Abstract:
A method of transmission of a signal using a ring network that includes a plurality of transmission devices, any one transmission device of the plurality of transmission devices being set as a first blocking portion, and any another transmission device of the plurality of transmission devices being set as a second blocking portion, the method includes setting first information to a first signal, and transmitting the first signal from a first transmission device to a second transmission device, wherein when a value of the first information is a first value, the first blocking portion passes the first signal and the second blocking portion blocks the first signal, and when the value of the first information is a second value, the second blocking portion passes the first signal and the first blocking portion blocks the first signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-084059, filed on Apr. 19, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    A technology described in the present specification is related to a method of transmission of a signal and a ring network. 
       BACKGROUND 
       [0003]    At present, in each of the Internet and data center networks, communication is performed by using the Ethernet (registered trademark). 
         [0004]    In the Ethernet, at a time of transmitting a signal, the signal is transmitted in accordance with a relationship between a destination and a transmitting port, stored in a table. In addition, in a case where a destination and a transmitting port are not established, the signal is transmitted by using a flooding method. Note that the flooding method is a method for transmitting a signal from all ports (all ports other than a receiving port in a case of relaying the signal). 
         [0005]    Note that, in a method for storing, in the table, the relationship between the destination and the transmitting port, a relationship between a receiving port and a transmission source (a source address) is stored at a time of receiving a signal from another transmission device. Note that the receiving port and the transmitting port may be provided in, for example, the same interface card, thereby enabling a correspondence relationship therebetween to be acquired. 
         [0006]    In addition, there is a ring network that couples, in a ring shape, transmission devices to configure the Ethernet. The ring network is a technology for forming an annular transmission path so as to string together the annular transmission path like beads and performing communication (end-to-end) between adjacent transmission devices, thereby providing signals (frames). 
         [0007]    In such a ring network in the Ethernet, in order to avoid the occurrence of a loop at a time of switching a path or in order to perform high-speed switching, Ethernet ring protection (hereinafter, described as ERP) specified by ITU-TG.8032 is widely prevalent. As a technical literature of the related art, there is ITU-T, “ITU-T G.8032/Y.1344”. 
       SUMMARY 
       [0008]    According to an aspect of the invention, a method of transmission of a signal using a ring network that includes a plurality of transmission devices, any one transmission device of the plurality of transmission devices being set as a first blocking portion, and any another transmission device of the plurality of transmission devices being set as a second blocking portion, the method includes receiving, by a first transmission device of the plurality of transmission devices, a first signal, setting, by the first transmission device, first information to the first signal, and transmitting, from the first transmission device to a second transmission device plurality of transmission devices, the first signal in which the first information is set, wherein when a value of the first information is a first value, the first blocking portion passes the first signal and the second blocking portion blocks the first signal, and when the value of the first information is a second value, the second blocking portion passes the first signal and the first blocking portion blocks the first signal. 
         [0009]    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. 
         [0010]    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 
         [0011]      FIG. 1  is a diagram illustrating an example of a ring network  10 . 
           [0012]      FIG. 2  is a diagram illustrating an example of a ring network  20 . 
           [0013]      FIG. 3  is a diagram illustrating an example of hardware configurations of transmission devices  100  in a first embodiment. 
           [0014]      FIG. 4  is a diagram illustrating an example of functional block configurations of the transmission devices  100  in the first embodiment. 
           [0015]      FIG. 5  is a diagram illustrating an example of a table stored in a storage unit  116  in the first embodiment. 
           [0016]      FIG. 6  is a diagram illustrating an example of a table stored in the storage unit  116  in a second embodiment. 
           [0017]      FIG. 7  is a diagram illustrating an example of a network  30  in which ring networks (Major-Rings) are coupled by a new ring network (a Sub-Ring). 
           [0018]      FIG. 8  is a diagram illustrating an example of a network  40  in a third embodiment. 
           [0019]      FIG. 9  is a diagram illustrating an example of functional block configurations of transmission devices  200  in the third embodiment. 
           [0020]      FIG. 10  is a diagram illustrating an example of a flowchart to when one of the transmission devices  200 , which serves as an owner, transmits, to corresponding ones of the transmission devices  200  within a ring network  23   a , a control signal for configuring a ring network  23   b.    
           [0021]      FIG. 11  is a diagram illustrating an example of a flowchart in control before the corresponding ones of the transmission devices  200  receive the control signal, thereby configuring the ring network  23   b.    
           [0022]      FIG. 12  is a diagram illustrating an example of a frame format of a signal. 
           [0023]      FIGS. 13A, 13B and 13C  are diagrams illustrating examples of communication paths of signals in the network  40 . 
           [0024]      FIG. 14  is a diagram illustrating an example of a network  50  in a first example of a modification to the third embodiment. 
           [0025]      FIG. 15  is a diagram illustrating an example of a network  60 . 
           [0026]      FIG. 16  is a diagram illustrating an example of a network  70  in a second example of a modification to the third embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    In the ERP specified in ITU-T G.8032/Y.1344, in order to avoid the occurrence of a loop of a signal within a network, a blocking port is installed. This is a technology for blocking some ports within a transmission device, thereby stopping transmission and reception of signals in a specific communication path. 
         [0028]    However, by using the above-mentioned blocking port in a ring network, a communication path that is included in coupled communication paths and that is to be blocked is put into a state of not being used. In addition, a communication path established by receiving a signal turns out not to use a path related to the blocking port. In this manner, the application of the blocking causes communication bands in coupled communication paths to be difficult to fully use between ring networks, and accordingly, it is difficult to effectively use communication bands within the ring networks. 
         [0029]      FIG. 1  illustrates an example of a configuration of the ring network  10 . The ring network  10  includes transmission devices  100   a ,  100   b ,  100   c , and  100   d . In addition, in the ring network  10 , by using a communication path  15   a  through which a signal is transmitted via the transmission devices  100   a ,  100   c ,  100   d , and  100   b  in this order and a communication path  15   b  through which a signal is transmitted via the transmission devices  100   a ,  100   b ,  100   d , and  100   c  in this order, signals are transmitted between the individual transmission devices  100 . 
         [0030]    In addition, between the transmission device  100   a  and the transmission device  100   b , blocking is performed by a port of the transmission device  100   a  (hereinafter, the port is described as a blocking port), and in order to stop congestion of signals within the ring network  10 , communication between the transmission device  100   a  and the transmission device  100   b  is stopped. Note that, in  FIG. 1 , a position of the blocking port is described as “A”. 
         [0031]    Note that, in a case of not being distinguished from one another, the transmission devices  100   a ,  100   b ,  100   c , and  100   d  are simply described as transmission devices  100 . 
         [0032]    In the ring network  10 , the blocking port A causes communication to be difficult to perform between the transmission device  100   a  and the transmission device  100   b . Therefore, a communication band between the transmission device  100   a  and the transmission device  100   b  is put into a state of being difficult to effectively use. 
         [0033]      FIG. 2  illustrates the ring network  20  serving as an example of a configuration of the present disclosure. By using pieces of information assigned to signals in addition to the configuration of the ring network  10 , the ring network  20  includes a blocking port in one of the transmission devices  100  for each of the pieces of information so that the pieces of information correspond to respective paths between different ones of the transmission devices  100 . 
         [0034]    Note that, by using the pieces of information assigned to the signals, a blocking port (B) and a blocking port (C) are installed, as blocking ports, in respective paths between the different ones of the transmission devices  100  in the ring network  20 . 
         [0035]    The blocking port (B) in the ring network  20  is located at the same position as that of the blocking port (A) in  FIG. 1  and corresponds to a port of the transmission device  100   a . The blocking port (C) in the ring network  20  is arranged between the transmission device  100   c  and the transmission device  100   d  and corresponds to a port within the transmission device  100   c . Note that each of the blocking port (B) and the blocking port (C) is not limited to this and only has to be installed so as to be located between different ones of the transmission devices  100 . 
         [0036]    Regarding the blocking port (B) and the blocking port (C), blocking ports are installed in respective different ones of the transmission devices  100 . Therefore, for each of the pieces of information assigned to the signals, a communication path to be subjected to blocking is different. 
         [0037]    Accordingly, it is possible to effectively utilize communication bands for establishing couplings between the transmission devices  100 . 
         [0038]    In this way, different blocking ports are applied by using the pieces of information assigned to the signals, thereby enabling the coupled communication bands to be effectively utilized. 
       First Embodiment 
       [0039]      FIG. 3  illustrates one of hardware configuration diagrams of the transmission devices  100  in the first embodiment. Note that the first embodiment will be described by using the ring network  20  in  FIG. 2 . The transmission devices  100  each include a control card  110 , interface cards  120   a  to  120   n , and a switch card  130 . Note that, in a case of not being distinguished from one another, the interface cards  120   a  to  120   n  are simply described as interface cards  120 . In addition, every time the number of, for example, communication paths is physically increased, the interface card  120  is additionally installed. 
         [0040]    The control card  110  is configured by a CPU  111  and a memory  112  and performs, on the interface cards  120  and the switch card  130 , control including transfers of signals (frames) and instructions for installing blocking ports, for example. 
         [0041]    The interface cards  120  each perform transmission and reception of signals to and from another device. A signal received by a corresponding one of the interface cards  120  is processed by the relevant interface card  120  and is transferred to the switch card  130 . In addition, in a case where a signal to be transmitted is transferred by the switch card  130 , a corresponding one of the interface cards  120  performs signal processing and performs transmission. Note that the signal processing in each of the interface cards  120  will be described later. 
         [0042]    In a case where a signal is transferred by a corresponding one of the interface cards  120 , the switch card  130  transfers, in accordance with control from the control card  110 , the signal to a corresponding one of the interface cards  120 , which serves as a transmission destination. 
         [0043]    Next,  FIG. 4  illustrates an example of functional block diagrams of the transmission devices  100  in the first embodiment. Note that since each of the transmission devices  100  is coupled to two of the transmission devices  100 , the functional block diagram of one of the transmission devices  100  in  FIG. 4  includes the two interface cards  120   a  and  120   b  corresponding to the number of physical communication paths. 
         [0044]    The transmission devices  100  each include a control unit  115 , a storage unit  116 , reception units  121   a  and  121   b , judgment units  122   a  and  122   b , filter units  123   a  and  123   b , transmission units  124   a  and  124   b , transmission processing units  125   a  and  125   b , filter units  126   a  and  126   b , and a switching unit  131 . 
         [0045]    Note that, in a case of not being distinguished from each other, the individual functions (the reception units  121   a  and  121   b , the judgment units  122   a  and  122   b , the filter units  123   a  and  123   b , the transmission units  124   a  and  124   b , the transmission processing units  125   a  and  125   b , and the filter units  126   a  and  126   b ) of the interfaces  120  are simply described only by using numeric characters (described as the reception units  121 , the judgment units  122 , the filter units  123 , the transmission units  124 , the transmission processing units  125 , and the filter units  126 ). 
         [0046]    The control unit  115  controls individual functions within the corresponding one of the transmission devices  100  by using information of the storage unit  116  and information of the judgment units  122 . The control unit  115  controls, for example, filtering conditions in the filter units  123  and the filter units  126 . 
         [0047]    The storage unit  116  stores therein information used for each destination, ports of output destinations, and a blocking port in a case of serving as an owner transmission device  100 . Note that the control unit  115  and the storage unit  116  correspond to the CPU  111  and the memory  112 , respectively, in the control card  110  in  FIG. 3 . 
         [0048]    Each of the reception units  121  receives and transfers a signal from another one of the transmission devices  100  to a corresponding one of the judgment units  122 . 
         [0049]    Each of the judgment units  122  judges based on information stored in a corresponding one of the received signals and sends a judgment result to the control unit  115 . Note that the information to be judged includes, for example, information of a Virtual Local Area Network (VLAN) and information of a control signal, stored in a frame format of a signal. 
         [0050]    In accordance with control from the control unit  115 , the filter units  123  each perform filtering on a signal output by a corresponding one of the judgment units  122 . The functions of the filter units  123  each correspond to control on a receiving side of a blocking port as described above and each stop a signal from a corresponding communication path in accordance with information judged by a corresponding one of the judgment units  122 . 
         [0051]    The transmission units  124  each transmit a signal transferred by a corresponding one of the transmission processing units  125 , to a corresponding one of the other coupled transmission devices  100 . In a case where the interface card  120   a  is coupled to another one of the transmission devices  100 , the transmission unit  124   a  transmits a signal to the coupled transmission device  100 , for example. 
         [0052]    The transmission processing units  125  each perform, on a signal output by a corresponding one of the filter units  126 , processing for transmission in accordance with control from the control unit  115 . Operations, administration, and maintenance (OAM) information is assigned to the signal output by the corresponding one of the filter units  126 , for example. 
         [0053]    The filter units  126  each perform filtering on a signal output by a corresponding one of the switching units  131 , in accordance with control from the control unit  115 . The functions of the filter units  126  each correspond to control on a transmitting side of a blocking port as described above and each stop outputting a signal to a corresponding communication path, in accordance with information judged by a corresponding one of the judgment units  122 . 
         [0054]    The switching unit  131  transfers a signal received by each of the interface cards  120  to a corresponding one of the interface cards  120 , which is to transmit the signal. Note that the switching unit  131  corresponds to the switch card  130  in  FIG. 3 . 
         [0055]    Note that the interface cards  120  and the switch card  130  are each configured by, for example, an integrated circuit such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA), or digital signal processing (DSP). 
         [0056]    In addition, control of the filter units  123  and the filter units  126  is performed by a corresponding one of the transmission devices  100 , which serves as an owner for each of pieces of information (for example, pieces of VLAN information) assigned to signals. 
         [0057]    Control performed by each of the transmission devices  100  in the ring network  20  will be described. Note that while control of a blocking port will be described by using the VLAN, there is no limitation to this. In addition, an owner for a signal to which VLAN information (first VLAN information) corresponding to the blocking port (B) is assigned is the transmission device  100   a , and an owner for a signal to which VLAN information (second VLAN information) corresponding to the blocking port (C) is assigned is the transmission device  100   c.    
         [0058]    Note that, in a case where a communication route is stored in the storage unit  116  when the corresponding one of the transmission devices  100  transmits a signal to another one of the transmission devices  100 , the signal is transmitted based on information stored in the storage unit  116 . 
         [0059]    In addition, in a case where no communication route is stored in the storage unit  116 , the corresponding one of the transmission devices  100  transmits a signal by using a flooding method. 
         [0060]    A case where the transmission device  100   a  transmits a signal to the transmission device  100   b  in the ring network  20  will be used and described. 
         [0061]    In a case where no signal paths are established when the transmission device  100   a  transmits, to the transmission device  100   b , signals to which the first VLAN information and the second VLAN information are assigned, the signals are transmitted by using the communication paths  15   a  and  15   b.    
         [0062]    The signal to which the first VLAN information is assigned is routed through the transmission device  100   c  and the transmission device  100   d  and is received by the transmission device  100   b . In addition, the relevant signal is subjected, based on the blocking port (B), to filtering by a corresponding one of the filter units  126  within the transmission device  100   a  between the transmission device  100   a  and the transmission device  100   b  and is not transmitted. 
         [0063]    The signal to which the second VLAN information is assigned is received by the transmission device  100   b  without being routed through another one of the transmission devices  100 . In addition, in a route routed through the transmission device  100   c  and the transmission device  100   d , in a case where the transmission device  100   c  tries to transmit a signal to the transmission device  100   d , the signal is blocked off by a corresponding one of the filter units  126  in the transmission device  100   c . Accordingly, the signal is not transmitted to the transmission device  100   d . Therefore, in the route routed through the transmission device  100   c  and the transmission device  100   d  in the second VLAN information, it is difficult for the transmission device  100   b  to receive the signal. 
         [0064]    In other words, the signal to which the first VLAN information is assigned is routed through the transmission device  100   c  and the transmission device  100   d  and is received by the transmission device  100   b , and the signal to which the second VLAN information is assigned is received by the transmission device  100   b  without being routed through another one of the transmission devices  100 . In this way, the two communication paths exist within the ring network  20 . 
         [0065]    In this case, by using the two received signals, the control unit  115  in the transmission device  100   b  selects one of the two communication paths as a communication path between the transmission device  100   a  and the transmission device  100   b  and stores, in the storage unit  116 , a port (a corresponding one of the interface cards  120 ), transmission destination information (the transmission device  100   a  in this case), and the VLAN information while associating the port (the corresponding one of the interface cards  120 ), the transmission destination information (the transmission device  100   a  in this case), and the VLAN information with one another. As a method for selection, a communication path having an earlier reception time or a communication path having a smaller number of the transmission devices  100  routed through is selected, for example. Note that, after establishing a path, transmission may be performed while assigning VLAN information different from the first VLAN information and the second VLAN information (alternatively, while assigning no VLAN information), and corresponding VLAN information does not have to be stored. 
         [0066]    In addition, in the same way, the transmission devices  100  that relay the signals along the paths each store, in the storage unit  116 , a port (one of the interface cards  120 , which corresponds to the port), transmission source information (the transmission device  100   a  in this case), and the VLAN information while associating the port (one of the interface cards  120 , which corresponds to the port), the transmission source information (the transmission device  100   a  in this case), and the VLAN information with one another. By doing so, a destination and a port (one of the interface cards  120 ) are stored while being associated with each other, thereby establishing a communication path. In addition, in a case of transmitting a signal having a destination for which a communication path is established, the signal is transmitted by using information in the storage unit  116 . 
         [0067]    After that, the transmission devices  100  each reference the storage unit  116  at a time of transmitting a signal and each transmit, from a corresponding one of the interface cards  120 , the signal to which the VLAN information is assigned in accordance with a destination. In addition, the VLAN information assigned by the transmission device  100  serving as a transmission source is not rewritten within the ring network  20 . In other words, the same VLAN information is used for each of signals in the same ring network  20 . 
         [0068]    In addition,  FIG. 5  illustrates an example of a storage table that is stored in the storage unit  116  and in which destinations, transmission destination communication paths (corresponding ones of the interface cards  120 ), and the pieces of VLAN information are stored while being associated with one another. 
         [0069]    Note that the storage table illustrated in  FIG. 5  indicates an example in which the transmission device  100   a  in the ring network  20  in  FIG. 2  receives signals having transmission sources corresponding to the respective transmission devices  100   b ,  100   c , and  100   d  and communication paths for the individual transmission devices  100  are established. 
         [0070]    In this way, by using the first VLAN information and the second VLAN information within the ring network  20 , it is possible to effectively utilize communication bands. 
       Second Embodiment 
       [0071]    The first embodiment indicates that, by using pieces of information assigned to signals, the communication bands of the ring network  20  are effectively utilized. In the second embodiment, a method for effectively utilizing bands by using communication paths will be illustrated. 
         [0072]    In the same way as in the first embodiment, the second embodiment will be described by using the ring network  20  in  FIG. 2 . 
         [0073]    Communication paths of signals will be described by using an example in which the transmission device  100   a  transmits signals to the transmission device  100   d.    
         [0074]    In a case where a communication path of a signal to which the first VLAN information is assigned is not established from the transmission device  100   a  to the transmission device  100   d , the transmission device  100   a  assigns, at a time of transmitting a first signal, the first VLAN information thereto and transmits the first signal to each of the communication paths  15   a  and  15   b . Note that since filtering is performed based on a corresponding one of the filter units  126 , transmission is not performed in the communication path  15   b . Accordingly, a communication route corresponding to the first VLAN information is established in the transmission device  100   d  and the transmission device  100   c.    
         [0075]    In addition, in a case where a communication path of a signal to which the second VLAN information is assigned is not established from the transmission device  100   a  to the transmission device  100   d , the transmission device  100   a  assigns, at a time of transmitting a second signal, the second VLAN information thereto and transmits the second signal to each of the communication paths  15   a  and  15   b . Note that since a signal in the direction of the transmission device  100   d  is subjected to filtering by a corresponding one of the filter units  126  in the transmission device  100   c , a signal that uses the communication path  15   a  and to which the second VLAN information is assigned is not received by the transmission device  100   d . Accordingly, a communication route corresponding to the second VLAN information is established in the transmission device  100   b  and the transmission device  100   d . Note that, regarding each of the transmission devices  100  that perform reception until blocking is performed by the blocking ports, a transmission source and a port may be stored while being associated with each other. This causes a period of time before a route setting of the entire ring network  20  to be shortened. 
         [0076]    Note that, in a case of receiving signals from the respective other transmission devices  100 , pieces of VLAN information and communication paths corresponding to the interface cards  120  of transmission destinations are stored, for each of destinations, in the storage unit  116  in a corresponding one of the transmission devices  100  while being associated with each other, as illustrated in  FIG. 6 . In other words, communication routes are established for each of the destinations. 
         [0077]    In addition, in a case of transmitting a subsequent signal, the VLAN information used for each signal is changed under a predetermined condition, and the signal is transmitted by the interface card  120  corresponding to a transmission destination communication path corresponding to the assigned VLAN information. In addition, regarding the predetermined condition, the pieces of VLAN information are alternately used and selected or and selected by using a random function, for example. Note that, by putting, for each of destinations by use of flags described in  FIG. 6 , a mark of the VLAN information to be subsequently used for the relevant destination, it is possible to select the interface card  120  corresponding to both the VLAN information and a transmission destination communication path, based on the flags and the destinations. 
         [0078]    As described above, by changing a communication route used every time a signal is transmitted, it is possible to effectively utilize communication bands. In addition, at a time of establishing a communication route, it is possible to reduce the number of times a duplicate copy of a signal is made. 
       Third Embodiment 
       [0079]    In the first and second embodiments, control in the single ring network  20  is described. In the third embodiment, an example of applying to a ring network that couples ring networks to each other will be described. 
         [0080]    As the ring network that couples ring networks to each other, a network obtained by coupling ring networks (Major-Rings) specified by ITU-T G.8032/Y.1344 by use of a new ring network (a Sub-Ring) will be used and be described. 
         [0081]      FIG. 7  illustrates an example of a configuration of the network  30  configured by a ring network (a Sub-Ring) coupling ring networks (Major-Rings) to each other. The network  30  includes a ring network (a Major-Ring)  21   a , a ring network (a Major-Ring)  21   b , and a ring network  22  (a Sub-Ring). Note that, in a case of not being distinguished from each other, the ring network  21   a  and the ring network  21   b  are simply described as ring networks  21 . 
         [0082]    The ring network  21   a  includes transmission devices  200   a  to  200   d . In addition, in the ring network  21   a , blocking between the transmission device  200   a  and the transmission device  200   b  is performed by a blocking port of the transmission device  200   a , and communication between the transmission device  200   a  and the transmission device  200   b  is stopped. Note that if it is assumed that the transmission device  200   a  is an owner of, for example, the ring network  21   a , a method for stopping communication is realized by installing a blocking port (C), based on control within the transmission device  200   a  serving as the owner. 
         [0083]    If it is assumed that a transmission device  200   g  is an owner of, for example, of the ring network  21   b , the ring network  21   b  is realized by installing a blocking port (E), based on control within the transmission device  200   g  serving as the owner. 
         [0084]    The ring network  22  includes transmission devices  200   c  to  200   f . In addition, in the ring network  22 , provision of a blocking port (D) is set on a port side coupled to the transmission device  200   e , based on control performed by the transmission device  200   c . Note that the ring network  22  is a ring network for coupling the ring network  21   a  and the ring network  21   b  to each other and provides a sub-ring (a Sub-Ring) indicated by G.8032. In addition, a communication path between the transmission device  200   c  and the transmission device  200   d  and a communication path between the transmission device  200   e  and the transmission device  200   f  each form a shape of being virtually included as a configuration element of a ring (are each the same line as that of a corresponding one of the Major-Rings), and these communication paths are called virtual links (Virtual Links: VLs). Note that, in a case of not being distinguished from one another, the transmission devices  200   a  to  200   h  are simply described as transmission devices  200 . 
         [0085]    In the network  30 , the blocking port (D) based on the ring network  22  inhibits communication between the device  200   c  and the transmission device  200   e  from being performed, and communication between the ring network  21   a  and the ring network  21   b  is only performed between the transmission device  200   d  and the transmission device  200   f . In such a ring network  22  that couples the ring networks  21  to each other, blocking makes it difficult to effectively utilize communication bands (between the transmission device  200   c  and the transmission device  200   e ). 
         [0086]    In addition, between the transmission device  200   c  and the transmission device  200   d  and between the transmission device  200   e  and the transmission device  200   f , communication paths of the respective ring network  21   a  and ring network  21   b  are used. Therefore, in a case where the ring network  22  controls, the ring network  21   a  and the ring network  21   b  are influenced, and accordingly, it is difficult to control. In the ring network  22 , it is desirable to install a blocking port between the transmission device  200   c  and the transmission device  200   e  or between the transmission device  200   d  and the transmission device  200   f.    
         [0087]      FIG. 8  illustrates an example of a configuration of the network  40  in the third embodiment. Note that, in  FIG. 8 , the same symbol is used for the same configuration as that of the network  30  in  FIG. 7 . 
         [0088]    The network  40  has a configuration in which ring networks  23   a  and  23   b  that each couple the ring network  21   a  and the ring network  21   b  to each other each perform different control, depending on information of a signal. Note that, in a case of not being distinguished from each other, the ring networks  23   a  and  23   b  are simply described as ring networks  23 . In addition, the ring network  23   a  and the ring network  23   b  use the same line, and different control is performed depending on information assigned to a signal. 
         [0089]    In the same way as the ring network  22  in  FIG. 7 , the ring network  23   a  installs and controls the blocking port (D) in the transmission device  200   c . The ring network  23   b  installs and controls a blocking port (D′) in the transmission device  200   d.    
         [0090]    In addition, the ring network  23   a  and the ring network  23   b  are distinguished by using, for example, a virtual local area network (VLAN). 
         [0091]    In this way, in the ring networks  23  to couple the ring network  21   a  and the ring network  21   b  to each other, the congestion of a signal (or a frame loop) is avoided by using a blocking port different for each of rings (each of the ring network  21   a  and the ring network  21   b ) corresponding to the VLAN information, and it is possible to effectively utilize bands. 
         [0092]    Next,  FIG. 9  illustrates an example of functional block diagrams of the transmission devices  200  in the third embodiment. Note that the functional block diagram of the transmission device  200  in  FIG. 9  illustrates a functional block diagram corresponding to each of the transmission device  200   c  to the transmission device  200   f  coupled to the ring networks  21  and the ring networks  23  in  FIG. 8 . Therefore, three interface cards  220   a  to  220   c  corresponding to the number of physical communication paths are included. In addition, in each of the other transmission devices  200 , functions corresponding to the interface cards  220  are increased or decreased as appropriate. In addition, the interface card  220   a  and the interface card  220   b  indicate couplings within the ring network  21   a , in other words, a case of being coupled to the transmission devices  200  that perform communication between Major-Rings, and are equal to the interface cards  120   a  and  120   b  in the transmission device  100  in  FIG. 4 . On the other hand, the interface card  220   c  indicates couplings within the ring networks  23 , in other words, a case of being coupled to the transmission devices  200  that perform communication between Sub-Rings. 
         [0093]    Since part of processing is different depending on whether between Major-Rings or between Sub-Rings, this is separated. In this regard, however, the interface card  220   c  may be used for each of all the interface cards  220 . 
         [0094]    In addition, a judgment may be performed for each of signals received by using the interface card  220   a  in place of the interface card  220   c.    
         [0095]    The corresponding one of the transmission devices  200  includes a control unit  215 , a storage unit  216 , reception units  221   a  to  221   c , judgment units  222   a  to  222   c , filter units  223   a ,  223   b ,  223   c - 1 , and  223   c - 2 , transmission units  224   a  to  224   c , transmission processing unit  225   a  to  225   c , filter units  226   a ,  226   b ,  226   c - 1 , and  226   c - 2 , a branching unit  227   c , and a switching unit  231 . 
         [0096]    Note that, in a case of not being distinguished from each other, the individual functions (the reception units  221   a  to  221   c , the judgment units  222   a  to  222   c , the filter units  223   a ,  223   b ,  223   c - 1 , and  223   c - 2 , the transmission units  224   a  to  224   c , the transmission processing units  225   a  to  225   c , and the filter units  226   a ,  226   b ,  226   c - 1 , and  226   c - 2 , and the branching unit  227   c ) in the individual interfaces  220  are simply described only by using numeric characters (described as reception units  221 , judgment units  222 , filter units  223 , transmission units  224 , transmission processing units  225 , filter units  226 , and a branching unit  227 ). In addition, in a case of not being distinguished from each other, the filter units  226   c - 1  and  226   c - 2  are simply described as filter units  226   c.    
         [0097]    In addition, since the control unit  215 , the storage unit  216 , the reception units  221 , the judgment units  222 , the filter units  223 , the transmission units  224 , the transmission processing units  225 , and the switching unit  231  are the same as the control unit  115 , the storage unit  116 , the reception units  121 , the judgment units  122 , the filter units  123 , the transmission units  124 , the transmission processing units  125 , and the switching unit  131  in  FIG. 4 , respectively, the descriptions thereof will be omitted. 
         [0098]    The filter units  226   a  and  226   b  are the same as the filter units  126  in  FIG. 4 . In accordance with control from the control unit  215 , the filter unit  226   c  performs filtering on a signal output by the branching unit  227   c . The functions of the filter units  226  each correspond to control on a transmitting side of a blocking port as described above and each stop outputting a signal to a corresponding communication path. 
         [0099]    The branching unit  227   c  makes duplicate copies (two in a case of  FIG. 6 ) of a signal transferred by the switching unit  231  and assigns different pieces of information (for example, VLANs) to the duplicate signals, thereby transferring the duplicate signals to the filter units  226   c  corresponding to the respective assigned pieces of information. Note that, as a method for assigning the different pieces of information (VLANs) to the signals, information of one of the two signals is set to information (a first VLAN) of a sent signal and information of the other is switched to information (a second VLAN) different from the information of the sent signal, for example. Note that switched information (a third VLAN) may be used without using the information (the first VLAN) of the sent signal. 
         [0100]    Next, by using  FIG. 10  and  FIG. 11 , a method for forming the ring networks  23  by use of two different VLANs will be described. 
         [0101]    Note that, in each of  FIG. 10  and  FIG. 11 , it is assumed that an owner in the ring network  23   a  in the network  40  in  FIG. 8  is the transmission device  200   c . In addition, in the following description, an explanation will be provided under the assumption that VLANs are used as information for distinguishing the ring networks  23 . 
         [0102]      FIG. 10  illustrates a flowchart to when one of the transmission devices  200 , which serves as an owner, transmits, to corresponding ones of the transmission devices  200  within the ring network  23   a , a control signal for configuring the ring network  23   b.    
         [0103]    In accordance with ITU-T G.8032, the ring network  23   a  to couple the ring network  21   a  and the ring network  21   b  to each other is configured. Each of the transmission devices  200  confirms whether the self-transmission device  200  is the transmission device  200  serving as an owner in the ring network  23   a  (step S 10 ). Note that the transmission device  200  to serve as the owner is preliminarily set under a predetermined condition. Note that, regarding a setting under the predetermined condition, a network administrator preliminarily performs the setting, as an example. 
         [0104]    In a case where the self-transmission device  200  is the owner (step S 10 : Yes), the presence or absence of failure information related to the self-transmission device  200  is examined (step S 11 ). Note that the presence or absence of failure information is examined based on the presence or absence of reception of a control signal between the coupled transmission devices  200 , as an example. In addition, in a case where the self-transmission device  200  is not the owner (step S 10 : No), the processing is terminated. 
         [0105]    In a case where no failure information exists in the self-transmission device  200  (step S 11 : Yes), it is examined whether NR information indicating the absence of failure information is received from one of the adjacent transmission devices  200  (one of the coupled transmission devices  200 ) by using the control signal (step S 12 ). 
         [0106]    In addition, in a case where the failure information exists in the self-transmission device  200  (step S 11 : No), the processing shifts to step S 14 . 
         [0107]    In a case where the NR information is received (step S 12 : Yes), a signal in which flags for configuring the ring network  23   b  are assigned to the control signal is transmitted (step S 13 ). Note that, as the flags for configuring the ring network  23   b , there are an owner instruction flag (a first flag) and a flag (a second flag) indicating the implementation of the ring network  23   b . Note that, as an example of the flag (the second flag) indicating the implementation, a VLAN Active Flag is cited. Note that, as an example of a method for storing the flags, an empty space in a frame format of the signal is used. 
         [0108]    In addition, in a case where no NR information is received (step S 12 : No), in other words, in a case where there is failure information related to another one of the transmission devices  200  configuring the ring network  23   a , the processing shifts to step S 14 . 
         [0109]    In a case where there is failure information in one of the transmission devices  200  configuring the ring network  23   a , a recovery is confirmed (step S 14 ), and in a case where there is no recovery, a recovery is waited for (step S 14 : No). Note that, during the waiting, the ring network  23   a  is defined as a ring network in which a blocking port is formed by one of the transmission devices  200  so that a communication path in which a failure occurs is subjected to blocking. 
         [0110]    In a case where failure information in each of the transmission devices  200  disappears (step S 14 : Yes), a reversion to the owner is performed (step S 15 ), and the processing operation in step S 11  is performed. Note that, as an example of the reversion to the owner, information indicating that failures disappear from the other transmission devices  200  is received, thereby performing the reversion, for example. Note that, regarding the information indicating that failures disappear from the other transmission devices  200 , it is possible to determine by receiving the NR information indicating the absence of a failure. 
         [0111]    In this way, the transmission device  200  to serve as the owner transmits, to the adjacent transmission devices  200 , the control signal for configuring the ring network  23   b.    
         [0112]      FIG. 11  is a flowchart illustrating an operation of each of the transmission devices  200  at a time of receiving a control signal having a flag. 
         [0113]    Upon receiving the signal, a corresponding one of the transmission devices  200  examines whether or not the signal is a signal having a flag (step S 20 ). Note that it is assumed that, as flags in the flowchart in  FIG. 11 , there are the first flag and the second flag, described in  FIG. 10 . In addition, the judgment unit  222  judges whether being a signal having a flag. 
         [0114]    In a case where a signal having a flag is received (step S 20 : Yes), it is examined whether the signal has the first flag (step S 21 ). 
         [0115]    In addition, in a case where no signal having a flag is received (step S 20 : No), the processing is terminated without performing control related to the configuration of the ring network  23   b.    
         [0116]    In a case where the first flag is received (step S 21 : Yes), a communication path from which the first flag is received is examined. Note that, as a method for examining, it is examined whether or not the signal is received from a line (a Virtual Link: VL) identical to those of the ring networks  23  and one of the ring networks  21  (step S 22 ). 
         [0117]    In addition, in a case where no first flag is received (step S 21 : No), in other words, in a case where only the second flag is received, a setting of the ring network  23   b  is performed (step S 25 ). Note that, regarding a setting of the ring network  23   b , a new VLAN different from the ring network  23   a  is assigned and is provided, for example. 
         [0118]    In a case where the signal is received from a line (a VL) identical to those of the ring networks  23  and one of the ring networks  21  (step S 22 : Yes), processing as the owner of the ring network  23   b  is performed (step S 23 ). The processing as the owner is a setting of a blocking port, as an example. 
         [0119]    In addition, in a case where the signal is not received from a line (a VL) identical to that of one of the ring networks  21  (step S 22 : No), the flags are removed (step S 24 ), and the processing is terminated. 
         [0120]    Note that, in a case where the owner processing is performed (step S 23 ) or processing for a setting of the ring network  23   b  (step S 25 ) is performed, the information of the second flag is assigned to a signal to be transmitted to a path of the ring networks  23 , different from a communication path from which reception is performed, and the signal is transmitted. 
         [0121]    By doing so, in all the transmission devices  200  configuring the ring networks  23 , a setting of a new VLAN becomes available. Therefore, a configuration of the ring networks  23 , which utilizes pieces of VLAN information, becomes available. This indicates a virtual duplex configuration utilizing VLANs, for example. 
         [0122]    In addition, as the assignment of a new VLAN, there are a method in which the transmission device  200  on a transmitting side preliminarily stores in the second flag and a method for storing in a signal at a time of coupling the ring network  23   b  in step S 23 , for example. 
         [0123]    Here, a storage position of a flag and so forth will be described by using an example of a frame format of a signal in  FIG. 12 . The frame format of a signal illustrated in  FIG. 12  is an automatic protection switching (APS) format specified by G.8032. 
         [0124]    By using two unused bits of Flags ( 0 ) in (a) in  FIG. 12 , the storage location of the flag is able to be realized. In addition, the NR information corresponds to R-APS specific information (32 octets) in (b) in  FIG. 12 . 
         [0125]    Next, an operation utilizing the ring networks  23  formed for respective pieces of VLAN information will be described by using  FIGS. 13A, 13B and 13C . Note that, regarding owners of respective ring networks, it is assumed that the transmission device  200   a  is an owner of the ring network  21   a , the transmission device  200   g  is an owner of the ring network  21   b , and the transmission device  200   c  is an owner of the ring network  23   a . In addition, regarding the ring network  23   b , the transmission device  200   d  becomes an owner, based on the flowchart in  FIG. 11 . 
         [0126]      FIGS. 13A, 13B and 13C  are diagrams for explaining a case of transmitting by using a flooding method. 
         [0127]      FIG. 13A  illustrates a case where the transmission devices  200  each transmit based on the flooding method and the transmission device  200   a  transmits a signal to the transmission device  200   g  or the transmission device  200   h . In other words,  FIG. 13A  illustrates directions of signals transmitted by the respective transmission devices  200  in a case where a communication path from the transmission device  200   a  to the transmission device  200   g  or the transmission device  200   h  is not established. 
         [0128]    In addition, solid lines in  FIG. 13A  indicate a case of transmission in only one direction, and dotted lines indicate a case of transmission in directions. 
         [0129]    Since the blocking port (C) exists in the direction of the transmission device  200   b , the transmission device  200   a  only transmits to the transmission device  200   c.    
         [0130]    Upon receiving a signal from the transmission device  200   a , the transmission device  200   c  transmits, to the transmission device  200   e , a signal to which the second VLAN information is assigned, and the transmission device  200   c  transmits the received signal to the transmission device  200   d.    
         [0131]    Here, by using  FIG. 9 , an operation of the transmission device  200   c  will be described. Note that, in the description, the interface card  220   a  in the functional block diagram in  FIG. 9  is coupled to the transmission device  200   a , the interface card  220   b  therein is coupled to the transmission device  200   d , and the interface card  220   c  therein is coupled to the transmission device  200   e.    
         [0132]    First, in a case where the reception unit  221   a  receives a signal, the judgment unit  222   a  judges the signal. Note that the judgment unit  222   a  references, for example, destination information of the signal and judges whether being established as a communication path. Since no blocking port exists on a transmission device  200   a  side, the filter unit  223   a  transfers the signal to the switching unit  231  without change. The switching unit  231  outputs the signal to the filter unit  226   b  (within the interface card  220   b ) and the branching unit  227   c  (within the interface card  220   c ). Note that since the signal is output to the two interface cards  220  at this time, the switching unit  231  makes and transfers duplicate copies of the signal to the respective two interface cards  220 . 
         [0133]    Since no blocking port exists on a transmission device  200   d  side, the filter unit  226   b  transfers the signal to the transmission processing unit  225   b  without change. The transmission processing unit  225   b  performs processing (for example, assignment of OAM information) for transmitting to the transmission device  200   d  and transfers to the transmission unit  224   b . The transmission unit  224   b  transmits the signal to the transmission device  200   d.    
         [0134]    In addition, in a case where the signal is transferred by the switching unit  231 , the branching unit  227   c  makes duplicate copies of the signal and assigns pieces of information of different VLANs thereto, thereby transferring the signals to the respective filter units  226   c - 1  and  226   c - 2 . 
         [0135]    The filter unit  226   c - 1  corresponds to the ring network  23   a  and includes the blocking port (D) on a transmission device  200   e  side in the ring network  23   a . Therefore, the filter unit  226   c - 1  does not transfer the signal to the transmission processing unit  225   c . On the other hand, the filter unit  226   c - 2  corresponds to the ring network  23   b , and no blocking port exists on a transmission device  200   e  side in the ring network  23   b . Therefore, the filter unit  226   c - 2  transfers the signal to the transmission processing unit  225   c . The transmission processing unit  225   c  performs processing (for example, assignment of OAM information) for transmitting to the transmission device  200   e  and transfers to the transmission unit  224   c . The transmission unit  224   c  transmits the signal to the transmission device  200   d  via the ring network  23   b.    
         [0136]    The processing explained in the above description is performed by the transmission device  200   c , and transmission from the transmission units  224  in the interface cards  220  other than the interface card  220  corresponding to the reception unit  221  that performs reception becomes available, based on the flooding method. 
         [0137]    Returning to  FIG. 13A , the transmission device  200   d  transmits, to the transmission device  200   b  and the transmission device  200   f , the signal received from the transmission device  200   c.    
         [0138]    Note that since the blocking port (C) exists, transmission of a signal from the transmission device  200   b  to the transmission device  200   a  is not performed. Note that since the transmission device  200   a  serving as an owner manages the blocking port (C) in the ring network  21   a , actually the signal is transmitted to the transmission device  200   a  by the transmission device  200   b  and a corresponding one of the filter units  223  performs blocking in a case where the transmission device  200   a  receives the signal. 
         [0139]    In addition, at a time of transmitting to the transmission device  200   f , the transmission device  200   d  performs control (control in the interface card  220   c ) described in the transmission device  200   c  and transmits by using the ring network  23   a . In other words, a signal is transmitted to the transmission device  200   f  by using the VLAN information (the first VLAN information corresponding to the ring network  23   a ) different from that at a time when the transmission device  200   c  transmits the signal to the transmission device  200   e.    
         [0140]    Upon receiving the signal from the transmission device  200   d , the transmission device  200   f  transmits the received signal to the transmission device  200   e  and the transmission device  200   h.    
         [0141]    Note that since the blocking port (E) exists, transmission of a signal from the transmission device  200   h  to the transmission device  200   g  is not performed. 
         [0142]    The transmission device  200   e  transmits, to the transmission device  200   g , the signal received from the transmission device  200   c . In addition, the signal from the transmission device  200   f  is not transmitted while being received. 
         [0143]    An operation of the transmission device  200   e  will be described in detail. Note that, in the functional block diagram in  FIG. 9 , regarding the transmission device  200   e , the interface card  220   a  is coupled to the transmission device  200   g , the interface card  220   b  is coupled to the transmission device  200   f , and the interface card  220   c  is coupled to the transmission device  200   c.    
         [0144]    Upon receiving a signal from the transmission device  200   c , the reception unit  221   c  transfers the signal to the judgment unit  222   c . The judgment unit  222   c  judges the signal and sends a result thereof to the control unit  215 . In addition, the judgment unit  222   c  transfers the signal to the filter unit  223   c - 1  or the filter unit  223   c - 2 , in accordance with the information of a VLAN, assigned to the signal. Note that the filter unit  223   c - 1  corresponds to a VLAN corresponding to the ring network  23   a  and the filter unit  223   c - 2  corresponds to a VLAN corresponding to the ring network  23   b . Note that since the signal is a signal received (received with the ring network  23   b  as a communication path) from the transmission device  200   c , the transmission device  200   e  transfers the signal to the filter unit  223   c - 2 . 
         [0145]    Since there is no control of a blocking port, the filter unit  223   c - 2  changes a VLAN and transfers to the switching unit  231 . 
         [0146]    In addition, in accordance with information (for example, VLAN information, a source address (a source address), and a destination address (a destination address)) from the judgment unit  222   c , the control unit  215  controls the filter unit  223   b  and the filter unit  226   b  or controls the switching unit  231 . Specifically, control is performed so that a source address and a destination address of the signal received by the reception unit  221   c  are stored in the storage unit  216  and so that the same signal (a signal having the same source address and the same destination address) is not transferred in a case of receiving the same signal from another interface. 
         [0147]    This avoids the congestion of the signal by controlling the filter unit  223   b  and the filter unit  226   b  or controlling the switching unit  231  in a case of receiving via the ring network  23   b  (a ring network utilizing a signal of a VLAN different from normal), in addition to normal control of a blocking port. 
         [0148]    Note that, in the control of the filter unit  226   b  or the switching unit  231 , one of the two only has to be controlled and in a case of controlling the switching unit  231 , control is performed so that a signal is only transferred to the interface card  220   a.    
         [0149]    In addition, in a case of controlling the filter unit  226   b , a corresponding signal is subjected to filtering, and no signal is transferred to the transmission processing unit  225   b.    
         [0150]    In a case where such control is performed and the reception unit  220   b  receives the signal from the transmission device  200   f , the signal is judged by the judgment unit  222   b , and, based on control from the control unit  215 , the signal is subjected to filtering by the filter unit  223   b , thereby enabling congestion to be avoided. 
         [0151]    In a case where, as described above, the transmission device  200   a  transmits the signal to the transmission device  200   g  or the transmission device  200   h  by using the flooding method, the transmission devices  200  that relay the signal along the paths and the transmission device  200  that receives the signal each store, in the storage unit  226 , a relationship between a port (one of the interface cards  220 ), a transmission source, and the VLAN information. 
         [0152]    In the same way, upon receiving, from each of the transmission device  200   g  and the transmission device  200   h , a signal addressed to the transmission device  200   a , communication paths are established, and in subsequent communication, a signal is transmitted by using a communication path illustrated in  FIG. 13B  or  FIG. 13C . Note that  FIG. 13B  and  FIG. 13C  are diagrams illustrating a case where communication routes are established at a time when the transmission device  200   a  transmits a signal to the transmission device  200   g  and at a time when the transmission device  200   a  transmits a signal to the transmission device  200   h , respectively. As understood from  FIG. 13B  and  FIG. 13C , it is understood that different communication paths are used in the ring networks  23  at a time of transmitting signals to the respective transmission device  200   g  and transmission device  200   h.    
         [0153]    In this way, by using pieces of VLAN information, communication paths in the ring networks  23  become able to be established. 
         [0154]    Note that, in a case where a failure related to one of the transmission devices  200  configuring the ring networks  23 , switching to a single piece of VLAN information is performed (alternatively, all pieces of VLAN information are erased), and furthermore, pieces of related information such as destinations and transmission destination communication paths, stored in the storage unit  216 , are erased. In other words, the virtual double coupling is cancelled, and an operation as a ring network in which a failure point is set to a blocking port is performed. 
         [0155]    As described above, in the third embodiment, by causing the ring networks  23  coupling the ring network  21   a  and the ring network  21   b  to each other to have a duplex configuration, it becomes possible to effectively utilize communication paths coupling the transmission devices  200  to each other. 
       First Example of Modification to Third Embodiment 
       [0156]    In the third embodiment, the ring networks  23  coupling the ring network  21   a  and the ring network  21   b  to each other is described in a case where the same number of transmission devices  200  are used in each of the ring networks  21 . 
         [0157]    Here, a case where a different number of transmission devices  200  are used in each of the ring networks  21  will be described. 
         [0158]      FIG. 14  illustrates the network  50  in the first example of a modification to the third embodiment. The network  50  includes the ring network  21   a , a ring network  21   c , a ring network  24   a , and a ring network  24   b . Note that the ring network  24   a  and the ring network  24   b  are distinguished from each other by using the VLAN information and are ring networks virtually having a duplex configuration. In other words, a network in which a physical line is used by VLANs is illustrated. In addition, in a case of not being distinguished from each other, the ring network  21   a  and the ring network  21   c  are simply described as ring networks  21 . In addition, in a case of not being distinguished from each other, the ring network  24   a  and the ring network  24   b  are simply described as ring networks  24 . Note that the same symbol is assigned to an element having the same configuration as that in the network  40 , and the description thereof will be omitted. In addition, in a case of not being distinguished from one another, the transmission devices  200   a  to  200   d  and transmission devices  200   i  to  200   k  are simply described as transmission devices  200 . 
         [0159]    The ring network  21   c  includes the transmission devices  200   i  to  200   k . Note that the transmission device  200   j  and the transmission device  200   k  are the same as the transmission device  200   g  and the transmission device  200   h , respectively, in  FIG. 8 . In addition, in the ring network  21   c , a corresponding port of the transmission device  200   j  is defined as a blocking port (G) between the transmission device  200   j  and the transmission device  200   k , and communication between the transmission device  200   j  and the transmission device  200   k  is stopped. 
         [0160]    The ring networks  24   a  and  24   b  include the transmission devices  200   c ,  200   d , and  200   i . In the ring networks  24   a  and  24   b , communication between the ring network  21   a  and the ring network  21   c  is considered to be performed between the transmission device  200   c  and the transmission device  200   i  or between the transmission device  200   d  and the transmission device  200   i.    
         [0161]    In addition, in the ring network  24   a , the transmission device  200   c  forms a blocking port (F) (corresponding to the first VLAN information) between the transmission device  200   c  and the transmission device  200   i , thereby stopping communication between the transmission device  200   c  and the transmission device  200   i . In addition, in the ring network  24   b , the transmission device  200   d  forms a blocking port (F′) (corresponding to the second VLAN information) between the transmission device  200   d  and the transmission device  200   i , thereby stopping communication between the transmission device  200   d  and the transmission device  200   i.    
         [0162]    Note that the transmission device  200   i  includes a total of four interface cards  220  including two interface cards  220  (each corresponding to the interface card  220   c  in  FIG. 9 ) that each perform communication between the Sub-Rings and two interface cards  220  (each corresponding to the interface card  220   a ) that each perform communication between the Major-Rings. 
         [0163]    As described in the third embodiment, in such a network  50 , by causing the Sub-Rings to have a duplex configuration, it is possible to effectively utilize bands that are unused due to blocking. 
       Second Example of Modification to Third Embodiment 
       [0164]    The third embodiment illustrates a method in which a different blocking point is used for each of the pieces of VLAN information in the Sub-Rings coupling the Major-Rings to each other, thereby effectively utilizing bands. 
         [0165]    In the second example of a modification to the third embodiment, a case where a different blocking point is applied to each of pieces of VLAN information in the Major-Rings will be described. 
         [0166]      FIG. 15  illustrates an example of a configuration of the network  60  configured by a ring network (a Major-Ring) coupling ring networks (Sub-Rings) to each other. The network  60  includes a ring network (the Major-Ring)  31 , a ring network  32  (one of the Sub-Rings), and a ring network  33  (the other of the Sub-Rings). In addition, it is assumed that each of the ring network  32  and the ring network  33  is further coupled to another Major-Ring. Accordingly, a line between transmission devices  300   a  and  300   b  and a line between transmission devices  300   g  and  300   h  are each the same line (a Virtual Link: VL) as that of a corresponding one of the other Major-Rings and are each difficult to use as a blocking port in a case of not being used by a blocking port by the corresponding one of the other Major-Rings. 
         [0167]    Note that the ring network  31  is configured by transmission devices  300   c  to  300   f , the ring network  32  is configured by the transmission devices  300   a  to  300   d , and the ring network  33  is configured by the transmission devices  300   e  to  300   h.    
         [0168]    In addition, regarding blocking ports of the respective ring networks, the ring network  31  includes a blocking port (corresponding to a blocking port (I)) in the transmission device  300   c , the ring network  32  includes a blocking port (corresponding to a blocking port (H)) in the transmission device  300   a , and the ring network  33  includes a blocking port (corresponding to a blocking port (J)) in the transmission device  300   e.    
         [0169]    Note that the transmission devices  300  each have the same configuration as those of the transmission devices  200  described in the third embodiment and the interface cards  220  vary depending on coupling states. 
         [0170]    Here,  FIG. 16  illustrates the network  70  in which a blocking port is applied to each of the ring networks  32  and  33  for each of different pieces of VLAN information by using the method described in the third embodiment. The network  70  includes the ring network  31 , ring networks  32   a  and  32   b , and ring networks  33   a  and  33   b . Note that the ring network  32   a  and the ring network  32   b  are obtained by configuring the ring network  32  in  FIG. 15  by using pieces of VLAN information and are simply described as ring networks  32  in a case of not being distinguished from each other. In addition, in the same way, the ring networks  33   a  and  33   b  are simply described as ring networks  33  in a case of not being distinguished from each other. In addition, regarding blocking ports of the individual ring networks, the ring network  32   a  includes a blocking port (corresponding to the blocking port (H)) in the transmission device  300   a , the ring network  32   b  includes a blocking port (corresponding to a blocking port (H′)) in the transmission device  300   b , the ring network  33   a  includes a blocking port (corresponding to the blocking port (J)) in the transmission device  300   e , and the ring network  33   b  includes a blocking port (corresponding to a blocking port (J′)) in the transmission device  300   f.    
         [0171]    As described above, in both the Sub-Rings installed in the Major-Ring, VLANs are used based on the method described in the third embodiment, and blocking ports of the respective VLANs are used for each of the Sub-Rings, thereby enabling communication bands to be effectively used. 
         [0172]    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 embodiments of the present invention have 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.