DESCRIPTION Title of Invention:
WIRELESS COMMUNICATION DEVICE AND WIRELESS COMMUNICATION
PROGRAM
Technical Field
[0001] The present invention relates to a technology for performing wireless
communication between wireless stations.
Background Art
[0002] A wireless mesh network is configured of a plurality of wireless devices for
the purpose of collecting various automation information or various sensor information
in a wireless manner.
In the wireless mesh network, routing protocol for low power and lossy networks (RPL), which is one of routing protocols, is used.
RPL is a routing protocol aimed at using low-power-consumption or low-resource terminals (for example, wireless sensors) in an unstable communication environment, and is standardized by IETF RFC 6550 (see Non-Patent Literature 1). [0003] RPL is formed to have a tree configuration called destination oriented directed acyclic graph (DODAG).
In RPL, a path control message called DODAG Information object (DIO) is used to construct an upstream-direction path.
Also, in RPL, a path control message called destination advertisement object (DAO) is used to construct a downstream-direction path.
Besides, in RPL, a path control message called DODAG Information solicitation (DIS) is used to search for an upstream path. [0004] A wireless station which performs path construction by the RPL protocol

constructs a path by transmitting and receiving a DIO and a DAO. More specifically, the wireless station constructs the path by transmitting/receiving path control messages to/from nearby wireless stations and selecting connection destinations in the path such that the connection destinations are as close as possible to a master station in the path.
A DIO is a message employed by a wireless station having constructed a route and serves to inform the existence of the wireless station. The DIO is transmitted by broadcast.
The DIO is transmitted from wireless stations including the master station. Each of the wireless stations except the master station selects an upstream path from among DIOs received from other wireless stations.
A DAO is a message for requesting connection of a downstream path and is transmitted by unicast to a wireless station selected as an upstream-side wireless station by a downstream-side wireless station. The upstream-side wireless station receives the DAO and sets the sender of the DAO as the downstream-side wireless station. [0005] For example, suppose that a master station #0 transmits a DIO by broadcast. Then, the DIO of the master station #0 reaches each of a slave station #1 and a slave station #2 existing within coverage of the master station #0. Each of the slave station #1 and slave station #2 selects the master station #0 as the upstream-side wireless station and transmits a DAO to the master station #0 by unicast. Thus, the slave station 1 and the slave station #2 complete path construction.
The slave station #1 and the slave station #2 that have completed the path construction transmit the DIOs by broadcast. The DIO of the slave station #2 reaches the master station #0, the slave station#l, and a slave station #3 that are within the coverage of the slave station #2. Only the DIO of the slave station #2 reaches the coverage of the slave station #3. Therefore, the slave station #3 transmits the DAO to

the slave station #2 by unicast, and completes path construction. The slave station #2 transmits the DAO of the slave station #3 to the master station #0 which is the upstream-side wireless station.
[0006] The upstream-side wireless station and the downstream-side wireless station are located within the communication ranges of each other. A communication range will be referred to as coverage.
The coverage of the master station #0 and the coverage of the slave station #3 do not overlap. Accordingly, the master station #0 and the slave station #3 cannot connect their paths directly. Therefore, the slave station #2 whose coverage overlaps the coverage of the master station #0 and the coverage of the slave station #3 serves as a relay.
[0007] Between wireless stations whose coverages overlap, carrier sensing is always performed before transmission of messages in order to avoid message collision as much as possible. Each wireless station confirms that the other wireless stations are not transmitting messages, and then starts transmitting a message.
[0008] Transmission of a DIO and a DAO by each wireless station is performed not only at the time of path construction after startup, but also periodically to enable detection of a path abnormality. This keeps the paths maintained.
If a DIO is not received from the upstream-side wireless station for a certain period of time, the downstream-side wireless station determines that it cannot communicate with the upstream-side wireless station. Then, the downstream-side wireless station changes the path by selecting a new upstream-side wireless station.
If a DAO from a station on the downstream path is not received for a certain period of time, a local station determines that it cannot communicate with that station on the downstream path, and deletes the downstream path leading to that station on the

downstream path. Citation List Patent Literature
[0009] Patent Literature 1: JP 2014-216726 A Non-Patent Literature
[0010] Non-Patent Literature 1: RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", IETF, RFC 6550, March, 2012. Summary of Invention Technical Problem
[0011] Patent Literature 1 discloses instructing a change in transmission interval in accordance with the number of wireless stations within coverage. Changing the transmission interval by each wireless station side reduces collision of messages.
It should be noted that the technique disclosed in Patent Literature 1 is a technique for avoiding a collision with a message transmitted from a wireless station existing within the same coverage. [0012] A wireless mesh network using RPL has the following problems.
Each wireless station cannot detect that a wireless station not located in the coverage is transmitting a message. Therefore, if a first wireless station in first coverage and a second wireless station in second coverage transmit messages simultaneously, the message of the first wireless station and the message of the second wireless station collide in a third wireless station included in both the first coverage and the second coverage.
[0013] Each wireless station performs DIO transmission when a periodic timer for DIO transmission expires. The downstream-side wireless station that has received a DIO from the upstream-side wireless station performs DAO transmission when a

periodic timer for DAO transmission expires. This operation is repeated periodically. Hence, each wireless station maintains the path.
However, when a DIO or DAO collision occurs, the maintenance of the path is suspended until the next opportunity.
[0014] It may be possible to keep maintaining the path by setting a long period for detecting a path abnormality in case a message transmission/reception failure occurs including such a collision.
However, if the detection period of the path abnormality is extended, the time required to detect the path abnormality also becomes long. If detection of the path abnormality is delayed, system restoration necessitated due to a route change will take extra time, and data communication cannot be performed correctly until system restoration.
More specifically, when a plurality of wireless stations use the same wireless station as the upstream-side wireless station, the plurality of wireless stations perform DAO transmission at the opportunity of DIO reception from the same wireless station. Since the plurality of wireless stations have different coverages, there is a possibility that a plurality of DAOs transmitted from different coverages may collide in the upstream-side wireless station. The more the downstream-side wireless stations, the higher the probability that DAOs transmitted from different coverages will collide in the upstream-side wireless station.
[0015] The collision probability can be reduced if a random number is applied to the DAO transmission period in a wide range. The RPL does not specifically define the DAO transmission period.
However, there is no definite means that applies a random number to the DAO transmission period in a wide range.

It is therefore necessary to set the range of random numbers to be applied to the DAO transmission period by assuming the maximum number of wireless stations that are to be terminated in the network.
As a result, the larger the number of wireless stations serving as relay stations, the longer the time required for a DAO from each slave station to reach the master station.
For example, when a DAO of the slave station #3 reaches the master station #0 via the slave station #2, the total of the period required for transmission of the DAO from the slave station #3 to the slave station #2 and the period required for transmission of the DAO from the slave station #2 to the master station #0 is the time required for the master station #0 to detect that the slave station #3 has completed the network connection.
[0016] The maximum necessary time required for a DAO from a slave station to reach a master station is determined by the following equation:
Maximum Necessary Time = Maximum Time of DAO Transmission Period
x (relay station count +1) where:
DAO transmission period is determined by random numbers; and
"+ 1" signifies DAO transmission from a slave station to the first relay station. [0017] That is, when the range of random numbers applied to the DAO transmission period is extended largely, the time required for path construction at startup and the time required for path change during operation increase in the whole network. [0018] It is an objective of the present invention to enable transmission of a message in an appropriate transmission period. Solution to Problem

[0019] A wireless communication device according to the present invention operates as a local station which is one of wireless stations.
The wireless communication device includes:
a storage unit to store local coverage information being information of wireless stations existing within coverage of the local station, and downstream coverage information being information of wireless stations existing within coverage of a downstream station which is one of the wireless stations existing within the coverage of the local station;
a reception unit to receive a downstream message;
a timer unit to start a downstream timer when the downstream message is received, the downstream timer being set with a downstream transmission period determined based on the local coverage information and the downstream coverage information which are stored in the storage unit; and
a transmission unit to transmit a message including the local coverage information stored in the storage unit, when the downstream timer expires. Advantageous Effects of Invention
[0020] According to the present invention, a message can be transmitted in an appropriate transmission period determined based on local coverage information and downstream coverage information. Brief Description of Drawings
[0021] Fig. 1 is a configuration diagram of a wireless communication device 100 in Embodiment 1.
Fig. 2 is a configuration diagram of a wireless communication system 200 in Embodiment 1.
Fig. 3 is a diagram illustrating a format of a DIO in Embodiment 1.

Fig. 4 is a diagram illustrating a format of a DODAG configuration in Embodiment 1.
Fig. 5 is a configuration diagram of a coverage management table 210 in Embodiment 1.
Fig. 6 is a flowchart of an operation of a master station in Embodiment 1.
Fig. 7 is a flowchart of an operation of a slave station in Embodiment 1.
Fig. 8 is a flowchart of an operation of the slave station in Embodiment 1.
Fig. 9 is a flowchart of an operation of the slave station in Embodiment 1.
Fig. 10 is a flowchart of update of a random number range (S300) in Embodiment 1.
Fig. 11 is a flowchart of update of the random number range (S300) in Embodiment 1.
Fig. 12 is a flowchart of timer start processing in Embodiment 1.
Fig. 13 is a flowchart of message generation processing in Embodiment 1.
Fig. 14 is a flowchart of reception processing in Embodiment 1.
Fig. 15 is a flowchart of transmission processing in Embodiment 1.
Fig. 16 is a flowchart of registration of an upstream station in Embodiment 1.
Fig. 17 is a flowchart of registration of a downstream station in Embodiment 1.
Fig. 18 is a flowchart of update of local coverage information in Embodiment 1.
Fig. 19 is a diagram illustrating a format of transit information in Embodiment 2.
Fig. 20 is a flowchart of reception processing in Embodiment 2.
Fig. 21 is a flowchart of message generation processing in Embodiment 2.
Fig. 22 is a configuration diagram of a wireless communication device 100 in

Embodiment 3.
Fig. 23 is a configuration diagram of a wireless communication device 100 in Embodiment 5.
Fig. 24 is a flowchart of reception processing in Embodiment 5.
Fig. 25 is a hardware configuration diagram of the wireless communication device 100 in Embodiments. Description of Embodiments
[0022] The same reference numeral denotes the same elements and equivalent elements throughout the embodiments and drawings. Description of the elements denoted by the same reference numeral will be omitted or simplified appropriately. Arrows in the drawings mainly illustrate flows of data or flows of processing. [0023] Embodiment 1.
An embodiment in which an appropriate DIO transmission period and an appropriate DAO transmission period are determined will be described referring to Figs. 1 to 18. [0024] * * * Description of Configuration * * *
A configuration of a wireless communication device 100 will be described referring to Fig. 1.
The wireless communication device 100 is a computer provided with hardware devices such as a processor 901, a memory 902, an auxiliary storage device 903, an RF circuit 904, a PHY control circuit 905, an antenna 906, and an oscillator 907. These hardware devices are connected to each other via signal lines. [0025] The processor 901 is an integrated circuit (IC) which performs computing processing and controls the other hardware devices. For example, the processor 901 is a central processing unit (CPU), a digital signal processor (DSP), or a graphics

processing unit (GPU).
The memory 902 is a volatile storage device. The memory 902 is also called a main storage device or a main memory. For example, the memory 902 is a random access memory (RAM). Data stored in the memory 902 is backed up in the auxiliary storage device 903 where necessary.
The auxiliary storage device 903 is a non-volatile storage device. For example, the auxiliary storage device 903 is a read only memory (ROM), a hard disk drive (HDD), or a flash memory. The data stored in the auxiliary storage device 903 is loaded to the memory 902 where necessary.
[0026] The RF circuit 904 is a circuit that performs wireless communication. RF is an abbreviation for Radio Frequency.
The PHY control circuit 905 is a circuit that controls a physical layer. PHY is an abbreviation for Physical Layer.
The antenna 906 is an antenna for implementing wireless communication.
The oscillator 907 is a circuit that generates a clock pulse. [0027] The wireless communication device 100 is provided with software elements such as a coverage management unit 110, a timer unit 120, a message generation unit 130, and a path management unit 140. The software elements are the elements implemented by software.
[0028] The auxiliary storage device 903 stores a wireless communication program that causes the computer to function as the coverage management unit 110, the timer unit 120, the message generation unit 130, and the path management unit 140. The wireless communication program is loaded to the memory 902 and executed by the processor 901.
The auxiliary storage device 903 further stores an operating system (OS). At

least part of the OS is loaded to the memory 902 and executed by the processor 901.
That is, the processor 901 executes a wireless communication program while executing the OS.
Data obtained by execution of the wireless communication program is stored in a storage device such as the memory 902, the auxiliary storage device 903, a register in the processor 901, and a cache memory in the processor 901. [0029] The memory 902 serves as a storage unit 191 to store data. Another storage device may serve as the storage unit 191 on behalf of the memory 902 or together with the memory 902.
The RF circuit 904 serves as a reception unit 192 to receive data. The RF circuit 904 also serves as a transmission unit 193 to transmit data. [0030] The wireless communication device 100 may be provided with a plurality of processors that replace the processor 901. The plurality of processors share the role of the processor 901.
[0031] The wireless communication program can be computer-readably stored in a non-volatile storage medium such as a magnetic disk, an optical disk, and a flash memory. The non-volatile storage medium is a non-transitory tangible medium. [0032] A configuration of a wireless communication system 200 will be described referring to Fig. 2.
The wireless communication system 200 is a system provided with a plurality of wireless stations.
Solid-line circles represent wireless stations. Reference numerals in the solid-line circles are station numbers to identify wireless stations.
The wireless communication device 100 serves as a wireless station. [0033] A wireless station #0 is a master station in the wireless communication system

200.
Wireless stations #1 to #9 are slave stations in the wireless communication system 200.
[0034] The range where an electric wave from a wireless station reaches, that is, the communication range of the wireless station, will be referred to as coverage.
Broken-line ellipses represent coverage of the wireless stations #0, coverage of the wireless station #1, coverage of the wireless station #5, and coverage of the wireless station #6.
[0035] Wireless stations existing in coverage will be referred to as intra-coverage stations.
A number of intra-coverage stations will be referred to as an intra-coverage station count.
A list of wireless stations existing in coverage will be referred to as intra-coverage station list.
The intra-coverage station count and the intra-coverage station list will be referred to as coverage information.
For example, the intra-coverage stations of the wireless station #6 are the wireless station #1, the wireless station #6, the wireless station #7, and the wireless station #9. That is, the intra-coverage station count of the wireless station #6 is 4. [0036] The intra-coverage stations that overlap between coverages will be referred to as overlapping stations.
The number of intra-coverage stations that overlap between coverages will be referred to as overlapping station count.
For example, the overlapping stations that overlap between the coverage of the wireless station #6 and the coverage of the wireless station #1 are the wireless station 1,

the wireless station #6, and the wireless station #7. That is, the overlapping station count between the coverage of the wireless station #6 and the coverage of the wireless station #1 is 3. [0037] Each wireless station operates as a local station.
The local station communicates directly with an intra-coverage station in the local station.
The local station communicates with a wireless station existing outside the coverage of the local station via an intra-coverage station of the local station.
For example, the wireless station #6 communicates with each of the wireless station #1, the wireless station #7, and the wireless station #9 directly. The wireless station #6 also communicates with the wireless station #0 via the wireless station #1. [0038] Thick broken lines represent communication paths.
In a communication path, a direction to approach the wireless station #0 will be referred to as upstream, and a direction to separate from the wireless station #0 will be referred to as downstream. An upstream-side communication path will be referred to as an upstream path. A downstream-side communication path will be referred to as a downstream path.
In the upstream path, a wireless station located next to the local station will be referred to as an upstream station.
In the downstream path, a wireless station located next to the local station will be referred to as a downstream station.
For example, the upstream station of the wireless station #6 is the wireless station #1. The downstream station of the wireless station #6 is the wireless station #9. [0039] Coverage information of the local station will be referred to as local coverage information. Coverage information of the upstream station will be referred to as

upstream coverage information. Coverage information of the downstream station will be referred to as downstream coverage information.
[0040] In the wireless communication system 200, a path control message for constructing the communication path is communicated.
A path control message transmitted in the downstream direction, that is, a downstream message, will be referred to as a DIO. The DIO is communicated by broadcast.
A path control message transmitted in the upstream direction, that is, an upstream message, will be referred to as a DAO. The DAO is transmitted to an upstream station as a destination by unicast. [0041 ] Fig. 3 illustrates a format of the DIO.
A format of the DIO is defined in Non-Patent Literature 1.
An option not defined in Non-Patent Literature 1 can be added to the DIO.
Coverage information is stored in the DIO as an additional option. [0042] Fig. 4 illustrates a format of a DODAG configuration which is one of options of the DIO.
Random number ranges are set in the field of DIOIntDoubl. and the field of DIOIntMin. That is, the random number ranges are stored in the DIO as a configuration option. [0043] A coverage management table 210 will be described referring to Fig. 5.
The coverage management table 210 is a table for managing the coverage information and the random number ranges. The coverage management table 210 is stored in the storage unit 191.
The coverage management table 210 has fields for wireless station, direction, intra-coverage station count, intra-coverage station list, first random number range, and

second random number range.
The field for wireless station indicates an identifier of the wireless station.
The field for direction indicates upstream or downstream.
The field for intra-coverage station count indicates the intra-coverage station count.
The field for intra-coverage station list indicates the intra-coverage station list.
The field for first random number range indicates the first random number range which is a random number range corresponding to the intra-coverage station count.
The field for second random number range indicates the second random number range which is a random number range corresponding to the intra-coverage station list.
The random number ranges are the ranges of random numbers used in determination of a period with which a DIO or DAO is transmitted.
A random number range associated with an upstream station will be referred to as an upstream random number range. The upstream random number range is used in determination of the period with which the DAO is transmitted.
A random number range associated with a downstream station will be referred to as a downstream random number range. The downstream random number range is used in determination of the period with which the DIO is transmitted. [0044] The coverage management table 210 of Fig. 5 is a coverage management table 210 of the wireless station #6 in the wireless communication system 200 of Fig. 2.
The intra-coverage station count of the local station #6 is 4. The intra-coverage station list of the local station #6 is (#1, #6, #7, #9). [0045] The upstream station of the local station #6 is the wireless station #1.

The intra-coverage station count of the upstream station #1 is 7. The intra-coverage station list of the upstream station #1 is (#0, #1, #2, #3, #4, #6, #7).
The first random number range of the upstream station #1 is a random number range corresponding to the sum total (11 stations) of the intra-coverage station counts of the local station #6 and upstream station #1.
The overlapping station count between the local station #6 and the upstream station #1 is 3 (#1, #6, #7).
The second random number range of the upstream station #1 is a random number range corresponding to a station count (8 stations) obtained by subtracting the overlapping station count (3 stations) from the sum total (11 stations) of the intra-coverage station counts of the local station #6 and upstream station #1. [0046] The downstream station of the local station #6 is the wireless station #9.
The intra-coverage station count of the downstream station #9 is 3. The intra-coverage station list of the downstream station #9 is (#6, #7, #9).
The first random number range of the downstream station #9 is a random number range corresponding to the sum total (7 stations) of the intra-coverage station counts of the local station #6 and downstream station #9.
The overlapping station count between the local station #6 and the downstream station #9 is 3 (#1, #6, #7).
The second random number range of the downstream station #9 is a random number range corresponding to a station count (4 stations) obtained by subtracting the overlapping station count (3 stations) from the sum total (7 stations) of the intra-coverage station counts of the local station #6 and downstream station #9. [0047] *** Description of Operation ***
An operation of the wireless communication device 100 corresponds to a

wireless communication method. A procedure of the wireless communication method
corresponds to a procedure of the wireless communication program.
[0048] An operation of the master station will be described referring to Fig. 6.
Referring to Fig. 6, the flow of the operation of the master station will mainly be described. The details of processes will be described later as operations of elements of a wireless station.
When the master station activates a wireless function, wireless communication is enabled in the master station. After activating the wireless function, the master station operates as follows. [0049] In step S101, the timer unit 120 starts a DIO timer.
The DIO timer is a timer in which a transmission period of a DIO is set.
The DIO timer will be referred to as a downstream timer. The DIO transmission period will be referred to as a downstream transmission period. [0050] In step SI 02, if the DIO timer expires, the timer unit 120 detects expiration of the DIO timer. Expiration of the timer signifies a lapse of time being set in the timer. For example, Trickle timer indicated in Non-Patent Literature 1 is used.
If the DIO timer expires, the processing proceeds to step SI 03.
If the DIO timer does not expire, the processing proceeds to step Sill. [0051] In step SI03, the message generation unit 130 generates a DIO, and the transmission unit 193 transmits the DIO by broadcast.
After step S103, the processing proceeds to step SI01. [0052] In step SI 11, if the DIO transmitted from a slave station reaches the antenna 906, the reception unit 192 receives the DIO.
If a DIO is received, the processing proceeds to step S300.
If a DIO is not received, the processing proceeds to step S121.

[0053] In step S300, the coverage management unit 110 performs update of the random number range.
After update of the random number range, the processing proceeds to step S121.
[0054] In step S121, if a DAO transmitted from a slave station reaches the antenna 906, the reception unit 192 receives the DAO.
If a DAO is received, the processing proceeds to step SI22.
If a DAO is not received, the processing proceeds to step SI02. [0055] In step S122, the path management unit 140 registers the sender of the DAO to path information as the downstream station.
The path information is information to which an upstream station and a downstream station are registered. The path information is stored in the storage unit 191.
After step SI22, the processing proceeds to step SI02. [0056] An operation of the slave station will be described referring to Figs. 7, 8, and 9.
Referring to Figs. 7 to 9, the flow of the operation of the slave station will mainly be described. The details of each process will be described later as a process of each corresponding element of the wireless station.
When the slave station activates a wireless function, wireless communication is enabled in the slave station. After activating the wireless function, the slave station operates as follows.
[0057] In step S201 (see Fig. 7), if a DIO transmitted from the master station or another slave station reaches the antenna 906, the reception unit 192 receives the DIO.
If a DIO is received, the processing proceeds to step S300.

If a DIO is not received, the processing proceeds to step S201. [0058] In step S300, the coverage management unit 110 performs update of the random number range.
After update of the random number range, the processing proceeds to step S211.
[0059] In step S211, the timer unit 120 starts the DIO timer. [0060] In step S212, the timer unit 120 starts a DAO timer.
The DAO timer is a timer in which a transmission period of a DAO is set.
The DAO timer will be referred to as an upstream timer. The DAO transmission period will be referred to as an upstream transmission period. [0061] In step S221, if the DIO timer expires, the timer unit 120 detects expiration of the DIO timer.
If the DIO timer expires, the processing proceeds to step S222.
If the DIO timer does not expire, the processing proceeds to step S231. [0062] In step S222, the message generation unit 130 generates a DIO, and the transmission unit 193 transmits the DIO by broadcast.
After step S222, the processing proceeds to step S223. [0063] In step S223, the timer unit 120 starts the DIO timer.
[0064] In step S231, if the DAO timer expires, the timer unit 120 detects expiration of the DAO timer.
If the DAO timer expires, the processing proceeds to step S232.
If the DAO timer does not expire, the processing proceeds to step S221. [0065] In step S232, the message generation unit 130 generates a DAO, and the transmission unit 193 transmits the DAO to the upstream station by unicast.
After step S232, the processing proceeds to step S241 (see Fig. 8).

[0066] A communication path is constructed by the processing of step S201 to step S232.
After that, the processing of step S241 and steps beyond is executed. [0067] In step S241 (see Fig. 8), if a DIO transmitted from the master station or another slave station reaches the antenna 906, the reception unit 192 receives the DIO.
If a DIO is received, the processing proceeds to step S300.
If a DIO is not received, the processing proceeds to step S251. [0068] In step S300, the coverage management unit 110 performs update of the random number range.
After update of the random number range, the processing proceeds to step S242. [0069] In step S242, the timer unit 120 determines whether the DAO timer is started.
If the DAO timer is started, the processing proceeds to step S251.
If the DAO timer is not started, the processing proceeds to step S243. [0070] In step S243, the timer unit 120 starts the DAO timer. [0071] In step S251, if a DAO transmitted from another slave station reaches the antenna 906, the reception unit 192 receives the DAO.
If a DAO is received, the processing proceeds to step S252.
If a DAO is not received, the processing proceeds to step S261 (see Fig. 9). [0072] In step S252, the path management unit 140 registers the sender of the DAO to the path information as the downstream station. [0073] In step S253, the timer unit 120 determines whether the DAO timer is started.
If the DAO timer is started, the processing proceeds to step S261 (see Fig. 9).
If the DAO timer is not started, the processing proceeds to step S254. [0074] In step S254, the timer unit 120 starts the DAO timer.

After step S254, the processing proceeds to step S261 (see Fig. 9). [0075] In step S261 (see Fig. 9), if the DIO timer expires, the timer unit 120 detects expiration of the DIO timer.
If the DIO timer expires, the processing proceeds to step S262.
If the DIO timer does not expire, the processing proceeds to step S271. [0076] In step S262, the message generation unit 130 generates a DIO, and the transmission unit 193 transmits the DIO by broadcast. [0077] In step S263, the timer unit 120 starts the DIO timer.
[0078] In step S271, if the DAO timer expires, the timer unit 120 detects expiration of the DAO timer.
If the DAO timer expires, the processing proceeds to step S272.
If the DAO timer does not expire, the processing proceeds to step S241 (see Fig. 8).
[0079] In step S272, the message generation unit 130 generates a DAO, and the transmission unit 193 transmits the DAO to the upstream station by unicast.
After step S272, the processing proceeds to step S241 (see Fig. 8). [0080] Update of the random number range (S300) will be described referring to Fig. 10.
Update of the random number range (S300) is executed mainly by the coverage management unit 110 when a DIO is received.
[0081 ] In step S301, the coverage management unit 110 determines whether the sender of the DIO is either an upstream station or a downstream station.
More specifically, the coverage management unit 110 determines whether the sender of the DIO is either a wireless station registered in the path information as an upstream station or a wireless station registered in the path information as a downstream

station.
If the sender of the DIO is an upstream station or a downstream station, the processing proceeds to step S311.
If the sender of the DIO is neither an upstream station nor a downstream station, the processing proceeds to step S302.
[0082] In step S302, the coverage management unit 110 determines whether the local station is a slave station.
More specifically, attribute information designating a master station or a slave station is stored in the storage unit 191 in advance. The coverage management unit 110 refers to the attribute information to determine whether the local station is a slave station.
If the local station is a slave station, the processing proceeds to step S303.
If the local station is a master station, the random number range is not updated, and update of the random number range ends.
[0083] In step S303, the coverage management unit 110 determines whether an upstream station is registered in the path information.
If an upstream station is unregistered, the processing proceeds to step S304.
If an upstream station is registered, the random number range is not updated, and update of the random number range ends.
[0084] In step S304, the coverage management unit 110 registers the sender of the DIO to the path information as the upstream station.
[0085] In step S311, the coverage management unit 110 updates the intra-coverage station count of the sender which is registered with the coverage management table 210 to the intra-coverage station count of the sender which is set in the DIO. [0086] In step S312, the coverage management unit 110 updates the first random

number range of the sender which is registered with the coverage management table
210.
[0087] More specifically, the coverage management unit 110 updates the first random
number range of the sender as follows.
First, the coverage management unit 110 acquires the intra-coverage station count of the local station and the intra-coverage station count of the sender from the coverage management table 210.
Subsequently, the coverage management unit 110 calculates the sum total of the intra-coverage station count of the local station and the intra-coverage station count of the sender. The calculated value will be referred to as a first sum total station count. The first sum total station count is expressed by the following equation:
First Sum Total Station Count = Intra-Coverage Station Count of Local Station
+ Intra-Coverage Station Count of Sender [0088] Subsequently, the coverage management unit 110 determines the first random number range on the basis of the first sum total station count. The smaller the first sum total station count, the narrower the first random number range. The larger the first sum total station count, the wider the first random number range.
The coverage management unit 110 then updates the first random number range of the sender which is registered with the coverage management table 210 to the determined first random number range.
[0089] After step S312, the processing proceeds to step S321 (see Fig. 11). [0090] In step S321 (see Fig. 11), the coverage management unit 110 determines whether an intra-coverage station list is stored in the DIO. That is, the coverage management unit 110 determines whether an intra-coverage station list exists in the DIO.

If an intra-coverage station list exists in the DIO, the processing proceeds to step S322.
If an intra-coverage station list does not exist in the DIO, the processing proceeds to step S331.
[0091] In step S322, the coverage management unit 110 updates the intra-coverage station list of the sender which is registered with the coverage management table 210 to the intra-coverage station list set in the DIO.
[0092] In step S323, the coverage management unit 110 updates the second random number range of the sender which is registered with the coverage management table 210.
[0093] More specifically, the coverage management unit 110 updates the second random number range of the sender as follows.
First, the coverage management unit 110 acquires the intra-coverage station list of the local station and the intra-coverage station list of the sender from the coverage management table 210.
Subsequently, the coverage management unit 110 merges the intra-coverage station list of the local station and the intra-coverage station list of the sender such that the wireless stations will not overlap. By this merging, a list of wireless stations existing in at least either the coverage of the local station or the coverage of the sender can be acquired. The acquired list will be referred to as a merge list.
Then, the coverage management unit 110 counts the wireless stations included in the merge list. The number of wireless stations included in the merge list will be referred to as a second sum total station count. The second sum total station count is expressed by the following equation:
Second Sum Total Station Count

= Intra-Coverage Station Count of Local Station
+ Intra-Coverage Station Count of Sender
- Overlapping Station Count between Local Station and Sender [0094] Subsequently, the coverage management unit 110 determines the second random number range on the basis of the second sum total station count. The smaller the second sum total station count, the narrower the second random number range. The larger the second sum total station count, the wider the second random number range.
The coverage management unit 110 then updates the second random number range of the sender which is registered with the coverage management table 210 to the determined second random number range.
[0095] In step S331, the coverage management unit 110 determines whether the sender of the DIO is a downstream station.
More specifically, the coverage management unit 110 determines whether the sender of the DIO is a wireless station registered in the path information as a downstream station.
If the sender of the DIO is a downstream station, the processing ends update of the random number range.
If the sender of the DIO is not a downstream station, the processing proceeds to step S332.
[0096] In step S332, the coverage management unit 110 accesses the path management unit 140.
The path management unit 140 registers the sender of the DIO to the path information as an upstream station. The detail of the registration will be described later.

[0097] Timer start processing will be described referring to Fig. 12.
Timer start processing is executed by the timer unit 120 when starting the DIO timer or DAO timer.
[0098] In step S401, the timer unit 120 acquires a random number range from the coverage management table 210.
[0099] More specifically, the timer unit 120 acquires the random number range as follows.
When starting the DIO timer, the timer unit 120 acquires the first random number range or second random number range of the downstream station from the coverage management table 210.
When starting the DAO timer, the timer unit 120 acquires the first random number range or second random number range of the upstream station from the coverage management table 210.
[0100] More specifically, the timer unit 120 acquires the first random number range or the second random number range as follows.
If a second random number range is set in the coverage management table 210, the timer unit 120 acquires the second random number range from the coverage management table 210.
If a second random number range is not set in the coverage management table 210, the timer unit 120 acquires the first random number range from the coverage management table 210.
[0101] In step S402, the timer unit 120 calculates the transmission period using a random number within the random number range. A random number within the random number range signifies a value (random number) selected randomly from the random number range.

[0102] The transmission period for the DIO timer is calculated by Trickle Algorithm (IETF RFC 6206) defined in Non-Patent Literature 1 with using a random number within the random number range.
[0103] The transmission period for the DAO timer is expressed by the following equation. Note that rand (random number range) signifies a random number within the random number range.
Transmission Period = rand {random number range} x unit time [0104] In step S403, the timer unit 120 sets the transmission period in the timer. [0105] In step S404, the timer unit 120 starts the timer. [0106] Message generation processing will be described referring to Fig. 13.
Message generation processing is executed by the message generation unit 130 when generating a DIO or DAO. [0107] In step S411, the message generation unit 130 generates a message main body.
The message main body is a part of an entire message excluding an option. [0108] More specifically, the message generation unit 130 generates the message main body as follows.
When generating a DIO, the message generation unit 130 generates the message main body using a format of the DIO.
When generating a DAO, the message generation unit 130 generates the message main body using a format of the DAO.
The format of the DIO and the format of the DAO are defined in the Non-Patent Literature 1. The format of the DIO and the format of the DAO are stored in the storage unit 191 in advance. [0109] When generating a DIO, the processing proceeds to step S412.
When generating a DAO, message generation processing ends.

[0110] In step S412, the message generation unit 130 generates an additional option for coverage information of the local station and attaches the generated additional option to the message main body.
[0111] More specifically, the message generation unit 130 generates the additional option for the coverage information of the local station as follows.
First, the message generation unit 130 acquires at least either the intra-coverage station count of the local station or the intra-coverage station list of the local station from the coverage management table 210 as coverage information of the local station.
Subsequently, using the format of an additional option, the message generation unit 130 generates an additional option in which the coverage information of the local station is set. The generated additional option is the additional option for the coverage information of the local station. The format of the additional option is stored in the storage unit 191 in advance.
[0112] In step S413, the message generation unit 130 generates a configuration option for the random number range and attaches the generated configuration option to the message main body.
[0113] More specifically, the message generation unit 130 generates the configuration option for the random number range as follows.
First, the message generation unit 130 acquires a first random number range or second random number range of the local station from the coverage management table 210.
Subsequently, using the format of the configuration option, the message generation unit 130 generates a configuration option in which the acquired random number range is set. The generated configuration option is the configuration option

for the random number range. The format of the configuration option is stored in the storage unit 191 in advance.
[0114] More specifically, the message generation unit 130 acquires the first random number range or the second random number range as follows.
If a second random number range is set in the coverage management table 210, the message generation unit 130 acquires the second random number range from the coverage management table 210.
If a second random number range is not set in the coverage management table 210, the message generation unit 130 acquires the first random number range from the coverage management table 210. [0115] Reception processing will be described referring to Fig. 14.
Reception processing is executed mainly by the reception unit 192 when a DIO or DAO reaches the antenna 906. The DIO or DAO reaches the antenna 906 as a modulated signal.
[0116] In step S421, the reception unit 192 receives the modulated signal via the antenna 906.
In step S422, the reception unit 192 demodulates the modulated signal. Hence, a message is obtained.
In step S423, the reception unit 192 outputs the message. The outputted message is the DIO or DAO.
[0117] In step S460, the coverage management unit 110 updates the coverage information of the local station.
Update of the coverage information of the local station will be described later. [0118] Transmission processing will be described referring to Fig. 15.
Transmission processing is executed by the transmission unit 193 when a DIO

or DAO is generated.
[0119] In step S431, the transmission unit 193 accepts a message. The accepted
message is a DIO or DAO.
In step S432, the transmission unit 193 modulates the message. Hence, a modulated signal is obtained.
In step S433, the transmission unit 193 transmits the modulated signal via the antenna 906. [0120] Registration of an upstream station will be described referring to Fig. 16.
Registration of an upstream station is executed by the path management unit 140 when a DIO is received.
[0121] In step S441, the path management unit 140 determines whether an upstream station is registered in the path information. That is, the path management unit 140 determines whether an upstream station has been registered.
If an upstream station has been registered, the processing proceeds to step S442.
If an upstream station has not been registered, the processing proceeds to step S444.
[0122] In step S442, the path management unit 140 determines whether an update condition of an upstream station is satisfied.
For example, the update condition is a condition that the received power of the DIO which is received from the sender is larger than the received power of the DIO which is received from an upstream station. The update condition can be determined arbitrarily.
If the update condition of an upstream station is satisfied, the processing proceeds to step S443.

If the update condition of an upstream station is not satisfied, the sender of the DIO is not registered as the upstream station, and the registration of an upstream station ends.
[0123] In step S443, the path management unit 140 updates the upstream station registered in the path information to the sender of the DIO.
After step S443, update of an upstream station ends. [0124] In step S444, the path management unit 140 registers the sender of the DIO to the path information as the upstream station.
After step S444, update of an upstream station ends. [0125] Registration of a downstream station will be described referring to Fig. 17.
Registration of a downstream station is executed by the path management unit 140 when a DAO is received.
[0126] In step S451, the path management unit 140 determines whether the DAO is a message addressed to the local station.
More specifically, the path management unit 140 refers to the destination of the DAO and determines whether the destination of the DAO is the local station.
If the DAO is a message addressed to the local station, the processing proceeds to step S452.
If the DAO is not a message addressed to the local station, the sender of the DAO is not registered as the downward station, and registration of a downstream station ends.
[0127] In step S452, the path management unit 140 determines whether a downstream station is registered in the path information. That is, the path management unit 140 determines whether a downstream station has been registered.
If a downstream station has been registered, the processing proceeds to step

S453.
If a downstream station has not been registered, the processing proceeds to step S454.
[0128] In step S453, the path management unit 140 updates the downstream station registered in the path information to the sender of the DAO.
[0129] In step S454, the path management unit 140 registers the sender of the DAO to the path information as the downstream station.
[0130] Update of the local coverage information (S460) will be described referring to Fig. 18.
Update of the local coverage information (S460) is executed by the coverage management unit 110 when a DIO or DAO is received.
[0131] In step S461, the coverage management unit 110 determines whether the sender of the message is registered in the intra-coverage station list of the local station in the coverage management table 210. That is, the coverage management unit 110 determines whether the sender of the message has been registered.
If the sender of the message has been registered, the coverage information of the local station is not updated, and update of the coverage information of the local station ends.
If the sender of the message has not been registered, the processing proceeds to step S462.
[0132] In step S462, the coverage management unit 110 adds 1 to the intra-coverage station count of the local station in the coverage management table 210. [0133] In step S463, the coverage management unit 110 adds the sender of the message to the intra-coverage station list of the local station in the coverage management table 210.

[0134] *** Supplement to Embodiment 1 ***
In Embodiment 1, the DIO in which the coverage information of the local station is stored as the additional option is transmitted by broadcast. Hence, the coverage information of the local station is informed to the other wireless stations.
The intra-coverage station list of the local station may be stored as the additional option to serve as the coverage information of the local station.
The size of an address (Prefix) indicating a station number differs depending on the system. When IPv6 is employed by the system, Prefix has a size of 128 bits. Hence, if the intra-coverage station count of the local station is large, the message length of the DIO becomes excessively long.
In view of this, the local station may employ a 16-bit short address as Prefix, so that the size of Prefix is reduced.
The local station may store its intra-coverage station list to the DIO in a plurality of batches, and inform the whole intra-coverage station list of the local station to the other wireless stations by transmitting the DIO a plurality of times.
The local station may generate an additional option for the intra-coverage list of the local station. Where necessary, the local station may transmit a DIO to which the additional option for the intra-coverage station list of the local station is attached. For example, when the intra-coverage station list is requested by another wireless station, or when the intra-coverage station list of the local station is updated, the local station transmits a DIO to which an additional option for the intra-coverage station list of the local station is attached. [0135] *** Effect of Embodiment 1 ***
The wireless station can learn coverage information of the upstream station or downstream station by receiving a DIO from the upstream station or downstream

station.
The wireless station can learn the random number range of the upstream station by receiving a DIO from the upstream station.
The local station widens or narrows the random number range in accordance with the intra-coverage station count of the local station in order to decrease collisions of the DIOs transmitted from different coverages. The local station determines the transmission period of the DIO using a random number within the random number range. Hence, the local station can transmit the DIO in an appropriate transmission period based on an appropriate random number range according to the intra-coverage station count of the local station. [0136] * * * Other Configurations * * *
The local station may acquire a DIO at an earlier stage by DIS transmission, as described in Non-Patent Literature 1. [0137] Embodiment 2.
An embodiment in which an appropriate validity period of a downstream path is notified will be described mainly on differences from Embodiment 1 referring to Figs. 19 to 21. [0138] *** Description of Configuration ***
Fig. 19 illustrates a format of transit information which is one of options of the DAO.
An effective validity period of the downstream path is set in the field of path lifetime. That is, the validity period of the downstream path is stored in the DAO as a transit option. [0139] *** Description of Operation ***
Reception processing will be described referring to Fig. 20.

Step S421 to step S423 have been described in Embodiment 1 (see Fig. 14).
Step S460 has been described in Embodiment 1 (see Fig. 18). [0140] In step S471, a path management unit 140 determines whether a received message is a DAO.
If a received message is a DAO, the processing proceeds to step S472.
If a received message is a DIO, reception processing ends. [0141] In step S472, the path management unit 140 acquires a validity period of the downstream path from the transit option of the DAO, and registers the validity period of the downstream path to the path information.
If a validity period of the downstream path is already registered in the path information, the path management unit 140 updates the validity period of the downstream path which is registered in the path information to the validity period of the downstream path which is acquired from the transit option of the DAO. [0142] Message generation processing will be described referring to Fig. 21.
Step S411 to step S413 have been described in Embodiment 1 (see Fig. 13). [0143] In step S414, a message generation unit 130 determines whether a generated message is a DAO.
If a generated message is a DAO, the processing proceeds to step S415.
If a generated message is a DIO, message generation processing ends. [0144] In step S415, the message generation unit 130 generates a transit option for a validity period, and attaches the generated transit option to the message main body. [0145] More specifically, the message generation unit 130 generates a transit option for a validity period as follows.
First, the message generation unit 130 determines a sufficient validity period which is a period not insufficient as a validity period of the downstream path.

Then, using a format of a transit option, the message generation unit 130 generates a transit option in which the sufficient validity period is set. The generated transit option is the transit option for the validity period. The format of the transit option is stored in a storage unit 191 in advance.
[0146] More specifically, the message generation unit 130 determines the sufficient validity period as follows. The sufficient validity period corresponds to an intra-coverage station count of the local station.
First, the message generation unit 130 acquires a validity period of the downstream path from the path information.
Subsequently, the message generation unit 130 acquires a first random number range or second random number range of an upstream station from a coverage management table 210.
Subsequently, using the acquired random number range, the message generation unit 130 calculates the maximum transmission period of the DAO in the local station. The maximum transmission period is expressed by the following formula. Note that a maximum random number is the maximum value of a random number in the random number range.
Maximum Transmission Period = Maximum Random Number x Unit Time [0147] Then, the message generation unit 130 adds the maximum transmission period to the validity period of the downstream path. The acquired time is the sufficient validity period. [0148] *** Supplement to Embodiment 2 ***
The validity period of the downstream path which is determined in the local station is a validity period that reflects an intra-coverage station count of an upstream station, a random number range of the upstream station, and the maximum transmission

period of the DAO in the local station.
The maximum transmission period of the DAO in the local station corresponds to the maximum transmission period of the DIO in the upstream station. [0149] In the wireless communication system 200 of Fig. 2, when the slave station #3 transmits a DAO to the slave station #2, the slave station #2 manages the validity period of the downstream path which is stored in the DAO, that is, the validity period of the downstream path which is determined by the slave station #3. Furthermore, when the slave station #2 transmits a DAO of the slave station #3 to the slave station #0, the slave station #2 adds a maximum transmission period of the DAO in the slave station #2 to the validity period of the downstream path which is determined by the slave station #3. [0150] *** Effect of Embodiment 2 ***
At the opportunity of reception of a DIO from the upstream station, the local station transmits a DAO to the upstream station. Hence, when the transmission period of the DIO in the upstream station increases, sometimes the validity period of the downstream path may be insufficient.
In view of this, when transmitting the DAO, the local station calculates the maximum time of the transmission period of the DIO in the upstream station, increases or decreases the validity period of the downstream-path in accordance with the calculated time, and transmits the DAO to the upstream station.
[0151] The local station can determine an appropriate transmission period of the DAO and an appropriate validity period of the downstream path in accordance with the intra-coverage station count of the local station.
[0152] Even when the relay count increases, the sum total of the validity periods of the downstream path of the sections existing between the master station and the slave station may be treated as the validity period of the downstream path in the local station.

Even when the random number range of the transmission period of the DIO or DAO fluctuates among the wireless stations, shortage of the validity period of the downstream path can be avoided. [0153] Embodiment 3.
An embodiment in which an appropriate transmission period for an application message is determined will be described mainly on differences from Embodiment 1 and Embodiment 2 referring to Fig. 22. [0154] * * * Description of Configuration * * *
A configuration of a wireless communication device 100 will be described referring to Fig. 22.
The wireless communication device 100 is provided with an application unit 150 in addition to the software elements described in Embodiment 1 (see Fig. 1).
A wireless communication program causes a computer to serve as a coverage management unit 110, a timer unit 120, a message generation unit 130, a path management unit 140, and the application unit 150.
A storage unit 191 stores an application executed by the application unit 150. The application refers to an application program. [0155] *** Description of Operation ***
The application unit 150 generates an application message by executing an arbitrary application.
The application message is a message generated by execution of the arbitrary application and is a message different from a path control message. [0156] In reception processing (see Fig. 14 or 20), a reception unit 192 receives the application message in addition to a DIO and a DAO.
The coverage management unit 110 executes update of the coverage

information of the local station (S400) when an application message is received as well. That is, the coverage management unit 110 registers a sender of the application message to the coverage information of the local station.
The timer unit 120 executes timer start processing (see Fig. 12) when an application message is generated as well. That is, the timer unit 120 starts an application timer. The application timer is a timer in which a transmission period of the application message is set. The transmission period of the application message is the same as a transmission period of the DIO or a transmission period of the DAO. More specifically, if the destination of the application message is a downstream station, the transmission period of the application message is the same as the transmission period of the DIO. If the destination of the application message is an upstream station, the transmission period of the application message is the same as the transmission period of the DAO.
When the application timer expires, the transmission unit 193 transmits the application message. [0157] *** Effect of Embodiment 3 ***
With Embodiment 3, collisions of application messages among different coverages can be decreased. [0158] Embodiment 4.
An embodiment in which an appropriate backoff of a MAC layer message is determined will be described mainly on differences from Embodiment 1 to Embodiment 3. [0159] *** Description of Operation ***
A transmission unit 193 transmits a MAC layer message. A MAC layer message is a message communicated in a MAC layer. A DIO, a DAO, and an

application message are contained in the MAC layer message. MAC is an abbreviation for Media Access Control.
In IEEE 802. 15. 4, a standby time called backoff is defined in order to reduce a probability that a plurality of wireless stations within the same coverage transmit MAC layer messages simultaneously. The backoff is expressed by the following formula. Note that backoff slot is a unit time for backoff:
Backoff = Backoff Slot x Random Number [0160] A timer unit 120 executes timer start processing (see Fig. 12) before the MAC layer message is transmitted as well. That is, the timer unit 120 calculates the backoff and starts a MAC layer timer. A MAC layer timer is a timer in which a backoff is set. The backoff is expressed by the following formula:
Backoff = Backoff Slot x rand (random number range) [0161] A backoff of a MAC layer message toward an upstream station is calculated using a random number range of the upstream station. A backoff calculated using the random number range of the upstream station is referred to as an upstream backoff.
A backoff of a MAC layer message toward a downstream station is calculated using a random number range of the downstream station. A backoff calculated using the random number range of the downstream station is referred to as a downstream backoff.
[0162] The transmission unit 193 transmits a MAC layer message when the MAC layer timer expires. [0163] *** Effect of Embodiment 4 ***
A probability that MAC layer messages are transmitted from different coverages to the same coverage simultaneously is reduced, and collisions of the MAC layer messages can be reduced.

[0164] Embodiments.
An embodiment to avoid construction of a communication path in which two wireless stations are in a state of hidden-terminal relation will be described mainly on differences from Embodiment 1 to Embodiment 4 referring to Fig. 23. [0165] *** Description of Configuration ***
A configuration of a wireless communication device 100 will be described referring to Fig. 23.
The wireless communication device 100 is provided with a hidden control unit 160 in addition to the software elements described in Embodiment 1 (see Fig. 1). The wireless communication device 100 may also be provided with the application unit 150 described in Embodiment 3 (see Fig. 22).
A wireless communication program causes a computer to serve as a coverage management unit 110, a timer unit 120, a message generation unit 130, a path management unit 140 (an application unit 150), and the hidden control unit 160. [0166] *** Description of Operation ***
Reception processing will be described referring to Fig. 24.
Step S471 and step S472 described in Embodiment 2 (see Fig. 20) may be added to reception processing.
[0167] Step S421 has been described in Embodiment 1 (see Fig. 14). [0168] In step S480, the reception unit 192 measures power of a received modulated signal. The measured power will be referred to as a received power of a sender.
The hidden control unit 160 stores the received power of the sender to a storage unit 191. More specifically, the hidden control unit 160 stores the received power of the sender to the storage unit 191 so as to be associated with a station number of the sender.

[0169] Step S422 and step S423 have been described in Embodiment 1 (see Fig. 14).
Step S460 has been described in Embodiment 1 (see Fig. 18). [0170] In step S481, the hidden control unit 160 determines whether a received message is a DAO.
If a received message is a DAO, the processing proceeds to step S482.
If a received message is not a DAO, the reception processing ends. [0171] In step S482, the hidden control unit 160 compares a received power sum of the received power of an upstream station and the received power of the sender with a power threshold value. [0172] More specifically, the hidden control unit 160 operates as follows.
First, the hidden control unit 160 acquires the received power of the upstream station and the received power of the sender from the storage unit 191. The received power of the upstream station is a received power of a DIO from the upstream station. The received power of the sender is a received power of a DAO from the sender.
Subsequently, the hidden control unit 160 calculates the sum of the received power of the upstream station and the received power of the sender. The calculated sum is the received power sum.
Then, the hidden control unit 160 compares the received power sum with the power threshold value. The power threshold value is a predetermined value.
If the received power sum exceeds the power threshold value, the processing proceeds to step S483.
If the received power sum is equal to or less than the power threshold value, reception processing ends. [0173] In step S483, the hidden control unit 160 rejects a request made by a DAO.
More specifically, the hidden control unit 160 generates a DAO-ACK message

indicating rejection. Then, a transmission unit 193 transmits the DAO-ACK message. The DAO-ACK message is a response message to the DAO. Rejection of a request made by a DAO signifies rejection of becoming an upstream station, that is, rejection of a path construction. [0174] *** Effect of Embodiment 5 ***
An outline and effect of Embodiment 5 will be described referring to as an example a relation among the slave station #1, the slave station #7, and the master station 0 in the wireless communication system 200 of Fig. 2.
If the sum of the received powers of the master station #0 and slave station #7, which are intra-coverage stations of the slave station #1, exceeds a threshold value, the slave station #1 estimates that the master station #0 and the slave station #7 hold a hidden-terminal relation.
A hidden-terminal relation is a relation in which two wireless stations are not intra-coverage stations with each other.
[0175] More specifically, the slave station #1 receives a DAO from the slave station #7 and determines whether the sum of the received power of the DAO from the slave station #7 and the received power of the DIO from the master station #0 exceeds the threshold value.
If the sum of the received powers exceeds the threshold value, the slave station #1 expresses rejection to the DAO by means of the DAO-ACK message to the slave station #7.
[0176] The slave station #7 gives up path construction with the slave station #1, searches for another path, and constructs a bypass path.
As a result, the slave station #7 is able to avoid construction of a communication path in which a state of hidden-terminal relation with the master station

#0 is held via the slave station #1. [0177] *** Other Configurations ***
Whether a hidden-terminal relation exists or not may be determined on the basis of a message transmission success rate instead of a received power. [0178] *** Effect of Embodiment ***
A wireless station can determine a transmission period of a DIO appropriately in accordance with an intra-coverage station count. Thus, an unnecessarily long period of time need not be spent for detection of an abnormality on the upstream path.
A downstream station can learn the maximum value of a transmission period of a DIO of an upstream station by receiving the DIO. Therefore, the downstream station can determine an appropriate validity period of a downstream path with respect to the transmission period of the DIO of the upstream station. Thus, an unnecessarily long period of time need not be spent for detection of an abnormality on the downstream path.
As the upstream station notifies the intra-coverage station count by the DIO, a local station can appropriately determine a random number range of a transmission period of a DAO a random number range of a transmission period of a DAO in accordance with the sum of the intra-coverage station counts of the upstream station and the local station.
As the random number range of the transmission period of the DAO is determined optimally, it is possible to reduce a possibility that a plurality of DAOs arriving from a plurality of coverages collide in the same wireless station.
In transmission of an application message as well, it is possible to reduce a possibility that a plurality of application messages arriving from a plurality of coverages collide in the same wireless station.

[0179] *** Supplement to Embodiments ***
In the embodiments, the function of the wireless communication device 100 may be implemented by hardware.
Fig. 25 illustrates a configuration of a case where the function of the wireless communication device 100 is implemented by hardware.
The wireless communication device 100 is provided with a processing circuit 990. The processing circuit 990 is also referred to as processing circuitry.
The processing circuit 990 is a dedicated electronic circuit that implements the coverage management unit 110, the timer unit 120, the message generation unit 130, the path management unit 140, the application unit 150, the hidden control unit 160, and the storage unit 191.
For example, the processing circuit 990 is a single circuit, a composite circuit, a programmed processor, a parallel-programmed processor, a logic IC, a GA, an ASIC, or an FPGA, or a combination of them. GA is an abbreviation for Gate Array. ASIC is an abbreviation for Application Specific Integrated Circuit. FPGA is an abbreviation for Field Programmable Gate Array.
[0180] The wireless communication device 100 may be provided with a plurality of processing circuits that replace the processing circuit 990. The plurality of processing circuits share the role of the processing circuit 990.
[0181] The embodiments exemplify preferred embodiments and are not intended to limit the technical scope of the present invention. Each embodiment may be practiced partly or by combination with another embodiment. The procedures described using flowcharts or the like may be changed where necessary. Reference Signs List [0182] 100: wireless communication device; 110: coverage management unit; 120:

timer unit; 130: message generation unit; 140: path management unit; 150: application unit; 160: hidden control unit; 191: storage unit; 192: reception unit; 193: transmission unit; 200: wireless communication system; 210: coverage management table; 901: processor; 902: memory; 903: auxiliary storage device; 904: RF circuit; 905: PHY control circuit; 906: antenna; 907: oscillator; 990: processing circuit

WE CLAIM:
[Claim 1] A wireless communication device which operates as a local station
being one of wireless stations, the wireless communication device comprising:
a storage unit to store local coverage information being information of wireless stations existing within coverage of the local station, and downstream coverage information being information of wireless stations existing within coverage of a downstream station which is one of the wireless stations existing within the coverage of the local station;
a reception unit to receive a downstream message;
a timer unit to start a downstream timer when the downstream message is received, the downstream timer being set with a downstream transmission period determined based on the local coverage information and the downstream coverage information which are stored in the storage unit; and
a transmission unit to transmit a message including the local coverage
information stored in the storage unit, when the downstream timer expires.
[Claim 2] The wireless communication device according to claim 1,
wherein the downstream message includes upstream coverage information being information of wireless stations existing within coverage of an upstream station which is one of the wireless stations existing within the coverage of the local station,
wherein the wireless communication device comprises a coverage management unit to store, when the downstream message is received, the upstream coverage information included in the downstream message to the storage unit,
wherein the timer unit starts an upstream timer when the downstream message is received, the upstream timer being set with an upstream transmission period determined based on the local coverage information and the upstream coverage

information which are stored in the storage unit, and
wherein the transmission unit transmits an upstream message when the
upstream timer expires.
[Claim 3] The wireless communication device according to claim 2,
wherein the reception unit receives an upstream message,
wherein the timer unit starts the upstream timer when the upstream message is received, and
wherein the transmission unit transmits an upstream message when the
upstream timer expires.
[Claim 4] The wireless communication device according to claim 3,
wherein each of the local coverage information, the upstream coverage
information, and the downstream coverage information which are stored in the storage
unit is at least either a wireless station count or a wireless station list.
[Claim 5] The wireless communication device according to claim 4,
wherein the coverage management unit determines an upstream random number range based on the local coverage information and the upstream coverage information, and determines a downstream random number range based on the local coverage information and the downstream coverage information, and
wherein the timer unit calculates the upstream transmission period using a
random number selected from the upstream random number range, and calculates the
downstream transmission period using a random number selected from the downstream
random number range.
[Claim 6] The wireless communication device according to claim 5,
the wireless communication device comprising a message generation unit to generate a message,

wherein the upstream message includes a validity period of a downstream path,
wherein the message generation unit calculates a maximum transmission period using the upstream random number range, calculates a sufficient validity period by adding the maximum transmission period to the validity period of the downstream path, the validity period of the downstream path being included in the upstream message, and generates a message including the sufficient validity period, and
wherein the transmission unit transmits the message including the sufficient
validity period.
[Claim 7] The wireless communication device according to claim 5,
wherein the timer unit starts an application timer in which the upstream transmission period or the downstream transmission period is set, and
wherein the transmission unit transmits an application message when the
application timer expires.
[Claim 8] The wireless communication device according to claim 5,
wherein the timer unit calculates a backoff using the upstream random number range or the downstream random number range, and starts a MAC layer timer in which the backoff is set, and
wherein the transmission unit transmits a MAC layer message when the MAC
layer timer expires.
[Claim 9] The wireless communication device according to claim 5,
the wireless communication device comprising a hidden control unit to reject a request made by the upstream message,
wherein the hidden control unit rejects the request made by the upstream message when a sum of a received power of the upstream message and a received power of the downstream message exceeds a power threshold value.

[Claim 10] A wireless communication program which causes a computer to
execute:
a timer process of starting a downstream timer when a downstream message is received, the downstream timer being set with a downstream transmission period determined based on local coverage information; and
a transmission process of transmitting a message including local coverage information when the downstream timer expires,
wherein the local coverage information is information of wireless stations existing within coverage of a local station, and
wherein the downstream coverage information is information of wireless stations existing within coverage of a downstream station which is one of the wireless stations existing within the coverage of the local station.