Patent Publication Number: US-10771373-B2

Title: Ad hoc network route construction system, node, and center node

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Priority Patent Application JP 2017-187838 filed Sep. 28, 2017, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to an ad hoc network route construction system including one center node and a plurality of nodes (next hop nodes), the nodes (next hop nodes), and the center node. 
     2. Description of Related Art 
     There is known an ad hoc network route construction system including one center node and a plurality of next hop nodes. 
     SUMMARY OF THE INVENTION 
     In the ad hoc network route construction system, a route search packet is flooded a plurality of times. Therefore, it is desirable to reduce the load on the network. Further, since an ad hoc network is a network that does not depend on an infrastructure including a dedicated base station, it is desirable to construct a new ad hoc network route in a short time. 
     According to an embodiment of the present disclosure, there is provided an ad hoc network route construction system, including: 
     one center node; and 
     a plurality of next hop nodes, in which 
     the center node
         generates a request packet including a data part and a header part, a MAC (Media Access Control) address of the center node and position information of the center node being described in the data part, a positive integer value being described in the header part as time to live, and   transmits the request packet to one or more next hop nodes located in an area where the one or more next hop nodes can communicate with the center node,       

     each of the next hop nodes
         receives the request packet,   if determining that a value of the time to live described in the header part of the request packet is 0,   generates a reply packet including a data part, all of MAC addresses and all pieces of position information described in the data part of the received request packet being described in the data part of the reply packet, and   transmits the reply packet to a request source node as a source of the received request packet,       

     each of the next hop nodes excluding the next hop node being a source of the reply packet further
         receives the reply packet from the request destination node, and   transmits the received reply packet to the request source node, and the center node further   receives one or more reply packets from one or more request destination nodes, and then   creates, at regular time intervals, a routing table based on all of MAC addresses and all pieces of position information described in data parts of the received one or more reply packets.       

     According to an embodiment of the present disclosure, there is provided a node (next hop node), included in an ad hoc network route construction system including one center node, and a plurality of nodes, the node including: 
     a packet control unit that
         receives, from the center node, a request packet including a data part and a header part, a MAC (Media Access Control) address of the center node and position information of the center node being described in the data part, a positive integer value being described in the header part as time to live;   if determining that a value of the time to live described in the header part of the request packet is 0,   generates a reply packet including a data part, all of MAC addresses and all pieces of position information described in the data part of the received request packet being described in the data part of the reply packet; and   transmits the reply packet to a request source node as a source of the received request packet.       

     According to an embodiment of the present disclosure, there is provided a center node, included in an ad hoc network route construction system including 
     one center node, and 
     a plurality of next hop nodes, in which 
     the center node
         generates a request packet including a data part and a header part, a MAC (Media Access Control) address of the center node and position information of the center node being described in the data part, a positive integer value being described in the header part as time to live, and   transmits the request packet to one or more next hop nodes located in an area where the one or more next hop nodes can communicate with the center node,       

     each of the next hop nodes
         receives the request packet,   if determining that a value of the time to live described in the header part of the request packet is 0,   generates a reply packet including a data part, all of MAC addresses and all pieces of position information described in the data part of the received request packet being described in the data part of the reply packet, and   transmits the reply packet to a request source node as a source of the received request packet,       

     each of the next hop nodes excluding the next hop node being a source of the reply packet further
         receives the reply packet from the request destination node, and   transmits the received reply packet to the request source node, and the center node further   receives one or more reply packets from one or more request destination nodes, and then   creates, at regular time intervals, a routing table based on all of MAC addresses and all pieces of position information described in data parts of the received one or more reply packets.       

     These and other objects, features and advantages of the present disclosure will become more apparent in light of the following detailed description of best mode embodiments thereof, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a hardware configuration of an information processing apparatus; 
         FIG. 2  shows a functional configuration of an ad hoc network route construction system; 
         FIG. 3  shows an operational flow of a center node; 
         FIG. 4  shows an operational flow of a next hop node; 
         FIG. 5  shows an operational sequence of the center node and the next hop node (including terminal node); 
         FIG. 6  shows an SFRREQ packet; 
         FIG. 7  shows a header of an existing RREQ packet; 
         FIG. 8  schematically shows a request transmission area and a request transmission disabled area; 
         FIG. 9  shows an SFRREP packet; 
         FIG. 10  shows a header of an existing RREP packet; and 
         FIG. 11  shows an operational sequence of the center node and the next hop node (including time to live zero node). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. 
     1. OVERVIEW OF AD HOC NETWORK ROUTE CONSTRUCTION SYSTEM 
     An ad hoc network is a network that does not depend on the infrastructure including a dedicated base station. Specifically, the ad hoc network includes a plurality of nodes. The plurality of nodes are connected to each other by using the relay functions of the terminal apparatuses (nodes) themselves by Wi-Fi (registered trademark), Bluetooth (registered trademark), ZigBee (registered trademark), or the like. 
     Typically, in order to newly construct a route of the ad hoc network, a specific (arbitrary) one of the plurality of terminal apparatuses (nodes) is set as a center node. The center node floods a request packet for route search to a plurality of nodes (next hop nodes) located in an area where the nodes can communicate with the center node. 
     The node that receives the request packet additionally describes, in a buffer of the request packet, terminal information of the node itself. The node floods the described request packet to a plurality of nodes (next hop nodes) located in an area where the nodes can communicate with the node. This flooding process is repeated many times for each hopping. 
     In the case where there is no next hop node, the node that receives the request packet determines that the node itself is a terminal node. The terminal node generates a reply packet in which the terminal information (terminal information of all of the plurality of nodes via which the request packet is transmitted) stored in the buffer of the request packet is described. The terminal node transmits the reply packet to the center node via the plurality of next hop nodes (all of the plurality of nodes via which the request packet is transmitted). 
     The center node calculates, based on terminal information of each node described in the received reply packet, respective routes between the nodes, and create a routing table. The center node supplies the created routing table to all of the nodes by flooding. Each of the nodes constructs a route based on the routing table. 
     As described above, typically, in order to newly construct a route of the ad hoc network, a process of flooding the request packet is repeated many times. Therefore, the load on the network is high. In view of the above-mentioned circumstances, according to this embodiment, the load on the network is reduced in the ad hoc network route construction system. Further, an ad hoc network does not depend on the infrastructure including a dedicated base station. Such an ad hoc network may be used at a time of a disaster, for example. Therefore it is desirable to construct a new ad hoc network route in real time and in a short time. In view of the aforementioned circumstances, according to the present embodiment, a new ad hoc network route is constructed in a short time. 
     2. HARDWARE CONFIGURATION OF INFORMATION PROCESSING APPARATUS 
       FIG. 1  shows a hardware configuration of an information processing apparatus. 
     A node is an information processing apparatus having the relay function by Wi-Fi (registered trademark), Bluetooth (registered trademark), ZigBee (registered trademark), or the like, and typically an indoor installation type (not mobile) terminal apparatus. For example, the information processing apparatus is, for example, a desktop personal computer, an image forming apparatus such as an MFP (Multifunction Peripheral), a facsimile, a television receiver, or a household appliance. 
     An information processing apparatus  10  as a node includes a controller circuit  11 , and a display device  12 , a network interface  13 , an operation device  15 , a storage device  16 , and a position information acquisition device  14  that are connected to the controller circuit  11  via a bus  17 . 
     An information processing apparatus  10  as a node includes a controller circuit  11 , and a display device  12 , a network interface  13 , an operation device  15 , a storage device  16 , and a position information acquisition device  14  that are connected to the controller circuit  11  via a bus  17 . 
     The controller circuit  11  includes a CPU (Central Processing Unit) and the like. The CPU of the control circuit  11  loads a program recorded in a ROM (Read Only Memory), which is an example of a non-transitory computer readable recording medium, in a RAM (Random Access Memory) and executes the program 
     The storage device  16  includes a ROM, a RAM, and a large-volume storage device such as an HDD (Hard Disk Drive). The ROM fixedly stores programs to be executed by the controller circuit  11 , data, and the like. The programs stored in the ROM are loaded to the RAM. 
     The display device  12  includes an LCD (Liquid Crystal Display), an organic EL (Electroluminescence) display, or the like. The display device  12  carries out operational processing based on information received from the controller circuit  11  and displays generated image signals on a screen. The display device  12  may be an external display device. 
     The operation device  15  includes a keyboard, a mouse, a touch panel, and/or various switches. The operation device  15  detects user operations and outputs operation signals to the controller circuit  11 . 
     The network interface  13  is an interface for communicating with another information processing apparatus  10  (node) via Wi-Fi (registered trademark), Bluetooth (registered trademark), ZigBee (registered trademark), or the like. 
     The position information acquisition device  14  acquires the present position information of the information processing apparatus  10  itself. For example, the position information acquisition device  14  measures, based on the cycle at which Wi-Fi (registered trademark) radio waves transmitted from a plurality of wireless LAN (Local Area Network) access points reach the information processing apparatus  10 , the position of the information processing apparatus  10  by using triangulation. Alternatively, the position information acquisition device  14  measures, based on the cycle at which radio waves from a Bluetooth (registered trademark) transmitter (beacon) reach the information processing apparatus  10 , the position of the information processing apparatus  10 . Further, the position information acquisition device  14  may measure the position of the information processing apparatus  10  by using a GPS (Global Positioning System). 
     3. FUNCTIONAL CONFIGURATION OF AD HOC NETWORK ROUTE CONSTRUCTION SYSTEM 
       FIG. 2  shows a functional configuration of an ad hoc network route construction system. 
     In the following description, the information processing apparatus  10  as a center node will be referred to as the “center node  10 C”. A plurality of nodes  10  other than the center node  10 C among a plurality of nodes included in the ad hoc network are referred to as the “next hop nodes  10 N” for convenience in some cases for the sake of distinguishing them from the center node. That is, an ad hoc network route construction system  1  includes the one center node  10 C and the plurality of next hop nodes  10 N. 
     The center node  10 C may be arbitrarily decided by a user (administrator). A terminal apparatus (not shown) such as a personal computer receives a user operation, and requests, via Wi-Fi (registered trademark), Bluetooth (registered trademark), ZigBee (registered trademark), or the like, the arbitrary information processing apparatus  10  to function as a center node. 
     The center node  10 C loads an information processing program recorded in a ROM, which is an example of a non-transitory computer readable recording medium, in a RAM and executes the program to thereby operate as the functional blocks, i.e., a packet control unit  101 C and a construction route management unit  102 C. 
     The next hop node  10 N loads an information processing program recorded in a ROM, which is an example of a non-transitory computer readable recording medium, in a RAM and executes the program to thereby operate as the functional blocks, i.e., a packet control unit  101 N and a construction route management unit  102 N. 
     The packet control unit  101 C of the center node  10 C generates an SFRREQ packet including a data part in which a MAC address and position information of the center node  10 C are described and a header part in which a positive integer value is described as the time to live. The packet control unit  101 C transmits (floods) the SFRREQ packets to the next hop nodes  10 N located in an area where the next hop nodes  10 N can communicate with the center node  10 C. 
     The packet control unit  101 N of each of the next hop nodes  10 N receives an SFRREQ packet. The packet control unit  101 N determines whether or not the time to live value described in the header part of the SFRREQ packet is 0. In the case of determining that the time to live value is not 0, the packet control unit  101 N determines whether or not there is the next hop node  10 N (request destination node) as the destination of the SFRREQ packet. In the case of determining that there is the request destination node, the packet control unit  101 N additionally describes, in the data part of the received SFRREQ packet, the MAC address and position information of the next hop node  10 N itself, and transmits the described SFRREQ packet to the next hop node  10 N (request destination node). In the case of determining that there is no request destination node, the packet control unit  101 N determines that the next hop node  10 N itself is a terminal node. In the case of determining that the next hop node  10 N itself is a terminal node, the next hop node  10 N generates an SFRREP packet including a data part in which all of the MAC addresses and all pieces of position information described in the data part of the received SFRREQ packet are described and including a header part in which the described time to live value is decremented, and transmits the SFRREP packet to the request source node as a source of the received SFRREQ packet. The next hop node  10 N other than the terminal node receives the SFRREP packet from the request destination node, and transmits the received SFRREP packet to the request source node. 
     In the case of determining that the time to live value described in the header part of the SFRREQ packet is 0, the packet control unit  101 N of the next hop node  10 N generates an SFRREP packet, and transmits the SFRREP packet to the request source node. Further, in the case of determining that there is a request destination node, the packet control unit  101 N additionally describes the MAC address and the position information of the next hop node  10 N itself in the data part of the received SFRREQ packet, and resets the time to live value described in the header part of the received SFRREQ packet. The packet control unit  101 N transmits the SFRREQ packet to the next hop node  10 N (request destination node). 
     The packet control unit  101 C of the center node  10 C receives SFRREP packets from one or more next hop nodes  10 N. 
     The construction route management unit  102 C of the center node  10 C creates, at regular time intervals, a routing table based on all of the MAC addresses and all pieces of position information described in the data parts of the one or more received SFRREP packets. The construction route management unit  102 C supplies, at regular time intervals, the created routing table to the next hop node  10 N. 
     The construction route management unit  102 N of the next hop node  10 N acquires the routing table created by the center node  10 C. 
     4. OPERATIONAL FLOW OF CENTER NODE AND NEXT HOP NODE 
       FIG. 3  shows an operational flow of the center node.  FIG. 4  shows an operation flow of the next hop node.  FIG. 5  shows an operational sequence of the center node and the next hop node (including terminal node). 
     A controller circuit  11 C of the center node  10 C transmits, via a network interface  13 C (Step S 101 ), HELLO packets to all the next hop nodes  10 N with which the center node  10 C can communicate. Hereinafter, the next hop nodes  10 N as destinations to which the center node  10 C transmits the HELLO packets will be referred to as the “first next hop node  10 N 1 ” for convenience. 
     A controller circuit  11 N of each of the first next hop node  10 N 1  receives the HELLO packet from the center node  10 C via a network interface  13 N. The controller circuit  11 N of the first next hop node  10 N 1  that receives the HELLO packet transmits the HELLO packet to the center node  10 C via the network interface  13 N. 
     The controller circuit  11 C of the center node  10 C receives the HELLO packets from the plurality of first next hop node  10 N 1  via the network interface  13 C (YES in Step S 102 ). 
     In the case of receiving the HELLO packets, the packet control unit  101 C of the center node  10 C generates SFRREQ packets as request packets. The packet control unit  101 C of the center node  10 C transmits, via the network interface  13 C, the generated SFRREQ packets by unicast to all of the plurality of first next hop node  10 N 1  (request destination nodes) as sources of the received HELLO packets (Step S 103 ). After transmitting the SFRREQ packets, the center node  10 C stands by to receive an SFRREP packet (to be described later). 
     Now, the “SFRREQ packet” will be described. The “SFRREQ packet” is a packet unique to this embodiment, which is obtained by improving an existing RREQ (Route Request) packet used in the AODV (Ad hoc On-Demand Distance Vector) protocol, and represents “Smart Flooding Route Request packet”. The “SFRREQ packet” stores information requisite for constructing a route of the ad hoc network. 
       FIG. 6  shows an SFRREQ packet.  FIG. 7  shows a header of an existing RREQ packet. 
     In a header (SFRREQ header) ( FIG. 6 ) of an SFRREQ packet according to this embodiment, the change point from the header (RREQ header) ( FIG. 7 ) of the existing RREQ packet is as follows. That is, in the SFRREQ header according to this embodiment, “destination MAC address”, “source MAC address”, and “time to live 2 (TTL 2 )” are described instead of “destination IP address”, “destination sequence number”, “source IP address”, and “source sequence number” in the existing RREQ header. The “destination MAC address” is a MAC address of a node as a destination of the SFRREQ packet. The “source MAC address” is a MAC address of a node as a source of the SFRREQ packet. In the case of Step S 103 , the MAC address of the center node  10 C is described as the “source MAC address”. The “time to live 2 (TTL 2 )” is a positive integer value (for example, “3”). The “time to live 2 (TTL 2 )” value is changed for every hopping (described later). 
     The data part of the SFRREQ packet includes an RREQ ID, a MAC address buffer, and a position information buffer. The “RREQ ID” is an ID (identifier) for uniquely identifying the SFRREQ packet. In the “MAC address buffer”, a MAC address of a node as a source of this SFRREQ packet is stored (additionally described for each hopping). In the case of Step S 103 , the MAC address of the center node  10 C is described. In the “position information buffer”, position information (information acquired by the position information acquisition device  14 ) of a node as a source of this SFRREQ packet is stored (additionally described for each hopping). In the case of Step S 103 , position information of the center node  10 C acquired by a position information acquisition device  14 C is described. 
     Now, the operational flow will be described again. The packet control unit  101 N of each of the first next hop node  10 N 1  (request destination node) that transmit the HELLO packets to the center node  10 C receives an SFRREQ packet from the center node  10 C via the network interface  13 N (YES in Step S 201 ). In the case of receiving the SFRREQ packet, the packet control unit  101 N of the first next hop node  10 N 1  determines whether or not the packet control unit  101 N has ever transmitted the SFRREQ packet (Step S 202 ). Assumption is made that the packet control unit  101 N of the first next hop node  10 N 1  determines that the packet control unit  101 N has never transmitted the SFRREQ packet (NO in Step S 202 ). In this case, the controller circuit  11 N of the first next hop node  10 N 1  transmits, via the network interface  13 N, a HELLO packet to all of the next hop nodes  10 N with which the first next hop node  10 N 1  can communicate. Hereinafter, the next hop node  10 N as a destination to which the first next hop node  10 N 1  transmits the HELLO packet will be referred to as the “second next hop node  10 N 2 ” for convenience. At this time, the controller circuit  11 N of the first next hop node  10 N 1  requests for a response HELLO packet in which position information of the second next hop node  10 N 2  as a response source is additionally described as a response HELLO packet to the HELLO packet (Step S 203 ). Hereinafter, the “response HELLO packet” represents the HELLO packet in which position information of a node as a response source additionally described. 
     Assumption is made that the packet control unit  101 N of the first next hop node  10 N 1  determines that the packet control unit  101 N has transmitted the SFRREQ packet (YES in Step S 202 ). In this case, the first next hop node  10 N 1  finishes the processing without transmitting the HELLO packet. In other words, each of the next hop nodes  10 N transmits the SFRREQ packet only one time. Accordingly, it is possible to reduce the possibility that each of the next hop nodes  10 N receives the SFRREQ packet from many next hop nodes  10 N, and reduce the load on the network. 
     The packet control unit  101 N of each of the first next hop node  10 N 1  performs the following processing by using the stand-by time from when transmitting the HELLO packet to when receiving the response Hello packet. That is, the packet control unit  101 N of the first next hop node  10 N 1  additionally describes the MAC address of the first next hop node  10 N 1  itself after the MAC address of the center node  10 C that is already described in the MAC address buffer in the data part of the SFRREQ packet received from the center node  10 C (YES in Step S 201 ). Further, the packet control unit  101 N of the first next hop node  10 N 1  additionally describes the position information of the first next hop node  10 N 1  itself subsequent to the position information of the center node  10 C that is already described in the position information buffer in the data part of the SFRREQ packet. In this way, the packet control unit  101 N of the first next hop node  10 N 1  updates the SFRREQ packet. 
     The packet control unit  101 N of the first next hop node  10 N 1  reads the TTL 2  value described in the header part of the SFRREQ packet. The packet control unit  101 N determines whether or not TTL 2  is 0 (Step S 206 ). In the case of determining that TTL 2  is not 0 (NO in Step S 206 ), the packet control unit  101 N decrements the TTL 2  value described in the header of the SFRREQ packet (Step S 207 ). Meanwhile, in the case of determining that TTL 2  is 0 (YES in Step S 206 ), the packet control unit  101 N describes a new positive integer value (for example, “3”) as the TTL 2  (resets TTL 2 ) (Step S 208 ). The described TTL 2  value may be the same as or different from the TTL 2  value of the SFRREQ packet (Step S 103 ) generated by the center node  10 C. 
     As described above, the packet control unit  101 N of the first next hop node  10 N 1  updates the SFRREQ packet during the standby time from transmission of the HELLO packet to reception of the response HELLO packet. 
     Hereinafter, (1) the case where TTL 2  is not 0 (NO in Step S 206 ) and (2) the case where TTL 2  is 0 (YES in Step S 206 ) will be described in order. 
     (1) Case where TTL 2  is not 0 (NO in Step S 206 ) 
     The packet control unit  101 N of the first next hop node  10 N 1  calculates the request transmission area and the request transmission disabled area of the first next hop node  10 N 1  based on the position information of the center node  10 C and the position information of the first next hop node  10 N 1  that are described in the position information buffer of the data part of the SFRREQ packet. Simply speaking, the packet control unit  101 N of the first next hop node  10 N 1  sets, as the request destination node, the node  10  located at the position away from the position of the center node  10 C as the source of the SFRREQ packet based on the position information of the center node  10 C and the position information of the first next hop node  10 N 1 . A method of calculating the request transmission area and the request transmission disabled area will be described below. 
       FIG. 8  schematically shows the request transmission area and the request transmission disabled area. 
     A communicable area A 1  has a distance range (schematically shown simply as a circle in  FIG. 8 ) within which the radio wave of the first next hop node  10 N 1  can reach. The packet control unit  101 N of the first next hop node  10 N 1  divides the communicable area A 1  where the first next hop node  10 N 1  itself can perform communication into two areas A 2  and A 3  by a straight line L 2  that is orthogonal to a line segment L 1  and passes through the position of the first next hop node  10 N 1  itself. The line segment L 1  connects the position of the center node  10 C as the source of the SFRREQ packet and the position of the first next hop node  10 N 1 . The packet control unit  101 N of the first next hop node  10 N 1  sets the area that does not include the position of the center node  10 C as the source of the SFRREQ packet as a request transmission area A 2  and the area that includes the position of the center node  10 C as a request transmission disabled area A 3  out of the two areas A 2  and A 3  obtained by dividing the communicable area A 1 . 
     Now, the operational flow will be described again. The controller circuit  11 N of each of the second next hops node  10 N 2  receives the HELLO packet from the first next hop node  10 N 1  via the network interface  13 N (NO in Step S 201 , and YES in Step S 204 ). The controller circuit  11 N of the second next hop node  10 N 2  that receives the HELLO packet generates a response HELLO packet obtained by additionally describing the position information (information acquired by a position information acquisition device  14 N) of the second next hop node  10 N 2  itself in the HELLO packet. The controller circuit  11 N of the second next hop node  10 N 2  transmits the response HELLO packet to the first next hop node  10 N 1  via the network interface  13 N (Step S 205 ). 
     The controller circuit  11 N of the first next hop node  10 N 1  receives (or has already received) the response HELLO packet from each of the second next hop nodes  10 N 2  via the network interface  13 N (YES in Step S 209 ). 
     The packet control unit  101 N of the first next hop node  10 N 1  reads the position information of the second next hop node  10 N 2  itself described in the response HELLO packet. The controller circuit  11 N of the first next hop node  10 N 1  determines whether each of the second next hop nodes  10 N 2  is located in the calculated request transmission area A 2  or the calculated request transmission disabled area A 3  (Step S 210 ). 
     The controller circuit  11 N of the first next hop node  10 N 1  transmits, via the network interface  13 N, the updated SFRREQ packet (Step S 211 ) by unicast to all of the second next hop nodes  10 N 2  (request destination nodes) located in the request transmission area A 2  (YES in Step S 210 ). Note that the “updated SFRREQ packet” is an SFRREQ packet in which the MAC address and the position information of the first next hop node  10 N 1  itself are additionally described, and in which the TTL 2  value is decremented (Step S 207 ). 
     The packet control unit  101 N of each of the second next hop nodes  10 N 2  (request destination nodes) receives the SFRREQ packet from the first next hop node  10 N 1  via the network interface  13 N (YES in Step S 201 ). In the case of receiving the SFRREQ packet, the second next hop node  10 N 2  itself becomes the request source node, and processing on and after Step S 202  is repeated. 
     Note that the second next hop node  10 N 2  updates the SFRREQ packet as follows. The packet control unit  101 N of the second next hop node  10 N 2  additionally described the MAC address of the second next hop node  10 N 2  itself subsequent to the MAC address of the center node  10 C and the subsequent MAC address of the first next hop node  10 N 1  that are already described in the MAC address buffer of the data part of the SFRREQ packet received from the first next hop node  10 N 1  (Yes in Step S 201 ). Further, the packet control unit  101 N of the second next hop node  10 N 2  additionally described the position information of the second next hop node  10 N 2  itself subsequent to the position information of the center node  10 C and the subsequent position information of the first next hop node  10 N 1  that are already described in the position information buffer of the data part of the SFRREQ packet. The packet control unit  101 N of the second next hop node  10 N 2  decrements (Step S 207 ) or resets (Step S 208 ) the TTL 2  value in the header part of the SFRREQ packet. 
     Note that the second next hop node  10 N 2  calculates the request transmission area and the request transmission disabled area as follows. The packet control unit  101 N of the second next hop node  10 N 2  divides the communicable area A 1  where the second next hop node  10 N 2  itself can perform communication into the two areas A 2  and A 3  by the straight line L 2  that is orthogonal to the line segment L 1  and passes through the position of the second next hop node  10 N 2  itself. The line segment L 1  connects the position of the first next hop node  10 N 1  as the source of the SFRREQ packet and the position of the second next hop node  10 N 2 . The packet control unit  101 N of the second next hop node  10 N 2  sets the area that does not include the position of the first next hop node  10 N 1  as the source of the SFRREQ packet as the request transmission area A 2  and the area that includes the position of the first next hop node  10 N 1  as the request transmission disabled area A 3  out of the two areas A 2  and A 3  obtained by dividing the communicable area A 1 . 
     Meanwhile, the controller circuit  11 N of the next hop node  10 N transmits the HELLO packet (Step S 203 ) but receives no response HELLO packet from another next hop node  10 N (NO in Step S 209 ) in some cases. Alternatively, the controller circuit  11 N of the next hop node  10 N determines that there is no other next hop node  10 N located in the request transmission area A 2  (NO in Step S 210 ) in some cases. In these cases, there is no node as a destination of the SFRREQ packet. Therefore, the controller circuit  11 N of the next hop node  10 N determines that the next hop node  10 N is a terminal node. Hereinafter, the next hop node  10 N as the terminal node will be referred to as the “terminal node  10 N 3 ” for convenience. 
     The packet control unit  101 N of the terminal node  10 N 3  generates an SFRREP packet as a reply packet (Step S 212 ). 
     Now, the “SFRREP packet” will be described. The “SFRREP packet” is a packet unique to this embodiment, which is obtained by improving an existing RREP (Route Reply) packet used in the AODV protocol, and represents “Smart Flooding Route Reply packet”. The “SFRREP packet” stores information requisite for constructing a route of the ad hoc network. 
       FIG. 9  shows an SFRREP packet.  FIG. 10  shows a header of the existing RREP packet. 
     In the header (SFRREP header) ( FIG. 9 ) of the SFRREP packet according to this embodiment, the change point from the header (RREP header) ( FIG. 10 ) of the existing RREP packet is as follows. That is, in the SFRREP header according to this embodiment, a “source MAC address” is described instead of a “destination IP address”, a “destination sequence number”, and a “source IP address” of the existing RREP header. The “source MAC address” is the MAC address of the terminal node  10 N 3  as a source of the SFRREP packet. 
     The data part of the SFRREP packet is completely the same as the data part of the SFRREQ packet acquired by the terminal node  10 N 3 . That is, the data part of the SFRREP packet includes the RREQ ID, the MAC address buffer, and the position information buffer. The “RREQ ID” is an ID for uniquely identifying the SFRREQ packet acquired by the terminal node  10 N 3 . In the “MAC address buffer”, the MAC addresses of nodes from the center node  10 C to the terminal node  10 N 3  are stored in the order of hopping. In the “position information buffer”, the pieces of position information of nodes from the center node  10 C to the terminal node  10 N 3  are stored in the order of hopping. 
     Now, the operation flow will be described again. The packet control unit  101 N of the terminal node  10 N 3  transmits the generated SFRREP packet to the center node  10 C by referring to the MAC addresses stored in the order of hopping in the MAC address buffer to trace the plurality of next hop nodes  10 N in the order of being newly stored in the MAC address buffer (in the order of hopping) (Step S 213 ). 
     Specifically, the packet control unit  101 N of the terminal node  10 N 3  transmits the generated SFRREP packet to the next hop node  10 N (e.g., the second next hop node  10 N 2 ) (request source node) as a source of the received SFRREQ packet. The packet control unit  101 N of the second next hop node  10 N 2  receives from the SFRREP packet from the terminal node  10 N 3  (request destination node), and transmits the received SFRREP packet to the first next hop node  10 N 1  (request source node). The packet control unit  101 N of the first next hop node  10 N 1  receives the SFRREP packet from the second next hop node  10 N 2  (request destination node), and transmits the received SFRREP packet to the center node  10 C (request source node). 
     (2) Case where TTL 2  is 0 (YES in Step S 206 ) 
     The case where TTL 2  is 0 means that the SFRREQ packet is hopped several times. A next hop node, which receives an SFRREQ packet in which TTL 2  is 0, will be referred to as “time to live zero node  10 N 4 ”, for convenience. The packet control unit  101 N of the time to live zero node  10 N 4  generates an SFRREP packet ( FIG. 9 ) as a reply packet (Step S 212 A). The “source MAC address” described in the header (SFRREP header) of the SFRREP packet is the MAC address of the time to live zero node  10 N 4 , which is the source of the SFRREP packet. The data part of the SFRREP packet is completely the same as the data part of the SFRREQ packet obtained by the time to live zero node  10 N 4 . The “RREQ ID” is an ID that uniquely identifies the SFRREQ packet obtained by the time to live zero node  10 N 4 . In the “MAC address buffer”, the MAC addresses of the nodes, from the center node  10 C to the time to live zero node  10 N 4 , are stored in the order of hopping. In the “position information buffer”, the position information of the nodes, from the center node  10 C to the time to live zero node  10 N 4 , are stored in the order of hopping. 
       FIG. 11  shows an operational sequence of the center node and the next hop node (including time to live zero node). 
     Steps S 101  to S 103  and Steps S 201  and S 202  of  FIG. 11  are similar to those steps of  FIG. 5  and are not shown. 
     The packet control unit  101 N of the time to live zero node  10 N 4  transmits the generated SFRREP packet to the center node  10 C by referring to the MAC addresses stored in the order of hopping in the MAC address buffer to trace the plurality of next hop nodes  10 N in the order of being newly stored in the MAC address buffer (in the order of hopping) (Step S 213 A). Its method is similar to the method described in Step S 213 . 
     By the way, the controller circuit  11 N of the time to live zero node  10 N 4  receives (or has already received) the response HELLO packet from each of the second next hop nodes  10 N 2  via the network interface  13 N (YES in Step S 209 A). 
     The packet control unit  101 N of the time to live zero node  10 N 4  reads the position information of the second next hop node  10 N 2  itself described in the response HELLO packet. The controller circuit  11 N of the time to live zero node  10 N 4  determines whether each of the second next hop nodes  10 N 2  is located in the calculated request transmission area A 2  or the calculated request transmission disabled area A 3  (Step S 210 A). Its method is similar to the method of Step S 210 . 
     The controller circuit  11 N of the time to live zero node  10 N 4  transmits, via the network interface  13 N, the updated SFRREQ packet (Step S 211 A) by unicast to all of the second next hop nodes  10 N 2  (request destination nodes) located in the request transmission area A 2  (YES in Step S 210 A). Note that the “updated SFRREQ packet” is an SFRREQ packet in which the MAC address and the position information of the time to live zero node  10 N 4  itself are additionally described, and in which the TTL 2  value is reset (Step S 208 ). 
     The packet control unit  101 N of each of the second next hop nodes  10 N 2  (request destination nodes) receives the SFRREQ packet from the first next hop node  10 N 1  via the network interface  13 N (YES in Step S 201 ). In the case of receiving the SFRREQ packet, the second next hop node  10 N 2  itself becomes the request source node, and processing on and after Step S 202  is repeated. 
     With reference to  FIG. 3  again, how the packet control unit  101 C of the center node  10 C receives SFRREP packets will be described. 
     The packet control unit  101 C of the center node  10 C receives SFRREP packets from all of the plurality of first next hop node  10 N 1  (request destination nodes) as destinations of SFRREQ packets via the network interface  13 C (Steps S 104  and S 105 ). Specifically, the packet control unit  101 C of the center node  10 C receives SFRREP packets from the plurality of first next hop node  10 N 1  within a particular time period (arbitrary time length) after receiving the SFRREP packet first. The packet control unit  101 C of the center node  10 C supplies the received SFRREP packet to the construction route management unit  102 C. 
     The construction route management unit  102 C of the center node  10 C puts the received plurality of SFRREP packets in a queue in the order of arrival, and stores information described in each SFRREP packet in a storage device  16 C in order for each SFRREP packet. Specifically, the construction route management unit  102 C stores, in received-data storage unit  104 C of the storage device  16 C, all of the MAC addresses described in the MAC address buffers and all pieces of position information described in the position information buffers, which are included in data parts of the SFRREP packets (Step S 106 ). 
     Next, the construction route management unit  102 C of the center node  10 C creates a routing table based on the MAC address and the position information stored in the received-data storage unit  104 C. Specifically, the construction route management unit  102 C assigns an IP address to each of the next hop nodes  10 N for each of the MAC addresses of the next hop nodes  10 N, and creates a routing table (Step S 107 ). The routing table represents the connection between the nodes  10 . The construction route management unit  102 C stores the created routing table in a construction route storage unit  103 C of the storage device  16 C. 
     The construction route management unit  102 C of the center node  10 C transmits, based on route information of the created routing table, a packet including the IP address and the routing table, to each of the next hop nodes  10 N (Step S 108 ). 
     The construction route management unit  102 N of each of the next hop nodes  10 N receives the packet including the IP address and the routing table via the network interface  13 N. The construction route management unit  102 N stores, in a received-data storage unit  105 N of a storage device  16 N, the IP address and the routing table included in the received packet. Each of the next hop nodes  10 N establishes a data link based on the IP address and the routing table to construct an ad hoc network. 
     The packet control unit  101 C of the center node  10 C creates a routing table based on the received new SFRREP packets (Step S 104 , Step S 105 ) (Step S 107 ) at regular time intervals (LOOP of Step S 109 ). The packet control unit  101 C of the center node  10 C transmits a packet including the IP address and the routing table, to each of the next hop nodes  10 N (Step S 108 ) at regular time intervals (LOOP of Step S 109 ). 
     5. CONCLUSION 
     Typically, there is known a technology in which each node transmits packets to surrounding next hop nodes by broadcasting without distinction to newly construct a route of an ad hoc network. According to this technology, the load on a network or the consumption of memory resources are increased as the number of nodes is increased. Further, in the case where manual input is necessary to search for a communication route, more time and effort are needed as the number of nodes is increased. 
     Meanwhile, according to this embodiment, all of the next hop nodes  10 N excluding the center node  10 C transmit the SFRREQ packet only one time (Steps S 202  and S 203 ). Accordingly, it is possible to reduce the possibility that each of the next hop nodes  10 N receives the SFRREQ packet from many next hop nodes  10 N. 
     Further, according to this embodiment, all of the next hop nodes  10 N excluding the center node  10 C transmit the SFRREQ packet to only the next hop node  10 N located in a part (request transmission area A 2 ) of the communicable area A 1  (Steps S 210  and S 211 ). Accordingly, it is possible to further reduce the possibility that each of the next hop nodes  10 N receives the SFRREQ packet from many next hop nodes  10 N. 
     Specifically, according to this embodiment, the next hop node  10 N requests, from one or more different next hop nodes  10 N, a response HELLO packet in which position information of the different next hop nodes  10 N as request sources is additionally described, as a response packet to the HELLO packet (Step S 203 ). Accordingly, since the next hop node  10 N can acquire the position information of the different next hop node  10 N before transmitting the SFRREQ packet, it is possible to reliably narrow the transmission target of the SFRREQ packet. 
     Further, according to this embodiment, the center node  10 C assigns an IP address to each of the next hop nodes  10 N, and creates a routing table (Step S 107 ). Accordingly, only the center node  10 C includes the construction route storage unit  103 C for storing information requisite for constructing a route, and each of the next hop nodes  10 N includes no construction route storage unit  103 C. Therefore, it is possible to reduce the resources consumed by each of the next hop nodes  10 N. Further, it is possible to reduce the amount of packets transmitted/received in the network, and reduce the load on the network. 
     As described above, according to this embodiment, it is possible to reduce the load on the network, reduce the memory resources consumed by each node, and eliminate the manual operation. 
     Further, according to the present embodiment, if a time to live value of a received request packet is 0, a next hop node transmits a reply packet to the center node. In addition, the next hop node resets the time to live of the request packet, and transmits the request packet to a terminal node. Further, after receiving a reply packet for the first time, the center node creates a routing table at regular time intervals. As a result, the time from transmission of a HELLO packet by a center node to the first reception of a reply packet is shorter than that of an assumption in which a request packet does not have time to live (i.e., only a terminal node generates a reply packet). In addition, the center node receives reply packets more frequently. Therefore the time from transmission of a HELLO packet by a center node to creation of the first routing table is shorter. As a result, the center node is capable of constructing a new ad hoc network route in real time and in a short time. An ad hoc network does not depend on the infrastructure including a dedicated base station. Such an ad hoc network may be used at a time of a disaster, for example. Therefore it is advantageous to construct a new ad hoc network route in real time and in a short time at a time of a disaster, for example. 
     Further, according to the present embodiment, if a time to live value of a received request packet is not 0, a next hop node decrements the time to live of the request packet, and transmits the request packet to another next hop node. Since the time to live of the request packet is decremented, the time from transmission of a HELLO packet by a center node to the first reception of a reply packet is shorter than that of an assumption in which a request packet does not have time to live (i.e., only a terminal node generates a reply packet). In addition, the center node receives reply packets more frequently. Therefore the time from transmission of a HELLO packet by a center node to creation of the first routing table is shorter. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.