Patent Description:
As one optical access system, a passive optical network (PON) system including an optical line terminal (hereinafter referred to as "OLT") and an optical network unit (hereinafter referred to as "ONU") has been proposed.

<FIG> is a diagram illustrating a configuration example of a conventional PON system <NUM>. The PON system <NUM> includes an OLT <NUM> and a plurality of ONUs <NUM>-<NUM> to <NUM>-N (N is an integer of <NUM> or more). The OLT <NUM> and the ONUs <NUM>-<NUM> to <NUM>-N are communicably connected via an optical multiplexer/demultiplexer <NUM> including an optical transmission line, an optical splitter, and the like. Note that the plurality of ONUs <NUM>-<NUM> to <NUM>-N will be simply referred to as "ONUs <NUM>" when they are not distinguished from one another.

In downlink communication that is communication from the OLT <NUM> to the ONU <NUM>, the OLT <NUM> transmits a downlink signal <NUM> that is continuous light toward the ONU <NUM>. Each ONU <NUM> selectively receives a signal in an arbitrary time slot allocated to each ONU <NUM> among the downlink signals <NUM> transmitted from the OLT <NUM> via the optical multiplexer/demultiplexer <NUM>.

In uplink communication that is communication from the ONU <NUM> to the OLT <NUM>, each ONU <NUM> transmits an uplink signal <NUM> that is a temporally intermittent burst signal in a time slot allocated in advance by the OLT <NUM> according to a band required by each ONU <NUM>.

In a case where the unauthenticated ONU <NUM> is communicably connected to the OLT <NUM>, authentication processing of the unauthenticated ONU <NUM> is executed. <FIG> is a diagram for illustrating authentication processing and uplink communication on a conventional time axis.

The authentication processing of the ONU <NUM> is periodically performed by the OLT <NUM>, for example, and searches for an unconnected ONU <NUM>. During this time, data communication in uplink communication cannot be performed. Specifically, the OLT <NUM> transmits a terminal authentication signal called a discovery gate to the ONU <NUM>. With the reception of the terminal authentication signal, the ONU <NUM> transmits an authentication request signal called a register request to the OLT <NUM>. Here, the unauthenticated ONU <NUM> is not time-synchronized with the OLT <NUM> or another ONU <NUM>. Therefore, if the unauthenticated ONUs <NUM> receive the terminal authentication signals and simultaneously transmit the authentication request signals to the OLT <NUM>, the terminal authentication signals may collide in the optical multiplexer/demultiplexer <NUM>. Therefore, each unauthenticated ONU <NUM> suppresses the collision of the terminal authentication signals by randomly delaying the time for transmitting the authentication request signal. Note that the time for randomly delaying the transmission timing of the authentication request signal is referred to as a random delay time.

Upon receiving the authentication request signal in a specific period called a discovery window, the OLT <NUM> authenticates the ONU <NUM> that has transmitted the authentication request signal and registers the signal. This specific period has a predetermined time width and is periodically provided on the time axis. For example, the specific period is set with a value obtained by adding a total value of a difference between a minimum value and a maximum value of a propagation delay between the OLT <NUM> and the ONU <NUM> and a maximum value of a random delay time allocated to the ONU <NUM>, a delay time of processing of the ONU <NUM> or the OLT <NUM>, and the like (see, for example, Non Patent Literature <NUM>).

<CIT> : discloses a method of registering subscriber terminal that is provided to improve line efficiency by reducing the width of a periodically-provided discovery window for inhibiting communication between registered optical network unit (ONU) and optical line terminal (OLT).

<CIT> : provides a method of registering or de-registering, by an optical line terminal (OLT), one or more optical network units (ONUs) in an optical communications network, the method including setting sub-channels for registration or de-registration among two or more sub-channels, and a size of a contention window in the one or more sub-channels; transmitting, to one or more ONUs, a registration information message including information of the one or more set sub-channels and the size of the contention window; receiving, through one of the set sub-channels, a registration or de-registration request message from the one or more ONUs, which receive the registration information message; and registering and de-registering the one or more ONUs.

<CIT>: discloses a method and an apparatus for setting a quiet window in a passive optical network system wherein, a first response time from a time after an optical line terminal (OLT) transmits a serial number request up to a time in which the OLT receives a first response signal to the serial number request is measured, and a second response time up to a time in which the OLT receives a final response signal to the serial number request is measured.

However, as illustrated in <FIG>, when the number of unauthenticated ONUs <NUM> increases, a collision with an authentication request signal transmitted from another ONU <NUM> occurs. For this reason, the time width of the specific period for receiving the authentication request signal may be increased in advance (see, for example, Non Patent Literature <NUM>). However, in the conventional OLT <NUM>, since the time width and the cycle of the specific period set once cannot be changed, the time zone in which the uplink communication cannot be performed increases depending on the time width of the specific period to be set, and the band utilization efficiency of the uplink communication decreases.

In view of the above circumstances, an object of the present invention is to provide a technique capable of improving band utilization efficiency of uplink communication.

An aspect of the present invention is an optical line terminal according to claim <NUM>.

An aspect of the present invention is a registration method according to claim <NUM>.

According to the present invention, band utilization efficiency of uplink communication can be improved.

The following is a description of an embodiment of the present invention, with reference to the drawings.

<FIG> is a diagram illustrating a configuration example of an optical communication system <NUM> in a first embodiment. The optical communication system <NUM> of the first embodiment is, for example, a PON system.

The optical communication system <NUM> includes an optical line terminal (OLT) <NUM> and a plurality of optical network units (ONUs) <NUM>-<NUM> to <NUM>-N. The OLT <NUM> and the ONUs <NUM>-<NUM> to <NUM>-N are communicably connected via an optical multiplexer/demultiplexer <NUM> including an optical transmission line, an optical splitter, and the like. Note that the plurality of ONUs <NUM>-<NUM> to <NUM>-N will be simply referred to as "ONU <NUM>" when they are not distinguished from one another. A direction from the ONU <NUM> to the OLT <NUM> is referred to as uplink, and a direction from the OLT <NUM> to the ONU <NUM> is referred to as downlink.

The OLT <NUM> performs uplink communication or downlink communication with the ONU <NUM>. A communication system of the uplink communication or the downlink communication is not particularly limited, and a time division multiplexing system or a wavelength division multiplexing system may be used. The OLT <NUM> transmits a downlink signal to the ONU <NUM> in the downlink communication. The OLT <NUM> receives an uplink signal from the ONU <NUM> in the uplink communication. In a case where an unauthenticated ONU <NUM> is communicably connected to the OLT <NUM>, for example, the OLT <NUM> performs authentication processing for the unauthenticated ONU <NUM> to register the ONU <NUM>. The authentication processing of the ONU <NUM> is periodically performed by the OLT <NUM>, for example, and involves searching for an unconnected ONU <NUM>. During this time, data communication in uplink communication cannot be performed. Here, when executing the authentication processing of the ONU <NUM>, the OLT <NUM> dynamically changes at least one of a time width Δt and a cycle T of a specific period called a discovery window. As a result, the OLT <NUM> can improve a decrease in the band utilization efficiency of the uplink communication.

Each ONU <NUM> selectively receives a signal in an arbitrary time slot allocated to each ONU <NUM> among the downlink signals transmitted from the OLT <NUM> via the optical multiplexer/demultiplexer <NUM>. In uplink communication that is communication from the ONU <NUM> to the OLT <NUM>, each ONU <NUM> transmits an uplink signal that is a temporally intermittent burst signal in a time slot allocated in advance by the OLT <NUM> according to a band required by each ONU <NUM>.

Hereinafter, a schematic configuration of the OLT <NUM> for executing the authentication processing according to the first embodiment will be described.

The OLT <NUM> includes a communication unit <NUM> and a control unit <NUM>. A part or all of the control unit <NUM> may be realized by, for example, a processor such as a central processing unit (CPU) executing a program stored in the storage unit <NUM>, or may be realized by using hardware such as a large scale integration (LSI) or an application specific integrated circuit (ASIC).

The communication unit <NUM> transmits and receives optical signals to and from the ONU <NUM>. For example, the communication unit <NUM> transmits a terminal authentication signal indicating an authentication request to the ONU <NUM>. For example, the communication unit <NUM> receives an authentication request signal from the ONU <NUM>.

When executing the authentication processing of the ONU <NUM>, the control unit <NUM> dynamically changes at least one of the time width Δt of a specific period that is a period for receiving an authentication request signal from a new ONU <NUM> via the communication unit <NUM> and the cycle T of the specific period. As an example, the control unit <NUM> dynamically changes at least one of the time width Δt and the cycle T based on the authentication statuses of the plurality of ONUs <NUM>. The authentication status of the plurality of ONUs <NUM> is information related to authentication of the ONUs <NUM>, and includes a change in time of the number of unauthenticated ONUs <NUM> (hereinafter, referred to as the "unauthenticated number"), the number of newly authenticated ONUs <NUM> (hereinafter, referred to as the "new authentication number"), and the like.

Hereinafter, an example of the configuration of the control unit <NUM> will be described. The control unit <NUM> includes, for example, a storage unit <NUM>, an authentication unit <NUM>, a calculation unit <NUM>, and a change unit <NUM>.

The storage unit <NUM> stores a set value of the time width Δt of the specific period (hereinafter referred to as a "first set value") and a set value of the cycle T (hereinafter referred to as a "second set value").

The authentication unit <NUM> reads the first set value and the second set value stored in the storage unit <NUM>. The authentication unit <NUM> receives an authentication request signal from an unauthenticated ONU <NUM> in a specific period in which the time width Δt is a first set value and the cycle T is a second set value. The authentication unit <NUM> does not receive an authentication request signal from an unauthenticated ONU <NUM> outside the specific period. In a case where the authentication request signal is received within the specific period, the authentication unit <NUM> authenticates the ONU <NUM> that is the transmission source of the received authentication request signal, and registers information of the authenticated ONU <NUM> in the storage unit <NUM>.

The calculation unit <NUM> acquires information on the authentication status of the ONU <NUM> from the authentication unit <NUM>. Then, the calculation unit <NUM> calculates at least one of the time width Δt and the cycle T based on the information on the authentication status of the ONU <NUM>.

The change unit <NUM> changes the setting of the first set value stored in the storage unit <NUM> to the calculated value of the time width Δt calculated by the calculation unit <NUM>. The change unit <NUM> changes the setting of the second set value stored in the storage unit <NUM> to the calculated value of the cycle T calculated by the calculation unit <NUM>. However, the change unit <NUM> only needs to change at least one of the first set value and the second set value, and does not need to change both set values. That is, the change unit <NUM> changes the setting of one or both of the first set value and the second set value.

The change unit <NUM> causes the communication unit <NUM> to transmit, to each ONU <NUM>, a terminal authentication signal including information on the first set value and the second set value whose settings have been changed. However, the present invention is not limited thereto, and for example, the change unit <NUM> may cause the communication unit <NUM> to transmit, to each ONU <NUM>, a terminal authentication signal including the maximum value of the random delay time corresponding to the first set value whose setting has been changed and the second set value whose setting has been changed. For example, the change unit <NUM> may set a random delay time of each unauthenticated ONU <NUM> corresponding to the first set value whose setting has been changed, and may cause the communication unit <NUM> to transmit a terminal authentication signal including the random delay time and the second set value to each ONU <NUM>.

In a case where the OLT <NUM> transmits the information on the first set value and the second set value to the ONU <NUM>, the information may be transmitted through a communication path different from the main signal of the uplink signal or the downlink signal. For example, the OLT <NUM> may transmit, to the ONU <NUM>, a control signal superimposed on a frequency different from the frequency of the main signal, or information on the first set value and the second set value whose settings have been changed using another communication path such as wireless communication.

Hereinafter, a schematic configuration of the ONU <NUM> for executing the authentication processing of the present embodiment will be described. The ONU <NUM> of the present embodiment includes a communication unit <NUM>, a control unit <NUM>, and a storage unit <NUM>.

The communication unit <NUM> transmits and receives optical signals to and from the communication unit <NUM>. For example, the communication unit <NUM> receives the terminal authentication signal from the communication unit <NUM>. The communication unit <NUM> transmits an authentication request signal to the communication unit <NUM> at a timing when a random delay time set in the storage unit <NUM> elapses after receiving the terminal authentication signal.

The control unit <NUM> stores the random delay time according to the terminal authentication signal received by the communication unit <NUM> in the storage unit <NUM>. For example, the control unit <NUM> changes the set value of the random delay time stored in the storage unit <NUM> to the random delay time according to the first set value or the maximum value of the random delay time and the second set value included in the terminal authentication signal.

A set value of the random delay time is stored in the storage unit <NUM>. This set value is updated by the control unit <NUM>.

Next, a method of calculating the time width Δt and the cycle T in the calculation unit <NUM> will be described. First, a method of calculating the time width Δt will be described.

The time width Δt includes a propagation delay amount and a random delay amount. The propagation delay amount is a time in consideration of a propagation delay between the OLT <NUM> and each ONU <NUM>, and for example, a difference between a minimum value and a maximum value of each propagation delay is set as an initial value. The random delay amount is the maximum value of the random delay time set in the next specific period (authentication phase). Further, the time width Δt may include a delay time of processing of the ONU <NUM> or the OLT <NUM>. For example, the calculation unit <NUM> changes the time width Δt by changing the value of the random delay amount on the basis of the information on the authentication status of the ONU <NUM>.

For example, the calculation unit <NUM> calculates the random delay amount such that the probability P of the number of ONUs <NUM> expected to be authenticated in one specific period is equal to or greater than a preset threshold Pth with respect to the current unauthenticated number n. For example, the probability P is a probability of the number of ONUs <NUM> expected to be authenticated in the next specific period with respect to the current unauthenticated number n.

The unauthenticated number n is a difference between a maximum value of the number of ONUs <NUM> that can be authenticated and the number of authenticated ONUs <NUM>. The maximum value of the number of ONUs <NUM> that can be authenticated may be set to an arbitrary value, or may be set from the number of users allocated to the OLT <NUM> from the operating system in which the service provision information and the like are described.

A probability P is calculated by the following equation (<NUM>) as an expected value E(n) of the number of ONUs <NUM> to be authenticated, for example. As a method of calculating the probability P, for example, the method described in Reference Literature <NUM> may be used.

(Reference Literature <NUM>: <NPL>. However, the present invention is not limited thereto, and for example, the probability P may be a value obtained by dividing the expected value E(n) by the current authentication number n. <NUM>] <MAT>.

In Equation (<NUM>), W is the maximum value of the random delay time of the current unauthenticated ONU <NUM>. In Equation (<NUM>), M is the signal length of the uplink signal, and n is the current unauthenticated number.

As an example, the calculation unit <NUM> has random delay information in which the probability P and the random delay amount are associated with each unauthenticated number n in advance. The random delay information may be determined experimentally or theoretically, for example. The random delay information may be a calculation formula, a table format, or another format.

<FIG> is a graph illustrating an example of random delay information. In the graph of <FIG>, the horizontal axis represents the random delay amount, and the vertical axis represents a value in which a probability P(E(n)/n) is expressed as a percentage. <FIG> illustrates a relationship between the probability P and the random delay amount for five unauthenticated numbers n1, n2, n3, n4, and n5 as an example of the random delay information. The magnitude relationship of the five unauthenticated numbers n is represented as n1<n2<n3<n4<n5. <FIG> is a diagram illustrating random delay information in a table format. As illustrated in <FIG>, in the random delay information, the random delay amount in which the probability P is equal to or larger than the threshold Pth increases as the unauthenticated number n increases. In other words, it is indicated that the random delay amount in which the probability P satisfies the threshold Pth is reduced as the unauthenticated number n decreases. Therefore, when the authentication processing is performed several times and the unauthenticated number n gradually decreases, the random delay amount becomes a small value.

The calculation unit <NUM> acquires information of the current unauthenticated number n from the authentication unit <NUM>, and calculates a random delay amount in which the probability P is the probability Pth or more in the acquired current unauthenticated number n from, for example, random delay information in a table format. As a result, the calculation unit <NUM> can calculate the time width Δt according to the authentication status of the ONU <NUM> as an example. For example, the calculation unit <NUM> calculates the time width Δt with the portion of the propagation delay amount as an initial value and the portion of the random delay amount as a calculated value. However, the present invention is not limited thereto, and the calculation unit <NUM> may change the time width Δt by varying a part of the propagation delay amount. For example, the calculation unit <NUM> may calculate the propagation delay amount on the basis of the maximum transmission distance set by the user carrier. In this case, the calculation unit <NUM> may calculate the time width Δt using a portion of the propagation delay amount as a calculated value and a portion of the random delay amount as a calculated value or the maximum value W. Note that the calculation value of the random delay amount calculated by the calculation unit <NUM> may be a random delay amount in which the probability P is equal to or greater than the probability Pth, but it is preferable that the random delay amount be small.

The cycle T of the specific period is calculated based on the time change of the new authentication number. For example, in a case where the new authentication number is equal to or smaller than a threshold value Uth at a preset time interval ΔK, the calculation unit <NUM> may widen the interval of the authentication phase, that is, calculate a cycle longer than the currently set cycle T (second set value) as the cycle T. The case where the new authentication number is equal to or smaller than the threshold Uth includes a case where the new authentication number is zero.

For example, in a case where the new authentication number U is equal to or less than the threshold value Uth in a time interval ΔK from a timing at which there is a new authentication number equal to or greater than the threshold value Uth until a certain time elapses, the calculation unit <NUM> calculates the cycle T of a specific period so that an interval of a next authentication phase becomes wider than an interval of a current authentication phase.

<FIG> is a diagram illustrating an example of a trend of a timing of new authentication of the ONU <NUM> in the past. As illustrated in <FIG>, in a case where the timing of the new authentication of the ONU <NUM> has periodicity, or the like, the calculation unit <NUM> may calculate the period T of the specific period in accordance with the tendency of the timing of the new authentication. For example, the calculation unit <NUM> may obtain in advance information indicating the tendency of the timing of new authentication of the ONU <NUM> for each predetermined period such as each day or each time, and calculate the cycle T so as to provide a specific period at the timing when the ONU <NUM> is newly authenticated and not to provide the specific period at the timing when the ONU <NUM> is not newly authenticated.

Next, a flow of a registration method of the ONU <NUM> in the first embodiment will be described. <FIG> is a diagram illustrating a flow of the registration method of the ONU <NUM> in the first embodiment. For example, the optical communication system <NUM> repeats steps S101 to S105 to change at least one of the time width Δt and the cycle T for each specific period. In the description using <FIG>, both the time width Δt and the cycle T are changed as an example.

The OLT <NUM> calculates the time width Δt and the cycle T of the specific period in accordance with the authentication status of the ONU <NUM> (step S101). The OLT <NUM> changes the setting of the first set value and the cycle T stored in the storage unit <NUM> to the calculated values calculated in step S101 (step S102). As a result, as illustrated in <FIG>, the time width Δt and the cycle T of the specific period of the present embodiment are not fixed values, and are dynamically changed, for example, for each specific period. Accordingly, the OLT <NUM> can improve a decrease in the band utilization efficiency of the uplink communication.

The OLT <NUM> transmits a terminal authentication signal including the calculated values of the time width Δt and the cycle T calculated in step S101 to the ONU <NUM> (step S103).

The ONU <NUM> receives the terminal authentication signal from the OLT <NUM> and transmits an authentication request signal with a random delay time corresponding to each calculated value included in the received terminal authentication signal (step S104). When the OLT <NUM> receives the authentication request signal within the specific period in which the time width Δt and the cycle T are changed in step S102, the OLT <NUM> registers (discovers) the ONU <NUM> that has transmitted the received authentication request signal (step S105).

As described above, the OLT <NUM> according to the first embodiment dynamically changes at least one of the time width Δt and the cycle T of the specific period that is a period for receiving an authentication request signal from a new ONU <NUM>. As a result, the OLT <NUM> can reduce the time zone in which the uplink communication cannot be performed and improve the band utilization efficiency of the uplink communication.

<FIG> is a diagram illustrating a modification example of the OLT <NUM> according to the first embodiment. As illustrated in <FIG>, the OLT <NUM> may further include a monitoring control unit <NUM>. Hereinafter, a modification example of the OLT <NUM> is referred to as an OLT 10A.

The OLT 10A includes the communication unit <NUM>, a control unit 12A, and the monitoring control unit <NUM>. The control unit 12A includes, for example, the storage unit <NUM>, the authentication unit <NUM>, a calculation unit 15A, and the change unit <NUM>.

In a case where a failure of the OLT 10A is detected, the monitoring control unit <NUM> transmits an initialization signal to the control unit 12A. The monitoring control unit <NUM> detects the restart of the OLT 10A, and transmits an initialization signal to the control unit 12A in a case where the restart is detected. In a case of detecting the interruption of at least one of the uplink signal and the downlink signal, the monitoring control unit <NUM> transmits an initialization signal to the control unit 12A.

The calculation unit 15A has the same function as the calculation unit <NUM>. In a case of receiving the initialization signal from monitoring control unit <NUM>, the calculation unit 15A executes the initialization operation. The initialization operation is an operation of changing the time width Δt of the specific period and the cycle T of the specific period to preset values. For example, in a case of receiving the initialization signal from the monitoring control unit <NUM>, the calculation unit 15A changes the time width Δt of the specific period to the maximum value of the settable time width Δt, and changes the cycle T of the specific period to the minimum value of the settable cycle. As a result, at the time of activation of the OLT <NUM> to which a large number of new ONUs <NUM> are predicted to be simultaneously connected, or the like, the OLT 10A can perform authentication of a larger number of new ONUs <NUM> by executing the initialization operation.

<FIG> is a diagram illustrating an example configuration of an optical communication system 1B according to a second embodiment. In the optical communication system 1B of the second embodiment, the OLT <NUM> and the ONU <NUM> communicate using a plurality of wavelengths. In the following description, parts having functions similar to those described in the first embodiment will be denoted by similar names and reference numerals, and a specific description regarding the functions will be omitted.

The optical communication system 1B includes an OLT 10B and a plurality of ONUs 20B-<NUM> to 20B-N. The OLT 10B and the ONUs 20B-<NUM> to 20B-N are communicably connected via an optical multiplexer/demultiplexer <NUM> including an optical transmission line, an optical splitter, and the like. Note that the plurality of ONUs 20B-<NUM> to 20B-N will be simply referred to as "ONU 20B" when they are not distinguished from one another.

For example, the OLT 10B and the plurality of ONUs 20B-<NUM> to 20B-N communicate with each other by a communication scheme using a wavelength division multiplexing technology. Therefore, the ONUs 20B-<NUM> to 20B-N transmit optical signals of different wavelengths. Note that it is not necessary for all the ONUs 20B-<NUM> to 20B-N to transmit optical signals having different wavelengths, and some of the ONUs may transmit optical signals having different wavelengths. In the following description, a case where all the ONUs 20B-<NUM> to 20B-N transmit optical signals of different wavelengths will be described as an example.

As illustrated in <FIG>, the OLT 10B includes a communication unit 11B and a control unit 12B. A part or all of the control unit 12B may be realized, for example, by a processor such as a CPU executing a program stored in the storage unit 13B, or may be realized by using hardware such as an LSI or an ASIC).

The communication unit 11B performs bidirectional communication with the ONU 20B using a wavelength division multiplexing technique. For example, the communication unit 11B includes a plurality of optical transceivers <NUM> and a multiplexer/demultiplexer <NUM>.

The plurality of optical transceivers <NUM> are connected to the control unit 12B. The plurality of optical transceivers <NUM> generate optical signals having different wavelengths and transmit the optical signals to the multiplexer/demultiplexer <NUM>. The plurality of optical transceivers <NUM> receive the optical signal demultiplexed and output for each wavelength from the multiplexer/demultiplexer <NUM>.

The multiplexer/demultiplexer <NUM> combines a plurality of optical signals having different wavelengths generated by the plurality of optical transceivers <NUM> to generate a downlink WDM signal. The downlink WDM signal is transmitted to the ONUs 20B-<NUM> to 20B-N. The multiplexer/demultiplexer <NUM> receives an uplink WDM signal obtained by multiplexing each uplink signal from each ONU 20B by the optical multiplexer/demultiplexer <NUM>. Then, the multiplexer/demultiplexer <NUM> demultiplexes the uplink WDM signal and outputs the demultiplexed signal to each optical transceiver <NUM>.

The control unit 12B has the same configuration as the control unit <NUM>. The control unit 12B may include the monitoring control unit <NUM> described above. Here, the control unit 12B may manage the number of ONUs 20B registered for each wavelength, and set the time width Δt and the cycle T of the specific period based on the authentication status of the managed ONU 10B. In this case, for example, the control unit 12B may set the unauthenticated number n to a value obtained by dividing the number of ONUs 20B in the entire optical communication system 1B by the number of wavelengths. In the OLT 10B, the maximum registration number of the ONUs 20B for each wavelength may be registered in advance. The number of ONUs 10B registered in the OLT 20B may be the sum of all wavelengths, and the control unit 12B may collectively control the time width ΔT and the cycle T.

Hereinafter, a schematic configuration of the ONU 20B for executing the authentication processing of the present embodiment will be described. The ONU <NUM> of the present embodiment includes a communication unit 21B, the control unit <NUM>, and the storage unit <NUM>.

The communication unit 21B transmits and receives optical signals to and from the communication unit 11B. For example, the communication unit <NUM> communicates with the communication unit 11B by a communication scheme using a wavelength division multiplexing technology.

Note that the method of calculating the time width Δt and the cycle T of the second embodiment is similar to that of the first embodiment, and thus the description thereof will be omitted. Since the flow of the registration method of the ONU 20B of the second embodiment is similar to that of the first embodiment, the description thereof will be omitted.

As described above, the OLT 10B in the second embodiment has the same effects as those of the first embodiment.

Some functions of the OLTs <NUM>, 10A, and 10B and the ONUs <NUM> and 20B in the above-described embodiments may be implemented by a computer. In that case, the program for achieving these functions may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read and executed by a computer system to achieve the functions. Note that the "computer system" mentioned herein includes hardware such as an OS and peripheral devices. Also, the "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM, or a CD-ROM, or a storage device such as a hard disk included in a computer system. Further, the "computer-readable recording medium" may include a medium that dynamically holds the program for a short time, such as a communication line in a case where the program is transmitted via a network such as the Internet or a communication line such as a telephone line, and a medium that holds the program for a certain period of time, such as a volatile memory inside a computer system serving as a server or a client in that case. Also, the above program may be for achieving some of the functions described above, may be formed with a combination of the functions described above and a program already recorded in a computer system, or may be formed with a programmable logic device such as an FPGA.

As above, the embodiments of the present invention have been described in detail with reference to the drawings.

Reference throughout the specification to a portion as "including," "having," or "comprising" a component means that the portion does not exclude other components but may further include other components unless specifically stated to the contrary.

In addition, the term ". unit" described in the specification means a unit that processes at least one function or operation, which may be embodied as hardware or software, or may be embodied in a combination of hardware and software.

Claim 1:
An optical line terminal (<NUM>, 10A, 10B) comprising:
a communication unit (<NUM>, 11B) configured to communicate with a plurality of optical network units; and
a control unit (<NUM>, 12A, 12B) configured to dynamically change at least one of a time width of a specific period that is a time width in the discovery window of authentication processing of the plurality of optical network units and a cycle of the specific period that is a cycle in the discovery window of authentication processing of the plurality of optical network units, the specific period being a period in which an authentication request signal is received from a new optical network unit via the communication unit,
characterized in that
the control unit (<NUM>, 12A, 12B) is adapted to change at least one of a time width of the specific period and a cycle of the specific period based on authentication statuses of the plurality of optical network units,
wherein the control (<NUM>, 12A, 12B) unit is adapted to change the time width so that a probability of the optical network unit that is expected to be authenticated in one specific period becomes equal to or greater than a preset threshold with respect to an unauthenticated number that is the number of unauthenticated optical network units.