Abstract:
A wireless communications connection control method that enables flexible reconstruction of a wireless communications network system structure. The wireless communications connection control method comprising the steps of retaining in advance a node registration table in which the inter-node distance and node types are associated with each node; issuing a joining notification, together with the inter-node distance, in response to when a new node joins the wireless communications system; refreshing the node registration table according to the joining notification, determining whether it is necessary to change the node type based on the inter-node distance stored in the node registration table, and changing the node type stored in the node registration table when it is determined that the type needs to be refreshed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a wireless communications connection control method for controlling wireless communications connections among a plurality of nodes included in a wireless communications system. 
         [0003]    2. Description of the Related Art 
         [0004]    Recently there has been demand for adapting wireless communications network systems to more diverse applications, and there has also been demand for making it easier to join and leave such systems as well as for building more flexible communications network structures within such systems. A sequence for allowing a wireless terminal (referred to below as “nodes”) to join a network, as shown in  FIG. 30 , is disclosed in “IEEE 802.15.4,” IEEE (Institute of Electrical and Electronics Engineers): Non-Patent Literature 1, for example. A child node MAC that seeks to join the network transmits a beacon request, receives a beacon from e.g. a receive-enabled parent node MAC, and transmits an association request to one parent node among the nodes that have sent a beacon. The parent node MAC returns an association response, and assigns the child node MAC a network address. By setting the network address that has been received, the child node MAC can join the communications network. 
       SUMMARY OF THE INVENTION 
       [0005]    In the technique disclosed in the Non-Patent Literature 1, the wireless communications network structure lacks flexibility because the network must be built by sequentially powering and starting up from wireless terminals located upstream in the network hierarchy in order to optimize the wireless communications network structure. In addition, after the wireless communications network has been built and started up, it has not been possible to rebuild the wireless communications network into an optimal structure, despite the ability to allow new additional nodes to join the wireless communications network as needed. When rebuilding, it has been necessary to temporarily power off the wireless terminals located downstream in the network hierarchy and near the added wireless terminals, and to then again sequentially power and start up from the wireless terminals located upstream. Problems have accordingly been presented in that constraints are imposed when building a wireless communications network and that the wireless communications network system that has been built is poorly adaptable to structural modification. An optimal wireless communications network structure thus cannot be achieved, and the network may suffer from poor system performance and stability. In view of the drawbacks noted above, an object of the present invention is to provide a wireless communications connection control method enabling flexible reconstruction of system structures that have already been built, without constraints such as power-up timing when a wireless communications network system is built. 
         [0006]    The wireless communications connection control method of the present invention is a wireless communications connection control method for controlling wireless communications connections between nodes in a wireless communications system including a plurality of nodes, wherein the nodes wirelessly communicate with each other based on node type, the method comprising the steps of having each of the nodes retain in advance a node registration table in which the inter-node distance and type of the node are associated with each node and retained; issuing a joining notification, together with the inter-node distance, in response to a new node joining the wireless communications system; refreshing the node registration table according to the joining notification; determining the necessity of changing the node type based on the inter-node distance stored in the node registration table; and changing the node type stored in the node registration table according to a determination that the type needs to be refreshed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a view showing a wireless communications system in an embodiment; 
           [0008]      FIG. 2  is a block diagram showing nodes; 
           [0009]      FIG. 3  is a view showing node information; 
           [0010]      FIG. 4  is a view showing a joining notification packet; 
           [0011]      FIG. 5  is a view showing a node registration table; 
           [0012]      FIG. 6  is a flow chart showing a communications connection control routine in the wireless communications connection control method; 
           [0013]      FIG. 7  is a first part of a sequence diagram showing the operations of a wireless connection control process; 
           [0014]      FIG. 8  is a second part of a sequence diagram showing the operations of a wireless connection control process; 
           [0015]      FIG. 9  is a third part of a sequence diagram showing the operations of a wireless connection control process; 
           [0016]      FIG. 10  is a view showing a wireless communications system along with a joining request and joining permission; 
           [0017]      FIG. 11  is a view showing a wireless communications system along with a broadcast radius; 
           [0018]      FIG. 12  is a view showing a wireless communications system along with a joining request and joining permission; 
           [0019]      FIG. 13  is a view showing a wireless communications system along with registration refresh; 
           [0020]      FIG. 14  is a view showing a wireless communications system along with a broadcast radius; 
           [0021]      FIG. 15  is a view showing a wireless communications system along with a re-joining request, joining permission, and registration refresh; 
           [0022]      FIG. 16  is a view showing a wireless communications system along with a broadcast radius and joining notification; 
           [0023]      FIG. 17  is a view showing a wireless communications system after re-connection is complete; 
           [0024]      FIG. 18  is a view showing the node registration table for a node T 09 ; 
           [0025]      FIG. 19  is a view showing the node information of the node T 09 ; 
           [0026]      FIG. 20  is a view showing the node registration table for a node T 04 ; 
           [0027]      FIG. 21  is a view showing the node registration table for a node T 05 ; 
           [0028]      FIG. 22  is a view showing the node information of the node T 05 ; 
           [0029]      FIG. 23  is a view showing the node registration table for a node T 05 ; 
           [0030]      FIG. 24  is a view showing the node registration table for a node T 04 ; 
           [0031]      FIG. 25  is a view showing the node registration table for a node T 08 ; 
           [0032]      FIG. 26  is a view showing the node registration table for a node T 08 ; 
           [0033]      FIG. 27  is a view showing the node registration table for a node T 05 ; 
           [0034]      FIG. 28  is a view showing the node information of the node T 08 ; 
           [0035]      FIG. 29  is a view showing the node registration table for a node T 08 ; and 
           [0036]      FIG. 30  is a sequence diagram showing the sequence by which a wireless terminal (node) joins a communications network in a conventional technique. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    Embodiments of the invention are described in detail below with reference to the accompanying drawings. 
         [0038]      FIG. 1  is a view showing a wireless communications system  1  according to the present embodiment. Eight wireless communications terminals (referred to below as “nodes”) are included in the wireless communications system  1 . Each of the nodes is assigned one extension address from among T 01  through T 08 . The nodes are referred to below as nodes T 01  through T 08  corresponding to the assigned extension addresses. The nodes T 01 , T 02 , through T 08  are assigned network addresses N 00 , N 11 , N 21 , N 22 , N 31 , N 32 , N 33 , and N 41 , respectively. The node T 01  assigned the network address N 00  is designated node T 01 :N 00  below. The other nodes are also similarly designated node T 02 :N 11 , node T 03 :N 21 , node T 04 :N 22 , node T 05 :N 33 , node T 06 :N 31 , node T 07 :N 32 , and node T 08 :N 41 . 
         [0039]    As shown in  FIG. 1 , the network topology of the wireless communications system  1  is a series structure in which the node T 01 :N 00  is the root. The nodes connected to each other by solid lines are nodes that communicate with each other (referred to below as communications connection nodes). For example, nodes T 01 :N 00 , T 03 :N 21 , and T 04 :N 22 , which are connected by solid lines to the node T 02 :N 11 , are communications connection nodes for the node T 02 :N 11 . In the following description, the communications connection nodes on the side near the node T 01 :N 00  (which is the root) side are referred to as parent nodes, and the nodes opposite that are farther away from the node T 01 :N 00  are referred to as child nodes. Nodes that are neither parent nodes nor child nodes are referred to as other nodes. For example, the node T 01 :N 00  is the parent node to the node T 02 :N 11 , and T 03 :N 21  and T 04 :N 22  are the child nodes. The rest are the other nodes for the node T 02 :N 11 . The network addresses can be calculated by Cskip as defined in IEEE802.15.4, for example, and it is possible to determine which nodes are the parent nodes, child nodes, and other nodes for each of the nodes based on the network addresses obtained by those calculations. 
         [0040]      FIG. 2  is a block diagram showing the node T 04 . The node T 04  includes a joining process component  11 , a node information setting component  12 , a joining notification transmission component  13 , a table refresh component  14 , a link quality determination component  15 , a node registration table  16 , a node type change component  17 , and a communication component  18 . 
         [0041]    The joining process component  11  has joining request transmission means for issuing a joining request to nodes that are to become parent nodes when the node joins the wireless communications system  1  or in response to a determination by the table refresh component  14  that the settings for communications connection nodes should be refreshed. The joining request is, for example, a beacon request signal. The joining process component  11  also has joining permission notification means for issuing notification of joining permission using a beacon signal, for example, in response to a joining request from a node that is to become a child node. 
         [0042]    After the joining request, the joining process component  11  issues a network address provision request to nodes that are to become parent nodes, by using an association request signal, for example. Using an association response signal, for example, the joining process component  11  issues to the relevant nodes the network addresses determined in response to the network address provision request from the nodes that are to become the child nodes. The joining process component  11  provides the network addresses from the parent nodes to the node information setting component  12 . 
         [0043]    The node information setting component  12  stores the information related to its own node (referred to below as “node information”). The node information includes an extension address, a network address, a network distance, and joining permission.  FIG. 3  is a view showing the node information for the node T 04 :N 22 . The extension address is a node-specific identifier, which is T 04  for the node T 04 :N 22 . The network address is an identifier on the network, and is the address provided from the parent node. The network address for the node T 04 :N 22  is N 22 . The network distance is the distance between nodes on the network topology, from the node T 01 :N 00  (the root) to the node itself. Here, the network distance is represented as the number of hops from the node T 01 :N 00  to the node itself. Because the node T 04 :N 22  is the 2nd hop from the node T 01 :N 00 , the network distance is 2. Because the node T 01 :N 00  is itself the root, its network distance is 0. Joining permission represents whether the node itself can or cannot connect to child nodes according to the criteria of “can join” or “cannot join.” 
         [0044]    The joining notification transmission component  13  issues joining notification, along with the node information stored in the node information setting component  12 , to nearby nodes when the wireless communications system  1  is newly joined or re-joined. The joining notification transmission component  13  issues joining notification by any transmission means among broadcasting to individual nodes in broadcast range, multicasting to individual nodes corresponding to the node information stored in the node registration table  16 , or unicasting to one of the nodes corresponding to the node information stored in the node registration table  16 . The joining notification transmission component  13  broadcasts the joining notification.  FIG. 4  is a view showing a joining notification packet. The packet comprises a packet header and packet payload. The packet header includes the packet type, the network address of the node itself (source address), data indicating broadcast transmission (destination address), and broadcast radius (radius). The packet period command includes the extension address of the node itself, network distance, and joining permission. 
         [0045]    The table refresh component  14  receives the joining notification and refreshes the node registration table  16  based on the node information contained in the joining notification. For example, when the extension address in the node information is T 09 , the table refresh component  14  searches the node registration table  16  to determine whether or not information related to the extension address T 09  is registered there, and when no information is registered, registers the node information included in the joining notification in the node registration table  16 . The table refresh component  14  also has node information deletion means for deleting the node information stored in the node registration table  16  after a predetermined period of time has passed since the point in time when the information was stored. Node information can be deleted after a predetermined period of time has passed so as to control the memory volume in the node registration table  16 . 
         [0046]    The link quality determination component  15  continuously monitors the wireless communication status of the transmission component  17  and determines the link quality for each node. The link quality is, for example, the wireless reception strength, correlation value, error rate, etc. The link quality is represented by a link quality value, such as 1: good; 2: fair; and 3; good. The link quality determination component  15  registers the resulting link quality value in the node registration table  16 . 
         [0047]    The node registration table  16  stores node information (extension address, network address, network distance, and joining permission), link quality, and node type (collectively referred to below as “registration information”) for each node.  FIG. 5  is a view showing the node registration table for the node T 04 :N 22 . The node registration table includes registration information corresponding to the node T 02 :N 11  (the parent node) and the nodes T 05 :N 33  and T 07 :N 22  (child nodes). In the case of the node T 02 :N 11 , for example, the extension address is T 02 , the network address is N 11 , the network distance is 1, the link quality is 1, the node type is parent node, and the joining permission is “can join.” Here, the node information, the link quality, and the node type are registered and refreshed by the table refresh component  14 , link quality determination component  15 , and node type change component  17 , respectively. 
         [0048]    The node type change component  17  selects the parent node based on the registration information stored in the node registration table  16 , and determines whether or not the node type setting should be refreshed. The node type change component  17  determines, for each node, the parent node selection reference values obtained by means of the product of the network distance and the link quality, for example. When a node has a parent node selection reference value smaller than the parent node selection reference value of the current parent node, the node type change component  17  selects that node as the parent node and determines that the node type settings should be refreshed. The node with the lower parent node selection reference value is more suitable as the parent node. Even when “parent node” is not in the node type of the node registration table  16 , the node type change component  17  will select a new parent node and will determine that the node type settings should be refreshed. The node type change component  17  may also determine parent node selection reference values based only on either just either the network distance or link quality. 
         [0049]    For example, when the current parent node is a node with a network distance of 2 and a link quality of 1, the parent node selection reference value of the current parent node is 2×1=2. When the registration information of a new node with a network distance of 1 and a link quality of 1 is added here, for example, the parent node selection reference value of the new node will be 1×1=1. In this case, the parent node selection reference value 1 of the new node is lower than the parent node selection reference value of 2 of the current parent node, and the new node will accordingly be selected as the new parent node. When the parent node selection reference value of the parent node is 2 and the parent node selection reference value of a new node is 2 or more, the parent node will not be changed. 
         [0050]    The node type change component  17  actually refreshes the node type settings when the joining process component  11  receives joining permission from the parent node. 
         [0051]    The communication component  18  carries out wireless communication between nodes for which the node type in the node registration table  16  is a parent node or child node. Each of the nodes T 01  through T 08  have the same structure as described above. 
         [0052]      FIG. 6  is a flow chart showing a communications connection control routine in the wireless communications connection control method.  FIGS. 7 ,  8 , and  9  are sequence diagrams showing the operations of the nodes T 01 , T 09 , T 04 , T 05 , and T 08  when the node T 09  newly joins the wireless communications system  1 . The sequence in  FIG. 8  is a continuation of the sequence in  FIG. 7 , and the sequence in  FIG. 9  is a continuation of the sequence in  FIG. 8 .  FIGS. 10 through 17  show the wireless communications system  1  along with notification signals and the like at this time. Appropriate reference will be made to  FIGS. 6 through 17  in the following description of the communications connection control process using the wireless communications connection control method when the node T 09  newly joins the wireless communications system  1 . 
         [0053]    First, the node T 09  issues a joining notification to T 01  on joining the wireless communications system  1  (Step S 201 ,  FIG. 10 ). In response to the joining notification, the node T 01 :N 00  gives joining permission along with its own extension, network address and the like to the node T 09  (Step S 202 ,  FIG. 10 ). The node T 09  registers the parent node T 01 :N 00  in the node registration table (Step S 203 ).  FIG. 18  shows the post-registration node registration table for the node T 09 . The extension address, network address, network distance, link quality, node type, and joining permission are registered as T 01 , N 00 , 0, 1, “parent node,” and “can join,” respectively. 
         [0054]    The node T 09  then issues an address provision request to the node T 01 :N 00  (Step S 204 ). The node T 01 :N 00  provides the network address N 12  to the node T 09  in response to the address provision request (Step S 205 ). The node T 09  refreshes its own node information to extension address T 09 , network address N 12 , a network distance of  1 , and joining permission of “can join,” as shown in  FIG. 19 . The node T 09 :N 12  issues a joining notification, including its own extension address of T 09 , network address of N 12 , network distance of 1, and joining permission of “can join,” to adjacent nodes within the broadcast radius indicated by the dashed line in  FIG. 11 ; i.e., to the nodes T 01 :N 00 , T 04 :N 22 , and T 05 :N 33  (step S 101 , Step S 206 ,  FIG. 11 ). 
         [0055]    The nodes T 04 :N 22  and T 05 :N 33  that receive the joining notification search their own node registration tables to determine if there is any registration information corresponding to the extension address T 09 . Because the node T 09 :N 12  has newly joined, no registration information corresponding to the extension address T 09  is registered in the node registration tables of the nodes T 04 :N 22  and T 05 :N 33 . The nodes T 04 :N 22  and T 05 :N 33  therefore register the node information for the node T 09 :N 12  in their own node registration tables (Step S 102 , Step S 207 , and S 208 ).  FIG. 20  shows the node registration table for the node T 04 :N 22 . In addition to the previously registered registration information corresponding to extension addresses T 02 , T 07 , and T 05 , the registration information corresponding to the extension address T 09  is added. The node type corresponding to the extension address T 09  at this point in time is set as “other node” by the node type change component  17 . The link quality of 1 in the node registration table is determined and registered by the link quality determination component  15 .  FIG. 21  shows the node registration table for the node T 05 :N 33 . In addition to the previously registered registration information corresponding to extension addresses T 04 , and T 08 , the registration information corresponding to the extension address T 09  is added. The node type corresponding to the extension address T 09  at this point in time is “other node.” 
         [0056]    The nodes T 04 :N 22  and T 05 :N 33  select parent node when their own node registration tables are refreshed (Step S 103 , Step S 209 , and S 210 ). The node T 04 :N 22  determines the parent node selection reference value by using the product of the network distance and link quality. The parent node selection reference value of the node T 02 :N 11  (which is currently the parent node) is 1×1=1, and the parent node selection reference value of the added node T 09 :N 12  is 1×1=1. Since the parent node selection reference value of both nodes is 1, the node T 04 :N 22  determines that there is no need to refresh the node type settings (Step S 104 , Step S 211 ), and the communications connection control process is complete. In the node T 05 :N 33 , the parent node selection reference value for the node T 04 :N 22  is 2×1=2 and the parent node reference value for the node T 09 :N 12  is 1×1=1. Because the parent node selection reference value of 1 for the node T 09 :N 12  is lower than the parent node selection reference value of 2 for the node T 04 :N 22 , the node T 04 :N 22  determines that the node type settings should be refreshed (Step S 104 , Step S 212 ). 
         [0057]    The node T 05 :N 33  issues a re-joining request and network address provision request, as shown in  FIG. 12 , to the node T 09 :N 12  that is to be established as the parent node (Step S 105 , Step S 213 ). The node T 09 :N 12  communicates joining permission to the node T 05 :N 33  and issues the network address N 23  (Step S 106 , Step S 214 ), as shown in  FIG. 12 . The node T 05 :N 33  refreshes its own node information, as shown in  FIG. 22 , to an extension address of T 05 , network address of N 23 , network distance of 2, and joining permission of “can join.” The node T 05 :N 23  refreshes the node type in its own node registration table (Step S 107 , Step S 215 ).  FIG. 23  shows the refreshed node registration table for the node T 05 :N 23 . The node types corresponding to the extension addresses T 04  and T 08  are changed to “other node,” and the node type corresponding to the extension address T 09  is changed to “parent node.” The dashed line between the node T 05 :N 23  and node T 04 :N 22  in  FIG. 13  and the dashed line between the node T 05 :N 23  and node T 08 :N 41  show that the node types in the registration table have been refreshed, so that the node T 04 :N 22  and node T 08 :N 41  become other nodes to the node T 05 :N 23 . 
         [0058]    The node T 05 :N 23  then issues a joining notification, including its own extension address T 05 , network address N 23 , network distance  2 , and “can join” joining permission, to the adjacent nodes in the broadcast radius indicated by the dashed line in  FIG. 14 ; i.e., to the nodes T 04 :N 22 , T 08 :N 41 , and T 09 :N 12  (Step S 101 , Step S 216 ,  FIG. 14 ). 
         [0059]    The node T 04 :N 22  that has received the joining notification searches its own node registration table to determine if there is any registration information corresponding to the extension address T 09  (Step S 217 ). The node T 04 :N 22  refreshes the registration data corresponding to the previously registered extension address T 09  (Step S 218 ,  FIG. 15 ).  FIG. 24  shows the refreshed node registration table for the node T 04 :N 22 . The node registration table (before being refreshed) for the node T 04 :N 22  is also shown in  FIG. 20 . Comparison of the node registration table before and after being refreshed reveals that the network address for the extension address T 05  is changed from N 33  to N 23 , the network distance is changed from 3 to 2, and the node type is changed from “child node” to “other node.” The node type is also set by the node type change component  17 . The dotted line between the node T 04 :N 22  and node T 05 :N 23  in  FIG. 15  shows that the node types in the registration table have been refreshed, so that the node T 05 :N 23  becomes the other node to the node T 04 :N 22 . 
         [0060]    The node T 04 :N 22  refreshes its own node registration table and selects the parent node (Step S 103 , Step S 219 ). The parent node selection reference value of the node T 02 :N 11 .(which is currently the parent node) is 1×1=1, and the parent node selection reference value of the refreshed extension address T 05  is 2×1=2. Since the parent node selection reference value 1 of the node T 02 :N 11  (the current parent node) is lower than the parent node selection reference value 2 of the refreshed extension address T 05 , the node T 04 :N 22  determines that there is no need to refresh the node type settings (Step S 104 , Step S 220 ), and the communications connection control process is concluded. 
         [0061]    The node T 08 :N 41  that has received the joining notification searches its own node registration table to determine if there is any registration information corresponding to the extension address T 09  (Step S 221 ). The node T 08 :N 41  refreshes the registration data corresponding to the previously registered extension address T 09  (Step S 222 ).  FIG. 25  shows the node registration table, before being refreshed, corresponding to the node T 08 :N 41 .  FIG. 26  shows the refreshed node registration table for the node T 08 :N 41 . Comparison of the node registration table before and after being refreshed reveals that the network address for the extension address T 05  is changed from N 33  to N 23 , the network distance is changed from 3 to 2, and the node type is changed from “parent node” to “other node.” 
         [0062]    The node T 08 :N 41  refreshes its own node registration table and selects the parent node (Step S 103 , Step S 223 ). As shown in  FIG. 26 , there is no “parent node” in the node types of the refreshed node registration table. The node T 08 :N 41  therefore issues a re-joining request, as shown in  FIG. 15 , to the node T 05 :N 23  which is to be established as the parent node (Step S 105 , Step S 224 ). The node T 05 :N 23  communicates the joining permission to the node T 08 :N 41 , as shown in  FIG. 15 , in response to the re-joining request (Step S 106 , Step S 225 ). The node T 08 :N 41  then issues a network address provision request to the node T 05 :N 23  (Step S 226 ), and the node T 05 :N 23  gives the network address N 33  to the node T 08 :N 41  in response to the provision request (Step S 227 ). 
         [0063]    The node T 05 :N 23  refreshes its own node registration table (Step S 228 ).  FIG. 27  shows the refreshed node registration table for the node T 05 :N 23 . The node registration table (before being refreshed) of the node T 08 :N 33  is also shown in  FIG. 23 . Comparison of the node registration tables before and after being refreshed reveals that the network address for the extension address T 08  is changed from N 41  to N 33 , the network distance is changed from 4 to 3, and the node type is changed from “other node” to “child node.”The node T 08 :N 33  refreshes its own node information, as shown in  FIG. 28 , to an extension address of T 08 , a network address of N 33 , a network distance of 3, and joining permission of “can join.” The node T 05 :N 23  also refreshes the node type of its own node registration table (Step S 107 , Step S 229 ).  FIG. 29  shows the refreshed node registration table for the node T 08 :N 33 . The node registration table for the node T 08 :N 33 , before being refreshed, is also shown in  FIG. 26 . Comparison of the node registration table before and after being refreshed reveals that the node type corresponding to the extension address T 05  is changed to “parent node.” 
         [0064]    The node T 08 :N 33  then issues a joining notification, including its own extension address T 08 , network address N 33 , network distance 3, and “can join” joining permission, to the adjacent node in the broadcast radius indicated by the dashed line in  FIG. 16 ; i.e., to the node T 05 :N 23  (Step S 101 , S 230 ,  FIG. 16 ). 
         [0065]    The node T 05 :N 23  that has received the joining notification searches its own node registration table to determine if there is any registration information corresponding to the extension address T 08  (Step S 231 ). The node T 04 :N 22  determines that it is not necessary to re-join because the registration information corresponding to the extension address T 08  that has already been registered and the node information in the joining notification from the node T 08 :N 33  are the same (Steps S 102  to S 104 , S 232 ). 
         [0066]      FIG. 10  shows the wireless communications system  1  after re-connection is complete. Before the node T 09  joined, the node T 05  was located 3 hops from the node T 01  (the root) through the nodes T 02  and T 04 , but after the node T 09  joined, the node T 05  is 2 hops from the node T 01  through the node T 09 . The number of hops from the node T 05  to the node T 01  (the root) can be deleted, allowing the wireless communications connection structure in the wireless communications system  1  to be optimized. 
         [0067]    As noted above, in the wireless communications connection control method of this embodiment, a node that newly joins or rejoins a wireless communications system issues a joining notification with its own node information to adjacent nodes, and the adjacent nodes that receive the joining notification select a parent node and determine whether or not to rejoin based on the network distance, link quality, or the like. The adjacent node that is determined to be in need of joining then issues a joining request to the parent node, and refreshes the node type and sets a new parent node when a notification of joining permission and a network address are given in response to the joining request. 
         [0068]    Such a process makes it possible to optimize wireless communications connection in the network topology of a wireless communications system through the autonomous deletion of the number of hops to the root node, without any need for the system administrator to manually process the joining and departing of the nodes, even when a node newly joins the wireless communications system. As a result, it is possible to eliminate conventional power-up timing constraints such as the need to sequentially power up from the root node to the subordinate nodes when building a wireless communications system, and wireless communications systems that have already been built up can be flexibly rebuilt. 
         [0069]    This application is based on Japanese Patent Application No. 2008-149285 which is hereby incorporated by reference.