JP3600512B2 - Optical transmission network and optical channel setting method - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical transmission network configuration method and an optical channel connection setting technique for performing communication between nodes in the optical transmission network. In particular, the present invention relates to a method of configuring an optical transmission network configured using a wavelength division multiplexing (WDM) technique, and a technique of setting an optical channel between nodes.
[0002]
[Prior art]
A method for setting an optical channel in a conventional optical transmission network will be described below. Here, an optical path network is assumed as an example of the optical transmission network. Regarding the optical path network, there are literatures such as "Kenichi Sato and Satoshi Okamoto" Development of Optical Path Layer Technology ", IEICE Autumn Conference SB-7-1, September 1992", and the like. Here, an optical path network using a WP having no wavelength conversion function will be described as an optical path network. Here, the optical path may be considered to be the same as the optical channel used in the claims.
[0003]
FIG. 5 shows a conventional optical transmission network, and FIG. 6 shows a procedure for setting / releasing an optical path by a backward signaling method using a packet as a control signal. In the optical transmission network of FIG. 5, adjacent nodes are connected by an optical fiber 107. Also, a description will be given using a backward method as a signaling method. The procedure for setting the optical path is as follows.
[0004]
The node that has issued the optical path setting request is the start node 105, and calculates an optimal path for setting the optical path to the target node (end node 106). The method of calculating the optimum route includes, for example, the Dijkstra method, but is not limited to a specific method (for the Dijkstra method, literature: A.V. 00791-9).
[0005]
When the path for setting the optical path is determined, the start node 105 transmits a signal for checking the use state of the wavelength on each link in the optical path setting path to the end node 106. Here, this signal is called a survey packet.
[0006]
The investigation packet is transferred hop-by-hob along the optical path setting path, and the wavelength at the link between the (n + 1) -th node to which the investigation packet is transferred next at the n-th node from the source node 105 Using the usage status and information on the wavelength usage status on the link from the start node 105 to the n-th node, the wavelength usage status on the link from the start node 105 to the (n + 1) -th node is determined.
[0007]
As a result, in the link between the n-th node and the (n + 1) -th node, all the wavelength-multiplexed wavelengths are used for an optical path or the like, so that the (n + 1) -th node is shifted from the start node. If there is no wavelength that can be used for setting an optical path in the link between the n-th node and the n-th node, the n-th node transmits a NACK packet to the source node 105 and receives the NACK packet from the source node. 105 repeats the process of the optical path setting route from the search.
[0008]
If there is a wavelength that can be used for setting an optical path in the link between the start node 105 and the (n + 1) th node, a check packet is transmitted to the (n + 1) th node. The above processing is sequentially performed on each node from the start node 105 to the end node 106 along the optical path setting route.
[0009]
The arrival of the investigation packet at the end node 106 means that at least one wavelength that can be used for setting an optical path exists between the start node 105 and the end node 106. The destination node 106 receiving the investigation packet selects one wavelength used for optical path setting from wavelengths available for optical path setting between the start point and the end point, and the investigation packet is forwarded to the start node 105. A signal for setting an optical path in the reverse direction on the same route as the route is transmitted.
[0010]
Here, this signal is called a setting packet. In the setting packet, information on the wavelength used for setting the optical path and the like are described. The setting packet is a process for setting an optical path between the (n-1) th node to which the next setting packet is transferred in the node located nth from the start node 105 on the optical path setting route. I do.
[0011]
If an optical path cannot be set between the n-th node and the (n-1) -th node using the wavelength selected by the end node 106, an optical path is set to the start node 105. Send a NACK packet to communicate the failure. At the same time, the optical path set from the n-th node for which the setting has failed to the end point node 106 is released.
[0012]
Upon receiving the NACK packet indicating the failure of the optical path setup, the start node 105 restarts the process from the route search. The above-described processing is sequentially performed for each node along the optical path setting path, and the start node 105 receives the setting packet transmitted from the end node 106, whereby the setting of the optical path succeeds. An optical path is set between the nodes 106.
[0013]
The start node 105 starts transferring data to the end node 106. When the data transfer is completed and the set optical path becomes unnecessary, the optical path release processing is started. The procedure for releasing the set optical path is as follows. The start node 105 transmits a release request packet, which is a signal for requesting release of the optical path, to the end node 106.
[0014]
The end point node 106 that has received the release request packet transmits a signal for releasing the optical path to the start point node 105. Here, this signal is called a release packet. The relay node that has received the release packet releases the set optical path.
[0015]
Upon receiving the release packet, the start-point node 105 transmits an ACK packet for transmitting the completion of release of the optical path setting to the end-point node 106. The end-point node 106 receives the ACK packet and performs the optical path release processing. Is completed. The optical path network used here and the optical channel used in the present invention may be considered to be the same.
[0016]
[Problems to be solved by the invention]
In the conventional method as described above, the size of the optical transmission network (WP network) configured by nodes having no wavelength conversion function increases (the number of nodes configuring the optical transmission network increases. For example, a domestic trunk network) And the like, and the success rate of setting the optical channel between the start node and the end node decreases, and the setting time of the optical channel increases.
[0017]
(References: Masaaki Mukoto, Satoshi Okamoto, "Proposal of Dynamic Optical Path Setup Protocol", IEICE PNI Research Group, May 2000, PNI2000-5 PP.34). In addition, in the recovery method at the time of an optical channel failure using the conventional signaling method, similarly, there is a problem that the setting time of the optical channel increases and it is difficult to recover from the failure in a short time.
[0018]
According to the invention, the above-mentioned problem is solved by the means as set forth in the claims.
[0021]
Claim 1 The invention of In an optical transmission network in which adjacent nodes are connected by an optical fiber through which an optical signal wavelength-multiplexed by the wavelength division multiplexing technique is propagated, the optical transmission network is an optical transmission network that is locally and functionally divided. Consists of a set of subnets, and A node having a function of connecting a subnet located in an arbitrary subnet in the optical transmission network does not propagate control / management information in the subnet to a node located in another different subnet, and , By controlling and managing information of other subnets not to be propagated to nodes located in the subnet, it is configured to control and manage any subnet independently of other subnets. Light transmission network It is.
[0022]
Claim 2 The present invention provides a method of setting an optical channel between a start node and an end node existing in different subnets in an optical transmission network according to claim 1, wherein the route for setting an optical channel is connected to adjacent subnets. An optical channel is set by parallel processing for each subnet located along the optical channel setting path between the start node and the end node, and an optical channel is set for each subnet by parallel processing. This is an optical channel setting method in which an optical channel is set between a start point node and an end point node by connecting channels at nodes having a function of connecting adjacent subnets.
[0023]
Claim 3 The invention of claim 2 In the described optical channel setting method, when setting of an optical channel in an arbitrary subnet fails, the optical channel is reset in the subnet.
[0024]
Claim 4 The invention of In an optical transmission network in which adjacent nodes are connected by an optical fiber through which an optical signal wavelength-multiplexed by the wavelength division multiplexing technique is propagated, the optical transmission network is an optical transmission network that is locally and functionally divided. It consists of a set of subnets, and In a process of recovering from a state in which communication cannot be performed due to a failure of an optical channel set between a start point node and an end point node, failure recovery is performed in a subnet including a failure point.
[0025]
As described above, the present invention divides a large-scale optical transmission network composed of nodes having no wavelength conversion function into a plurality of small-scale optical transmission networks (with a small number of nodes), and divides the optical transmission network. Constitute a large-scale optical transmission network. This divided small-scale optical transmission network is called a subnet here.
[0026]
Adjacent subnets are connected by a node having a function of connecting subnets such as a wavelength conversion function. Here, a node for connecting adjacent subnets is called a gateway node.
[0027]
When an optical channel is set between a start node and an end node located on different subnets, the optical channel is set over a plurality of subnets. As described earlier in the section "Problems to be Solved by the Invention", setting an optical channel between a start point and an end point increases the setting time.
[0028]
Therefore, in the present invention, when an optical channel to be set extends over a plurality of subnets, an optical channel is set in parallel for each subnet. Also, an optical channel setting route is set from an arbitrary subnet to an adjacent subnet so as to always pass through a gateway node.
[0029]
Accordingly, the optical channel setting sections include a start node to a gateway node, a gateway node to a gateway node, and a gateway node to an end node. By connecting the optical channels set in these sections at the gateway node, the start and end points are set. An optical channel is set in between.
[0030]
The same applies when an optical channel fails. The optical channel set by the above means is maintained and controlled between the start point and the end point and for each subnet, and when the existing optical channel is interrupted due to a failure in an arbitrary subnet, the failure is recovered in the corresponding subnet. I do.
[0031]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows an optical transmission network according to the configuration of the present invention. In the configuration example shown in FIG. 1, the optical transmission network is divided into four subnets 103, each of which is composed of six nodes 101 and a plurality of gateway nodes 104. Adjacent nodes are connected by an optical fiber 107.
[0032]
Each subnet is assumed to be a WP network constituted by nodes having no wavelength conversion function, but is constituted by a VWP network constituted by nodes having a wavelength conversion function or a WP / VWP mixed network in which these are mixed. It is also possible. Adjacent subnets 103 are connected by a gateway node 104, and the gateway node exists over two or more subnets.
[0033]
The gateway node 104 is equipped with a function required to connect adjacent subnets. The functions include a wavelength conversion function. In addition, information that is maintained and managed independently of the other subnets, such as the wavelength usage status of each link in the subnet, such as information on the used wavelength of each link in the subnet, is not propagated to other subnets. Perform subnet management independently.
[0034]
The optical channel 102 set between the start node 105 and the end node 106 on different subnets is set from the start node 105 to the end node 106 via the gateway node 104 at least once. FIG. 2 is a diagram showing a recovery method when an optical channel fails in the optical transmission network constructed according to the present invention.
[0035]
In FIG. 2, when the active optical channel 102 fails in the subnet including the gateway nodes 104-1 and 104-2, the optical channel is not reset between the start node 105 and the end node 106. Perform recovery within the subnet including the failure point. In FIG. 2, failure recovery is performed by setting the optical channel 201 between the gateway nodes within the subnet including the gateway nodes 104-1 and 104-2.
[0036]
[First embodiment]
FIG. 3 shows an optical channel setting procedure based on backward signaling according to the first embodiment of the present invention in the optical transmission network shown in FIG. Here, it is assumed that the start node 105 and the end node 106 belong to different subnets. This applies to other embodiments described later.
[0037]
In this embodiment, a packet is used as a control signal for processing such as setting or canceling of an optical channel. However, if wavelength information, node information, route information, and the like necessary for setting are described, a signal is used. Is not limited to a packet. For example, it is also possible to use an overhead byte of a layer 1 signal.
[0038]
These control packets are transferred using a dedicated optical channel always terminated between adjacent nodes, for example, an optical section monitoring channel or the like. Further, here, a WP network in which each subnet is configured by a node having no wavelength conversion function is assumed, but the present invention is not limited to this, and a VWP configured by a node having a wavelength conversion function is not limited thereto. The present invention is similarly applicable to a network or a subnet constituted by a mixed network of both.
[0039]
The node that first receives the optical channel setting request becomes the start node 105, and searches for an optimal route for setting the optical channel 102 to the target node (end node 106). As the optimum route, for example, a route including the largest number of wavelengths that can be used for setting an optical channel between the start node 105 and the end node 106 is considered, but the number of hops and the distance between the start point and the end point are the smallest (short). A route or a route obtained by using all of them as parameters may be used.
[0040]
The method of calculating the optimum route is, for example, the Dijkstra method, but is not limited to a specific method. The start node 105 obtains an optical channel setting route by calculation based on the route information, including the gateway nodes 104-1 and 104-2 that pass through the route to the end node 106 located on a different subnet. When the route is determined, the start node 105 transmits a signal to the end node 106 to check the usage status of the wavelength on each link in the optical channel setting route.
[0041]
This signal is referred to herein as a survey packet. The investigation packet is transferred hop-by-hop along the optical channel setting path. The investigation packet is transferred to the end point node 106. First, the available wavelength of the optical channel setting path from the start point node 105 to the gateway node 104-1 passing first is examined.
[0042]
In the n-th node from the start node 105, the wavelength use status on the link between the (n + 1) -th node to which the investigation packet is transferred next, and the link between the start node 105 and the n-th node Is used to determine the wavelength usage status in the link from the start node 105 to the (n + 1) th node.
[0043]
As a result, all the wavelengths multiplexed in the link between the n-th node and the (n + 1) -th node are used from the start node 105 to (n + 1) for the reason that the wavelengths are used for an optical channel or the like. If there is no wavelength that can be used for the optical channel setting in the link between the n-th node, the n-th node transmits a NACK packet to the source node 105, and repeats the process from the path search.
[0044]
If there is a wavelength that can be used for optical channel setting in the link between the start node 105 and the (n + 1) -th node, an investigation packet is transmitted to the (n + 1) -th node. The fact that the first gateway node 104-1 has received the investigation packet means that there is at least one wavelength that can be used for optical channel setting between the source node 105 and the gateway node 104-1.
[0045]
The gateway node 104-1 selects one wavelength used for the optical channel setting from the wavelength information available for the optical channel setting, and in the opposite direction to the source node 105 on the same route as the route on which the investigation packet was transferred. , And transmits a signal for setting an optical channel. Here, this signal is called a setting packet. The setting packet describes information on the wavelength used for setting the optical channel, and the like.
[0046]
By transmitting the packet set by the gateway node 104-1 to the start node 105, the start node 105 to the gateway node 104-independent of the subnet located downstream (in the investigation packet transmission direction) on the set path. The setting of the light channel is started between the two.
[0047]
The setting packet performs processing for setting an optical channel between the (n-1) th node to which the setting packet is to be transferred next at the node positioned nth from the start node 105 on the optical channel setting path. . If the optical channel cannot be set between the n-th node and the (n-1) -th node using the wavelength selected by the gateway node 104-1, the setting of the n-th optical channel has failed. An optical channel set between the node and the gateway node 104-1 is released, and at the same time, a NACK packet for transmitting an optical channel setting failure is transmitted to the start node 105.
[0048]
The source node 105 that has received the NACK packet indicating the optical channel setting failure performs a route search, and performs the process of resetting the optical channel between the source node and the gateway node 104-1 again. At this time, the start node 105 transmits the investigation packet not to the end node 106 but to the gateway node 104-1.
[0049]
At the same time as transmitting a setting packet for setting an optical channel between the start node 105 and the gateway node 104-1 to the start node 105, the gateway node 104-1 has initialized the usable wavelength information of the previously received investigation packet. Thereafter, the status of the wavelength use in the link with the next node is checked, and if there is an available wavelength, it is forwarded to the end node 106. If there is no usable wavelength, the gateway node 104-1 recalculates the optical channel setting path to the end point node 106 and tries to reset it.
[0050]
Investigation of the wavelength use status by the investigation packet for setting the optical channel between the gateway node 104-1 and the gateway node 104-2 and between the gateway node 104-2 and the end point node 106 is the same as described above. When there is no usable wavelength between the gateway node 104-2 and the gateway node 104-2 and the end node 106, the NACK packet returned to the start node 105 is in the same subnet as the node that transmitted the NACK packet. The NACK packet is processed by the upstream gateway node on the optical channel setting path located at the position No. 1 and is not relayed toward the source node 105.
[0051]
The gateway node that has received the NACK packet recalculates the optical channel setting path to the destination node 106, and transfers the investigation packet to the destination node 106. The processing is the same as the method described above.
[0052]
The gateway node 104-2 and the destination node 106 that have received the investigation packet arbitrarily select one wavelength that can be used for the optical channel setting on the optical channel setting path in each subnet, and respectively transmit the setting packet to the gateway node 104-1. , To the gateway node 104-2 to set an optical channel. The setting of the optical channel is the same as that between the start node 105 and the gateway node 104-1.
[0053]
The gateway node that has received the setting packet confirms that the optical channel has been set in the downstream subnet belonging to itself, connects to the optical channel set in the upstream subnet, and connects the optical node between the start node 105 and the end node 106. The channel 102 is opened.
[0054]
The connection of each optical channel in the gateway node is performed using, for example, an identifier for identifying the optical channel to be connected. The setting of the optical channel between the start node 105 and the end node 106 is confirmed by the following procedure. In the first subnet on the optical channel setting path, when the start node 105 receives the setting packet from the gateway node 104-1, the setting of the optical channel between the start node 105 and the gateway node 104-1 is completed.
[0055]
The start-point node 105 that has received the setting packet transmits a setting confirmation packet, which is a signal, to the end-point node 106 to check the setting between the start and end points of the optical channel. The gateway node 104-1 that has received the setting confirmation packet transmits a setting confirmation packet if an optical channel is set between the gateway node 104-1 and the gateway node 104-2 in the next adjacent subnet on the optical channel setting path. To the gateway node 104-2.
[0056]
If the optical channel has not been set, the gateway node 104-1 transmits a NACK packet to the source node 105. Upon receiving the NACK packet, the start node 105 transmits a setting confirmation packet again after a predetermined time has elapsed, and performs the above processing in the order of subnets. Upon receiving the setting confirmation packet, the end node 106 transmits an ACK packet indicating that the setting of the optical channel 102 has been completed between the start point and the end point to the start node 105, and the start node 105 receives the ACK packet. By doing so, an optical channel is set between the start node 105 and the end node 106. The setting confirmation packet and the ACK of the setting confirmation packet may be transmitted using the set optical channel.
[0057]
The start node 105 repeats the above-described processing until an ACK packet is received. However, if an ACK packet is not received after a certain period of time, the optical channel setting between the start node 105 and the end node 106 has failed. And release the corresponding optical channel. The data transmission ends, and the setting of the unused optical channel is released. The procedure for releasing the optical channel is the same as that of the above-described conventional example.
[0058]
[Second embodiment]
FIG. 4 shows an optical channel setting procedure by backward signaling according to the second embodiment of the present invention in the optical transmission network shown in FIG. In this embodiment, a packet is used as a control signal for processing such as setting or canceling of an optical channel. However, if wavelength information, node information, route information, and the like necessary for setting are described, a signal is used. Is not limited to a packet, and it is also possible to use, for example, an overhead byte of a layer 1 signal.
[0059]
These control packets are transferred using a dedicated optical channel always terminated between adjacent nodes, for example, an optical section monitoring channel or the like. Here, each subnet is assumed to be a WP network constituted by nodes having no wavelength conversion function. However, the present invention is not limited to this, and the present invention provides a VWP network constituted by nodes having a wavelength conversion function. Alternatively, the present invention can be similarly applied to a subnet constituted by a network in which both are mixed.
[0060]
The node that first receives the optical channel setting request becomes the start node 105, and searches for an optimal route for setting the optical channel 102 to the target node (end node 106). However, since the end node 106 is located on a different subnet from the start node 105, the start node 105 calculates not the route to the end node 106 but the optimal route to the end node 106 based on the route information.
[0061]
As the optimal route, for example, a route that includes the largest number of wavelengths that can be used for optical channel setting can be considered. It may be a route. The method of calculating the optimum route includes, for example, the Dijkstra method, but is not limited to a specific method.
[0062]
When the route is determined, the start node 105 transmits a signal to the gateway node 104-1 for checking the usage status of the wavelength on each link in the optical channel setting route. Here, this signal is called a survey packet. The investigation packet is transferred hop-by-hop along the optical channel setting path.
[0063]
At the n-th node from the start node 105, the wavelength usage state on the link between the (n + 1) -th node to which the investigation packet is transferred next, and the wavelength on the link between the start node 105 and the n-th node Using the information on the wavelength usage status, the wavelength usage status on the link from the start node 105 to the (n + 1) th node is obtained.
[0064]
As a result, in the link between the n-th node and the (n + 1) -th node, all of the wavelength-multiplexed wavelengths are used for an optical channel or the like, so that (n + 1) If there is no wavelength that can be used for the optical channel setting in the link between the n-th node, the n-th node transmits a NACK packet to the source node 105, and repeats the process from the path search.
[0065]
If there is a wavelength that can be used for optical channel setting in the link between the start point node 105 and the (n + 1) th node, an investigation packet is transmitted to the (n + 1) th node. The fact that the gateway node 104-1 has received the investigation packet means that there is at least one wavelength that can be used for optical channel setting between the source node 105 and the gateway node 104-1.
[0066]
The gateway node 104-1 selects one wavelength used for the optical channel setting from the wavelength information usable for the optical channel setting, and transmits the light in the reverse direction on the same route as the route on which the investigation packet was transferred to the source node 105. Transmit a signal to set a channel. Here, this signal is called a setting packet.
[0067]
The setting packet describes information on the wavelength used for setting the optical channel, and the like. At the same time, the gateway node 104-1 calculates an optical channel setting path to the destination node 106, which is the final destination node of the investigation packet. In the case of the optical transmission network shown in FIG. 1, the destination node 106 is not included in the subnet including the gateway node 104-1.
[0068]
Therefore, the gateway node 104-1 calculates the optimum route to the gateway node 104-2 in order to transmit the investigation packet to the gateway node 104-2 that can set the optimum route to the end point node 106. After the determination of the optimum route, the status of the wavelength used in the link with the next node is checked, and if there is an available wavelength, it is transmitted to the gateway node 104-2.
[0069]
If there is no usable wavelength, the gateway node 104-1 recalculates the optical channel setting path to the gateway node 104-2 and tries to reset it. By repeatedly executing the above processing procedure, the investigation packet transmitted from the start node 105 reaches the end node 106. Each of the gateway nodes 104-1 and 104-2 and the end point node 106 immediately set an optical channel in the subnet after receiving the investigation packet.
[0070]
The setting packet is transmitted to the source node 105 or the gateway node in the same subnet. The setting of the optical channel in each subnet is the same as in the first embodiment. When the start node 105 or the upstream gateway node receives the setting packet, an optical channel has been set in the subnet.
[0071]
After that, the connection is established by the gateway node that relays the optical channel set to each subnet, thereby completing the setting of the optical channel 102 between the start node 105 and the end node 106. This processing procedure is the same as in the first embodiment. When the data transmission is completed, the setting of the unused optical channel is released. The procedure for releasing the optical channel is the same as in the above-described conventional example.
[0072]
【The invention's effect】
As described above, according to the optical transmission network and the optical channel setting method of the present invention, in a large-scale optical transmission network, an optical channel can be set in parallel on a plurality of subnets. Compared with the method, the success rate of the optical channel setting is increased, and the setting of the optical channel can be performed in a short time. Also, by providing a subnet, even in the event of a failure, the effect of the failure can be prevented from being propagated to all nodes in the network, so that there is an advantage that recovery from the failure can be performed in a short time.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating an example of the configuration of an optical transmission network according to the present invention.
FIG. 2 is a diagram illustrating a failure recovery method in an optical transmission network according to the present invention.
FIG. 3 is a diagram illustrating an optical channel setting / releasing method using backward signaling according to the first embodiment;
FIG. 4 is a diagram illustrating an optical channel setting / releasing method using backward signaling according to a second embodiment.
FIG. 5 is a diagram illustrating an example of a configuration of a conventional optical transmission network.
FIG. 6 is a diagram illustrating a conventional optical channel setting / releasing method using backward signaling.
[Explanation of symbols]
101 nodes
102,201 optical channel
103 subnet
104-1, 104-2 Gateway node
105 Start node
106 End node
107 Optical fiber

Claims (4)

In an optical transmission network in which adjacent nodes are connected by an optical fiber through which an optical signal wavelength-multiplexed by the wavelength division multiplexing technique is propagated,
The optical transmission network is composed of a set of subnets, which are regional and functionally divided optical transmission networks,
Node having a function of connecting the subnet located in any subnet in the optical transmission network is not propagated to the node located in the control and management information in the other different subnets within the subnet, Also, an optical transmission network that controls and manages an arbitrary subnet independently of another subnet by not transmitting control / management information of another different subnet to a node located in the subnet. 2. The method according to claim 1, wherein an optical channel is set between a source node and a destination node existing in different subnets.
It is determined that the route for setting the optical channel always passes through a node having a function of connecting adjacent subnets,
An optical channel is set by parallel processing for each subnet located along the optical channel setting path between the start node and the end node,
By connecting an optical channel set by parallel processing for each subnet at a node having a function of connecting adjacent subnets,
An optical channel setting method, comprising: setting an optical channel between a start node and an end node. 3. The optical channel setting method according to claim 2 , wherein when setting of an optical channel in an arbitrary subnet fails, the optical channel is reset within the subnet. In an optical transmission network in which adjacent nodes are connected by an optical fiber through which an optical signal wavelength-multiplexed by the wavelength division multiplexing technique is propagated,
The optical transmission network is composed of a set of subnets, which are regional and functionally divided optical transmission networks,
When the optical channel set between the start node and the end node in the optical transmission network is in a state where communication cannot be performed due to a failure, in the process of recovering from the state, a failure recovery is performed in a subnet including a failure point. An optical transmission network characterized in that:

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