Method for registering optical network unit in telecommunications network and optical network unit therefor

In a telecommunications network including an optical line terminal (OLT) having optical subscriber units (OSUs) and an optical network unit (ONU) connected to the OLT, bandwidth allocation signals are periodically transmitted at a first period from the respective OSUs to the ONU, and an acknowledgement request signal is transmitted from at least one of the OSUs to the ONU at a second period having a value corresponding to the first period multiplied by a constant. If an ONU is registered in none of the OSUs, the first period is determined from reception intervals of the assignment signals, and a second period is calculated from the first period. Transmissive wavelength of a variable-wavelength (VW) filter in the ONU is periodically changed at a third period longer than the second period. In the OSU having transmitted the request signal, the unregistered ONU having received the request signal is registered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of registering an unregistered optical network unit (ONU) in a telecommunications network. The present invention also relates to a telecommunications network comprising an optical line terminal (OLT) including a plurality of optical subscriber units (OSUs) and an ONU connected to the OLT. Furthermore, the present invention relates to an ONU for use in communication through a connection to an OLT.

2. Description of the Background Art

A telecommunications network which links a communication station possessed by a communication common carrier to customer premises may be called as an access network. In recent years, with the trend that communications capacities have increased, optical access networks enabling huge amounts of information to be transmitted by means of optical communications are becoming the mainstream of access networks.

As a form of optical access network, passive optical networks (PONs) are available. The PON is configured to have an OLT installed in a communication station, plural ONUs installed on respective subscriber premises and an optical splitter. The OLT and ONUs are interconnected to each other via the optical splitter with optical fiber lines.

The interconnection between an OLT and an optical splitter can be implemented by a single-core optical fiber, which is shared by plural ONUs. Optical splitters are, in general, inexpensive passive elements, so that PONs are superior in economics and in maintenanability. For those reasons, PONS are being rapidly introduced.

In a PON, signals transmitted by ONUs toward an OLT, which may be referred to as upstream optical signals, are first combined by an optical splitter and then transmitted to the OLT. On the contrary, signals transmitted by the OLT toward the ONUs, which may be referred to as downstream optical signals, are first split by the optical splitter and then transmitted to the respective ONUs. In order to prevent upstream and downstream optical signals from interfering with each other, the upstream and downstream optical signals are assigned to wavelengths that are different therebetween.

PONs are implemented by a variety of multiplexing techniques, such as time division multiplexing (TDM) assigning short time slots to subscriber terminals, wavelength division multiplexing (WDM) assigning different wavelengths specific to subscriber terminals, code division multiplexing (CDM) assigning different codes specific to subscriber terminals. In those multiplexing techniques, PONs based on the TDM, i.e. TDM-PON, currently enjoy the widest acceptance. Among TDM-PONs, time division multiple access (TDMA) is prevailing, in which an OLT manages timing at which ONUs transmit upstream optical signals so as to prevent upstream signals transmitted from the different ONUs from colliding with each other.

One of the PON systems based on Ethernet (trademark) may be referred to as Ethernet-PON, which includes a PON system based upon Gigabit (1×109bit/sec) Ethernet, that is, GE-PON. The GE-PON is standardized, for example, by IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.3ah and IEEE 802.3av.

In order to establish communications between an OLT and ONUS in a GE-PON system, it is necessary to register the ONUS in the OLT. Generally in GE-PON systems, the single OLT has plural ONUS connected thereto. Thus, when an ONU not yet registered in the OLT is newly registered in the OLT, the registration has to be performed without affecting communications between the OLT and other ONUs already registered. For that aim, the IEEE 802.3ah and IEEE 802.3av Standards regulate a procedure of detecting an unregistered ONU and then registering that OLT. The procedure may be referred to as a discovery sequence.

In the discovery sequence, an OLT periodically broadcasts a discovery gate massage notifying all ONUS of transmitting timing. The discovery gate message is transmitted to every ONU, regardless of whether or not the ONU is already registered in the OLT. A time interval at which a discovery gate message is transmitted from the OLT may be referred to as a discovery period. After an ONU newly connected to the PON system, although not registered, is powered on and then prepared for reception of signals, it will periodically receive discovery gate messages.

If unregistered one of the ONUS receives a discovery gate message, it transmits a registration request to the OLT to request that the ONU be registered in the OLT. The registration request contains a media access control (MAC) address, which is the identification of the ONU.

In the OLT, a discovery window is set. The OLT opens a discovery window, in which it waits for reception of a registration request from an unregistered ONU.

The OLT receives a registration request to thereby recognize the MAC address of the unregistered ONU. The OLT in turn transmits a register message, which contains a logical link identifier (LLID) on the PON system, to the ONU having the recognized MAC address.

Subsequently to the transmission of the register message, the OLT transmits a gate message notifying the ONU of a transmission bandwidth and timing and allowing the ONU to transmit upstream optical signals.

On receiving the gate message, the unregistered ONU transmits a register acknowledgement (ACK) to the OLT. When the OLT receives the register ACK, registration of the new ONU is completed, namely, the discovery sequence thus finishes. After the new ONU is registered, normal communications between the OLT and the newly registered ONU will start.

At the present time, a PON system based upon both TDM and WDM, i.e. TDM/WDM-PON, is proposed. In the TDM/WDM-PON, an OLT may include a plurality of optical subscriber units (OSUs).

In the TDM/WDM-PON, OSUs are assigned to transmission wavelengths that are different OSU by OSU. The OSUs transmit downstream optical signals on the transmission wavelengths specifically assigned thereto. On the contrary, the ONUS transmit upstream optical signals on the transmission wavelengths and timing notified on the downstream optical signal by the OSU.

In the TDM/WDM-PON, ONUS are grouped correspondingly to plural OSUs so that each ONU is to be managed dedicatedly by either one of the plural OSUs, thus reducing the number of ONUs managed by each OSU. This can expand the service bandwidths provided to subscriber units.

In the TDM/WDM-PON thus configured, it is sufficient for each ONU to be registered in either one of the plural OSUs. Furthermore, as described previously, in the TDM/WDM-PON each ONU communicates at a wavelength appropriate for an OSU associated therewith. It is thus preferable for the ONUS to have the wavelengths thereof variably available for transmission and reception.

As disclosed in Japanese patent laid-open publication No. 2011-004270 to Fujiwara et al., an ONU in the TDM/WDM-PON includes a variable-wavelength (VW) filter. In this prior art system, the ONU, when unregistered, sweeps the passable, or transmissive, wavelength of its VW filter in order to wait for receipt of a discovery gate message.

When the unregistered ONU receives a downstream optical signal, it measures the optical power of the optical signal. When the measured optical power is less than a predetermined value, or threshold, the VW filter shifts its transmissive wavelength to a different wavelength.

On the contrary, when the measured optical power is equal to or greater than the predetermined value, the unregistered ONU makes a decision as to whether or not the downstream optical signal is a discovery gate message. If a discovery gate is detected in the optical signal, a discovery sequence will proceed as the processes described above.

If no discovery gate is detected, the ONU waits for receipt of a discovery gate message for a predetermined period of time, i.e. wait time. If no discovery gate message is received during the period of time, namely, the period of time is over, then the VW filter shifts its transmissive wavelength again to another different wavelength.

In the unregistered ONU, the VW filter repeats the shift of its transmissive wavelength until it receives a discovery gate message, thus searching for a transmissive wavelength on which it can receive a discovery gate message.

A wait time during which an unregistered ONU awaits a discovery gate message at a transmissive wavelength of its VW filter is set longer than a discovery period. That is, the wait time TT-FLof an unregistered ONU at each transmissive wavelength with respect to a discovery period TDISis set by following Expression (1):
TT-FL>TDIS(1)

The condition defined by Expression (1) ensures that a discovery gate message is sent at least once within the wait time at each transmissive wavelength. This makes it possible to correctly determine, at each transmissive wavelength set, whether or not a discovery gate message can be received. The VW filter thus sequentially, e.g. incrementally or decrementally, switches its transmissive wavelength, the unregistered ONU thereby being able to reliably receive a discovery gate message.

In the TDM/WDM-PON, the discovery period TDISis usually set equal to a dynamic bandwidth allocation (DBA) period multiplied by a constant α, where α is greater than unity. The DBA period is a period of time in which an OSU transmits a bandwidth allocation signal, such as a gate message in the aforementioned IEEE Standards, notifying an ONU of a transmission bandwidth and timing. The constant α is a value set in advance as a parameter during the operation of the system. Hence, when the DBA period is rendered to change, the discovery period TDISis changed accordingly.

When the discovery period TDISis varied, the wait time TT-FLof the unregistered ONU at each transmissive wavelength may become shorter than the discovery period TDIS. In this case, a relation given by following Expression (2) is developed between the wait time TT-FLand the discovery period TDIS:
TT-FL<TDIS(2)

Under the condition given by above Expression (2), the transmissive wavelength of a VW filter is changed at a transmission interval at which a discovery gate message is sent. That may cause a condition to continue in which the wavelength of a discovery gate is not coincident with the transmissive wavelength of a VW filter. Consequently, it is highly likely that it takes a long time until a discovery sequence commences.

In order to maintain the condition defined by Expression (1) whenever the DBA period changes, the OSU would be adapted to notify the ONUS of the change of the DBA period. However, the ONU, when not activated, could not receive a notice of the change of the DBA period. Therefore, the ONU, when started up, would not set the wait time TT-FLappropriate for the change of the DBA period.

In order to maintain the relation given by Expression (1), a maximum assumption method would be introduced in which the possible maximum DBA period is considered so as to set the wait time TT-FLof an unregistered ONU at each transmissive wavelength. In the maximum assumption method, an unregistered ONU would calculate the discovery period TDISon the basis of the maximum value of the DBA period. Then, the wait time TT-FLwould be set so that Expression (1) could hold at the calculated discovery period TDIS. Thence, if the DBA period is changed, the wait time TT-FLwould be prevented from becoming shorter than the discovery period TDIS. The relation given by Expression (1) would thus be maintained.

In the maximum assumption method, however, the wait time TT-FLwould have to be set to a large value, which would require an extensive time until the wait time expires. Accordingly, when the DBA period is not maximal, if the wavelength of a discovery gate message is not coincident with the transmissive wavelength of the VW filter, it would then be highly likely that discovery gate messages the unregistered ONU fails to receive would repetitively be transmitted. For this reason, the unregistered ONU would inefficiently receive discovery gate messages, and hence it would take a long time until the discovery sequence is started and completed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved method of registering an unregistered optical network unit (ONU) in a telecommunications network. It is also an object of the invention to provide an improved telecommunications network capable of registering an unregistered ONU in an optical line terminal (OLT). Furthermore, it is also an object of the invention to provide an improved ONU connectable to such a telecommunications network.

In accordance with the present invention, a method of registering an ONU in a telecommunications network which includes an OLT having a plurality of OSUs and at least one ONU connected to the OLT, the ONU including a VW filter, the plurality of OSUs transmitting a signal having different wavelengths assigned to the respective OSUs, comprises steps of periodically transmitting by the plurality of OSUs a bandwidth allocation signal at a first period to each of the ONUS connected to the OLT; transmitting by at least one of the OSUs an acknowledgement request signal to the ONU at a second period having a value corresponding to the first period multiplied by a constant; determining by the unregistered ONU, if the ONU is registered in none of the OSUs, the first period for receiving the bandwidth allocation signals on the basis of reception intervals of the bandwidth allocation signals, and then calculating a second period on the basis of the first period; periodically changing by the unregistered ONU a transmissive wavelength of the VW filter in the unregistered ONU at a third period longer than the calculated second period; receiving the acknowledgement request signal by the unregistered ONU, and registering by the OSU having transmitted the acknowledgement request signal the unregistered ONU having received the acknowledgement request signal.

Also in accordance with the present invention, a telecommunications network comprises an OLT having a plurality of OSUs and at least one ONU connected to the OLT. The plurality of OSUs comprises a signal generator producing a bandwidth allocation signal for notifying the ONU of a transmission bandwidth of a signal to be transmitted from the ONU to the OLT; and an optical signal transmitter transmitting an optical signal containing the bandwidth allocation signal, different transmission wavelengths being assigned to the optical signals transmitted from the respective OSUs. The ONU comprises an optical signal receiver having a VW filter having a transmissive wavelength changeable to receive the optical signal having a transmission wavelength; a signal reader reading the bandwidth allocation signal contained in the received optical signal; a signal interval detector detecting a reception interval of the bandwidth allocation signal for determining a first period; a period calculator calculating a second period on the basis of the determined first period; and a VW filter controller periodically changing the transmissive wavelength of the VW filter at a third period longer than the calculated second period.

Further in accordance with the present invention, an ONU for use in communication through a connection to an OLT comprises: an optical signal receiver having a VW filter having a transmissive wavelength changeable to receive an optical signal having a transmission wavelength transmitted to the ONU; a signal reader reading a bandwidth allocation signal contained in the received optical signal, the bandwidth allocation signal notifying the ONU of a transmission bandwidth of a signal to be transmitted from the ONU to the OLT; a signal interval detector detecting a reception interval of the bandwidth allocation signal to determine a first period; a period calculator calculating a second period on the basis of the determined first period; and a VW filter controller periodically changing the transmissive wavelength of the VW filter at a third period longer than the calculated second period.

In accordance with the present invention, the wait time TT-FLsatisfying the condition of above Expression (1) can be reliably set in the ONU.

Also in accordance with the present invention, the time taken until a timeout occurs in the ONU can be shortened so that the discovery sequence may be quickly started and completed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the positional relationships between the constituent components are shown as simplified to the extent that the present invention can easily be understood. Also note that numerical values conditioned in the following preferred embodiments are merely exemplified. Accordingly, the present invention may not be restricted to the following specific embodiments.

One embodiment of a telecommunications network system according to the present invention may be implemented into an optical network system, more specifically to a TDM/WDM-PON, which is a PON system based upon both TDM and WDM and adapted for registering an ONU. Accordingly, the schematic configuration of the embodiment for use in the TDM/WDM-PON will first be described by referring toFIG. 1.

In the embodiment, a TDM/WDM-PON10based upon both TDM and WDM includes an optical line terminal (OLT)12and a plurality of ONUs14. The TDM/WDM-PON10further includes an optical transmission channel16for optically connecting the OLT12to the respective ONUs14.

The optical transmission channel16includes an optical splitter18for splitting a downstream optical signal and combining upstream optical signals. The optical transmission channel16includes an optical fiber line20connecting the OLT12to the optical splitter18and optical fiber lines22a-22dconnecting the splitter18to the ONUs14a-14d, respectively.

The OLT12functions to produce a downstream optical signal including both a downstream data signal received from an upper or global layer network, not shown, and a downstream control signal for controlling operations of the ONUs14to transmit the produced optical signal to the ONUs14. The OLT12is connected by a communication line24to the upper network.

The OLT12includes a plurality of optical subscriber units (OSUs)26for producing a downstream optical signal including both a downstream data signal received from the upper network and a downstream control signal for managing the ONUs14to transmit the produced signal to the ONUs14. InFIG. 1, the plurality of OSUs26are also shown as a group28of OSUs26a-26dfor convenience.

The embodiment shown inFIG. 1includes four ONUs14ato14dand four OSUs26ato26d. However, the number of the ONUs14and the OSUs26may not be limited to the specific value of the embodiment. In the illustrative embodiment, the ONUs14ato14dmay be the same as each other and generally be designated just with a reference numeral14. The OSUs26ato26dmay also be the same as each other and generally be designated simply with a reference numeral26. The configurations and functions of the ONUs14and OSUs26will be described in further detail later.

The OLT12further includes a switching element30for establishing a communication path between the upper network and the OSUs26, and a controller32for controlling the OSUs26. The OLT12also includes an M/D34adapted for multiplexing downstream optical signals of different wavelengths supplied from the OSUs26and demultiplexing an upstream combined optical signal received from the optical transmission channel16. The M/D34is connected by the optical fiber line20to the optical splitter18.

The ONU14is adapted to produce an upstream optical signal including both an upstream data signal received from a user terminal, not shown, connected to the ONU14and an upstream control signal for making a bandwidth request to send the produced optical signal toward the OSUs26.

In the TDM/WDM-PON10, the respective ONUs14may be registered in either one of the plurality of OSUs26. Furthermore, the respective OSUs26are as signed to wavelengths different from each other. The OSUs26serve to transmit downstream optical signals on the assigned wavelengths. The OSUs26also serve to receive upstream optical signals on the assigned wavelengths.

One of the ONUs14, when being registered in either one of the OSUs26ato26d, transmits an upstream optical signal on a wavelength which the one OSU26can accept. In order to prevent upstream optical signals transmitted from other ONUs14already registered in that one OSU26from overlapping with each other, the respective ONUs14registered in the identical OSU26are assigned to transmission timing different therebetween.

In the TDM/WDM-PON10, as described earlier, the plurality of ONUs14may be grouped correspondingly to the plurality of OSUs26ato26dso that each ONU14is to be managed or controlled dedicatedly to either one of the plurality of ONUs14. This reduces the number of ONUs14managed by each OSU26, and hence the service bandwidths provided to respective subscribers can be expanded.

In the OLT12, the switching element30is connected by the communication line24to the upper network to communicate with the upper network on the line24. The switching element30is also connected by a connection line36to the controller32and by communication lines38a-38dto the OSUs26a-26d, respectively.

The switching element30is adapted to distribute downstream data signals to the OSUs26a-26don the basis of a transmission plan, described later, decided by the controller32to deliver the distributed signals to the destination OSUs26. The switching element30is further adapted to transmit upstream data signals supplied from the OSUs26a-26dto the upper network.

Furthermore, the switching element30functions to provide the controller32with information such as the destinations and traffic of the downstream data signals received from the upper network.

The controller32in the OLT12is adapted to manage information on the ONUs14registered in the OSUs26, that is, PON link information. The controller32stores the PON link information in a memory40such as a random access memory which the PON link information can be read out from and written into.FIG. 1shows that the memory40is connected by a connection line42to the controller32. However, the memory40may be directly or indirectly connected to other constituent elements such as OSUs36and further may be included in the controller32per se.

The controller32is further adapted to decide a transmission plan of downstream signals on the basis of information received from the switching element30, the PON link information and so on. The controller32may receive from the switching element30information such as the destination and traffic of downstream data signals.

The controller32communicates with the switching element30on the connection line36and the group28of OSUs on a connection line44. The controller32may communicate individually with the respective OSUs26ato26d. Due to that configuration, the controller32can give a notice of a transmission plan to the switching element30and the OSUs26.

Furthermore, the controller32is adapted to produce a wavelength assigning signal for assigning the OSUs26a-26dto different wavelengths of downstream optical signals. The wavelength assigning signal is delivered to the OSUs26on the line44. In the TDM/WDM-PON10, as previously described, communications between the OSU26and the ONU14are performed on wavelengths specific thereto. Accordingly, the controller32is adapted to cause the OSUs26to conduct assignment of respectively different wavelengths of downstream optical signals.

In addition, the controller32is adapted to select, on the basis of a transmission plan, one of the OSUs26in which unregistered one of the ONUS14a-14dis to newly be registered. The controller32in turn instructs that OSU26to produce a wavelength notice signal. The wavelength notice signal serves as instructing that ONU14such as to allow the ONU14to transmit upstream optical signals on a wavelength assigned to the OSU26to which the ONU14will perform a discovery sequence and to receive downstream optical signals on a wavelength assigned to the same OSU26.

The M/D34, which is connected by communication lines46a-46dto OSUs26a-26d, respectively, functions to multiplex downstream optical signals on different wavelengths fed from the OSUs26to transmit the multiplexed signal over the optical transmission channel16to the ONUS14. The M/D34further functions to demultiplex upstream optical signals wavelength- and time-multiplexed by the optical splitter18into optical signals on respective wavelengths to transfer the demultiplexed signals to the OSUs26associated with the respective wavelengths.

The configuration of the OSU26will be described with reference toFIG. 2. In the instant embodiment, the OSU26includes an electric signal processor50for processing electric signals and an optical signal processor52for processing optical signals.

The electric signal processor50includes an interface54for transmitting and receiving data signals to and from the upper network, an electric signal transmitter56for transmitting an electric signal to the optical signal processor52, and an electric signal receiver58for receiving an electric signal from the optical signal processor52. The electric signal processor50further includes a controller60for controlling the interface54, the transmitter56and the receiver58. The controller60may further control the optical signal processor52.

The optical signal processor52includes an optical signal transmitter62for converting an electric signal received from the electric signal processor50to a corresponding optical signal to transmit the resultant optical signal to the ONU14, and an optical signal receiver64for receiving an optical signal from the ONUs14to convert the optical signal to a corresponding electric signal. Thus, the optical signal transmitter62and the optical signal receiver64may be thought of as an electric-to-optical (E/O) converter62and an optical-to-electric (O/E) converter64, respectively.

The optical signal processor52further includes an M/D66such as a WDM filter adapted for multiplexing and demultiplexing optical signal(s).

The interface54, the electric signal transmitter56and the electric signal receiver58per se may be similar in structure to those of any conventional OLTs in PONS. Thus, a detailed description of the structure of those constituent elements is omitted.

The interface54is connected by the communication line38to the switching element30. Due to that constitution, the interface54serves to transmit and receive upstream and downstream data signals to and from the upper network via the switching element30. The interface54is also connected by the connection line44to the controller32. The interface54further has its output68connected to an input of the electric signal transmitter56to transmit a downstream data signal received from the upper network to the transmitter56. In addition, the interface54has its input70connected to an output of the electric signal receiver58to receive an upstream data signal from the receiver58.

The electric signal transmitter56further has its input72connected to an output of the controller60and its output74connected to an input of the optical signal transmitter62. The electrical signal transmitter56produces a downstream electrical signal from a downstream data signal received from the interface54in response to a downstream control signal received from the controller60. The downstream electrical signal is delivered to the optical signal transmitter62on the line74.

The optical signal transmitter62may include a tunable laser diode (TLD) that is adapted to vary the wavelength of an optical signal. The optical signal transmitter62converts a downstream electrical signal received from the electric signal transmitter56into a corresponding downstream optical signal. The optical signal transmitter62has its output76connected to an input of the M/D66to transmit the converted optical signal to the M/D66.

The M/D66is connected by the communication line46to the M/D34in the OLT12to transmit a downstream optical signal produced by the optical signal transmitter62via the M/D34and the optical transmission channel16to the ONUs14. Furthermore, the M/D66in the OSU26has its output78connected to an input of the optical signal receiver64to deliver an upstream optical signal received over the optical transmission channel16via the M/D34to the optical signal receiver64.

In the TDM/WDM-PON10, upstream optical signals are transmitted on wavelength bands that are different from those on which downstream optical signals are transmitted. By means of the difference in wavelength, upstream and downstream optical signals can be separated from each other by the M/D66. Thus, upstream and downstream optical signals can be separately processed, i.e. multiplexed and demultiplexed by the M/D66.

The optical signal receiver64includes an opto-electric conversion element such as a photodiode (PD). The optical signal receiver64functions as converting an upstream optical signal supplied from the M/D66on the communication line78into a corresponding upstream electrical signal. The photodiode is adapted to be able to receive upstream optical signals at least on wavelength bands which the ONUs14a-14dcan set up for communications with the OLT12.

The optical signal receiver64is connected by a communication line80to the electrical signal receiver58to deliver the converted upstream electrical signals to the electric signal receiver58.

The electric signal receiver58splits an upstream electrical signal received from the optical signal receiver64into an upstream data signal and an upstream control signal. The upstream data signal is sent out to the upper network via the interface54and the switching element30.

The electric signal receiver58has its output82connected to an input of the controller60to deliver the upstream control signal produced by the receiver58to the controller60.

The controller60may be similar in essential configuration to that used in any conventional OLTs in PONs. Thus, only the significant configuration and functions of the controller60will be described in detail.

The controller60is connected by a control line86to the interface54to intercommunicate with the interface54. Furthermore, the controller60includes a signal generator88for producing a downstream control signal.

For example, the downstream control signal may be a bandwidth allocation signal, such as a gate signal, which is formed on the basis of a transmission plan decided by the controller32and bandwidth requests received from the ONUs14for notifying the ONUs14of a transmission bandwidth and transmission timing for upstream optical signals. The downstream control signal may also be a discovery gate message for use in a discovery sequence.

The downstream control signal contains information giving instructions on the transmission wavelength of upstream optical signals and the reception wavelength of downstream optical signals.

As already described, in the TDM/WDM-PON10, each ONU14communicates with one of the OSUs26which has that ONU14registered on a wavelength specific thereto. For example, if the ONU14ais registered in the OSU26a, the ONU14acommunicates with the OSU26aon a wavelength that is specific to the OSU26a.

Thus, the controller60instructs the ONUS14by a downstream control signal so as to allow the ONUS14to send upstream optical signals on wavelengths respectively specific to the destination OSUs26and to receive downstream optical signals on wavelengths respectively specific to the source OSUs26. The downstream control signal is first delivered to the electric signal transmitter56on the line72.

The controller60includes a signal reader90for reading information on the ONUS14such as MAC addresses and requested bandwidths contained in the upstream control signal.

The controller60further includes an ONU register92for registering the ONUS14which are made communicable through a discovery sequence. The ONU register92is further adapted to give a notice of PON link information as to which ONUS14are registered in the OSUs26to the controller32via the interface54. In other words, the ONU register92informs the controller32which one of the ONUS14a PON link has been established from the OSU26in which that ONU register92is involved. Of course, the ONU register92may inform the controller32of any information on the ONUS14.

The controller60includes a wavelength setter94for setting the wavelength of a downstream optical signal produced in the optical signal transmitter62on the basis of a wavelength setting signal supplied from the controller32.

The controller60may be adapted to serve, as a whole, as the signal generator88, signal reader90, ONU register92and wavelength setter94. In that case, the controller60may be implemented by a computer on which program sequences stored in the memory40shown inFIG. 1are run so as to produce downstream control signals, readout information on the ONUs14, register the ONUs14and set the wavelengths of downstream optical signals. The results of processing by the program sequences performed on the controller60may be appropriately stored in the memory40or the like. In this regard, the illustrative embodiment of the controller60is depicted and described as configured by separate functional blocks, such as the signal generator88. It is however to be noted that such a depiction and a description do not restrict those blocks to an implementation only in the form of hardware but may partially or entirely be implemented by software as described above. That may also be the case with illustrative embodiments which will be described below. In this connection, the word “circuit” or “unit” may be understood not only as hardware, such as an electronics circuit, but also as a function that may be implemented by software installed and executed on a computer.

Next, the configuration of each ONU14will be described by referring toFIG. 3. In the illustrative embodiment shown inFIG. 3, the ONU14includes an optical signal processor102for processing an optical signal and an electric signal processor104for processing an electric signal.

The optical signal processor102includes an M/D106for multiplexing and demultiplexing optical signal(s), an optical signal receiver108for receiving an optical signal from the OLT12to convert the optical signal to a corresponding electric signal, and an optical signal transmitter110for converting an electric signal processed by the electric signal processor104to a corresponding optical signal to transmit the converted optical signal to the OLT12. Thus, the optical signal receiver108and the optical signal transmitter110may be thought of as an O/E converter108and an E/O converter110, respectively.

The electric signal processor104includes an electric signal receiver112for receiving an electric signal from the optical signal receiver108, an interface114for transmitting and receiving data signals to and from user terminals, not shown, a buffer116, and an electric signal transmitter118for feeding an electric signal to the optical signal transmitter110. The electric signal processor104further includes a controller120for controlling the receiver112, the interface114, the buffer116and the transmitter118, and further the optical signal processor102.

The electrical signal receiver112, the interface114, the buffer116and the electrical signal transmitter118and may be similar in configuration to those of conventional ONUs. Thus, a detailed description of the structure of those constituent units is omitted.

The M/D106may include any arbitrary type of multi/demultiplexer such as a WDM filter. The M/D106is connected by the optical fiber line22to the OLT12to receive an optical downstream signal from the OLT12. As already described, in the TDM/WDM-PON, downstream and upstream optical signals are transmitted on optical bands that are different between downstream and upstream. By means of the difference in wavelength, upstream and downstream optical signals can be separated from each other by the M/D106. Thus, upstream and downstream optical signals can be separately processed, i.e. multiplexed and demultiplexed by the M/D106.

The M/D106has its output122connected to an input of the optical signal receiver108to transmit a downstream optical signal received from the OLT12to the optical signal receiver108. The optical signal receiver108converts the downstream optical signal provided from the M/D106into a corresponding downstream electric signal.

The optical signal receiver108includes a VW filter124. The VW filter124can variably or adaptively set its passable, or transmissive, wavelength for a downstream optical signal received by the optical signal receiver108according to the content of a notice provided from the controller120. The downstream optical signal the optical signal receiver108receives is thus filtered by the VW filter124into the transmissive wavelength thus set. Namely, only the optical signal component having its wavelength matching with the transmissive wavelength set by the controller120is allowed to pass the filter124. It will be described later in detail how the controller120controls the transmissive wavelength of the VW filter124.

The optical signal receiver108also includes a opto-electric conversion element (O/E element)126such as a photodiode. The O/E element126is arranged so as to receive an optical signal passing the VW filter124. For example, the VW filter124may have its output128connected to an input of the O/E element126. The O/E element126is so arranged to be able to receive downstream optical signals at least on a wavelength band that is settable by the OSU26. The O/E element126converts the downstream optical signal having passed the VW filter124into a corresponding downstream electric signal.

The optical signal receiver108is connected by a communication line132to the electric signal receiver112. The downstream electric signal converted by the O/E element126is delivered to the electric signal receiver112on the line132.

Furthermore, the optical signal receiver108may include a power detector134for measuring the optical power intensity of a downstream optical signal the receiver108has received.

The electrical signal receiver112separates the downstream electric signal thus received into a downstream data signal and a downstream control signal. The receiver112is connected by a communication line134to the interface114to transmit the downstream data signal to a user terminal via the interface114. The receiver112is also connected by a communication line136to the controller120to transfer the downstream control signal to the controller120.

The interface114functions as transmitting a downstream data signal to and receiving an upstream data signal from a user terminal, not shown, on a communication line138to thereby communicate with the user terminal. The interface114also communicates with the controller120on a communication line140to thereby be controlled by the controller120.

The interface114is connected by a communication line142to the buffer116. The buffer116receives an upstream data signal supplied from the user terminal via the interface114to temporarily store the received upstream data signal in the buffer116.

The buffer116is connected by a communication line144to the controller120. The buffer116also has its output146connected to an input of the electric signal transmitter118. The buffer116gives a notice on the amount of data stored therein, i.e. buffering amount, to the controller120. The buffer114reads out the stored upstream data signal in response to an instruction issued from the controller120to deliver the read-out upstream data signal to the electric signal transmitter118.

The electrical signal transmitter118has its input148connected to an output of the controller120and its output152connected to an input of the optical signal transmitter110. The electrical signal transmitter118produces an upstream electric signal from the upstream data signal received from the buffer116in response to an upstream control signal received from the controller120. The transmitter118delivers the produced upstream electric signal to the optical signal transmitter110.

The optical signal transmitter110converts the delivered upstream electric signal into a corresponding upstream optical signal. The optical signal transmitter110may include a TLD adapted to vary the wavelength of the converted optical signal. The wavelength of the upstream optical signal is set on the basis of on the content of a notice given from the controller120.

The optical signal transmitter110has its output154connected to an input of the M/D106. The upstream optical signal produced by the optical signal transmitter110is sent via the M/D106over the optical transmission channel16to the OSUs26.

The controller120includes a signal reader162for reading information which is necessary to establish communication with the OSUs26and contained in a downstream control signal. The information read by the signal reader162may include instructions on the transmission bandwidth, transmission timing and transmission wavelength of an upstream optical signal and the reception wavelength of a downstream optical signal.

The controller120includes a signal generator164for producing an upstream control signal to inform the OSUs26of a buffering amount received from the buffer116. The upstream control signal produced by the signal generator164is delivered to the electric signal transmitter118on the line148.

The controller120includes a transmission setter166for setting conditions such as the wavelength, transmission timing and transmission wavelength of an upstream optical signal on the basis of information read by the signal reader162, and giving instructions on the set conditions via the electric signal transmitter118to the optical signal transmitter110.

The controller120includes a VW filter controller168for controlling the VW filter124to set and modify its transmissive wavelength. The controller120is connected by a communication line170to the optical signal receiver108so as to control the VW filter124in the receiver108. The VW filter controller168can set the wait time TT-FLfor a discovery gate message sent from the OSU26to thereby control timing at which the transmissive wavelength is to shift.

The controller120includes an interval detector172for detecting a time interval at which bandwidth allocation signals, such as gate messages, transmitted from the OSU26are received. As described previously, the period of bandwidth allocation signals corresponds to the dynamic bandwidth allocation (DBA) period. Therefore, the interval detector172can determine a DBA period on the basis of a detected interval at which the bandwidth allocation signals are received.

The controller120includes a discovery period calculator, or TDIScalculator,174adapted for calculating a discovery period TDISby multiplying a DBA period by a constant α defined as a parameter as described earlier. If the controller120in one of the ONUS14which is not registered yet in any of the OSUs26is running, the controller120periodically varies the transmissive wavelength of its VW filter124on the basis of the discovery period TDIScalculated by the discovery period calculator174such that the wait time TT-FLat each transmissive wavelength of the filter124is rendered longer than the discovery period TDIS, that is to say, such that above-mentioned Expression (1) becomes satisfied.

Now, in operation, an ONU will be registered in the following fashion, according to the present illustrative embodiment as applied to the standard discovery sequence described earlier.

The discovery sequence will start, for example, whenever unregistered one of ONUS14is connected to the optical network system. When an ONU14already registered in either of the OSUs26a-26dis restarted, this ONU14is once deregistered and again to be registered through a discovery sequence.

In the instant illustrative embodiment, it is assumed that the OSUs26a,26b,26cand26dtransmit downstream optical signals having wavelengths λ1, λ2, λ3 and λ4, respectively. The respective OSUs26ato26dperiodically transmit bandwidth allocation signals, more specifically gate messages, at a DBA period that is common to every ONU14. The allowable range within which DBA periods are settable by the OSUs26is preset at the onset of operation of the system, and is notified to the ONUS14. At least one of the OSUs26transmits an acknowledgement request signal, which may be a discovery gate message in the present embodiment, toward every ONU14in a discovery period TDIS, which is set to a times of the DBA period.

In the embodiment, the ONU14, when registered in none of the OSUs26, sequentially changes the transmissive wavelength of its VW filter124and adjusts the wait time TT-FLon each wavelength to thereby wait for a discovery gate message from the OSU26.

Now, with reference toFIG. 4, the VW filter controller168in the unregistered ONU14first sets the transmissive wavelength of its VW filter124to a certain wavelength, which may or may not be predetermined, and has the wait time TT-FLset for a discovery gate message to be supplied from the OSU26(step S1).

The transmissive wavelength of the VW filter124is set according to an instruction notified from the VW filter controller168. In the instant embodiment, the TDM/WDM-PON10includes the four OSUs,26ato26d, to which respectively different transmissive wavelengths λ1, λ2, λ3 and λ4 are assigned. Therefore, the transmissive wavelength of the VW filter124is set to either one of the wavelengths λ1, λ2, λ3 and λ4.

The discovery period calculator174in the unregistered ONU14in question multiplies the constant α by an arbitrary DBA period within the allowable range available for the TDM/WDM-PON10to thereby obtain a discovery period TDISas an expected discovery period. Under the condition of Expression (1), i.e. TT-FL>TDIS, the wait time TT-FLis set larger than the expected discovery period TDIS. As described previously, the constant α is a preset parameter, of which the ONUS14are informed.

The unregistered ONU14receives the gate message from the OSU26(step S2). As described previously, the respective OSUs26ato26dperiodically transmit gate messages at the DBA period to every ONU14. Accordingly, whichever the transmissive wavelength of the VW filter124is set in step S1among the wavelengths λ1, λ2, λ3 and λ4, the unregistered ONU14can receive a gate message through the VW filter124.

FIG. 5shows the frame configuration of a gate message stipulated in IEEE 802.3av described earlier. In the figure, as with a general format for use in showing frames, numerals shown rightward the respective fields indicate the amount of information applied to the corresponding fields in octets. For example, the notation “ 0/4” shown on the right side of the fields “Grant#1 Start time” through “Grant#4 Start time” indicates that 0 or 4 octets can be applied to the fields. Such is the case with the notation “ 0/2” on the right side of the fields “Grant#1 length” through “Grant#4 Length”. In this embodiment, the amount of information of the field “Reserved/Pad” is between 15 and 39 octets, inclusive.

Types of frame are distinguishable by operation codes (Opcodes). The gate message is identified by the field “Opcode=0x0002”. When the unregistered ONU14receives a gate message, the signal reader162in the unregistered ONU14reads the field on the opcode contained in the gate message to recognize a reception of the gate message.

The discovery period TDISis usually set to, for example, about 100 times as long as the DBA period. That is to say, the constant α turns out to be 100. As previously described, the unregistered ONU14sets the wait time TT-FLto a value larger than the expected discovery period TDIS. The unregistered ONU14may thus receive two or more gate messages on the transmissive wavelength of the VW filter124set in step S1within the wait time TT-FL.

Then, the unregistered ONU14updates the wait time TT-FL(step S3). Because the unregistered ONU14may receive two or more gate messages in step S2, the interval detector172can measure an interval, at which gate messages are received, as a DBA period. The discovery period calculator174multiplies the DBA period by the constant α to thereby be able to calculate and determine the discovery period TDIS.

The VW filter controller168or the like in the unregistered ONU14updates the wait time TT-FLon the basis of the determined discovery period TDIS. In case where the wait time TT-FLset in step S1is shorter than the determined discovery period TDIS, the wait time TT-FLis modified so that it becomes longer than the discovery period TDISso as to satisfy the condition given by Expression (1).

On the contrary, even when the wait time TT-FLset in step S1is longer than the determined discovery period TDIS, the wait time TT-FLis modified to its minimum value within a possible range dependent upon the accuracy in reception of the ONU14to the extent that the condition of Expression (1) becomes satisfied.

After updating the wait time TT-FLin step S3, the unregistered ONU14waits for a discovery gate message while periodically changing the transmissive wavelength of the VW filter124per the updated wait time TT-FL. The waiting for a discovery gate message will be described below.

When the DBA period, which is a time interval in which bandwidth allocation signals can be received, is modified during waiting for an acknowledgement request signal such as discovery gate message, the unregistered ONU14updates the wait time TT-FLagain on the basis of the modified DBA period.

The signal reader162in the unregistered ONU14makes a determination as to whether or not the ONU14receives a discovery gate message (step S4).

If the signal reader162determines that the ONU14receives a discovery gate message (Yes, in step S4), the unregistered ONU14will conduct a discovery sequence, described previously, with either one of the OSUs26ato26dwhich is indicated during the discovery sequence. Thus, subsequent, normal operation for registering the unregistered ONU14in the OLT12will be performed.

On the contrary, if the signal reader162of the ONU14fails to receive a discovery gate message, the control advances to step S5in which the ONU14stands by without proceeding to a discovery sequence, and passes the loop of steps S4and S5until the wait time TT-FLupdated in step S3expires.

More specifically, the controller120in the unregistered ONU14makes a decision, in step S5, as to whether or not the wait time TT-FLexpires. If the controller120determines that the wait time TT-FLhas not yet expired (No, in step S4), the registering control goes back to step S4. Thus, the signal reader162will proceed again to the decision loop, steps S4and S5, to await reception of a discovery gate message.

If the controller120determines that the wait time TT-FLexpires (Yes, in step S4), the VW filter controller168changes and sets the transmissive wavelength of the VW filter124to a different wavelength (step S6). In this embodiment, the transmissive wavelength is sequentially changed whenever the wait time TT-FLhas expired without receiving any discovery sequence. More specifically, the transmissive wavelength of the VW filter124may be changed in the order of λ1, λ2, λ3 and λ4. After having changed the transmissive wavelength of the VW filter124, the registering control goes back to step S4. Thus, the signal reader162will again proceed to making a determination on reception of a discovery gate message.

As described so far, in the instant embodiment according to the present invention, the unregistered ONU14may calculate and determine the discovery period TDIS, on the basis of the interval at which bandwidth allocation signals can be received. Therefore, the ONU14can reliably set the wait time TT-FLwhich satisfies the condition given by Expression (1).

Furthermore, because the ONU14in the present embodiment is adapted to calculate the discovery period TDIS, the wait time TT-FLcan be set to its possible minimum value while satisfying the condition given by Expression (1). Consequently, it can take a shortened period of time until it is timed out while the wavelength of a discovery gate message remains inconsistent with the transmissive wavelength of the VW filter124. As a result, the discovery sequence can be started and completed in a shorter time.

In the TDM/WDM-PON10, there may be a time when one or more of the OSUs26ato26dis/are not in operation. Under the circumstances, it is possible that the transmissive wavelength of the VW filter124is set in step S1to a transmission wavelength that has already been allotted to one of the OSUs26that is not in operation. In this case, the unregistered ONU14cannot receive a bandwidth allocation signal in step S2. Accordingly, the system is adapted such that the ONU14may skip steps S2and S3to transfer to step S4whenever the unregistered ONU14fails to receive in step S2a bandwidth allocation signal from the OSU26within the wait time TT-FL. When the ONU14receives a bandwidth allocation signal during a period of waiting for receipt of an acknowledge request signal in steps S4-S6, the ONU14may change the wait time TT-FL.

FIG. 6shows a control flow in which the ONU14waits for receipt of an acknowledge request signal such as a discovery gate message, regardless of receipt of a bandwidth allocation signal such as a gate message. In the flowchart, step S2is changed to as a decision step. Furthermore, according to the flowchart, the control is modified such that the waiting operations of an acknowledge request signal shown as steps S4-S6may be performed while awaiting receipt of a gate message.

In the embodiment in which the optical signal receiver162includes the power detector134, the system may be adapted such that, the transmissive wavelength of the VW filter124may be changed even sufficiently before the wait time TT-FLset in step S1expires when the optical signal receiver108fails to receive a gate message during a predetermined period of time in step S2, for example, when the power detector134cannot sense any optical power intensity in a predetermined period of time.

In one instance, the predetermined period of time is preferably equal to the maximum value of a DBA period applicable to the OSU26. If the unregistered ONU14receives no gate message on a transmissive wavelength until the maximum time of the DBA period expires, more specifically, if the power detector134cannot sense any significant optical intensity, it may be expected that the OSU26to which a transmission wavelength coincident with the transmissive wavelength is assigned would not be in operation. Accordingly, once it is determined that no gate message was received during the period corresponding to the maximum value of the DBA period, the OSU26thus determined inactive changes the transmissive wavelength, whereby the wait time for the unregistered ONU14can be shortened.

FIG. 7is a flowchart for use in understanding an operation in which, when the ONU14does not receive any gate message within the predetermined time (step S21), the VW filter controller168immediately changes the transmissive wavelength of the VW filter124(step S22).

Alternatively, the system may be adapted such that the wait time for receiving a gate message from the inactive OSU26in step S22may be set to twice as much as the maximum value of the DBA period applicable to the OSU26. The ONU14can thus determine with more precision that the OSU26associated with the selected transmissive wavelength of the VW filter124is inactive because the transmissive wavelength of the VW filter124is changed only in a case where a gate message is not detected in two successive DBA periods. Of course, the wait time for receiving a gate message may be arbitrarily set in terms of quick reception and precise detection of a gate message.

In the illustrative embodiment, when the wait time TT-FLis set in the way described above, it will be described by referring toFIG. 8how a sequence of operations proceeds until the unregistered ONU14receives a discovery gate message.

For illustration, the OSU26anow sends a discovery gate message on wavelength λ1 in the discovery period TDISof 100 ms, for example. Unregistered one of the ONUS14performs steps S2and S3after the transmissive wavelength is set to the wavelength λ1 subsequently to the startup of the ONU14, and the wait time TT-FLis set to 200 ms to satisfy the condition given by Expression (1).

The unregistered ONU14further waits for a discovery gate message while changing the transmissive wavelength of the VW filter124to λ1, λ2, λ3 and λ4 in turn in steps S4-S6described above. For changing the transmissive wavelength of the VW filter124, it takes a wavelength conversion time Ttransof 100 ms, for example.

In the embodiment shown inFIG. 8, the worst condition is assumed, under which at the moment that the wavelength is changed from λ1 to λ2 after the unregistered ONU14is booted up, the first discovery gate message is transmitted.

The number N of transmissions of discovery gate messages, required until the unregistered ONU14can receive as shown inFIG. 8, may be given by following Expression (3):
N={TT-FL×(nλ−1)+Ttrans×(nλ)}/TDIS+1  (3),
where the term nλis the number of transmissive wavelengths that are set when the VW filter124operates to sweep the transmission wavelength of downstream optical signals. In the present embodiment, the term nλis equal to “4” because four wavelengths λ1, λ2, λ3 and λ4 are set as the transmissive wavelengths of the VW filter124.

In the present embodiment, when the 11th discovery gate message is transmitted to the unregistered ONU14, the ONU14in which the transmissive wavelength of the VW filter124is set to λ1 may receive the discovery gate message. Accordingly, it takes 1100 ms at the maximum until the unregistered ONU14receives the discovery gate message.

In comparison, a sequence of operations performed until the unregistered ONU14receives a discovery gate message in a case where the wait time TT-FLis set using the earlier-described conventional maximum assumption method will be described by referring toFIG. 9.

It is now assumed that the range of a set value of the DBA period is 1 to 10 ms and that the constant α is 100. Thus, possible values of the discovery period TDISmay be 100 to 1000 ms. It is also assumed that the unregistered ONU14sets the wait time TT-FLto 2000 ms on the basis of the maximum value (1000 ms) of the discovery period TDIS.

The remaining in this example shown inFIG. 9are similar to those shown inFIG. 8. It is then derived from Expression (3) in the maximum assumption method that the unregistered ONU14would receive the 65th discovery gate message. Accordingly, it takes 6500 ms at the maximum until the unregistered ONU14receives the discovery gate message.

As can be definite from above, with the illustrative embodiment of the present invention, since the wait time TT-FLis set in the way described so far, it takes much shorter time until a discovery gate message is received than with the conventional maximum assumption method.

The entire disclosure of Japanese patent application No. 2013-054815 filed on Mar. 18, 2013, including the specification, claims, accompanying drawings and abstract of the disclosure, is incorporated herein by reference in its entirety.

While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the present invention to accomplish the advantages described so rat.