Abstract:
An optical ring system having: a wavelength demultiplexer to which wavelength-multiplexed optical signal to be sent through an optical fiber from a previous node of multiple nodes is input and in which optical signal with each wavelength assigned to itself is demultiplexed; an optical ring device which is disposed in a predetermined node of the multiple nodes to the each wavelength assigned and which is composed of a failure existence judging part which terminates an overhead of each optical signal with a wavelength demultiplexed by the wavelength demultiplexer and judges whether a failure occurs in regard to a wavelength in a previous section through which optical signal with the assigned wavelength is sent, and a switching part which, when the failure existence judging means determines the occurrence of failure, selects a path that allows optical signal with the wavelength to be transmitted to the previous node while avoiding the previous section incurring the failure; and a wavelength multiplexer which multiplexes optical signal to be output from the optical ring part and then outputs it to an optical fiber connected to a next node.

Description:
FIELD OF THE INVENTION 
     This invention relates to an optical ring system that is composed of multiple nodes connected in the form of a ring, and more particularly to, an optical ring system to process optical signal with multiple wavelengths. 
     BACKGROUND OF THE INVENTION 
     Owing to an increase in amount of communications caused by the popularization of the Internet etc. and the emergence of wideband data service, the enhancement of transfer capacity in backbone network has been desired increasingly. Some of Routers and ATM (asynchronous transfer mode) switches as a device for data service are already provided with a wideband interface at a transfer rate of Gbit/sec, and therefore it is difficult to connect such a device to an existing synchronous network. So, a technology to connect to the network while skipping “a device to do the time-division multiplexing of low-order group of signal into high-order group of signal”, an interface of the existing synchronous network, or a WDM (wavelength division multiplexing) technology having a transfer performance more than Gbit/sec to each wavelength has been required. 
     FIG. 1 shows a prior point-to-point wavelength multiplexing transmission system. First to N-th optical wavelength transmitters  11   1 ,  11   2 , . . . ,  11   N , respectively, convert first to N-th optical signal into optical signal of intrinsic wavelength λ 1  to λ N , and then output it to an optical wavelength multiplexer  12 . The optical wavelength multiplexer  12  multiplexes these optical signal and then outputs it to transmission line  13  connected on the output side. An optical amplifier  14 , suitably provided on the transmission line  13 , conducts the recovery of deteriorated light in optical signal with wavelength λ 1 , to λ N  multiplexed. 
     An optical wavelength demultiplexer  15  is input optical signal multiplexed from transmission line  16 , and demultiplexes it into former optical signal of wavelength λ 1  to λ N , and then corresponding first to N-th optical wavelength receivers  16   1 ,  16   2 , . . . ,  16   N  reproduces the original signal. 
     Such a point-to-point wavelength multiplexing transmission system as shown in FIG. 1 is equipped with no protection function to protect the transmission of signal when it is subject to a failure such as disconnection of optical fiber to form the transmission lines  13 ,  16 , and malfunction of the optical transmitter/receiver. 
     The simplest method to provide the optical wavelength multiplexing protection function is to detect a failure at each optical terminal node to provide the protection at multiplexing level of all wavelengths, i.e. in unit of one optical fiber. 
     FIG. 2 shows an example of optical ring system that employs such a protection method suggested so far. This optical ring system, which is disclosed in Japanese patent application laid-open No. 6-61986 (1994), comprises a four-fiber ring which is of four optical fibers  31  to  34  connected in the form of ring among a master station  21  and first to third slave stations  22  to  24 . Of the four fibers, two optical fibers  31 ,  32  compose work line, i.e. actually-working line, and the remaining two optical fibers  33 ,  34  compose protection line, i.e. backup line. 
     The optical ring system in FIG. 2 is not subject to failure. In FIG. 2, master clock output from a clock-supplying device  25 , which is disposed in the master station  21 , is supplied sequentially from the first to third slave stations  22  to  24 . 
     FIG. 3 shows a case that the work line between the master station  21  and the first slave station  22  in the optical ring system is subject to a failure  41 . When the failure  41  occurs on the work line composed of the first and second optical fibers  31   1 ,  32   1 , the transmission line is switched so that two optical fibers  33   1 ,  34   1  to compose the protection line in this section can supply the master clock. Then, between the first slave station  22  and the master station  21 , the transmission line through the second slave station  23  and the third slave station  24  supplies the master clock like that in FIG.  2 . 
     FIG. 4 shows a case that, between the master station  21  and the first slave station  22  in this optical ring system, not only the work line but also the protection line is subject to a failure  42 . When the failure  42  occurs on both the work line composed of the first and second optical fibers  31   1 ,  32   1  and the protection line composed of the third and fourth optical fibers  33   1 ,  34   1 , the master clock is supplied through optical fiber  32   4  in the direction from the master station  21  to the third slave station  24 . 
     Although the transmission of master clock is explained in this example. the first to fourth optical fibers  31  to  34  respectively transmit multiplexing optical signal with multiple wavelengths λ 1  to λ N , and when optical signal with more than one of the wavelengths is subject to a failure, the switching of transmission line is conducted in like manner described above. 
     Thus, in the case that, as shown in FIG. 1, optical signal with multiple wavelengths λ 1  to λ N  is multiplexed in one transmission line, even when the transmission line of partial wavelength is subject to a failure, the protection of signal transmission line is conducted by unit of one optical fiber. This means that due to failure of one wavelength, the transmission line of optical signal to the remaining wavelengths has to be switched. 
     In recent years, optical amplification technology and wavelength multiplexing technology have been developed abruptly. Along with this, the multiplexing number N of optical signal to be transmitted through one optical fiber has increased. Under this background, if even for a failure concerning one wavelength the switching of optical signal to all the remaining wavelengths must be conducted, then optical signal to the wavelengths operated normally may be influenced by an instantaneous shut-off by the switching operation to switch into the protection line. Also, due to the rerouting required when conducting the switching operation of transmission line, delay in transmission of signal occurs, and as a whole the efficiency in use of wavelength band lowers. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an optical ring system that when a failure occurs as to only part of wavelengths in point-to-point wavelength multiplexing transmission system, the protection function is conducted to only a wavelength subject to the failure. 
     According to the invention, an optical ring system, comprises: 
     a wavelength demultiplexing means to which wavelength-multiplexed optical signal to be sent through an optical fiber from a previous node of multiple nodes composing a ring network is input and in which optical signal with each wavelength assigned to itself is demultiplexed; 
     an optical ring means which is provided in a predetermined node of the multiple nodes to the each wavelength assigned and which is composed of a failure existence judging means which terminates an overhead of each optical signal with a wavelength demultiplexed by the wavelength demultiplexing means and judges whether a failure occurs in regard to a wavelength in a previous section through which optical signal with the assigned wavelength is sent, and a switching means which, when the failure existence judging means determines the occurrence of failure, selects a path that allows optical signal with the wavelength to be transmitted to the previous node while avoiding the previous section incurring the failure; and 
     a wavelength multiplexing means which multiplexes optical signal to be output from the optical ring means and then outputs it to an optical fiber connected to a next node composing the ring network. 
     In this invention, the optical ring means is provided for each wavelength in each node, and after being demultiplexed by the wavelength demultiplexing means, optical signal with a wavelength assigned to the optical ring means is input. The optical ring means terminates the overhead of optical signal assigned and determines whether a failure occurs. In regard to optical signal with the assigned wavelength, when a failure is detected in the previous section, the switching means is controlled to offer the recovery from failure. Optical signal passed through the switching means is multiplexed again with optical signal with the other wavelength by the wavelength multiplexing means when sent to the next node. Thus, the optical ring system of this invention is configured as if the ring network is provided for each wavelength. Therefore, when a failure occurs as to only part of wavelengths in point-to-point wavelength multiplexing transmission system, the protection function can be conducted to only a wavelength subject to the failure. 
     Also, according to another aspect of the invention, an optical ring system, comprises: 
     a wavelength demultiplexing means to which wavelength-multiplexed optical signal to be sent through an optical fiber from a previous node of multiple nodes composing a ring network is input and in which optical signal with each wavelength assigned to itself is demultiplexed; 
     an optical ring means which is provided in a predetermined node of the multiple nodes to the each wavelength assigned and which is composed of a failure existence judging means which terminates an overhead of each optical signal with a wavelength demultiplexed by the wavelength demultiplexing means and judges whether a failure occurs in regard to a wavelength in a previous section through which optical signal with the assigned wavelength is sent, a switching means which, when the failure existence judging means determines the occurrence of failure, selects a path that allows optical signal with the wavelength to be transmitted to the previous node while avoiding the previous section incurring the failure, and a wavelength changing means which changes the wavelength of optical signal to be output from the switching means; and 
     a wavelength multiplexing means which multiplexes optical signal be output from the optical ring means and then outputs it to an optical fiber connected to a next node composing the ring network. 
     In this aspect of the invention, the optical ring means is provided for each wavelength in each node, and after being demultiplexed by the wavelength demultiplexing means, optical signal with a wavelength assigned to the optical ring means is input. The optical ring means terminates the overhead of optical signal assigned and determines whether a failure occurs. In regard to optical signal with the assigned wavelength, when a failure is detected in the previous section, the switching means is controlled to offer the recovery from failure. Optical signal passed through the switching means is multiplexed again with optical signal with the other wavelength by the wavelength multiplexing means when sent to the next node. Thus, the optical ring system of this invention is configured as if the ring network is provided for each wavelength. Therefore, when a failure occurs as to only part of wavelengths in point-to-point wavelength multiplexing transmission system, the protection function can be conducted to only a wavelength subject to the failure. In addition, by the wavelength changing means, the output-side wavelength is changed. Therefore, this system can be flexibly applied even to a system where wavelength assignment to optical signal is predetermined. 
     According to another aspect of the invention, an optical ring system, comprises: 
     a wavelength demultiplexing means to which wavelength-multiplexed optical signal to be sent through an optical fiber from a previous node of multiple nodes composing a ring network is input and in which optical signal with each wavelength assigned to itself is demultiplexed; 
     an optical ring means which is provided in a predetermined node of the multiple nodes to the each wavelength assigned and which is composed of a failure existence judging means which terminates an overhead of each optical signal with a wavelength demultiplexed by the wavelength demultiplexing means and judges whether a failure occurs in regard to a wavelength in a previous section through which optical signal with the assigned wavelength is sent, a switching means which, when the failure existence judging means determines the occurrence of failure, selects a path that allows optical signal with the wavelength to be transmitted to the previous node while avoiding the previous section incurring the failure, and a bandwidth narrowing means which narrows the bandwidth of optical signal to be output from the switching means; and 
     a wavelength multiplexing means which multiplexes optical signal to be output from the optical ring means and then outputs it to an optical fiber connected to a next node composing the ring network. 
     In this aspect of the invention, the optical ring means is provided for each wavelength in each node, and after being demultiplexed by the wavelength demultiplexing means, optical signal with a wavelength assigned to the optical ring means is input. The optical ring means terminates the overhead of optical signal assigned and determines whether a failure occurs. In regard to optical signal with the assigned wavelength, when a failure is detected in the previous section, the switching means is controlled to offer the recovery from failure. Optical signal passed through the switching means is multiplexed again with optical signal with the other wavelength by the wavelength multiplexing means when sent to the next node. Thus, the optical ring system of this invention is configured as if the ring network is provided for each wavelength. Therefore, when a failure occurs as to only part of wavelengths in point-to-point wavelength multiplexing transmission system, the protection function can be conducted to only a wavelength subject to the failure. In addition, by the bandwidth narrowing means, the bandwidth of optical signal output is narrowed. Therefore, the interaction between optical signals in multiplexing can be reduced, thereby enhancing the efficiency and quality in multiplexing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in conjunction with the appended drawings, wherein: 
     FIG. 1 is a schematic block diagram showing the conventional point-to-point wavelength multiplexing transmission system, 
     FIG. 2 is a network diagram showing a case that no failure occurs in the conventional optical ring system, 
     FIG. 3 is a network diagram showing a case that a failure occurs on the work line between the master station and the first slave station in the conventional system in FIG. 2, 
     FIG. 4 is a network diagram showing a case that a failure occurs not only on the work line but also on the protection line between the master station and the first slave station in the conventional system in FIG. 2, 
     FIG. 5 is a network diagram showing the schematic composition of an optical ring system in a first preferred embodiment according to the invention, 
     FIG. 6 is an illustration showing the concept of wavelength multiplexing, 
     FIG. 7 is a block diagram showing the composition of one node connected with four optical fibers to compose the optical ring system in the first embodiment, 
     FIG. 8 is a block diagram showing the detailed composition of an optical ring device in the first embodiment, 
     FIG. 9 is an illustration showing a connection pattern of a switching section in the first embodiment in normal state that no failure occurs, 
     FIG. 10 is an illustration showing a first example of connection pattern of the switching section in the first embodiment in state that a failure in communication is detected, 
     FIG. 11 is an illustration showing a second example of connection pattern of the switching section in the first embodiment in state that a failure in communication is detected, 
     FIG. 12 is an illustration showing a connection pattern of the switching section of a through node in the first embodiment, 
     FIG. 13 is a network diagram illustrating a four-fiber ring network for specific wavelength λ j  to which the BWPSR system is applied, in a second preferred embodiment according to the invention, 
     FIG. 14 is a network diagram illustrating a case that, in the second embodiment, first and second work-line fibers incur a failure between a second node and a third node, 
     FIG. 15 is a network diagram illustrating a case that, in the second embodiment, all fibers incur a failure between the second node and third node, 
     FIG. 16 is a network diagram illustrating two ring networks with different routes combined, in a third preferred embodiment according to the invention, 
     FIG. 17 is a block diagram showing a conventional operation that optical signal from each client is wavelength-multiplexed and then output to the optical fiber, and 
     FIG. 18 is a block diagram showing an operation that, due to wavelength converter disposed in the optical ring device, optical signal from each client is wavelength-converted without being multiplexed on time axis, in the first to third embodiments of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments will be explained below. 
     First Embodiment 
     FIG. 5 shows a schematic diagram showing the whole composition of an optical ring system in the first preferred embodiment according to the invention. In this example, for simplification of illustration, only the system composition concerning two representative wavelengths λ i  and λ j  is shown. 
     The optical ring system in this embodiment comprises first to fourth nodes  101 ,  102 ,  103  and  104 . In the first to fourth nodes  101 ,  102 ,  103  and  104 , wavelength multiplexing/demultiplexing sections  132 ,  133  to multiplex/demultiplex wavelength of optical signal are disposed. In the first to third nodes  101 ,  102  and  103 , an optical ring device  131   i  for wavelength λ i  is disposed. Also, as shown by dotted lines in FIG. 5, in the second to fourth nodes  102 ,  103  and  104 , an optical ring device  131   j  for wavelength λ j  is disposed. To the optical ring system, clients  135  are connected. The clients  135  is, for example, SDH (synchronous digital hierarchy)/SONET (synchronous optical network) device, IP router and ATM device. The client devices can communicate with each other through the optical ring system. There are some cases that it is not necessary to connect the client  135  to the system. In node  104 , for wavelength λ i , signal can be made to pass through by connecting between the wavelength multiplexing/demultiplexing sections  132 ,  133  by through connection  114   i . Similarly, in node  101 , for wavelength λ j , signal can be made to pass through by connecting between the wavelength multiplexing/demultiplexing sections  132 ,  133  by through connection  114   j . 
     Node  104  is a node that yields a transmission line to make wavelength λ i  pass through, and is not provided with the optical ring device  131  and client  135  connected therewith. Node  101  yields a transmission line to make wavelength λ j  pass through. 
     Considering independently each wavelength, the optical ring system in this embodiment is thus provided with four nodes, and the four optical fibers  141  to  144  are connected therebetween in the form of a ring. Namely, the optical ring system in this embodiment is composed of four-fiber ring. Of the four fibers, two optical fibers  141 ,  142  compose work line, i.e. actually-working line, and the remaining two optical fibers  143 ,  144  compose protection line, i.e. backup line. Each of these optical fibers  141  to  144  is not provided for each wavelength, but provided for wavelength-multiplexed optical signal. 
     FIG. 6 illustrates the concept of wavelength multiplexing. Herein, one optical fiber is represented as optical fiber  140 . The optical fiber  140  in this embodiment can be regarded as the assembling (Σλ) of, in total, 32 transmission lines from transmission line  151   1  for first wavelength λ 1  to transmission line  151   32  for 32 nd  wavelength λ 32 . In this embodiment, a ring structure is given to each wavelength. This can be understood as if there exist 32 ring structures up and down and four optical fibers  141  to  144  wavelength-multiplexed are connected to connect 32 optical ring systems, respectively, up an down. In this embodiment, numerical value “32” is used replaced by “N” in explanations below. 
     FIG. 7 shows the composition of one node connected with four optical fibers to compose the optical ring system in FIG.  5 . The other nodes are also composed similarly. One node is provided with from optical ring device  131   1  for first wavelength λ 1  to optical ring device  131   N  for N-th wavelength λ N  that are disposed corresponding to first wavelength λ 1  to N-th (32nd) wavelength λ N . Here, for simplification of explanation, it is assumed that the composition to pass through a specific wavelength like connection  111   j  in FIG. 5 is not employed and the optical ring device  131  is provided to all the wavelengths. 
     In the node, four wavelength multiplexing sections  161  to  164  and four wavelength demultiplexing sections  165  to  168  are disposed corresponding to the four optical fibers  141  to  144 . In operation, the first wavelength multiplexing section  161  multiplexes optical signals  171  with multiple wavelengths output from the optical ring device  131   1  for first wavelength λ 1  to the optical ring device  131   N  for N-th wavelength λ N , and then outputs output wavelength multiplexed signal  181 . This output wavelength multiplexed signal  181  is sent out to the optical fiber  141   1  as work line in FIG.  5 . 
     In like manner, the second wavelength multiplexing section  162  multiplexes optical signals  172  with multiple wavelengths output from the optical ring device  131   1  for first wavelength λ 1  to the optical ring device  131   N  for N-th wavelength λ N , and then outputs output wavelength multiplexed signal  182 . This output wavelength multiplexed signal  182  is sent out to the optical fiber  143   1  as protection line in FIG.  5 . 
     The third wavelength multiplexing section  163  is disposed on the opposite side of the first and second wavelength multiplexing sections  161 ,  162  in the first to N-th optical ring device  131   1  to  131   N . In operation, the third wavelength multiplexing section  163  multiplexes optical signals  173  with multiple wavelengths output from the optical ring device  131   1  for first wavelength λ 1  to the optical ring device  131   N  for N-th wavelength λ N , and then outputs output wavelength multiplexed signal  183 . This output wavelength multiplexed signal  183  is sent out to the optical fiber  142   4  as work line in FIG. 5, which is in the direction reverse to output wavelength multiplexed signal  181 . 
     Also, the fourth wavelength multiplexing section  164  is disposed on the opposite side of the first and second wavelength multiplexing sections  161 ,  162  in the first to N-th optical ring device  131   1  to  131   N . In operation, the fourth wavelength multiplexing section  164  multiplexes optical signals  174  with multiple wavelengths output from the optical ring device  131   1  for first wavelength λ 1  to the optical ring device  131   N  for N-th wavelength λ N , and then outputs output wavelength multiplexed signal  184 . This output wavelength multiplexed signal  184  is sent out to the optical fiber  144   4  as protection line in FIG. 5, which is in the same direction as output wavelength multiplexed signal  183 . 
     On the other hand, the first wavelength demultiplexing section  165  receives input wavelength multiplexed signal  185  from optical fiber  142   1  as work line in FIG. 5, demultiplexing the input wavelength multiplexed signal  185  into optical signals  175  with the respective multiple wavelengths, then inputting them to the corresponding optical ring devices  131   1  to  131   N  for first wavelength λ 1  to N-th wavelength λ N . 
     Also, the second wavelength demultiplexing section  166  receives input wavelength multiplexed signal  186  from optical fiber  144   1  as protection line in FIG. 5, demultiplexing the input wavelength multiplexed signal  186  into optical signals  176  with the respective multiple wavelengths, then inputting them to the corresponding optical ring devices  131   1  to  131   N  for first wavelength λ 1  to N-th wavelength λ N . 
     The third wavelength demultiplexing section  167  is disposed on the opposite side of the first and second wavelength demultiplexing sections  165 ,  166 . It receives input wavelength multiplexed signal  187  from optical fiber  141   4  as work line in FIG. 5, demultiplexing the input wavelength multiplexed signal  187  into optical signals  177  with the respective multiple wavelengths, then inputting them to the corresponding optical ring devices  131   1  to  131   N  for first wavelength λ 1  to N-th wavelength λ N . 
     The fourth wavelength demultiplexing section  168  is disposed on the same side as the third wavelength demultiplexing section  167 . It receives input wavelength multiplexed signal  188  from optical fiber  143   4  as protection line in FIG. 5, demultiplexing the input wavelength multiplexed signal  188  into optical signals  178  with the respective multiple wavelengths, then inputting them to the corresponding optical ring devices  131   1  to  131   N  for first wavelength λ 1  to N-th wavelength λ N . 
     Tributary side signals  191  are input/output between the SDH/SONET devices  135  in FIG.  5  and the corresponding optical ring devices  131   1  to  131   N  for first wavelength λ 1  to N-th wavelength λ N . The optical ring devices  131   1  to  131   N  input/output tributary side signal  191  to the west or east. Herein, west means the left direction in the drawing and east means the right direction in the drawing. 
     FIG. 8 shows the detailed composition of optical ring device  131  in FIG.  7 . As shown in FIG. 7, the optical ring device  131  is provided to each wavelength in each node. In FIG. 8, for example, optical ring device  131   1  to wavelength λ i  is explained below. 
     From the west side of the optical ring device  131   i , optical signal  175   i  to wavelength λ i  demultiplexed by the first wavelength demultiplexing section  165  in FIG.  7  and optical signal  176   i  to wavelength λ i  demultiplexed by the second wavelength demultiplexing section  166  in FIG. 7 are input thereto. These signals are wavelength-converted by corresponding input wavelength converters  201 ,  202 , then overhead-terminated by corresponding overhead terminating sections  203 ,  204 . Thus, they are input to a switching section  205  while their overhead is removed. The overhead information stored into the overhead is given away to a switch controller  206  to control the switching section  205 . 
     From the east side of the optical ring device  131   i , optical signal  177   i  to wavelength λ i  demultiplexed by the third wavelength demultiplexing section  167  in FIG.  7  and optical signal  178   i  to wavelength λ i  demultiplexed by the fourth wavelength demultiplexing section  168  in FIG. 7 are input thereto. These signals are wavelength-converted by corresponding input wavelength converters  208 ,  209 , then overhead-terminated by corresponding overhead terminating sections  211 ,  212 . Thus, they are input to the switching section  205  while their overhead is removed. The over head information stored into the overhead is given away to the switch controller  206  to control the switching section  205 . 
     On the other hand, optical signal  171   i  to be output to the west side is first output from the switching section  205 , then input to an overhead generator  214 , where the overhead information sent from the switch controller  206  is, as an overhead, added to the signal. Then, it is input to an output wavelength converter  215 , and is then output as the optical signal  171   i  to wavelength λ i . 
     In like manner, optical signal  172   i  to be output to the west side is first output from the switching section  205 , then input to an overhead generator  216 , where the overhead information sent from the switch controller  206  is, as an overhead, added to the signal. Then, it is input to an output wavelength converter  217 , and is then output as the optical signal  172   i  to wavelength λ i . 
     The same applies to the east side. Namely, optical signal  173   i  to be output to the east side is first output from the switching section  205 , then input to an overhead generator  218 , where the overhead information sent from the switch controller  206  is, as an overhead, added to the signal. Then, it is input to an output wavelength converter  219 , and then is output as the optical signal  173   i  to wavelength λ i . Also, optical signal  174   i  to be output to the east side is first output from the switching section  205 , then input to an overhead generator  221 , where the overhead information sent from the switch controller  206  is, as an overhead, added to the signal. Then, it is input to an output wavelength converter  222 , and is then output as the optical signal  174   i  to wavelength λ i . 
     Thus, the optical ring device  131   i  is provided with the input and output wavelength converters  201 ,  202 ,  215 ,  217 ,  208 ,  209 ,  219  and  222 . Therefore, it can convert a demultiplexed wavelength input from corresponding one of four optical fibers  141  to  144  into arbitrary one and then input it to the switching section  205 , and it can output converting the wavelength of optical signal output from the switching section  205  into arbitrary wavelength. Namely, optical signal can be input/output changing the wavelength of optical signal to coincide with a wavelength that is already used by an existing node. Also, by changing input wavelength range into further narrowed wavelength range, narrow-band light necessary for wavelength multiplexing can be obtained. 
     The switching section  205  is further connected with two overhead terminating sections  231 ,  232  and two overhead generating sections  233 ,  234 . First and second tributary side signals  191   1 ,  191   2  are input to the overhead terminating sections  231  and  232 , respectively. After incurring the overhead processing, they are input to the switching section  205 . The processed overhead information is sent to the switch controller  206 . Signals input from the switching section  205  to the overhead generating sections  233 ,  234  are provided with overheads based on the overhead information sent from the switch controller  206 , and then are output as third and fourth tributary side signals  191   3 ,  191   4 , respectively. 
     The protection operation in occurrence of failure etc. is conducted by the switch controller  206  to control the switching section  205 . Namely, the switch controller  206  collects information of failure from the respective overhead terminating sections  203 ,  204 ,  211 ,  212 ,  231  and  232 , and then, according to the content, switching the tributary side signal  191  and the west side or east side signal, it allows optical signal to be transmitted avoiding the position of failure. In this embodiment, the optical ring device  131  is provided to each wavelength. Therefore, the optical ring device  131  to each wavelength adds overhead information for conducting the switching control to the corresponding overhead generating sections  214 ,  216 ,  218 ,  221 ,  233  and  234 . This allows the optical ring device  131  in the other node to conduct the switching control. 
     The control operation of the switch controller  206  in this optical ring system thus composed will be explained below, taking cases of normal state and failure. 
     [Switching Control In Normal State] 
     FIG. 9 shows a connection pattern of the switching section  205  in normal state that no failure occurs. In this state, failure of wavelength to conduct the add/drop processing of signal line in the optical ring system is not detected. “add” means to set signal path in the optical ring system so that signal received on the tributary side is transmitted to the neighboring optical ring system. “drop” means to set signal path in the optical ring system so that signal received from the neighboring optical ring system is transmitted to the tributary side. In this state, where no failure is detected, a pair  251  of first and third tributary side signals  191   1 ,  191   3  is, as shown in FIG. 9, connected to the west-work side optical fibers  141 ,  142 . Also, a pair  252  of second and fourth tributary side signals  191   2 ,  191   4  is connected to the east-work side optical fibers  141 ,  142 . 
     In this connection pattern, for example, first tributary side signal  191   1  is input to the switching section  205  after the overhead is removed by the overhead terminating section  231 , then provided with an overhead by the overhead generating section  214 , wavelength-converted by the output wavelength converter  215 , multiplexed by the wavelength multiplexing section  161 , output to the optical fiber  141 . 
     The two west-protection side optical fibers  143 ,  144  shown by dotted lines in FIG. 9 are not connected by the switching section  205 . The same applies to the two east-protection side optical fibers  143 ,  144 . 
     Although in FIG. 9 the switching section  205  at one node is shown,the ring protection is, as shown in FIG. 5, formed by disposing this at each node except the through node. 
     [Switching Control (example 1) In Failure] 
     FIG. 10 shows a first example of connection pattern of the switching section  205  in a case that a failure in communication is detected. In the first example, failure  261  occurs not only at the west-work side optical fibers  141 ,  142  but also at the west-protection side optical fibers  143 ,  144 . The overhead terminating sections  203 ,  204  in FIG. 8 each are detecting the failure. 
     When the switch controller  206  in FIG. 8 receives overhead information to indicate the occurrence of communication failure on the west-work and west-protection sides, it controls the switching section  205  to have the connection pattern shown in FIG.  10 . Namely, a pair  251  of first and third tributary side signals  191   1 ,  191   3  is connected switching from the west-work side optical fibers  141 ,  142  to the east-protection side optical fibers  143 ,  144 . Then, the switch controller  206  sends this switching execution information to the overhead generating section  221  so as to write it into the overhead of optical signal  174  in FIG.  8 . 
     In FIG. 10, shown is the case that the failure occurs on both the west-work side and the west-protection side. However, the same applies to a case that a failure occurs on both the east-work side and the east-protection side. Namely, in this case, a pair  252  of second and fourth tributary side signals  191   2 ,  191   4  is connected to the west-protection side optical fibers  143 ,  144  instead of being connected to the east-work side optical fibers  141 ,  142 . Also in this case, the overhead terminating sections  211 ,  212  in FIG. 8 each are detecting the failure, the switch controller  206  conducts the switching control based on this detection. In response to this, the switching section  205  conducts the above switching. Also in this case, the switch controller  206  sends this switching execution information to the overhead generating section  216  so as to write it into the overhead of optical signal  172  in FIG.  8 . This operation is called path-ring switch mode, where the recovery from failure can be made by switching signal to the direction opposite to the side that the failure is detected. 
     [Switching Control (example 2) In Failure] 
     FIG. 11 shows a second example of connection pattern of the switching section  205  in a case that a failure in communication is detected. In the second example, failure  262  occurs only at the west-work side optical fibers  141 ,  142 . The overhead terminating section  203  in FIG. 8 is detecting the failure. 
     When the switch controller  206  in FIG. 8 receives overhead information to indicate the occurrence of communication failure on the west-work side, it controls the switching section  205  to have the connection pattern shown in FIG.  11 . Namely, a pair  251  of first and third tributary side signals  191   1 ,  191   3  is connected switching to the west-protection side optical fibers  143 ,  144 . Then, the switch controller  206  sends this switching execution information to the overhead generating section  216  so as to write it into the overhead of optical signal  172  in FIG.  8 . 
     In FIG. 11, shown is the case that the failure occurs on the west-work side. However, the same applies to a case that a failure occurs on the east-work side. Namely, in this case, a pair  252  of second and fourth tributary side signals  191   2 ,  191   4  is connected to the east-protection side optical fibers  143 ,  144 . Also in this case, the overhead terminating section  211  in FIG. 8 is detecting the failure, the switch controller  206  conducts the switching control based on this detection. In response to this, the switching section  205  conducts the above switching. Also in this case, the switch controller  206  sends this switching execution information to the overhead generating section  221  so as to write it into the overhead of optical signal  174  in FIG.  8 . This operation is called path-span switch mode, where the recovery from failure can be made by switching signal to the protection side in the same direction as the work side that the failure is detected. Here, the protection side in the same direction means that it is switched to the west-protection side, for example, when a failure occurs on the west-work side. 
     FIG. 12 shows a connection pattern of the switching section  205  in a through node. In FIG. 5, as described earlier, in the through node  104  to wavelength λ i  and the through node  101  to wavelength λ j , in fact, the switching section  205  does not conduct the switching operation. In the through state, the switching section  205  connects a pair  251  of first and third tributary side signals  191   1 ,  191   3  to the west-work side optical fibers  141 ,  142 . Also, a pair  252  of second and fourth tributary side signals  191   2 ,  191   4  is connected to the east-work side optical fibers  141 ,  142 . Further, the two west-protection side optical fibers  143 ,  144  are directly connected to the corresponding two east-protection side optical fibers  143 ,  144 . 
     Therefore, such a connection control may be fixedly conducted by the switch controller  206  of optical ring device  131  in the through node, or by using only optical fibers without using any optical ring device  131 . 
     Second Embodiment 
     Meanwhile, in wavelength multiplexing network, a failure such as disconnection of optical fiber and malfunction of optical transmitter/receiver may occur. To cope with this, the protection function, as a failure-recovering means, explained in FIGS. 10 and 11 in the first embodiment is necessary for the optical ring system or optical ring network. An optical ring network equipped with the protection function can have a BWPSR (bidirectional wavelength switched ring) system. Point to detect a failure is a node to terminate a wavelength path. Therefore, failure can be detected by an optical ring device disposed in the optical ring network. 
     In BWPSR system, unit for switching of signal is literally a wavelength path. So, protection wavelengths are, in advance, provided to form a protection wavelength path to be used when a failure occurs in ring network, and are shared between multiple work wavelength paths. Such an optical ring network can be composed not only by four fibers but also by two fibers. For example, explanation below is given to a four-fiber ring. 
     FIG. 13 illustrates a four-fiber ring network for specific wavelength λ j  to which the BWPSR system is applied. This network is composed of first to third optical ring devices  301  to  303 , a first work-line fiber  321  and a first protection-line fiber  331  to be used for the clockwise data transfer among multiple nodes  311 ,  312 ,  313 , . . . , and a second work-line fiber  322  and a second protection-line fiber  332  to be used for the counterclockwise data transfer among multiple nodes  311 ,  312 ,  313 , . . . . 
     In the four-fiber ring BWPSR system, when a failure occurs, a node to each wavelength that terminates a work path where the failure occurs switches the path into a protection path, thereby the recovery from failure is conducted in unit of wavelength path. For example, in FIG. 13, concerned one of nodes  312 ,  314  and  316  to handle wavelength λ i  conducts the switching to protection path. 
     Thus, when one fiber in four-fiber ring where two wavelengths λ i  and λ j  are multiplexed as in the first embodiment in FIG. 5 incurs a failure, optical rings for wavelength λ i  and wavelength λ j  each conduct the recovery from failure. 
     FIG. 14 shows a case that in the second embodiment the first and second work-line fibers incur a failure between the second node and the third node. In FIG. 14, like the case in FIG. 13, there is also provided a four-fiber ring network to specific wavelength λ j . In FIG. 14, like parts are indicated like reference numerals used in FIG.  13  and their explanations are omitted herein. The failure on the first work-line fiber  321  is detected by the second node  312  that terminates the wavelength path. Also, the failure on the second work-line fiber  322  is detected by the fourth node  314  that terminates the wavelength path since the data transfer is conducted counterclockwise. 
     In this example, the failure occurs only on the first and second work-line fibers  321 ,  322 . Therefore, nodes  312 ,  314  to terminate the work path extending through the failure position come into the path-span switch mode, where the protection path is set in the same direction as the work path to recover the failure. Thus, when failure occurs only on the work path through the work line, it operates as a path-span switch to switch into the same direction as the path being set. 
     In contrast, FIG. 15 illustrates a case that not only the first and second work-line fibers but also the protection-line fibers incur a failure between the second node and third node. In FIG. 15, like the case in FIGS. 13 and 14, there is also provided a four-fiber ring network to specific wavelength λ j . In FIG. 15, like parts are indicated like reference numerals used in FIG.  13  and their explanations are omitted herein. 
     Thus, when all fibers  321 ,  322 ,  331  and  332  incur a failure between the second and third nodes, the second and third nodes  312 ,  314  to terminate the work path extending through the failure position detect the failure and come into the path-ring switch mode, where the protection path is set in the direction opposite to the work path to recover the failure. 
     Although in FIGS. 14 and 15 the recovery from failure is explained about specific wavelength λ j , such recovery can be independently conducted to each wavelength in optical ring devices (which are, in FIGS. 13 to  15 , shown only to wavelength λ j ) that are disposed to each wavelength. Also, when all fibers are interrupted at a specific section, i.e., when the work line and protection line are interrupted to all wavelengths, it operates as a path-ring switch to switch into the direction opposite to the work path being set. 
     Third Embodiment 
     FIG. 16 illustrates a case that two ring networks with different routes are combined. As explained in the first embodiment in FIG.  8 . when provided with dedicated optical ring devices  131  that have wavelength converter section such as input wavelength converters  201 ,  202  and output wavelength converters  215 ,  217 , the multiplexing of ring can be realized by combining multiple networks with different wavelengths. 
     In FIG. 16, ( a ) shows a first ring network using wavelength λ i . The first ring network is formed connecting first to fifth optical ring device  401  to  405  by four-fiber transmission line  411  for wavelength λ i . Here, the four-fiber transmission line  411  means collectively transmission lines with wavelength λ i  in four optical fibers  141  to  144  in FIG.  5 . 
     On the other hand, FIG.  16 ( b ) shows a second ring network using wavelength λ j . The second ring network is formed connecting first, third, fourth and sixth optical ring devices  401 ,  403 ,  404  and  406  by four-fiber transmission line  412  for wavelength λ j . Here, the four-fiber transmission line  412  means collectively transmission lines with wavelength λ j  in four optical fibers  141  to  144  in FIG.  5 . 
     FIG.  16 ( c ) illustrates a configuration that the first and second ring networks are combined each other. By this combination, two wavelengths λ i , λ j  make an entry into the first, third and fourth optical ring devices  401 ,  403  and  404 . However, the different wavelengths can be accommodated by the wavelength conversion to use the input wavelength converter and output wavelength converter provided for each wavelength. 
     FIGS. 17 and 18 illustrate the merit of an output wavelength converter, as an example, to be disposed in the optical ring device. Meanwhile, FIG. 17 shows a conventional operation that optical signal from each client is wavelength-multiplexed and then output to the optical fiber. In the conventional operation, output data obtained from client  501  is selected by a switch  503  in a SDH/SONET device (see FIG. 5)  502 , multiplexed on time axis by an optical signal multiplexer  504 , passing through an optical signal interface  505 , converted into a pre-assigned wavelength λ N  by a wavelength converter  512  in an optical wavelength multiplexing add/drop section  511 , multiplexed with the other wavelengths by a wavelength multiplexer  513 , output to optical fiber  514 . 
     In contrast with this, as shown in FIG. 18, the above embodiments of this invention have the composition that the wavelength converter is disposed in the optical ring device. Namely, output data obtained from client  501  is selected by a switch  512  in the optical ring device  521 , converted into a pre-assigned wavelength λ N  by a wavelength converter  522  without being multiplexed on time axis, multiplexed with the other wavelengths by the wavelength multiplexer  513 , output to optical fiber  514 . 
     Thus, in the embodiments of this invention, the output-side wavelength is made to be narrow-band by the wavelength converter  522  disposed in the optical ring device  521 . Therefore, it is not necessary to conduct the time-axis multiplexing, thereby the circuit composition until the output of signal can be simplified. Also, due to the simplified circuit composition, the protection topology of ring network can be constructed at a lower cost. 
     Although in the above embodiments the four-fiber ring connecting four optical fibers in the form of a ring is employed, the invention is not limited to such a composition. Alternately, two-fiber ring connecting two optical fibers may be applied, or a ring network formed by more than four optical fibers can be applied. 
     Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth.