Abstract:
A system and method for managing link status in Gigabit Ethernet passive optical network (GE-PON) units (ONUs) is disclosed. The GE-PON comprises one or more ONUs allocated data transmission periods, respectively. Each of the ONUs sends a report signal and a data signal in a corresponding one of the allocated data transmission periods. The report signal contains a signal to request allocation of a bandwidth for data signal transmission in a next window period. The GE-PON further comprises an optical splitter having its one side connected with the ONUs and its other side connected with at least one optical communication channel. The optical splitter switches an input signal to a desired destination. The GE-PON further comprises an optical line terminal (OLT) for determining whether the report signal is received, identifying a specific one of the ONUs having sent the report signal when the report signal is received, determining whether the data signal is received from the specific ONU in a specific one of the data transmission periods allocated to the specific ONU, and discriminating and managing a link status of the specific ONU in accordance with the determination made.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority, pursuant to 35 U.S.C. §119, to that patent application entitled “OPTICAL LINE TERMINAL FOR MANAGING LINK STATUSES OF OPTICAL NETWORK UNITS AND GIGABIT ETHERNET PASSIVE OPTICAL NETWORK EMPLOYING THE SAME,” filed in the Korean Intellectual Property Office on Jan. 20, 2004 and assigned Serial No. 2004-4177, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a Gigabit Ethernet passive optical networks (GE-PON) consisting of one optical line terminal (OLT) provided at the service provider side and a plurality of optical network units (ONUs) provided at the user side, and more particularly to a method for managing link status between the OLT and the ONUs.  
         [0004]     2. Description of the Related Art  
         [0005]     Nowadays, the expansion of public networks, such as wireless networks, and very high-speed communication networks, enables mass data to be shared online. It is the current reality that the offline sharing of data through low-priced mass storage media, such as compact discs (CDs) or digital versatile discs (DVDs), is also used very widely. Therefore, users are provided with numerous types of data shared online and offline.  
         [0006]     A passive optical network (PON) is a communication network system that transfers signals to end users over an optical cable network. The typical PON consists of an optical line terminal (OLT) installed in a communication company in communication with a plurality of optical network units (ONUs) installed near subscribers. Typically, an OLT can support a maximum of 32 ONUs.  
         [0007]     The PON can provide a bandwidth of 1 Gbps in the upstream and downstream directions in one stand-alone system, and this bandwidth can be allocated to a plurality of PON users. The PON may also be used as a trunk between a large-scale system, such as a cable TV system, and an Ethernet network for a neighboring building or home employing a coaxial cable.  
         [0008]     In the conventional PON, an OLT transmits a signal to an ONU via an optical cable, referred to as downstream transmission. The ONU receives the signal transmitted from the OLT, processes it in a predetermined manner and then transfers the processed result to an end user. The ONU, which, in this case, is a transfer system for the service subscriber side, is an optical network termination unit that provides a service interface to the end user.  
         [0009]     The ONU accommodates FTTC (Fiber To The Curb), FTTB (Fiber To The Building), FTTF (Fiber To The Floor), FTTH (Fiber To The Home), FTTO (Fiber To The Office) methods of transmission. Thus, the ONU is implemented to provide high service accessibility to the subscriber. The ONU functions to interconnect a cable that is connected with the subscriber to transmit an analog signal downstream and optical equipment that is connected with the OLT to transmit and receive optical signals. As a result, the ONU performs an optical/electrical conversion operation to convert an optical signal from the OLT into an electrical signal, in a downstream transmission and transmit the converted electrical signal to the subscriber, and an electrical/optical conversion operation to convert an electrical signal from the subscriber into an optical signal and transmit the converted optical signal to the OLT, in an upstream transmission.  
         [0010]      FIG. 1  illustrates a conventional downstream data transmission structure of a Gigabit Ethernet passive optical network (GE-PON), and  FIG. 2  illustrates a conventional upstream data transmission structure of the Gigabit Ethernet passive optical network. As shown in  FIGS. 1 and 2 , the Gigabit Ethernet passive optical network (GE-PON) has a structure where one OLT  10  is connected with a plurality of ONUs  20 ,  22  and  24  in a tree form via an optical splitter  15 . The GE-PON is an optical access network that is more inexpensive and efficient than an AON (Activity-On-Node) network.  
         [0011]     An earlier version of the GE-PON, an asynchronous transfer mode passive optical network (ATM-PON) has been developed and standardized. The ATM-PON transmits ATM cells in the form of a block with a desired size in the upstream or downstream direction. Another PON, an Ethernet passive optical network (E-PON), transmits packets of different sizes.  
         [0012]     Downstream data transmission is now described with reference to  FIG. 1 . In the downstream transmission, the OLT  10  broadcasts data to be transmitted to the ONUs  20 ,  22  and  24 . The optical splitter  15  receives the data broadcast from the OLT  10  and transmits the received data to each of the ONUs  20 ,  22  and  24 . The ONUs  20 ,  22  and  24 , each detect data to be transferred to a corresponding one of users  30 ,  32  and  34  from the data transmitted from the optical splitter  15  and transfers only the detected data to the corresponding user  30 ,  32  or  34 .  
         [0013]     Upstream data transmission is now described with reference to  FIG. 2 . In the upstream transmission, data from the users  30 ,  32  and  34  are transferred to the ONUs  20 ,  22  and  24 , respectively. The ONUs  20 ,  22  and  24  transmit the data from the users  30 ,  32  and  34  to the optical splitter  15  according to a transmission permission convention from the OLT  10 , respectively. The ONUs  20 ,  22  and  24  each transmit, upstream, the received data in a time slot set in a TDM (Time Division Multiplexing) manner. Therefore, no data collision resulting from the upstream data transmission occurs in optical splitter  15 .  
         [0014]     OLT  10  monitors a link status of each of the ONUs  20 ,  22  and  24  to notify a network manager of a change in a connection, e.g., a registration, with each of the ONUs  20 ,  22  and  24 . As a result, the network manager checks the link status of each of the ONUs  20 ,  22  and  24 , monitored and provided by the OLT  10 , and performs a network task corresponding to the change.  
         [0015]      FIG. 3  is a block diagram illustrating an architecture for the monitoring of link status of ONUs by an OLT in a GE-PON. In this illustrative example, OLT  40  is connected with a maximum of 32 ONUs  60 ,  62 , . . . ,  66  and  68  in a tree form via an optical splitter  50 . The OLT  40  is provided at the service provider side and the ONUs  60 ,  62 ,  66  and  68  are provided at the user side.  
         [0016]     OLT  40  includes an optical transmitter  44  for transmitting an optical signal to the ONUs  60 ,  62 ,  66  and  68  via the optical splitter  50 , an optical receiver  46  for receiving optical signals transmitted from the ONUs  60 ,  62 ,  66  and  68  via the optical splitter  50 , and a network management unit  42  for managing the transmission and reception of optical signals. The network management unit  42  is a network management group that performs operation, administration and maintenance (OAM) functions of the network.  
         [0017]     OLT  40  monitors the link status of the ONUs  60 ,  62 ,  66  and  68  using a conventional link status monitoring method based on receive signal power loss employed in a point to point architecture.  
         [0018]     However, since the GE-PON has a point to multipoint architecture using the optical splitter  50 , the conventional link status monitoring method employed in the point to point architecture has a disadvantage in that it cannot accurately discriminate among and determine the link status of each of the ONUs  60 ,  62 ,  66  and  68 .  
         [0019]     In conventional monitoring methods, the ONUs  60 ,  62 ,  66  and  68  each turn on a laser diode and maintain a standby mode, in order to transmit data in a time slot allocated by the OLT  40 . That is, even while any one of the ONUs  60 ,  62 ,  66  and  68  transmits data, the others must always turn on the laser diodes and maintain the standby mode, such as a power save mode. However, OLT  40  receives a signal corresponding to data actually transmitted from any one of the ONUs  60 ,  62 ,  66  and  68  and signals transmitted from the other ONUs in the standby mode. As a result, OLT  40  cannot accurately recognize the link status of the ONUs  60 ,  62 ,  66  and  68  from the signals transmitted.  
         [0020]      FIG. 4  represents a timing diagram illustrating a conventional method for managing link status on the basis of receive signal power losses based on the transmission laser power levels of the ONUs. As shown, a window size 70 is a period in which ONUs registered in an OLT transmit signals in time slots allocated by the OLT. The ONUs each change the operation mode of a laser diode from a standby mode to a data transmission mode in a corresponding one of the allocated time slots. As a result, the ONUs each raise the transmission power level of the laser diode to a desired level before a data transmission period  72 ,  74  or  76  in the corresponding time slot and then maintain the desired power level for the respective data transmission period  72 ,  74  or  76 . The ONUs transmit the maintained power levels in the data transmission periods  72 ,  74  and  76 , respectively.  
         [0021]     OLT  40  receives the signals transmitted from the ONUs in the data transmission periods  72 ,  74  and  76  allocated to respective ONUs. For the signal transmission of these ONUs, the power level of the laser diode of each of the ONUs is raised to the desired level before the data transmission period and lowered to a level corresponding to the standby mode after the data transmission period. For this reason, the laser diodes of the ONUs have power loss periods  82 ,  84  and  86  before and after the data transmission periods  72 ,  74  and  76 .  
         [0022]     However, the OLT also receives signals with the power level corresponding to the standby mode from two neighbor ones of the ONUs in each of the power loss periods  82 ,  84  and  86 , so it cannot accurately discriminate the link statuses of the ONUs.  
       SUMMARY OF THE INVENTION  
       [0023]     The present invention has been made in view of the above problems, and it is an object of the present invention to provide a device that is capable of more accurately managing link status of optical network units (ONUs) and a Gigabit Ethernet passive optical network (GE-PON) employing the same.  
         [0024]     In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of an optical line terminal (OLT) for managing link status of optical network units (ONUs) connected therewith, comprising an optical transmitter for transmitting to the ONUs information about data transmission periods allocated to the respective ONUs, an optical receiver for receiving a report signal and data signal sent from each of the ONUs in each of the allocated data transmission periods, the report signal containing bandwidth allocation request information, a link status discriminator for determining whether the report signal is received by the optical receiver, identifying a specific one of the ONUs having sent the report signal if the report signal is received, determining whether the data signal is sent from the specific ONU and received by the optical receiver in a specific one of the data transmission periods allocated to the specific ONU, and discriminating a link status of the specific ONU in accordance with the determination and a network management unit for performing an operation based on the link status of the specific ONU discriminated by the link status discriminator.  
         [0025]     The link status discriminator is adapted to identify the specific ONU having sent the report signal by an LLID (Logical Link Identification) associated with a specific ONU contained in the report signal and discriminate a type of the received signal by option code information contained in the report signal, the option code information being indicative of the type of the received signal.  
         [0026]     The link status discriminator may determine the link status of the specific ONU to be normal if the report signal is received at a start time of the specific data transmission period and the data signal is received from the specific ONU for the specific data transmission period.  
         [0027]     The link status discriminator may determine that power loss has occurred in the specific ONU if the report signal is received at the start time of the specific data transmission period and the data signal is received from the specific ONU for only part of the specific data transmission period. In this case, the link status discriminator may determine the link status of the specific ONU to be abnormal if no report signal from the specific ONU is received in a next window period.  
         [0028]     Preferably, the link status discriminator is adapted to determine that an error has occurred in an operation of any one of the ONUs corresponding to a certain one of the data transmission periods if no report signal is received at a start time of the certain data transmission period and the data signal is received for the certain data transmission period.  
         [0029]     The link status discriminator may determine that an error has occurred in at least one of an operation and link status of any one of the ONUs corresponding to a certain one of the data transmission periods if no report signal is received at a start time of the certain data transmission period and no data signal is received for the certain data transmission period.  
         [0030]     In accordance with another aspect of the present invention, there is provided a Gigabit Ethernet passive optical network (GE-PON) comprising one or more ONUs allocated data transmission periods, respectively, each of the ONUs sending a report signal and a data signal in a corresponding one of the allocated data transmission periods, the report signal containing a signal to request allocation of a bandwidth for data signal transmission in a next window period, an optical splitter having its one side connected with the ONUs and its other side connected with at least one optical communication channel, the optical splitter switching an input signal to a desired destination, and an OLT for determining whether the report signal is received, identifying a specific one of the ONUs having sent the report signal if the report signal is received, determining whether the data signal is received from the specific ONU in a specific one of the data transmission periods allocated to the specific ONU, and discriminating and managing a link status of the specific ONU in accordance with the determination.  
         [0031]     In a feature of the present invention, an OLT identifies a report signal sending ONU on the basis of a report signal that is sent in a data transmission period allocated in the previous window period, determines whether there is data received from the report signal sending ONU in the data transmission period, discriminates a link status of that ONU in accordance with the determination, and performs an operation corresponding to a result of the discrimination. Therefore, the OLT can more accurately discriminate and manage link statuses of ONUs registered therein. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0033]      FIG. 1  is a view showing a downstream data transmission structure of a Gigabit Ethernet passive optical network (GE-PON);  
         [0034]      FIG. 2  is a view showing an upstream data transmission structure of the GE-PON;  
         [0035]      FIG. 3  is a block diagram showing an architecture for the monitoring of link statuses of ONUs by an OLT in a GE-PON;  
         [0036]      FIG. 4  is a timing diagram illustrating a conventional method for managing link statuses of ONUs on the basis of receive signal power losses based on the transmission laser power levels of the ONUs;  
         [0037]      FIG. 5  is a chart illustrating a conventional procedure for communication with an OLT by an ONU;  
         [0038]      FIG. 6  is a block diagram showing an embodiment of a GE-PON for managing link statuses of ONUs, according to the present invention;  
         [0039]      FIG. 7  is a timing diagram illustrating a method for discriminating the link status of ONUs shown in  FIG. 6  according to the present invention; and  
         [0040]      FIG. 8  is a view showing the format of a report signal which is transmitted from an ONU according to the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]     Embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements may be denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a variety of specific elements such as constituent elements of various concrete circuits are shown. The description of such elements has been made only for a better understanding of the present invention. Those skilled in the art will appreciate that the present invention can be implemented without using the above-mentioned specific elements. For purposes of clarity, detailed description of known functions and configurations incorporated herein are omitted when it makes the subject matter of the present invention unclear.  
         [0042]      FIG. 5  is a chart illustrating a conventional procedure for communication with an OLT by an ONU. Upon power-up, the OLT, denoted by the reference numeral  92 , broadcasts a gate signal to all ONUs connected via an optical communication medium to discover them (S 110 ). The OLT  92  sends the gate signal to the ONUs at intervals of a predetermined time until it receives a registration request signal (S 120 ). In response to the gate signal sent from the OLT  92 , the ONU, denoted by the reference numeral  94 , sends the registration request signal to the OLT  92  (S 130 ).  
         [0043]     When the OLT  92  receives the registration request signal sent from the ONU  94 , then it registers the ONU  94  and allocates an LLID (Logical Link IDentification) thereto. The OLT  92  then sends information about the registration of the ONU  94  and information about the LLID allocated thereto to the ONU  94  (S 140 ). Upon receiving the registration information and LLID information, the ONU  94  sends registration acknowledgement information to the OLT  92  (S 150 ). When the OLT  92  receives the registration acknowledgement information from the ONU  94 , it sends information permitting transmission of a data transmission request signal (S 160 ) to the ONU  94 . At this time, the transmission permission information contains information about a time slot allocated to the ONU  94 . The ONU  94  receives the transmission permission information and sends a bandwidth allocation request signal to the OLT  92  to request it to allocate a bandwidth for data transmission (S 170 ). Here, the bandwidth allocation request signal is called a report signal. This report signal contains the LLID allocated to the ONU  94  and information about the type of the signal being sent.  
         [0044]     When the OLT  92  receives the report signal sent from the ONU  94 , it allocates the data transmission bandwidth to the ONU  94  (S 180 ). The ONU  94  transmits data in the allocated transmission bandwidth (S 190 ). At this time, the ONU  94  sends a report signal to the OLT  92  to request it to allocate a bandwidth for data transmission in the next window period. This report signal contains the LLID allocated to the ONU  94  and information about the type of the signal being sent.  
         [0045]     In the present invention, the OLT monitors a link status of the ONU using a bandwidth for data transmission allocated to the ONU in the previous window period and a report message sent from the ONU in the allocated bandwidth.  
         [0046]      FIG. 6  is a block diagram showing an embodiment of a Gigabit Ethernet passive optical network (GE-PON) for managing link statuses of ONUs, according to the present invention. As shown, the GE-PON has a structure where one OLT  100  is connected with a maximum of 32 ONUs  320 ,  330 ,  340 , . . . ,  360  and  370  in a tree form via an optical splitter  200 . The optical splitter  200  is adapted to switch signals received from the OLT  100  and the ONUs  320 ,  330 ,  340 ,  360  and  370  to corresponding destinations. The OLT  100  includes an optical transmitter  120 , an optical receiver  140 , a network management unit  160 , and a link status discriminator  180 . The optical transmitter  120  acts to transmit an optical signal to the ONUs  320 ,  330 ,  340 ,  360  and  370  via the optical splitter  200 . The optical receiver  140  acts to receive optical signals transmitted from the ONUs  320 ,  330 ,  340 ,  360  and  370  via the optical splitter  200 . The network management unit  160  acts to manage the transmission and reception of optical signals and operations based on the link statuses of the ONUs  320 ,  330 ,  340 ,  360  and  370 . The link status discriminator  180  is adapted to discriminate the link status of ONUs  320 ,  330 ,  340 ,  360  and  370  on the basis of the signals received by the optical receiver  140  and output the discrimination results to the network management unit  160 .  
         [0047]     The present embodiment will now be described with reference to one of the ONUs  320 ,  330 ,  340 ,  360  and  370  as an example. When the ONU  320 , for example, completes a registration procedure with the OLT  100 , it sends a report signal requesting the allocation of a bandwidth for data transmission, to the OLT  100 , in every window period (See  FIG. 5 ). The OLT  100  allocates a bandwidth for data transmission by the ONU  320  in the next window period. The OLT  100  sends a gate signal containing information about the allocated transmission bandwidth to the ONU  320 . Here, the transmission bandwidth is a period in which the ONU  320  is permitted to transmit data in the next window. In this connection, the transmission bandwidth information contains a bandwidth start time and bandwidth length.  
         [0048]     The link status discriminator  180  discriminates a link status of the ONU  320  on the basis of the start time of a transmission bandwidth allocated to the ONU  320  in the previous window period and a report signal received from the ONU  320  in the allocated transmission bandwidth. More specifically, the link status discriminator  180  checks the LLID contained in a report signal to determine which one of the ONUs  320 ,  330 ,  340 ,  360  and  370  has sent the report signal. The link status discriminator  180  also checks signal type information contained in a received signal to determine whether the received signal is a report signal. When the received signal is a report signal, the link status discriminator  180  determines which one of the ONUs has sent the report signal, by checking an LLID contained in the report signal. Where the report signal sending ONU is determined to be the same as one allocated the data transmission bandwidth (period), the link status discriminator  180  determines that data received in the allocated period has been sent from the allocated ONU.  
         [0049]     Therefore, in the case where data is received from the report signal sending ONU in the allocated data transmission period, the link status discriminator  180  determines the link status of that ONU to be normal.  
         [0050]     However, if either no data is received from the report signal sending ONU in the allocated data transmission period, only data is received in the allocated period with no report signal received, or any signal is not received in the allocated period, the link status discriminator  180  determines the link status of that ONU to be abnormal.  
         [0051]     The link status discriminator  180  transfers the discrimination information about the link status of each ONUs to the network management unit  160 . The network management unit  160  receives the link status discrimination information from the link status discriminator  180  and provides a visual or aural indication of the received information so that a network manager can confirm that information.  
         [0052]     As described above, the OLT identifies a report signal sending ONU on the basis of a report signal that is sent in a data transmission period allocated in the previous window period, determines whether there is data received from the report signal sending ONU in the data transmission period, discriminates a link status of that ONU in accordance with the determination, and performs an operation corresponding to a result of the discrimination. Therefore, the OLT can more accurately discriminate and manage link statuses of ONUs registered therein.  
         [0053]      FIG. 7  is a timing diagram illustrating a method for discriminating the link status of the ONUs shown in  FIG. 6  according to the invention. In this case, each of the ONUs changes the operation mode of a laser diode from a standby mode to a data transmission mode in a corresponding time slots allocated by the OLT. As a result, the ONUs selectively raise the transmission power level of the laser diode to a desired level before a data transmission period  320 . 1 ,  330 . 1 ,  340 . 1  and  350 . 1  in the corresponding time slot, which was allocated in the previous window period, and maintains the desired power level for the data transmission period  320 . 1 ,  330 . 1 ,  340 . 1  and  350 . 1 . The ONUs transmit signals to the OLT with the maintained power levels in the respective data transmission periods  320 . 1 ,  330 . 1 ,  340 . 1  and  350 . 1 .  
         [0054]     The ONUs each send a report signal to the OLT at the start time of an allocated data transmission period  320 . 1 ,  330 . 1 ,  340 . 1  and  350 . 1 . Thereafter, each ONUs transmits data during the allocated data transmission period  320 . 1 ,  330 . 1 ,  340 . 1  and  350 . 1 .  
         [0055]     For example, in the allocated data transmission period  320 . 1  of the ONU 1   320 , the OLT  100  receives a report signal sent from the ONU 1   320  at the start time of the data transmission period a allocated in the previous window period and a data signal transmitted from the ONU 1   320  for the data transmission period a. As a result, the OLT  100  determines the link status of the ONU 1   320  allocated the data transmission period  3201 . 1  to be normal, i.e., expected report and expected data received. In the data transmission period  330 . 1  allocated to ONU 2   330 , the OLT  100  receives a report signal sent from the ONU 2   330  at the start time of the data transmission period  330 . 1  allocated in the previous window period. At this time, if the OLT  100  receives a data signal transmitted from the ONU 2   330  for a limited period of the data transmission period  330 . 1 , it determines the remaining duration,  330 . 21 , of the data transmission period  330 . 1  to be a power loss period  330 . 12  of the ONU 2   330 . In this case, if there is no report signal received from the ONU 2   330  in the next window period, the OLT  100  determines the link status of the ONU 2   330  to be abnormal.  
         [0056]     In the data transmission period  340 . 1  allocated in the previous window period to ONU 3   340 , the OLT  100  receives no report signal from the ONU 3   340  at the start time of the data transmission period  340 . 1 . In this case, the OLT  100  recognizes that there is no power loss in the ONU 3   340  since a data signal is received for the data transmission period  340 . 1 , but there is a problem in the ONU 3   340 . If the received data signal is not a normal data frame, the OLT  100  determines that the received data signal has been transmitted under the influence of optical interference of the ONU 3   340  with another ONU. In this case, the OLT  100  searches for an ONU with the same phenomenon as that of the ONU 3   340 . If the OLT  100  detects ONUs with the same phenomenon, it can recognize the ONUs as those having caused interference in the data signal received in the allocated data transmission period  340 . 1 .  
         [0057]      FIG. 7  also illustrates that in a data transmission period  350 . 1 , allocated to the ONU 4   350 , there is neither a report signal nor a data signal received from the ONU 4   350 . In this case, the OLT  100  determines that a problem has occurred in the operation and/or the link with of the ONU 4   350 .  
         [0058]     As further shown in  FIG. 7 , the power loss periods  320 . 12 ,  330 . 12 ,  340 . 12  are present between the data transmission periods allocated to the ONUs. Since there are no report signal and data signal received in the data transmission period allocated to the ONU 4   350 , this data transmission period becomes a power loss period  350 . 12  of the ONU 4   350 .  
         [0059]      FIG. 8  illustrates an exemplary format of a report signal which is transmitted from an ONU. As shown in this drawing, the report signal has an LLID of a corresponding ONU, and an option (OP) code indicative of a signal type. The LLID is contained in a preamble. Information indicating that the sent signal is a report signal is contained in an option code. An option code corresponding to the report signal is “00-03”.  
         [0060]     Accordingly, the OLT  100  identifies a signal sending ONU on the basis of an LLID contained in a preamble of a signal sent therefrom and determines on the basis of an option code of the sent signal whether the sent signal is a report signal.  
         [0061]     According to the present invention, an OLT identifies a report signal sending ONU on the basis of a report signal that is sent in a data transmission period allocated in the previous window period, determines whether there is data received from the report signal sending ONU in the data transmission period, discriminates a link status of that ONU in accordance with the determination, and performs an operation corresponding to a result of the discrimination. Therefore, the OLT can more accurately discriminate and manage link status of ONUs registered therein.  
         [0062]     Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.