Optical communication system, station-side device, home-side device, and optical communication method

Abstract

This optical communication system comprises: a station-side device that has an optical transceiver of a subcarrier multiplexing scheme; a plurality of home-side devices that each have an optical transceiver of a subcarrier multiplexing scheme; and an optical distribution network that connects the station-side device and the plurality of home-side devices. The station-side device transmits a plurality of downlink subcarriers to the plurality of home-side devices via a time division scheme, receives an uplink subcarrier from at least one of the plurality of home-side devices via the subcarrier multiplexing scheme, and demodulates the uplink subcarrier. At least one of the plurality of home-side devices receives the plurality of downlink subcarriers from the station-side device, demodulates a demodulatable downlink subcarrier among the plurality of downlink subcarriers, and transmits the uplink subcarrier to the station-side device. The total number of downlink subcarriers that the station-side device can transmit is less than the total number of uplink subcarriers that the station-side device can receive.

Description

Optical communication system, station side device, home side device, and optical communication method

[0001] This application claims priority to Japanese Patent Application No. 2024-134866, filed on August 13, 2024, and incorporates by reference all of the contents of that application.

[0002] Patent Documents 1 and 2 describe an optical transceiver employing a subcarrier multiplexing method (hereinafter also referred to as a "subcarrier multiplexing method") that generates multiple subcarriers (subcarriers) with different wavelengths using a single light source through digital signal processing and transmits the generated subcarriers to an optical fiber. Specifically, Patent Document 1 describes an optical communication system in which one optical line terminal employing the above-mentioned optical transceiver and multiple optical line terminals are connected in a two-core P2MP (Point To Multi Point) connection configuration, and subcarrier access multiplexing is performed.

[0003] Patent Document 2 describes an optical communication system in which one optical line terminal and one optical network terminal using the above-mentioned optical transceiver are connected via a single-core P2P (Point To Point) and single-core bidirectional transmission is performed by separating the transmitting and receiving subcarriers. In this way, the subcarrier multiplexing optical transceiver can be installed in each of the optical line terminal and optical network terminal connected in a connection topology such as P2MP or P2P.

[0004] US Patent Application Publication No. 2021 / 0021365 US Patent Application Publication No. 2022 / 0376812

[0005] The optical communication system disclosed herein includes an optical line terminal (OLT) having a subcarrier multiplexing optical transceiver, multiple optical network units (ONUs) each having a subcarrier multiplexing optical transceiver, and an optical distribution network connecting the ONU to the multiple ONUs. The ONU transmits multiple downstream subcarriers to the multiple ONUs using a time division multiplexing method, and receives and demodulates upstream subcarriers from at least one of the multiple ONUs using a subcarrier multiplexing method. At least one of the multiple ONUs receives the multiple downstream subcarriers from the ONU, demodulates demodulatable downstream subcarriers from the multiple downstream subcarriers, and transmits the upstream subcarriers to the ONU. In the ONU, the total number of downstream subcarriers that can be transmitted is smaller than the total number of upstream subcarriers that can be received.

[0006] Fig. 1 is a block diagram showing an example of the configuration of an optical communication system. Fig. 2 is a block diagram showing an example of the configuration of an optical transceiver of a station-side device. Fig. 3 is a block diagram showing an example of the configuration of an optical transceiver of a home-side device. Fig. 4 is an explanatory diagram showing a comparative example of subcarrier allocation. Fig. 5 is an explanatory diagram showing an example of subcarrier allocation. Fig. 6 is an explanatory diagram showing an example of bandwidth adjustment in a HUB and a LEAF.

[0007] In an optical communication system including one optical line terminal and multiple optical network units, it may be desirable to increase the number of optical network units that can be accommodated in one optical line terminal (i.e., the number of optical network units that can be connected to one optical line terminal) in response to an increase in the number of users. However, for example, in a method in which one optical line terminal is allocated one subcarrier each equally for upstream communication and downstream communication, the maximum number of optical network units that can be accommodated in one optical line terminal is limited to half or less of the total number of subcarriers that the optical line terminal can use (see FIG. 4).

[0008] In view of the above-described conventional problems, an object of the present disclosure is to increase the maximum number of optical network units that can be accommodated by one optical line terminal.

[0009] According to the present disclosure, it is possible to increase the maximum number of optical network units that can be accommodated by an optical line terminal.

[0010] The following provides an outline of embodiments of the present disclosure.

[0011] (1) An optical communication system according to an embodiment of the present disclosure includes an optical line terminal (OLT) having a subcarrier multiplexing optical transceiver, multiple optical network units (ONUs) having subcarrier multiplexing optical transceivers, and an optical distribution network connecting the ONU to the multiple ONUs. The ONU transmits multiple downstream subcarriers to the multiple ONUs using a time division multiplexing method, and receives and demodulates upstream subcarriers from at least one of the multiple ONUs using a subcarrier multiplexing method. At least one of the multiple ONUs receives the multiple downstream subcarriers from the ONU, demodulates demodulatable downstream subcarriers from the multiple downstream subcarriers, and transmits the upstream subcarriers to the ONU. In the ONU, the total number of downstream subcarriers that can be transmitted is smaller than the total number of upstream subcarriers that can be received.

[0012] (2) In the optical communication system of (1) above, the optical line terminal may include a bandwidth control unit that shapes the downstream transmission bandwidth to correspond to the total number of the plurality of downstream subcarriers.

[0013] (3) In the optical communication system of (1) or (2) described above, at least one of the plurality of home devices may have a bandwidth control unit that shapes the bandwidth of the upstream transmission to correspond to at least one of the upstream subcarriers.

[0014] (4) In the optical communication system of (1) above, the optical distribution network may be configured by a single-core bidirectional optical fiber.

[0015] (5) An optical line terminal according to an embodiment of the present disclosure is connected to a plurality of optical network units via an optical distribution network, and includes a subcarrier multiplexed optical transceiver and a control unit that controls communication by the optical transceiver. The control unit controls the optical transceiver to transmit a plurality of downstream subcarriers to the plurality of optical network units using a time division multiplexing method and to receive and demodulate upstream subcarriers from at least one of the plurality of optical network units using a subcarrier multiplexing method. The total number of downstream subcarriers that the optical transceiver can transmit is smaller than the total number of upstream subcarriers that it can receive.

[0016] (6) The optical line terminal of (5) above may further include a bandwidth control unit that shapes the downstream transmission bandwidth to correspond to the total number of downstream subcarriers.

[0017] (7) In the optical line terminal of (5) above, the optical distribution network may be configured by a single-core bidirectional optical fiber.

[0018] (8) An optical network unit according to an embodiment of the present disclosure is connected to a light source device via an optical distribution network and includes a subcarrier multiplexing optical transceiver and a control unit that controls communication via the optical transceiver. The number of optical network units is two or more. In at least one of the optical network units, the control unit controls the optical transceiver to receive multiple downstream subcarriers from the light source device, demodulate demodulatable downstream subcarriers from the multiple downstream subcarriers, and transmit upstream subcarriers to the light source device. The total number of downstream subcarriers that one optical network unit can receive is less than the total number of upstream subcarriers that all of the optical network units can transmit.

[0019] (9) The optical network unit of (8) above may further include a bandwidth control unit that shapes the bandwidth of the upstream transmission so that it corresponds to at least one of the upstream subcarriers.

[0020] (10) In the optical network unit of (8) above, the optical distribution network may be configured by a single-core bidirectional optical fiber.

[0021] (11) An optical communication method according to an embodiment of the present disclosure is performed between an optical line terminal and multiple optical network units connected by an optical distribution network, and includes the steps of: the optical line terminal transmitting multiple downstream subcarriers to the multiple optical network units in a time division multiplexing manner; at least one of the multiple optical network units receiving the multiple downstream subcarriers from the optical line terminal and demodulating demodulatable downstream subcarriers from the multiple downstream subcarriers; at least one of the multiple optical network units transmitting upstream subcarriers to the optical line terminal; and the optical line terminal receiving and demodulating the upstream subcarriers in a subcarrier multiplexing manner. The total number of downstream subcarriers that the optical line terminal can transmit is smaller than the total number of upstream subcarriers that it can receive.

[0022] (12) The optical communication method of (11) above may further include a step of shaping the downstream transmission bandwidth in the optical line terminal so that the bandwidth corresponds to the total number of downstream subcarriers.

[0023] (13) The optical communication method of (11) or (12) above may further include a step of shaping the bandwidth of upstream transmission in the optical network unit so that the bandwidth corresponds to at least one of the upstream subcarriers.

[0024] (14) In the optical communication method of (11) above, the optical distribution network may be configured by a single-core bidirectional optical fiber.

[0025] In the optical communication system (1), optical line terminal (5), and optical communication method (11) of this embodiment, the total number of downstream subcarriers that can be transmitted by the optical line terminal is less than the total number of upstream subcarriers that can be received. In the optical line terminal (8) of this embodiment, the total number of downstream subcarriers that can be received by one optical line terminal is less than the total number of upstream subcarriers that can be transmitted by all optical line terminals. Therefore, the optical line terminal can accommodate a number of optical line terminals corresponding to the number of available upstream subcarriers, and can accommodate more than half the total number of available subcarriers. Therefore, the maximum number of optical line terminals that can be accommodated by the optical line terminal can be increased compared to a conventional optical line terminal in which one subcarrier is allocated equally between the upstream communication and the downstream communication of the optical line terminal. Furthermore, in the optical communication system, optical line terminal, optical line terminal, and optical communication method of this embodiment, the optical line terminal and multiple optical line terminals are connected by an optical distribution network composed of optical fiber. Therefore, compared to connections via wireless or other means, high-speed, large-capacity communication is possible, communication stability is improved, and security is strengthened.

[0026] In the optical communication system (2), optical line terminal (6), and optical communication method (12) of this embodiment, the optical line terminal shapes the downstream transmission bandwidth to correspond to the total number of downstream subcarriers, thereby reducing downstream communication problems caused by buffer overflow in the optical line terminal.

[0027] In the optical communication system (3), optical network unit (9), and optical communication method (13) of this embodiment, at least one of the optical network units shapes the upstream transmission bandwidth to correspond to at least one upstream subcarrier. This reduces upstream communication problems caused by buffer overflow in the optical network unit.

[0028] In the optical communication system (4), the optical line terminal (7), the optical network unit (10), and the optical communication method (14) of this embodiment, the optical distribution network is configured with a single-core bidirectional optical fiber. In this way, both downstream and upstream subcarriers can be transmitted over a single optical fiber, thereby improving the efficiency of optical fiber usage and significantly reducing the number of required optical fibers.

[0029] Hereinafter, details of embodiments of the present disclosure will be described with reference to the drawings. Note that at least some of the embodiments described below may be combined in any manner.

[0030] [Configuration Example of Optical Communication System] FIG. 1 is a block diagram showing a configuration example of an optical communication system 200. As shown in FIG. 1, in the optical communication system 200, an optical line terminal 1 and multiple optical network terminals 2 are connected by an optical fiber line 3. Although only one optical network terminal 2 is shown in FIG. 1, an optical network terminal 2 (not shown) is connected to each branch line 5. In this embodiment, communication from the optical line terminal 1 to the optical network terminal 2 is referred to as "downlink communication," and communication from the optical network terminal 2 to the optical line terminal 1 is referred to as "uplink communication." Downlink communication includes "downlink transmission" and "downlink reception." Uplink communication includes "uplink transmission" and "uplink reception." Subcarriers used in downstream communication are also referred to as "downlink subcarriers" or "downlink optical signals." Subcarriers used in upstream communication are also referred to as "uplink subcarriers" or "uplink optical signals." Subcarriers may be indicated using SCi (i is an identification number). Subcarriers SCi are a general term for downstream subcarriers and upstream subcarriers.

[0031] The optical line 3 is configured, for example, by a single-core bidirectional transmission ODN (Optical Distribution Network) in which both downstream optical signals and upstream optical signals are transmitted through a single optical fiber. Specifically, the optical line 3 includes a trunk line 4 extending upstream and a plurality of branch lines 5 branching from the trunk line 4 and extending downstream. Both the trunk line 4 and the branch lines 5 are single-core (single optical fiber) optical transmission paths through which both upstream optical signals and downstream optical signals are transmitted.

[0032] The trunk line 4 and branch lines 5 are connected by an optical coupler 6 that branches optical power. The optical coupler 6 is a passive optical component that does not have a wavelength selection function. The trunk line 4 may be branched into multiple branch lines by another optical coupler provided therein. The branch line 5 may be further branched by another optical coupler provided therein.

[0033] The optical line terminal 1 is installed, for example, in a central office of a telecommunications carrier. The optical line terminal 2 is installed, for example, in the building of a user who uses a communication service. Note that the terms "optical line terminal" and "optical line terminal" indicate the relative positions of the devices and do not limit the installation locations of the devices. A controller 10 is connected to the optical line terminal 1. The controller 10 is, for example, a management computer operated by a communications administrator. The controller 10 may be connected to the optical line terminal 1 via a management network such as a local area network (LAN).

[0034] The optical line terminal 1 (specifically, a switch 12 described later) is connected to an upper network 7, such as a core network, via an upper PHY (Physical Layer) device (not shown). Each optical line terminal 2 (specifically, a switch 22 described later) is connected to a lower network 8, such as an in-home LAN in a user's home, via a lower PHY device (not shown). The optical transceiver 11 of the optical line terminal 1 and the optical transceiver 21 of the optical line terminal 2 are both optical transceivers that transmit signals using subcarrier multiplexing (SCM).

[0035] In the SCM (Sub Carrier Multiplexing) system, a transmitting optical transceiver modulates a baseband signal into an electrical signal (subcarrier) with a width of several GHz, converts the modulated electrical signal into an optical signal, and transmits the converted optical signal to a receiving optical transceiver. Meanwhile, the receiving optical transceiver receives the optical signal, extracts the subcarrier assigned to itself from the received optical signal by filtering, and demodulates it.

[0036] The multiple optical network units 2 exclusively use different subcarriers. The optical line terminal 1 stores information about which optical network unit 2 can demodulate which subcarrier. The optical line terminal 1 broadcasts downstream subcarriers assigned to the optical network units 2 under its control. The multiple optical network units 2 receive the downstream subcarriers and demodulate the downstream subcarriers within the demodulatable range.

[0037] The optical line terminal 1 is capable of demodulating upstream subcarriers from the optical network terminals 2 subordinate thereto, and performs subcarrier multiplexing reception processing for upstream communications. At least one of the optical network terminals 2 transmits upstream subcarriers to the optical line terminal 1. The optical line terminal 1 demodulates the upstream subcarriers from the optical network terminals 2.

[0038] 1, in this embodiment, the optical line terminal 1 is sometimes referred to as a "HUB," the optical network terminal 2 as a "LEAF," and the optical transceivers 11 and 21 as "TRx." "HUB" refers to a device on the line concentrator side of an optical line. "LEAF" refers to a device on the end terminal side of an optical line.

[0039] In this embodiment, when simply referring to "wavelength," it includes a wavelength in the broad sense that can be considered a subcarrier, a unit of an optical path in an optical gateway network, and multiple subcarriers included in a predetermined wavelength band. Specifically, wavelength includes the following concepts: Wavelength in the narrow sense: A wavelength that has a wavelength range corresponding to a wavelength grid and is a unit of an optical path Subcarrier: A narrow range of wavelengths that can be divided into multiple parts and multiplexed within a wavelength in the narrow sense Wavelength in the broad sense: A wavelength that includes a wavelength in the narrow sense, a subcarrier, and multiple subcarriers

[0040] 1, optical line terminal 1 includes optical transceiver 11, switch 12, control unit 13, and circulator 14. In FIG. 1, circulator 14 is disposed outside optical transceiver 11 of optical line terminal 1, but circulator 14 may be housed within the housing of optical transceiver 11 or the housing of optical line terminal 1.

[0041] The optical transceiver 11 of the optical line terminal 1 is an optical module that converts optical signals to and from electrical signals, and may be, for example, a CFP (Centum Form-factor Pluggable) digital coherent optical transceiver conforming to the MSA (Multi-Source Agreement) standard. The optical transceiver 11 may also be an SFP (Small Form Factor Pluggable) optical transceiver. The SFP type is a general term for SFP, SFP+, SFP28, QSFP, QSFP28, and their upwardly compatible pluggable optical modules.

[0042] The optical transceiver 11 has a function for switching the wavelength λk of the optical signal, but the subcarriers used for transmission and reception may be fixed or switchable. At least one carrier (main carrier) with the same wavelength λk is used for transmission and reception between the optical transceiver 11 and the optical network unit 2. Specifically, the optical transceiver 11 uses, for communication with the optical network unit 2, subcarriers assigned to the optical network unit 2 under its control, from the subcarrier range (subcarriers before and after the center frequency) that it can demodulate.

[0043] The frequency range that the optical transceiver 11 of the optical line terminal 1 can modulate and demodulate is a frequency range that includes all subcarriers assigned to the optical network terminals 2 under its control. The subcarrier switching function can be realized, for example, by a subcarrier multiplexing (SCM) coherent optical transceiver ( FIG. 2 ). The subcarrier switching function can be used, for example, to allocate subcarriers to logical channels (e.g., VLANs (Virtual LANs)) in the upper network 7.

[0044] The circulator 14 of the optical line terminal 1 is an optical device having three ports, such that an optical signal input to any port is output from the next port. The trunk line 4 is connected to one port of the circulator 14, and single-core optical fibers 4u and 4d leading to the optical transceiver 11 are connected to the other two ports. The optical fiber 4u is a transmission path for upstream optical signals, and the optical fiber 4d is a transmission path for downstream optical signals. Two-core optical fibers leading to the optical transceiver 11 may be connected to the other two ports of the circulator 14. The circulator 14 may be an optical device having two ports. In this case, the trunk line 4 is connected to one port of the circulator 14, and a single-core optical fiber leading to the optical transceiver 11 for bidirectional transmission is connected to the other port. The circulator 14 may be an optical device having four or more ports. In this case, multiple trunk lines 4 may be connected to the circulator 14. A circulator may not be provided.

[0045] The optical fibers 4u and 4d are connected to the optical transceiver 11 of the optical line terminal 1. The optical transceiver 11 is electrically connected to a predetermined port of the switch 12. Specifically, the optical transceiver 11 is connected to the switch 12 via one interface (a port of the switch 12).

[0046] The switch 12 of the optical line terminal 1 is an integrated circuit, such as an LSI (Large Scale Integration), that has relay functions for the L2 and L3 layers. The switch 12 connects to the optical transceiver 11 as a single port and performs relay processing for the L2 and L3 layers for user frames. A control unit 13 is electrically connected to a predetermined port of the switch 12. The switch 12 and the control unit 13 may be implemented in a single integrated circuit, such as a SoC (System on a Chip).

[0047] The control unit 13 of the optical line terminal 1 is an arithmetic processing unit including, for example, a CPU (Central Processing Unit) 13A and a memory 13B. The memory 13B includes a non-volatile memory in which a predetermined control program and data required for executing the program (e.g., setting information such as subcarriers used by the optical transceiver 11) are recorded, and a volatile memory in which the control program and the like are temporarily loaded.

[0048] The control unit 13 may include a field-programmable gate array (FPGA) or an application-specific integrated circuit (ASIC). The control unit 13 may be configured with at least one of an FPGA and an ASIC. The control unit 13 communicates with the switch 12 via, for example, Ethernet, and with the optical transceiver 11 via serial communication such as I2C (Inter-Integrated Circuit).

[0049] The control unit 13 can acquire predetermined setting information from the controller 10 through control communication with the controller 10. Based on the acquired setting information, the control unit 13 performs various setting processes for the optical line terminal 1 and the optical network terminal 2. For example, the control unit 13 can perform initial setting of the subcarriers used by the optical transceiver 11, setting the quality of service (QoS) of the switch 12, setting the bandwidth of each port of the switch 12, setting a virtual LAN (VLAN), and the like.

[0050] The controller 10 performs management communication with the control unit 13 of the optical line terminal 1. Specifically, the controller 10 transmits setting information of the optical line terminal 1 and setting information of the optical network unit 2 to the control unit 13 of the optical line terminal 1. If the received setting information is setting information for the optical transceiver 11 of the optical line terminal 1, the control unit 13 controls the optical transceiver 11 of the optical line terminal 1 in accordance with the received setting information via serial communication such as I2C.

[0051] The control unit 13 of the optical line terminal 1 transfers the setting information for the optical network terminal 2 to the optical network terminal 2 by control communication via the optical line 3. That is, if the received setting information is setting information for the optical transceiver 21 of the optical network terminal 2, the control unit 13 of the optical line terminal 1 generates an Ethernet ("Ethernet" is a registered trademark) control frame addressed to the optical network terminal 2 and including the received setting information, and outputs the control frame to the switch 12.

[0052] The control frame is transmitted to the control unit 23 of the optical network unit 2 via Ethernet communication. The control unit 23 of the optical network unit 2 controls its own optical transceiver 21 according to the received setting information via serial communication such as I2C. If the TRxs can perform control communication using a dedicated control channel (control wavelength), the setting information of the optical transceiver 21 of the optical network unit 2 may be transmitted via that control communication.

[0053] 1, the optical network unit 2 includes an optical transceiver 21, a switch 22, a control unit 23, and a circulator 24. In FIG. 1, the circulator 24 is disposed outside the optical transceiver 21 of the optical network unit 2, but the circulator 24 may be housed within the housing of the optical transceiver 21 or the housing of the optical network unit 2.

[0054] The optical transceiver 21 of the optical network unit 2 may be the same optical device as the optical transceiver 11 of the optical line terminal 1. The modulation / demodulation range of the optical transceiver 21 may be the same as that of the optical line terminal 1. The modulation / demodulation range of the optical transceiver 21 may be limited to a range close to the subcarrier set for the optical transceiver 21. This has the advantage of making the digital signal processing performance reasonable and achieving economy.

[0055] The circulator 24 of the optical network unit 2 is an optical device with three ports, in which an optical signal input to any port is output from the next port. A branch line 5 is connected to one port of the circulator 24, and single-core optical fibers 5u and 5d leading to the optical transceiver 21 are connected to the other two ports. The optical fiber 5u is a transmission path for upstream optical signals, and the optical fiber 5d is a transmission path for downstream optical signals. Two-core optical fibers leading to the optical transceiver 21 may be connected to the other two ports of the circulator 24. The circulator 24 may be an optical device with two ports. In this case, the branch line 5 is connected to one port of the circulator 24, and a single-core optical fiber leading to the optical transceiver 21 for bidirectional transmission is connected to the other port. The circulator 24 need not be provided.

[0056] The optical fibers 5u and 5d are connected to the optical transceiver 21 of the optical network unit 2. The optical transceiver 21 is electrically connected to a predetermined port of the switch 22. Specifically, the optical transceiver 21 is connected to the switch 22 via one interface (a port of the switch 22).

[0057] The switch 22 of the optical network unit 2 may be the same as the switch 12 of the optical line terminal 1, for example. The control unit 23 of the optical network unit 2 may be the same as the optical line terminal 1, for example, and may include a CPU 23A and a memory 23B. The control unit 23 is electrically connected to a predetermined port of the switch 22. The switch 22 and the control unit 23 may be implemented on a single integrated circuit such as an SoC.

[0058] The memory 23B includes a non-volatile memory that stores a predetermined control program and data required for executing the program (e.g., setting information such as the subcarrier used by the optical transceiver 21), and a volatile memory that temporarily stores the control program, etc. The control unit 23 communicates with the switch 22 via, for example, Ethernet, and with the optical transceiver 21 via serial communication such as I2C.

[0059] When the switch 22 receives an Ethernet control frame addressed to the optical network unit (home device) 2, it transfers the received control frame to the control unit 23. The control unit 23 performs various setting processes for the optical network unit 2 via serial communication such as I2C based on the setting information included in the received control frame. For example, the control unit 23 may perform initial setting of the subcarrier used by the optical transceiver 21, QoS setting of the switch 22, bandwidth setting of each port of the switch 22, VLAN setting, etc.

[0060] If the TRxs can perform control communication using a dedicated control channel (control wavelength), the setting information of the optical transceiver 21 of the optical network unit 2 can be transmitted by the control communication. Also, some or all of the settings of the optical network unit 2 may be performed directly by a controller (not shown) without going through the optical line terminal 1.

[0061] [Optical Transceiver of Optical Line Terminal Device] Figure 2 is a block diagram showing an example of the configuration of the optical transceiver 11 of the optical line terminal device 1. The optical transceiver 11 in Figure 2 is an SCM coherent optical transceiver. Since the wavelength λ (m) is a physical quantity obtained by dividing the speed of light c (m / s) by the frequency f (Hz), in the following explanation, "subcarrier" can also be referred to as frequency. The meanings of "TSm" and "RSm" in Figure 2 are as follows:

[0062] m: Identification number of the electrical signal within the optical transceiver 11. TSm: An electrical signal with identification number m that is the source of the downstream optical signal. Hereinafter, this will also be referred to as the "transmission signal." TSm is generated by dividing a high-speed signal from the interface into multiple low-speed subcarriers.

[0063] RSm: An electrical signal with identification number m based on an upstream optical signal. Hereinafter, this is also referred to as a "received signal." RSm is an electrical signal corresponding to each low-speed subcarrier, and is multiplexed onto a high-speed signal and output to an interface. Note that in upstream communication, inverse MUX may also be performed on a LEAF basis to allocate bandwidth for multiple subcarriers.

[0064] The optical transceiver 11 includes a light source 101, an optical splitter 102, a first signal processing unit (DSP) 103, a DAC (Digital-to-Analog Converter) 104, an optical transmitting unit (SCM) 105, an optical receiving unit (SCM) 106, an ADC (Analog-to-Digital Converter) 107, a second signal processing unit (DSP) 108, a processor 110, a multiplexer (MUX) 111, and a demultiplexer (DEMUX) 112. A downstream optical signal output from the optical transmitting unit 105 is transmitted to the downstream optical fiber 4d as a multiplexed optical signal Oout. An upstream optical signal from the upstream optical fiber 4u is input to the optical receiving unit 106 as a multiplexed optical signal Oin.

[0065] The processor 110 may be an integrated circuit such as an MPU (Micro Processing Unit). The first signal processing unit 103 and the second signal processing unit 108 may be a digital signal processor (DSP). The light source 101 is, for example, a semiconductor laser diode. The laser diode is, for example, a diode with a tunable emission wavelength, but the emission wavelength may also be fixed. The wavelength of the light source 101 corresponds to the wavelength of the subcarrier transmitted and received by the optical line terminal 1.

[0066] The optical splitter 102 splits the output light of the light source 101 in two directions. One of the split lights is sent to the optical transmitter 105 as transmission light. The other of the split lights is sent to the optical receiver 106 as local light. The local light is used for coherent detection in the optical receiver 106. In this way, the light source 101 is used for both transmission and reception processing, and functions as both a transmission light source and a local light source. This allows for the optical transceiver 11 to be made smaller, consume less power, and be less expensive. Instead of providing the optical splitter 102, a transmission light source and a local light source may be provided separately.

[0067] An electrical signal input from the interface of the switch 12 is separated into transmission signals (electrical signals) TSm by a demultiplexer 112 and input to the first signal processing unit 103. The first signal processing unit 103 generates a drive signal for the optical transmitting unit 105 according to the transmission signals (electrical signals) TSm by digital signal processing. The correspondence between the identification number m of the transmission signal TSm and the identification number i of the subcarrier SCi is specified in advance by the processor 110.

[0068] The second signal processing unit 108 performs predetermined digital signal processing on the input signal from the ADC 107 and outputs a received signal (electrical signal) RSm to the multiplexer 111. The multiplexer 111 multiplexes the received signal (electrical signal) RSm into one electrical signal and outputs it to the interface of the switch 12. The correspondence between the identification number m of the received signal RSm and the identification number i of the subcarrier SCi is specified in advance by the processor 110.

[0069] Therefore, the first signal processing unit 103 and the second signal processing unit 108 can transmit and receive at least one subcarrier SCi. This allows multiple subcarriers SCi to be shared or separated. The digital signal processing by the second signal processing unit 108 may include at least one of dispersion compensation, sampling phase synchronization, adaptive equalization, frequency offset compensation, carrier phase recovery, and error correction decoding.

[0070] In this way, the first signal processing unit 103 and the second signal processing unit 108 can perform transmission processing or reception processing for the transmission signal TSm and reception signal RSm for each identification number m corresponding to the subcarrier SCi. Note that the transmission processing and reception processing may be performed simultaneously.

[0071] The DAC 104 converts the digital drive signal generated by the first signal processing unit 103 into an analog drive signal. The analog drive signal is amplified by a driver amplifier or the like and output to the optical transmitting unit 105. The ADC 107 converts the analog electrical signal corresponding to the modulation method such as the power and phase of the signal light input from the optical receiving unit 106 into a digital electrical signal, and outputs the converted digital electrical signal to the second signal processing unit 108.

[0072] The optical transmitter 105 is driven by a drive signal having a signal waveform corresponding to the transmission signal TSm, and modulates the transmission light with a transmission data signal. The optical transmitter 105 outputs the modulated multiplexed optical signal Oout as a downstream optical signal. Modulation methods that may be used include multilevel phase shift keying (PSK) and multilevel quadrature amplitude modulation (QAM). Furthermore, multiplexing such as polarization multiplexing and orthogonal frequency division multiplexing (OFDM) may be performed on a single wavelength.

[0073] The upstream optical signal is input as a multiplexed optical signal Oin to the optical receiving unit 106. The optical receiving unit 106 mixes the local light and the upstream communication multiplexed optical signal Oin using, for example, a 90-degree hybrid mixer, and separates them into multiple optical signals including I and Q components.

[0074] The optical receiving unit 106 includes a conversion unit such as a PD-TIA (Photodiode-Trans Impedance Amplifier) ​​array. The conversion unit converts the separated optical signals into electrical signals corresponding to the received optical power. The converted electrical signals are converted into digital signals by an ADC 107 and output to a second signal processing unit 108.

[0075] The processor 110 is, for example, a one-chip microcontroller that controls the components included in the optical transceiver 11. The microcontroller may be, for example, a microprocessing unit (MPU), a logic circuit such as an FPGA or a complex programmable logic device (CPLD), or a combination of these.

[0076] The processor 110 is capable of serial communication with the control unit 13 acting as the master. Therefore, the processor 110 can execute predetermined slave processing led by the control unit 13. The predetermined slave processing includes, for example, processing for notifying the control unit 13 of the received optical power monitored by the optical receiving unit 106. If the second signal processing unit 108 has a function such as forward error correction (FEC), the slave processing also includes processing for notifying the control unit 13 of the monitored bit error rate.

[0077] The processor 110 can control whether to use all or a portion (e.g., half) of the multiple subcarriers SCi in accordance with an instruction from the control unit 13. Specifically, the processor 110 generates a control signal CS1 that causes modulation of the subcarrier SCi corresponding to the transmission identification number m instructed by the control unit 13, and outputs the generated control signal CS1 to the optical transmitting unit 105. Similarly, the processor 110 generates a control signal CS2 that causes demodulation of the subcarrier SCi corresponding to the reception identification number m instructed by the control unit 13, and outputs the generated control signal CS2 to the optical receiving unit 106.

[0078] [Optical Transceiver of Optical Network Unit] Figure 3 is a block diagram showing an example configuration of the optical transceiver 21 of the optical network unit 2. The optical transceiver 21 in Figure 3 is an SCM coherent optical transceiver. Since the wavelength λ (m) is a physical quantity obtained by dividing the speed of light c (m / s) by the frequency f (Hz), in the following description, the term "subcarrier" can also be referred to as a frequency. The meanings of "TSn" and "RSn" in Figure 3 are as follows:

[0079] n: Identification number of the electrical signal within the optical transceiver 21. TSn: An electrical signal with identification number n that is the source of the upstream optical signal. Hereinafter, this is also referred to as the "transmit signal." TSn is generated by inverse multiplexing a high-speed signal from the interface onto multiple low-speed subcarriers.

[0080] RSn: An electrical signal with identification number n based on a downstream optical signal. Hereinafter, this is also referred to as a "received signal." RSn is an electrical signal corresponding to each low-speed subcarrier, and is multiplexed onto a high-speed signal and output to an interface. When multiple subcarriers are assigned to downstream communication, inverse MUX may be performed to assign a bandwidth equivalent to the number of subcarriers.

[0081] The optical transceiver 21 includes a light source 201, an optical splitter 202, a first signal processing unit (DSP) 203, a DAC 204, an optical transmission unit (SCM) 205, an optical receiving unit (SCM) 206, an ADC 207, a second signal processing unit (DSP) 208, a processor 210, a multiplexer (MUX) 211, and a demultiplexer (DEMUX) 212. A multiplexed optical signal Oout output from the optical transmission unit 205 is transmitted to an optical fiber 5u for upstream signals. A downstream optical signal from an optical fiber 5d for downstream signals is input to the optical receiving unit 206 as a multiplexed optical signal Oin.

[0082] The processor 210 may be an integrated circuit such as an MPU. The first signal processing unit 203 and the second signal processing unit 208 may be a DSP. The light source 201 is, for example, a semiconductor laser diode. The laser diode is, for example, a diode with a tunable emission wavelength, but the emission wavelength may also be fixed. The wavelength of the light source 201 corresponds to the wavelength of the carrier transmitted and received by the optical network unit 2.

[0083] The optical splitter 202 splits the output light of the light source 201 in two directions. One of the split lights is sent to the optical transmitting unit 205 as transmission light. The other of the split lights is sent to the optical receiving unit 206 as local light. The local light is used for coherent detection in the optical receiving unit 206. In this way, the light source 201 is used for both transmission and reception processing, and functions as both a transmission light source and a local light source. This allows for the optical transceiver 21 to be made smaller, consume less power, and cost less. Instead of providing the optical splitter 202, a transmission light source and a local light source may be provided separately.

[0084] An electrical signal input from the interface of the switch 22 is separated into transmission signals (electrical signals) TSn by a demultiplexer 212 and input to a first signal processing unit 203. The first signal processing unit 203 generates a drive signal for the optical transmitting unit 205 according to the transmission signals (electrical signals) TSn by digital signal processing. The correspondence between the identification number n of the transmission signal TSn and the identification number i of the subcarrier SCi is specified in advance by the processor 210.

[0085] The second signal processing unit 208 performs predetermined digital signal processing on the input signal from the ADC 207 and outputs a received signal (electrical signal) RSn to the multiplexer 211. The multiplexer 211 multiplexes the received signal (electrical signal) RSn into one electrical signal and outputs it to the interface of the switch 22. The correspondence between the identification number n of the received signal RSn and the identification number i of the subcarrier SCi is specified in advance by the processor 210.

[0086] Therefore, the first signal processing unit 203 and the second signal processing unit 208 can transmit and receive at least one subcarrier SCi. This allows multiple subcarriers SCi to be shared or separated. The digital signal processing by the second signal processing unit 208 may include at least one of dispersion compensation, sampling phase synchronization, adaptive equalization, frequency offset compensation, carrier phase recovery, and error correction decoding.

[0087] In this way, the first signal processing unit 203 and the second signal processing unit 208 can perform transmission processing or reception processing for the transmission signal TSn and reception signal RSn for each identification number n corresponding to the subcarrier SCi. Note that the transmission processing and reception processing may be performed simultaneously.

[0088] The DAC 204 converts the digital drive signal generated by the first signal processing unit 203 into an analog drive signal. The analog drive signal is amplified by a driver amplifier or the like and output to the optical transmitting unit 205. The ADC 207 converts the analog electrical signal corresponding to the power of the signal light input from the optical receiving unit 206 into a digital electrical signal, and outputs the converted digital electrical signal to the second signal processing unit 208.

[0089] The optical transmitter 205 is driven by a drive signal having a signal waveform corresponding to the transmission signal TSn, and modulates the transmission light with the transmission data signal. The optical transmitter 205 outputs the modulated multiplexed optical signal Oout as an upstream optical signal. Modulation methods that may be used include multilevel PSK and multilevel QAM. Furthermore, multiplexing such as polarization multiplexing and orthogonal frequency division multiplexing (OFDM) may be performed for a single wavelength.

[0090] The downstream optical signal is input as a multiplexed optical signal Oin to the optical receiving unit 206. The optical receiving unit 206 mixes the local light and the upstream communication multiplexed optical signal Oin using, for example, a 90-degree hybrid mixer, and separates them into optical signals of multiple systems each including an I component and a Q component.

[0091] The optical receiving unit 206 includes a conversion unit such as a PD-TIA array. The conversion unit converts the separated optical signals into electrical signals corresponding to the received optical power. The converted electrical signals are converted into digital signals by an ADC 207 and output to a second signal processing unit 208.

[0092] The processor 210 is, for example, a one-chip microcontroller that controls the components included in the optical transceiver 21. The microcontroller may be, for example, an MPU, a logic circuit such as an FPGA or a CPLD, or a combination of these.

[0093] The processor 210 is capable of serial communication with the control unit 23 as the master. Therefore, the processor 210 can execute predetermined slave processing led by the control unit 23. The predetermined slave processing includes, for example, processing for notifying the control unit 23 of the received optical power monitored by the optical receiving unit 206. If the second signal processing unit 208 has a function such as forward error correction (FEC), the slave processing also includes processing for notifying the control unit 23 of the monitored bit error rate.

[0094] The processor 210 generates a control signal CS3 for modulating the subcarrier SCi corresponding to the identification number n for transmission instructed by the control unit 23, and outputs the generated control signal CS3 to the optical transmitting unit 105. Similarly, the processor 210 generates a control signal CS4 for demodulating the subcarrier SCi corresponding to the identification number n for reception instructed by the control unit 23, and outputs the generated control signal CS2 to the optical receiving unit 206.

[0095] In this embodiment, the transmission signal TSn and reception signal RSn of the optical transceiver 21 of the optical network unit 2 are part of the channels included in the reception signal RSm and transmission signal TSm of the optical transceiver 11 of the optical station unit 1, respectively, and are signals corresponding to channels (excluding multicast channel signals) exclusively assigned to other optical network units 2. Therefore, the first signal processing unit 203 and the second signal processing unit 208 only need to process the subcarriers SCi corresponding to the transmission and reception channels.

[0096] [Comparative Example of Subcarrier Allocation] Fig. 4 is an explanatory diagram showing a comparative example of subcarrier allocation. The meanings of the parameters in Fig. 4 are as follows. As an example, the carrier width of subcarrier SCi is assumed to be 4 GHz (equivalent to a data rate of 25 Gbps). A predetermined guard band (e.g., 0.3 GHz) is provided between each subcarrier SCi.

[0097] BW: The frequency width (e.g., 68.5 GHz) that the optical transceiver 11 of the HUB can modulate and demodulate. N: The total number of subcarriers SCi included in the frequency width BW (hereinafter referred to as the "total number of carriers"). In this comparative example, N = 16. fmin: The minimum frequency that the optical transceiver 11 of the HUB can modulate and demodulate. fmax: The maximum frequency that the optical transceiver 11 of the HUB can modulate and demodulate. fc: The center frequency of the frequency width BW.

[0098] When subcarrier multiplexing is performed using a single-core bidirectional optical line (ODN) (optical line) in which both downstream and upstream optical signals are transmitted over a single optical fiber, it is desirable to use different subcarriers SCi for upstream and downstream communications to minimize the effects of reflected light. Therefore, if one subcarrier SCi is assigned equally to each of the upstream and downstream communications of a hub for one LEAF, the maximum number of LEAFs that can be accommodated in one hub is eight. For example, in the comparative example of Figure 4, 16 subcarriers SCi are assigned as follows:

[0099] HUB: SC1 to SC8 are used for downstream transmission (subcarrier multiplexing). SC9 to SC16 are used for upstream reception (subcarrier multiplexing). LEAF1: SC1 is used for downstream reception. SC9 is used for upstream transmission. LEAF2: SC2 is used for downstream reception. SC10 is used for upstream transmission. LEAF3: SC3 is used for downstream reception. SC11 is used for upstream transmission. LEAF4: SC4 is used for downstream reception. SC12 is used for upstream transmission.

[0100] LEAF5: SC5 is used for downstream reception. SC13 is used for upstream transmission. LEAF6: SC6 is used for downstream reception. SC14 is used for upstream transmission. LEAF7: SC7 is used for downstream reception. SC15 is used for upstream transmission. LEAF8: SC8 is used for downstream reception. SC16 is used for upstream transmission.

[0101] [Communication Policy Applied to HUB and LEAF] As described above, the method of equally allocating one subcarrier SCi each to the upstream communication and downstream communication of one HUB has the problem that the maximum number of LEAFs that can be accommodated in one HUB is limited to half the total number N (= 16) of subcarriers that the HUB can use. In this embodiment, in order to be able to accommodate LEAFs that exceed half of the total number N of subcarriers, multiple subcarriers SCi are allocated unequally to the upstream communication and downstream communication of the HUB, and multiple subcarriers SCi are shared in downstream communication using TDM (Time Division Multiplexing).

[0102] Specifically, the following "communication policy" is adopted for the HUB and each LEAF. The parameters included in the communication policy have the following meanings: SCj: downstream subcarrier (j is an identification number) SCk: upstream subcarrier (k is an identification number, where k≠j) Nd: total number of SCj (total number of downstream subcarriers that the HUB can transmit simultaneously, i.e., total number of downstream subcarriers allocated by the HUB) Nu: total number of SCk (total number of upstream subcarriers that the HUB can receive simultaneously, i.e., total number of upstream subcarriers allocated by the HUB)

[0103] (Communication policy for the HUB: First communication policy) The HUB allocates multiple SCj and multiple SCk unequally so that Nd < Nu. In downstream communication, the HUB transmits multiple SCj using a time division method. In upstream communication, the HUB receives multiple SCk using a subcarrier multiplexing method.

[0104] (Communication policy for LEAF: second communication policy) In downstream communication, the LEAF receives multiple SCj. In upstream communication, the LEAF transmits one of multiple SCk. In the above downstream communication, the switch 22 of the LEAF may select frames containing the VLAN ID to which it (and the client device) belongs, for example, based on the VLAN ID, and discard frames containing other VLAN IDs.

[0105] [Example of Subcarrier Allocation] Figure 5 is an explanatory diagram showing an example of subcarrier allocation. The parameters in Figure 5 have the same meanings as in Figure 4. In this embodiment, N = 16. The example in Figure 5 is an example in which Nd = 4 and Nu = 12 in the first and second communication policies described above. Specifically, four subcarriers SCj (j = 1 to 4) and 12 subcarriers SCk (k = 5 to 16) are allocated as follows:

[0106] The HUB broadcasts SC1 to SC4 to all LEAFs using time division multiplexing (time division transmission of multiple subcarriers), and receives SC5 to SC16 via subcarrier multiplexing. The LEAFs receive and transmit as follows: LEAF1: Uses SC1 to SC4 for downstream reception. Uses SC5 for upstream transmission. LEAF2: Uses SC1 to SC4 for downstream reception. Uses SC6 for upstream transmission. LEAF3: Uses SC1 to SC4 for downstream reception. Uses SC7 for upstream transmission. LEAF4: Uses SC1 to SC4 for downstream reception. Uses SC8 for upstream transmission.

[0107] LEAF5: SC1 to SC4 are used for downstream reception. SC9 is used for upstream transmission. LEAF6: SC1 to SC4 are used for downstream reception. SC10 is used for upstream transmission. LEAF7: SC1 to SC4 are used for downstream reception. SC11 is used for upstream transmission. LEAF8: SC1 to SC4 are used for downstream reception. SC12 is used for upstream transmission.

[0108] LEAF9: SC1 to SC4 are used for downstream reception. SC13 is used for upstream transmission. LEAF10: SC1 to SC4 are used for downstream reception. SC14 is used for upstream transmission. LEAF11: SC1 to SC4 are used for downstream reception. SC15 is used for upstream transmission. LEAF12: SC1 to SC4 are used for downstream reception. SC16 is used for upstream transmission.

[0109] As described above, if the multiple subcarriers SCi available to the HUB are unevenly allocated so that Nd < Nu, and the multiple subcarriers SCj used for downstream communication are shared by TDM, it becomes possible to accommodate more than half the total number of subcarriers N (12 in the illustrated example). Specifically, by applying the above-mentioned first and second communication policies, the number of LEAFs that can be accommodated per HUB can be increased to Nu (= N - Nd).

[0110] [Changing the Number of Accommodated LEAFs] In the optical communication system 200 of this embodiment, the difference between Nu and Nd may be changed to gradually increase the number of accommodated LEAFs that can be connected to the HUB. Specifically, according to the optical communication system 200 of this embodiment, the number of accommodated LEAFs Nu can be increased stepwise in accordance with an increase in the number of users, for example, as shown in the transition from operation state 1 to operation state 4 below.

[0111] Operational state 1: If less than eight LEAFs are sufficient, set Nd=8 and Nu=8 (equal allocation). Operational state 2: If it is desired to increase the number of LEAFs to 10, set Nd=6 and Nu=10 (unequal allocation).

[0112] Operational state 3: When it is desired to increase the number of LEAFs to 12, Nd=4 and Nu=12 (unequal allocation). Operational state 4: When it is desired to increase the number of LEAFs to 14, Nd=2 and Nu=14 (unequal allocation).

[0113] In the optical communication system 200 of this embodiment, if the number of users decreases compared to before, the number of LEAFs that can be accommodated Nu may be gradually reduced, for example, by returning operational state 4 to operational state 3 or returning operational state 3 to operational state 2.

[0114] [Bandwidth Adjustment in HUB and LEAF] Figure 6 is an explanatory diagram showing an example of bandwidth adjustment in a HUB and a LEAF. As shown in Figure 6, the maximum bandwidths for the upper network 7 and the optical transceiver 11 in the switch 12 of the optical line terminal 1 (HUB) are each 400 Gbps. Also, the maximum bandwidths for the lower network 8 and the optical transceiver 21 in the switch 22 of the optical network terminal 2 (LEAF) are each 100 Gbps.

[0115] The bandwidth of the HUB and LEAF switches 12, 22 is typically set to the same bandwidth on both the upstream and downstream sides of the switches 12, 22. In this embodiment (the example in FIG. 5), the HUB transmits four downstream subcarriers (SC1 to SC4), so the downstream bandwidth of the optical transceiver 11 is 25 Gbps x 4 = 100 Gbps. This reduces downstream communication problems caused by buffer overflow compared to leaving the downstream bandwidth of the switch 12 at 400 Gbps.

[0116] Therefore, the switch 12 of the HUB shapes the bandwidth of downstream transmission to the optical transceiver 11 to 100 Gbps. On the other hand, the switch 12 adopts the maximum bandwidth of 400 Gbps for upstream communication to the higher-level network 7 and passes the upstream frames through without shaping.

[0117] In this way, the switch 12 of the HUB functions as a "bandwidth control unit" that shapes the downstream transmission bandwidth to a bandwidth equivalent to four downstream subcarriers (100 Gbps in this embodiment). The above-mentioned bandwidth adjustment is not limited to being performed by the switch 12, but may also be performed by another signal processing unit included in the HUB.

[0118] In this embodiment (the example in FIG. 5), the LEAF transmits one upstream subcarrier, so the bandwidth available for upstream transmission by the optical transceiver 21 is 25 Gbps x 1 = 25 Gbps. This reduces the likelihood of upstream communication problems caused by buffer overflow compared to when the upstream bandwidth of the switch 22 is left at 100 Gbps.

[0119] Therefore, the switch 22 of the LEAF shapes the bandwidth of upstream transmissions to the optical transceiver 21 to 25 Gbps. On the other hand, the switch 22 adopts the maximum bandwidth of 100 Gbps for downstream transmissions to the lower network 8, and allows downstream frames to pass through without shaping.

[0120] In this way, the switch 22 of the LEAF functions as a "bandwidth control unit" that shapes the bandwidth of the upstream transmission to a bandwidth equivalent to one upstream subcarrier (25 Gbps in this embodiment). Note that the above-mentioned bandwidth adjustment is not limited to being performed by the switch 22, but may also be performed by another signal processing unit included in the LEAF.

[0121] [Other Modifications] The embodiments disclosed herein are illustrative in all respects and are not restrictive. The scope of the present invention is not limited to the above-described embodiments, but includes all modifications within the scope of equivalents to the configurations described in the claims.

[0122] In the above-described embodiment, the optical line 3 may be a line configuration in which, for example, a ROADM (Reconfigurable Optical Add / Drop Multiplexer) network, which is a type of core network, is interposed in the middle of the trunk line 4. In this case, since a core network is usually two-core, a connection configuration in which a circulator is interposed at the connection point between the trunk line 4 and the core network to convert between one core and two cores may be adopted.

[0123] In the above-described embodiment, some of the LEAFs connected to the optical line 3, which is a single-core bidirectional transmission ODN, may include an optical network unit 2 that receives downstream signals by subcarrier multiplexing.

[0124] In the above-described embodiment, the minimum unit of the number of subcarriers assigned to each LEAF is not limited to 1, but may be 2. When subcarriers are assigned to each LEAF in units of 2, the maximum number of LEAFs that can be accommodated is 4 in the comparative example of Fig. 4, but in the embodiment of Fig. 5, this can be increased to a maximum of 6.

[0125] In the above-described embodiments, the present invention can be realized not only as a system and apparatus having the above-described characteristic configuration, but also as a program for causing a computer to execute such characteristic configuration. Furthermore, the present invention can be realized as a semiconductor integrated circuit that achieves part or all of the system and apparatus.

[0126] Each process (each function) in the above-described embodiments is achieved by a processing circuit (circuitry) including one or more processors. The processing circuit may be configured with an integrated circuit or the like that combines one or more processors, one or more memories, various analog circuits, and various digital circuits. The one or more memories store programs (instructions) that cause the one or more processors to execute each process. The one or more processors may execute each process according to a program read from one or more memories, or according to a logic circuit designed in advance to execute each process. The processor may be a CPU, GPU, DSP, FPGA, ASIC, or any other processor suitable for computer control. Note that multiple physically separated processors may cooperate with each other to execute each process. For example, processors installed in multiple physically separated computers may cooperate with each other to execute each process via a network such as a LAN (Local Area Network), a WAN (Wide Area Network), or the Internet. The program may be installed into memory from an external server device or the like via a network, or may be distributed in a state stored on a recording medium such as a CD-ROM (Compact Disc Read Only Memory), a DVD-ROM (Digital Versatile Disc Read Only Memory), or a semiconductor memory, and then installed into the memory from the recording medium.

[0127] REFERENCE SIGNS LIST 1 optical line (hub) 2 optical fiber (lead optical fiber) 3 optical line (optical distribution network) 4 trunk 4d optical fiber (for downstream communication) 4u optical fiber (for upstream communication) 5 branch 5d optical fiber (for downstream communication) 5u optical fiber (for upstream communication) 6 optical coupler 7 upper network 8 lower network 10 controller 11 optical transceiver of optical line device 12 switch (bandwidth control unit) of optical line device 13 control unit of optical line device 13A CPU 13B memory 14 circulator (upper side) 21 optical transceiver of optical line device 22 switch (bandwidth control unit) of optical line device 23 control unit of optical line device 23A CPU 23B memory 24 circulator (lower side) 101 light source 102 optical branching unit 103 first signal processing unit (DSP) 104 DAC 105 Optical transmitting unit 106 Optical receiving unit 107 ADC 108 Second signal processing unit (DSP) 110 Processor 111 Multiplexer (MUX) 112 Demultiplexer (DEMUX) 200 Optical communication system 201 Light source 202 Optical branching unit 203 First signal processing unit (DSP) 204 DAC 205 Optical transmitting unit 206 Optical receiving unit 207 ADC 208 Second signal processing unit (DSP) 210 Processor 211 Multiplexer (MUX) 212 Demultiplexer (DEMUX)

Claims

1. An optical communications system comprising: a station side device having a subcarrier multiplexing optical transceiver; a plurality of home network side devices having subcarrier multiplexing optical transceivers; and an optical distribution network connecting the station side device and the plurality of home network side devices, wherein the station side device transmits a plurality of downstream subcarriers to the plurality of home network side devices using a time division method, receives upstream subcarriers from at least one of the plurality of home network side devices using a subcarrier multiplexing method and demodulates them, at least one of the plurality of home network side devices receives the plurality of downstream subcarriers from the station side device, demodulates downstream subcarriers that can be demodulated from the plurality of downstream subcarriers, and transmits the upstream subcarriers to the station side device, and the total number of downstream subcarriers that can be transmitted in the station side device is less than the total number of upstream subcarriers that can be received.

2. The optical communication system according to claim 1, wherein the optical line terminal includes a bandwidth control unit that shapes the downstream transmission bandwidth to correspond to the total number of the plurality of downstream subcarriers.

3. An optical communication system as described in claim 1 or claim 2, wherein at least one of the plurality of home devices has a bandwidth control unit that shapes the upstream transmission bandwidth to correspond to at least one of the upstream subcarriers.

4. The optical communication system according to claim 1, wherein said optical distribution network is constructed of a single-core bidirectional optical fiber.

5. A station-side device connected to a plurality of home-side devices via an optical distribution network, comprising: an optical transceiver using a subcarrier multiplexing method; and a control unit that controls communication by the optical transceiver, wherein the control unit controls the optical transceiver to transmit a plurality of downstream subcarriers to the plurality of home-side devices using a time-division method, and to receive and demodulate upstream subcarriers from at least one of the plurality of home-side devices using a subcarrier multiplexing method, and wherein the total number of downstream subcarriers that the optical transceiver can transmit is less than the total number of upstream subcarriers that it can receive.

6. The optical line terminal according to claim 5, further comprising a bandwidth control unit that shapes the downstream transmission bandwidth to correspond to the total number of downstream subcarriers.

7. The optical line terminal according to claim 5, wherein said optical distribution network is constructed of a single-core bidirectional optical fiber.

8. A home appliance connected to a station device via an optical distribution network, comprising: a subcarrier multiplexing optical transceiver; and a control unit that controls communications by the optical transceiver; wherein there are two or more home appliances; and in at least one of the home appliances, the control unit controls the optical transceiver to receive multiple downstream subcarriers from the station device, demodulate downstream subcarriers that can be demodulated from the multiple downstream subcarriers, and transmit upstream subcarriers to the station device; and wherein the total number of downstream subcarriers that can be received by one home appliance is less than the total number of upstream subcarriers that can be transmitted by all of the home appliances.

9. The optical network unit according to claim 8, further comprising a bandwidth control unit that shapes the bandwidth of upstream transmission so that it corresponds to at least one of the upstream subcarriers.

10. The optical network unit according to claim 8, wherein the optical distribution network is configured by a single-core bidirectional optical fiber.

11. An optical communication method performed between a station side device and multiple home network devices connected by an optical distribution network, comprising: a step in which the station side device transmits multiple downstream subcarriers to the multiple home network devices using a time division method; a step in which at least one of the multiple home network devices receives the multiple downstream subcarriers from the station side device and demodulates demodulatable downstream subcarriers from the multiple downstream subcarriers; a step in which at least one of the multiple home network devices transmits upstream subcarriers to the station side device; and a step in which the station side device receives and demodulates the upstream subcarriers using a subcarrier multiplexing method, wherein the total number of downstream subcarriers that the station side device can transmit is less than the total number of upstream subcarriers that it can receive.

12. The optical communication method according to claim 11, further comprising the step of: in said optical line terminal, shaping the downstream transmission bandwidth so that it corresponds to the total number of said downstream subcarriers.

13. The optical communication method according to claim 11 or 12, further comprising a step in the optical network unit shaping the upstream transmission bandwidth so that it corresponds to at least one of the upstream subcarriers.

14. The optical communication method according to claim 11, wherein said optical distribution network is constructed of a single-core bidirectional optical fiber.

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