Patent Publication Number: US-2015071635-A1

Title: Apparatus and method for effective design of a communication network enabling large-capacity transmission

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-188567 filed on Sep. 11, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to apparatus and method for effective design of a communication network enabling large-capacity transmission. 
     BACKGROUND 
     With an increase in communication demand because of the widening use of cloud services, smartphones, and so on, optical networks utilizing wavelength division multiplexing (WDM) have come into widespread use. Wavelength division multiplexing is a technology for transmitting multiplexed optical signals having different wavelengths. 
     With wavelength division multiplexing, for example, optical signals with 88 wavelengths and a transmission speed of 40 Gbps may be multiplexed and transmitted as a wavelength-multiplexed optical signal (hereinafter referred to as a “multiplexed optical signal”). One known example of wavelength division multiplexing transmission equipment utilizing WDM is reconfigurable optical add-drop multiplexer (ROADM) equipment. 
     Although the transmission capacities of wavelength division multiplexing transmission equipment are increasing, the transmission capacities of optical fibers for transmitting multiplexed optical signals are limited. For example, the wavelength bands of light that propagates through optical fibers are limited because of the physical properties of the optical fibers. Examples of the wavelength bands include the conventional band (C band) and the long band (L band). 
     In recent years, with anticipation of an increase in future communication demand, attempts are being made to realize coherent transmission by applying a polarization multiplexing (dual polarization) system or a multilevel modulation system, such as quaternary phase-shift keying (QPSK) used for wireless communication, to wavelength division multiplexing transmission equipment. In order to increase the communication capacity, a multilevel modulation system for a larger amount of data and a higher-density frequency multiplexing technology are used. However, the communication capacity is approaching Shannon&#39;s theoretical limit. 
     Thus, in network design, a scheme for providing an optical fiber cable accommodating a plurality of optical fibers between the same nodes is conceivable to increase the transmission capacity between pieces of wavelength division multiplexing transmission equipment. An optical fiber cable accommodates a plurality of optical fibers (for example, hundreds to thousands of optical fibers) within its sheath. With respect to network design, for example, Japanese National Publication of International Patent Application No. 2005-032076 discloses a scheme for designing optimum paths in an optical network. 
     SUMMARY 
     According to an aspect of the invention, an apparatus designs a communication route for each of requested communication channels by selecting, with higher priority, first transmission paths that provide connections between particular nodes in a network in which a wavelength-multiplexed optical signal is transmitted than second transmission paths that provide connections between three or more nodes in the network, and assigns, for each communication channel, a wavelength included in the wavelength-multiplexed optical signal. When a first number of wavelengths assigned to the communication channels passing through the first transmission paths between the particular nodes is greater than a second number of wavelengths assigned to the communication channels passing through the second transmission paths between the particular nodes, the apparatus modifies, the designed communication routes and the assigned wavelengths so that one of two communication channels to which a same wavelength is assigned is routed from the first transmission path to the second transmission path via any of the particular nodes and other one of the two communication channels is routed from the second transmission path to the first transmission path via any of the particular nodes. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of a network in which transmission paths and nodes are made redundant; 
         FIG. 2  is a diagram illustrating an example of a network in which transmission paths are made redundant; 
         FIG. 3  is a diagram illustrating an example of a network in which transmission paths between particular nodes are made redundant; 
         FIG. 4  is a diagram illustrating an example of wavelength division multiplexing transmission equipment at a general node; 
         FIG. 5  is a diagram illustrating an example of wavelength division multiplexing transmission equipment at a local node; 
         FIG. 6  is a diagram illustrating an example of a configuration of a network design apparatus, according to an embodiment; 
         FIG. 7  is a diagram illustrating an example of a function configuration of a central processing unit (CPU) and information stored in a hard disk drive (HDD), according to an embodiment; 
         FIG. 8  is a diagram illustrating an example of demand information, according to an embodiment; 
         FIG. 9  is a diagram illustrating an example of a turn-back route; 
         FIG. 10  is a diagram illustrating an example of a network in which transmission paths between particular nodes are made redundant; 
         FIG. 11  is a diagram illustrating an example of design of communication routes, according to an embodiment; 
         FIGS. 12A and 12B  are diagrams illustrating an example of communication routes before modification and an example of communication routes after the modification, according to an embodiment; 
         FIG. 13  is a diagram illustrating an example of the communication routes after the modification, according to an embodiment; 
         FIG. 14  is a diagram illustrating an example of modifications of communication routes, according to an embodiment; 
         FIG. 15  is a diagram illustrating an example of an operational flowchart for a network design method, according to an embodiment; 
         FIG. 16  is a diagram illustrating an example of an operational flowchart for communication-route design processing, according to an embodiment; 
         FIG. 17  is a diagram illustrating an example of an operational flowchart for communication-route and wavelength modification processing, according to an embodiment; and 
         FIG. 18  is a diagram illustrating an example of costs for respective network configurations, according to an embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     The wavelength division multiplexing transmission equipment at each node has components (for example, wavelength selective switches and optical amplifiers) for pathways corresponding to the number of optical fibers. Thus, when a plurality of optical fibers are provided between nodes, there is a problem in that the equipment cost increases. In this case, since transmission route candidates corresponding to the number of optical fibers exist during selection of transmission routes that provide connection between predetermined nodes, there is also a problem in that the network design becomes complicated. 
       FIG. 1  is a diagram illustrating an example of a network in which transmission paths and nodes are made redundant. This network includes nodes A to J and nodes a to j provided in exchanges  90 . Although a case in which a network to be designed is a ring network is described in this example, the embodiment is not limited thereto, and the network may be a network having another architecture, such as a linear or mesh network. 
     The nodes A to J are connected to each other through first transmission paths  910 , and the nodes a to j are connected through second transmission paths  911 . Thus, the nodes A to J and the nodes a to j are independent from each other in the network. The first transmission paths  910  and the second transmission paths  911  each include a pair of optical fibers that transmit light in directions opposite to each other. The first transmission paths  910  and the second transmission paths  911  are accommodated in the same optical fiber cables (communication cables)  91 . 
     At the nodes A to J and a to j, respective pieces of wavelength division multiplexing transmission equipment, such as ROADMs, are provided. Thus, each piece of the wavelength division multiplexing transmission equipment at the nodes A to J wavelength-multiplexes an optical signal λin 0 , input (inserted) from an external network (not illustrated), with another optical signal and transmits the resulting signal to the adjacent node as a multiplexed optical signal. Each piece of the wavelength division multiplexing transmission equipment at the nodes A to J also splits (branches) an optical signal λout 0  from a multiplexed optical signal transmitted from the adjacent node and outputs the resulting signals to an external network. Each piece of the wavelength division multiplexing transmission equipment at the nodes a to j also transmits an optical signal λin 1 , input from an external network, to the adjacent node as a multiplexed optical signal and splits an optical signal λout 1  from a multiplexed optical signal transmitted from the adjacent node. A network management apparatus (not illustrated) sets, for pieces of the wavelength division multiplexing transmission equipment at the nodes A to J and a to j, the wavelengths of optical signals that are inserted and the wavelengths of optical signals that are branched. 
     Thus, in the network in this example, a communication channel may be provided between arbitrary nodes (except between the nodes A to J and the nodes a to j). The pieces of wavelength division multiplexing transmission equipment at the nodes A to J are connected to the corresponding first transmission paths  910 , and the pieces of wavelength division multiplexing transmission equipment at the nodes a to j are connected to the corresponding second transmission paths  911 , thus providing two pathways (that is, transmission paths connected to the adjacent nodes). 
     In the network in this example, the exchanges  90  are connected to each other through the optical fiber cables  91 . Thus, the network has a transmission capacity twice as large as that of a network in which the nodes are not made redundant. However, since the nodes in the exchanges  90  are also made redundant, the equipment cost and the operating cost are also twice as high as those in a network in which the nodes are not made redundant. In the network in this example, since the nodes A to J and the nodes a to j are connected through the individual transmission paths  910  and  911 , a requested communication channel is distributed to either of the two transmission paths  910  and  911  in the network design. Thus, when this network is used to provide a communication service, there is the inconvenience that optical signals of customers that receive the communication service provided using the different transmission paths  910  and  911  are not inter-connectable in the form of light without converting the optical signals into electrical signals, since two sub-networks respectively including the different transmission paths  910  and  911  are independent from each other. 
     Accordingly, in order to reduce the number of nodes, the nodes A to J and the nodes a to j may be integrated together in the exchanges  90  to configure a network in which the transmission paths are made redundant.  FIG. 2  is a diagram illustrating an example of a network in which transmission paths are made redundant. In  FIG. 2 , elements that are the same as or similar to those in  FIG. 1  are denoted by the same reference numerals, and descriptions thereof are not given hereinafter. 
     In the network in this example, exchanges  90  are provided with respective nodes A to J. The nodes A to J are connected to each other through first transmission paths  910  and second transmission paths  911 . Thus, each piece of the wavelength division multiplexing transmission equipment provided in the nodes A to J has four pathways. 
     In this example network, although the number of nodes in each exchange  90  is reduced to one, the number of pathways at each wavelength division multiplexing transmission equipment increases, and thus cost is not reduced sufficiently. In addition, since each of the nodes A to J is connected to both the first transmission paths  910  and the second transmission paths  911 , a single network is formed. Thus, in this network, the inconvenience related to the interconnection described above with reference to  FIG. 1  does not occur. 
     However, since two candidate transmission paths  910  and  911  exist for each of the nodes A to J, design of a communication route for a communication channel is complicated. For example, when a communication channel P is requested between the nodes G and J, the number of communication-route candidates for the communication channel P is 8 (=2×2×2), since two candidate transmission paths exist between the nodes G and H, two between the nodes H and I, and two between the nodes I and J. Hence, it is desired that the communication route design be simplified. 
     Also, in the networks illustrated in  FIGS. 1 and 2 , the nodes A to J and the nodes a to j are connected to each other through the optical fiber cables  91  accommodating the plurality of optical fibers. Thus, for example, when any of the optical fiber cables  91  is broken, failures may occur in the plurality of the optical fibers accommodated therein at the same time. When failures occur in the plurality of optical fibers at the same time, a problem arises in that the multiple failures make it difficult to re-establish communication channels. 
     For example, in  FIG. 2 , when the optical fiber cable  91  between the node I and the node J is broken (see mark ×), failures occur in both of the first transmission path  910  and the second transmission path  911  in the section. In this case, multiple failures occur in a communication route R that originates at the node G, turns back at the node J, and reaches the node I. Thus, it is desirable that the network using the optical fiber cables  91  be designed so as to avoid multiple failures. 
       FIG. 3  is a diagram illustrating an example of a network in which transmission paths between particular nodes are made redundant. In  FIG. 3 , elements that are the same as or similar to those in  FIG. 1  are denoted by the same reference numerals, and descriptions thereof are not given hereinafter. 
     In the network in this example, only particular nodes A, D, and I are connected to first transmission paths  910 , and other nodes B, C, E to H, and J are connected to only second transmission paths  911 . In exchanges in which the nodes B, C, E to H, and J are provided, the first transmission paths  910  are coupled to each other via optical connectors  900 . The first transmission paths  910  may also be coupled to each other via optical amplifiers, instead of the optical connectors  900 . 
     According to this configuration, the second transmission paths  911  provide connections between all (three or more) the nodes A to J in the network, and the first transmission paths  910  provide connections between the particular nodes A, D, and I in the network. This makes it easier to selectively use the first transmission paths  910  and the second transmission paths  911 , thus simplifying the design of communication routes. If this network is compared to a railroad, the first transmission paths  910  correspond to local lines, and the second transmission paths  911  correspond to express lines. The particular nodes A, D, and I correspond to express train stations, and the other nodes B, C, E to H, and J correspond to regular stations. In the following description, the nodes A, D, and I are referred to as “general nodes”, and the nodes B, C, E to H, and J are referred to as “local nodes”. Also, the first transmission paths  910  are referred to as “sub transmission paths”, and the second transmission paths  911  are referred to as “main transmission paths”. 
       FIG. 4  is a diagram illustrating an example of the wavelength division multiplexing transmission equipment at the general nodes A, D, and I. Although  FIG. 4  illustrates the configuration of the wavelength division multiplexing transmission equipment at the general node D, the configurations of the wavelength division multiplexing transmission equipment at the other general nodes A and I are also substantially the same. 
     The wavelength division multiplexing transmission equipment has four multiplexers  72   a  and  72   b , four demultiplexers  71   a  and  71   b , and an optical switch  70 . Each of the demultiplexers  71   a  and  71   b  demultiplexes an input multiplexed optical signal by splitting optical signals with different wavelengths and outputs the resulting optical signals to the optical switch  70 . The demultiplexers  71   a  are connected to the corresponding adjacent general nodes A and I through the sub transmission paths  910 , and the demultiplexers  71   b  are connected to the corresponding adjacent local nodes C and E through the main transmission paths  911 . 
     The optical switch  70  switches between destinations to which optical signals are to be output. The optical switch  70  outputs multiplexed optical signals, input from the demultiplexers  71   a  and  71   b , or optical signals λin, input from an external network, to the multiplexers  72   a  and  72   b  corresponding to the pathways to which the optical signals are to be output. The optical switch  70  also outputs only optical signals λout to be branched to an external network, out of the optical signals being included in optical signals that have been split according to the wavelengths by the demultiplexers  71   a  and  71   b.    
     Each of the multiplexers  72   a  and  72   b  multiplexes optical signals with different wavelengths. Each of the multiplexers  72   a  and  72   b  multiplexes optical signals input from the optical switch  70  to generate a multiplexed optical signal and outputs the multiplexed optical signal. The multiplexers  72   a  are connected to the corresponding adjacent general nodes I and A through the sub transmission paths  910 , and the multiplexers  72   b  are connected to the corresponding adjacent local nodes E and C through the main transmission paths  911 . 
       FIG. 5  is a diagram illustrating an example of the wavelength division multiplexing transmission equipment at the local nodes B, C, E to H, and J. Although  FIG. 5  illustrates the configuration of the wavelength division multiplexing transmission equipment at the local node F, the configurations of the wavelength division multiplexing transmission equipment at the other local nodes B, C, E, G, H, and J are also substantially the same. 
     The wavelength division multiplexing transmission equipment has two multiplexers  62 , two demultiplexers  61 , and an optical switch  60 . Each demultiplexer  61  demultiplexes an input multiplexed optical signal by splitting optical signals with different wavelengths and outputs the resulting optical signals to the optical switch  60 . The demultiplexers  61  are connected to the corresponding adjacent local nodes E and G through the main transmission paths  911 . 
     The optical switch  60  switches between destinations to which optical signals are to be output. The optical switch  60  outputs multiplexed optical signals, input from the demultiplexers  61 , or optical signals λin, input from an external network, to the multiplexers  62  corresponding to the pathways to which the optical signals are to be output. The optical switch  60  also outputs only optical signals λout to be branched to an external network, out of the optical signals being included in optical signals that have been split according to the wavelengths by the demultiplexers  61 . 
     Each multiplexer  62  multiplexes optical signals with different wavelengths. Each multiplexer  62  multiplexes optical signals input from the optical switch  60  to generate a multiplexed optical signal and outputs the multiplexed optical signal. The multiplexers  62  are connected to the corresponding adjacent local nodes E and G through the main transmission paths  911 . 
     As described above, the number of pathways at each piece of the wavelength division multiplexing transmission equipment at the general nodes A, D, and I is 4 and the number of pathways at each piece of the wavelength division multiplexing transmission equipment at the local nodes B, C, E to H, and J is 2. Thus, the total number of multiplexers  72   a  and  72   b  and demultiplexers  71   a  and  71   b  in each piece of the wavelength division multiplexing transmission equipment at the general nodes A, D, and I is 8, and the total number of multiplexers  62  and demultiplexers  61  in the wavelength division multiplexing transmission equipment at the local nodes B, C, E to H, and J is 4. 
     Hence, the general nodes A, D, and I have a larger number of optical components than the local nodes B, C, E to H, and J, and thus involve a higher equipment cost than that of the local nodes B, C, E to H, and J. However, in the network illustrated in  FIG. 3 , since the general nodes A, D, and I are particular nodes, not all of the nodes, the equipment cost is reduced compared with the network in  FIG. 2  in which all of the nodes are general nodes. For example, in order to design the network illustrated in  FIG. 3 , a network design apparatus according to the embodiment performs communication-route design and wavelength assignment for each requested communication channel. 
       FIG. 6  is a diagram illustrating an example of a configuration of a network design apparatus, according to an embodiment. The network design apparatus is, for example, a computer apparatus such as a server. The network design apparatus includes a CPU  10 , a read only memory (ROM)  11 , a random access memory (RAM)  12 , an HDD  13 , a communication processing unit  14 , a portable-storage-medium drive  15 , an input processing unit  16 , and an image processing unit  17 . 
     The CPU  10  is a computational processor and performs network design processing in accordance with a network design program. The CPU  10  is communicably coupled to the aforementioned elements  11  to  17  through a bus  18 . The network design apparatus  1  is not limited to an apparatus that operates on software. The CPU  10  may also be replaced with other hardware, such as an integrated circuit for a specific application. 
     The RAM  12  is used as a working memory for the CPU  10 . The ROM  11  and the HDD  13  are used to store therein, for example, the network design program, which causes the CPU  10  to operate. The communication processing unit  14  is, for example, a network card and communicates with external apparatuses and equipment through a network, such as a local area network (LAN). 
     The portable-storage-medium drive  15  is equipment that writes information to and reads information from a portable storage medium  150 . Examples of the portable storage medium  150  include a Universal Serial Bus (USB) memory, a recordable compact disc (CD-R), and a memory card. The network design program may also be stored in/on the portable storage medium  150 . 
     The network design apparatus further includes input equipment  160  for performing an operation for inputting information and a display  170  for displaying images. The input equipment  160  is, for example, a keyboard, a mouse, and so on. Information input using the input equipment  160  is output to the CPU  10  via the input processing unit  16 . The display  170  is, for example, a liquid-crystal display that displays images. Image data from the CPU  10  is output and displayed on the display  170  via the image processing unit  17 . The input equipment  160  and the display  170  may also be replaced with equipment, such as a touch panel having those functions. 
     The CPU  10  executes programs stored in the ROM  11 , the HDD  13 , or the like or programs read from the portable storage medium  150  by the portable-storage-medium drive  15 . The programs include not only an operating system (OS) but also the aforementioned network design program. The programs may also include a program downloaded via the communication processing unit  14 . 
     Upon executing the network design program, the CPU  10  realizes multiple functions.  FIG. 7  is a diagram illustrating an example of the functions of the CPU  10  and information stored in the HDD  13 , according to an embodiment. 
     The CPU  10  includes a communication-route designing unit  100 , a wavelength assigning unit  101 , and a modification processing unit  102 . The HDD  13  also stores therein topology information  130 , demand information  131 , transmission path information  133 , communication route information  134 , and wavelength assignment information  135  in connection with the communication-route designing unit  100 , the wavelength assigning unit  101 , and the modification processing unit  102 . The storage of the information  130  to  135  is not limited to the HDD  13  and may also be the ROM  11  or the portable storage medium  150 . 
     The topology information  130 , the demand information  131 , and the transmission path information  133  are design information indicating conditions for designing the network. For example, the topology information  130 , the demand information  131 , and the transmission path information  133  may be input via the input equipment  160  by an operator or may also be downloaded from a network via the communication processing unit  14 . 
     The topology information  130  indicates a topology of a network (see  FIG. 3 ) to be designed, that is, the relationship of connections of the nodes A to J through links. The topology information  130  is composed, for example, by associating identifiers of a pair of nodes connected through each link in the network with an identifier of the link. 
     The demand information  131  indicates the contents of requests for communication channels to be established in the network. The demand information  131  includes, for example, information identifying a pair of nodes serving as termination points (a start point and an end point) of each communication channel, and the number of wavelengths used for each of the communication channels. Each pair of nodes that serve as the termination points of a communication channel is a combination of a node to which an optical signal λin is inserted and a node at which an optical signal λout is branched. 
     The transmission path information  133  indicates the configuration of transmission paths that provide connections between the nodes A to J in the network. The transmission path information  133  is composed by associating the number of optical fibers with a pair of nodes that serve as termination points with respect to each of the main transmission paths  911  that provide connections between all (three or more) the nodes A to J and each of the sub transmission paths  910  that provide connections between the general nodes A, D, and I. 
     The communication-route designing unit  100  reads the topology information  130 , the demand information  131 , and the transmission path information  133  and selects the sub transmission paths  910  with higher priority than the main transmission paths  911  to thereby design a communication route for each requested communication channel. Design processing of communication routes will be described below. 
       FIG. 8  is a diagram illustrating an example of the contents of the demand information  131 .  FIG. 8  illustrates a linearly expanded form of the network illustrated in  FIG. 3 . In this example, the upper limit of the number of wavelengths assignable to each transmission path is assumed to be 4. 
     A communication channel P 1  is requested between the nodes A and D, and the number of wavelengths is 3 (see “×3” in the parentheses, which notation also applies to the following). A communication channel P 2  is requested between the nodes D and I, and the number of wavelengths is 3. A communication channel P 3  is requested between the nodes I and A, and the number of wavelengths is 2. A communication channel P 4  is requested between the nodes B and D, and the number of wavelengths is 2. A communication channel P 5  is requested between the nodes E and G, and the number of wavelengths is 1. 
     A communication channel P 6  is requested between the nodes G and H, and the number of wavelengths is 1. A communication channel P 7  is requested between the nodes I and J, and the number of wavelengths is 1. A communication channel P 8  is requested between the nodes C and J, and the number of wavelengths is 1. A communication channel P 9  is requested between the nodes F and A, and the number of wavelengths is 2. 
     In  FIG. 8 , each numeral indicated in a circle represents the total number of optical signals λin and λout inserted into or branched at a corresponding one of the nodes A to J. For example, in the case of the node A, since three optical signals of the communication channel P 1 , two optical signals of the communication channel P 3 , and two optical signals of the communication channel P 9  are inserted or branched, the total number of optical signals λin and λout is 7. Also, in the case of the node G, since an optical signal of the communication channel P 5  and an optical signal of the communication channel P 6  are inserted or branched, the total number of optical signals λin and λout is 2. 
     In this example, the nodes A, D, and I at which the total number of optical signals λin and λout is 5 or more are referred to as general nodes, and the nodes B, C, E to H, and J at which the total number of optical signals λin and λout is 4 or less are referred to as local nodes. Thus, by determining the general nodes and the local nodes depending on the total number of optical signals λin and λout in accordance with the demand information  131 , the communication-route designing unit  100  may efficiently design communication routes for the communication channels P 1  to P 9 . 
     That is, since each general node is connected to both of the main transmission paths  911  and the sub transmission paths  910 , the number of candidates of routes of the optical signals λin and λout is larger than that of the local node. This makes it possible to flexibly provide a communication route. When the largest number of wavelengths of optical signals transmitted to the main transmission path  911  and the sub transmission path  910  is assumed to be 4, the total number of optical signals λin and λout at each of the general nodes A, D, and I exceeds 4. Thus, the optical signals λin and λout are separately transmitted to the main transmission path  911  and the sub transmission path  910 . 
     The communication-route designing unit  100  divides the communication channels P 1  to P 9  indicated by the demand information  131  into two groups, depending upon whether or not the sub transmission paths  910  are usable. More specifically, the communication-route designing unit  100  determines whether or not any of links L 1  to L 3  that provide connections between the general nodes exist in each of the sections of the communication channels P 1  to P 9 , and divides the communication channels P 1  to P 9  into two groups in accordance with the result of the determination. The link L 1  is a link between the general nodes A and D, the link L 2  is a link between the general nodes D and I, and the link L 3  is a link between the general nodes A and I. 
     In this example, the link L 1  exists in the section of the communication channel P 1  (between the nodes A and D), the link L 2  exists in the sections of the communication channel P 2  (between the nodes D and I) and the communication channel P 8  (between the nodes C and J), and the link L 3  exists in the sections of the communication channel P 3  (between the nodes A and I) and the communication channel P 9  (between the nodes A and F). Thus, the communication channels P 1  to P 3 , P 8 , and P 9  belong to the group that is allowed to use the sub transmission paths  910 , and the other communication channels P 4  to P 7  belong to the group that is not allowed to use the sub transmission paths  910 . 
     With respect to the group that is allowed to use the sub transmission paths  910 , the communication-route designing unit  100  designs communication routes including the sub transmission paths  910 . For example, the communication-route designing unit  100  selects a combination of the sub transmission path  910  between the general nodes D and I, the main transmission path  911  between the local nodes C and D, and the main transmission path  911  between the local nodes I and J as a communication route for the communication channel P 8 . 
     With respect to the group that is not allowed to use the sub transmission paths  910 , the communication-route designing unit  100  designs a communication route including only the main transmission path(s)  911 . For example, the communication-route designing unit  100  selects a combination of the main transmission path  911  between the local nodes E and F and the main transmission path  911  between the local nodes F and G as a communication route for the communication channel P 5 . 
     The communication-route designing unit  100  does not design a communication route through which an optical signal is turned back at a general node to an input-source node, as described above with reference to  FIG. 2 , in order to avoid occurrence of multiple failures.  FIG. 9  is a diagram illustrating an example of a turn-back route in a network. 
     For example, it is assumed that a communication channel P 10  is requested between the general node D and the local node H. In this case, the communication-route designing unit  100  is not allowed to select, as a communication route R 10  for the communication channel P 10 , a combination of the sub transmission path  910  between the general nodes D and I and the main transmission path  911  between the general node I and the local node H. If the communication route R 10  is permitted, multiple failures due to a break of the optical fiber cable  91  may occur between the general nodes D and I, since the optical fibers of the main transmission paths  911  and the optical fiber of the sub transmission path  910  are accommodated in the same optical fiber cable  91 . 
     In order to avoid design of a communication route through which an optical signal is turned back toward an input-source node, optical cross-connect equipment for transmitting wavelength-multiplexed optical signals may also be used as the wavelength division multiplexing transmission equipment (see  FIG. 4 ) provided at the general nodes A, D, and I. In this case, since the optical switches  70  regulate the pathways to which the optical signals are output, the wavelength-multiplexed optical signals are transmitted in only one direction d in the ring of the network to prohibit a turn-back communication route. A system that is different from the optical cross-connect equipment may also be used to transmit the wavelength-multiplexed optical signals in only one direction d in the ring of the network. 
     However, such a turn-back communication route may be permitted, unless the optical fibers of the main transmission path  911  and the optical fibers of the sub transmission path  910  are accommodated in the same optical fiber cable  91 .  FIG. 10  is a diagram illustrating another example of a network in which transmission paths between particular nodes are made redundant. 
     The network in this example is logically the same as the example network illustrated in  FIG. 3 . However, a turn-back communication route is permitted, since main transmission paths  911  and sub transmission paths  910  are installed independently from each other. Needless to say, the direction in which multiplexed optical signals are transmitted may also be limited in a certain direction d in the ring of the network in this example by using the above-described system to prohibit a turn-back communication route. Although an example in which a turn-back communication route is not permitted is described below for convenience of description, the network in this example is not excluded from a design target. 
     Next, details of design of communication routes will be described in conjunction with another example of the network.  FIG. 11  is a diagram illustrating an example of design of communication routes in a network, according to an embodiment. 
     In the network in this example, general nodes A, C, E, G, and I are connected to each other through sub transmission paths  910  and are connected to local nodes B, D, F, H, and J through main transmission paths  911 . The local nodes B, D, F, H, and J are connected to the general nodes A, C, E, G, and I through the main transmission paths  911 . In the network in this example, a communication channel P 11  is requested between the local nodes B and H, and a communication channel P 12  is requested between the local nodes D and F. The number of wavelengths of each of the communication channels P 11  and P 12  is 1 (see “×1”). 
     Since a link that provides connection between the general nodes C and G exists in the section of the communication channel P 11 , the communication channel P 11  belongs to the group that is allowed to use the sub transmission paths  910 . Thus, the communication-route designing unit  100  generates following routes (1) to (3) as candidates of a communication route for the communication channel P 11 . 
     Route (1): a combination of the main transmission path  911  between the local nodes B and C, the sub transmission path  910  between the general nodes C and E, the sub transmission path  910  between the general nodes E and G, and the main transmission path  911  between the local nodes G and H. 
     Route (2): a combination of the main transmission path  911  between the local nodes B and C, the main transmission path  911  between the local nodes C and D, the main transmission path  911  between the local nodes D and E, the sub transmission path  910  between the general nodes E and G, and the main transmission path  911  between the local nodes G and H. 
     Route (3): a combination of the main transmission path  911  between the local nodes B and C, the sub transmission path  910  between the general nodes C and E, the main transmission path  911  between the local nodes E and F, the main transmission path  911  between the local nodes F and G, and the main transmission path  911  between the local nodes G and H. 
     Of routes (1) to (3), the communication-route designing unit  100  selects, as a communication route R 11  for the communication channel P 11 , route (1) in which the number of sub transmission paths  910  is the largest. That is, since the number of sub transmission paths  910  in route (1) is 2 and the number of sub transmission paths  910  in each of routes (2) and (3) is 1, route (1) is selected as the communication route R 11  for the communication channel P 11 . 
     On the other hand, since any link that provides connection between general nodes does not exist in the section of the communication channel P 12 , the communication channel P 12  belongs to the group that is not allowed to use the sub transmission paths  910 . Thus, the communication-route designing unit  100  selects, as a communication route for the communication channel P 12 , a combination of the main transmission path  911  between the local nodes D and E and the main transmission path  911  between the local nodes E and F. 
     As described above, the communication-route designing unit  100  uses the sub transmission path(s)  910  as much as possible, to design a communication route. That is, the communication-route designing unit  100  selects the sub transmission paths  910  with higher priority than the main transmission paths  911  to design a communication route for each requested communication channel. This is because the main transmission paths  911  have a higher degree of freedom of inserting and branching optical signals, since they are connected to all of the nodes A to J, whereas the sub transmission paths  910  have a lower degree of freedom of inserting and branching optical signals, since they are connected to only the particular nodes (general nodes). 
     According to such a scheme for selecting communication routes, the communication channel P 11  belonging to the group that is allowed to use the sub transmission paths  910  and the communication channel P 12  belonging to the group that is not allowed to use the sub transmission paths  910  are configured so that the transmission paths in the communication route R 11  for the communication channel P 11  and the transmission paths in the communication route R 12  for the communication channel P 12  are different from each other. This makes it possible to assign the same wavelength to the communication channels P 11  and P 12 , thus making it possible to save wavelength resources. 
     Referring back to  FIG. 7 , the communication-route designing unit  100  generates, as a design result, communication route information  134  indicating a communication route for each of the requested communication channels and writes the communication route information  134  to the HDD  13 . The communication route information  134  includes, for example, a combination of the identifiers each selected from at least one of the main transmission path(s)  911  and the sub transmission path(s)  910 , in association with each communication channel. 
     The wavelength assigning unit  101  reads the topology information  130 , the demand information  131 , the transmission path information  133 , and the communication route information  134  and assigns, for each communication channel, wavelengths included in a wavelength-multiplexed optical signal. The wavelength assigning unit  101  generates, as a result of the assignment, wavelength assignment information  135  indicating one or more wavelengths assigned to each of the requested communication channels, and writes the wavelength assignment information  135  to the HDD  13 . 
     When the number of wavelengths assigned to communication channels that go through the sub transmission path  910  between the general nodes is larger than the number of wavelengths assigned to communication channels that go through the main transmission paths  911  between the general nodes, the modification processing unit  102  modifies the communication routes and the assignment of wavelengths. The communication routes and the assignment of wavelengths are modified so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path  910  to the main transmission path  911  via any of the general nodes and the other communication channel is routed from the main transmission path  911  to the sub transmission path  910  via any of the general nodes. 
     Since the communication-route designing unit  100  selects the sub transmission paths  910  with higher priority than the main transmission paths  911 , as described above, there is a possibility that the number of wavelengths in the sub transmission path  910  becomes larger than the number of wavelengths in the main transmission path  911  between the general nodes. In this case, when the number of wavelengths in the sub transmission path  910  exceeds an upper-limit number of wavelengths that are assignable, a network is not configurable. 
     Thus, the modification processing unit  102  modifies the communication routes designed by the communication-route designing unit  100  and the wavelengths assigned by the wavelength assigning unit  101  so that the number of wavelengths in the main transmission path  911  and the number of wavelengths in the sub transmission path  910  are balanced. The modification processing unit  102  reflects the modified communication routes in the communication route information  134  and reflects the modified wavelengths in the wavelength assignment information  135 . The communication-route and wavelength modifications will be described below in conjunction with a specific example. 
       FIGS. 12A and 12B  are diagrams illustrating an example of communication routes in a network before and after the modification, according to an embodiment.  FIG. 12A  illustrates communication routes before the modification, and  FIG. 12B  illustrates communication routes after the modification. The topology of the network is the same as that of the network illustrated in  FIG. 11 . 
     In the network in this example, a communication channel P 13  is requested between the nodes B and H, and a communication channel P 14  is requested between the nodes D and I. Since a link that provides connection between general nodes exists in the sections of each of the communication channels P 13  and P 14 , the communication channels P 13  and P 14  belong to the group that is allowed to use the sub transmission paths  910 . The number of wavelengths of each of the communication channels P 13  and P 14  is 1 (see “×1”). 
     As described above, the communication-route designing unit  100  selects the sub transmission paths  910  with higher priority to design a communication route. Thus, before the modification, the communication-route designing unit  100  selects, as a communication route R 13  for the communication channel P 13 , a combination of the main transmission path  911  between the nodes B and C, the sub transmission path  910  between the nodes C and E, the sub transmission path  910  between the nodes E and G, and the main transmission path  911  between the nodes G and H. The communication-route designing unit  100  also selects, as a communication route R 14  for the communication channel P 14 , a combination of the main transmission path  911  between the nodes D and E, the sub transmission path  910  between the nodes E and G, and the sub transmission path  910  between the nodes G and I. 
     Consequently, the communication route R 13  for the communication channel P 13  uses the same sub transmission path  910  between the nodes E and G as that in the communication route R 14  for the communication channel P 14 . Thus, the wavelength assigning unit  101  assigns different wavelengths λ1 and λ2 to the communication channels P 13  and P 14 . As a result, between the nodes E and G, the number of wavelengths (“2” (λ1 and λ2) in this example) assigned to the communication channels P 13  and P 14  that go through the sub transmission path  910  becomes larger than the number of wavelengths (“0” in this example) assigned to a communication channel that goes through the main transmission path  911 . 
     If the upper-limit number of wavelengths in each of the main transmission path  911  and the sub transmission path  910  is assumed to be 1, the network is not configurable, since the wavelengths are not sufficient. Accordingly, by utilizing the state in which the main transmission path  911  and the sub transmission path  910  connected to the node G have an available path X for a communication route for the wavelength λ1, the modification processing unit  102  modifies the communication routes R 13  and R 14  so that the same wavelength is assigned to the communication channels P 13  and P 14 . The reason why there is the available path X for a communication route for the wavelength λ1 is that design of a turn-back communication route is prohibited, as described above with reference to  FIG. 9 . 
     The modification processing unit  102  checks whether or not it is possible to modify the communication routes R 13  and R 14  at the general nodes E or G positioned at the opposite ends of a section of the sub transmission path  910  through which both the communication routes R 13  and R 14  pass so that two communication routes having the same wavelength pass through the main transmission path  911  and the sub transmission path  910 , respectively. That is, the modification processing unit  102  determines whether or not it is possible to modify the communication routes R 13  and R 14  so that one of the communication channels P 13  and P 14  having the same wavelength is routed from the sub transmission path  910  to the main transmission path  911  via the general node E or G and the other one of the communication channel P 13  and P 14  is routed from the main transmission path  911  to the sub transmission path  910  via the general node E or G. 
     Although the route modification may be performed at any of the general nodes E and G,  FIG. 12B  illustrates a case of the general node G. The modification processing unit  102  modifies the communication route R 14  for the communication channel P 14  so that it passes through the main transmission path  911  between the nodes E and F and the main transmission path  911  between the nodes F and G. As a result, a communication route R 14   a  after the modification is a combination of the main transmission path  911  between the nodes D and E, the main transmission path  911  between the nodes E and F, the main transmission path  911  between the nodes F and G, and the sub transmission path  910  between the nodes G and I. 
     Since the communication route R 14   a  after the modification does not include the same transmission path  910  or  911  as that in the communication route R 13  for the communication channel P 13 , the modification processing unit  102  changes the wavelength of the communication channel P 14  from λ2 to λ1, which is the same as that of the communication channel P 13 . As a result, one of the two communication channels P 13  and P 14  to which the same wavelength λ1 is assigned is routed from the sub transmission path  910  to the main transmission path  911  via the general node G, and the other one of the two communication channels P 13  and P 14  is routed from the main transmission path  911  to the sub transmission path  910  via the general node G. That is, the two communication routes P 13  and P 14  having the same wavelength λ1 are modified at the general node G so as to interchange the main transmission path  911  and the sub transmission path  910 . In this example, although a case in which the modification processing unit  102  performs wavelength modification so that both of the wavelengths of the two communication channels P 13  and P 14  become λ1 has been described, the wavelength modification may also be made so that both of the wavelengths become λ2. 
     In this example, although the modification processing unit  102  performs route modification at the general node G so that the communication channels P 13  and P 14  having the same wavelength λ1 interchange the main transmission path  911  and the sub transmission path  910 , the modification processing unit  102  may also perform the route modification at the general node E in the same manner.  FIG. 13  is a diagram illustrating an example of communication routes in a network after the modification, according to an embodiment. 
     The modification processing unit  102  modifies the communication route R 13  for the communication channel P 13  so that it passes through the main transmission path  911  between the nodes E and F and the main transmission path  911  between the nodes F and G. Thus, a communication route R 13   a  after the modification is a combination of the main transmission path  911  between the nodes B and C, the sub transmission path  910  between the nodes C and E, the main transmission path  911  between the nodes E and F, the main transmission path  911  between the nodes F and G, and the main transmission path  911  between the nodes G and H. 
     Since the communication route R 13   a  after the modification does not include the same transmission path  910  or  911  as that in the communication route R 14  for the communication channel P 14 , the modification processing unit  102  changes the wavelength of the communication channel P 14  from λ2 to λ1, which is the same as that of the communication channel P 13 . As a result, one of the two communication channels P 13  and P 14  to which the same wavelength λ1 is assigned is routed from the sub transmission path  910  to the main transmission path  911  via the general node E, and the other one of the communication channel P 13  and P 14  is routed from the main transmission path  911  to the sub transmission path  910  via the general node E. That is, the two communication channels P 13  and P 14  having the same wavelength λ1 are modified at the general node E so as to interchange the main transmission path  911  and the sub transmission path  910 . In this example, although a case in which the modification processing unit  102  performs wavelength modification so that both of the wavelengths of the two communication channels P 13  and P 14  become λ1 has been described, the wavelength modification may also be made so that both of the wavelengths become λ2. 
     Next, the modification processing performed by the modification processing unit  102  will be further described in detail in conjunction with another specific example.  FIG. 14  is a diagram illustrating an example of communication route modifications in a network, according to an embodiment. 
     The network in this example has nodes A to O. The nodes A, C, F, H, J, M, and O are general nodes, and the nodes B, D, E, G, I, K, L, and N are local nodes. In the network in this example, a communication channel P 21  is requested between the nodes B and I, and a communication channel P 22  is requested between the nodes D and K. Also, a communication channel P 23  is requested between the nodes E and L, and a communication channel P 24  is requested between the nodes G and N. 
     In  FIG. 14 , character G 1  indicates communication routes and assigned wavelengths before the modification. Character G 2  indicates communication routes and assigned wavelengths after the modification as a comparative example, and character G 3  indicates communication routes and assigned wavelengths in the embodiment. 
     The communication routes R 21  to R 24 , R 21   a , R 22   a , and R 24   a  are represented by lines that extend horizontally at vertical positions z0 and z1. The lines that extend horizontally at vertical position z1 represent the sub transmission paths  910 , and the lines that extend horizontally at vertical position z0 represent the main transmission paths  911 . 
     Two numerals indicated in brackets (see “[1/2]” and so on) indicate, between the general nodes, the number of wavelengths in the communication channel(s) that go through the sub transmission path  910  and the number of wavelengths in the communication channel(s) that go through the main transmission path  911 . For example, with respect to the communication routes before the modification (see character G 1 ) between the general nodes F and H, “[3/1]” indicates that the number of wavelengths in the communication channels P 21  to P 23  passing through the sub transmission path  910  is 3, and the number of wavelengths in the communication channel P 24  passing through the main transmission path  911  between the general nodes F and H is 1. 
     First, a reference is made to the communication routes before the modification. The communication route R 21  for the communication channel P 21  is a combination of the main transmission path  911  between the nodes B and C, the sub transmission path  910  between the nodes C and F, the sub transmission path  910  between the nodes F and H, and the main transmission path  911  between the nodes H and I. The communication route R 22  for the communication channel P 22  is a combination of the main transmission path  911  between the nodes D and E, the main transmission path  911  between the nodes E and F, the sub transmission path  910  between the nodes F and H, the sub transmission path  910  between the nodes H and J, and the main transmission path  911  between the nodes J and K. The communication route R 23  for the communication channel P 23  is a combination of the main transmission path  911  between the nodes E and F, the sub transmission path  910  between the nodes F and H, the sub transmission path  910  between the nodes H and J, the main transmission paths  911  between the nodes J and K, and the main transmission paths  911  between the nodes K and L. The communication route R 24  for the communication channel P 24  is a combination of the main transmission path  911  between the nodes G and H, the sub transmission path  910  between the nodes H and J, the sub transmission path  910  between the nodes J and M, and the main transmission path  911  between the nodes M and N. 
     A wavelength λ1 is assigned to the communication channel P 21 , and a wavelength λ2 is assigned to the communication channel P 22 . A wavelength λ3 is assigned to the communication channel P 23 , and a wavelength λ1 is assigned to the communication channel P 24 . 
     Between the general nodes F and H, the number of wavelengths assigned to the communication channels P 21  to P 23  that pass through the sub transmission path  910  is 3, and the number of wavelengths assigned to the communication channel P 24  that pass through the main transmission path  911  is 1. Between the general nodes H and J, the number of wavelengths assigned to the communication channels P 22  to P 24  that pass through the sub transmission path  910  is 3, and the number of wavelengths assigned to the communication channel P 21  that passes through the main transmission path  911  is 1. 
     Thus, since the number of wavelengths assigned to the communication channels P 22  to P 24  that pass through the sub transmission paths  910  between the nodes F and H and between the nodes H and J is larger than the number of wavelengths assigned to the communication channel P 21  that passes through the main transmission paths  911  between the nodes F and H and between the nodes H and J, the modification processing unit  102  causes the above two numbers of wavelengths to be balanced. 
     In the comparative example (see character G 2 ), the communication route R 22  for the communication channel P 22  has been modified from the sub transmission paths  910  to the main transmission paths  911  between the general nodes F and H and between the general nodes H and J. Thus, the communication route R 22   a  after the modification is a combination of only the main transmission paths  911  that provide connections between the nodes D and K. 
     As a result of such a modification to the communication route R 22 , the number of wavelengths in the communication channels P 22  to P 24  that pass through the sub transmission paths  910  between the general nodes F and H and between the general nodes H and J is 2, and the number of wavelengths in the communication channel P 21  that passes through the main transmission paths  911  is 2. As a result, the numbers of wavelengths between the general nodes F and H and between the general nodes H and J become balanced. 
     However, since the communication route R 22   a  after the modification uses the same main transmission path  911  as that of the communication route R 23  for the communication channel P 23 , wavelength blocking occurs (see character B). Consequently, the communication channels P 22  and P 23  use the mutually different wavelengths λ2 and λ3, and the number of wavelengths in the entire network becomes 3, which is the same as the number of wavelengths before the modification. 
     In contrast, in the embodiment (see character G 3 ), the communication route R 21  for the communication channel P 21  has been modified from the sub transmission path  910  to the main transmission paths  911  between the general nodes F and H. Also, the communication route R 24  for the communication channel P 24  has been modified from the sub transmission path  910  to the main transmission paths  911  between the general nodes H and J. 
     Thus, the communication route R 21   a  after the modification is a combination of the main transmission path  911  between the nodes B and C, the sub transmission path  910  between the nodes C and F, the main transmission path  911  between the nodes F and G, the main transmission path  911  between the nodes G and H, and the main transmission path  911  between the nodes H and I. Also, the communication route R 24   a  after the modification is a combination of the main transmission path  911  between the nodes G and H, the main transmission path  911  between the nodes H and I, the main transmission path  911  between the nodes I and J, the sub transmission path  910  between the nodes J and M, and the main transmission path  911  between the nodes M and N. 
     As a result of such modifications to the communication route R 21  and R 24 , the number of wavelengths in the communication channels P 22  and P 23  that pass through the sub transmission paths  910  between the general nodes F and H and between the general nodes H and J is 2, and the number of wavelengths in the communication channels P 21  and P 22  that pass through the main transmission paths  911  is 2. As a result, the numbers of wavelengths between the general nodes F and H and between the general nodes H and J become balanced. 
     In this case, since the communication routes R 21   a  and R 22  do not use the same transmission path  910  or  911  and thus no wavelength blocking occurs, the communication channels P 21  and P 22  may use the same wavelength λ1. Similarly, since the communication routes R 23  and R 24   a  do not use the same transmission path  910  or  911  and thus no wavelength blocking occurs, the communication channels P 23  and P 24  can use the same wavelength λ2. 
     That is, the communication route R 21  and the wavelengths are modified so that one of the communication channels P 21  and P 22  to which the same wavelength λ1 is assigned is routed from the sub transmission path  910  to the main transmission path  911  via the general node F and the other one of the communication channels P 21  and P 22  is routed from the main transmission path  911  to the sub transmission path  910  via the general node F (character X 1 ). Also, the communication route R 24  and the wavelengths are modified so that one of the communication channels P 23  and P 24  to which the same wavelength λ2 is assigned is routed from the sub transmission path  910  to the main transmission path  911  via the general node J and the other one of the communication channels P 23  and P 24  is routed from the main transmission path  911  to the sub transmission path  910  via the general node J (character X 2 ). 
     According to the embodiment, as described above, it is possible not only to make the number of wavelengths balance with each other but also to reduce the number of wavelengths in the entire network from 3 to 2, unlike the comparative example. 
     Next, a description will be given of the operation of the network design apparatus.  FIG. 15  is a diagram illustrating an example of an operational flowchart for a network design method, according to an embodiment. 
     First, in step St 1 , an operator inputs design information to the network design apparatus via the input equipment  160  or the communication processing unit  14 . The design information includes the topology information  130 , the demand information  131 , and the transmission path information  133 . The design information is stored in the HDD  13 . 
     Next, in step St 2 , based on the topology information  130 , the demand information  131 , and the transmission path information  133 , the communication-route designing unit  100  designs a communication route for each of the requested communication channels. In this case, the communication-route designing unit  100  selects, with higher priority, the sub transmission paths  910  that provide connections between particular nodes (general nodes) in the network than the main transmission paths  911  that provide connections between all (three or more) nodes in the network, as described above. 
     For example, the communication-route designing unit  100  generates a model for a mixed integer programming problem for communication routes and obtains a solution thereof to determine the communication route. The mixed integer programming problem is an analysis method for obtaining a maximum value or a minimum value of an objective function under one or more constraints. 
     In step St 3 , the CPU  10  sets a variable k at 0. The variable k indicates the number of times the wavelength assigning unit  101  has executed wavelength assignment. 
     In step St 4 , the wavelength assigning unit  101  assigns, for each communication channel, wavelengths included in a wavelength-multiplexed optical signal in the network. In this case, for example, the wavelength assigning unit  101  generates a model for the mixed integer programming problem for wavelengths and obtains a solution to execute wavelength assignment. The constraint for the mixed integer programming problem is that, for example, the same wavelength is not assignable to communication channels that pass through the same main transmission path  911  or sub transmission path  910 . In other words, the constraint is that the same wavelength is not assignable to communication channels that share at least part of the communication routes. 
     In step St 5 , the modification processing unit  102  determines whether or not the number of wavelengths assigned to the communication channel(s) that pass through the sub transmission path  910  between the general nodes and the number of wavelengths assigned to the communication channel(s) that pass through the main transmission paths  911  between the general nodes are balanced with each other. When the numbers of wavelengths are balanced with each other (YES in step St 5 ), the network design apparatus outputs a design result in step St 6  and then ends the processing. In this case, the design result may not only be stored in the HDD  13  as the communication route information  134  and the wavelength assignment information  135  but also be displayed on the display  170 . 
     On the other hand, when the numbers of wavelengths are not balanced with each other (NO in step St 5 ), the process proceeds to step St 7  in which the CPU  10  determines whether or not the variable k has reached a predetermined value Kmax. When the variable k has reached the predetermined value Kmax (YES in step St 7 ), the network design apparatus issues a notification indicating that the design has failed in step St 8  and ends the processing. The failure notification is, for example, displayed on the display  170 . Since the number of times the wavelength assigning unit  101  executes the wavelength assignment is limited to the predetermined value Kmax, as described above, the network design apparatus is prohibited from permanently repeating design of a network to which wavelengths are not assignable. 
     When the variable k is smaller than the predetermined value Kmax (NO in step St 7 ), the CPU  10  adds “1” to the variable k in step St 9 . Next, in step St 10 , the modification processing unit  102  modifies the communication routes designed by the communication-route designing unit  100  and the wavelengths assigned by the wavelength assigning unit  101 . The communication-route and wavelength modifications are performed so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path  910  to the main transmission path  911  and the other one of the two communication channels is routed from the main transmission path  911  to the sub transmission path  910 . 
     In order to reflect information of the modified communication routes and wavelengths, the modification processing unit  102  updates the communication route information  134  and the wavelength assignment information  135 . In step St 4 , the wavelength assigning unit  101  executes the wavelength assignment again, based on the updated the communication route information  134  and wavelength assignment information  135 . Thereafter, the process in step St 5  and the subsequent processes are repeated. The network design is performed in the manner described above. 
     Next, a description will be given of the communication-route design processing (step St 2  in  FIG. 15 ).  FIG. 16  is a diagram illustrating an example of an operational flowchart for communication-route design processing, according to an embodiment. 
     First, in step St 21 , the communication-route designing unit  100  reads the demand information  131  from the HDD  13  and selects a requested communication channel. Next, in step St 22 , the communication-route designing unit  100  reads the topology information  130  and the transmission path information  133  from the HDD  13 , and generates communication-route candidates by selecting combination candidates of the transmission paths  910  and  911  for each section (a pair of nodes) of the communication channel, as described above with reference to  FIG. 11 . 
     Next, in step St 23 , based on the topology information  130 , the communication-route designing unit  100  determines whether or not a link between general nodes exists in the section of the selected communication channel. When a link between general nodes exists (YES in step St 23 ), the communication-route designing unit  100  classifies the selected communication channel into the group that is allowed to use the sub transmission paths  910 . In step St 24 , the communication-route designing unit  100  selects, as a communication route, a communication-route candidate including the largest number of the sub transmission paths  910  among the generated communication-route candidates. In the case of the example illustrated in  FIG. 11 , the communication-route designing unit  100  selects candidate route (1) among candidate routes (1) to (3). 
     On the other hand, when a link between general nodes does not exist (NO in step St 23 ), the communication-route designing unit  100  classifies the selected communication channel into the group that is not allowed to use the sub transmission paths  910 . In step St 25 , the communication-route designing unit  100  selects, as a communication route, a candidate including only the main transmission paths  911 . 
     Next, in step St 26 , based on the demand information  131 , the communication-route designing unit  100  determines whether or not there is an unselected communication channel. When there is an unselected communication channel (YES in step St 26 ), the communication-route designing unit  100  selects the unselected communication channel in step St 21  and executes the process in step St 22  again. When there is no unselected communication channel left (NO in step St 26 ), the communication-route designing unit  100  ends the processing. The communication-route design processing is executed in the manner described above. 
     Next, a description will be given of the communication-route and wavelength modification processing (step St 10  in  FIG. 15 ).  FIG. 17  is a diagram illustrating an example of an operational flowchart for communication-route and wavelength modification processing, according to an embodiment. 
     First, in step St 31 , based on the topology information  130 , the modification processing unit  102  selects a pair of adjacent general nodes. In the case of the example illustrated in  FIG. 12A , the modification processing unit  102  selects one of a pair of the nodes A and C, a pair of the nodes C and E, a pair of the nodes E and G, a pair of the nodes G and I, and a pair of the nodes I and A. 
     In step St 32 , the modification processing unit  102  determines whether or not the number of wavelengths assigned to the communication channel(s) that pass through the sub transmission path(s)  910  between the selected general nodes is larger than the number of wavelengths assigned to the communication channel(s) that pass through the main transmission path(s)  911  between the selected general nodes. When the number of wavelengths in the sub transmission path  910  is larger than the number of wavelengths in the main transmission path(s)  911  (YES in step St 32 ), the process proceeds to step St 33  in which the modification processing unit  102  determines whether or not the main transmission path  911  and the sub transmission path  910  of the communication channels having the same wavelength are interchangeable at any of the general nodes. 
     That is, the modification processing unit  102  determines whether or not the communication routes and the wavelengths are able to be modified so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path  910  to the main transmission path  911  via any of the general nodes and the other one of the two communication channels is routed from the main transmission path  911  to the sub transmission path  910  via any of the general nodes. In the case of the example illustrated in  FIG. 12A , the modification processing unit  102  makes the determination with respect to any of the general nodes E and G. 
     When the interchange of the main transmission path  911  and the sub transmission path  910  is possible (YES in step St 33 ), the process proceeds to step St 34  in which the modification processing unit  102  updates the communication route information  134  and the wavelength assignment information  135 . When the interchange of the main transmission path  911  and the sub transmission path  910  is not possible (NO in step St 33 ) or when the number of wavelengths in the sub transmission path  910  is smaller than or equal to the number of wavelengths in the main transmission path(s)  911  (NO in step St 32 ), the modification processing unit  102  does not perform the above-described update. 
     In step St 35 , based on the topology information  130 , the modification processing unit  102  determines whether or not there is an unselected pair of general nodes. When there is an unselected pair of general nodes (YES in step St 35 ), the modification processing unit  102  selects the unselected pair of general nodes in step St 31  and then executes the process in step St 32  again. When an unselected pair of general nodes does not exist (NO in step St 35 ), the modification processing unit  102  ends the processing. The communication-route and wavelength modification processing is executed in the manner described above. 
     Next, a description will be given of the cost of nodes in a network to be designed.  FIG. 18  is a diagram illustrating an example of costs for respective network configurations, according to an embodiment. 
     The costs illustrated in  FIG. 18  are calculated based on the total number of demultiplexers  71   a ,  71   b , and  61  and multiplexers  72   a ,  72   b , and  62  (“the total number of multiplexers and demultiplexers”) illustrated in  FIGS. 4  and  5 . The wavelength division multiplexing transmission equipment (a ROADM or the like) installed at each node has optical amplifiers for the respective pathways in order to compensate for loss of optical power of multiplexed optical signals, which is caused by the demultiplexers and the multiplexers. The demultiplexers, the multiplexers, and the optical amplifiers are expensive, thus greatly affecting the equipment cost. In practice, the equipment cost also includes fixed costs that do not depend on the number of pathways, such as the cost of a power source unit. 
     Since the wavelength division multiplexing transmission equipment at each general node has four demultiplexers  71   a  and  71   b  and four multiplexers  72   a  and  72   b  for four pathways, as illustrated in  FIG. 4 , the number of multiplexers and demultiplexers is 8. On the other hand, since the wavelength division multiplexing transmission equipment at each local node has two demultiplexers  61  and two multiplexers  62  for two pathways, as illustrated in  FIG. 5 , the number of multiplexers and demultiplexers is 4. 
     Thus, in the case of a network configuration in which ten nodes (corresponding to local nodes), each having two pathways, are provided, the number of multiplexers and demultiplexers is 40. The “relative cost” in  FIG. 18  indicates, when the cost of this network is assumed to be 1.0 (reference value), the costs of other network configurations. All of the network configurations are assumed to be ring networks. 
     In the case of a network configuration (see  FIG. 3 ) in which seven nodes (local nodes), each having two pathways, are provided and three nodes (general nodes), each having four pathways, are provided, the number of multiplexers and demultiplexers is 52. Thus, the relative cost in this network configuration is 1.3, which is given by the ratio of the multiplexers and demultiplexers (52/40). 
     In the case of a network configuration (see  FIG. 1 ) in which 20 nodes (corresponding to local nodes), each having two pathways, are provided, the number of multiplexers and demultiplexers is 80. Thus, the relative cost of this network configuration is 2.0, which is given by the ratio of the multiplexers and demultiplexers (80/40). 
     In the case of a network configuration (see  FIG. 2 ) in which ten nodes (corresponding to general nodes), each having four pathways, are provided, the number of multiplexers and demultiplexers is 80. Thus, the relative cost of this network configuration is 2.0, which is given by the ratio of the multiplexers and demultiplexers (80/40). 
     Hence, the equipment cost is reduced by 35% when the network illustrated in  FIG. 3  is used, compared with a case in which the networks illustrated in  FIGS. 1 and 2  are used. Even when compared with the simple network in which ten nodes, each having two pathways, are provided, the network illustrated in  FIG. 3  also makes it possible to reduce an increase in the equipment cost up to about 30(%). 
     As described above, the network design apparatus includes the communication-route designing unit  100 , the wavelength assigning unit  101 , and the modification processing unit  102 . The communication-route designing unit  100  designs a communication route for each requested communication channel, by selecting, with higher priority, the sub transmission paths  910  that provide connections between particular nodes (general nodes) in the network in which a wavelength-multiplexed optical signal is transmitted, than the main transmission paths  911  that provide connections between all (three or more) nodes in the network. 
     The wavelength assigning unit  101  assigns, for each communication channel, wavelengths included in a wavelength-multiplexed optical signal. When the number of wavelengths assigned to the communication channel(s) that pass through the sub transmission path  910  between the particular nodes is larger than the number of wavelengths assigned to the communication channel(s) that pass through the main transmission paths  911  between the particular nodes, the modification processing unit  102  modifies the communication routes designed by the communication-route designing unit  100  and the wavelengths designed by the wavelength assigning unit  101 . The communication-route and wavelength modifications are performed so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path to the main transmission path via any of the particular nodes and the other one of the two communication channels is routed from the main transmission path to the sub transmission path via any of the particular nodes. 
     According to the configuration described above, the main transmission paths  911  provide connections between three or more nodes in the network, and the sub transmission paths  910  provide connections between particular nodes (general nodes) in the network. Accordingly, the transmission paths between the particular nodes are made redundant, thus making it possible to increase the transmission capacity of the network while reducing the cost. 
     The communication-route designing unit  100  also selects the sub transmission paths  910  with higher priority than the main transmission paths  911  to design a communication route for each requested communication channel. Thus, as many communication routes as possible may be concentrated through the sub transmission paths  910  whose degree of freedom of inserting and branching optical signals is lower than the main transmission paths  911 . 
     Since the wavelength assigning unit  101  assigns, for each communication channel, wavelengths included in a wavelength-multiplexed optical signal, the number of wavelengths used is determined for each of the main transmission paths  911  and the sub transmission paths  910 . 
     When the number of wavelengths in the sub transmission path(s)  910  between particular nodes is larger than the number of wavelengths in the main transmission path  911 , the modification processing unit  102  modifies the communication routes designed by the communication-route designing unit and the wavelengths assigned by the wavelength assigning unit. Accordingly, even if the number of wavelengths in the main transmission path  911  becomes larger than the number of wavelengths in the main transmission path  911  as a result of selecting the sub transmission path  910  with higher priority to design communication routes, the number of wavelengths in the sub transmission path  910  and the number of wavelengths in the sub transmission path  910  may be made balanced with each other. 
     In this case, since the communication-route and wavelength modifications are performed so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path to the main transmission path via any of the particular nodes and the other one of the two communication channels is routed from the main transmission path to the sub transmission path via any of the particular nodes, the number of wavelengths in the transmission paths  910  and  911  is reduced. Hence, the equipment cost at each node is reduced. 
     Accordingly, the network design apparatus according to the embodiment makes it possible to effectively design a network that allows large-capacity transmission. 
     Also, the network design method according to the embodiment is a method for causing a computer to execute processes (1) to (3) below. 
     Process (1): a communication route for each requested communication channel is designed by selecting, with higher priority, sub transmission paths  910  that provide connections between particular nodes in a network in which a wavelength-multiplexed optical signal is transmitted than main transmission paths  911  that provide connections between three or more nodes in the network. 
     Process (2); wavelengths included in the wavelength-multiplexed optical signal are assigned for each communication channel. 
     Process (3): when the number of wavelengths assigned to the communication channel that passes through the sub transmission path  910  between the particular nodes is larger than the number of wavelengths assigned to the communication channel that passes through the main transmission path  911  between the particular nodes, the communication routes designed in the process for designing the communication paths and the wavelengths assigned in the process for assigning the wavelengths are modified. The communication-route and wavelength modifications are performed so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path  910  to the main transmission path  911  via any of the particular nodes and the other one of the two communication channels is routed from the main transmission path  911  to the sub transmission path  910  via any of the particular nodes. 
     The network design method according to the embodiment offers advantages that are the same as or similar to those described above, since it is applied to a configuration that is the same as or similar to that of the above-described network design apparatus. 
     Also, the network design program according to the embodiment is a program for causing a computer to execute processing (1) to (3) below. 
     Processing (1): a communication route for each requested communication channel is designed by selecting, with higher priority, sub transmission paths  910  that provide connections between particular nodes in a network in which a wavelength-multiplexed optical signal is transmitted than main transmission paths  911  that provide connections between three or more nodes in the network. 
     Processing (2): wavelengths included in the wavelength-multiplexed optical signal are assigned for each communication channel. 
     Processing (3): when the number of wavelengths assigned to the communication channel that goes through the sub transmission path  910  between the particular nodes is larger than the number of wavelengths assigned to the communication channel that goes through the main transmission path  911  between the particular nodes, the communication routes designed in the processing for designing the communication paths and the wavelengths assigned in the processing for assigning the wavelengths are modified. The communication-route and wavelength modifications are performed so that one of the two communication channels to which the same wavelength is assigned is routed from the sub transmission path  910  to the main transmission path  911  via any of the particular nodes and the other communication channel is routed from the main transmission path  911  to the sub transmission path  910  via any of the particular nodes. 
     The network design program according to the embodiment offers operational effects that are the same as or similar to those described above, since it is applied to a configuration that is the same as or similar to that of the above-described network design apparatus. 
     Although the contents of the present disclosure have been specifically described above with reference to the preferred embodiments, it is apparent to those skilled in the art that various modification and changes are possible based on the basic technical spirit and the teaching of the present disclosure. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.