Field of the Invention
The present invention relates to a multi-hop communication system.
Background of the Invention
Conventionally, there is known a multi-hop communication in which, if communication cannot be directly performed between communication terminals, which are intended to transfer information therebetween, existing on a communication network, communication can be made by relaying the communication using other communication terminals. The multi-hop communication is used especially in a wireless network, one of communication networks. The multi-hop communication is also used in a PLC network, i.e., a communication network built through the use of a technology of power line communication (hereinafter briefly referred to as "PLC").
In this communication network, it is sometimes the case that the communication terminals previously capable of making communication become unable to make communication due to the connection/disconnection of the communication terminals or the change in communication environments, as a

result of which the network topology of the communication network undergoes a change. In order to perform good communication between the respective communication terminals, it is therefore necessary to build a communication route between the respective communication terminals in the event that the network topology of the communication network is changed (see, e.g., JP2006-67557A and JP2008-244679A).
As communication route building methods, there is available, e.g., a method in which a communication route between communication terminals is built by exchanging route information between the communication terminals, searching for usable communication routes and selecting one route having a good communication quality from the usable communication routes.
Also available is a method in which a communication route between communication terminals is built by selecting a communication route in view of not only a communication quality but also a hop count of the communication route.
In recent years, there is available a communication network in which a plurality of parent terminals is installed on a region-by-region basis and in which the respective parent terminals make communication with a plurality of child terminals existing around the parent terminals. It has been proposed that the aforementioned multi-hop communication is used in such a communication

network to perform communication between the parent terminals and the child terminals. Each of the parent terminals arranged on a region-by-region basis acquires specified information directly from the respective child terminals existing around each of the parent terminals or indirectly from the respective child terminals using other child terminals as relay terminals.
In the communication network employing the parent terminals and the child terminals, one of the parent terminals builds communication routes between itself and the child terminals, whereby a plurality of child terminals is set under one parent terminal. In the parent terminals, the information (route information) on the communication routes built between the parent terminals and the child terminals under the parent terminals needs to be held in a memory as, e.g., a route table. However, depending on the capacity of the memory for storing the route table, there is an upper limit in the number of route information capable of being stored in the memory (record number). In other words, there is an upper limit in the number of child terminals capable of building communication routes with one parent terminal.
In the conventional communication network in which communication routes are built pursuant to the communication quality of the communication routes and the hop count, however, it is likely that the child terminals may concentrate on a specific parent terminal. If the residual

record number of the specific parent terminal on which the child terminals concentrate becomes equal to 0, a child terminal newly installed near the specific parent terminal needs to build a communication route with a distant parent terminal. However, the communication route built between the child terminal and the distant parent terminal may sometimes become unusable due to the deteriorated communication quality or the increased hop count. This may possibly generate an isolated child terminal incapable of making communication with any parent terminal.
As stated above, the conventional communication network in which the communication routes are built pursuant to the communication quality of the communication routes and the hop count suffers from a problem in that there may be generated a child terminal incapable of making communication with any parent terminal.
Summary of the Invention
In view of the above, the present invention provides a multi-hop communication system by which the generation of a child terminal incapable of making communication with any parent terminal can be suppressed with no likelihood of concentration of a plurality of child terminals under a specific parent terminal.
According to a first embodiment of the present

invention, there is provided a multi-hop communication system, including:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, the child terminals configured to build communication routes with one of the parent terminals,
wherein each of the child terminals is configured to decide one of the parent terminals with which the child terminals build the communication routes, based on a residual number of communication routes capable of being built between each of the parent terminals and the child terminals, a hop count of each of communication routes existing between the child terminals and each of the parent terminals and a communication quality of each of the communication routes existing between the child terminals and each of the parent terminals.
According to a second embodiment of the present invention, there is provided a multi-hop communication system, including:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, the child terminals configured to build communication routes with one of the parent terminals,
wherein each of the parent terminals is configured to send a hello packet which contains residual record number information indicating a residual number of communication

routes capable of being built between each of the parent terminals and the child terminals and which notifies existence of each of the parent terminals,
each of the child terminals building the communication routes with one of the parent terminals is configured to send a hello packet which contains information on a residual record number of each of the parent terminals building communication routes with the child terminals and which notifies existence of the child terminals, and
each of the child terminals not building communication routes with one of the parent terminals is configured, upon receiving a plurality of hello packets within a specified time, to calculate a communication cost for the parent terminal serving as a source of the hello packet and a communication cost for the parent terminal with which the child terminals serving as sources of the hello packet builds the communication routes, based on a residual record number of each of the parent terminals, a hop count of each of the communication routes existing between the child terminals and the parent terminals and a communication quality of each of the communication routes existing between the child terminals and the parent terminals, and to build a communication route with the parent terminal which remains lowest in the communication cost.
In the present invention, each of the child terminals not building communication routes with one of the parent

terminals is configured, upon receiving a plurality of hello packets within a specified time, to select one parent terminal in which a hop count of each of the communication routes existing between the parent terminal and the child terminals is equal to or smaller than a specified threshold value, from the parent terminal serving as a source of the hello packet and the parent terminal with which the child terminals serving as sources of the hello packet builds the communication routes, to calculate a communication cost for the selected parent terminal based on the residual record number of each of the parent terminals and the communication quality of each of the communication routes existing between the child terminals and the parent terminals, and to build a communication route between each of the child terminals and the parent terminal which remains lowest in the communication cost. The specified threshold value of the hop count is set with respect to each of the parent terminals to become larger as the residual record number of each of the parent terminals grows larger.
Preferably, each of the child terminals not building communication routes with one of the parent terminals is configured, upon receiving a plurality of hello packets within a specified time, to select one parent terminal in which a communication quality level of each of the communication routes existing between the parent terminal and the child terminals is equal to or higher than a

specified threshold value, from the parent terminal serving as a source of the hello packet and the parent terminal with which the child terminals serving as sources of the hello packet builds the communication routes, to calculate a communication cost for the selected parent terminal based on the residual record number of each of the parent terminals and the hop count of each of the communication routes existing between the child terminals and the parent terminals, and to build a communication route between each of the child terminals and the parent terminal which remains lowest in the communication cost. The specified threshold value of the communication quality level is set with respect to each of the parent terminals to become smaller as the residual record number of each of the parent terminals grows larger.
In addition, each of the child terminals not building communication routes with one of the parent terminals is configured, upon receiving a plurality of hello packets within a specified time, to calculate a communication cost for the parent terminal serving as a source of the hello packet and a communication cost for the parent terminal with which the child terminals serving as sources of the hello packet builds the communication routes, by giving weights to the residual record number of each of the parent terminals, the hop count of each of the communication routes existing between the child terminals and the parent terminals and the

communication quality of each of the communication routes existing between the child terminals and the parent terminals. The weight given to the residual record number is changed in such a direction that the communication cost becomes higher as the hop count grows larger.
According to a sixth embodiment of the present invention, there is provided a multi-hop communication system, including:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, the child terminals configured to build communication routes with one of the parent terminals, the parent terminals including a first parent terminal and a second parent terminal,
wherein each of the child terminals building a communication route with the first parent terminal is configured, if a residual number of the communication routes capable of being built between the first parent terminal and the child terminals becomes equal to or smaller than a first threshold value, to build a communication route between each of the child terminals and the second parent terminal in which the residual number of the communication routes capable of being built between the second parent terminal and the child terminals exceeds a second threshold value.
According to a seventh embodiment of the present invention, there is provided a multi-hop communication

system, including:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, the child terminals configured to build communication routes with one of the parent terminals, the parent terminals including a first parent terminal and a second parent terminal,
wherein each of the parent terminals is configured to send a hello packet which contains residual record number information indicating a residual number of communication routes capable of being built between each of the parent terminals and the child terminals and which notifies existence of each of the parent terminals,
each of the child terminals building the communication routes with one of the parent terminals is configured to send a hello packet which contains information on a residual record number of each of the parent terminals building communication routes with the child terminals and which notifies existence of the child terminals,
each of the child terminals not building communication routes with one of the parent terminals is configured, upon receiving a plurality of hello packets within a specified time, to calculate a communication cost for the parent terminal serving as a source of the hello packet and a communication cost for the parent terminal with which the child terminals serving as sources of the hello packet

builds the communication routes, based on at least a communication quality of each of the communication routes existing between the child terminals and the parent terminals, and to build a communication route with the parent terminal which remains lowest in the communication cost, and
each of the child terminals building a communication route with the first parent terminal is configured to build a communication route with the second parent terminal, if the residual record number of the first parent terminal pursuant to the hello packet received from the first parent terminal or the child terminals building communication routes with the first parent terminal becomes equal to or smaller than a first threshold value and if the residual record number of the second parent terminal pursuant to the hello packet received from the second parent terminal or the child terminals building communication routes with the second parent terminal exceeds a second threshold value.
Each of the child terminals building the communication route with the first parent terminal is configured, if each of the child terminals serves as a relay terminal between other child terminals and the parent terminals, to continuously build the communication route with the first parent terminal even when the residual record number of the first parent terminal becomes equal to or smaller than the first threshold value.

Preferably, each of the child terminals building the communication route with the first parent terminal is configured, if each of the child terminals serves as a relay terminal between other child terminals and the parent terminals, to increase the first threshold value as the number of other child terminals using each of the child terminals as a relay terminal grows smaller.
Moreover, each of the child terminals building the communication route with the first parent terminal is configured, if each of the child terminals serves as a relay terminal between other child terminals and the parent terminals, to reduce the second threshold value as the number of other child terminals using each of the child terminals as a relay terminal grows smaller.
Each of the child terminals building the communication route with the first parent terminal is configured, if the residual record number of the first parent terminal becomes equal to or smaller than the first threshold value and if the residual record number of the second parent terminal exceeds the second threshold value, to send a parent terminal change notifying packet to the first parent terminal and to build a communication route with the second parent terminal. The first parent terminal which has received the parent terminal change notifying packet is configured to increment the residual record number of the first parent terminal.

Preferably, each of the child terminals building the communication route with the first parent terminal is configured, if the residual record number of the first parent terminal becomes equal to or smaller than the first threshold value and if the residual record number of the second parent terminal exceeds the second threshold value, to send a parent terminal change feasibility packet to the first parent terminal. The first parent terminal which has received the parent terminal change feasibility packet is configured, if the residual record number of the first parent terminal is equal to or smaller than the first threshold value, to send a parent terminal change permission packet to the child terminals serving as sources of the parent terminal change feasibility packet and to increment the residual record number of the first parent terminal. Each of the child terminals which has received the parent terminal change permission packet is configured to build a communication route with the second parent terminal.
According to an eleventh embodiment of the present invention, there is provided a multi-hop communication system, including:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, the child terminals configured to build communication routes with one of the parent terminals,
wherein each of the parent terminals is configured, if

a residual number of communication routes capable of being built between the parent terminals and the child terminals becomes equal to or smaller than a specified threshold value and if a communication route is built between the parent terminals and new child terminals, to ask a communication route change to the child terminals which is building communication routes with each of the parent terminals and which is capable of building communication routes with other parent terminals.
According to a twelfth embodiment of the present invention, there is provided a multi-hop communication system, including:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, the child terminals configured to build communication routes with one of the parent terminals,
wherein each of the parent terminals is configured to send a hello packet which contains residual record number information indicating a residual number of communication routes capable of being built between each of the parent terminals and the child terminals and which notifies existence of each of the parent terminals,
each of the child terminals building the communication routes with one of the parent terminals is configured to send a hello packet which contains information on a residual record number of each of the parent terminals building

communication routes with the child terminals and which notifies existence of the child terminals,
each of the child terminals not building communication routes with one of the parent terminals is configured to determine whether the parent terminal having a residual record number exceeding a specified threshold value exists among the parent terminal serving as a source of the hello packet and the parent terminal with which the child terminals serving as sources of the hello packet build the communication routes, to build a communication route with one of the parent terminals having the residual record number exceeding the threshold value if there exists the parent terminals having the residual record number exceeding the threshold value, and to send a joining failure notifying packet to one or more parent terminals among the parent terminal serving as a source of the hello packet and the parent terminal with which the child terminals serving as sources of the hello packet build the communication routes if the parent terminals having the residual record number exceeding the threshold value do not exist,
each of the parent terminals which has received the joining failure notifying packet is configured to send a parent terminal change request packet to the child terminals building communication routes with each of the parent terminals, and
each of the child terminals which has received the

parent terminal change request packet and which has received the hello packet containing the residual record number information of the parent terminals not building communication routes with the child terminals is configured to build a communication route with one of the parent terminals having the residual record number exceeding the threshold value among the parent terminals not building communication routes with the child terminals.
Each of the child terminals building the communication routes with the parent terminals is configured to receive the hello packet containing the residual record number information of the parent terminal not building communication routes with the child terminals and to send a topology notifying packet containing the residual record number information included in the hello packet to the parent terminal building the communication routes with the child terminals. The parent terminal which has received the topology notifying packet is configured, upon receiving the joining failure notifying packet, to identify, based on the topology notifying packet, the child terminals which have received the hello packet from the parent terminals having the residual record number exceeding the threshold value and to send the parent terminal change request packet to only the identified child 'terminals corresponding in number to the child terminals serving as sources of the joining failure notifying packet.

Preferably, each of the child terminals building the communication routes with the parent terminals is configured to receive the hello packet containing the residual record number information of the parent terminal not building communication routes with the child terminals and to send a topology notifying packet containing the residual record number information included in the hello packet to the parent terminal building the communication routes with the child terminals. The parent terminal which has received the topology notifying packet is configured, upon receiving the joining failure notifying packet, to identify, based on the topology notifying packet, the child terminals which have received the hello packet from the parent terminals having the residual record number exceeding the threshold value and to send the parent terminal change request packet to the identified child terminals with a priority given to the child terminal which is larger in the number of the parent terminals whose residual record number exceeds the threshold value.
In addition, the parent terminal which has received the joining failure notifying packet is configured to send the parent terminal change request packet to the child terminal which does not serve as a relay terminal between the parent terminal and other child terminals, among the child terminals building communication routes with the parent terminal.

The parent terminal which has received the joining failure notifying packet is configured to send the parent terminal change request packet to the child terminals building communication routes with the parent terminal with a priority given to the child terminal which is smaller in the number of other child terminals using the child terminal as a relay terminal.
According to the present invention, there is provided a child terminal for use in the multi-hop communication system recited above.
According to the present invention, there is provided a parent terminal for use in the multi-hop communication system recited above.
As set forth above, the present invention provides an effect by which the generation of a child terminal incapable of making communication with any parent terminal can be suppressed with no likelihood of concentration of a plurality of child terminals under a specific parent terminal.
Brief Description of the Drawings
The objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings.

Fig. 1 is a schematic diagram showing a wireless network made up of a multi-hop communication system according the present invention.
Fig. 2 is a block diagram showing the configuration of a communication terminal employed in the multi-hop communication system.
Fig. 3 is a table diagram illustrating the configuration of a communication-capable-terminal management table used in the multi-hop communication system.
Figs. 4A and 4B are table diagrams illustrating the configurations of communication route tables used in the multi-hop communication system.
Fig. 5 is a table diagram illustrating the configuration of a residual record number table used in the multi-hop communication system.
Fig. 6 is a sequence diagram illustrating a communication sequence of the multi-hop communication system.
Figs. 7A and 7B are diagrams illustrating the formats of communication packets used in the multi-hop communication system.
Fig. 8 is a sequence diagram illustrating the configuration of a residual record number table according to a fifth embodiment.
Fig. 9 is a view illustrating the resetting of registration-capable record number in the residual-record-

number table.
Fig. 10 is a table diagram illustrating the configuration of a record-cost table according to sixth and eleventh embodiments.
Fig. 11 is a table diagram illustrating the configuration of a subordinate terminal table according to an eighth embodiment.
Fig. 12 is a sequence diagram showing a communication sequence according to a tenth embodiment.
Fig. 13 is a table diagram illustrating the configuration of a child topology table according to a twelfth embodiment.
Fig. 14 is a sequence diagram showing a communication sequence according to the twelfth embodiment.
Fig. 15 is a table diagram illustrating the configuration of a child topology table according to a thirteenth embodiment.
Fig. 16 is a table diagram illustrating the configuration of a child topology table according to a fourteenth embodiment.
Fig. 17 is a sequence diagram showing a communication sequence according to a sixteenth embodiment.
Detailed Description of the Preferred Embodiments
Certain embodiments of the present invention will now

be described in detail with reference to the accompanying drawings which form a part of the subject specification. Throughout the drawings, identical or similar components will be designated by like reference symbols and will not be described repeatedly.
(First Embodiment)
Fig. 1 is a schematic diagram of a wireless network made up of a multi-hop communication system according to the present embodiment. The wireless network is used in a dwelling unit group which includes a plurality of dwelling units X. Communication terminals 1 serving as parents are installed within the dwelling unit group on a region-by-region basis (e.g., every 500 m in radius). Communication terminals 2 serving as children are installed in the respective dwelling units X. In the following description, the communication terminals 1 serving as parents will be referred to as "parent terminals 1" and the communication terminals 2 serving as children will be referred to as "child terminals 2". If there is a need to individually identify the parent terminals 1, reference symbols 1-1, 1-2, 1-3, etc. will be used to designate the parent terminals 1. If there is a need to individually identify the child terminals 2, reference symbols 2-1, 2-2, 2-3, etc. will be used to designate the child terminals 2.
The child terminals 2 serve to wirelessly send the specified information on the respective dwelling units X to

one of the parent terminals 1. The parent terminals 1 serve to wirelessly acquire the specified information on the respective dwelling units X from the child terminals 2 and to send the acquired specified information to an upper management device (not shown) through the use of optical fiber links or the like. For example, the parent terminals 1 may acquire meter reading information, such as an electric power consumption amount, a gas consumption amount, and a tap water consumption amount in the respective dwelling units X, from the child terminals 2. This makes it possible to form a remote meter reading system. In addition, the parent terminals 1 may send and receive pre-set specified information to and from the child terminals 2. This makes it possible to form a remote monitoring system for monitoring the conditions of devices arranged within the respective dwelling units X or a remote control system for controlling the conditions of devices arranged within the respective dwelling units X.
In the present wireless network, the parent terminals 1 and the child terminals 2 send and receive wireless signals to and from each other by virtue of multi-hop communication. More specifically, in the present wireless network, communication is directly or indirectly performed between the parent terminals 1 and the child terminals 2. The child terminals 2 incapable of making direct communication with the parent terminals 1 perform

communication with the parent terminals 1 by causing other child terminals 2 existing within a communication distance to sequentially relay communication packets.
Fig. 2 is a block diagram showing one of communication terminals A. In the present embodiment, the same communication terminals A are used as the parent terminals 1 and the child terminals 2. For example, the communication terminals A serve as the parent terminals 1 if they are set to become parents by a setting means such as a jumper switch or a changeover switch. The communication terminals A serve as the child terminals 2 if they are set to become children. In the following description, the parent terminals 1 and the child terminals 2 will be referred to as "communication terminals A" if there is no need to distinguish the parent terminals 1 and the child terminals 2 from each other.
Each of the communication terminals A includes a memory unit 10, a control unit 20, and a wireless communication interface unit 30.
The memory unit 10 is formed of a nonvolatile memory such as a ROM or the like, a rewritable nonvolatile memory such as an EEPROM or the like, or a volatile memory such as a RAM or the like. The memory unit 10 includes a table storage unit 101 for storing link information on a communication route and adjacent terminals capable of making communication (the parent terminals 1 or the child terminals 2 capable of making direct communication). The memory unit

10 also stores various kinds of programs, such as a control program for operating the communication terminals A or other programs and various kinds of information required in executing the respective programs.
In the present embodiment, unique terminal IDs are allotted to the parent terminals 1 and the child terminals 2. In the memory unit 10 of each of the communication terminals A, the ID allotted to the subject terminal is also stored. In the present embodiment, terminal IDs VM1", "M2", "M3", etc. are allotted in advance to the parent terminals 1-1, 1-2, 1-3, etc. If the below-mentioned communication route is built, terminal IDs "Tl", "T2", "T3", etc. are allotted to the child terminals 2-1, 2-2, 2-3, etc. by the parent terminals 1.
A device ID such as a serial number (production number) or a MAC address is allotted in advance to each of the communication terminals A. The device ID is stored in advance in the memory unit 10 of each of the communication terminals A. By adding the device ID, communication control is performed with respect to the communication packets sent and received by the communication terminals A.
A communication-capable-terminal management table TBI shown in Fig. 3 is stored in the table storage unit 101 of each of the parent terminals 1 and the child terminals 2. A communication route table TB21 shown in Fig. 4A is stored in the table storage unit 101 of each of the parent terminals

1. A communication route table TB22 shown in Fig. 4B and a residual record number table TB3 shown in Fig. 5 are stored in the table storage unit 101 of each of the child terminals 2.
As shown in Fig. 3, the communication-capable-terminal management table TBI stores, in a tabular form, the information (communication-capable-terminal information) on the communication terminals A (adjacent terminals) capable of making direct communication with specific communication terminals A without going through other communication terminals A. More specifically, the communication-capable-terminal management table TBI is provided with individual fields for the adjacent terminal ID, the terminal kind, the receiving link communication quality, the sending link communication quality, and the link communication quality.
In the communication-capable-terminal management table TBI, the adjacent terminal IDs denote terminal IDs allotted to the communication terminals A (hereinafter referred to as "adjacent terminals A") capable of making direct communication with the subject terminal. The terminal kind indicates the kind of the adjacent terminals A ("parent" indicating the parent terminals 1 and "child" indicating the child terminals 2). The receiving link communication quality indicates the communication quality of communication links extending from the adjacent terminals A to the subject terminal. The sending link communication quality indicates

the communication quality of communication links extending from the subject terminal to the adjacent terminals A. The link communication quality indicates the communication quality of communication links between the adjacent terminals A and the subject terminal.
For example, a communication quality value SQ is used to indicate the link communication quality in a communication link between two communication terminals A and A capable of making direct communication with each other. The communication quality value SQ is indicated by integer values of ten steps or twenty steps which grow smaller as the received signal intensity between two communication terminals A and A capable of making direct communication with each other becomes larger. In other words, the attenuation of communication packets becomes smaller and the communication state becomes better as the integer values of the communication quality value SQ grow smaller.
Since noise levels are different in the communication packet receiving places, the bidirectional communication qualities in the communication link differ from one another. The bidirectional communication qualities in the communication link include the receiving link communication quality and the sending link communication quality. The receiving link communication quality is the received signal intensity in the link between two communication terminals A and A capable of making direct communication with each

other, when the subject terminal A receives communication packets from the other communication terminal A. The sending link communication quality is the received signal intensity in the link between two communication terminals A and A capable of making direct communication with each other, when the subject terminal A sends communication packets to the other communication terminals A and when other communication terminal A receives the communication packets.
The communication-capable-terminal management table TBI is provided with individual fields for the receiving link communication quality and the sending link communication quality of the communication terminals A. With a view to assure communication certainty (reliability) when communication is performed between the communication terminals A and A, the receiving link communication quality or the sending link communication quality, whichever is poor in the communication state {whichever is larger in the value of the link communication quality), is employed as the link communication quality between two communication terminals A and A.
The sum of the link communication qualities of the respective communication links making up the communication routes between the parent terminals 1 and the child terminals 2 is employed as the below-mentioned route communication quality which is the evaluation of the

communication quality of the communication routes between the parent terminals 1 and the child terminals 2.
While the aforementioned communication quality value SQ is associated with the received signal intensity, it may be possible to calculate the communication quality value SQ in association with other factors such as a signal-noise ratio, an error vector magnitude, a bit error rate, and a packet error rate, in place of the received signal intensity.
Next, the communication routes between the parent terminals 1 and the child terminals 2 are formed of one or more communication links. The communication . route table TB21 (see Fig. 4A) held by each of the parent terminals 1 stores, in a tabular form, the information (communication route information) on the communication routes between the parent terminals 1 and the child terminals 2 remaining under the parent terminals 1. More specifically, the communication route table TB21 is provided with individual fields for the terminal IDs, the route communication quality, the hop count, and the hop destination.
In the communication route table TB21 held by each of the parent terminals 1, the terminal IDs are the terminal IDs allotted to the child terminals 2 that build communication routes with respect to each of the parent terminals 1. The route communication quality indicates the communication quality in the communication routes leading to

the child terminals 2 registered on the terminal ID field. The hop count indicates the hop count in the communication routes leading to the child terminals 2 registered on the terminal ID field. The hop destination indicates the communication terminals A of the destinations of the respective hops in the communication routes leading to the child terminals 2 registered on the terminal ID field.
In the communication route table TB21, the row unit composed of a plurality of fields is a record. The number of rows is called a record number (In other words, the row number in the communication route table TB21 is the record number). The upper limit of the record number is decided by the memory capacity of the table storage unit 101. There is also an upper limit in the number of the child terminals 2 capable of building communication routes with one parent terminal 1.
In the following description, the value obtained by subtracting the number of currently registered records from the upper value of the number of records capable of being registered on the communication route table TB21 of each of the parent terminals 1 will be referred to as "residual record number". The residual record number corresponds to the residual number of the communication routes that can be built between each of the parent terminals 1 and the child terminals 2. In other words, the number of the child terminals 2 capable of building communication routes with

the parent terminal 1 becomes larger as the residual record number grows larger. The number of the child terminals 2 capable of building communication routes with the parent terminal 1 becomes smaller as the residual record number grows smaller.
Next, the communication route table TB22 (see Fig. 4B) held by each of the child terminals 2 stores, in a tabular form, the information (communication route information) on the communication routes between the child terminals 2 and the parent terminals 1 capable of making communication with the child terminals 2. More specifically, the communication route table TB22 is provided with individual fields for the terminal IDs, the route communication quality, the hop count, and the hop destination.
In the communication route table TB22 held by each of the child terminals 2, the terminal IDs are the terminal IDs allotted to the parent terminals 1 capable of making communication with the child terminals 2. The route communication quality indicates the communication quality in the communication routes leading to the parent terminals 1 registered on the terminal ID field. The hop count indicates the hop count in the communication routes leading to the parent terminals 1 registered on the terminal ID field. The hop destination indicates the communication terminals A of the destinations of the respective hops in the communication routes leading to the parent terminals 1

registered on the terminal ID field.
In the communication route tables TB21 and TB22, the hop count is the number of the communication terminals A existing in the communication route extending from the subject terminal to a destination terminal. For example, the hop count is three in case of the communication route where the child terminal 2-4 makes communication with the parent terminal 1 through the child terminals 2-3 and 2-1. The terminal IDs of the communication terminals A arranged between the subject terminal and a destination terminal are registered on the hop destination field in the arrangement order. The terminal IDs of the communication terminals A registered on the terminal ID field are finally registered on the hop destination field.
Next, the residual record number table TB3 (see Fig. 5) held by each of the child terminals 2 stores, in a tabular form, the information on the residual record number of the parent terminal 1. More specifically, the residual record number table TB3 is provided with individual fields for the terminal IDs and the residual record number.
In the residual record number table TB3 held by each of the child terminals 2, the terminal IDs are terminal IDs allotted to the parent terminals 1. The residual record number indicates the residual record number of the parent terminals 1 registered on the terminal ID field.
Next, the wireless communication interface unit 30 is

a communication interface circuit for making communication with other communication terminals A using a wireless signal.
The control unit 20 is a device that controls the respective parts of the communication terminals A to thereby control the overall operations of the communication terminals A. The control unit 20 is formed of, e.g., a microprocessor and peripheral circuits thereof. The control unit 20 includes a table processing unit 201, a communication processing unit 2 02, and a sending timer unit 2 03. The control unit 20 performs a communication route building process for directly or indirectly building communication routes between the child terminals 2 and the parent terminals 1.
The table processing unit 201 manages the registration contents of the respective tables stored in the table storage unit 101 of the memory unit 10. The communication processing unit 202 sends and receives the communication packets to and from other communication terminals A through the use of the wireless communication interface unit 30. By performing the below-mentioned operation, the communication processing unit 202 performs a communication route building process for building communication routes between the parent terminals 1 and the child terminals 2. The sending timer unit 2 03 is a clock means for measuring the lapse of a specified time. The sending timer unit 203 notifies the

communication processing unit 202 of arrival of the sending timing of various kinds of communication packets at a specified time interval.
With reference to a sequence shown in Fig. 6, description will now be made on how to build a communication route in the present wireless network.
If the communication terminals A are started up, the sending timer unit 203 of the control unit 20 of each of the communication terminals A begins to measure the time in order to send a hello packet (hereinafter referred to as "H packet") . Upon the time-up, the sending timer unit 203 notifies the communication processing unit 202 to the effect that it is now the sending timing of the H packet. Responsive to this notification, the communication processing unit 202 sends the H packet to the wireless network through the use of multi-destination communication.
The H packet is a communication packet by which each of the communication terminals A notifies other communication terminals A of existence of the subject terminal. Fig. 7A illustrates the format of the H packet which includes a source terminal ID section, a destination terminal ID section, an operation code section, a terminal kind section, a communication route section, and a residual record number section.
The source terminal ID section of the H packet holds +the terminal ID of the communication terminal A which has

sent the H packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes the destination of the H packet. In case of the H packet, a multi-destination communication code "BC" such as broad cast or the like is held in the destination terminal ID section. The operation code section holds the code of the H packet. The terminal kind section holds the information for identifying which of the parent terminal 1 and the child terminal 2 is the communication terminal A that has sent the H packet.
The communication route section of the H packet holds the communication route information indicating the communication routes from the child terminals 2 to the parent terminal 1 and the route quality information indicating the communication qualities of the communication routes. The communication route information is indicated in such a way as to sequentially enumerate the terminal IDs of the communication terminals A arranged in the communication route extending from the child terminals 2 to the parent terminal 1. The route quality information is indicated by the sum of the link communication qualities within the communication routes. The sum of the link communication qualities will be called a route communication quality.
For example, it is assumed that the child terminal 2-4 builds a communication route between itself and the parent terminal 1 via the child terminals 2-3 and 2-1 and further

that the route communication quality is equal to 17. In this case, the communication route section of the H packet sent by the child terminal 2-4 holds "T4-T3-T1->M1:17". The hop count of the communication route is equal to "3". The communication route section of the H packet sent by the parent terminal 1 remains "null" (or data-free), in which case the hop count is equivalent to "0" and the route communication quality is equivalent to "0".
If the communication terminal A which has sent the H packet is the parent terminal 1, the residual record number section of the H packet holds the residual record number of the parent terminal 1 which is the source of the H packet. If the communication terminal A which has sent the H packet is the child terminal 2, the residual record number section of the H packet holds the residual record number of the parent terminal 1 with which the child terminal 2 as the source of the H packet builds a communication route.
In order to reduce the communication traffic of the wireless network, the H packet is sent by each of the communication terminals A in case where the communication terminal A is set as the parent terminal 1 and in case where the communication terminal A is the child terminal 2 building a communication route leading to the parent terminal 1. The H packet sending process is performed at a specified time interval after the startup.
With regard to the communication packets other than

the H packet, upon receiving the communication packets, the communication processing unit 2 02 of the communication terminal A calculates the receiving link communication quality and discriminates the kind of the received communication packets by referring to the operation code thereof.
More specifically, as shown in Fig. 6, the parent terminals 1-1 and 1-2 send H packets (SI and S2). In the child terminal 2-1 which has received the H packet sent by the parent terminal 1-1, the communication processing unit 2 02 determines the kind of the communication terminal A which has sent the H packet, by referring to the terminal kind section of the received H packet. If it is determined that the communication terminal A which has sent the H packet is the parent terminal 1, the communication processing unit 202 determines that the child terminal 2-1 can make direct communication with the parent terminal 1-1 and further that there exists a communication route of "child terminal 2-1 -► parent terminal 1-1". Then, the table processing unit 201 of the child terminal 2-1 takes out the terminal ID and the terminal kind by referring to the source terminal ID section and the terminal kind section of the received H packet. Then, a new record is provided in the communication-capable-terminal management table TBI of the table storage unit 101. The terminal ID and the terminal kind thus taken out and the receiving link communication

quality thus calculated are registered on the adjacent terminal ID section, the terminal kind section, and the receiving link communication quality section, respectively. Then, the table processing unit 201 of the child terminal 2-1 registers the receiving link communication quality of the parent terminal 1-1, as a tentative link communication quality, on the link communication quality section of the communication-capable-terminal management table TBI.
In this regard, the reason for registering the receiving link communication quality as a tentative link communication quality is that the sending link communication quality is unknown at the present time while the link communication quality becomes the receiving link communication quality or the sending link communication quality, whichever is poor (whichever is larger in numerical value).
In the child terminal 2-1 which has received the H packet sent by the parent terminal 1-1, the communication processing unit 202 acquires the residual record number of the parent terminal 1-1 which has sent the H packet, by referring to the residual record number section of the received H packet. Then, the table processing unit 201 of the child terminal 2-1 registers the residual record number of the parent terminal 1-1 on the residual record number table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of the parent terminal 1-

If the child terminal 2-1 further receives the H packet from the parent terminal l-!2 within a specified time period after the child terminal 2-1 has received the H packet from the parent terminal 1-1, a new record is provided in the communication-capable-terminal management table TBI in the same manner as stated above. Then, the terminal ID and the terminal kind taken out from the H packet of the parent terminal 1-2 and the receiving link communication quality calculated as above are respectively registered on the adjacent terminal ID section, the terminal kind section, and the receiving link communication quality section of the communication-capable-terminal management table TBI. Then, the table processing unit 201 of the child terminal 2-1 registers the receiving link communication quality of the parent terminal 1-2, as a tentative link communication quality, on the link communication quality section of the communication-capable-terminal management table TBI.
In the child terminal 2-1 which has received the H packet sent by the parent terminal 1-2, the communication processing unit 202 acquires the residual record number of the parent terminal 1-2 which has sent the H packet, by referring to the residual record number section of the received H packet. Then, the table processing unit 201 of the child terminal 2-1 registers the residual record number

of the parent terminal 1-2 on the residual record number table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of the parent terminal 1-2.
Then, in the child terminal 2-1 which has directly-received the H packets from the parent terminals 1-1 and 1-2, the communication processing unit 2 02 determines whether a terminal ID is allotted to the subject terminal, by-referring to the memory unit 10. If the terminal ID is not allotted to the subject terminal, the child terminal 2-1 performs the following process to decide the parent terminal 1 which is asked to allot the terminal ID (namely, which is used to build a communication route).
First, in the child terminal 2-1 which has directly received the H packets from the parent terminals 1-1 and 1-2, the communication processing unit 202 calculates communication costs of individual communication routes existing between the child terminal 2-1 and the parent terminals 1-1 and 1-2.
In the child terminals 2, the communication cost of the communication route between the child terminal 2 and the parent terminal 1 is calculated pursuant to the route communication quality of the communication route (the sum of the link communication qualities) , the hop count of the communication route, and the residual record number of the parent terminal 1.

The calculation formula of the communication cost is represented by: communication cost = Ka * [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] + Kb * [the hop count pursuant to the communication route information included in the communication route section of the H packet + 1] + Kc * [1/the residual record number of the residual record number table TB3] . Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kb is a weighting coefficient of the hop count of the communication route. Kc is a weighting coefficient of the residual record number of the parent terminal 1.
Since the communication quality value SQ is used as the route communication quality, the route communication quality becomes better as the value thereof grows smaller. The hop count becomes better as the value thereof grows smaller. The residual record number becomes better as the value thereof grows larger. For that reason, in view of the relationship between the magnitude of the respective elements (the route communication quality, the hop count and the residual record number) and the level of the communication cost, the weighting coefficient Kc is multiplied by the inverse number of the residual record number.

The communication cost calculated as above is the communication quality level in the communication route between the parent terminal 1 and the child terminal 2. The communication quality level in the communication route becomes higher as the value of the communication cost grows smaller.
In this regard, the communication route section of the H packet sent by the parent terminal 1 becomes "null" (or data-free) . The hop count is equivalent to "0" and the route communication quality is equivalent to "0". Accordingly, the communication cost between the parent terminal 1 and the child terminal 2 which has directly received the H packet from the parent terminal 1 is represented by: communication cost = Ka x [the link communication quality of the communication-capable-terminal management table TBI] + Kb * [1] + Kc * [1/the residual . record number of the residual record number table TB3].
Then, the communication processing unit 202 of the child terminal 2-1 calculates the communication cost with respect to the parent te rminals 1—1 and 1—2 which are the sources of the H packets. Then, the communication processing unit 2 02 sends an address request packet asking allocation of the terminal ID to the parent terminal 1-1 or 1-2, whichever is lower in the communication cost (S3). In the present embodiment, it is assumed that the communication cost of the parent terminal 1-1 is lower than the

communication cost of the parent terminal 1-2 and further that the child terminal 2-1 sends the address request packet the parent terminal 1-1. If the child terminal 2-1 receives the H packet from only one parent terminal 1, the child terminal 2-1 asks the parent terminal 1 to allot a terminal ID.
In the configuration described above, the child terminal 2 selects the parent terminal 1 as the destination of the address request packet based on the tentative link communication quality. However, the parent terminal 1 as the destination of the address request packet may be decided by the following method. First, upon receiving H packets from a plurality of parent terminals 1, the child terminal 2 selects two or more parent terminals 1, which are higher in the tentative link communication quality, and investigates bidirectional route communication qualities between the child terminal 2 and the parent terminals 1 thus selected. Then, the child terminal 2 sends an address request packet to the parent terminal 1 which is lowest in the communication cost found from the bidirectional route communication qualities.
As set forth above, if the child terminal 2 receives H packets from a plurality of parent terminals 1, the child terminal 2 calculates the communication costs between the child terminal 2 and the respective parent terminals 1 based on the communication qualities of the communication routes,

the hop count of the communication routes, and the residual record number of the parent terminal 1. Then, the child terminal 2 builds a communication route between itself and one of the parent terminals 1 which is lowest in the communication cost (namely, the parent terminal 1 which is highest in the communication quality level of the communication route).
In the present embodiment, the communication cost is calculated in view of the residual record number of the parent terminal 1 as well as the route communication quality of the communication route and the hop count of the communication route. In other words, the communication cost is increased if the residual record number of the parent terminal 1 is small. The communication cost is decreased if the residual record number of the parent terminal 1 is large. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
As shown in Fig. 7B, the address request packet sent by the child terminal 2 includes, e.g., a source terminal ID section, a destination terminal ID section, a sending communication route section, an operation code section, a distribution address section, an adjacent terminal section, and a link communication quality section.
The source terminal ID section of the address request

packet holds the terminal ID of the communication terminal A which has sent the address request packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes a destination of the address request packet. In case of the child terminal 2 whose terminal ID is not set as yet, a "reserved terminal ID" is set in the source terminal ID section.
The sending communication route section holds the information on the communication route used in sending the address request packet. This information is indicated in such a way as to sequentially enumerate terminal IDs along the communication route extending from the child terminal 2 as a source to the parent terminal 1 as a destination. The communication terminal A which has received the address request packet determines the communication terminal A as a destination (a forwarding destination or a relaying destination) of the address request packet based on the sending communication route section of the address request packet thus received. The communication terminal A which has received the address request packet forwards (or relays) the address request packet to the destination communication terminal A.
The operation code section holds an address request code.
The distribution address section holds "null" (or vacant data) in case of the address request packet.

In case of the address request packet, the adjacent terminal section holds the terminal ID of the source communication terminal A of the H packet received by the child terminal 2.
The link communication quality section holds the link communication quality between the child terminal 2 and the communication terminal A as a source of the H packet received by the child terminal 2. In case of the child terminal 2 whose terminal ID is not set as yet, the link communication quality section of the address request packet holds the receiving link communication quality as a tentative link communication quality.
The address request packet sent from the child terminal 2-1 to the parent terminal 1-1 will be described in more detail. A "reserved terminal ID" is held in the source terminal ID section. "Ml" is held in the destination terminal ID section. "Reserved terminal ID —» Ml" is held in the sending communication route section. In addition, an "address request code" is held in the operation code section. "Null" is held in the distribution address section. "Ml" is held in the adjacent terminal section. For example, "4" is held in the link communication quality section.
In the parent terminal 1-1 which has received the address request packet sent by the child terminal 2-1, the communication processing unit 202 allots a terminal ID to

the child terminal 2-1 which has sent the address request packet. In the present embodiment, it is assumed that a terminal ID "T1" is allotted to the child terminal 2-1.
The communication processing unit 202 of the parent terminal 1-1 takes out the adjacent terminal ID "Ml" and the link communication quality "4" included in the received address request packet. Since the terminal ID "Ml" is held in the adjacent terminal section of the address request packet, the communication processing unit 202 of the parent terminal 1-1 determines that the child terminal 2-1 allotted with the terminal ID "Tl" is capable of making direct communication with the parent terminal 1-1 and further that a communication route "Ml—►Tl" has been built between the parent terminal 1-1 and the child terminal 2-1. In the parent terminal 1-1, the link communication quality "4" held in the address request packet is a sending link
communication quality of parent terminal 1-1 — child terminal 2-1.
The communication processing unit 202 of the parent terminal 1-1 compares the receiving link communication quality calculated upon receiving the address request packet with the sending link communication quality. The link communication quality which is poor in the communication state (which is larger in the numerical value) is used as the link communication quality between the parent terminal 1-1 and the child terminal 2-1. For example, if the

receiving link communication quality calculated upon receiving the address request packet is "5", the sending link communication quality "4" and the receiving link communication quality "5" are compared with each other. Thus, the link communication quality is set equal to "5". Since the parent terminal 1-1 is capable of making direct communication with the child terminal 201 having the terminal ID "Tl", the link communication quality "5" becomes the route communication quality between the parent terminal 1-1 and the child terminal 2-1.
In the parent terminal 1-1, the table processing unit 201 registers the adjacent terminal ID "Tl" allotted to the child terminal 2-1, the terminal kind "child", the receiving link communication quality "5", the sending link communication quality "4", and the link communication quality "5" on the communicatioh-capable-terminal management table TBI. The receiving link communication quality "5" is the communication quality in the link of child terminal 2-1 —* parent terminal 1-1. The sending link communication quality "4" is the communication quality in the link of parent terminal 1-1 —► child terminal 2-1.
In the parent terminal 1-1, the table processing unit 201 registers the terminal ID "Tl" allotted to the child terminal 2-1, the route communication quality "5", the hop count "1", and the first hop WT1" on the communication route table TB21.

Then, the parent terminal 1-1 sends an address response packet to the child terminal 2-1 which has sent the address request packet (S4).
As shown in Fig. 7B, the address response packet includes, e.g., a source terminal ID section, a destination terminal ID section, a sending communication route section, an operation code section, a distribution address section, an adjacent terminal section, and a link communication quality section. In other words, the address response packet is formed of the same format as the aforementioned address request packet.
The source terminal ID section of the address response packet holds the terminal ID of the communication terminal A which has sent the address response packet. The destination terminal ID section holds the terminal ID of the communication terminal A which becomes a destination of the address response packet. In case where the address response packet is sent to the child terminal 2 whose terminal ID is not set as yet, a code "BC" for multi-destination communication such as broadcast or the like is held in the destination terminal ID section.
The sending communication route section holds the information on the communication route used in sending the address response packet. This information is indicated in such a way as to sequentially enumerate terminal IDs along the communication route extending from the source parent

terminal 1 to the destination child terminal 2. The communication terminal A which has received the address response packet determines the communication terminal A serving as a destination (a forwarding destination or a relaying destination) of the address response packet based on the sending communication route section of the received address response packet and forwards (or relays) the address response packet to the destination communication terminal A thus determined. In case of the address response packet, the terminal ID of the child terminal 2 serving as a destination is not set as yet. For that reason, a code "BC" for multi-destination communication such as broadcast or the like is registered at the end (tail-end) of the sending communication route section.
The operation code section holds an address response code.
In case of the address response packet, the distribution address section holds the terminal ID allotted to the child terminal 2 serving as a source of the address request packet.
The adjacent terminal section holds the terminal ID of the source communication terminal A of the H packet received by the child terminal 2.
The link communication quality section holds the link communication quality between the child terminal 2 and the communication terminal A serving as a source of the H packet

received by the child terminal 2.
The address response packet prepared by the parent terminal 1-1 will be described in more detail. "Ml" is held in the source terminal ID section. "BC" is held in the destination terminal ID section. "Ml-BC" is held in the sending communication route section. In addition, an "address response code" is held in the operation code section. "T1" is held in the distribution address section. "Ml" is held in the adjacent terminal section. "5" is held in the link communication quality section.
In the child terminal 2-1 which has received the address response packet sent by the parent terminal 1-1, the communication processing unit 202 takes out the distribution address "Tl", the adjacent terminal ID "Ml", and the link communication cost "5" held in the received address response packet.
Then, the child terminal 2-1 sets "Tl" as the terminal ID of the subject terminal. In the child terminal 2-1, the table processing unit 201 registers the sending link communication cost "5" on the communication-capable-terminal management table TBI in a corresponding relationship with the adjacent terminal ID "Ml".
The communication processing unit 202 of the child terminal 2-1 compares the receiving link communication quality "4" corresponding to the adjacent terminal ID "Ml" with the sending link communication quality "5" within the

communication-capable-terminal management table TBI. Then, the link communication quality which is poor in the communication state (which is larger in the numerical value) is used as the link communication quality between the parent terminal 1-1 and the child terminal 2-1. In this case, the link communication quality is renewed by the link communication quality "5".
As stated above, it is determined that the child terminal 2-1 can build the communication route of "child terminal 2-1 —► parent terminal 1-1". Thus, the link communication quality "5" becomes the route communication quality between the parent terminal 1-1 and the child terminal 2-1. In other words, the child terminal 2-1 builds a communication route of hop count "1" and route communication quality "5" in between the child terminal 2-1 and the parent terminal 1-1. Accordingly, the table processing unit 201 of the child terminal 2-1 registers the terminal ID "Ml" of the parent terminal 1, the route communication quality "5", the hop count "1" and the first hop "Ml" on the communication route table TB22.
In case where the parent terminal 1 (parent terminal 1-1) and the child terminal 2 (child terminal 2-1) can make direct communication with each other as set forth above, three steps, namely a step for the parent terminal 1 to send the H packet, a step for the child terminal 2 to send the address request packet, and a step for the parent terminal 1

to send the address response, are carried out. Then, the terminal ID (communication address) of the child terminal 2 is set by carrying out these steps. The communication route between the parent terminal 1 and the child terminal 2 capable of making direct communication with each other is built in two ways.
Next, the child terminal 2-1 building a communication route leading to the parent terminal 1-1 receives a notice from the sending timer unit 203. If the sending timing of the H packet arrives, the child terminal 2-1 sends an H packet to the wireless network by multi-destination communication (S5).
The terminal ID "Tl" of the communication terminal 2-1 which has sent the H packet is held in the source terminal ID section of the H packet sent by the child terminal 2-1. The terminal ID of the communication terminal A which becomes a destination of the communication packet is held in the destination terminal ID section. In case of the H packet, a code "BC" for multi-destination communication such as broadcast or the like is held in the destination terminal ID section. The code of the H packet is held in the operation code section. The information for identifying which of the parent terminal 1 and the child terminal 2 is the communication terminal A that has sent the H packet is held in the terminal kind section.
The communication route information indicating the

communication route extending from the child terminal 2-1 to the parent terminal 1-1 and the route quality information indicating the communication quality of the communication route are held in the communication route section of the H packet sent by the child terminal 2-1. In the present embodiment, the terminal ID of the child terminal 2-1 is "Tl". The terminal ID of the parent terminal 1-1 is "Ml". The route communication quality of the communication route built between the child terminal 2-1 and the parent terminal 1-1 is "5". Therefore, WT1->M1:5" is held in the communication route section.
The residual record number of the parent terminal 1-1 with which the child terminal 2-1 builds a communication route is held in the residual record number section of the H packet sent by the child terminal 2-1.
In the child terminal 2, e.g., the child terminal 2-2, which has received the H packet sent by the child terminal 2-1 building a communication route leading to the parent terminal 1-1, the communication processing unit 202 determines the communication terminal A which has sent the H packet, by referring to the terminal kind section of the received H packet. If it is determined that the communication terminal A which has sent the H packet is the child terminal 2, the communication processing unit 202 refers to the communication route section of the received H packet. Since the communication route "Tl—>M1:5" leading to

the parent terminal 1-1 is included in the communication route section of the H packet sent by the child terminal 2-1, the communication processing unit 202 determines that the child terminal 2-2 can make communication with the parent terminal 1-1. In other words, the communication processing unit 202 determines that it is possible to build a communication route of "child terminal 2-2 —► child terminal 2-1 -> parent terminal 1-1".
Then, the table processing unit 201 of the child terminal 2-2 takes out the terminal ID and the terminal kind by referring to the source terminal ID section and the terminal kind section of the H packet received from the child terminal 2-1. Then, a new record is provided in the communication-capable-terminal management table TBI of the table storage unit 101. The terminal ID and the terminal kind thus taken out and the receiving link communication quality calculated as above are registered on the adjacent terminal ID section, the terminal kind section, and the receiving link communication quality section, respectively. Then, the table processing unit 201 of the child terminal 2-2 registers the receiving link communication quality, as a tentative link communication quality, on the link communication quality section of the communication-capable-terminal management table TBI.
In the child terminal 2-2 which has received the H packet sent by the child terminal 2-1, the communication

processing unit 202 acquires the residual record number of the parent terminal 1-1 with which the child terminal 2-1 builds a communication route, by referring to the residual record number of the received H packet. Then, the table processing unit 201 of the child terminal 2-2 registers the residual record number of the parent terminal 1-1 on the residual record number table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of the parent terminal 1-1.
It is assumed that the child terminal 2-3 has already built a communication link with the parent terminal 1-2. It is also assumed that the child terminal 2-2 has received an H packet from the child terminal 2-3 within a specified time period after the child terminal 2-2 has received an H packet from the child terminal 2-1 (S6). In the child terminal 2-2 which has received the H packet sent by the child terminal 2-3, the communication processing unit 202 determines the communication terminal A which has sent the H packet, by referring to the terminal kind section of the received H packet. Then, if it is determined that the communication terminal A which has sent the H packet is the child terminal 2, the communication processing unit 202 refers to the communication route section of the received H packet. Since the communication route leading to the parent terminal 1-2 is included in the communication route section of the H packet sent by the child terminal 2-3, the communication

processing unit 202 determines that the child terminal 2-2 can make communication with the parent terminal 1-2. In other words, the communication processing unit 202 determines that it is possible to build a communication route of "child terminal 2-2 — child terminal 2-3 —» parent terminal 1-2".
In the same manner as mentioned above, the table processing unit 201 of the child terminal 2-2 registers the terminal ID and the terminal kind taken out from the H packet of the child terminal 2-3 and the receiving link communication quality calculated as above, on the adjacent terminal ID section, the terminal kind section, and the receiving link communication quality section of the communication-capable-terminal management table TBI. Then, the table processing unit 201 of the child terminal 2-2 registers the receiving link communication quality, as a tentative link communication quality, on the link communication quality section of the communication-capable-terminal management table TBI.
In the child terminal 2-2 which has received the H packet sent by the child terminal 2-3, the communication processing unit 202 acquires the residual record number of the parent terminal 1-2 with which the child terminal 2-3 builds a communication route, by referring to the residual record number section of the received H packet. Then, the table processing unit 201 of the child terminal 2-2

registers the residual record number of the parent terminal 1-2 on the residual record number table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of the parent terminal 1-2.
As described above, in the child terminal 2-2 which has directly received the H packet from a plurality of communication terminals A (the child terminals 2-1 and 2-3 in the present embodiment) , the communication processing unit 2 02 determines whether a terminal ID is allotted to the subject terminal, by referring to the memory unit 10. If no terminal ID is allotted to the subject terminal, the child terminal 2-2 performs the following process to decide the parent terminal 1 which is asked to allot a terminal ID.
First, in the child terminal 2-2 which has directly received the H packets from the child terminals 2-1 and 2-3, the communication processing unit 202 calculates communication costs of individual communication routes extending through the child terminals 2-1 and 2-3.
In the child terminals 2, the communication cost of the communication route between the child terminal 2 and the parent terminal 1 is calculated pursuant to the route communication quality of the communication route (the sum of the link communication qualities), the hop count of the communication route, and the residual record number of the parent terminal 1.
The calculation formula of the communication cost is

represented by: communication cost = Ka * [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] + Kb x [the hop count pursuant to the communication route information included in the communication route section of the H packet + 1] + Kc * [1/the residual record number of the residual record number table TB3]. Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kb is a weighting coefficient of the hop count of the communication route. Kc is a weighting coefficient of the residual record number of the parent terminal 1.
The communication cost calculated as above is the communication quality level in the communication route between the parent terminal 1 and the child terminal 2. The communication quality level in the communication route becomes higher as the value of the communication cost grows smaller.
Then, the communication processing unit 202 of the child terminal 2-2 calculates communication costs with respect to the respective communication routes extending through the child terminals 2-1 and 2-3 as the sources of the H packets (namely, the respective communication routes leading to the parent terminals 1-1 and 1-2) . Then, the communication processing unit 202 sends an address request

packet asking allocation of the terminal ID to the parent terminal 1-1 or 1-2, whichever is lower in the communication cost (namely, whichever is higher in the communication quality level) (S7). In the present embodiment, it is assumed that the communication cost of the communication route of the parent terminal 1-1 extending through the child terminal 2-1 is lower than the communication cost of the communication route of the parent terminal 1-2 extending through the child terminal 2-3 and further that the child terminal 2-2 sends the address request packet through the child terminal 2-1. If the child terminal 2-2 receives the H packet from only one communication terminal A, the child terminal 2-2 asks allocation of the terminal ID using the communication route extending through the communication terminal A.
As set forth above, if the child terminal 2 receives H packets from a plurality of communication terminals A, the child terminal 2 calculates the communication costs of the communication routes extending from the child terminal 2 to the parent terminal 1 through the respective communication terminals A based on the route communication qualities of the communication routes, the hop count of the communication routes, and the residual record number of the parent terminal 1. Then, the child terminal 2 builds a communication route between itself and the parent terminal 1 through one of the communication terminals A forming a

communication route which is lowest in the communication cost.
In the present embodiment, the communication cost is calculated in view of the residual record number of the parent terminal 1 as well as the route communication quality of the communication route and the hop count of the communication route. In other words, the communication cost is increased if the residual record number of the parent terminal 1 is small. The communication cost is decreased if the residual record number of the parent terminal 1 is large. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
The address request packet sent by the child terminal 2-2 through the child terminal 2-1 will be described in more detail. A "reserved terminal ID" is held in the source terminal ID section. "T1" is held in the destination terminal ID section. "Reserved terminal ID —> Tl -> Ml" is held in the sending communication route section. In addition, an "address request code" is held in the operation code section. "Null" is held in the distribution address section. "xi" is held in the adjacent terminal section. For example, "3" is held in the link communication quality section.
In the child terminal 2-1 which has received the

address request packet sent by the child terminal 2-2, the communication processing unit 202 determines that the next destination of the address request packet is the parent terminal 1-1, by referring to the sending communication route section of the address request packet. Then, the communication processing unit 202 renews the destination terminal ID section with the terminal ID "Ml" of the parent terminal 1-1 as the next destination. Thereafter, the communication processing unit 202 sends an address request packet (S8) .
The address request packet sent by the child terminal 2-1 will be described in more detail. "xi" is held in the source terminal ID section. "Ml" is held in the destination terminal ID section. "Reserved terminal ID —» Tl —* Ml" is held in the sending communication route section. In addition, an "address request code" is held in the operation code section. "Null" is held in the distribution address section. "T1" is held in the adjacent terminal section. "3" is held in the link communication quality section.
In the parent terminal 1-1 which has received the address request packet relayed by the child terminal 2-1, the communication processing unit 202 allots a terminal ID to the child terminal 2-2 which has sent the address request packet. In the present embodiment, it is assumed that a terminal ID "T2" is allotted to the child terminal 2-2.
The communication processing unit 202 of the parent

terminal 1-1 takes out the adjacent terminal ID "Tl" and the link communication quality "3" which are included in the received address request packet. Since the terminal ID "Tl" is held in the adjacent terminal section of the address request packet, the communication processing unit 202 of the parent terminal 1-1 determines that the child terminal 2-2 allotted with a terminal ID "T2" can make communication with the parent terminal 1-1 through the child terminal 2-1 of terminal ID "Tl". Then, the communication processing unit 202 determines that there has been built a communication route (Ml—»T1—*T2) extending from the parent terminal 1-1 to the child terminal 2-2 via the child terminal 2-1.
The communication processing unit 202 of the parent terminal 1-1 determines that the link communication quality "3" taken out from the link communication quality section included in the received address request packet is the link communication quality between the child terminals 2-1 and 2-2. The communication processing unit 202 calculates the route communication quality "8" of the communication route extending between the parent terminal 1-1 and the child terminal 2-3, by adding the link communication quality "3" to the route communication quality "5" of the communication route extending from the parent terminal 1-1 to the child terminal 2-1.
In the parent terminal 1-1, the table processing unit 201 registers the terminal ID "T2" allotted to the child

terminal 2-2, the route communication quality "8", the hop count "2", the first hop "Tl", and the second hop "T2" on the communication route table TB21.
Then, the parent terminal 1-1 sends an address response packet (S9).
The address response packet prepared by the parent terminal 1-1 will be described in more detail. "Ml" is held in the source terminal ID section. «T1" is held in the destination terminal ID section. "Ml—►Tl—►BC" is held in the sending communication route section. In addition, an "address response code" is held in the operation code section. "T2" is held in the distribution address section. "Tl" is held in the adjacent terminal section. "3" is held in the link communication quality section.
In the child terminal 2-1 which has received the address response packet sent by the parent terminal 1-1, the communication processing unit 202 determines that the next destination of the address response packet is the child terminal 2-2, by referring to the sending communication route section of the address response packet. Then, the communication processing unit 202 renews the destination terminal ID section with the code "BC" for multi-destination communication as the next destination. Thereafter, the communication processing unit 202 sends an address response packet (S10).
The address response packet sent by the child terminal

2-1 will be described in more detail. "Tl" is held in the source terminal ID section. "BC" is held in the destination terminal ID section. "M1->T1—>BC" is held in the sending communication route section. In addition, an "address response code" is held in the operation code section. "T2" is held in the distribution address section. "Tl" is held in the adjacent terminal section. "3" is held in the link communication quality section.
Then, in the child terminal 2-2 which has received the address response packet relayed by the child terminal 2-1, the communication processing unit 202 takes out the distribution address "T2", the adjacent terminal ID "Tl", and the link communication quality "3" held in the received address response packet.
Then, the child terminal 2-2 sets WT2" as the terminal ID of the child terminal 2-2. In the child terminal 2-2, the table processing unit 201 registers the sending link communication quality "3" on the communication-capable-terminal management table TBI in a corresponding relationship with the adjacent terminal ID "Tl".
The communication processing unit 202 of the child terminal 2-2 compares the receiving link communication quality corresponding to the adjacent terminal ID "Tl" with the sending link communication quality within the communication-capable-terminal management table TBI. Then, the link communication quality which is poor in the

communication state (which is larger in the numerical value) is used as the link communication quality between the child terminals 2-1 and 2-2. In the present embodiment, the link communication quality is set equal to "3".
As described above, it is determined that the child terminal 2-2 can build a communication route of "child terminal 2-2 —> child terminal 2-1 -» parent terminal 1-1". Thus, the table processing unit 201 of the child terminal 2-2 reads out the link communication quality between the child terminals 2-2 and 2-1 from the communication-capable-terminal management table TBI. The table processing unit 2 01 adds the link communication quality "3" between the child terminals 2-2 and 2-1 to the route communication quality "5" between the child terminal 2-1 and the parent terminal 1-1 held in the communication route section of the H packet received from the child terminal 2-1. Then, the addition result "8" is used as the route communication quality between the child terminal 2-2 and the parent terminal 1-1.
In other words, the child terminal 2-2 builds a communication route of "T2—>-Tl->Ml" between itself and the parent terminal 1-1 with a hop count "2" and a route communication quality "8". Accordingly, the table processing unit 201 of the child terminal 2-2 registers the terminal ID "Ml" of the parent terminal 1-1, the communication cost "8", the hop count "2", the first hop

"Tl", and the second hop "Ml" on the communication route table TB22.
In case where the parent terminal 1-1 and the child terminal 2-2 can make indirect communication with each other through the child terminal 2-1 as set forth above, three steps, namely a step for the child terminal 2-1 to send the H packet, a step for the child terminal 2-2 to send the address request packet, and a step for the parent terminal 1-1 to send the address response packet, are carried out. Then, the terminal ID (communication address) of the child terminal 2-2 is set by carrying out these steps. The communication route between the parent terminal 1-1 and the child terminal 2-2 capable of making direct communication with each other is built in two ways.
As described above, in the present embodiment, either the child terminal 2 capable of making direct communication with the parent terminal 1 or the child terminal 2 capable of making indirect communication with the parent terminal 1 builds a communication route in view of the residual record number of the parent terminal 1 as well as the route communication quality of the communication route and the hop count of the communication route. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.

By causing the child terminal 2 which has built a communication route between itself and the parent terminal 1 to send an H packet, it is possible to build a communication route having a hop count of 3 or more between the parent terminal 1 and the child terminal 2 in the same manner as mentioned above.
(Second Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
In the present embodiment, if a child terminal 2 not allotted with a terminal ID receives H packets directly from a plurality of communication terminals A, the child terminal 2 performs the following process to decide a parent terminal 1 which is asked to allot a terminal ID to the child terminal 2.
First, in the present embodiment, the communication processing unit 202 of the child terminal 2 which has received H packets from a plurality of communication terminals A determines whether the hop count of the communication route employing each of the communication terminals A as the sources of the H packets is no greater than a specified threshold value Rl. The hop count of this communication route is given by: [the hop count pursuant to

the communication route information included in the communication route section of the H packet + 1].
Then, the communication cost of each of the communication routes extending through the respective communication terminals A is calculated with respect to only the communication terminal A, among the communication terminals A serving as the sources of the H packets, whose hop count found as above is no greater than the specified threshold value Rl.
The threshold value Rl varies on a route-by-route basis depending on the residual record number of the parent terminal 1 making up the communication routes. More specifically, the threshold value Rl in each of the communication routes is set greater as the residual record number of the parent terminal 1 making up the communication routes grows larger. For example, the threshold value Rl is represented by: Rl = a ˟ residual record number + b, where "a" and "b" are integers.
For example, in the child terminal 2-2 which has directly received H packets from a plurality of child terminals 2-1 and 2-3, the communication processing unit 2 02 calculates the threshold value Rl in the communication route extending through each of the child terminals 2-1 and 2-3. In the present embodiment, it is assumed that the residual record number of the parent terminal 1 with which the child terminal 2-1 builds a communication route is larger than the

residual record number of the parent terminal 1 with which the child terminal 2-3 builds a communication route. In this case, the threshold value Rl in each of the communication routes extending through the child terminal 2-1 is set larger than the threshold value Rl in each of the communication routes extending through the child terminal 2-3.
In general, in the area where the distribution density of the child terminals 2 is high, the child terminals 2 installed near the parent terminal 1 are easy to come under the parent terminal 1. The residual record number of the parent terminal 1 gets reduced. The threshold value Rl becomes lower. In other words, the child terminals 2 positioned farther from the parent terminal 1 are consequently hard to come under the parent terminal 1 (The child terminals 2 positioned farther from the parent terminal 1 come under another parent terminal 1 adjoining the child terminals 2) . On the other hand, in the area where the distribution density of the child terminals 2 is low, the residual record number of the parent terminal 1 has room to spare if only the child terminals 2 installed near the parent terminal 1 are taken into account. Thus, the threshold value Rl grows larger. In other words, the child terminals 2 installed farther from the parent terminal 1 are consequently easy to come under the parent terminal 1.
Accordingly, regardless of the distribution density of

the child terminals 2, it is possible to make uniform the number of the child terminals 2 that come under each of the parent terminals 1. In the area where the distribution density of the child terminals 2 is low, it is possible to reduce the number of the parent terminals 1.
The calculation formula of the communication cost is represented by: communication cost = Ka ˟ [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] + Kc x [1/the residual record number of the residual record number table TB3]. Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kc is a weighting coefficient of the residual record number of the parent terminal 1.
The communication cost calculated as above is the communication quality level in the communication route between the parent terminal 1 and the child terminal 2. The communication quality level in the communication route becomes higher as the value of the communication cost grows smaller.
Then, the communication processing unit 202 of the child terminal 2 which has received the H packet sends an address request packet asking allocation of a terminal ID through the communication terminal A having the lowest

communication cost (or to the communication terminal A having the lowest communication cost).
As described above, if the child terminal 2 receives H packets from a plurality of communication terminals A, the child terminal 2 calculates the communication costs of the communication routes extending from the child terminal 2 to the parent terminal 1 through the respective communication terminals A based on the route communication qualities of the communication routes, the hop count of the communication routes and the residual record number of the parent terminal 1. Then, the child terminal 2 builds a communication route between itself and the parent terminal 1 through one of the communication terminals A forming a communication route which is lowest in the communication cost.
(Third Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
In the present embodiment, if a child terminal 2 not allotted with a terminal ID receives H packets directly from a plurality of communication terminals A, the child terminal 2 performs the following process to decide a parent terminal 1 which is asked to allot a terminal ID to the child terminal 2.

First, in the present embodiment, the communication processing unit 202 of the child terminal 2 which has received H packets from a plurality of communication terminals A determines whether the route communication quality (the sum of the link communication qualities) of the communication route employing each of the communication terminals A as the sources of the H packets is no greater than a specified threshold value R2. In the present embodiment, the communication quality value SQ is used as the route communication quality. As the integer value of the communication quality value SQ grows smaller, the attenuation of communication packets becomes smaller and the communication state becomes better. In other words, the determination as to whether the route communication quality is no greater than the specified threshold value R2 is equivalent to the determination as to whether the level of the communication quality of the communication route is no less than a specified threshold value R3.
The route communication quality of the communication route is given by: [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI].
Then, the communication cost of each of the communication routes extending through the respective communication terminals A is calculated with respect to only

the communication terminal A, among the communication terminals A serving as the sources of the H packets, whose route communication quality found as above is no greater than the specified threshold value R2 (namely, whose level of the communication quality of the communication route is no less than the specified threshold value R3).
The threshold value R2 varies on a route-by-route basis depending on the residual record number of the parent terminal 1 making up the communication routes. More specifically, the threshold value R2 in each of the communication routes is set greater as the residual record number of the parent terminal 1 making up the communication routes grows larger (In other words, the threshold value R3 in each of the communication routes is set smaller as the residual record number of the parent terminal 1 making up the communication routes grows larger). For example, the threshold value R2 is represented by: R2 = a x residual record number + b, where "a" and "b" are integers.
For example, in the child terminal 2-2 which has directly received H packets from a plurality of child terminals 2-1 and 2-3, the communication processing unit 202 calculates the threshold value R2 in the communication route extending through each of the child terminals 2-1 and 2-3. In the present embodiment, it is assumed that the residual record number of the parent terminal 1 with which the child terminal 2-1 builds a communication route is larger than the

residual record number of the parent terminal 1 with which the child terminal 2-3 builds a communication route. In this case, the threshold value R2 in each of the communication routes extending through the child terminal 2-1 is set larger than the threshold value R2 in each of the communication routes extending through the child terminal 2-3.
With the configurations described above, the threshold value R2 grows larger. This is because, in the area where the distribution density of the child terminals 2 is low, the residual record number of the parent terminal 1 has room to spare if only the child terminals 2 installed near the parent terminal 1 are taken into account. In general, the distance between the child terminals 2 is long in the area where the distribution density of the child terminals 2 is low. Thus, the link communication quality tends to become worse. However, the communication links slightly poor in the communication quality are also used. This makes it easy to build communication routes between the child terminals 2 and the parent terminal 1.
Accordingly, regardless of the distribution density of the child terminals 2, it is possible to make uniform the number of the child terminals 2 that come under each of the parent terminals 1. In the area where the distribution density of the child terminals 2 is low, it is possible to reduce the number of the parent terminals 1.

The calculation formula of the communication cost is represented by: communication cost = Kb ˟ [the hop count pursuant to the communication route information included in the communication route section of the H packet +1] + Kc x [1/the residual record number of the residual record number table TB3]. Kb is a weighting coefficient of the hop count of the communication route. Kc is a weighting coefficient of the residual record number of the parent terminal 1.
The communication cost calculated as above is the communication quality level in the communication route between the parent terminal 1 and the child terminal 2. The communication quality level in the communication route becomes higher as the value of the communication cost grows smaller.
Then, the communication processing unit 202 of the child terminal 2 which has received the H packet sends an address request packet asking allocation of a terminal ID through the communication terminal A having the lowest communication cost (or to the communication terminal A having the lowest communication cost).
As described above, if the child terminal 2 receives H packets from a plurality of communication terminals A, the child terminal 2 calculates the communication costs of the communication routes extending from the child terminal 2 to the parent terminal 1 through the respective communication terminals A based on the route communication qualities of

the communication routes, the hop count of the communication routes, and the residual record number of the parent terminal 1. Then, the child terminal 2 builds a communication route between itself and the parent terminal 1 through one of the communication terminals A forming a communication route which is lowest in the communication cost.
(Fourth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
In the present embodiment, if a child terminal 2 not allotted with a terminal ID receives H packets directly from a plurality of communication terminals A, the child terminal 2 calculates the communication costs of the respective communication routes in the same manner as in the first embodiment to decide a parent terminal 1 which is asked to allot a terminal ID to the child terminal 2.
The calculation formula of the communication cost is represented by: communication cost = Ka x [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] + Kb x [the hop count pursuant to the communication route

information included in the communication route section of the H packet + 1] + Kc ˟ [1/the residual record number of the residual record number table TB3]. Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kb is a weighting coefficient of the hop count of the communication route. Kc is a weighting coefficient of the residual record number of the parent terminal 1.
The communication cost calculated as above is the communication quality level in the communication route between the parent terminal 1 and the child terminal 2. The communication quality level in the communication route becomes higher as the value of the communication cost grows smaller.
In the present embodiment, the weighting coefficient Kc of the residual record number of the parent terminal 1 becomes larger as the hop count of the communication route grows larger. For example, the weighting coefficient Kc is represented by: Kc = a X the hop count of the communication route + b, where "a" and "b" are integers. In other words, the weight added to the residual record number varies in such a direction that the communication cost becomes higher as the hop count grows larger.
For example, it is assumed that there are a communication route having a large hop count and a communication route having a small hop count and further

that the residual record number of the parent terminal 1 remains the same in the two communication routes. In this case, the communication route having a small hop count is higher in the communication route building probability than the communication route having a large hop count. This is because the weighting coefficient Kc of the residual record number of the parent terminal 1 varies depending on the hop count of the communication route as set forth above. In other words, regardless of the residual record number of the parent terminal 1, it becomes easier for the child terminal 2 to build a communication route with the parent terminal 1 as the child terminal 2 is positioned nearer to the parent terminal 1. This configuration is proper even in terms of physical arrangement.
(Fifth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
In the parent terminal 1 of the present embodiment, the record number capable of being registered on the communication route table TB21 of the parent terminal (namely, the number of the communication routes capable of being built between the parent terminal 1 and the child terminals 2) is set with respect to each range of the hop

count. For example, if the upper limit of the record number capable of being registered on the communication route table TB21 is "100", the record number "20" capable of being registered on the communication route table TB21 is set with respect to each of the ranges of hop number "1", "2", "3", "4" and "5 or more". In other words, the upper limit "100" of the record number capable of being registered on the communication route table TB21 of the parent terminal 1 is divided into five fractions in a corresponding relationship with the respective ranges of hop number of the communication route.
Then, in the parent terminal 1, the value obtained by subtracting the currently registered record number from the record number "20" capable of being registered on the communication route table TB21 of the parent terminal 1 is used as the residual record number of each of the ranges of hop count.
In case where the parent terminal 1 has sent an H packet, the residual record number of the parent terminal 1 as a source of the H packet is held in the residual record number section of the H packet with respect to each of the ranges of hop count. In case where the child terminal 2 has sent an H packet, the residual record number of the parent terminal 1 with which the child terminal 2 as a source of the H packet builds a communication route is held in the residual record number section of the H packet with respect

to each of the ranges of hop count.
The table storage unit 101 of the child terminal 2 stores the residual record number table TB3 shown in Fig. 8. The residual record number table TB3 stores the information on the residual record number of the parent terminal 1 in a tabular form. More specifically, the residual record number table TB3 is provided with individual fields for the terminal ID and the residual record number in each of the ranges of hop count.
In the residual record number table TB3 held by the child terminal 2, the terminal ID is the one allotted to the parent terminal 1. The residual record number in each of the ranges of hop count indicates the residual record number of the parent terminal 1 registered in the terminal ID field, with respect to each of the ranges of hop count.
In the child terminal 2 which has received the H packet, the communication processing unit 202 acquires the residual record number in each of the ranges of hop count of the parent terminal 1 by referring to the residual record number section of the received H packet. Then, the table processing unit 201 of the child terminal 2 registers the residual record number of the parent terminal 1-1 on the residual record number table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of the parent terminal 1 with respect to each of the ranges of hop count.

In case where the child terminal 2 not allotted with the terminal ID has directly received H packets from a plurality of communication terminals A, the child terminal 2 calculates the communication cost of each of the communication routes in view of the residual record number in each of the ranges of hop count.
The calculation formula of the communication cost is represented by: communication cost = Ka ˟ [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] + Kb x [the hop count pursuant to the communication route information included in the communication route section of the H packet + 1] + Kc * [1/the residual record number of the residual record number table TB3 corresponding to the hop count] . Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kb is a weighting coefficient of the hop count of the communication route. Kc is a weighting coefficient of the residual record number of the parent terminal 1.
The second term of the right side of the above calculation formula, [the hop count pursuant to the communication route information included in the communication route section of the H packet + 1] , is the hop count of the communication route. The third term of the

right side of the above calculation formula, [the residual record number of the residual record number table TB3 corresponding to the hop count], is the residual record number in the range of hop count to which the hop count of the communication route belongs.
For example, it is assumed that a child terminal 2 (small in the hop count of the communication route) positioned near a certain parent terminal 1 newly joins the wireless network. However, it is sometimes the case that another child terminal 2 large in the hop count of the communication route has already built a communication route with the parent terminal 1 and, therefore, the newly joining child terminal 2 cannot build a communication route with the parent terminal 1.
In the present embodiment, the residual record number is secured with respect to each of the ranges of hop count. Moreover, the communication cost is calculated in view of the residual record number in each of the ranges of hop count. Accordingly, it becomes easy to avoid a situation that the child terminal 2 existing near the parent terminal 1 cannot build a communication route with the closest parent terminal 1.
In case where the upper limit of the record number capable of being registered on the communication route table TB21 is "100", it is assumed that, as indicated by "o" in Fig. 9, the record number "20" capable of being registered

on the communication route table TB21 is initially set with respect to each of the ranges of hop count wl", "2", "3", "4" and "5 or more". However, the record number actually registered on the communication route table TB21 may sometimes vary in each of the ranges of hop count as indicated by "•" in Fig. 9.
Accordingly, every time a specified time period lapses, the communication processing unit 202 of the parent terminal 1 may change the record number capable of being registered on the communication route table TB21, depending on the record number which has already been registered with respect to each of the ranges of hop count. For example, as indicated by "A" in Fig. 9, the record number may be changed to a value substantially proportional to the actual record number with respect to each of the ranges of hop count. For instance, the record number capable of being registered on the communication route table TB21 is represented by: record number = a * registered record number + b, where "a" and "b" are integers.
The record number actually registered with respect to each of the ranges of hop count varies depending on the actual environment. For that reason, the record number capable of being registered with respect to each of the ranges of hop count is changed in conformity with the distribution state of the actual record number. Since the record number capable of being registered can be set in

conformity with the actual environment, it becomes easy to avoid a situation that the child terminal 2 cannot build a communication route with the parent terminal 1.
(Sixth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the first embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
A record-cost table TB3 (see Fig. 10) held by the child terminal 2 stores, in a tabular form, the information on the residual record number of the parent terminal 1 and the information on the communication cost between the child terminal 2 and the parent terminal 1. More specifically, the record-cost table TB3 is provided with individual fields for the terminal ID, the residual record number, and the communication cost.
In the record-cost table TB3 held by the child terminal 2, the terminal ID is the one allotted to the parent terminal 1. The residual record number indicates the residual record number of the parent terminal 1 registered on the terminal ID field. The communication cost indicates the communication cost of a communication route existing between the child terminal 2 and the parent terminal 1 registered on the terminal ID field.
In the child terminal 2, the communication cost of the

communication route existing between the child terminal 2 and the parent terminal 1 is calculated based on the route communication quality of the communication route (the sum of the link communication qualities) and the hop count of the communication route.
The calculation formula of the communication cost is represented by: communication cost = Ka ˟ [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] + Kb ˟ [the hop count pursuant to the communication route information included in the communication route section of the H packet + 1] . Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kb is a weighting coefficient of the hop count of the communication route.
The communication cost calculated as above is the communication quality level in the communication route between the parent terminal 1 and the child terminal 2. The communication quality level in the communication route becomes higher as the value of the communication cost grows smaller.
In this regard, the communication route section of the H packet sent by the parent terminal 1 becomes "null" (or data-free) . The hop count is equivalent to "0" and the route communication quality is equivalent to "0".
-85-

Accordingly, the communication cost between the parent terminal 1 and the child terminal 2 which has directly received the H packet from the parent terminal 1 is represented by: communication cost Ka x [the link communication quality of the communication-capable-terminal management table TBI] + Kb x [1].
Then, the communication processing unit 202 of the child terminal 2-1 calculates the communication cost with respect to the parent terminals 1-1 and 1-2 which are the sources of the H packets. Then, the table processing unit 201 of the child terminal 2-1 registers the communication costs of the parent terminals 1-1 and 1-2 on the record-cost table TB3 of the table storage unit 101 in a corresponding relationship with the respective terminal IDs of the parent terminals 1-1 and 1-2.
In the child terminal 2, the communication cost of the communication route between the child terminal 2 and the parent terminal 1 is calculated based on the route communication quality of the communication route (the sum of the link communication qualities) and the hop count of the communication route.
The calculation formula of the communication cost is represented by: communication cost = Ka x [the route communication quality included in the communication route section of the H packet + the link communication quality of the communication-capable-terminal management table TBI] +

Kb x [the hop count pursuant to the communication route information included in the communication route section of the H packet + 1] . Ka is a weighting coefficient of the route communication quality of the communication route (the sum of the link communication qualities). Kb is a weighting coefficient of the hop count of the communication route.
Then, the communication processing unit 202 of the child terminal 2-2 calculates the communication costs with respect to the respective communication routes extending through the child terminals 2-1 and 2-3 as the sources of the H packets (namely, the respective communication routes leading to the parent terminals 1-1 and 1-2) . Then, the table processing unit 201 of the child terminal 2-2 registers the communication costs of the parent terminals 1-1 and 1-2 on the record-cost table TB3 of the table storage unit 101 in a corresponding relationship with the respective terminal IDs of the parent terminals 1-1 and 1-2.
As stated above, if the child terminal 2 receives H packets from a plurality of communication terminals A, the child terminal 2 calculates the communication costs of the communication routes extending from the child terminal 2 to the parent terminal 1 through the respective communication terminals A based on the route communication qualities of the communication routes and the hop count of the communication routes. Then, the child terminal 2 builds a communication route between itself and the parent terminal 1

through one of the communication terminals A forming a communication route which is lowest in the communication cost.
As described above, in the present embodiment, either the child terminal 2 capable of making direct communication with the parent terminal 1 or the child terminal 2 capable of making indirect communication with the parent terminal 1 builds a communication route in view of the route communication quality of the communication route and the hop count of the communication route. In case where the child terminal 2 builds a communication route between itself and the parent terminal 1 for the first time after startup, a task of building a communication route with one of the parent terminals 1 is preferentially performed without giving consideration to the residual record number included in the H packet.
Next, description will be made on a process to be carried out after each of the child terminals 2 has built the communication route between itself and one of the parent terminals 1 in the aforementioned manner.
First, for the sake of reducing the communication traffic of the wireless network, H packets are sent from the respective communication terminals A in case where the communication terminal A is set as a parent terminal 1 and in case where the communication terminal A is a child terminal 2 that builds a communication route leading to the

parent terminal 1. The H packet sending process in the respective communication terminals A is carried out at a specified time interval with respect to each of the communication terminals A.
Accordingly, even after the child terminal 2 has built the communication route between itself and the parent terminal 1 in the aforementioned manner, the child terminal 2 continues to receive H packets (see Fig. 7A) from the parent terminals 1 and the child terminals 2 with which the child terminal 2 can make direct communication (to which the child terminal 2 is adjacent). If the communication terminal A which has sent the H packet is a parent terminal
1, the residual record number section of the H packet holds the residual record number of the parent terminal 1 serving as the source of the H packet. If the communication terminal A which has sent the H packet is a child terminal
2, the residual record number section of the H packet holds the residual record number of the parent terminal 1 with which the child terminal 2 serving as the source of the H packet builds a communication route.
In the child terminal 2 which has built the communication route with a certain parent terminal 1 and then has received the H packet, the communication processing unit 202 acquires the residual record number included in the received H packet. In other words, the communication processing unit 202 can acquire not only the residual record

number of the parent terminal 1 that builds a communication route with the subject terminal at the present time but also the residual record number of another parent terminal 1 existing within the wireless network. Then, the table processing unit 201 of the child terminal 2 registers the residual record number of the parent terminal 1 on the record-cost table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of another parent terminal 1 existing within the wireless network. In this regard, the parent terminal 1 whose residual record number is registered on the record-cost table TB3 is capable of directly or indirectly building a communication route between itself and the child terminal 2. The record-cost table TB3 is renewed with the latest residual record number every time the H packet is received.
In the child terminal 2 which has built the communication route with a certain parent terminal 1 and then has received the H packet, the communication processing unit 202 calculates the communication costs of the respective communication routes based on the route communication quality and the hop count in the same manner as mentioned above. The table processing unit 2 01 of the child terminal 2 registers the respective communication costs on the record-cost table TB3 of the table storage unit 101 in a corresponding relationship with the terminal ID of another parent terminal 1 existing within the wireless

network.
The communication processing unit 202 of the child terminal 2 building a communication route between itself and the parent terminal 1 monitors the record-cost table TB3 and determines whether to change over the parent terminal 1 building a communication route with the subject terminal.
For example, the communication processing unit 202 of the child terminal 2-2 building a communication route between itself and the parent terminal 1-1 monitors the residual record number of the parent terminal 1-1 and compares the residual record number of the parent terminal 1-1 with a threshold value Kl (a first threshold value). If the residual record number of the parent terminal 1-1 exceeds the threshold value Kl, the communication route built between the child terminal 2-2 and the parent terminal 1-1 is maintained.
However, if the residual record number of the parent terminal 1-1 becomes equal to or smaller than the threshold value Kl, the communication processing unit 202 of the child terminal 2-2 compares the residual record number of other parent terminals 1 registered on the record-cost table TB3 with a threshold value K2 (a second threshold value). Then, the communication processing unit 202 selects one parent terminal 1 which is smallest in the communication cost registered on the record-cost table TB3, e.g., the parent terminal 1-2, from one or more other parent terminals 1

whose residual record number exceeds the threshold value K2. The threshold value Kl and the threshold value K2 may be equal to each other or may differ from each other.
Then, the communication processing unit 202 of the child terminal 2-2 sends an address request packet to the parent terminal 1-2 in order to build a new communication route between the child terminal 2-2 and the parent terminal 1-2. Thereafter, the child terminal 2-2 receives an address response packet from the parent terminal 1-2 in the same manner as mentioned above and sets a new terminal ID allotted by the parent terminal 1-2 to the subject terminal. In other words, the child terminal 2-2 changes over the parent terminal 1 building a communication route with the subject terminal from the parent terminal 1-1 to the parent terminal 1-2.
As described above, if the residual record number of the parent terminal 1 with which the child terminal 2 builds a communication route becomes small, the child terminal 2 builds a new communication route between itself and another parent terminal 1 having the residual record number to spare. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
Even if the residual record number of the parent terminal 1-1 becomes equal to or smaller than the threshold

value Kl, the child terminal 2-2 maintains the communication route built between the child terminal 2-2 and the parent terminal 1-1 as long as the residual record number of all other parent terminals 1 registered in the record-cost table TB3 is equal to or smaller than the threshold value K2.
(Seventh Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the sixth embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
First, the child terminal 2 of the present embodiment building a communication route between itself and the parent terminal 1 has a recognition as to whether the subject terminal serves as a relay terminal within the wireless network. In the present embodiment, the child terminal 2 that relays a communication packet in the communication between the parent terminal 1 and the child terminal 2 will be referred to as "relay terminal".
More specifically, the communication packet sent and received by the communication terminals A contains the information on the communication route of the communication packet (e.g., the communication route section of the H packet shown in Fig. 7A, the sending communication route section of the address request packet and the address +response packet shown in Fig. 7B, etc.). The information on

the communication route is indicated in such a way as to sequentially enumerate the terminal IDs of the communication terminals A arranged in the communication route. The child terminal 2 can determine whether the subject terminal is a relay terminal or not, by referring to the communication packets received from other communication terminals A. In other words, if the terminal ID of the subject terminal does not exist at the tail-end but exists midway in the communication route, the subject terminal is regarded as a relay terminal.
As stated above, if the communication terminal A relays and transmits the communication packet received from other communication terminals A, the communication terminal A determines that the subject terminal is a relay terminal. However, it is not always the case that the communication terminal A continues to be a relay terminal thereafter. For example, the communication terminal A may not serve as a relay terminal if other child terminals 2 are broken away from the wireless network or if a communication route is changed to another communication route along with a change in a transmission line condition. Accordingly, it may be determined that the subject terminal serves as a relay terminal until a specified time Yl lapses after relaying and transmitting the communication packet received from other communication terminals A. More specifically, the counting of the time Yl is started upon relaying and transmitting the

communication packet. If a new communication packet is relayed and transmitted until the lapse of the time Yl, the counting of the time Yl is restarted. On the other hand, if no new communication packet is relayed and transmitted until the lapse of the time Yl, it is determined that the subject terminal does not serve as a relay terminal.
Even if the residual record number of the parent terminal 1 with which the subject terminal builds a communication route becomes equal to or smaller than the threshold value Kl, the child terminal 2 which has determined that the subject terminal serves as a relay terminal maintains the communication route built between the child terminal 2 and the parent terminal 1.
In case where the child terminal 2 serving as a relay terminal within the wireless network has changed over the parent terminal 1 building a communication route, the communication routes of other child terminals 2 (hereinafter referred to as "subordinate terminals") using the child terminal 2 as a relay terminal may possibly be lost. In addition, if the child terminal 2 as a relay terminal changes over the parent terminal 1 building a communication route, it is likely that the communication routes of the subordinate terminals may be changed and further the child terminals 2 may concentrate under a specific parent terminal 1. Moreover, the subordinate terminals which have lost the communication routes may possibly fail to build

communication routes if a new parent terminal 1 with which the subordinate terminals can make communication does not exist.
However, the aforementioned problems are not posed in the present embodiment. This is because, regardless of the residual record number of the parent terminal 1 building a communication route, the child terminal 2 serving as a relay terminal maintains the communication route built between the child terminal 2 and the parent terminal 1 as stated above. In other words, it is possible to maintain communication reliability within the wireless network.
(Eighth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the sixth embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
First, the child terminal 2 of the present embodiment building a communication route between itself and the parent terminal 1 has a recognition as to whether the subject terminal serves as a relay terminal within the wireless network. In the present embodiment, the child terminal 2 that relays a communication packet in the communication between the parent terminal 1 and the child terminal 2 will be referred to as "relay terminal".
More specifically, the communication packet sent and

received by the communication terminals A contains the information on the communication route of the communication packet (e.g., the communication route section of the H packet shown in Fig. 7A, the sending communication route section of the address request packet and the address response packet shown in Fig. 7B, etc.). The information on the communication route is indicated in such a way as to sequentially enumerate the terminal IDs of the communication terminals A arranged in the communication route. The child terminal 2 can determine whether the subject terminal is a relay terminal or not, by referring to the communication packets received from other communication terminals A. Moreover, the child terminal 2 can grasp the number of other child terminals 2 (hereinafter referred to as "subordinate terminals") using the subject terminal as a relay terminal, by referring to the communication packets received from other communication terminals A. The number of the subordinate terminals of the child terminal 2 not serving as a relay terminal is "0".
The subordinate terminals may not serve as relay terminals if other child terminals 2 are removed from the wireless network or if a communication route is changed to another communication route along with a change in a transmission line condition. Accordingly, the child terminal 2 may determine that other child terminal 2 serve as relay terminals until a specified time Y2 lapses after

relaying and transmitting the communication packets received from other child terminals 2. More specifically, a subordinate terminal table TB4 shown in Fig. 11 is stored in the table storage unit 101 of the child terminal 2. The terminal IDs of the subordinate terminals and the final relaying time of the communication packets received from the subordinate terminals are registered on the subordinate terminal table TB4. The child terminal 2 determines that the child terminals 2 whose terminal IDs are registered on the subordinate terminal table TB4 are subordinate terminals. The records of the subordinate terminals may be deleted at the time when the time Y2 has lapsed from the final relaying time of the communication packets.
The child terminal 2 of the present embodiment changes the threshold values Kl and K2 depending on the number of the subordinate terminals that use the subject terminal as a relay terminal.
For example, if the number of the subordinate terminals grows larger, the probability for the child terminal 2 to change over a communication route is reduced by decreasing the threshold value Kl and increasing the threshold value K2. On the other hand, if the number of the subordinate terminals grows smaller, the probability for the child terminal 2 to change over a communication route is increased by increasing the threshold value Kl and decreasing the threshold value K2. The threshold values Kl

and K2 may be changed either continuously or step by step depending on the change in the number of the subordinate terminals.
In case where the child terminal 2 serving as a relay terminal within the wireless network has changed over the parent terminal 1 building a communication route, the communication routes of the subordinate terminals may possibly be lost. In addition, if the child terminal 2 as a relay terminal changes over the parent terminal 1 building a communication route, it is likely that the communication routes of the subordinate terminals may be changed and further that the child terminals 2 may concentrate under a specific parent terminal 1. Moreover, the subordinate terminals which have lost the communication routes may possibly fail to build communication routes if a new parent terminal 1 with which the subordinate terminals can make communication does not exist.
It is thinkable to solve the aforementioned problems by allowing the child terminal 2 serving as a relay terminal to maintain the communication route built between the child terminal 2 and the parent terminal 1 regardless of the residual record number of the parent terminal 1 building a communication route with the subject terminal. However, this method may make it impossible to suppress generation of a child terminal 2 incapable of making communication with the parent terminal 1.

In the present embodiment, however, the child terminal 2 serving as a relay terminal changes the probability of changing over the parent terminal 1 with which the subject terminal builds a communication route, depending on the number of the subordinate terminals that use the subject terminal as a relay terminal. Accordingly, it is possible to maintain communication reliability within the wireless network and to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
It may be possible to employ either a configuration in which both the threshold values Kl and K2 are changed depending on the number of the subordinate terminals or a configuration in which only one of the threshold values Kl and K2 is changed depending on the number of the subordinate terminals.
(Ninth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the sixth embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
First, it is assumed that the child terminal 2-2 builds a communication route between itself and the parent terminal 1-1. Then, the communication processing unit 202

of the child terminal 2-2 monitors the residual record number of the parent terminal 1-1 and compares the residual record number of the parent terminal 1-1 with a threshold value Kl (a first threshold value). If the residual record number of the parent terminal 1-1 exceeds the threshold value Kl, the communication route built between the child terminal 2-2 and the parent terminal 1-1 is maintained.
However, if the residual record number of the parent terminal 1-1 becomes equal to or smaller than the threshold value Kl, the communication processing unit 2 02 of the child terminal 2-2 compares the residual record number of other parent terminals 1 registered on the record-cost table TB3 with a threshold value K2 (a second threshold value). Then, the communication processing unit 202 selects one parent terminal 1 which is smallest in the communication cost registered on the record-cost table TB3, e.g., the parent terminal 1-2, from one or more other parent terminals 1 whose residual record number exceeds the threshold value K2. The threshold value Kl and the threshold value K2 may be equal to each other or may differ from each other.
Then, the communication processing unit 2 02 of the child terminal 2-2 sends a parent terminal change notifying packet to the parent terminal 1-1 and sends an address request packet to the parent terminal 1-2.
In the parent terminal 1-1 which has received the parent terminal change notifying packet from the child

terminal 2-2, the table processing unit 201 deletes the communication route information on the child terminal 2-2 from the communication route table TB21 (see Fig. 4A) and increments the residual record number by 1.
The child terminal 2-2 which has sent the address request packet to the parent terminal 1-2 receives an address response packet from the parent terminal 1-2 in the same manner as mentioned above and sets a new terminal ID allotted by the parent terminal 1-2 to the subject terminal. In other words, the child terminal 2-2 changes over the parent terminal 1 building a communication route with the subject terminal from the parent terminal 1-1 to the parent terminal 1-2.
On the other hand, if the child terminal 2-2 sends only the address request packet to the parent terminal 1-2 without sending the parent terminal change notifying packet to the parent terminal 1-1, time is required for the parent terminal 1-1 to detect the child terminal 2-2 broken away from the subject terminal. For example, the child terminal 2 sends a routing protocol packet such as a hello packet or a route notifying packet every specified time. The route notifying packet stores the information on a communication route built between the subject terminal and the parent terminal 1. The route notifying packet is sent to the parent terminal 1 with which the subject terminal builds a communication route. Just when the parent terminal 1-1

fails to receive a routing protocol packet such as a hello packet or a route notifying packet from the child terminal 2-2 for a specified time period, the parent terminal 1-1 can detect the child terminal 2-2 broken away from the subject terminal. In other words, the parent terminal 1-1 detects the breakaway of the child terminal 2-2 when a certain period of time lapses after the child terminal 2-2 is actually broken away from the subject terminal. Consequently, there may be a situation that the number of the child terminals 2 under the parent terminal 1-1 is unnecessarily decreased or the number of the child terminals 2 under the parent terminal 1-2 is unnecessarily increased until the parent terminal 1-1 detects the breakaway of the child terminal 2-2.
In the present embodiment, as described above, the child terminal 2 that changes over the parent terminal 1 building a communication route sends a parent terminal change notifying packet to the old parent terminal 1 building the current communication route and sends an address request packet to a new parent terminal 1. Accordingly, the old parent terminal 1 can detect on a real time basis that the child terminal 2 is broken away from the subject terminal and can immediately send a hello packet in which the residual record number of the communication route table TB21 is changed to a latest value. In other words, it is possible to notify the child terminal 2 of an increase in

the residual record number and to prevent an unnecessarily large number of child terminals 2 from breaking away from the subject terminal. That is to say, it is possible to prevent an unnecessary decrease in the number of the child terminals 2 coming under the old parent terminal 1 or an unnecessary increase in the number of the child terminals 2 coming under the new parent terminal 1. It is therefore possible to make uniform the number of the child terminals 2 coming under the parent terminal 1.
(Tenth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the sixth embodiment. The same configurations will be designated by like reference symbols with no description made thereon. With reference to a sequence shown in Fig. 12, description will now be made on how to build a communication route in the present wireless network.
First, it is assumed that the child terminal 2-2 builds a communication route between itself and the parent terminal 1-1. Then, the communication processing unit 202 of the child terminal 2-2 monitors the residual record number of the parent terminal 1-1 and compares the residual record number of the parent terminal 1-1 with a threshold value Kl (a first threshold value). If the residual record number of the parent terminal 1-1 exceeds the threshold value Kl, the communication route built between the child

terminal 2-2 and the parent terminal 1-1 is maintained.
However, if the residual record number of the parent terminal 1-1 becomes equal to or smaller than the threshold value Kl, the communication processing unit 202 of the child terminal 2-2 compares the residual record number of other parent terminals 1 registered on the record-cost table TB3 with a threshold value K2 (a second threshold value). Then, the communication processing unit 202 selects one parent terminal 1 which is smallest in the communication cost registered on the record-cost table TB3, e.g., the parent terminal 1-2, from one or more other parent terminals 1 whose residual record number exceeds the threshold value K2. The threshold value Kl and the threshold value K2 may be equal to each other or may differ from each other.
The communication processing unit 202 of the child terminal 2-2 sends a parent terminal change feasibility packet to the parent terminal 1-1 (S21) .
In the parent terminal 1-1 which has received the parent terminal change feasibility packet from the child terminal 2-2, the communication processing unit 202 sends a parent terminal change permission packet to the child terminal 2-2 serving as a source of the parent terminal change feasibility packet if the residual record number of the subject terminal in the communication route table TB21 is equal to or smaller than the threshold value Kl (S22) . The table processing unit 201 of the parent terminal 1-1

deletes the communication route information on the child terminal 2-2 from the communication route table TB21 (see Fig. 4A) and increments the residual record number by 1.
Then, the child terminal 2-2 which has received the parent terminal change permission packet from the parent terminal 1-1 sends an address request packet to the parent terminal 1-2 (S23). Thereafter, the child terminal 2-2 receives an address response packet from the parent terminal 1-2 in the same manner as mentioned above and sets a new terminal ID allotted by the parent terminal 1-2 to the subject terminal (S24). In other words, the child terminal 2-2 changes over the parent terminal 1 building a communication route with the subject terminal from the parent terminal 1-1 to the parent terminal 1-2.
In the present embodiment, as described above, the child terminal 2 that changes over the parent terminal 1 building a communication route sends a parent terminal change feasibility packet to the old parent terminal 1 building the current communication route. The old parent terminal 1 which has received the parent terminal change feasibility packet sends a parent terminal change permission packet to the child terminal 2 if the residual record number of the subject terminal is equal to or smaller than the threshold value Kl. After receiving the parent terminal change permission packet from the old parent terminal 1, the child terminal 2 sends an address request packet to a new

parent terminal 1. Since the old parent terminal 1 sends the parent terminal change permission packet pursuant to the residual record number of the subject terminal, it is possible to detect on a real time basis that the child terminal 2 is broken away from the subject terminal. It is also possible to prevent an unnecessarily large number of child terminals 2 from breaking away from the subject terminal.
In case of the ninth embodiment, it is possible to know the increase in the residual record number of the old parent terminal 1 as the child terminal 2 receives a hello packet from the old parent terminal 1. However, if the hop count of the communication route built between the child terminal 2 and the old parent terminal 1 is large, a certain degree of time is required for the information on the latest residual record number of the old parent terminal 1 to be transferred to the child terminal 2.
On the other hand, in the present embodiment, the parent terminal 1 determines the changeability of the communication route of the child terminal 2 on a real time basis depending on the residual record number of the subject terminal. Accordingly, it is possible to prevent an unnecessary decrease in the number of the child terminals 2 coming under the old parent terminal 1 or an unnecessary increase in the number of the child terminals 2 coming under the new parent terminal 1. It is therefore possible to make

uniform the number of the child terminals 2 coming under the parent terminal 1.
(Eleventh Embodiment)
Just like the sixth embodiment, each of the child terminals 2 continues to acquire the information on the residual record number and communication cost of other parent terminals 1 existing within the wireless network, even after each of the child terminals 2 builds a communication route between itself and one of the parent terminals 1.
However, if the residual record number of all the parent terminals 1 capable of making communication with the subject terminal is "0", there occurs a situation that the child terminal 2 (e.g., a newly-joined child terminal) which has not yet built a communication route with any one of the parent terminals 1 cannot build a communication route between itself and the parent terminal 1.
Based on the H packet received from the communication terminal A, the child terminal 2 stores the residual record number of the parent terminal 1 capable of making communication with the child terminal 2 on the record-cost table TB3. If the residual record number of one of the parent terminals 1 exceeds a threshold value "0" in the record-cost table TB3, the child terminal 2 builds a communication route between itself and one of the parent terminals 1 whose residual record number exceeds the

threshold value "0" and whose communication cost is lowest as described above.
However, if the residual record number of all the parent terminals is "0" in the record-cost table TB3 and if the child terminal 2 cannot build a communication route with any one of the parent terminals 1, the child terminal 2 carries out the following process. In this regard, the threshold value "0" of the residual record number corresponds to a specified threshold value of the present invention.
First, it is assumed that the child terminal 2-3 cannot build a communication route with any one of the parent terminals 1. The child terminal 2-3 sends a joining failure notifying packet to all the parent terminals 1 allotted with the terminal IDs stored in the record-cost table TB3 of the subject terminal (namely, the parent terminals 1 which have received the residual record information through the H packet). The parent terminals 1 serving as the destinations of the joining failure notifying packet are the terminals whose residual record number is "0".
The parent terminals 1 which have received the joining failure notifying packet, e.g., the parent terminals 1-1 and 1-2, send a parent terminal change request packet to the child terminal 2 that builds a communication route between itself and the subject terminal.

The communication processing unit 202 of the child terminal 2 which has received the parent terminal change request packet identifies not only the parent terminals 1 (1-1 and 1-2) with which the subject terminal builds communication routes at the present time but also the parent terminals 1 whose residual record number is equal to or larger than the threshold value "0". In other words, the child terminal 2 which has received the parent terminal change request packet identifies not only the parent terminals 1 (1-1 and 1-2) with which the subject terminal builds communication routes at the present time but also the parent terminals 1 which can build communication routes between themselves and the subject terminal. Then, the communication processing unit 2 02 of the child terminal 2 which has received the parent terminal change request packet sends an address request packet to not only the parent terminals 1 which have built communication routes between themselves and the subject terminal but also the parent terminals 1 which can build communication routes between themselves and the subject terminal and which remain lowest in the communication cost. In the present embodiment, it is assumed that the child terminal 2 existing under the parent terminal 1-1 sends an address request packet to the parent terminal 1-3 and further that the child terminal 2 existing under the parent terminal 1-2 sends an address request packet to the parent terminal 1-4.

The respective child terminals 2 which have sent the address request packet under the parent terminals 1-1 and 1-
2 receive an address response packet from the parent
terminals 1-3 and 1-4 in the same manner as mentioned above
and set new terminal IDs allotted by the parent terminals 1-
3 and 1-4 to the subject terminals. In other words, the
child terminals 2 which have sent the address request packet
change over the parent terminals 1 building communication
routes with the subject terminals from the parent terminals
1-1 and 1-2 to the parent terminals 1-3 and 1-4.
Accordingly, the number of the child terminals 2 under the parent terminals 1-1 and 1-2 is reduced. In the present embodiment, the child terminal 2 sends a routing protocol packet such as a route notifying packet or the like every specified time. The route notifying packet stores the information on a communication route built between the subject terminal and the parent terminal 1. The route notifying packet is sent to the parent terminal 1 with which the subject terminal builds a communication route. Just when the parent terminal 1 fails to receive a routing protocol packet such as a route notifying packet or the like from the child terminal 2 under thereof for a specified time period, the parent terminal 1 can detect the child terminal 2 broken away from the subject terminal. Accordingly, each of the table processing units 201 of the parent terminals 1-1 and 1-2 deletes the record of the broken-away child

terminal 2 from the communication route table TB21 (see Fig. 4A) . Thus, the residual address number of the subject terminal is increased.
If the child terminal 2-3 which has not yet built a communication route receives a hello packet, the residual record number of the parent terminals 1-1 and 1-2 is increased in the record-cost table TB3 of the subject terminal. Accordingly, as described above, it becomes possible to build a communication route between the child terminal 2-3 and the parent terminal 1-1 or 1-2, whichever is lower in the communication cost.
As stated above, the child terminal 2 incapable of building a communication route between itself and the parent terminal 1 increases the residual record number of the parent terminal 1 capable of making communication with the subject terminal and then builds a communication route between itself and the parent terminal 1. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
The child terminal 2 incapable of building a communication route with any one of the parent terminals 1 may send a joining failure notifying packet to only one of the parent terminals 1 which is allotted with terminal IDs in the record-cost table TB3 of the subject terminal and

which remains lowest in the communication cost.
(Twelfth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the eleventh embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
The present embodiment is characterized by the configuration for selecting the child terminal 2 that changes the parent terminal 1 to be used in building a communication route.
First, the child terminal 2 building a communication route between itself and the parent terminal 1 stores the information on the H packet received after building the communication route in the record-cost table TB3 (see Fig. 5). In other words, the child terminal 2 holds not only the information on the parent terminal 1 building a communication route at the present time but also the information on other parent terminals 1 with which the child terminal 2 can make communication.
The child terminal 2 building a communication route sends a topology notifying packet, every specified time, to the parent terminal 1 that builds a communication route at the present time. The topology notifying packet contains not only the information on the parent terminal 1 currently building a communication route with the child terminal 2

serving as the source of the topology notifying packet but also the information on the terminal IDs and the residual record number of other parent terminals 1 capable of making communication with the child terminal 2.
The parent terminal 1 which has received the topology notifying packet from the child terminal 2 existing under thereof prepares a child topology table TB4 of the table storage unit 101 based on the topology notifying packet. As shown in Fig. 13, the child topology table TB4 is provided with individual fields for the terminal ID and the number of other parent terminals 1 capable of making communication.
In the child topology table TB4 held by the parent terminal 1, the terminal ID is the one allotted to the child terminal 2 existing under thereof, which has built a communication route with the subject terminal. The number of other parent terminals 1 capable of making communication denotes the number of other parent terminals 1 with which the child terminals 2 registered on the terminal ID field can make communication and whose residual record number exceeds a threshold value "0".
In other words, by referring to the child topology table TB4, the parent terminal 1 recognizes the number of other parent terminals 1 with which the child terminals 2 existing under thereof can make communication. That is to say, the parent terminal 1 can recognize the number of the parent terminals 1 which can be changed over by the child

terminals 2 existing under thereof.
With reference to a sequence shown in Fig. 14, description will now be made on how to build a communication route in the present wireless network.
When the residual record number of the subject terminal is "0", upon receiving a joining failure notifying packet from the child terminal 2 which has not yet built a communication route (S21), the communication processing unit 2 02 of the parent terminal 1 refers to the child topology table TB4. Then, the communication processing unit 202 takes out, from the child terminals 2 existing under thereof, the child terminal 2 having a communication route through which the child terminal 2 can make communication with other parent terminals 1 (namely, the child terminal 2 capable of making communication with one or more other parent terminals 1). Moreover, the communication processing unit 2 02 detects the number of the child terminals 2 serving as the sources of the joining failure notifying packet based on the received joining failure notifying packet. The number of the child terminals 2 thus detected is used as a change target number. Then, the communication processing unit 202 sends a parent terminal change request packet from the child terminal 2 having a communication route through which the child terminal 2 can make communication with other parent terminals, to only the same number of child terminals 2 as the change target number (S22) . At this time, the

child terminals 2 to which the parent terminal change request packet is to be sent are selected from the child topology table TB4 in the descending order of the number of other parent terminals 1 with which each of the child terminals 2 can make communication.
Thereafter, just like the eleventh embodiment, a step for the child terminal 2 to send an address request packet (S23) and a step for the parent terminal 1 to send an address response packet (S24) are performed between the parent terminal 1 and the child terminal 2. The child terminal 2 which has received the address response packet changes over the parent terminal 1 that builds a communication route with the subject terminal. Accordingly, the parent terminal 1 which has received the joining failure notifying packet can increase the residual record number of the subject terminal.
As stated above, the child terminal 2 incapable of building a communication route between itself and the parent terminal 1 increases the residual record number of the parent terminal 1 capable of making communication with the subject terminal and then builds a communication route between itself and the parent terminal 1. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.

In the present embodiment, the parent terminal 1 whose residual record number is "0" sends a parent terminal change request packet to only a minimum number of child terminals 2 whose communication routes need to be changed, among the child terminals 2 existing under the parent terminal 1 and having communication routes through which the child terminals 2 can make communication with other parent terminals 1. Accordingly, it is possible to minimize the , change of network configurations and to reduce the communication traffic.
In addition, the parent terminal 1 whose residual record number is "0" sends a parent terminal change request packet to the child terminals 2 provided with a larger number of other parent terminals 1 that can be changed over. It is therefore possible to increase the communication qualities of the changed communication routes as far as possible.
(Thirteenth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the eleventh embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
The present embodiment is characterized by the configuration for selecting the child terminal 2 that changes the parent terminal 1 to be used in building a

communication route.
First, the child terminal 2 of the present embodiment building a communication route between itself and the parent terminal 1 has a recognition as to whether the subject terminal serves as a relay terminal within the wireless network. In the present embodiment, the child terminal 2 that relays a communication packet in the communication between the parent terminal 1 and the child terminal 2 will be referred to as "relay terminal".
More specifically, the communication packet sent and received by the communication terminals A contains the information on the communication route of the communication packet (e.g., the communication route section of the H packet shown in Fig. 7A, the sending communication route section of the address request packet, and the address response packet shown in Fig. 7B, etc.). The information on the communication route is indicated in such a way as to sequentially enumerate the terminal IDs of the communication terminals A arranged in the communication route. The child terminal 2 can determine whether the subject terminal is a relay terminal or not, by referring to the communication packets received from other communication terminals A. In other words, if the terminal ID of the subject terminal does not exist at the tail-end but exists midway in the communication route, the subject terminal is regarded as a relay terminal.

As stated above, if the communication terminal A relays and transmits the communication packet received from other communication terminals A, the communication terminal A determines that the subject terminal is a relay terminal. However, it is not always the case that the communication terminal A continues to be a relay terminal thereafter. For example, the communication terminal A may not serve as a relay terminal if other child terminals 2 are broken away from the wireless network or if a communication route is changed to another communication route along with a change in a transmission line condition. Accordingly, it may be determined that the subject terminal serves as a relay terminal until a specified time Yl lapses after relaying and transmitting the communication packet received from other communication terminals A. More specifically, the counting of the time Yl is started upon relaying and transmitting the communication packet. If a new communication packet is relayed and transmitted until the lapse of the time Yl, the counting of the time Yl is restarted. On the other hand, if no new communication packet is relayed and transmitted until the lapse of the time Yl, it is determined that the subject terminal does not serve as a relay terminal.
Then, the child terminal 2 building a communication route sends a topology notifying packet, every specified time, to the parent terminal 1 that builds a communication route at the present time. The topology notifying packet

indicates whether the child terminal 2 serving as the source of the topology notifying packet is a relay terminal or not.
The parent terminal 1 which has received the topology notifying packet from the child terminal 2 existing under thereof can determine, based on the topology notifying packet, whether the child terminal 2 existing under thereof is a relay terminal or not. The determination result is stored in a child topology table TB5 of the table storage unit 101. As shown in Fig. 15, the child topology table TB5 is provided with individual fields for the terminal ID and the relay terminal. The parent terminal 1 can determine whether the child terminal 2 existing under thereof is a relay terminal or not, by referring to the communication route table TB21 of the subject terminal. Therefore, the parent terminal 1 does not necessarily require the topology notifying packet sent from the child terminal 2 existing under the thereof.
In the child topology table TB5 held by the parent terminal 1, the terminal ID is the one allotted to the child terminal 2 existing under the parent terminal 1 and building a communication route with the subject terminal. If the child terminal 2 registered on the terminal ID field is a relay terminal, the relay terminal field is indicated by symbol "O". If not, the relay terminal field is indicated by symbol "X".
In other words, the parent terminal 1 can determine

whether each of the child terminals 2 existing under thereof is a relay terminal or not by referring to the child topology table TB5.
When the residual record number of the subject terminal is "0", upon receiving a joining failure notifying packet from the child terminal 2 which has not yet built a communication route, the communication processing unit 202 of the parent terminal 1 refers to the child topology table TB5. Then, the communication processing unit 202 sends a parent terminal change request packet to the child terminal 2 existing under the parent terminal 1 but not serving as a relay terminal.
Thereafter, just like the eleventh embodiment, a step for the child terminal 2 to send an address request packet and a step for the parent terminal 1 to send an address response packet are performed between the parent terminal 1 and the child terminal 2. The child terminal 2 which has received the address response packet changes over the parent terminal 1 that builds a communication route with the subject terminal. Accordingly, the parent terminal 1 which has received the joining failure notifying packet can increase the residual record number of the subject terminal.
As stated above, the child terminal 2 incapable of building a communication route between itself and the parent terminal 1 increases the residual record number of the parent terminal 1 capable of making communication with the

subject terminal and then builds a communication route between itself and the parent terminal 1. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
In case where the child terminal 2 serving as a relay terminal within the wireless network has changed over the parent terminal 1 building a communication route, the communication routes of other child terminals 2 (hereinafter referred to as "subordinate terminals") using the child terminal 2 as a relay terminal may possibly be lost. In addition, if the child terminal 2 as a relay terminal changes over the parent terminal 1 building a communication route, it is likely that the communication routes of the subordinate terminals may be changed and further that the child terminals 2 may concentrate under a specific parent terminal 1.
However, the aforementioned problems are not posed in the present embodiment. This is because the child terminal 2 serving as a relay terminal maintains the current communication route as stated above. In other words, it is possible to maintain communication reliability within the wireless network.
Just like the twelfth embodiment, the destinations of the parent terminal change request packet may be further

limited based on the number of the child terminals 2 serving as the sources of the joining failure notifying packet or the number of other parent terminals 1 capable of making communication with the child terminals 2, which are registered on the child topology table TB4. (Fourteenth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the eleventh embodiment. The same configurations will be designated by like reference symbols with no description made thereon.
The present embodiment is characterized by the configuration for selecting the child terminal 2 that changes the parent terminal 1 to be used in building a communication route.
First, the child terminal 2 of the present embodiment building a communication route between itself and the parent terminal 1 has a recognition as to whether the subject terminal serves as a relay terminal within the wireless network. In the present embodiment, the child terminal 2 that relays a communication packet in the communication between the parent terminal 1 and the child terminal 2 will be referred to as "relay terminal".
More specifically, the communication packet sent and received by the communication terminals A contains the information on the communication route of the communication

packet (e.g., the communication route section of the H packet shown in Fig. 7A, the sending communication route section of the address request packet, and the address response packet shown in Fig. 7B, etc.). The information on the communication route is indicated in such a way as to sequentially enumerate the terminal IDs of the communication terminals A arranged in the communication route. The child terminal 2 can determine whether the subject terminal is a relay terminal or not, by referring to the communication packets received from other communication terminals A. In other words, if the terminal ID of the subject terminal does not exist at the tail-end but exists midway in the communication route, the subject terminal is regarded as a relay terminal. Moreover, the child terminal 2 can grasp the number of other child terminals 2 (hereinafter referred to as "subordinate terminals") using the subject terminal as a relay terminal, by referring to the communication packets received from other communication terminals A.
The child terminal 2 building a communication route sends a topology notifying packet, every specified time, to the parent terminal 1 building a communication route at the present time. The topology notifying packet indicates whether the child terminal 2 serving as the source of the topology notifying packet is a relay terminal or not. In case where the child terminal 2 serving as the source of the topology notifying packet is a relay terminal, the topology

notifying packet also contains the information on the number of the subordinate terminals.
The parent terminal 1 which has received the topology notifying packet from the child terminal 2 existing under thereof can determine, based on the topology notifying packet, whether the child terminal 2 existing under thereof is a relay terminal or not. Moreover, the parent terminal 1 can determine the number of the subordinate terminals. The determination result is stored in a child topology table TB6 of the table storage unit 101. As shown in Fig. 16, the child topology table TB6 is provided with individual fields for the terminal ID, the relay terminal, and the subordinate terminal number. The parent terminal 1 can also determine whether the child terminal 2 existing under thereof is a relay terminal or not, by referring to the communication route table TB21 of the subject terminal. Moreover, the parent terminal 1 can also determine the number of the subordinate terminals. Therefore, the parent terminal 1 does not necessarily require the topology notifying packet sent from the child terminal 2 existing under thereof.
In the child topology table TB6 held by the parent terminal 1, the terminal ID is the one allotted to the child terminal 2 existing under the parent terminal 1 and building a communication route with the subject terminal. If the child terminal 2 registered on the terminal ID field is a relay terminal, the relay terminal field is indicated by

symbol "0". If not, the relay terminal field is indicated by symbol "X". if the child terminal 2 registered on the terminal ID field is a relay terminal, the number of the subordinate terminals thereof is registered on the subordinate terminal number field. If the child terminal 2 registered on the terminal ID field is not a relay terminal, "0" is registered on the subordinate terminal number field.
In other words, the parent terminal 1 can determine whether each of the child terminals 2 existing under thereof is a relay terminal or not by referring to the child topology table TB6.
When the residual record number of the subject terminal is "0", upon receiving a joining failure notifying packet from the child terminal 2 which has not yet built a communication route, the communication processing unit 202 of the parent terminal 1 refers to the child topology table TB6. Then, the communication processing unit 202 gives a higher priority to each of the child terminals 2 existing under the parent terminal 1 as the number of the subordinate terminals of each of the child terminals 2 becomes smaller.
Moreover, the communication processing unit 202 of the parent terminal 1 which has received the joining failure notifying packet detects the number of the child terminals 2 serving as the sources of the joining failure notifying packet based on the received joining failure notifying packet. The number of the child terminals 2 thus detected

is used as a change target number.
Then, the communication processing unit 202 of the parent terminal 1 which has received the joining failure notifying packet sends a parent terminal change request packet to only the child terminals 2 just as much as the change target number in the order of higher priorities by referring to the child topology table TB6.
Thereafter, just like the eleventh embodiment, a step for the child terminal 2 to send an address request packet and a step for the parent terminal 1 to send an address response packet are performed between the parent terminal 1 and the child terminal 2. The child terminal 2 which has received the address response packet changes over the parent terminal 1 that builds a communication route with the subject terminal. Accordingly, the parent terminal 1 which has received the joining failure notifying packet can increase the residual record number of the subject terminal.
As stated above, the child terminal 2 incapable of building a communication route between itself and the parent terminal 1 increases the residual record number of the parent terminal 1 capable of making communication with the subject terminal and then builds a communication route between itself and the parent terminal 1. It is therefore possible to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2

under a specific parent terminal 1.
In case where the child terminal 2 serving as a relay terminal within the wireless network has changed over the parent terminal 1 building a communication route, the communication routes of other child terminals 2 (hereinafter referred to as "subordinate terminals") using the child terminal 2 as a relay terminal may possibly be lost. In addition, if the child terminal 2 as a relay terminal changes over the parent terminal 1 building a communication route, it is likely that the communication routes of the subordinate terminals may be changed and further that the child terminals 2 may concentrate under a specific parent terminal 1.
However, in the present embodiment, the propriety of sending the parent terminal change request packet is decided depending on the number of the subordinate terminals of the child terminals 2 existing under the parent terminal 1 whose residual record number is "0". Accordingly, it is possible to maintain communication reliability within the wireless network and to suppress generation of a child terminal 2 incapable of making communication with a parent terminal 1, with no likelihood of concentration of child terminals 2 under a specific parent terminal 1.
Just like the twelfth embodiment, the destinations of the parent terminal change request packet may be further limited based on the number of other parent terminals 1

capable of making communication with the child terminals 2, which are registered on the child topology table TB4.
(Fifteenth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of the eleventh to fourteenth embodiments. The same configurations will be designated by like reference symbols with no description made thereon.
In the present embodiment, the child terminal 2-2 which has received the parent terminal change request packet from the parent terminal 1-1 building a communication route with the subject terminal at the present time operates as follows.
The child terminal 2-2 which has received the parent terminal change request packet selects one parent terminal 1 which is smallest in the communication cost registered on the record-cost table TB3, e.g., the parent terminal 1-2, from one or more parent terminals 1 whose residual record number exceeds a threshold value w0".
Then, the communication processing unit 202 of the . child terminal 2-2 sends a parent terminal change notifying packet to the parent terminal 1-1 building a communication route with the subject terminal at the present time and then sends an address request packet to the parent terminal 1-2 which remains lowest in the communication cost.
In the parent terminal 1-1 which has received the

parent terminal change notifying packet from the child terminal 2-2, the table processing unit 201 deletes the communication route information on the child terminal 2-2 from the communication route table TB21 (see Fig. 4A) and increments the residual record number by 1. Thus, the parent terminal 1-1 can increase the residual record number of the subject terminal.
The child terminal 2-2 which has sent the address request packet to the parent terminal 1-2 receives an address response packet from the parent terminal 1-2 in the same manner as mentioned above and sets a new terminal ID allotted by the parent terminal 1-2 to the subject terminal. In other words, the child terminal 2-2 changes over the parent terminal 1 building a communication route with the subject terminal from the parent terminal 1-1 to the parent terminal 1-2.
Then, if the residual record number of the parent terminal 1-1 exceeds the threshold value "0", the parent terminal 1-1 sends a parent terminal change stop packet to all the child terminals 2 existing under the parent terminal 1-1. The child terminal 2 which has received the parent terminal change stop packet ceases to send the parent terminal change notifying packet and maintains the current communication route built between itself and the parent terminal 1-1. The parent terminal change stop packet may be formed into an independent communication packet.

Alternatively, parent terminal change stop data may be stored within, e.g., an H packet.
On the other hand, if the child terminal 2-2 sends only the address request packet to the parent terminal 1-2 without sending the parent terminal change notifying packet to the parent terminal 1-1, time is required for the parent terminal 1-1 to detect the child terminal 2-2 broken away from the subject terminal. For example, the child terminal 2 sends a routing protocol packet such as a route notifying packet or the like every specified time. The route notifying packet stores the information on a communication route built between the subject terminal and the parent terminal 1. The route notifying packet is sent to the parent terminal 1 with which the subject terminal builds a communication route. Just when the parent terminal 1-1 fails to receive a routing protocol packet such as a route notifying packet or the like from the child terminal 2-2 for a specified time period, the parent terminal 1-1 can detect the child terminal 2-2 broken away from the subject terminal. In other words, the parent terminal 1-1 detects the breakaway of the child terminal 2-2 when a certain period of time lapses after the child terminal 2-2 is actually broken away from the subject terminal. Consequently, there may be a situation that the number of the child terminals 2 existing under the parent terminal 1-1 is unnecessarily decreased or the number of the child

terminals 2 existing under the parent terminal 1-2 is unnecessarily increased until the parent terminal 1-1 detects the breakaway of the child terminal 2-2.
In the present embodiment, as described above, the child terminal 2 that changes over the parent terminal 1 building a communication route sends a parent terminal change notifying packet to the old parent terminal 1 building the current communication route and sends an address request packet to a new parent terminal 1. If the residual record number of the old parent terminal 1 gets increased, the old parent terminal 1 sends a parent terminal change stop packet to all the child terminals 2 existing under thereof. Accordingly, the old parent terminal 1 can detect on a real time basis that the child terminal 2 is broken away from the subject terminal and can prevent an unnecessarily large number of child terminals 2 from breaking away from the subject terminal. That is to say, it is possible to prevent an unnecessary decrease in the number of the child terminals 2 coming under the old parent terminal 1 or an unnecessary increase in the number of the child terminals 2 coming under the new parent terminal 1. It is therefore possible to make uniform the number of the child terminals 2 coming under the parent terminal 1.
(Sixteenth Embodiment)
The configurations of the multi-hop communication system of the present embodiment remain the same as those of

the eleventh through fourteenth embodiments. The same configurations will be designated by like reference symbols with no description made thereon. With reference to a sequence shown in Fig. 17, description will now be made on how to build a communication route in the present wireless network.
In the present embodiment, the child terminal 2-2 which has received the parent terminal change request packet from the parent terminal 1-1 building a communication route with the subject terminal at the present time operates as follows.
The child terminal 2-2 which has received the parent terminal change request packet selects one parent terminal 1 which is smallest in the communication cost registered on the record-cost table TB3, e.g., the parent terminal 1-2, from one or more parent terminals 1 whose residual record number exceeds a threshold value "0" (S31) .
Then, the communication processing unit 2 02 of the child terminal 2-2 sends a parent terminal change feasibility packet to the child terminal 1-1 building a communication route with the subject terminal at the present time (S32).
In the parent terminal 1-1 which has received the parent terminal change feasibility packet, if the residual record number of the subject terminal in the communication route table TB21 is "0" (smaller than a threshold value) ,

the communication processing unit 202 sends a parent terminal change permission packet to the child terminal 2-2 serving as the source of the parent terminal change feasibility packet (S33). In addition, the table processing unit 201 of the parent terminal 1-1 deletes the communication route information on the child terminal 2-2 from the communication route table TB21 (see Fig. 4A) and increments the residual record number by 1. Thus, the parent terminal 1-1 can increase the residual record number of the subject terminal.
Then, the child terminal 2-2 which has received the parent terminal change permission packet from the parent terminal 1-1 sends an address request packet to the parent terminal 1-2 (S34). Thereafter, the child terminal 2-2 receives an address response packet from the parent terminal 1-2 in the same manner as mentioned above and sets a new terminal ID allotted by the parent terminal 1-2 to the subject terminal (S35). In other words, the child terminal 2-2 changes over the parent terminal 1 building a communication route with the subject terminal from the parent terminal 1-1 to the parent terminal 1-2.
In the present embodiment, as described above, the child terminal 2 that changes over the parent terminal 1 building a communication route sends a parent terminal change feasibility packet to the old parent terminal 1 building the current communication route. If the residual

record number of the subject terminal is smaller than the threshold value, the old parent terminal 1 which has received the parent terminal change feasibility packet sends a parent terminal change permission packet to the child terminal 2. After receiving the parent terminal change permission packet from the old parent terminal 1, the child terminal 2 sends an address request packet to a new parent terminal 1. Since the old parent terminal 1 sends the parent terminal change permission packet pursuant to the residual record number of the subject terminal, the old parent terminal 1 can detect on a real time basis that the child terminal 2 is broken away from the subject terminal and can prevent an unnecessarily large number of child terminals 2 from breaking away from the subject terminal.
For example, in the configuration of the fifteenth embodiment, it is likely that the child terminal 2 may change the communication route at a subtle timing overlapping with the sending and receiving timing of the parent terminal change stop packet (or the H packet carrying parent terminal change stop data). However, in the present embodiment, the parent terminal 1 determines, on a real time basis, the propriety of changing the communication route of the child terminal 2, depending on the residual record number of the subject terminal. Accordingly, it is possible to prevent an unnecessary decrease in the number of the child terminals 2 existing under the old parent terminal 1

or an unnecessary increase in the number of the child terminals 2 existing under the new parent terminal 1. It is therefore possible to make uniform the number of the child terminals 2 coming under the parent terminal 1.
In the respective embodiments described above, the threshold value compared with the residual record number is set equal to "0". However, the threshold value may be an integer equal to or larger than "1".
In the respective embodiments described above, the parent terminal 1 and the child terminal 2 may make up a PLC network, i.e., a communication network built through the use of a technology of power line communication. Even in this case, it is possible to obtain the same effects as mentioned above.
While certain preferred embodiments of the present invention have been described above, the present invention is not limited to these specific embodiments but may be changed or modified in many different forms without departing from the scope of the invention defined in the appended claims. Such changes or modifications shall be construed to fall within the scope of the invention.

1. A multi-hop communication system, comprising:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, each of the child terminals configured to build a communication route with one of the parent terminals,
wherein each of the child terminals is configured to decide one of the parent terminals with which the corresponding child terminal builds the communication route, based on a residual number of communication routes capable of being built between each of the parent terminals and the corresponding child terminal, a hop count of each of communication routes existing between the corresponding child terminal and each of the parent terminals, and a communication quality of each of the communication routes existing between the corresponding child terminal and each of the parent terminals.
2. A multi-hop communication system, comprising:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, each of the child terminals configured to build the communication route with one of the parent terminals,
wherein each of the parent terminals is configured to send a hello packet which contains residual record number

information indicating a residual number of communication routes capable of being built between each of the parent terminals and the child terminals and which notifies existence of the corresponding parent terminal,
each of the child terminals building the communication routes with the parent terminals is configured to send a hello packet which contains information on a residual record number of the parent terminal building communication routes with the corresponding child terminal and which notifies existence of the corresponding child terminal, and
each of the child terminals not building communication routes with the parent terminals is configured, upon receiving a plurality of the hello packets within a specified time, to calculate a communication cost for each of the parent terminals serving as a source of the hello packets and a communication cost for the parent terminal with which the child terminals serving as sources of the hello packet build the communication routes, based on a residual record number of each of the parent terminals, a hop count of each of the communication routes existing between the corresponding child terminal and the parent terminals and a communication quality of each of the communication routes existing between the corresponding child terminal and the parent terminals, and to build a communication route with the parent terminal which remains lowest in the communication cost.

3. The multi-hop communication system of claim 2, wherein each of the child terminals not building communication routes with the parent terminals is configured, upon receiving a plurality of the hello packets within a specified time, to select among the parent terminal serving as a source of the hello packet and the parent terminals with which the child terminals serving as sources of the hello packets build the communication routes, the parent terminals in which a hop count of each of the communication routes existing between the parent terminals and the corresponding child terminal is equal to or smaller than a specified threshold value, to calculate communication costs for the selected parent terminals based on the residual record number of each of the selected parent terminals and the communication quality of each of the communication routes existing between the corresponding child terminal and the selected parent terminals, and to build a communication route between the corresponding child terminal and the parent terminal which remains lowest in the communication cost, and
the specified threshold value of the hop count is set with respect to each of the parent terminals to become larger as the residual record number of each of the parent terminals grows larger.

4. The multi-hop communication system of claim 2, wherein
each of the child terminals not building communication
routes with the parent terminals is configured, upon
receiving a plurality of the hello packets within a
specified time, to select among the parent terminal serving
as a source of the hello packet and the parent terminals
with which the child terminals serving as sources of the
hello packet build the communication routes, the parent
terminals in which a communication quality level of each of
the communication routes existing between the parent
terminals and the corresponding child terminal is equal to
or higher than a specified threshold value, to calculate
communication costs for the selected parent terminals based
on the residual record number of each of the selected parent
terminals and the hop count of each of the communication
routes existing between the corresponding child terminal and
the selected parent terminals, and to build a communication
route between the corresponding child terminal and the
parent terminal which remains lowest in the communication
cost, and
the specified threshold value of the communication quality level is set with respect to each of the parent terminals to become smaller as the residual record number of each of the parent terminals grows larger.
5. The multi-hop communication system of claim 2, wherein

each of the child terminals not building communication routes with the parent terminals is configured, upon receiving a plurality of the hello packets within a specified time, to calculate a communication cost for the parent terminal serving as a source of the hello packet and a communication cost for each of the parent terminals with which the child terminals serving as sources of the hello packets build the communication routes, by giving weights to the residual record number of each of the parent terminals, the hop count of each of the communication routes existing between the corresponding child terminal and the parent terminals and the communication quality of each of the communication routes existing between the corresponding child terminal and the parent terminals, and
the weight given to the residual record number is changed in such a direction that the communication cost becomes higher as the hop count grows larger.
6. A multi-hop communication system, comprising:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, each of the child terminals configured to build communication routes with one of the parent terminals, the parent terminals including a first parent terminal and a second parent terminal,
wherein each of the child terminals building a

communication route with the first parent terminal is configured, if a residual number of the communication route capable of being built between the first parent terminal and the corresponding child terminal becomes equal to or smaller than a first threshold value, to build a communication route between the corresponding child terminal and the second parent terminal in which the residual number of the communication route capable of being built between the second parent terminal and the corresponding child terminal exceeds a second threshold value.
7. A multi-hop communication system, comprising:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, each of the child terminals configured to build communication routes with one of the parent terminals, the parent terminals including a first parent terminal and a second parent terminal,
wherein each of the parent terminals is configured to send a hello packet which contains residual record number information indicating a residual number of communication routes capable of being built between the corresponding parent terminal and the child terminals and which notifies existence of the corresponding parent terminal,
each of the child terminals building the communication routes with the parent terminals is configured to send a

hello packet which contains information on a residual record number of the parent terminal building communication routes with the corresponding child terminal and which notifies existence of the corresponding child terminal,
each of the child terminals not building communication routes with the parent terminals is configured, upon receiving a plurality of the hello packets within a specified time, to calculate a communication cost for the parent terminal serving as a source of the hello packet and a communication cost for each of the parent terminals with which the child terminals serving as sources of the hello packets build the communication, routes, based on at least a communication quality of each of the communication routes existing between the corresponding child terminal and the parent terminals, and to build a communication route with the parent terminal which remains lowest in the communication cost, and
each of the child terminals building a communication route with the first parent terminal is configured to build a communication route with the second parent terminal, if the residual record number of the first parent terminal pursuant to the hello packet received from the first parent terminal or the child terminals building the communication routes with the first parent terminal becomes equal to or smaller than a first threshold value and if the residual record number of the second parent terminal pursuant to the

hello packet received from the second parent terminal or the child terminals building the communication routes with the second parent terminal exceeds a second threshold value.
8. The multi-hop communication system of claim 7, wherein each of the child terminals building the communication route with the first parent terminal is configured, if the corresponding child terminal serves as a relay terminal between other child terminals and the parent terminals, to continuously build the communication route with the first parent terminal even when the residual record number of the first parent terminal becomes equal to or smaller than the first threshold value.
9. The multi-hop communication system of claim 7, wherein each of the child terminals building the communication route with the first parent terminal is configured, if the corresponding child terminal serves as a relay terminal between other child terminals and the parent terminals, to increase the first threshold value as the number of other child terminals using the corresponding child terminals as a relay terminal grows smaller.
10. The multi-hop communication system of claim 7 or 9, wherein each of the child terminals building the communication route with the first parent terminal is

configured, if the corresponding child terminal serves as a relay terminal between other child terminals and the parent terminals, to reduce the second threshold value as the number of other child terminals using the corresponding child terminal as a relay terminal grows smaller.
11. The multi-hop communication system of claim 7, wherein
each of the child terminals building the communication route
with the first parent terminal is configured, if the
residual record number of the first parent terminal becomes
equal to or smaller than the first threshold value and if
the residual record number of the second parent terminal
exceeds the second threshold value, to send a parent
terminal change notifying packet to the first parent
terminal and to build a communication route with the second
parent terminal, and
the first parent terminal which has received the parent terminal change notifying packet is configured to increment the residual record number of the first parent terminal.
12. The multi-hop communication system of claim 7, wherein
each of the child terminals building the communication route
with the first parent terminal is configured, if the
residual record number of the first parent terminal becomes
equal to or smaller than the first threshold value and if
the residual record number of the second parent terminal

exceeds the second threshold value, to send a parent terminal change feasibility packet to the first parent terminal,
the first parent terminal which has received the parent terminal change feasibility packet is configured, if the residual record number of the first parent terminal is equal to or smaller than the first threshold value, to send a parent terminal change permission packet to the child terminals serving as sources of the parent terminal change feasibility packet and to increment the residual record number of the first parent terminal, and
each of the child terminals which has received the parent terminal change permission packet is configured to build a communication route with the second parent terminal.
13. A multi-hop communication system, comprising:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, each of the child terminals configured to build a communication route with one of the parent terminals,
wherein each of the parent terminals is configured, if a residual number of communication routes capable of being built between the parent terminals and the child terminals becomes equal to or smaller than a specified threshold value and if a communication route is built between the parent terminals and new child terminals, to ask a communication

route change to the child terminals which are building communication routes with the corresponding parent terminal and which are capable of building communication routes with other parent terminals.
14. A multi-hop communication system, comprising:
a plurality of parent terminals and a plurality of child terminals making multi-hop communication with each other, each of the child terminals configured to build communication routes with one of the parent terminals,
wherein each of the parent terminals is configured to send a hello packet which contains residual record number information indicating a residual number of communication routes capable of being built between the corresponding parent terminal and the child terminals and which notifies existence of the corresponding parent terminal,
each of the child terminals building the communication routes with the parent terminals is configured to send a hello packet which contains information on a residual record number of the parent terminal building communication routes with the corresponding child terminal and which notifies existence of the corresponding child terminal,
each of the child terminals not building communication routes with the parent terminals is configured to determine whether, among the parent terminals serving as a source of the hello packets and the parent terminal with which the

child terminals serving as sources of the hello packet build the communication routes, the parent terminal or terminals having a residual record number exceeding a specified threshold value exists, to build a communication route with one of the parent terminals having the residual record number exceeding the threshold value if there exist the parent terminals having the residual record number exceeding the threshold value, and to send a joining failure notifying packet to one or more parent terminals among the parent terminals serving as a source of the hello packets and the parent terminal with which the child terminals serving as sources of the hello packet build the communication routes if the parent terminals having the residual record number exceeding the threshold value do not exist,
each of the parent terminals which has received the joining failure notifying packet is configured to send a parent terminal change request packet to the child terminals building communication routes with the corresponding parent terminal, and
each of the child terminals which has received the parent terminal change request packet and which has received the hello packet containing the residual record number information of the parent terminals not building communication routes with the corresponding child terminal is configured to build a communication route with one of the parent terminals having the residual record number exceeding

the threshold value among the parent terminals not building communication routes with the corresponding child terminal.
15. The multi-hop communication system of claim 14, wherein
each of the child terminals building the communication
routes with the parent terminals is configured to receive
the hello packet containing the residual record number
information of the parent terminal not building
communication route with the corresponding child terminal
and to send a topology notifying packet containing the
residual record number information included in the hello
packet to the parent terminal building the communication
routes with the corresponding child terminal, and
the parent terminal which has received the topology notifying packet is configured, upon receiving the joining failure notifying packet, to identify, based on the topology notifying packet, the child terminals which have received the hello packets from the parent terminals having the residual record number exceeding the threshold value and to send the parent terminal change request packet to only the identified child terminals corresponding in number to the child terminals serving as sources of the joining failure notifying packet.
16. The multi-hop communication system of claim 14, wherein
each of the child terminals building the communication

routes with the parent terminals is configured to receive the hello packet containing the residual record number information of the parent terminal not building communication routes with the corresponding child terminal and to send a topology notifying packet containing the residual record number information included in the hello packet to the parent terminal building the communication routes with the corresponding child terminal, and
the parent terminal which has received the topology notifying packet is configured, upon receiving the joining failure notifying packet, to identify, based on the topology notifying packet, the child terminals which have received the hello packet from the parent terminals having the residual record number exceeding the threshold value and to send the parent terminal change request packet to the identified child terminals with a priority given to the child terminal which is larger in the number of the parent terminals whose residual record number exceeds the threshold value.
17. The multi-hop communication system of claim 14, wherein the parent terminal which has received the joining failure notifying packet is configured to send the parent terminal change request packet to the child terminals which do not serve as a relay terminal between the parent terminals and other child terminals, among the child terminals building

communication routes with the corresponding parent terminal.
18. The multi-hop communication system of claim 14, wherein the parent terminal which has received the joining failure notifying packet is configured to send the parent terminal change request packet to the child terminals building communication routes with the corresponding parent terminal with a priority given to the child terminal which is smaller in the number of other child terminals using the corresponding child terminal as a relay terminal.
19. A child terminal for use in the multi-hop communication system recited in any one of claims 1, 2, 6, 7, 13 and 14.
20. A parent terminal for use in the multi-hop communication system recited in any one of claims 1, 2, 6, 7, 13 and 14.