Wireless communication device, wireless communication method, and computer program product

A wireless communication device performs a wireless communication with a communication target device via an intermediate device. A route-information obtaining unit obtains communication route information indicating a communication route to the communication target device. A quality-information obtaining unit obtains quality information indicating quality of a signal received by a device included in the communication route indicated by the communication route information. A route selecting unit selects an optimum communication route to the communication target device based on the quality information.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese priority documents 2007-167313 filed in Japan on Jun. 26, 2007 and 2008-158301 filed in Japan on Jun. 17, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication technology implemented in a mesh network.

2. Description of the Related Art

In recent years, the use of a mesh network for performing wireless communication is being extensively studied. The mesh network is one of the network forms that include a star network, a bus network, a ring network, and the like. More particularly, the mesh network is a reticulate network in which each of a plurality of communication devices can mutually communicate with all other communication devices via a plurality of communication routes. Thus, even if a problem occurs in one of the communication routes, the communication can be performed via the rest of the communication routes. Such a feature facilitates in configuring a failure-resistant mesh network. Moreover, to maintain the communication speed in a mesh network, a communication route can be selected based on the data communication traffic. A common example of a mesh network is a power distribution network or the Internet network.

In a mesh network used for a wireless communication (hereinafter, “wireless mesh network”), direct communication of wireless signals between two communication devices is possible if the communication devices are arranged in a direct communicable range. However, even if the communication devices are arranged out of the direct communicable range, the wireless signals can be communicated by arranging one or more intermediate communication devices therebetween. Furthermore, because large amount of transmission power is necessary to perform direct communication between two distant communication devices, the communication performed via the intermediate communication devices is useful from an energy saving perspective. Meanwhile, such a wireless mesh network can be easily incorporated with a wired network by connecting any one of the communication devices to the wired network. That eliminates the need of a conventional configuration in which an access point, which is connected to the wired network, needs to be installed at the center of the communicable range of a wireless network. Instead, by using a communication device that is arranged near an end limit of the communicable range as the access point for connection with the wired network, the cabling can be restricted to the minimum.

Moreover, it is also possible to easily expand the communicable range of the wireless mesh network. Conventionally, when an access point is installed at the center of the communicable range, the communicable range can be expanded by enhancing the output capacity from the access point or by installing additional access points. Generally, to enhance the output capacity from an access point, it is necessary to replace the device thereat. Similarly, while installing additional access points, it is necessary to lay the communication line up to the additional access points. On the other hand, in the wireless mesh network, the communicable range can be easily expanded by newly arranging a communication device at the desired place for communication outside the current communicable range and supplying power to the newly arranged communication device.

In such a wireless mesh network, a significant factor while performing communication is the selection of a suitable communication route from among a plurality of communication routes. For example, Japanese Patent Application Laid-Open No. 2006-186565 discloses a mesh network in which a separate communication route is allotted for transmission of each signal to prevent overlapping of communication routes. More particularly, consider a case when an intermediate communication device is transmitting a first signal and receives a request for transmission of a second signal, which has a higher priority than the first signal. Then, according to Japanese Patent Application Laid-Open No. 2006-186565, the intermediate communication device transmits the second signal and informs a source communication device of the first signal about the subsequent communication route for the second signal. Based on that information, the first signal is transmitted via another communication route and overlapping of the communication routes is prevented. Such a configuration reduces the throughput of the mesh network in entirety.

Generally, a communication route is selected based on the number of intermediate communication devices arranged therein. The number of intermediate communication devices corresponds to the number of hops taken while transmitting a wireless signal via a communication route. Because an overhead time is generated when each intermediate communication device in a communication route transmits a wireless signal, it is desirable that a communication route having the least number of intermediate communication devices is selected such that the transmission is completed at the earliest. Moreover, the actual time for transmission of a wireless signal from one communication device to another communication device depends on the distance therebetween. By taking into consideration the transmission speed of a wireless signal, the overhead time generated at each intermediate communication device affects the actual time for transmission more than the distance between the source communication device and the destination communication device does.

However, as the distance between two intermediate communication devices in a communication route increases, the transmitted wireless signal gets weaker thereby reducing the reception power of the communication device that receives the wireless signal. Moreover, there is also a possibility that the noise increases during the transmission. Thus, longer the distance between the communication devices, more is the possibility that the signal quality degrades. Consequentially, a signal to noise ratio (SNR) at the communication device that receives the signal also degrades. If an error rate for the wireless signal is at a correctable level, then the actual time for transmission is not substantially affected. However, when the error rate reaches an uncorrectable level, then it becomes necessary to retransmit the signal thereby increasing the actual time for transmission. Thus, to sum up, selecting a communication route having the least number of intermediate communication devices does not guarantee speedy communication of a wireless signal.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a wireless communication device that performs a wireless communication with a communication target device via an intermediate device. The wireless communication device includes a route-information obtaining unit that obtains communication route information indicating a communication route to the communication target device; a quality-information obtaining unit that obtains quality information indicating quality of a signal received by a device included in the communication route indicated by the communication route information; and a route selecting unit that selects an optimum communication route to the communication target device based on the quality information.

Furthermore, according to another aspect of the present invention, there is provided a wireless communication method that is configured to be executed in a wireless communication device that performs a wireless communication with a communication target device via an intermediate device. The wireless communication method includes obtaining communication route information indicating a communication route to the communication target device; obtaining quality information indicating quality of a signal received by a device included in the communication route indicated by the communication route information; and selecting an optimum communication route to the communication target device based on the quality information.

Moreover, according to still another aspect of the present invention, there is provided a computer program product including a computer-usable medium having computer-readable program codes embodied in the medium for performing a wireless communication in a wireless communication device that performs the wireless communication with a communication target device via an intermediate device. The computer-readable program codes when executed cause a computer to execute obtaining communication route information indicating a communication route to the communication target device; obtaining quality information indicating quality of a signal received by a device included in the communication route indicated by the communication route information; and selecting an optimum communication route to the communication target device based on the quality information.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described in detail below with reference to the accompanying drawings. The present invention is not limited to these exemplary embodiments.

FIG. 1is a schematic diagram of a wireless mesh network system1according to a first embodiment of the present invention. The wireless mesh network system1includes six nodes100to600. Each of the nodes100to600is a wireless communication device that mutually communicates with every other node to form a reticulate network.FIG. 2is a schematic diagram for explaining a functional configuration of the node100. Each of the nodes200to600has an identical functional configuration as that of the node100. As shown inFIG. 2, the node100includes a user interface (I/F)101, a wireless communicating unit102, an information storing unit103, and a controller110. The controller110includes a main control unit111, a communication control unit112, a storage medium control unit113, a communication route selecting unit114, a communication route obtaining unit116, and a quality information obtaining unit117.

The user I/F101includes an operating unit (not shown) such as a keyboard or a mouse that can be used by a user to operate the node100and a displaying unit (not shown) such as a monitor on which the status of the node100is displayed. The wireless communicating unit102includes a communication antenna (not shown) that is used for communication with the other nodes200to600. The information storing unit103is a storage medium such as a disk type storage device (e.g., a hard disk drive (HDD) or an optical disk drive) or a non volatile memory (e.g., a read only memory (ROM) or a non volatile random access memory (NV-RAM)). A position database is stored in the information storing unit103. The position database includes position information of the nodes100to600.

FIG. 3is a table for explaining an exemplary position database stored in the information storing unit103. The position database includes an identification ID, a node name, and position information of each of the nodes100to600. The position of each of the nodes100to600is represented as coordinates with respect to an origin0. By using the position database, it is possible to obtain the positions of the nodes200to600with respect to the node100. Meanwhile, it is also possible to represent an absolute position of each of the nodes100to600by storing the corresponding latitude and the corresponding longitude as the coordinates.

The controller110is configured by a combination of hardware components and software programs. More particularly, the software programs include a control program such as firmware, which is stored in the information storing unit103and loaded in a dynamic random access memory (DRAM); while the hardware components include a software control unit, an integrated circuit, and the like that are controlled by a central processing unit (CPU). The controller110controls the node100in entirety.

The main control unit111controls all the constituent elements in the controller110and issues commands thereto. When the wireless communicating unit102receives a wireless signal or a command from another device, the communication control unit112transmits that wireless signal or command to the main control unit111. On the other hand, the main control unit111controls the communication control unit112to access other devices via the wireless communicating unit102. The storage medium control unit113receives a command from the main control unit111and accordingly stores information in the information storing unit103or reads information from the information storing unit103. The communication route obtaining unit116receives a command from the main control unit111and accordingly obtains a plurality of communication routes via which a wireless signal can be transmitted to another node. More particularly, the communication route obtaining unit116obtains the communication routes by using the position database. The quality information obtaining unit117receives a command from the main control unit111and accordingly obtains information about the quality of a wireless signal received by intermediate nodes in each of the obtained communication routes and a destination node by using the position database. The communication route selecting unit114receives a command from the main control unit111and, based on the quality information obtained by the quality information obtaining unit117, selects a communication route for transmitting a wireless signal to other nodes.

FIG. 4is a schematic diagram of two exemplary communication routes A and B via which a wireless signal can be transmitted from the node100to the node600. The communication route A includes four inter-node communication paths a, b, c, and d (shown in full lines inFIG. 4), while the communication route B includes two inter-node communication paths e and f (shown in dashed lines inFIG. 4).

FIG. 5is a flowchart for explaining a process of selecting a communication route performed by the node100. First, a communication command is generated in the node100for transmitting a wireless signal to the node600(Step S501). Then, the communication route obtaining unit116receives a command from the main control unit111and controls the storage medium control unit113to read the position information of the node600from the position database stored in the information storing unit103(Step S502). The main control unit111then issues a command to the communication route obtaining unit116and the communication route selecting unit114for selecting a communicating route via which the wireless signal can be transmitted to the node600(Step S503). Subsequently, the communication route obtaining unit116obtains a plurality of communication routes (i.e., the communication routes A and B) by selecting corresponding intermediate nodes (i.e., the nodes200,300, and400in the communication route A, and the node500in the communication route B) via which the wireless signal can be transmitted to the node600(Step S504).

The quality information obtaining unit117calculates the total expected time for transmission via each communication route (Step S505). The description of calculating the expected time for transmission is given below in detail. Meanwhile, it is determined whether the expected time for transmission is calculated for each communication route obtained at Step S504(Step S506). If it is determined that the expected time for transmission is not yet calculated for each communication route (No at Step S506), the process of calculating the expected time for transmission is repeated for all the remaining communication routes (Step S505). If it is determined that the expected time for transmission is calculated for each communication route (Yes at Step S506), the communication route selecting unit114selects a communication route that requires the least total expected time for transmission (Step S507).

An expected time for transmission TAnecessary to transmit a wireless signal to the node600via the communication route A is obtained by using Equation (1).
TA=ta+tb+tc+td+3×tOH(1)

where, tato tdrepresent expected time for transmission in the inter-node communication paths a, b, c, and d, respectively; while tOHrepresents an overhead time generated when each of the intermediate nodes200,300, and400in the communication route A transmits the wireless signal. Thus, the expected time for transmission TAvia the communication route A is calculated by adding the overhead time at each intermediate node to the sum of the expected time for transmission from the node100to the node200, from the node200to the node300, from the node300to the node400, and from the node400to the node600.

Similarly, an expected time for transmission TBnecessary to transmit a wireless signal to the node600via the communication route B is obtained by using Equation (2).
TB=te+tf+tOH(2)

where, teand tfrepresent expected time for transmission in the inter-node communication paths e and f, respectively, while tOHrepresents an overhead time generated when the intermediate node500in the communication route B transmits the wireless signal. Thus, the expected time for transmission TBfrom the node100to the node600via the communication route B is calculated by adding the overhead time tOHat the intermediate node500to the sum of the expected time for transmission from the node100to the node500and from the node500to the node600.

In other words, if a source node is numbered to be 0-th, a destination node is numbered to be N-th, and intermediate nodes between the source node and the destination node are sequentially numbered to be 1-st, 2-nd, 3-rd, . . . , (N−1)-th, then a total expected time for transmission T necessary to transmit a wireless signal from the source node to the destination node via the intermediate nodes can be obtained by using Equation (3).

where, n represents the number of inter-node communication paths in the communication route and tnrepresents the expected time for transmission in each of the n number of inter-node communication paths. Meanwhile, the total expected time for transmission T is directly proportional to the total number of intermediate nodes N and to the expected time for transmission tnin each of the n number of inter-node communication paths.

The expected time for transmission tnin each of the n number of inter-node communication paths is obtained by using Equation (4).

where, Vnrepresents signal transfer efficiency in the n number of inter-node communication paths. COrepresents volume of information to be transmitted from the source node (node100inFIG. 4) to the destination node (node600inFIG. 4). CRnrepresents a retransmission retry count with respect to the wireless signal at the n number of inter-node communication paths. Thus, the expected time for transmission tnin each of the n number of inter-node communication paths is obtained by calculating the sum of the expected time for transmission of the information (COVn) and the expected time for retransmission of the wireless signal (CRn/Vn). If, e.g., a retransmission retry count CRein the inter-node communication path e substantially increases, then the expected time for transmission tein the inter-node communication path e also increases substantially. In that case, even though the communication route B includes less number of intermediate nodes than the communication route A, there is a possibility that the expected time for transmission TBvia the communication route B is longer than the expected time for transmission TAvia the communication route A. That happens because the expected time for transmission tein the inter-node communication path e increases. In other words, by selecting the communication route A, which includes more number of intermediate nodes, over the communication route B, which includes less number of intermediate nodes, there is a possibility that the total expected time for transmission T decreases.

The retransmission retry count CRndepends on the SNR at each intermediate node and the destination node, which receive the wireless signal. More particularly, the retransmission number CRnis inversely proportional to the SNR at each node that receives the wireless signal. Thus, when an error rate at such a node degrades, there is possibility that the signal quality degrades to an uncorrectable level. In that case, the wireless signal needs to be retransmitted. For that reason, error correctability is determined, e.g., for each packet of the signal transmission such that the signal retransmission can be performed on a packet basis. The SNR at the node that receives the wireless signal varies according to a reception power value Prthat is obtained by using Equation (5).

where, Ptrepresents a transmission power value at a node that transmits the wireless signal, λ represents a radio wavelength, while dnrepresents distance between a node that transmits the wireless signal and a node that receives the wireless signal in each of the n number of inter-node communication paths. Equation (5) indicates that the signal transmission gain of the node that transmits the wireless signal and the node that receives the wireless signal is equal to one. As given in Equation (5), the reception power value Pris inversely proportional to the square of the distance dn. In other words, the reception power value Prof the node that receives the wireless signal decreases as the distance dnfrom the node that transmits the wireless signal increases. As a result, the SNR at the node that receives the wireless signal degrades.

For that reason, while obtaining the expected time for transmission tnin each of the n number of inter-node communication paths, the quality information obtaining unit117first calculates the distance dnof each of the n number of inter-node communication paths based on the position database obtained at Step S502. Based on the calculated distance dn, the quality information obtaining unit117obtains the reception power value Prand the SNR of the node that receives the wireless signal. More particularly, because the SNR is proportional to the reception power value Pr, the SNR is obtained after obtaining the reception power value Prby using the proportionality relation. Subsequently, by using the inverse relationship with the SNR, the quality information obtaining unit117obtains the retransmission retry count CRnand then obtains the expected time for transmission tnby using the retransmission retry count CRn. Thus, the quality information obtaining unit117obtains the information about the quality of the wireless signal at each node that receives the wireless signal in the form of the distance dn, the reception power value Pr, the SNR, and the CRnto calculate the expected time for transmission tnin each of the n number of inter-node communication paths. Subsequently, the quality information obtaining unit117obtains the total expected time for transmission T from the source node to the destination node by using Equation (3).

Meanwhile, as described above with reference toFIG. 4, the overhead time tOHin the communication route A is longer than the overhead time tOHin the communication route B (see Equation (1) and Equation (2)). On the other hand, the distance dnof each of the inter-node communication paths a, b, c, and d in the communication route A is shorter than the distance dnof either of the inter-node communication paths e and f in the communication route B. Thus, if the reception power value Prat either one of the nodes500and600in the communication route B decreases due to the longer distance dnof the inter-node communication paths e and f (see Equation (5)), then the SNR in the inter-node communication paths e and f degrades thereby increasing the retransmission retry count CReand a retransmission retry count CRf, respectively (see Equation (4)). As a result, the expected time for transmission teand tfin the inter-node communication paths e and f, respectively, also increases. For that reason, at Step S507, the communication route selecting unit114selects the communication route A, which requires less expected time for transmission than the communication route B.

Meanwhile, even with the longer distance dnof the inter-node communication paths e and f, if the expected time for transmission TBdoes not exceed the expected time for transmission TA, then the communication route selecting unit114selects the communication route B at Step S507. In this way, based on the expected time for transmission via a plurality of communication routes, a communication route can be selected via which the transmission of a wireless signal can be completed at the earliest.

In the above description, the quality information obtaining unit117calculates the total expected time for transmission T via each communication route (see Equation (3)) such that the communication route selecting unit114can select a communication route that requires the least total expected time for transmission T. Instead, the quality information obtaining unit117can be configured to calculate a signal transfer efficiency value in each of the n number of inter-node communication paths based on the corresponding distance dn. The signal transfer efficiency value can be obtained based on the sum of COand CRn. In that case, the quality information obtaining unit117can be configured to calculate the average of the signal transfer efficiency values in the n number of inter-node communication paths to obtain the signal transfer efficiency value of the entire communication route. Subsequently, the communication route selecting unit114can be configured to compare the signal transfer efficiency values of all the communication routes and select a communication route having the highest signal transfer efficiency value. Such a configuration also enables to select a communication route via which the transmission of a wireless signal can be completed at the earliest. Meanwhile, the communication route selecting unit114can be configured to have a threshold for the distance dnor the SNR such that a communication route in which the distance dnor the SNR of an inter-node communication path exceeds the corresponding threshold can be excluded while comparing the communication routes.

Moreover, in the above description, the total expected time for transmission T is obtained by using the total number of nodes N and the expected time for transmission tnin each of the n number of inter-node communication paths. However, it is also possible to obtain the total expected time for transmission T without using the total number of nodes N, i.e., by using only the distance dnof each of the n number of inter-node communication paths or the expected time for transmission tnin each of the n number of inter-node communication paths. Such a method is effective when the overhead time tOHis negligible as compared the expected time for transmission tnin each of the n number of inter-node communication paths.

Furthermore, instead of configuring each node in the wireless mesh network system1to be a wireless communication device that can receive and transmit a wireless signal (seeFIG. 2), some of the nodes in the wireless mesh network system1can be configured to only relay a wireless signal without being able to receive or transmit a wireless signal. The configuration of such relay nodes can be simplified by omitting constituent elements such as the communication route selecting unit114and the like that are necessary in a wireless communication device. Moreover, instead of storing in advance the position database in the information storing unit103, each node can be configured to include a position identifying mechanism such as a global positioning system (GPS) to detect the positions of other nodes. Thus, even if a new node is added to the wireless mesh network system1, there is no need to manually update the position database in each existing node.

Meanwhile, each of the nodes100to600can be an information processing device (e.g., a personal computer (PC) or a cellular phone) or an image processing device (e.g., a printer, a scanner, or a multifunction product having printing and scanning functions).

Moreover, in the above description, the coordinates of the nodes100to600with respect to the origin0are stored in the position database such that the node100can obtain the positions of the nodes200to600therefrom. Instead, it is also possible to store a distance database in the information storing unit103such that the node100can obtain distances from each of the nodes200to600.FIG. 6is a table of an exemplary distance database stored in the information storing unit103. The distance database includes the distance of each of the nodes200to600from the node100. As described above at Step S505, the quality information obtaining unit117calculates the total expected time for transmission T via each communication route based on the distance of the corresponding inter-node communication paths. Thus, when the distance database is stored in the information storing unit103, the communication route obtaining unit116can be configured to obtain the communication routes at Step S504by using the distance database. Moreover, if a communication route database is stored in the information storing unit103, then the communication route obtaining unit116can be configured to obtain the communication routes at Step S504by using the communication route database. In such cases, the quality information obtaining unit117does not need to calculate the distances between the nodes by referring to the position database. That reduces the processing time. Meanwhile, it is also possible to store an identical position database in each of the nodes100to600such that each node can obtain the positions of the other nodes. Such a configuration is simpler than storing a separate distance database in each of the nodes100to600. As described above, the position database or the distance database is recorded in advance in each of the nodes100to600(seeFIGS. 3 and 6). It is not complicated to manually update the position database and the distance database when there are only a few nodes in the wireless mesh network system1. However, as the number of nodes increases, it becomes practically difficult to manually update the information. More particularly, the process of separately updating the distance database in each node is complicated. On the other hand, in the case of using a GPS, it is necessary to arrange a GPS in each node thereby increasing the manufacturing cost of the wireless mesh network system1. To prevent such problems, the wireless mesh network system1can be configured such that each node therein can use a corresponding communication function to independently obtain the positions of other nodes or the distances from the other nodes.

For example, the physical layer of an ultra wide band (UWB) communication such as WiMedia has a distance measuring function that is a high precision function having an error range up to several tens of centimeters. Moreover, although having a lower precision than the UWB, a wireless local area network (LAN) communication protocol such as the IEEE 802.11 standard also has a distance measuring function that obtains an inter-node distance by measuring corresponding reception power value. Thus, if nodes having a similar distance measuring function are used, then it is possible to build a position database as shown inFIG. 3or a distance database as shown inFIG. 6.

FIG. 7is a schematic diagram for explaining a functional configuration of a node700according to a second embodiment of the present invention. According to the second embodiment, each node in the wireless mesh network system1is able to independently obtain the positions of other nodes or the distances with the other nodes. Each of the nodes200to600according to the second embodiment has an identical functional configuration as that of the node700. A controller710in the node700includes a distance measuring unit115in addition to the constituent elements descried in the first embodiment. The distance measuring unit115operates in tandem with the communication control unit112and, based on a result of communication with each of the other nodes200to600, obtains the distances therefrom.

FIG. 8is a flowchart for explaining a process performed by the node700to independently measure the distances from the nodes200to600. To obtain those distances, first, the main control unit111controls the communication control unit112to transmit a signal via the wireless communicating unit102and identify nodes that are arranged in the communicable range of the node700(Step S801). When no node is identified to be in the communicable range of the node700(No at Step S801), the process is completed. When one or more nodes are identified to be in the communicable range of the node700(Yes at Step S801), the distance measuring unit115measures the distance from each identified node (Step S802). Subsequently, the main control unit111transmits information regarding the measured distance to each identified node (Step S803). Upon receiving the information, each identified node performs a predetermined operation described hereinafter and transmits back an end signal to the node700. The main control unit111then determines whether each identified node has transmitted an end signal (Step S804). When it is determined that each identified node has transmitted an end signal (Yes at Step S804), the process of measuring the distances is completed.

FIG. 9is a flowchart for explaining the operations performed by the nodes (one or more of the nodes200to600) that are identified to be in the communicable range of the node700at Step S801. The description below is given with reference to the node200. First, the node200receives information regarding the measured distance from the node700(Step S901). Then, the main control unit111therein controls the corresponding communication control unit112to transmit a signal via the corresponding wireless communicating unit102such that nodes arranged in the communicable range of the node200are identified (Step S902). When no node is identified to be in the communicable range of the node200(No at Step S902), the node200transmits back an end signal to the node700(Step S906) and the process is completed. When one or more nodes are identified to be in the communicable range of the node200(Yes at Step S902), the distance measuring unit115in the node200measures the distance from each identified node (Step S903). Subsequently, the main control unit111transmits information regarding the measured distance to each identified node (Step S904). Upon receiving the information, each identified node performs an identical operation to that performed by the node200and transmits back an end signal to the node200. The main control unit111then determines whether an end signal is transmitted from each identified node (Step S905). If it is determined that each identified node has transmitted an end signal (Yes at Step S905), the main control unit111controls the communication control unit112and transmits an end signal to the node700via the wireless communicating unit102(Step906), and the process is completed.

In this way, first, the node700functions as a parent node and transmits information regarding the measured distance to the node200that functions as a child node. Subsequently, the node200functions as a parent node and transmits information regarding the measuring distance to other child nodes. In this way, each of the nodes200to700in the wireless mesh network system1can independently measure the distances with the other nodes. Meanwhile, the wireless mesh network system1can also be configured such that more than two nodes are defined as standard nodes in which the position database is recorded in advance. Due to such a configuration, the distance information and the position information can be obtained by using a triangulation method. On the other hand, if no standard nodes are defined in advance, then any of the nodes having the position database can be used to obtain the position information by using a triangulation method.

However, in the abovementioned case, if the positions of the standard nodes are varied, it becomes difficult to obtain precise position information. Such a problem occurs when a portable node such as a notebook computer or a cellular phone is used. To solve the problem, a portability flag can be stored in the portable node to make sure that the portable node is not defined as a standard node. In this way, each node in the wireless mesh network system1can independently obtain the positions of the other nodes or the distances from the other nodes. Thus, even if a node is newly arranged in the communicable range of the wireless mesh network system1, the other nodes are able to automatically obtain the position and the distance of the newly arranged node. As a result, the newly arranged node can easily be incorporated in a communication route.

On the other hand, when a node, which is compatible to the wireless mesh network system1and has a communication function to independently obtain the positions of other nodes or the distances from the other nodes, is newly arranged in the communicable range of the wireless mesh network system1, then that node automatically gets incorporated in the wireless mesh network system1. However, such a configuration is not desirable from the perspective of information security. To solve such a problem, as shown inFIG. 10, a communicable range D is defined for the wireless mesh network system1and the information regarding the communicable range D is recorded in the information storing unit103of each of the nodes100to600. Thus, in that case, if the node700is arranged in the communicable range of the node600but outside the communicable range D, then the nodes100to600consider obtaining the position information of the node700. As a result, the node700is not incorporated in the communicable range D unless a system administrator of the wireless mesh network system1allows that to happen.

Meanwhile, in the above description with reference toFIGS. 1 and 3, only the coplanar positions of the nodes100to600are taken into considerations. In addition, it is also possible to take into consideration the heightwise positions of the nodes100and600. By taking into consideration the heightwise positions, it becomes possible to exclude from a communication route a node that is arranged coplanarly close to the other nodes but, e.g., on a different floor partitioned by the ceiling (e.g., the node700shown inFIG. 11). In such a case, the wireless signal gets jammed thereby degrading the signal quality. To take into consideration the heightwise positions of the nodes, a three-dimensional communicable range D can be defined for the wireless mesh network system1.

Moreover, by taking into consideration the heightwise positions, it is also possible to select a communication route that includes nodes arranged at an elevated position near the ceiling because the region near the ceiling is generally free of obstacles such as office equipments or furniture.

As described above, according to the first embodiment, the total expected time for transmission T is calculated by using the distance dnof each of the n number of inter-node communication paths. That enables exclusion of a communication route including distantly arranged intermediate nodes. However, even if the intermediate nodes in a communication route are not distantly arranged, the signal transfer efficiency can degrade because of obstacles in the communication route. Such a communication route is unsuitable for speedy communication. Thus, as described hereinafter, it is also necessary to take into consideration the physical environment surrounding the communication routes.FIG. 12is a schematic diagram of a plurality of communication routes in the wireless mesh network system1according to a third embodiment of the present invention. InFIG. 12, the constituent elements having reference numerals identical to those inFIG. 1have identical configuration as described in the first embodiment. As shown inFIG. 12, a node1300can transmit a wireless signal to the node600via the communication route A, the communication route B, and a communication route C that includes three inter-node communication paths e, g, and d. Moreover, it is assumed that an obstacle Q lies in the inter-node communication path f in the communication route B.

FIG. 13is a schematic diagram for explaining a functional configuration of the node1300. Each of the nodes200to600has an identical functional configuration as that of the node1300. As shown inFIG. 13, the node1300includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller1310. The controller1310includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route selecting unit114, the quality information obtaining unit117, and a communication route obtaining unit1316. Except for the communication route obtaining unit1316, the constituent elements in the node1300have identical configuration as described in the first embodiment.

To select a communication route via which the node1300can transmit a wireless signal to the node600, first, the total expected time for transmission T via each of the communication routes A, B, and C is obtained by using the number of corresponding intermediate nodes and the position database. In that case, generally, it is likely that the communication route B is selected because of having the least total expected time for transmission. However, because of the obstacle Q in the inter-node communication path f, there is a possibility that the actual time for transmission exceeds the total expected time for transmission TBby a large amount. In other words, because of the obstacle Q in the inter-node communication path f, the wireless signal gets weaker thereby reducing the reception power value Prof the node600. As a result, the actual time for transmission in the inter-node communication path f exceeds the expected time for transmission tf, which is obtained based on a distance dfof the inter-node communication path f. Thus, there is a possibility that the wireless signal is not transmitted at the earliest via the communication route B. To solve such a problem, the communication route obtaining unit1316is configured to exclude any communication route that includes an inter-node communication path with an obstacle.

According to the third embodiment, in addition to the position database, an obstacle database is also stored in the information storing unit103. The obstacle database includes information regarding the positions of obstacles lying in the communicable range of the wireless mesh network system1.FIG. 14is a table for explaining an exemplary obstacle database stored in the information storing unit103. The obstacle database includes an identification ID, an obstacle name, and an interference range of each obstacle that causes degradation in the signal quality. The interference range of each obstacle is a range in which the obstacle interferes with the wireless communication. The interference range is represented as a coordinate range with respect to the origin0.

FIG. 15is a flowchart for explaining a process of selecting a communication route performed by the node1300. First, a communication command is generated in the node1300for transmitting a wireless signal to the node600(Step S1501). Then, the communication route obtaining unit1316receives a command from the main control unit111and controls the storage medium control unit113to read the position information of the node600from the position database stored in the information storing unit103(Step S1502).

The main control unit111then issues a command to the communication route obtaining unit1316and the communication route selecting unit114for selecting a communicating route from the node1300to the node600(Step S1503). Subsequently, the communication route obtaining unit1316obtains a plurality of communication routes (i.e., the communication routes A, B, and c) by selecting corresponding intermediate nodes via which the wireless signal can be transmitted to the node600(Step S1504). The communication route obtaining unit1316then refers to the obstacle database in the information storing unit103and excludes the communication routes in which an obstacle recorded in the obstacle database is lying (Step S1505). That is, with reference toFIG. 12, the communication route obtaining unit1316excludes the communication route B in which the obstacle Q is lying. Subsequently, similar to Steps S505to S507inFIG. 5, a communication route that requires the least total expected time for transmission is determined in Steps S1506to S1508.

In this way, by excluding the communication routes with an obstacle, a communication route can be efficiently selected via which the transmission of a wireless signal can be completed at the earliest.

In the abovementioned process of selecting a communication route, it is also possible to take into consideration the effect of the obstacles while obtaining the total expected time for transmission at Step S1506. When a wireless signal in interfered by an obstacle in an inter-node communication path, the reception power value Prof the node that receives the wireless signal in the inter-node communication path decreases, resulting in the degradation of the SNR and increase in the retransmission retry count CRnof the node. Consequentially, the expected time for transmission tnin the inter-node communication path increases thereby increasing the total expected time for transmission T of the communication route including the inter-node communication path with the obstacle. To solve such a problem, at Step S1506, the communication route selecting unit114takes into consideration the interference of the obstacle Q on the wireless signal in the inter-node communication path f while obtaining the SNR at the node600, and the retransmission retry count CRfand the expected time for transmission tfin the inter-node communication path f. Accordingly, the communication route selecting unit114obtains the expected time for transmission TBand compares that with the expected time for transmission of the other communication routes to select a communication route for transmission.

Meanwhile, although all communication routes with an obstacle are excluded at Step S505, there is a possibility that some of the obstacles do not cause a substantial degradation in the signal quality. Thus, to make sure that only those communication routes are excluded in which an obstacle substantially degrades the signal quality, the effect of the obstacles is taken into consideration while obtaining the expected time for transmission of the communication routes.

As described above in the first embodiment, the total expected time for transmission T of a communication route is obtained by using the distance dnof each of the n number of inter-node communication paths. Moreover, as described above in the third embodiment, the expected time for transmission tnin each of the n number of inter-node communication paths is obtained by taking into consideration the effect of a corresponding obstacle. In both cases, the expected time for transmission tnis obtained by using the reception power value Pr, the SNR, and the retransmission retry count CRnof the corresponding destination node. However, as described hereinafter, the expected time for transmission tncan also be obtained by using information regarding the signal quality, irrespective of the distance between the nodes in a communication route or whether an obstacle lies in the communication route.

Given below is the description of a method of selecting a communication route in the wireless mesh network system1according to a fourth embodiment of the present invention. The description is given with reference to the wireless mesh network system1shown inFIG. 12except that the wireless mesh network system1includes a node1600instead of the node1300.

FIG. 16is a schematic diagram for explaining a functional configuration of the node1600. Each of the nodes200to600has an identical functional configuration as that of the node1600. As shown inFIG. 16, the node1600includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller1610. The controller1610includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route obtaining unit116, a communication route selecting unit1614, and a quality information obtaining unit1617. Except for the communication route selecting unit1614and the quality information obtaining unit1617, the constituent elements in the node1600have identical configuration as described in the first embodiment.

According to the fourth embodiment, a signal quality database is stored in the information storing unit103.FIG. 17is a table for explaining an exemplary signal quality database stored in the information storing unit103. The signal quality database includes an identification ID, a node name, and a reception power ratio of each of the nodes200to600. The reception power ratio is a ratio of the transmission power value of the node1600and the reception power value of each of the nodes200to600. For example, inFIG. 17, the node1600is transmits a wireless signal at a transmission power value of Pt100while the node200receives the wireless signal at a reception power value of Pr100-200. Thus, the reception power ratio of the node200is Pr100-200/Pt100.

The quality information obtaining unit1617obtains the reception power ratios from the signal quality database and calculates the total expected time for transmission via each communication route by using the reception power ratios. Based on such total expected time for transmission, the communication route selecting unit1614selects a communication route via which the transmission of a wireless signal can be completed at the earliest.

Given below is the description of a process of selecting a communication route performed by the node1600. That process is by and large identical to the process explained with reference toFIG. 5. However, with reference to Step S502, the communication route obtaining unit116reads information from the signal quality database instead of the position database. Subsequently, with reference to Step S505, the quality information obtaining unit1617calculates the total expected time for transmission T via each communication route by using the reception power ratios in the signal quality database. More particularly, it is possible to obtain the SNR and the retransmission retry count CRnof a node by using the reception power ratio of that node. Accordingly, the expected time for transmission tncan be obtained by using Equation (4) and the total expected time for transmission T can be obtained by using Equation (3). Meanwhile, with reference to Step S504, communication routes can be obtained by using information stored in advance or by using the reception power ratios of the nodes200to600.

In this way, by using the reception power ratios to obtain the expected time for transmission via a plurality of communication routes, a communication route can be selected via which the transmission of a wireless signal can be completed at the earliest.

Meanwhile, in addition to the node1600, each of the nodes200to600can also be configured to independently obtain the signal quality information. More particularly, in each of the nodes200to600, the communication control unit112can be configured to identify the corresponding transmission power value Ptand the corresponding reception power value Prbased on which the main control unit111can obtain the signal quality information.

As described with reference to Equation (5), the reception power of a node that receives a wireless signal varies according to the transmission power of a node that transmits the wireless signal. Thus, even if the distance between those two nodes is long, there is still a possibility that a sufficiently large transmission power facilitates in securing a reception power that is sufficient to speedily complete the transmission of the wireless signal. On the other hand, from the perspective of energy saving, it is desirable to reduce the transmission power as much as possible. Considering such conditions, given below is the description according to a fifth embodiment of the present invention in which the transmission power of a node is controlled.

The description is given with reference to the wireless mesh network system1shown inFIG. 12except that the wireless mesh network system1includes a node1800instead of the node1300.

FIG. 18is a schematic diagram for explaining a functional configuration of the node1800. Each of the nodes200to600has an identical functional configuration as that of the node1800. As shown inFIG. 18, the node1800includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller1810. The controller1810includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route obtaining unit116, the quality information obtaining unit117, a communication route selecting unit1814, and a transmission power control unit1815. Except for the communication route selecting unit1814and the transmission power control unit1815, the constituent elements in the node1800have identical configuration as described in the first embodiment.

The transmission power control unit1815receives a command from the main control unit111and accordingly controls the transmission power value of the node1800while transmitting a wireless signal. For example, when the distance between the node1800and a destination node for the wireless signal is short, the transmission power control unit1815reduces the transmission power value Ptto save energy. On the other hand, when the total number of intermediate nodes N is small and the distance dnof the inter-node communication paths is long, the transmission power control unit1815increases the transmission power value Ptsuch that the expected time for transmission tndoes not increase. That prevents the decrease in the reception power value Prand the degradation of the SNR of the destination node thereby speeding up the transmission.

FIG. 19is a flowchart for explaining a process of selecting a communication route performed by the node1800. The process is described for an energy saving mode and a speed-up mode of the node1800controlled by the transmission power control unit1815. The process from the generation of a communication command (Step S1901) up to the obtainance of a plurality of communication routes (Step S1904) is identical to the process from Steps S501to S504inFIG. 5.

Upon obtaining a plurality of communication routes (Step S1904), it is determined whether the node1800is in the speed-up mode (Step S1905). When the node1800is determined to be in the speed-up mode (Yes at Step S1905), the communication route selecting unit1814selects a communication route that includes the least number of intermediate nodes (Step S1906). The transmission power control unit1815then controls the transmission power value at a high level (Step S1907) and the process is completed.

FIG. 20is a table for explaining the information that is included in a wireless signal transmitted, e.g., from the node1800to the node600. The wireless signal includes destination node information, communication route information, transmission power information, and reception power information. The destination node information is indicated by, e.g., an internet protocol (IP) address of the destination node. The communication route information is information regarding the selected communication route from among the communication routes A, B, and C, and is represented by the nodes therein. For example, the communication route A is represented as “node1800-node200-node300-node400-node600”.

The transmission power information is information regarding the transmission power value specified for each node at the time of transmission. Moreover, the transmission power value is specified with respect to a standard power value. Meanwhile, it is also possible to specify the standard power value as the actual transmission power value. Each source node transmits the transmission power information to a destination node. At Step S1906inFIG. 19, the communication B is selected as the communication route that includes the least number of intermediate nodes. Thus, the communication route information regarding the communication route B (node1800-node500-node600) is generated. At Step S1907, the transmission power value larger than the standard power value is set in the transmission power information.

At Step S1905, when the node1800is determined not to be in the speed-up mode (No at Step S1905), it is determined whether the node1800is in the energy saving mode (Step S1908). When the node1800is determined to be in the energy saving mode (Yes at Step S1908), then the communication route selecting unit1814selects a communication route that includes the shortest inter-node communication paths (Step S1909). Herein, a communication route that includes the shortest inter-node communication paths means either a communication route in which the distance between two nodes in each intermediate communication path is shortest or a communication node having the least average distance. Thus, at Step S1909, the communication route A is selected and the communication route information thereof is generated as “node1800-node200-node300-node400-node600”. The transmission power control unit1815then controls the transmission power value at a low level (Step S1910) and the process is completed. That is, at Step S1910, the transmission power value smaller than the standard power value is set in the transmission power information.

When the node1800is determined to be neither in the speed-up mode (No at Step S1905) nor in the energy saving mode (No at Step S1908), then the communication route selecting unit1814calculates the total expected time for transmission for all communication routes (Steps S1911and S1912) and selects a communication route that requires the least total expected time for transmission (Step S1913).

When a communication route is selected, the wireless communicating unit102is activated based on the information generated in the communication control unit112(seeFIG. 20). That is, when the node1800is in the speed-up mode, the transmission power value from the wireless communicating unit102is set higher than the standard power value for transmitting a wireless signal via the communication route B. Upon receiving the wireless signal along with the transmission power information, the intermediate node500transfers the wireless signal to the node600after setting the transmission power value based on the received transmission power information. Although the distance deof the intermediate communication path e is longer than that of the intermediate communication paths a to d in the communication route A and the obstacle Q lies in the inter-node communication path f (seeFIG. 12), the reception power value Prof the node600can be maintained at a high level because the intermediate node500sets the transmission power value at a high level. Moreover, because the communication route B includes the least number of intermediate nodes, the overhead time tOHin the communication route B is minimized thereby speeding up the transmission. Meanwhile, each transmission method generally has an independent standard regarding the upper limit of the transmission power value. Thus, in the abovementioned case, the transmission power value cannot be set higher than the upper limit. However, the abovementioned process can be performed when the standard power value is set to be smaller than the upper limit for the purpose of saving energy.

On the other hand, when the node1800is in the energy saving mode, the transmission power value from the wireless communicating unit102is set lower than the standard power value and a wireless signal is transmitted via the communication route A. That is, the intermediate nodes200,300, and500receive a wireless signal along with the transmission power information and, after setting the transmission power value based on the received transmission power information, transmit the wireless signal to the node600. Because each of the inter-node communication paths a, b, c, and d is short, the rate of reduction in the reception power value at each of the nodes200,300,500, and600is small. Thus, by using the energy saving mode, it is possible to transmit the wireless signal at a low transmission power value.

In this way, according to the fifth embodiment, on one hand, it is possible to save energy by controlling the transmission power at each node; while on the other hand, it is possible to speed up the transmission even when a communication route has a high rate of degradation in the signal quality.

Meanwhile, instead of controlling the transmission power of a node that transmits a wireless signal, the abovementioned object can also be achieved by controlling a signal reception gain at each node that receives the wireless signal. When a node is controlled to transmit a wireless signal with a transmission power value higher than the standard power value, it is assumed that the corresponding communication route has a high rate of degradation in the signal quality. However, there is a possibility that the inter-node communication paths in the communication route are short and without any obstacle. As a result, the signal quality does not degrade substantially. In that case, transmitting a wireless signal at a high transmission power value leads to wastage of energy. To avoid such a problem, the transmission power value can be determined based on the distances of the inter-node communication paths in a communication route as well as the reception power ratios of the nodes therein.

As described above, each intermediate node transmits a wireless signal based on the transmission power information generated by a source node. Instead, each intermediate node can be configured to independently determine the transmission power based on the distance of the corresponding inter-node communication path as well as the reception power ratio of the subsequent node.

Given below is the description of the wireless mesh network system1according to a sixth embodiment of the present invention in which the distance database is updated automatically.

The description is given with reference to the wireless mesh network system1shown inFIG. 12except that the wireless mesh network system1includes a node2100instead of the node1300.

FIG. 21is a schematic diagram for explaining a functional configuration of the node2100. Each of the nodes200to600has an identical functional configuration as that of the node2100. As shown inFIG. 21, the node2100includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller2110. The controller2110includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route selecting unit114, the communication route obtaining unit116, the quality information obtaining unit117, a distance measuring unit2115, and a distance updating unit2118. Except for the distance measuring unit2115and the distance updating unit2118, the constituent elements in the node2100have identical configuration as described in the first embodiment.

According to the sixth embodiment, a distance database identical to that shown inFIG. 6is stored in the information storing unit103. In the distance database, the distance of each of the nodes200to600from the node2100is recorded. Moreover, it is assumed that at least one of the nodes200to600is arranged at a fixed position.

The distance measuring unit2115measures the distance from the node arranged at the fixed position by using any known method proposed in a communication protocol. Subsequently, the distance updating unit2118determines whether the distance from the fixed position of the node is identical to that recorded in the distance database. When the distance from the fixed position of the node is determined to be different, then the distance updating unit2118updates the distance database with the new distance. Meanwhile, the process of selecting a communication route is identical to the process described in the first embodiment.

FIG. 22is a flowchart for explaining a process of updating the distance database. The process is repeated at a regular time interval in the node2100as well as in each of the nodes200to600.

In the node2100, first, the distance measuring unit2115measures the distance from a node arranged at a fixed position (Step S2101). Subsequently, the distance updating unit2118determines whether the distance from the fixed position of the node is different than that recorded in the distance database (Step S2102). When the distance from the fixed position of the node is determined to be the same (No at Step S2102), then the distance updating unit2118certifies that the node2100is not moved from the previous recorded position and the process is completed.

When the distance from the fixed position of the node is determined to be different (Yes at Step2102), then the distance updating unit2118certifies that the node2100is moved to a new position and obtains new distances from each of the nodes200to600(Step S2103). Subsequently, the distance updating unit2118updates the distance database with the new distances (Step S2104).

In this way, the distance database is updated when the node2100is moved to a new position.

Meanwhile, instead of updating the distance database, it is also possible to update a position database identical to that shown inFIG. 3.

Given below is the description of the wireless mesh network system1according to a seventh embodiment of the present invention in which a position database is updated by deleting the position information of a node that is determined to be lying outside the communicable range D.

The description is given with reference to the wireless mesh network system1shown inFIG. 12except that the wireless mesh network system1includes a node2300instead of the node1300.

FIG. 23is a schematic diagram for explaining a functional configuration of the node2300. Each of the nodes200to600has an identical functional configuration as that of the node2300. As shown inFIG. 23, the node2300includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller2310. The controller2310includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route selecting unit114, the communication route obtaining unit116, the quality information obtaining unit117, a distance measuring unit2315, and a position updating unit2318. Except for the distance measuring unit2315and the position updating unit2318, the constituent elements in the node2300have identical configuration as described in the first embodiment.

According to the seventh embodiment, a position database identical to that shown inFIG. 3and the information regarding the communicable range D as shown inFIG. 10is stored in the information storing unit103. The position database includes an identification ID, a node name, and position information of the node2300as well as each of the nodes200to600.

The distance measuring unit2315measures the distance from each of the nodes200to600by using any known method proposed in a communication protocol. Subsequently, the position updating unit2318determines whether any of the distances is longer than a predetermined distance. When a distance is determined to be longer than the predetermined distance, the position updating unit2318certifies that the corresponding node is lying outside the communicable range D and deletes the position information of that node from the position database. Meanwhile, the process of selecting a communication route is identical to the process described in the first embodiment.

FIG. 24is a flowchart for explaining a process of updating the position database.

First, the distance measuring unit2315measures the distance from one of the nodes200to600(Step S2401). Then, the position updating unit2318determines whether the distance is longer than a predetermined distance (Step S2402). When the distance is determined to be longer than the predetermined distance (Yes at Step S2402), the position updating unit2318certifies that the corresponding node is lying outside the communicable range D (Step S2403) and deletes the position information of that node from the position database (Step S2404).

When the distance is determined to be equal to or shorter than the predetermined distance (No at Step S2402), the position database is not updated. The process from Step S2401to Step S2404is repeated with respect to each of the nodes200to600(Step S2405).

In this way, the position database is updated when a node is moved outside the communicable range D.

Meanwhile, instead of updating the position database, it is also possible to update a distance database identical to that shown inFIG. 6.

Given below is the description of the wireless mesh network system1according to an eighth embodiment of the present invention in which heightwise distances of the nodes are taken into consideration while selecting a communication route (seeFIG. 11) and, based on the heightwise distances, a different communication protocol is implemented for transmission.

The description is given with reference to the wireless mesh network system1shown inFIG. 12except that the wireless mesh network system1includes a node2500instead of the node1300.

FIG. 25is a schematic diagram for explaining a functional configuration of the node2500. Each of the nodes200to600has an identical functional configuration as that of the node2500. As shown inFIG. 25, the node2500includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller2510. The controller2510includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route obtaining unit116, the quality information obtaining unit117, and a communication route selecting unit2514. Except for the communication route selecting unit2514, the constituent elements in the node2500have identical configuration as described in the first embodiment.

According to the eighth embodiment, a distance database is stored in the information storing unit103. The distance database includes information regarding a coplanar distance and a heightwise distance from each of the nodes200to600.FIG. 26is a table for explaining an exemplary distance database stored in the information storing unit103.

The communication route selecting unit2514selects a communication route by performing the process described in the first embodiment. Subsequently, the communication route selecting unit2514determines whether the heightwise distance from the subsequent node in the selected communication route is equal to or longer than a predetermined distance. If the heightwise distance from the subsequent node is determined to be equal to or longer than the predetermined distance, the communication route selecting unit2514selects the IEEE 802.11a standard (having a usable frequency of 5.2 gigahertz) as a high frequency communication protocol for communication that has a higher frequency than a predetermined frequency (e.g. 3.0 gigahertz). On the other hand, if the heightwise distance from the subsequent node is determined to be shorter than the predetermined distance, the communication route selecting unit2514selects either one of the IEEE 802.11b standard and the IEEE 802.11g (having a usable frequency of 2.4 gigahertz) as a low frequency communication protocol for communication that has a lower frequency than the predetermined frequency. Subsequently, the wireless communicating unit102transmits a wireless signal to the subsequent node by using the selected communication protocol. Meanwhile, the high frequency communication protocol is not limited to the IEEE 802.11a standard, and the low frequency communication protocol is not limited to the IEEE 802.11b standard or the IEEE 802.11g standard. For example, consider a case when the subsequent node is arranged at an elevated position near the ceiling and the heightwise distance from the subsequent node is equal to or longer than the predetermined distance. Then, it can be assumed that no obstacle lies in the communication route up to the subsequent node. In that case, it is desirable to use a high frequency communication protocol that has a non-diffracting property for transmitting a wireless signal. On the other hand, consider a case when the subsequent node is arranged at about the same level and the heightwise distance from the subsequent node is shorter than the predetermined distance. Then, it can be assumed that an obstacle such as a table or a chair lies in the communication route up to the subsequent node. In that case, it is desirable to use a low frequency communication protocol that has a diffracting property for transmitting a wireless signal.

FIG. 27is a flowchart for explaining a process of selecting a communication route according to the eighth embodiment. The process from the generation of a communication command (Step S2701) up to the selection of a communication route (Step S2707) is identical to the process from Steps S501to S507inFIG. 5. Upon selecting a communication route (Step S2707), the communication route selecting unit2514reads from the distance database the heightwise distance from the subsequent node in the selected communication route and determines whether that heightwise distance is equal to or longer than a predetermined distance (Step S2708). When the heightwise distance from the subsequent node is determined to be equal to or longer than the predetermined distance (Yes at Step S2708), the communication route selecting unit2514selects the IEEE 802.11a standard as a high frequency communication protocol for communication that has a higher frequency than a predetermined frequency (Step S2709). When the heightwise distance from the subsequent node is determined to be shorter than the predetermined distance (No at Step S2708), the communication route selecting unit2514selects either one of the IEEE 802.11b standard and the IEEE 802.11g as a low frequency communication protocol for communication that has a lower frequency than the predetermined frequency (Step S2710). Subsequently, the wireless communicating unit102transmits a wireless signal to the subsequent node by using the selected communication protocol.

In this way, by taking into consideration the heightwise distances of the nodes, the wireless signal can be transmitted speedily.

Given below is the description of the wireless mesh network system1according to a ninth embodiment of the present invention. According to the ninth embodiment, a communication route is selected by using signal quality information that includes a reception power ratio of each node corresponding to a plurality of communication protocols. Moreover, the transmission power is controlled based on the reception power ratios.

The description is given with reference to the wireless mesh network system1shown inFIG. 12except that the wireless mesh network system1includes a node2800instead of the node1300.

FIG. 28is a schematic diagram for explaining a functional configuration of the node2800. Each of the nodes200to600has an identical functional configuration as that of the node2800. As shown inFIG. 28, the node2800includes the user I/F101, the wireless communicating unit102, the information storing unit103, and a controller2810. The controller2810includes the main control unit111, the communication control unit112, the storage medium control unit113, the communication route obtaining unit116, a communication route selecting unit2814, a transmission power control unit2815, a quality information obtaining unit2817, and a protocol determining unit2818. Except for the communication route selecting unit2814, the transmission power control unit2815, the quality information obtaining unit2817, and the protocol determining unit2818, the constituent elements in the node2800have identical configuration as described in the first embodiment.

According to the ninth embodiment, a signal quality database is stored in the information storing unit103.FIG. 29is a table for explaining an exemplary signal quality database stored in the information storing unit103. The signal quality database includes an identification ID, a node name, and a reception power ratio of each of the nodes200to600with respect to a plurality of wireless LAN communication protocols such as the IEEE 802.11a standard, the IEEE 802.11b standard, and the IEEE 802.11g standard. The reception power ratio is a ratio of the transmission power value of the node2800and the reception power value of each of the nodes200to600.

The protocol determining unit2818determines a wireless LAN communication protocol being currently implemented for receiving a wireless signal (hereinafter, “reception protocol”) as well as a wireless LAN communication protocol implemented for transmitting a wireless signal (hereinafter, “transmission protocol”).

The quality information obtaining unit2817obtains from the signal quality database the reception power ratios corresponding to the reception protocol and calculates the total expected time for transmission via each communication route by using the reception power ratios.

The transmission power control unit2815receives a command from the main control unit111and controls the transmission power value based on the transmission protocol. That is, the transmission power control unit2815varies the transmission power value depending on the transmission protocol. The process of controlling the transmission power is identical to that described in the fifth embodiment.

FIG. 30is a flowchart for explaining a process of selecting a communication route according to the ninth embodiment. First, a communication command is generated in the node2800for transmitting a wireless signal to the node600(Step S3001). Then, the protocol determining unit2818determines which of the IEEE 802.11a standard, the IEEE 802.11b standard, and the IEEE 802.11g standard is implemented as the wireless LAN communication protocol (Step S3002a). Subsequently, the quality information obtaining unit2817obtains from the signal quality database the reception power ratios corresponding to the implemented wireless LAN communication protocol (Step S3002b). The subsequent process of obtaining a plurality of communication routes (Step S3003) and determining whether the node2800is in the speed-up mode (Step S3005) is performed in an identical manner to that described in the fifth embodiment.

When a communication route that includes the least number of intermediate nodes is selected (Step S3006), the protocol determining unit2818determines the transmission protocol (Step S3007). The transmission power control unit2815then varies the transmission power value depending on the transmission protocol (Step S3008) and controls the transmission power value at a high level (Step S3009).

When the node2800is determined not to be in the speed-up mode (No at Step S3005), it is determined whether the node2800is in the energy saving mode (Step S3010). When the node2800is determined to be in the energy saving mode (Yes at Step S3010), then the protocol determining unit2818determines the transmission protocol (Step S3011). The transmission power control unit2815then varies the transmission power value depending on the transmission protocol (Step S3012), selects a communication route that includes the shortest inter-node communication paths (Step S3013), and controls the transmission power value at a low level (Step S3014).

In this way, the total expected time for transmission is calculated by taking into consideration the reception power corresponding to the implemented wireless LAN communication protocol. Thus, a communication route can be selected via which the transmission of a wireless signal can be completed at the earliest. Moreover, irrespective of whether a communication route has a high rate of degradation in the signal quality, it is possible to save energy as well as speed up the transmission depending on the implemented wireless LAN communication protocol.

Meanwhile, if the transmission protocol determined at step S3007is different than the reception protocol, then the transmission power control unit2815can be configured not to control the transmission power value.

Thus, according to an aspect of the present invention, a communication route can be selected from among a plurality of communication routes in a wireless mesh network system such that a wireless signal can be transmitted at the earliest.