Abstract:
An optical wavelength division multiplexing network has a multi-level structure where a plurality of optical network units (ONUs) are connected to a lowest-level network. A node apparatus connected to networks other than the lowest-level network includes (a) passive optical components to branch optical signals from a higher-level network to a lower-level network, and couple optical signals from the lower-level network to the higher-level network, and (b) optical amplifiers for the optical signals. A node apparatus connected to the lowest-level network includes (a) an optical multiplexer/de-multiplexer to de-multiplex optical signals from the higher-level network, selectively output an optical signal to each ONU, and multiplex wave-length specific optical signals from the ONUs into a multiplexed optical signal, and (b) optical amplifiers for the optical signals. The node apparatuses provide an optical communication path between the higher-level network and the lower-level (or lowest-level) network without converting the optical signals into electrical signals.

Description:
STATEMENT OF RELATED APPLICATION  
       [0001]     The present application is a divisional application of U.S. patent application Ser. No. 09/785,402, filed Feb. 20, 2001 (now allowed), in the name of the same inventors, which in turn claims the benefit of priority based on Japanese Patent Application Nos. 2000-043293, filed Feb. 21, 2000, and 2000-240232, filed Aug. 8, 2000, all commonly owned herewith. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical wavelength division multiplexing network which multiplexes and transmits optical signals having a plurality of different wavelengths.  
         [0004]     2. Description of the Related Art  
         [0005]      FIG. 25  shows an example of the constitution of a conventional optical wavelength division multiplexing network. The network shown in  FIG. 25  has a ring structure comprising two or three layers. The network of  FIG. 25  will be explained in separate sections comprising a network ( 1 )  11 , a network ( 2 )  12 , a network ( 3 )  13 , a network ( 4 )  14 , and a network ( 5 )  15 . The network ( 1 )  11  has a ring structure, and is provided at the highest level. The network ( 1 )  11  comprises at least one center node  21  and two or more (three in  FIG. 1 ) remote nodes  22 ,  23 , and  24 . The network ( 2 )  12  is a ring network comprising a node (# 4 )  24 , which is one of the remote nodes of the network ( 1 )  11 , and is provided below the network ( 1 )  11 . The network ( 3 )  13  comprises a tree-shaped structure centered on a node (# 41 )  25 , which is one of the nodes of the network ( 2 )  12 , and is provided below the network ( 2 )  12 . The network ( 4 )  14  comprises a ring-shaped structure centered on a node (# 3 )  23 , which is one of the remote nodes of the network ( 1 )  11 , and is provided below the network ( 1 )  11 . The network ( 5 )  15  comprises a tree-shaped structure centered on a node  27 , which is one of a plurality of nodes  23 ,  26 ,  27 , and  28  of the network ( 4 )  14 , and can conceivably be provided below the network ( 4 )  14 . Optical network units (ONU) (they are also called optical service units.)  51  to  56  comprise the subscribers of each home, business office, and the like, and are provided in the network ( 3 )  13  or the network ( 5 )  15 .  
         [0006]     In  FIG. 25 , the nodes of network ( 1 )  11 , the network ( 2 )  12 , and the network ( 4 )  14 , are connected by optical fiber transmission paths  60 ,  60 , . . . which comprise a plurality of optical fibers. The ONUs are connected to the nodes of the networks ( 3 )  13  and ( 5 )  15  by optical fiber transmission paths  70 ,  70 , . . . which comprise a single optical fiber. Equipment for signal termination electrically processes transmission signals, which have been converted from optical signals to electrical signals, and are provided at the nodes  21  to  28 .  
         [0007]     In this explanation, traffic from subscribers in the network ( 3 )  13  or the network ( 5 )  15  is assumed to be 1.5 Mb/s. The traffic from the subscribers is multiplexed at the subscriber office (node  25  or node  27 ), and transmitted to the node (# 4 )  24  in the network ( 2 )  12  or the remote node (# 3 )  23  in the network ( 1 )  11  at a higher transmission speed of, for example, 52 Mb/s. At the node (# 41 )  25  and the node  27 , traffic sent from other nodes in the network ( 2 )  12  or the network ( 4 )  14  is combined with the multiplexed traffic from the subscribers, and transmitted to the next node in the network ( 2 )  12  or the network ( 4 )  14  at an even high transmission speed. Transmission speed conversion and the like is also carried out at the nodes in the network ( 1 )  11 . That is, electrical processing is carried out at each node.  
         [0008]     In conventional optical networks such as that shown in  FIG. 25 , when starting a new high-speed access service for users at a speed of, for example, approximately 150 Mb/s or 1 Gb/s, transmission apparatuses which carry out electrical processing for multiplexing the traffic must be provided at each node, since there are several users belonging to the network ( 3 )  13  or the network ( 5 )  15 . Consequently, the initial expenditure is considerable. Moreover, depending on the region, there may be fewer users per node, leading to a drawback of expenditure efficiency.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, it is an object of this invention to provide an optical wavelength division multiplexing network which can carry out large-capacity optical access services with a simpler constitution. It is a more specific object of this invention to provide the optical wavelength division multiplexing network which enables initial expenditure to be reduced in large-capacity optical access services using ONU.  
         [0010]     In order to solve the problems mentioned above, a first aspect of this invention provides an optical wavelength division multiplexing network having a structure comprising at least two layers, a highest level network being a ring network which comprises at least one center node and two or more remote nodes which are joined by at least two optical fibers; in the case where the layered structure comprises three or more layers, excepting the lowest level network the intermediate level network comprising a ring having the node belonging to the highest level network as its center node, nodes belonging to the ring network being joined by at least two optical fibers; the lowest level network comprising a star network centered around an access node which multiplexes traffic from one or a plurality of optical network units (ONU), the ONU and the access node being directly joined by at least one optical fiber; the remote nodes amplifying optical wavelength division multiplexing signals which are transmitted on an optical fiber comprising the higher level network which the remote nodes belong to, branching the signals to an optical fiber comprising the lower level network, and coupling optical wavelength division multiplexing signals, input from an optical fiber comprising the lower level network, to optical wavelength division multiplexing signals transmitted on an optical fiber comprising the higher level network, and amplifying the coupled signals; the access node amplifying the optical wavelength division multiplexing signals transmitted from the optical fibers which comprise the higher level network which the access node is connected to, selecting optical signals having wavelengths which correspond to the ONU, and outputting the selected signals to the ONU; multiplexing the optical signals transmitted from the ONU, dividing the multiplexed signals in a plurality of directions, amplifying the divided signals, and transmitting the amplified signals to an optical fiber comprising a higher level network which the access node is connected to; and the center node belonging to the highest level network and the ONU establishing a direct communication path by using lights of different wavelengths, the optical signals being amplified, branched, and routed at the remote nodes and the access node provided therebetween.  
         [0011]     Furthermore, a first aspect of the center node comprising the optical wavelength division multiplexing network according to the first aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to optical signals at each wavelength; a plurality of optical receivers which convert the optical signals which have been de-multiplexed by the optical de-multiplexers to electrical signals; a plurality of selectors which selectively output either of the outputs from the plurality of optical receivers; a signal termination section which performs predetermined electrical processing to the electrical signals which have been selected by the selectors, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; and a plurality of optical multiplexers which multiplex the optical signals output from the optical senders, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0012]     A second aspect of the center node comprising the optical wavelength division multiplexing network according to the first aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to optical signals at each wavelength; a plurality of optical switches which select one of the optical signal which have been de-multiplexed by the optical de-multiplexers; a plurality of optical receivers which convert the optical signals which have been selected by the optical switches to electrical signals; a signal termination section which performs predetermined electrical processing to the electrical signals which have been output from the optical receivers, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; and a plurality of optical multiplexers which multiplex the optical signals output from the optical senders, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0013]     A third aspect of the center node comprising the optical wavelength division multiplexing network according to the first aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to a plurality of optical signals at each wavelength; a plurality of optical switches which select one of the plurality of optical signals which have been de-multiplexed by the optical de-multiplexers; a plurality of optical receivers which convert the optical signals which have been selected by the optical switches to electrical signals; a signal termination section which performs predetermined electrical processing to the electrical signals which have been output from the optical receivers, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the plurality of electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; a plurality of optical dividers which divide the optical signals output from the optical senders in a plurality of directions; and a plurality of optical multiplexers which multiplex the plurality of optical signals output from the optical dividers, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0014]     A remote node comprising the optical wavelength division multiplexing network according to the first aspect described above comprising: passive optical components which branch optical signals transmitted on an optical fiber comprising a higher level network to an optical fiber comprising a lower level network, and couple optical signals input from an optical fiber comprising the lower level network to optical signals transmitted on an optical fiber comprising the higher level network; and optical amplifiers which amplify the optical signals input to the passive optical components and the optical signals output from the passive optical components.  
         [0015]     An access node comprising the optical wavelength division multiplexing network according to the first aspect described above comprising: an optical switch which selects one of the optical signals which are input from optical fibers comprising a higher level network; a first optical amplifier which amplifies, among the optical signals which are input from the optical fibers comprising the higher level network, at least the optical signal selected by the optical switch; an optical multiplexer/de-multiplexer which, based on the optical signal selected by the optical switch, selects an optical signal having a wavelength which corresponds to the ONU, outputs the selected signal to the ONU, and multiplexes the optical signals transmitted from the ONU; an optical divider which divides the optical signal, multiplexed by the optical multiplexer/de-multiplexer, into a plurality of directions, and transmits the divided signals to the optical fibers comprising the higher level network; and a second optical amplifier which amplifies the optical signals which are transmitted to the optical fibers comprising the higher level network.  
         [0016]     A second aspect of this invention provides an optical wavelength division multiplexing network having a structure comprising at least two layers, a highest level network being a ring network which comprises at least one center node and two or more remote nodes which are joined by at least two optical fibers; a lowest level network comprising a star network centered around an access node which multiplexes traffic from one or a plurality of optical network units (ONU), the ONU and the access node being directly joined by at least one optical fiber; an immediately higher level network of the lowest level network being a ring network comprising at least one the access node connected by at least two fibers, traffic from the access nodes being multiplexed at a center node in the ring network which the access node belongs to, and connected by the center node to an even higher level network; the remote node amplifying and branching optical wavelength division multiplexing signals which are transmitted on an optical fiber comprising the higher level network which the remote node belongs to, de-multiplexing and receiving only optical signals at wavelengths corresponding to the ONU, electrically processing the optical signals, and transmitting the processed signals at a predetermined wavelength to optical fibers comprising a lower level network; de-multiplexing and receiving only optical signals among the optical wavelength division multiplexing signals, input along the optical fibers comprising the lower level network, which are at wavelengths corresponding to the ONU, electrically processing the optical signals, converting the processed signals to optical signals at wavelengths which were allocated beforehand, and coupling the converted signals to optical wavelength division multiplexing signals transmitted on optical fibers comprising the higher level network; the access node provided between the remote node and the ONU amplifying the optical wavelength division multiplexing signals which are transmitted on the optical fibers comprising the higher level network which the access node is connected to, selecting optical signals which correspond to the ONU and outputting the selected signals thereto; and multiplexing the optical signals from the ONU, dividing the multiplexed signal in a plurality of directions, amplifying the divided signals, and transmitting the amplified signals on optical fibers comprising the higher level network which the access node is connected to; and optical signals having different wavelengths being transmitted between the ONU and the remote node in the higher level network, which is the center node in the ring network comprising the access node, the access node provided between the remote node and the ONU amplifying and routing the optical signals.  
         [0017]     A first aspect of the center node comprising the optical wavelength division multiplexing network according to the second aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to optical signals at each wavelength; a plurality of optical receivers which convert the optical signals which have been de-multiplexed by the optical de-multiplexers to electrical signals; a plurality of selectors which selectively output either of the outputs from the plurality of optical receivers; a signal termination section which performs predetermined electrical processing to the electrical signals which have been selected by the selectors, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; and a plurality of optical multiplexers which multiplex the optical signals output from the optical senders, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0018]     A second aspect of the center node comprising the optical wavelength division multiplexing network according to the second aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to optical signals at each wavelength; a plurality of optical switches which select one of the optical signal which have been de-multiplexed by the optical de-multiplexers; a plurality of optical receivers which convert the optical signals which have been selected by the optical switches to electrical signals; a signal termination section which performs predetermined electrical processing to the electrical signals which have been output from the optical receivers, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; and a plurality of optical multiplexers which multiplex the optical signals output from the optical senders, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0019]     A third aspect of the center node comprising the optical wavelength division multiplexing network according to the second aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to a plurality of optical signals at each wavelength; a plurality of optical switches which select one of the plurality of optical signals which have been de-multiplexed by the optical de-multiplexers; a plurality of optical receivers which convert the optical signals which have been selected by the optical switches to electrical signals; a signal termination section which performs predetermined electrical processing to the electrical signals which have been output from the optical receivers, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the plurality of electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; a plurality of optical dividers which divide the optical signals output from the optical senders in a plurality of directions; and a plurality of optical multiplexers which multiplex the plurality of optical signals output from the optical dividers, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0020]     A remote node comprising the optical wavelength division multiplexing network according to the second aspect described above comprising: passive optical components which branch optical signals transmitted on optical fibers comprising the higher level network, and couple input optical signals to optical signals transmitted on optical fibers comprising the higher level network; optical amplifiers which amplify the optical signals input to the passive optical components and the optical signals output from the passive optical components; and an equipment for signal termination which de-multiplexes only the optical signals among those divided by the passive optical components at wavelengths corresponding to the ONU, receives and electrically processes the optical signals at each wavelength, and transmits the processed signals at a predetermined wavelength, and in addition, de-multiplexes only the optical signals among those input along the optical fibers comprising a lower level network which are at wavelengths corresponding to the ONU, receives and electrically processes the optical signals at each wavelength, converts the processed signals to optical signals at a wavelength allocated beforehand, and transmits the converted signals to the passive optical components.  
         [0021]     An access node comprising the optical wavelength division multiplexing network according to the second aspect described above comprising: an optical switch which selects one of the optical signals which are input from optical fibers comprising a higher level network; a first optical amplifier which amplifies, among the optical signals which are input from the optical fibers comprising the higher level network, at least the optical signal selected by the optical switch; an optical multiplexer/de-multiplexer which, based on the optical signal selected by the optical switch, selects an optical signal having a wavelength which corresponds to the ONU, outputs the selected signal to the ONU, and multiplexes the optical signals transmitted from the ONU; an optical divider which divides the optical signal, multiplexed by the optical multiplexer/de-multiplexer, into a plurality of directions, and transmits the divided signals to the optical fibers comprising the higher level network; and a second optical amplifier which amplifies the optical signals which are transmitted to the optical fibers comprising the higher level network.  
         [0022]     A third aspect of this invention provides an optical wavelength division multiplexing network having a structure comprising at least three layers, a highest level network being a ring network comprising at least one center node and two or more remote nodes which are joined by at least four optical fibers; an intermediate level network being a ring network having a node belonging to the higher level network as a center node thereof, access nodes belonging to the ring network being joined by at least four optical fibers; a lowest level network comprising a star network centered around an access node which multiplexes traffic from optical network units (ONU), the ONU and the access node being directly joined by at least one optical fiber; the remote node amplifying optical wavelength division multiplexing signals transmitted on the optical fibers comprising a higher level node which the remote node belongs to, branching the signals to optical fibers comprising a lower level network, and coupling optical wavelength division multiplexing signals which are input from optical fibers comprising the lower level network to optical wavelength division multiplexing signals transmitted on optical fibers comprising the higher level network, thereby amplifying the coupled signals; the access node amplifying optical wavelength division multiplexing signals transmitted on optical fibers comprising a higher level network, which the access node belongs to, branching the amplified signals to a lower level network for outputting the branched signals to the ONU; multiplexing optical signals transmitted from the ONU, dividing the multiplexed signals in a plurality of directions, coupling the divided signal to optical wavelength division multiplexing signals transmitted on optical fibers comprising a higher level network which the access node is connected to, and amplifying the coupled signals; and the center node belonging to the highest level network and the ONU establishing a direct communication path by using lights of different wavelengths, the optical signals being amplified, branched, or routed, at the remote nodes and the access nodes provided therebetween.  
         [0023]     A fourth aspect of this invention provides an optical wavelength division multiplexing network having a structure comprising at least three layers, a highest level network being a ring network comprising at least one center node and two or more remote nodes which are joined by at least two optical fibers; an intermediate level network being a ring network having a node belonging to the higher level network as a center node thereof, access nodes belonging to the ring network being joined by at least four optical fibers; a lowest level network comprising a star network centered around an access node which multiplexes traffic from optical network units (ONU), the ONU and the access node being directly joined by at least one optical fiber; the remote nodes amplifying optical wavelength division multiplexing signals transmitted on the optical fibers comprising a higher level network which the remote nodes belong to, branching the signals to optical fibers comprising a lower level network, and coupling optical wavelength division multiplexing signals which are input from optical fibers comprising the lower level network to optical wavelength division multiplexing signals transmitted on optical fibers comprising the higher level network, and amplifying the coupled signals; the access node amplifying optical wavelength division multiplexing signals transmitted on optical fibers comprising a higher level network, which the access node belongs to, branching them to a lower level network for outputting the branched signals to the ONU; multiplexing optical signals transmitted from the ONU, dividing them in a plurality of directions, coupling the divided signals to optical wavelength division multiplexing signals transmitted on optical fibers comprising a higher level network which the access node is connected to, and amplifying the coupled signals; and the center node belonging to the highest level network and the ONU establishing a direct communication path by using lights of different wavelengths, the optical signals being only amplified, branched, or routed, at the remote nodes and the access node provided therebetween.  
         [0024]     A center node comprising the optical wavelength division multiplexing network according to the third and fourth aspects described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to optical signals at each wavelength; a plurality of optical receivers which convert the optical signals which have been de-multiplexed by the optical de-multiplexers to electrical signals; a plurality of selectors which selectively output either of the outputs from the plurality of optical receivers; a signal termination section which performs predetermined electrical processing to the electrical signals which have been selected by the selectors, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; and a plurality of optical multiplexers which multiplex the optical signals output from the optical senders, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0025]     A remote node comprising the optical wavelength division multiplexing network according to the third aspect described above comprising: passive optical components which branch optical signals transmitted on optical fibers comprising a higher level network to optical fibers comprising a lower level network, and in addition, couple optical signals input from optical fibers comprising the lower level network to optical signals transmitted on optical fibers comprising the higher level network; and optical amplifiers which amplify optical signals which are input to, and output from, the passive optical components; wherein both ends of the loop of optical fibers comprising the lower level network are opened by using optical terminators.  
         [0026]     An access node comprising the optical wavelength division multiplexing network according to the third and fourth aspects described above comprising: first passive optical components which branch optical signals transmitted on optical fibers comprising a higher level network to a lower level network; an optical switch which selects one of the optical signals which have been branched by the first passive optical components; an optical multiplexer/de-multiplexer which transmits the optical signals selected by the optical switch toward the ONU, and multiplexes the optical signals transmitted from the ONU; an optical divider which divides the optical signals multiplexed by the optical multiplexer/de-multiplexer in a plurality of directions; second passive optical components which couple optical signals divided by the optical divider to optical signals transmitted on optical fibers comprising the higher level network; and optical amplifiers which amplify the optical signals which are input to and output from the first and second passive optical components.  
         [0027]     A remote node comprising the optical wavelength division multiplexing network according to the fourth aspect described above comprising: passive optical components which branch optical signals transmitted on optical fibers comprising a higher level network to optical fibers comprising a lower level network, and in addition, couple optical signals input from optical fibers comprising the lower level network to optical signals transmitted on optical fibers comprising the higher level network; and optical amplifiers which amplify optical signals transmitted on the optical fibers comprising the higher level network; wherein one end of the loop of optical fibers comprising the lower level network is opened by using optical terminators.  
         [0028]     A fifth aspect of this invention provides an optical wavelength division multiplexing network having a structure comprising at least two layers, a highest level network comprising a ring network having at least one center node and two or more remote nodes, which are joined by at least four optical fibers; intermediate level networks excepting the lowest level network comprising a ring network having a node belonging to the higher level network as a center node, and at least one node belonging to the intermediate level ring networks being joined by at least four optical fibers; the lowest level network comprising a star network centered around an access node belonging to the ring network which is provided immediately thereabove, the access node being joined to at least one optical network unit (ONU) by at least two optical fibers; the remote nodes amplifying optical wavelength division multiplexing signals transmitted on the optical fibers comprising a higher level node which the remote nodes belong to, branching the signals to optical fibers comprising a lower level network; and coupling optical wavelength division multiplexing signals which are input from optical fibers comprising the lower level network to optical wavelength division multiplexing signals transmitted on optical fibers comprising the higher level network; the access node amplifying optical wavelength division multiplexing signals transmitted on optical fibers comprising a higher level network which the access node is connected to, branching the amplified signals to a lower level network, amplifying the divided signals, and outputting the amplified signals to the ONU; multiplexing and amplifying optical signals transmitted from the ONU, dividing the amplified signals in a plurality of directions, coupling the divided signals to optical wavelength division multiplexing signals transmitted on optical fibers comprising a higher level network which the access node is connected to, and amplifying the coupled signals; and the center node belonging to the highest level network transmitting data by using different wavelengths allocated to the ONU, the ONU transmitting the data to the center node by using optical signals having the same wavelengths as the allocated wavelengths; and the access nodes and the remote nodes provided between the center node and the ONU only amplifying and dividing, or routing, the optical signals.  
         [0029]     A center node comprising the optical wavelength division multiplexing network according to the fifth aspect described above comprising: a plurality of optical de-multiplexers which de-multiplex optical wavelength division multiplexing signals, input from optical fibers comprising the highest level network, to optical signals at each wavelength; a plurality of optical receivers which convert the optical signals which have been de-multiplexed by the optical de-multiplexers to electrical signals; a plurality of selectors which selectively output either of the outputs from the plurality of optical receivers; a signal termination section which performs predetermined electrical processing to the electrical signals which have been selected by the selectors, and outputs a plurality of groups of electrical signals; a plurality of optical senders which convert the electrical signals output from the signal termination section to a plurality of optical signals having different wavelengths; and a plurality of optical multiplexers which multiplex the optical signals output from the optical senders, and output the multiplexed signals to optical fibers comprising the highest level network.  
         [0030]     A remote node comprising the optical wavelength division multiplexing network according to the fifth aspect described above comprising: first passive optical components which branch optical signals transmitted on optical fibers comprising a higher level network to optical fibers comprising a lower level network; second passive optical components which couple optical signals input from optical fibers comprising the lower level network to optical signals transmitted on optical fibers comprising the higher level network; and optical amplifiers which amplify optical signals which are input to, and output from, the first and second passive optical components; wherein both ends of the loop of optical fibers comprising the lower level network are opened by using optical terminators.  
         [0031]     An access node comprising the optical wavelength division multiplexing network according to the fifth aspect described above comprising: first passive optical components which branch optical signals transmitted on optical fibers comprising a higher level network to a lower level network; an optical switch which selects one of the optical signals which have been branched by the first passive optical components; a first optical amplifier which amplifies, among the optical signals which have been branched by the first passive optical components, at least the optical signal selected by the optical switch; a second passive optical component which distributes the optical signals amplified by the first optical amplifier to the ONU, and multiplexes the optical signals transmitted from the ONU; a second optical amplifier which amplifies the optical signals multiplexed by the second passive optical component; an optical divider which divides the optical signal, amplified by the second optical amplifier, into a plurality of directions; a third passive optical component which couples the optical signals branched by the optical divider to an optical signal transmitted on optical fibers comprising the higher level network; and a third optical amplifier which amplifies the optical signals which are transmitted on the optical fibers comprising the higher level network.  
         [0032]     In this invention, the center node belonging to the highest level network and the ONU establish a direct communication path by using lights of different wavelengths. At the nodes therebetween, the signals are amplified, branched, and routed in their optical format without being electrically processed. In other words, the center node of the network which is the final multiplexing destination of the traffic can be directly linked to the user by an optical signal at a certain wavelength. No electrical processing is performed at the nodes in between. The users and the center node are directly joined by optical signals at different wavelengths. In this case, only the center node multiplexes traffic from users, and carries out electrical processing such as communicating with other users in the regional network, distributing traffic to the core network, and the like. Therefore, according to this invention, it is possible to provide an optical wavelength division multiplexing network which can carry out large-capacity access services with a simpler constitution.  
         [0033]     Furthermore, according to a system switching method in the optical wavelength division multiplexing network of this invention, when an optical fiber (working fiber), which is being used in transmitting a down signal from the center node to the ONU in the higher level network, becomes severed, an access node belonging to a remote node provided downstream than the severance point as seen from the center node, switches from the working fiber side to an optical fiber side (protection fiber) which is not presently in use, the down signal being received after transmission along the protection fiber; when a working fiber for transmitting an up signal from the ONU to the center node in the higher level network has become severed, for an access node belonging to remote node where the severance point on the working fiber to the center node is located, the center node switches from the working fiber to a protection fiber, and receives the up signal from the protection fiber; and when an optical cable in the intermediate level network has become severed, an access node, among the access nodes connected to the intermediate level network, which is provided downstream than the severance point for the optical signal transmitted on the severed fiber switches from the working fiber to the protection fiber and thereby receives the down signal; and at the access node provided downstream, the center node switches from the working fiber to the protection fiber and thereby receives the up signal from the protection fiber. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  is a block diagram showing the entire constitution of the optical wavelength division multiplexing network in the embodiments of this invention;  
         [0035]      FIG. 2  is a block diagram showing the constitution of a first embodiment of this invention;  
         [0036]      FIG. 3  is a block diagram showing the constitution of a second embodiment of this invention;  
         [0037]      FIG. 4  is a block diagram showing one example of the constitution of a center node  21   a  shown in FIGS.  1  to  3  and FIGS.  7  to  11 ;  
         [0038]      FIG. 5  is a block diagram showing another example of the constitution of the center node  21   a  shown in FIGS.  1  to  3  and FIGS.  7  to  11 ;  
         [0039]      FIG. 6  is a block diagram showing yet another example of the constitution of the center node  21   a  shown in FIGS.  1  to  3  and FIGS.  7  to  11 ;  
         [0040]      FIG. 7  is a block diagram showing the constitution of a third embodiment of this invention;  
         [0041]      FIG. 8  is a block diagram showing the constitution of a fourth embodiment of this invention;  
         [0042]      FIG. 9  is a block diagram showing the constitution of a fifth embodiment of this invention;  
         [0043]      FIG. 10  is a block diagram showing the constitution of a sixth embodiment of this invention;  
         [0044]      FIG. 11  is a block diagram showing the constitution of a seventh embodiment of this invention;  
         [0045]      FIG. 12  is a block diagram showing the constitution of an eighth embodiment of this invention;  
         [0046]      FIG. 13  is a block diagram showing an optical multiplexer/de-multiplexer  115  shown in these diagrams when it is arranged as an AWG;  
         [0047]      FIG. 14  is a diagram showing one example of the relationship between the wavelength of the AWG  115  of  FIG. 13  and the input/output ports;  
         [0048]      FIG. 15  is a block diagram showing the constitution of a ninth embodiment of this invention;  
         [0049]      FIG. 16  is a block diagram showing the constitution of the ninth embodiment of this invention;  
         [0050]      FIG. 17  is a block diagram showing the constitution of the ninth embodiment of this invention;  
         [0051]      FIG. 18  is a block diagram showing the constitution of the center node  21   b  of FIGS.  15  to  17 ;  
         [0052]      FIG. 19  is a block diagram showing the constitution of a tenth embodiment of this invention;  
         [0053]      FIG. 20  is a block diagram showing the constitution of the tenth embodiment of this invention;  
         [0054]      FIG. 21  is a block diagram showing the constitution of the center node  21   c  of FIGS.  19  to  20 ;  
         [0055]      FIG. 22  is a block diagram showing the constitution of an eleventh embodiment of this invention;  
         [0056]      FIG. 23  is a block diagram showing the constitution of the center node  21   d  of  FIG. 22 ;  
         [0057]      FIG. 24  is a block diagram showing the constitution of a twelfth embodiment of this invention;  
         [0058]      FIG. 25  is a block diagram showing one example of the constitution of a conventional optical wavelength division multiplexing network;  
         [0059]      FIG. 26  is a block diagram showing the constitution of a thirteenth embodiment of this invention;  
         [0060]      FIG. 27  is a block diagram showing the constitution of a fourteenth embodiment of this invention; and  
         [0061]      FIG. 28  is a block diagram showing the constitution of a fifteenth embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0062]     Preferred embodiments of the optical wavelength division multiplexing network according to this invention will be explained with reference to the drawings.  
       Embodiment 1  
       [0063]      FIG. 1  is a block diagram showing the entire constitution of the optical wavelength division multiplexing network in this embodiment and in embodiments subsequently explained. As shown in  FIG. 1 , parts of the constitution which are identical to those in  FIG. 25  are represented by the same reference numerals. Parts of the constitution which correspond to those in  FIG. 25  are specified by adding the letter “a” to the end of the reference numerals shown in  FIG. 25 .  
         [0064]     The first embodiment will be explained with reference to  FIGS. 1 and 2 .  FIG. 2  is a block diagram showing the constitutions of the network ( 1 )  11   a,  the network ( 2 )  12   a,  and the network ( 3 )  13   a,  shown in  FIG. 1 . This embodiment uses a two-fiber bi-directional ring constitution, in which the nodes of the networks ( 1 )  11   a,  ( 2 )  12   a,  etc., are connected by pairs of optical fibers which transmit optical signals in differing directions. For example, in the network ( 1 )  11   a,  the nodes are connected by optical fibers ( 1 )  62  and ( 2 )  61 , and optical fibers ( 1 )  64  and ( 2 )  63 , which transmit optical signals in differing directions. In the network ( 2 )  12   a,  the nodes are connected by optical fibers  65  and  66 , and optical fibers  67  and  68 , which transmit optical signals in differing directions.  
         [0065]     In the example shown in  FIG. 2 , ONU  51 ,  52 , and  53  are joined by optical signal transmitters comprising the linear optical fibers  65 ,  66 ,  67 ,  68 , and the like, which extend from the node  24   a  belonging to the network ( 1 )  11   a  having the center node  21   a.  In  FIG. 2 , the curved solid lines represent the fibers which are being used as working fibers (optical fibers  61 ,  62 ,  67 ,  68 , etc.), and the dotted lines represent the fibers which are being used as protection fibers (optical fibers  63 ,  64 ,  65 ,  66 , etc.). The optical networks in this embodiment, and in the embodiments described subsequently, are characterized in that, as shown in  FIG. 1 , the optical signals are not electrically processed (i.e. processing performed when multiplexing traffic, such as converting the transmission speed) at the remote nodes and access nodes (offices, telephone stations) other than the center node ( 21   a ) belonging to the ring network (network ( 1 )  11   a ) at the highest level.  
         [0066]     This feature will be explained in detail. The constitution of the center node  21   a  shown in  FIGS. 1 and 2  will be explained with reference to FIGS.  4  to  6 . In FIGS.  4  to  6 , signal lines for transmitting optical signal are represented by thick lines, and signal lines for transmitting electrical signals are represented by fine lines.  
         [0067]     In the constitution shown by way of example in  FIG. 4 , the center node  21   a  of  FIG. 2  comprises an optical de-multiplexer  201  which de-multiplexes a wavelength division multiplexing signal, which is input from the remote node  22   a  via the optical fiber ( 2 )  61  and has n different wavelengths from λn+1 to λ 2   n,  to n optical signals at each wavelength, an optical de-multiplexer  202  which de-multiplexes a wavelength division multiplexing signal, which is input from the remote node  24   a  via the optical fiber ( 1 )  64  and has n different wavelengths from λn+1 to λ 2   n,  to n optical signals at each wavelength, n optical receivers  221   —   n+ 1 to  221 _ 2   n  and optical receivers  222   —   n+ 1 to  222 _ 2   n  which convert the n optical signals de-multiplexed by the optical de-multiplexers  201  and  202  to electrical signals, n selectors  231 _ 1  to  231   —   n  which selectively output either one of the outputs from the optical receivers  221   —   n+ 1 to  221 _ 2   n  and the optical receivers  222   —   n+ 1 to  222 _ 2   n,  an equipment for signal termination  241  which performs predetermined electrical processing to the electrical signals output from the selectors  231 _ 1  to  231   —   n  and outputs two groups of n electrical signals, n optical senders  251 _ 1  to  251   —   n  and optical senders  252 _ 1  to  252   —   n  which convert the electrical signals output from the equipment for signal termination  241  to optical signals having n different wavelengths from λ 1  to λn, an optical multiplexer  211  which multiplexes the optical signals output from the optical senders  251 _ 1  to  251   —   n  and outputs them to the optical fiber ( 1 )  62 , and an optical multiplexer  212  which multiplexes the optical signals output from the optical senders  252 _ 1  to  252   —   n  and outputs them to the optical fiber ( 2 )  63 .  
         [0068]     In the constitution shown by way of example in  FIG. 5 , the center node  21   a  of  FIG. 2  comprises an optical de-multiplexer  201 , identical to that in the constitution shown in  FIG. 4 , which de-multiplexes a wavelength division multiplexing signal, which is input from the remote node  22   a  via the optical fiber ( 2 )  61  and has n different wavelengths from λn+1 to λ 2   n,  to n optical signals at each wavelength, an optical de-multiplexer  202  which de-multiplexes a wavelength division multiplexing signal, which is input from the remote node  24   a  via the optical fiber ( 1 )  64  and has n different wavelengths from λn+1 to λ 2   n,  to n optical signals at each wavelength, n optical switches  261   —   n+ 1 to  261 _ 2   n  which selectively output either of the n optical signals output from the optical de-multiplexers  201  and  202 , n optical receivers  271 _ 1   n +1 to  271 _ 2   n  which convert the n optical signals output from the optical switches  261   —   n+ 1 to  261 _ 2   n  to electrical signals, an equipment for signal termination  241  which performs predetermined electrical processing to the electrical signals output from the optical receivers  271   —   n+ 1 to  271 _ 2   n  and outputs two groups of n electrical signals, n optical senders  251 _ 1  to  251   —   n  and optical senders  252 _ 1  to  252   —   n  which convert the electrical signals output from the equipment for signal termination  241  to optical signals having n different wavelengths from λ 1  to λn, an optical multiplexer  211  which multiplexes the optical signals output from the optical senders  251 _ 1  to  251   —   n  and outputs them to the optical fiber ( 1 )  62 , and an optical multiplexer  212  which multiplexes the optical signals output from the optical senders  252 _ 1  to  252   —   n  and outputs them to the optical fiber ( 2 )  63 .  
         [0069]     In the constitution shown by way of example in  FIG. 6 , the center node  21   a  of  FIG. 2  comprises an optical de-multiplexer  201 , identical to that in the constitution shown in  FIG. 5 , which de-multiplexes a wavelength division multiplexing signal, which is input from the remote node  22   a  via the optical fiber ( 2 )  61  and has n different wavelengths from λn+1 to λ 2   n,  to n optical signals at each wavelength, an optical de-multiplexer  202  which de-multiplexes a wavelength division multiplexing signal, which is input from the remote node  24   a  via the optical fiber ( 1 )  64  and has n different wavelengths from λn+1 to λ 2   n,  to n optical signals at each wavelength, n optical switches  261   —   n+ 1 to  261 _ 2   n  which selectively output either of the n optical signals output from the optical de-multiplexers  201  and  202 , n optical receivers  271   —   n+ 1 to  271 _ 2   n  which convert the n optical signals output from the optical switches  261   —   n+ 1 to  261 _ 2   n  to electrical signals, an equipment for signal termination  241   a  which performs predetermined electrical processing to the electrical signals output from the optical receivers  271   —   n+ 1 to  271 _ 2   n  and outputs one group of n electrical signals, n optical senders  281 _ 1  to  281   —   n  which convert the electrical signals output from the equipment for signal termination  241   a  to optical signals having n different wavelengths from λ 1  to λn, n optical dividers  291 _ 1  to  291   —   n  which divide into two the optical signals output from the n optical senders  281 _ 1  to  281   —   n , an optical multiplexer  211  which multiplexes the n optical signals output from the optical dividers  291 _ 1  to  291   —   n  and outputs them to the optical fiber ( 1 )  62 , and an optical multiplexer  212  which multiplexes the n optical signals output from the optical dividers  291 _ 1  to  291   —   n  and outputs them to the optical fiber ( 2 )  63 .  
         [0070]     The center node  21   a  having one of the constitutions shown in FIGS.  4  to  6  for example divides an electrical signal, which is to be transmitted to the remote node  22   a,  into two, and modulates two light sources (the optical senders  251 _ 1  and  252 _ 1 ) which have an oscillating frequency of wavelength λ 1  by using the two optical signals. The center node  21   a  also modulates one light source (the optical sender  281 _ 1 ), and the optical dividers  291 _ 1  divides the signal therefrom. One of the divided signals is input to the optical fiber ( 1 )  62 , and the other is input to the optical fiber ( 2 )  63 . Similarly, the center node  21   a  modulates a light source having an oscillating frequency of wavelength λ 2  and generates one group of optical signals by using an electrical signal which is to be transmitted to the remote node  23   a.  One of the divided signals is multiplexed with the optical signal having a wavelength λ 1  and is input to the optical fiber ( 1 )  62 , and the other is multiplexed with the optical signal having a wavelength λ 1  and is input to the optical fiber ( 2 )  63 . The n optical wavelength division multiplexing signals are input to the two optical fibers in the same manner. In this network, two wavelengths are allocated to the optical path which joins the center node  21   a  and the ONU  51  to  53 . One wavelength is allocated when transmitting from the center node  21   a  to an ONU, and one wavelength is allocated when transmitting from the ONU to the center node  21   a.  Therefore, when the total number of ONU in the regional network of this example is one hundred, two hundred wavelengths are used.  
         [0071]     One of the optical wavelength division multiplex signals which are output from the center node  21   a  is transmitted, for example, counterclockwise, and the other signal is transmitted clockwise. That is, the optical wavelength division multiplex signals are transmitted from the center node  21   a  to the remote nodes  22   a,    23   a  and  24   a  counterclockwise at the optical fibers ( 1 )  62  and  64 , and clockwise at the optical fibers ( 2 )  61  and  63 . Consequently, as shown in  FIG. 2 , optical wavelength division multiplex signals from two optical fibers are input to the remote nodes.  
         [0072]     Optical amplifiers  101 ,  103 ,  102 , and  104  which amplify the optical signals which are input/output by using the optical fibers ( 1 ) and ( 2 ), and optical couplers (or optical circulators)  105  and  106  which couple light input from the optical fibers  65  or  68 , comprising the network ( 2 )  12   a,  to the optical signal which is transmitted along the optical fiber ( 1 ) or the optical fiber ( 2 ), and divides the light which is output to the optical fiber  66  or  67 , are provided at the remote node  24   a  belonging to the network ( 1 )  11   a.  An optical switch  114  for dealing with severed fibers, an AWG (arrayed waveguide grating)  115  comprising an optical multiplexer/de-multiplexer, optical amplifiers  111 ,  111 ,  111 , and  111 , and an optical divider  113  are provided at the remote node  25   a  which becomes the access node.  FIG. 13  and  FIG. 14  (Table 1) show one example of the relationship between the wavelength and input/output ports when using an AWG. For example, with regard to the WDM (wavelength division multiplex) signals which are input from the input port  1 , a signal having a wavelength of λ 1  is output from the output port  1 . Conversely, when a signal having a wavelength of λ 10  is input from the output port  1 , the signal having a wavelength of λ 1  is output from the input port  7 . Therefore, the wavelength division multiplex signals can be de-multiplexed and multiplexed simultaneously by using the AWG. Here, λ 1  to λ 6 , λ 10  to λ 15  represent different optical wavelengths arranged in sequence according to wavelength.  
         [0073]     The center node  21   a  transmits signals constantly to the optical fiber ( 1 )  62  and the optical fiber ( 2 )  63  toward the ONU in the network. As a consequence, the same signal is transmitted along two paths and input to, for example, the optical switch  114  of the access node  25   a.  The optical switch  114  shown in  FIG. 2  is set so as to select the optical signal which has been transmitted on the working fiber  67 . The optical switch  114  selects only optical signals from the working fiber  67 , and outputs them to the optical multiplexer/de-multiplexer  115 . All of the for example one hundred optical signals which have been transmitted from the center node  21  a toward all the ONU  51 ,  52 , and  53 , are input into the optical multiplexer/de-multiplexer  115 . The optical multiplexer/de-multiplexer  115  selects only the corresponding wavelength and transmits this signal to the corresponding ONU  51 ,  52 , and  53 .  
         [0074]     A wavelength which has not been used in transmission from the center node  21   a  is used for the optical signals to be transmitted from the ONU  51 ,  52 , and  53  toward the center node  21   a.  The signals from the ONU  51 ,  52 , and  53  are multiplexed by the optical multiplexer/de-multiplexer  115 , and joined to the two optical transmission paths (represented by the solid and dotted lines) by using the optical divider  113  such as an optical coupler. After being amplified by the respective optical amplifiers  111 , the signals are transmitted to the remote node  24   a.  Since the remote node  24   a  does not perform electrical processing, the signals from the ONU  51 ,  52 , and  53  are received at the center node  21   a  from two paths comprising the optical fiber ( 1 )  64  and the optical fiber ( 2 )  61 . The center node  21   a  receives the signals transmitted from the ONU  51 ,  52 , and  53 , and its own transmitted signal, and extracts only the signals from the ONU by using the optical de-multiplexers ( 201  and  202  in FIGS.  4  to  6 ). The signals from ONU which have been de-multiplexed by the optical de-multiplexers ( 201  and  202 ) are converted to electrical signals by the optical receivers (OR). From the converted electrical signals, a selector selects the electrical signals which corresponded to the working fiber. The selected signals are electrically processed, and distributed as signals to be transmitted within the network and signals to be transmitted to a network at a higher level. That is, no electrical processing is carried out at the access node and the remote nodes.  
         [0075]     Subsequently, the operation when a fiber has become severed at position AA′ in the network ( 1 )  11   a  of  FIG. 2  will be explained. The center node  21   a  is transmitting signals constantly toward the ONU in the network on both the optical fibers ( 1 )  62  and ( 2 )  63 . Therefore, the same signal which has been transmitted along the two paths is input into the optical switch  114  of the access node  25   a  belonging to the network ( 2 )  12   a.  The optical switch  114  shown in  FIG. 2  is set so as to select the optical signal transmitted on the working fiber  67 . However, when the fiber has been severed at AA′, the optical switch  114  detects the severance of an input signal and automatically switches so as to select the signal which has been transmitted on the protection fiber  66 , represented by the dotted line. On the other hand, the signals transmitted from the ONU  51 ,  52 , and  53  to the center node  21   a  are divided by the optical divider  113  and always output to the working and protection paths comprising the fibers  68  and  65 . Since the remote node  24   a  does not perform electrical processing, the signals from the ONU  51 ,  52 , and  53  are normally received at the center node  21   a  from two paths comprising the optical fibers  61  and  64 . When a fiber is severed, the selectors switch so that the signal which has been received from the working fiber will be received from the protection fiber  64 .  
         [0076]     Similarly, in the case where there is an access node belonging to the remote node (e.g. a remote node such as node  30   a,  represented by the chained line) provided downstream than to the signal being transmitted along the optical fiber ( 1 ), the optical switch switches from working to protection. The signal is then transmitted by using the protection path shown by the dotted line. In the case where there is an access node belonging to the remote node  22   a,  provided upstream with regard to the signal being transmitted along the optical fiber ( 1 ), the optical switch does not switch and the signal is transmitted along the working path. At the center node  21   a,  the selectors select each direction which a signal is input in at each wavelength, and the signals are transmitted.  
         [0077]     Subsequently, an example will be explained in the case where the fiber has been severed at point BB′ of the network ( 2 )  12   a  shown in  FIG. 2 .  
         [0078]     At the access node  25   a  which is connected to the remote node  24   a,  the optical switch  114  is switched to the direction shown by the dotted line. The switches at the other access nodes continue to input the signals in the working state, and consequently do not switch. At the center node  21   a,  the selectors select each direction which a signal is input in at each wavelength, and the signals are transmitted.  
         [0079]     As described above, the optical signal is only electrically processed at the center node  21   a  and the ONU  51 ,  52 , and  53 , even when a fiber has been severed.  
       Embodiment 2  
       [0080]      FIG. 3  shows an embodiment comprising a double ring. This embodiment differs from that shown in  FIG. 2  in that (i) a plurality of access nodes are connected to the remote node, (ii) the access nodes are connected in a ring, and particularly (iii) this embodiment comprises an optical band pass filter which prevents optical signals at the wavelengths allocated to the ONU  54 ,  55 , and  56 , which belong to the ring network ( 4 )  14   a  comprising the remote node  23   a,  from passing around the ring network. An optical band pass filter  301  which passes only wavelengths allocated for transmission from the remote node  23   a  to the center node  21   a,  and optical band pass filter  302  which passes only wavelengths allocated for transmission from the center node  21   a  to the remote node  23   a,  are connected to the input and output terminals of the optical coupler  105  in the remote node  23   a.  In addition, the optical band pass filters  301  and  302  are connected to the input and output terminals of an optical coupler  106 . The constitution of these, and the operation of the optical switches at the access nodes in the case where the fiber becomes severed at the point AA′, are the same as in the first embodiment. The constitution of the center node  21   a  is the same as that shown in FIGS.  4  to  6 . Incidentally, the remote node  23   a  comprises the same elements as the internal constitution of the remote node  24   a  shown in  FIG. 2 . As shown by the access node ( 2 )  27   a,  the optical amplifiers  111 ,  111 ,  111 , and  111  in the access nodes ( 1 )  26   a  to ( 3 )  28   a  are arranged so that the optical fiber transmission paths form a ring, in the same manner as the remote node  23   a.    
         [0081]     Subsequently, the operation in the case where the fiber has become severed at point BB′ will be explained. The optical switch at the access node ( 3 )  28   a  does not switch, since communication is possible by using the working fiber shown by the solid line. Furthermore, the selector which corresponds to the wavelength allocated to the access node ( 3 )  28   a  does not switch at the center node  21   a.  On the other hand, at the access nodes ( 2 )  27   a  and ( 1 )  26   a  which are provided downstream than the access node ( 3 )  28   a,  the optical signal from the working fiber is severed. Consequently, the optical switch  114  switches to the protection fiber shown by the dotted line. The signals from the ONU  54 ,  55 , and  56  are transmitted along the protection fiber to the center node  21   a  via the remote node  23   a.  At the center node  21   a,  the selector selects a signal which corresponds to the signal from the protection fiber. This has no effect on the access nodes corresponding to the remote node  22   a  and the remote node  24   a.    
         [0082]     In this network, the optical signal is electrically processed only at the center node and the ONU, even when a fiber has been severed.  
       Embodiment 3  
       [0083]      FIG. 7  shows an embodiment comprising a two-fiber unidirectional ring. This embodiment is characterized in that the transmission direction of the optical signal from the center node  21   a  to the remote node  24   b  (corresponding to the remote node  24   a  of  FIG. 2 ) is the same as the transmission direction of the optical signal from the remote node  24   b  to the center node  21   a.    FIG. 7  shows the constitution of the remote node  24   b  and the access node  25   a  at this time. This constitution differs from that shown in  FIG. 2  in that the up and down signals from the access node  25   a  are input to identical optical couplers  105   b  and  106   b,  provided at the remote node  24   b.  The optical fibers  67   b  and  68   b  (corresponding to the optical fibers  67  and  68  of  FIG. 2 ) are connected to the optical coupler  105   b,  and the optical fibers  65   b  and  66   b  (corresponding to the optical fibers  65  and  66  of  FIG. 2 ) are connected to the optical coupler  106   b.  Here, the network ( 1 )  11   b  comprising the center node  21   a  is arranged as a two-fiber unidirectional ring which corresponds to the network ( 1 )  11   a  of  FIG. 2 .  
         [0084]     The operation when the optical fiber has been severed at point AA′ will be explained. The optical signal from the optical fiber ( 1 )  62  can be received at the access nodes which are connected at a lower level than the remote nodes  22   b  and  23   b  (corresponding to the remote nodes  22   a  and  23   a  of  FIG. 2 ). Therefore, the optical switches which are provided at the access nodes do not switch to the protection fiber. In transmitting from the access node to the center node  21   a,  an optical divider, comprising an optical coupler or the like, divides the signal into two. The optical fiber ( 2 )  61  is the protection fiber, and connects one of the divided signals to the center node  21   a.  The selector at the center node  21   a  selects the signal received from the optical fiber ( 2 )  61 . On the other hand, at the access node  25   a  which is connected at a lower level than the remote node  24   b,  the optical signal becomes severed. Consequently, the optical switch  114  switches to the protection system. In transmitting from the access node  25   a  to the center node  21   a,  only the signal in the divided output of the optical multiplexer/de-multiplexer  115  which is connected to the optical fiber ( 1 )  64  is transmitted counterclockwise along the optical fiber ( 1 )  64  to the center node  21   a.  Since the center node  21   a  has already selected the signal which was received from the optical fiber ( 1 )  64 , the selectors do not change its signal selection. Therefore, when the cable is severed at the point AA′, the transmission path of the two-fiber unidirectional ring network becomes the same as that in the bi-directional ring.  
         [0085]     Subsequently, the operation when the cable is severed at the point BB′ will be explained. At the access node  25   a  connected to the remote node  24   b,  the optical switch  114  switches to the protection system when the cable is severed. The signal from the access node  25   a  to the remote node  24   b  is transmitted along the fibers  65   b  and  66   b,  represented by dotted lines, and connects to the protection optical fibers ( 2 )  61  and  63  in the remote node  24   b.  The signal is transmitted clockwise along the optical fibers ( 2 )  61  and  63 . The selector at the center node  21   a  selects the signal which is received from the optical fiber ( 2 )  61 . The signals corresponding to the remote nodes  22   b  and  23   b  are not switched by the access nodes connected thereto, nor are they subject to the change in signal selection by the selectors at the center node  21   a.    
         [0086]     In the network described above, the optical signal is only electrically processed at the center node and the ONU even in the case where a fiber has been severed.  
       Embodiment 4  
       [0087]     In the constitution shown in  FIG. 8 , the network ( 1 )  11   b  (corresponding to the network ( 1 )  11   a  of  FIG. 3 ) comprising the center node  21   a  is a two-fiber unidirectional ring, and the lower level network ( 4 )  14   b  (corresponding to the network ( 4 )  14   a  of  FIG. 3 ) also comprises a ring. This constitution differs from that shown in  FIG. 3  in that the up and down signals from the access node are input to identical optical couplers  105   b  and  106   b,  provided at the remote node  23   b  (corresponding to the remote node  23   a  of  FIG. 3 ), as in the remote node  24   b  of  FIG. 7 . Another important difference to  FIG. 7  is that the provision of band-pass filters which prevent optical signals at the wavelengths allocated to the ONT  54 ,  55 , and  56  in the ring network ( 4 )  14   b  comprising the remote node  23   b,  from passing around the ring network. An optical band-pass filter  301  which passes only wavelengths allocated for transmission from the remote node  23   b  to the center node  21   a,  and optical band-pass filter  302  which passes only wavelengths allocated for transmission from the center node  21   a  to the remote node  23   b,  are connected to the input and output terminals of the optical coupler  105   b  and the optical coupler  106   b.  As in the previous embodiments, the optical switches of the access nodes and the selectors of the center nodes are normally set so as to select the signals from the working fiber, shown by the solid line.  
         [0088]     The operation when the optical fiber has been severed at the point AA′ will be explained. The optical signal from the optical fiber ( 1 )  62  can be received at the access node which is connected at a lower level than the remote node  22   b.  Therefore, the optical switches which are provided at the access node lower level than the remote node  22   b  do not switch to the protection fiber. In transmitting from the access node to the center node  21   a,  an optical divider, comprising an optical coupler or the like, divides the signal into two. One of the divided signals is transmitted clockwise to the center node  21   a  along the optical fiber ( 2 )  61 , which comprises the protection fiber. The selector at the center node  21   a  selects the signal received from the optical fiber ( 2 )  61 . On the other hand, at the access node which is connected at a lower level than the remote nodes  23   b  and  24   b,  the optical signal becomes severed. Consequently, the optical switch switches to the protection system. In transmitting from the access node to the center node, only the signal in the divided output of the optical multiplexer/de-multiplexer which is connected to the optical fiber ( 1 )  64  is transmitted counterclockwise along the optical fiber ( 1 )  64  to the center node  21   a.  Since the center node  21   a  has already selected the signal which is received from the optical fiber ( 1 )  64 , the selectors do not change its signal selection. Therefore, when the cable is severed at the point AA′, the transmission path of the two-fiber unidirectional ring network becomes the same as that in the bi-directional ring.  
         [0089]     Subsequently, the operation when the cable is severed at the point BB′ will be explained. Since the optical signal is not cut-off at the access node ( 3 )  28   a  connected to the remote node  23   b,  the optical switch  114  does not switch. Since the signal is transmitted to the center node  21   a  along the working fiber, the selector in the receiving section of the center node  21   a  does not change its signal selection. On the other hand, the optical signal is cut-off at the access nodes ( 2 )  27   a  and ( 1 )  26   a.  Therefore, when the cable is severed, the optical switch  114  switches to the protection system, and the optical signal is received from the protection fiber. The signals from the access nodes  26   a  and  27   a  to the remote node  23   b  are transmitted along the fibers represented by dotted lines, and connect to the protection optical fibers ( 2 )  61  and  63  through the optical coupler  106   b  in the remote node  23   b.  The signals are transmitted clockwise along the optical fibers ( 2 )  61  and  63 . The selector at the center node  21   a  selects the signal which was received from the optical fiber ( 2 )  61 . The signals corresponding to the remote nodes  22   b  and  24   b  are not switched at the access nodes connected thereto, nor are they subject to the change in signal selection by the selectors at the center node  21   a.    
         [0090]     In the network described above, the optical signal is only electrically processed at the center node and the ONU even in the case where a fiber has been severed.  
       Embodiment 5  
       [0091]      FIG. 9  shows an embodiment in which an optical signal is converted to an electrical signal by using transponders (in  FIG. 9 , optical amplifiers/senders)  121 ,  121 ,  121 , and  121 , in a remote node  24   c  corresponding to the remote node  24   a  of  FIG. 2 . At the remote node  24   c,  optical de-multiplexers in the transponders  121 ,  121 ,  121 , and  121  de-multiplex only the wavelengths which correspond to the ONU  51 ,  52 , and  53  belonging to lower levels. Signals at each wavelength are received, equalized, identified, reproduced, and retransmitted using appropriate wavelengths. Signals from the access node  25   a  are similarly processed, converted to predetermined wavelengths, and transmitted to the center node  21   a.  Signals can be multiplexed and de-multiplexed by using an optical multiplexer/de-multiplexer such as an AWG.  
         [0092]     In the example shown in  FIG. 9 , remote nodes  26   c  and  27   c  have the same constitution as the remote node  24   c,  and are provided in the ring network comprising the center node  21   a  and the remote node  24   c.  A center node  71  and a plurality of remote nodes  72 ,  72 , . . . are provided in the higher level ring network comprising the center node  21   a.    
       Embodiment 6  
       [0093]      FIG. 10  shows another embodiment in which an optical signal is converted to an electrical signal by using transponders  121 ,  121 ,  121 , and  121 , in a remote node  23   c  corresponding to the remote node  23   a  of  FIG. 3 . The constitution is the same as that shown in  FIG. 9 , with the exception that the access nodes ( 1 ) to ( 3 ) and the remote node  23   c  are connected in a ring.  
         [0094]     In the example shown in  FIG. 10 , remote nodes  22   c  and  24   c  have the same constitution as the remote node  23   c,  and are provided in the ring network comprising the center node  21   a  and the remote node  23   c.  A center node  71  and a plurality of remote nodes  72 ,  72 , . . . are provided in the higher level ring network comprising the center node  21   a.    
       Embodiment 7  
       [0095]      FIG. 11  shows an embodiment in which communication between the access node  25   c  (corresponding to the access node  25   a  of  FIG. 2 ) and the ONU  51 ,  52 , and  53 , is doubled by using radio communication (radio receiver/sender  130 ,  131 ,  132 , and  133 ). When communication is doubled by using radio, all the paths which join the ONU  51 ,  52 , and  53  to the center node  21   a  can be doubled inexpensively.  FIG. 11  shows only a two-fiber unidirectional ring, but this constitution can be applied to all networks in the embodiments of this invention. The operations when the optical cable becomes severed at points AA′ and BB′ (not shown in  FIG. 11 ) are the same as those described in the first embodiment.  
       Embodiment 8  
       [0096]      FIG. 12  shows an embodiment in which the optical multiplexer/de-multiplexer  115   a  is provided at a remote terminal  29  near the user, instead of at the access node  25   d  (corresponding to the access node  25   a  of  FIG. 2 ). In this embodiment, the constitution of the network above the access node  25   d  can be applied in all of the embodiments of this invention. By providing the optical multiplexer/divider nearer to the ONU, the cost of establishing the path can be reduced. The operations when the optical cable becomes severed at points AA′ and BB′ (not shown in  FIG. 12 ) are the same as those described in the first embodiment.  
       Embodiment 9  
       [0097]     FIGS.  15  to  17  show an embodiment wherein, at the remote nodes (offices) which belong to the lower level ring network comprising the access node and the higher level ring network, both ends of two loop-like optical fibers (one working fiber and one protection fiber) which join the access nodes belonging to the lower level ring network are open (specifically, between the optical terminators  1509  and  1510  of the working fiber, and between the optical terminators  1609  and  1610  of the protection fiber). Instead of providing an optical multiplexer/de-multiplexer having wavelength selectability at the access nodes or the above remote nodes (offices), the ONU themselves have an optical de-multiplexing function. Further, the wavelength division multiplexing signals which are transmitted along the two optical fibers used in the ring networks are all bi-directional, and bi-directional optical amplifiers are used in the remote nodes and the access nodes. FIGS.  15  to  17  show a case where, in the higher level ring network comprising the center node  21   b,  the optical signal from the center node  21   b  to the remote node  1504  is transmitted in the opposite direction to the optical signal from the remote node  1504  to the center node  21   b,  i.e. a bi-directional ring. In  FIG. 15 , fibers  1501 ,  1503 ,  1511 ,  1513 ,  1514 ,  1516 ,  1522 , and  1524  are working fibers, and in  FIG. 16 , fibers  1601 ,  1603 ,  1611 ,  1613 ,  1614 ,  1616 ,  1622 , and  1624  are protection fibers. Firstly, signal transmission on the working fibers  1501 ,  1503 ,  1511 ,  1513 ,  1514 ,  1516 ,  1522 , and  1524  will be explained. A signal is transmitted counterclockwise from the center node  21   b  to the remote node  1504 . A fiber coupler  1505  is provided at the remote node  1504 , and divides the signal, which is then transmitted to the lower level ring network comprising the access node  1517 . In the access node  1517  shown in  FIG. 17 , the received optical wavelength division multiplexing signal is divided by the fiber coupler  1518  and received. One end of the fiber coupler  1518  functions as an optical terminator  1520 . The optical wavelength division multiplexing signal which was divided at the access node  1517  is led to an optical switch  1702  by a circulator  1701 , and then led to a star coupler  1709  by another optical circulator  1705 . The star coupler  1709  distributes the signal to the ONU  1706 ,  1707 , and  1708 . The ONU  1706 ,  1707 , and  1708  de-multiplex and receive only signals at wavelengths allocated to the ONU. It is a major feature of this embodiment that the ONU  1706 ,  1707 , and  1708  have the ability to de-multiplex signals. No optical de-multiplexers having wavelength selectability, such as an AWG, are provided in the access node  1517  and the remote node  1504 . Instead, the ONU  1706 ,  1707 , and  1708  themselves are able to de-multiplex wavelengths.  
         [0098]     In this example, the optical circulators  1701 ,  1703 , and  1705  comprise optical circuits in which an optical signal which has been input from a port ( 1 ) is output from a port ( 2 ), an optical signal which has been supplied from the port ( 2 ) is output from a port ( 3 ), and an optical signal which has been supplied from the port ( 3 ) is output from the port ( 1 ).  
         [0099]     As in the embodiments described above, when transmitting from the ONU  1706 ,  1707 , and  1708  to the center node  21   b,  the ONU  1706 ,  1707 , and  1708  use predetermined wavelengths. In contrast to the above embodiments, the signals transmitted from the ONU  1706 ,  1707 , and  1708  are multiplexed by the star coupler  1709 . Thereafter, an optical circulator  1705 , an optical divider  1704 , and another optical circulator  1701  transmit the signal in the opposite direction to that received on the optical fiber.  
         [0100]     Another important feature of this embodiment is that both ends of the looped optical fibers  1514 ,  1516 ,  1522 , and  1524  which join the access nodes  1515 ,  1517 , and  1523 , to the remote node  1504  are opened by optical terminators  1509  and  1510  in the remote node  1504 . This is to prevent the optical signals from passing around the lower level ring network which the access node  1517  belongs to.  
         [0101]     Subsequently, in the protection system shown in  FIG. 16 , the signals are transmitted in the opposite direction to that in  FIG. 15 . The open ends of the loop in the remote node  1504  (optical terminators  1609  and  1610 ) are also provided at opposite positions. The operation of the optical switch  1702  provided at the access node  1517  is the same as the embodiments described above. The access node in this embodiment comprises an optical circulator, but an optical coupler may alternatively be used.  
         [0102]     In FIGS.  15  to  17 , reference numerals  1505 ,  1518 ,  1605 , and  1618  represent two-by-two optical fiber couplers, reference numerals  1506 ,  1507 ,  1508 ,  1519 ,  1521 , and  1606 ,  1607 ,  1608 ,  1619 , and  1621  represent bi-directional optical amplifiers, reference numerals  1502  and  1512  represent remote nodes, reference numerals  1515  and  1523  represent access nodes, and reference numeral  1620  represents an optical terminator. The solid-line arrows show the direction of the optical signals which are transmitted from the center node toward the ONU, and the broken-line arrows show the direction of the optical signals which are transmitted from the ONU toward the center node. The operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in FIGS.  15  to  17 ) are the same as that already described in the second embodiment.  
         [0103]      FIG. 18  shows one example of the constitution of the center node  21   b  according to this embodiment. In the constitution of the center node  21   b  shown in  FIG. 18 , parts which are identical to those in the constitution of the center node  21   a  shown in  FIG. 4  are represented by the same reference numerals, and will not be explained further. The center node  21   b  shown in  FIG. 18  comprises an optical terminator  1801  which terminates the working fiber  1501  shown in  FIG. 15 , an optical circulator  1802  connected to the protection fiber  1601  of  FIG. 16 , an optical amplifier  1803  having an input terminal connected to the port ( 3 ) of an optical circulator  1802 , an optical amplifier  1804  having an output terminal connected to the port ( 1 ) of the optical circulator  1802 , an optical terminator  1805  which terminates the protection fiber  1613  shown in  FIG. 16 , an optical circulator  1806  which is connected to the working fiber  1513  of  FIG. 15 , an optical amplifier  1807  having an input terminal connected to the port ( 3 ) of the optical circulator  1806 , and an optical amplifier  1808  having an output terminal connected to the port ( 1 ) of the optical circulator  1806 . In this case, the output of the optical amplifier  1803  connects to the input of the optical de-multiplexer  202 , the input of the optical amplifier  1804  connects to the output of the optical multiplexer  212 , the output of the optical amplifier  1807  connects to the input of the optical de-multiplexer  201 , and the input of the optical amplifier  1808  connects to output of the optical multiplexer  211 . Incidentally, the optical amplifiers  1803 ,  1804 ,  1807 , and  1808  need only be provided as necessary.  
       Embodiment 10  
       [0104]      FIGS. 19 and 20  show embodiments of the present invention.  FIGS. 19 and 20  show the case where, in the higher level ring network comprising the center node  21   c,  the direction of the optical signal which transmits data from the center node  21   c  to the remote node  1904  is the same as the direction of the optical signal which transmits data from the remote node  1904  to the center node  21   c,  i.e. the network is a unidirectional ring.  FIG. 19  shows a working fiber, and  FIG. 20  shows a protection fiber.  
         [0105]     As in FIGS.  15  to  17 , solid-line arrows and broken-line arrows are used to represent examples of the directions of signals transmitted from the center node  21   c  via the remote node # 2  ( 1904 ) to the ONU belonging to the access node  2  ( 1517 ).  FIGS. 19 and 20  differ from FIGS.  15  to  17  in that (i) bi-directional optical amplifiers are not needed in the higher level ring network, and (ii) the remote node  1904  comprises optical circulators  1909  and  2009 .  FIG. 21  shows an example of the constitution of the center node used here.  
         [0106]     In  FIGS. 19 and 20 , reference numerals  1905  and  2005  represent two-by-two optical fiber couplers, reference numerals  1907  and  2007  represent bi-directional optical amplifiers, reference numerals  1906 ,  1908 ,  2006 , and  2008  represent (unidirectional) optical amplifiers, reference numerals  1910  and  2010  represent optical terminators, and reference numerals  1909  and  2009  represent optical circulators which connect the port ( 1 ) to the optical fiber couplers  1905  and  2005 , and connect the port ( 2 ) to the port ( 3 ). The operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in FIGS.  19  to  20 ) are the same as that already described in the second embodiment. The center node  21   c  shown in  FIG. 21  comprises an optical amplifier  2101  which inputs signals from the working fiber  1501  and outputs signals to the optical de-multiplexer  202 , an optical amplifier  2102  which outputs signals to the protection fiber  1601  and inputs signals from the optical multiplexer  212 , an optical amplifier  2103  which inputs signals from the optical fiber  1613  and outputs signals to the optical de-multiplexer  201 , and an optical amplifier  2104  which outputs signals to the optical fiber  1513  and inputs signals from the optical multiplexer  211 . The optical amplifiers  2101  to  2104  may be provided where necessary.  
       Embodiment 11  
       [0107]      FIG. 22  shows an embodiment comprising a three-layered optical network in which the highest level network is a ring network comprising one center node and two or more remote nodes, which are joined by four optical fibers. The intermediate level network comprises a ring network having a node belonging to the higher level network as its center node. Access nodes belonging to the ring network are joined by four optical fibers. The lowest level network comprises a star network centered around an access node, which multiplexes traffic from ONU. The ONU and access node are each directly joined by one optical fiber. The center node belonging to the highest level network and the ONU establish a direct communication path by using lights of different wavelengths. The optical signals are not electrically processed, but are amplified, branched, or routed at the remote nodes and the access node provided therebetween. In addition, at the node belonging to the intermediate level ring network, both ends of the four looped optical fibers (two of the optical fibers corresponding to working fibers, and two corresponding to protection fibers) which join together the access nodes belonging to the lower level ring network, are open (between optical terminators  2203   g  and  2203   h,  and between  2203   i  and  2203   j  on the working fiber; between optical terminators  2203   q  and  2203   r,  and between  2203   s  and  2203   t  on the protection fiber). Furthermore, the access nodes and the remote nodes do not comprise optical multiplexer/de-multiplexers having the ability to select wavelengths. Instead, the ONU themselves having a wavelength de-multiplexing function. The block  2201   a  enclosed by the chain line comprises two working fibers, and the block  2201   b  comprises two protection fibers. The access node  2206  connects to four optical fibers. In the case shown in  FIG. 22 , the access node  2206  comprises an optical coupler  2206   i,  but an optical circulator may be used instead, as shown in  FIG. 17 .  FIG. 22  shows a bi-directional ring network comprising two working fibers and two protection fibers, the signals to the ONU being transmitted in the opposite direction from signals transmitted from the ONU. This embodiment is characterized in that (i) both ends of the optical fibers which connect the access node in a ring are open at the remote node, preventing the signals from passing around the loop, and (ii) no optical multiplexer/de-multiplexer having wavelength selectability is used at the remote node and the access node. Instead, the ONU are able to select wavelengths for transmitting and receiving.  
         [0108]     In  FIG. 22 , the solid lines represent optical fibers which are used in communication between the center node  21   d  and the access node  2206 . The same applies in an embodiment subsequently described in  FIG. 24 . Reference numeral  21   d  represents the center node, reference numerals  2202 ,  2203 ,  2204  represent remote nodes, reference numerals  2212   a,    2213   a,    2219   a,  and  2220   a  represent ring-shaped optical fibers for working, reference numerals  2214   a,    2215   a,    2217   a,  and  2218   a  represent ring-shaped optical fibers for working, reference numerals  2212   b,    2213   b,    2219   b,  and  2220   b  represent ring-shaped optical fibers for protection, reference numerals  2214   b,    2215   b,    2217   b,  and  2218   b  represent ring-shaped optical fibers for protection, reference numerals  2203   a  and  2203   b  represent two-by-two fiber couplers, reference numerals  2203   c,    2203   d,    2203   e,  and  2203   f  represent optical amplifiers, reference numerals  2203   g,    2203   h,    2203   i,  and  2203   j  represent optical terminators where the fiber loop is open, reference numerals  2203   k  and  2203   l  represent two-by-two fiber couplers, reference numerals  2203   m,    2203   n,    2203   o,  and  2203   p  represent optical amplifiers, and reference numerals  2203   q,    2203   r,    2203   s,  and  2203   t  represent optical terminators where the fiber loop is open. Reference numerals  2205 ,  2206 , and  2207  represent access nodes, reference numeral  2208  represents a star coupler, reference numerals  2209 ,  2210 , and  2211  represent ONU, reference numerals  2212  and  2213  represent protection optical fibers, reference numerals  2206   a,    2206   b,    2206   j,  and  2206   k  represent two-by-two fiber couplers, reference numerals  2206   c,    2206   d,    2206   e,    2203   f,    2206   l,    2206   m,    2206   n,  and  2206   o  represent optical amplifiers, reference numeral  2206   g  represents an optical switch, and reference numerals  2206   h  represents an optical multiplexer/de-multiplexer. The solid-line arrows show the direction of the optical signals which are transmitted from the center node toward the ONU, and the broken-line arrows show the direction of the optical signals which are transmitted from the ONU toward the center node. The operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in  FIG. 22 ) are the same as that already described in the second embodiment.  
         [0109]      FIG. 23  shows one example of the constitution of the center node in this embodiment. The center node  21   d  comprises an optical amplifier  2301  which inputs signals from the ring-shaped optical fiber  2215   b  and outputs signals to the optical de-multiplexer  202 , an optical amplifier  2302  which outputs signals to the ring-shaped optical fiber  2214   b  and inputs signals from the optical multiplexer  212 , an optical amplifier  2303  which inputs signals from the optical fiber  2212   a  and outputs signals to the optical de-multiplexer  201 , and an optical amplifier  2304  which outputs signals to the optical fiber  2213   a  and inputs signals from the optical multiplexer  211 . The optical amplifiers  2301  to  2304  may be provided where necessary.  
       Embodiment 12  
       [0110]      FIG. 24  shows an embodiment of the present invention which provides an optical wavelength division multiplexing network comprising at least three layers. The highest level network comprises a ring network having at least one center node and two or more remote nodes which are joined by two optical fibers. The intermediate level network comprises a ring network having a node belonging to the higher level network as its center node. Access nodes belonging to the ring network are joined by four optical fibers. The lowest level network comprises a star network centered around an access node, which multiplexes traffic from ONU. The ONU and the access node are directly joined by one optical fiber. The center node belonging to the highest level network and the ONU establish a direct communication path by using lights of different wavelengths. The optical signals are not electrically processed at the remote nodes and the access nodes provided between the center node and the ONU. Instead, only the optical signals are amplified, divided, or routed. At a node (an office) belonging to the intermediate level ring network, one end of the four looped optical fibers (two for working, and two for protection) which join the access nodes belonging to the lower level ring network, are open (optical terminators  2403   d  and  2403   e  on the working fiber; optical terminators  2403   i  and  2403   j  on the protection fiber). Furthermore, the access nodes and the remote nodes (offices) do not comprise optical multiplexer/de-multiplexers having the ability to select wavelengths. Instead, the ONU themselves having a wavelength de-multiplexing function. The block  2401   a  enclosed by the chain line comprises the working fiber, and the block  2401   b  comprises the protection fiber. Four optical fibers connect the access nodes shown in the blocks  2401   a  and  2401   b.  In  FIG. 24 , the access node  2406  comprises an optical coupler  2406   i,  but an optical circulator may be used instead, as shown in  FIG. 17 .  FIG. 24  shows a unidirectional ring network comprising two working/protection fibers, the signals to the ONU being transmitted in the same direction as signals transmitted from the ONU.  
         [0111]     In  FIG. 24 , reference numeral  21   e  represents the center node, reference numerals  2402 ,  2403 ,  2404  represent remote nodes, reference numerals  2412   a,    2418   a,  and  2419   a  represent ring-shaped optical fibers for working, reference numerals  2414   a,    2417   a,  and  2420   a  represent ring-shaped optical fibers for working, reference numerals  2412   b,    2418   b,  and  2419   b  represent ring-shaped optical fibers for protection, reference numerals  2414   b,    2417   b,  and  2420   b  represent ring-shaped optical fibers for protection, reference numerals  2403   a  and  2403   f  represent two-by-two fiber couplers, reference numerals  2403   b,    2403   c,    2403   g,  and  2403   h  represent optical amplifiers, reference numerals  2403   d,    2403   e,    2403   i,  and  2403   j  represent optical terminators where the fiber loop is open, reference numerals  2405 ,  2406 , and  2407  represent access nodes, reference numeral  2408  represents a star coupler, reference numerals  2409 ,  2410 , and  2411  represent ONU, reference numerals  2412  and  2413  represent optical fibers for working, reference numerals  2406   a,    2406   b,    2406   j,  and  2406   k  represent two-by-two fiber couplers, reference numerals  2406   c,    2406   d,    2406   e,    2406   f,    2406   l,    2406   m,    2406   n,  and  2406   o  represent optical amplifiers, reference numeral  2406   g  represents an optical switch, and reference numerals  2406   h  represents an optical multiplexer/de-multiplexer. The solid-line arrows show the direction of the optical signals which are transmitted from the center node toward the ONU, and the broken-line arrows show the direction of the optical signals which are transmitted from the ONU toward the center node. Incidentally, the constitution of the center node  21   e  may be the same as, for example, that in  FIG. 23 . The operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in  FIG. 24 ) are the same as that already described in the second embodiment.  
       Embodiment 13  
       [0112]      FIG. 26  shows an embodiment of a network. Two fibers which are usually used as working fibers are shown on the left side of  FIG. 26 , and the remaining two fibers which are used as protection fibers are shown on the right side. That is, the block  2601   a  enclosed by the chain line comprises the two working fibers, and the block  2601   b  comprises the two protection fibers. Four optical fibers connect the access node  2606 .  
         [0113]     In  FIG. 26 , the solid lines on the left side (working) of the diagram, and the thick dotted lines on the right (protection) side of the diagram represent optical fibers which are used in communications between the center node  21   f  and the access node  2606 . The same applies in embodiments shown in  FIGS. 27 and 28 , which will be explained later. Reference numeral  21   f  represent the center node, reference numerals  2602 ,  2603 , an  2604  represent the remote nodes which, with the center node  21   f,  comprise the higher level ring network, reference numerals  2612   a,    2613   a,    2614   a,    2615   a,    2617   a,    2618   a,    2619   a,  and  2620   a  represent ring optical fibers for working, reference numerals  2612   b,    2613   b,    2614   b,    2615   b,    2617   b,    2618   b,    2619   b,  and  2620   b  represent ring optical fibers for protection, reference numerals  2603   a  and  2603   b  represent two-by-two couplers, reference numerals  2603   c,    2603   d,    2603   e,    2603   f,    2603   u,  and  2603   v  represent optical amplifiers, reference numerals  2603   g,    2603   h,    2603   i,  and  2603   j  represent optical terminators where the fiber loop is open, reference numerals  2603   k  and  2603   l  represent two-by-two fiber couplers, reference numerals  2606   m,    2606   n,    2606   o,    2606   p,    2606   w,  and  2606   x  represent optical amplifiers, reference numerals  2603   q,    2603   r,    2603   s,  and  2603   t  represent optical terminators where the fiber loop is open. Reference numerals  2605 ,  2606 , and  2607  represent access nodes which, with the remote node  2603 , comprise the lower level ring network, reference numerals  2609 ,  2610 , and  2611  represent ONU, reference numerals  2612  and  2613  represent optical fibers for protection, reference numerals  2606   a,    2606   b,    2606   j,  and  2606   k  represent two-by-two fiber couplers, reference numerals  2606   c,    2606   d,    2606   e,    2606   f,    2606   l,    2606   m,    2606   n,    2606   o,    2606   p,  and  2606   q  represent optical amplifiers, reference numeral  2606   g  represents an optical divider, reference numerals  2606   h  represents an AWG, and reference numerals  2606   i  represents an optical switch. Incidentally, the constitution of the center node  21   f  may be the same as, for example, that in  FIG. 23 .  
         [0114]      FIG. 26  shows communication from the center node  21   f,  via the remote node  2603 , to the ONU  2609 ,  2610 , and  2611  belonging to the access node  2606 . The working network shown on the left side of  FIG. 26  will be explained. The center node  21   f  allocates wavelengths to the ONU  2609 ,  2610 , and  2611  belonging to the access node  2606 , and transmits signals to the remote nodes  2602 ,  2603 , and  2604  in the higher level network by using the fiber  2613   a.  At the remote nodes  2602 ,  2603 , and  2604 , the optical couplers branch the optical wavelength division multiplexing signals which have been transmitted. Taking the remote node  2603  by way of example, the remote node  2603  transmits the branched signals to the access nodes  2605 ,  2606 , and  2607  belonging to the lower level ring network. At the access nodes  2605 ,  2606 , and  2607  belonging to the lower level ring network, the optical wavelength division multiplexing signals transmitted from the center node  21   f  are divided by using an optical coupler. Taking the access node  2606  by way of example, the optical wavelength division multiplexing signals from the center node  21   f  which have been divided by the optical coupler  2606   a  are divided by the AWG  2606   h,  separated into the allocated wavelengths, and received at the ONU  2609 ,  2610 , and  2611 . The ONU  2609 ,  2610 , and  2611  transmit to the AWG  2606   h  by using the same wavelength as that received. The AWG  2606   h  is connected to each ONU  2609 ,  2610 , and  2611  by two optical fibers, one for receiving down signals and one for up signals to the center node. The wavelengths allocated to the ONU  2609 ,  2610 , and  2611  are such that they are not output from adjacent output ports of the AWG  2606   h.  The optical fibers which transmit the up signal are connected to ports adjacent to the down signal output port of the AWG  2606   h.  As a consequence, the signals from the ONU  2609 ,  2610 , and  2611  are multiplexed and transmitted from the access node  2606  to the center node  21   f.  The signals transmitted from the access node  2606  to the center node  21   f  are coupled by the optical coupler  2606   b,  and transmitted to the remote node  2603 . In the same way, up signals from the ONU belonging to the lower level network are multiplexed at the remote node  2603 , and transmitted by using the optical fiber  2612   a  to the center node  21   f.  In this embodiment, the optical wavelength division multiplexing signals are transmitted on the fibers  2613   a  and  2612   a  in the higher level network in opposite directions.  
         [0115]     At the access node  2606 , the optical switch  2606   i  is provided in the input section for the down signal to the AWG  2606   h,  in order to switch to the protection system in the case where the fiber becomes severed. Furthermore, the optical divider  2606   g  is provided in the output section for the up signal from the AWG  2606   h,  in order to transmit the up signal on both the working and protection fibers. As already explained, the working and protection signals are selected in the center node  21   f.    
         [0116]     This embodiment is characterized in that (i) there is no switching at the remote nodes when the fiber becomes severed, and (ii) the optical signal is not electrically processed at the nodes (offices) provided between the ONU and the center node. Incidentally, the operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in  FIG. 26 ) are the same as that already described in the second embodiment.  
       Embodiment 14  
       [0117]      FIG. 27  shows an embodiment of the present invention. This embodiment differs from that shown in  FIG. 26  in that, in the higher level ring network, the up and down signals are transmitted in the same direction.  
         [0118]     In  FIG. 27 , the block  2701   a  enclosed by the chain line comprises the two working fibers, and the block  2701   b  comprises the two protection fibers. Four optical fibers connect the access node  2706 . The reference numeral  21   g  represents the center node, reference numerals  2702 ,  2703 , and  2704  represent remote nodes comprising, with the center node  21   g,  the higher level ring network, reference numerals  2712   a,    2713   a,    2714   a,    2715   a,    2717   a,    2718   a,    2719   a,  and  2720   a  represent ring-shaped optical fibers for working, reference numerals  2712   b,    2713   b,    2714   b,    2715   b,    2717   b,    2718   b,    2719   b  and  2720   b  represent ring-shaped optical fibers for protection, reference numerals  2703   a  and  2703   b  represent two-by-two fiber couplers, reference numerals  2703   c,    2703   d,    2703   e,    2703   f,    2703   u,  and  2703   v  represent optical amplifiers, reference numerals  2703   g,    2703   h,    2703   i,  and  2703   j  represent optical terminators where the fiber loop is open, reference numerals  2703   k  and  2703   l  represent two-by-two fiber couplers, reference numerals  2703   m,    2703   n,    2703   o,    2703   p,    2703   w,  and  2703   x  represent optical amplifiers, and reference numerals  2703   q,    2703   r,    2703   s,  and  2703   t  represent optical terminators where the fiber loop is open. Reference numerals  2705 ,  2706 , and  2707  represent access nodes which, with the remote node  2703 , comprise the lower level ring network, reference numerals  2709 ,  2710 , and  2711  represent ONU, reference numerals  2712  and  2713  represent optical fibers for protection, reference numerals  2706   a,    2706   b,    2706   j,  and  2706   k  represent two-by-two fiber couplers, reference numerals  2706   c,    2706   d,    2706   e,    2706   f,    2706   l,    2706   m,    2706   n,    2706   o,    2706   p  and  2706   q  represent optical amplifiers, reference numeral  2706   g  represents an optical divider, reference numeral  2706   h  represents an AWG, and reference numeral  2706   i  represents an optical switch. Incidentally, the constitution of the center node  21   g  may be the same as, for example, the center node shown in  FIG. 21 . The operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in  FIG. 27 ) are the same as that already described in the second embodiment.  
       Embodiment 15  
       [0119]      FIG. 28  shows an embodiment of the present invention. With the exception of the constitution of the access node, this embodiment is identical to that of  FIG. 26 . In  FIG. 28 , the block  2801   a  enclosed by the chain line comprises the two working fibers, and the block  2801   b  comprises the two protection fibers. Four optical fibers connect the access node  2806 . The reference numeral  21   h  represents the center node, reference numerals  2802 ,  2803 , and  2804  represent remote nodes comprising, with the center node  21   h,  the higher level ring network, reference numerals  2812   a,    2813   a,    2814   a,    2815   a,    2817   a,    2818   a,    2819   a,  and  2820   a  represent ring-shaped optical fibers for working, reference numerals  2812   b,    2813   b,    2814   b,    2815   b,    2817   b,    2818   b,    2819   b  and  2820   b  represent ring-shaped optical fibers for protection, reference numerals  2803   a  and  2803   b  represent two-by-two fiber couplers, reference numerals  2803   c,    2803   d,    2803   e,    2803   f,    2803   u,  and  2803   v  represent optical amplifiers, reference numerals  2803   g,    2803   h,    2803   i,  and  2803   j  represent optical terminators where the fiber loop is open, reference numerals  2803   k  and  2803   l  represent two-by-two fiber couplers, reference numerals  2803   m,    2803   n,    2803   o,    2803   p,    2803   w,  and  2803   x  represent optical amplifiers, and reference numerals  2803   q,    2803   r,    2803   s,  and  2803   t  represent optical terminators where the fiber loop is open. Reference numerals  2805 ,  2806 , and  2807  represent access nodes which, with the remote node  2803 , comprise the lower level ring network, reference numerals  2809 ,  2810 , and  2811  represent ONU, reference numerals  2812  and  2813  represent optical fibers for protection, reference numerals  2806   a,    2806   b,    2806   j,  and  2806   k  represent two-by-two fiber couplers, reference numerals  2806   c,    2806   d,    2806   e,    2806   f,    2806   l,    2806   m,    2806   n,    2806   o,    2806   p,  and  2806   q  represent optical amplifiers, reference numeral  2806   g  represents an optical switch, reference numeral  2806   h  represents an optical coupler, reference numeral  2806   i  represents an optical coupler, and reference numerals  2806   y  and  2806   z  represent star couplers. Incidentally, the constitution of the center node  21   h  may be the same as, for example, the center node shown in  FIG. 23 .  
         [0120]     One feature of this embodiment is that, star couplers which are not dependent on wavelength are used in distributing signals to the ONU belonging to the access nodes, and in multiplexing signals from the ONU. This is effective when the number of ONU varies from access node to access node. Furthermore, the ONU in this embodiment must be capable of selecting wavelengths. This embodiment also has the advantages that (i) there is no switching at the remote nodes when the fiber becomes severed, and (ii) the optical signal is not electrically processed at the nodes (offices) provided between the ONU and the center node. Incidentally, the operations in the cases where the optical cable becomes severed at the points AA′ and BB′ (not shown in  FIG. 28 ) are the same as that already described in the second embodiment.  
         [0121]     According to the embodiments described above, it is possible to reduce initial expenditure when realizing a large-capacity access service by using ONU. Further, when increasing the number of ONU, only the transmission apparatuses at the center node need be increased, achieving an easily expandable network.