Abstract:
To realize a highly reliable monitor control signal network at low costs, a data transmission system comprises a first network to optically transmit a data signal and a monitor control signal, first and second nodes connecting to the first network, and a second network capable of transmitting the monitor control signal. The first node comprises a demultiplexer to demultiplex the data signal and the monitor control signal out of the signal light from the first network, a signal distributor having a plurality of input ports and a plurality of output ports to distribute the data signal demultiplexed by the demultiplexer into designated output port, a controller to control the signal distributor and to monitor each part in the first node, a first selector to select one of the monitor control signal demultiplexed by the demultiplexer and the other monitor control signal entered from the second network and to apply the selected one to the controller, a second selector to output the monitor control signal output for the second node output from the controller toward one of the first network and the second networks, and a multiplexer to multiplex the monitor control signal output for the first network output from the second selector and the data signal output from the signal distributor and to output the multiplexed signals onto the first network.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority to Japanese Patent Application No. 2002-094478, filed Mar. 29, 2002, the entire contents of which are incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    This invention generally relates to a data transmission system, a transmission method of an optical network monitor control signal, and a node.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a large-scale network, a system to remotely monitor each node connecting to the network is indispensable.  
           [0004]    As a configuration to transmit a monitor signal, a control signal, and a status signal (generically referred to hereinafter as a “monitor control signal”) between a monitor center and each node, one typical type is to multiplex a monitor control signal on a network to transmit a data signal itself, hereinafter referred to as a “signal network”, and the other is to provide another external network besides the signal network to transmit a monitor control signal through the external network. Such external networks used as a monitor control network comprise for example public networks and/or private networks.  
           [0005]    In an optical network to transmit a WDM optical signal of 10 Gb/s per wavelength, for example, a monitor control signal is 2 Mbit/s or 155 Mbit/s.  
           [0006]    Generally, each node comprises a cross connector or an add/drop multiplexer to transmit a signal with another node and with a transceiver terminal, and a controller having functions to transmit/receive a monitor control signal and to control the cross connector or the add/drop multiplexer according to the monitor signal or the control signal.  
           [0007]    In one configuration, a controller disposed on a specific node functions as a monitor center, and in the other configuration, a monitor center is provided besides a controller disposed on each node. In the latter, one possible case is to connect the monitor center with the controller of each node and the other possible case is to connect the monitor center with a controller of a specific node and to transmit/receive a monitor control signal to/from controllers of the other nodes through the controller of the specific node.  
           [0008]    Main four roles of the monitor control signal are (1) monitoring an alarm signal generated from each part and informing the monitor center when an alarm signal is detected, (2) monitoring continuity and normality of signal transmission between adjacent nodes, (3) crossconnect-controlling (provisioning) of an optical path, and (4) signaling about relief (protection/restoration) protocol when any optical path has a fault.  
           [0009]    A fault in the monitor control network gives a bad influence to the operation and maintenance of the signal network. Accordingly, the monitor control network needs to have the durability for fault and reliability equal to or even more than those of the signal network.  
           [0010]    When an external network is used as the monitor control network, the reliability of the monitor control network depends on that of the external network to be used. Furthermore, the monitor control signal depends on the paths, the path control, the bandwidth, and the state of convergence of the external network to be used. Therefore, such an external network is unsuitable for a swift supervision/control because not only it has a long delay time but also it is unstable.  
           [0011]    On the other hand, when a signal network is used as the monitor control network, namely in a case that a monitor control signal is transmitted through a signal network, a fault of the signal network becomes exactly a fault of the monitor control network. When fault occur in a plurality of links, several nodes are isolated in consequence. In such case, it is impossible to monitor and control the isolated nodes. Generally, a monitor control signal is assigned one wavelength and therefore it also leads to the waste of resources.  
         SUMMARY OF THE INVENTION  
         [0012]    A data transmission system according to the present invention comprises a first network to optically transmit a data signal and a monitor control signal, first and second nodes connecting to the first network, and a second network capable of transmitting the monitor control signal. In particular, the first node comprises a demultiplexer to demultiplex the data signal and the monitor control signal out of the signal light from the first network, a signal distributor having a plurality of input ports and a plurality of output ports to distribute the data signal demultiplexed by the demultiplexer to a designated output port, a controller to control the signal distributor and to monitor each part in the first node, a first selector to select one of the monitor control signal demultiplexed by the demultiplexer and the other monitor control signal entered from the second network and to apply the selected one to the controller, a second selector to output the monitor control signal for the second node output from the controller toward one of the first network and the second network, and a multiplexer to multiplex the monitor control signal for the first network output from the second selector and the data signal output from the signal distributor and to output the multiplexed signals for the first network.  
           [0013]    With the above configuration, a signal network (the first network) transmits the monitor control signal in the normal operation while an external network (the second network) transmits the monitor control signal when the signal network has a fault and therefore the system can keep even more reliability for the monitor control signal compared to that for a data signal. Transmission delay of the monitor control signal also becomes minimal. Furthermore, since it is not necessary to enhance the reliability of the external network as much as that of the signal network, the signal network can be monitored and controlled at low costs.  
           [0014]    A data transmission system according to the present invention comprises a first network to optically transmit a data signal and a monitor control signal, first and second nodes connecting to the first network, a first data transceiver terminal to transmit/receive a data to/from the first node, a second data transceiver terminal to transmit/receive a data to/from the second node, and a second network capable of transmitting the monitor control signal. In particular, the first node comprises a signal distributor having a demultiplexer to demultiplex the data signal and the monitor control signal out of the signal light input from the first network, a signal distributor having a plurality of input ports including first and second monitor control signal input ports and a plurality of output ports including first and second monitor control signal output ports to distribute the data signal and the monitor control signal demultiplexed by the demultiplexer into respective designated output ports, a controller to control the signal distributor and to monitor each part in the first node, a first selector to select one of the monitor control signal output from the first monitor control signal output port of the signal distributor and the other monitor control signal input from the second network and to apply the selected one to the controller, a second selector to output the monitor control signal for the second node output from the controller toward one of the first monitor control signal input port of the signal distributor and the second network, and a multiplexer to multiplex the monitor control signal from the second monitor control signal output port of the signal distributor and the data signal output from the signal distributor and to output the multiplexed signals to the first optical network. The signal distributor connects the first monitor control signal input port to the second monitor control signal output port and the second monitor control signal input port to the first monitor control signal output port when the first network is normally operating.  
           [0015]    The above configuration makes it possible that a signal network (the first network) transmits a monitor control signal in normal operation while an external network (the second network) transmits the monitor control signal when the signal network has a fault, and therefore the system can keep even more reliability for a monitor control signal than that for a data signal. Transmission delay of a monitor control signal also becomes minimal. Moreover, it is not necessary to strengthen the reliability of an external network as much as that of the signal network, and thus a signal network can be monitored/controlled at low costs. Furthermore, since a monitor control signal is distributed by a signal distributor, a channel change in the signal network becomes easy. At the same time, continuity can be checked with a loop back test and a fault part can be cut off easily.  
           [0016]    A transmission method according to the present invention is a method to transmit a monitor control signal for a signal network to optically transmit a data signal and a plurality of nodes connecting to the signal network, comprising steps of transmitting the monitor control signal through the signal network in normal operation and transmitting the monitor control signal through an external network when the signal network has a fault.  
           [0017]    The above configuration makes it possible that an optical network transmits a monitor control signal in normal operation while an external network transmits the monitor control signal when the optical network has a fault, and thus the system can keep even more reliability for the monitor control signal than that for the data signal. Transmission delay of a monitor control signal also becomes minimal. Moreover, it is not necessary to strengthen the reliability of the external network as much as that of the optical network, and so the signal network can be monitored/controlled at low costs.  
           [0018]    A node according to the present invention connects a first network to optically transmit a data signal and a monitor control signal and a second network capable of transmitting the monitor control signal. The node comprises a demultiplexer to demultiplex the data signal and the monitor control signal out of the signal light input from the first network, a signal distributor having a plurality of input ports and a plurality of output ports to distribute the data signal demultiplexed by the demultiplexer to respective predetermined output port, a controller to control the signal distributor and to monitor each part, a first selector to select one of the monitor control signal demultiplexed by the demultiplexer and the other monitor control signal input from the second network and to apply the selected one to the controller, a second selector to output the monitor control signal for another node output from the controller toward one of the first network and the second network, and a multiplexer to multiplex the monitor control signal for the first network output from the second selector and the data signal from the signal distributor and to output the multiplexed signal for the first optical network.  
           [0019]    The above configuration makes it possible to switch over a signal network (a first network) and an external network (a second network) as the transmission path of the monitor control signal at low costs. The signal network transmits the monitor control signal in normal operation while the external network transmits the monitor control signal when the signal network has a fault, and thus the system can keep even more reliability for the monitor control signal than that for the data signal. Transmission delay of the monitor control signal also becomes minimal. Moreover, it is not necessary to strengthen the reliability of the external network as much as that of the signal network, and thus the signal network can be monitored/controlled at low costs.  
           [0020]    A node according to the present invention connects a first network to optically transmit a data signal and a monitor control signal and a second network capable of transmitting the monitor control signal. The node comprises a demultiplexer to demultiplex the data signal and the monitor control signal out of the signal light input from the first network, a signal distributor having a plurality of input ports including first and second monitor control signal input ports and a plurality of output ports including first and second monitor control signal output ports to distribute the data signal and the monitor control signal demultiplexed by the demultiplexer to respective predetermined output ports, a controller to control the signal distributor and to monitor each part, a first selector to select one of the monitor control signal from the first monitor signal output port of the signal distributor and the other monitor control signal input from the second network and to apply the selected one to the controller, a second selector to output the monitor control signal for another node output from the controller toward one of the first monitor control signal input port of the signal distributor and the second network, and a multiplexer to multiplex the monitor control signal output from the second monitor signal output port of the signal distributor and the data signal from the signal distributor and to output the multiplexed signals for the first network. The signal distributor connects the first monitor control signal input port with the second monitor control output port and the second monitor control signal input port with the first monitor control signal output port when the first network is normally operating.  
           [0021]    The above configuration makes it possible to switch over a signal network (a first network) and an external network (a second network)as the transmission path of the monitor control signal at low costs. Also, the signal network (the first network) transmits a monitor control signal in normal operation while the external network (the second network) transmits the monitor control signal when the signal network has a fault, and thus the system can keep even more reliability for the monitor control signal than that for the data signal. Transmission delay of a monitor control signal also becomes minimal. Moreover, it is not necessary to strengthen the reliability of the external network as much as that of the signal network and therefore the signal network can be monitored/controlled at low costs. Furthermore, since a monitor control signal is distributed by the signal distributor, a channel change in the signal network becomes easier. At the same time, continuity can be checked with a loop back test and a fault part can be cut off easily. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0022]    The above and other objects, features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings, in which:  
         [0023]    [0023]FIG. 1 shows a schematic block diagram of an embodiment according to the invention;  
         [0024]    [0024]FIG. 2 shows an example of network configurations according to the embodiment;  
         [0025]    [0025]FIG. 3 shows another configuration example of node B;  
         [0026]    [0026]FIG. 4 is a diagram showing internal connection of an optical crossconnector  120  in normal operation; and  
         [0027]    [0027]FIG. 5 shows a diagram showing internal connection of the optical crossconnector  120  when a loop back test is performed. 
     
    
     DETAILED DESCRIPTION  
       [0028]    Embodiments of the invention are explained below in detail with reference to the drawings.  
         [0029]    [0029]FIG. 1 shows a schematic block diagram of an embodiment according to the present invention, and FIG. 2 shows an example of network configurations according to the embodiment. In FIG. 1, a solid line with an arrow expresses an optical signal, and a broken line with an arrow expresses an electric signal. The example is shown on the assumption that nodes A, B, and C are connected on a ring or mesh type optical network in this order. That is, the node A connects to the node B through optical fibers  10  and  12 , and the node B connects to the node C through optical fibers  14  and  16 . The optical fibers  10 ,  12 ,  14 , and  16  form optical transmission lines of a signal network. The nodes A, B, and C are all connected to an external network  18  which works as a spare monitor control network to transmit a monitor control signal when the signal network has a fault. Although the nodes A and C connect to respective nodes through a pair of optical fibers, the nodes are omitted in FIG. 1. Although the signal network can be any type in mesh, ring, and line, it is generally a mesh or ring type.  
         [0030]    The nodes A, B, and C connect to data transceiver terminals  19 A,  19 B, and  19 C respectively. Each of the nodes A, B, and C transmits/receives a data to/from adjacent nodes of two directions and therefore four signal lines are provided between the data transceiver terminals  19 A,  19 B,  19 C and their respective corresponding nodes A, B, and C.  
         [0031]    The internal configuration and operation of the node B is described below. The node B comprises, as main elements, an optical crossconnector  20 , a controller  22  to control the optical crossconnector  20 , wavelength demultiplexers (DMUX)  24 ,  25 , and wavelength multiplexers (MUX)  28 ,  30 .  
         [0032]    The wavelength demultiplexer  24  demultiplexes a WDM signal light entered through the optical fiber  10  from the node A into respective wavelengths. The wavelength demultiplexer  26  demultiplexes a WDM signal light entered through the optical fiber  16  from the node C into respective wavelengths. In this embodiment, one wavelength is assigned for transmission of a monitor control signal. A data signal light demultiplexed by the wavelength demultiplexer  24 ,  26  enters a corresponding transponder  32 , a monitor control signal light demultiplexed by the wavelength demultiplexer  24  enters a photoelectric converter  34 , and a monitor control light demultiplexed by the wavelength demultiplexer  26  enters a photoelectric converter  36 .  
         [0033]    The transponder  32  converts the input signal light into a signal light of another wavelength (e.g. 1.3 μm band) and applies to the optical crossconnector  20 . The transponder  32  comprises a photoelectric converter to convert an input signal light into an electrical signal for a while and an electro-optical converter to convert the electrical signal into an optical signal again. The transponder  32  can further comprise an electric signal processor to perform reshaping of a signal waveform and the other processes.  
         [0034]    Each output light from the optical crossconnector  20  enters a corresponding transponder  38  having the same function with the transponder  32 . Each transponder  38  converts the input signal light into a signal light of another wavelength and applies to the wavelength multiplexers  28  and  30 .  
         [0035]    Needless to say, the optical crossconnector  20  connects to a data transceiver terminal  19 B through add ports and drop ports.  
         [0036]    A photodiode  40  is disposed at each input of the optical crossconnector  20  so as to monitor whether an input light exists, and a photodiode  42  is disposed at each output of the optical crossconnector  20  so as to monitor whether an output light exists. That is, a portion of the output light from each transponder  32  enters the corresponding photodiode  40 , and a portion of each output light from the optical crossconnector  20  enters the corresponding photodiode  42 . Each output from the photodiodes  40  and  42  enters a controller  22 . Accordingly, the controller  22  can monitor each input port whether an input signal light exists and each output port whether an output signal light exists.  
         [0037]    A selector  44  selects any one of the monitor control signals output from the photoelectric converters  34 ,  36  and the external network  18  and applies the selected one to the controller  22 . A selector  46  applies the monitor control signal output for another node from the controller  22  to any one of the external network  18  and electro-optical converters  48  and  50 . The selectors  44  and  46  are practically interlocked. That is, when the selector  44  selects the external network  18 , the selector  46  also selects the external network  18 . When the selector  44  selects the output from the photoelectric converter  34 , the selector  46  selects the electro-optical converter  48 . When the selector  44  selects the output from the photoelectric converter  36 , the selector  46  selects the electro-optical converter  50 .  
         [0038]    The selectors  44  and  46  can be switches controlled by the controller  22 . It is also applicable that the selector  44  is a switch to change input ports according to a signal input while the selector  46  is a switch to change to a corresponding connecting state according to the connecting state of the selector  44 . When the monitor control signal is an IP based signal, it is possible to use a router as the selector  44 ,  46 . When the monitor control signal is imparted virtual LAN (what is called VLAN) identifier per path, a switching hub corresponding to the VLAN can be used as the selector  44 ,  46 . See below for further details about the operation using a router and a switching hub.  
         [0039]    The wavelength multiplexer  28  multiplexes the output signal light (the monitor control signal light for the node A) from the electro-optical converter  48  and the signal light for the node A in the output light from the transponder  38  and outputs the multiplexed signal light for the node A through the optical fiber  12 . Also, the wavelength multiplexer  30  multiplexes the output signal light (the monitor control signal light for the node C) from the electro-optical converter  50  and the signal light for the node C in the output light from the transponder  38  and outputs the multiplexed signal light for the node C through the optical fiber  14 .  
         [0040]    The controller  22  controls the switching of the optical crossconnector  20  for protection/restoration etc. according to the monitor control signal from the selector  44  and the output from each of the diodes  40  and  42 . The controller  22  also informs a supervisor about its monitored result of each part including the output condition of the photodiodes  40 ,  42  and the connecting condition or switching operation of the optical crossconnector  20  through a monitor control signal.  
         [0041]    The internal configuration of the node C is basically identical to that of the node B.  
         [0042]    In the embodiment, the transmission configuration for a data signal is identical to that of an existing mesh or ring type network. A flow of a data signal between the nodes A, B, and C is briefly explained below.  
         [0043]    The node A outputs a data signal light (a WDM signal light) onto the optical fiber  10  for the node B, the node C, or a node beyond those nodes. The data signal light propagates on the optical fiber  10  and enters the wavelength demultiplexer  24  in the node B. The node C outputs a data signal light (a WDM signal light) onto the optical fiber  16  for the node B, node A, or a node beyond those nodes. The data signal light propagates on the optical fiber  16  and enters the wavelength demultiplexer  26  of the node B.  
         [0044]    The wavelength demultiplexer  24  demultiplexes the WDM signal light input from the optical fiber  10  into respective wavelengths, and the wavelength demultiplexer  26  demultiplexes the WDM signal light input from the optical fiber  16  into respective wavelengths. Each data signal light demultiplexed by the wavelength demultiplexers  24  and  26  enters a corresponding input port of the optical crossconnector  20  through a corresponding transponder  32 .  
         [0045]    The optical crossconnector  20  drops a signal light for the data transceiver terminal  19 B in the input data signal light and applies the dropped signal light to the data transceiver terminal  19 B through a drop port. The optical crossconnector  20  also applies the other input data signal lights and the data signal light input through an add port from the data transceiver terminal  19 B to the wavelength multiplexers  28  and  30  through the transponder  38  via an output port assigned by the controller  22 . The controller  22  controls the optical crossconnector  20  to output which data signal light from which output port.  
         [0046]    The wavelength multiplexer  28  multiplexes the input data signal lights. The WDM signal light from the wavelength multiplexer  28  enters the node A through the optical fiber  12 . Also, the wavelength multiplexer  30  multiplexes the input data signal lights. The WDM signal light from the wavelength multiplexer  30  enters the node C through the optical fiber  14 .  
         [0047]    Next, the transmission configuration of a monitor control signal light is explained below. In the embodiment, a monitor control signal transmits on an optical network on which a data signal propagates, namely on the optical fibers  10 ,  12 ,  14 , and  16  in normal operation.  
         [0048]    Assuming that the optical fiber  10  for example breaks and leads the data communication between the node A and node B disabled, a monitor control signal sent from the node A to the node B does not enter the controller  22  and output levels of all photodiodes  40  to monitor optical power of respective output light from the wavelength demultiplexer  24  reduce. Owing to the above operation, the controller  22  can detects the occurrence of faults in the optical fiber  10 . Once a fault is detected, the controller  22  controls the selectors  44  and  46  so that a monitor control signal for the node A is transmitted to the node A through the external network  18 . That is, the selector  46  outputs the monitor control signal for the node A output from the controller  22  onto the external network  18 . The external network  18  transmits the monitor control signal to the node A. When the monitor control signal from the node B enters from the external network  18 , the node A transmits a monitor control signal for the node B through the external network  18 . The selector  44  in the node B transmits the monitor control signal entered through the external network  18  from the node A toward the controller  22 .  
         [0049]    The optical fibers  14  and  16  between the nodes B and C are normal, and the selectors  44  and  46  use the optical fibers  14  and  16  for the transmission between the nodes B and C regardless of a fault in the optical fiber  10 .  
         [0050]    It is also applicable to control the selectors  44  and  46  separately so that when a transmission fault is occurred in one of a pair of optical fibers connecting between nodes, a monitor control signal propagates on the remained normal optical fiber. In that case, the controller  22  is required to control the selectors  44 ,  46  finely and thus loaded with an increased burden. A switching devices to interlock each other such as the selectors  44 ,  46  are easily available at low-cost as one unit, and therefore it is profitable to simply interlock the selectors  44  and  46  in order to cut down system costs.  
         [0051]    As described above, an IP router or switching hub in Ethernet (Trademark) can be used as the selectors  44 ,  46 .  
         [0052]    When an IP router is utilized as the selectors  44  and  46 , the controller  22  rewrites a routing table of the selector  46  according to whether or not a fault exists in a signal network. Although the selector  44  is enough as far as it simply has a function to transfer an input signal to the controller  22 , generally a single IP router replaces the part composed of the selectors  44  and  46 .  
         [0053]    When a fixed path (e.g. a label switched path (LSP) using MPLS) for a monitor control signal is structured in each of the external network  18  and the internal network having the optical fibers  10 - 16  and the redundancy in the ratio of 1 to 1 is set between the paths, it is possible to get the high speed protection even if having a fault. However, to realize this configuration, all routers in the external network  18  need to have an MPLS function.  
         [0054]    It is also applicable that the external network  18  and the internal network having the optical fibers  10 ˜ 16  are distinguished and paths of a monitor control signal are switched according to whether a fault exists in the internal network, using a virtual local area network (VLAN). In this case, a switching hub is used as each of the selectors  44  and  46 . The controller  22  adds to a monitor control signal a VLAN tag which is different per path and sends to the selector  46  (the switching hub). On the other hand, the selector  46  sends the monitor control signal except for the VLAN tag to the electro-optical converter  48  or  50  or the external network  18 . Accordingly, the VLAN avoids a waste of band caused by broadcast signals.  
         [0055]    Although not shown in FIG. 1, it is also applicable to dispose photodiodes to detect whether input signal lights in the photoelectric converters  34  and  36  exist and photodiodes to detect whether output signal lights from the electro-optical converters  48  and  50  and outputs from those photodiodes are applied to the controller  22 . With this configuration, the controller  22  can monitor a fault in the route of the monitor control signal light propagating through the signal network.  
         [0056]    [0056]FIG. 3 shows a modified configuration of node B. In the configuration shown in FIG. 3, the monitor control signal passes an optical crossconnector. Owing to this configuration, a spare wavelength in the internal optical network is used for the transmission of the monitor control signal. Also, the continuity of the monitor control signal can be checked by looping back the monitor control signal.  
         [0057]    A controller  122  controls the optical crossconnector  120 , monitors each part in the node B and informs the other nodes or a monitor center of the monitor control signal.  
         [0058]    A wavelength demultiplexer  124  demultiplexes a WDM signal light input from the node A through the optical fiber  10  into respective wavelengths. A wavelength demultiplexer  126  demultiplexes a WDM signal light input from the node C through the optical fiber  16  into respective wavelengths. In the embodiment shown in FIG. 3, one wavelength is assigned for the transmission of the monitor control signal. The data signal lights demultiplexed by the wavelength demultiplexers  124  and  126  enter respectively input ports #5˜#m of the optical crossconnector  120  through corresponding transponders  132 . The monitor signal lights demultiplexed by the wavelength demultiplexers  124  and  126  directly enter respectively input ports #3 and #4 of the optical crossconnector  120 . The configuration and function of the transponder  132  are identical to those of the transponder  32 .  
         [0059]    The optical crossconnector  120  comprises two output ports #1 and #2 for outputting monitor control signal lights to be transmitted for the controller  122 , two output ports #3 and #4 for outputting monitor control signal lights for the nodes A and C, and a plurality of output ports #5˜#m for outputting data signal lights for the nodes A and C. The output port #3 directly connects to a wavelength multiplexer  128 , and the output port #4 directly connects to a wavelength multiplexer  130 . Each of the output ports #5˜#m connects to the wavelength multiplexer  128  or  130  through a corresponding transponder  138  having a function identical to that of the transponder  38 .  
         [0060]    Needless to say, the optical crossconnector  120  connects to a data transceiver terminal  19 B through add ports and drop ports.  
         [0061]    The two output ports #1 and #2 and two input ports #1 and #2 are used for transmission with the controller  122 . Photoelectric converters  134  and  136  connect to the output ports #1 and #2 of the optical crossconnector  120  respectively, and output lights from electro-optical converters  148  and  150  enter the input ports #1 and #2 of the optical crossconnector  120  respectively.  
         [0062]    A photodiode  140  is disposed on each of the input ports #3˜#m of the optical crossconnector  120 , except for the two input ports #1 and #2 to which output lights from the electro-optical converters  148  and  150  input, so as to monitor whether an input light exists. Also, a photodiode  142  is disposed on each of the output ports #3˜#m of the optical crossconnector  120 , except for the two output ports #1 and #2 to which the photoelectric converters  134  and  136  connect, so as to monitor whether an output light exists. The reason why not monitor the output lights from the electro-optical converters  148  and  150  and the input light of the photoelectric converters  134  and  136  is to reduce the costs and that there is little necessity. When a general-purpose type is desired, it is applicable to dispose the photodiodes  140 ,  142  on the input ports #1, #2 and the output ports #1, #2 of the optical crossconnector  120  connecting to those converters.  
         [0063]    The photodiodes  140  monitor whether any monitor control signal light having entered the optical crossconnector  120  from the wavelength demultiplexers  124  and  126  exists, and whether any data signal light having output from the transponder  132  and entered the optical crossconnector  120  exists. The photodiodes  142  monitor whether any data signal light and monitor control signal light output from the optical crossconnector  120  for the nodes A and C exist. Each output from the photodiodes  140  and  142  is applied to the controller  122 . With this configuration, the controller  122  can monitor the data signal lights and the monitor control lights input/output to/from the optical crossconnector  120 .  
         [0064]    The selector  144  selects any one of the monitor control signals output from the photoelectric converters  134 ,  136  and the external network  18  and applies the selected one to the controller  122 . The selector  146  applies the monitor control signal output from the controller  122  for the other nodes to any one of the photoelectric converters  134 ,  136  and the external network  18 . The selectors  144  and  146  are practically interlocked. That is, when the selector  144  selects the external network  18 , the selector  146  also selects the external network  18 . When the selector  144  selects the output from the photoelectric converter  134 , the selector  146  selects the electro-optical converter  148 . When the selector  144  selects the output from the photoelectric converter  136 , the selector  146  selects the electro-optical converter  150 . The function of the selector  144 ,  146  can be identical to that of the selector  44 ,  46 . That is, similarly to the selectors  44  and  46 , it is possible to use various switching devices like an IP router and switching hub as the selectors  144  and  146 .  
         [0065]    The wavelength multiplexer  128  multiplexes a monitor signal light for the node A output from the output port #3 of the optical crossconnector  120  and a data signal light for the node A in the output light from the transponder  138  and outputs the multiplexed lights for the node A through the optical fiber  12 . Also, the wavelength multiplexer  130  multiplexes a monitor control light for the node C output from the output port #4 of the optical crossconnector  120  and a data signal light for the node C in the output light from the transponder  138  and outputs the multiplexed lights for the node C through the optical fiber  14 .  
         [0066]    Furthermore, although the basic function of the controller  122  is almost identical to that of the controller  22 , the optical crossconnector  120  is used for transmission of a monitor control signal and thus the controller  122  has an additional function for this purpose.  
         [0067]    The transmission of the data signal light is identical to the embodiment shown in FIG. 1. The switching between the internal and external networks for a monitor control signal is also fundamentally identical to the embodiment shown in FIG. 1. Connection check of a monitor control signal which is a characteristic function of the embodiment shown in FIG. 3 is described below.  
         [0068]    [0068]FIG. 4 shows an internal connection of an optical crossconnector in a normal operation, namely when a monitor control signal transmits on an internal network on which a data signal light propagates. To make it easily understandable, connecting paths of a monitor control signal alone are illustrated. As clearly understandable from FIG. 4, the controller  122  connects the input port #1 of the optical crossconnector  120  to the output port #3, input port #2 to the output port #4, input port #3 to the output port #1, and input port #4 to the output port #2 in a normal operation.  
         [0069]    In the internal connecting condition shown in FIG. 4, the monitor control signal output from the node A enters the controller  122  through the optical fiber  10 , the wavelength demultiplexer  124 , the input port #3 and the output port #1 of the optical crossconnector  120 , the photoelectric converter  134 , and the selector  144 . Similarly, The monitor control signal output from the node C enters the controller  122  through the optical fiber  16 , the wavelength demultiplexer  126 , the input port #4 and the output port #2 of the optical crossconnector  120 , the photoelectric converter  136 , and the selector  144 .  
         [0070]    The monitor control signal for the node A output form the controller  122  enters the node A through the selector  146 , electro-optical converter  148 , the input port #1 and the output port #3 of the optical crossconnector  120 , the wavelength multiplexer  128 , and the optical fiber  12 . Similarly, the monitor control signal for the node C output from the controller  122  enters the node C through the selector  146 , the electro-optical converter  150 , the input port #2 and the output port #4 of the optical crossconnector  120 , the wavelength multiplexer  130 , and the optical fiber  14 .  
         [0071]    Such function is identical to the embodiment shown in FIG. 1 that a monitor control signal can be transmitted through an external network  18  when the signal network has a fault. Furthermore, in this embodiment, when devices such as a light emission element and a photodetector have a fault instead of a fault in the optical fibers  10 ˜ 16 , it is possible to transmit a monitor control signal on the other channel in the optical fibers  10 ˜ 16 . This is the very merit a monitor control signal enters the optical crossconnector  120 . That is, the high-speed protection/restoration by the optical crossconnector  120  can be also used for monitor control signal and a spare band for a monitor control signal can be obtained beforehand.  
         [0072]    [0072]FIG. 5 shows an internal connection of the optical crossconnector  120  at continuity check through a loop back method. When the continuity is checked through a loop back method, the controller  122  connects the input port #1 of the optical crossconnector  120  to the output port #1, the input port #2 to the output port #2, the input port #3 to the output port #3, and the input port #4 to the output port #4.  
         [0073]    In the internal connecting condition shown in FIG. 5, a signal output from the electro-optical converter  148  enters the controller  122  through the input port #1 and the output port #1 of the optical crossconnector  120 , the photoelectric converter  134 , and the selector  144 . Also, a signal output from the electro-optical converter  150  enters the controller  122  through the input port #2 and the output port #2 of the optical crossconnector  120 , the photoelectric converter  136 , and the selector  144 . According to this operation, the controller  122  can detects whether the photoelectric converters  134  and  136 , the selectors  144  and  146 , and the electro-optical converters  148  and  159  operate normally and whether those connecting paths have problems.  
         [0074]    Furthermore, as shown in FIG. 5, by connecting the input port #3 to the output port #3 and the input port #4 to the output port #4, the monitor control signal from the node A can be directly transferred to the node C and the monitor control signal from the node C can be directly transferred to the node A. With this operation, a monitor control signal can be transferred between the nodes A and C bypassing the controller  122  in the node B. This can use for check of connecting conditions and detection of fault points.  
         [0075]    In the embodiment shown in FIG. 3, a monitor control signal can be distributed by the optical crossconnector  120  to a desired optical path and a desired wavelength channel and thus it is possible, for example, to change channels (optical carrier wavelengths) on the optical fibers  10 ,  12 ,  14 , and  16 . That is, the high-speed protection/restoration by the optical crossconnector  120  can be also used for a monitor control signal. In other words, a spare band for a monitor control signal can be obtained beforehand.  
         [0076]    The present invention is also applicable for nodes located on a terminal end in a linear type network. In this case, in FIGS. 1 and 3, for example, the configuration of the node B is used and the configuration element corresponding to the node C should be omitted.  
         [0077]    As readily understandable from the aforementioned explanation, according to the invention, a monitor control signal can be transmitted with reliability equivalent to that of a data signal light. In a normal operation, since the monitor control signal transmits on an optical network for the data signal light, there is little transmission delay and also a delay time is stable. When the signal network has a fault, an external network can transmit the monitor control signal and thus the monitor control signal can even obtain the reliability higher than that of the data signal light.  
         [0078]    While the invention has been described with reference to the specific embodiment, it will be apparent to those skilled in the art that various changes and modifications can be made to the specific embodiment without departing from the spirit and scope of the invention as defined in the claims.