Patent Publication Number: US-2007116467-A1

Title: Passive optical network

Description:
CLAIM OF PRIORITY  
      This application claims priority under 35 U.S.C. §  119  to an application entitled “Passive Optical Network,” filed in the Korean Intellectual Property Office on Nov. 23, 2005 and assigned Serial No. 2005-112350, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a passive optical network (PON), and in particular, to a point-to-multi-point Ethernet PON (EPON) for monitoring normality/abnormality thereof.  
      2. Description of the Related Art  
      An optical time domain reflectometer (OTDR) is used for monitoring normality/abnormality of an optical fiber or optical cable. It calculates the return time and the intensity of light by inputting pulse type light into a target optical fiber, and it detects light reflected and returned due to diffusion at a specific position of the optical fiber where abnormality occurs. Further, the abnormality type can be determined based on the calculated return time and intensity of light. The OTDR can monitor the entire configuration by being connected to one end of an optical fiber or optical cable, thereby reducing the time and cost required to monitor the network. In detail, the OTDR can provide information such as a loss per unit length, evaluation of a splice and connector, the calculation of an abnormality occurrence position, and so on.  
      Monitoring an optical communication subscriber network(EPON) using the OTDR has been suggested, e.g., an in-service or active fiber testing method in which the OTDR is inserted into an existing optical subscriber network. A typical network management system controls a complex network to maximize the efficiency and productivity of the network and performing a realtime network monitoring and control to optimize the performance of the network. However, if the OTDR is applied to an EPON using a point-to-multi-point scheme, instead of a conventional point-to-point scheme, the cost and time loss increases. That is, since a plurality of optical network units (ONUs) are linked to a single optical line terminal (OLT) in a conventional optical subscriber network, the conventional optical subscriber network must be monitored in realtime by incorporating an expensive OTDR thereto. Furthermore, a separate manager for managing the OTDR is required.  
      Accordingly, there is a need for a network, which overcomes the problems associated with the prior art.  
     SUMMARY OF THE INVENTION  
      The present invention relates to an Ethernet passive optical network (EPON) including devices for performing a realtime monitoring of the EPON at low cost.  
      According to one aspect of the present invention, there is provided a point-to-multi-point passive optical network (PON) comprising: an optical line terminal (OLT) including an optical transceiver for generating a downstream optical signal and a monitoring signal and for detecting an upstream optical signal; a plurality of optical network units (ONUs) for detecting the downstream optical signal, reflecting the monitoring signal to the OLT, and transmitting a data-modulated upstream optical signal in a designated time slot; and an optical fiber for connecting the ONUs and the OLT. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
       FIG. 1  is a configuration of a point-to-multi-point PON according to an embodiment of the present invention;  
       FIG. 2  is a block diagram of an optical transceiver of  FIG. 1 ; and  
       FIG. 3  is a block diagram of each ONU of  FIG. 1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiments of the present invention will be described herein below with reference to the accompanying drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail.  
       FIG. 1  is a configuration of a point-to-multi-point PON  100  according to an embodiment of the present invention. As shown the point-to-multi-point PON  100  includes an optical line terminal (OLT)  110  including an optical transceiver (OLT PMD)  130  for generating a downstream optical signal and a monitoring signal (1490 nm) and for detecting an upstream optical signal (1310 nm), a plurality of optical network units (ONUs)  160 - 1  to  160 -n for detecting the downstream optical signal, reflecting the monitoring signal to the OLT  110 , and transmitting a data-modulated upstream optical signal in a designated time slot, an optical splitter  150  located between the OLT  110  and the ONUs  160 - 1  to  160 -n, and an optical fiber  101  for coupling the OLT  110  and the ONUs  160 - 1  to  160 -n.  
      The OLT  110  includes an optical detector (OTDR receiver)  120  for detecting the monitoring signal reflected by each of the ONUs  160 - 1  to  160 -n, a tap coupler  112 , which is located between the optical transceiver  130  and the ONUs  160 - 1  to  160 -n, outputs the monitoring signal reflected by each of the ONUs  160 - 1  to  160 -n to the optical detector  120 , and outputs the upstream optical signal to the optical transceiver  130 . The OLT  110  further includes a media access controller (MAC)  111  for outputting the downstream optical signal to the ONUs  160 - 1  to  160 -n and monitoring normality/abnormality of the PON  100  using the monitoring signal detected by the optical detector  120 .  
      For the upstream optical signal and the downstream optical signal, different wavelength bands can be used. For example, when 1490 nm is used for a wavelength band of the downstream optical signal, 1310 nm can be used for a wavelength band of the upstream optical signal. The downstream optical signal is transmitted to each of the ONUs  160 - 1  to  160 -n, and the OLT  110  can identify each of the ONUs  160 - 1  to  160 -n since each upstream optical signal is transmitted in the corresponding time slot. That is, in an optical subscriber network according to the embodiment, a time division multiplexing access (TDMA) scheme in which a time slot is designated can be applied to each of ONUs.  
      In detail, in the embodiment, a master/slave TDMA scheme of an asynchronous transfer mode PON (ATM-PON) can be applied, the scheme in which the OLT  110  plays a role of designating a time slot to each of the ONUs  160 - 1  to  160 -n and each of the ONUs  160 - 1  to  160 -n plays a role of a slave for requesting the OLT  110  for a needed time slot. Here, a multi point control protocol (MPCP) can be used. The MPCP can use five new MAC control frames (MPCPDUs: MPCP data units), ‘GRANT’ and ‘REPORT’ of which are used the most.  
      The MAC  111  determines normality/abnormality between the MAC  111  and each of the ONUs  160 - 1  to  160 -n from the amplitude of the monitoring signal detected by the optical detector  120  and the time taken until the reflected light returns, then calculates an abnormality occurrence position when the abnormality occurs. The MAC  111 , as a master, collects time slots which the ONUs  160 - 1  to  160 -n request, designates an appropriate time slot to each of the ONUs  160 - 1  to  160 -n, and, if necessary, can control the optical transceiver  130  to generate the monitoring signal.  
      The MAC  111  designates a time slot indicating an available upstream transmission start time and a transmission duration to each of the ONUs  160 - 1  to  160 -n using ‘GRANT’ and provides to each of the ONUs  160 - 1  to  160 -n a chance for transmitting ‘REPORT’ by periodically transmitting ‘GRANT’ to each of the ONUs  160 - 1  to  160 -n.  
      ‘GRANT’ transmitted by the OLT  110  includes ‘Discovery GRANT’ for providing a chance for an unregistered ONU to be registered, ‘Forced Report GRANT’ for informing an ONU in an idle state when there is no data in an upstream buffer, a data state, and ‘Data GRANT’ for general data transmission. Note that different types of ‘GRANT’ can be identified using a flag field.  
       FIG. 2  is a block diagram of the optical transceiver  130  shown in  FIG. 1 . As shown, the optical transceiver  130  includes a downstream transmitter  137  for generating the downstream optical signal, an upstream receiver  138  for detecting the upstream optical signal, and a wavelength selection coupler  131 . The optical transceiver  130  is a single device and is connected to the optical fiber  101  via an optical connector (not shown) of the OLT  110 .  
      The wavelength selection coupler  131  is coupled to the tap coupler  112 , outputs the upstream optical signal input through the tap coupler  112  to an optical receiver  133 , and outputs the downstream optical signal generated by a light source  132  to the tap coupler  112 . If a coupling ratio of the tap coupler  112  is 8:2, a 1 dB loss occurs in the coupling of the downstream optical signal, and a 7 dB loss occurs in the coupling of the monitoring signal having a pulse pattern to the optical detector  120 .  
      The downstream transmitter  137  includes the light source  132  for generating the downstream optical signal, a downstream transmitter circuit  134  for driving the light source  132 , and an optical isolator  136  for preventing an unnecessary optical signal from being input to the light source  132 . The upstream receiver  138  includes the optical receiver  133  and an upstream receiver circuit  135  for amplifying a signal detected by the optical receiver  133 .  
      The optical isolator  136  prevents a deterioration of the light source  132  by preventing the monitoring signal generated by the light source  132  from being input to the light source  132  again.  
      For the light source  132 , a semiconductor laser or a semiconductor optical amplifier may be used, and for the optical receiver  133 , a photo diode may be used. The MAC  111  controls the downstream transmitter  137  to generate the downstream optical signal and the monitoring signal having a pulse pattern. In addition, if necessary, the MAC  111  controls the downstream transmitter  137  to generate a downstream optical signal according to a time division scheme.  
      The optical detector  120  includes a filter  124  for passing only a predetermined wavelength of the monitoring signal, a first amplifier  123  for pre-amplifying the monitoring signal input from the filter  124 , a photo diode  122  for detecting an electrical signal from the amplified monitoring signal, and a second amplifier  121  for amplifying the electrical signal detected by the photo diode  122  and transmitting the amplified electrical signal to the MAC  111 . The optical detector  120  detects the amplitude of the monitoring signal and outputs the detected amplitude of the monitoring signal and a detection time to the MAC  111 .  
      For the first amplifier  123 , a semiconductor optical amplifier may be used, and for the photo diode  122 , a pin or avalanche photo diode may be used.  
       FIG. 3  is a block diagram of each ONU  160  of  FIG. 1 . As shown, each ONU  160  includes an upstream transmitter  167 , a downstream receiver  168 , a wavelength selection coupler  161  for outputting the upstream optical signal to the OLT  110  and outputting the downstream optical signal to the downstream receiver  168 , and a separate MAC  164  for confirming a time slot designated by ‘GRANT’ input from the OLT  110  and generating ‘REPORT’ including a clock.  
      The upstream transmitter  167  includes a light source  162  for generating a data-modulated upstream optical signal in a designated time slot and an upstream transmitter circuit  165  for driving the light source  162 . The downstream receiver  168  includes a downstream optical receiver  163  for detecting the downstream optical signal and a downstream receiver circuit  166  for amplifying a signal detected by the downstream optical receiver  163 .  
      Each of the ONUs  160 - 1  to  160 -n transmits ‘REPORT’ for informing the OLT  110  about the amount of data to be transmitted using a time slot designated by ‘GRANT.’ ONUs unregistered in the OLT  110  among the ONUs  160 - 1  to  160 -n can use MPCPDUs, such as ‘REGISTER_REQ’ for performing a registration provided by ‘GRANT’ of the OLT  110  and ‘REGISTER_ACK’ for terminating the registration process. If a plurality of unregistered ONUs simultaneously transmit ‘REGISTER_REQ’ for registration to the OLT  110 , the ‘REGISTER_REQs’ transmitted by the unregistered ONUs may be collided each other. Thus, each of the unregistered ONUs transmits ‘REGISTER_REQ’ at a random time to minimize the collision.  
      The OLT  110  recognizes the unregistered ONUs from the ‘REGISTER_REQs’ received from the unregistered ONUs and simultaneously transmits ‘REGISTER’ and ‘GRANT’ for registration to the unregistered ONUs, and each of the unregistered ONUs, which has received ‘REGISTER’ and ‘GRANT’ terminates the registration process (synchronization) by transmitting ‘REGISTER_ACK’ to the OLT  110 .  
      All the ONUs  160 - 1  to  160 -n and the OLT  110  must operate based on a reference clock so that upstream optical signals in the respective time slots according to ‘GRANT’ can be normally transmitted without collision. The point-to-multi-point PON  100  defines a reference clock of the ONUs  160 - 1  to  160 -n in the MAC  111  of the OLT  110  and performs synchronization by transmitting the reference clock together when the OLT  110  transmits ‘GRANT’ to the ONUs  160 - 1  to  160 -n. Thus, the ONUs  160 - 1  to  160 -n are synchronized by the reference clock while performing the registration process with the OLT  110  and transmits clock information to the OLT  110  through ‘REPORT.’  
      The OLT  110  and each of the ONUs  160 - 1  to  160 -n are separated from each other by a distance according to a set position of each of the ONUs  160 - 1  to  160 -n, and accordingly, an information difference according to a transmission delay time of the reference clock occurs. To compensate for the transmission delay time, the OLT  110  can prevent the collision between upstream optical signals by always measuring a distance from each of the ONUs  160 - 1  to  160 -n and allocating a time slot compensated by the distance between the OLT  110  and each of the ONUs  160 - 1  to  160 -n to each of the ONUs  160 - 1  to  160 -n. A round trip time (RTT) between the OLT  110  and each of the ONUs  160 - 1  to  160 -n can be calculated by a difference between a clock included in ‘REPORT’ received from each of the ONUs  160 - 1  to  160 -n and the reference clock designated to the OLT  110 .  
      The optical detector  1 . 20  does not operate in the PON  100  in a normal operation state but operates when the PON  100  is changed to an OTDR mode by a control of the MAC 111. Since the OLT  110  and each of the ONUs  160 - 1  to  160 -n are located separately from each other by a set distance, each of the ONUs  160 - 1  to  160 -n always measures and compensates for a distance with the OLT  110 . Thus, an operational state of each of the ONUs  160 - 1  to  160 -n can be electrically observed. In addition, the MAC  164  of each of the ONUs  160 - 1  to  160 -n may monitor an optical transmission link state of the PON  100  in realtime by being periodically changed to the OTDR mode. That is, if an OTDR signal of any one of the ONUs  160 - 1  to  160 -n is not received for a long time, the OLT  110  determines that one of three abnormal states described below occurs and confirms normality/abnormality with the ONU whose OTDR signal has not been received as well as an abnormality occurrence position, and an abnormality type.  
      The three abnormal states are: firstly, abnormality on a line between each of the ONUs  160 - 1  to  160 -n and the OLT  110 ; secondly, abnormality of each of the ONUs  160 - 1  to  160 -n and the OLT  110 ; and thirdly, an operation stop state due to non-use of each of the ONUs  160 - 1  to  160 -n for a long time. The abnormality due to non-use of each of the ONUs  160 - 1  to  160 -n for a long time can be determined according to whether each of the ONUs  160 - 1  to  160 -n responses and is not determined as an actual abnormal state.  
      As an example, a case where abnormality occurs between a specific ONU  160  and the OLT  110  in the PON  100  will now be described. Since the specific ONU  160  and the OLT  110  continuously manages the PON  100  using the RTT, the OLT  110  detects normality/abnormality with respect to the specific ONU  160 . If abnormality with the specific ONU  160  is detected, the OLT is changed to the OTDR mode by the MAAC  111 , and the optical transceiver  130  generates a monitoring signal. The monitoring signal is transmitted to the ONUs  160 - 1  to  160 -n, reflected at an abnormality occurrence position between the OLT  110  and the specific ONU  160 , and returned to the OLT  110 .  
      The optical detector  120  of the OLT  110  detects the reflected and returned monitoring signal and informs the MAC  111  of the detection result. Thereafter, the MAC  111  can find the abnormality occurrence position by calculating the RTT of the monitoring signal.  
      As described above, according to embodiments of the present invention, by generating a monitoring signal used by an OTDR using an optical transceiver for generating an optical signal in an EPON, management and monitoring of the EPON is easy, and a configuration of the EPON is simplified, thereby being effective in the terms of cost, time, and human operation.  
      While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.