Patent Publication Number: US-2022224595-A1

Title: Optical communication device and control method

Description:
TECHNICAL FIELD 
     The present invention relates to an optical communication device, a control method and a control program. 
     BACKGROUND ART 
     There has been known a communication system including a Passive Optical Network (PON) system that is an optical communication system. The PON system includes an optical communication device (referred to also as a “master station device”) installed in a station of a telecommunications carrier and a plurality of optical communication devices (referred to also as “slave station devices”) installed on the subscribers&#39; side. The master station device is referred to as an Optical Line Termination (OLT). The slave station device is referred to as an Optical Network Unit (ONU). 
     In communication systems, IP over Ethernet (IPoE) (registered trademark) is used as a technology for connecting to a host network such as the Internet. Further, a communication system includes a Dynamic Host Configuration Protocol (DHCP) server. The DHCP server assigns an Internet Protocol (IP) address to a client device connected to the ONU. The client device is capable of connecting to the host network by using the IP address. 
     Furthermore, in communication systems, there are cases where a configuration in the system has become redundant in order to increase the reliability of the system. For example, there has been proposed a technology regarding a redundant configuration (see Patent Reference 1 and Non-patent Reference 1). 
     PRIOR ART REFERENCE 
     Patent Reference 
     
         
         Patent Reference 1: Japanese Patent Application Publication No. 2017-175176 
       
    
     Non-Patent Reference 
     
         
         Non-patent Reference 1: ITU-T Recommendation G.983.1 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     Incidentally, the OLT has stored information regarding the client device connected to each ONU. The OLT monitors unauthorized use of the IP address assigned to the client device by using the information. 
     Further, when an ONU has a redundant configuration, the OLT stores information regarding the client device connected to the ONU as an operational system. Incidentally, in the convention, when an ONU as a standby system is switched to the operational system, the OLT deletes the information regarding the client device connected to the operational-system ONU. Then, the OLT generates information regarding the client device connected to the standby-system ONU. In this case, the communication system does not connect the client device to the host network until the information is generated. That is because the OLT cannot carry out the monitoring by using the information. 
     As above, the client device cannot connect to the host network until the information is generated. Thus, there is a problem in that the client device cannot connect to the host network for a long time. 
     An object of the present invention is to let the client device connect to a network in a short time. 
     Means for Solving the Problem 
     A communication system includes a first slave station device as a slave station device as an operational system, a second slave station device as a slave station device as a standby system, a master station device that connects to a network, and a client device that connects to the first slave station device and the second slave station device and connects to the network via the first slave station device and the master station device by using a first address. There is provided an optical communication device according to an aspect of the present invention as the master station device in the communication system. The optical communication device includes a storage unit that stores management information indicating that the first address has been assigned to the client device that connects to the first slave station device and the second slave station device, a detection unit that detects occurrence of a failure to the first slave station device, a communication control unit that connects to the second slave station device when the occurrence of the failure to the first slave station device is detected and controls communication performed by the client device so that the client device connects to the network via the second slave station device and the master station device, and a monitoring unit that monitors unauthorized use of the first address based on the management information both when the client device connects to the network via the first slave station device and the master station device and when the client device connects to the network via the second slave station device and the master station device. 
     Effect of the Invention 
     According to the present invention, it is possible to let the client device connect to the network in a short time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a communication system in a first embodiment. 
         FIG. 2  is a sequence diagram showing an IP address assigning process in the first embodiment. 
         FIG. 3  is a diagram showing a configuration of hardware included in an OLT in the first embodiment. 
         FIG. 4  is a functional block diagram showing a configuration of the OLT in the first embodiment. 
         FIG. 5  is a diagram showing a concrete example of a management table in the first embodiment. 
         FIG. 6  is a functional block diagram showing a configuration of ONUs in the first embodiment. 
         FIG. 7  is a sequence diagram showing an example of a process executed in the communication system in the first embodiment. 
         FIG. 8  is a functional block diagram showing a configuration of ONUs in a second embodiment. 
         FIG. 9  is a functional block diagram showing a configuration of ONUs in a third embodiment. 
         FIG. 10  is a diagram showing an example of an ONU authentication table in the third embodiment. 
         FIG. 11  is a flowchart showing a process regarding ONU authentication and UNI blockage when an ONU redundant configuration is not employed in the third embodiment. 
         FIG. 12  is a flowchart showing a process regarding the ONU authentication and the UNI blockage when a new ONU has been added when the ONU redundant configuration is employed in the third embodiment. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments will be described below with reference to the drawings. The following embodiments are just examples and a variety of modifications are possible within the scope of the present invention. 
     First Embodiment 
       FIG. 1  is a diagram showing a communication system in a first embodiment. The communication system includes an OLT  100 , an ONU  200 , an ONU  300  and a client device  500 . Further, the communication system may include a switch  400 , a client device  500 , a DHCP server  600 , ONUS  700  and  701 , and client devices  800  and  801 . 
     A system including the OLT  100 , the ONU  200 , the ONU  300 , the ONU  700  and the ONU  701  is referred to also as a PON system. The OLT  100  connects to the ONU  200 , the ONU  300  and the ONUS  700  and  701  via a splitter  10 . 
     In the PON system, control based on time division multiplexing of upstream signals is executed. Incidentally, the upstream signal is an optical signal transmitted by an ONU to the OLT  100 . Thanks to the control based on the time division multiplexing, collision of optical signals can be prevented. 
     The OLT  100  is referred to also as a master station device. Further, the OLT  100  is referred to also as an optical communication device. The OLT  100  executes a control method. The OLT  100  connects to the Internet  20 . The Internet  20  is referred to also as a network. 
     Each of the OLT  100 , the ONU  200 , the ONU  300 , the ONU  700  and the ONU  701  measures the number of frames transmitted and received. By using the number of frames, a system administrator can analyze the occurrence status of an error frame, the presence/absence of a network attack, identification of an occupying user, and so forth. 
     The client devices  500 ,  501 ,  800  and  801  are referred to also as DHCP client devices. Each of the client devices  500 ,  501 ,  800  and  801  connects to the Internet  20  by using an IP address assigned thereto. For example, the client device  800  connects to the Internet  20  via the ONU  700 . The client device  801  connects to the Internet  20  via the ONU  701 . 
     Here, each ONU is referred to also as a slave station device. Further, the ONU  200  is assumed to be an ONU as an operational system. The ONU  200  is referred to also as a first slave station device. The ONU  300  is assumed to be an ONU as a standby system. The ONU  300  is referred to also as a second slave station device. In the first embodiment, the number of standby-system ONUS is assumed to be one. However, the number of standby-system ONUS can also be two or more. 
     The client devices  500  and  501  connect to the ONUS  200  and  300 . Specifically, the client devices  500  and  501  connect to the ONUS  200  and  300  via the switch  400 . 
     When the ONU  200  is an operational-system ONU, the client devices  500  and  501  connect to the Internet  20  via the ONU  200  and the OLT  100 . For example, the client device  500  connects to the Internet  20  via the ONU  200  and the OLT  100  by using the IP address assigned to the client device  500 . Incidentally, the IP address is referred to also as a first address. 
     When a failure occurs to the ONU  200 , the ONU  300  switches to the operational-system ONU. Then, the client devices  500  and  501  connect to the Internet  20  via the ONU  300  and the OLT  100 . 
     The switch  400  relays the communication between the operational-system ONU and the client devices  500  and  501 . In  FIG. 1 , the number of client devices connected to the switch  400  is two. However, the number of client devices connected to the switch  400  can also be three or more. 
     The DHCP server  600  manages the IP addresses used for the connection to the Internet  20 . The DHCP server  600  executes the assigning of IP addresses and releasing of IP addresses based on the DHCP. 
     Next, the assigning of the IP address will be described below. 
       FIG. 2  is a sequence diagram showing an IP address assigning process in the first embodiment.  FIG. 2  illustrates a case where the DHCP server  600  assigns an IP address to the client device  500 . In  FIG. 2 , illustration of the OLT  100 , the ONU  200  and the switch  400  is left out. 
     (Step ST 101 ) The client device  500  transmits a DHCP discover message to the DHCP server  600 . The DHCP discover message is a message requesting the assignment of the IP address. 
     (Step ST 102 ) The DHCP server  600  transmits a DHCP offer message to the client device  500 . The DHCP offer message is a message including an IP address that the client device  500  can use. 
     (Step ST 103 ) When the IP address included in the DHCP offer message has no problem, the client device  500  transmits a DHCP request message to the DHCP server  600 . The DHCP request message is a message officially requesting the DHCP server  600  to assign the IP address. 
     (Step ST 104 ) The DHCP server  600  transmits a DHCP ack message to the client device  500 . The DHCP ack message is a message for making the client device  500  set the IP address. 
     The client device  500  makes the setting of the IP address according to information indicated by the DHCP ack message. Accordingly, the client device  500  can connect to the Internet  20  by using the IP address. 
     Next, the main hardware configuration of the OLT  100  will be described below. 
       FIG. 3  is a diagram showing the configuration of hardware included in the OLT in the first embodiment. The OLT  100  includes a processor  101 , a volatile storage device  102  and a nonvolatile storage device  103 . 
     The processor  101  controls the whole of the OLT  100 . For example, the processor  101  is a Central Processing Unit (CPU), a Field Programmable Gate Array (FPGA) or the like. The processor  101  can also be a multiprocessor. The OLT  100  may also be implemented by a processing circuitry or implemented by software, firmware or a combination of software and firmware. Incidentally, the processing circuitry may be either a single circuit or a combined circuit. 
     The volatile storage device  102  is main storage of the OLT  100 . The volatile storage device  102  is a Random Access Memory (RAM), for example. The nonvolatile storage device  103  is auxiliary storage of the OLT  100 . The nonvolatile storage device  103  is a flash memory, for example. 
     Each of the ONUs  200 ,  300 ,  700  and  701 , the client devices  500 ,  501 ,  800  and  801  and the DHCP server  600  includes a processor, a volatile storage device and a nonvolatile storage device similarly to the OLT  100 . 
     Next, functional blocks included in the OLT  100  and the ONUs  200  and  300  will be described below. 
       FIG. 4  is a functional block diagram showing the configuration of the OLT in the first embodiment. The OLT  100  includes a storage unit  110 , a communication control unit  120 , a message processing unit  130 , a redundancy management unit  140 , a detection unit  150  and a monitoring unit  160 . 
     The storage unit  110  is implemented as a storage area secured in the volatile storage device  102  or the nonvolatile storage device  103 . 
     Part or all of the communication control unit  120 , the message processing unit  130 , the redundancy management unit  140 , the detection unit  150  and the monitoring unit  160  may be implemented by the processor  101 . Part or all of the communication control unit  120 , the message processing unit  130 , the redundancy management unit  140 , the detection unit  150  and the monitoring unit  160  may be implemented as modules of a program executed by the processor  101 . For example, the program executed by the processor  101  is referred to also as a control program. The control program has been recorded in a record medium, for example. 
     The storage unit  110  stores a management table. The management table will be described here. 
       FIG. 5  is a diagram showing a concrete example of the management table in the first embodiment. The management table  111  is referred to also as management information. The management table  111  includes items of No., ONU IDENTIFIER (ID), OPERATION/STANDBY, OPERATION PARAMETER and OPERATION INFORMATION. 
     The item of No. indicates an identifier of each ONU. For example, No.  1  represents the ONU  200 . No.  2  represents the ONU  300 . Incidentally, illustration of ONUS corresponding to Nos.  3  and  4  is left out in  FIG. 1 . 
     The item of ONU ID indicates an identifier representing a combination of an operational-system ONU and a standby-system ONU. For example, an identifier representing a combination of the ONU  200  and the ONU  300  is N1. 
     As above, in the management table  111 , the ONU  200  and the ONU  300  are managed by using one ONU ID. 
     The item of OPERATION/STANDBY indicates whether each ONU is in the state of an operational system or a standby system. For example, the management table  111  indicates that the ONU  200  is an operational system. Further, for example, the management table  111  indicates that the ONU  300  is a standby system. 
     The item of OPERATION PARAMETER indicates parameters previously set to each ONU. 
     The item of OPERATION INFORMATION includes items of STATISTICAL INFORMATION and DEVICE INFORMATION. The item of STATISTICAL INFORMATION indicates the number of frames transmitted and received by each ONU. Here, for example, the redundancy management unit  140  acquires the number of frames transmitted and received by the ONU  200  from the ONU  200  and registers the frame counts in the management table  111 . 
     The item of DEVICE INFORMATION indicates information regarding a client device connected to each ONU. Specifically, the item of DEVICE INFORMATION includes items of AUTHENTICATION INFORMATION and IP ADDRESS. 
     The item of AUTHENTICATION INFORMATION indicates the Media Access Control (MAC) address of the client device connected to each ONU, the date and time of authentication of the client device, and a user ID of the user using the client device. 
     The item of IP ADDRESS indicates the IP address assigned to the client device connected to each ONU. 
     For example, the MAC address a is assumed to be the MAC address of the client device  500 . The IP address A is assumed to be the IP address assigned to the client device  500 . According to the relationship between the MAC address and the IP address, the device information indicates that the IP address A has been assigned to the client device  500 . 
     As above, the management table  111  indicates, for example, that the IP address A has been assigned to the client device  500  connecting to the ONU  200  and the ONU  300 . Further, for example, the management table  111  may also be represented as indicating a correspondence relationship between the identifier representing the combination of the ONU  200  and the ONU  300  and the information indicating that the IP address A has been assigned to the client device  500 . 
     The communication control unit  120  has an Optical/Electrical (O/E) conversion function. Further, the communication control unit  120  executes transmission and reception of optical signals and electric signals. 
     The message processing unit  130  executes snooping. For example, the message processing unit  130  acquires the DHCP request message transmitted from the client device  500 . The message processing unit  130  acquires the MAC address of the client device  500 , information regarding the operational-system ONU to which the client device  500  is connected, and so forth from the DHCP request message. The message processing unit  130  stores the acquired information in the storage unit  110 . 
     The redundancy management unit  140  manages the operational-system ONU  200  and the standby-system ONU  300 . Further, the redundancy management unit  140  updates the management table  111  depending on a parameter that is set to the operational-system ONU and the state of the operational-system ONU. 
     The detection unit  150  detects occurrence of a failure to the operational-system ONU  200 . Here, when the occurrence of a failure to the ONU  200  is detected, the communication control unit  120  connects to the ONU  300 . The communication control unit  120  controls the communication performed by the client device so that the client device connects to the Internet  20  via the ONU  300  and the OLT  100 . Accordingly, the client device can connect to the Internet  20 . 
     For example, the monitoring unit  160  monitors the unauthorized use of the IP address assigned to the client device  500  based on the management table  111  both when the client device  500  connects to the Internet  20  via the ONU  200  and the OLT  100  and when the client device  500  connects to the Internet  20  via the ONU  300  and the OLT  100 . For example, it is assumed here that the client device  800  transmits a frame to the Internet  20 . The frame includes the MAC address of the client device  800  and the IP address assigned to the client device  500 . The monitoring unit  160  acquires the frame via the communication control unit  120 . The monitoring unit  160  refers to the management table  111  and compares the combination of the MAC address of the client device  500  and the IP address assigned to the client device  500  with the combination of the MAC address of the client device  800  and the IP address assigned to the client device  500 . By the comparison, the monitoring unit  160  detects the unauthorized use of the IP address assigned to the client device  500 . Incidentally, the unauthorized use is referred to also as spoofing. 
       FIG. 6  is a functional block diagram showing the configuration of ONUS in the first embodiment. The ONU  200  includes a PON control unit  210  and a User Network Interface (UNI) unit  220 . 
     Part or all of the PON control unit  210  and the UNI unit  220  may be implemented by a processor included in the ONU  200 . Part or all of the PON control unit  210  and the UNI unit  220  may be implemented as modules of a program executed by the processor included in the ONU  200 . 
     The ONU  300  includes a PON control unit  310  and a UNI unit  320 . Part or all of the PON control unit  310  and the UNI unit  320  may be implemented by a processor included in the ONU  300 . Part or all of the PON control unit  310  and the UNI unit  320  may be implemented as modules of a program executed by the processor included in the ONU  300 . 
     Here, the function of the PON control unit  210  and the function of the PON control unit  310  are the same as each other. Further, the function of the UNI unit  220  and the function of the UNI unit  320  are the same as each other. Therefore, the description with reference to  FIG. 6  will be given of the PON control unit  210  and the UNI unit  220 . Then, the description is omitted for the PON control unit  310  and the UNI unit  320 . 
     The PON control unit  210  has the  0 /E conversion function. Further, the PON control unit  210  executes transmission and reception of optical signals and electric signals. 
     The UNI unit  220  communicates with the client devices  500  and  501  via the switch  400 . 
     Next, a process executed in the communication system will be described below. 
       FIG. 7  is a sequence diagram showing an example of a process executed in the communication system in the first embodiment. Incidentally, the client device  501 , the ONUS  700  and  701  and the client devices  800  and  801  are left out in  FIG. 7 . 
     Here, the ONU  300  has been set in a state of being incapable of transmitting upstream signals in order to hold down the power consumption while the ONU  300  stays on standby as the standby system. The state of being incapable of transmitting upstream signals is referred to as an optical shutdown state. Incidentally, the ONU  300  in the optical shutdown state is capable of receiving downstream signals. The downstream signal is an optical signal transmitted by the OLT  100  to an ONU. Further, in the ONU  300 , the UNI unit  320  has been blocked off so as not to receive a frame from the client device  500 . 
     (Step ST 111 ) The client device  500  transmits the DHCP discover message to the DHCP server  600 . 
     (Step ST 112 ) The DHCP server  600  transmits the DHCP ack message to the client device  500 . 
     Accordingly, the client device  500  can connect to the Internet  20  by using the IP address assigned by the DHCP server  600 . 
     Incidentally, illustration of the transmission and reception of the DHCP offer message and the DHCP request message is left out between the step ST 111  and the step ST 112 . 
     (Step ST 113 ) A failure occurs to the ONU  200 . For example, the failure is an abnormality occurring in the ONU  200  or a malfunction of the ONU  200 . 
     (Step ST 114 ) The detection unit  150  of the OLT  100  detects the occurrence of a failure to the ONU  200 . For example, the detection unit  150  of the OLT  100  detects the occurrence of a failure to the ONU  200  when the power of an upstream signal transmitted by the ONU  200  is lower than a threshold value. 
     (Step ST 115 ) The communication control unit  120  of the OLT  100  commands the ONU  200  to shift to the optical shutdown state. Further, the communication control unit  120  of the OLT  100  commands the ONU  200  to block off the UNI unit  220 . The communication control unit  120  of the OLT  100  disconnects the logical link between the OLT  100  and the ONU  200 . This disables the communication between the OLT  100  and the ONU  200 . Further, since the logical link has been disconnected, the communication control unit  120  of the OLT  100  cancels the authentication of the ONU  200 . 
     (Step ST 116 ) The communication control unit  120  of the OLT  100  commands the ONU  300  to cancel its optical shutdown state and to cancel the state of blocking off the UNI unit  320 . 
     The communication control unit  120  of the OLT  100  establishes the logical link between the OLT  100  and the ONU  300 . Since the logical link has been established, the communication control unit  120  of the OLT  100  authenticates the ONU  300 . Accordingly, the ONU  300  switches to the operational system. 
     Incidentally, information corresponding to the ONU  200  registered in the management table  111  will be used as information corresponding to the ONU  300 . For example, the operation information registered when the ONU  200  was the operational system will be handled as operation information regarding the ONU  300 . In other words, the operation information regarding the ONU  200  is handed over as the operation information regarding the ONU  300 . 
     It is also possible for the OLT  100  to acquire the MAC address from the client device  500  again. 
     (Step ST 117 ) The communication control unit  120  of the OLT  100  controls the communication performed by the client device  500  so that the client device  500  connects to the Internet  20  via the ONU  300  and the OLT  100 . Specifically, the communication control unit  120  of the OLT  100  establishes the logical link between the OLT  100  and the ONU  300 . After the logical link with the ONU  300  is established, the communication control unit  120  of the OLT  100  authenticates the client device  500 . 
     Accordingly, the client device  500  can connect to the Internet  20 . 
     (Step ST 118 ) The ONU  200  is replaced with a new ONU. The new ONU is referred to as the ONU  200 . 
     (Step ST 119 ) The communication control unit  120  of the OLT  100  commands the ONU  300  to shift to the optical shutdown state. Further, the communication control unit  120  of the OLT  100  commands the ONU  300  to block off the UNI unit  320 . The communication control unit  120  of the OLT  100  disconnects the logical link between the OLT  100  and the ONU  300 . This disables the communication between the OLT  100  and the ONU  300 . Further, since the logical link has been disconnected, the communication control unit  120  of the OLT  100  cancels the authentication of the ONU  300 . 
     (Step ST 120 ) The communication control unit  120  of the OLT  100  commands the ONU  200  to cancel its optical shutdown state and to cancel the state of blocking off the UNI unit  220 . 
     The communication control unit  120  of the OLT  100  establishes the logical link between the OLT  100  and the ONU  200 . Since the logical link has been established, the communication control unit  120  of the OLT  100  authenticates the ONU  200 . Accordingly, the ONU  200  switches to the operational system. 
     (Step ST 121 ) The communication control unit  120  of the OLT  100  authenticates the client device  500 . 
     Incidentally, in the conventional technology, when an ONU as a standby system is switched to the operational system, the OLT deletes the information regarding the client device connected to the operational-system ONU. Then, the OLT generates information regarding the client device connected to the standby-system ONU. In this conventional method, the client device cannot connect to the Internet until the information is generated. Thus, there is a problem in that the client device cannot connect to the Internet for a long time. 
     According to the first embodiment, the OLT  100  uses the information corresponding to the ONU  200  registered in the management table  111  as information corresponding to the ONU  300 . Thus, when the ONU  300  as the standby system is switched to the operational system, the OLT  100  does not delete information regarding the client devices  500  and  501  connected to the ONU  200  that has been registered in the management table  111 . Namely, when the ONU  300  as the standby system is switched to the operational system, the OLT  100  does not delete the device information in the management table  111 . Further, the OLT  100  does not register information regarding the client devices  500  and  501  newly connected to the ONU  300  in the management table  111 . Therefore, the OLT  100  is capable of letting the client devices  500  and  501  connect to the Internet  20  in a short time. 
     Second Embodiment 
     Next, a second embodiment will be described below. The following description will be given mainly of features different from those in the first embodiment, in which description will be omitted for features in common with the first embodiment.  FIGS. 1 to 7  will be referred to in the description of the second embodiment. 
     In the first embodiment, a case where the ONU includes the UNI unit was described. In the second embodiment, a case where the ONU is of the Small Foam-factor Pluggable (SFP) type will be described. 
       FIG. 8  is a functional block diagram showing the configuration of ONUS in the second embodiment. First, the communication system includes the OLT  100 , a switch  410  and the client device  500 . The communication system may include the client device  501 , the DHCP server  600 , the ONUS  700  and  701  and the client devices  800  and  801 . 
     The switch  410  includes an ONU  200   a  of the SFP type and an ONU  300   a  of the SFP type. The ONU  200   a  is implemented as a module of an optical transceiver. The optical transceiver is referred to also as a first optical transceiver. The ONU  300   a  is implemented as a module of an optical transceiver. The optical transceiver is referred to also as a second optical transceiver. Specifically, the ONU  200   a  and the ONU  300   a  are inserted in a cage of the switch  410  and used. Incidentally, the ONU  200   a  is the operational system. The ONU  300   a  is the standby system. 
     Each component in  FIG. 8  that is the same as a component shown in  FIG. 6  is assigned the same reference character as in  FIG. 6 . The ONU  200   a  differs from the ONU  200  in including no UNI unit. The ONU  300   a  differs from the ONU  300  in including no UNI unit. 
     The ONU  200   a  and the ONU  300   a  cannot be blocked off since they include no UNI unit. Therefore, a mirroring function is enabled by the switch  410 . For example, when the ONU  200   a  is the operational system and the ONU  300   a  is the standby system, a port of the ONU  300   a  is designated as the destination of the mirroring of a port of the ONU  200   a . Accordingly, the switch  410  duplicates frames received by the ONU  200   a  from the client device for the ONU  300   a . For example, frames transmitted by the client device  500  are received by the ONU  200   a . Further, the frames are duplicated by the switch  410  and the duplicated frames are received by the ONU  300   a . Here, when the ONU  300   a  is the standby system, the ONU  300   a  is in the optical shutdown state. Thus, the ONU  300   a  does not transmit the duplicated frames to the OLT  100 . Accordingly, the OLT  100  receives only the frames transmitted by the ONU  200   a.    
     Incidentally, no logical link has been established between the OLT  100  and the ONU  300   a . Therefore, the ONU  300   a  does not receive downstream signals. 
     According to the second embodiment, even in cases where the ONU  200   a  and the ONU  300   a  are of the SFP type, the OLT  100  is capable of letting the client devices  500  and  501  connect to the Internet  20  in a short time. 
     Third Embodiment 
     Next, a third embodiment will be described below. The following description will be given mainly of features different from those in the first embodiment, in which description will be omitted for features in common with the first embodiment.  FIGS. 1 to 7  will be referred to in the description of the third embodiment. 
       FIG. 9  is a functional block diagram showing the configuration of ONUS in the third embodiment. Each component in  FIG. 9  that is the same as a component shown in  FIG. 6  is assigned the same reference character as in  FIG. 6 . 
     The ONU  200  further includes a setting unit  230 . For example, the setting unit  230  may be implemented by a rotary switch. The setting unit  230  sets an ONU ID to the ONU  200 . 
     The ONU  300  further includes a setting unit  330 . For example, the setting unit  330  may be implemented by a rotary switch. The setting unit  330  sets an ONU ID to the ONU  300 . Incidentally, the ONU ID set to the ONU  200  and the ONU ID set to the ONU  300  are the same as each other. 
     After the logical link has been established between the OLT  100  and the ONU  200 , the communication control unit  120  acquires the ONU ID and the MAC address of the ONU  200  from the PON control unit  210 . Further, after the logical link has been established between the OLT  100  and the ONU  300 , the communication control unit  120  acquires the ONU ID and the MAC address of the ONU  300  from the PON control unit  310 . 
     The communication control unit  120  registers the ONU ID and the MAC address of the ONU  200  in an ONU authentication table. Further, the communication control unit  120  registers the ONU ID and the MAC address of the ONU  300  in the ONU authentication table. Here, a concrete example of the ONU authentication table will be shown below. 
       FIG. 10  is a diagram showing an example of the ONU authentication table in the third embodiment. The ONU authentication table  112  is stored in the storage unit  110 . The ONU authentication table  112  includes items of ONU ID, MAC ADDRESS OF OPERATIONAL-SYSTEM ONU, and MAC ADDRESS OF STANDBY-SYSTEM ONU. 
     Here, the MAC address of the ONU  200  is assumed to be XA1. The MAC address of the ONU  300  is assumed to be YA1. 
     For example, the fact that the ONU ID indicating the combination of the ONU  200  and the ONU  300  is N1 is registered in the ONU authentication table  112 . As in this example, the ONU  200  and the ONU  300  are managed by using the same ONU ID. 
     Next, a description will be given of a process regarding the ONU authentication and the UNI blockage executed by the OLT  100  when the ONU redundancy is not employed. 
       FIG. 11  is a flowchart showing the process regarding the ONU authentication and the UNI blockage when the ONU redundant configuration is not employed in the third embodiment. Namely, this flowchart is a flowchart showing the process regarding the ONU authentication and the UNI blockage when the ONU redundancy is not employed and an additional ONU is connected to the OLT. For example, the process of  FIG. 11  is started when the logical link has been established between the OLT  100  and the ONU  700 . 
     (Step S 11 ) The communication control unit  120  receives the ONU ID and the MAC address from the ONU  700 . 
     (Step S 12 ) The communication control unit  120  judges whether or not the acquired MAC address exists in the ONU authentication table  112 . 
     When the MAC address exists in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 15 . When the MAC address does not exist in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 13 . 
     (Step S 13 ) The communication control unit  120  judges whether or not the acquired ONU ID exists in the ONU authentication table  112 . 
     When the ONU ID exists in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 17 . When the ONU ID does not exist in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 14 . 
     (Step S 14 ) The communication control unit  120  registers the acquired ONU ID and MAC address in the ONU authentication table  112 . 
     (Step S 15 ) The communication control unit  120  authenticates the ONU  700 . 
     (Step S 16 ) The communication control unit  120  transmits a blockage cancellation command to the ONU  700 . Then, the communication control unit  120  ends the process. 
     (Step S 17 ) The communication control unit  120  transmits a blockage command to the ONU  700 . Accordingly, the ONU  700  blocks off its UNI unit. Further, the communication control unit  120  commands the ONU  700  to shift to the optical shutdown state. 
     Here, the system administrator can modify the registered contents of the ONU authentication table  112  by using a terminal device. Incidentally, illustration of the terminal device is left out in  FIG. 9 . 
     Next, a description will be given of a method of determining whether a newly added ONU should operate as the operational system or operate as the standby system when the new ONU is added to the communication system in a state in which the OLT  100  has authenticated one or more ONUS. Here, the new ONU is assumed to be the ONU  300 . 
       FIG. 12  is a flowchart showing a process regarding the ONU authentication and the UNI blockage when a new ONU has been added when the ONU redundant configuration is employed in the third embodiment. For example, the process of  FIG. 12  is started when the logical link has been established between the OLT  100  and the ONU  300 . 
     (Step S 21 ) The communication control unit  120  receives the ONU ID and the MAC address from the ONU  300 . 
     (Step S 22 ) The communication control unit  120  judges whether or not the acquired MAC address exists in the ONU authentication table  112 . 
     When the MAC address exists in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 24 . When the MAC address does not exist in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 23 . 
     (Step S 23 ) The communication control unit  120  judges whether or not an ONU ID overlapping with the acquired ONU ID exists in the ONU authentication table  112 . 
     When an overlapping ONU ID exists in the ONU authentication table  112 , the communication control unit  120  advances the process to step S 25 . When no overlapping ONU ID exists in the ONU authentication table  112 , the communication control unit  120  registers the ONU ID and the MAC address in the ONU authentication table  112 . Then, the communication control unit  120  advances the process to the step S 24 . 
     (Step S 24 ) The communication control unit  120  authenticates the ONU  300  as the operational system. 
     (Step S 25 ) The communication control unit  120  transmits the blockage command to the ONU  300 . Accordingly, the ONU  701  blocks off its UNI unit. Further, the communication control unit  120  commands the ONU  300  to shift to the optical shutdown state. 
     According to the third embodiment, the OLT  100  can manage the ONU  200  and the ONU  300  by using the same ONU ID. 
     Features in the embodiments described above can be appropriately combined with each other. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       10 : splitter,  20 : Internet,  100 : OLT,  101 : processor,  102 : volatile storage device,  103 : nonvolatile storage device,  110 : storage unit,  111 : management table,  112 : ONU authentication table,  120 : communication control unit,  130 : message processing unit,  140 : redundancy management unit,  150 : detection unit,  160 : monitoring unit,  200 ,  200   a ,  300 ,  300   a ,  700 ,  701 : ONU,  210 : PON control unit,  220 : UNI unit,  230 : setting unit,  310 : PON control unit,  320 : UNI unit,  330 : setting unit,  400 ,  410 : switch,  500 ,  501 ,  800 ,  801 : client device,  600 : DHCP server.