Optical Transmission System

INCORPORATION BY REFERENCE

This application claims priority based on a Japanese patent application, No. 2013-250483 filed on Dec. 3, 2013, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an optical transmission system that suppresses the expansion of the scale of an optical switch matrix of each node and the increase of the number of links and realizes the securement of an alternative route in which the number of passed nodes is optimized in an optical transmission network where a path including an active signal line and plural alternative routes including preliminary signal lines exist in one or plural ring networks configured by plural nodes each of which has an optical switch and switching to the alternative route is performed when the active signal line is unavailable.

BACKGROUND OF THE INVENTION

As a result of broadening a band by optical fiber communication, the low-priced communication of mass digital information is enabled. New service that applies mass digital information further promotes broadening a band and the traffic of the Internet grows at a high rate such as approximately double in two years. An optical fiber network where mass data is communicated at high speed in relatively long distance of some or more km which a basic trunk, a metropolitan area network and an access line cover has been developed. Afterward, it is considered that optical link technology that uses optical link is also further effective in extremely short distance between information communication (ICT) devices such as a server in a data center (several meters to several hundreds meters) or in the information communication device (several to several tens centimeters).

For network topology that plural nodes are connected via an optical fiber, a transmission network has been configured in the complex combination of a linear chain network, a mesh network, a ring network, a multi-ring network and a standby line access network are complexly combined in addition to a point-to-point network which is the most simple. In such complex transmission networks, it is estimated that damage when failure occurs in a transmission line because of disaster and disconnection is inscrutable. Then, nowadays, the configuration of a network that can also maintain required communication in a sudden change of communication environment such as the suspension of a network in disaster and others is demanded. For a representative example of a network in which plural transmission paths exist based upon the abovementioned background, the mesh network can be given. In the mesh network, when an active signal line is unavailable, switching to an alternative route is performed and at the time of disaster, the securement of a transmission path is also enabled.

The mesh network has configuration that a signal input to each node is output in at least three directions to secure an alternative route and the scale of a matrix in an optical switch for switching paths increases. In the optical switch in which the scale of the matrix increases as described above, loss increases because the number of interfering paths for switching paths increases and the number of cross points between waveguides increases in the case of a waveguide type. Besides, when such an optical switch is a mechanical type, an operational range widens and loss increases by the quantity of an operational error as the scale of the matrix increases. In the meantime, in the optical switch in which the scale of the matrix increases, as the number of ports increases, the number of wires increases. This reason is that plural wires are prepared every time a network is increased, and as an installation cost greatly increases, the optical switch has poor scalability and a huge cost is required for the restoration of a transmission network. Moreover, when the alternative route is long, transit in the optical switch is frequent and loss applied to a transmitted signal by the optical switch also increases.

For a transmission network in which the securement of plural paths between different nodes is enabled, a transmission network configured by plural rings each of which is acquired by connecting ends of the mesh network is well-known (for example, refer to International Publication WO 2007/018164).

SUMMARY OF THE INVENTION

Then, a transmission network where a low-loss and general purpose optical switch is used for an optical switch of each node and switching to a preliminary signal line is also enabled in the optical switch in which the scale of a matrix is inhibited when the transmission of a signal in an active signal line is turned impossible is being researched. In such a transmission network, it is demanded that concretely, the following three requirements are met. The first requirement is that the securement of plural paths between different nodes is enabled. The second requirement is that the scale of the matrix in the optical switch located in each node is required to be inhibited. The third requirement is that an alternative route switched from the active path turned impossible is required to be the route in which the number of passed nodes is inhibited to inhibit a passage count.

The International Publication WO 2007/018164 can meet the first requirement because the securement of plural paths between different nodes is enabled.

Referring to FIGS. 15A to 15C, the second and third requirements in the International Publication WO 2007/018164 will be described below. As shown in FIG. 15A, a path from a cluster switch to a node C in normal operation is a path via a node F. At this time, when link between the node C and the node F is disconnected, one of alternative routes from the cluster switch to the node C is a route via a node E and a node B. Accordingly, the scale of a matrix in an optical switch provided to the node B is as shown in FIG. 15B. That is, as configuration disclosed in the International Publication WO 2007/018164 is configuration that a signal input to each node is output in at least three directions (for example, signal input 9 is output in three directions of signal output 6, signal output 10 and signal output 11 in FIG. 15A), the scale of a matrix in the switch for switching paths increases and the number of links increases. Accordingly, the International Publication WO 2007/018164 does not meet the second requirement. Besides, as shown in FIG. 15C, the path from the cluster switch to the node C in normal operation is the path via the node F and a passage count of the optical switches is three times. At this time, when the link between the node C and the node F is disconnected, a passage count of optical switches is four times in the path via the node E and the node B from the cluster switch to the node C as one of the alternative routes to the node C. That is, as the passage count of the optical switches increases in the alternative route, an effect which the loss of the optical switches has on a transmitted signal increases. Accordingly, the International Publication WO 2007/018164 does not meet the third requirement.

As described above, as all the first to third requirements cannot be met in the related art, the related art has a problem that a transmission network where a transmission line can be certainly secured when an active signal line fails cannot be provided.

An object of the present invention is to settle the abovementioned problem and to provide an optical transmission system that inhibits the expansion of the scale of an optical switch matrix in each node and the increase of the number of wires and realizes the securement of an alternative route in which the number of passed nodes is optimized.

The disclosed optical transmission system includes at least one ring network configured by plural nodes. Each node is provided with an optical switch having connection configuration that the output in at least two directions of a signal input to the node is allowed and the output of the optical switch functions as input to another node included in the plural nodes.

According to the present invention, the expansion of the scale of the matrix in the optical switch of the node and the number of links are inhibited and the securement of an alternative route in which the number of passed nodes is optimized is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show an example of the configuration of a network in an optical transmission system equivalent to a first embodiment, FIG. 1A showing network link configuration, and FIG. 1B showing the configuration of an optical switch;

FIG. 2 shows an example of a node transit frequency in an active path and an alternative route in the first embodiment;

FIG. 3 shows an example of the configuration of a network in an optical transmission system equivalent to a second embodiment;

FIG. 4 is a flowchart showing switching operation by a controller;

FIGS. 5A and 5B show an example of the configuration of a network in an optical transmission system equivalent to a third embodiment, FIG. 5A showing network wiring configuration, and FIG. 5B showing the configuration of an optical switch;

FIGS. 6A and 6B show an example of the configuration of a network in an optical transmission system equivalent to a fourth embodiment, FIG. 6A showing network link configuration, and FIG. 6B showing the configuration of an optical switch;

FIG. 7 shows an example of a node transit frequency in an active path and an alternative route in the fourth embodiment;

FIG. 8 shows an example of the configuration of a network in an optical transmission system equivalent to a fifth embodiment;

FIG. 9 shows an example of a node transit frequency in an active path and an alternative route in the fifth embodiment;

FIG. 10 shows another example of a node transit frequency in the active path and the alternative route in the fifth embodiment;

FIG. 11 shows an example of the configuration of a network in an optical transmission system equivalent to a sixth embodiment;

FIG. 12 shows another example of a node transit frequency in an active path and an alternative route in the sixth embodiment;

FIG. 13 shows an example of the configuration of a network in an optical transmission system equivalent to a seventh embodiment;

FIGS. 14A and 14B show an example of the configuration of a network in an optical transmission system equivalent to an eighth embodiment, FIG. 14A showing network link configuration, and FIG. 14B showing multicore fiber link; and

FIGS. 15A to 15C are explanatory drawings for explaining requirements with which an optical transmission system should be provided, FIG. 15A showing network wiring configuration, FIG. 15B showing the configuration of an optical switch, and FIG. 15C showing a path in normal operation and a route when failure occurs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An optical transmission system described as embodiments below is an optical transmission system including at least one ring network configured by plural nodes. Each node is provided with an optical switch having such the configuration of connection that the output in at least two directions of a signal input to the node is allowed and the output of the optical switch functions as input to another node included in the plural nodes. By such configuration, the expansion of the scale of an optical switch matrix of each node and the increase of the number of links are suppressed and the securement of an alternative route in which the number of passed nodes is optimized is realized.

The embodiments of the optical transmission system will be described referring to the drawings below. In the drawings, an optical fiber is used for an optical transmission medium. In the description of the drawings, the same reference numeral is allocated to the same component and the duplicate description is possibly omitted.

First Embodiment

Referring to FIGS. 1A and 1B, the configuration of an optical transmission system in this embodiment will be described below. FIGS. 1A and 1B show the configuration of links in a transmission network 1 (network configuration) in this embodiment and the scale of a matrix in an optical switch provided to each node.

This embodiment includes a cluster switch which is an external connection interface and plural ring networks (in a frame shown by a dotted line) as shown in FIG. 1A. The first ring network 2 includes three nodes (D, E, F), the second ring network 3 also includes three nodes (A, B, C), and the nodes in each ring network are connected via a transmission line. As for connection between the ring networks, the nodes in each pair of A and D, B and E, and C and F are connected via a transmission line. The three nodes (D, E, F) that configure the first ring network 2 are connected to the cluster switch.

In normal operation, access from the cluster switch to the node A is made in a path via the node D. Access from the cluster switch to the node B is made in a path via the node E, access from the cluster switch to the node C is made in a path via the node F, access from the cluster switch to the node D is made in its path, access from the cluster switch to the node E is made in its path, and access from the cluster switch to the node F is made in its path.

Next, failure in the access path to the node C (shown as x in FIG. 1A) will be taken up. The path to the node C in normal operation is a path via the node F from the cluster switch. At this time, when wiring between the node C and the node F is disconnected, the cluster switch selects the path to the node D in response to the detection of the disconnection of the wiring for one of alternative routes to the node C and access from the cluster switch to the node C is made in a path via the node D and the node A. At this time, the scale of a matrix in the optical switch provided to the node A is shown in FIG. 1B. That is, the scale of the matrix has only to have optical switch configuration that the output in only two directions of a wire 7 and a wire 8 of a signal input to the node A via a wire 5 is allowed. Similarly, the scale of a matrix in the optical switch provided to the node B has the similar configuration to the configuration shown in FIG. 1B though numbers of links are different. The scale of a matrix in each optical switch provided to the nodes C, D, E, F also has the similar configuration to that shown in FIG. 1B though numbers of links are different.

Next, a node transit frequency in an active path in normal operation and in an alternative route when failure occurs will be taken up. As shown in FIG. 2, the path in normal operation from the cluster switch to the node C is the path via the node F and a transit frequency of optical switches is three times. When the wire between the node C and the node F is disconnected, one of the alternative routes from the cluster switch to the node C is the path via the node D and the node A and a passage count of optical switches is four times. Accordingly, in the case of the network wiring configuration shown in FIGS. 1A and 1B, the transmission network considering the passage count of optical switches of four times is required to be designed.

According to this embodiment, as the plural or all the nodes in the ring network form the pair with the plural or all the nodes in the ring network except the abovementioned ring network, the nodes in the same pair are connected via the transmission line and the optical switch has the configuration of connection that the output in at least two directions of a signal input to the one node is allowed, a low-loss and general purpose optical switch can be applied as the optical switch installed in each node and required for switching operation and the transmission network in which the preliminary signal line can be certainly secured as a switched destination is provided.

As described above, as the optical switch provided to each node has configuration that the output in only two directions of an input signal is allowed, the securement of the two paths of the path in normal operation and the alternative route when failure occurs can be realized using the low-loss and general purpose optical switch in which the scale of the matrix is suppressed.

Second Embodiment

Referring to FIG. 3, the configuration of a transmission network 1 in this embodiment will be described below. FIG. 3 shows the configuration of the transmission network in this embodiment. This embodiment relates to the transmission network when the number of nodes is not limited. An optical switch provided to each node is similar to that in the first embodiment.

As shown in FIG. 3, this embodiment shows a case that plural ring networks A, B, C, D, E are provided and the number of nodes in each ring network is at least 12. In FIG. 3, a cluster switch is omitted. The nodes in the ring network are connected via a transmission line. Between the ring networks, each node forms a pair as shown in FIG. 3 and the nodes in the same pair are connected via a transmission line. Each transmission line includes an active signal line, a preliminary signal line and a control signal line.

A controller is connected to the node 1 in the ring network A as shown in FIG. 3. The controller integrates a result of monitoring a situation of each transmission line in each node using the control signal lines and issues a switched destination instruction to each node in view of the situations of the transmission lines. In the ring network, the result of monitoring or information of the switched destination instruction is transmitted using the clockwise control signal line and the counterclockwise control signal line as shown in FIG. 3 and each node shares the information. Hereby, this embodiment can cope with multiple failures on the transmission lines. Between the ring networks, the result of monitoring or the information of the switched destination instruction is also transmitted using the control signal line as shown in FIG. 3 and the ring networks share the information.

Next, switching operation in the transmission network configured as described above will be described. FIG. 4 is a flowchart showing switching operation in the transmission network in the second embodiment. Normality confirmatory operation of a redundant path corresponding to an operation path will be described below.

An operation path including an active signal line by a switching device of each node is normally operated (a step ST21), normality/abnormality is judged in a signal detection unit and a signal monitoring unit of each node in normality confirmatory operation of the operation path, and a result of the judgment is notified a controller (a step ST22). At this time, when the result of the judgment is normality, normal operation on the operation path (the step ST21) is continued. In the meantime, when abnormality is identified, the node which is identified as abnormal transmits a monitoring signal to a preliminary signal line and monitors a state of the preliminary signal line (a step ST23). Concretely, in normality confirmatory operation of the preliminary signal line by the switching device, the node which is identified as abnormal first transmits a monitoring signal to the preliminary signal line to select a redundant path corresponding to the operation path (in a case that plural preliminary signal lines exist). According to the monitoring signal transmitted to the preliminary signal line, monitoring is performed in each node (a step ST24). The normality of the preliminary signal line is judged in the controller that receives results of monitoring in each node and the preliminary signal line which is identified as normal is selected as a switched destination from the abnormal operation path (a step ST25). Next, according to the controller, an optical switch is connected to the preliminary signal line (a step ST26). As described above, operation by the preliminary signal line is started (the step ST26) and normality/abnormality is judged in the signal detection unit and the signal monitoring unit of each node (a step ST27). At this time, as the normality of the preliminary signal line is confirmed before switching, the preliminary signal line is never identified as abnormal, the operation by the preliminary signal line is necessarily judged normal (a step ST28), and switching operation is finished.

As described above, according to this embodiment, as the monitoring signal is transmitted to the preliminary signal line beforehand and its state can be monitored, the normality of the redundant path using the preliminary signal line can be confirmed before failure occurs in the operation path and switching is executed and double failure that the preliminary signal line for functioning as a protection is not normally operated can be prevented when failure occurs in the operation path.

According to this embodiment, as the optical switch has connection configuration that the output in at least two directions (the active path by the active signal line and the redundant path by the preliminary signal line) of a signal input to one node is allowed, the optical transmission system where plural preliminary signal lines as a switching destination are provided, the preliminary signal line the normality of which is confirmed is selected out of the plural preliminary signal lines and the preliminary signal line can be secured as the switching destination is provided.

Besides, this embodiment provides the optical transmission system where the signal detection unit and the signal monitoring unit respectively provided to each node share the information of a result of monitoring a state of the preliminary signal line by a monitoring signal using a control signal transmitted in the control signal line when the active signal line is turned unusable in the transmission network connected by the active signal line, the preliminary signal line and the control signal line, thereby, switching from the active signal line to the preliminary signal line is performed between the plural nodes with the nodes interlocked even if the switching of the path is required in the plural nodes to secure the transmission path and the switching operation can be completed in short time.

Moreover, according to this embodiment, as sharing the information acquired by monitoring between the nodes in the one or plural ring networks is performed by executing in the two counterclockwise and clockwise directions and between (among) the ring networks using the control signal line, the information of a result of monitoring a situation of the paths including the information of specifying a location of failure in the line is shared among units provided with the optical switch installed in the respective nodes and the operation of the optical switch in the respective nodes is switched according to the switching destination instruction based upon situation monitoring results with each operation interlocked, the optical transmission system where operation for switching to the alternative path based upon the results of monitoring is also securely completed without disconnecting a control signal in the case of complex failure such as the disconnection of plural fibers is provided.

Further, according to this embodiment, as the plural nodes connected by the active signal line, the preliminary signal line and the control signal line and the transmission system including them include at least the ring network or a mesh network (each node has three or more-direction paths), the optical transmission system where the reliable and safe transmission path can be secured in a network in which a station and homes are star-connected such as Fiber To The Home (FTTH) to an equipment link network such as a data center is provided.

As described above, as the optical transmission system has the configuration that the output in only two directions of the active signal line and the preliminary signal line of an input signal to the optical switch provided to each node is allowed, the securement of the two paths of the path in normal operation and the alternative route when failure occurs can be realized using the low-loss and general purpose optical switch the scale of the matrix of which is suppressed.

Third Embodiment

Referring to FIGS. 5A and 5B, the configuration of a transmission network in this embodiment will be described below. FIGS. 5A and 5B show the link configuration of the transmission network in this embodiment and the scale of a matrix in an optical switch provided to each node.

This embodiment includes a cluster switch which is an external connection interface and multiple ring networks (in a dotted frame) as shown in FIG. 5A. The first ring network includes three nodes (D, E, F), the second ring network also includes three nodes (A, B, C), and the nodes in each ring network are connected via a transmission line. As for connection between the ring networks, each pair of A and D, B and E, and C and F is formed and the nodes in the pair are connected via a transmission line. Connection with the cluster switch is made between the cluster switch and any of the three nodes (D, E, F) included in the first ring network.

A rack where multiple servers are housed is provided to each node as shown in FIG. 5A and server information (information transmitted/received to/from the server connected to another node by the server) is input/output via an input-output signal line that connects the node and the server.

An optical switch, an optical switch control/signal monitoring unit and a signal detection unit are provided to each node as shown in FIG. 5B. The optical switch enables access to the server provided to each node (switching from a wire 5 to an input-output wire) in addition to the switching of paths (switching from the wire 5 to a wire 7 or switching from the wire 5 to a wire 8).

The optical switch control/signal monitoring unit controls the optical switch based upon link monitoring information in the signal detection unit. The optical switch control/signal monitoring unit is connected to each node via a control signal line.

Accordingly, this transmission network can be applied to an access network like a business area network such as a data center including a metropolitan area network and wiring in a residential area such as Fiber To The Home (FTTH).

Fourth Embodiment

Referring to FIGS. 6A and 6B, the configuration of a transmission network in this embodiment will be described below. FIGS. 6A and 6B show the wiring configuration of the transmission network in this embodiment and the scale of a matrix in an optical switch provided to each node.

This embodiment includes a cluster switch which is an external connection interface and plural ring networks (in a dotted frame) as shown in FIG. 6A. The first ring network includes three nodes (D, E, F), the second ring network also includes three nodes (A, B, C), and the nodes in each ring network are connected via a transmission line. As for connection between the ring networks, a pair of the two nodes in each ring network, D and B, and C and E is formed and the nodes in the pair are connected via a transmission line. Connection with the cluster switch is made between the cluster switch and any of the three nodes (D, E, F) included in the first ring network and between the cluster switch and the node A in the second ring network.

In normal operation, access is made from the cluster switch to the node A. Access from the cluster switch to the node B is made via the node D, access from the cluster switch to the node C is made via the node E, access from the cluster switch to the node D is directly made, access from the cluster switch to the node E is directly made, and access from the cluster switch to the node F is directly made.

Next, failure (shown as x in FIG. 6A) in the access path to the node C will be taken up. The path from the cluster switch to the node C in normal operation is a path via the node E. At this time, when a link between the node C and the node E is disconnected, one of alternative routes to the node C is a route via the node A. At this time, the scale of a matrix in the optical switch provided to the node A is as shown in FIG. 6B. That is, the optical switch has only to have configuration that the output in only two directions of the link 7 and the link 8 of a signal input to the node A via the link 5 is allowed. Similarly, the scale of a matrix in the optical switch provided to the node B is also as shown in FIG. 6B. The scale of a matrix in each optical switch provided to the nodes C, D, E, F is also as shown in FIG. 6B.

Next, the number of passed nodes in the active path in normal operation and in the alternative route when failure occurs will be taken up. As shown in FIG. 7, the path from the cluster switch to the node C in normal operation is a path via the node E and a passage count of optical switches are three times. At this time, when the wire between the node C and the node E is disconnected, an alternative route from the cluster switch to the node C is a route via the node A and a passage count of the optical switches is three times. Accordingly, in the case of the network link configuration shown in FIGS. 6A and 6B, the transmission network considering the passage count of the optical switches as three times can be designed.

Accordingly, as this embodiment has the configuration that the output in only two directions of a signal input to the optical switch provided to each node is allowed, the route in which the number of passed nodes is suppressed to suppress an optical switch passage count can be selected in not only the operation path but in the alternative route using the low-loss and general purpose optical switch the scale of the matrix of which is suppressed.

Fifth Embodiment

Referring to FIG. 8, the configuration of a transmission network in this embodiment will be described below. FIG. 8 shows the configuration of the transmission network in this embodiment. In FIG. 8, the basic configuration is similar to those shown in FIGS. 1A, 1B, 6A and 6B and this embodiment will be mainly described with locations different from those in FIGS. 1A, 1B, 6A and 6B.

This embodiment includes a cluster switch which is an external connection interface and multiple ring networks (in a dotted frame) as shown in FIG. 8. The ring network A1 includes six nodes (25, 26, 27, 28, 29, 30), the ring network A2 also includes six nodes (10, 20, 21, 22, 23, 24), the ring network A3 also includes six nodes (13, 14, 15, 16, 17, 18), the ring network A4 also includes six nodes (7, 8, 9, 10, 11, 12), and the ring network A5 also includes six nodes (1, 2, 3, 4, 5, 6). The nodes in each ring network are connected via a transmission line. As for connection between (among) each ring network, a pair of the node 2 and the node 7, a set of the node 3, the node 8 and the node 13, a set of the node 4, the node 9, the node 14 and the node 19, a set of the node 5, the node 10, the node 15, the node 20 and the node 25, a set of the node 6, the node 11, the node 16, the node 21 and the node 26, a set of the node 12, the node 17, the node 22 and the node 27, a set of the node 18, the node 23 and the node 28 and a pair of the node 24 and the node 29 are formed out of the five nodes in each ring network, and the nodes in each pair (each set) are connected via each transmission line. Connection with the cluster switch is made between the cluster switch and the node 1 in the ring network A5, the node 7 in the ring network A4, the node 13 in the ring network A3, the node 19 in the ring network A2 or any of the six nodes (25, 26, 27, 28, 29, 30) in the ring network A1.

In normal operation, as shown in FIG. 9, an access path from the cluster switch to the node 10 is a path via the node 25, the node 20 and the node 15. Accordingly, a transit frequency of the nodes is five times.

Failure shown as x in FIG. 8 in the access path between the node 15 and the node 10 will be taken up below. When a wire between the node 10 and the node 15 is disconnected, an alternative route 1 to the node 10 is a path via the node 26, the node 21, the node 16 and the node 11 from the cluster switch to the node 10 and a passage count of the nodes is six times. In the meantime, for an alternative route to the node 10, an alternative route 2 is also conceivable. The alternative route 2 is a path via the node 7, the node 12, the node 11 from the cluster switch to the node 10 and a passage count of the nodes is five times.

Accordingly, in the case of the network link configuration shown in FIG. 8, correspondence to the alternative route when failure occurs is enabled by designing the transmission network considering the passage count of the optical switches provided to each node in normal operation.

Another example will be taken up below. In normal operation, as shown in FIG. 10, an access path from the cluster switch to the node 11 is a path via the node 26, the node 21 and the node 16 from the cluster switch to the node 11. Accordingly, a passage count of the nodes is five times. At this time, when a wire between the node 11 and the node 16 is disconnected, an alternative route 1 to the node 11 is a path via the node 27, the node 22, the node 17 and the node 12 from the cluster switch to the node 11 and a passage count of the nodes is six times. In the meantime, for the alternative route to the node 11, an alternative route 2 is conceivable in addition to the abovementioned route. The alternative route 2 is a path via the node 7 and the node 12 from the cluster switch to the node 11 and a passage count of the nodes is four times.

Accordingly, in the case of the network link configuration shown in FIG. 8, the path the signal loss of which is reduced can be selected as the alternative route, compared with the transit path of the optical switches provided to each node in normal operation.

As described above, in the design of the transmission network, if the number of the ring networks and the number of nodes in the ring network are considered, transmission loss in the operation path or in the alternative route can be reduced.

Sixth Embodiment

Referring to FIG. 11, the configuration of a transmission network in this embodiment will be described below. FIG. 11 shows the configuration of the transmission network in this embodiment. In FIG. 11, the basic configuration is similar to that shown in FIGS. 1A and 1B and this embodiment will be mainly described with locations different from those in FIGS. 1A and 1B.

This embodiment includes a cluster switch which is an external connection interface and plural ring networks (in a dotted frame) as shown in FIG. 11. The ring network A1 includes six nodes (25, 26, 27, 28, 29, 30), the ring network A2 also includes six nodes (19, 20, 21, 22, 23, 24), the ring network A3 also includes six nodes (13, 14, 15, 16, 17, 18), the ring network A4 also includes six nodes (7, 8, 9, 10, 11, 12), and the ring network A5 also includes six nodes (1, 2, 3, 4, 5, 6). The nodes in each ring network are connected via each transmission line. As for connection among each ring network, a set of the node 1, the node 7, the node 13, the node 19 and the node 25, a set of the node 2, the node 8, the node 14, the node 20 and the node 26, a set of the node 3, the node 9, the node 15, the node 21 and the node 27, a set of the node 4, the node, 10, the node 16, the node 22 and the node 28, a set of the node 5, the node 11, the node 17, the node 23 and the node 29 and a set of the node 6, the node 12, the node 18, the node 24 and the node 30 are formed out of all the nodes in each ring network, and the nodes in the set are connected via each transmission line. Connection with the cluster switch is made between the cluster switch and any of the six nodes (25, 26, 27, 28, 29, 30) in the ring network A1.

In normal operation, as shown in FIG. 12, an access path to the node 10 is a path via the node 28, the node 22 and the node 16 from the cluster switch to the node 10. Accordingly, a passage count of the nodes is five times.

Failure (shown as x in FIG. 11) in the access path to the node 10 will be taken up below. At this time, when a wire between the node 10 and the node 16 is disconnected, an alternative route to the node 10 is a path via the node 29, the node 23, the node 17 and the node 11 from the cluster switch to the node 10 as shown in FIG. 12 and a passage count of the nodes is six times.

When failure occurs in an access path to another node (7, 8, 9, 11, 12) in the ring network A4 to which the node 10 belongs, a passage count of the nodes in the alternative route when the failure occurs is six times, compared with a passage count of the nodes in the path in normal operation is five times.

Accordingly, in the case of the network link configuration shown in FIG. 11, correspondence to a signal transmitted in the alternative line when failure occurs is enabled by designing the transmission network considering a passage count of the optical switches acquired by adding once to the passage count of the optical switches provided to each node in normal operation.

Seventh Embodiment

Referring to FIG. 13, the configuration of a transmission network in this embodiment will be described below. FIG. 13 shows the configuration of the transmission network in this embodiment. In FIG. 13, the basic configuration is similar to that shown in FIG. 3 and this embodiment will be mainly described with locations different from those in FIG. 3.

This embodiment relates to the transmission network when the number of nodes is not limited. An optical switch provided to each node is similar to the optical switch in the second embodiment.

As shown in FIG. 13, this embodiment includes multiple ring networks A, B, C, D, E and shows a case that the number of nodes in each ring network is at least 12. In this embodiment, a cluster switch is omitted. The nodes in the ring network are connected via each transmission line. Among ring networks, as shown in FIG. 13, 11 nodes in the ring network form sets and the nodes in the same set are connected via each transmission line. The transmission line includes an active signal line, a preliminary signal line and a control signal line.

Accordingly, as shown in FIG. 13, total 16 nodes including 12 nodes in the ring network A, one node in the ring network B, one node in the ring network C, one node in the ring network D and one node in the ring network E are connected to a cluster switch.

As described above, access to the ring network E is enabled without passing the ring networks A, B, C, D. Similarly, access to the ring network D is enabled without passing the ring networks A, B, C, access to the ring network C is enabled without passing the ring networks A, B, and access to the ring network B is enabled without passing the ring network A.

Eighth Embodiment

Referring to FIGS. 14A and 14B, the configuration of a transmission network in this embodiment will be described below. FIGS. 14A and 14B show the configuration of the transmission network in this embodiment. In FIGS. 14A and 14B, the basic configuration is similar to that shown in FIG. 1 and this embodiment will be mainly described with locations different from those in FIGS. 1A and 1B.

A multicore fiber can be used for an active signal line and a preliminary signal line which are transmission lines that connect nodes. FIGS. 14A and 14B show the configuration that three cores are allocated to the active signal line and three cores are allocated to the preliminary signal line out of seven cores that configure the multicore fiber. The residual one core may be also allocated to a control signal line that connects an optical switch control/signal monitoring unit.

As described above, the multicore fiber can be used for the active signal line and the preliminary signal line that connect nodes.

The example that a signal is transmitted and received between nodes has been described, however, the present invention is not limited to this example and can be applied to anything such as an optical switch installed in each node of a network and a transmission network for transmitting/receiving a signal in the transmission network except between nodes.

Optical Transmission System

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