Central-office termination apparatus of adaptively changing connection to subscriber-terminal terminator and path switching method

A central-office termination apparatus includes plural termination devices and a termination device distributor. Each termination device includes a buffer, a subscriber-terminal terminator distributor and a scheduler. The buffer has a through queue and one or more switching queues. The subscriber-terminal terminator distributor transfers a packet meant for one subscriber-terminal terminator under switching to the switching queue, and transfers another packet meant for another subscriber-terminal terminator not under switching to the through queue. The scheduler reads out packets from the switching and through queues. The termination device distributor transfers a received packet to a termination device having the subscriber-terminal terminator registered for which that packet is meant.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a central-office termination apparatus capable of adaptively changing connection to a subscriber-terminal terminator and a path switching method of changing connection between a subscriber-terminal terminator and central-office termination apparatuses.

2. Description of the Background Art

Recently, in order to provide high-speed broadband service to private premises, services, called an FTTH (Fiber To The Home), using optical fiber transmission lines in transmission channels has been developed. In the provision of the broadband services using the FTTH, an optical access network called a passive optical network (PON) for subscriber terminals is frequently applied.

The PON is configured so that a single central-office termination apparatus, e.g. an optical line terminal (OLT) and a plurality of subscriber-terminal terminators, e.g. optical network units (ONUs) are connected by means of a single optical cable split by an optical passive device called an optical splitter or coupler. In the PON, the optical fiber or OLT is commonly used by a plurality of subscriber terminals to economically provide the FTTH services.

As a sort of PON, there is so-called 10-Gigabit Ethernet (Trademark) PON (10G-EPON) as referred to in IEEE (Institute of Electrical and Electronics Engineers), Std 802.3av-2009. In that sort of PON, a Time Division Multiple Access (TDMA) technique is applied to communication from ONUs to an OLT, i.e. upstream communication, in order to avoid collision of signals transmitted from the ONUs with each other. The PON using the TDMA technique is often called a TDM-PON.

In response to prospective increase of communication demand in optical access networks, research and development are conducted on a WDM (Wavelength Division Multiplexing)/TDM-PON as one of the next generation PONs using transmission rate exceeding 10 Gbps. The WDM/TDM-PON constructs a plurality of TDM-PONs on a single PON infrastructure by the WDM technique, as refer to Japanese patent laid-open publication No. 2011-55407. By means of the TWDM-PON, the transmission capacity of the PON infrastructure can be increased.

The OLT in the TWDM-PON disclosed in Japanese patent laid-open publication No. 2011-55407 is provided with a plurality of optical transmitter/receivers and a controller for controlling the TWDM-PON. The optical transmitter/receivers are connected to a plurality of ONUs via an optical coupler.

In the upstream communication, receiving wavelengths are fixedly allocated to the optical transmitter/receivers of the OLT so as not to overlap with each other between the optical transmitter/receivers. In that solution, the transmission wavelengths of optical transmitter/receivers of the ONU are adapted changeable, thereby adaptively switching connections between the optical transmitter/receivers of the OLT and the ONU. In a downstream communication from the OLT to the ONU, likewise the upstream communication, the transmission wavelengths are fixedly allocated to the optical transmitter/receivers of the OLT, and the reception wavelengths of the optical transmitter/receivers of the ONU are adapted changeable, thereby adaptively switching connections between the optical transmitter/receivers of the OLT and the ONU. Thus, the TWDM-PON is advantageous in load distribution responsive to traffic fluctuation, higher reliability by path switching diversity during failure and power saving by sleep of the optical transmitter/receivers and device circuitry during low load.

In the TWDM-PON, when the connections of the OLT to the ONUs are adaptively switched in the downstream communication, the switching of the optical transmitter/receivers of the OLT and the switching of the reception wavelengths of the ONU are carried out. During a switching period of time until the reception wavelength of the ONU is switched to a new wavelength, the ONU cannot receive a packet of the downstream communication, i.e. downstream packet. However, in a multimedia application, it is preferable that packet loss would not occur during the switching period of time, and thus uninterrupted switching process is required.

Therefore, in order to avoid packet loss in the downstream communication during the switching period of time, it is necessary that the ONUs are adapted for buffering packets meant for that ONU currently under switching.

In a solution proposed for buffering inputted packets and switching a communication path, a buffer is arranged at the stage in front of a switch for switching a path and the switch switches the path in accordance with the destinations of inputted packets, refer to Japanese patent laid-open publication No. 229404/1998.

In the TWDM-PON, the OLT identifies downstream packets in terms of respective ONUs, and then, distributes the packets to optical transmitter/receivers to which the transmission wavelengths corresponding to the reception wavelengths of the respective ONUs are allocated.

When the configuration disclosed in Japanese patent laid-open publication No. 229404/1998 is applied to carrying out uninterrupted path switching in the TWDM-PON, it would be necessary to provide the buffers at the stage in front of the switch correspondingly in number to the ONUs contained in the TWDM-PON. Therefore, if a lot of ONUs are contained in the system, circuit scale is increased. Since the buffer needs its capacity sufficient for storing packets receivable during the switching period of time, if the time for the switching is lengthened, it would be necessary to increase the buffer capacity. The increase of the circuit scale or the buffer capacity would cause a problem in implementing the system.

By contrast, in a system to which a common buffer scheme commonly using a buffer is applied, as the number of the contained ONUs is increased, information for managing addresses of the buffer would be increased. Therefore, if a lot of ONUs are contained in the system, a memory with large capacity is required specifically for managing the addresses, thus also causing a problem in implementing the system.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an OLT capable of adaptively changing connection to an ONU, and a path switching method of adaptively changing connection between an ONU and an OLT, free from packet loss even when many ONUs are contained in the system, without increase of circuit scale.

In accordance with the present invention, a central-office termination apparatus, or an optical line terminal (OLT), comprises a plurality of termination devices, or optical subscriber units (OSUs), and an OSU distributor. Each of the OSUs includes a buffer, a subscriber-terminal terminator distributor, or an optical network unit (ONU) distributor, and a scheduler. The buffer includes a through queue and one or more switching queues. The ONU distributor transfers a packet destined for one ONU under switching to the switching queue, and transfers another packet destined for another ONU not under switching to the through queue. The scheduler reads out packets from the switching and through queues. The OSU distributor transfers a received packet to an OSU having the ONU registered for which the packet is destined.

Further in accordance with the invention, a path switching method to be performed in a telecommunications network provided with the above-described OLT comprises the following steps. The OSU distributor changes an OSU, to which a packet destined for the ONU under switching is to be sent, from the OSU on which the ONU under switching is registered, to an OSU on which the ONU under switching is to be registered. The ONU distributor provided in the OSU on which the ONU under switching is to be registered references a learning table, and transfers, when the ONU under switching has been learned, a packet destined for the ONU to a process queue that has been learned, or transfers, when the ONU under switching has not been learned, the packet destined for the ONU to a process queue not in use to register the process queue on the learning table. After a packet destined for the ONU under switching and stored in the OSU on which the ONU under switching is registered has run out, the OSU to which the OSU on which the ONU under switching has been registered is switched starts transmission of a packet destined for the ONU under switching. After the packet destined for the ONU under switching and stored in the process queue has run out, the ONU under switching is released from the learning table, and a packet is transmitted via the through queue.

According to the present invention, the OLT and path switching method can restrain or minimize the number of the ONUs on which the switching can simultaneously be carried out, and provide the switching queues holding packets during the switching fewer in number than the ONUs contained in the system.

In addition, since the number of the queues is reduced, the memory capacity required for address management of the buffer can be reduced. Accordingly, uninterruptedly switchable OLTs can be economically provided.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An illustrative embodiment of the present invention will be described with reference to the figures. The figures conceptually or schematically show the constituent elements to the extent to which the invention is understandable. The preferred embodiment of the present invention will be described with the numerical or other conditions as preferred exemplification. Therefore, the present invention is not to be understood restrictive to the specific embodiment or examples. Those skilled in the art may change or modify the illustrative embodiment so as to accomplish the advantages of the present invention without departing from the scope and spirit of the invention. In the figures, part of the circuitry not directly relevant to understanding of the present invention is not shown, and detailed description thereof will not be made in order to avoid redundancy.

With reference toFIG. 1, an example of the configuration of a TWDM-PON including an OLT of the present invention will be described.FIG. 1is a schematic block diagram showing the TWDM-PON. The TWDM-PON is an optical access network using a PON system. In the TWDM-PON, downstream signals from the OLT to ONUs and upstream signals from the ONUs to the OLT are transmitted and received. The upstream and downstream signals include data signals transmitted/received between an upper-side or host network, not shown, connected to the OLT and a user terminal, not shown, connected to the ONU, and a control signal for use in establishing a PON link. Hereinafter, the data signal (often referred to as a downstream packet) contained in the downstream signal will be mainly described, and the control signals contained in the upstream and downstream signals will not so often focused.

The TWDM-PON 10 includes a single OLT100, a plurality of ONUs300ato300m, where m is an integer equal to or more than two, and an optical splitter400serving as an optical passive element. The OLT100and optical splitter400, as well as the ONU300ato300mand optical splitter400are respectively interconnected by optical fiber lines30ato30n, and32ato32m.

The OLT100includes an LLID (Logical Link Identification) identifier110, an OSU (Optical Subscriber unit) distributor120, a plurality of OSUs200ato200n, where n is an integer equal to or more than two, a plurality of optical transmitter130ato130nand an OLT controller140, which are interconnected as illustrated.

The LLID identifier110is connected to the OSU distributor120. The LLID identifier110reads information of a downstream packet12inputted from the upper-side network to identify an ONU for which the packet is to be destined. For instance, as the identifying information of a downstream packet12, a virtual LAN (VLAN) ID (VID) contained in a packet (frame) of Ethernet (Trademark) may be used. Note that signals, data or packets are designated with the reference numerals of connections on which they are conveyed. The LLID identifier110has an LLID identification table112for use in correlating a VID with an LLID. Basically, LLIDs are correlated with connected ONUs on a one-to-one basis. Therefore, the LLID identifier110can use the LLID identification table112to identify a destined ONU300from the VID of a downstream packet12. The LLID identifier110adds to the downstream packet the LLID correlated with the ONU300for which the packet is destined and transfers as the downstream packet14to the OSU distributor120.

The OSU distributor120is connected to the plurality of OSUs200ato200nthrough signal lines16ato16n. In the TWDM-PON of the illustrative embodiment, each of the ONUs300ato300nmay be registered on one of the plurality of OSUs200ato200n. The OSU distributor120has an LLID allocation table122for use in correlating the LLIDs with the OSUs. The OSU distributor120references the LLID allocation table122with the LLID of the received downstream packet14to identify an OSU200on which the ONU300for which the packet is destined is recorded. The OSU distributor120transfers the downstream packet14to the identified OSU200.

The OSUs200ato200nare respectively connected to the optical transmitters130ato130nthrough signal lines18ato18non a one-to-one basis. To the optical transmitters130ato130n, wavelengths λ1 to λn, different from each other, are fixedly allocated respectively. The optical transmitters130ato130nare connected to the ONUs300ato300mvia the optical splitter400.

The downstream packet16inputted to the OSU200is transmitted to the ONU300, for which the packet is destined, via the connected optical transmitter130on the wavelength allocated to that transmitter. Since the OSUs200ato200nare respectively connected to the optical transmitter130ato130nvia signal lines18ato18non a one-to-one basis, the transmission wavelength of downstream packets is determined in accordance with the OSU on which the ONU for which the packet is destined is recorded. Therefore, in the following description, the wavelengths allocated to the optical transmitters130ato130nconnected to the OSUs200ato200nare often called the wavelengths allocated to the OSUs200ato200n.

Each of the OSUs200ato200nincludes an ONU distributor210, a buffer220, a scheduler250and a control signal generator260, which are interconnected as shown inFIG. 2. The buffer220includes a single through queue222and one or more switching queues224ato224k, where k is an integer equal to or more than two, arranged in parallel to each other. The number k of the switching queues is correspondent with the number of paths or routes that can simultaneously be switched. It is then preferable that a plurality of switching queues224are arranged. If the number k of the switching queues224is increased, the circuit scale is increased. It is therefore preferable that the number k of the switching queues224is set less than the number of the ONUs300registerable on the OSUs200to restrain or minimize the increase of the circuit scale.

When the destination of packets16is not an ONU under switching, namely, not an ONU that is being registered on an OSU different from the OSU on which the ONU is currently registered, the ONU distributor210transfers packets such as packets20to the through queue222. By contrast, when the destination of packets16is an ONU under switching, the ONU distributor210transfers the packets as one of the packets22ato22kto any of the switching queues224ato224k. It is determined by referencing a learning table212to which of the plurality of switching queues the ONU distributor210should transfer a packet meant for an ONU under switching. When an ONU under switching is unlearned, i.e. that ONU is not registered on, or included in, the learning table, the ONU distributor210transfers an unused switching queue224to that ONU. In turn, the ONU distributor210registers or records the switching queue224to which the packet has been sent on the learning table212, and sets the queue as finished in learning. When the ONU under switching is set as finished in learning, the ONU distributor210transfers the packet to the switching queue224registered on the learning table212.

The control signal generator260generates a control signal such as gate signal24for use in establishing a PON link. Switching of the reception wavelength in the ONU can be directed by using the control signal24.

The through queue222and the plurality of switching queues224ato224kare responsive to packets, when inputted, to issue one of corresponding transmission requests26, and28ato28kto the scheduler250. The scheduler250is operative in response to a transmission request from the queues222and224ato224kor a request for transmitting a control signal24to arrange its output and transmits a downstream signal18to the ONU via the optical transmitter.

Referring back toFIG. 1, the OLT controller140controls the entire OLT and the OSU installed in the OLT. For instance, the OLT controller140carries out updating of the LLID identification table112and LLID allocation table122. The OLT controller140also carries out monitoring of traffic pas sing through the OLT and determines the timing and content of the path switching. In addition, the OLT controller140monitors readout of the OSU from the scheduler250.

Moreover, when changing the OSU on which an ONU under switching will be registered, the OLT controller140releases the old OSU currently under switching, i.e. the OSU having the ONU registered so far, registers the ONU on a new OSU to which the registration is to be switched or shifted, and notifies the new OSU of the fact that that OSU is registered as a new OSU to which the ONU under switching is registered. The notification may be directed to the OSU directly by the OLT controller140or may be added to the packet, which is in turn transmitted to that OSU.

Each of the ONUs300ato300mincludes an optical receiver330for receiving downstream packets. The optical receiver330has its reception wavelength variably set so as to receive downstream packets on a wavelength allocated to the OSU on which the ONU is thus registered.

The TWDM-PON 10 of the illustrative embodiment may be configured, except for the configuration described above, in a similar way to conventional TWDM-PONs.

With reference toFIG. 3, it will be described how to switch paths or routes.FIG. 3is a schematic block diagram useful for understanding the path switching method in the illustrative embodiment. In the disclosure, like components are designated with the same reference numerals. With reference toFIG. 4also, the LLID identification table112comprises the storage fields for storing data of the ONUs36made in association with data of the LLIDs34. In the illustrative embodiment, such data may be numeral keys specific to the LLIDs or ONUs. For instance, the LLID of the first ONU300ais set to a numeral key “10”. More specifically, on the LLID identification table112,FIG. 4, in a line, or record, including the field of the LLID34set to a numeral key “10”, a numeral key “1” is allocated to the ONU36.

At the time point I, the first ONU300ais registered on the first OSU200a. More specifically, as shown inFIG. 5A, on the LLID allocation table122, in a line including the field of the LLID38set to the numeral key “10”, the numeral key “1” is allocated to the OSU40. Therefore, the OSU distributor120transfers the packet14as a packet16afor the first ONU300ato the first OSU200a. This packet14will be transmitted in the form of downstream signal32aon the wavelength λ1 to the first ONU300avia the through queue222, scheduler250and first optical transmitter130ain the first OSU200a.

Now, the OSU, on which the first ONU300aunder switching is to be registered, is switched or shifted from the first OSU200ato the second OSU200b. Such a switching may be carried out, for example, for the purpose of distributing communication load or traffic.

At the time point II, as shown inFIG. 5B, the OLT controller140updates the LLID allocation table122so as to allocate the OSU40in a line, or entry, including the field of the LLID38set to the numeral key “10” to the numeral key “2”. After updating the LLID allocation table122, the packet12destined for the first ONU300atransmitted from the upper-side network to the OLT100is transferred accordingly to the second OSU200bas a packet16b.

At that time, since the first ONU300ahas its wavelength set to λ1, it cannot receive the downstream signal from the second OSU200bon the reception wavelength is λ2. Accordingly, it is necessary to the first OSU200ato direct the first ONU300ato switch its reception wavelength to λ2.

However, there is a possibility that packets destined for the first ONU300aremain stored in the first OSU200awhile untransmitted. Moreover, the switching of the reception wavelength in the first ONU300amay require a significant period of time.

Accordingly, it is preferable to take into account a time that would be taken for the wavelength switching in the ONU300aand a time that would be taken until packets remaining stored in the first OSU200ahave run out to direct the first ONU300aso as to render the wavelength switching timed with those times.

After the time point II, the second OSU200breceives the packet meant for the first ONU300a, and then decides whether or not the first ONU300ais an ONU under switching. At the time point II, when the LLID allocation table122is updated, the OLT controller140notifies the second OSU200bthat the first ONU300ais an ONU under switching. By this notification, the ONU distributor210in the second OSU200brecognizes that the first ONU300ais an ONU under switching.

Subsequently, at the time point III, the ONU distributor210decides whether or not the first ONU300ahas been learned in the learning table212. In the instant example, the first ONU300ais not registered as shown inFIG. 6A, and hence the distributor210decides that the first ONU300ais unlearned. The ONU distributor210transfers the packet16bmeant for the unlearned ONU to one of the unused switching queues to make the learning table212learn that switching queue. In this example, as an unused switching queue, the first switching queue224ais chosen. For choosing a switching queue, any of the unused switching queues may be selected in a priority of ascending order of the queue numbers or at random, for example.

After the first switching queue224ais chosen, the packet16bmeant for the first ONU300ais transferred to the switching queue224a #1, and, as shown inFIG. 6B, the fields of the LLID42and queue44in a blank line on the table212have the numeral keys “10” and “1” respectively recorded. Consequently, the first ONU300ahas its state indicated as learned. After the first ONU300ahas become the learned state, the ONU distributor210references the learning table212to transfer the packet meant for the first ONU300ato the first switching queue224a.

Subsequently, at the time point IV, the reception wavelength of the first ONU300ais switched from λ1 to λ2. The timing of the wavelength switching is adjusted so that packets16ameant for the first ONU300aremaining stored in the first OSU200ahave run out by the time point IV. After the switching of the reception wavelength in the first ONU300ais completed, a control signal such as gate signal is transmitted from the second OSU200bto the first ONU300a, and then a response from the first ONU300ais received. Thus, the link of the second OSU200bto the first ONU300ais established.

After that, at the time point V, the scheduler250allows the packets16bstored in the first switching queue224ato be transmitted toward the first ONU300a.

At the time point VI, the learning entry is released. The scheduler250adjusts the timing of reading out the queue so that the packets16bstored in the first switching queue224arun out by the time point VI. The release of the learning entry is carried out by deleting, as shown inFIG. 6C, the values “10” and “1” specifying the first ONU300aand the first switching queue224a, respectively, from the learning table212. The first ONU300ais set to an object not under switching.

After the learning entry is released and the first ONU300ais set to an object not under switching, the packets16bmeant for the first ONU300ais transferred to the optical transmitter130bvia the through queue222and scheduler250in the second OSU200b.

If the OSU, on which the second ONU300bunder switching is to be registered, is switched from the first OSU200ato the second OSU200bduring the path switching of the first ONU300a, it is also possible to carry out the path switching by the above-described steps. In such a case, since the first switching queue224ahas been used, the second switching queue224bmay be used as an unused switching queue. The OLT controller140monitors how the path switching proceeds, and restrains the number of the ONUs on which wavelength switching can simultaneously be carried out so as not to exceed the number of the switching queues.

In accordance with the OLT and the path switching method of the present invention, it is possible to restrain the number of the ONUs on which the switching can simultaneously be carried out and to provide the switching queues for holding packets during the switching fewer in number than the ONUs provided in the system. Since the number of queues is thus reduced, it is possible to save the memory capacity required for address management of the buffer. Accordingly, it is possible to economically provide uninterruptedly switchable OLTs.

Although the example is described above in which each ONU under switching uses one switching queue, the present invention may not be restricted to that specific example. For instance, the path switching of a plural ONU having wavelength switching time equal to each other can be carried out by using one switching queue.

The illustrative embodiment described so far is directed to the TWDM-PON system. The OLT and the path switching method of the present invention may however not be restricted to the TWDM-PON system. The OLT and the path switching method can be applied in general telecommunications networks to path switching intended for load distribution or uninterrupted switching of a redundant or diversity path when failure occurs.

The entire disclosure of Japanese patent application No. 2013-180982 filed on Sep. 2, 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 embodiment, it is not to be restricted by the embodiment. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.