Patent Publication Number: US-9432877-B2

Title: Information sharing system, communication apparatus, control method and computer program

Description:
TECHNICAL FIELD 
     The present invention relates to an information sharing system, a communication device, a control method, and a computer program. In particular, the present invention relates to an information sharing system in which information is shared between communication devices which form an ad hoc network, a communication device that has a means of wirelessly connecting with another communication device in order to communicate with it without going through an access point, a control method for controlling the relevant communication devices, and a program for the relevant communication devices. 
     Priority is claimed on Japanese Patent Application No. 2009-279846, filed Dec. 9, 2009, the contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     The widespread popularity of the Internet in recent years has been closely linked with the development of wireless communication technology and with the popularity of mobile telephones and mobile communication terminals in vehicles and the like that have a wireless communication function that enables them to connect to the Internet. Accordingly, services for sharing information contents (a generic term indicating images, music, video, text, messages and the like) that are based on an ad hoc network which forms a network without any communication infrastructure being required between multiple mobile communication terminals are showing genuine promise, and their usefulness as information transmission devices in cities or in disaster areas is anticipated. 
     However, because this type of ad hoc network is based on wireless communication, unlike the Internet which is formed by an interconnected wire network, the communication performance (i.e., the communication zone) is considerably limited by the environment surrounding the mobile communication terminals, and by obstacles and radio wave interference and the like. In addition, the communication topology of a formed ad hoc network is changed if the mobile communication terminals move, and route disconnections and the like occur frequently. As a result, it is not possible to hope for stable communication such as that obtainable from a wire network. 
     In information sharing that takes place when the communication performance (i.e., the communication zone) is limited, and in a communication environment where the communication path cannot be guaranteed, it is effective for information to be exchanged beforehand at a time when communication with other mobile communication terminals is possible. One method that has been proposed to achieve this is a method in which, when the movements of two mobile communication terminals cause them to come into mutual contact, they exchange information with each other, and synchronize the information they are holding (see, for example, Non-patent document 1). 
     In Non-patent document 1, the “Epidemic” method for synchronizing held information between all mobile communication terminals is proposed. The Epidemic method is characterized in that information is synchronized based on summary vectors (referred to below as ‘SV’) which are messages that summarize the information held by a terminal and notify other mobile communication terminals about this information. 
     In the Epidemic method, when a mobile communication terminal within a network detects another mobile communication terminal, they exchange summary vectors with each other, and ascertain the contents of the SV of the other mobile communication terminal. Based on the received SV, each mobile communication terminal then examines any contents that it does not itself hold, and attempts to acquire those contents. For example, as is shown in  FIG. 17 , a terminal T 1  and a terminal T 2  that have come into mutual contact as a result of moving their positions exchange SV with each other, and the terminal T 2  verifies any differential contents by comparing the SV of the terminal T 1  with its own SV and determining whether or not the terminal T 1  has any contents that it does not itself hold. At this time, because the terminal T 2  does not hold the contents B, it acquires the contents B by transmitting an acquisition request for the contents B to the terminal T 1 . The information held by all the terminals is synchronized as a result of all of the terminals within a network repeating this operation. 
     As has been described above, the Epidemic method is a method in which a comparison of the contents held by each terminal is made by means of the SV so that each terminal confirms whether there are any contents it does not itself hold, and then exchanges contents. These SV can also be used for replacing information within the network (i.e., terminal position coordinates and held contents information) with the information shown by the SV, and for verifying information existing on the network, and for transmitting requests to terminals within the network. 
     PRIOR ART DOCUMENTS 
     
         
         [Non-patent document 1] “Epidemic Routing for Partially-Connected Ad Hoc Networks”, A. Vandat and D. Becker, Tech. Report CS-200006, Duke University, April 2000 
       
    
     DISCLOSURE OF INVENTION 
     However, according to the method described in Non-patent document 1, SV are transmitted extremely frequently and a problem occurs in that there is a huge increase in the bandwidth load. For example, as is shown in  FIG. 18A , if each terminal holds completely different information, and also holds different SV, then when one of the terminals transmits an SV, the other mobile communication terminals detect the differences in the SV and transmit their own SV. Furthermore, each terminal again detects the differences in the SV transmitted by the other mobile communication terminals, and once again transmits their own SV. The other mobile communication terminals then transmit an SV in response to the SV that have been transmitted once again, and this operation is repeated until each terminal has transmitted its own SV numerous times. As a result of this, the bandwidth load becomes extremely heavy, and ends up obstructing other communication. 
     Moreover, for example, as a result of multiple terminals performing an operation together, the information held by each terminal becomes very similar, and groups of terminals having the same SV are formed ( FIG. 18B ). Consequently, when another mobile communication terminal which is holding different information becomes detected as a result of that terminal moving its position, all of the terminals within the group end up sending the same SV all at once to the detected terminal, so that the bandwidth load becomes extremely heavy. 
     As is described above, in the SV transmission control of the existing technology, because no consideration is given to the content of SV that are exchanged with other mobile communication terminals, mobile communication terminals have frequently exchanged duplicate SV. Moreover, as the density of the terminals within an environment increases, there is a likelihood that the number of SV transmissions will increase. 
     Here, consideration will be given again to the distribution of mobile communication terminals (i.e., people) in disaster areas and the like where it can be anticipated that an ad hoc network will be used. It is likely that rescue operations will cause mobile communication terminals (i.e., rescue workers) to congregate in large numbers in rescue sites and operate in groups, and that the density of mobile communication terminals will increase in localized areas. It is also likely that groups of mobile communication terminals that are holding similar information will appear. In cases such as this, under the existing technology, the above-described situation ends up occurring, and there is a possibility that the efficiency of the SV exchanges will deteriorate, and a further possibility that information sharing in the ad hoc network will no longer function. 
     The present invention has, for example, the following aspects. 
     A first aspect is an information sharing system in which information is shared between communication devices that form an ad hoc network, wherein there are provided multiple communication devices that wirelessly connect with other communication devices in order to communicate with them without going through an access point, and wherein each communication device has: an internal deduction section that, using a reception history of notification messages received from the other communication devices, deduces which information is currently being held by each of the other communication devices; a duplication determination section that determines whether or not the information that is currently being held by another communication device and that was deduced by the internal deduction section matches information held by its own host device; and an information exchange section that, when the duplication determination section determines that the information does not match, transmits a notification message that contains the information held by its own host communication device. 
     A second aspect is a communication device that wirelessly connects with other communication devices in order to communicate with them without going through an access point, and that includes: an internal deduction section that, using a reception history of notification messages received from the other communication devices, deduces which information is currently being held by a particular other communication device; a duplication determination section that determines whether or not the information that is currently being held by another communication device and that was deduced by the internal deduction section matches information held by its own host device; and an information exchange section that, when the duplication determination section determines that the information does not match, transmits a notification message that contains the information held by its own host communication device. 
     A third aspect is a control method for controlling a communication device that wirelessly connects with other communication devices in order to communicate with them without going through an access point, and that includes: an internal deduction stage in which, using a reception history of notification messages received from the other communication devices, a deduction is made as to which information is currently being held by a particular other communication device; a duplication determination stage in which whether or not the information that is currently being held by another communication device and that was deduced by the internal deduction section matches information held by its own host device is determined; and an information exchange stage in which, when it is determined in the duplication determination stage that the information does not match, a notification message that contains the information held by its own host communication device is transmitted. 
     A fourth aspect is a computer program that is stored on a computer-readable non-temporary recording medium of a communication device that wirelessly connects with other communication devices in order to communicate with them without going through an access point, and that includes: an internal deduction command that, using a reception history of notification messages received from the other communication devices, deduces which information is currently being held by a particular other communication device; a duplication determination command that determines whether or not the information that is currently being held by another communication device and that was deduced by the internal deduction section matches information held by its own host device; and an information exchange command that, when the duplication determination section determines that the information does not match, transmits a notification message that contains the information held by its own host communication device. 
     Note that in the above-described aspects, not all of the necessary features have been exemplified and, for example, it is also possible to form sub-combinations of these feature groups. 
     According to the above-described aspects, because a terminal determines whether or not transmission information is duplicated based on information held by itself and by other communication terminals, it is possible to reduce transmissions of duplicated information to other communication terminals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing an example of an application environment of an information sharing system  100  according to an embodiment. 
         FIG. 2  is a view showing specific examples of summary vectors. 
         FIG. 3  is a structural view of a first embodiment. 
         FIG. 4A  is a view showing an image of a summary vector reception history and internal deduction results. 
         FIG. 4B  is a view showing an image of a summary vector reception history and internal deduction results. 
         FIG. 5  is a view showing a deduction method  1  for deducing the summary vectors of another mobile communication terminal. 
         FIG. 6  is a view showing a deduction method  2  for deducing the summary vectors of another mobile communication terminal. 
         FIG. 7  is a view showing a deduction method  3  for deducing the summary vectors of another mobile communication terminal. 
         FIG. 8  is a view showing an operational flowchart of the first embodiment. 
         FIG. 9  is a view showing an actual operating example 1. 
         FIG. 10  is a view showing an actual operating example 2. 
         FIG. 11  is a sequence diagram showing back-off calculation results. 
         FIG. 12  is a view showing an actual operating example 3. 
         FIG. 13  is a view showing an actual operating example 4. 
         FIG. 14  is a view showing an actual operating example 5. 
         FIG. 15  is a view showing the structure of a second embodiment. 
         FIG. 16  is a view showing an operational flowchart of the second embodiment. 
         FIG. 17  is a view showing an outline of the conventional technology. 
         FIG. 18A  is a view showing the conventional technology. 
         FIG. 18B  is a view showing the conventional technology. 
         FIG. 19  is a view showing an example of the hardware structure of a mobile communication terminal  110 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will now be described. The following embodiments are not limiting of the invention according to the range of the claims. In addition, not all combinations of the features described in the embodiments are necessarily essential component elements. 
       FIG. 1  shows an example of an application environment of an information sharing system  100  according to an embodiment. The information sharing system  100  is provided with multiple mobile communication terminals  110   a ,  110   b ,  110   c , . . . (hereinafter, these will be generically referred to as mobile communication terminals  110 ). The information sharing system  100  is achieved as a result of a large number of the mobile communication terminals  110  congregating together, with each one being connected to the others via wireless communication L 1  so as to construct an ad hoc network. 
       FIG. 2  shows specific examples of summary vectors. A summary vector (SV) is a message that sends notification of the information held by a mobile communication terminal  110 . Moreover, summary vectors (SV) not only provide notification about the contents held by their own host terminal, but summary vector is also a generic term for the summary information of contents held by other mobile communication terminals  110  within a network, for example, information in list form that is formed from multiple elements such as data listed in hash tables and position information and the like. In addition, in the present embodiment, when an event such the following 1˜4 is detected as a result of a particular host terminal exchanging SV with other mobile communication terminals  110 , that host terminal transmits an SV. 
     1. When a new mobile communication terminal  110  is detected. 
     2. When the host terminal is holding information that is not held by a received SV. 
     3. When a received SV is holding information that is older than that held by the host terminal. 
     4. Periodic transmissions after regular time intervals. 
     The contents of an SV in the present embodiment are described as the intra-network contents information  102  shown in  FIG. 2 . Moreover, if, for example, the summary information of the SV held by the mobile communication terminal  110   a  and the summary information of the SV held by the mobile communication terminal  110   b  are taken as X and Y respectively, then these SV are written as SV (X) and SV (Y). Moreover, when the terminal  110   a  receives an SV from the terminal  110   b , and the contents of the SV are updated, then in order to make it understood that the terminal  110   a  is holding two items of information, namely, the SV (X) of the terminal  110   a  and the SV (Y) of the terminal  110   b , the SV of the terminal  110   a  are written separately as SV (X, Y). 
       FIG. 3  is a structural view of the first embodiment. The mobile communication terminal  110  includes a wireless communication function section  11  that performs wireless communication with other mobile communication terminals  110   b ,  110   c , . . . , an information management section  12  that manages the information held by its own host terminal, and an exchange control section  13  that controls information exchanges. 
     The wireless communication function section  11  is provided with an information exchange section  111 . The information exchange section  111  has the functions of receiving information (i.e., SV, contents acquisition requests and responses, contents information, and position information for other mobile communication terminals) from other mobile communication terminals via the wireless communication function section  11 , and of transmitting contents information held by its own host terminal and also transmitting SV at periodic intervals. Received information is sent to a data storage section  121  of the information management section  12 , and is saved in the data storage section  121 . When an SV is received, it is saved in the data storage section  121  after passing through an exchange detection section  131  of the exchange control section  13 . 
     When transmitting information, the information exchange section  111  reads from the data storage section  121  located in the information management section  12  the information that is to be transmitted, and then transmits the information. When transmitting an SV, in addition to transmitting SV at periodic intervals, when the information exchange section  111  receives a transmission notification from a duplication determination section  133  of the exchange control section  13 , it reads the SV of its own host terminal from the data storage section  121 , and transmits the SV. 
     The information management section  12  is provided with the data storage section  121 . The data storage section  121  has the functions of updating, saving, and deleting contents and SV created by its own host terminal, contents acquired via the information exchange section  111 , and SV received via the exchange detection section  131 . When the data storage section  121  receives a new SV, it integrates the new SV with the saved SV in its own host terminal. For example, the data storage section  121  compares the received SV with the summary information within the SV of its own host terminal, and if summary information having a more recent update date is contained within the received SV, the data storage section  121  overwrites the relevant summary information within the SV of its own host terminal, while if newly obtained summary information is contained within the received SV, it updates the SV of its own host terminal with this newly obtained summary information as a new addition. When the data storage section  121  receives notification for an information transmission from the information exchange section  111 , it sends the relevant information to the information exchange section  111 . 
     The exchange control section  13  is provided with an exchange detection section  131 , an adjacent terminal management section  132 , the duplication determination section  133 , an internal deduction section  134 , and a back-off calculation section  135 . When the exchange detection section  131  receives an SV from another mobile communication terminal from the information exchange section  111  of the wireless communication functions section  11 , the exchange detection section  131  has the functions of comparing the received SV with the SV of its own host terminal and determining whether or not an exchange of SV is required. If it determines that an exchange of SV is required, it then has the function of notifying this determination to the duplication determination section  133 . The exchange detection section  131  determines that an exchange is necessary if the host terminal of the duplication determination section  133  holds summary information which is not in the received SV, or holds summary information which has a more recent update date that that of its own host terminal. For example, if the SV of its own host terminal is holding summary information that is not in the received SV, the exchange detection section  131  determines that the SV of its own host terminal is provided with a greater quantity of intra-network summary information, and exchanges SV with the mobile communication terminal which is the source of the SV transmission. If the exchange detection section  131  detects that the summary information in the received SV is older than the summary information in the SV within its own host terminal, it determines that the SV of its own host terminal contains the newer summary information, and determines that an SV exchange is necessary in order to send notification to the mobile communication terminal which is the source of the SV transmission. 
     Moreover, once the determination about the SV exchange has ended, irrespective of whether or not an exchange is to be performed or not performed, the exchange detection section  131  sends the received SV to the data storage section  121  of the information management section  12 , and updates the SV of its own host terminal. If the exchange detection section  131  determines that the exchange is to be performed, it also sends notification to start exchanging SV to the duplication determination section  133 , and the duplication determination section  133  determines whether or not the contents of the SV transmitted by the other mobile communication terminal duplicate those transmitted by its own host terminal. Until notification announcing that the transmission is completed is received from the duplication determination section  133 , the exchange detection section  131  recognizes that an SV transmission waiting period is still in force (i.e., that a back-off period is still in force), and sends subsequently received SV not only to the data storage section  121 , but also to the adjacent terminal management section  132 , and performs processing to save the SV reception history starting from the determination to transmit the SV of its own host terminal up until the SV is transmitted. 
     As is shown in  FIGS. 4A and 4B , the adjacent terminal management section  132  has the functions of saving the reception histories of SV from other mobile communication terminals that arrive from the exchange detection section  131  ( FIG. 4A ), and of saving the internal deduction results of SV of other mobile communication terminals that are deduced by its own host terminal ( FIG. 4B ). For example, when an SV is newly received from the exchange detection section  131 , firstly, the reception history of the SV is saved. If the received SV was received from the terminal  110   b  at the time 13:00:00, and the content of the summary information was [X], then as is shown in  FIG. 4A , the transmitting terminal ID is recorded as being [ 110   b ], and the content of the SV received from that terminal is recorded as being [X]. Next, using the internal deduction section  134 , the adjacent terminal management section  132  acquires the new internal deduction results shown in  FIG. 4B  from the SV reception history and the previous internal deduction results, and then saves these. 
     The duplication determination section  133  has the functions of determining whether or not another mobile communication terminal has transmitted an SV for contents that are duplicated in its own host terminal by the time that its own host terminal transmitted the SV, and of determining whether or not the other mobile communication terminal is storing the same summary information as its own host terminal. When it receives notification from the exchange detection section  131  to transmit an SV from its own host terminal, the duplication determination section  133  starts transmission processing to transmit an SV from its own host terminal. 
     Firstly, the duplication determination section  133  decides the transmission timing to transmit the SV from its own host terminal using the back-off calculation section  135 . Next, at the time of the SV transmission, the duplication determination section  133  reads the internal deduction results for the SV of other mobile communication terminals from the adjacent terminal management section  132 , and decides whether or not a transmission should be made based on the internal deduction result and on the SV of its own host terminal (i.e., the information possessed by its own host terminal). If the duplication determination section  133  decides that a transmission should be made, it sends notification of this to the information exchange section  111 , and transmits the SV of its own host terminal. 
     At this time, the duplication determination section  133  makes the decision of whether or not to transmit an SV on the affirmative side if, among the SV of the other mobile communication terminals in the internal deduction results (i.e., the information possessed by the other mobile communication terminals), there is one or more SV, or a number of SV above a threshold value, of the other mobile communication terminals that does not contain all of the information of the SV of its own host terminal (i.e., information possessed by its own host terminal) which is to be transmitted. If the information of the SV of its own host terminal is contained in all of the SV in the internal deduction results, then because the other mobile communication terminals already hold the same information as that held by the host terminal, the duplication determination section  133  determines that it is not necessary to transmit the SV of its own host terminal, and discontinues the transmitting of the SV of its own host terminal. When the transmission control has ended, the duplication determination section  133  sends back an SV transmission control completion notification to the exchange detection section  131 . It is also possible for the decision of whether or not to transmit an SV to be made each time an SV is received from another mobile communication terminal, and for the transmitting of the SV to be discontinued without waiting until the SV transmission timing of the host terminal. 
     The internal deduction section  134  has the function of internally deducing within its own host terminal which of the current SV (i.e., possessed information) may be thought to be held by other mobile communication terminals from the SV reception history saved in the adjacent terminal management section  132 . When the internal deduction section  134  receives from the adjacent terminal management section  132  the reception histories for SV from the other mobile communication terminals, and the internal deduction results for the SV held by the other mobile communication terminals that were calculated previously, it deduces which SV may be thought to be currently held by the other mobile communication terminals. Several methods may be considered as the method for deducing the SV held by the other mobile communication terminals. 
     An example of a deduction method is shown in  FIG. 5 . Because the sum total (X, Y) of the contents of the SV received in the SV reception history  111  corresponds to the information that was transmitted within the communication range of its own host terminal, it is assumed that all of the other mobile communication terminals located within the communication range of the host terminal are also holding the same information, and the deduction is simple as is shown in the internal deduction results  121  shown in  FIG. 5 . 
     A method in which the SV is deduced based on common portions of the SV of the mobile communication terminals  110  located in the SV reception history may also be considered. An SV is received from another mobile communication terminal, and the saving of the SV reception history is started. For example, as is shown in the SV reception history  112  shown in  FIG. 6 , the reception history up until the time when the SV of the other mobile communication terminals was deduced is then taken as being, firstly, that an SV (X) has been received from the mobile communication terminal  110   b , and, next, that an information SV (X,Y) has been received from the mobile communication terminal  110   c.    
     At this time, the summary information [X] transmitted by the mobile communication terminal  110   b  is contained in the SV received from the mobile communication terminal  110   c , and it can be understood that there are common portions to both the summary information in the SV of the mobile communication terminal  110   b  and the summary information in the SV of the mobile communication terminal  110   c . As a consequence, it can be thought that the mobile communication terminal  110   c  which has common portions with the mobile communication terminal  110   b  transmitted an SV after first receiving the SV transmitted by the mobile communication terminal  110   b . From this, a method may be considered in which it is determined that, in the same way, the mobile communication terminal  110   b  has received the SV (X, Y) transmitted by the mobile communication terminal  110   c , and as is shown by the internal deduction results  122  in  FIG. 6 , the SV of the mobile communication terminal  110   b  is deduced to be SV (X, Y). 
     Furthermore, a method may also be considered in which, if an SV reception history up until the internal deduction calculation which includes the periodic SV transmissions and receptions has been saved in the data storage section  121 , then a deduction may be made based on any changes from the past history. For example, as is shown in the reception history  113  in  FIG. 7 , the SV (Y) is received from the mobile communication terminal  110   c  as a result of the initial receiving of a periodic SV. Thereafter, the mobile communication terminal  110   b  transmits the SV (X), and the mobile communication terminal  110   c  subsequently transmits the SV (X, Y). At this time, it is understood that the SV of the mobile communication terminal  110   c  changes from [X] before the transmission by the mobile communication terminal  110   b  to [X, Y] after the transmission by the mobile communication terminal  110   b . In addition to this change in the SV, the fact that the mobile communication terminal  110   c  has transmitted an SV after the mobile communication terminal  110   b  indicates that the mobile communication terminal  110   c  has already received the SV from the mobile communication terminal  110   b  and detected any differences therein, and has only then transmitted its own SV. From this, a method can be considered in which it is determined that the mobile communication terminal  110   b  and the mobile communication terminal  110   c  are located in positions where they are able to communicate with each other, and that the mobile communication terminal  110   b  is also receiving the SV (X, Y) transmitted by the mobile communication terminal  110   c , and in which, as is shown in the internal deduction results  123  in  FIG. 7 , the SV of the mobile communication terminal  110   b  can be deduced to be the SV (X, Y). 
     In the above-described method, if the [X] of the summary information is updated to [X′], then it is not treated as the same information, but is treated as separate information. For example, using an example in which the prediction method which is based on common portions is employed, firstly, the SV (X, Y) is received from the mobile communication terminal  110   b , and the SV (X′, Y) is then received from the mobile communication terminal  110   c . At this time, the items of summary information [X] that have mutually different creation times are treated as being different from each other, and the summary information [Y] is recognized as being a common portion. The mobile communication terminal  110   b  and the mobile communication terminal  110   c  are determined to be within a range where they can communicate with each other, and it is deduced that the mobile communication terminal  110   b  and the mobile communication terminal  110   c  both possess the SV (X′, Y). 
     As has been described above, the internal deduction results for the SV of other mobile communication terminals that were calculated by the internal deduction section  134  are sent back to the adjacent terminal management section  132 , and the internal deduction results for the SV of the other mobile communication terminals that are being saved in the adjacent terminal management section  132  are updated. 
     The back-off calculation section  135  has the function of deciding the transmission timings of the SV of its own host terminal. When the back-off calculation section  135  receives a notification from the duplication determination section  133  to start calculating a back-off period, it randomly calculates a transmission wait time (Formula (1)). In the calculation of the transmission wait time, position information for the terminals is exchanged in the information exchange section  111 , and if position information can be used from the data storage section  121 , the positional relationship between terminals and their movement speed may be read from the data storage section  121 , and used for the calculation (Formula (2)). After the movement speed has been calculated from the position information, if the SV are to be transmitted with priority given in sequence to the terminals having the fastest movement speed, it is possible to then set a large correction coefficient α, or to set a small reference value for deciding the increase or decrease in the random numerical values. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             back 
                             ⁢ 
                             
                               - 
                             
                             ⁢ 
                             off 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             period 
                           
                           = 
                             
                           ⁢ 
                           
                             random 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             numerical 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             value 
                             * 
                             slot 
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                           ⁢ 
                           
                             random 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             numerical 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             value 
                             * 
                           
                         
                       
                     
                     
                       
                         
                             
                           ⁢ 
                           
                             
                               ( 
                               
                                 1 
                                 - 
                                 
                                   α 
                                   * 
                                   v 
                                   ⁢ 
                                   
                                     / 
                                   
                                   ⁢ 
                                   V 
                                 
                               
                               ) 
                             
                             * 
                             slot 
                           
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       slot 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       transmission 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       interval 
                     
                     , 
                     
                       
 
                     
                     ⁢ 
                     
                       α 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       correction 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       coefficient 
                     
                     , 
                     
                       
 
                     
                     ⁢ 
                     
                       V 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       reference 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       value 
                     
                     , 
                     
                       
 
                     
                     ⁢ 
                     
                       v 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       velocity 
                     
                   
                 
               
               
                 
                   
                     
                       
                         ( 
                         1 
                         ) 
                       
                     
                   
                   
                     
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
       FIG. 8  shows an operational flowchart of the first embodiment. Hereinafter, SV transmission control that is based on information held between mobile communication terminals  110  in the present embodiment will be explained in detail. As the method that is used to deduce the SV of another mobile communication terminal, a method that is based on common portions with the SV of the terminals in the SV reception history shown in  FIG. 6  is employed. 
     The mobile communication terminals  110   a  to  110   d  shown in  FIG. 9  form an ad hoc network in which communication is possible between each terminal and all of the other terminals. As is shown in  FIG. 9 , the mobile communication terminals  110   a  to  110   d  respectively hold SV for the summary information X, Y, Z, Z. At this time, each of the mobile communication terminals  110  starts exchanging SV with the other mobile communication terminals in order to ascertain information over the network. 
     Firstly, each mobile communication terminal  110  monitors differences in the SV between the mobile communication terminals  110  by transmitting its own SV at periodic intervals in broadcast mode to the other mobile communication terminals  110 . At this time, when an SV is received from another mobile communication terminal  110 , a determination is made by the exchange detection section  131  of the exchange control section  13  as to whether or not its own host terminal needs to transmit an SV ( FIG. 8 : steps S 100 , S 101 ). The SV is then sent to the data storage section  121  and the SV of its own host terminal is updated. 
     In the exchange detection section  131 , firstly, a determination is made as to whether or not its own host terminal has already received another SV, and whether SV transmission control notification has been sent to the duplication detection section  133 , and whether or not it is currently in a back-off period and awaiting a completion notification response ( FIG. 8 : step S 100 ). 
     If it is not currently in a back-off period, the exchange detection section  131  confirms any differences in the summary information between the received SV and the SV of its own host terminal, and if there are none in the received SV, or if its own host terminal holds new summary information, or if a new terminal is detected, the exchange detection section  131  starts transmission processing for the SV of its own host terminal ( FIG. 8 : steps starting from step S 102 ). 
     Next, the actual flow from the situation shown in  FIG. 9  until the completion of the exchange of SV between the mobile communication terminals  110   a  to  110   d  will be explained using  FIG. 8  to  FIG. 14 . 
     Firstly, the mobile communication terminal  110   a  transmits an SV (X) as a periodic SV transmission in order to give notification of its own SV to the other mobile terminals  110   b  to  110   d . The mobile communication terminals  110   b  to  110   d  that have received the SV (X) from the mobile communication terminal  110   a  start processing in their exchange detection sections  131 , based on the contents of the received SV, to determine whether or not they need to transmit their own SV ( FIG. 8 : steps S 100 , S 101 ). 
     In step S 100 , because the exchange detection sections  131  of the mobile communication terminals  110   b  to  110   d  are not currently sending notification for SV transmission control to the duplication detection sections  133 , and are not in a back-off period awaiting a completion notification response, in step S 100  in  FIG. 8 , it is determined that they are not in a back-off period and the routine moves to step S 101 . 
     In step S 101 , any difference between the SV of their own host terminal and the received SV is confirmed, and the exchange detection sections  131  determine whether or not it is necessary to transmit the SV of their own host terminal. As is shown in the table in  FIG. 9 , if a difference is confirmed when they compare the SV of their own host terminal with the received SV (X), the exchange detection sections  131  establish that there is summary information that is not in the SV (X) of the mobile communication terminal  110   a , for example, the summary information Y in the case of the mobile communication terminal  110   b , and the summary information Z in the case of the mobile communication terminals  110   c  and  110   d , and establish that their own host terminals are holding summary information that should be notified to the mobile communication terminal  110   a.    
     As a result, the exchange detection sections  131  of the respective mobile communication terminals  110   b  to  110   d  determine that the SV of their own host terminals do need to be transmitted, and send notification to the duplication determination sections  133  telling them to perform SV transmission control for their own host terminals. Consequently, each of the mobile communication terminals  110   b  to  110   d  begins the SV transmission processing which starts from step S 102  shown in  FIG. 8 . 
     Here, up until the exchange detection sections  131  that sent the notification to the duplication determination sections  133  telling them to perform SV transmission control receive a response from the duplication determination sections  133  stating that the SV transmission processing of their own host terminals has been completed, they not only send SV received thereafter to the data storage section  121  and update the SV of their own host terminals, but they also send the SV to the adjacent terminal management section  132  and save a temporary SV reception history ( FIG. 10 ). 
     Firstly, when the adjacent terminal management sections  132  receive an SV from the exchange detection section  131  that had determined that transmission of that SV was necessary, the processing of step S 102  in  FIG. 8  is started. 
     In step S 102 , the adjacent terminal management sections  132  record the ID of the transmission source terminal ( 110   a ) and the content of the received SV (X), and, as in the SV reception history shown in  FIG. 10 , save the SV received from their exchange detection sections  132  as a reception history. Thereafter, the internal deduction results ( FIG. 10 ) are calculated using the internal deduction sections  134  ( FIG. 8 : step S 102 ). The internal deduction results are obtained by the terminals themselves deducing the current SV of the other mobile communication terminals. 
     In the processing of step S 102  in  FIG. 8 , in the initial calculation of the internal deduction results, other than the contents of the received SV and the transmission source terminal thereof, each terminal does not hold information about the other mobile communication terminals  110  or the SV information of those terminals. Because of this, the received SV information (i.e., the transmission source terminal ID and the contents of the SV) are recorded as they are as internal deduction results, as is shown in  FIG. 10 . After the initial time, the adjacent terminal management sections  132  update and record the internal deduction results using the internal deduction section  134  each time an SV is received from the exchange detection section  131 . 
     Meanwhile, when the data storage sections  121  receive the SV from the exchange detection sections  131 , they start the processing of step S 103  in  FIG. 8 . 
     In step S 103 , the data storage sections  121  verify any discrepancies between the update times of the summary information in the SV of their own host terminals and of the summary information in the received SV, and whether or not there are differences between the two information contents, and also update the summary information within the SV of their own host terminals, as is shown in  FIG. 10 . Because this time they have received the summary information X which is not held by their own host terminals, the mobile communication terminals  110   b  to  110   d  add the summary information X to the SV of their own host terminals, and the mobile communication terminal  110   b  updates its SV to the SV (X, Y), while the mobile communication terminals  110   c  and  110   d  update their SV to the SV (X, Z) ( FIG. 8 : step S 103 ). 
     Next, when the duplication determination sections  133  of the mobile communication terminals  110   b  to  110   d  receive notification from the exchange detection sections  131  instructing them to perform SV transmission control, they begin the processing starting from step S 104  in  FIG. 8  in order for the SV of their own host terminals to be transmitted. 
     In step S 104 , in order to decide the timings of the SV transmissions of their own host terminals, the duplication determination sections  133  decide the times for transmitting the SV using their back-off calculation sections  135  ( FIG. 8 : step S 104 ). Thereafter, the routine moves to step S 105 . 
     This transmission times may be decided using random delay times (Formula (1)) or by using a calculation (Formula (2)) that is based on the terminal movement speed. If the transmission time is calculated based on the terminal movement speed, then thought may be given to hastening the transmission time for terminals having a fast movement speed so that their SV can be transmitted before they end up moving out of the communication range. 
     Next, during the back-off period up until the SV transmission timing shown in step S 105 , if the mobile communication terminals  110   b  to  110   d  receive another new SV, the exchange detection section  131  of each terminal determines that their own host terminal is currently in a back-off period via the processing of step S 100  in  FIG. 8 . As a result, when an SV is newly received, the processing of step S 109  and step S 110  in  FIG. 8  is executed, and until the current SV transmission processing is ended, the SV of their own host terminals are no longer transmitted even if a new SV is received. 
     The transmission timings of each terminal that were set at this time as well as the subsequent SV transmission sequences are shown in  FIG. 11 , and the SV transmission sequence, which is based on the back-off period decisions, are in the sequence of the mobile communication terminal  110   b , then the mobile communication terminal  110   c , and then the mobile communication terminal  110   d.    
     After the mobile communication terminal  110   a  has transmitted an SV, as is described above, the mobile communication terminals  110   b  through  110   d  enter a back-off period in order for their own SV transmissions to be performed ( FIG. 8 : step S 105 ). Next, as is shown in  FIG. 11 , the mobile transmission terminal  110   b , which has the earliest transmission timing from among the mobile transmission terminals  110   b  through  110   d , finishes the processing of step S 105  in  FIG. 8 , and starts the processing of step S 106  ( FIG. 12 ). 
     In step S 106 , at the moment when the SV transmission timing arrives, the duplication determination section  133  of the mobile communication terminal  110   b  reads the internal deduction results from the adjacent terminal management section  132 , and compares the deduced SV of the other mobile communication terminals with the SV of its own host terminal, and verifies whether or not there are any differences in the summary information that need to be transmitted to the other mobile communication terminals ( FIG. 8 : step S 106 ). 
     As a result of this, the mobile communication terminal  110   b  is able to ascertain that it is holding summary information [Y] in its own SV (X, Y) that is not in the deduced SV (X) of the mobile communication terminal  110   a . Accordingly, the mobile communication terminal  110   b  determines that its own SV contains a greater quantity of summary information compared to the mobile communication terminal  110   a , and decides to transmit its own SV (X, Y) in order to provide notification of this to the mobile communication terminal  110   a  ( FIG. 8 : step S 107 ). 
     Next, the duplication determination section  133  sends an SV transmission notification to the information exchange section  111 . As a result, the SV of the mobile communication terminal  110   b  is transmitted ( FIG. 8 : step S 108 ), and the SV transmission processing of the mobile communication terminal  110   b  is ended. 
     At this time, the duplication determination section  133  notifies the exchange detection section  131  that the SV transmission has been completed, and cancels the back-off period that was initially set, and also stops subsequent transfers of the SV to the adjacent terminal management section  132  and stops the internal deduction calculations. 
     Next, because the mobile communication terminal  110   a , the mobile communication terminal  110   c , and the mobile communication terminal  110   d  that received the SV from the mobile communication terminal  110   b  have received a new SV from another mobile communication terminal, a determination is started in the exchange detection section  131  of each of these terminals as to whether or not it is necessary for them to transmit an SV ( FIG. 8 : steps S 100  and S 101 ). 
     The mobile communication terminal  110   a , firstly, verifies in its exchange detection section  131  whether or not it is currently in a back-off period ( FIG. 8 : step S 100 ), and because currently no back-off period is provided, it determines that it is not in a back-off period, and the routine moves to the next step S 101 . 
     In step S 101 , the exchange detection section  131  verifies any differences between the SV of its own host terminal and the received SV, and determines whether or not it is necessary to transmit the SV of its own host terminal. In step S 101 , although there is a difference between the received SV (X, Y) and the SV of its own host terminal (X), because its own host terminal is not holding any summary information which is not in the received SV, the exchange detection section  131  of the mobile communication terminal  110   a  determines that it is not necessary for it to transmit an SV, and the processing performed by the exchange detection section  131  is ended. The received SV is then sent to the data storage section  121  of the information management section  12 , and the SV of the mobile communication terminal  110   a  is updated. As is shown in  FIG. 12 , because the new summary information Y has been received, the SV is now (X, Y). 
     The mobile communication terminal  110   c  and the mobile communication terminal  100   d , firstly, verify in their exchange detection sections  131  whether or not they are currently in a back-off period ( FIG. 8 : step S 100 ). Because they have sent notification for SV transmission control to the duplication determination sections  133 , but the exchange detection sections  131  have not yet received a response indicating that this has ended, they determine that they are currently in a back-off period, and the routine moves to step S 109  in  FIG. 8 . 
     In step S 109 , the exchange detection sections  131  send the received SV to their data storage section  121  and adjacent terminal management section  132 , and in the same way as when they received the SV from the mobile communication terminal  110   a , the adjacent terminal management sections  132  update their SV reception history, and then, as is shown in  FIG. 12 , update the internal deduction results for the SV of the other mobile communication terminals using their internal deduction section  134  ( FIG. 8 : step S 109 ). The routine then moves to step S 110 , and because the new summary information Y was received, the SV of their own host terminals are updated in the manner shown in  FIG. 12 . 
     Here, in the calculations of the second and subsequent internal deductions ( FIG. 8 : step S 109 ), the internal deduction section  134  performs these using the SV reception histories from the other mobile communication terminals that are recorded in the adjacent terminal management section  132 . 
     As is shown in  FIG. 12 , hitherto, the mobile communication terminal  110   c  and the mobile communication terminal  110   d  have firstly received the SV (X) from the mobile communication terminal  110   a , and thereafter received the SV (X, Y) from the mobile communication terminal  110   b . As a result, the reception histories from the mobile communication terminal  110   a  and the mobile communication terminal  110   b  are recorded in the reception histories saved by the adjacent terminal management section  132 . 
     At this time, the mobile communication terminal  110   c  and the mobile communication terminal  110   d  regard the summary information in the SV in the reception histories as one element, and when the intersections (SV (X) ∩ SV (X, Y)) are calculated, then it can be seen that the summary information X exists as summary information that is common to the two SV histories. Aside from the periodic SV transmission, the SV transmissions take place when summary information that is not held by the other mobile communication terminals is held by these terminals. Namely, after it has been determined that the mobile communication terminal  110   a  and the mobile communication terminal  110   b  are both holding summary information that is not held by another mobile communication terminal, the summary information held by these is transmitted. 
     Consequently, it might be considered that, originally, the mobile communication terminal  110   b  was not holding the summary information X, and that, only after it had received the SV (X) from the mobile communication terminal  110   a , did it transmit the SV (X, Y). Alternatively, it might be considered that the mobile communication terminal  110   b  received the SV in advance from the mobile communication terminal  110   a  at a point when the two mobile communication terminals were able to communicate with each other. Accordingly, it can be deduced that the mobile communication terminal  110   a  also received the SV (X, Y) transmitted by the mobile communication terminal  110   b , and that the mobile communication terminal  110   a  is holding the summary information X, Y. If no portions that are common to both SV exist in the SV reception histories, then it is determined that no SV has been received, and the deduced held information is not updated. 
     Here, it is not absolutely essential for the SV internal deduction results of other mobile communication terminals to have been deduced completely accurately. The internal deductions are intended to limit the sending in a single transmission of a large quantity of information having duplicated contents, and information that is wrongly identified with information already held by another mobile communication terminal can be exchanged during the occasion of the next periodic SV transmission. 
     Step S 109 , which is the processing of the internal deduction sections  134 , is then ended, and, as is shown in  FIG. 12 , the mobile communication terminal  110   c  and the mobile communication terminal  110   d  update their own internal deduction results. After the SV of their own host terminals have been updated in the data storage sections  121  ( FIG. 8 : step S 110 ), the mobile communication terminal  110   c  and the mobile communication terminal  110   d  finish their processing for the SV reception from the mobile communication terminal  110   b.    
     After the mobile communication terminal  110   b  has transmitted the SV, as is described above, the mobile communication terminal  110   a , the mobile communication terminal  110   c , and the mobile communication terminal  110   d  perform processing on the newly received SV. Next, as is shown in  FIG. 11 , the mobile communication terminal  110   c , which has the earlier transmission timing out of the mobile communication terminal  110   c  and the mobile communication terminal  110   d , ends step S 105  in  FIG. 8 , and starts the processing of the next step S 106  ( FIG. 13 ). 
     In step S 106 , at the point in time when the SV of its own host terminal is to be transmitted, the duplication determination section  133  of the mobile communication terminal  110   c  reads an internal deduction  102  from the adjacent terminal management section  132 , and compares the SV of the other mobile communication terminals with the SV of its own host terminal so as to verify any differences between them ( FIG. 8 : step S 106 ). 
     As a result, the mobile communication terminal  110   c  ascertains that its own SV (X, Y, Z) holds the summary information Z that is not in the SV (X, Y) of the mobile communication terminal  110   a  and the mobile communication terminal  110   b . Accordingly, the mobile communication terminal  110   c  determines that its own SV contains a greater quantity of summary information compared to the SV of the mobile communication terminal  110   a  and the mobile communication terminal  110   b , and decides to transmit its own SV (X, Y, Z) in order to provide notification of this to the mobile communication terminal  110   a  and the mobile communication terminal  110   b.    
     Next, the duplication determination section  133  sends notification of the SV transmission to the information exchange section  111 , and the SV of the mobile communication terminal  110   c  is transmitted ( FIG. 8 : step S 108 ). 
     At this time, the duplication determination section  133  notifies the exchange detection section  131  that the SV transmission is completed, and cancels the back-off period that was initially set, and also stops subsequent transfers of the SV to the adjacent terminal management section  132  and stops the calculation of the internal deduction results. 
     Next, because the mobile communication terminal  110   a , the mobile communication terminal  110   b , and the mobile communication terminal  110   d  that received the SV from the mobile communication terminal  110   c  have received a new SV from another mobile communication terminal, a determination is started in the exchange detection section  131  of each of these terminals as to whether or not it is necessary for them to transmit an SV ( FIG. 8 : steps S 100  and S 101 ). 
     The mobile communication terminal  110   a , in the same way as when it received the SV from the mobile communication terminal  110   b  (described above), determines via its exchange detection section  131  that it is currently not in a back-off period ( FIG. 8 : step S 100 ), and also compares the received SV with the SV of its own host terminal and, because there is no summary information that needs to be transmitted by its own host terminal ( FIG. 8 : step S 101 ), sends the received SV to the data storage section  121 , and updates the SV of its own host terminal. As is shown in  FIG. 13 , because the new summary information Z was received, the SV of the mobile communication terminal  110   a  becomes the SV (X, Y, Z). 
     The mobile communication terminal  110   b , firstly, verifies via its exchange detection section  131  whether or not it is currently in a back-off period ( FIG. 8 : step S 100 ). Because it has received notification from the duplication determination section  133  indicating that the SV transmission control has been completed, it determines that it is not in a back-off period, and the routine moves to the next step S 101 . 
     In step S 101 , although there is a difference between the received SV (X, Y, Z) and the SV of its own host terminal (X, Y), because its own host terminal is not holding any summary information which is not in the received SV, the exchange detection section  131  determines that it is not necessary for it to transmit an SV ( FIG. 8 : step S 101 ), and the processing performed by the exchange detection section  131  is ended. The received SV is then sent to the data storage section  121 , and the SV of its own host terminal is updated. As is shown in  FIG. 13 , because the new summary information Z has been received, the SV of the mobile communication terminal  110   b  is updated to the SV (X, Y, Z). 
     The mobile communication terminal  110   d , firstly, verifies in its exchange detection section  131  whether or not it is currently in a back-off period ( FIG. 8 : step S 100 ). Because it has sent notification for SV transmission control to the duplication determination section  133 , but the exchange detection section  131  has not yet received a response indicating that this has ended, it determines that it is currently in a back-off period, and the routine moves to step S 109  in  FIG. 8 . 
     In step S 109 , the exchange detection section  131  sends the received SV to its data storage section  121  and adjacent terminal management section  132 , and in the same way as when it received the SV from the mobile communication terminal  110   b , the adjacent terminal management section  132  updates its SV reception history, and then, as is shown in  FIG. 13 , updates the internal deduction results for the SV of the other mobile communication terminals using its internal deduction section  134  ( FIG. 8 : step S 109 ), and, as is shown in  FIG. 13 , updates the SV of its own host terminal ( FIG. 8 : step S 110 ). 
     Here, in the same way as for the calculation methods described above, in the calculations of the internal deductions ( FIG. 8 : step S 109 ), by discovering common portions in the summary information within the SV from the SV reception history, the reception situation of the other mobile communication terminals is deduced. Because the mobile communication terminal  110   d  ascertains from the reception history shown in  FIG. 13  that the summary information SV (X) and the summary information SV (X, Y) received from the mobile communication terminal  110   a  and the mobile communication terminal  110   b  are contained within the summary information SV (X, Y, Z) received from the mobile communication terminal  110   c , it determines that the mobile communication terminal  110   c , the mobile communication terminal  110   a , and the mobile communication terminal  110   b  are in positions where they are able to communicate with each other. In addition, the mobile communication terminal  110   d  determines that the contents transmitted by the mobile communicate terminal  110   c  are being received by the mobile communication terminal  110   a  and the mobile communication terminal  110   b  and, as is shown in  FIG. 13 , the internal deduction results for the SV held by the mobile communication terminal  110   a , the mobile communication terminal  110   b , and the mobile communication terminal  110   c  are updated to the SV (X, Y, Z) 
     After the mobile communication terminal  110   c  has transmitted the SV, as is described above, the mobile communication terminal  110   a , the mobile communication terminal  110   b , and the mobile communication terminal  110   d  perform processing on the newly received SV. Next, as is shown in  FIG. 11 , the mobile communication terminal  110   d , which is the last terminal to transmit an SV, ends step S 105  in  FIG. 8 , and starts the processing of the next step S 106  ( FIG. 14 ). 
     In step S 106 , at the point in time when the SV of its own host terminal is to be transmitted, the duplication determination section  133  reads from the adjacent terminal management section  132  an internal deduction result that shows the SV of the other mobile communication terminals at the current point in time, and makes a comparison in order to ascertain whether there are any differences between these and the current SV of its own host terminal, and then determines whether or not it should transmit an SV ( FIG. 8 : step S 107 ). 
     As a result, as is shown in  FIG. 14 , the duplication determination section  133  ascertains that the SV (X, Y, Z) of the mobile communication terminal  110   d  matches the SV (X, Y, Z) of all of the other mobile communication terminals  110   a  through  110   c  in the internal deduction, and that there is no summary information needing to be sent by its own host terminal. The duplication determination section  133  of the mobile communication terminal  110   d  then determines that it is not necessary for it to transmit an SV, and ends its own SV transmission control. 
     At this time, the duplication determination section  133  notifies the exchange detection section  131  that the SV transmission is completed, and cancels the back-off period that was initially set, and also stops subsequent transfers of the SV to the adjacent terminal management section  132  and stops the calculation of the internal deduction results. 
     When the SV transmissions by the mobile communication terminals  110   a  through  110   d  have ended, as is shown in  FIG. 14 , the synchronizing of the summary information between the mobile communication terminals  110   a  through  110   d  is completed, and all of the mobile communication terminals  110  are holding the SV (X, Y, Z). Thereafter, the mobile communication terminals employ periodic SV transmissions to monitor whether their summary information is synchronized with that of the other mobile communication terminals, and if a difference in the SV is generated between themselves and another mobile communication terminal, the above-described synchronization processing is executed once again. 
     If the internal deduction to deduce the SV of another mobile communication terminal is not carried out accurately, and summary information exists that cannot be shared with other mobile communication terminals in a single summary information synchronization processing operation, then the missing portion of the summary information is supplemented at the time of the next periodic SV exchange. 
     After the SV transmissions have been completed, if the Epidemic method is being used, each mobile communication terminal starts to make reception requests for contents that it lacks based on the SV, and starts the synchronizing of the contents information. If position information is used in the SV, then the synchronizing of the information is completed at the same time as the transmitting of the SV is completed. 
     By employing the above-described method it is possible to reduce transmissions of SV that are duplicated between mobile communication terminals  110 . Consequently, it is possible to solve the problem points in the existing technology that occur in situations of a high terminal density, or when one terminal is holding similar information to the other terminals. 
     In the present embodiment, a description has been given of when the network which is formed by the terminals is a wireless network, however, the present invention may also be used in other networks in which information is transmitted to all of the terminals within a communicable range, in the same way as in a wireless network. For example, in an interconnected wire network, the present invention may be used for information sharing within a broadcast domain (or a collision domain). 
       FIG. 15  shows the structure of a second embodiment. In the first embodiment a system was described in which a terminal only set a back-off period once when transmitting its own SV. In contrast to this, in the second embodiment, a back-off period is set once again at the initially set SV transmission timing, and the back-off period is calculated based on a degree of similarity that shows the proportion of elements in the information held by a particular terminal that are in common with the SV of the other mobile communication terminals. Specifically, the second embodiment differs from the first embodiment in that, instead of the back-off calculation section  135  of the exchange control section  13  of the first embodiment which was shown in  FIG. 3 , the exchange control section  13  has a recursive back-off calculation section  136 . 
     The duplication determination section  133  has the same functions as in the first embodiment, however, in the present embodiment, it may also make a new determination as to whether or not an SV transmission should be made (described below). The duplication determination section  133  makes the decision of whether or not to transmit an SV on the affirmative side if, when the degree of similarity between the SV to be transmitted by a particular terminal and the SV of all of the other mobile communication terminals in the internal deduction results is calculated, the average thereof is equal to or less than a threshold value. If it is more than the threshold value, then it is determined by that particular terminal that all of the other mobile communication terminals are already holding the same SV as itself, and that terminal may discontinue the transmission of its own SV. 
     The calculation of the degree of similarity may be made based on how many items of summary information are contained in common within all the SV. For example, if the SV is taken as one set, and the summary information within the SV of a terminal taken as one element within that set so as to form the sets (SVa and SVb), it is possible to use a degree of similarity that is calculated by dividing the number of common elements in the set SVa and the set SVb by the total number of elements in at least one of these sets (i.e., Jaccard&#39;s coefficient).
 
[Formula 2]
 
 j=|SV   a   ∩SV   b   |/|SV   a   ∪SV   b |  Jaccard&#39;s coefficient
 
     Moreover, instead of discontinuing the transmission, it is also possible to decide the SV transmission timing by providing a repeat back-off period using the recursive back-off calculation section  136 . By doing this, it is possible to give priority to the transmissions of SV from terminals that have a greater number of differences in their summary information from among the mobile communication terminals  110 , and, in one transmission, to make up for the large number of differences in the summary information between the mobile communication terminals  110 . 
     The recursive back-off calculation section  136  has the functions of reading the SV of its own host terminal from the data storage section  121 , and the internal deduction results for the SV of the other mobile communication terminals from the adjacent terminal management section  132 , and of calculating a back-off period based on the SV of its own host terminal and the SV of the other mobile communication terminals. If a transmission is to be made immediately, it sends notification to the duplication determination section  133  that there is to be no back-off period. 
     The calculation of the back-off period that is based on SV is made based on the correlation between the SV held by its own host terminal and the received SV. For example, the calculation may be a simple one as is shown by Formula (3).
 
[Formula 3]
 
back-off period=random numerical value*(β+γ*correlation of holding information)*slot  (3)
 
slot: transmission interval, β, γ: correction coefficient
 
     The correlation between the held information may also be determined using Jaccard&#39;s coefficient in the same way as the above-described calculation of the degree of similarity. 
     Formula (3) makes it possible to adjust the time distribution acquired by a terminal using the settings of the correction coefficient. 
     If the transmission time is to be brought forward, the value of β is decreased, while if the transmission time is to be delayed, the value of β is increased. 
     If the effects of the correlation of the held information on one side are to be strengthened, the value of γ may be increased, while if the effects are to be weakened, the value of γ may be decreased. By setting the back-off period based on correlations between held information, it is possible to transmit SV from terminals having the most differences in their summary information, and it is possible in one transmission to make up for the large number of differences in the summary information. 
     Moreover, it is also possible to make the calculation without using Formula (3), and to instead provide a threshold value for the correlation value derived using Jaccard&#39;s coefficient, and when this threshold value is exceeded to make the calculation using the back-off period calculation Formulas (1) and (2) that were used in the first embodiment. 
       FIG. 16  shows an operational flowchart of the second embodiment. The second embodiment is characterized in that processing for a step S 111  is added between step S 107  and step S 108  of the first embodiment. The processing performed by the exchange detection section  131 , the adjacent terminal management section  132 , the duplication determination section  133 , and the internal deduction section  134 , which are described in steps S 100  through S 103  and steps S 105  through S 110 , are the same as the operations performed by the respective function sections  131  through  134  of the first embodiment, and, therefore, any description thereof is omitted here. 
     Only those points of difference when the recursive back-off calculation section  136  of the present embodiment performs the processing of steps S 104  and S 111  will be described using the flow starting from the situation shown in  FIG. 9 , which was described in the first embodiment, up until the point when synchronization of the SV has been completed. 
     Firstly, the mobile communication terminal  110   a  transmits its own SV as a periodic SV transmission. The mobile communication terminal  110   b , the mobile communication terminal  110   c , and the mobile communication terminal  11   d  which receive the SV from the mobile communication terminal  110   a  perform the processing of steps S 100  through S 103  shown in  FIG. 16 , and in the next step S 104 , they calculate the back-off period using their recursive back-off calculation section  136 . 
     In step S 104 , the recursive back-off calculation section  136  reads the SV of its own host terminal from the data storage section  121 , and the internal deduction results from the adjacent terminal management section  132 . It then calculates an SV transmission timing based on Formula (3), and sends notification of this to the duplication determination section  133 . 
     As a result of this back-off period being calculated, in the same way as in the first embodiment, the SV transmission sequence is decided in the manner shown in  FIG. 11 , with the transmission sequence being set as, firstly, the mobile communication terminal  110   b , then the mobile communication terminal  110   c , and then the mobile communication terminal  110   d.    
     After the mobile communication terminal  110   a  has transmitted the SV, as is shown in  FIG. 11 , the mobile communication terminal  110   b , which has the earliest transmission timing from among the mobile communication terminal  110   b , the mobile communication terminal  110   c , and the mobile communication terminal  110   d , is the next to start transmitting its SV. 
     When the mobile communication terminal  110   b  has finished the steps S 105  and S 106  shown in  FIG. 16 , because it is holding summary information that is not held by the mobile communication terminal  110   a , it decides in step S 107  to transmit its own SV (X, Y). The routine then moves to the processing of step S 111 . 
     In step S 111 , the duplication detection section  133  of the mobile communication terminal  110   b  has already made the decision to transmit the SV of its own host terminal, however, it now determines whether or not to set a repeat back-off period. 
     In the resetting of the back-off period ( FIG. 16 : step S 111 ), a comparison is made by determining the internal deductions of the SV of the other mobile communication terminals and the average of the Jaccard&#39;s coefficient of the SV of its own host terminal. If the average value is low (i.e., if there are many differences in the held information, a similarity of less than 80%), then transmission takes place immediately, while if the average value is high (i.e., if there are few differences, a similarity of 80% or more), transmission does not take place immediately and a back-off period is once again provided, and communication opportunities are allocated to the other mobile communication terminals. 
     In this manner, the reason for setting a repeat back-off period is because, even if a particular terminal initially has a large number of summary information differences compared to the other mobile communication terminals, there are cases in which terminals that have already transmitted their SV ahead of that particular terminal have already transmitted the same summary information. 
     When a particular terminal is transmitting its SV, if the differences in the summary information compared to the other mobile communication terminals have decreased, then by providing a repeat back-off period, it is possible to present a communication opportunity to the next terminal having a large number of differences in the summary information that is due to transmit. As a result, it is possible using only a small number of transmissions to make up for a large number of differences in the summary information between a particular terminal and the terminals around it. 
     Thereafter, the processing is continued in the same way as in the first embodiment, and when the mobile communication terminal  110   c  and the mobile communication terminal  110   d  perform their transmissions, they synchronize SV while making the determination about the resetting of the back-off period. 
     Moreover, as was described in the first embodiment, each time a new SV is received, the determination as to whether or not the terminal should transmits its own SV, as well as the resetting of the back-off period may be performed. By resetting the back-off period each time an SV is received, it is possible to dynamically adjust the transmission timings such that the terminal having the greatest number of differences in the summary information at that point in time compared to the other mobile communication terminals is able to transmit its SV first. 
     As a result of the above, in the present embodiment, because back-off periods are set based on differences in the held information between the mobile communication terminals  110 , it is possible to give priority to the transmission of the SV from the terminal that has the greatest number of differences in its summary information compared to the other mobile communication terminals. As a result, compared to the first embodiment, it is possible using only a small number of transmissions to make up for a large number of differences in the summary information. 
     In the present embodiment, a description has been given of when the network which is formed by the multiple mobile communication terminals  110  is a wireless network, however, the present invention may also be used in other networks in which information is transmitted to all of the terminals within a communicable range, in the same way as in a wireless network. For example, in an interconnected wire network, the present invention may be used for information sharing within a broadcast domain (or a collision domain). 
       FIG. 19  shows an example of the hardware structure when the mobile communication terminal  110  is formed by an electronic information processing device such as a computer or the like. The mobile communication terminal  110  is provided with a CPU (Central Processing Unit) peripheral portion, an input/output portion, and a legacy input/output portion. The CPU peripheral portion has a CPU  902 , RAM (random Access Memory)  903 , a graphic controller  904 , and a display unit  905  that are mutually connected together via a host controller  901 . The input/output portion has a communication interface  907 , a hard disk drive  908 , and a CD-ROM (Compact Disk Read Only Memory) drive  909  that are connected to the host controller  901  via an input/output controller  906 . The legacy input/output portion has ROM (Read Only Memory)  910 , a flexible disk drive  911 , and an input/output chip  912  that are connected to the input/output controller  906 . 
     The host controller  901  connects together the RAM  903 , the CPU  902  that accesses the RAM  903  at a high transmission rate, and the graphic controller  904 . The CPU  902  operates based on programs stored in the ROM  910  and the RAM  903  so as to control the various portions. The graphic controller  904  acquires image data created by the CPU  902  and the like on a frame buffer provided in the RAM  903 , and displays this image data on the display unit  905 . Instead of this, it is also possible for the graphic controller  904  to internally include a frame buffer for storing image data created by the CPU  902  and the like. 
     The input/output controller  906  connects together the host controller  901 , the hard disk drive  908 , which is a comparatively high-speed input/output device, the communication interface  907 , and the CD-ROM drive  909 . The hard disk drive  908  stores programs and data used by the CPU  902 . The communication interface  907  connects with other mobile communication terminals  110  and exchanges programs and data. The CD-ROM drive  909  reads programs and data from a CD-ROM  992 , and then supplies them via the RAM  903  to the hard disk drive  908  and the communication interface  907 . 
     The ROM  910 , the flexible disk drive  911 , and the input/output chip  912 , which is a comparatively low-speed input/output device, are connected to the input/output controller  906 . The ROM  910  stores boot programs that are executed by the mobile communication terminal  110  when it is activated, and programs and the like that depend on the hardware of the mobile communication device  110 . The flexible disk drive  911  reads programs and data from a flexible disk  993 , and then supplies them via the RAM  903  to the hard disk drive  908  and the communication interface  907 . The input/output chip  912  is used to connect the flexible disk drive  911 , or to connect various types of input/output devices via a parallel port, a serial port, a keyboard port, a mouse port, or the like. 
     The programs executed by the CPU  902  are stored on a recording medium such as the flexible disk  993 , the CD-ROM  992 , or an IC card or the like, and are supplied by a user. The programs stored on the recording medium may be either compressed or uncompressed. The programs are installed from the recording medium onto the hard disk drive  908 , and are then read in the RAM  903  and executed by the CPU  902 . The programs executed by the CPU  902  are made to function as the wireless communication function section  11 , the information management section  12 , the exchange control section  13 , the information exchange section  111 , the data storage section  121 , the exchange detection section  131 , the adjacent terminal management section  132 , the duplication determination section  133 , the internal deduction section  134 , the back-off calculation section  135 , and the recursive back-off calculation section  136  that were described above in association with  FIG. 1  through  FIG. 16 . 
     The aforementioned programs may also be stored on an external recording medium. As the recording medium it is possible to use, in addition to the flexible disk  993  and the DC-ROM  992 , an optical recording medium such as a DVD (Digital Versatile Disk) or a PD (Phase Disk), a magneto-optical recording medium such as an MD (Mini Disk), a tape medium, or semiconductor memory such as an IC card or the like. It is also possible to use as the recording medium a storage medium such as a hard disk or RAM that is provided in a server system that is connected to a dedicated communication network or to the Internet, and to supply the information sharing system  100  via this network as a program. 
     The respective embodiments of the present invention have been described and illustrated above, however, the range of technology of the present invention is in no way limited to the range described in these embodiments. It is to be understood that it is evident to one skilled in the art that various modifications and improvements may be added to the above-described embodiments. Embodiments created by such modifications and improvements are also to be included in the range of technology of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The above-described embodiments can be applied, for example, to an information sharing system that shares information contents over an ad hoc network between multiple mobile communication terminals. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
           100  Information sharing system 
           110  Mobile communication terminal 
         L 1  Wireless communication 
           11  Wireless communication function section 
           111  Information exchange section 
           12  Information management section 
           121  Data storage section 
           13  Exchange control section 
           131  Exchange detection section 
           132  Adjacent terminal management section 
           133  Duplication determination section 
           134  Internal deduction section 
           135  Back-off calculation section 
           136  Recursive back-off calculation section 
           901  Host controller 
           902  CPU 
           903  RAM 
           904  Graphic controller 
           905  Display unit 
           906  Input/output controller 
           907  Communication interface 
           908  Hard disk drive 
           909  CD-ROM drive 
           910  ROM 
           911  Flexible disk drive 
           912  Input/output chip 
           992  CD-ROM 
           993  Flexible disk