Abstract:
In a wireless network system for interconnecting a wireless network and another network different from the wireless network, the wireless network system includes: a plurality of wireless nodes for establishing the wireless network; a control device connected to the other network; and a plurality of gateways for interconnecting the wireless network and the other network, wherein the wireless node searches a path from a gateway candidate list, selects a gateway on an optimal path based on information collected in searching the path, and transfers data to the control device through the gateway on the optimal path.

Description:
This application is based on and claims priority from Japanese Patent Application No. 2006-298793, filed on Nov. 2, 2006, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates to a wireless network system for interconnecting a wireless network and another network different from the wireless network, and more particularly to a wireless network system for selecting an optimum one of a plurality of gateways, thereby enabling an interconnection between networks. 
     2. Related Art 
     The related-art documents related to a wireless network system for interconnecting a conventional wireless network and another network different from the conventional wireless network are as follows:
     Japanese Patent Unexamined Documents: JP-A-2000-196674, JP-A-2003-258704, JP-T-2004-514339, JP-A-2005-167609 and JP-A-2005-268988.   

       FIG. 6  is a block diagram showing an example of a conventional wireless network system. In  FIG. 6 , wireless nodes  1 ,  2 ,  3 ,  4 ,  5  and  6  establish a mesh type of a wireless network and carry out a communication through the wireless network, A gateway  7  interconnects the wireless network to an Internet Protocol (IP) network, A control device  8  such as a computer carries out a communication through the IP network, and an IP network  100  is shown. 
     The wireless node  1  is interconnected to adjacent wireless nodes  3 ,  4  and  2  using wireless lines “WN 01 ”, “WN 02 ” and “WN 03 ” as shown in  FIG. 6 . Similarly, wireless nodes  2 ,  3 ,  4 ,  5  and  6  are also interconnected to the adjacent wireless nodes properly using wireless lines “WN 04 ”, “WN 05 ”, “WN 07 ”, “WN 08 ” and “WN 09 ” as shown in  FIG. 6 . 
     Furthermore, the wireless nodes  2 ,  4  and  6  are interconnected to the gateway  7  using wireless lines “WN 10 ”, “WN 11 ” and “WN 12 ” as shown in  FIG. 6 , respectively. Then, a wireless network shown in a broken line of  FIG. 6  is established. 
     In addition, the gateway  7  is interconnected to the IP network  100 , and also the control device  8  is interconnected to the IP network  100 . 
     Moreover,  FIG. 7  is a block diagram showing a specific example of the wireless node. In  FIG. 7 , wireless communication means  9  carries out a communication through a wireless network (a wireless line), calculation control means  10  controls a whole wireless node such as a Central Processing Unit (CPU), storing means  11  stores data and information for a communication path search, and storing means  12  stores a program to control the wireless node. Furthermore, the wireless communication means  9 , the calculation control means  10 , the storing means  11  and storing means  12  constitute a wireless node  50 . 
     The wireless communication means  9  is interconnected to a wireless network (a wireless line), and an input/output of the wireless communication means  9  is interconnected to the calculation control means  10 . Moreover, the storing means  11  and the storing means  12  are also interconnected to the calculation control means  10 . 
     The calculation control means  10  reads and executes a stored program from the storing means  12  to control the whole wireless node, and the wireless communication means  9  is controlled based on information stored in the storing means  11  to establish a wireless network together with the adjacent wireless node, thereby transferring data from each other. Thus, the calculation control means  10  is operated as the wireless node. 
       FIG. 8  is a block diagram showing a specific example of the gateway. In  FIG. 8 , wireless communication means  13  carries out a communication through a wireless network (a wireless line), calculation control means  14  controls a whole gateway such as a CPU, communication means  15  carries out a communication through an IP network (not shown), storing means  16  stores a program to control the gateway, and storing means  17  stores information for a protocol conversion. Moreover, the wireless communication means  13 , the calculation control means  14 , the communication means  15 , the storing means  16  and the storing means  17  constitute a gateway  51 . 
     The wireless communication means  13  is interconnected to a wireless network (a wireless line), and an input/output of the wireless communication means  13  is interconnected to the calculation control means  14 . Moreover, the communication means  15  is interconnected to the IP network and has an input/output interconnected to the calculation control means  14 . Furthermore, the storing means  16  and the storing means  17  are interconnected to the calculation control means  14 . 
     The calculation control means  14  reads and executes the program stored in the storing means  16  to control the whole gateway, and controls the wireless communication means  13  based on the information stored in the storing means  17  to establish a wireless network together with the adjacent wireless node, thereby transferring data from each other. On the other hand, the calculation control means  14  controls the communication means  15  to carry out a data transfer together with the IP network  100 . 
     Furthermore, the calculation control means  14  converts a protocol of data received through the wireless communication means  13  or the communication means  15  based on the information for a protocol conversion which is stored in the storing means  17  and transfers the data subjected to the protocol conversion through the communication means  15  or the wireless communication means  13 . Thus, the calculation control means  14  is operated as the gateway. 
     An operation according to the conventional example shown in  FIG. 6  will be described with reference to  FIGS. 9 ,  10 ,  11  and  12 .  FIG. 9  is a flowchart for explaining an operation of each wireless node,  FIGS. 10 and 12  are diagrams for explaining a data transfer, and  FIG. 11  is a flowchart for explaining an operation of the gateway  7 . 
     At “S 001 ” in  FIG. 9 , the wireless node (more specifically, the calculation control means  10 ) determines whether to transfer data to the control device  8  (the IP node) through the IP network  100 . 
     At “S 001 ” in  FIG. 9 , if the wireless node determines to transfer the data, the wireless node (more specifically, the calculation control means  10 ) searches a transfer path to the gateway  7  and establishes a transfer path at “S 002 ” in  FIG. 9  and the wireless node (more specifically, the calculation control means  10 ) transfers the data to the gateway  7  using the established transfer path at “S 003 ” in  FIG. 9 . 
     For example, if the wireless node  1  (more specifically, the calculation control means in the wireless node  1 ) determines to transfer the data, the wireless node  1  searches and establishes a transfer path using wireless lines “WN 02 ” and “WN 11 ” in  FIG. 10  and the wireless node  1  (more specifically, the calculation control means in the wireless node  1 ) transfers the data to the gateway  7  as shown in “SD 21 ” of  FIG. 10 . 
     Meanwhile, at “S 101 ” in  FIG. 11 , the gateway  7  (more specifically, the calculation control means  14 ) determines whether the data is received through a wireless network or not. 
     At “S 101 ” in  FIG. 11 , if the gateway  7  determines that the data is received through the wireless network, the gateway  7  (more specifically, the calculation control means  14 ) converts a protocol of the received data at “S 102 ” in  FIG. 11  and the gateway  7  (more specifically, the calculation control means  14 ) transfers the data to the control device  8  (the IP node) through the IP network  100  at “S 103 ” in  FIG. 11 . 
     For example, if the gateway  7  (more specifically, the calculation control means in the gateway  7 ) determines that the date is received, the gateway  7  (more specifically, the calculation control means in the gateway  7 ) transfers the data to the control device  8  through the IP network  100  as shown in “SD 31 ” of  FIG. 12 . 
     As a result, it is possible to interconnect a wireless network and another network different from the wireless network by providing the gateway between the wireless network and the IP network, converting the protocol of the received data and transferring the data subjected to the protocol conversion through the gateway. 
     Moreover,  FIG. 13  is a block diagram showing another example of the conventional wireless network system, in which the gateway is particularly doubled. In  FIG. 13 ,  1  to  6  and  8  are the same reference numerals as those in  FIG. 6 , and a gateway  18  (Primary Gateway) for a normal operation interconnects a wireless network and an IP network, a gateway  19  (Secondary Gateway) for a backup interconnects the wireless network to the IP network, and a IP network  101  is shown. 
     The wireless node  1  is interconnected to adjacent wireless nodes  3 ,  4  and  2  using wireless lines “WN 41 ”, “WN 42 ” and “WN 43 ” in  FIG. 13 . Similarly, wireless nodes  2 ,  3 ,  4 ,  5  and  6  are also interconnected to the adjacent wireless nodes by properly using wireless lines “WN 44 ”, “WN 45 ”, “WN 46 ”, “WN 47 ”, “WN 48 ” and “WN 49 ” in  FIG. 13 . 
     Furthermore, the wireless nodes  2 ,  4  and  6  are interconnected to the gateway  18  using wireless lines “WN 50 ”, “WN 52 ” and “WN 54 ” in  FIG. 13  respectively, and the wireless nodes  2 ,  4  and  6  are interconnected to the gateway  19  using wireless lines “WN 51 ”, “WN 53 ” and “WN 55 ” in  FIG. 13  respectively. Thus, a wireless network shown in a broken line of  FIG. 13  is established. 
     In addition, the gateways  18  and  19  are interconnected to the IP network  101 , and also the control device  8  is interconnected to the IP network  101 . 
     An operation according to the conventional example shown in  FIG. 13  will be described with reference to  FIGS. 14 ,  15 ,  16 ,  17 ,  18  and  19 .  FIG. 14  is a flowchart for explaining an operation of each wireless node.  FIGS. 15 ,  17  and  19  are diagrams for explaining a data transfer.  FIG. 16  is a flowchart for explaining an operation of the gateway  18  for a normal operation.  FIG. 18  is a flowchart for explaining an operation of the gateway  19  for a backup. 
     It is assumed that the gateway  18  for a normal operation and the gateway  19  for a backup are treated as an identical network address. Moreover, it is assumed that specific structures of the wireless nodes  1  to  6  and the gateways  18  and  19  are the same as those in  FIGS. 7 and 8 . 
     At “S 201 ” in  FIG. 14 , the wireless node (more specifically, the calculation control means  10 ) determines whether to transfer the data to the control device  8  (the IP node) through the IP network  101  or not. 
     At “S 201 ” in  FIG. 14 , if the wireless node determines to transfer the data, the wireless node (more specifically, the calculation control means  10 ) searches a transfer path to the gateway  18  for a normal operation and establishes the transfer path at “S 202 ” in  FIG. 14  and the wireless node (more specifically, the calculation control means  10 ) transfers the data to the gateway  18  for a normal operation using the established transfer path at “S 203 ” in  FIG. 14 . 
     For example, if the wireless node  1  (more specifically, the calculation control means in the wireless node  1 ) determines to transfer the data, the wireless node  1  (more specifically, the calculation control means in the wireless node  1 ) searches and establishes a transfer path using wireless lines “WN 42 ” and “WN 52 ” in  FIG. 15 , and transfers the data to the gateway  18  for a normal operation as shown at “SD 61 ” in  FIG. 15 . 
     As described above, the gateway  18  for a normal operation and the gateway  19  for a backup are treated as the identical network address. Therefore, at the same time, the data are also transferred from the wireless node  1  to the gateway  19  for a backup through wireless lines “WN 42 ” and “WN 53 ” in  FIG. 15  as shown in “SD 62 ” in  FIG. 15 . 
     Meanwhile, at “S 301 ” in  FIG. 16 , the gateway  18  for a normal operation (more specifically, the calculation control means  14 ) determines whether the data is received through a wireless network or not. 
     At “S 301 ” in  FIG. 16 , if the gateway  18  determines that the data is received through the wireless network, the gateway  18  for a normal operation (more specifically, the calculation control means  14 ) converts the protocol of the received data at “S 302 ” in  FIG. 16  and the gateway  18  for a normal operation (more specifically, the calculation control means  14 ) transfers the data to the control device  8  (the IP node) through the IP network  101  at “S 303 ” in  FIG. 16 . 
     For example, if the gateway  18  for a normal operation (more specifically, the calculation control means in the gateway  18 ) determines that the data are received, the gateway  18  for a normal operation (more specifically, the calculation control means in the gateway  18 ) transfers the data to the control device  8  through the IP network  101  as shown in “SD 71 ” in  FIG. 17 . 
     At “S 401 ” in  FIG. 18 , moreover, the gateway  19  for a backup (more specifically, the calculation control means  14 ) determines whether the data are received through the wireless network or not. 
     At “S 401 ” in  FIG. 18 , if the gateway  19  determines that the data are received through the wireless network, the gateway  19  for a backup (more specifically, the calculation control means  14 ) determines whether the gateway  18  for a normal operation is in a normal operation or not at “ 402 ” in  FIG. 18 . 
     At “S 402 ” in  FIG. 18 , if the gateway  19  determines that the gateway  18  for a normal operation is not in the normal operation (an abnormal operation), the gateway  19  for a backup (more specifically, the calculation control means  14 ) converts the protocol of the received data at “S 403 ” in  FIG. 18  and the gateway  19  for a backup (more specifically, the calculation control means  14 ) transfers the data through the IP network  101  to the control device  8  (the IP node) at “S 404 ” in  FIG. 18 . 
     For example, if the gateway  19  for a backup (more specifically, the calculation control means in the gateway  19 ) determines that the data are received, the gateway  19  for a backup (more specifically, the calculation control means in the gateway  19 ) transfers the data to the control device  8  through the IP network  101  as shown in “SD 81 ” of  FIG. 19 . 
     As a result, by providing two gateways, that is, the gateway for a normal operation and the gateway for a backup between the wireless network and the IP network, it is possible to double the gateway and to enhance the reliability. 
     However, in the conventional example shown in  FIG. 6 , a communication load of the gateway  7  becomes greater because all communications are carried out through the gateway  7 . There has been a problem in that the wireless network cannot be interconnected to the IP network when a fault is generated in the gateway  7 . 
     Since an area in which each wireless node can carry out the communication is limited and only one gateway is provided, moreover, there has been a problem in that setting position of the gateway and that of the wireless node are naturally restricted. 
     In the conventional example shown in  FIG. 13 , furthermore, a higher reliability than that in the conventional example shown in  FIG. 6  is obtained because the gateway is doubled. In the normal operation state, however, all communications are carried out via the gateway  18  for a normal operation in the same manner as in the conventional example shown in  FIG. 6 . For this reason, a problem of an increase in the communication load cannot be solved. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments provide a wireless network system capable of selecting an optimum one of a plurality of gateways, thereby carrying out an interconnection between networks. 
     In order to achieve the foregoing object, a first aspect of the present invention is directed to a wireless network system for interconnecting a wireless network and another network different from the wireless network, comprising: 
     a plurality of wireless nodes for establishing the wireless network; 
     a control device connected to the other network; and 
     a plurality of gateways for interconnecting the wireless network and the other network, 
     wherein the wireless node searches a path from a gateway candidate list, selects a gateway on an optimal path based on information collected in searching the path, and transfers data to the control device through the gateway on the optimal path. 
     Consequently, it is possible to select an optimum one of a plurality of gateways, thereby interconnecting the networks. Moreover, it is possible to handle a problem of an increase in a communication load and to enhance the degree of freedom of setting position of the gateway and that of the wireless node. 
     A second aspect of the present invention is directed to the wireless network system according to the first aspect of the present invention, wherein the wireless node comprises: 
     wireless communication means for carrying out a communication through the wireless network; 
     storing means for storing the data, the gateway candidate list, a path search result list, a program for controlling the wireless node and information for searching a communication path; and 
     calculation control means for selecting a gateway candidate to be used from the gateway candidate list, searching a path for the gateway candidate, selecting a gateway on an optimal path based on information collected in searching the path, establishing a transfer path to the gateway on the optimal path, and transferring data to the gateway on the optimal path by using the established transfer path. 
     Consequently, it is possible to select an optimum one of a plurality of gateways, thereby carrying out an interconnection between the networks. In addition, it is possible to handle a problem of an increase in a communication load and to enhance the degree of freedom of setting position of the gateway and that of the wireless node. 
     A third aspect of the present invention is directed to the wireless network system according to the second aspect of the invention, wherein the wireless node retransfers data to a gateway on a second optimal path when detecting a failure in the data transfer to the gateway on the optimal path. 
     Consequently, it is possible to enhance a reliability of a data transfer. 
     A fourth aspect of the present invention is directed to the wireless network system according to the first aspect of the invention, wherein the gateway comprises: 
     wireless communication means for carrying out a communication through the wireless network; 
     storing means for storing a program for controlling the gateway and information for a protocol conversion; 
     communication means for carrying out a communication through the other network; and 
     calculation control means for converting protocol of data received through the wireless network and transferring the data to the control device via the other network. 
     Consequently, it is possible to select an optimum one of a plurality of gateways, thereby carrying out an interconnection between the networks. Moreover, it is possible to handle of a problem of an increase in a communication load and to enhance the degree of freedom of setting position of the gateway and that of the wireless node. 
     A fifth aspect of the present invention is directed to the wireless network system according to the first aspect of the present invention, wherein the other network is an IP network. 
     Consequently, it is possible to select an optimum one of a plurality of gateways, thereby carrying out an interconnection between the networks. Moreover, it is possible to handle of a problem of an increase in a communication load and to enhance the degree of freedom of setting position of the gateway and that of the wireless node. 
     A sixth aspect of the present invention is directed to the wireless network system according to the first aspect of the present invention, wherein the wireless network system is applied to a plant control system. 
     Consequently, it is possible to improve a robustness of the wireless network, thereby enhancing the reliability of the plant control system. 
     A seventh aspect of the present invention is directed to the wireless network system according to the first aspect of the present invention, wherein the wireless network system is applied to a building automation system. 
     Consequently, it is possible to improve a robustness of the wireless network, thereby enhancing the reliability of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an example of a wireless network system in accordance with the present invention; 
         FIG. 2  is a flowchart for explaining an operation of each wireless node; 
         FIG. 3  is a diagram for explaining a data transfer; 
         FIG. 4  is a flowchart for explaining an operation of a gateway; 
         FIG. 5  is a diagram for explaining a data transfer; 
         FIG. 6  is a block diagram showing an example of a conventional wireless network system; 
         FIG. 7  is a block diagram showing a specific example of the wireless node; 
         FIG. 8  is a block diagram showing a specific example of the gateway; 
         FIG. 9  is a flowchart for explaining an operation of each wireless node; 
         FIG. 10  is a diagram for explaining a data transfer; 
         FIG. 11  is a flowchart for explaining an operation of the gateway; 
         FIG. 12  is a diagram for explaining a data transfer; 
         FIG. 13  is a block diagram showing another example of a conventional wireless network system; 
         FIG. 14  is a flowchart for explaining an operation of each wireless node; 
         FIG. 15  is a diagram for explaining a data transfer; 
         FIG. 16  is a flowchart for explaining an operation of a gateway for a normal operation; 
         FIG. 17  is a diagram for explaining a data transfer; 
         FIG. 18  is a flowchart for explaining an operation of a gateway for a backup; and 
         FIG. 19  is a diagram for explaining a data transfer. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     According to the invention, the following advantages can be obtained: 
     According to the first, second, fourth and fifth aspects of the present invention, a plurality of gateways are provided between a wireless network and an IP network. Then, a wireless node searches a path from a gateway candidate list, and a gateway on an optimal path is selected to transfer data based on information collected in searching the path. Consequently, it is possible to select an optimum one of a plurality of gateways, thereby carrying out an interconnection between the networks. 
     Moreover, the wireless node selects a gateway on the optimal path based on a load situation of each gateway in a path search result list. Consequently, the increased communication loads can be distributed into a plurality of gateways, so that a countermeasure can be taken against the increase in the communication load. Furthermore, since the plurality of gateways are disposed, it is possible to enhance the degree of freedom of setting position of the gateway and that of the wireless node. 
     According to the third aspect of the present invention, furthermore, in the case where a failure is detected in the data transferred to the selected gateway on the optimal path, the data are retransferred to a gateway on a second optimal gateway. Consequently, it is possible to enhance a reliability of a data transfer. 
     According to the sixth aspect of the invention, by the application to the plant control system, it is possible to improve the robustness of the wireless network and to enhance the reliability of plant control system. 
     According to the seventh aspect of the invention, by the application to the building automation system, it is possible to improve the robustness of the wireless network and to enhance the reliability of the system. 
     The present invention will be described below in detail with reference to the drawings.  FIG. 1  is a block diagram showing an example of a wireless network system in accordance with the present invention. In  FIG. 1 , wireless nodes  1  to  6  and  8  have the same reference numerals as those in  FIG. 13 , and gateways  20 ,  21 ,  22  and  23  interconnect a wireless network to an IP network, and an IP network  102  is shown. 
     The wireless node  1  is interconnected to adjacent wireless nodes  3 ,  4  and  2  using wireless lines “WN 111 ”, “WN 102 ” and “WN 103 ” in  FIG. 1 . Similarly, wireless nodes  2 ,  3 ,  4 ,  5  and  6  are also interconnected to the adjacent wireless nodes by properly using wireless lines “WN 104 ”, “WN 105 ”, “WN 106 ”, “WN 107 ”, “WN 108 ” and “WN 109 ” in  FIG. 1 . 
     Furthermore, the wireless nodes  2  and  4  are interconnected to the gateway  20  using wireless lines “WN 110 ” and “WN 111 ” in  FIG. 1  respectively, and the wireless nodes  2  and  4  are interconnected to the gateway  21  using wireless lines “WN 112 ” and “WN 113 ” in  FIG. 1  respectively. 
     Similarly, the wireless nodes  6  and  5  are interconnected to the gateway  22  using wireless lines “WN 1114 ” and “WN 1115 ” in  FIG. 1  respectively, and the wireless nodes  6  and  5  are interconnected to the gateway  23  using wireless lines “WN 116 ” and “WN 117 ” in  FIG. 1  respectively. Thus, a wireless network shown in a broken line of  FIG. 1  is established. 
     In addition, the gateways  20 ,  21 ,  22  and  23  are interconnected to the IP network  102 , and also the control device  8  is interconnected to the IP network  102 . 
     An operation according to the example shown in  FIG. 1  will be described with reference to  FIGS. 2 to 5 .  FIG. 2  is a flowchart for explaining an operation of each wireless node,  FIGS. 3 and 5  are diagrams for explaining a data transfer, and  FIG. 4  is a flowchart for explaining an operation of the gateways  20  to  23 . 
     It is assumed that specific structures of the wireless nodes  1  to  6  and the gateways  20  to  23  are the same as those in  FIGS. 7 and 8  and storing means  11  of the wireless node stores a gateway candidate list and a path search result list in addition to data and information for a communication path search. 
     At “S 501 ” in  FIG. 2 , the wireless node (more specifically, calculation control means  10 ) determines whether to transfer data to a control device  8  (an IP node) through the IP network  102  or not. 
     At “S 501 ” in  FIG. 2 , if the wireless node determines to transfer data, the wireless node (more specifically, the calculation control means  10 ) selects a gateway candidate to be used from the gateway candidate list at “S 502 ” in  FIG. 2  and the wireless node (more specifically, the calculation control means  10 ) searches a path for the gateway candidate at “S 503 ” in  FIG. 2 . 
     For example, the calculation control means  10  constituting the wireless node selects a gateway candidate to be used from the gateway candidate list stored in the storing means  11  and searches a path for the gateway candidate, and stores a search result such as the collected information in the path search result list in the storing means  11 . 
     In searching the path, the information stored in the path search result list include “a final destination address”, “a next hop address”, “a situation of a communication load of a gateway” and “a path cost to a final destination”, for example, and an index such as a signal receiving strength, a bit error rate in a radio wave modulation or the number of hops is used for calculating “a path cost to a final destination”. 
     Next, at “S 504 ” in  FIG. 2 , the wireless node (more specifically, the calculation control means  10 ) selects a gateway on an optimal path based on the path search result list. 
     For example, the optimal path means a path in which a communication condition is optimum in a wireless network, and a gateway which can be reached with the smallest number of hops, a gateway on the lowest communication error rate, and a gateway on a small communication load are selected as the gateway on the optimal path. 
     At “S 505 ” in  FIG. 2 , the wireless node (more specifically, the calculation control means  10 ) establishes a transfer path to the gateway on the optimal path. At “S 506 ” in  FIG. 2 , the wireless node (more specifically, the calculation control means  10 ) transfers the data to the gateway on the optimal path by using the established transfer path. 
     For example, if the wireless node  1  (more specifically, the calculation control means in the wireless node  1 ) determines to transfer the data and also determines that transfer of the data to the gateway  22  through a transfer path using the wireless lines “WN 109 ”, “WN 108 ” and “WN 114 ” in  FIG. 1  is on the optimal path, a transfer path using the wireless lines “WN 109 ”, “WN 108 ” and “WN 114 ” in  FIG. 3  is established and the wireless node  1  (more specifically, the calculation control means in the wireless node  1 ) transfers the data to the gateway  22  as shown in “SD 91 ” of  FIG. 3 . 
     Meanwhile, at “S 601 ” in  FIG. 4 , the gateway  22  determined to be on the optimal path (more specifically, calculation control means  14 ) determines whether the data are received through a wireless network or not. 
     At “S 601 ” in  FIG. 4 , if the gateway  22  determines that the data are received through the wireless network, the gateway  22  (more specifically, the calculation control means  14 ) converts a protocol of the received data at “S 602 ” in  FIG. 4  and the gateway  22  (more specifically, the calculation control means  14 ) transfers the data to the control device  8  (the IP node) through the IP network  102  at “S 603 ” in  FIG. 4 . 
     For example, if the gateway  22  (more specifically, the calculation control means in the gateway  22 ) determines that the data are received, the gateway  22  (more specifically, the calculation control means in the gateway  22 ) transfers the data to the control device  8  through the IP network  102  as shown in “SD 101 ” in  FIG. 5 . 
     As a result, a plurality of gateways are provided between the wireless network and the IP network, and the wireless node searches a path from the gateway candidate list and selects a gateway on an optimal path to transfer data based on the information collected in the path search. Consequently, it is possible to select an optimum one of a plurality of gateways, thereby carrying out an interconnection between the networks. 
     Moreover, the wireless node selects the gateway on the optimal path based on the situation of the communication load of each of the gateways in the path search result list. Consequently, the increased communication loads can be distributed into a plurality of gateways, so that a countermeasure can be taken against the increase in the communication loads. Furthermore, the plurality of gateways are provided. Therefore, it is possible to enhance the degree of freedom of setting position of the gateway and that of the wireless node. 
     Although storing means is divided into two parts for the brief description with reference to  FIGS. 7 and 8 , it is a matter of course that the present invention may be implemented by one storing means. 
     While the interconnection between the wireless network and the IP network is illustrated in the description of the example shown in  FIG. 1 , moreover, it is a matter of course that networks other than an IP network may be employed, such as “Foundation Fieldbus” (registered trademark), “BACnet” (registered trademark) and “wireless Local Area Network (LAN)” to be controlling networks. 
     In the description of the example shown in  FIG. 1 , it is possible to apply any standard wireless network, irrespective of a simple notation of the wireless network. For example, it is a matter of course that “ZigBee (IEEE802.15.4)” (registered trademark) may also be applied. 
     Moreover, a path search algorithm is not restricted but any path search algorithm may be used. 
     In the case where a failure is detected in a data transferred to the selected gateway on the optimal path, moreover, the wireless node may retransfer the data to a gateway on a second optimal path. In this case, it is possible to enhance the reliability of the data transfer. 
     Furthermore, it is preferable to apply the example shown in  FIG. 1  to the following systems: 
     (1) Plant control system; and 
     (2) Building automation system. 
     (1) In the plant control system, a large number of obstacles (facilities) are present in a plant building in which a wireless node is provided, and a radio wave hindrance is apt to be caused. Therefore, the wireless node selects an optimal path including an obstacle avoidance and a radio wave hindrance avoidance so that the robustness of the wireless network can be implemented and the reliability of the plant control system can be enhanced. 
     (2) In the building automation system, in the case where an illuminator or a switch is set to be a wireless node, a large number of obstacles (apparatuses or equipment) are also present inside a building where the wireless node is disposed, and a radio wave hindrance is also apt to be caused. The wireless node selects an optimal path including an obstacle avoidance and a radio wave hindrance avoidance so that the robustness of the wireless network can be implemented and the reliability of the system can be enhanced. 
     While there has been described in connection with the exemplary embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the present invention. It is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.