Patent Publication Number: US-6987967-B2

Title: Communication apparatus and communication method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of application Ser. No. 09/940,579, filed Aug. 29, 2001 now U.S. Pat. No. 6,876,850, the entire contents being incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to network systems, communication apparatus, and communication control methods wherein various kinds of information is transmitted, for example, through radio signals, among a plurality of communication terminals. 
   2. Description of the Related Art 
   Conventionally, a network system, for example, in which wireless transmitters are incorporated in personal computers or audio-video devices so as to transmit information among these plural personal computers or audio-video devices is known. In such network system, employed generally is a method in which various types of transmission managements are performed under the control of one central control station, and a plurality of terminal stations are controlled. In this case, a communicating station operating as a control station is set so as to subordinate other communicating stations to the control station to form a network while employing a plurality of wireless communication devices. 
   In the case where a network is formed employing a plurality of wireless communication devices, a wireless communication control method and the like has been considered wherein a predetermined wireless communication device is specified as the control station of the network, and, for example, a periodic transmission frame cycle is set, or access control of an information transmission region is performed, based on the control of the control station. 
   In recent years, a wireless communication device has been developed wherein so called an adhoc wireless network can be constructed in which a specific wireless communication device is not determined as the control station from the beginning, and an arbitrary wireless communication device is set as the control station of the wireless network as the need arises so that information can be communicated without any access points. For example, in Japanese Patent Application Laid-Open No. 2000-082989, disclosed is a wireless communication method where so called an adhoc wireless network is constructed in which information can be communicated without any access points while a control station mode and a terminal station mode are switched alternately. 
   However, in this conventional network system, it is supposed that some inconvenience occurs in a central control station, and the function of the central control station is damaged. In this case, in the conventional network system, a necessity occurs wherein the operating condition of the network is once reset, and another terminal station becomes the central control station so that a wireless network is newly constructed once again. Thus, in the conventional network system, in order to perform this reset (reconstruction of the new wireless network), data transmitted on the network operating so far is temporarily suspended, taking an enormous time period until the network is operated once again. 
   In order to solve the problem, in Japanese Patent Application Laid-Open No. 2000-151618, the present applicant has proposed a system in which another terminal station in the network is operated as a central control station automatically when some inconvenience occurs in the central control station (master control station) in the network. In this proposed system, the central control station needs to specify in advance a terminal station which would be the central control station when an inconvenience occurs in the master control station as a slave control station. However, for example, in a case where a common power supply is employed for the master control station and the slave control station, when only one slave control station is prepared in the network, and when an inconvenience occurs in both stations at the same time, such problem cannot be dealt with. 
   SUMMARY OF THE INVENTION 
   The present invention was developed considering such circumstances, and it is an object of the present invention to provide communication devices and communication methods by which stable information transmissions can be performed without stopping the operating condition even when some inconvenience occurs in a central control station. 
   A communication apparatus, according to the present invention, which employs a plurality of communication devices to construct a wireless network to be a control station transmitting management information in the wireless network, is characterized by comprising a communication means transmitting information among the plurality of communication devices, a determining means determining as to whether, when the control station disappears from the wireless network, in order that any one of other communication devices is newly operated as a control station, the other communication devices have a control function as a control station candidate or not, a set means setting priority order for each of control station candidates when there exist a plurality of communication devices to be the control station candidates, and a notification means notifying the wireless network of priority order information of the control station candidates. 
   That is, for the time when the master control station becomes incapable of communication, priority order is set in advance for a plurality of slave control station candidates in the wireless network. 
   A communication apparatus, according to the present invention, which is in a wireless network, is characterized by comprising a communication means transmitting information among the other communication devices, a means receiving priority order information representing stand-by times different in each communication device from the control station and setting communication devices as control station candidates in a memory, and a means performing communication control as a control station in the wireless network when the stand-by time elapses. 
   That is, when the master control station becomes incapable of communication, it is set that a plurality of slave control stations perform communication control as control stations sequentially based on preset priority order. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an explanatory view illustrating an example of a network system structure according to one embodiment of the present invention; 
       FIG. 2  is a block diagram illustrating an example of a communication terminal structure according to one embodiment of the present invention; 
       FIG. 3  is an explanatory view illustrating an example of a frame structure according to one embodiment of the present invention; 
       FIG. 4  is an explanatory view illustrating a communicable region of one communication terminal (terminal  101 ) in a network system according to one embodiment of the present invention; 
       FIG. 5  is an explanatory view illustrating a communicable region of one communication terminal (terminal  102 ) in a network system according to one embodiment of the present invention; 
       FIG. 6  is an explanatory view illustrating a communicable region of one communication terminal (terminal  103 ) in a network system according to one embodiment of the present invention; 
       FIG. 7  is an explanatory view illustrating a communicable region of one communication terminal (terminal  104 ) in a network system according to one embodiment of the present invention; 
       FIG. 8  is an explanatory view illustrating a communicable region of one communication terminal (terminal  105 ) in a network system according to one embodiment of the present invention; 
       FIG. 9  is an explanatory view illustrating a communicable region of one communication terminal (terminal  106 ) in a network system according to one embodiment of the present invention; 
       FIG. 10  is an explanatory view illustrating a communicable region of one communication terminal (terminal  107 ) in a network system according to one embodiment of the present invention; 
       FIG. 11  is an explanatory view illustrating a communicable region of one communication terminal (terminal  107 ) in a network system according to one embodiment of the present invention; 
       FIG. 12  is an explanatory view illustrating a communicable region of one communication terminal (terminal  105 ) in a network system according to one embodiment of the present invention; 
       FIG. 13  is an explanatory view illustrating a communicable region of one communication terminal (terminal  103 ) in a network system according to one embodiment of the present invention; 
       FIG. 14  is a flow chart explaining operations of a communication terminal as a master control station according to one embodiment of the present invention; 
       FIG. 15  is a flow chart explaining operations of the time when a communication terminal is set as a slave control station according to one embodiment of the present invention; 
       FIG. 16  is a flow chart explaining operations of a communication terminal as a slave control station according to one embodiment of the present invention; 
       FIG. 17  is a chart illustrating network stand-by time for each communicating station according to one embodiment of the present invention, and  FIG. 17A  corresponds to a communication device  211 ,  FIG. 17B  a communication device  212 ,  FIG. 17C  a communication device  213 ,  FIG. 17D  a communication device  214 ,  FIG. 17E  a communication device  215 ,  FIG. 17F  a communication device  216 , and  FIG. 17G  a communication device  217 ; 
       FIG. 18  is a chart illustrating transition states of changes in network stand-by time for each communicating station according to one embodiment of the present invention, and  FIG. 18A  corresponds to the communication device  211 ,  FIG. 18B  the communication device  212 ,  FIG. 18C  the communication device  213 ,  FIG. 18D  the communication device  214 ,  FIG. 18E  the communication device  215 ,  FIG. 18F  the communication device  216 , and  FIG. 18G  the communication device  217 ; 
       FIG. 19  is a chart illustrating confirmation states of changes in network stand-by time for each communicating station according to one embodiment of the present invention, and  FIG. 19A  corresponds to the communication device  211 ,  FIG. 19B  the communication device  212 ,  FIG. 19C  the communication device  213 ,  FIG. 19D  the communication device  214 ,  FIG. 19E  the communication device  215 ,  FIG. 19F  the communication device  216 , and  FIG. 19G  the communication device  217 ; 
       FIG. 20  is a chart illustrating a transmission sequence example of control station candidate information according to one embodiment of the present invention; 
       FIG. 21  is a view illustrating an example of the structure of a control station candidate specifying packet according to one embodiment of the present invention; and 
       FIG. 22  is a view illustrating an example of the structure of a priority order confirming packet according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Network systems to which the present invention is applied are explained as embodiments of the present invention below referring to drawings. 
   1) Embodiment 1 
   A network system  1  is, for example, composed of eight communication terminals  100 ,  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107  as shown in  FIG. 1 . The respective communication terminals  100  to  107  perform wireless communications of signals obtained by modulating, for example, carrier waves of 5 GHz, and mutually transmit information. Here, the communication terminal  100  becomes a central control station (master control station) and directly controls communications of all peripheral stations  101  to  107  in a radio wave attainable region  10  of the communication terminal  100 . When the communication terminal  100  cannot perform operations as the central control station, the communication terminals  103 ,  105 ,  107  are here prepared as communication terminals (slave control stations) which can be the central control stations. Here, a radio wave attainable region  13  of the communication terminal  103 , a radio wave attainable region  15  of the communication terminal  105 , and a radio wave attainable region  17  of the communication terminal  107  do not cover all communication terminals in the network system  1 . However, transmitting with a terminal outside of a radio wave attainable region is possible through a relay transmission as described later on. 
     FIG. 2  is a diagram illustrating the respective communication terminals  100  to  107  and a configuration connected to the communication terminals. Here, two wireless communication devices  20 A,  20 B are shown. The respective wireless communication devices  20 A,  20 B are provided with transmission control management sections  22 A,  22 B to which management information storing sections  21 A,  21 B are connected, and wireless transmission processing is performed under the control of the transmission control management sections  22 A,  22 B. The data transmission processing between devices  27 A,  27 B connected to the respective wireless communication devices  20 A,  20 B is performed in interface sections  23 A,  23 B. High frequency transmission processing sections  25 A,  25 B are connected to the interface sections  23 A,  23 B via coding/decoding sections  24 A,  24 B, and sending and receiving of radio signals are performed through antennas  26 A,  26 B connected to the high frequency transmission processing sections  25 A,  25 B. Various information devices such as a personal computer unit, an audio-video device, or the like, are employed as the devices  27 A,  27 B connected to the wireless communication devices  20 A,  20 B. 
   The management information storing sections  21 A,  21 B store network management information, operating program, and the like, such as information necessary as a master control station and a slave control station (the details are set forth later on). These management information storing sections  21 A,  21 B also store and hold data related to the priority order that a slave control station described later on needs. 
   The transmission control management sections  22 A,  22 B govern and control the interface sections  23 A,  23 B, the coding/decoding sections  24 A,  24 B, and the high frequency transmission processing sections  25 A,  25 B based on the information stored in the management information storing sections  21 A,  21 B. The interface sections  23 A,  23 B send/receive data to/from the devices  27 A,  27 B connected to the wireless communication devices such as a personal computer unit and an audio-video device. The connections between the interface sections  23 A,  23 B and the devices  27 A,  27 B are bridges in conformity to a bus line standardized, for example, as IEEE (The Institute of Electrical and Electronics Engineers) 1394 format. 
   The coding/decoding sections  24 A,  24 B perform coding of data transmitted via the network and decoding of data received via the network. The high frequency transmission processing sections  25 A,  25 B perform modulation/demodulation processing or the like of data transmitted via the antennas  26 A,  26 B. 
   The wireless communication devices  20 A,  20 B are not limited to ones with the structures as shown in  FIG. 2 , and any other structures can be employed as far as they can realize the present invention. Although only two wireless transmission devices are shown in  FIG. 2 , for example, in the case of the network structure shown in  FIG. 1 , the network system  1  is formed preparing eight wireless transmission devices. However, it is not necessary that all the wireless transmission devices in the network system  1  are connected to devices (the devices  27 A,  27 B in  FIG. 2 ) such as a personal computer unit or an audio-video device. The transmission control management sections  22 A,  22 B do not have to make all the wireless transmission devices in the network system  1  perform control operations necessary as a master control station or a slave control station, and for example, a means or a program for performing control operations are not necessary to be prepared for some number of wireless transmission devices in the network system  1 . In the explanation below, all wireless transmission devices in the network system  1  can be a master control station or a slave control station. 
   An example of a frame structure of data transmitted among the respective communication terminals  100  to  107  is shown in  FIG. 3 . 
   The transmission among the respective communication terminals  100  to  107  is performed in setting a frame period here. One frame is, for example, set to 4 ms, and a media information transmission region and a control information transmission region are provided in one frame so that this frame structure is repeated. Real data exchanged among the respective communication terminals  100  to  107  is transmitted in the media information transmission region. The control information transmission region is provided with a section in which descending control information that a master station transmits control information to the respective communication terminals is transmitted and a section in which all communication terminals on the network transmit ascending control information. Another frame structure may be set wherein a fixed time slot divided for each station is prepared between the ascending control information transmitting sections, and the ascending control information is sent/received to/from all stations on the network so that mutual connection condition is confirmed through information of connection link relationship among respective stations. 
   In the network system  1  of such structure, a master control station functioning as a central control station performing management of the information transmission among the respective communication terminals is set. In the network system  1  shown in  FIG. 1 , for example, the communication terminal  100  is set as the master control station (hereafter, this communication terminal  100  may also be called the master control station  100 ). This master control station  100  transmits management information in the section in which the descending control information described above is transmitted and controls transmission of data by the respective communication terminals. The peripheral communication terminals  101  to  107  exist in the radio wave attainable region  10  of the master control station  100 . That is, the master control station  100  can directly communicate with the peripheral communication terminals  101  to  107 . 
   In the network system  1  having such structure, when an inconvenience occurs in the master control station  100  and the master control station  100  does not carry out the function as the central control station, a plurality of slave control stations carrying out the function as the central control station are set as a substitute for the master control station  100 . In the network system  100  shown in  FIG. 1 , for example, three communication terminals  103 ,  105 ,  107  are set as the slave control stations (hereafter, these communication terminals  103 ,  105 ,  107  may also be called the slave control stations). In this case, priority order is set for the case where the communication terminals  103 ,  105 ,  107  carry out the function as the central control stations among the three communication terminals  103 ,  105 ,  107 . This priority order is determined by the master control station  100  instructing one terminal to be a slave control station, and the order of the stations are stored in the respective slave control stations  103 ,  105 ,  107  themselves. 
   These slave control station  103 ,  105 ,  107  transmit the management information during the section in which the descending control information described above is transmitted and control transmission of data by the respective communication terminals when the function as the central control station is not carried out since an inconvenience occurs in the master control station  100 . However, processing to transmit the management information in accordance with the priority order described above is supposed to be performed, and when a state where transmission control is correctly performed by any one of the slave control stations occurs, other slave control stations do not carry out the transmission control and wait. 
   Here, in the network system  1 , the master control station  100  specifies three communication terminals as slave control stations in the order of most excellent communication terminals in an information transmission condition with other peripheral stations among the peripheral communication terminals  101  to  107  with which the master control station  100  can directly communicate. The communication terminal excellent in the information transmission condition herein is determined, for example, from the number of communication terminals capable of direct communication in the network and communication quality with the respective communication terminals. Specifically, for example, in the order from a communication terminal with which the number of communication terminals capable of direct communication in the network is the largest, one is set as a slave control station with a high priority order, and when the number of communication terminals capable of direct communication in the network is the same, a communication terminal for which the communication quality with other communication terminals at the time is judged as being excellent is selected as a slave control station with a high priority order. The communication quality herein is determined from, for example, the electric power of a received signal, error rate, or the like. 
   For a determination as to whether there is a connection with a peripheral station, which is necessary in making the above decisions, for example, techniques described in Japanese Patent Application No. Hei 10-47416 (Counterpart application in U.S. Ser. No. 09/252,807 and Japanese Patent Application No. Hei 10-258855 (Counterpart application in U.S. Ser. No. 09/392,739 that the present applicant has proposed may be employed wherein respective stations on a network mutually send/receive ascending control information in a fixed time slot to grasp a station existing around a station itself. 
   Here, decision processing for selecting a slave control station in the network system  1  of the present example is explained. When a case where the communication terminal  101  is set as the central control station is supposed, the communication terminals  100 ,  102 ,  106 ,  107  exist in a radio wave attainable region  11 , and the communication terminal  101  can communicate with these four stations as shown in  FIG. 4 . 
   The communication terminals  100 ,  101 ,  103 ,  107  exist in a radio wave attainable region  12 , and the communication terminal  102  can communicate with these four stations as shown in  FIG. 5 . 
   The communication terminals  100 ,  102 ,  104 ,  105 ,  107  exist in a radio wave attainable region  13 , and the communication terminal  103  can communicate with these five stations as shown in  FIG. 6 . 
   The communication terminals  100 ,  103 ,  105  exist in a radio wave attainable region  14 , and the communication terminal  104  can communicate with these three stations as shown in  FIG. 7 . 
   The communication terminals  100 ,  103 ,  104 ,  106 ,  107  exist in a radio wave attainable region  15 , and the communication terminal  105  can communicate with these five stations as shown in  FIG. 8 . 
   The communication terminals  100 ,  101 ,  105 ,  107  exist in a radio wave attainable region  16 , and the communication terminal  106  can communicate with these four stations as shown in  FIG. 9 . 
   The communication terminals  100 ,  101 ,  102 ,  103 ,  105 ,  106  exist in a radio wave attainable region, and the communication terminal  107  can communicate with these six stations as shown in  FIG. 10 . 
   As a result, in the network system  1 , other than the central control station  100 , since the communication terminal  107  can connect with the largest number of communication terminals (can connect with six stations), the communication terminal  107  is specified as a slave control station whose priority order is the highest. 
   Since the number of communication terminals capable of connection regarding the communication terminal  103  and the communication terminal  105  is large (communication with five stations is possible) next to that of the communication terminal  107 , these two communication terminals  103 ,  105  are also specified as the slave control stations. However, since the number of stations capable of connection is the same in the two communication terminals  103 ,  105 , the priority order of a terminal in which it is determined that the communication quality between each communication terminal  103 ,  105  and other communication terminals is excellent is set to a higher order. Here, while it is supposed that it is determined that the communication quality with peripheral stations regarding the communication terminal  105  is more excellent than that regarding the communication terminal  103 , the communication terminal  105  is specified as second slave control station in the priority order, and the communication terminal  103  is specified as third slave control station in the priority order. 
   The decision regarding the communication quality for determining the order between the slave control station  103  and the slave control station  105  may be made from other factors. For example, either station in the two communication terminals  103 ,  105  which can directly communicate excellently with the master control station  100  (that is, a station that is supposed to be closer to the master control station) may be set to a higher order in the priority order. Also, in the case where the number of stations capable of direct connection is the same, a communication control station with a newer address may be simply specified as a slave control with a higher priority order without determining the communication quality as described above. 
   When the master control station  100  selects three slave control stations  103 ,  105 ,  107  as described above, the master control station  100  informs the entire network that the communication terminals  103 ,  105 ,  107  are specified as slave control stations. This notification may be made through a broadcast transmission by descending control information. In order to confirm the condition, confirmation information may be transmitted by ascending control information. At this notification time, information regarding the priority order of the three slave control stations  103 ,  105 ,  107  may also be notified. 
   In the notified three slave control stations  103 ,  105 ,  107 , based on the priority order of a station itself specified at the time, a stand-by time of the time the master control station comes to a state where communication is not possible is registered. Here, the stand-by time for the communication terminal  107  that is first in the priority order is set to 1 second, the stand-by time for the communication terminal  105  that is second in the priority order is set to 2 seconds, and the stand-by time for the communication terminal  103  that is third in the priority order is set to 3 seconds. In the case where there are four or more slave control stations, each one second of stand-by time may be added further each time the order is lowered. 
     FIG. 11  illustrates a communication state in the network of the time the communication terminal  107  that is a slave control station becomes the central control station when the master control station  100  becomes incapable of communication. At this time, since the descending control information transmitted from the slave control station  107  does not directly reach the communication terminal  104 , another communication terminal  103  or  105  needs to relay and transmit it. The transmission of ascending control information and/or real data is also needed to be relayed similarly. 
     FIG. 12  illustrates a communication state in the network of the time the communication terminal  105  that is a slave control station becomes the central control station when the master control station  100  becomes incapable of communication. At this time, since the descending control information transmitted from the slave control station  105  does not directly reach the communication terminals  101 ,  102 , other communication terminals  103 ,  107  need to relay and transmit it. The transmission of ascending control information and/or real data is also needed to be relayed similarly. 
     FIG. 13  illustrates a communication state in the network of the time the communication terminal  103  that is a slave control station becomes the central control station when the master control station  100  becomes incapable of communication. At this time, since the descending control information transmitted from the slave control station  103  does not directly reach the communication terminals  101 ,  106 , other communication terminals  102  and  105  or  107  need to relay and transmit it. The transmission of ascending control information and/or real data is also needed to be relayed similarly. 
   With respect to processing incorporating a communication terminal existing outside of a radio wave attainable region of a control station into a network by relay and transmission, for example, it is possible to employing a technique shown in Japanese Patent Application No. Hei 10-258855 submitted by the present applicant and incorporate such terminal into the network as a hidden terminal station. 
   Next, operations of the master control station  100  are explained employing a flow chart shown in  FIG. 14 . 
   The master control station  100  first forms network common information from a collection result of connection information of the network and information of a specified slave control station at step S 11 . Then, at step S 12 , a broadcast transmission is performed on the network in the descending control information communication section. 
   Thereafter, the master control station  100  receives the ascending information sent from the peripheral communication terminals  101  to  107  at step S 13 . Connection condition of the network is then grasped at step S 14 . 
   The master control station  100  specifies and registers as slave control stations three communication terminals in the order from a station in which the number of connection links is the largest among the peripheral communication terminals  101  to  107  at step S 15 . 
   Next, operations in the communication terminals  101  to  107  other than the master control station  100  are explained employing a flow chart shown in  FIG. 15 . 
   The communication terminals  101  to  107  receive the descending information sent from the central control station (the master control station  100 ) at step S 21 . Confirmation operation of network information of the descending control information is then performed at step S 22 . 
   Here, as a result of analysis of the information, when specifying as slave control stations is performed by the master control station at step S 23 , operation to register as slave control stations is performed proceeding to step S 24 . At this time, at step S 25 , from a specified priority order, the stand-by time until the time of performing control operation as a central control station is registered. This registration is performed by storing it, for example, in the management information storing sections  21 A,  21 B of the wireless transmission devices  20 A,  20 B shown in  FIG. 2  under the control of the transmission control management sections  22 A,  22 B. When the registration as a slave control station is performed, the confirmation information may be sent to the master control station (central control station), employing the ascending control information and the like. 
   When it is determined that the number of communication terminals with which that slave control station can directly communicate is larger than that of the master control station from network connection condition determined at the time of specifying a slave control station, a change request of a control station may be sent to the master control station so that the slave control station becomes the central control station. 
   Next, operations in the communication terminals (slave control stations  103 ,  105 ,  107 ) registered as slave control stations are explained employing a flow chart shown in  FIG. 16 . 
   The communication terminals as the slave control stations first try to receive the descending control information sent from the central control station (the master control station  100 ) at step S 31 . 
   When the descending control signal can be received at step S 31 , network information is confirmed with peripheral communication terminals at step S 32 . At step S 33 , it is determined whether or not specifying as a slave control station is cancelled from the master control station, and when the specifying as a slave control station is cancelled, the specifying as a slave control stations is cancelled so that a station performs an operation as a usual peripheral terminal station at step S 34 . When the specifying is not cancelled, an operation as a slave control station is performed as it is. 
   When the descending control information cannot be received at step S 31 , the ascending control information is received at step S 35 , and connection condition of the network is grasped at step S 36 . Then, at step S 37 , it is determined whether or not an inconvenience occurs in the master control station. With respect to the determination as to whether or not an inconvenience occurs in the master control station herein, for example, when the descending management information periodically transmitted from the master control station cannot be received, it is determined that an inconvenience is occurring in the master control station. However, since it can also be supposed that the descending management information cannot be received from about one time to several times for some reason even when the master control station operates correctly, it is preferred that it is determined that an inconvenience is occurring in the master control station when the descending management information cannot be received for some time continuously. Further, when the master control station cannot be recognized in all other peripheral terminal stations from the connection condition of the network grasped at step S 36  (that is, the signal from the master control station cannot be received), it may be determined that an inconvenience is occurring in the master control station. 
   When it is determined that an inconvenience is occurring in the master control station, at step S 38 , it is determined whether or not another station which can operate as the control station is generated so that the descending management information can be received. Here, when the descending management information comes to be received, the station does not perform the operation as the central control station (that is, does not perform management of information transmission by sending of the descending management information) to perform the operation as a peripheral terminal station at step S 39 . 
   When the descending management information cannot be received at step S 38 , at step S 40 , it is determined whether or not the stand-by time registered in the station has elapsed since an inconvenience occurs in the master control station. For example, in the slave control station  107  that is first in the priority order, it is determined whether 1 second has elapsed since an inconvenience occurs in the master control station. In the slave control station  105  that is second in the priority order, it is determined whether 2 seconds has elapsed since the inconvenience occurs in the master control station. In the slave control station  103  that is third in the priority order, it is determined whether 3 seconds has elapsed since the inconvenience occurs in the master control station. When it is determined that the stand-by time registered in the station has not elapsed in the determining at step S 40 , the step returns to the determining of step S 38 . 
   When it is determined that the stand-by time registered in the station has not elapsed in the determining of step S 40 , an operation necessary as the central control station is performed. Specifically, periodic transmission of the descending management information is started to carry out the management of information transmission in the network at the station. 
   In the network system  1  of the embodiment of the present invention as described above, when some inconvenience occurs in the master control station  100  functioning as the central control station, a slave control station functions as a central control station, and stable information transmission can be performed without stopping the operating condition of the network. 
   In this case, since it is set that a slave control station which becomes a central control station at the time an inconvenience occurs in the master control station specifies a plurality of communication terminals in the network, in a state where at least one among the specified slave control stations normally operates, the operating condition at the time can be maintained without resetting the network, and the possibility that the operating condition of the network can be maintained can be enhanced compared with a case where only one is specified as a slave control station. In this case, since priority order is set in the plurality of slave control stations and the plurality of slave control stations are operated as central control stations after standing by only each different stand-by time set in accordance with the priority order, the plurality of slave control stations do not start to operate as the central control stations at the same time, whereby a situation where a plurality of central control stations exist in one network can be avoided reliably. 
   In the embodiment described above, although three communication terminals in the network are specified as slave control stations, two or four or more communication terminals may be given priority order to be specified as slave control stations. In this case, when all communication terminals in the network has a network structure in which all communication terminals can be control stations, all the communication terminals may be given priority order to be specified as slave control stations. 
   In the embodiment described above, when a terminal to be specified as a slave control station is selected, although a determination is made from the number of terminals with which the terminal can communicate or the communication quality, the determination may be made from another factor. For example, even in a structure where a corresponding communication terminal can operate as a slave control station, when it is not preferable for the corresponding communication terminal to be a control station from the structure and/or the operating condition of the corresponding communication terminal, the communication terminal may be ruled out from candidates of the terminals specified as slave control stations. 
   Specifically, for example, a corresponding communication terminal may be specified as a slave control station only when being operated by supply of commercial AC power, and the terminal may not be specified as a slave control station when an incorporated battery is employed as a power supply so as to restrain the consumption of a battery. Or, one communication terminal in which a battery is set as the power supply among a plurality of specified slave control stations may be specified as one slave control station (however, it is preferred to specify the communication terminal whose priority order is low as a slave control station), so that the operating condition of the network may be maintained even when a service interruption occurs wherein the supply of AC power to some number of communication terminals in the network system is temporarily stopped. 
   In the embodiment described above, although the stand-by time set based on the priority order is of one second interval time such as 1 second, 2 seconds, 3 seconds, the present invention is not limited to such stand-by time. In a structure in which an inconvenience in a control station can be detected faster, a shorter time may be set as the stand-by time. 
   Further, slave control stations may be operated in accordance with the order in a process in which differences are provided in the stand-by time other than the process in which slave control stations are operated based on the set priority order. 
   2) Embodiment 2 
   Next, a second embodiment of the present invention is described. 
     FIG. 17  is a chart illustrating stand-by time for each communicating station. 
   In the drawing, a communication device  214  to be an existing control station shown in  FIG. 17D  selects communication devices  211 ,  215 ,  216  to be control station candidates each shown in  FIG. 17A ,  FIG. 17E , and  FIG. 17F , and  FIG. 17  illustrates a state where priority order P 1 , P 2 , P 3  is set among them. 
   Here, for the sake of convenience, it is specified that the communication device  215  is the control station candidate of priority order P 1  representing first priority order, the communication device  211  is the control station candidate of priority order P 2  representing second priority order, and the communication device  216  is the control station candidate of priority order P 3  representing third priority order, and reconstruction start times T 2 , T 4 , T 6  each of which is from the time of reconstructing the wireless network to the time of starting operations as control stations are set. 
   The communication device  215  of the control station candidate of priority order P 1  sets 2 units as the reconstruction start time T 2  with respect to a predetermined base unit T 1 , the communication device  211  of the control station candidate of priority order P 2  sets 4 units as the reconstruction start time T 4  with respect to the predetermined base unit T 1 , and the communication device  216  of the control station candidate of priority order P 3  sets 6 units as the reconstruction start time T 6  with respect to the predetermined base unit T 1 . 
   By this, in a case where the communication device  214  which is an existing control station disappears from the wireless network, the communication device  215  of priority order P 1  of the control station candidate obeys the priority order and performs the operation as a control station prior to other communication devices. 
   Other communication devices  212 ,  213 ,  217  which do not have specified priority order are in a state where specifying of priority order is not performed, and since the communication devices  212 ,  217  are provided with control functions necessary to operate as control stations, the communication devices  212 ,  217  are allowed to start operating as control stations simultaneously after a predetermined simultaneous cancel time TR has elapsed. 
   Since the communication devices  213  is not provided with the control function necessary to operate as a control station, it is set that the communication devices  213  is subordinate to a communication device to be a control station candidate to form the wireless network. 
   At the predetermined simultaneous cancel time TR, for example, maximumly 15 communication devices other than the control station can be provided, and when considering that T 2  is set as a reconstruction start time in each communication device, a reconstruction start time can be set to 30T (=15×2T). 
     FIG. 18  is a chart illustrating transition states of changes in stand-by time of each communicating station.  FIG. 18  illustrates transition states of a case where the priority order for each communication device set in  FIG. 17  described above is respectively changed. 
   The drawing shows a case where priority order P 2  representing second priority order of the communication device  211  is changed to priority order P 1  representing first priority order, priority order P 1  representing first priority order of the communication device  215  is changed to priority order P 2  representing second priority order, and no priority order of the communication device  217  is newly set to priority order P 4  representing fourth priority order. There is no change in priority order P 3  representing third priority order of the communication device  216 . 
   Here, although specifying reconstruction start times is performed in even number units T 2 , T 4 , T 6  with respect to the base unit T 1  according to priority order in  FIG. 17  described above, to avoid a state where the existing setting of reconstruction start times T 2 , T 4 , T 6  are duplicated, in  FIG. 19 , specifying reconstruction start times is performed in odd number units T 1 , T 3 , T 5 , T 7  with respect to the base unit T 1  so as not to overlap with those of  FIG. 17 . 
   That is, the communication device  211  of the control station candidate of priority order P 1  sets  1  unit of the base unit T 1  as the reconstruction start time T 1 , the communication device  215  of the control station candidate of priority order P 2  sets 3 units of the base unit T 1  as the reconstruction start time T 3 , the communication device  216  of the control station candidate of priority order P 3  sets 5 units for the predetermined base unit T 1  as the reconstruction start time T 5 , and the communication device  217  of the control station candidate of priority order P 4  sets 7 units of the base unit T 1  as the reconstruction start time T 7 . 
   Another communication device  212  which does not have specified priority order is in a state where specifying of priority order is not performed, and since being provided with the control function necessary to operate as a control station, the communication device  212  is allowed to start operating as a control station simultaneously after a predetermined simultaneous cancel time TR has elapsed. 
   Since the communication devices  213  is not provided with the control function necessary to operate as a control station, it is set that the communication devices  213  is subordinate to a communication device to be a control station candidate to form the wireless network. 
     FIG. 19  is a chart illustrating confirmation states of changes in stand-by time for each communicating station.  FIG. 19  illustrates confirmation states of a case where the priority order for each communication device set in the transition states of  FIG. 18  described above are respectively confirmed in order to correspond to changes on next priority order. Therefore, in a case where the priority order is changed over again from confirmation states of  FIG. 19 , after shift to the transition states of  FIG. 18  is once performed, shift to the confirmation states of  FIG. 19  is performed once again. 
   Here, in the confirmation states of  FIG. 19 , specifying reconstruction start times is performed in even number units T 2 , T 4 , T 6 , T 8  with respect to the base unit T 1  similarly to that of  FIG. 17 , although specifying reconstruction start times is performed in odd number units T 1 , T 3 , T 5 , T 7  with respect to the base unit T 1  according to priority order in the transition states of  FIG. 18  described above. 
   That is, the communication device  211  of the control station candidate of priority order P 1  sets 2 unit of the base unit T 1  as the reconstruction start time T 2 , the communication device  215  of the control station candidate of priority order P 2  sets 4 units of the base unit T 1  as the reconstruction start time T 4 , the communication device  216  of the control station candidate of priority order P 3  sets 6 units for the predetermined base unit T 1  as the reconstruction start time T 6 , and the communication device  217  of the control station candidate of priority order P 4  sets 8 units of the base unit T 1  as the reconstruction start time T 8 . 
   Another communication device  212  which does not have specified priority order is in a state where specifying of priority order is not performed, and since the communication device  212  is provided with the control function necessary to operate as a control station, the communication device  212  is allowed to start operating as a control station simultaneously after a predetermined simultaneous cancel time TR has elapsed. 
   Since the communication devices  213  is not provided with the control function necessary to operate as a control station, it is set that the communication devices  213  is subordinate to a communication device to be a control station candidate to form the wireless network. 
     FIG. 20  is a chart illustrating a transmission sequence of control station candidate information. 
     FIG. 20  illustrates an operation sequence in which the communication device  214  that is an existing control station determines priority order of each communication device for the communication devices  211 ,  215 ,  216  to be control station candidates to inform the network of selection result. 
   First, at step S 51 , priority order information is transmitted via a broadcast from the communication device  214  that is an existing control station to each communication device on the network. Specifically, the priority order information is priority order P 1  to P 4  for communication devices of each control station candidate shown in  FIG. 17 ,  FIG. 18 ,  FIG. 19  described above, the reconstruction start times T 1  to T 8  for reconstructing the wireless network, and the like. 
   At step S 52 , the communication device  211  to which priority order is given through the priority order information transmitted via the broadcast sends back receipt confirmation information for the priority order information to the communication device  214  that is an existing control station. Specifically, by the sending back of the receipt confirmation information from the communication device  211 , it becomes clear that the communication device  211  recognizes the priority order P 1  to P 4  of the priority order information of the stations in the network and the reconstruction start times T 1  to T 8 . 
   At step S 53 , similarly, the communication device  215  to which priority order is given through the priority order information transmitted via the broadcast sends back the receipt confirmation information for the priority order information to the communication device  214  that is an existing control station. Specifically, by the sending back of the receipt confirmation information from the communication device  215 , it becomes clear that the communication device  215  recognizes the priority order P 1  to P 4  of the priority order information of the stations in the network and the reconstruction start times T 1  to T 8 . 
   At step S 54 , similarly, the communication device  216  to which priority order is given through the priority order information transmitted via the broadcast sends back the receipt confirmation information for the priority order information to the communication device  214  that is an existing control station. Specifically, by the sending back of the receipt confirmation information from the communication device  216 , it becomes clear that the communication device  216  recognizes the priority order P 1  to P 4  of the priority order information of the stations in the network and the reconstruction start times T 1  to T 8 . 
   Here, regarding the communication devices  212 ,  213 ,  217  which do not have specified priority order, reduction in transmission traffic can be achieved by omitting sending back of the receipt confirmation of the priority order. 
     FIG. 21  is a view illustrating an example of the structure of a control station candidate specifying packet. 
     FIG. 21  corresponds to priority order information transmitted via the broadcast from the communication device  214  that is an existing control station to each communication device on the network at step S 51  of  FIG. 20  described above and is the one showing the structure of the control station candidate specifying packet as a control packet by which an existing control station gives priority order to specify a control station candidate. The control station candidate specifying packet of  FIG. 21  is the information transmitted employing an asynchronous transmission region (ASY)  238  shown in  FIG. 19 . 
   In the drawing, the control station candidate specifying packet has and is composed of a packet type  241  for specifying a packet, a transmission source communicating station ID  242  specifying an information transmission source communication device, and a reception destination communicating station ID  243  specifying an information reception destination communication device. 
   When the broadcast transmission is performed, a broadcast specifying code (3Fh) is substituted in the field thereof. 
   Following those, in the control station candidate specifying packet, stand-by time  244 - 1  and control station candidate communicating station ID  244 - 2  are specified as information  244  of priority order P 1 , stand-by time  245 - 1  and control station candidate communicating station ID  245 - 2  are specified as information  245  of priority order P 2 , and stand-by time  246 - 1  and control station candidate communicating station ID  246 - 2  are specified as information  246  of priority order P 3 . The stand-by times correspond to T 1  to T 8 , and the control station candidate communicating station IDs correspond to the communication devices  211  to  213 ,  215  to  217 . 
   The control station candidate specifying packet is constructed wherein existing control station specifies only the respective stations of the number of communicating stations provided with control functions which are fit to be control station candidates and a reserve region  247  is added with a CRC (Cyclic Redundancy Check)  248  at the end for a future expansion of a time when specifying priority order P 5  or later is performed in remaining region. 
   Another information may be added or unnecessary information may be curtailed to or from the fields as the need arises. 
     FIG. 22  is a view illustrating the structure of a priority order confirming packet. 
     FIG. 22  illustrates the structure of the priority order confirming packet as a control packet of the receipt confirmation sent back from a control station candidate to the existing control station. The priority order confirming packet of  FIG. 22  is the information transmitted employing the asynchronous transmission region (ASY)  238  shown in  FIG. 19 . 
   In the drawing, the priority order confirming packet has and is composed of a packet type  251  for specifying a packet, a transmission source communicating station ID  252  specifying an information transmission source communication device, and a reception destination communicating station ID  253  specifying an information reception destination communication device. 
   Following those, the priority order confirming packet sets priority order  254  specified for control station candidates, the stand-by time  255  thereof, and specified communicating station ID  256  of a station specified as a control station candidate, and further, remaining region is structured as a reserve region  257  for further expansion while a CRC  258  is added at the end thereof. 
   Another information may be added or unnecessary information may be curtailed to or from the fields as the need arises. 
   Although the embodiments described above are examples in which the present invention is applied to a network in which wireless communication is performed, as far as a network needing a similar central control station is employed, the present invention can also be applied to a network in which a plurality of communication terminals are connected by wired signal lines. 
   Although the present embodiments described above are examples in which the present invention is applied to the wireless 1394 format, the present invention is not limited to this, and it is needless to say that the present invention can be applied to other wireless networks. 
   According to the present invention, when the master control station becomes incapable of communication, a plurality of slave control stations try to manage information transmission as control stations in the set order one after another, and the slave control stations managing the information transmission thereafter manage the information transmission among the respective communication terminals. Therefore, since any one of the plurality of slave control stations prepared can manage the information transmission, even when the master control station becomes incapable of communication, the operating condition of the network can be continued. Here, since the order of trying to manage the information transmission is set among the plural slave control stations, the plural slave control stations do not operate simultaneously, whereby control condition is not be disturbed. 
   Regarding management means of the plural control stations, in the setting of the order by which management of the information transmission is performed at the time the master control station is incapable of communication, by providing the time difference from the time when the master control station becomes incapable of communication to the time managing the information transmission is started, there are time differences to manage based on the set order, and managing only by one of the plural slave control stations can be performed excellently. 
   Regarding management means of the master control station, by setting the order of specifying the slave control stations based on information transmission condition of the respective communication terminals, for example, a communication terminal most excellent in the information transmission condition can be set as a slave control station of higher order, and a communication terminal having a poorer information transmission condition can be set as a slave control station of lower order, whereby setting of order of slave control stations can be performed in a most preferable state. 
   At the time of setting the order of specifying the slave control stations based on the information transmission condition of the respective communication terminals as described above, the information transmission condition can be determined from the number of communication terminals capable of direct communication in the network or the communication quality with the respective communication terminals, whereby decision on the information transmission condition can be performed excellently. 
   Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications could be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.