Signal strength equalization method and relay node

Abstract

A signal intensity leveling method for classifying a plurality of user devices accommodated in the same relay node such that the user devices for which the optical signal intensities are close to one another are classified into the same class, setting a signal transfer path so that the optical signals transmitted from the user devices of the same class are collected together in the same transmission line, and collectively adjusting the signal intensities of the optical signals collected in the same transmission line without separating the optical signals by wavelength.　

Description

[Technical field]

[0001] The present invention relates to a signal strength equalization method and a relay node. [Background technology]

[0002] In recent years, studies are being conducted on transmitting optical signals throughout the entire network (for example, Non-Patent Document 1). Fig. 10 shows the configuration of an optical communication system that constitutes an all-optical network utilizing WDM (Wavelength Division Multiplexing). In the optical communication system shown in Fig. 10, transmitting terminals 200-1 to 200-3 and receiving terminals 300-1 to 300-3 are optically connected end-to-end without photoelectric conversion. Optical signals transmitted from each transmitting terminal 200 are input to one of relay nodes 350, where route switching is performed for each wavelength, and then transferred to receiving terminal 300.

[0003] 11 is a diagram showing a configuration example of a conventional relay node 350. The relay node 350 includes a wavelength multiplexer / demultiplexer 351, an optical SW 352, a wavelength multiplexer / demultiplexer 353, a wavelength multiplexer / demultiplexer 354, an optical amplifier 355, and an optical amplifier 356. The wavelength multiplexers / demultiplexers 351, 353, and 354 are formed of, for example, arrayed waveguide gratings (AWGs). An optical signal input to the relay node 350 is demultiplexed by wavelength by the wavelength multiplexer / demultiplexer 351, and a desired output destination is switched by the optical SW 352. Thereafter, the optical signal is wavelength-multiplexed again by the wavelength multiplexer / demultiplexer 353 or 354, transmitted to the next relay destination, and finally transferred to a corresponding device (for example, receiving terminals 300-1 to 300-3). When the loss in the node or transmission line is greater than a specified value, optical amplifiers 355 and 356 are provided after the wavelength division multiplexer 353 or 354 as shown in FIG. 11 to amplify the optical signal.

[0004] Here, the problem of intensity variations in an optical communication system constituting an all-optical network will be described with reference to Fig. 12. Consider a case in which transmitting terminals 200-1, 200-2 (shown as Tx1 and Tx2 in Fig. 12) communicate with receiving terminals 300-1, 300-2 (shown as Rx1 and Rx2 in Fig. 12). Here, it is assumed that transmitting terminal 200-1 is located farther from the relay node than transmitting terminal 200-2, and has a larger transmission loss. If the transmission power of each transmitting terminal 200 is equal, when each optical signal transmitted from each transmitting terminal 200 reaches optical amplifier 355-1, there is an intensity difference ΔP in the level of each optical signal. in The change in signal strength versus transmission distance (level diagram) is shown in the lower part of Figure 12.

[0005] In optical communication systems, if the signal strength entering a receiver is lower than the minimum receiving sensitivity, the bit error rate increases due to the effects of thermal noise generated within the receiver. Therefore, in order to realize long-distance transmission with large losses, it is effective to use an optical amplifier to compensate for the decrease in signal strength. Amplified spontaneous emission (ASE) emitted by an optical amplifier is optical noise and can degrade the signal receiving characteristics. For example, even if the strength of the optical signal entering the receiver is amplified to a high level by an optical amplifier, if the ASE strength relative to the optical signal is large, i.e., the optical signal to noise ratio (OSNR) is low, the bit error rate will decrease.

[0006] To avoid this, it is important to keep the input optical signal strength at all the optical amplifiers (for example, optical amplifiers 355-1 to 355-3 in FIG. 12) that the signal passes through sufficiently higher than the ASE level emitted by the optical amplifiers. minBy positioning the optical amplifier so that the optical amplifier output is as high as possible, it is possible to avoid the degradation of reception characteristics caused by ASE. As described above, in order to avoid the effects of ASE, it is important to keep the signal strength high within the transmission path. However, if the optical signal strength is too high, waveform degradation occurs due to nonlinear optical effects, which increases the bit error rate. The signal strength affected by nonlinear optical effects is called P max Generally, in order to transmit error-free in a repeater optical amplification system, the optical power in the transmission section must always be above the "upper limit due to nonlinear effects (P max ) and "Lower bound by OSNR (P min It is important to design the transmission line so that it is between

[0007] If the loss between the optical amplifiers is ΔL, the range of possible signal levels between the transmitting terminal 200-1 and the transmitting terminal 200-2 is ΔP in +ΔL. In order to transmit both of these signals error-free, the following conditions must be met: (Condition):P max -P min >ΔP in +ΔL

[0008] Therefore, to expand the allowable distance between nodes (ΔL>>), the intensity difference ΔP in It is important to reduce

[0009] One method of leveling signal strength to reduce the intensity difference is to use a variable optical attenuator (VOA) (see, for example, Non-Patent Document 2). Fig. 13 is a diagram showing a configuration example of a relay node 350a using a variable optical attenuator. In the relay node 350a shown in Fig. 13, a VOA is placed in front of the ports for each wavelength of the wavelength multiplexers / demultiplexers 353 and 354, and the attenuation amount is controlled independently for each VOA, thereby leveling the signal level for each wavelength.

[0010] In an optical communication system as shown in FIG. 12, it is important to keep the OSNR high. To achieve this, it is effective to increase the signal strength at the point of incidence into the optical amplifier. On the other hand, in the method shown in FIG. 13, a strong signal is made weaker by the VOA, which reduces the OSNR of the strong signal. On the other hand, when an optical amplifier 360 is used before the signal is flattened, as in the relay node 350b shown in FIG. 14, the strong signal can be amplified without attenuating it, and deterioration of the OSNR can be avoided. However, when wavelength-multiplexed signals with different intensities are amplified by the same amplifier, the gain of the weak signal may decrease. The reason is as follows.

[0011] Consider the example of a fiber optical amplifier shown in FIG. 15. In the fiber optical amplifier, pumping light irradiated from a pumping light generating LD 380 is input to an optical fiber serving as an amplification medium 370 together with an optical signal to be amplified, and the power of the pumping light is converted into an optical signal in the amplification medium 370. An optical fiber serving as the amplification medium 370 is used that is appropriate for the wavelength band to be amplified. For example, in the C-band around 1550 nm, an optical fiber doped with erbium is often used. The gain of an optical amplifier depends on the strength of the input optical signal. For example, when an optical signal relatively strong relative to the intensity of the pumping light is input, the power of the pumping light propagating through the amplification medium 370 is reduced due to the energy conversion from the pumping light to the optical signal, and as a result, the gain is reduced. This is called gain saturation. On the other hand, when a signal weak relative to the intensity of the pumping light is input, the attenuation of the pumping light due to the energy conversion is small, and therefore a high gain can be obtained. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] H. Kawahara et al., “Optical Full-mesh Network Technologies Supporting the All-Photonics Network”, NTT Technical Review, Vol. 18 No.5, May 2020, pp. 24-29. [Non-Patent Document 2] Hisato Uetsuka, “AWG Technologies for Dense WDM Applications”, IEEE J. Sel. Top. Quantum Electron., Vol. 10, No. 2, March / April 2004. [Non-Patent Document 3] S. Koenig et al., “Rival Signals in SOA Reach-Extended WDM-TDM-GPON Converged with RoF”, OFC2011, OWT1, 2011. Summary of the Invention [Problem to be solved by the invention]

[0013] When flattening signals with intensity variations and transmitting them to a relay fiber, the weaker the signal, the higher the gain required, while the stronger the signal, the smaller the gain required. At first glance, the above gain saturation does not seem to be a problem. However, when wavelength-multiplexed signals with intensity differences are amplified by the same amplifier as shown in Figure 14, the stronger signal causes attenuation of the pump light, resulting in a decrease in the gain of the weaker signal. Thus, in the past, there was a problem in that it was not possible to flatten the intensity differences of optical signals transmitted from each user device accommodated in the same relay node without reducing the OSNR.

[0014] In view of the above circumstances, an object of the present invention is to provide a technique capable of flattening the intensity difference between optical signals transmitted from user devices accommodated in the same relay node without reducing the OSNR. [Means for solving the problem]

[0015] One aspect of the present invention is a signal strength equalization method that classifies multiple user equipment devices accommodated in the same relay node so that user equipment devices with similar optical signal strengths are in the same class, sets a signal transfer path for concentrating optical signals transmitted from user equipment devices of the same class within the same optical transmission path, and adjusts the signal strength of the optical signals concentrated within the same optical transmission path all at once without dividing them by wavelength.

[0016] One aspect of the present invention is a signal strength equalization method that classifies multiple user equipment accommodated in the same relay node into the minimum number of classes such that user equipment with similar optical signal strengths are in the same class and that gain reduction of weak signals due to gain saturation does not occur, sets a signal forwarding path for concentrating optical signals transmitted from user equipment of the same class within the same optical transmission path, and concentrates optical signals transmitted from user equipment of the same class within the same optical transmission path.

[0017] One aspect of the present invention is a relay node that includes an allocation unit that classifies a plurality of user equipment accommodated therein so that user equipment with similar optical signal strengths are in the same class, and sets a signal transfer path for concentrating optical signals transmitted from user equipment of the same class within the same optical transmission path, and an adjustment unit that adjusts the signal strength of the optical signals concentrated within the same optical transmission path by the allocation unit all at once without dividing them by wavelength.

[0018] One aspect of the present invention is a relay node that includes an allocation unit that classifies multiple user equipment accommodated in the same relay node into the minimum number of classes such that user equipment with similar optical signal strengths are in the same class and that does not cause gain reduction of weak signals due to gain saturation, sets signal forwarding paths for concentrating optical signals transmitted from user equipment of the same class within the same optical transmission path, and concentrates optical signals transmitted from user equipment of the same class within the same optical transmission path. Effect of the Invention

[0019] According to the present invention, it is possible to flatten the intensity difference between optical signals transmitted from user devices accommodated in the same relay node without reducing the OSNR. [Brief description of the drawings]

[0020] [Figure 1] 1 is a diagram illustrating an example of the configuration of an optical communication system according to a first embodiment. [Diagram 2] FIG. 2 is a diagram illustrating a configuration example of a terminal relay node in the first embodiment. [Diagram 3] FIG. 2 is a diagram illustrating an example of the configuration of a user device and a user management terminal in the first embodiment. [Figure 4] FIG. 2 is a diagram illustrating an example of the configuration of a management device in the first embodiment. [Diagram 5] FIG. 2 is a diagram for explaining an overview of the operation of the optical communication system in the first embodiment. [Figure 6] FIG. 4 is a sequence diagram for explaining a process flow of the optical communication system in the first embodiment. [Figure 7] FIG. 11 is a diagram illustrating an example of the configuration of an optical communication system according to a second embodiment. [Figure 8] FIG. 13 is a diagram illustrating an example of the configuration of an optical communication system according to a third embodiment. [Figure 9] FIG. 13 is a diagram illustrating an example of the configuration of an optical communication system according to a fourth embodiment. [Figure 10] FIG. 1 is a diagram showing a configuration of an optical communication system that constitutes an all-optical network utilizing WDM technology. [Figure 11] FIG. 1 is a diagram illustrating an example of the configuration of a conventional relay node. [Figure 12] FIG. 1 is a diagram for explaining the problem of wavelength variation in an optical communication system that constitutes an all-optical network. [Figure 13] FIG. 13 is a diagram illustrating a configuration example of a relay node using a variable optical attenuator. [Figure 14] FIG. 13 is a diagram illustrating another example of the configuration of a relay node using a variable optical attenuator. [Figure 15]FIG. 1 is a diagram illustrating an example of the configuration of a fiber optical amplifier. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) 1 is a diagram showing an example of the configuration of an optical communication system 100 in the first embodiment. The optical communication system 100 includes a plurality of user devices 10-1 to 10-9, a plurality of optical splitters 20-1 to 20-3, an end relay node 30, one or more relay nodes 45, an end relay node 50, a plurality of optical splitters 60, a plurality of user devices 70, and a management device 80. In the following description, a case in which the number of user devices 10 and user devices 70 is nine will be described as an example, but the number of user devices 10 and user devices 70 may be two or more.

[0022] In the following description, for convenience of explanation, the user devices 10-1 to 10-9 may be denoted on the drawings with the symbols A to I. Furthermore, among the user devices 10-1 to 10-3, the user devices 10-1, 10-2, and 10-3 are closest to the end relay node 30 in this order, among the user devices 10-4 to 10-6, the user devices 10-4, 10-5, and 10-6 are closest to the end relay node 30 in this order, and among the user devices 10-7 to 10-9, the user devices 10-7, 10-8, and 10-9 are closest to the end relay node 30 in this order.

[0023] The user devices 10-1 to 10-3 are connected to the optical splitter 20-1 via optical transmission paths, the user devices 10-4 to 10-6 are connected to the optical splitter 20-2 via optical transmission paths, and the user devices 10-7 to 10-9 are connected to the optical splitter 20-3 via optical transmission paths. The optical transmission paths are, for example, optical fibers.

[0024] Furthermore, each optical splitter 20 is connected to a terminal relay node 30 via an optical transmission line, and the terminal relay node 30 is connected to a terminal relay node 50 via one or more relay nodes 45. For simplicity, in Fig. 1, it is assumed that the terminal relay node 30 and the terminal relay node 50 are connected via a one-to-one optical transmission line, and only optical amplification is performed in the relay node 45 between the terminal relay node 30 and the terminal relay node 50.

[0025] An optical signal transmitted from the user device 10-1 is wavelength-multiplexed with optical signals transmitted from the user devices 10-2 and 10-3 connected to the optical splitter 20-1, and input to the terminal relay node 30. However, there is variation in signal strength depending on the distance from the terminal relay node 30.

[0026] The user devices 10 and 70 are equipped with wavelength-tunable optical transmitters and receivers as optical transceivers. Therefore, the user devices 10 and 70 can communicate at any wavelength. The wavelengths used by the user devices 10 and 70 for communication are assigned by a management device 80. The optical transceivers may be optical transceivers with an AMCC (Auxiliary Management and Control Channel) function. In this case, the wavelengths used by the user devices 10 and 70 are controlled via control signals superimposed by the AMCC. The user devices 10 and 70 are, for example, ONUs (Optical Network Units) installed in subscriber premises.

[0027] The optical splitter 20 multiplexes or branches the input optical signals. For example, a plurality of user devices 10 are connected to the optical splitter 20, and the optical splitter 20 multiplexes the optical signals transmitted from each of the user devices 10. A terminal relay node 30 is connected to the optical splitter 20, and the optical splitter 20 branches the optical signals output from the terminal relay node 30 and outputs the branched optical signals to each of the user devices 10.

[0028] The terminal relay node 30 is a relay device that transfers optical signals between the user equipment 10 and the user equipment 70. The terminal relay node 30 is provided at a position that is closer to the user equipment 10 than the other relay nodes. The terminal relay node 30 uniforms the signal strength of optical signals transmitted from the user equipment 10 belonging to the same class and outputs the signals to the relay node 45. Here, the user equipment 10 belonging to the same class is a group of user equipment having similar optical signal strengths.

[0029] Depending on the installation location of the user equipment 10, even if each user equipment 10 transmits an optical signal with the same transmission power, the strength of the optical signal input to the end relay node 30 is different. For example, the user equipment 10-1 is closer to the end relay node 30 than the user equipments 10-2 and 10-3. Therefore, when the end relay node 30 receives the optical signals transmitted by each of the user equipments 10-1 to 10-3, the signal strength of the optical signal transmitted from the user equipment 10-1 is greater than the signal strength of the optical signals transmitted from the user equipments 10-2 and 10-3. In this way, the strength of the optical signal is based on the distance between the user equipment 10 and the end relay node 30.

[0030] The relay node 45 amplifies the optical signal between the end relay node 30 and the end relay node 50 .

[0031] The terminal relay node 50 is a relay device that transfers optical signals between the user equipment 10 and the user equipment 70. The terminal relay node 50 is provided at a position that is closer to the user equipment 70 than the other relay nodes. The terminal relay node 50 uniforms the signal strength of optical signals transmitted from the user equipment 70 belonging to the same class and outputs the signals to the relay node 45.

[0032] The optical splitter 60 multiplexes or branches the input optical signals. For example, a plurality of user devices 70 are connected to the optical splitter 60, and the optical splitter 60 multiplexes the optical signals transmitted from each of the user devices 70. A terminal relay node 50 is connected to the optical splitter 60, and the optical splitter 60 branches the optical signals output from the terminal relay node 50 and outputs the branches to each of the user devices 70.

[0033] The management device 80 controls at least the user equipment 10, 70, controls the optical switches provided in the terminal relay nodes 30, 50, and groups the user equipment 10, 70. Here, the control of the user equipment 10, 70 includes, for example, allocation of emission wavelengths to the user equipment 10, 70, instructions to stop light, instructions to change wavelengths, etc. The control of the optical switches includes, for example, setting connections between ports of the optical switches and setting optical paths, etc.

[0034] 2 is a diagram showing a configuration example of end relay nodes 30, 50 in the first embodiment. End relay node 30 includes a plurality of splitters 31-1 to 31-m (m is an integer of 2 or more), a plurality of circulators 32-1 to 32-m, a path switching unit 33, a plurality of optical amplifiers 34-1 to 34-n (n is an integer of 2 or more), a plurality of circulators 35-1 to 35-n, a path switching unit 36, a user management terminal 37, an optical switch control unit 38, an optical amplifier control unit 39, and a plurality of optical amplifiers 40-1 to 40-m.

[0035] The splitters 31-1 to 31-m multiplex or split the input optical signals. For example, the splitters 31-1 to 31-m split the optical signal transmitted from the user device 10 and output the split signal to the user management terminal 37 and the circulators 32-1 to 32-m.

[0036] The circulators 32-1 to 32-m have at least three or more ports. In the following description, it is assumed that the circulators 32-1 to 32-m have three ports. The first ports of the circulators 32-1 to 32-m are connected to the splitters 31-1 to 31-m. The second ports of the circulators 32-1 to 32-m are connected to the path switching unit 33. The third ports 53-3 of the circulators 32-1 to 32-m are connected to the optical amplifiers 40-1 to 40-m.

[0037] An optical signal input to a first port of each of the circulators 32-1 to 32-m is output from a second port. An optical signal input to a second port of each of the circulators 32-1 to 32-m is output from a third port. An optical signal input to a third port of each of the circulators 32-1 to 32-m is output from a first port.

[0038] The path switching unit 33 is composed of a plurality of wavelength multiplexers / demultiplexers 331-1 to 331-m, an optical SW 332, and a plurality of wavelength multiplexers / demultiplexers 333-1 to 333-n. The wavelength multiplexer / demultiplexer 331 is composed of an AWG or the like. The wavelength multiplexer / demultiplexer 331 multiplexes or demultiplexes the input optical signal for each wavelength.

[0039] The optical SW332 is an optical switch having a plurality of first ports and a plurality of second ports. An optical signal input to a certain port of the optical SW332 is output from another port. For example, an optical signal input to a first port of the optical SW332 is output from a second port. The optical SW332 has a connection relationship set between the first port and the second port under the control of the optical switch control unit 38.

[0040] The wavelength multiplexer / demultiplexer 333 is configured by an AWG etc. The wavelength multiplexer / demultiplexer 333 multiplexes or demultiplexes the input optical signals for each wavelength.

[0041] The optical amplifiers 34-1 to 34-n amplify the optical signals with an amplification factor controlled by an optical amplifier control unit 39.

[0042] The circulators 35-1 to 35-n each have at least three ports. In the following description, it is assumed that the circulators 35-1 to 35-n each have three ports. The first ports of the circulators 35-1 to 35-n are connected to the optical amplifiers 34-1 to 34-m. The second ports of the circulators 35-1 to 35-n are connected to the optical transmission lines. The third ports of the circulators 35-1 to 35-n are connected to the path switching unit 36.

[0043] An optical signal input to a first port of each of the circulators 35-1 to 35-n is output from a second port. An optical signal input to a second port of each of the circulators 35-1 to 35-n is output from a third port. An optical signal input to a third port of each of the circulators 35-1 to 35-n is output from a first port.

[0044] The path switching unit 36 ​​is composed of a plurality of wavelength multiplexers / demultiplexers 361-1 to 361-n, an optical SW 362, and a plurality of wavelength multiplexers / demultiplexers 363-1 to 363-n. The wavelength multiplexer / demultiplexer 361 is composed of an AWG or the like. The wavelength multiplexer / demultiplexer 361 multiplexes or demultiplexes the input optical signal for each wavelength.

[0045] The optical SW362 is an optical switch having a plurality of first ports and a plurality of second ports. An optical signal input to a certain port of the optical SW362 is output from another port. For example, an optical signal input to the first port of the optical SW362 is output from the second port. The optical SW362 has a connection relationship set between the first port and the second port under the control of the optical switch control unit 38.

[0046] The wavelength multiplexer / demultiplexer 363 is configured by an AWG etc. The wavelength multiplexer / demultiplexer 363 multiplexes or demultiplexes the input optical signals for each wavelength.

[0047] The user management terminal 37 is a terminal for managing the status of the user equipment 10, 70 connected to the end relay node 30, 50. When the user equipment 10, 70 is newly connected to the end relay node 30, 50, the user management terminal 37 first communicates with the user equipment 10, 70 to authenticate the user and confirm the destination terminal. The user management terminal 37 notifies the user equipment 10, 70 of information on the assigned wavelength notified by the management device 80.

[0048] The optical switch control unit 38 controls the connection between the ports of the optical SW 332 and 362. For example, the optical switch control unit 38 controls the connection between the ports of the optical SW 332 so that optical signals transmitted from user devices 10 of the same class are input to the same wavelength multiplexer / demultiplexer 333. Controlling the connection between ports means setting a route so that a certain port is connected to another port.

[0049] The optical amplifier control unit 39 controls the amplification factor of each of the optical amplifiers 34 and 40 based on the gain notified by the management device 80 .

[0050] The optical amplifiers 40-1 to 40-m amplify the optical signals with the amplification factor controlled by the optical amplifier control unit 39.

[0051] For example, assume that the user device 10 is newly connected to the terminal relay node 30. In this case, after the authentication of the user device 10 and the allocation of wavelengths to the user device 10 are completed, the user device 10 transmits an optical signal to the opposite device via the path 75. The optical signal input to the terminal relay node 30 is divided by the circulator 32 according to the signal traveling direction. The optical signal transmitted from the user device 10 to the user device 70 is transmitted via an arbitrary optical transmission path by the path switching unit 33.

[0052] 3 is a diagram for explaining an example of the configuration of the user devices 10, 70 and the user management terminal 37 in the first embodiment. Note that only the configuration necessary for the explanation is shown in FIG. 3. First, the configuration of the user devices 10, 70 will be explained. Since the user devices 10, 70 have the same configuration, the user device 10 will be explained here as an example. The user device 10 includes a transceiver unit 11, a user authentication unit 12, an allocated wavelength detection unit 13, and an allocated wavelength setting unit 14.

[0053] The transmitter / receiver 11 is a wavelength-tunable optical transmitter / receiver serving as an optical transceiver. The transmitter / receiver 11 communicates with the terminal relay node 30. For example, when the user device 10 newly connects to the terminal relay node 30, the transmitter / receiver 11 transmits an optical signal with a wavelength for initial connection to the terminal relay node 30. The transmitter / receiver 11 receives wavelength information notified from a user management terminal 37 of the terminal relay node 30.

[0054] The user authentication unit 12 performs authentication for connection to the end relay node 30. For example, the user authentication unit 12 transmits preset authentication information to the end relay node 30 via the transmission / reception unit 11.

[0055] The assigned wavelength detector 13 detects wavelength information from an optical signal transmitted from the terminal relay node 30 .

[0056] The assigned wavelength setting unit 14 sets the wavelength indicated by the detected wavelength information as the wavelength to be used by the transmitter / receiver 11 for transmission and reception.

[0057] Next, we will explain an example of the configuration of the user management terminal 37. The user management terminal 37 includes a transmitting / receiving unit 371, a user authentication unit 372, and an assigned wavelength transmitting unit 373. The transmitter / receiver 371 is a wavelength-tunable optical transmitter / receiver serving as an optical transceiver. The transmitter / receiver 371 communicates with the user device 10. For example, the transmitter / receiver 371 receives authentication information transmitted from the user device 10. For example, the transmitter / receiver 371 transmits wavelength information notified from the management device 80.

[0058] The user authentication unit 372 authenticates the newly connected user device 10 based on the authentication information received by the transmission / reception unit 371. If the newly connected user device 10 is authenticated (authentication OK), the user authentication unit 372 transmits information about the newly connected user device 10 to the management device 80. For example, the user authentication unit 372 transmits to the management device 80 information indicating which port of the optical SW the newly connected user device 10 is connected to, information about the signal strength of the optical signal transmitted from the user device 10, and the like. If the newly connected user device 10 is not authenticated (authentication NG), the user authentication unit 372 notifies the user device 10 of an error.

[0059] The assigned wavelength transmitting unit 373 transmits the wavelength information notified by the management device 80 to the user device 10 via the transmitting / receiving unit 371 .

[0060] 4 is a diagram illustrating an example of the configuration of a management device 80 in the first embodiment. The management device 80 includes a transmitting / receiving unit 81, a storage unit 82, a wavelength / route allocation unit 83, and a light intensity adjustment information calculation unit 84.

[0061] The transmitter / receiver 81 communicates with the end relay node 30, the relay node 45, and the end relay node 50. For example, the transmitter / receiver 81 receives information on a user device newly connected to the end relay node 30 or 50 from the end relay node 30 or 50. Examples of the information on the newly connected user device include a transmission distance, a reaching optical intensity, and authentication information.

[0062] The storage unit 82 stores information about the user device received by the transmission / reception unit 81. The storage unit 82 stores, for example, the transmission distance, the optical intensity, and authentication information of the user device newly connected to the terminal relay node 30 or 50, information indicating which port of the optical SW the user devices 10 and 70 are connected to, and information about the wavelengths assigned to the user devices 10 and 70.

[0063] The wavelength / route allocation unit 83 performs classification based on the information stored in the memory unit 82, and then determines user wavelengths and route allocation (which wavelengths are to be output to which optical transmission paths) using a predetermined algorithm. The wavelength / route allocation unit 83 transmits the determined user wavelengths and route information to be set in the optical SW to the transceiver unit 81. The wavelength / route allocation unit 83 classifies multiple user devices 10 or 70 accommodated in the same terminal relay node 30 or 50 so that user devices with similar optical signal strengths are in the same class.

[0064] The optical intensity adjustment information calculation unit 84 calculates the gain required for the optical amplifiers 34 and 40 included in the terminal relay node 30 or 50, based on the user wavelength and route information determined by the wavelength / route allocation unit 83 and the information such as the signal intensity obtained from the storage unit 82. However, when the signal intensity is adjusted by a VOA instead of an optical amplifier, the attenuation to be applied to the VOA is estimated.

[0065] Next, the assigned wavelength determined in the management device 80 and the algorithm for determining the route in the optical SW will be described. Fig. 5 is a diagram for explaining an outline of the operation of the optical communication system 100 in the first embodiment. For simplicity, it is assumed here that the optical signals transmitted from the user devices 10-1 to 10-9 are transmitted through the same end relay nodes 30, 50 and relay node 45. In other words, it is assumed that the optical signal transmitted from the user device 10-1 arrives at the end relay node 50 closest to the user device 70, no matter which optical transmission path connected to the end relay node 30 the optical signal is output from.

[0066] Further, for the sake of simplicity, only communication in the direction from the user device 10 to the user device 70 will be considered. Therefore, of the functional units included in the end relay node 30, only the functional units necessary for the explanation are shown.

[0067] Depending on the distance from the end relay node 30 to the user equipment 10, there is a difference in the signal strength of the optical signal when it reaches the end relay node 30. In Fig. 5, the management device 80 classifies each user equipment 10 into one of "near," "medium," and "far" classes in order of the distance between the user equipment 10 and the end relay node 30, and the closer the user equipment 10 is to the end relay node 30, the smaller the transmission path loss is and the greater the signal strength of the optical signal when it reaches the end relay node 30.

[0068] FIG. 5 shows an example in which management device 80 classifies user devices 10-1, 10-7, and 10-8 into a "near" class, user devices 10-2, 10-4, and 10-6 into a "medium" class, and user devices 10-3, 10-5, and 10-9 into a "far" class based on the distance between user device 10 and terminal relay node 30.

[0069] By devising the wavelength allocation for each user device 10 and the inter-port connection of the optical SW 332, it is possible to bundle optical signals from user devices 10 of a close distance class into the same optical transmission path. In the example shown in Fig. 5, the "near" class is assigned to the wavelength multiplexer / demultiplexer 331-1, the "middle" class is assigned to the wavelength multiplexer / demultiplexer 331-2, and the "far" class is assigned to the wavelength multiplexer / demultiplexer 331-n.

[0070] When the allocation shown in FIG. 5 is performed, the optical switch control unit 38 controls the connection between the ports of the optical SW 332 so that the optical signals transmitted from the user devices 10-1, 10-7, and 10-8 belonging to the "near" class are input to the wavelength multiplexer / demultiplexer 333-1. Furthermore, the optical switch control unit 38 controls the connection between the ports of the optical SW 332 so that the optical signals transmitted from the user devices 10-2, 10-4, and 10-6 belonging to the "middle" class are input to the wavelength multiplexer / demultiplexer 333-2. Furthermore, the optical switch control unit 38 controls the connection between the ports of the optical SW 332 so that the optical signals transmitted from the user devices 10-3, 10-5, and 10-9 belonging to the "far" class are input to the wavelength multiplexer / demultiplexer 333-n. The wavelength / route allocation unit 83 of the management device 80 may switch the wavelengths based on the result of classification so that the user devices 10 belonging to the same class do not use the same wavelength.

[0071] As a result of the optical switch control unit 38 controlling the connection between the ports of the optical SW 332 as described above, optical signals with approximately the same signal intensity are input to each wavelength multiplexer / demultiplexer 333. In this case, the intensity of each wavelength bundled in each path is equalized. Furthermore, by appropriately setting the gain of the optical amplifier 34, it is possible to make the intensity of all optical signals constant at the time of output to the relay node 45. Although the example shown in FIG. 5 shows the optical amplifier 34, a VOA may be used instead of the optical amplifier 34.

[0072] 5 shows an example in which there are three output ports, but if the number of user devices 10 accommodated in the terminal relay node 30 increases and becomes insufficient, an output port will be added. In this case, the gain of the optical amplifier 40 installed at the output port can be set according to the distribution of the user devices 10.

[0073] Fig. 6 is a sequence diagram for explaining the flow of processing in the optical communication system 100 in the first embodiment. Note that in the explanation of Fig. 6, a case where the signal intensity is flattened in the end relay node 30 will be explained. The wavelength / route allocation unit 83 of the management device 80 classifies each user device 10 accommodated in the terminal relay node 30 based on the information stored in the storage unit 82 (step S101). Here, the method of classification performed by the wavelength / route allocation unit 83 will be specifically described. For example, as a method of classification performed by the wavelength / route allocation unit 83, it is possible to classify signals arriving from each user device 10 by intensity at equal intervals in a grid. For example, the wavelength / route allocation unit 83 classifies user devices 10 with a received optical intensity of 0 to -10 dBm into a near class, user devices 10 with a received optical intensity of -10 to -20 dBm into a medium class, and user devices 10 with a received optical intensity of -20 to -30 dBm into a far class. The range of the received optical intensity for classification is appropriately changed according to the operation. Furthermore, the wavelength / route allocation unit 83 assigns different wavelengths so that the wavelengths used by user devices belonging to the same class do not overlap.

[0074] The strength of the signal arriving from the user device 10 may be assumed to be equal to the signal strength at the time of initial connection (before wavelength assignment), or the distance may be estimated from a transmission delay time due to RTT (Round-Trip Time) or the like, and the received optical strength may be estimated from a fiber loss per unit length that is stored in advance. Note that the above classification method is just one example, and other methods may be used as long as the classification can be performed based on the received strength of the optical signal.

[0075] The wavelength / route allocation unit 83 transmits classification information indicating the result of classification and route information to the end relay node 30 via the transmission / reception unit 81 (step S102). The route information transmitted here includes information for setting the route of the optical SW 332 so that optical signals transmitted from the user devices 10 belonging to the same class are concentrated in the same optical transmission path. For example, in the case of the example shown in FIG. 5, information for setting the route in the optical SW 332 is included so that optical signals transmitted from the user devices 10-1, 10-7, and 10-8 are output to the wavelength multiplexer / demultiplexer 333-1. This allows the signal intensity of the optical signals multiplexed by the wavelength multiplexer / demultiplexer 333-1 and concentrated in one optical transmission path to be adjusted collectively at the rear of the wavelength multiplexer / demultiplexer 333-1 without being divided by wavelength. The optical switch control unit 38 of the end relay node 30 controls the connection between the ports of the optical SW (for example, the optical SW 332) based on the classification information and route information notified from the management device 80 (step S103).

[0076] Specifically, the optical switch control unit 38 controls the connection between ports so that optical signals transmitted from user devices 10 of the same class are input to the same wavelength multiplexer / demultiplexer 333. The connection between the ports of the optical SW is switched under the control of the optical switch control unit 38. Each user device 10 transmits an optical signal (step S104). The optical signals transmitted from each user device 10 are multiplexed by the optical splitter 20 and input to the terminal relay node 30 as a multiplexed signal. The multiplexed signal input to the terminal relay node 30 is input to the first port of the circulator 32.

[0077] For example, a multiplexed signal input to a first port of a circulator 32-1 of a terminal relay node 30 is output from a second port of the circulator 32-1. Since a wavelength multiplexer / demultiplexer 331-1 of a path switching unit 33 is connected to the second port of the circulator 32-1, the multiplexed signal output from the second port of the circulator 32-1 is input to the wavelength multiplexer / demultiplexer 331-1 of the path switching unit 33. The wavelength multiplexer / demultiplexer 331-1 of the path switching unit 33 demultiplexes the input multiplexed signal for each wavelength (step S105). The optical signals demultiplexed for each wavelength by the wavelength multiplexer / demultiplexer 331-1 are output to the wavelength multiplexer / demultiplexer 333 by the optical SW 332.

[0078] Similarly, the multiplexed signals input to the first ports of the circulators 32-2 to 32-m of the terminal relay node 30 are output from the second ports of the circulators 32-2 to 32-m. Since the wavelength multiplexers / demultiplexers 331-2 to 331-m of the path switching unit 33 are connected to the second ports of the circulators 32-2 to 32-m, the multiplexed signals output from the second ports of the circulators 32-2 to 32-m are input to the wavelength multiplexers / demultiplexers 331-2 to 331-m of the path switching unit 33. The wavelength multiplexers / demultiplexers 331-2 to 331-m of the path switching unit 33 demultiplex the input multiplexed signals for each wavelength. The optical signals demultiplexed for each wavelength by the wavelength multiplexers / demultiplexers 331-2 to 331-m are output to the wavelength multiplexers / demultiplexers 333-2 to 333-n by the optical SW 332.

[0079] Each wavelength multiplexer / demultiplexer 333 multiplexes the input optical signals (step S106). The multiplexed signal multiplexed by each wavelength multiplexer / demultiplexer 333 is amplified by the optical amplifier 40 (step S107).

[0080] According to the optical communication system 100 configured as above, the management device 80 classifies the multiple user devices 10 accommodated in the same terminal relay node 30 so that the user devices 10 with similar optical signal strength are in the same class, sets a signal transfer path for concentrating optical signals transmitted from the user devices 10 of the same class in the same optical transmission path in the terminal relay node 30, and adjusts the signal strength of the optical signals concentrated in the same optical transmission path all at once without dividing them by wavelength. For example, the terminal relay node 30 switches the connection between the ports of the optical SW 332 so that the optical signals transmitted from the user devices 10 of the same class are input to the same wavelength multiplexer / demultiplexer 333, multiplexes the optical signals by class using the wavelength multiplexer / demultiplexer 333, and adjusts the signal strength of the multiplexed optical signal. As a result, the signal strength of the optical signals of each wavelength concentrated in the same optical transmission path is flattened. Therefore, it is possible to flatten the intensity difference of the optical signals transmitted from each user device accommodated in the same relay node without reducing the OSNR.

[0081] A modification of the first embodiment will be described. 1 shows a configuration in which optical signals are wavelength-multiplexed using optical splitters 20, 60 in the access sections between the user equipment 10 and the terminal relay node 30 and between the user equipment 70 and the terminal relay node 50. In the access sections, wavelength multiplexing may not be performed. That is, each user equipment 10 may be directly connected to the terminal relay node 30, and each user equipment 70 may be directly connected to the terminal relay node 50.

[0082] Second embodiment In the first embodiment, the distance between the user device and the end relay node is classified into three classes, "near," "middle," and "far," in order of decreasing distance, and the signal strength is flattened for each class. Here, the greater the total number of classes, the smaller the wavelength variation after flattening can be. However, if the total number of classes is greater than the scale of the user devices accommodated by the end relay node, the number of user devices classified into one class may decrease, resulting in an increase in the number of devices such as optical amplifiers. In the second embodiment, in order to avoid such a problem, for example, the minimum number of classes is set so that the gain reduction of a weak signal due to gain saturation does not occur, and after rough flattening, fine flattening is performed using a wavelength multiplexer and a VOA arranged at the rear of the optical amplifier.

[0083] FIG. 7 is a diagram showing a configuration example of an optical communication system 100a in the second embodiment. The basic system configuration in the second embodiment is similar to that in the first embodiment. The difference in the second embodiment is that the number of classes classified in the management device 80 is the minimum number of classes (e.g., two classes) that does not cause a gain reduction of a weak signal due to gain saturation, and the configuration of the end relay nodes 30a and 50a. Since the end relay nodes 30a and 50a have the same configuration, the end relay node 30a will be taken as an example for explanation. In the following explanation, as shown in FIG. 7, the configuration difference from the first embodiment will be explained.

[0084] The terminal relay node 30a in the second embodiment newly includes a plurality of wavelength multiplexers / demultiplexers 41-1 to 41-p (p is an integer equal to or greater than 1), a plurality of VOAs 42-1 to 42-p, and a plurality of wavelength multiplexers / demultiplexers 43-1 to 43-p at the rear stage of each optical amplifier 34. The wavelength multiplexers / demultiplexers 41-1 to 41-p multiplex or demultiplex the input optical signals for each wavelength. The VOAs 42-1 to 42-p are provided in the same number as the number of wavelengths demultiplexed by the wavelength multiplexers / demultiplexers 41-1 to 41-p. The VOAs 42-1 to 42-p adjust the signal intensity for each wavelength. The wavelength multiplexers / demultiplexers 43-1 to 43-p multiplex or demultiplex the input optical signals for each wavelength.

[0085] According to the optical communication system 100a configured as above, when the number of classes is reduced to two, "near" and "far", compared to the first embodiment, the signal intensity of the optical signal after passing through the optical amplifier 40 varies more. Since the signal intensity is finely adjusted by the VOA at the rear stage, it is possible to flatten it. In a conventional configuration in which the intensity is made uniform by the VOA and then optical amplification is performed, the optical intensity at the time of input to the optical amplifier decreases, so the OSNR decreases. On the other hand, when the intensity is made uniform by the VOA after optical amplification, the OSNR decrease can be prevented, but the gain of the weak signal decreases. In contrast, in the optical communication system 100a, in a configuration with a high OSNR in which a VOA is installed at the rear stage of the optical amplification, it is possible to suppress the decrease in the gain of the weak signal by making the signal level uniform by classification so that the decrease in the gain of the weak signal does not occur.

[0086] A modification of the second embodiment will be described. In the above-mentioned configuration, the terminal repeater node 30a concentrates optical signals transmitted from user devices 10 belonging to the same class in the same optical transmission path, and then fine-tunes each wavelength by the VOA. On the other hand, if the signal variation after the minimum classification is within the system tolerance, the terminal repeater node 30a does not need to fine-tune each wavelength by the VOA. In this configuration, for example, in the terminal repeater node 30a, a path not including the wavelength multiplexer / demultiplexer 41, the VOA 42, and the wavelength multiplexer / demultiplexer 43 may be provided after the optical SW 332, so that optical signals transmitted from user devices 10 belonging to a class whose signal variation is within the system tolerance may be concentrated in the same optical transmission path, and transferred to the repeater node 45 via a path not including the wavelength multiplexer / demultiplexer 41, the VOA 42, and the wavelength multiplexer / demultiplexer 43.

[0087] (Third embodiment) In the third embodiment, a configuration in which the function of the terminal relay node in the first embodiment is added to a reconfigurable optical add / drop multiplexer (ROADM) will be described.

[0088] Fig. 8 is a diagram showing a configuration example of an optical communication system 100b in the third embodiment. The basic system configuration in the third embodiment is similar to that in the first embodiment. The difference in the third embodiment is that the terminal relay node 30 or 50 is provided in the ROADM 90. Fig. 8 shows a case where the terminal relay node 30 is provided in the ROADM 90.

[0089] In Fig. 8, a clockwise route will be described as an example. An optical switch such as a WSS (Wavelength Selective Switch) 91 selects "through" or "drop" for each wavelength of an optical signal that enters a ROADM 90. In the through route, the signal is sent directly to the next ROADM 90. In the "drop" route, the signal is sent to a user equipment 10 under the control of the ROADM 90. Meanwhile, an optical signal sent from a user equipment 10 is input to a terminal relay node 30 and undergoes the same processing as in the first embodiment.

[0090] The optical communication system 100b configured as above can also be applied to a topology such as a ring configured with ROADMs.

[0091] (Fourth embodiment) In the first embodiment, for simplicity, a configuration in which two terminal relay nodes are connected one-to-one is assumed. In the fourth embodiment, a configuration in which a 1:multiple connection is described. In this case, the wavelengths constituting the multiplexed signal after flattening by the method of the first embodiment are not necessarily signals that should be output to the same path. Therefore, in the fourth embodiment, an optical switch is further added after the optical amplifier, and signals with the same path are output to the same optical transmission path.

[0092] Fig. 9 is a diagram showing a configuration example of an optical communication system 100c in the fourth embodiment. Note that the basic system configuration in the fourth embodiment is similar to that in the first embodiment. The differences in the fourth embodiment are that a plurality of end relay nodes 50 are provided, and the configuration of the end relay node 30c. Note that, although Fig. 9 shows a case where there is one end relay node 30c and a plurality of end relay nodes 50, the reverse may also be true. In the following description, the configuration that differs from the first embodiment will be described.

[0093] The end relay nodes 50-1 and 50-2 are provided at different locations. For example, the end relay node 50-1 is provided at the location of the path A of the end relay node 30c, and the end relay node 50-2 is provided at the location of the path B of the end relay node 30c.

[0094] The terminal relay node 30c in the fourth embodiment newly includes a plurality of wavelength multiplexers / demultiplexers 95-1 to 95-p, an optical SW 96, and a plurality of wavelength multiplexers / demultiplexers 97-1 to 97-q (q is an integer of 2 or more) at the rear stage of each optical amplifier 34. The wavelength multiplexers / demultiplexers 95-1 to 95-p multiplex or demultiplex the input optical signals for each wavelength. The optical SW 96 is an optical switch having a plurality of first ports and a plurality of second ports. An optical signal input to a certain port of the optical SW 96 is output from the other port. For example, an optical signal input to the first port of the optical SW 96 is output from the second port. The optical SW 96 has a connection relationship between the first port and the second port set by the control of the optical switch control unit 38. The wavelength multiplexers / demultiplexers 97-1 to 97-q multiplex or demultiplex the input optical signals for each wavelength.

[0095] The optical switch control unit 38 performs the same process as in the first embodiment. Furthermore, the optical switch control unit 38 controls the connection between the ports of the optical SW 96. For example, the optical switch control unit 38 controls the connection between the ports of the optical SW 96 so that optical signals to be transferred to the same path are input to the same wavelength multiplexer / demultiplexer 97.

[0096] For example, the optical switch control unit 38 controls the connection between the ports of the optical SW 96 so that the optical signal to be transferred to path A is input to the wavelength multiplexer / demultiplexer 97-1, and controls the connection between the ports of the optical SW 96 so that the optical signal to be transferred to path B is input to the wavelength multiplexer / demultiplexer 97-q.

[0097] According to the optical communication system 100c configured as above, it is also possible to apply the system to a configuration in which terminal relay nodes are connected in a one-to-multiple relationship.

[0098] A modified example common to the first to fourth embodiments will be described. The management device 80 may be integrated with the end relay nodes 30 , 30 a , 30 b , 30 c and 50 .

[0099] Some of the functional units of the terminal relay nodes 30, 30a, 30b, 30c, and 50 and the management device 80 in the above-described embodiment may be realized by a computer. In this case, a program for realizing the functions may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read into a computer system and executed to realize the functions. Note that the term "computer system" as used herein includes hardware such as an OS (Operating System) and peripheral devices.

[0100] In addition, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs (Read Only Memory), and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, the term "computer-readable recording medium" may also include those that dynamically hold a program for a short period of time, such as a communication line when transmitting a program via a network such as the Internet or a communication line such as a telephone line, and those that hold a program for a certain period of time, such as a volatile memory inside a computer system that serves as a server or client in such a case. Furthermore, the above-mentioned program may be for realizing part of the above-mentioned functions, or may be capable of realizing the above-mentioned functions in combination with a program already recorded in the computer system, or may be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0101] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0102] The present invention can be applied to an optical communication system using an optical switch. [Explanation of symbols]

[0103] 10, 10-1 to 10-9, 70...user device, 20, 20-1 to 20-3, 60...optical splitter, 30, 50...terminal relay node, 31, 31-1 to 31-m...optical splitter, 32, 32-1 to 32-m, 35, 35-1 to 35-n...circulator, 33, 36...route switching unit, 34, 34-1 to 34-n, 38...optical switch control unit, 39...optical amplifier control unit, 40, 40-1 to 40-m...optical amplifier, 42-1 to 42-p...VOA, 45...relay node, 80...management device, 81...transmitting / receiving unit, 82...storage unit, 83...wavelength / route allocation unit, 84...optical intensity adjustment information calculation unit, 90...ROADM, 91...WSS, 37...user management terminal, 41-1~41-p, 43-1~43-p, 95-1~95-p, 97-1~97-q, 331, 331-1~331-m, 333, 333-1~333-n, 361, 361-1~361-n, 363, 363-1~363-m...Wavelength multiplexer / demultiplexer, 96, 332, 362…optical SW

Claims

1. classifying a plurality of user devices accommodated in the same relay node such that user devices having similar optical signal strengths are in the same class; A signal transfer path is set for concentrating optical signals transmitted from user devices of the same class within the same optical transmission path, and the signal strength of the optical signals concentrated within the same optical transmission path is adjusted collectively without being divided by wavelength. Signal strength flattening method.

2. determining the similarity of optical signal strength between the user devices according to the distance between each user device and the relay node, and classifying the user devices so that the user devices having similar optical signal strength are in the same class; 2. The method of claim 1 .

3. Allocating different wavelengths between user devices belonging to the same class; 3. A method for equalizing signal intensity according to claim 1 or 2.

4. classifying a plurality of user equipments accommodated in the same relay node into a minimum number of classes such that user equipments having similar optical signal strengths are in the same class and such that a gain reduction of a weak signal due to gain saturation does not occur; setting a signal transfer path for concentrating optical signals transmitted from user devices of the same class in the same optical transmission path, and concentrating optical signals transmitted from user devices of the same class in the same optical transmission path; Signal strength flattening method.

5. adjusting the signal strength of the optical signals concentrated in the same optical transmission line for each class collectively without dividing them by wavelength, and then adjusting the signal strength of the optical signals for each wavelength, and concentrating the optical signals of each wavelength whose signal strength has been adjusted, in the same optical transmission line; A method for signal intensity equalization according to any one of claims 1 to 4.

6. forwarding the optical signal whose signal strength has been adjusted to a path to which a destination relay node is connected; Signal strength flattening method. A method for signal intensity equalization according to any one of claims 1 to 3 and 5.

7. an allocation unit that classifies a plurality of accommodated user devices so that user devices having similar optical signal strengths are in the same class, and sets a signal transfer path for concentrating optical signals transmitted from user devices of the same class in the same optical transmission path; an adjustment unit that collectively adjusts the signal intensity of the optical signals concentrated in the same optical transmission line by the allocation unit without dividing the optical signals by wavelength; A relay node comprising:

8. an allocation unit that classifies a plurality of user equipments accommodated in the same relay node into a minimum number of classes such that user equipments having similar optical signal strengths are in the same class and a gain reduction of a weak signal due to gain saturation does not occur, sets a signal transfer path for concentrating optical signals transmitted from user equipments of the same class in the same optical transmission path, and concentrates optical signals transmitted from user equipments of the same class in the same optical transmission path; A relay node comprising:

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