Abstract:
A ring network is composed of an even-number of optical fibers wherein each pair of optical fibers forms a bidirectional transmission line and a plurality of nodes coupled through the optical fibers in ring topology. In this ring system, each of the nodes includes the even-number of Add/Drop circuits coupled to the optical fibers, respectively, and the even-number of optical transceivers each including an optical transmitter and an optical receiver. A switch changes a combination of an Add/Drop circuit and each of the optical transmitter and the optical receiver.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an optical ring network, and in particular to a ring network having a protection capability using Wavelength-Division Multiplexing (WDM) technology. 
     2. Description of the Related Art 
     In an optical WDM ring network using a wavelength for services and another wavelength for protection, duplicate signals are generated by a node to travel over an optical fiber in opposite directions. In each node of the ring network, it is determined whether a signal can be received from another node on the wavelength for services. If a node receives the signal on the wavelength for services, the node selects the service wavelength to receive a transmission signal. 
     In case of a cable cut or a node failure occurring at a location, however, the node cannot receive the signal on the service wavelength from another node which is located between the failure location and the node in the downstream direction. In this case, the node selects the protection wavelength to receive the duplicate transmission signal traveling in the opposite direction. Therefore, data transmission can be performed as normal even if transmission failures occur. Such an optical WDM ring network has been disclosed in Japanese Patent Unexamined Publication No. 6-97950. 
     Other survivable WDM ring networks have been proposed by A. F. Elrefale (ICC&#39;93 Geneva. 1993, Geneva, paper 48.7) and by R. E. Wagner et al. (OEC&#39;94, 1994, Japan, 14C3-1). 
     However, the conventional ring network as described above has disadvantages in that the number of assigned wavelengths is twice that of nodes because a pair of wavelengths for service and protection are assigned to each node. This results in broadened necessary bandwidth. Further, a wavelength selector provided at each node has the increased number of wavelengths to be selected. Furthermore, since duplicate signals are traveling in opposite directions, a transmission line cannot be used effectively. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an optical WDM ring network which can effectively use wavelength components thereof to achieve high performance. 
     Another object of the present invention is to provide an optical WDM ring network which can ensure reliable data transmission in case of line or node failures. 
     According to the present invention, in a ring network that includes a plurality of transmission media and a plurality of nodes coupled through the transmission media in ring topology, each of the nodes includes a transceiver for transmitting and receiving a plurality of signals and a switch for changing a path for each signal which is transmitted and received to and from one of the transmission media through the path. 
     Further, the present invention may be applied to a ring network including an even-number of optical fibers wherein each pair of optical fibers forms a bidirectional transmission line and a plurality of nodes coupled through the optical fibers in ring topology. In this ring system, each of the nodes includes the even-number of interfaces coupled to the optical fibers, respectively, and the even-number of optical transceivers each comprising an optical transmitter and an optical receiver, wherein each of the optical transmitter and the optical receiver is coupled to a selected one of the interfaces The node is further provided with a switch for changing a combination of an interface and each of the optical transmitter and the optical receivers 
     Since the switch can select one of the transmission media for transmitting or receiving each signal, all wavelength components in the ring network are usable for services without the need of setting wavelengths for protection. Therefore, the wavelength components can be used effectively, resulting in reduced wavelength band and the reduced number of wavelengths to be selected in each node. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an optical WDM ring network according to a first embodiment of the present invention; 
     FIGS. 2A-2E are spectrum diagrams for explanation of operation of an Add/Drop circuit in the first embodiment; 
     FIG. 3A is a schematic diagram showing a fiber grating in the Add/Drop circuit of FIG.  2 : 
     FIG. 3B is a diagram showing a reflection characteristic of the fiber grating; 
     FIG. 3C is a diagram showing a passband characteristic of the fiber grating; 
     FIG. 4 is a block diagram showing a 4×4 matrix switch in the first embodiment of FIG. 2; 
     FIG. 5A is a block diagram showing a first example of an optical transmitter in the first embodiment of FIG. 2; 
     FIG. 5B is a block diagram showing a second example of an optical transmitter in the first embodiment of FIG.  2 : 
     FIG. 5C is a block diagram showing a third example of an optical transmitter in the first embodiment of FIG. 2; 
     FIG. 6A is a diagram showing a normal operation of the first embodiment of FIG. 2; 
     FIG. 6B is a diagram showing an operation of the first embodiment of FIG. 2 in the case of service fiber cut; 
     FIG. 6C is a diagram showing an operation of the first embodiment of FIG. 2 in the case of service and protection fiber cut; 
     FIG. 6D is a diagram showing an operation of the first embodiment of FIG. 2 in the case of node failure: 
     FIG. 7 is a diagram showing another normal operation of the ring network of FIG. 2; 
     FIG. 8 is a block diagram showing an optical WDM ring network according to a second embodiment of the present invention; 
     FIG. 9A is a diagram showing a normal operation of the second embodiment of FIG. 8; 
     FIG. 9B is a diagram showing an operation of the second embodiment of FIG. 8 in the case of service fiber cut; 
     FIG. 9C is a diagram showing an operation of the second embodiment of FIG. 8 in the case of service and protection fiber cut; 
     FIG. 9D is a diagram showing an operation of the second embodiment of FIG. 8 in the case of node failure; 
     FIG. 10 is a block diagram showing an optical WDM ring network according to a third embodiment of the present invention: 
     FIG. 11 is a block diagram showing an optical WDM ring network according to a fourth embodiment of the present invention; 
     FIG. 12A is a diagram showing a normal operation of the fourth embodiment of FIG. 11; 
     FIG. 12B is a diagram showing an operation of the fourth embodiment of FIG. 11 in the case of fiber cut; 
     FIG. 12C is a diagram showing an operation of the fourth embodiment of FIG. 11 in the case of node failure: 
     FIG. 13 is a diagram showing another normal operation of the fourth embodiment of FIG. 11; 
     FIG. 14 is a block diagram showing an optical WDM ring network according to a fifth embodiment of the present invention; 
     FIG. 15A is a diagram showing a normal operation of the fifth embodiment of FIG.  14 : 
     FIG. 15B is a diagram showing an operation of the fifth embodiment of FIG. 14 in the case of fiber cut; 
     FIG. 15C is a diagram showing an operation of the fifth embodiment of FIG. 14 in the case of node failure; and 
     FIG. 16 is a block diagram showing the control circuit of a node according to the first embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     Ref erring to FIG. 1, there is shown a WDM ring network having a plurality of nodes NODE 1 , NODE 2 , . . . , NODEn which are optically connected in a ring topology through four fibers F 1 -F 4 . The four fibers may be classified under service and protection bidirectional fiber pairs. Here, the fibers F 1  and F 2  are a service bidirectional fiber pair and the fibers F 3  and F 4  are a protection bidirectional fiber pair. 
     The nodes NODE 1 , NODE 2 , . . . , NODEn on the ring have different receiving wavelengths previously assigned thereto. As shown in the figure, the receiving wavelengths λ 1 -λ n  are assigned to the nodes NODE 1 , NODE 2 , . . . , NODEn, respectively. Therefore, the signal on the wavelength λ 1 , for example, cannot be received at the nodes but the node NODE 1 . Further, each node can drop and receive the signal on the assigned wavelength from any fiber and can select a transmitting wavelength from λ 1 -λ n  depending on which node a signal should be transmitted to, as will be described later. 
     Each node has the same basic circuit configuration as shown in FIG.  1 . Taking the node NODE 3  as an example, it is provided with four optical Add/Drop circuits  101 - 104  which are inserted on the four fibers F 1 -F 4 , respectively. In each optical Add/Drop circuit, a signal on the assigned wavelength (here, λ 3 ) is dropped and output to a 4×4 matrix switch  105  for reception and, on the other hand, a now signal to be added on a selected wavelength is received from a 4×4 matrix switch  106  for transmission. 
     The 4×4 matrix switch  105  selectively connects the optical Add/Drop circuits  101 - 104  to the optical receivers of line terminals T 1 -T 4  depending on a selection control signal S DR . Similarly, the 4×4 matrix switch  106  selectively connects the optical Add/Drop circuits  101 - 104  to the optical transmitters of the line terminals T 1 -T 4  depending on a selection control signal S ADD . 
     Each of the line terminals T 1 -T 4  is provided with a tunable optical transmitter  301 , an optical receiver  302 , a multiplexer/demultiplexer  303  and a low-speed interface  304 . The tunable optical transmitter  301  can be set to a selected wavelength and the optical receiver  302  is fixed to the corresponding wavelength (here, λ 3 ). 
     Add/Drop Circuit 
     Referring to FIGS. 2A-E, each of the optical Add/Drop circuits  101 - 104  is composed of optical amplifiers  201  and  202 , an optical combiner  203 , an optical circulator  204  and a fiber grating  205 . Input WDM signals on the wavelengths λ 1 -λ n  as shown in FIG. 2A are amplified by the optical amplifier  201  and then pass through the optical circulator  204  to the fiber grating  205 . The fiber grating  205  reflects the components of the assigned wavelength (here, λ 3 ) and the remaining wavelength components as shown in FIG. 2B pass through the fiber grating  205 . The reflected wavelength components as shown in FIG. 2C go back to the optical circulator  204  which guides them to the 4×4 matrix switch  105 . 
     On the other hand, when receiving the passing wavelength components from the fiber grating  205  and further a new signal to be added on a selected wavelength (here, A;) as shown in FIG. 2D from the 4×4 matrix switch  106 , the optical combiner  203  combines them and then the optical amplifier  202  amplifies the combined wavelength components as shown in FIG.  2 E. 
     Fiber Grating 
     Referring to FIG. 3A, the fiber grating  205  is composed of an index grating which is formed by strongly exposing a core of a single-mode fiber in a predetermined pattern. Among input WDM signals on the wavelengths λ 1 -λ n , only the signal on a specific wavelength λ 1  which is matched to the predetermined pattern of the index grating is reflected and goes back to the optical circulator  204 . The remaining signals travel through the fiber grating  205  to the optical combiner  203 . The passing or reflecting bandwidth of the fiber grating  205  may be set within the range from 0.5 nm to 1.0 nm as shown in FIGS. 3B and 3C. 
     4×4 Matrix Switch 
     Referring to FIG. 4, each of the 4×4 matrix switches  105  and  106  is provided with six 2×2 optical switches SW 1 -SW 6  which are arranged in a matrix on a substrate and each may be made of LiNbO 3  and the like. The 2×2 optical switches SW 1 -SW 6  are connected through waveguides as follows. The two output terminals of the 2×2 optical switches SW 1  are connected to the first input terminal of the 2×2 optical switches SW 3  and the first input terminal of the 2×2 optical switches SW 4 , respectively. Similarly, the two output terminals of the 2×2 optical switches SW 3  are connected to the first input terminal of the 2×2 optical switches SW 5  and the first input terminal of the 2×2 optical switches SW 6 , respectively. The two output terminals of the 2×2 optical switches SW 2  are connected to the second input terminal of the 2×2 optical switches SW 3  and the second input terminal of the 2×2 optical switches SW 4 , respectively. Similarly, the two output terminals of the 2×2 optical switches SW 4 are connected to the second input terminal of the 2×2 optical switches SW 5  and the second input terminal of the 2×2 optical switches SW 4 , respectively. 
     The respective 2×2 optical switches SW 1 -SW 4  perform switching operations to form paths from input terminals to output terminals depending on the selection control signal. Therefore, each of input signals S I1 -S I4  can be output as one of output signals S 01 -S 04 . 
     Optical Transmitter 
     As described above, the optical transmitter  301  can select a transmitting wavelength from λ 1 -λ n  depending on which node a signal should be transmitted to. Several examples of such an optical transmitter can be considered as shown in FIGS. 5A-5C. 
     Referring to FIG. 5A, the optical transmitter  301  is composed of an LD controller  401 , a tunable laser diode (LD)  402  and an external modulator  403 . The LD controller  401  controls the output wavelength of the tunable laser diode  402  depending on a wavelength selection signal. The external modulator  403  performs the intensity modulation of the laser light of a selected wavelength λ x  (x=1, 2, . . . , n) received from the tunable laser diode  402  depending on transmission data. 
     Referring to FIG. 5B, the optical transmitter  301  is composed of n laser diodes LD 1 -LD N , a n:1 optical switch  404  and the external modulator  403 . The laser diodes LD 1 -LD N  output n laser lights of wavelengths λ 1 -λ n , respectively. The n:1 optical switch  404  selects one of the laser lights of wavelengths λ 1 -λ n  depending on the wavelength selection signal. The external modulator  403  performs the intensity modulation of the selected laser light of a selected wavelength λ x  (x=1, 2, . . . , n) depending on transmission data 
     Referring to FIG. 5C, the optical transmitter  301  is composed of n laser diodes LD 1 -LD N , n drivers DRV 1 -DRV N , and an optical WDM coupler  405 , and the external modulator  403 . The drivers DRV 1 -DRV N  are controlled by the wavelength selection signals such that a selected one of the laser diodes LD 1 -LD N  is driven to output the laser light of the corresponding wavelength λ x  to the optical WDM coupler  405 . The external modulator  403  performs the intensity modulation of the laser light of a selected wavelength λ x  (x=1, 2, . . . , n) depending on transmission data. 
     Operation 
     For simplicity, operations in the ring network will be described in the case where the node NODE 1  communicates with the node NODE 3 . 
     Referring to FIG. 6A, under normal conditions, the node NODE 1  communicates with the node NODE 3  through the node NODE 2 , the clockwise service fiber F 1  and the counterclockwise service fiber F 2 . The node NODE 1  transmits an optical signal on wavelength λ 3  onto the clockwise service fiber F 1  and the node NODE 3  receives the optical signal on wavelength λ 3  from the clockwise service fiber F 1 . On the other hand, the node NODE 3  transmits an optical signal on wavelength λ 1  onto the counterclockwise service fiber F 2  and the node NODE 1  receives the optical signal on wavelength λ 1  from the counterclockwise service fiber F 2 . The receiving/transmitting and add/drop operations of each node are as described before. 
     Referring to FIG. 6B, in the case of service fiber cut occurring between the node NODE 2  and the node NODE 3 , the node NODE 1  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal on wavelength λ 3  is transmitted onto the clockwise protection fiber F 3  and an optical signal on wavelength λ 1  is received from the counterclockwise protection fiber F 4 . Similarly, the node NODE 3  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal on wavelength λ 1  is transmitted onto the counterclockwise protection fiber F 4  and an optical signal on wavelength λ 3  is received from the clockwise protection fiber F 3 . 
     Referring to FIG. 6C, in the case of service and protection fiber cut occurring between the node NODE 2  and the node NODE 3 , the node NODE 1  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal on wavelength λ 3  is transmitted onto the counterclockwise protection fiber F 4  and an optical signal on wavelength λ 1  is received from the clockwise protection fiber F 3 . Similarly, the node NODE 3  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal on wavelength λ 1  is transmitted onto the clockwise protection fiber F 3  and an optical signal on wavelength A 2  is received from the counterclockwise protection fiber F 4 . 
     Referring to FIG. 6D, in the case of failure of the node NODE 2 , the respective nodes NODE 1  and NODE 3  change the respective connection states of the 4×4 optical switches  105  and  106  as in the case of FIG.  6 C. 
     As described above, the 4×4 optical switches  105  and  106  selectively connects the optical Add/Drop circuits  101 - 104  to the optical receivers and the optical transmitters of line terminals T 1 -T 4 , all wavelength components in the ring network are usable for services without the need of setting wavelengths for protection. Therefore, the wavelength components can be used effectively, resulting in reduced wavelength band and the reduced number of wavelengths to be selected in each node. 
     Further, even in the case of node failure and/or fiber cut as shown in FIGS. 6B-6D, the both sides can select another pair of optical fibers for protection to ensure communications between two nodes as normal. 
     As shown in FIG. 7, by controlling the 4×4 optical switches  105  and  106 , another communications mode may be set under normal conditions. More specifically, the respective nodes NODE 1 , NODE 2 , NODE 3  and NODEn change the respective connection states of the 4×4 optical switches  105  and  106  such that high-priority communication between the nodes NODE 1  and NODE 3  through the service fibers F 1  and F 2  and further low-priority communications between the nodes NODE 1  and NODE 2  and between the nodes NODE 2  and the node NODEn through the protection fibers F 3  and F 4 . In this communication mode, when service fiber cuts or node failures occur, the higher-priority communications are protected as described above but the lower-priority communications are possibly broken down. In other words, the lower-priority communications may be performed through protection fibers based on the premise of the possibility of breakdown. Therefore, data transmission can be performed with a higher degree of efficiency. 
     Second Embodiment 
     Referring to FIG. 8, there is shown a WDM ring network where circuit blocks similar to those previously described with reference to FIG. 1 are denoted by the same reference numerals and their details will be described as necessary. 
     Each node has the same basic circuit configuration as shown in FIG.  8 . Taking the node NODE 3  as an example, it is provided with four optical Add/Drop circuits  101 - 104  which are inserted on the four fibers F 1 -F 4 , respectively. In each optical Add/Drop circuit, a signal on the assigned wavelength (here, λ 2 ) is dropped and output to a 4×4 matrix switch  105  for reception and, on the other hand, a new signal to be added on a selected wavelength is received from a 4×4 matrix switch  106  for transmission. 
     The 4×4 matrix switch  105  selectively connects the optical Add/Drop circuits  101 - 104  to the optical receivers of line terminals T 1 -T 4  depending on a selection control signal S DR . Similarly, the 4×4 matrix switch  106  selectively connects the optical Add/Drop circuits  101 - 104  to the optical transmitters of the line terminals T 1 -T 4  depending on a selection control signal S ADD . 
     Each of the line terminals T 1 -T 4  is provided with a tunable optical transmitter  301 , an optical receiver  302 , a multiplexer/demultiplexer  303  and a low-speed interface  304 . The tunable optical transmitter  301  can be set to a selected wavelength and the optical receiver  302  is fixed to the corresponding wavelength (here, λ 3 ) as described before. 
     Further, each node has a routing function which is implemented by 2×2 optical switches  110 - 113  as shown in FIG.  8 . More specifically, the 2×2 optical switch  110  is provided between two routes corresponding to the clockwise service fiber F 1  and the clockwise protection fiber F 3 , respectively, to select one from the two routes corresponding to the clockwise service fiber F 1  and the counterclockwise protection fiber F 2 . The 2×2 optical switch  111  is provided between two opposite routes corresponding to the clockwise service fiber F 1  and the counterclockwise protection fiber F 4 , respectively, to select one therefrom The 2×2 optical switch  112  is provided between two opposite routes corresponding to the counterclockwise service fiber F 2  and the clockwise protection fiber F 3 , respectively, to select one therefrom. The 2×2 optical switch  113  is provided between two routes corresponding to the counterclockwise service fiber F 2  and the counterclockwise protection fiber F 4 , respectively, to select one therefrom. The respective locations of the 2×2 optical switches  110 - 113  are not limited to this embodiment as shown in FIG.  8 . Another arrangement may be possible if the same function can be performed. 
     Operation 
     For simplicity, operations in the ring network will be described in the case where the node NODE 1  communicates with the node NODE 3 . 
     Referring to FIG. 9A, under normal conditions, the node NODE 1  communicates with the node NODE 3  through the node NODE 2 , the clockwise service fiber F 1  and the counterclockwise service fiber F 2 . The node NODE 1  transmits an optical signal on wavelength λ 3  onto the clockwise service fiber F 1  and the node NODE 3  receives the optical signal on wavelength λ 3  from the clockwise service fiber F 1 . On the other hand, the node NODE 3  transmits an optical signal on wavelength λ 1  onto the counterclockwise service fiber F 2  and the node NODE 1  receives the optical signal on wavelength λ 1  from the counterclockwise service fiber F 2 . Since the receiving wavelength of the node NODE 2  is set to λ 2 , none of the optical signals on wavelengths λ 1  and λ 3  is not received. The receiving/transmitting and add/drop operations of each node are as described before. 
     Referring to FIG. 9B, in the case of service fiber cut occurring between the node NODE 2  and the node NODE 3 , the node NODE 2  changes the respective connection states of the 2×2 optical switches  110  and  113  such that the clockwise service fiber F 1  is optically connected to the clockwise protection fiber F 3  and the counterclockwise protection fiber F 4  is optically connected to the counterclockwise service fiber F 2 . Therefore, an optical signal on wavelength λ 3  is transferred from the clockwise service fiber F 1  to the clockwise protection fiber F 3  and an optical signal on wavelength λ 1  is transferred from the counterclockwise protection fiber F 4  to the counterclockwise service fiber F 2 . 
     On the other hand, the node NODE 3  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal of wavelength λ 1  is transmitted onto the counterclockwise protection fiber F 4  and an optical signal on wavelength λ 3  is received from the clockwise protection fiber F 3 . 
     Referring to FIG. 9C, in the case of service and protection fiber cut occurring between the node NODE 2  and the node NODE 3 , the node NODE 2  changes the respective connection states of the 2×2 optical switches  111  and  112  such that the clockwise service fiber F 1  is optically connected to the counterclockwise protection fiber F 4  and the clockwise protection fiber F 3  is optically connected to the counterclockwise service fiber F 3 . Therefore, an optical signal on wavelength λ 3  is transferred from the clockwise service fiber F 1  to the counterclockwise protection fiber F 4  and an optical signal on wavelength λ 1  is transferred from the clockwise protection fiber F 3  to the counterclockwise service fiber F 2 . 
     On the other hand, the node NODE 3  changes the respective connection states of the 4×4 optical switches  105  and  106  such that the optical signal of wavelength λ 1  is transmitted onto the clockwise protection fiber F 3  and the optical signal on wavelength λ 3  is received from the counterclockwise protection fiber F 4 . 
     Referring to FIG. 9D, in the case of failure of the node NODE 2 , the node NODE 1  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal on wavelength λ 3  is transmitted onto the counterclockwise protection fiber F 4  and an optical signal on wavelength λ 1  is received from the clockwise protection fiber F 3 . Similarly, the node NODE 3  changes the respective connection states of the 4×4 optical switches  105  and  106  such that an optical signal on wavelength λ 1  is transmitted onto the clockwise protection fiber F 3  and an optical signal on wavelength λ 3  is received from the counterclockwise protection fiber F 4 . 
     As described above, the 4×4 optical switches  105  and  106  selectively connects the optical Add/Drop circuits  101 - 104  to the optical receivers and the optical transmitters of line terminals T 1 -T 4 , all wavelength components in the ring network are usable for services without the need of setting wavelengths for protection. Therefore, the wavelength components can be used effectively, resulting in reduced wavelength band and the reduced number of wavelengths to be selected in each node. 
     Further, since a routing function is implemented by the 2×2 optical switches  110 - 113 , data transmission can be performed with a higher degree of efficiency. 
     As in the case of the first embodiment, by controlling the 4×4 optical switches  105  and  106  of each node, the communications mode as shown in FIG. 7 may be set under normal conditions. 
     Third Embodiment 
     Referring to FIG. 10, there is shown a WDM ring network where circuit blocks similar to those previously described with reference to FIG. 1 are denoted by the same reference numerals and their details will be described as necessary. 
     Each node has the same basic circuit configuration as shown in FIG.  10 . Taking the node NODE 3  as an example, it is provided with four optical Add/Drop circuits  101 - 104  which are inserted on the four fibers F 1 -F 4 , respectively. In each optical Add/Drop circuit, a signal on the assigned wavelength (here, λ 3 ) is dropped and output to an optical transceiver  107  and, on the other hand, a new signal to be added on a selected wavelength is received from the optical transceiver  107 . 
     The optical transceiver  107  is provided with four tunable optical transmitters indicated by reference numerals  301 - 1  to  301 - 4 , respectively, and four optical receivers indicated by reference numerals  302 - 1  to  302 - 4 , respectively. As described before, each tunable optical transmitter can be set to a selected wavelength and each optical receiver is fixed to the assigned wavelength (here, λ 3 ). The respective pairs of transmitter and receiver are connected to multiplexer/demultiplexer circuits  303  which are in turn connected to a low-speed interface  304  through a path switch  305  which is formed with electrical circuits. 
     In the third embodiment, the path switch  305  is provided in place of the 4×4 matrix switches  105  and  106 . The function of the path switch  305  is the same as that of the 4×4 matrix switches  105  and  106 . In other words, four electrical signals to be processed in each node can be selectively connected to four arbitrary pairs of transmitter and receiver. Therefore, the operations of the third embodiment are the same as those of the first embodiment as shown in FIGS. 6A-6D. 
     Since the path switch  305  which is formed with electrical circuits is employed, there is no need to mount two multiinput/output optical switches  105  and  106  on the node. Therefore, the circuit configuration of the optical system is simplified. 
     Fourth Embodiment 
     Referring to FIG. 11, there is shown a WDM ring network where circuit blocks similar to those previously described with reference to FIG. 1 are denoted by the same reference numerals and their details will be described as necessary. 
     The WDM ring network has a plurality of nodes NODE 1 , NODE 2 , . . . NODEn which are optically connected in a ring topology through a clockwise fiber P 1  and a counterclockwise fiber F 2 . Each node has the same basic circuit configuration as shown in FIG.  11 . Taking the node NODE 3  as am example, it is provided with two optical Add/Drop circuits  101  and  102  which are inserted on the two fibers F 1  and F 2 , respectively. In each optical Add/Drop circuit, a signal on the assigned wavelength (here, λ 3 ) is dropped and output to a 2×2 matrix switch  108  for reception and, on the other hand, a new signal to be added on a selected wavelength is received from a 2×2 matrix switch  109  for transmission. 
     The 2×2 matrix switch  108  selectively connects the optical Add/Drop circuits  101  and  102  to the optical-receivers of line terminals T 1  and T 2  depending on a selection control signal S DR . Similarly, the 2×2 matrix switch  109  selectively connects the optical Add/Drop circuits  101  and  102  to the optical transmitters of the line terminals T 1  and T 2  depending on a selection control signal S ADD . 
     Each of the line terminals T 1  and T 2  is provided with a tunable optical transmitter  301 , an optical receiver  302 , a multiplexer/demultiplexer  303  and a low-speed interface  304 . The tunable optical transmitter  301  can be set to a selected wavelength and the optical receiver  302  is fixed to the corresponding wavelength (here, λ 3 ) as described before. 
     For simplicity, operations in the ring network will be described in the case where the node NODE 1  communicates with the node NODE 3 . 
     Referring to FIG. 12A, under normal conditions, the node NODE 1  communicates with the node NODE 3  through the node NODE 2 , the clockwise fiber F 1  and the counterclockwise fiber F 2 . The node NODE 1  transmits an optical signal on wavelength λ 3  onto the clockwise fiber F 1  and the node NODE 3  receives the optical signal on wavelength λ 3  from the clockwise fiber F 1 . On the other hand, the node NODE 3  transmits an optical signal on wavelength λ 1  onto the counterclockwise fiber F 2  and the node NODE 1  receives the optical signal on wavelength λ 1  from the counterclockwise fiber F 2 . The receiving/transmitting and add/drop operations of each node are as described before. 
     Referring to FIG. 12B, in the case of fiber cut occurring between the node NODE 2  and the node NODE 3 , the node NODE 1  changes the respective connection states of the 2×2 optical switches  108  and  109  such that an optical signal on wavelength λ 3  is transmitted onto the counterclockwise fiber F 2  and an optical signal on wavelength λ 1  is received from the clockwise fiber F 1 . Similarly, the node NODE 3  changes the respective connection states of the 2×2 optical switches  108  and  109  such that an optical signal on wavelength λ 1  is transmitted onto the clockwise fiber F 1  and an optical signal on wavelength λ 3  is received from the counterclockwise fiber F 2 . 
     Referring to FIG. 12C, in the case of failure of the node NODE 2 , the respective nodes NODE 1  and NODE 3  change the respective connection states of the 2×2 optical switches  108  and  109  as in the case of FIG.  12 B. 
     As described above, even in the case of node failure and/or fiber cut, communications between two nodes can be ensured as normal. 
     As shown in FIG. 13, by controlling the 2×2 optical switches  108  and  109 , another communications mode may be set under normal conditions. More specifically, the respective nodes NODE 1  and NODE 3  change the respective connection states of the 2×2 optical switches  108  and  109  such that a higher-priority signal on wavelength λ 3  is transferred from the node NODE 1  to the node NODE 3  through the node NODE 2  traveling over the clockwise fiber F 1  and a higher-priority signal on wavelength λ 1  is transferred from the node NODE 3  to the node NODE 1  through the node NODE 2  traveling over the counterclockwise fiber F 2 . Further, the respective nodes NODE 1  and NODEn change the respective connection states of the 2×2 optical switches  108  and  109  such that a lower-priority signal on wavelength λ n  is transferred from the node NODE 1  to the node NODEn traveling over the counterclockwise fiber F 2  and a lower-priority signal on wavelength λ 1  is transferred from the node NODE 3 n to the node NODE 1  traveling over the clockwise fiber F 1 . In this communication mode, when fiber cuts or node failures occur, the higher-priority communications are protected as described above but the lower-priority communications are possibly broken down. In other words, the lower-priority communications may be performed based on the premise of the possibility of breakdown. 
     Fifth Embodiment 
     Referring to FIG. 14, there is shown a WDM ring network where circuit blocks similar to those previously described with reference to FIG. 11 are denoted by the same reference numerals and their details will be described as necessary. 
     Each node has the same basic circuit configuration as shown in FIG.  11 . Taking the node NODE 3  as an example, it is provided with two optical Add/Drop circuits  101  and  102  which are inserted on the two fibers F 1  and F 2 , respectively. In each optical Add/Drop circuit, a signal on the assigned wavelength (here, λ 3 ) is dropped and output to a 2×2 matrix switch  108  for reception and, on the other hand, a new signal to be added on a selected wavelength is received from a 2×2 matrix switch  109  for transmission. 
     The 2×2 matrix switch  108  selectively connects the optical Add/Drop circuits  101  and  102  to the optical receivers of line terminals T 1  and T 2  depending on a selection control signal S DR . Similarly, the 2×2 matrix switch  109  selectively connects the optical Add/Drop circuits  101  and  102  to the optical transmitters of the line terminals T 1  and T 2  depending on a selection control signal S ADD . 
     Each of the line terminals T 1  and T 2  is provided with a tunable optical transmitter  301 , an optical receiver  302 , a multiplexer/demultiplexer  303  and a low-speed interface  304 . The tunable optical transmitter  301  can be set to a selected wavelength and the optical receiver  302  is fixed to the corresponding wavelength (here, λ 3 ) as described before. 
     Further, each node has a routing function which is implemented by 2×2 optical switch  114  as shown in FIG.  14 . More specifically, the 2×2 optical switch  114  is provided between two opposite routes corresponding to the clockwise fiber F 1  and the counterclockwise fiber F 2 , respectively, to select one from the two opposite routes. The location of the 2×2 optical switch  114  is not limited to this embodiment as shown in FIG.  14 . Another arrangement may be possible if the same function can be performed. 
     Operation 
     For simplicity, operations in the ring network will be described in the case where the node NODE 1  communicates with the node NODE 3 . 
     Referring to FIG. 15A, under normal conditions, the node NODE 1  communicates with the node NODE 3  through the node NODE 2 , the clockwise fiber F 1  and the counterclockwise fiber F 2 . The node NODE 1  transmits an optical signal on wavelength λ 2  onto the clockwise fiber F 1  and the node NODE 3  receives the optical signal on wavelength λ 3  from the clockwise fiber F 1 . On the other hand, the node NODE 3  transmits an optical signal on wavelength λ 1  onto the counterclockwise fiber F 2  and the node NODE 1  receives the optical signal on wavelength λ 1  from the counterclockwise fiber F 2 . Since the receiving wavelength of the node NODE 2  is set to λ 2 , none of the optical signals on wavelengths λ 1  and λ 3  is not received. The receiving/transmitting and add/drop operations of each node are as described before. 
     Referring to FIG. 15B, in the case of fiber cut occurring between the node NODE 2  and the node NODE 3 , the node NODE 2  changes the respective connection states of the 2×2 optical switch  114  such that the clockwise fiber F 1  is optically connected to the counterclockwise fiber F 2 . Therefore, optical signals on wavelength λ 3  and λ 1  are transferred from the clockwise fiber F 1  to the counterclockwise fiber F 2 . 
     On the other hand, the node NODE 3  changes the respective connection states of the 2×2 optical switches  108  and  109  such that the optical signal of wavelength λ 1  is transmitted onto the clockwise fiber F 1  and the optical signal on wavelength λ 2  is received from the counterclockwise fiber F 2 . 
     Referring to FIG. 15C, in the case of failure of the node NODE 2 , the node NODE 1  changes the respective connection states of the 2×2 optical switches  108  and  109  such that an optical signal on wavelength λ 3  is transmitted onto the counterclockwise fiber F 2  and an optical signal on wavelength Al is received from the clockwise fiber F 1 . Similarly, the node NODE 3  changes the respective connection states of the 2×2 optical switches  108  and  109  such that an optical signal on wavelength λ 1  is transmitted onto the clockwise fiber F 1  and an optical signal on wavelength λ 3  is received from the counterclockwise fiber F 2 . 
     As in the case of the fourth embodiment, by controlling the 2×2 optical switches  108  and  109  of each node, the communications mode as shown in FIG. 13 may be set under normal conditions. 
     Control System 
     Taking the first embodiment as shown in FIG. 1 as an example, a control system for each node will be described hereinafter. 
     Referring to FIG. 16, in addition to circuit elements as shown in FIG. 1, each node is further provided with a control processor  501 , a program memory  502  and a data transceiver  503  for communicating with a network manager (not shown). The control processor  501  controls the operations of the node as shown in FIGS. 6A-6D according to a control program stored in the program memory  502 . More specifically, when a failure is detected in a node, the control processor  501  thereof transmits the failure indication signal to the network manager. The network manager decided the optimal restoration plan based on failure indication signals polled from nodes, and then sends the restoration plan data to the nodes where the signal rerouting is needed. In the nodes where the restoration plan data is received through the data transceiver  503 , the control processor  501 , depending on the restoration plan, outputs the selection control signals S DR  and S ADD  to the 4×4 matrix switches  105  and  106 , respectively, and further outputs wavelength selection signals S T1 -S T4  to the optical transmitters Tx 1 -Tx 4  which are provided in the line terminals T 1 -T 4 , respectively. Any restoration plan may be obtained not only by the network manager but also by the control processor using inter-node data communications. 
     In a control system of the second embodiment as shown in FIG. 8, in addition to the selection control signals S DR  and S ADD  and the wavelength selection signals S T1 -S T4 , the control processor  501  outputs switch control signals to the 2×2 optical switches  110 - 113 , respectively. In the second embodiment, any failure such as fiber cut and node failure may be usually detected by inter-node communications. Similarly, control systems for the third to fifth embodiments can be formed. 
     The present invention is not limited to the first to third embodiments having two bidirectional transmission lines each consisting of two fibers. Three or more bidirectional transmission lines may be used depending on the amount of transmission data. The number of Add/drop circuits and line terminals needs to be equal to the total number of fibers included in the transmission lines and further the number of input/output terminals of optical switch is also equal to the total number of fibers. Furthermore, the fourth and fifth embodiments can be applied to one of three or more bidirectional transmission lines.