensure communication paths in the upstream direction. Further, it is desirable if each of the ONUs is configured so as to be able to detect when the ONU itself is in a continuous emission state and to automatically discontinue the force emission state. Further, generally speaking, because each of the ONUs is installed at a residence or the like of a subscriber, it is desirable to be able to perform the processes described above, with a minimum additional circuit to a general-purpose ONU. 
     To address these demands, the ONU  1  according to the present embodiment is provided with a continuous emission state detecting unit that detects whether the upstream line in the system of its own is in a continuous emission state. Also, according to the present embodiment, when the ONU  1  detects that a continuous emission state is present, the ONU  1  judges if the ONU  1  itself is the one being in a continuous emission state. When the ONU  1  has determined that it is the ONU  1  itself that is in a continuous emission state, the ONU  1  performs an emission force-quit process so as to ensure a communication path in the upstream direction. 
     Returning to the description of  FIG. 4 , for example, in the situation illustrated in  FIG. 4 , by monitoring whether each of the ONUs  1 - 1  to  1 - 3  is in a “state where it is not possible to receive any unicast frame for a predetermined period of time, although a discovery gate was received”, it is possible to detect whether any of the ONUs  1 - 1  to  1 - 3  is in an abnormal emission state where the communications of the upstream signals are hindered. 
     In general-purpose ONUs, a circuit that detects whether a discovery gate has been received is already installed so as to perform the discovery process. Also, it is possible to judge whether no unicast frame has been received for the predetermined period of time, by using a means for performing regular receiving processes. For example, let us assume that, if no unicast frame has been received for the predetermined period of time, a judgment result shows “no unicast frame has been received”. Each of the ONUs is able to judge whether an abnormality has occurred in the upstream communication by judging whether an upstream communication abnormality detection condition shown below in Expression (1) is satisfied. 
       An upstream communication abnormality detection condition=“Discovery gates have been received in a predetermined cycle” AND “No unicast frame has been received”  (1)
 
     In Expression (1) shown above, confirming that discovery gates have been received in the predetermined cycle corresponds to confirming that the downstream communication is normal. Also, confirming that no unicast frame has been received corresponds to confirming that the upstream communication is not normal. 
     To further simplify the circuit configuration, it is also possible to judge whether an abnormality has occurred in the upstream communication by using a communication abnormality detection condition shown below in Expression (2). 
       Another upstream communication abnormality detection condition=“The downstream signal optical input is normal” AND “No unicast frame has been received”  (2)
 
     It is possible to judge whether the downstream signal optical input is normal, by using a regular receiving function of the ONUs. When Expression (2) shown above is used, determining that the downstream signal optical input is normal corresponds to determining that the downstream communication is normal. It should be noted that, however, when Expression (2) shown above is used, it is not possible to distinguish the situation where discovery gates are not properly transmitted due to a failure of the OLT  20 . Thus, it is preferable to use the condition in Expression (1) as long as the circuit scale of the ONUs is within a restriction range. 
     With the arrangement described above, it is possible to determine that an abnormality has occurred in the upstream communication; however, it is still not possible to identify which ONU is in an abnormal state. In the present embodiment, for the purpose of, in the following stage, identifying the ONU that is in a continuous light-emission abnormal state (hereinafter, a “continuous emission abnormal state”) and realizing the emission force-quit process on the identified ONU, each of the ONUs exercises emission force-quit control and each of the ONUs monitors whether the abnormal state in the upstream communication is cancelled by the control. In this situation, if the ONUs performed the emission force-quit processes at the same time, it would not be possible to determine which ONU&#39;s emission force-quit control has contributed to the recovery of the upstream communication. Thus, it is necessary to ensure that the ONUs do not perform the emission force-quit processes at the same time. 
     To ensure that the ONUs do not perform the emission force-quit processes at the same time, it is necessary to, for example, calculate an emission-quit starting time by using a number unique to each of the ONUs such as an identifier so that the emission-quit starting times do not overlap one another among the ONUs. As a specific embodiment example, it is possible to use a Logical Link Identification (LLID) provided by the OLT for each of the ONUs through an auto discovery process. 
     The Logical Link Identification is a number that is unique to each ONU and is assigned to the ONU by the OLT when the ONU link-up process is performed. Each of the ONUs multiplies an emission force-quit time period that is set in advance by a predetermined value calculated based on the LLID assigned to the ONU, so as to obtain a multiplied time period. Further, each of the ONUs sets a starting point at a time (i.e., a continuous emission abnormal state detection time) at which an abnormality in the upstream communication is detected based on Expression (1) or (2) shown above and determines a time at which the multiplied time period has elapsed since the starting point as the emission force-quit starting time. With this arrangement, the ONUs are able to perform the emission force-quit processes without overlapping one another. 
       FIG. 5  is a sequence chart of an example of a method for controlling the emission force-quit processes according to the present embodiment. In the present example also, it is assumed that three ONUs (i.e., the ONUs  1 - 1  to  1 - 3 ) are connected, like in the examples shown in 
       FIGS. 3 and 4 . Also, as for the LLIDs, it is assumed that LLID # 0  (the value of the LLID number is “0”) is assigned to the ONU  1 - 1 , LLID # 2  (the value of the LLID number is “2”) is assigned to the ONU  1 - 2 , and LLID # 3  (the value of the LLID number is “3”) is assigned to the ONU  1 - 3 . It is also assumed that the transmission cycle of the discovery gates transmitted by the OLT  20  is 1 second and that the emission force-quit time period of each of the ONUs is 5 seconds. The transmission cycle of the discovery gates and the emission force-quit time period mentioned here are only examples. The present invention is not limited to these examples, and the time periods may be set to any number of seconds. 
     First, based on the upstream communication abnormality detecting condition defined in Expression (1) or (2) shown above, each of the ONUs  1 - 1  to  1 - 3  detects (DET) that an abnormality has occurred in the upstream communication i.e., that a continuous emission abnormal state is present (steps S 51 , S 52 , and S 53 ). In this situation, the difference in the timing with which the continuous emission abnormal state is detected among the ONUs  1 - 1  to  1 - 3  (i.e., the time periods between the occurrence of the abnormality and the detection) depends on the length of the optical fiber being connected and an operation clock deviation of each of the ONUs; however, generally speaking, the difference is 1 millisecond or shorter. Thus, in the present example, the difference is considered to be negligible in controlling the emission force-quit processes. Consequently, it is assumed that all of the ONUs  1 - 1  to  1 - 3  detect the continuous emission abnormal state at a time T 1 . 
     When having detected the continuous emission abnormal state, each of the ONUs  1 - 1  to  1 - 3  calculates an 
     ONU emission force-quit starting time T 2  at which the emission force-quit process is to be started, by using Expression (3) shown below. 
       The ONU emission force-quit starting time T2 =T1+(LLID number+1)*emission force-quit time period   (3)
 
     For example, for the ONU  1 - 1 , T 2  is calculated as T2=T1+1*5 by using Expression (3) shown above. Thus, the ONU  1 - 1  forcibly stops the light emission (force-quits the light emission) for 5 seconds from the time T 2 , which is 5 seconds later than T 1  (step S 54 ). Similarly, the ONU  1 - 2  performs the emission force-quit process for 5 seconds from a time T 3 , which is 15 seconds later than T 1  (step S 55 ). The ONU  1 - 3  performs the emission force-quit process for 5 seconds from a time T 4 , which is 20 seconds later than T 1  (step S 56 ). As a result, the times at which the ONUs  1 - 1  to  1 - 3  perform the emission force-quit processes do not overlap one another. 
     According to the present embodiment, to ensure that the ONUs perform the emission force-quit processes without overlapping one another, the emission force-quit starting times are determined by using the LLIDs; however, it is also acceptable to determine the emission force-quit starting times based on any other information unique to each of the ONUs. For example, it is also acceptable to calculate the emission force-quit starting times by using Media Access Control (MAC) addresses of the ONUs. 
     Next, an operation that is performed after the emission force-quit processes are performed in the manner described above, so as to identify the ONU that is in the continuous emission abnormal state based on the result of the emission force-quit processes and to cancel the communication abnormal state in the upstream direction will be explained.  FIGS. 6-1  and  6 - 2  are flowcharts of an example of the process to identify the ONU that is in a continuous emission abnormal state and the communication abnormality eliminating operation. 
     In the following sections, an example in which the number of ONUs is three (i.e., the ONUs  1 - 1  to  1 - 3 ), like in the examples shown in  FIGS. 3 ,  4 , and  5 , will be explained. Further, it is assumed that the order in which the ONUs  1 - 1  to  1 - 3  perform the emission force-quit processes is determined based on the unique values respectively corresponding to the ONUs, as explained with reference to the example shown in  FIG. 5 . In the example shown in  FIGS. 6-1  and  6 - 2 , it is assumed that, after the continuous abnormal state is detected, the ONU  1 - 1  first performs the emission force-quit process, and subsequently, the ONU  1 - 2  and ONU  1 - 3  perform the emission force-quit processes in the state order, like in the example shown in  FIG. 5 . 
     First, each of the ONUs  1 - 1  to  1 - 3  detects the continuous emission abnormal state (step S 61 ). After having detected the continuous emission abnormal state, each of the ONUs calculates the emission force-quit starting time as explained above. First, the ONU  1 - 1  performs the emission force-quit process (step S 62 ). After the ONU  1 - 1  performs the emission force-quit process, the ONUs  1 - 1  to  1 - 3  judge whether the continuous emission abnormal state has been cancelled (step S 63 ). More specifically, for example, when it has become possible to receive a unicast frame within a predetermined time period since a reception of a discovery gate, it is determined that the continuous emission abnormal state has been cancelled. 
     When the ONUs  1 - 1  to  1 - 3  determine that the continuous emission abnormal state has been cancelled (step S 63 : Yes), the ONUs  1 - 1  to  1 - 3  determine that the ONU  1 - 1  is the ONU that caused the continuous emission abnormal state and further keep the ONU  1 - 1  in the force emission state so as to secure an upstream communication path (step S 64 ). More specifically, when it is determined that the continuous emission abnormal state has been cancelled, the ONU  1 - 1  continues to be in the force emission state, and each of the ONUs  1 - 2  and  1 - 3  does not perform the emission force-quit process even if the emission force-quit starting time calculated for itself has arrived. Further, the ONU  1 - 1  illuminates a Light Emitting Diode (LED) thereof to notify that the ONU  1 - 1  itself is an abnormal ONU (step S 65 ). It is determined that the process to identify and isolate the ONU that is in the abnormal emission state has been completed, and the process is ended (step S 66 ). 
     The LED is illuminated when each of the ONUs is provided with the function to illuminate the LED; however, if the ONUs were not provided with the function, the LED illumination process would not necessarily have to be performed. Further, it is possible to use any method to illuminate the LED. The method for illuminating the LED for the purpose of having the abnormality recognized is determined in advance, so that the LED can be illuminated by using the method. 
     On the contrary, if the ONUs  1 - 1  to  1 - 3  determine, at step S 63 , that the continuous emission abnormal state has not been cancelled (step S 63 : No), the ONU  1 - 1  checks to see if the emission force-quit time period has expired (step S 67 ). If the ONU  1 - 1  determines that the emission force-quit time period has expired (step S 67 : Yes), the ONU  1 - 1  discontinues the emission force-quit process (step S 68 ). On the contrary, if the ONU  1 - 1  determines that the emission force-quit time period has not expired (step S 67 : No), the ONU  1 - 1  returns to step S 63 . At step S 67 , the ONUs  1 - 2  and  1 - 3  do not perform the process, but stand by while the process at step S 67  is being performed. When the ONU  1 - 1  performs the process at step S 63 , the ONUs  1 - 2  and  1 - 3  also perform the process at step S 63  at the same time. 
     When the ONU  1 - 1  discontinues the emission force-quit process at step S 68 , the ONU  1 - 2  performs the emission force-quit process based on the emission force-quit starting time of its own, as an emission force-quit process to follow (step S 69 ). Further, in the same manner as at step S 63 , the ONUs  1 - 1  to  1 - 3  judge whether the continuous emission abnormal state has been cancelled (step S 70 ). When the ONUs  1 - 1  to  1 - 3  determine that the continuous emission abnormal state has been cancelled (step S 70 : Yes), the ONUs  1 - 1  to  1 - 3  determine that the ONU  1 - 2  is the ONU that caused the continuous emission abnormal state and further keep the ONU  1 - 2  in the force emission state so as to secure an upstream communication path (step S 71 ). Further, the ONU  1 - 2  illuminates an LED to notify that the ONU  1 - 2  itself is an abnormal ONU (step S 72 ). It is determined that the process to identify and isolate the ONU that is in the abnormal emission state has been completed, and the process is ended (step S 66 ). 
     On the contrary, if the ONUs  1 - 1  to  1 - 3  determine that the continuous emission abnormal state has not been cancelled (step S 70 : No), the ONU  1 - 2  checks to see if the emission force-quit time period has expired (step S 73 ). If the ONU  1 - 2  determines that the emission force-quit time period has expired (step S 73 : Yes), the ONU  1 - 2  discontinues the emission force-quit process (step S 74 ). On the contrary, if the ONU  1 - 2  determines that the emission force-quit time period has not expired (step S 73 : No), the ONU  1 - 2  returns to step S 70 . At step S 73 , the ONUs  1 - 1  and  1 - 3  do not perform the process, but stand by while the process at step S 73  is being performed. When the ONU  1 - 2  performs the process at step S 70 , the ONUs  1 - 1  and  1 - 3  also perform the process at step S 73  at the same time. 
     When the ONU  1 - 2  discontinues the emission force-quit process at step S 74 , the ONU  1 - 3  performs the emission force-quit process based on the emission force-quit starting time of its own, as an emission force-quit process to follow (step S 75 ). Further, in the same manner as at step S 63 , the ONUs  1 - 1  to  1 - 3  judge whether the continuous emission abnormal state has been cancelled (step S 76 ). When the ONUs  1 - 1  to  1 - 3  determine that the continuous emission abnormal state has been cancelled (step S 76 : Yes), the ONUs  1 - 1  to  1 - 3  determine that the ONU  1 - 3  is the ONU that caused the continuous emission abnormal state and further keep the ONU  1 - 3  in the force emission state so as to secure an upstream communication path (step S 77 ). Further, the ONU  1 - 3  illuminates an LED to notify that the ONU  1 - 3  itself is an abnormal ONU (step S 78 ). It is determined that the process to identify and isolate the ONU that is in the abnormal emission state has been completed, and the process is ended (step S 66 ). 
     On the contrary, if the ONUs  1 - 1  to  1 - 3  determine that the continuous emission abnormal state has not been cancelled (step S 76 : No), the ONU  1 - 3  checks to see if the emission force-quit time period has expired (step S 79 ). If the ONU  1 - 3  determines that the emission force-quit time period has expired (step S 79 : Yes), the ONU  1 - 3  discontinues the emission force-quit process (step S 80 ). Because there is a possibility that the cause of the abnormal emission state may not be the ONUs, the process is ended without identifying the ONU causing the abnormality (step S 81 ). In this situation, to notify the user that no ONU has been identified as the cause of the abnormality, a notifying process may be performed through an illumination of an LED or by using other means. 
     On the contrary, if the ONU  1 - 3  determines, at step S 79 , that the emission force-quit time period has not expired (step S 79 : No), the ONU  1 - 3  returns to step S 76 . At step S 79 , the ONUs  1 - 1  and  1 - 2  do not perform the process, but stand by while the process at step S 79  is being performed. When the ONU  1 - 3  performs the process at step S 76 , the ONUs  1 - 1  and  1 - 2  also perform the process at step S 76  at the same time. 
     As a result of the processes described above, it is possible to identify the ONU that is in the continuous emission abnormal state and to recover the upstream communication by causing the identified ONU to perform the emission force-quit process. The operation was explained above, with reference to  FIGS. 3 ,  4 ,  5 ,  6 - 1 , and  6 - 2 , while using the example in which the number of ONUs is three. It should be noted, however, that there is no limitation to the number of ONUs. The operation should be performed in the same manner as many times as the number of ONUs. 
     Next, returning to the description of  FIG. 1 , a procedure in the operation related to the emission force-quit process performed by the ONU  1  according to the present embodiment will be explained. In  FIG. 1 , only the constituent elements that are related to the emission force-quit process and the connection relationship related to the process are shown. Thus, other constituent elements used for performing regular communication are not shown in the drawing. First, the optical transmitting and receiving unit  2  receives, as optical signals, the discovery gate  10  transmitted from the OLT  20  via the optical fiber  14  and the unicast frames  11  to  13  transmitted by the OLT  20  to the ONUs. The optical transmitting and receiving unit  2  converts the received optical signals into the electric signals and sends the electric signals to the received frame detecting unit  4 . Also, when data is to be transmitted, the optical transmitting and receiving unit  2  converts transmission data, which is an electric signal, into an optical signal and outputs the optical signal to the optical fiber  14 . 
     The optical input detecting unit  3  judges whether the optical signals transmitted from the OLT  20  are received at a signal level equal to or higher than a predetermined level (i.e., an optical input level that allows the optical transmitting and receiving unit  2  to properly reconstruct the optical signals into electric signals), that is to say, judges whether the optical signals are received at a normal optical input level. The optical input detecting unit  3  then notifies the erroneous emission detecting unit  5  of the judgment result. 
     Based on each of the electric signals output from the optical transmitting and receiving unit  2 , the received frame detecting unit  4  judges whether the signal is the discovery gate  10  transmitted from the OLT  20  or the unicast frames  11  to  13  transmitted from the OLT  20  to the ONUs and notifies the erroneous emission detecting unit  5  of the judgment result. 
     When being notified by the optical input detecting unit  3  that the signal is received at a normal optical input level, the erroneous emission detecting unit  5  judges whether discovery gates  10  are received in the predetermined cycle, based on the notification from the received frame detecting unit  4  indicating that a discovery gate  10  is received. Further, if the erroneous emission detecting unit  5  determines that the discovery gates  10  are received in the predetermined cycle, and also, no notification indicating that the unicast frames  11  to  13  have been received was issued by the received frame detecting unit  4  within a predetermined time period since the reception of a discovery gate  10 , the erroneous emission detecting unit  5  detects that there is a possibility that an abnormality may have occurred in one or more of the ONUs (including the ONU of its own) connected to mutually the same OLT  20  so that the one or more ONUs are in a continuous emission state, and the communication in the upstream direction is disabled (i.e., a continuous emission abnormal state). When the continuous emission abnormal state is detected, the erroneous emission detecting unit  5  so notifies the LED controlling unit  8  and the emission force-quit controlling unit  6 . 
     On the contrary, if a notification indicating that the unicast frames  11  to  13  have been received was issued by the received frame detecting unit  4  within the predetermined time period since the reception of a discovery gate  10 , the erroneous emission detecting unit  5  determines that the continuous emission abnormal state has been cancelled and so notifies the LED controlling unit  8  and the emission force-quit controlling unit  6 . 
     In the present example, when the optical input detecting unit  3  notifies that the signal is received at a normal optical input level, the normality of the downstream communication is judged by judging whether the discovery gates  10  are received in the predetermined cycle, based on the notifications from the received frame detecting unit  4  indicating that the discovery gates  10  were received. In other words, the normality of the downstream communication is judged based on the conditions in both Expressions (1) and (2) above; however, the present invention is not limited to this example. Alternatively, it is acceptable to judge the normality of the downstream communication by judging whether the discovery gates  10  are received in the predetermined cycle as defined in Expression (1) above. Further, it is also acceptable to judge the normality of the downstream communication based on a notification from the optical input detecting unit  3  as defined in Expression (2) above, instead of judging whether the discovery gates  10  are received in the predetermined cycle. 
     The emission force-quit controlling unit  6  calculates the emission force-quit starting time according to an expression such as Expression (3) shown above, based on a unique ID such as the LLID of its own, so that the emission force-quit time periods of the ONUs do not overlap one another. When the calculated emission force-quit starting time has arrived, the emission force-quit controlling unit  6  instructs the emission force-quit unit  7  to perform the emission force-quit process, which is to stop the light emission for the predetermined emission force-quit time period. Further, if the emission force-quit controlling unit  6  receives a notification from the erroneous emission detecting unit  5  indicating that the continuous emission abnormal state is cancelled while the emission force-quit unit  7  is performing the emission force-quit process, the emission force-quit controlling unit  6  determines that the ONU of its own is the ONU that is in the continuous emission abnormal state. Further, the emission force-quit controlling unit  6  keeps instructing the emission force-quit unit  7  to continue performing the emission force-quit process even after the emission force-quit time period expires. The emission force-quit controlling unit  6  also notifies the LED controlling unit  8  that the ONU of its own is determined to be the ONU that is in the continuous emission abnormal state. 
     When having received the notification from the emission force-quit controlling unit  6  indicating that the ONU of its own is determined to be the ONU that is in the continuous emission abnormal state, the LED controlling unit  8  illuminates an LED to indicate that the ONU itself is an abnormal ONU. 
     It is desirable to provide the emission force-quit controlling unit  6  with the function to, once the ONU of its own has been determined to be the ONU that is in the continuous emission abnormal state, stop the electric power supply to the ONU, keep the emission force-quit state even after the electric power is turned on again, and keep the LED illuminated. However, this function is not essential to the present invention. 
     In the present embodiment, the example in which the communication method defined by the IEEE Std 802.3-2005 or the IEEE 802.3av is adopted is explained; however, the present invention is not limited to this example. It is possible to similarly apply the operation according to the present embodiment to any communication method by which predetermined signals are transmitted from the OLT  20  to the ONUs in a predetermined cycle, and each of the ONUs responds to the signals. In such a situation, instead of judging whether the discovery gates  10  are regularly received, it is judged whether the predetermined signals that are regularly transmitted are received regularly. 
     As explained above, according to the present embodiment, the erroneous emission detecting unit  5 , the emission force-quit controlling unit  6 , and the emission force-quit unit  7  are added to a general-purpose ONU defined by the IEEE Std 802.3-2005 or the IEEE 802.3ay. When the discovery gates are received regularly, and also, no unicast frame has been received within the predetermined time period since the reception of a discovery gate  10 , the erroneous emission detecting unit  5  determines that the continuous emission abnormal state is present. Thus, it is possible to detect continuous emission abnormalities, while keeping the circuit added to the general-purpose ONU minimum. 
     Further, the emission force-quit controlling unit  6  instructs that the emission force-quit process should be started in such a manner that the emission force-quit process time periods of the ONUS do not overlap one another. Accordingly, the emission force-quit unit  7  forcibly stops the light emission based on the instruction. Further, if the emission force-quit controlling unit  6  is notified by the erroneous emission detecting unit  5  that the continuous emission abnormal state is cancelled while the emission force-quit process is being performed, the emission force-quit controlling unit  6  determines that the ONU of its own is the cause of the continuous emission abnormal state and keeps the emission force-quit state. As a result, because it is possible to identify the ONU causing the continuous emission abnormality and to forcibly stop the light emission of the identified ONU, it is possible to quickly recover from the abnormal state. 
     INDUSTRIAL APPLICABILITY 
     As explained above, the optical subscriber terminating device and the abnormality detecting method according to an aspect of the present invention are useful in a PON system and are suitable for, in particular, a PON system in which processes are performed with handshakes between an OLT and ONUs.