Patent Publication Number: US-2021168022-A1

Title: Communication apparatus and communication method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a communication device and a communication method. 
     BACKGROUND ART 
     Ethernet (registered trademark) is a wired network protocol used worldwide, and is adopted in many standard network devices. 
     Especially, Ethernet (registered trademark) with its physical layer defined by an optical interface employing an optical transmission technique has been used exclusively for Point-to-Point (P2P) communications, and has been widespread as an interface enabling long-range high-speed communications. 
     When the above P2P network topology is used in an access network that accommodates a plurality of network devices, especially for performing long-distance transmission in particular, a high installation cost of the optical fiber and a large occupation area of station devices are required. In view of this issue, a Point-to-Multipoint (P2MP) communication system using a Passive Optical Network (PON) topology has been used as a PON system for an access network. In the PON system, an optical fiber and an optical line terminal (OLT) are shared among a plurality of optical network units (ONUs) so that the installation cost of the optical fiber and the occupation area of the OLT can be reduced. This configuration of the PON system has been defined by the Institute of Electrical and Electronics Engineers (IEEE). 
     In known PON systems, a Discovery process is executed for establishing initial connection between the OLT and the ONU. The Discovery process is a method in which an OLT newly registers an ONU. The initial connection between the OLT and the ONU is performed on the basis of this registration (see Non Patent Literature 1). 
       FIG. 9  is a diagram illustrating a network configuration in which an ONU is newly connected to a PON system. As illustrated in  FIG. 9 , the OLT is already connected to at least one connected ONU (hereinafter referred to as “connected ONU”). At least one ONU yet to be connected (hereinafter referred to as “unconnected ONU”) establishes an initial connection with the OLT through the Discovery process. 
       FIG. 10  is a block diagram illustrating a configuration of a communication system in which the Discovery process is executed according to a known technique.  FIG. 11  is a sequence diagram illustrating a flow of processing for the Discovery process according to the known technique.  FIG. 12  is a flowchart illustrating a flow of processing for the Discovery process according to the known technique. The flow of the processing for the Discovery process according to the known technique will be described with reference to  FIGS. 10 to 12 . 
     The Discovery process is executed with none of the connected ONUs transmitting data. When the Discovery process starts, a message processing unit of the OLT transmits a Discovery_Gate signal to all the ONUs via a data signal transmission processing unit, a data signal transmission unit, and a data signal optical transmission unit. 
     A message processing unit of the unconnected ONU receives the Discovery_Gate signal via a data signal optical reception unit, a data signal reception unit, and a data signal reception processing unit. 
     Upon receiving the Discovery_Gate signal, the message processing unit of the unconnected ONU transmits a Register_Request signal to the OLT via a data signal transmission processing unit, a data signal transmission unit, and a data signal optical transmission unit. This process is implemented through burst transmission of an optical signal to the OLT via an optical signal control instruction unit, an optical signal output control unit, and an optical signal control unit of the unconnected ONU. 
     The message processing unit of the OLT receives the Register_Request signal via a data signal optical reception unit, a data signal reception unit, and a data signal reception processing unit. Upon receiving the Register_Request signal, the message processing unit of the OLT recognizes an identifier described in the Discovery_Gate signal. The message processing unit of the OLT transmits a Register signal to the unconnected ONU via the data signal transmission processing unit, the data signal transmission unit, and the data signal optical transmission unit. 
     The message processing unit of the unconnected ONU receives the Register signal via the data signal optical reception unit, the data signal reception unit, and the data signal reception processing unit. Upon receiving the Register signal, the message processing unit of the unconnected ONU transmits a Register_ACK signal to the OLT via the data signal transmission processing unit, the data signal transmission unit, and the data signal optical transmission unit. 
     The message processing unit of the OLT receives the Register_ACK signal via the data signal optical reception unit, the data signal reception unit, and the data signal reception processing unit. 
     Through the processing described above, the Discovery process is completed, whereby the initial connection is completed with the unconnected ONU registered in the OLT. 
     CITATION LIST 
     Non Patent Literature 
     
         
         Non Patent Literature 1: “IEEE Standard for Ethernet SECTION FIVE”, IEEE Std 802.3TM-2015″, IEEE Computer Society, pp. 310-343, 675-709, 2015 Non Patent Literature 2: “IEEE Standard for Ethernet SECTION FOUR”, IEEE Std 802.3TM-2015”, IEEE Computer Society, pp. 321-323, 2015 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In a known technique, a type of Ethernet (registered trademark) with a physical layer defined by an optical interface is used for Point-to-Point (P2P) type long-range high-speed communications or the like. Use of the P2P network topology in an access network involves a risk of an increase in the optical fiber installation cost. Thus, a PON system using the PON topology for P2MP has been used for access networks. 
     In the PON system, the Discovery process is executed for establishing the initial connection between the OLT and the ONU. When the PON system is implemented using an Ethernet (registered trademark) device that is a general-purpose device, an ONU using a 10 G Ethernet (registered trademark) device in particular transmits an optical signal before being controlled by the OLT. Thus, the optical signal transmitted by the ONU collides with the optical signal transmitted by another ONU. This results in a failure to establish the initial connection. Furthermore, due to the limitation imposed by a link failure notification function that is a required standard of 10 G Ethernet (registered trademark), the Discovery process cannot be executed in a state where the optical signals have not been transmitted from all the connected ONUs yet. All things considered, there is a problem in that the initial connection cannot be established with the known technique or similar. 
     The present disclosure has been made in view of the above, and an object of some aspects of the present disclosure is to provide a technique enabling the initial connection to be established with standard Ethernet (registered trademark) devices alone, in a configuration in which a plurality of network devices are connected to each other by the PON topology. 
     Means for Solving the Problem 
     One aspect of the present disclosure is a communication device including a first communication device and a plurality of second communication devices in an optical access system in which the first communication device and the plurality of second communication devices communicate through a time division multiple access scheme, the first communication device including: an Ethernet (registered trademark) controller configured to implement a communication as an Ethernet communication; a link failure detection unit configured to detect, when the plurality of second communication devices perform an initial connection to a network in the optical access system, whether a link failure has occurred based on collision detection of an optical signal transmitted by each of the plurality of second communication devices; and a signal processing unit configured to output, when the link failure detection unit detects that the link failure has not occurred, an initial connection start notification for causing initiation of a processing of the initial connection. 
     One aspect of the present disclosure is a communication device including a first communication device and a plurality of second communication devices in an optical access system in which the first communication device and the plurality of second communication devices communicate through a time division multiple access scheme, the second communication device including: an Ethernet (registered trademark) controller configured to implement a communication as an Ethernet communication; and a first suspension instruction unit configured to output, when the plurality of second communication devices perform an initial connection to a network in the optical access system, a first suspension instruction for causing suspension of an output of an optical signal to the first communication device for a predetermined time period. 
     One aspect of the present disclosure is the communication device described above, further including a first transmission instruction unit configured to output, when an initial connection start notification for causing initiation of a processing of the initial connection transmitted by the first communication device is not received over the predetermined time period after connected to the network, a first transmission instruction for causing transmission of an idle signal to the first communication device. 
     One aspect of the present disclosure is the communication device described above, further including a retransmission instruction unit configured to output, when a link failure occurrence notification indicating an occurrence of a link failure that is transmitted by the first communication device in response to the transmission of the idle signal to the first communication device is received, a retransmission instruction for causing suspension of an output of the optical signal to the first communication device and retransmit the idle signal after an optional time period elapses. 
     One aspect of the present disclosure is the communication device described above, further including a second suspension instruction unit configured to output, when a link failure occurrence notification indicating occurrence of a link failure is received after an initial connection start notification for causing initiation of a processing of the initial connection is received while the communication device has been connected to the network, a second suspension instruction for causing suspension of an output of the optical signal to the first communication device. 
     One aspect of the present disclosure is the communication device described above, further including a second transmission instruction unit configured to, when a link failure occurrence notification indicating an occurrence of a link failure is received while the communication device has been connected to the network, transmit a second transmission instruction for causing suspension of an output of the optical signal to the first communication device, and, when the link failure occurrence notification is received while the communication device has not been connected to the network, transmit after elapse of an optional time period an initial connection response to the first communication device that is a response to the first communication device for an initial connection start notification for causing initiation of a processing of the initial connection. 
     One aspect of the present disclosure is a communication method in an optical access system in which a first communication device and a plurality of second communication devices communicate through a time division multiple access scheme, the communication method performed by a computer of the first communication devices and including: implementing a communications as an Ethernet (registered trademark) communication; detecting, when the plurality of second communication devices perform an initial connection to a network in the optical access system, whether a link failure has occurred based on collision detection of an optical signal transmitted by each of the plurality of second communication devices; and outputting, when it is detected that the link failure has not occurred, an initial connection start notification for causing initiation of a processing of the initial connection. 
     One aspect of the present disclosure is a communication method in an optical access system in which a first communication device and a plurality of second communication devices communicate through a time division multiple access method, the communication method performed by a computer of the plurality of second communication devices and including: implementing a communication as an Ethernet (registered trademark) communication; and outputting, when the plurality of second communication devices perform an initial connection to a network in the optical access system, a suspension instruction for causing suspension of an output of an optical signal to the first communication device for a predetermined time period. 
     Effects of the Invention 
     Some aspects of the present disclosure enables the initial connection to be performed with standard Ethernet (registered trademark) devices alone, in a configuration in which a plurality of network devices are connected to each other by the PON topology. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a functional configuration of a communication system  1  according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram illustrating a flow of the process of connecting the first device in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
         FIG. 3  is a diagram illustrating a flow of the process of simultaneously connecting the first ONU and the second ONU or more in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
         FIG. 4  is a diagram illustrating a flow of the process of connecting one new ONU in a state where at least one ONU is connected, in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
         FIG. 5  is a diagram illustrating a flow of the process of connecting two new ONUs in a state where at least one ONU is connected, in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
         FIG. 6  is a flowchart illustrating a flow of processing in the Discovery process by the communication system  1 . 
         FIG. 7  is a flowchart illustrating a flow of processing in the Discovery process by the communication system  1 . 
         FIG. 8  is a flowchart illustrating a flow of processing in the Discovery process by the communication system  1 . 
         FIG. 9  is a diagram illustrating a network configuration in which an ONU is newly connected to a PON system. 
         FIG. 10  is a block diagram illustrating a configuration of a communication system in which the Discovery process is executed according to a known technique. 
         FIG. 11  is a sequence diagram illustrating a flow of processing for the Discovery process according to the known technique. 
         FIG. 12  is a flowchart illustrating a flow of processing for the Discovery process according to the known technique. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. 
     Functional Configuration of Communication System 
       FIG. 1  is a diagram illustrating a functional configuration of a communication system  1  according to an embodiment of the present disclosure. As illustrated in  FIG. 1 , the communication system  1  is configured to include an ONU  10  and an OLT  20 . The communication system  1  is an optical access system in which the OLT  20  (first communication device) and a plurality of the ONUs  10  (second communication devices) communicate under a time division multiple access scheme. 
     The ONU  10  is configured to include an arithmetic processing unit  11 , an Ethernet controller  13 , and an optical signal transceiver  15 . 
     The arithmetic processing unit  11  is configured to include a processor such as a Central Processing Unit (CPU), for example. As illustrated in  FIG. 1 , the arithmetic processing unit  11  includes a message processing unit  111 , a data signal transmission processing unit  112 , an optical signal control instruction unit  113 , a data signal reception processing unit  114 , an initial connection start standby instruction unit  115 , an initial connection start processing unit  116 , an idle signal retransmission instruction unit  117 , an initial connection response standby processing adding unit  118 , a link failure processing unit  119 , and an initial connection response standby processing unit  120 . 
     Note that the message processing unit  111 , the data signal transmission processing unit  112 , the optical signal control instruction unit  113 , the data signal reception processing unit  114 , the initial connection start standby instruction unit  115 , the initial connection start processing unit  116 , the idle signal retransmission instruction unit  117 , the initial connection response standby processing adding unit  118 , the link failure processing unit  119 , and the initial connection response standby processing unit  120  are software programs executed by a processor such as a CPU. 
     The message processing unit  111  outputs a control start instruction and a control end instruction to each of the data signal transmission processing unit  112  and the optical signal control instruction unit  113 . 
     When the control start instruction is input, the data signal transmission processing unit  112  starts transmission processing on data sent from a request source and in transmission standby (hereinafter referred to as “transmission standby data”). Note that the transmission standby data is stored in a temporary storage medium (not illustrated) provided by the arithmetic processing unit  11  or another functional block of the ONU  10 . The data signal transmission processing unit  112  outputs a data signal transmission instruction for causing the transmission of the data signal as well as data on which transmission processing has been executed, to a data signal optical transmission unit  151  of the optical signal transceiver  15  via a data signal transmission unit  131  of the Ethernet controller  13 . 
     When the control start instruction is input, the optical signal control instruction unit  113  outputs an optical signal control instruction for causing optical signal output control, to an optical signal output control unit  132  of the Ethernet controller  13 . 
     The data signal reception processing unit  114  acquires a data signal based on the optical signal received by a data signal optical reception unit  153 , from a data signal reception unit  133  via a link failure detection unit  134 .
 
The initial connection start standby instruction unit  115  outputs an instruction to the optical signal control instruction unit  113  to prevent the output of the optical signal for a predetermined time period when network connection is established.
 
     When no initial connection start notification message is received over a predetermined time period after the network connection has been established, the initial connection start processing unit  116  outputs an instruction to the optical signal control instruction unit  113  for causing output of an idle signal. 
     The idle signal retransmission instruction unit  117  suspends output of the optical signal when a link failure notification is received in response to the transmission of the idle signal. Then, when a random (appropriate) time period elapses, the idle signal retransmission instruction unit  117  outputs an instruction for causing re-output of the idle signal to the optical signal control instruction unit  113 . 
     When the initial connection start notification message is received before the idle signal is re-output, the initial connection response standby processing adding unit  118  outputs an instruction for causing transmission of an initial connection response message to the data signal transmission processing unit  112  after waiting again for the random (appropriate) time period to elapse. 
     When the link failure notification is received after the reception of the initial connection start notification message with the network connection already established, the link failure processing unit  119  outputs an instruction for suspending output of the optical signal, to the optical signal control instruction unit  113 . 
     The initial connection response standby processing unit  120  suspends the optical signal output when a link failure notification is received in response to the transmission of the idle signal. Then, after a random (appropriate) time period has elapsed, the initial connection response standby processing unit  120  outputs an instruction for causing transmission of an initial connection response message, to the data signal transmission processing unit  112 . 
     The Ethernet controller  13  is a circuit implementing the communications as Ethernet (registered trademark) communications in particular, among circuits that enable data communications under Media Access Control (MAC) and physical (PHY) layer protocols and are installed in a network interface and a network device, for example. As illustrated in  FIG. 1 , the Ethernet controller  13  is configured to include the data signal transmission unit  131 , the optical signal output control unit  132 , the data signal reception unit  133 , and the link failure detection unit  134 . 
     The data signal transmission unit  131  outputs an electrical signal based on the data signal input from the data signal transmission processing unit  112 , to the data signal optical transmission unit  151 . Here, the optical signal output control unit  132  controls an optical signal control unit  152  on the basis of an electrical signal output from the data signal transmission unit  131  to the data signal optical transmission unit  151 , to control the output of the optical signal transmitted from the data signal optical transmission unit  151 . 
     The data signal reception unit  133  acquires an electrical signal based on the optical signal received by the data signal optical reception unit  153  from the link failure detection unit  134 , and outputs a data signal based on the acquired electrical signal to the data signal reception processing unit  114 . 
     The link failure detection unit  134  acquires an electrical signal based on the optical signal received by the data signal optical reception unit  153 , and detects the presence or absence of a link failure notification. The link failure detection unit  134  outputs the acquired link failure notification to the link failure processing unit  119 . 
     The link failure detection unit  134  outputs a data signal based on the acquired electrical signal to the data signal reception unit  133 . 
     The optical signal transceiver  15  is a device having a physical medium dependent unit function that can output an electrical signal as an optical signal. Specifically, the optical signal transceiver  15  is an optical module such as, for example, a Small Form-factor Pluggable (SFP) (Mini-GBIC)/SFP+, or an optical module mounted on a printed circuit board. As illustrated in  FIG. 1 , the optical signal transceiver  15  is configured to include the data signal optical transmission unit  151 , the optical signal control unit  152 , and the data signal optical reception unit  153 . 
     Under the control by the optical signal control unit  152 , the data signal optical transmission unit  151  transmits an optical signal based on the electrical signal input from the data signal transmission unit  131 , to the OLT  20  via the network. 
     Under the control by the optical signal output control unit  132 , the optical signal control unit  152  switches ON/OFF an optical pulse on the basis of the values (0 and 1) of the electrical signal input to the data signal optical transmission unit  151 . Thus, an optical signal is transmitted to the OLT  20 . With the configuration described above, the ONU  10  can transmit the data signal to the OLT  20  as an optical burst signal without using an optical line termination device.
 
The data signal optical reception unit  153  receives the optical signal transmitted from the OLT  20 . The data signal optical reception unit  153  outputs an electrical signal based on the received optical signal, to the link failure detection unit  134 .
 
     The OLT  20  is configured to include an arithmetic processing unit  21 , an Ethernet controller  23 , and an optical signal transceiver  25 . 
     The arithmetic processing unit  21  is configured to include a processor such as a CPU, for example. As illustrated in  FIG. 1 , the arithmetic processing unit  21  is configured to include a message processing unit  211 , a data signal transmission processing unit  212 , a data signal reception processing unit  213 , and an idle signal processing unit  214 . 
     The message processing unit  211  outputs a control start instruction and a control end instruction to the data signal transmission processing unit  212  and the data signal reception processing unit  213 . 
     When the control start instruction is input, the data signal transmission processing unit  212  starts transmission processing on the transmission standby data. Note that the transmission standby data is stored in a temporary storage medium (not illustrated) provided by the arithmetic processing unit  21  or another functional block of the OLT  20 . The data signal transmission processing unit  212  outputs a data signal transmission instruction for causing the transmission of the data signal as well as data on which transmission processing has been executed, to the data signal optical transmission unit  251  of the optical signal transceiver  25  via the data signal transmission unit  231  of the Ethernet controller  23 . 
     The data signal reception processing unit  213  acquires a data signal based on the optical signal received by the data signal optical reception unit  252 , from the data signal reception unit  233  via a link failure detection unit  232 . 
     For the initial connection, the idle signal processing unit  214  receives the idle signal, and outputs an instruction for transmitting the initial connection start notification message, to the data signal transmission processing unit  212  in a state with no link failure. 
     The Ethernet controller  23  is a circuit implementing the communications as Ethernet (registered trademark) communications in particular, among circuits that enable data communications under MAC and physical (PHY) layer protocols, and are installed in a network interface and a network device, for example. As illustrated in  FIG. 1 , the Ethernet controller  23  is configured to include the data signal transmission unit  231 , the data signal reception unit  232 , and the link failure detection unit  233 . 
     The data signal transmission unit  231  outputs an electrical signal based on the data signal input from the data signal transmission processing unit  212 , to the data signal optical transmission unit  251 . 
     The data signal reception unit  232  acquires an electrical signal based on the optical signal received by the data signal optical reception unit  252  from the link failure detection unit  233 , and outputs a data signal based on the received electrical signal to the data signal reception processing unit  213 .
 
The link failure detection unit  233  acquires an electrical signal based on the optical signal received by the data signal optical reception unit  252 , and detects the presence or absence of link failures. The link failure detection unit  233  outputs the acquired electrical signal to the data signal reception unit  232 .
 
     The optical signal transceiver  25  is a device having a physical medium dependent unit function that can output an electrical signal as an optical signal. Specifically, the optical signal transceiver  25  is an optical module such as, for example, an SFP/SFP+, or an optical module mounted on a printed circuit board. As illustrated in  FIG. 1 , the optical signal transceiver  25  is configured to include the data signal optical transmission unit  251  and the data signal optical reception unit  252 . 
     The data signal optical transmission unit  251  transmits an optical signal based on the electrical signal input from the data signal transmission unit  231 , to the ONU  10  via the network. The data signal optical reception unit  252  receives the optical signal transmitted from the OLT  10 . The data signal optical reception unit  252  outputs an electrical signal based on the received optical signal, to the link failure detection unit  233 . 
     With the configuration described above, the initial connection is established. 
     Now, with reference to  FIGS. 2 to 8 , processes in the Discovery process by the communication system  1  will be described including: a process of connecting the first ONU; a process of simultaneously connecting the first ONU and the second ONU or more; a process of connecting one new ONU in a state where at least one ONU is connected; and a process of connecting two new ONUs in a state where at least one ONU is connected. 
     Process of connecting first ONU 
       FIG. 2  is a diagram illustrating the flow of the process of connecting the first device in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
     The unconnected ONU  10  to be connected to the network as the first ONU is connected to the network (step S 10 ), and then enters initial connection start standby without outputting the optical signal due to the instruction from the initial connection start standby instruction unit  115  (S 11 ). 
     When no initial connection start notification message is received for a predetermined time period, the unconnected ONU  10  transmits the idle signal due to the instruction from the initial connection start standby instruction unit  115  (step S 12 ).
 
Upon receiving the idle signal, the OLT  20  transmits the initial connection start notification message in response to an instruction from the idle signal processing unit  214  (step S 13 ).
 
     Upon receiving the initial connection start notification message, the unconnected ONU  10  transmits the initial connection response message due to the instruction from the message processing unit  111  (step S 14 ). 
     Upon receiving the initial connection response message, the OLT  20  transmits an initial connection completion notification message due to the instruction from the message processing unit  211  (step S 15 ).
 
When the unconnected ONU  10  receives the initial connection completion notification message, the initial connection is completed.
 
     Process of simultaneously connecting first ONU and second ONU or more  FIG. 3  is a diagram illustrating the flow of the process of simultaneously connecting the first ONU and the second ONU or more in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
     The unconnected ONUs  10  to establish the initial connection with the network as the first ONU and the second ONU or more are connected to the network (step S 20 ), and then enter the initial connection start standby without outputting the optical signal due to the instruction from the initial connection start standby instruction unit  115  (S 21 ). 
     When no initial connection start notification message is received for a predetermined time period, the unconnected ONUs  10  transmit the idle signal to the OLT  20  due to the instruction from the initial connection start standby instruction unit  115  (step S 22 ).
 
The reception of the idle signals transmitted from the two unconnected ONUs  10  by the OLT  10  is a collision event. Thus, the link failure detection unit  233  of the OLT  20  detects a link failure (step S 23 ).
 
     Then, the data signal optical transmission unit  251  of the OLT  20  transmits the link failure notification to the unconnected ONUs  10  (step S 24 ). 
     Upon receiving the link failure notification, the unconnected ONUs  10  recognizes that the link failure has occurred due to the transmission of the idle signals and thus suspend output of the optical signals (step S 25 ).
 
After a random (appropriate) time period corresponding to a device unique value has elapsed (step S 26 ), the unconnected ONUs  10  retransmit the idle signal to the PLT  20  due to the instruction from the idle signal retransmission instruction unit  117  (step S 27 ).
 
     Upon receiving the idle signal from the first unconnected ONU  10 , the OLT  20  transmits the initial connection start notification message to the unconnected ONU  10  in response to an instruction from the idle signal processing unit  214  (step S 28 ). 
     Upon receiving the initial connection start notification message, the unconnected ONU  10  transmits the initial connection response message to the OLT  20  due to the instruction from the message processing unit  111  (step S 29 ).
 
Upon receiving the initial connection response message, the OLT  20  transmits an initial connection completion notification message to the unconnected ONU  10  due to the instruction from the message processing unit  211  (step S 30 ).
 
When the unconnected ONU  10  receives the initial connection completion notification message, the initial connection is completed.
 
     On the other hand, the unconnected ONU  10  that has received the initial connection start notification message before transmitting the idle signal waits again until the random (appropriate) time period corresponding to the device unique value elapses (step S 31 ), and then transmits the initial connection response message to the OLT  20  due to the instruction from the initial connection response standby processing adding unit  118  (step S 32 ). 
     Upon receiving the initial connection response message, the OLT  20  transmits an initial connection completion notification message to the unconnected ONU  10  due to the instruction from the message processing unit  211  (step S 33 ).
 
When the unconnected ONU  10  receives the initial connection completion notification message, the initial connection is completed.
 
     It is possible to keep establishing the initial connection for the third unconnected ONU  10  and after to be connected, by repeating the process described above. 
     The process ends when the initial connection is completed for all the unconnected ONUs  10 . 
     Process of newly connecting one ONU in state where at least one ONU is connected  FIG. 4  is a diagram illustrating the flow of the process of connecting one new ONU in a state where at least one ONU is connected, in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
     The unconnected ONU  10  to be newly connected to the network is connected to the network (step S 40 ), and then enters initial connection start standby without outputting the optical signal due to the instruction from the initial connection start standby instruction unit  115  (S 41 ). 
     Upon receiving the initial connection start notification message transmitted due to the instruction from the message processing unit  211  of the OLT  20  within a predetermined time period (step S 42 ), the unconnected ONU  10  starts transmitting the idle signal to the OLT  20  due to the instruction from the initial connection start standby instruction unit  115  (step S 43 ).
 
The link failure detection unit  233  of the OLT  20  that has received the idle signal detects the link failure (step S 44 ).
 
     Then, the data signal optical transmission unit  251  of the OLT  20  transmits the link failure notification to the at least one connected ONU  10  and the unconnected ONU  10  (step S 45 ). 
     Upon receiving the link failure notification, all of the at least one connected ONU  10  suspend output of the optical signal due to the instruction from the link failure processing unit  119  (step S 46 ).
 
When the optical signal output thus suspends, the unconnected ONU  10  transmits the initial connection response message to the OLT  20  as a response to the initial connection start message, due to the instruction from the message processing unit  111  (step S 47 ).
 
     Upon receiving the initial connection response message, the OLT  20  transmits an initial connection completion notification message to the unconnected ONU  10  due to the instruction from the message processing unit  211  (step S 48 ). 
     When the unconnected ONU  10  receives the initial connection completion notification message, the initial connection is completed. 
     Process of newly connecting two ONU in state where at least one ONU is connected  FIG. 5  is a diagram illustrating the flow of the process of connecting two new ONUs in a state where at least one ONU is connected, in the Discovery process by the communication system  1  according to an embodiment of the present disclosure. 
     The unconnected ONUs  10  to simultaneously establish the initial connection with the network as the first ONU and the second ONU or more to be newly connected to the network are connected to the network (step S 50 ), and then enter the initial connection start standby without outputting the optical signal due to the instruction from the initial connection start standby instruction unit  115  (S 51 ). 
     Upon receiving the initial connection start notification message transmitted due to the instruction from the message processing unit  211  of the OLT  20  within a predetermined time period (step S 52 ), the unconnected ONU  10  starts transmitting the idle signal to the OLT  20  due to the instruction from the initial connection start standby instruction unit  115  (step S 53 ). 
     The link failure detection unit  233  of the OLT  20  that has received the idle signal detects the link failure (step S 54 ). 
     Then, the data signal optical transmission unit  251  of the OLT  20  transmits the link failure notification to all of the at least one connected ONU  10  and all of the two or more unconnected ONUs  10  (step S 55 ).
 
Upon receiving the link failure notification, the connected ONUs  10  suspend output of the optical signal due to the instruction from the link failure processing unit  119  (step S 56 ).
 
     Still, transmission of the idle signals from two or more unconnected ONUs  10  continues, and thus the link failure continues to occur. If the link failure continues to occur for a predetermined time period, all of the two or more unconnected ONUs  10  suspend transmission of the optical signal to the OLT  20  due to the instruction from the initial connection response standby processing adding unit  118  (step S 57 ). 
     The unconnected ONUs  10  transmit the idle signal when the respective random (appropriate) time periods elapse (step S 58 ), and then transmits the initial connection response message to the OLT  20  due to the instruction from the message processing unit  111  (step S 59 ). 
     Upon receiving the initial connection response message, the OLT  20  transmits an initial connection completion notification message to the unconnected ONU  10  that has transmitted the initial connection response message, due to the instruction from the message processing unit  211  (step S 60 ). 
     When the unconnected ONU  10  receives the initial connection completion notification message, the initial connection is completed.
 
This flow is repeated until the initial connection is completed for all the unconnected ONUs  20 .
 
       FIG. 6  is a flowchart illustrating a flow of processing in the Discovery process by the communication system  1 . 
     If there is at least one connected ONU  10  (Yes in ACT 001 ), the connected ONU  10  transmits the idle signal to the OLT  20  (ACT 002 ). 
     Next, initial connection processing A illustrated in  FIG. 7  is executed (ACT 003 ). 
     The OLT  20  transmits the initial connection start notification message to the unconnected ONU  10  (ACT 101 ). 
     The unconnected ONU  10  receives the initial connection start notification message transmitted from OLT  20  (ACT 102 ). 
     The description will be given by referring back to  FIG. 6 . 
     The unconnected ONU  10  transmits the idle signal to the OLT  20  (ACT 004 ).
 
The OLT  20  detects the link failure and transmits the link failure notification to all the connected ONUs  10  (ACT 005 ).
 
All the connected ONUs  10  suspend output of the optical signal (ACT 006 ).
 
If there are two or more unconnected ONUs  10  (Yes in ACT 007 ), the ONUs  10  wait until the respective random (appropriate) time periods elapse (ACT 008 ). If the number of unconnected ONU  10  is less than two (if there is one unconnected ONU  10 ) (No in ACT 007 ), the ONU  10  does not wait for the elapse of random (appropriate) time period as described above.
 
Then, initial connection processing B illustrated in  FIG. 8  is repeated by the number of unconnected ONUs  10  (ACT 017 ).
 
     The unconnected ONU  10  transmits the initial connection response message to the OLT  20  (ACT 201 ). 
     The OLT  20  receives the initial connection response message from the unconnected ONU  10  (ACT 202 ).
 
The OLT  20  transmits the initial connection completion notification message to the unconnected ONU  10  (ACT 203 ).
 
The unconnected ONU  10  receives the initial connection completion notification message transmitted from the OLT  20  (ACT 204 ).
 
The description will be given by referring back to  FIG. 6 .
 
If the number of connected ONU  10  is less than one (if there is no connected ONU  10 ) (Yes in ACT 001 ), the unconnected ONU  10  enters initial connection start standby without outputting the optical signal (ACT 009 ).
 
The unconnected ONU  10  transmits the idle signal to the OLT  20  (ACT 010 ).
 
If the number of unconnected ONU  10  is less than two (if the number of unconnected ONU  10  is one) (No in ACT 011 ), the processing proceeds to ACT 016  described below. If the number of unconnected ONUs  10  is two or more (Yes in ACT 011 ), the OLT  20  detects the link failure and transmits the link failure notification to all the connected ONUs  10  (ACT 012 ).
 
     All the unconnected ONUs  10  suspend output of the optical signal (ACT 013 ). 
     The ONU  10  waits until the random (appropriate) time period elapses (ACT 014 ).
 
One of the unconnected ONUs  10  transmits the idle signal to the OLT  20  (ACY 015 ).
 
Then, the initial connection processing A described above with reference to  FIG. 7  is executed (ACT 016 ).
 
Then, the initial connection processing B described above with reference to  FIG. 8  is repeated by the number of unconnected ONUs  10  (ACT 017 ).
 
Then, the processing in the flowcharts illustrated in  FIGS. 6 to 8  ends.
 
     As described above, when an Ethernet (registered trademark) device (ONU) is newly connected to the PON system, the control for preventing the optical signal transmission until the initial connection starts after the network connection is required to prevent collision between an optical signal transmitted by the ONU and an optical signal transmitted by another ONU. Furthermore, the initial connection needs to be enabled even when the link failure notification function is activated. 
     In the present disclosure, control is performed so that the ONU  10  including an Ethernet (registered trademark) device refrains from transmitting the optical signal when it is connected to the OLT  20 , and control is performed so that the connected ONU  10  suspends output of the optical signal upon receiving the link failure notification. Thus, the present disclosure enables the initial connection to be performed with standard Ethernet (registered trademark) devices alone, in a configuration in which a plurality of network devices are connected to each other by the PON topology. 
     The embodiments of the present disclosure have been described above in detail with reference to the drawings. However, specific configurations are not limited to those embodiments, and include any design or the like within the scope not departing from the gist of the present disclosure. 
     Note that a part or all of the ONU  10  and the OLT  20  according to the embodiments described above may be realized by a computer. In that case, this configuration may be enabled by recording a program for implementing such control functions on a computer-readable recording medium and causing a computer system to read the program recorded on the recording medium for execution. 
     Note that it is assumed that the “computer system” mentioned here refers to a computer system built into the ONU  10  and the OLT  20 , and the computer system includes an OS and hardware components such as a peripheral apparatus. The “computer-readable recording medium” refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, and a CD-ROM, and a storage apparatus such as a hard disk installed in a computer system. 
     Moreover, the “computer-readable recording medium” may include a medium that dynamically retains a program for a short period of time, such as a communication line that is used to transmit the program over a network such as the Internet or over a communication line such as a telephone line, and may also include a medium that retains a program for a certain period of time, such as a volatile memory within the computer system for functioning as a server or a client in such a case. Furthermore, the program may be configured to realize some of the functions described above, and also may be configured to be capable of implementing the functions described above in combination with a program already recorded in the computer system. 
     A part or all of the ONU  10  and the OLT  20  in the embodiments described above may be realized as an integrated circuit such as a Large Scale Integration (LSI). Each function block of the ONU  10  and the OLT  20  may be individually realized as processors, or a part or all thereof may be integrated into processors. Furthermore, a circuit integration technique is not limited to the LSI, and a part of or all of the transfer apparatus may be enabled with a dedicated circuit or a general-purpose processor. In a case that with advances in semiconductor technology, a circuit integration technology with which an LSI is replaced appears, an integrated circuit based on the technology may be used. 
     REFERENCE SIGNS LIST 
     
         
           1  Communication system 
           10  Optical network unit (ONU) 
           11  Arithmetic processing unit 
           13  Ethernet controller 
           15  Optical signal transceiver 
           20  Optical line terminal (OLT) 
           21  Arithmetic processing unit 
           23  Ethernet controller 
           25  Optical signal transceiver 
           111  Message processing unit 
           112  Data signal transmission processing unit 
           113  Optical signal control instruction unit 
           114  Data signal reception processing unit 
           115  Initial connection start standby instruction unit 
           116  Initial connection start processing unit 
           117  Idle signal retransmission instruction unit 
           118  Initial connection response standby processing adding unit 
           119  Link failure processing unit 
           120  Initial connection response standby processing unit 
           131  Data signal transmission unit 
           132  Optical signal output control unit 
           133  Data signal reception unit 
           134  Link failure detection unit 
           151  Data signal optical transmission unit 
           152  Optical signal control unit 
           153  Data signal optical reception unit 
           211  Message processing unit 
           212  Data signal transmission processing unit 
           213  Data signal reception processing unit 
           214  Idle signal processing unit 
           231  Data signal transmission unit 
           232  Data signal reception unit 
           233  Link failure detection unit 
           251  Data signal optical transmission unit 
           252  Data signal optical reception unit