Abstract:
A wireless communication system is configured such that, during a period in which wireless communication is being performed between first and second wireless access points, wireless communication in a first wireless communication sub-network and a second wireless communication sub-network is made to stand by, and during the time other than the period in which wireless communication is being performed, wireless communication in at least one of the first wireless communication sub-network and the second wireless communication sub-network is performed.

Description:
[0001]    This application claims the priority of Japanese Patent Application No. JP 2012-202214, filed Sep. 14, 2012, the disclosure of which is expressly incorporated by reference herein in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wireless communication device, a wireless communication system, and a wireless communication control method. 
         [0004]    2. Background Art 
         [0005]    In recent years, with a rise in awareness of environmental problems and institutionalization, there is an increasing technical need for energy management. On the side of consumers who consume energy, an energy management system (EMS), such as a home energy management system (HEMS) in which energy management is performed at home, a building energy management system (BEMS) in which energy management is performed at an apartment house or a building, or a city energy management system (CEMS) in which energy management is performed in an area of a community, is expected. On the side of electric power companies which generate energy, a smart meter or an advanced metering infrastructure (AMI) in which automatic reading of a power meter is performed, or a smart grid for stabilizing a power distribution/transmission system is required. There is also a study on the concept of a smart city in which these techniques are collectively applied in a city area. 
         [0006]    In order to realize an energy-related management system, it becomes essential to secure the infrastructure of communication means. As the communication means, there are various options, such as wireless communication, wired communication, optical communication, and power line communication, and optimum means is selected depending on an environment which is constructed as the infrastructure. Of these, wireless communication is excellent from the viewpoint of ease of installation and maintenance, low-cost devices, ease of expansion, and the like, and is highly expected. 
         [0007]    When wireless communication is used as the communication infrastructure means, an installation environment should also be taken into consideration and optimization is required depending on the purposes. For example, in regard to the smart meter in the AMI, there is a need for allowing multiple wireless terminals to perform communication regularly (every 30 minutes or the like) in a comparatively close area within several 100 m or 1 km. In the smart grid, when emergency communication is required, it is necessary to perform communication at a high speed (low latency or short time) over a long distance of about 10 km. Examples of emergency communication include short-circuiting due to electric wire breakage, an emergency stop instruction of power distribution, and the like. 
         [0008]    Of the background, in the related art, as disclosed in the following patent literatures, wireless techniques for optimally realizing an AMI wireless network have been developed, and multiple demonstration experiments have been carried out. Practically, however, read data from the AMI wireless network should be transmitted to the electric power company through the smart grid wireless network, and control information or an emergency instruction from the electric power company should be transmitted through the smart grid wireless network and the AMI wireless network. For this reason, a wireless communication scheme which can integrate a plurality of wireless networks having different requested specifications or features, such as the smart grid wireless network and the AMI wireless network, in a single wireless network is required. 
         [0009]    JP-A-2012-015897 discloses a technique which constructs an optimum communication root on the basis of link metric information between wireless nodes in a mesh network centering on a wireless gateway. 
         [0010]    JP-A-2009-188469 discloses a technique which performs transmission/reception only for a given period of time in a given cycle in an ad-hoc network and a mesh network to enable a network with low power consumption in an asynchronized communication scheme. 
       SUMMARY OF THE INVENTION 
       [0011]    An object of the invention is to enable wireless communication between a plurality of independent ad-hoc wireless networks and multi-hop wireless networks in a wireless communication network. 
         [0012]    In the related art, in one ad-hoc or multi-hop wireless network, a technique for optimizing communication reliability, power consumption, the number of connected transceivers, low cost, and the like depending on the purposes has been studied and developed. However, when there is a plurality of ad-hoc or multi-hop wireless networks, and communication should be performed between the wireless networks, since there is no common communication protocol between the wireless networks, it is not possible to realize wireless communication between the wireless networks. 
         [0013]    That is, the invention has been accomplished in consideration of the problems in the related art, and an object of the invention is to provide a wireless communication device, a wireless communication system, and a wireless communication control method which enable the realization of a plurality of ad-hoc/multi-hop networks (sub-networks) in a wireless communication network and an ad-hoc/multi-hop network (main network) connecting the sub-networks using a single wireless protocol. Another object of the invention is to prevent wireless communication of sub-networks from affecting communication quality of other main networks and sub-networks in realizing the above-described object. 
         [0014]    A wireless communication device, a wireless communication system, and a wireless communication control method of the invention for solving the above-described problems have the following means. 
         [0015]    The features of the invention are as follows. 
         [0016]    An access point (AP) is arranged in at least one of wireless communication devices in each sub-network, and a main network is configured between the APs to realize communication between a plurality of sub-networks. Since the main network is a main system, and there is a high probability that high-priority communication is occurred, a communication opportunity between the APs is preferentially secured in accordance with a wireless application specification. Besides, each AP allocates the time other than the communication opportunity between the main networks to the communication time between the sub-networks to which the AP belongs. 
         [0017]    When the AP performs communication between the main networks, the start time of carrier sense (a structure in which radio power strength around the wireless device is measured to determine the presence/absence of a transmission radio wave, and if there is a transmission radio wave, radio wave transmission is delayed to avoid packet collision) immediately before radio wave transmission is set to be smaller than the start time of carrier sense of the wireless communication devices in the sub-network, thereby placing priority on communication between the main networks by the AP. 
         [0018]    According to the invention, it is possible to provide a wireless communication device, a wireless communication system, and a wireless communication control method which enable the realization of a plurality of ad-hoc/multi-hop networks (sub-networks) in a wireless communication network and an ad-hoc/multi-hop network (main network) connecting the sub-networks using a single wireless protocol. It is also possible to provide a wireless communication network system which enables wireless communication in which main network communication by the AP is placed priority and prevents wireless communication of one sub-network from affecting communication quality of other main networks and sub-networks. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a network configuration example according to the invention. 
           [0020]      FIG. 2  shows a data transmission procedure (in case of CSMA). 
           [0021]      FIG. 3  shows a data transmission procedure (in case of TDMA). 
           [0022]      FIG. 4  shows a transceiver configuration of an AP and a wireless node ND in Example 1. 
           [0023]      FIG. 5  shows a communication time chart example (main network) (when a sub-network is asynchronous (CSMA)). 
           [0024]      FIG. 6  shows a communication time chart example (sub-network  11 ) (when a sub-network is asynchronous (CSMA)). 
           [0025]      FIG. 7  is an operation flowchart (1) of an AP of Example 1. 
           [0026]      FIG. 8  shows a transceiver configuration (2) of an AP and a wireless node ND in Example 2. 
           [0027]      FIG. 9  shows a communication time chart example (main network) (when a sub-network is synchronous (TDMA)). 
           [0028]      FIG. 10  shows a communication time chart example (sub-network  11 ) (when a sub-network is synchronous (TDMA)). 
           [0029]      FIG. 11  is an operation flowchart (2) of an AP in Example 2. 
           [0030]      FIG. 12  shows packet priority control (1) by reduction in carrier sense time (in case of CSMA). 
           [0031]      FIG. 13  shows packet priority control (2) by reduction in carrier sense time (in case CSMA). 
           [0032]      FIG. 14  shows packet priority control (1) by reduction in carrier sense time (in case of TDMA). 
           [0033]      FIG. 15  shows packet priority control (2) by reduction in carrier sense time (in case of TDMA). 
           [0034]      FIG. 16  shows a communication time chart example (sub-networks  11  and  12 )-1 (when each sub-network independently performs communication by TDMA). 
           [0035]      FIG. 17  shows a communication time chart example (sub-networks  11  and  12 )-2 (when each sub-network independently performs communication by TDMA). 
           [0036]      FIG. 18  is an operation flowchart ( 3 ) of an AP in Example  5 . 
           [0037]      FIG. 19  shows a communication time chart example (sub-networks  11  and  12 )-3 (when each sub-network independently performs communication by TDMA). 
           [0038]      FIG. 20  shows a communication time chart example (sub-networks  11  and  12 )-1 (when each sub-network independently performs communication by an asynchronized scheme (CSMA)). 
           [0039]      FIG. 21  shows a communication time chart example (sub-networks  11  and  12 )-2 (when each sub-network independently performs communication by an asynchronized scheme (CSMA)). 
           [0040]      FIG. 22  is an operation flowchart (4) of an AP in Example 8. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    Hereinafter, a wireless communication device, a wireless communication system, and a wireless communication control method according to the invention will be described in detail referring to an embodiment shown in the drawings. 
         [0042]      FIG. 1  shows a wireless communication network configuration of this example. A wireless communication network of this example includes a server  100 , wireless access points APs  101 ,  102 , and  103 , and wireless nodes NDs  111  to  113  and  121  to  126 , and NDs  111  to  113  double as wireless rooters. The AP  101  and the NDs  111 ,  121 , and  122  configure a sub-network  11 , the AP  102  and the NDs  112 ,  123 , and  124  configure a sub-network  12 , the AP  103  and the NDs  113 ,  125 , and  126  configure a sub-network  13 , and the server  100  and the APs  101 ,  102 , and  103  configure a main network  1 . In this example, it is assumed that wireless communication is performed along a path shown in  FIG. 1 . 
         [0043]    The network configuration example shown in  FIG. 1  assumes a communication network which is associated with a smart grid power distribution network and an AMI network, and the main network  1  is required for communication over a long distance (equal to or smaller than 10 km) with low latency (for example, 10 s) mainly with multiple hops as the smart grid wireless network. 
         [0044]    The server  100  installed in a substation or the like is connected to the higher level of the main network  1 , and in the server  100 , power read information from each sub-network is collected through the AP installed in an electric pole, and signals, such as a power control signal and an emergency instruction, according to the read information are transmitted to a controlled device, such as a switch, through the AP. The ND assumes a smart meter, and a sub-network covers a close distance (about 100 m to 1 km) at a communication interval of 30 minutes as an AMI wireless network. The power read information is collected from the smart meter to the AP and the server by the ad-hoc/mesh network, and the AP preferentially transmits control information from the server to a controlled device, such as a household electrical appliance. 
         [0045]    Hereinafter, a communication procedure by a career sense multiple access (CSMA) scheme and a communication procedure by a time division multiple access (TDMA) scheme which are commonly used in Examples will be described. In general, since the CSMA scheme is a simple protocol, while functional mounting is eased, packet collision is likely to occur. While the TDMA scheme is complicated and requires the number of man-hour for functional mounting, packet management can be performed, thereby avoiding packet collision. 
       Data Transmission/Reception Procedure by CSMA Scheme 
       [0046]    In the CSMA scheme, carrier sense is performed immediately before data transmission, and radio power strength is measured to check whether or not the other wireless device is transmitting data. If no radio wave of the other wireless device is detected, data is transmitted as scheduled. 
         [0047]    If a radio wave of the other wireless device is detected, scheduled data transmission is cancelled, and retransmission is performed in the next transmission opportunity. Accordingly, it is possible to avoid packet collision due to the matching of the data transmission timing. 
         [0048]    In the CSMA scheme, the period in which carrier sense is performed is set to be random in each transmission opportunity. 
         [0049]      FIG. 2  shows an example of a data transmission/reception procedure in the career sense multiple access (CSMA) scheme. In  FIG. 2 , a receiver  200   a  is kept in a reception standby state ( 201 ), and prepares for data reception. If transmitting data is generated ( 202 ), a transmitter  200   b  immediately performs carrier sense ( 203 ). In the example of  FIG. 2 , since a transceiver  200   c  is in data transmission ( 204 ) when transmitting data of the transmitter  200   b  is generated ( 202 ), a transmission radio wave is detected by carrier sense ( 203 ) of the transmitter  200   b  ( 205 ). 
         [0050]    For this reason, the transmitter  200   b  avoids scheduled data transmission ( 206 ), stands by the completion of data transmission of the transceiver  200   c,  performs carrier sense ( 207 ), and then perform data transmission again ( 209 ). Since the receiver  200   a  is in the reception standby state ( 201 ), data transmission ( 209 ) of the transmitter can be received. The transmitter  200   b  is changed to a reception standby state ( 212 ) in a short time after data transmission ( 209 ) and stands by a response (Ack. This means an acknowledge signal. The same applies to the following) from the receiver  200   a.  If data reception ( 210 ) is completed, the receiver  200   a  immediately transmits Ack ( 211 ), the transmitter  200   b  receives Ack ( 212 ), and communication is completed. 
         [0051]    In the CSMA mode, during a period other than carrier sense, data transmission, Ack transmission, and the data transmission avoidance period, reception standby is performed to prepare for data reception. 
       Data Transmission/Reception Procedure by TDMA Scheme 
       [0052]    In the TDMA scheme, transmission/reception of a packet for synchronization (Adv: Advertise) between the transmitter and the receiver is preformed regularly to perform time synchronization between the transmitter and the receiver, and communication is performed while matching the communication timing. 
         [0053]    For this reason, as in the CSMA scheme, there is an advantage in that packet collision or transmission delay due to a transmission radio wave of the other transceiver in the host system does not occur. 
         [0054]      FIG. 3  shows an example of a data transmission/reception procedure in the time division multiple access (TDMA) scheme. In the example of  FIG. 3 , in timeslots  317  and  318 , a receiver  300   a  performs a reception operation, and a transmitter  300   b  performs a transmission operation. The start time and the end time of the timeslot are shared by the receiver  300   a  and the transmitter  300   b  by sharing of a packet for time synchronization. 
         [0055]    The receiver performs reception standby for a given time in the timeslots  317  and  318  (( 301 ), ( 308 )), when there is no data reception ( 301 ), is changed to the next timeslot directly, and when there is data reception ( 308 ), transmits Ack at the end of the timeslot ( 314 ). 
         [0056]    If transmitting data is generated ( 302 ), for example, the transmitter  300   b  performs carrier sense over a given period in the timeslots  317  and  318  (( 303 ), ( 309 )), when no radio wave is detected ( 311 ), performs data transmission ( 310 ), and when a radio wave is detected ( 305 ), avoids scheduled data transmission ( 306 ). 
         [0057]    Reception standby is performed in a short time at the end of the timeslot (( 307 ), ( 316 )), and Ack is received from the receiver  300   a.  In the example shown in  FIG. 3 , while the transmitter  300   b  attempts data transmission to the receiver  300   a  in the timeslot  317 , since a radio wave ( 304 ) from the other system is detected ( 305 ), data retransmission ( 313 ) is performed in the timeslot  318  as the next transmission/reception opportunity. 
         [0058]    A repetition cycle of communication having a plurality of timeslots is referred to as a superframe. 
         [0059]    Hereinafter, Examples will be described in detail referring to the drawings. While the carrier sense time immediately before data transmission is not shown unless particularly required, in Examples, it is assumed that carrier sense immediately before data transmission is performed. 
       EXAMPLE 1 
       [0060]    This example illustrates an example of the invention when transmission/reception of a packet for time synchronization (Adv) is performed between the server  100  and the AP  101 ,  102 , or  103  in the main network of  FIG. 1 , whereby time synchronization is established. 
         [0061]      FIG. 4  shows a configuration example of a transceiver of an AP and an ND in this example. The configuration may be common to the AP and the ND. The transceiver has an antenna  401 , a transmission and reception circuit  402 , a communication control unit  403 , an external device IF  404 , a clock  405 , and a main network communication time and destination list ( 406 ) stored in a memory. The communication control unit  403  instructs the transmission and reception circuit  402  to perform data transmission/reception processing on the basis of the time of the clock  405  in accordance with the main network communication time and destination list  406  to be referenced by the communication control unit  403 , and performs transmission/reception of signals from the antenna  401 . 
         [0062]      FIG. 5  shows an example of a communication timing chart in the main network  1 . In this example, while communication by the TDMA mode is performed in the main network  1 , communication in the CSMA mode is performed in the sub-network  11 ,  12 , or  13 . That is, it is assumed that power read information which is aggregated in the AP through the AMI wireless network performing asynchronized communication by the CSMA scheme is collected by the smart grid wireless network of the TDMA scheme. 
         [0063]    In  FIG. 5 , the server  100  and the APs  101 ,  102 , and  103  set a communication allocation period for the main network  1  ( 501  to  518 ), communication to a sub-network is stopped in the main network communication period ( 501  to  518 ), and communication with an AP registered in advance is performed at the time registered in advance in the main network communication time and destination list  406 . The list  406  may be registered in advance and used, or may be rewritten through communication and used. Data reception/Ack return from the server  100  is performed in the main network  1  communication period ( 514 ), and the AP  101  transmits data to the AP  102  and receives Ack in the main network  1  communication period ( 501 ). In the main network communication period ( 508 ), data from the AP  102  is received and Ack is returned. 
         [0064]    In the main network communication period ( 509 ), the packet for time synchronization (Adv) is transmitted to the AP  102  to perform time synchronization. Communication by the CSMA scheme is performed using an unoccupied time between the main network communication periods ( 501 ,  508 , and  509 ) as a sub-network  11  communication period. 
         [0065]    For the server  100 , the AP  102 , and the AP  103 , similarly, communication with an AP is performed in the main network  1  communication period. For the AP  102  and the AP  103 , the unoccupied time between the main network communication periods is allocated to the communication periods of the sub-networks  12  and  13 . 
         [0066]      FIG. 6  shows an example of a communication time chart of the sub-network  11  communication period of the AP  101  in  FIG. 5 . 
         [0067]    While  FIG. 6  shows an example of communication of the sub-network  11  in the CSMA mode, a polling scheme in which each ND transmits data to the AP  101  in response to a data request (Req.) from the AP  101  such that communication of the sub-network can be stopped in the main network  1  communication period is used. 
         [0068]    In the polling scheme, the data request (Req.) is transmitted to an ND, and only the ND which receives Req. can transmit data. The AP  101  transmits Req. to the next ND first when data reception from the ND is completed. Description will be provided on the basis of the example of  FIG. 6 . The AP  101  receives data from the server  100  in the main network communication period  514 , and performs communication with the AP  102  in the main network communication period  501 . Next, Req. ( 111   r ) is transmitted to the ND  111 , and the ND  111  which receives Req. ( 111   r ) transmits data ( 111   d ) to the AP  101 . After reception of data ( 111   d ) is confirmed, the AP  101  transmits Req. ( 121   r ) to the ND  121  through the ND  111 , and acquires data ( 121   d ) of the ND  121 . Thereafter, after data ( 122   d ) of the ND  122  is received, the AP  101  performs communication of data and Adv with the AP  102  in the main network  1  communication periods  508  and  509 , and performs communication of data and Adv to the server  100  in the main network communication periods  516  and  518 . 
         [0069]      FIG. 7  is an operation flowchart of an AP in this example. If power is applied ( 701 ), an AP is changed to reception standby ( 702 ) and repeats a reception state ( 702 ) and Adv reception presence/absence determination ( 703 ) until the packet for synchronization (Adv) is received ( 703 ). If the Adv packet is received, the clock  405  is corrected ( 704 ) on the basis of the time included in the Adv packet. 
         [0070]    Thereafter, the procedure is changed to a loop of communication processing, and it is confirmed whether or not the main network communication timing is reached ( 705 ) referring to the main network communication time and destination AP list  406  which is referenced to by the communication control unit  403 . If the main network communication timing is reached, data transmission/reception with the AP in the main network is performed in accordance with the list  406 , and if the main network communication timing is not reached, the Req. packet is transmitted to the ND of the sub-network  11 ,  12 , or  13  ( 707 ). 
         [0071]    If the reception of data from the ND of the sub-network  11 ,  12 , or  13  is completed, the procedure is changed again to main network communication timing determination ( 705 ). 
       EXAMPLE 2 
       [0072]    In this example, a case where communication in the sub-network  11 ,  12 , or  13  in Example 1 is performed by the TDMA scheme will be described. 
         [0073]      FIG. 8  shows a transceiver configuration example of an AP and an ND when this example is carried out. Although  801  to  805  are the same as in Example 1, in this example, a communication control unit  803  references a superframe table  806 . A superframe is defined as a repetition unit of communication having a plurality of timeslots. The superframe table  806  describes the operation of each timeslot, the type of transmission or reception of each timeslot, and address information of a communication party. The communication control unit  803  references the superframe table  806  when the power is turned on, and repeats the operation defined in the superframe table  806 . 
         [0074]      FIG. 9  shows a superframe configuration of the server  100  and the APs  101 ,  102 , and  103  and a communication time chart in the main network  1 . In the superframes of the server  100  and each AP, the main network communication timeslots ( 901  to  918 ) are allocated to perform data transmission/reception. In the AP, it is assumed that other timeslots are the communication timeslots of the sub-networks  11 ,  12 , and  13 . For example, as shown in  FIG. 9 , in the superframe configuration of the AP  101 , data communication with the server  100  is allocated in the timeslot Nos. 1 and 11, data communication to the AP  102  is allocated in the timeslot No. 2, communication with the ND in the sub-network  11  is allocated in the timeslots Nos. 3 to 9, 14, and 15, data reception from the AP  102  is allocated in the timeslot No. 10, and Adv reception from the server  100  and Adv transmission to the AP  102  are allocated in the timeslots Nos. 12 and 13. The superframe configuration, the communication timing, and the transmission/reception ND of  FIG. 9  are an example for illustrating this example and may be changed. 
         [0075]      FIG. 10  shows a communication time chart in the sub-network communication timeslot of  FIG. 9  using the sub-network  11  belonging to the AP  101  as an example. 
         [0076]    In this example, since the sub-network uses the TDMA scheme, communication of the sub-network  11  does not affect communication of the main network  1 . For this reason,  FIG. 10  shows an example where the ND spontaneously transmits data to the AP  101  at a set time. 
         [0077]    The ND  111  transmits data ( 111   d ) to the AP  101  in the timeslot No. 3, receives data ( 121   d ) from the ND  121  in the timeslot No. 4, and transfers data ( 121   d ) to the AP  101  in the timeslot No. 5. Data ( 122   d ) of the ND  122  is received by the ND  111  in the timeslot No. 6, and is transferred to the AP  101  in the timeslot No. 7. 
         [0078]    The packet for time synchronization (Adv) is broadcasted from the AP  101  to the ND in the timeslots No. 14 and 15. The superframe configuration, the communication timing, and the transmission/reception ND of  FIG. 10  are an example for illustrating this example, and may be changed. 
         [0079]      FIG. 11  is an operation flowchart of an AP for realizing this example. 
         [0080]    If the power is turned on ( 1101 ), the AP is changed to reception standby ( 1102 ) and receives the packet for synchronization (Adv) ( 1103 ). If the packet for synchronization (Adv) is received, the superframe table  806  is confirmed ( 1105 ), and a communication schedule is determined. Thereafter, it is confirmed whether or not the present time is the main network communication timing described in the superframe table  806  ( 1106 ), when the main network communication timing is reached, signal transmission/reception with the AP in the main network is performed ( 1107 ), and when the main network communication timing is not reached, communication with the ND in the sub-network is performed as described in the superframe table  806 . 
       EXAMPLE 3 
       [0081]    In this example, a method which, when transmitting emergency data or a device control command from the server  100  of  FIG. 1  to the ND through the main network  1  and the sub-network in Example 1, preferentially transmits emergency data or a device control command in the sub-network and send emergency data or a device control command to the destination ND with low latency will be described. Specifically, it is assumed that priority data is transmitted from the AP  101  to the ND at an arbitrary timing during communication of  FIG. 6 . 
         [0082]      FIGS. 12 and 13  illustrate a structure of packet priority control in the CSMA scheme. 
         [0083]      FIG. 12  shows an example of a communication flow when priority control is not performed in transmitting data from the AP  101  to the ND  111 . In  FIG. 12 , the AP  111  generates emergency (or) control transmitting data ( 1201 ) in data transmission of the ND  111 , and the other transceiver  1200  in the system generates transmitting data ( 1219 ). 
         [0084]    The other transceiver ( 1200 ) may be the ND in the same sub-network  11  as the ND  111 , or may be the AP or the ND in the other sub-network. In this case, both the AP  101  and the other transceiver  1200  avoid data communication ( 1204 ) and ( 1214 ) with carrier sense ( 1202 ) and ( 1212 ), and enter a retransmission process simultaneously after data transmission of the ND  111  ends. 
         [0085]    While carrier sense is performed immediately before data transmission in the retransmission process, the carrier sense time is set to be random so as to give the same transmission probability to all transceivers. 
         [0086]    Accordingly, as shown in  FIG. 12 , there is the difference in the carrier sense time between the AP  101  and the other transceiver  1200 .  FIG. 12  shows a case where the carrier sense time ( 1206 ) of the AP  101  is set to be greater than the carrier sense time ( 1216 ) of the other transceiver. In this case, since data transmission ( 1217 ) of the other transceiver  1200  is transmitted ahead of emergency (or) control transmitting data of the AP  101  having high priority, a transmission radio wave is detected ( 1207 ) in carrier sense ( 1205 ) of the AP  101 , and transmission of priority data of the AP  101  is avoided again. 
         [0087]    Accordingly, in  FIG. 13 , in carrier sense ( 1305 ) of the AP  101 , the shortest carrier sense time ( 1306 ) which is smaller than the normal carrier sense time ( 1316 ) is set as the carrier sense time, whereby data transmission ( 1308 ) of the AP  101  can be preferentially performed earlier than the other transceiver  1300 , and packet collision with the other transceiver  1300  can be avoided. That is, in emergency or at the time of transmission of high-priority data, such as control data, the carrier sense time at the time of transmission is set to be shortest compared to the transceiver in the system, thereby performing priority control. 
       EXAMPLE 4 
       [0088]    In this example, a method which, when transmitting priority data, such as emergency data or a device control command, from the server  100  of  FIG. 1  to the AP through the main network  1  in Example 2, sends priority data to the destination AP in the main network with low latency will be described. 
         [0089]    There is a case in which communication between the AP  102  and the AP  103  is delayed due to radio wave interference from the ND in the sub-network  11  during communication between the AP  102  and the AP  103  in  FIG. 1 . 
         [0090]    Specifically, when the AP  102  transmits priority data to the AP  103  in a communication allocation period ( 903 ) (timeslot No. 4) of the main network  1  of the AP  102  of the  FIG. 9 , as in the timeslot No. 4 of  FIG. 10 , the ND  121  in the sub-network  11  transmits a radio wave simultaneously with the AP  102 , and there is a possibility that collision with priority data from the AP  102  to the AP  103  occurs. 
         [0091]      FIG. 14  shows an example where packet collision occurs. 
         [0092]    The ND  121  and the AP  102  perform carrier sense ( 1403 ) and ( 1409 ) immediately before transmission when executing the generation of transmitting data ( 1402 ) and priority transmitting data ( 1408 ) in the timeslot ( 1401 ). 
         [0093]    In the TDMA scheme, since the carrier sense time ( 1404 ) and ( 1403 ) is constant, both the ND  121  and the AP  102  determine that no radio wave is detected ( 1405 ) in carrier sense, and thus transmit data simultaneously ( 1406 ) and ( 1411 ). For this reason, packet collision ( 1414 ) occurs. 
         [0094]    Accordingly, when performing communication between the AP  102  and the AP  103 , as shown in  FIG. 15 , the shortest carrier sense time ( 1509 ) is used in carrier sense ( 1510 ) of the transmission side (AP  102 ), whereby priority data transmission ( 1512 ) of the AP  102  can start in the middle of carrier sense ( 1503 ) of the ND  121 , and data transmission of the ND  121  can be thus avoided ( 1506 ). 
         [0095]    For this reason, it is possible to reduce transmission delay of the main network due to interference of a communication radio wave of the sub-network. 
       EXAMPLE 5 
       [0096]    This example illustrates an example of the invention when time synchronization by the transmission/reception of the packet for time synchronization (Adv) is not established between the server  100  and the AP  101 ,  102 , or  103  in the main network of  FIG. 1 , and each sub-network independently performs communication by the TDMA scheme. A transceiver configuration of an AP and an ND in this example is shown in  FIG. 8 , and is the same as in Example 2. 
         [0097]      FIG. 16  shows a superframe configuration and a communication time chart of the sub-network  11  and the sub-network  12 . Transmission/reception allocation in a superframe and a communication party are as shown in  FIG. 16 . In each sub-network, the ND transmits and transfers data to the AP in an allocated timeslot, and the AP receives data from the ND in a determined timeslot of a superframe. 
         [0098]    In this example, since the sub-network  11  and the sub-network  12  independently uses the TDMA scheme, the time between the sub-network  11  and the sub-network  12  is asynchronous, and the superframe length is different. 
         [0099]    For this reason, it is not possible to configure the main network by connecting the sub-networks together. 
         [0100]    Accordingly, the sub-network  11  and the sub-network  12  are connected together by means shown in  FIG. 17  to configure the main network. The superframes of the AP  101  and the AP  102  describe the transmission/reception timeslots of the ND  111  and the ND  112 , all unoccupied timeslots are allocated to “reception (R)” and stored in the superframe table  806  ( 1701 ,  1707  to  1722 ). 
         [0101]    Besides, for example, when transmitting data addressed to the AP  102  is generated ( 1701 ) in the AP  101 , the communication control unit  803  rewrites the superframe table  806  so as to transmit (S) ( 1702 ,  1703 ) data to the AP  102  successively after the next timeslot ( 1702 ). 
         [0102]    When this happens, data transmitted to the AP  102  in the timeslot ( 1702 ) undergoes communication failure because the AP  102  is not in the reception (R) mode of data from the AP  101 . Meanwhile, in regard to transmitting data which is transmitted to the AP  102  in the timeslot ( 1703 ), since the AP  102  is in the reception (R) mode, transmission/reception is established. 
         [0103]    Accordingly, communication through the main network can also be performed between the sub-networks which independently configure a wireless network by the TDMA scheme. 
         [0104]    However, similarly to the timeslot ( 1702 ), when a timeslot allocated to data reception from the ND  111  is written to transmission (S), the AP  101  performs carrier sense for a radio wave of data ( 111   d ), and there is a possibility that transmission to the AP  101  in the same timeslot is cancelled. For this reason, the shortest carrier sense time ( 1509 ) in  FIG. 15  described in Example 4 is set at the time of transmission of the AP ( 101 ), whereby transmitting data addressed to the AP  102  can be reliably transmitted while placing priority over radio wave transmission from the ND  111 . 
         [0105]      FIG. 18  is an operation flowchart of an AP in this example. 
         [0106]    If the power is turned on ( 1401 ), the AP confirms the superframe table ( 1402 ). Next, the carrier sense time is set to be shortest ( 1403 ), and the procedure is changed to a communication mode. In the communication mode, the presence/absence of a communication request for the main network (AP) is confirmed ( 1404 ). When there is a communication request, the superframe table  806  is rewritten by the communication control unit  803 , and successive transmission to the main network (AP) is performed ( 1406 ) until Ack is receivable. When there is no communication request for the main network (AP), transmission/reception with the ND of the sub-network is performed ( 1407 ) in accordance with the superframe table  806 . 
       EXAMPLE 6 
       [0107]    This example illustrates an example of the invention when time synchronization by transmission/reception of the packet for time synchronization (Adv) is not established between the server  100  and the AP  101 , the AP  102 , or the AP  103  in the main network of  FIG. 1 , and each sub-network independently performs communications by the TDMA scheme in association with Example 5. 
         [0108]      FIG. 19  shows a communication time chart in this example. 
         [0109]    The superframe configuration in  FIG. 19  is the same as  FIG. 16  in Example 5. 
         [0110]    In this example, main network communication timeslots ( 1902  to  1908 ) are arranged cyclically in the superframes of the AP  101  and the AP  102 . In the example of  FIG. 19 , the AP  101  arranges the main network communication timeslots in the timeslot ( 1902 ) and the timeslot ( 1903 ), and the AP  102  arranges the main network communication timeslots in the timeslots ( 1904 ), ( 1905 ), ( 1906 ), and ( 1907 ). 
         [0111]    If transmitting data addressed to the AP  102  is generated ( 1901 ), while the AP  101  transmits data to the AP  102  in the timeslot ( 1902 ), since the AP  102  is not allocated to the main network communication timeslot in this timeslot, communication failure ( 1909 ) occurs. 
         [0112]    However, if the AP  101  retransmits the same data in the timeslot ( 1903 ) ( 1910 ), the AP  102  is also allocated to the main network communication timeslot ( 1907 ) in this timeslot, whereby communication can be successful. 
       EXAMPLE 7 
       [0113]    This example illustrates an example of the invention when time synchronization by transmission/reception of the packet for time synchronization (Adv) is not established between the server  100  and the AP  101 ,  102 , or  103  in the main network of  FIG. 1 , and each sub-network independently performs communication by the CSMA scheme. 
         [0114]      FIG. 20  shows a communication time chart in this example. Each ND spontaneously performs carrier sense by the CSMA scheme over a random period, and when no radio wave is detected, transmits or transfers data to the AP  101  or the AP  102 . 
         [0115]    In the example of  FIG. 20 , during the carrier sense period ( 2001 ) and ( 2002 ) of the ND  122  and the carrier sense period ( 2003 ) of the ND  124 , carrier sense is performed, and as a result, since the transmission radio waves of data ( 121   d ), data ( 122   d ), and data ( 123   d ) are detected, immediate data transmission is put off. 
         [0116]    While the AP  101  transmit data addressed to the AP  102  immediately after carrier sense is performed in carrier sense ( 2004 ), if data ( 111   d ) of the ND ( 111 ) is detected in carrier sense ( 2004 ), data transmission addressed to the AP  102  is put off until the transmission of data ( 111   d ) of the ND  111  is completed, and retransmission is attempted. 
         [0117]    In the retransmission ( 2005 ), since no transmission radio wave is detected by carrier sense ( 2006 ), transmission of data ( 101   d ) addressed to the AP  102  is performed, and Ack ( 102 ) is received from the AP  102 . 
       EXAMPLE 8 
       [0118]    In this example, data communication of the main network or means for transmitting high-priority data, such as emergency information or control information, while giving priority over normal power monitoring information main network will be described in association with Example 6. 
         [0119]      FIG. 21  is a communication time chart when, similarly to  FIG. 20 , both the main network and the sub-network perform communication by the CSMA scheme. 
         [0120]    In the example of the  FIG. 20 , since the carrier sense time immediately before the AP  101  transmits data ( 101   d ) to the AP  102  is greater than the carrier sense time of the ND  111 , data addressed to the AP  102  is retransmitted. However, transmission delay occurs in high-priority data. 
         [0121]    Accordingly, in this example, as shown in  FIG. 13  described in Example 3, the shortest carrier sense time ( 2101 ) in which, at the time of priority data transmission, the previous carrier sense time is smaller than the time which can be set to be random is introduced. With this, it becomes possible to suppress data transmission of the ND  111  having low priority as shown in  FIG. 21  and to transmit priority data ( 101   d ) with minimum latency. 
         [0122]      FIG. 22  is an operation flowchart of the AP in this example. If the power is turned on ( 2201 ), the AP confirms the presence/absence of a high-priority communication request ( 2202 ). When a high-priority data transmitting request is generated, the carrier sense time during transmission is set to be shortest ( 2203 ). It stands by the end of transmission of the other wireless device in communication ( 2204 ), and after the transmission ends, carrier sense is performed ( 2205 ). The condition that no radio wave is detected by carrier sense is confirmed, and high-priority data is transmitted ( 2206 ). 
         [0123]    When there is no high-priority communication request in ( 2202 ), the presence/absence of a normal priority communication request is confirmed ( 2207 ). If there is no normal priority communication request, reception standby is performed. 
         [0124]    If there is a normal priority communication request, first, the carrier sense time is set to be random ( 2208 ), it stands by the end of transmission of the other wireless device ( 2209 ), carrier sense is performed, and if no radio wave is detected, data is transmitted ( 2211 ). 
       Supplementary Note 
       [0125]    (1) An example where the main network  1  and the sub-networks  12 ,  13 , and  14  in all of Examples perform communication by different communication frequency channels to avoid mutual interference and to realize wireless communication with low latency is included in Examples. 
         [0126]    (2) An example where a synchronized scheme/asynchronized scheme is switched depending on a system utilization location and traffic of wireless communication such that a synchronized scheme is used at a location, such as a city area or a residential area, where the density of wireless communication devices is high, and an asynchronized scheme is used at a location, such as a mountain area, where the density of wireless communication devices is low is included in Examples.