Abstract:
The present invention relates to prevention of erroneous connection in an optical network and, in particular, it provides an optical communication node having route switching function as well as an optical network system using the same in a wavelength division multiplex (WDM) network system. There is provided an optical communication node that is connected to a specific optical path in an optical network, the optical communication node having: an external network that is placed thereunder; and an interruption part for interrupting connection between the optical network and the external network, wherein the interruption part interrupts the connection until a sequence of changing a route is completed when the route setting of the optical path is changed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical communication node and an optical network and, in particular, it relates to prevention of erroneous connection of optical communication nodes and optical networks having route switching functions in a wavelength division multiplex (WDM) network system. 
   2. Description of the Related Art 
   In recent years, as communication on the Internet, image transmission and the like becomes widespread, an WDM system that is suitable for large capacity and high speed data transmission using optical signals has been introduced. First, the WDM system has been introduced to long distance networks where the WDM system had much economic merit. At present, as its installation cost has been reduced due to the maturity of the technology, the WDM system is being introduced also in intracity core rings. 
     FIG. 1  shows an example of a metro ring network. 
   Conventionally, intra-company LANs between buildings and a metro-oriented system between suburb systems have been oriented to a ring network. As shown in  FIG. 1 , a metro domain is divided into one domain that is comprised of metro access networks A and B close to the subscriber side, such as inter-building connection in urban districts, and the other domain that is comprised of a metro core network (that is also referred to as a metro interoffice, or a metro IOF) that interconnects the metro access networks in the urban districts or is connected to a long distance network. 
   Today, WDM is applied also to the metro access network, wherein a passive OADM (optical add drop multiplexing) method is the mainstream method. 
     FIGS. 2A and 2B  show an example of the metro access network using the passive OADM method.  FIG. 2A  shows an example of a two fiber ring network that is comprised of 4 nodes A-D, while  FIG. 2B  shows an example of a one-way intra-device configuration in each of the nodes A-D. 
   In  FIG. 2B , among WDM signals of 32 wavelengths (λ 1 -λ 32 ) that are transmitted on a fiber  10  in any one direction, a band signal having wavelengths λ 1 -λ 3  is dropped by a fixed wavelength filter  11  such as a dielectric multilayer filter, and then demultiplexed into light signals of each wavelength λ 1 -λ 3  by an optical branching filter  12  at the next stage. After undergoing processes such as optical amplification and wavelength conversion by corresponding transponders  13 - 15 , each wavelength signal λ 1 -λ 3  is output to an external network device such as a router installed in the node. 
   On the other hand, a signal that is input from the external network is converted into wavelength signals λ 4 -λ 6  by the corresponding transponders  23 - 25 , respectively. These are multiplexed into a signal of wavelengths λ 4 -λ 6  by an multiplexer  22  and then added to the WDM signal of 32 wavelengths (λ 1 -λ 32 ) transmitted on the fiber  10  via a fixed wave length filter  21 . 
   As described above, according to the passive OADM method, any light signal of a particular wavelength or wavelength group can be readily added to or dropped from the WDM signal. However, since each node uses dedicated parts such as the fixed wavelength filters  11  and  21  that are adjusted for the corresponding devices, a signal path route (the wavelength or wavelength group to be used) assigned for each node is fixed. 
   As a result, in network design according to the passive OADM method, it is necessary to determine transmission routes and transmission capacity in advance and dispose filters for the determined conditions. Further, when the transmission routes or the transmission capacity are changed after the service is started, it is necessary to disconnect the network and terminate the service, and then add filters that correspond to new conditions. 
   In the future, it is expected that optical WDM ring networks will be introduced into the metro core network that is located between the access networks and the long distance networks. As shown in  FIG. 1 , the metro core network is based on the ring configuration because of the ease of the protection technique and large-scale networks are constructed by connecting the ring configurations in a multistage manner. Also, the metro core network is connected to the rings of the metro access networks such as the passive OADM method as shown in  FIG. 2A  using unoccupied nodes, or is connected to the long distance networks. 
   In such a case, when data signals concentrated from the access networks are exchanged between the nodes, or line capacity is increased so as to connect to the long distance networks, for example, a technique for changing and switching line routes becomes important. It is expected that the paths (line routes) will be changed more frequently in the future and the switching will have to be performed automatically for the three reasons described below:
         (1) As the network size and demand for the network is increased, the line route setting will be changed more frequently,   (2) In order to provide IP over WDM, a protection feature will be provided at the side of the WDM system. In this case, due to bit-rate independence, it is necessary to perform wavelength switching in an optical layer and complete the switching within a short time (50-100 ms) corresponding to a SONET (Synchronous Optical Network) ring, and   (3) In the future, as a wavelength time sharing service is provided, the line route will be switched more frequently. Also in this case, the switching must be completed within the short time mentioned above.       

   In consideration of the above facts, it is necessary to support features for flexibly accommodating disconnection of networks, suspension of service and the like due to the change of transmission routes or transmission capacity of the metro access networks and for performing switching of connection destinations remotely while preventing erroneous connection. 
     FIG. 3  schematically describes operation of a ring network having path protection features according to an OSPPR (Optical Shared Protection Path Ring) method. 
   In  FIG. 3 , one signal is transmitted on one fiber of a two fiber ring in a clockwise direction and the other signal is transmitted on the other fiber in a counterclockwise direction. Here, a group of wavelength signals transmitted on each fiber is divided into work signals and protection signals. For example, in the group of the wavelength signals in the clockwise direction, even wavelengths are assigned to the work signals and odd wavelengths are assigned to the protection signals. On the other hand, in the group of the wavelength signals in the counterclockwise direction, odd wavelengths are assigned to the work signals and even wavelengths are assigned to the protection signals. 
   In normal communication, a transmitting end node A transmits a signal to a receiving end node D on a work path λ x  in the clockwise direction shown in a solid line in the figure by using the work signal. The receiving end node D receives the signal by selecting the work path. On the other hand, the corresponding protection path λ x  in the counterclockwise direction, that is shown as a dotted line in the figure, is idle. 
   Here, if any line failure, such as a break of the line, occurs in the work path, the transmitting end node A switches the path to the side of the protection path to continue transmission of the signal. After that, the receiving end node D also switches the path to the side of the protection path to continue reception of the signal. Here, it is to be noted that the signal interrupted by the line failure and the like must be recovered within 50-100 ms by the protection action. 
   In this connection, so as to improve operational efficiency of such configuration, path sharing is typically implemented by providing a signal PCA (protection channel access) path having a lower priority appropriately on the route for protection. In this case, when the failure occurs at the work side, the PCA signal is stopped at the protection side and a protection signal having a higher priority is inserted. 
   The line route switching occurs frequently and not only when the failure of the line route occurs but also when the line is operated normally due to the wavelength time-sharing service and the like. 
   In the switching of the line route setting described above, a network management system must manage the switching procedure and the switching timing and provide instructions appropriately but, conventionally, as shown below, there have been problems in that the erroneous connection of the paths might occur when the line route setting was switched, or operation of the instruction system for preventing such erroneous connection might be delayed. 
     FIGS. 4A and 4B  show an example of the erroneous connection that may occur at the time of protection operation. 
     FIG. 4A  shows a normal communication condition. Here, a signal is transmitted in a counterclockwise direction from a transmitting end node D to a receiving end node B via a work path λ 1 , and a PCA signal having a lower priority is transmitted in a clockwise direction from a transmitting end node A to a receiving end node C via its corresponding protection path λ 1 . 
     FIG. 4B  shows a case wherein a line failure such as a break of a fiber occurs in the work path λ 1  between the node C and the node D. In this case, the line route is switched to the protection route λ 1  by the protection operation. As a result, the signal from the transmitting end node D is transmitted to the receiving end node B through the node A in the clockwise direction. The relay node A terminates transmission of the own PCA signal and passes the signal from the transmitting end node D, and the receiving end node B switches the receiving route so as to receive the signal from the protection path λ 1 . 
   In the case described above, relative switching timing of the line routes between the nodes A-D, or switching sequence of them will become a problem. When the switching is started from the receiving end node B, after the clockwise route is switched from the through mode to the drop mode at the node B and until the relay node A stops the transmission of the PCA signal and then is switched to the through mode, the PCA signal from the node A is erroneously connected to the receiving end node B and output to an external network. 
   On the contrary, when the switching is started from the transmitting end node D, the transmitting end node D first switches the line route to the clockwise direction. Then, after the relay node A stops the transmission of the PAC signal and then switches the route to the through mode and until the receiving end node B switches the route in the clockwise direction from the through mode to the drop mode, the signal from the transmitting end node D is erroneously connected to the node C that is the receiving end node of the PCA signal and output to the external network. 
     FIG. 5  shows an exemplary node configuration for describing the action of the erroneous connection mentioned above more specifically. 
   First, the node configuration in  FIG. 5  will be described briefly. A WDM signal input from an optical fiber  31  in a clockwise route is amplified by an optical preamplifier  37 , and then demultiplexed into each wavelength signal (λ 1 -λ n ) to be output by an optical branching filter  38 . Further, a portion of the input signal is input to an optical supervisory channel (OSC)  33  by an optical branching filter  32 . 
   The optical supervisory channel  33  converts the input signal into an electric signal to give it to a processing/controlling section  34  at the next stage. The processing/controlling section  34  checks communication conditions of the optical fiber  31  in the clockwise route relying on the input signal and the like, and if there is any failure, performs switch control inside the node and the like. Further, a pilot signal and others to be given to the node at the next stage are output via an optical supervisory channel  35  and an optical multiplexing section  36  at the output side. 
   Next, there will be given a description of the switching action in the node, wherein a work signal λ 1  (w) in the clockwise direction that is demultiplexed by the optical branching filter  38  is input to a 2×2 switch  39 . If the 2×2 optical switch  39  is set to an add/drop mode, it drops the input work signal λ 1  (w) and outputs it to an external network  59  via an optical coupler  45  at the next stage and transponders  47  and  48  in a redundant configuration. 
   On the other hand, a signal from the external network  59  is input via either one of transponders  51  or  52 , which are configured redundantly, and a 1×2 optical switch  50  to the 2×2 switch  39 , which, in turn, adds the signal and outputs it as one wave in a WDM signal to the optical fiber  31  via an optical attenuator  40 , an optical multiplexer  41 , an optical postamplifier  42  and an optical multiplexer  36  at the next stage. 
   Alternatively, if the 2×2 optical switch  39  is set to a through mode, it passes the input work signal λ 1  (w) and outputs it to the optical fiber  31  in the clockwise direction as one wave in the WDM signal via the optical attenuator  40 , the optical multiplexer  41 , the optical postamplifier  42  and the optical multiplexer  36  at the next stage. Its corresponding protection signal λ 1  (p), that is input from an optical fiber  43  in a counterclockwise route, is handled similarly by a 2×2 optical switch  44 . 
   Here, in the lower central part of the figure, it is to be noted that there is also shown an interface with the external network  59  of a direct connection type wherein the 2×2 optical switch is connected to the external network  59  directly, in place of the one of a transponder type wherein an optical signal is once converted into an electric signal and then converted into a predetermined optical signal. In this case, a WDM optical transmitter/receiver is incorporated into network devices out of the ring. 
   Next, based on the premise of the node configuration described above, the process of the erroneous connection shown in  FIG. 4B  will be described more specifically. As a result of the line failure occurring between the node C and the node D, if the receiving end node B switches the setting of the 2×2 optical switch  44  from the through mode to the add/drop mode to turn on an optical coupler  46 , during the time when the relay node A switches the 2×2 optical switch  44  from the add/drop mode to the through mode, the PCA signal from the node A is output to the external network  59  of the node B. 
   On the other hand, if the transmitting end node D sets the 2×2 optical switch  44  to the add/drop mode and turns on the 1×2 optical switch  49  to start transmission in the clockwise line route, and then the relay node A switches the 2×2 optical switch  44  from the add/drop mode to the through mode, during the time when the receiving end node B switches the 2×2 optical switch  44  from the through mode to the add/drop mode, the signal from the node D is output to the external network  59  of the node C. 
   Further, if each node is instructed on switching procedures from the network management side in order to prevent the erroneous connection described above, there is a problem in that a load at the network management side may become excessively large and the time that is necessary for switching the line route in the entire node may be prolonged. 
   SUMMARY OF THE INVENTION 
   Therefore, in view of the above problem, it is an object of the present invention to provide a WDM ring network system that can prevent erroneous connection from a drop path of a node to an external network at the time of switching of a line route. 
   Further, it is another object of the present invention to provide a WDM ring network system that enables quick control and direction of network management in path switching and satisfies communication quality required for the ring network system that becomes increasingly large-scale. 
   In the large-scale WDM ring network system such as a metro core ring network, it enables support of prevention of the erroneous connection at the time of line route switching, and features for protection and for switching connection destinations remotely in connection with wavelength time sharing service that have been supported by a conventional SONET ring. 
   According to the present invention, there is provided an optical communication node that is connected to a specific optical path in an optical network, said optical communication node having: an external network that is placed thereunder; and an interruption means for interrupting connection between said optical network and said external network, wherein said interruption means interrupts the connection till a sequence of changing a route is completed when the route setting of said optical path is changed. 
   Said interruption means further interrupts the connection or releases the interruption in response to instructions from other optical communication nodes. Said optical communication node may further have an adding means for adding a receiving end node identifier to a transmitted signal, and a detecting means for detecting said receiving end node identifier included in a received signal, wherein said detecting means releases the interruption by said interruption means when said detecting means detects said receiving end node identifier of its own node. 
   Said optical communication node may further have a dummy signal means for generating and outputting a dummy signal, wherein said dummy signal means sends the dummy signal to said optical path and/or said external network to which signal transmission is stopped by said interruption means. 
   Still further, according to the present invention, there is provided an optical network system that interrupts at least one signal either from an external optical network to the input side of said optical network or from said optical network to the output side of the external optical network till a sequence of changing a route is completed when the route setting of an optical path is changed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below with reference to the accompanying drawings. 
       FIG. 1  is a diagram showing an example of a metro ring network; 
       FIG. 2A  is a diagram showing an example of a metro access network that is comprised of 4 nodes A-D according to a passive OADM method; 
       FIG. 2B  is a diagram showing an example of a one-way intra-device configuration in each of the nodes A-D shown in  FIG. 2 ; 
       FIG. 3  is a diagram for schematically describing operation of a ring network having path protection features according to a BPSR method; 
       FIG. 4A  is a diagram showing an example of a state before erroneous connection occurs at the time of protection switching; 
       FIG. 4B  is a diagram showing an example of a state after the erroneous connection occurs at the time of the protection switching; 
       FIG. 5  is a diagram showing an exemplary ring node configuration; 
       FIG. 6  shows a first embodiment of the present invention; 
       FIG. 7  is a diagram showing an exemplary line switching sequence in the first embodiment of the present invention; 
       FIG. 8  is a diagram showing an example of a specific circuit configuration ( 1 ) in the first embodiment; 
       FIG. 9  is a diagram showing an example of another circuit configuration ( 2 ) in the first embodiment; 
       FIG. 10  is a diagram showing a second embodiment of the present invention; 
       FIG. 11  is a diagram showing an exemplary line switching sequence in the second embodiment of the present invention; 
       FIG. 12  is a diagram showing an example of a specific circuit configuration ( 1 ) in the second embodiment; 
       FIG. 13  is a diagram showing an example of another circuit configuration ( 2 ) in the second embodiment; 
       FIG. 14  is a diagram showing a third embodiment of the present invention; 
       FIG. 15  is a diagram showing an exemplary signal format using digital wrapper technology; 
       FIG. 16  is a diagram showing an example of a specific circuit configuration ( 1 ) at the side of a transmitting end node in the third embodiment; 
       FIG. 17  is a diagram showing an example of another circuit configuration ( 2 ) at the side of the transmitting end node in the third embodiment; 
       FIG. 18  is a diagram showing an example of a specific circuit configuration ( 1 ) at the side of a receiving end node in the third embodiment; 
       FIG. 19  is a diagram showing an example of another circuit configuration ( 2 ) at the side of the receiving end node in the third embodiment; 
       FIG. 20  is a diagram showing an exemplary configuration of a pilot transmitting section; 
       FIG. 21  is a diagram showing an exemplary configuration of a pilot signal receiving section; 
       FIG. 22  is a diagram showing an example of a mesh network configured by using optical cross connect devices; 
       FIG. 23  is a diagram showing an exemplary line route switching of the mesh network when a failure occurs; and 
       FIG. 24  is a diagram showing an exemplary configuration of the optical cross connect device. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 6  shows a first embodiment of the present invention. 
   In  FIG. 6 , each of the nodes D, A and B on a line route that is switched when any failure occurs is provided with each of the interruption means  63 ,  62  and  61  for preventing a signal from being output to the outside, respectively. 
     FIG. 7  shows an exemplary line switching sequence in the first embodiment of the present invention. In this example, the line switching sequence is started from the receiving end node B at the downstream side that detects a failure first. 
   In step S 101  of  FIG. 7 , when the receiving end node B detects an abnormality of a work signal λ 1  (w), the interruption means  61  interrupts the work signal λ 1  (w) as well as its corresponding protection signal λ 1  (p) output from each drop path to an external network  59 . In this case, the node B either outputs a dummy signal such as an AIS (alarm indication signal) to the external network  59 , or outputs no signal. 
   Then, the line route is switched from the work side to the protection side to switch intra-device paths from a through mode to an add/drop mode. It prevents erroneous connection to the node C in subsequent processes. Then, a switching signal is transmitted to each upstream relay node residing on the line route at the protection side (the node A in this example), and an APS (auto protection switching) that is a switching control signal is transmitted to the transmitting end node D. Here, an optical supervisory channel (OSC) is used for transferring signals for path switching between the nodes. 
   In step S 102 , according to the switching signal received by each relay node, the intra-device paths are switched, if necessary. At this time, the node A interrupts connection with the external network  59  temporarily by means of the interruption means  62  so as to avoid signal leakage when the intra-device path is switched. In this example, the relay node A switches the clockwise route from the add/drop mode to the through mode. Further, it notifies the transmitting source node B of such switching. 
   In step S 103 , when the transmitting end node D receives the APS signal, the node D interrupts connection with the external network  59  temporarily by using the interruption means  63  so as to avoid leakage when the intra-device path is switched. Then, the signal transmission route is switched from the counterclockwise route at the work side to the clockwise route at the protection side. Then, the clockwise route is set to the add/drop mode. 
   In step S 104 , the transmitting end node D notifies the receiving end node B of completion of the path setting. In step S 105 , after the receiving end node B confirms receipt of a normal switching signal from each relay node (the relay node A in this example) and a path setting completion signal from the transmitting end node D, said interruption means  61  is released so as to permit the protection signal of a wavelength λ 1  (p) to be output from the drop path to the external network  59 . As described above, the interruption means  61  in the receiving end node B plays an important role in preventing the erroneous connection in this example. 
     FIG. 8  shows an example of a specific circuit configuration ( 1 ) in the first embodiment. 
   In  FIG. 8 , in a supervisory control section for optical signals, in addition to an optical supervisory channel  33  at the input side, a processing/controlling section  34 , and an optical supervisory channel  35  that are similar to the ones in the conventional case, a control section  71  is provided newly for controlling the interruption means of the present invention based upon the switching control signal through the supervisory signal from the upstream supervisory control section. 
   Further, in each of transponders  47  and  48  at the drop side, an AIS generating section  72  and a switching section  73  that constitutes the interruption means of the present invention are newly provided. The AIS generating section  72  generates a dummy signal that consists of an alternating pattern of “1” and “0” values, and the switching section  73  selects a signal from either the conventional signal processing section or the AIS generating section  72  and outputs it to the external network  59 . Here, when the switching section  73  selects the side of the AIS generating section  72 , the connection to the external network  59  is interrupted. 
   With reference to the operation of the first embodiment, when a failure occurs in the counterclockwise line route, the signal processing section  34  detects the failure and the control section  71  controls the switching section  73  to select the side of the AIS generating section  72 . Then, due to control by a 1×2 optical switch  53 , a signal that is dropped from a 2×2 optical switch  44  at the work side is switched to a signal that is dropped from a 2×2 optical switch  39  at the protection side. 
     FIG. 9  shows an example of another circuit configuration ( 2 ) in the first embodiment. 
   In  FIG. 9 , instead of the transponders including the AIS generating sections  72  and the switching sections  73  shown in  FIG. 8 , simple shutter cards  74  and  75  only for controlling passage/interruption of optical signals are used. Since the shutter cards are used in this example, the present invention can also be applied to the direct connection configuration shown in  FIG. 5 . 
   A 1×2 optical switch or an optical attenuator section  77  in the shutter card  74  interrupts a signal to the external network  59  in response to an instruction from the controlling section  71 . Further, by connecting a transmitter for generating an AIS signal to one side of the 1×2 optical switch  77 , a dummy signal may be output to the external network  59  at the time of interruption. The first embodiment operates similarly to the case shown in  FIG. 8 . 
     FIG. 10  shows a second embodiment of the present invention. 
     FIG. 11  shows an exemplary line switching sequence in the second embodiment of the present invention. In this example, until a predetermined route setting is completed, a transmitting end node suspends output of a signal, that passes through the route switching section and has a wavelength, to be switched. 
   In step S 201  in  FIG. 11 , when a receiving end node B detects abnormality of a work signal λ 1  (w), an interruption means  61  interrupts the work signal λ 1  (w) as well as its corresponding protection signal λ 1  (p) output from each drop path to an external network  59 . In this case, the node B outputs a dummy signal such as an AIS signal to the external network  59 . 
   Here, it is to be noted that it is not absolutely necessary that the interruption means  61  interrupts the signal λ 1  to the external network  59  in this example. Then, a request to stop transmission is sent to the transmitting end node of the signal to be dropped at the node B, and to the transmitting end node of the signal passing through the node B. In this example, the request to stop the transmission is sent to the transmitting end nodes D and A. An optical supervisory channel (OSC) is used for transferring control signals between the nodes. 
   In step S 202 , the transmitting end node D, and the relay node A that is disposed between the receiving end node B and the transmitting end node D and sends an PCA signal, receive said request to stop the transmission. The nodes D and A either interrupt the light transmitted from themselves by means of the respective interruption means  63  and  62 , or transmit the dummy signal instead. Then, the nodes D and A notify the receiving end node B of completion of the process to stop the transmission. 
   Further, though not shown in  FIG. 10 , when the nodes D and A further have another transmitting end node that transmits a signal to be dropped, in or passing through the nodes D and A themselves, the nodes D and A send a request to stop the transmission to the another node and receive notification of completion of the process to stop the transmission following the procedure similar to the method described above. In this case, after the nodes D and A receive the notification of completion of the process to stop the transmission from another node, the receiving end node B is notified of completion of the process to stop the transmission in said nodes D and A. 
   In step S 203 , the receiving end node B, which has received the notification of completion of the process to stop the transmission from the relay node A and the transmitting end node D, performs route switching in itself, and at the same time, instructs the nodes A and D to switch the route sequentially. In this example, as each of the nodes A, B and D performs route switching on the line route where there is no transmission signal or dummy signal, erroneous connection does not occur at the time of the route switching, as a matter of course. 
   In steps S 204  and S 205 , the receiving end node B, which has received the notification of completion of the route switching from the relay node A and the transmitting end node D, releases the interruption means  61  in itself, and at the same time, notifies the nodes A and D of release of the request to stop the transmission. As a result, the interruption means  63  and  62  are released and the transmitting end node D starts signal transmission. As described above, the interruption means  63  and  62  in the transmitting end nodes D and A play an important role in preventing the erroneous connection in this example. 
     FIG. 12  shows an example of a specific circuit configuration ( 1 ) in the second embodiment. 
   In  FIG. 12 , a supervisory control section for optical signals is configured similarly to the ones in  FIGS. 8 and 9 . In this embodiment, in each of transponders  50  and  52  at the add side, an AIS generating section  72  and a switching section  73  that constitutes the interruption means of the present invention are newly provided. Further, the switching section  73  outputs an add signal that selects either a signal processing section or the AIS generating section  72 . Here, if the switching section  73  selects the AIS generating section  72 , it comes in the interrupted state. 
   With reference to the operation of the second embodiment, when a failure occurs in the counterclockwise line route, the signal processing section  34  in the receiving end node B detects it and sends an APS signal to the transmitting end node D. The signal processing section  34  in the transmitting end node D, in turn, detects it and the control section  71  controls the switching section  73  to allow the AIS generating section  72  to output a dummy signal. Then, the 2×2 optical switch  39  at the protection side is switched to the add/drop mode and said dummy signal is input as an add signal. 
     FIG. 13  shows an example of another circuit configuration ( 2 ) in the second embodiment. 
   Just as in the case of  FIG. 9  described above, the configuration in  FIG. 13  differs from the one in  FIG. 12  only in that the transponders  50  and  52  at the add side are substituted by simple shutter cards  80  and  81  only for controlling passage/interruption of optical signals, wherein a 1×2 optical switch or an optical attenuator section  82  is also similar to the one in  FIG. 9 . As the shutter cards are used in this example, the present invention can also be applied to the direct connection configuration shown in  FIG. 5 , and this example operates similarly to the one shown in  FIG. 12 . 
     FIG. 14  shows a third embodiment of the present invention. 
   In the first and second embodiments described above, the optical supervisory channel (OSC) is used for transferring signals for path switching, and each node transfers the control signals independently therebetween so as to control the interruption means  61 - 63  and path switches in each node. Therefore, though the problem of the erroneous connection is solved by the first and second embodiments, when the network scale becomes larger or paths across a plurality of ring networks are provided, the time for switching the line route is increased, which may result in degradation of communication quality. 
   In this embodiment, this problem is solved by configuring so that the control of interruption means  61 - 63  can be performed easily and quickly. For such purpose, an identifier is predetermined for each node and a transmitting end node transmits a signal with the identifier included in the transmitted signal. The receiving end node reads the identifier included in the received signal and, if the signal is destined for the receiving end node itself, the receiving end node releases the interruption means to connect the drop path to an external network  59 . On the contrary, if the signal is not destined for the receiving end node itself, the receiving end node interrupts connection with the external network by means of the interruption means. 
     FIG. 15  shows an exemplary signal format using digital wrapper technology. Here, an OTU (optical channel transport unit) k frame structure is used, wherein the receiving end identifier of the present invention is set in each of SAPI (source access point identifier) and DAPI (destination access point identifier) locations in a header part of the OTUk frame structure and a transmitted signal is inserted into its data signal part. 
   For example, when the transponders are used, the transponder at the transmitting side performs coding using the digital wrapper technology to set the identifier of the receiving end node in said header part. The transponder at the receiving side performs decoding accordingly and outputs the received signal if the signal is destined for the receiving end node itself. If the signal is destined for the other node, the receiving end node either interrupts the output or outputs a dummy signal to the external network. 
   Further, as an alternative method other than the digital wrapper, when the transponders are used in the transmitting end node and the receiving end node, a descrambling pattern that is provided for each receiving end node by the signal processing section may be assigned as a unique pattern and used as said receiving end node identifier. In this case, by associating the scrambling pattern that is provided by the signal processing section of the transmitting end node with the scrambling pattern of the receiving end node, a similar process can be performed in a way easier than said digital wrapper. 
   In this connection, when  2 R transponders that do not perform retiming are used, or a WDM interface is provided at the side of network devices in the external network  59  to connect to the ring network directly without interposing the transponders (the direct connection configuration), the data signal itself cannot be manipulated such as by the digital wrapper technology described above and the like. In the present invention, in order to add receiving end identification information to the transmitted signal also in such case, a low speed pilot signal is superimposed on the main signal to be transmitted by amplitude modulation (AM), phase modulation (PSK) and the like. 
     FIG. 16  shows an example of a specific circuit configuration ( 1 ) at the side of a transmitting end node in the third embodiment. 
   According to this embodiment, in transponders  51  and  52  at the add side, a pilot signal transmitting section  85  is added to the conventional configuration. Instead of the digital wrapper signal shown in  FIG. 15 , the pilot signal transmitting section  85  adds the receiving end identification information to the transmitted signal as the low speed modulating signal (pilot signal) such as AM, PSK and the like so as to generate and transmit the modulated transmission signal. Here, it is to be noted that the supervisory control section for optical signals shown in the first and second embodiments are not shown since it is not concerned with the operation of this embodiment. 
     FIG. 20  shows an exemplary configuration of the pilot transmitting section. An amplitude modulation signal including the receiving end identification information is generated by a coder  92 , to which the information about the destination node (receiving end node) is input, and a low speed (f 0 ) oscillator  93 , and then superimposed on the transmission signal by an optical modulator  91 . The optical modulator is selected according to the modulation method: for example, when an amplitude modulation signal is superimposed, a MZ interference type modulator or an optical attenuator is used. Further, the signal is passed through a plurality of rings, the pilot signal may be terminated once at a junction between the rings and another pilot signal that is assigned to the termination node of the next ring may be superimposed again. 
   With reference to the operation of the third embodiment, the transmitting end node D transmits a signal in which a pilot signal including identifier information of the receiving end node B is superimposed on a transmission signal from the external network  59 . In this embodiment, said pilot signal is added to the 2×2 optical switch  39  via the 1×2 optical switch  49  and output to the clockwise line route. 
     FIG. 17  shows an example of another circuit configuration ( 2 ) at the side of the transmitting end node in the third embodiment. 
   In  FIG. 17 , shutter cards  80  and  81  are used instead of the transponders. The pilot signal transmitting section  85  is also provided here, which operates similarly to the case shown in  FIG. 16 . This embodiment is suitable for the direct connection configuration shown in  FIG. 5 . 
     FIG. 18  shows an example of a specific circuit configuration ( 1 ) at the side of receiving end node in the third embodiment. 
   Though an AIS generating section  72  and a switching section  73  that constitutes the interruption means of the present invention are provided in each of the transponders  47  and  48  at the drop side in this embodiment, a pilot signal receiving section  84  for controlling the switching section  73  is further provided in this example. The pilot signal receiving section  84  decodes the pilot signal (modulation signal) superimposed on the received signal and, if the information about the receiving end identifier specifies the receiving end node itself, controls the switching section  73  to select the side of the signal processing section. As a result, the interruption state is released. On the contrary, if the information specifies the node other than the receiving end node itself, the pilot signal receiving section  84  selects the side of the AIS generating section  72  to enter or maintain the interrupted state. 
     FIG. 21  shows an exemplary configuration of the pilot signal receiving section  84 . 
   In  FIG. 21 , a portion of the input signal is branched by a branch means  94  such as a coupler and detected by a photo diode (PD)  95 . A signal generated in a mixer  97  by mixing the detected signal and a local oscillator frequency signal from an oscillator  96  that has the same frequency (f 0 ) as the one at the transmitting side is decoded to the destination node information (receiving end identifier) by an decoder  98 . A control circuit  99  compares the decoded value with an identifier of its own node and, if a match is found, controls the switching section  73  to release the interruption state. 
   With reference to the operation of the third embodiment, the receiving end node B decodes the pilot signal included in the drop signal from the 2×2 optical switch  39  in the clockwise line route by the pilot signal receiving section  84  in the transponders. From the decoded result, the pilot signal receiving section  84  recognizes that the receiving end identifier information specifies its own node, and therefore controls the switching section  73  to select the side of the signal processing section. On the other hand, the relay node A recognizes that the signal is not destined for the relay node A itself from said decoded result, and therefore controls the switching section  73  to select the side of the AIS generating section  72 . 
     FIG. 19  shows an example of another circuit configuration ( 2 ) at the side of the receiving end node in the third embodiment. 
   In  FIG. 19 , shutter cards  74  and  75  are used instead of the transponders. The pilot signal transmitting section  85  is also provided here, which operates similarly to the case shown in  FIG. 18 , other than in that the pilot signal transmitting section  85  performs control to interrupt the signal to the external network  59  by means of the 1×2 optical switch or optical attenuator  77 . With reference to the third embodiment, this example operates similarly to the case shown in  FIG. 18 . This embodiment is suitable for the direct connection configuration. 
   When the first to third embodiments are applied to the connection between a plurality of ring networks, the path switching sequence according to each of the embodiments may be performed independently in each of the ring networks. 
     FIGS. 22-24  show an embodiment in which the present invention is applied to a mesh network configured by using optical cross connect (OXC) devices. 
     FIG. 22  shows an example of the mesh network by using the optical cross connect devices  101 ,  FIG. 23  shows an exemplary line route switching of the mesh network when a failure occurs, and  FIG. 24  shows an exemplary configuration of the optical cross connect device  101 , respectively. 
   In the mesh network shown in  FIG. 22 , at the time of a break of a fiber or failure of a device, an alternative path route is retrieved from a database and path switching is performed. At this time, the search of the alternative path is started from optical paths that are not used and then optical paths having a lower priority. Here, though not shown, optical amplifiers may be inserted between the optical cross connect devices  101  as optical repeaters. Further, an optical ADM (OADM) device  102  is used for connecting with the external network  59 . 
   A switch  111  in the optical cross connect device  101  shown in  FIG. 24  is an optical matrix switch in a 1+1 redundant configuration branched by couplers  113 . Here, though transponders  112  and  114  are shown in the figure, shutter cards may be used instead of the transponders  112  and  114 . Further, even the shutter cards may not be provided and said switch  111  may act as the shutter cards. 
   In  FIG. 23 , the four optical cross connect devices  101  form logical ring nodes as nodes A-D, respectively. First, a case where the first embodiment of the present invention is applied to this network will be described. When a failure occurs between the node C and the node D due to a break of a fiber or failure of a device and the like, the connection to the downstream of the path to be switched in the receiving end node C is interrupted first. Next, the connection is switched to the nodes B and A at the upstream side successively. Then, the switching is performed in the most upstream node D and after the switching in all nodes is completed, the connection to the downstream side is released in the receiving end node C. 
   The interruption may be implemented by using the transponders  114  shown in  FIG. 24 , or the shutter cards  74  shown in  FIG. 9 , or equivalents, instead of the transponders  114 . Alternatively, the interruption may be implemented similarly by combining to a port having no connection in the matrix switch  111 . The above description can be applied to the case according to the second embodiment of the present invention. 
   In the case of the third embodiment of the present invention, as described above, an identifier may be included in a signal, or alternatively, a pilot signal may be used. First, the case in which the identifier is included in the signal will be described. In this example, the identifier that corresponds to an intra-office interface of a specific port in the receiving end node C is inserted at the transmitting side of an intra-office interface of the transmitting end node D. When the receiving end node C detects the identifier of the own node at the receiving side of the intra-office interface, it outputs the signal to the external network. Further, information about the node through which the signal is to be passed may be included, and in this case, when the relay node detects its abnormality, the signal may be interrupted at the corresponding node. 
   On the other hand, when the pilot signal is used, the configuration shown in  FIG. 16  or  FIG. 17  described above, which is the embodiment using the transponders or the direct connection, respectively, may be used at the transmitting side of the intra-office interface in the transmitting end node D, and the configuration shown in  FIG. 17  or  FIG. 18  may be used at the receiving side of the intra-office interface in the receiving end node C. If the receiving end node C does not receive a predetermined data signal including the receiving end identification information, the output to the external network is interrupted. 
   As described above, according to the present invention, a WDM network that enables high-speed route switching without occurrence of erroneous connection is provided. As a result, prevention of the erroneous connection at the time of line route switching, a feature for switching destinations remotely in wavelength time sharing service and the like can be provided in a metro core ring network and the like.