CN109818670B

CN109818670B - TWDM-PON system with multipoint fault protection capability

Info

Publication number: CN109818670B
Application number: CN201910134306.9A
Authority: CN
Inventors: 吴晨光; 赵景隆; 王文革; 赵豫京; 戚晓勇; 李功明; 张毓琪; 邵奇; 郭亚琴; 甘朝钦
Original assignee: State Grid Corp of China SGCC; Information and Telecommunication Branch of State Grid Henan Electric Power Co Ltd; University of Shanghai for Science and Technology
Current assignee: State Grid Corp of China SGCC; Information and Telecommunication Branch of State Grid Henan Electric Power Co Ltd; University of Shanghai for Science and Technology
Priority date: 2019-02-22
Filing date: 2019-02-22
Publication date: 2020-09-15
Anticipated expiration: 2039-02-22
Also published as: CN109818670A

Abstract

The invention provides a TWDM-PON system with multipoint fault protection capability, which adopts a star-ring topology structure to effectively realize fault isolation and avoid the situation of 'pulling and moving the whole body', more importantly, through the design of the internal structures of a ring network and a far-end node RN, the network can realize fault recovery even if only one far-end node RN can normally work, N:1 comprehensive protection of a feeder line optical fiber is realized, the reliability of the network is greatly improved, in addition, signals are directly transmitted to a target optical network unit ONU along the star network, compared with the ring structure, other nodes are not needed, the transmission time delay and link loss are reduced, the network unit ONU adopts a bus topology structure, the working optical fiber and the protection optical fiber transmit signals simultaneously, a coupler is used instead of an optical switch along the way, and the automatic protection without switching of the ONU is realized, and meanwhile, the network cost is also reduced.

Description

TWDM-PON system with multipoint fault protection capability

Technical Field

The invention relates to the field of optical communication, in particular to a TWDM-PON system with multipoint fault protection capability.

Background

With the emergence of numerous emerging applications, such as high-definition television, virtual reality, and the like, network traffic is increasing explosively, and user bandwidth requirements are increasing dramatically. The WDM-PON and the TWDM-PON in the convergence-access integrated environment are considered to be effective solutions for future broadband access. The realization of the ultra-large bandwidth brings people's thinking about another problem, and once a network fails, mass data can be lost. In addition, many real-time services like telemedicine cannot tolerate long-term network failures. Therefore, it is important to improve the fault protection capability of the network.

In the existing protection scheme for feeder optical fibers, backup protection is usually adopted, or the natural "self-healing capability" of a ring structure is utilized to realize fault recovery. However, the backup protection causes problems of large system redundancy, high network cost, serious waste of bandwidth resources and the like, and the protection capability for multi-point faults is very limited. The ring structure greatly increases the reliability of the network, but even for the dual-fiber ring with the strongest reliability, the network cannot resist more than two dual-fiber faults occurring at the same time. In addition, in the ring structure, signals are sequentially transmitted along the ring network, and when any point fails, the normal operation of the whole network is affected, and fault isolation cannot be realized. Meanwhile, the transmission mode can increase the transmission distance of signals, thereby increasing the transmission delay and the link loss. Therefore, it is of great significance to research the optical network system with multi-point fault protection capability and capable of realizing fault isolation.

It is also worth noting that, in the existing protection scheme of the ONU side of the optical network unit, when a fault occurs, the corresponding optical switch is required to perform protection switching to implement fault recovery. In this process, many ONUs that can normally operate originally are affected, that is, a situation of "pulling and moving the whole body" occurs, which is a service that has a relatively high requirement on communication quality, such as: high quality video conferencing is unacceptable.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide an optical network system with multipoint fault protection capability, so that the optical network system can resist multipoint optical fiber faults occurring at the same time for feeder optical fibers, simultaneously realize fault isolation and avoid mutual interference, and for distributed optical fibers, when a fault occurs, a protection signal can be automatically transmitted to a target ONU without switching an optical switch, so that 'switching-free automatic protection' is realized.

The technical scheme for solving the problem is that an optical line terminal OLT is located at a central node of a network, and is respectively connected to N remote nodes RN through N feeder optical fibers to form a star network, and meanwhile, the remote nodes RN are connected together in a cross mode through the feeder optical fibers to form a double-fiber ring structure. The remote node RN is connected to M optical network units ONU through a distribution optical fiber, and the topological structure of the ONU end is of a bus type, which is characterized in that:

the optical line terminal OLT is composed of N optical transmitters, N optical receivers, a first 1 x N arrayed waveguide grating AWG, a second 2 x N arrayed waveguide grating AWG, an optical circulator, a bidirectional erbium-doped fiber amplifier EDFA and a1 x N optical splitter, wherein in the OLT, the N optical transmitters are connected to the first 1 x N arrayed waveguide grating AWG, the N optical receivers are connected to the second 1 x N arrayed waveguide grating AWG, the output of the first 1 x N arrayed waveguide grating AWG is connected to the optical circulator, the output of the optical circulator is connected to the bidirectional erbium-doped fiber amplifier EDFA, the output of the bidirectional erbium-doped fiber amplifier EDFA is connected to the 1 x N optical splitter, and the N outputs of the 1 x N optical splitter are respectively connected to N far-end nodes RN through N feeder fibers; the remote node RN is composed of a3 x 1 optical switch, a first and a second 21 x 2 power distributors, a tunable filter, a first and a second 2 waveguide array gratings AWG, N1 x 1 optical switches and a l x 2 optical coupler, wherein in the remote node RN, the 3 x 1 optical switch is connected to the first 1 x 2 power distributor, the output of the first 1 x 2 power distributor is divided into two paths, one path is connected to the tunable filter, the output of the tunable filter is connected to the second 1 x 2 power distributor, and the outputs of the second 1 x 2 power distributor are connected to the optical network unit ONU through distribution optical fibers; the other output of the first 1 × 2 power divider is connected to a first waveguide array grating AWG, the outputs of the first waveguide array grating AWG are respectively connected to N1 × 1 optical switches, the outputs of the N1 × 1 optical switches are connected to a second waveguide array grating AWG, and the output of the second waveguide array grating AWG is connected to a1 × 2 optical coupler;

the optical network unit ONU is composed of a coupler, a1 x 2 optical splitter, an optical receiver and a reflective semiconductor optical amplifier RSOA, wherein in the optical network unit ONU, the output of the coupler is connected to the 1 x 2 optical splitter, the output of the 1 x 2 optical splitter is divided into two paths, one path is connected to the optical receiver, and the other path is connected to the reflective semiconductor optical amplifier RSOA.

Preferably, the specific method for implementing uplink and downlink service transmission in the network in the normal operating mode is as follows: in the TWDM-PON system shown, signals are transmitted from the optical line terminal OLT to each remote node RN in the form of wavelength division multiplexing and from the remote node RN to each optical network unit ONU in the form of time division multiplexing, and N optical transmitters generate N optical signals λ carrying respective downstream information at the optical line terminal OLT₁λ₂...λ_N-1λ_NThen N optical signals are multiplexed by a1 x N arrayed waveguide grating AWG wavelength, then pass through an optical circulator, then pass through a bidirectional erbium-doped fiber amplifier EDFA for amplification, and then are transmitted to a1 x N optical splitter, the signals are divided into N paths from the N paths and transmitted to a far-end node RN along a feeder fiber, in the far-end node RN, the signals are transmitted to a1 x 2 optical splitter after passing through a3 x 1 optical switch, the signals are divided into two parts, one part is used for signal reception, the other part is used for fault protection, and the part of signals used for downlink signal reception is selected by a tunable filter and transmitted to the RN_iWavelength signal lambda required by a drop-off ONU_iThen the optical fiber is selected by a 1-to-2 optical splitterThe selected wavelength signal is divided into two paths, one path is transmitted to the RN in sequence along the forward direction of the working distribution optical fiber_iThe other path of each ONU is transmitted to the RN from the opposite direction along the protection distribution optical fiber_iThe two signals are combined into one path in the ONU through a1 x 2 coupler, then the signal is divided into two parts through a1 x 2 optical splitter, one part is connected by an optical receiver, the other part is transmitted to a reflective semiconductor optical amplifier RSOA for the remodulation of an uplink signal, and the remodulated uplink wavelength lambda is carried out in the reflective semiconductor optical amplifier RSOA_iThe uplink signals from the N RNs are sent back to the remote node RN again through a 1X 2 optical splitter and a 1X 2 coupler, the two same uplink signals from the distribution optical fiber are combined into one path through the 1X 2 optical splitter in the remote node RN, then the combined uplink signals are sent back to the optical line terminal OLT through the feeder optical fiber through a tunable filter, the 1X 2 optical splitter and a 3X 1 optical switch in the OLT, and the uplink signals lambda from the N RNs are sent back to the optical line terminal OLT in the OLT₁′λ₂′...λ_NThe signals are combined into one path through a1 × N optical splitter, then sequentially pass through a bidirectional erbium-doped fiber amplifier (EDFA) and an optical circulator to reach a1 × N Arrayed Waveguide Grating (AWG), and are transmitted to an optical receiver to be received after being demultiplexed.

Preferably, the method for implementing the protection function is as follows: s1, when the feeder optical fiber has a fault and at least one far-end node RN at two adjacent sides of the fault optical fiber can work normally, N can be realized through a ring network: 1 full protection, assuming connection to RN_iThe feeder optical fiber of (1) is in fault, and a remote node RN_iThe inner 3 x 1 optical switch carries out protection switching, the port 1 is connected to the port 2, and the remote node RN_i-1OS in internal 1-by-1 optical switch_iProtection switching is carried out, the switch is closed, and N downlink signals lambda generated by the OLT are generated₁λ₂...λ_N-1λ_NReach each remote node RN, RN through feeder optical fiber_iCannot receive a downstream signal belonging to it due to a fault in the feeder fibre connected to it, but contains an RN_iDownstream wavelength λ_iOf the multiplexed signal lambda₁λ₂...λ_N-1λ_NCan reach RN_i-1In RN_i-1Chinese and Western lettersThe signal is divided into two parts after passing through a 3X 1 optical switch and a 1X 2 optical splitter, and one part is used for RN_i-1Receiving self signal, another part demultiplexing RN by a1 × N arrayed waveguide grating AWG_iOf the downstream wavelength λ i, wavelength λ_iThen through the OS in the 1 x 1 optical switch _i1 × N arrayed waveguide grating AWG, output from 3 ports of 1 × 2 optical coupler, and transmitted via RN_i-1And RN_iBetween the feeder optical fiber to the RN_iAt a remote node RN_iIn the method, a signal reaches a1 x 2 optical splitter through a3 x 1 optical switch after protection switching, then the transmission mode of the signal is the same as the normal working mode, and meanwhile, RN (relay node)_iOf the upstream signal lambda_i' also by protecting the switched 3 x 1 optical switch, RN_i-1And RN_iBetween the feeder optical fiber to the RN_i-1In RN_i-1Medium, up wavelength lambda_i' sequentially pass through 1 × 2 optical coupler, 1 × N arrayed waveguide grating AWG, and OS in 1 × 1 optical switch_iAfter reaching 1 × 2 optical splitter, it is connected with RN_i-1Of the upstream wavelength lambda_i-1The combined wave is transmitted to an optical line terminal OLT (1), and then the transmission mode of the signal is the same as the normal working mode;

s2, when a feeder optical fiber fails and two far-end nodes RN at two adjacent sides of the failed optical fiber cannot work normally, N can be realized through a ring network formed by the feeder optical fiber: 1 full protection, assuming connection RN_i、RN_i+1、RN_i+2All feeder fibers of RNi failed₊₁Two adjacent remote node RNs can not work normally, so that failure protection can not be provided for the two remote node RNs, but the two remote node RNs still can pass through the RNs firstly_i-1Making RN_i、RN_i+1、RN_i+2The three fault points realize fault recovery, and the specific method is as follows: n-path downlink signal lambda generated by OLT₁λ₂...λ_N-1λ_NCan reach RN through feeder optical fiber_i-1At a remote node RN_i-1In the method, a signal is divided into two parts after passing through a 3X 1 optical switch and a 1X 2 optical splitter, and one part is used for RN_i-1Receiving self signal, another part demultiplexing RN by a1 × N arrayed waveguide grating AWG_iDownstream wavelength ofλ_i，RN_i+1Of the downstream wavelength lambda_i+1And RN_i+2Of the downstream wavelength lambda_i+2Wavelength λ_i、λ_i+1、λ_i+2Then, the optical signal passes through a1 × 1 optical switch, a1 × N arrayed waveguide grating AWG, is output from a3 port of a1 × 2 optical coupler and passes through RN_i-1And RN_iBetween the feeder optical fiber to the RN_iAt a remote node RN_iIn the method, a signal is divided into two paths after reaching a1 x 2 optical splitter through a3 x 1 optical switch after protection switching, and one path is used for RN_iReceiving self signal, another path demultiplexing RN by a1 × N arrayed waveguide grating AWG_i+1Of the downstream wavelength lambda_i+1And RN_i+2Of the downstream wavelength lambda_i+2Wavelength λ_i+1、λ_i+2Then, the optical signal passes through a1 × 1 optical switch, a1 × N arrayed waveguide grating AWG, is output from a3 port of a1 × 2 optical coupler and passes through RN_iAnd RN_i+1Between the feeder optical fiber to the RN_i+1At a remote node RN_i+1In the method, a signal is divided into two paths after reaching a1 x 2 optical splitter through a3 x 1 optical switch after protection switching, and one path is used for RN_i+1Receiving self signal, another path demultiplexing RN by a1 × N arrayed waveguide grating AWG_i+2Of the downstream wavelength lambda_i+2Wavelength λ_i+2Then, the optical signal passes through a1 × 1 optical switch, a1 × N arrayed waveguide grating AWG, is output from a3 port of a1 × 2 optical coupler and passes through RN_i+1And RN_i+2Between the feeder optical fiber to the RN_i+2At a remote node RN_i+2In the method, a signal reaches a1 x 2 optical splitter through an optical switch after protection switching, then the transmission mode of the signal is the same as the normal working mode, and meanwhile, RN (relay node) transmits the signal to a Relay Node (RN)_i+2Of the upstream signal lambda_i+2' also by protecting the switched 3 x 1 optical switch, RN_i+2And RN_i+1Between the feeder optical fiber to the RN_i+1In RN_i+1Medium, up wavelength lambda_i+2The' optical coupler and the RN sequentially pass through a1 x 2 optical coupler, a1 x N arrayed waveguide grating AWG and a1 x 1 optical switch, reach a1 x 2 optical splitter and then are connected with the RN_i+1Of the upstream wavelength lambda_i+1The 'combined wave' passes through the 3 x 1 optical switch and RN after protection switching_i+1And RN_iBetween the feeder optical fiber to the RN_iAt RN_iMedium, up wavelength lambda_i+2′、λ_i+1The' optical coupler and the RN sequentially pass through a1 x 2 optical coupler, a1 x N arrayed waveguide grating AWG and a1 x 1 optical switch, reach a1 x 2 optical splitter and then are connected with the RN_iOf the upstream wavelength lambda_iThe 'combined wave' passes through the 3 x 1 optical switch and RN after protection switching_iAnd RN_i-1Between the feeder optical fiber to the RN_i-1In RN_i-1Medium, up wavelength lambda_i+2′、λ_i+1′、λ_iThe' optical coupler and the RN sequentially pass through a1 x 2 optical coupler, a1 x N arrayed waveguide grating AWG and a1 x 1 optical switch, reach a1 x 2 optical splitter and then are connected with the RN_i-1Of the upstream wavelength lambda_i-1' Combined wave, upstream signal lambda after combined wave_i+2′、λ_i+1′、λ_i′、λ_i-1Transmitting the signal to an optical line terminal OLT, wherein the transmission mode of the signal is the same as the normal working mode;

s3, when the distribution optical fiber is in fault, the network can automatically realize fault recovery without the need of optical switch for protection switching. The transmission mode of signals from the optical line terminal OLT to the remote node RN is the same as the normal working mode, and due to the middle fault of the working distribution optical fiber, the ONU behind the ONU2₃...ONU_M-1、ONU_MNo downstream signal can be obtained from the working distribution fiber, but the other downstream signal split by the 1 x 2 optical splitter can be transmitted to the ONU sequentially from the opposite direction along the protection distribution fiber_M、ONU_M-1...ONU₃. The transmission mode of the signal after reaching the ONU is the same as the normal working mode.

Compared with the prior art, the invention has the following obvious prominent substantive characteristics and remarkable technical progress:

(1) the network realizes that N:1, the network can realize fault recovery even if only one remote node RN can normally work, thereby greatly improving the reliability of the network;

(2) by utilizing the natural structural advantage of star topology, transmission links of each remote node RN are not interfered with each other, and other RNs which normally work cannot be influenced by a fault, so that fault isolation is effectively realized;

(3) the signal is directly transmitted to the target optical network unit ONU along the star network, and compared with a ring type or bus type structure, the signal transmission method does not need other nodes, thereby reducing transmission delay and link loss;

(4) the ONU end adopts a double-bus type topological structure, only a coupler is used without an optical switch along the way, when the working optical fiber fails, the signal in the protection optical fiber can be automatically coupled to the target ONU without switching, and the non-switching automatic protection is realized;

(5) the working optical fiber and the protection optical fiber transmit signals simultaneously, so that the waste of optical fiber resources caused by idling of the protection optical fiber in a normal network working mode is avoided. Meanwhile, the coupler is lower than the optical switch in cost and loss, and the overall cost of the network and the signal-to-noise ratio of the signal at the receiving end are improved by using the coupler.

Drawings

Fig. 1 is a schematic diagram of a star-ring type TWDM-PON system according to the present invention.

Fig. 2 is a schematic diagram of the internal structure of the optical line terminal OLT according to the present invention.

Fig. 3 is a schematic diagram of an internal structure of a remote node RN according to the present invention.

Fig. 4 is a schematic diagram illustrating a connection method between remote nodes RN according to the present invention.

Fig. 5 is a schematic diagram of an internal structure of an optical network unit ONU according to the present invention.

Fig. 6 is a schematic diagram of an internal structure of a corresponding RN in the feeder fiber discrete fault protection mode according to the present invention.

Fig. 7 is a schematic diagram of an internal structure of a corresponding RN in the feeder optical fiber continuous fault protection mode according to the present invention.

Fig. 8 is a schematic diagram of a distribution optical fiber fault of the present invention.

Detailed Description

The foregoing and other technical matters, features and effects of the present invention will be apparent from the following detailed description of the embodiments, which is to be read in connection with the accompanying drawings of fig. 1 to 8. The structural contents mentioned in the following embodiments are all referred to the attached drawings of the specification.

Exemplary embodiments of the present invention will be described below with reference to the accompanying drawings.

In the first embodiment, a TWDM-PON system with multi-point fault protection capability, referring to fig. 1, an optical line terminal OLT1 is connected to N remote nodes RN3) through a feeder fiber 2 to form a star structure, so that transmission links of the remote nodes RN do not interfere with each other, and a fault does not affect other RNs that normally operate, thereby effectively achieving fault isolation, the remote nodes RN3) are connected to each other through a feeder fiber 4) to form a dual-fiber ring structure, each remote node RN3) is connected to M optical network units ONU6) through a distribution fiber 5), compared with a ring structure, no other nodes need to be accessed, thereby reducing transmission delay and link loss, the optical network unit ONU6 is connected in series from two directions through the distribution fiber 7, the distribution fiber 8, and the optical coupler 9, thereby forming a dual-bus structure, thereby achieving N:1 total protection of the feeder fiber, the network can realize fault recovery even under the condition that only one remote node RN can normally work, and the reliability of the network is greatly improved;

referring to fig. 2, the optical line termination OLT1) comprises N optical transmitters 10, N optical receivers 11, first and second 21 × N arrayed waveguide gratings AWG12, 13, an optical circulator 14, a bidirectional erbium doped fiber amplifier EDFA15 and a1 × N optical splitter 16, wherein in OLT1, the N optical transmitters 10 are connected to the first 1 × N arrayed waveguide grating AWG12, the N optical receivers 11 are connected to the second 1 × N arrayed waveguide grating AWG13, the output of the first 1 × N arrayed waveguide grating 12 is connected to the optical circulator 14, the output of the optical circulator 14 is connected to a bidirectional erbium doped fiber amplifier EDFA15, the output of the bidirectional erbium doped fiber amplifier EDFA15 is connected to a1 × N optical splitter 16, and the 1 × N optical splitter 16 is connected to N remote nodes RN3 via N optical fiber feeders 2;

referring to fig. 3, the far end node RN3 includes a3 × 1 optical switch 17, first and second 21 × 2

power splitters

18 and 19, a tunable filter 20, first and second 2 waveguide array gratings AWG21 and 22, N1 × 1 optical switches 23, and a1 × 2 optical coupler 24, where in the far end node RN3, the 1 port of the 3 × 1 optical switch 17 is connected to the first 1 × 2 power splitter 18, and the 1 port of the first 1 × 2 power splitter 18The 2 port is connected to the tunable filter 20, the output of the tunable filter 20 is connected to the second 1 x 2 power splitter 19, and the 2 and 3 ports of the second 1 x 2 power splitter 19 are connected to the ONU6 through the distribution fibers 7 and 8, respectively); the 3 ports of the first 1 x 2 power divider 18 are connected to the first waveguide array grating AWG21, the outputs of the first waveguide array grating AWG21 are connected to the N1 x 1 optical switches 23, the outputs of the N1 x 1 optical switches 23 are connected to the second waveguide array grating AWG22, the outputs of the second waveguide array grating AWG22 are connected to the 1 x 2 optical coupler 24, the design of the remote node RN3 realizes modularization, and each RN is modularized_iModule pair has 5

external ports

25, 26, 27, 28, 29, RN_iIs connected to OLT1, RN_iAre connected to the RN respectively at

ports

26, 28_i-1

Port

29, 27, RN of_iAre connected to the RN respectively_i+1The connection mode between the

ports

28 and 26 and the RN can be specifically seen in fig. 4;

referring to fig. 5, the ONU6 includes a coupler 30, a1 × 2 optical splitter 31, an optical receiver 32, and a reflective semiconductor optical amplifier RSOA33, in the ONU6, the output of the coupler 30 is connected to the 1 × 2 optical splitter 31, the port 1 of the 1 × 2 optical splitter 31 is connected to the optical receiver 32, the port 2 of the 1 × 2 optical splitter 31 is connected to the reflective semiconductor optical amplifier RSOA33, the ONU6 is also modularized, and each ONU 30 is designed to be modularized_jThe module has 2

external ports

34, 35, ONU_iAre connected to the far end node RN3 by distribution fibres 7, 8 and an optical coupler 9, respectively.

In the second embodiment, on the basis of the first embodiment, referring to fig. 1, fig. 2, fig. 3, fig. 4, and fig. 5, in the normal operating mode, the specific method for implementing uplink and downlink service transmission in the network is as follows: in the TWDM-PON system shown, signals are transmitted in a wavelength division multiplexed form from the optical line terminal OLT1 to each remote node RN3 and in a time division multiplexed form from the remote node RN3 to each optical network unit ONU6, and at the optical line terminal OLT1, N optical transmitters 10 generate N optical signals λ carrying respective downstream information₁λ₂...λ_N-1λ_NThen N wayThe optical signal is multiplexed by a1 × N arrayed waveguide grating AWG12 wavelength, passes through the optical circulator 14, is amplified by a bidirectional erbium-doped fiber amplifier EDFA15 and then is transmitted to a1 × N optical splitter 16, the signal is divided into N paths from the N paths and is transmitted to a far-end node RN3 along a feeder fiber 2, in the far-end node RN3, the signal is transmitted to a1 × 2 optical splitter 18 after passing through a3 × 1 optical switch 17, the signal is divided into two parts, the signal output from a port 3 of the 1 × 2 optical splitter 18 is used for fault protection, the signal output from a port 2 of the 1 × 2 optical splitter 18 is continuously transmitted backwards for receiving a downstream signal, and the part of the signal used for receiving the downstream signal is selected by a tunable filter 20 to be transmitted to the RN 3971_iThe wavelength signal λ i required by the drop-hanging ONU then passes through a1 x 2 optical splitter 19 to split the selected wavelength signal into two paths, one path being sequentially transmitted to the RN along the forward direction of the distribution optical fiber 7_iThe other path of each ONU in the lower part is transmitted to the RN from the opposite direction along the distribution optical fiber 8_iThe two signals are combined into one path in an ONU6 through a 1-2 coupler 30, then the signal is divided into two parts through a 1-2 optical splitter 31, one part is connected with an optical receiver 32, the other part is transmitted to a reflective semiconductor optical amplifier RSOA33 for the remodulation of an uplink signal, and the remodulated uplink wavelength lambda is carried out in the reflective semiconductor optical amplifier RSOA33_i' the signal is sent back to the remote node RN3 again by the 1 x 2 optical splitter 31 and the 1 x 2 coupler 30, the two identical upstream signals from the distribution optical fibers 7 and 8 are combined into one path by the 1 x 2 optical splitter 19 in the remote node RN3, and then sent back to the optical line terminal OLT1 by the feeder optical fiber 2 by the tunable filter 20, the 1 x 2 optical splitter 18 and the 3 x 1 optical switch 17, and the upstream signal λ from the N RNs is sent back to the optical line terminal OLT1₁′λ₂′...λ_NThe signals are combined into one path by a1 × N optical splitter 16, then sequentially pass through a bidirectional erbium-doped fiber amplifier EDFA15 and an optical circulator 14 to reach a1 × N arrayed waveguide grating AWG13, and are transmitted to an optical receiver 11 to be received after being demultiplexed.

In a third embodiment, on the basis of the first embodiment, the method for implementing the protection function includes: s1, referring to fig. 6, when the feeder fiber 2 is failed, and the failed fiber is adjacent to both sidesWhen at least one remote node RN can normally operate, N:1 full protection, assuming connection to RN_iIn a feeder line fiber fault, a remote node RN_iThe inner 3 x 1 optical switch 17 performs protection switching, the port 1 is connected to the port 2, and the remote node RN_i-1OS in 1-by-1 optical switch 23 inside_iProtection switching is carried out, the switch is closed, and N-path downstream signals lambda generated by the OLT1₁λ₂...λ_N-1λ_NTo each remote node RN, RN via feeder fibre 2_iCannot receive a downstream signal belonging to it due to a fault in the feeder fibre connected to it, but contains an RN_iDownstream wavelength λ_iOf the multiplexed signal lambda₁λ₂...λ_N-1λ_NCan reach RN_i-1In RN_i-1In 3, the signal is divided into two parts after passing through a 3X 1 optical switch 17 and a 1X 2 optical splitter 18, and the signal output from a port 2 of the 1X 2 optical splitter 18 is used for RN_i-1The self signal is received, the signal output from the port 3 of the 1X 2 optical splitter 18 is demultiplexed into RN by a 1X N array waveguide grating AWG21_iOf the downstream wavelength lambda_iWavelength λ_iThen through the OS in the 1 x 1 optical switch 23_i1 × N AWG22, output from 3 ports of 1 × 2 optical coupler 24 via RN_i-1And RN_iBetween the feeder fibres 4 to the RN_iAt a remote node RN_iIn 3, the signal reaches the 1 x 2 optical splitter 18 through the optical switch 17 after protection switching, and then the transmission mode of the signal is the same as the normal working mode;

at the same time, RN_iOf the upstream signal lambda_i' also by protecting the switched optical switch 17, RN_i-1And RN_iBetween the feeder fibres 4 to the RN_i-1In RN_i-1In 3, the upstream wavelength λ_i' pass through the OS in the 1 x 2 optical coupler 24, the 1 x N arrayed waveguide grating AWG22, the 1 x 1 optical switch 23 in sequence_iAfter reaching 1 x 2 optical splitter 18, it is connected to RN_i-1Of the upstream wavelength lambda_i-1The combined wave is transmitted to an optical line terminal OLT1 through an optical switch 17, and then the transmission mode of the signal is the same as the normal working mode;

s2, referring to fig. 7, when the feeder fiber 2 fails and neither of the two remote nodes RN on the two adjacent sides of the failed fiber can work normally, we can still perform N through the ring network formed by the feeder fibers 4: 1 full protection, as shown in FIG. 7, connect RN_i、RN_i+1、RN_i+2Has a fault in the feeder optical fiber, RN_i+1Two adjacent remote node RNs can not work normally, so that failure protection can not be provided for the two remote node RNs, but the two remote node RNs still can pass through the RNs firstly_i-1Making RN_i、RN_i+1、RN_i+2The three fault points realize fault recovery, and the specific method is as follows: n-path downstream signal lambda generated by OLT1₁λ₂...λ_N-1λ_NCan reach RN through feeder fiber 2_i-1At a remote node RN_i-1In 3, the signal is divided into two parts after passing through a 3X 1 optical switch 17 and a 1X 2 optical splitter 18, and the signal output from a port 2 of the 1X 2 optical splitter 18 is used for RN_i-1The self signal is received, the signal output from the port 3 of the 1X 2 optical splitter 18 is demultiplexed into RN by a 1X N array waveguide grating AWG21_iOf the downstream wavelength lambda_i，RN_i+1Of the downstream wavelength lambda_i+1And RN_i+2Of the downstream wavelength lambda_i+2Wavelength λ_i、λ_i+1、λ_i+2Then, the optical signal passes through the 1 × 1 optical switch 23, the 1 × N arrayed waveguide grating AWG22, and is output from the 3 port of the 1 × 2 optical coupler 24 via the RN_i-1And RN_iBetween the feeder fibres 4 to the RN_iAt a remote node RN_iIn 3, the signal reaches the 1 x 2 optical splitter 18 through the optical switch 17 after protection switching and is divided into two paths, and the signal output from the port 2 of the 1 x 2 optical splitter 18 is used for RN_iThe self signal is received, the signal output from the port 3 of the 1X 2 optical splitter 18 is demultiplexed into RN by a 1X N array waveguide grating AWG21_i+1Of the downstream wavelength lambda_i+1And RN_i+2Of the downstream wavelength lambda_i+2Wavelength λ_i+1、λ_i+2Then, the optical signal passes through the 1 × 1 optical switch 23, the 1 × N arrayed waveguide grating AWG22, and is output from the 3 port of the 1 × 2 optical coupler 24 via the RN_iAnd RN_i+1Between the feeder fibres 4 to the RN_i+1At a remote node RN _i+13, the letterThe signal reaches the 1X 2 optical splitter 18 through the optical switch 17 after protection switching and is divided into two paths, and the signal output by the port 2 of the 1X 2 optical splitter 18 is used for RN_i+1The self signal is received, the signal output from the port 3 of the 1X 2 optical splitter 18 is demultiplexed into RN by a 1X N array waveguide grating AWG21_i+2Of the downstream wavelength lambda_i+2Wavelength λ_i+2Then, the optical signal passes through the 1 × 1 optical switch 23, the 1 × N arrayed waveguide grating AWG22, and is output from the 3 port of the 1 × 2 optical coupler 24 via the RN_i+1And RN_i+2Between the feeder fibres 4 to the RN_i+2At a remote node RN_i+2In 3, the signal reaches the 1 x 2 optical splitter 18 through the optical switch 17 after protection switching, and then the transmission mode of the signal is the same as the normal working mode;

at the same time, RN_i+2Of the upstream signal lambda_i+2' also by protecting the switched optical switch 17, RN_i+2And RN_i+1Between the feeder fibres 4 to the RN_i+1In RN_i+1In 3, the upstream wavelength λ_i+2' the optical signal goes through the 1 × 2 optical coupler 24, the 1 × N arrayed waveguide grating AWG22, and the 1 × 1 optical switch 23 in sequence, reaches the 1 × 2 optical splitter 18, and then is transmitted to the RN_i+1Of the upstream wavelength lambda_i+1The 'combined wave' passes through the optical switch 17, RN_i+1And RN_iBetween the feeder fibres 4 to the RN_iIn RN_iIn 3, the upstream wavelength λ_i+2′、λ_i+1' the optical signal goes through the 1 × 2 optical coupler 24, the 1 × N arrayed waveguide grating AWG22, and the 1 × 1 optical switch 23 in sequence, reaches the 1 × 2 optical splitter 18, and then is transmitted to the RN_iOf the upstream wavelength lambda_iThe 'combined wave' passes through the optical switch 17, RN_iAnd RN_i-1Between the feeder fibres 4 to the RN_i-1In RN_i-1In 3, the upstream wavelength λ_i+2′、λ_i+1′、λ_i' the optical signal goes through the 1 × 2 optical coupler 24, the 1 × N arrayed waveguide grating AWG22, and the 1 × 1 optical switch 23 in sequence, reaches the 1 × 2 optical splitter 18, and then is transmitted to the RN_i-1Of the upstream wavelength lambda_i-1' Combined wave, upstream signal lambda after combined wave_i+2′、λ_i+1′、λ_i′、λ_i-1' to the optical line terminal OLT1 through the optical switch 17, after which the signals are transmitted in the same way as in the normal operation mode; it can be seen that onlyIf a remote node RN in the network can work normally, the network can realize fault recovery;

s3, referring to fig. 8, 3 and 5, if the distribution optical fibers 7 and 8 connected to the ONU have a fault, the network can automatically realize fault recovery without performing protection switching by the optical switch, the transmission mode of the signal from the optical line terminal OLT1 to the remote node RN3 is the same as the normal operation mode, and the ONU after the ONU2 has a fault in the middle of the distribution optical fiber 7₃…ONU_M-1、ONU_MNo downstream signal can be obtained from the distribution fibre 7, but the other downstream signal split by the 1 x 2 optical splitter 19 can be delivered to the ONU sequentially from the opposite direction along the distribution fibre 8_M、ONU_M-1…ONU₃The transmission mode of the signal after reaching 0NU is the same as the normal operation mode.

When the invention is used specifically, the connection mode of OLT and RN adopts a star-ring combined topological structure, and the natural advantages of a star structure are utilized, the transmission links of RNs are not interfered with each other, and a fault does not affect other RNs which normally work, so that fault isolation can be effectively realized, secondly, signals are directly transmitted to the target RN along a star network, compared with the ring structure, other nodes are not needed, so that transmission delay and link loss are reduced, in addition, the ring structure is utilized and the internal design of the RN nodes is combined, so that any RN in the network can provide protection for other RNs when the network fails, namely, even if only one RN can normally work, the network can also realize fault recovery, and N of the feeder optical fiber is realized: 1 comprehensive protection, ONU's connected mode adopts double bus type topological structure, one is as work optic fibre, another is as protection optic fibre, two optic fibres are established ties ONU from two directions respectively, can make the ONU figure of being protected reach the biggest like this, moreover, work optic fibre and protection optic fibre transmit signal simultaneously, avoided protection optic fibre to cause the optical fiber wasting of resources because of idling, only use the coupler along the way and not have the photoswitch, when work optic fibre trouble, need not to switch over just can be automatic with the signal coupling in the protection optic fibre to purpose ONU, realize not having switching automatic protection.

Claims

1. A TWDM-PON system with multi-point fault protection capability is characterized in that an optical line terminal OLT (1) is connected to N far-end nodes RN (3) through feeder fibers (2) to form a star-shaped structure, the far-end nodes RN (3) are connected with each other through the feeder fibers (4) to form a double-fiber ring structure, each far-end node RN (3) is connected to M optical network units ONU (6) through distribution fibers (5), the optical network units ONU (6) are connected in series from two directions through distribution fibers (7), distribution fibers (8) and an optical coupler (9) to form a double-bus type structure, and the TWDM-PON system is characterized in that the optical line terminal OLT (1) comprises N optical transmitters (10), N optical receivers (11), a first 1 × N array waveguide grating (12), a second 1 AWG N array waveguide grating (13), an optical circulator (14), A bidirectional erbium-doped fiber amplifier EDFA (15) and a1 × N optical splitter (16), in the OLT (1), N optical transmitters (10) are connected to a first 1 × N arrayed waveguide grating AWG (12), N optical receivers (11) are connected to a second 1 × N arrayed waveguide grating AWG (13), the output of the first 1 × N arrayed waveguide grating AWG (12) is connected to an optical circulator (14), the output of the optical circulator (14) is connected to a bidirectional erbium-doped fiber amplifier EDFA (15), the output of the bidirectional erbium-doped fiber amplifier EDFA (15) is connected to a1 × N optical splitter (16), and the 1 × N optical splitter (16) is connected to N far-end nodes RN (3) through N feeder fibers (2);

the remote node RN (3) comprises a3 x 1 optical switch (17), a first 1 x 2 power distributor (18), a second 1 x 2 power distributor (19), a tunable filter (20), a first 1 x N arrayed waveguide grating AWG (21), a second 1 x N arrayed waveguide grating AWG (22), N1 x 1 optical switches (23) and a1 x 2 optical coupler (24), in the remote node RN (3), 1 port of the 3 x 1 optical switch (17) is connected to the first 1 x 2 power distributor (18), 2 ports of the first 1 x 2 power distributor (18) are connected to the tunable filter (20), the output of the tunable filter (20) is connected to the second 1 x 2 power distributor (19), 2 ports and 3 ports of the second 1 x 2 power distributor (19) are respectively connected to the ONU (6) through distribution optical fibers (7 and 8), the 3 ports of the first 1 x 2 power divider (18) are connected to a first waveguide array grating AWG (21), the outputs of the first waveguide array grating AWG (21) are connected to N1 x 1 optical switches (23), respectively, the outputs of the N1 x 1 optical switches (23) are in turn connected to a fourth waveguide array grating AWG (21), respectivelyThe output of the second waveguide array grating AWG (22) is connected to a1 x 2 optical coupler (24), the design of a far-end node RN (3) realizes modularization, and each RN is_iThe module leaves 5 ports (25, 26, 27, 28, 29) outwards, RN_iIs connected to the OLT (1), RN_iAre connected to the RN respectively_i-1Port (29, 27), RN_iAre connected to the RN respectively_i+1Ports (28, 26);

the optical network unit ONU (6) comprises a coupler (30), a1 x 2 optical splitter (31), an optical receiver (32) and a reflective semiconductor optical amplifier RSOA (33), in the optical network unit ONU (6), the output of the coupler (30) is connected to the 1 x 2 optical splitter (31), the port 1 of the 1 x 2 optical splitter (31) is connected to the optical receiver (32), the port 2 of the 1 x 2 optical splitter (31) is connected to the reflective semiconductor optical amplifier RSOA (33), the ONU design of the optical network unit ONU (6) is modularized, and each ONU is modularized_jThe module is provided with 2 ports (34, 35) outwards, and the ONU_jAre connected to the remote node RN (3) by distribution fibres (7, 8) and an optical coupler (9), respectively.

2. The TWDM-PON system according to claim 1, wherein in the normal operation mode, the specific method for implementing uplink and downlink traffic transmission in the network is as follows:

the signals are transmitted from an optical line terminal OLT (1) to each remote node RN (3) in a form of wavelength division multiplexing, and are transmitted from the remote node RN (3) to each optical network unit ONU (6) in a form of time division multiplexing, and N optical transmitters (10) generate N optical signals lambda carrying respective downlink information at the optical line terminal OLT (1)₁λ₂…λ_N-1λ_NThen N optical signals are subjected to wavelength multiplexing through a1 x N arrayed waveguide grating AWG (12), then pass through an optical circulator (14), are amplified through a bidirectional erbium-doped fiber amplifier EDFA (15), and then are transmitted to a1 x N optical splitter (16), and the signals are divided into N paths from the N paths and transmitted to a far-end node RN (3) along a feeder fiber (2); in the remote node RN (3), the signal passes through the 3 x 1 optical switch (17)The signal is transmitted to a 1-to-2 optical splitter (18), the signal is divided into two parts, the signal output from the port 3 of the 1-to-2 optical splitter (18) is used for fault protection, and the signal output from the port 2 of the 1-to-2 optical splitter (18) is continuously transmitted backwards for receiving a downstream signal; the portion of the signal used for downlink signal reception is passed through a tunable filter (20) to select the RN_iWavelength signal lambda required by a drop-off ONU_iThen, the selected wavelength signal is divided into two paths by a 1-to-2 optical splitter (19), and one path is transmitted to the RN in sequence along the forward direction of the distribution optical fiber (7)_iThe other path of each ONU is transmitted to the RN from the opposite direction along a distribution optical fiber (8)_iEach ONU under the control; the two paths of signals are combined into one path in an ONU (6) through a 1-2 coupler (30), and then the signal is divided into two parts through a 1-2 optical splitter (31), wherein one part is connected with an optical receiver (32), and the other part is transmitted to a reflective semiconductor optical amplifier RSOA (33) for the remodulation of an uplink signal; in a reflective semiconductor optical amplifier RSOA (33), an uplink wavelength λ after being remodulated_iThe signals are sent back to a far-end node RN (3) again through a1 × 2 optical splitter (31) and a1 × 2 coupler (30), two paths of same uplink signals from distribution optical fibers (7, 8) are combined into one path through a1 × 2 optical splitter (19) at the far-end node RN (3), and then the signals are sent back to an optical line terminal OLT (1) through a tunable filter (20), a1 × 2 optical splitter (18) and a3 × 1 optical switch (17) through a feeder optical fiber (2); in OLT (1), upstream signals λ from N RNs₁′λ₂′...λ_NThe signals are combined into one path through a1 × N optical splitter (16), then sequentially pass through a bidirectional erbium-doped fiber amplifier EDFA (15) and an optical circulator (14) to reach a1 × N arrayed waveguide grating AWG (13), and the signals are transmitted to an optical receiver (11) to be received after being demultiplexed.

3. The TWDM-PON system having multi-point fault protection capability according to claim 2, wherein the method for implementing the protection function is as follows:

s1, when the feeder optical fiber (2) has a fault and at least one far-end node RN at two adjacent sides of the fault optical fiber can work normally, the ring network formed by the feeder optical fiber (4) can be usedAnd (3) carrying out complexation on the mixture by N:1 full protection, in case of failure of feeder fiber connected to RNi, 3 x 1 optical switch (17) in far end node RNi performs protection switching, port 1 is connected to port 2, OSi in 1 x 1 optical switch (23) in far end node RNi-1 performs protection switching, the switch is closed, and N downlink signals λ generated by OLT (1)₁λ₂…λ_N-1λ_NTo each remote node RN via a feeder fibre (2), RNi cannot receive its downstream signal due to a fault in the feeder fibre to which it is connected, but contains RNi downstream wavelength λ_iOf the multiplexed signal lambda₁λ₂...λ_N-1λ_NCan reach RNi-1, in RNi-1(3), the signal is divided into two parts after passing through a 3X 1 optical switch (17) and a 1X 2 optical splitter (18), the signal output by a port 2 of the 1X 2 optical splitter (18) is used for receiving the self signal of RNi-1, and the signal output by a port 3 of the 1X 2 optical splitter (18) is demultiplexed by a 1N arrayed waveguide grating AWG (21) to obtain RNi downstream wavelength lambda_iWavelength λ_iThen, the optical signal passes through OSi in a1 x 1 optical switch (23), a1 x N arrayed waveguide grating AWG (22), is output from a3 port of a1 x 2 optical coupler (24), and is transmitted through RN_i-1And RN_iBetween the two feeder optical fibres (4) to RN_iAt a remote node RN_i(3) In the method, a signal reaches a1 x 2 optical splitter (18) through an optical switch (17) after protection switching, and then the transmission mode of the signal is the same as that of a normal working mode;

at the same time, RN_iOf the upstream signal lambda_i' also by protecting the switched optical switch (17), RN_i-1And RN_iBetween the two feeder optical fibres (4) to RN_i-1In RN_i-1(3) Medium, up wavelength lambda_i' sequentially pass through OS in 1 x 2 optical coupler (24), 1 x N arrayed waveguide grating AWG (22), 1 x 1 optical switch (23)_iAfter reaching 1 x 2 optical splitter (18), the signal is connected with RN_i-1Of the upstream wavelength lambda_i-1The combined wave is transmitted to an optical line terminal OLT (1) through an optical switch (17), and then the transmission mode of the signal is the same as the normal working mode;

s2, when the feeder optical fiber (2) has a fault and neither of the two far-end nodes RN at the two adjacent sides of the fault optical fiber can work normally, the ring formed by the feeder optical fiber (4) can still pass throughThe network carries out N:1 overall protection, connect RN_i、RN_i+1、RN_i+2Has a fault in the feeder optical fiber, RN_i+1Two adjacent remote node RNs can not work normally, so that failure protection can not be provided for the two remote node RNs, but the two remote node RNs still can pass through the RNs firstly_i-1Making RN_i、RN_i+1、RN_i+2The three fault points realize fault recovery, and the specific method is as follows: n-path downlink signal lambda generated by OLT (1)₁λ₂...λ_N-1λ_NCan reach RN through feeder optical fiber (2)_i-1At a remote node RN_i-1(3) In the optical fiber grating, a signal is divided into two parts after passing through a 3X 1 optical switch (17) and a 1X 2 optical splitter (18), the signal output by a port 2 of the 1X 2 optical splitter (18) is used for receiving RNi-1 self signals, and the signal output by a port 3 of the 1X 2 optical splitter (18) is demultiplexed into RNi downstream wavelengths lambda through a 1X N arrayed waveguide grating AWG (21)_iRNi +1 downstream wavelength λ_i+1And a downstream wavelength λ of RNi +2_i+2Wavelength λ_i、λ_i+1、λ_i+2Then, the optical signal passes through a1 × 1 optical switch (23), a1 × N arrayed waveguide grating AWG (22), and is output from a 3-port of a1 × 2 optical coupler (24) via RN_i-1And RN_iBetween the two feeder optical fibres (4) to RN_iAt a remote node RN_i(3) In the optical fiber coupler, a signal reaches a1 x 2 optical splitter (18) through a protection switched optical switch (17) and then is divided into two paths, the signal output by a port 2 of the 1 x 2 optical splitter (18) is used for receiving RNi self signals, and the signal output by a port 3 of the 1 x 2 optical splitter (18) is demultiplexed into RNi +1 downstream wavelength lambda through a1 x N arrayed waveguide grating AWG (21)_i+1And a downstream wavelength λ of RNi +2_i+2Wavelength λ_i+1、λ_i+2Then, the optical signal passes through a1 × 1 optical switch (23), a1 × N arrayed waveguide grating AWG (22), and is output from a 3-port of a1 × 2 optical coupler (24) via RN_iAnd RN_i+1Between the two feeder optical fibres (4) to RN_i+1At a remote node RN_i+1(3) In the optical switch, the signal reaches a 1X 2 optical splitter (18) through a protection switch (17) and then is divided into two paths, the signal output by a port 2 of the 1X 2 optical splitter (18) is used for receiving RNi +1 self signal, and the signal output by a port 3 of the 1X 2 optical splitter (18) is optically guided through a 1N array waveguideThe grating AWG (21) demultiplexes RNi +2 downstream wavelength lambda_i+2Wavelength λ_i+2Then, the optical signal passes through a1 × 1 optical switch (23), a1 × N arrayed waveguide grating AWG (22), and is output from a 3-port of a1 × 2 optical coupler (24) via RN_i+1And RN_i+2Between the two feeder optical fibres (4) to RN_i+2At a remote node RN_i+2(3) In the method, a signal reaches a1 x 2 optical splitter (18) through an optical switch (17) after protection switching, and then the transmission mode of the signal is the same as that of a normal working mode;

at the same time, RN_i+2Of the upstream signal lambda_i+2' also by protecting the switched optical switch (17), RN_i+2And RN_i+1Between the two feeder optical fibres (4) to RN_i+1In RN_i+1(3) Medium, up wavelength lambda_i+2The optical fiber' sequentially passes through a1 x 2 optical coupler (24), a1 x N arrayed waveguide grating AWG (22) and a1 x 1 optical switch (23), reaches a1 x 2 optical splitter (18) and then is connected with RN_i+1Of the upstream wavelength lambda_i+1The combined wave passes through an optical switch (17) and RN_i+1And RN_iBetween the two feeder optical fibres (4) to RN_iIn RN_i(3) Medium, up wavelength lambda_i+2′、λ_i+1The optical fiber' sequentially passes through a1 x 2 optical coupler (24), a1 x N arrayed waveguide grating AWG (22) and a1 x 1 optical switch (23), reaches a1 x 2 optical splitter (18) and then is connected with RN_iOf the upstream wavelength lambda_iThe combined wave passes through an optical switch (17) and RN_iAnd RN_i-1Between the two feeder optical fibres (4) to RN_i-1In RN_i-1(3) Medium, up wavelength lambda_i+2′、λ_i+1′、λ_iThe optical fiber' sequentially passes through a1 x 2 optical coupler (24), a1 x N arrayed waveguide grating AWG (22) and a1 x 1 optical switch (23), reaches a1 x 2 optical splitter (18) and then is connected with RN_i-1Of the upstream wavelength lambda_i-1' Combined wave, upstream signal lambda after combined wave_i+2′、λ_i+1′、λ_i′、λ_i-1The signal is transmitted to an optical line terminal OLT (1) through an optical switch (17), and then the transmission mode of the signal is the same as the normal working mode;

s3, when the distribution optical fiber (7, 8) connected with ONU is in fault, the network can automatically realize fault recovery without the need of optical switch for protection switching, the signal is far from the optical line terminal OLT (1)The transmission mode of the end node RN (3) is the same as the normal working mode, and due to the middle fault of the distribution optical fiber (7), the ONU behind the ONU2₃…ONU_M-1、ONU_MNo downstream signal can be obtained from the distribution fiber (7), but the other downstream signal split by the 1 x 2 optical splitter (19) can be transmitted to the ONUs in sequence from the opposite direction along the distribution fiber (8)_M、ONU_M-1…ONU₃And the transmission mode of the signal after reaching the ONU is the same as the normal working mode.