GB2612689A

GB2612689A - Optical add/drop multiplexer branching unit, communication system, and method for signal transmission

Info

Publication number: GB2612689A
Application number: GB2213275.7A
Authority: GB
Inventors: Wang Yan; Yang Li; Shao Yue; Xu Changwu
Original assignee: HMN Technologies Co Ltd
Current assignee: HMN Technologies Co Ltd
Priority date: 2021-09-22
Filing date: 2022-09-12
Publication date: 2023-05-10
Anticipated expiration: 2042-09-12
Also published as: JP7325580B2; CN113572564A; JP2023046243A; GB2612689B; GB202213275D0; CN113572564B

Abstract

An optical system has a bidirectional trunk line between two trunk terminals 1,2 and one or more branch terminals 3 connected to the trunk line via optical add drop multiplexer branching units (OADM-BU) 4. The signal output to a terminal (e.g. branch output 32) includes optical signals destined for that terminal A2,B2 on particular sub-bands/wavelengths. On other sub-bands a garbled signal is output, where optical signals A1,B1 destined for other terminals are overlaid such that they interfere with each other via co-channel crosstalk. The interference should make the garbled signal unrecoverable, maintaining security of those signals. The overlaid signals may have a power level that negates the need for dummy load light on the output. The OADM may be constructed from components including splitters, combiners, filters or wavelength selective switches.

Description

OPTICAL ADD/DROP MULTIPLEXER BRANCHING UNIT, COMMUNICATION SYSTEM, AND METHOD FOR SIGNAL TRANSMISSION

FIELD

[0001] This application relates to the technical field of submarine communication, and in particular, to an optical add/drop multiplexer branching unit, a communication system, and a method for signal transmission.

BACKGROUND

[0002] A signal transmission path in a submarine fiber communication system includes a trunk line and branch line, where the trunk line refers to a signal transmission path between two trunk stations located on the land, and the branch line refers to a signal transmission path between a trunk station and a branch terminal station that are located on the land. As shown in FIG. 1, signals output by various terminal stations need to be split or combined by using an optical add/drop multiplexer branching unit (Optical Add/Drop multiplexer Branching Unit, OADM BU), to obtain a processed signal transmitted to a target terminal station. A terminal station A and a terminal station B are trunk stations, and a signal transmission path between the terminal station A and the terminal station B is a trunk line. A terminal station C is a branch terminal station, and a signal transmission path between the OADIVI BU and the terminal station C is a branch line.

[0003] FIG. 2 is the schematic diagram illustrating that a terminal station A transmits a signal to a terminal station B. As shown in FIG. 2, transmission signals between the terminal stations may be classified into four types, which respectively are express (pass-through) signals that are represented by using rectangles filled with horizontal lines, and are used to carry communication services between trunk terminal stations; drop (downloading) signals that are represented by using rectangles with diagonal lines from the upper right to the lower left, and are used to carry communication services between a trunk and a branch; add (insertion) signals that are represented by using rectangles filled with diagonal squares, and are used to carry communication services from a branch to a trunk; and loading (load) signals that are represented by using black rectangles. The loading signal is a load signal transmitted by a trunk and a branch terminal station. The load signal is a signal used for power balance, without carrying services. The load signal may be noise light, continuous light, or the like. Generally, carrier wavelengths of the Drop signal and the Add signal are the same.

[0004] As shown in FIG. 2, after entering the OADM BU through an input interface, a signal output by the terminal station A is split into two paths by a coupler (Splitter), one path of which is downloaded to the branch line, so that the Drop signal is received and recovered by the terminal station C; and the other path of which passes through a band-stop filter (Band Block Filter, BBF), so that the downloading signal is blocked, and the pass-through signal and the branch Add signal are left and are output to the terminal station B by an output interface after being combined by a coupler (Combiner). When there is a repeater on the branch line, to balance input power of the repeater, the Loading signal is input together with the Add signal.

The Loading signal is filtered by a band-pass filter (Band Pass Filter, BPF) in a propagation direction of the insertion signal, and the Add signal passing through the band-pass filter is output after being combined with the pass-through signal.

[0005] Referring to FIG. 2, for one pair of fibers on the trunk line, two corresponding pairs of fibers need to be configured on the branch line, one of which completes signal downloading (Drop) and signal uploading (Add) from the terminal station A to the terminal station C by using the OADM BU, and the other of which completes signal downloading (Drop) and signal uploading (Add) from the terminal station B to the terminal station C by using the OADM BU. That is to say, for a pair of fibers on a trunk line that need to support bidirectional communication between the terminal station A and the terminal station B and the terminal station C at the same time, the branch line needs to be correspondingly configured with fiber pairs whose quantity is twice of that of fiber pairs on the trunk line. Doubling the quantity of the fiber pairs on the branch line, on one hand, may greatly increase costs; and on the other hand, may cause that the quantity of fiber pairs required by the branch line exceeds a maximum quantity of fiber pairs that can be accommodated by an existing submarine cable and an underwater optical repeater (for example, an existing submarine cable system can support 16 fiber pairs at most). As a result, there are no matching underwater products available for the branch.

[0006] In addition, the OADM BU shown in FIG. 2 also has a problem in information transmission security. As shown in FIG. 2, a pass-through signal carrying communication information between the terminal station A and the terminal station B may be downloaded to the terminal station C together with a downloading signal. As a result, a communication signal between the terminal station A and the terminal station B may be monitored from the terminal station C, and thus anti-eavesdropping cannot be achieved.

[0007] FIG. 3 provides a schematic signal transmission diagram illustrating that a branch line simultaneously supports, by using only one fiber pair, a terminal station A and a terminal station B to perform bidirectional communication with a terminal station C. A service bandwidth is divided into three subbands of a, B, and c that are respectively used to carry communication services between the terminal station A and the terminal station C, communication services between the terminal station A and the terminal station B, and communication services between the terminal station B and the terminal station C. After optical signals aE, BE, and cE output by the terminal station B enter the OADM BU node, service subbands aE and BE are respectively blocked by a filter B (for blocking the subband B) and a filter A (for blocking the subband a), and a subband cE transmitted to the terminal station C is split out. Moreover, after optical signals aw, By, and cw output by the terminal station A enter the OADM BU node, a subband cw is blocked by a filter C; and a subband aw transmitted to the terminal station C and a subband Bw whose destination terminal station is the terminal station B are left, and enter a downlink (Drop) fiber A of the branch line after being combined with the subband cE by using an optical multiplexer A. In this way, services of two trunk stations are transmitted to a branch terminal station by using one fiber. A process of transmission in an opposite direction, that is, from the terminal station C to the terminal station A and the terminal station B is that after optical signals as, Bs, and cs output by an uplink (Add) fiber B on the branch line pass through the filter B, a subband Bs is blocked, and a subband as transmitted to the terminal station B and a subband cs transmitted to the terminal station A are split out and are transmitted to the terminal station B and the terminal station A after being combined, by using an optical multiplexer B, with a subband Bw, on a fiber on the trunk line, that is from the terminal station A and is filtered out by using the filter C and a filter D (which are configured to block a subband a and a subband c). After a signal transmitted to the terminal station A passes through the filter B again, the subband Bw is blocked, and the subbands as and cs are split out and are transmitted to the terminal station A after being combined with a subband BE from the terminal station B by using an optical multiplexer C. In this way, signal transmission from a branch terminal station to two trunk stations is completed on the branch line by using one fiber [0008] In a submarine cable communication system, an optical repeater works in a constant pump current mode, and is generally in a deep saturation working state. Therefore, input power changes within a certain range, but output power may remain unchanged, thereby presenting a self-healing capability. However, this may also bring in a problem: when a quantity of channels/wavelengths input to the repeater is reduced, output power of remaining wavelengths may be increased, and thus nonlinear transmission penalties may be caused. In the signal transmission process shown in FIG. 3, to balance single-wavelength output power of the repeater, a service subband of a non-target terminal station is combined with a useful service subband to form a service signal of an integral bandwidth to be transmitted the target terminal station. Specifically, trunk services (services carried by the Bw subband) is transmitted to the branch terminal station together with the branch services (aw and cE), so that the branch terminal station can receive a trunk signal Although the branch terminal station may be configured with a filtering device to split out a signal to be received, because a terminal station device is difficult to be replaced, a signal transmitted between terminal stations on the trunk is still at a risk of being eavesdropped. Similarly, in addition to trunk services (Bw or BE), branch services (as and cs) are also received by the trunk station, so that a branch signal is also at a risk of being monitored by the trunk station [0009] To avoid the risk of monitoring between terminal stations, compared with a case of performing forward transmission (uplink) and reverse transmission (downlink) between two bidirectional communication terminal stations by using a same service wavelength, uplink and downlink service signals for communication between the trunk station and the branch terminal station may be respectively carried on different wavelengths/subbands. FIG. 4 is a schematic diagram of a signal transmission process between terminal stations. As shown in FIG. 4, a transmission spectrum of the terminal station is divided into three subbands of X, Y, Z. The subband Y (YA and YB) is merely used to carry the communication services between the terminal station A and the terminal station B. Moreover, only some of the communication services are carried on the subband Y, and the rest of the communication services are carried on the subband X or Z (XA for west to east, or ZB for east to west). Meanwhile, subbands ZA and Zc are further used to carry an uplink service signal between the terminal station A and the terminal station C and a downlink service signal between the terminal station B and the terminal station C, respectively; and subbands Xc and XB respectively carry a downlink service signal between the terminal station A and the terminal station C and an uplink service signal between the terminal station B and the terminal station C. Spectral widths of the subbands X and Z need to be set to be the same, but starting points and end points of wavelengths may be different. A subband Yc is a load signal transmitted by the terminal station C, and is used for power balance of the branch line, without carrying services.

[0010] After a subband service is configured as above, service signals transmitted to the terminal station C from the terminal station A and from the terminal station B are respectively loaded to different subbands. Therefore, the service signals may be wavelength-multiplexed, within the OADM BU, with the subband Yc that serves as a load signal and then be output to a same fiber on the branch line for transmission. Meanwhile, after a branch service signal (such as ZA) transmitted from the terminal station A to the terminal station C is downloaded, the subband (Zc) carrying the branch service signal is reused to carry an insertion signal transmitted from the terminal station C to the terminal station B. Therefore, the newly carried signal may not have a wavelength collision with a straight-through signal (such as XA+YA) transmitted on the trunk line. In this way, the same quantity of fiber pairs on the branch line may be used as on the trunk line, so that the terminal station A and the terminal station B perform bidirectional communication with the terminal station C at the same time, and the service signal would not be transmitted to a non-destination terminal station, thereby ensuring security of information transmission.

[0011] However, by adopting the signal transmission mode as shown in FIG. 4, uplink and downlink services of a same terminal station are loaded to different subbands. In this case, complexity of system configuration and network management is increased, and especially for a scenario where a plurality of OADMs/ROADMs are cascaded, the system configuration and network management may be more complex.

SUMMARY

[0012] This application provides a communication system, an optical add/drop multiplexer branching unit, and a method for signal transmission, to make sure that a service signal output by a terminal station can only be received by a target terminal station on the basis of reducing a quantity of fiber pairs of a branch terminal station.

[0013] According to a first aspect, an embodiment of this application provides a communication system, comprising a plurality of terminal stations, wherein the plurality of terminal stations comprise two trunk stations and at least one branch terminal station, a service signal is transmitted between the two trunk stations, a service signal is transmitted between the at least one branch terminal station and the two trunk stations, and a combined signal output by each terminal station among the plurality of terminal stations is processed by using a corresponding optical add/drop multiplexer branching unit, to obtain a processed combined signal that is transmitted to a peer terminal station, [0014] the combined signal output by each terminal station comprises a plurality of service signals, and the plurality of service signals are carried on different subbands, and [0015] the optical add/drop multiplexer branching unit comprises a plurality of splitters, a plurality of filters, and a plurality of combiners, and is configured to: [0016] each processed combined signal that is obtained after being processed by the plurality of splitters, the plurality of filters, and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal [0017] In an implementation, the combined signal output by each terminal station further comprises a load signal, and the load signal is used to balance optical power of the plurality of service signals in the combined signal.

[0018] In an implementation, a sum of optical power spectrum density of the at least two non-target service signals is equal to optical power spectrum density of the target service signal.

[0019] In an implementation, the at least two non-target service signals have equal optical power.

[0020] In an implementation, at least one of the subbands includes an idle subband, the idle subband is used to carry a redundant signal corresponding to a first target subband, wherein the redundant signal refers to a service signal exceeding a bandwidth of the first target subband.

[0021] In an implementation, the idle subband is adjacent to or is spaced apart from the first target subband.

[0022] In an implementation, if the at least one branch terminal station merely communicates with one of the trunk stations, the subbands are divided based on a service signal transmitted between the at least one branch terminal station and one of the trunk stations and a service signal transmitted between the two trunk stations, wherein the service signal transmitted between the at least one branch terminal station and one of the trunk stations and the service signal transmitted between the two trunk stations are carried on different subbands.

[0023] In an implementation, the plurality of filters are reconfigurable wavelength 25 blockers.

[0024] In an implementation, the plurality of filters are band-stop filters with a same parameter or different parameters, or the plurality of filters are combinations of band-stop filters and a band-pass filters with a same parameter or different parameters.

[0025] According to a second aspect, an embodiment of this application provides a communication system, comprising a plurality of terminal stations that include two trunk stations and at least one branch terminal station, wherein a service signal is transmitted between the two trunk stations, a service signal is transmitted between the at least one branch terminal station and the two trunk stations, and a combined signal output by each terminal station among the plurality of terminal stations is processed by using a corresponding optical add/drop multiplexer branching unit, to obtain a processed combined signal that is transmitted to a peer terminal station; [0026] the combined signal output by each terminal station comprises a plurality of service signals, and the plurality of service signals are carried on different subbands; and [0027] the optical add/drop multiplexer branching unit comprises a plurality of wavelength selective switches and a plurality of combiners, the wavelength selective switches are configured to split and filter the combined signal, and the optical add/drop multiplexer branching unit is configured to: [0028] each processed combined signal that is obtained after being processed by the plurality of wavelength selective switches and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.

[0029] According to third aspect, an embodiment of this application provides an optical add/drop multiplexer branching unit, comprising a plurality of splitters, a plurality of filters, and a plurality of combiners, wherein the plurality of splitters are respectively connected to a signal output interface of a corresponding terminal station, and the plurality of combiners are respectively connected to a signal input interface of the corresponding terminal station; and [0030] the plurality of splitters, the plurality of filters, and the plurality of combiners are configured to: [0031] a processed combined signal that is obtained by processing a combined signal, input to the optical add/drop multiplexer branching unit, by using the plurality of splitters, the plurality of filters, and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.

[0032] According to a fourth aspect, an embodiment of this application provides an optical add/drop multiplexer branching unit, comprising a plurality of wavelength selective switches and a plurality of combiners, wherein input ends of the plurality of wavelength selective switches are respectively connected to a signal output interface of a corresponding terminal station, output ends of the plurality of wavelength selective switches are respectively connected to input ends of the plurality of combiners, output ends of the plurality of combiners are respectively connected to a signal input interface of the corresponding terminal station, and the wavelength selective switches are configured to split and filter the combined signal, and [0033] the plurality of wavelength selective switches and the plurality of combiners are configured to [0034] a processed combined signal that is obtained by processing a combined signal, input to the optical add/drop multiplexer branching unit, by using the plurality of wavelength selective switches and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.

[0035] According to a fifth aspect, an embodiment of this application provides a method for signal transmission, used in the communication system in the first aspect or the second aspect, where the method comprises: [0036] dividing an overall bandwidth into a specified quantity of subbands by using the terminal station; [0037] carrying, by the terminal station, a to-be-output service signal and a to-be-output load signal on different subbands, to obtain a combined signal; [0038] processing the combined signal by the optical add/drop multiplexer branching unit, to obtain a processed combined signal; and [0039] distributing, by the optical add/drop multiplexer branching unit, the processed combined signal to a peer terminal station of the terminal station.

[0040] In view of the above, this application provides a communication system, an optical add/drop multiplexer branching unit, and a method for signal transmission, and may process service signals carried on different subbands, output by various terminal stations by using the optical add/drop multiplexer branching unit, to support a trunk line and a branch line to use a same quantity of fiber pairs. Meanwhile, the target service signal and the non-target service signals in the processed combined signal obtained after being processed by the optical add/drop multiplexer branching unit are carried on different subbands, and the non-target service signals are carried on a same subband, so as to form co-channel crosstalk on the subband. In this way, the terminal station receiving the processed combined signal cannot obtain service information from the non-target service signals, thereby ensuring security of the service information transmitted between the terminal stations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] To more clearly describe the technical solutions of this application, the accompanying drawings to be used in the embodiments are briefly illustrated below. Obviously, those skilled in the art can also derive other accompanying drawings according to these accompanying drawings without an effective effort.

[0042] FIG. 1 is a schematic structural diagram of an existing submarine fiber communication system according to this application; [0043] FIG. 2 is a schematic diagram illustrating that a signal is transmitted from an existing terminal station A to an existing terminal station B according to this application; [0044] FIG. 3 is a schematic signal transmission diagram illustrating that an existing branch line simultaneously supports a terminal station A and a terminal station B to perform bidirectional communication with a terminal station C by using only one fiber pair according to this application; [0045] FIG. 4 is a schematic signal transmission diagram illustrating that an existing terminal station A and an existing terminal station B perform bidirectional communication with a terminal station C according to this application [0046] FIG. 5 is a schematic structural diagram of a communication system according to an embodiment of this application; [0047] FIG. 6 is a schematic diagram of signal transmission in a communication system according to an embodiment of this application; [0048] FIG. 7 is a schematic structural diagram of an initial subband according to an embodiment of this application; [0049] FIG. 8 is a schematic structural diagram of a reconstructed subband according to an embodiment of this application; [0050] FIG. 9 is a schematic structural diagram of a reconstructed subband according to another embodiment of this application; [0051] FIG. 10 is a schematic diagram of signal transmission in a communication system according to an embodiment of this application; [0052] FIG. 11 is a schematic diagram of signal transmission in a communication system according to another embodiment of this application; and [0053] FIG. 12 is a schematic diagram of signal transmission in a communication system according to still another embodiment of this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0054] The technical solutions in the embodiments of the present disclosure are clearly and completely described below in combination with the accompanying drawings in the embodiments of the present disclosure. Obviously, the described embodiments are merely some embodiments of the present disclosure and are not all embodiments. According to the embodiments in the present disclosure, all other embodiments derived by those skilled in the art without an effective effort belong to the protection scope of the present disclosure.

[0055] FIG. 5 is a schematic structural diagram of a communication system according to an embodiment of this application. The communication system may be applied to submarine fiber communication, and may also be applied to other communication scenarios, which are not enumerated herein. As shown in FIG. 5, the communication system includes a first trunk station 1, a second trunk station 2, N branch terminal stations 3, and M cascaded optical add/drop multiplexer branching units 4, wherein N=M. A signal transmission path between the first trunk station 1 and the second trunk station 2 is a trunk line, and signal transmission paths between the M cascaded optical add/drop multiplexer branching units 4 and the N branch terminal stations 3 are branch lines. Signals output by the first trunk station 1, the second trunk station 2, and the N branch terminal stations 3 all need to pass through corresponding optical add/drop multiplexer branching units among the M cascaded optical add/drop multiplexer branching units 4, to obtain processed signals and transmit the processed signals to a target terminal station.

[0056] Signals transmitted between various terminal stations in FIG. 5 are described by using FIG. 6 as an example. As shown in FIG. 6, the communication system includes the first trunk station 1, the second trunk station 2, the branch terminal station 3 (which may be any one among the N branch terminal stations in FIG. 5), and the optical add/drop multiplexer branching unit 4 (which may be any one among the Iv( optical add/drop multiplexer branching units in FIG. 5). In this embodiment, it is defined that service signals between the terminal stations have corresponding relationships with signal transmission directions, and the signal transmission direction may be described by using output of a signal from one terminal station to another terminal station. As shown in FIG. 6, (an arrow in the figure indicates a signal transmission direction), a service signal transmitted from the first trunk station 1 to the second trunk station 2 is a first service signal Al (hereinafter referred to as Al), a service signal transmitted from the first trunk station 1 to the branch terminal station 3 is a second service signal A2 (hereinafter referred to as A2), a service signal transmitted from the second trunk station 2 to the first trunk station I is a third service signal B! (hereinafter referred to as B1), a service signal transmitted from the second trunk station 2 to the branch terminal station 3 is a fourth service signal B2 (hereinafter referred to as B2), a service signal transmitted from the branch terminal station 3 to the first trunk station I is a fifth service signal C I (hereinafter referred to as CI), and a service signal transmitted from the branch terminal station to the second trunk station 2 is a sixth service signal C2 (hereinafter referred to as C2).

[0057] Embodiment] [0058] In this embodiment, in a submarine fiber communication system shown in FIG. 6, a signal transmission is performed between the branch terminal station 3and the first trunk station 1 and the second trunk station 2, respectively.

[0059] Before a service signal is output from each terminal station, the service signal first needs to be loaded on a corresponding frequency band. Therefore, an overall bandwidth is first divided into three subbands (a first subband, a second subband, and a third subband) according to a preset bandwidth allocation ratio by the terminal station. For example, the first subband is used to carry the signals (A2 and Cl) transmitted between the first trunk station 1 and the branch terminal station 3, the second subband is used to carry the signals (Al and B1) transmitted between the first trunk station I and the second trunk station 2, and the third subband is used to carry the signals (B2 and C2) transmitted between the second trunk station 2 and the branch terminal station 3. That is, each subband carries service signals between two terminal stations that serve as a target terminal station for each other. According to the foregoing carrying relationships between the subbands and the service signals, a first combined signal output by the first trunk station I includes the signal A2 carried on the first subband, the service signal Al carried on the second subband, and a first load signal Li (hereinafter referred to as Li) that is carried on the third subband and is used to balance optical power. A second combined signal output by the second trunk station 2 includes a second load signal L2 (hereinafter referred to as L2) that is carried on the first subband and is used to balance optical power the service signal B1 carried on the second subband, and the service signal B2 carried on the third subband.

A third combined signal output by the branch terminal station 3 includes the service signal Cl carried on the first subband, a third load signal L3 (hereinafter referred to as L3) that is carried on the second subband and is used to balance optical power, and the service signal C2 carried on the third subband. As shown in FIG. 6, after the first combined signal, the second combined signal, and the third combined signal are processed by using the optical add/drop multiplexer branching unit 4, a fourth combined signal transmitted to the first trunk station 1, a fifth combined signal transmitted to the second trunk station 2, and a sixth combined signal transmitted to the branch terminal station 3 are obtained, respectively. The fourth combined signal, the fifth combined signal, and the sixth combined signal all include a target service signal and at least two non-target service signals. The target service signal refers to a service signal whose destination terminal station is consistent with a receiving terminal station, and the non-target service signal refers to a service signal whose destination terminal station is inconsistent with the receiving terminal station. A filter in the optical add/drop multiplexer branching unit 4 merely blocks subbands on which load signals in the first combined signal, the second combined signal, arid the third combined signal are carried, that is, respective service signals may be completely reserved by filtering merely the load signals, thereby reserving the complete target service signal, and facilitating subsequent crosstalk processing between the non-target service signals. Specifically, the target service signal and the at least two non-target service signals need to satisfy the following condition. the target service signal and the at least two non-target service signals are carried on different subbands, and the at least two non-target service signals are carried on a same subband. In this way, co-channel crosstalk may be generated between the at least two non-target service signals that are located on the same subband, thus a garbled signal for which service information is difficult to be recovered may be formed on the subband.

[0060] Hence, it may be ensured that a quantity of fiber pairs of the branch terminal station 3 is consistent with that of fiber pairs of the trunk station, and transmission of the service signal can be completed without increasing the quantity of the fiber pairs. Meanwhile, in the combined signal received by each terminal station, a garbled signal is formed due to the crosstalk between the non-target service signals. Therefore, each terminal station may merely obtain service information carried in the target service signal, and cannot obtain service information carried in the non-target service signal, thereby ensuring security of service signal transmission, and thus preventing each terminal station from monitoring a non-target terminal station (which is a terminal station that does not serve as a destination terminal station of a transmitted service signal).

[0061] For example, the subbands may be classified according to a quantity of service signals transmitted between the terminal stations (a quantity of service signals between the trunk stations is relatively large, and a quantity of service signals between the trunk station and the branch terminal station is relatively small), where a relatively wide subband is allocated for the service signal transmitted between the trunk stations, and a relatively narrow subband is allocated for the service signal transmitted between the trunk station and the branch terminal station. It is assumed that the overall bandwidth corresponds to a frequency band 0-15, where the first subband corresponds to a frequency band 0-3, the second subband corresponds to a frequency band 3-12, and the third subband corresponds to a frequency band 12-15. Wavelengths ranges of the three subbands are different.

[0062] As shown in FIG. 6, the optical add/drop multiplexer branching unit 4 includes a first splitter 411, a first filter 412, a second filter 413, a first combiner 414, a second splitter 421, a third filter 422, a fourth filter 423, a second combiner 424, a third splitter 431, a fifth filter 432, a sixth filter 433, and a third combiner 434.

[0063] The first splitter 411, the second splitter 421, and the third splitter 431 may respectively allocate the first combined signal, the second combined signal, and the third combined signal according to an optical power ratio, and a split ratio is irrelevant to a wavelength. Each splitter has at least one input port and at least two output ports, to split an input combined signal into a plurality of split signals for output, and transmit the split signals to different routing paths in the optical add/drop multiplexer branching unit 4, respectively.

[0064] The first filter 412, the second filter 413, the third filter 422, the fourth filter 423, the fifth filter 432, and the sixth filter 433 may be fixed filters, or may be reconfigurable filters, such as reconfigurable wavelength blockers (Wavelength Blockers, WBs). Each filter may block a specified subband, and meanwhile attenuate optical power of a service signal carried on the specified subband. Setting of each filter needs to satisfy the following condition: a sum of optical power spectrum density of at least two non-target service signals in a subsequent combined signal formed by using the combiner is enabled to be equal to optical power spectrum density of each target service signal, to ensure power balance in signals with different wavelengths in the combined signal.

[0065] In some embodiments, when there is crosstalk between the at least two non-target service signals, crosstalk quality is affected by optical power of each non-target service signal.

That is to say, as the optical power of non-target service signals each is closer, the crosstalk quality is higher, thereby the transmission security of the non-target service signal being higher. Hence, to improve the crosstalk quality, optical power of each non-target service signal in the same combined signal, after being attenuated by the filter, may be equal. For example, a same combined signal includes two non-target service signals, and the filter is configured to attenuate optical power of each non-target service signal to half of original optical power (that is, 3 dB).

In this way, the attenuated optical power of the two non-target service signals is equal, and crosstalk quality on the same subband is the highest.

[0066] The first combiner 414, the second combiner 424, and the third combiner 434 may combine filtered split signals, where the combiner is wavelength independent. The combiner generally has at least two input ports and at least one output port. In this embodiment, each combiner includes two input ports and one output port, to receive two split signals by using the two input ports, and transmit combined signals combined into one path to a corresponding terminal station by using the output port.

[0067] A process of splitting and transmitting the combined signals, output by various terminal stations, in the optical add/drop multiplexer branching unit 4 is described with reference to FIG. 6 (an rectangle in odd-form formed by dotted curves and solid straight lines in FIG. 6 represents the overall bandwidth, where each small rectangle within the rectangle represents a subband, the width of each small rectangle represents the bandwidth of the subband, and the height of each small rectangle represents optical power spectrum density of a service signal carried on the subband), and details are as below.

[0068] Regarding the first trunk station 1: [0069] A signal transmission path is formed by an output interface 11 of the first trunk station 1 and the first splitter 411 through a fiber, and the first splitter 411 receives the first combined signal (the signal A2 carried on the first subband+ the service signal Al carried on the second subband+ the first load signal Ll carried on the third subband) output by the first trunk station I. The first splitter 411 splits the first combined signal into two signals (a first split signal and a second split signal) with equal power according to the split ratio ( the split ratio of 1:1 is used as an example for all splitters in this embodiment; and in other embodiments, other split ratios may also be used as required). The first split signal and the second split signal include same signals as the first combined signal with different optical power only. The first split signal is to be transmitted to the second trunk station 2, and the second split signal is to be transmitted to the branch terminal station 3.

[0070] The first split signal passes through the first filter 412, and the first filter 412 is configured to completely block the third subband, and attenuate optical power of the service signal carried on the first subband to half of original optical power (in this embodiment, an example of attenuating optical power to half of original optical power, that is, enabling attenuated optical power of non-target service signals whose optical power needs to be attenuated to be equal is used for all filters; and in other embodiments, other attenuation manners may also be used as required). In other words, the first load signal L1 is filtered, and the service signals Al and A2 are passed, where optical power of the service signal A2 is halved (the filtered first split signal includes the service signal A2 that is carried on the first subband with halved optical power and the service signal Al carried on the second subband). The first split signal processed by using the first filter 412 may be continued to be transmitted to the second trunk station 2 [0071] The second split signal passes through the second filter 413, and the second filter 413 is configured to completely block the third subband, and attenuate optical power of the service signal carried on the second subband to half of original optical power. In other words, the first load signal Li is filtered, and the service signals Al and A2 are passed, where optical power of the service signal Al is halved (the filtered second split signal includes the service signal A2 carried on the first subband and the service signal Al that is carried on the second subband with halved power). The second split signal processed by using the second filter 413 may be continued to be transmitted to the branch terminal station 3.

[0072] Regarding the second trunk station 2: [0073] A signal transmission path is formed by an output interface 21 of the second trunk station 2 and the second splitter 421 through a fiber, and the second splitter 421 receives the second combined signal (the second load signal L2 carried on the first subband+ the service signal B1 carried on the second subband+ the service signal B2 carried on the third subband) output by the second trunk station 2. The second splitter 421 splits the second combined signal into two signals (a third split signal and a fourth split signal) with equal power according to the split ratio. The third split signal and the fourth split signal include same signals as the second combined signal with different optical power only. The third split signal is to be transmitted to the first trunk station 1, and the fourth split signal is to be transmitted to the branch terminal station 3.

[0074] The third split signal passes through the third filter 422, and the third filter 422 is configured to completely block the first subband, and attenuate optical power of the service signal carried on the third subband to half of original optical power. In other words, the second load signal L2 is filtered, and the service signals B1 and B2 are passed, where optical power of the service signal B2 is halved (the filtered third split signal includes the service signal B1 carried on the second subband and the service signal B2 that is carried on the third subband with halved power). The third split signal processed by using the third filter 422 may be continued to be transmitted to the first trunk station 1.

[0075] The fourth split signal passes through the fourth filter 423, and the fourth filter 423 is configured to completely block the first subband, and attenuate the optical power of the service signal carried on the second subband to half of the original optical power. In other words, the second load signal L2 is filtered, and the service signals B1 and B2 are passed, where optical power of the service signal B1 is halved (the filtered fourth split signal includes the service signal B1 that is carried on the second subband with halved power and the service signal B2 carried on the third subband). The fourth split signal processed by using the fourth filter 423 may be continued to be transmitted to the branch terminal station 3.

[0076] Regarding the branch terminal station 3: [0077] A signal transmission path is formed by an output interface 31 of the branch terminal station 3 and the third splitter 431 through a fiber, and the third splitter 431 receives the third combined signal (the service signal Cl carried on the first subband+ the third load signal L3 carried on the second subband+ the second load signal C2 carried on the third subband) output by the branch terminal station 3. The third splitter 431 splits the third combined signal into two signals (a fifth split signal and a sixth split signal) with equal power according to the split ratio. The fifth split signal and the sixth split signal include same signals as the third combined signal with different optical power only. The fifth split signal is to be transmitted to the first trunk station I, and the sixth split signal is to be transmitted to the second trunk station 2.

[0078] The fifth split signal passes through the fifth filter 432, and the fifth filter 432 is configured to completely block the second subband, and attenuate optical power of the service signal C2 to half of original optical power. In other words, the third load signal L3 is filtered, and the service signals CI and C2 are passed, where the optical power of the service signal C2 is halved (the filtered fifth split signal includes the service signal Cl carried on the first subband and the service signal C2 that is carried on the third subband with halved power). The fifth split signal processed by using the fifth filter 432 may be continued to be transmitted to the first trunk station I. [0079] The sixth split signal passes through the sixth filter 433, and the sixth filter 433 is configured to completely block the second subband, and attenuate optical power of the service signal Cl to half of original optical power. In other words, the third load signal L3 is filtered, and the service signals CI and C2 are passed, where the optical power of the service signal C1 is halved (the filtered sixth split signal includes the service signal CI that is carried on the first subband with halved power and the service signal C2 carried on the third subband). The sixth split signal processed by using the sixth filter 433 may be continued to be transmitted to the second trunk station 2.

[0080] Implementations of the first filter 412 and the second filter 413 are exemplarily described below. In this embodiment of this application, the first filter 412 and the second filter 413 may be band-stop filters, or may be combinations of band-stop filters and a band-pass filters. For example, the first filter 412 may be implemented by two band-stop filters with different models. One of the band-stop filters is configured to completely block (suppress) the third subband, while the other subbands (the first subband and the second subband) are passed. Another band-stop filter after this band-stop filter is configured to attenuate the optical power of the service signal carried on the first subband to half of the original optical power, but allow the second subband to completely pass through. For another example, the second filter 413 may be implemented by a band-stop filter and a band-pass filter. The band-pass filter is configured to allow only the first subband and the second subband to pass through, while the subband of another wavelength (such as the third subband) is blocked. The band-stop filter after the band-pass filter is configured to attenuate the optical power of the service signal carried on the first subband to half of the original optical power, but allow the second subband to completely pass through.

[0081] It should be noted that the implementations of the first filter 412 and the second filter 413 may refer to each other, and the implementation of each filter may also be selected according to the foregoing content, and the description in details are omitted herein. Models of the filters may be the same or may be different, and may be designed according to actual requirements.

[0082] A process of combining combined signals, received by various terminal stations, in the optical add/drop multiplexer branching unit 4 is described based on the foregoing process. Details are as follows.

[0083] Regarding the first tnink station 1: [0084] The first combiner 414 receives the filtered third split signal (the service signal B1 carried on the second subband and the service signal B2 that is carried on the third subband with halved power) and the filtered fifth split signal (the service signal Cl carried on the first subband and the service signal C2 that is carried on the third subband with halved power), and combines the filtered third split signal and the filtered fifth split signal into the fourth combined signal. The fourth combined signal includes the service signal Cl carried on the first subband, the service signal B1 carried on the second subband, and the service signals B2 and C2 that are carried on the third subband. When the service signals B2 and C2 are combined, both service signals are carried on the third subband, and the frequency bands are completely overlapped. Therefore, co-channel crosstalk is formed during combining, which causes a data stream carried on the third subband to be completely scrambled, thus forming a garbled signal. Moreover, the garbled signal cannot be recovered.

[0085] A signal transmission path is formed by the first combiner 414 and an input interface 12 of the first trunk station 1 through a fiber, and the fourth combined signal is transmitted to the first trunk station 1 through the signal transmission path. Therefore, the first trunk station 1 may receive the service signal B1 transmitted by the second trunk station 2 and the service signal Cl transmitted by the branch terminal station 3. Meanwhile, co-channel crosstalk occurs when the service signal B2 that is to be transmitted to the branch terminal station 3 by the second trunk station 2 and the service signal C2 that is to be transmitted to the second trunk station 2 by the branch terminal station 3 are combined, and a garbled service signal is formed.

In this case, although being received by the first trunk station 1, the garbled service signal still cannot be recovered, thereby ensuring security of a communication service between the second trunk station 2 and the branch terminal station 3.

[0086] Regarding the second trunk station 2: [0087] The second combiner 424 receives the filtered first split signal (the service signal A2 that is carried on the first subband with halved power and the service signal Al carried on the second subband) and the filtered sixth split signal (the service signal C1 that is carried on the first subband with halved power and the service signal C2 carried on the third subband), and combines the filtered first split signal and the filtered sixth split signal into the fifth combined signal. The fifth combined signal includes the service signals A2 and Cl that are carried on the first subband, the service signal Al carried on the second subband, and the service signal C2 carried on the third subband. When the service signals A2 and Cl are combined, both service signals are carried on the first subband, and the frequency bands are completely overlapped. Therefore, co-channel crosstalk is formed during combining, which causes a data stream carried on the first subband to be completely scrambled. Moreover, such a garbled service signal cannot be recovered.

[0088] A signal transmission path is formed by the second combiner 424 and an input interface 22 of the second trunk station 2 through a fiber, and the fifth combined signal is transmitted to the second trunk station 2 through the signal transmission path. Therefore, the second trunk station 2 may receive the service signal Al transmitted by the first trunk station 1 and the service signal C2 transmitted by the branch terminal station 3. Meanwhile, co-channel crosstalk occurs when the service signal A2 that is to be transmitted to the branch terminal station 3 by the first trunk station I and the service signal CI that is to be transmitted to the first trunk station 1 by the branch terminal station 3 are combined, and a garbled service signal is formed. In this case, although being received by the second trunk station 2, the garbled service signal still cannot be recovered, thereby ensuring security of a communication service between the first trunk station 1 and the branch terminal station 3.

[0089] Regarding the branch terminal station 3: [0090] The third combiner 434 receives the filtered second split signal (the service signal A2 carried on the first subband and the service signal Al that is carried on the second subband with halved power) and the filtered fourth split signal (the service signal B1 that is carried on the second subband with halved power and the service signal B2 carried on the third subband), and combines the filtered second split signal and the filtered fourth split signal into the sixth combined signal The sixth combined signal includes the service signal A2 carried on the first subband, the service signals Al and B1 that are carried on the second subband, and the service signal B2 carried on the third subband. When the service signals Al and B1 are combined, both service signals are carried on the second subband, and the frequency bands are completely overlapped. Therefore, co-channel crosstalk is formed during combining, which causes a data stream carried on the second subband to be completely scrambled. Moreover, such a garbled service signal cannot be recovered.

[0091] A signal transmission path is formed by the third combiner 434 and an input interface 32 of the branch terminal station 3 through a fiber, and the sixth combined signal is transmitted to the branch terminal station 3 through the signal transmission path. Therefore, the branch terminal station 3 may receive the service signal A2 transmitted by the first trunk station 1 and the service signal B2 transmitted by the second trunk station 2. Meanwhile, co-channel crosstalk occurs when the service signal M that is to be transmitted to the second trunk station 2 by the first trunk station 1 and the service signal B1 that is to be transmitted to the first trunk station 1 by the second trunk station 2 are combined, and a garbled service signal is formed. In this case, although being received by the branch terminal station 3, the garbled service signal still cannot be recovered, thereby ensuring security of a communication service between the first trunk station 1 and the second trunk station 2.

[0092] It may be known from the foregoing technical solutions that the non-target service signals in the combined signal received by each terminal station may be interfered on a same subband, so as to prevent the terminal station from obtaining the service information in the non-target service signal, thereby ensuring security of service signals transmitted among various terminal stations, [0093] Embodiment 2 [0094] In Embodiment 1, subbands are classified merely according to a preset condition (for example, a historical quantity of service signals), and such roughly classified subbands may be referred to as initial subbands. However, during actual use, the quantity of service signals between terminal stations changes dynamically. If the initial subbands are always used for signal transmission, the dynamic change of the quantity of the service signals may be difficult to be adapted to. During use, the initial subband may usually be divided into two parts, that is, an occupied bandwidth (which is really used for signal transmission) and an idle subband (which is not yet used for signal transmission). To adapt to the dynamic change of the quantity of the service signals of each terminal station, that is, once a quantity of signals carried on a certain initial subband exceeds a bandwidth corresponding to the initial subband, idle subbands of other initial subbands may be used to transmit a redundant signal corresponding to the exceeded bandwidth. In this case, the idle subbands may be re-classified by each filter. Hence, the filter in this embodiment specifically refers to a reconfigurable filter, such as a WB. The idle subband is defined as a fourth subband. The fourth subband is used to carry a redundant signal corresponding to a first subband, a second subband, or a third subband, where the fourth subband may be adjacent to an initial subband corresponding to the redundant signal, or may be spaced apart from the initial subband corresponding to the redundant signal.

[0095] For example, regarding the initial subband shown in FIG. 7, it is assumed that an overall bandwidth corresponds to a frequency band 0-15, where the first subband corresponds to a frequency band 0-3, the second subband corresponds to a frequency band 3-12, and the third subband corresponds to a frequency band 12-15. During actual use, the first subband is completely occupied by a signal transmitted between the first trunk station 1 and the branch terminal station 3, and the second subband is completely occupied by a signal transmitted between the first trunk station 1 and the second trunk station 2. And merely a part of the bandwidth in the third subband is occupied by a signal transmitted between the second trunk station 2 and the branch terminal station 3. As shown in FIG. 7, for example, merely bandwidth 12-13 is occupied. Thus, the bandwidth 13-15 in the third subband is an idle subband. When a quantity of signals transmitted between the first trunk station I and the branch terminal station 3 is increased, the idle subband may be used to transmit the increased signals. In this case, as shown in FIG. 8, the idle subband is reset to the fourth subband, and the fourth subband is spaced apart from the first subband. The fourth subband is used to carry the signal transmitted between the first trunk station 1 and the branch terminal station 3. Alternatively, when a quantity of signals transmitted between the first trunk station 1 and the second trunk station 2 is increased, the idle subband may be used to transmit the increased signals. In this case, in the subband classification manner shown in FIG. 8, the idle subband is reset to the fourth subband, and the fourth subband is spaced apart from the second subband, which is used to carry the signal transmitted between the first trunk station 1 and the second trunk station 2.

[0096] In some embodiments, if only the bandwidth 13-15 in the third subband is occupied, the bandwidth 12-13 in the third subband is an idle subband. In this case, when a quantity of signals transmitted between the first trunk station 1 and the branch terminal station 3 is increased, the idle subband may be used to transmit the increased signals. In this case, as shown in FIG. 9, the idle subband is reset to the fourth subband, and the fourth subband is spaced apart from the first subband, which is used to carry the signal transmitted between the first trunk station 1 and the branch terminal station 3. Alternatively, when a quantity of signals transmitted between the first trunk station 1 and the second trunk station 2 is increased, the idle subband may be used to transmit the increased signals. In this case, in the subband classification manner shown in FIG. 9, the idle subband is reset to the fourth subband, and the fourth subband is spaced apart from the second subband, which is used to carry the signal transmitted between the first trunk station 1 and the second trunk station 2.

[0097] For a case in which the fourth subband is adjacent to the subband corresponding to the redundant signal, after the subbands are re-classified, for a signal transmission process between the terminal stations, reference may be directly made to Embodiment 1, and details are omitted herein.

[0098] For a case in which the fourth subband is spaced apart from the subband corresponding to the redundant signal, take the fourth subband as an example which is used to carry the signal transmitted between the first trunk station 1 and the second trunk station 2, and is spaced apart from the second subband. For ease of description, the service signals Al and B1 that are carried on the second subband are represented by using A1-1 and B1-1, and redundant service signals carried on the fourth subband are represented by using A1-2 and B1-2. For service signals carded on other subbands, reference may be made to Embodiment I. In this case, for a signal transmission process between the terminal stations, reference may be made to FIG. 10. For splitting, filtering, and combining processes of the service signals on various subbands, reference may all be made to Embodiment 1 and the description in details are omitted herein.

[0099] Embodiment 3 [00100] The optical add/drop multiplexer branching unit 4 provided in accordance with Embodiment 1, may further enable the branch terminal station 3 to perform signal transmission with only one trunk station of the first trunk station 1 and the second trunk station 2. In this case, the initial subbands (the first subband, the second subband, and the third subband) in Embodiment 1 may be reconstructed by various filters, to combine a subband that does not need to carry a signal (there is no signal transmission relationship between terminal stations) with an adjacent subband, and obtain two reconstructed subbands. The reconstructed subbands are used to carry corresponding signals for signal transmission. Hence, the filter in this embodiment specifically refers to a reconfigurable filter, such as a WB.

[00101] For example, signal transmission between the first trunk station 1 and the branch terminal station 3 is blocked, and signal transmission between the second trunk station 2 and the branch terminal station 3 is maintained. In this case, through subband reconstruction, the first subband and the second subband may be combined, and the third subband may be maintained.

The combined first subband and second subband may be referred to as a fifth subband. In some embodiments, an original bandwidth of the third subband may remain unchanged, or bandwidths of the fifth subband and the third subband may be readjusted according to actual requirements. In this case, the third subband is still used to carry a signal between the second trunk station 2 and the branch terminal station 3, and the fifth subband is used to carry a signal between the first trunk station 1 and the second trunk station 2. In this case, there is no subband used to carry a signal between the first trunk station I and the branch terminal station 3. In some embodiments, the third subband may alternatively be used to carry a signal between the first trunk station 1 and the second trunk station 2, and the fifth subband may be used to carry a signal between the second trunk station 2 and the branch terminal station 3. For example, the third subband is used to carry the signal between the second trunk station 2 and the branch terminal station 3, and the fifth subband is used to carry the signal between the first trunk station 1 and the second trunk station 2. For a signal transmission process between the terminal stations, reference may be made to FIG. 11. For splitting, filtering, and combining processes of service signals on various subbands, reference may all be made to Embodiment 1, and the

description in details are omitted herein.

[00102] Based on the reconstructed subband in this embodiment, if the branch terminal station 3 is required again to transmit signals to the first trunk station 1 and the second trunk station 2, the current subband may be reconstructed again, to split any current subband. A part of the split subband is used to carry a service signal currently carried on the split subband, and the remaining subband is used to carry a newly added service signal (a service signal corresponding to a trunk station that establishes a signal transmission relationship with the branch terminal station again). For splitting, filtering, and combining processes of a service signal on the subband reconstructed again, reference may all be made to Embodiment 1, and the description in details are omitted herein.

[00103] It may be known from the foregoing technical solutions that by loading the service signals communicated between the two trunk stations and between the two trunk stations and the branch terminal station to different subbands for transmission, a problem of wavelength collisions is avoided, and signals on the corresponding subbands may be combined and transmitted on one fiber. Therefore, a trunk section and a branch section can use a same quantity of fiber pairs, to support the two trunk stations to communicate with the branch terminal station, thereby reducing complexity and costs of a submarine cable system.

[00104] Meanwhile, during a process of combining the service signals, non-target service signals of each terminal station are all carried on a same subband, and the various non-target service signals respectively come from different terminal stations. Therefore, crosstalk may be formed between the non-target service signals, to form an irrecoverable garbled signal. In this way, it is ensured that service information carried in the non-target service signal cannot be obtained by the terminal station, thereby ensuring transmission security of the service information between the terminal stations. The scrambling scheme does not require an additional device such as a demultiplexer/multiplexer or a coupler. Therefore, transmission performance of the system would not be degraded due to intermediate-level insertion loss changes of an OADM/ROADM.

[00105] Moreover, connectivity (blocking or connection) between the terminal stations may be adjusted by reconfiguring the subbands, without redesigning a network physical layer. In other words, through changing subband settings of the WB merely by using a command control signal distributed by network management, communication bandwidths between various terminal stations may be flexibly allocated according to changes in service requirements, so as to support the branch terminal station to communicate with one or two trunk stations. In this way, co-existence of bandwidth shortage and wavelength idleness is effectively avoided, thereby improving bandwidth utilization of the system, and lowering requirements for accuracy of service prediction at an initial stage of network construction.

[00106] Embodiment 4 [00107] In this embodiment, in a submarine fiber communication system as shown in FIG. 12, the branch terminal station 3 respectively performs signal transmission with the first trunk station 1 and the second trunk station 2. Before each terminal station outputs a service signal, the service signal first needs to be loaded on a corresponding frequency band. For a specific loading process (subband classification or subband reconstruction), reference may be directly made to Embodiment 1, Embodiment 2 or Embodiment 3, and the description in details are omitted herein.

[00108] As shown in FIG. 12, the optical add/drop multiplexer branching unit 4 in this embodiment includes a first wavelength selective switch (Wavelength Selectable Switch, WSS) 415, the first combiner 414, a second wavelength selective switch 425, the second combiner 424, a third wavelength selective switch 435, and the third combiner 434. Each wavelength selective switch WSS may be a 1zN (N>2) wavelength selective switch, and has a plurality of output ports, where N refers to a quantity of the output ports.

[00109] First, the first wavelength selective switch 415, the second wavelength selective switch 425, and the third wavelength selective switch 435 may respectively allocate a first combined signal, a second combined signal, and a third combined signal according to a ratio, and a split ratio is irrelevant to a wavelength. Each wavelength selective switch WSS has an input port and at least two output ports, to split an input combined signal into a plurality of split signals for output. The split signals are respectively transmitted to different routing paths in the optical add/drop multiplexer branching unit 4.

[00110] Second, the first wavelength selective switch 415, the second wavelength selective switch 425, and the third wavelength selective switch 435 may further be dynamically reconstructed, and may be used as reconfigurable wavelength blockers. Each wavelength selective switch WSS may block a specified subband, and meanwhile attenuate optical power of a service signal carried on the specified subband. Setting of each wavelength selective switch WSS needs to satisfy the following condition: a sum of optical power spectrum density of at least two non-target service signals in a subsequent combined signal formed by using a combiner is enabled to be equal to optical power spectrum density of each target service signal, to ensure power balance in signals with different wavelengths in the combined signal.

[00111] For a process of combining filtered split signals by using the first combiner 414, the second combiner 424, and the third combiner 434, reference may be directly made to Embodiment 1, and the description in details are omitted herein.

[00112] It may be known from the foregoing content that each wavelength selective switch WSS can perform splitting and filtering, and may be used to replace with a splitter and a filter on an output path of the splitter. Details are as follows.

[00113] Regarding the first trunk station [00114] A signal transmission path is formed by an output interface 11 of the first trunk station 1 and the first wavelength selective switch 415 through a fiber, and the first wavelength selective switch 415 receives the first combined signal (the service signal A2 carried on a first subband + the service signal Al carried on a second subband+ the first load signal Li carried on a third subband) output by the first trunk station E The first wavelength selective switch 415 splits the first combined signal into two signals (a first split signal and a second split signal) with equal power according to the split ratio ( the split ratio of 1:1 is used as an example for all splitters in this embodiment; and in other embodiments, other split ratios may also be used as required). The first split signal and the second split signal include same signals as the first combined signal with different optical power only. The first split signal is to be transmitted to the second trunk station 2, and the second split signal is to be transmitted to the branch terminal station 3.

[00115] The first wavelength selective switch 415 is further configured to filter the first split signal. The first wavelength selective switch 415 may be reconfigured to completely block the third subband, and attenuate optical power of a service signal carried on the first subband to half of original optical power (in this embodiment, an example of attenuating optical power to half of original optical power, that is, enabling attenuated optical power of non-target service signals whose optical power needs to be attenuated to be equal is used for all filters; and in other embodiments, other attenuation manners may also be used as required). In other words, the first load signal Li is filtered, and the service signals Al and A2 are passed, where optical power of the service signal A2 is halved (the filtered first split signal includes the service signal A2 that is carried on the first subband with halved optical power and the service signal Al carried on the second subband). The first split signal processed by using the first wavelength selective switch 415 may be continued to be transmitted to the second trunk station 2 through a first output port 4151.

[00116] The first wavelength selective switch 415 is further configured to filter the second split signal. The first wavelength selective switch 415 may be reconfigured to completely block the third subband, and attenuate optical power of a service signal carried on the second subband to half of original optical power. In other words, the first load signal Lt is filtered, and the service signals Al and A2 are passed, where optical power of the service signal Al is halved (the filtered second split signal includes the service signal A2 carried on the first subband and the service signal Al that is carried on the second subband with halved power). The second split signal processed by using the first wavelength selective switch 415 may be continued to be transmitted to the branch terminal station 3 through a second output port 4152.

[00117] In other words, the first wavelength selective switch 415 may be used to replace with a first splitter 411, a first filter 412, and a second filter 413.

[00118] Regarding the second trunk station 2: [00119] A signal transmission path is formed by an output interface 21 of the second trunk station 2 and the second wavelength selective switch 425 through a fiber, and the second wavelength selective switch 425 receives the second combined signal (the second load signal L2 carried on the first subband+ the signal B1 carried on the second subband+ the signal B2 carried on the third subband) output by the second trunk station 2. The second wavelength selective switch 425 splits the second combined signal into two signals (a third split signal and a fourth split signal) with equal power according to the split ratio. The third split signal and the fourth split signal include same signals as the second combined signal with different optical power only. The third split signal is to be transmitted to the first trunk station 1, and the fourth split signal is to be transmitted to the branch terminal station 3.

[00120] The second wavelength selective switch 425 is further configured to filter the third split signal. The second wavelength selective switch 425 may be reconfigured to completely block the first subband, and attenuate optical power of a service signal carried on the third subband to half of original optical power. In other words, the second load signal L2 is filtered, and the service signals B1 and B2 are passed, where optical power of the service signal B2 is halved (the filtered third split signal includes the service signal B1 carried on the second subband and the service signal B2 that is carried on the third subband with halved power). The third split signal processed by using the second wavelength selective switch 425 may be continued to be transmitted to the first trunk station 1 through a third output port 4251.

[00121] The second wavelength selective switch 425 is further configured to filter the fourth split signal. The second wavelength selective switch 425 may be reconfigured to completely block the first subband, and attenuate the optical power of the service signal carried on the second subband to half of the original optical power. In other words, the second load signal L2 is filtered, and the service signals B1 and B2 are passed, where optical power of the service signal B1 is halved (the filtered fourth split signal includes the service signal B t that is carried on the second subband with halved power and the service signal B2 carried on the third subband). The fourth split signal processed by using the second wavelength selective switch 425 may be continued to be transmitted to the branch terminal station 3 through a fourth output port 4252.

[00122] In other words, second wavelength selective switch 425 may be used to replace the second splitter 42t, the third filter 422, and the fourth filter 423.

[00123] Regarding the branch terminal station 3: [00124] A signal transmission path is formed by an output interface 31 of the branch terminal station 3 and the third wavelength selective switch 435 through a fiber, and the third wavelength selective switch 435 receives the third combined signal (the service signal CI carried on the first subband+ the third load signal L3 carried on the second subband+ the service signal C2 carried on the third subband) output by the branch terminal station 3. The third wavelength selective switch 435 splits the third combined signal into two signals (a fifth split signal and a sixth split signal) with equal power according to the split ratio. The fifth split signal and the sixth split signal include same signals as the third combined signal with different optical power only. The fifth split signal is to be transmitted to the first trunk station 1, and the sixth split signal is to be transmitted to the second trunk station 2.

[00125] The third wavelength selective switch 435 is further configured to filter the fifth split signal. The third wavelength selective switch 435 may be reconfigured to completely block the second subband, and attenuate optical power of the service signal C2 to half of original optical power. In other words, the third load signal L3 is filtered, and the service signals Cl and C2 are passed, where the optical power of the service signal C2 is halved (the filtered fifth split signal includes the service signal Cl carried on the first subband and the service signal C2 that is carried on the third subband with halved power). The fifth split signal processed by using the third wavelength selective switch 435 may be continued to be transmitted to the first trunk station I through a fifth output port 4351.

[00126] The third wavelength selective switch 435 is further configured to filter the sixth split signal. The third wavelength selective switch 435 may be reconfigured to completely block the second subband, and attenuate optical power of the service signal Cl to half of original optical power. In other words, the third load signal L3 is filtered, and the service signals CI and C2 are passed, where the optical power of the service signal CI is halved (the filtered sixth split signal includes the service signal Cl that is carried on the first subband with halved power and the service signal C2 carried on the third subband). The sixth split signal processed by using the third wavelength selective switch 435 may be continued to be transmitted to the second trunk station 2 through a sixth output port 4352.

[00127] In other words, the third wavelength selective switch 435 may be used to replacethe third splitter 431, the fifth filter 432, and the sixth filter 433.

[00128] For a process of combining the combined signals received by various terminal stations in the optical add/drop multiplexer branching unit 4, reference may be directly made to Embodiment I, and the description in details are omitted herein [00129] It should be noted that the wavelength selective switch WSS may be dynamically reconstructed, and therefore is applicable to the subband classification processes in Embodiment 2 and Embodiment 3.

[00130] It may be known from the foregoing technical solutions that in this embodiment, the wavelength selective switch WSS is selected to replace the splitter and the filter. The wavelength selective switch WSS has higher integration degree, and is easy for mounting and remote control.

[00131] Objectives, technical solutions, and beneficial effects of the present disclosure are further described in detail in the foregoing specific implementations. It should be understood that merely specific implementations of the present disclosure are described above, which are not used to limit the protection scope of the present disclosure. Any modification, equivalent replacement, improvement, and the like made based on the technical solutions of the present disclosure shall fall within the protection scope of the present disclosure.

Claims

C LA I NI S1. A communication system, comprising a plurality of terminal stations, wherein the plurality of terminal stations comprise two trunk stations and at least one branch terminal station, a service signal is transmitted between the two trunk stations, a service signal is transmitted between the at least one branch terminal station and the two trunk stations, and a combined signal output by each terminal station among the plurality of terminal stations is processed by using a corresponding optical add/drop multiplexer branching unit, to obtain a processed combined signal that is transmitted to a peer terminal station; the combined signal output by each terminal station comprises a plurality of service signals, and the plurality of service signals are carried on different subbands; and the optical add/drop multiplexer branching unit comprises a plurality of splitters, a plurality of filters, and a plurality of combiners, and is configured to: each processed combined signal that is obtained after being processed by the plurality of splitters, the plurality of filters, and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.
2 The communication system according to claim 1, wherein the combined signal output by each terminal station further comprises a load signal, and the load signal is used to balance optical power of the plurality of service signals in the combined signal
3 The communication system according to claim 1, wherein a sum of optical power spectrum density of the at least two non-target service signals is equal to optical power spectrum density of the target service signal
4. The communication system according to claim 1, wherein the at least two non-target service signals have equal optical power.
5. The communication system according to claim 1, wherein at least one of the subbands comprises an idle subband, the idle subband is used to carry a redundant signal corresponding to a first target subband, wherein the redundant signal refers to a service signal exceeding a bandwidth of the first target subband
6. The communication system according to claim 5, wherein the idle subband is adjacent to or is spaced apart from the first target subband.
7 The communication system according to claim 1, wherein if the at least one branch terminal station merely communicates with one of the trunk stations, the subbands are divided based on a service signal transmitted between the at least one branch terminal station and one of the trunk stations and a service signal transmitted between the two trunk stations, wherein the service signal transmitted between the at least one branch terminal station and one of the trunk stations and the service signal transmitted between the two trunk stations are carried on different subbands.
8. The communication system according to claim 7, wherein the plurality of filters are reconfigurable wavelength blockers.
9. The communication system according to claim 1, wherein the plurality of filters are band-stop filters with a same parameter or different parameters, or the plurality of filters are combinations of band-stop filters and band-pass filters with a same parameter or different parameters.
A communication system, comprising a plurality of terminal stations, wherein the plurality of terminal stations comprise two trunk stations and at least one branch terminal station, a service signal is transmitted between the two trunk stations, a service signal is transmitted between the at least one branch terminal station and the two trunk stations, and a combined signal output by each terminal station among the plurality of terminal stations is processed by using a corresponding optical add/drop multiplexer branching unit, to obtain a processed combined signal that is transmitted to a peer terminal station, the combined signal output by each terminal station comprises a plurality of service signals, and the plurality of service signals are carried on different subbands, and the optical add/drop multiplexer branching unit comprises a plurality of wavelength selective switches and a plurality of combiners, the wavelength selective switches are configured to split and filter the combined signal, and the optical add/drop multiplexer branching unit is configured to.each processed combined signal that is obtained after being processed by the plurality of wavelength selective switches and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.
11. An optical add/drop multiplexer branching unit, comprising a plurality of splitters, a plurality of filters, and a plurality of combiners, wherein the plurality of splitters are respectively connected to a signal output interface of a corresponding terminal station, and the plurality of combiners are respectively connected to a signal input interface of the corresponding terminal station; and the plurality of splitters, the plurality of filters, and the plurality of combiners are configured to: a processed combined signal that is obtained by processing a combined signal, input to the optical add/drop multiplexer branching unit, by using the plurality of splitters, the plurality of filters, and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.
12. An optical add/drop multiplexer branching unit, comprising a plurality of wavelength selective switches and a plurality of combiners, wherein input ends of the plurality of wavelength selective switches are respectively connected to a signal output interface of a corresponding terminal station, output ends of the plurality of wavelength selective switches are respectively connected to input ends of the plurality of combiners, output ends of the plurality of combiners are respectively connected to a signal input interface of the corresponding terminal station, and the wavelength selective switches are configured to split and filter the combined signal; and the plurality of wavelength selective switches and the plurality of combiners are configured to: a processed combined signal that is obtained by processing a combined signal, input to the optical add/drop multiplexer branching unit, by using the plurality of wavelength selective switches and the plurality of combiners comprises a target service signal and at least two non-target service signals, wherein the target service signal and the at least two non-target service signals are carried on different subbands, the at least two non-target service signals are carried on a same subband, and transmission directions of the at least two non-target service signals are opposite, wherein the target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is consistent with a terminal station that receives the processed combined signal, and the non-target service signal refers to a service signal, in the processed combined signal, for which a corresponding destination terminal station is inconsistent with the terminal station that receives the processed combined signal.
13. A method for signal transmission, used in the communication system according to any one of claims 1 to 10, wherein the method comprises: dividing an overall bandwidth into a specified quantity of subbands by using the terminal station, carrying, by the terminal station, a to-be-output service signal and a to-be-output load signal on different subbands, to obtain a combined signal, processing the combined signal by the optical add/drop multiplexer branching unit, to obtain a processed combined signal, and distributing, by the optical add/drop multiplexer branching unit, the processed combined signal to a peer terminal station of the terminal station.