Patent Publication Number: US-2022217609-A1

Title: Node, communication system, and communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application No. PCT/JP2020/036978 filed on Sep.29, 2020, which claims priority to Japanese Patent Application No. 2019-180830, filed on Sep. 30, 2019. The entire disclosures of these applications are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a node that performs multi-hop communication, a 
     communication system including a node that performs multi-hop communication, and a communication method for the communication system. 
     BACKGROUND ART 
     ITU-T G. 9905 (A non-patent literature) discloses a communication processing method for sending information from an individual node to a destination node in a communication system that performs multi-hop communication. In the communication system disclosed in ITU-T G 9905 (A non-patent literature), if an individual node does not have information of a communication path to a destination node, it selects a communication path requiring the smallest cost to perform communication from this node to a master node and sends information. 
     SUMMARY 
     Anode according to a first aspect is a node that performs multi-hop communication. The node includes a relay node determiner configured to determine a relay node from a plurality of relay node candidates having path information of a path to a master node. The relay node is configured to perform relaying. At least when communication frequencies of the relay node candidates are within a predetermined range, the relay node determiner is configured to determine the relay node so that a relay node candidate with a higher communication frequency is more likely to be determined as the relay node than a relay node candidate with a lower communication frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a known communication system. 
         FIG. 2  is a diagram illustrating the configuration of a node. 
         FIG. 3  is a diagram illustrating a communication system. 
         FIG. 4  is a diagram illustrating examples of a node table and a neighbor node table. 
         FIG. 5  is a flowchart illustrating a processing method for establishing a communication path and generating path information. 
         FIG. 6  is a flowchart illustrating a communication processing method. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     (1) Overview of Communication Processing in Known Communication System  200   
       FIG. 1  is a schematic view of a known communication system  200  including plural nodes  2 . The nodes  2  are not restricted to a particular type, but they are at least communication devices that perform communication by using a multi-hop routing program. For the sake of description, the plural nodes  2  are each appended with one of letters M, A, B, C, X, and Y so as to be distinguished from each other. Hereinafter, an overview of communication processing for sending information from the node  2 X to the node  2 Y in the known communication system  200  will be described below. 
     Among the plural nodes  2 M,  2 A,  2 B,  2 C,  2 X, and  2 Y shown in  FIG. 1 , the node  2 M is a master node. The nodes  2 A,  2 B,  2 C,  2 X, and  2 Y other than the node  2 M are slave nodes. In  FIG. 1 , the lines connecting the nodes  2 M,  2 A,  2 B,  2 C,  2 X, and  2 Y each indicate a communication path through which communication can be performed with one hop. It is assumed that, in the communication system  200 , each of the plural nodes  2 A,  2 B,  2 C,  2 X, and  2 Y, which are slave nodes, has already established a communication path to the node  2 M. Processing for establishing a communication path will be discussed later when the communication system  100  is discussed. 
     The nodes  2 A,  2 B,  2 C,  2 X, and  2 Y having established a communication path each store information about the established communication path as path information of its own node. The path information includes information on the address of the master node, the address of a relay node which serves to perform relaying to the master node, the number of hops until the master node, the communication cost required until the master node, etc. In the present disclosure, the communication cost indicates a value of the communication quality between nodes. As the communication cost is smaller, the communication quality between nodes is higher. The communication cost is a value which is set based on the communication strength, for example, between nodes. It is preferable that each node execute communication processing to minimize the communication cost. The communication cost between nodes is represented by the number indicated near the line connecting these nodes, as shown in  FIG. 1 . For example, the communication cost between the node  2 M and the node  2 A is 6. 
     In the following description of the present disclosure, a node having a smaller number of hops from the master node in the communication path is assumed to be a higher node, while a node having a greater number of hops from the master node in the communication path is assumed to be a lower node. It is assumed that the master node stores all items of path information of the slave nodes of the master node and that a slave node stores all items of path information of the lower nodes of this slave node. More specifically, in  FIG. 1 , the node  2 M, which is the master node, stores path information of the nodes  2 A,  28 ,  2 C,  2 X, and  2 Y, which are slave nodes, while each of the nodes  2 A,  2 B,  2 C,  2 X, and  2 Y, which are slave nodes, stores path information of its own node and path information of its lower nodes. 
     The node  2 X is a sending node that stores information to be sent to the node  2 Y. The node  2 Y is a destination node that receives the information from the node  2 X. 
     Then sending information to the node  2 Y, the node  2 X first determines whether path information of the node  2 X or path information of its lower nodes includes information of the communication path to the node  2 Y. In this example, the items of path information stored in the node  2 X have no information of the communication path to the node  2 Y. The node  2 X thus sends the information to the node  2 B in accordance with the path information of the node  2 X (communication cost 5). 
     As information of communication paths, the node  2 X stores information of the communication path for performing communication with the node  2 M via the node  2 A and information of the communication path for performing communication with the node  2 M via the node  2 B. The communication cost of the communication path to the node  2 M via the node  2 A is 9, while that via the node  2 B is 7. The node  2 X accordingly selects the information of the communication path for performing communication with the node  2 M via the node  2 B having a smaller communication cost as path information of the node  2 X, and sends the information. 
     Upon receiving the information from the node  2 X, as in the node  2 X, the node  2 B determines whether the path information stored in the node  2 B includes information of the communication path to the node  2 Y. The path information stored in the node  2 B does not include information of the communication path to the node  2 Y and thus sends the information to the node  2 M in accordance with the path information of the node  2 B (communication cost 2). 
     The path information stored in the node  2 M includes information of the communication path to the node  2 Y. The node  2 M having received the information from the node  2 B thus sends the information to the node  2 Y via the node  2 A and the node  2 C in accordance with the path information of the path to the node  2 Y (communication cost 6+3+3). 
     Communication processing for sending the information from the node  2 X to the node  2 Y has been completed as described above. The communication cost required for completing this processing is 19. 
     In the known communication system  200 , a sending node selects a node to send information in accordance with whether path information stored in the sending node includes information of the communication path to a destination node. However, if, for example, the node  2 X sends information to the node  2 A, the information can be sent from the node  2 X to the node  2 Y via the node  2 A and the node  2 C, thereby completing communication processing. In this case, the communication cost required for completing the processing is 9. It is thus possible to complete communication processing with a smaller communication cost than that in the above-described known communication system  200 . 
     A communication system  100  according to the present disclosure executes communication processing by using a node  1  having the following characteristics, thereby contributing to decreasing the number of nodes which perform relaying to a destination node and accordingly reducing the communication cost required for completing communication processing. 
     (2) Configuration of Node  1   
       FIG. 2  is a diagram illustrating the configuration of the node  1  according to the present disclosure. The node  1  is not restricted to a particular type, but it is at least a communication device that performs communication by using a multi-hop routing program, The communication device is an outdoor unit or an indoor unit of an air conditioner, a ventilator, or one of various sensors, for example.  FIG. 3  is a schematic diagram of the communication system  100  including plural nodes  1  according to the present disclosure. For the sake of description, the plural nodes  1  shown in  FIG. 3  are each appended with one of letters M, A, C, X, and Y. The configurations of nodes  1 M,  1 A,  1 B,  1 C,  1 X, and  1 Y shown in  FIG. 3  are the same as that of the node  1 . Hence, among the plural nodes  1  having similar function units, for distinguishing one node  1  from the other nodes  1 , the node  1  will be described by appending a corresponding one of letters in, a, b, c, x, and y to the function units of the node  1 . 
     The node  1  shown in  FIG. 2  includes a communication unit  10 , a processing unit  20 , and a storage unit  30  as major elements. 
     (2-1) Communication Unit  10   
     The node  1  can perform multi-hop wireless communication using a wireless signal with another node in the communication system  100  via the communication unit  10 . A wireless signal includes plural items of information. The communication unit  10  can function as cost information communicators  11  and  12  or path information communicators  13  and  14  as a result of performing communication. 
     If the node  1  has path information of the node  1 , which will be discussed later, the communication unit  10  broadcasts a wireless signal at predetermined time intervals so as to search for another node (neighbor node) with which the node  1  can communicate with one hop. The communication unit  10  also receives a wireless signal broadcast from a neighbor node so as to recognize the presence of the neighbor node and to establish a communication path with this neighbor node. 
     (2-1-1) Cost Information Communicators  11  and  12   
     The cost information communicators (cost obtainers)  11  and  12  respectively include a cost information receiver  11  that receives information about the communication cost from another node and a cost information sender  12  that sends information about the communication cost to another node. 
     The communication cost, which indicates the communication quality between nodes in the communication system  100 , includes a first cost, a second cost, and a third cost. The first cost is a communication cost from a certain node to a relay node candidate, which will be discussed later. The second cost is a communication cost from a relay node candidate to a master node. The second cost may be a communication cost adjusted by a cost adjuster  24  of another node. The cost adjuster  24  will be discussed later in detail. The third cost is the sum of the first cost and the second cost and is a communication cost from a certain node to the master node. 
     More specifically, in the communication system  100  shown in  FIG. 3 , when a relay node candidate of the node  1 X is the node  1 A, for example, the cost information receiver  11   x  of the node  1 X obtains the communication cost from the node  1 A to the node  1 M as the second cost. Meanwhile, the cost information sender  12   a  of the node  1 A inputs information about the third cost of the node  1 A into a wireless signal and sends this wireless signal. In other words, the third cost of the node  1 A, which is a relay node candidate, is obtained by the node  1 X as the second cost. 
     The first cost is calculated as a result of the communication unit  10   x  of the node  1 X and the communication unit  10   a  of the node  1 A exchanging wireless signals. 
     The first cost and the second cost obtained by the cost information communicators  11  and  12  are stored in a neighbor node table  32  of the storage unit  30 , which will be discussed later. The third cost calculated based on the first cost and the second cost is stored in a node table  31  of the storage unit  30 . 
     (2-1-2) Path Information Communicators  13  and  14   
     The path information communicators  13  and  14  respectively include a path information receiver  13  that receives information about path information from another node and a path information sender  14  that sends information about path information to another node. 
     The path information is information of a communication path from a certain node to a master node and includes information on the address of the master node, the address of a relay node which serves to perform relaying to the master node, the number of hops until the master node, the communication cost required until the master node, etc. 
     The path information receiver  13  receives from another node path information of this node and path information of a lower node of this node. 
     The path information sender  14  sends path information of the node  1  generated by a path information generator  23  of the processing unit  20 , which will be discussed later, and path information of a lower node of the node  1  to another node. (2-2) Processing Unit  20   
     The processing unit  20  includes function units, such as a relay node candidate determiner  21 , a relay node determiner  22 , a path information generator  23 , and a cost adjuster  24 . The processing unit  20  is constituted by an integrated circuit including a central processing unit (CPU), for example. The individual function units are implemented as a result of the processing unit  20  executing a preinstalled computer program, for example. The individual function units will be discussed below. 
     (2-2-1) Relay Node Candidate Determiner  21   
     The relay node candidate determiner  21  executes processing for determining a neighbor node as a relay node candidate based on a wireless signal received by the communication unit  10 . The relay node candidate is a master node or a node which stores information of the communication path (path information) until the master node, among the nodes with which the node  1  can communicate with one hop. 
     (2-2-2) Relay Node Determiner  22   
     The relay node determiner  22  executes processing for determining a relay node which performs relaying, from among the relay node candidates determined by the relay node candidate determiner  21 . 
     If plural relay node candidates are determined by the relay node candidate determiner  21  and at least when the communication frequencies of the relay node candidates are within a predetermined range, the relay node determiner  22  determines the relay node so that a relay node candidate having a higher communication frequency is more likely to be determined as the relay node than a relay node candidate having a lower communication frequency. 
     The communication frequency of a relay node candidate is determined based on the number of lower nodes of this relay node candidate, for example. More specifically, if the number of lower nodes of each relay node candidate is within a predetermined range, a relay node candidate having more lower nodes is more likely to be determined as the relay node than a relay node candidate having fewer lower nodes. If the number of lower nodes of each relay node candidate is within the predetermined range, a relay node candidate having more lower nodes has a higher communication frequency than a relay node candidate having fewer lower nodes and is thus more likely to be determined as the relay node. The relay node determiner  22  may determine the relay node by using information other than the communication frequency, For example, the relay node determiner  22  may determine the relay node, based on the communication cost (third cost), which will be discussed later. 
     When the relay node is determined by the relay node determiner  22 , the path information generator  23 , which will be discussed below, generates path information of the node  1  based on the determination made by the relay node determiner  22 . (2-2-3) Path Information Generator  23   
     When the relay node is determined by the relay node determiner  22 , the path information generator  23  generates path information of the node  1 . The path information generated by the path information generator  23  is stored in the node table  31  of the storage unit  30 , which will be discussed later. The generated path information is also sent to the master node via the relay node. This allows the master node and the relay node positioned between the node  1  and the master node to obtain the path information of the node  1 . 
     (2-2-4) Cost Adjuster  24   
     When the path information receiver  13  has received path information indicating that the node  1  is a relay node from another node, the cost adjuster  24  adjusts the third cost, which is the communication cost from the node  1  to the master node. 
     More specifically, the cost adjuster  24  multiplies the third cost by a predetermined weighting factor, based on the number of lower nodes of the node  1 , thereby adjusting the third cost. The weighting factor is a factor that increases the possibility that a node having more lower nodes will be determined as the relay node than a node having fewer lower nodes when the number of lower nodes of the node  1  is within a predetermined range. In a specific example, the weighting factor is calculated by the following equation (1), In equation (1), the number of lower nodes of the node  1  is substituted into x. 
         y= 0.01 x   2 −0.2 x+ 1   (1)
 
     The number of lower nodes of the node is determined based on the number of items of path information of lower nodes received by the path information receiver  13 . 
     The third cost adjusted by the cost adjuster  24  is stored in the node table  31 . 
     An explanation will be given of a case in which path information is generated by each of the node  1 A and the node  1 B in the communication system  100  shown in  FIG. 3 . It is assumed that, in the communication system  100 , the communication path between the node  1 X and each of the node  1 A and the node  1 B is not yet established. 
     A description will first be given of a case in which the communication path between the node  1 A and the node  1 M is established and path information is generated. The third cost of the node  1 A (communication cost between the node  1 A and the node  1 M) is 6, as shown in  FIG. 3 . The lower nodes of the node  1 A are the node  1 C and the node  1 Y, and the number of lower nodes of the node  1 A is two. 2 is thus substituted into x in the above-described equation (1), and the weighting factor is calculated as 0.64. As a result of multiplying the third cost by the weighting factor, the adjusted third cost of the node  1 A results in 3.84. 
     A description will now be given of case in which the communication path between the node  1 B and the node  1 M is established and path information is generated. The third cost of the node  1 B (communication cost between the node  1 B and the node  1 M) is 2, as shown in  FIG. 3 . The number of lower nodes of the node  1 B is 0. 0 is thus substituted into x in the above-described equation (1), and the weighting factor is calculated as 1. As a result of multiplying the third cost by the weighting factor, the adjusted third cost of the node  1 B results in 2. 
     (2-3) Storage Unit  30   
     The storage unit  30  stores various items of information to be used by the node  1  to determine the relay node. For example, the storage unit  30  of the node  1  stores node information of the node  1 , information about the communication frequency which is required when the node  1  is a relay node candidate, and information about the priority to be used when the node  1  is a relay node candidate. 
     The node information of the node  1  at least includes identification information of the node  1 , system identification information of the system to which the node  1  belongs, and type information of the node  1 . The identification information of the node  1  is preset information for identifying each of the plural nodes. The system identification information of the node  1  is preset information for identifying the system to which the node  1  belongs. Each system includes one or plural nodes. The type information of the node  1  is information indicating information about the type, such as whether the node  1  is a master node and whether the node  1  is a parent node. 
     The information about the communication frequency which is required when the node  1  is a relay node candidate is a communication frequency actual value or a communication frequency estimated value of the node  1 . The communication frequency actual value or the communication frequency estimated value may be calculated based on the number of lower nodes of a relay node candidate, the signal transmission frequency of the relay node candidate, or a predetermined existing method, for example. 
     The information about the priority to be used when the node  1  is a relay node candidate is information determined based on the node information, the information about the communication frequency, or the third cost, for example, and is information concerning the priority regarding whether the node  1  is determined as a relay node. 
     The storage unit  30  also includes a node table  31  storing information about the node  1  and a neighbor node table  32  storing information about a neighbor node of the node  1 . If the node  1  has plural neighbor nodes, the neighbor node table  32  is created for each of the neighbor nodes. For example, a node table  31   x  and neighbor node tables  32   x  of the node  1 X in the communication system  100  are those shown in  FIG. 4 . 
     In the node table  31  of the node  1 , items of information, such as path information of the node  1  and the communication cost from the node  1  to the master node (third cost), are stored. 
     In the neighbor node table  32  of the node  1 , items of information, such as path information of a neighbor node of the node  1 , the communication cost from the neighbor node to the node  1  (first cost), the communication cost from the neighbor node to the master node (second cost), and the communication cost from the node  1  to the master node (third cost), arc stored. The communication cost from the neighbor node to the master node (second cost) is information of the third cost of the neighbor node anal is a value adjusted by the cost adjuster of this neighbor node. 
     Information other than the above-described items of information may he stored in the node table and the neighbor node tables. 
     (3) Processing for Establishing Communication Path with Neighbor Node by Node  1 X 
     An explanation will be given, with reference to  FIGS. 3 and 5 , of processing for establishing a communication path with a neighbor node by the node  1 X in the communication system  100  of the present disclosure,  FIG. 5  is a flowchart illustrating a processing method in the communication system  100 . It is assumed that the node  1 A and the node  1 B are present as the neighbor nodes of the node  1 X and communication paths other than that between the node  1 X and the node  1 A and that between the node  1 X and the node  1 B have already been established. 
     In step S 11 , the communication unit  10   x  of the node  1 X receives a wireless signal sent from the node  1 A and that from the node  1 B. The node  1 A has path information of the path from the node  1 A to the node  1 M, while the node  1 B has path information of the path from the node  1 B to the node  1 M, and the node  1 A and the node  1 B each broadcast a wireless signal at predetermined time intervals. The wireless signal sent from the node  1 A includes path information of the path from the node  1 A to the node  1 M and also includes information about the communication cost from the node  1 A to the node  1 M (third cost). The wireless signal sent from the node  1 B includes path information of the path from the node  1 B to the node  1 M and also includes information about the communication cost from the node  1 B to the node  1 M (third cost). The third cost of the node  1 A and that of the node  1 B may be the communication cost adjusted by the above-described cost adjusters  24  ( 24   a  and  24   b ). Upon receiving the wireless signals, the node  1 X recognizes that the node  1 A and  1 B are present as neighbor nodes of the node  1 X. 
     In step S 12 , the relay node candidate determiner  21   x  of the node  1 X determines the node  1 A and the node  1 B as relay node candidates and creates neighbor node tables  32   x.  The neighbor node table  32   x  is created for each of the node  1 A and the node  1 B, as shown in  FIG. 4 . 
     As shown in  FIG. 4 , in the neighbor node table  32   x  for the node  1 A, path information of the node  1 A and the communication cost from the node  1 A to the node  1 M (third cost of the node  1 A) are stored, based on the information received in step S 11 . The communication cost from the node  1 A to the node  1 M (third cost of the node  1 A) is stored as the second cost of the node  1 X. 
     Likewise, the information of the node  1 B received in step S 11  is also stored in the neighbor node table  32   x  for the node  1 B. 
     In step S 13 , the cost information communicators  11   x  and  12   x  of the node  1 X exchange wireless signals with each of the node  1 A and the node  1 B so as to obtain the first cost of the node  1 A and that of the node  1 B. The first cost of the node  1 A and that of the node  1 B obtained by the cost information communicators  11   x  and  12   x  are each stored in a region of the neighbor node table  32   x  where the first cost is stored. 
     In step S 14 , the node  1 X calculates the third cost based on the first cost and the second cost of the node  1 A stored in the neighbor node table  32   x  and calculates the third cost based on the first cost and the second cost of the node  1 B stored in the neighbor node table  32   x  and stores the third cost of the node A and that of the node B in the neighbor node tables  32   x.    
     In step S 15 , the relay node determiner  22   x  of the node  1 X determines the relay node based on the third cost of the node  1 A and that of the node  1 B stored in the neighbor node tables. The relay node determiner  22   x  determines the relay node candidate having the smallest third cost as the relay node. In this example, the relay node determiner  22   x  determines the node  1 A as the relay node of the node  1 X. 
     In step S 16 , the path information generator  23   x  of the node  1 X creates path information of the node  1 X based on the determination of the relay node determiner  22   x  made in step S 15 . The path information created by the path information generator  23   x  is stored in the node table  31   x  of the storage unit  30   x.    
     In step S 17 , the path information of the node  1 X created in step S 16  is sent to the node  1 M via the node  1 A. This allows the node  1 A and the node  1 M to obtain the path information of the node  1 X. 
     When the node  1 A has obtained the path information of the node  1 X in step S  17 , the cost adjuster  24   a  of the node  1 A adjusts the third cost of the node  1 A in step S 18 . The cost adjuster  24   a  adjusts the third cost by multiplying the third cost of the node  1 A by the predetermined weighting factor, as described above. In the present embodiment, the weighting factor is calculated by substituting the number of lower nodes of the node  1 A into equation (1). The third cost adjusted by the cost adjuster  24   a  of the node  1 A is stored in the node table  31   a  as the communication cost from the node  1 A to the node  1 M (third cost). The number of lower nodes stored in the node table  31   a  is updated to 1. 
     Then, processing for establishing a communication path with a neighbor node by the node  1 X has been completed, and the communication path between the node  1 X and the node  1 A is established. 
     (4) Communication Processing for Sending Information Based on Path Information 
     A description will now be given of processing for sending information from the node  1 X to the node  1 Y by using the path information of the path of the node  1 X established in steps S 11  through S 16  described above.  FIG. 6  is a flowchart illustrating an example of processing for sending information based on path information. 
     First, in step S 21 , the node  1 X judges whether information of the communication path until the node  1 Y is included in path information of the node  1 X or path information of the lower nodes stored in the storage unit  30   x.    
     If the result of the judgement in step S 21  indicates that information of the communication path until the node  1 Y is not included in the path information stored in the storage unit  30   x  of the node  1 X, in step S 22 , the node  1 X sends information to the node  1 A in accordance with the path information of the node  1 X generated in step S 16 . The communication cost from the node  1 X to the node  1 A is 3. 
     If the result of the judgement in step  521  indicates that information of the communication path until the node  1 Y is included in the path information stored in the storage unit  30   x  of the node  1 X, the node  1 X sends information to the node  1 Y in accordance with the path information. 
     Then, in step S 23 , upon receiving the information, the node  1 A judges whether information of the communication path until the node  1 Y is included in path information of the node  1 A or path information of a lower node stored in the storage unit  30   a.    
     Since the path until the node  1 Y is included in the path information stored in the node  1 A, in step S 24 , the node  1 A sends the information to the node  1 Y via the node  1 C in accordance with the path information of the path until the node  1 Y. The communication cost from the node  1 A to the node  1 C is 3, and the communication cost from the node  1 C to the node  1 Y is 3. 
     Then, communication processing for sending information from the node  1 X to the node  1 Y has been completed. The communication cost required for this communication processing is 9. 
     (5) Characteristics 
     (5-1) 
     A node  1  according to the present disclosure is a node  1  that performs multi-hop communication. A communication system according to the present disclosure is a communication system that performs multi-hop communication and includes the node  1  of the present disclosure. The node  1  includes a relay node determiner  22 . The relay node determiner  22  determines a relay node that performs relaying, from among relay node candidates. The relay node candidates have path information of a path to a master node. At least when the number of lower nodes of each of the relay node candidates is within a predetermined range, the relay node determiner  22  determines the relay node so that a relay node candidate having a larger number of lower nodes is more likely to be determined as the relay node than a relay node candidate having a smaller number of lower nodes. 
     The node  1  according to the present disclosure further includes a cost information receiver  11  and a cost information sender  12 , which serve as cost obtainers, that obtain a first cost, a second cost, and a third cost. The first cost is a cost from a node to a relay node candidate. The second cost is a cost which is adjusted based on the number of lower nodes of a relay node candidate. The second cost is a cost from a relay node candidate to the master node. The third cost is a cost from the node to the master node and is determined based on the first cost and the second cost. The relay node determiner  22  determines the relay node based on the third cost. 
     The second cost is a cost adjusted by multiplying a cost which is from a relay node candidate to the master node and which is stored in a relay node candidate by a predetermined weighting factor. When the number of lower nodes of a relay node candidate is within a predetermined range, the weighting factor serves as a factor that decreases the second cost. 
     The node  1  according to the present disclosure has the above-described characteristics and selects the relay node by using the weighting factor based on the number of lower nodes. With this arrangement, the node  1  is able to determine the relay node so that a relay node candidate having a larger number of lower nodes is more likely to be determined as the relay node than a relay node candidate having a smaller number of lower nodes. The node  1  can thus contribute to decreasing the number of nodes that perform relaying to a destination node. It is thus possible to reduce the communication cost required for completing communication processing in the communication system  100  which includes the node  1 . 
     (5-2) 
     The node  1  according to the present disclosure further includes a path information generator  23 , a path information receiver  13 , and a path information sender  14 . The path information generator  23  generates path information of a path from the node  1  to the master node via a relay node. The path information receiver  13  receives, from a lower node of the node  1 , path information of a path from the lower node to the master node, The path information sender  14  sends the path information generated by the path information generator  23  and the path information received by the path information receiver  13  to the relay node. The number of lower nodes of a relay node candidate is determined based on the number of items of path information received by this relay node candidate. 
     This enables the node  1  to determine the number of lower nodes of each of the relay node candidates and to select the relay node based on the number of lower nodes. 
     (5-3) 
     If the number of lower nodes of a relay node candidate is not within the predetermined range, the relay node determiner  22  of the node  1  of the present disclosure determines the relay node so that this relay node candidate is less likely to be determined as the relay node. 
     The cost adjuster  24  of the node  1  of the present disclosure adjusts the second cost by using the weighing factor based on equation (1). Equation (1) is a function that calculates the weighting factor so that the weighting factor decreases the second cost when the number of lower nodes of a relay node candidate is within the predetermined range. Equation (1) is also a function that calculates the weighting factor so that the weighting factor increases the second cost when the number of lower nodes of a relay node candidate is not within the predetermined range. The relay node determiner  22  determines the relay node by using the third cost which is calculated based on the second cost adjusted by the cost adjuster  24 . 
     When the number of lower nodes of a relay node candidate is not within the predetermined range, the node  1  determines the relay node so that this relay node candidate is less likely to be determined as the relay node. This can contribute to reducing the concentration of a load on a specific relay node candidate. 
     (5-4) 
     A communication method according to the present disclosure is a communication method for the communication system  100  that performs multi-hop communication. The communication method includes a relay node determining step S 15 . The relay node determining step S 15  determines a relay node that performs relaying, from among relay node candidates haying path information of a path to a master node. At least when the number of lower nodes of each of the relay node candidates is within a predetermined range, the relay node determining step S 15  determines the relay node so that a relay node candidate haying a larger number of lower nodes is more likely to be determined as the relay node than a relay node candidate having a smaller number of lower nodes. 
     By executing communication processing by using the above-described node  1 , the communication system  100  implements the above-described communication method. This method enables the node  1  to determine the relay node so that a relay node candidate having a larger number of lower nodes is more likely to be determined as the relay node than a relay node candidate having a smaller number of lower nodes. The node  1  can thus contribute to decreasing the number of nodes that perform relaying to a destination node. It is thus possible to reduce the communication cost required for completing communication processing in the communication system  100  which includes the node  1 . 
     (6) Modified Examples 
     (6-1) 
     If plural relay node candidates having the same third cost are found, the relay node determiner  22  of the node  1  of the present disclosure may determine the relay node candidate having the smallest second cost as the relay node. 
     If there are plural relay node candidates having the same third cost, the relay node candidate having a smaller second cost performs relaying. This can contribute to reducing the number of nodes that perform relaying to a destination node, thereby achieving efficient communication. 
     (6-2) 
     If the communication frequency of a relay node candidate is higher than or equal to a predetermined threshold, the relay node determiner  22  of the node  1  of the present disclosure determines the relay node so that this relay node candidate is less likely to be determined as the relay node. 
     More specifically, if the communication frequency of a relay node candidate is higher than or equal to the predetermined threshold, the node  1  according to the present disclosure may use a predetermined weighting factor to determine the relay node so that this relay node candidate is less likely to he determined as the relay node. 
     The node  1  can thus contribute to reducing the concentration of a load on a specific relay node candidate. 
     (6-3) 
     For the sake of description, all nodes included in the communication system  100  according to the present disclosure are nodes  1 . However, sonic of the nodes included in the communication system  100  may be nodes having different characteristics from the node  1 . For example, some of the nodes included in the communication system  100  may be nodes  2 . 
     (6-4) 
     The communication system  100  according to the present disclosure is a. communication system that performs multi-hop communication. However, the communication system  100  may also include a communication method other than multi-hop communication. For example, the node  1  may include communication means using a wired medium, such as power line communication. 
     (6-5) 
     In the above-described processing for establishing a communication path, the relay node determiner  22  determines the relay node based on the communication cost (third cost). Alternatively, at least when the communication frequency estimated values of the relay node candidates are within a predetermined range, the relay node determiner  22  may determine the relay node so that a relay node candidate having a higher communication frequency estimated value is more likely to be determined as the relay node than a relay node candidate having a lower communication frequency estimated value. Alternatively, at least when the communication frequency actual values of the relay node candidates are within a predetermined range, the relay node determiner  22  may determine the relay node so that a relay node candidate having a higher communication frequency actual value is more likely to be determined as the relay node than a relay node candidate having a lower communication frequency actual value. 
     When the communication frequency estimated value or the communication frequency actual value of a relay node candidate is not within the predetermined range, the processor  20  may execute adjusting processing by multiplying the communication frequency estimated value or the communication frequency actual value by a predetermined weighting factor so that this relay node candidate is less likely to he determined as the relay node. This can contribute to reducing the concentration of a load on a specific relay node candidate. 
     (6-6) 
     The node  1  according to the present disclosure has information concerning the priority for determining a relay node candidate as the relay node. 
     The information concerning the priority is information determined based on node information, information about the communication frequency, or the third cost, for example. The relay node determiner  22  may determine the relay node based on the information concerning the priority. 
     For example, it is possible to set the priority of the master node to be the highest priority. Additionally, if a node different from the node  1  belongs to the same system as the node  1 , if identification information of this different node indicates that the different node is a parent node of the node  1 , and if the different node is included in the relay node candidates, the relay node determiner  22  may set the priority of this different node to be a higher priority, 
     Setting the priority in this manner enables the relay node determiner  22  to determine a relay node candidate as the relay node based on the priority. 
     (7) 
     The embodiment of the present disclosure has been described above. It is to be understood that various changes may be made to modes and details of the embodiment without departing from the spirit and scope of the present disclosure defined by the following claims.