Abstract:
The present invention relates to a relay device realizing multicast communications efficient and effective in mobile communications. The relay device transmitting a multicast user data packet containing a content to a plurality of base stations comprises a storing unit storing, in a storing unit, information about a multicast group, information about the base station and information about the zone in a way that associates these items of information with each other, a determining unit determining, when receiving a LEAVE packet via the base station, whether or not it is required to transmit a query packet about a leave target multicast group by comparing the base station stored in the way being associated with the leave target multicast group and with the zone to which the via-base-station belongs with the base station relaying the received LEAVE packet, and a transmitting unit transmitting the user data packet containing the query packet to each of the base stations belonging to the zone on the basis of a result of the determination by the determining unit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a relay device, a wireless communication system and a multicast relay method that realize multicast communications efficient and effective in mobile communications. 
         [0003]    2. Description of the Related Art 
         [0004]    WiMAX (Worldwide Interoperability for Microwave Access) is defined as a fixed wireless communication technology based on IEEE802.16. Standardization of the WiMAX technology for providing a fast wireless communication service is in progress. The WiMAX Forum® is an industry-led organization established as an organization for promoting the WiMAX technology. Non-Patent document 1 given below is a document issued by the WiMAX Forum® as a technical overview about the WiMAX technology. 
         [0005]    Non-Patent document 1 describes a purport of supporting a multi BS (Base Station)-MBS (Multicast Broadcast Service) in the WiMAX technology. Specifically, the wireless communication system using the WiMAX technology is requested to provide, through multicast communications, a variety of contents such as a news program, a sports program, a weather forecast and traffic information. 
         [0006]    IGMP (Internet Group Management Protocol) and MLD (Multicast Listener Discovery) are employed as multicast group management protocols in a way that participates in and leaves a multicast group in the multicast communications using an IP (Internet Protocol) protocol. The IGMP is the multicast group management protocol that supports IPv4, and the MLD is the multicast group management protocol that supports IPv6. 
         [0007]    The following document is a conventional art related document that discloses a system etc for supporting the multicast communications. Patent document 1 given below discloses a wireless multicast system etc. The wireless multicast system is that a router storing associative information between information representing wireless access point equipment that transmits the packet to the multicast group and information representing the multicast group, specifies the access point equipment that should establish a connection to a wireless terminal by transmitting the associative information to the wireless terminal. 
         [0008]    Further, Patent document 2 given below discloses an IP packet multicast method. According to this method, in relay nodes having a hierarchical structure, a high-order node storing low-order node information in response to a participation request given from the low-order node and deletes the information in response to a cancellation request. 
         [0009]    The Patent document 1 is a “Japanese Patent Application Laid-Open Publication No. 2004-15435”. 
         [0010]    The Patent document 2 is a “Japanese Patent Application Laid-Open Publication No. 2002-94562”. 
         [0011]    The Non-Patent document 1 is a “WiMAX Forum, “A Technical Overview and Performance Evaluation”, Mobile WiMAX-Part 1, Feb. 16, 2006”. 
         [0012]    A wireless multicast system disclosed in Patent document 1 described above is not, however, suited to a system that supports mobile communications because of making the wireless terminal desiring to receive the packet of the predetermined multicast group select the should-be-connected wireless access point equipment. 
         [0013]    Further, according to a multicast communication method disclosed in Patent document 2 given above, each of the base station (RNC) manages the information on the participating multicast group regarding each of the connected mobile terminals, and hence such a problem arises that the information managed in each base station increases in quantity and needs synchronizing between the respective base stations, resulting in intricacy of the system architecture. 
         [0014]    Further, Non-Patent document 1 does not disclose a specific method of supporting the multi BS-MBS in the WiMAX technology. 
       SUMMARY OF THE INVENTION 
       [0015]    It is an object of the present invention to provide a relay device, a wireless communication system and a multicast relay method that realize multicast communications efficient and effective in mobile communications. 
         [0016]    The present invention adopts the following configurations in order to solve the problems. Namely, the present invention is related to a relay device transmitting a user data packet, containing a content to be distributed to a mobile terminal, in which to set information about a multicast group associated with the content, to a plurality of base stations managed in the way of being grouped into zones, that comprises a storing unit storing, in a storing unit, information about a multicast group in which the mobile terminal participates, information about the base station performing wireless communications with the mobile terminal and information about the zone to which the base station belongs in a way that associates these items of information with each other, an extracting unit extracting, when receiving a LEAVE packet of a request for leaving the multicast group from the mobile terminal via any one of the plurality of base stations, the information about the base station, which is stored in the way of being associated with the leave target multicast group and with the zone to which the via-base-station belongs from the storing unit, a determining unit determining whether or not it is required to transmit a query packet for checking existence of the mobile terminal continuing the participation in the leave target multicast group by comparing the base station extracted by the extracting unit with the base station that relays the received LEAVE packet, and a transmitting unit transmitting the user data packet containing the query packet to each of the base stations belonging to the zone whose information is stored together with the information about the leave target multicast group in the storing unit on the basis of a result of the determination made by the determining unit. 
         [0017]    In the relay device according to the present invention, the information on the multicast group in which the mobile terminal participates is managed together with the information on the base station performing the wireless communications with the mobile terminal and with the information on the zone to which the base station belongs. In the case of receiving the LEAVE message from the mobile terminal, the base station stored in the storing unit in connection with the leave target multicast group and the zone to which the base station relaying the received LEAVE message belongs, is compared with the base station relaying the LEAVE message, and it is determined based on a result of this comparison whether or not the query packet needs transmitting to the target zone related to the leave target multicast group. 
         [0018]    With this scheme, according to the present invention, if determined to be different as the result of this comparison, for example, it is determined that other mobile terminals participating in the leave target multicast group still exist in this zone, the query packet about the leave target multicast group can not be transmitted to each of the base stations belonging to this zone. 
         [0019]    Therefore, according to the present invention, it is possible to reduce the futile transmission and reception of the message, for managing the multicast group, such as the query packet in that situation, and to perform the multicast communications efficient and effective in the system targeted at the mobile terminals on the premise that these terminals are to move. 
         [0020]    Further, the relay device according to the present invention may further comprise a updating unit updating, when receiving a JOIN packet of a request for participating in the multicast group from the mobile terminal via any one of the plurality of base stations, the information on the base station that is stored in the storing unit in the way of being associated with the zone to which the via-base-station belongs and with the participation target multicast group, with the information on the base station that relays the JOIN packet. 
         [0021]    Namely, according to the present invention, the participation status of the mobile terminal in the multicast group is managed in a way that sequentially updates the information about the base station relaying the JOIN message for every zone to which the base station belongs each time the JOIN message is relayed. 
         [0022]    With this scheme, according to the present invention, if the base station relaying the received LEAVE message is different from the base station stored in the storing unit in the way of being associated with the leave target multicast group and the zone, it can be determined that other base stations wirelessly connected to other mobile terminals participating in the multicast group exist in this zone. This case eliminates a necessity of sending the query packet to this zone, and therefore the futile messages can be reduced. 
         [0023]    Further, the relay device according to the present invention may further comprise a deleting unit deleting from the storing unit, if a response packet is not received when the transmitting unit has transmitted the user data packet containing the query packet, a record containing the leave target multicast group, the base station that relays the LEAVE packet and the zone to which the base station belongs in the way of their being associated with each other. 
         [0024]    Still further, the relay device according to the present invention may further comprise a transferring unit checking, when the record is deleted by the deleting unit, existence of other records stored in the storing unit in the way of being associated with the multicast group contained in the deleted record, transferring the received LEAVE packet to a high-order device if no other records exist, and transferring none of the received LEAVE packet if other records exist. 
         [0025]    According to the present invention, the existence of the mobile terminals under other zones, which participate in the leave target multicast group related to the received LEAVE packet, can be checked based on the information stored in the storing unit, and hence the LEAVE packet can be transferred to the high-order device only in the case where none of the mobile terminals participate in that multicast group in all of the zones. 
         [0026]    Therefore, according to the present invention, the futile messages between the relay device and the high-order device can be eliminated, and hence the highly efficient multicast communications can be realized. 
         [0027]    Yet further, the present invention is an invention related to a wireless communication system comprising the relay device and the base station device, and is also an invention related to a multicast relay method by which a computer etc is made to actualize any one of the functions of the relay device. Still further, the present invention may also be a program that realizes any one of the functions described above, and may further be a readable-by-computer storage medium recorded with such a program. 
         [0028]    According to the present invention, it is possible to provide the relay device, the wireless communication system and the multicast relay method that realize the multicast communications efficient and effective in the mobile communications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]      FIG. 1  is a view illustrating a system architecture of a multi BS-MBS system in the present embodiment; 
           [0030]      FIG. 2  is a diagram illustrating a functional configuration of an ASN gateway; 
           [0031]      FIG. 3  is a diagram showing an example of a MBS zone information table within a MBS zone management database; 
           [0032]      FIG. 4  is a diagram showing a multicast group information table within the multicast group management database; 
           [0033]      FIG. 5  is a diagram showing an example of a tunnel information table within a tunnel information database; 
           [0034]      FIG. 6  is a diagram showing a JOIN message sent to an ASN gateway from a base station; 
           [0035]      FIG. 7  is a diagram showing the JOIN message sent to an edge router from the ASN gateway; 
           [0036]      FIG. 8  is a diagram showing a membership query (Group Specific) message sent to the base station from the ASN gateway; 
           [0037]      FIG. 9  is a diagram showing a membership report message sent to the ASN gateway from the base station as a response to the GS-query message; 
           [0038]      FIG. 10  is a diagram showing a LEAVE message sent to the ASN gateway from the base station; 
           [0039]      FIG. 11  is a diagram showing the LEAVE message sent to the edge router from the ASN-gateway; 
           [0040]      FIG. 12  is a diagram showing multicast data transmitted from a CSN and distributed to mobile terminals via the edge router, the ASN gateway and the base station; 
           [0041]      FIG. 13  is a diagram showing a process when receiving the JOIN message; 
           [0042]      FIG. 14  is a diagram showing a process when sending the GS-query message; 
           [0043]      FIG. 15  is a diagram showing a first process pattern when receiving a LEAVE message; and 
           [0044]      FIG. 16  is a diagram showing a second process pattern when receiving the LEAVE message. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment  
       [0045]    A multi BS-MBS system (which will hereinafter simply be referred to as a MBS system) in an embodiment of the present invention will hereinafter be described with reference to the drawings. It should be noted that a configuration in the following embodiment is an exemplification, and the present invention is not limited to the configuration in the embodiment. Further, the following discussion will deal with only a multicast communication service by the MBS system in the present embodiment, however, the MBS system may include other functions. 
       System Architecture and Device Configuration  
       [0046]    A system architecture of the MBS system in the present embodiment will be explained with reference to  FIG. 1 .  FIG. 1  is a view illustrating the system architecture of the MBS system in the present embodiment. 
         [0047]    The MBS system in the present embodiment is built up by connecting a CSN (Connectivity Service Network)  10  and an ASN (Access Service Network)  15  to each other via an edge router  20 . Note that the MBS system in the present embodiment may be configured to include a plurality of CSNs  10  and a plurality of ASNs  15  respectively, and the individual networks may be connected with no intermediary of the edge router  20 . 
       CSN  
       [0048]    The CSN  10  is configured by connecting a router, an AAA (Authentication Authorization Accounting) proxy server, a user database, an interworking gateway, etc, respectively (unillustrated). The CSN  10  provides an IP (Internet Protocol) connection service etc for a WiMAX subscriber (e.g., a mobile terminal  60 ) provided with a WiMAX service. Further, the CSN  10  provides a variety of contents such as a news program, a sport program, a weather forecast and traffic information as the WiMAX service through the multicast communications. The respective contents to be provided are assigned multicast addresses different from each other, and the CSN  10  manages a relative table between the respective contents and the multicast addresses. This relative table may be retained adjustably beforehand in the memory etc of the CSN  10 . The description of the communication service provided by the CSN  10  is about a service based on the multicast communications, however, the MBS system can provide other types of communication services. 
         [0049]    The CSN  10  is connected to the ASN  15  via the edge router  20 . This connection involves utilizing, e.g., the IP. The CSN  10  transmits, to the edge router  20 , a should-be-provided content as a multicast packet having a setting of a multicast address assigned with respect to the content. 
       Edge Router  
       [0050]    The edge router  20  is connected to a plurality of ASN gateways  30  under management and manages information on the multicast communications for distributing the content transmitted from the CSN  10 . The edge router  20  treats each of the ASN gateways  30  under the management as a host participating in a multicast group. The edge router  20  independently processes a multicast group management message, thereby controlling a procedure for each ASN gateway  30  to participate in or leave the predetermined multicast group. 
       ASN  
       [0051]    The ASN  15  is configured by connecting the ASN gateway  30  and the base stations  50 . The mobile terminal  60  performs wireless communication with the base station  50  covering a communication-enabled area where the mobile terminal  60  exists, thereby connecting the MBS system according to the present embodiment and thus getting provided with the WiMAX service. Each of the devices constructing the ASN  15  will hereinafter be described. 
       ASN Gateway  
       [0052]    The ASN gateway  30  is a gateway device including a CPU (Central Processing Unit), a memory, an input/output interface, etc. The ASN gateway  30  is connected to the plurality of base stations  50  under management. An example in  FIG. 1  is that the ASN gateway  30  is connected to each of base stations BS 1 , BS 2 , BS 3 , BS 4  and BS 5 . The ASN gateway  30  groups the base stations  50  under the management into predetermined MBS zones, thus controlling the multicast communications according to the MBS zone. The example in  FIG. 1  shows that the base stations BS 1 , BS 2  and BS 3  are managed as a MBS zone # 1 , and the base stations BS 4  and BS 5  are managed as a MBS zone # 2 . 
         [0053]    A device configuration of the ASN gateway  30  will hereinafter be explained with reference to  FIG. 2 .  FIG. 2  is a block diagram illustrating the functional configuration of the ASN gateway  30 . The ASN gateway  30  has a high-order communication unit  201 , a multicast control unit  202 , a MBS zone management unit  203 , a multicast group management unit  204 , a tunneling control unit  208 , a MBS zone management database  205 , a multicast group management database  206 , a tunnel information database  209 , etc. 
         [0054]    The high-order communication unit  201  controls the multicast communications with the edge router  20 . To be specific, the high-order communication unit  201  receives the multicast packet containing the content information transmitted from the edge router  20 , and transfers the received multicast packet to the multicast control unit  202 . Further, the high-order communication unit  201  transmits a packet for multicasting to the edge router  20 . Note that the high-order communication unit  201  may process data other than the multicast packet. 
         [0055]    The multicast control unit  202  works in cooperation with the high-order communication unit  201 , the MBS zone management unit  203 , the multicast group management unit  204  and the tunneling control unit  208 , thereby controlling the multicast communications for distributing the content information transmitted from the CSN  10  to the mobile terminals  60 . Each mobile terminal  60  needs to participate in the predetermined multicast group in order to receive distribution of a desired content. The multicast control unit  202  controls the procedure for each mobile terminal  60  to participate in or leave the predetermined multicast group. For instance, the multicast control unit  202  controls IGMP (Internet Group Management Protocol) or MLD (Multicast Listener Discovery) utilized as the procedure described above. The multicast control unit  202  processes a JOIN message (membership report message), a LEAVE message, a membership query (Group Specific) message (which will hereinafter be referred to as a GS-query message), a membership query (General) message (which will hereinafter be termed a G-query message), etc as multicast group management messages used by the protocol given above. 
         [0056]    Moreover, the multicast control unit  202  processes the multicast data containing the content information transmitted from the CSN  10 . The multicast control unit  202  transfers, in the multicast group management message and the multicast data, the multicast packet that should be transmitted to the edge router  20  to the high-order communication unit  201  and transfers the multicast packet that should be transmitted to the mobile terminal  60  to the tunneling control unit  208  together with the information about the MBS zone serving as a transmission destination. In-depth descriptions of the multicast group management message and the multicast data, which are to be processed in the multicast control unit  202 , will be made later on. 
         [0057]    The MBS zone management unit  203  refers to the MBS zone management database  205  and thus manages the information on the zones to which the respective base stations  50  belong. The MBS zone management unit  203  refers to a MBS zone information table within the MBS zone management database  205 .  FIG. 3  is a diagram showing an example of the MBS zone information table within the MBS zone management database  205 . The MBS zone information table retains pieces of information on the MBS zones to which the respective base stations belong. The MBS zone information table may be previously set up and may also be updated dynamically through the communications. 
         [0058]    The multicast group management unit  204  manages, based on the multicast group management message processed by the multicast control unit  202 , the respective pieces of information about the multicast groups in which the mobile terminals  60  within the MBS zones participate with respect to every MBS zone. This management involves making use of the multicast group management database  206 . 
         [0059]      FIG. 4  is a diagram showing a multicast-group information table within the multicast group management database  206 . The multicast group management unit  204  retains, for every multicast group and every MBS zone, a multicast address, MBS zone information and last responder information in the multicast group information table. What is retained as the last responder information is information about the base station that relayed last a request for the participation (JOIN message) in the same multicast group within the same MBS zone. 
         [0060]    The tunneling control unit  208  controls how a GRE (Generic Routing Encapsulation) tunnel is generated and cancelled, and processes the multicast communications with the base station  50  via the GRE tunnel. Specifically, the tunneling control unit  208  transmits the multicast packet sent from the multicast control unit  202  to the predetermined base station  50  via the GRE tunnel. At this time, the tunneling control unit  208  similarly, if the multicast packet is either the multicast group management message or the multicast data, transmits the multicast packet to the predetermined base station  50  via the GRE tunnel. Further, the tunneling control unit  208  receives a frame sent via the GRE tunnel from the base station  50  and transfers the received frame data to the multicast control unit  202 . In the MBS system, when the multicast group management message and the multicast data are transferred and received between the CSN  10  and the mobile terminal  60 , the communications between the base station  50  and the ASN gateway  30  involve using the GRE tunnel. Note that details of the GRE tunneling protocol are as defined by RFC2784 and RFC2890, and its description is herein omitted. 
         [0061]    The tunneling control unit  208  at the initial operation time generates, based on the information in the tunnel information database  209 , a downlink GRE tunnel utilized for the multicast communications flowing to each base station  50 . The downlink GRE tunnels utilized for the communications flowing to the base stations  50  from the ASN gateway  30  are generated by, e.g., a numerical value obtained from multiplying the number of the base stations  50  connected to the ASN gateway  30  by the number of the contents to be provided. 
         [0062]      FIG. 5  is a diagram showing an example of a tunnel information table within the tunnel information database  209 . The tunnel information table stores items of information about the should-be-generated GRE tunnels. The items of information stored in this tunnel information table are a GRE key defined as an identifier, a multicast address utilizing the GRE tunnel and should-be-generated destination MBS zone information (ZONE# 1 , ZONE# 2 , ZONE# 3  in  FIG. 5 ) with respect to every GRE tunnel. 
         [0063]    The tunneling control unit  208 , when receiving the should-be-transmitted multicast packet and the information on the transmitting destination MBS zone from the multicast control unit  202 , requests the MBS zone management unit  203  to acquire the information about the base stations  50  belonging to the transmitting destination MBS zone. The tunneling control unit  208  copies the multicast packet by a number corresponding to the acquired number of the base stations  50 . The tunneling control unit  208  attaches a predetermined header such as a GRE header to the copy of the multicast packet, and thereafter transmits the thus-generated frame via the GRE tunnel established between each of the base stations  50  and the ASN gateway  30 . 
         [0064]    At this time, the tunneling control unit  208  sets timing information associated with the transmitting destination MBS zone in this GRE header. The timing information is used for the individual base stations  50  belonging to the same MBS zone to simultaneously wirelessly transmit the same message sent from the ASN gateway  30 . Further, the timing information is generated from maximum delay time in each MBS zone, which is calculated based on processing delay time, communication delay time, etc collected from each of the base stations  50  belonging to the individual MBS zones by, e.g., an unillustrated function unit. It should be noted that the present invention does not limit a method of generating the timing information, and this timing information may be enough if used for the same multicast data to be simultaneously transmitted through the wireless communications from the base stations belonging to the same MBS zone. 
         [0065]    Each of the messages processed by the multicast control unit  202  will hereinafter be described with reference to  FIGS. 6 through 12 . 
         [0066]      FIG. 6  is a diagram illustrating the JOIN message sent to the ASN gateway  30  from the base station  50 . The JOIN message is a message sent, on the occasion of participating in the multicast group, from the mobile terminal  60  in a way that assigns a multicast address of a want-to-participate-in multicast group, and a message type of the JOIN message is the same as of a membership report message.  FIG. 6  illustrates a frame structure when the base station  50  receiving the JOIN message from the mobile terminal  60  transmits this JOIN message via the GRE tunnel. The frame is formed by attaching predetermined headers  302  (the GRE header, an IP header and a L2 (Layer-2) header) to a membership report packet  301  sent from the mobile terminal  60 . A multicast address of the want-to-participate-in multicast group is entered in a group address field of the membership report packet  301 . 
         [0067]    The multicast control unit  202 , upon receiving the JOIN message, refers to a SA (Sender Address) field of the IP header and thus specifies the sender base station. The multicast control unit  202  acquires the MBS zone, to which the specified base station belongs, from the MBS zone management unit  203 . The multicast control unit  202  extracts the multicast address from the group address field in an IGMP part of the received membership report packet  301 . The multicast control unit  202  requests the multicast group management unit  204  to check whether or not the extracted multicast address has already been stored in the multicast group information table. 
         [0068]    If the multicast address has already been stored in the multicast group information table with respect to the MBS zone, the multicast control unit  202  transfers, to the multicast group management unit  204 , the respective pieces of information about the sender base station  50  and the MBS zone to which the sender base station  50  belongs. The multicast group management unit  204  updates, based on these pieces of information, the last responder information into the information about the sender base station  50  in a record of the target multicast address and the target MBS zone within the multicast group information table. 
         [0069]    Note that if only the records of the different MBS zones exist even when the target multicast address is stored, the multicast group management unit  204  adds a new record to the multicast group information table and thus registers these pieces of information. 
         [0070]    Whereas if the multicast address is not yet set in the multicast group information table, the multicast control unit  202  extracts the membership report packet  301  from the frame, then sets an IP address of the self-device (the ASN gateway  30 ) in the SA field of the IP header part of the extracted packet, and transfers this packet to the high-order communication unit  201  so as to forward the same packet to the edge router  20 .  FIG. 7  is a diagram showing the JOIN message sent to the edge router  20  from the ASN gateway  30 . As shown in  FIG. 7 , the membership report packet, in which the ASN gateway  30  is set as Sender, is attached with the L2 header and is then transmitted to the edge router  20 . 
         [0071]    Next,  FIG. 8  is a diagram illustrating a membership query (Group Specific) message transmitted to the base station  50  from the ASN gateway  30 . The GS-query message is a message sent for checking existence of the mobile terminal  60  (receiving the multicast message of which a specified multicast address is designated) participating in a specified multicast group. The ASN gateway  30  sends the GS-query message related to each multicast group set in the multicast group information table to each of the MBS zones set in the same table. 
         [0072]    At this time, the multicast control unit  202  generates a GS-query packet  701  as illustrated in  FIG. 8 . In the GS-query packet  701 , an existence checking target multicast address is set in a group address field of the IGMP part and in a DA (Destination Address) field of the IP header part. The multicast control unit  202  transfers the generated GS-query packet  701  to the tunneling control unit  208 . The tunneling control unit  208  copies the GS-query packet  701 , corresponding to the number of the base stations belonging to the transmitting destination MBS zone, then attaches predetermined headers  702  to each of the copies of the GS-query packet  701 , and transmits each of the thus-generated frames to the individual base stations  50  via the GRE tunnels. At this time, the tunneling control unit  208  sets the timing information associated with the transmitting destination MBS zone in the GRE header part of the headers  702 . 
         [0073]      FIG. 9  is a diagram showing the membership report message sent to the ASN gateway  30  from the base station  50  as a response to the GS-query message. The membership report message is a message sent from the mobile terminal  60  if the mobile terminal  60  receiving the GS-query message continues to participate in the multicast group set therein.  FIG. 9  illustrates a frame structure on such an occasion that the base station  50  receiving the membership report message from the mobile terminal  60  sends this message via the GRE tunnel. This frame is formed by attaching predetermined headers  802  (a GRE header, an IP header and a L2 header) to a membership report packet  801  transmitted from the mobile terminal  60 . A multicast address continuing its participation is set in a DA field and in a group address field of the membership report packet  801 . 
         [0074]    The multicast control unit  202 , upon receiving the membership report message, checks the existence of the mobile terminal  60  continuing the participation about the multicast address set in the group address field of the IGMP part of the membership report packet  801 . Thereafter, the multicast control unit  202  transfers the membership report packet  801  to the tunneling control unit  208 . The tunneling control unit  208  attaches predetermined headers to the membership report packet  801 , and transmits the thus-generated frame via the GRE tunnels to the individual base stations  50  belonging to the MBS zone, which are registered in the multicast group. Owing to this scheme, the multicast control unit  202  recognizes that the mobile terminals  60  existing in the same MBS zone and participating in the same multicast group have no necessity of sending the membership report message any more. 
         [0075]    While on the other hand, the multicast control unit  202 , if receiving none of the membership report message for a predetermined period of time, recognizes that there exist none of the mobile terminals  60  participating the multicast group in that MBS zone, and notifies the multicast group management unit  204  of this purport. The multicast group management unit  204  deletes the record related to the multicast group about the MBS zone concerned. Note that the multicast control unit  202  may retransmit, in the case of recognizing that any mobile terminal  60  participating in the multicast group does not exist, the GS-query message a plural number of times. 
         [0076]    Next,  FIG. 10  is a diagram illustrating a LEAVE message sent to the ASN gateway  30  from the base station  50 . The LEAVE message is a message sent from the mobile terminal  60  on the occasion of leaving the participating multicast group.  FIG. 10  illustrates a frame structure on such an occasion that the base station  50  receiving the LEAVE message from the mobile terminal  60  sends the LEAVE message via the GRE tunnel. This frame is formed by attaching predetermined headers  902  (a GRE header, an IP header and a L2 header) to a LEAVE packet  901  transmitted from the mobile terminal  60 . A multicast address of a want-to-leave is entered in a group address field of the LEAVE packet  901 . 
         [0077]    The multicast control unit  202 , when receiving the LEAVE message, specifies the sender base station by referring to the SA field of the IP header. The multicast control unit  202  acquires the MBS zone, to which the specified base station belongs, from the MBS zone management unit  203 . The multicast control unit  202  extracts the multicast address from the group address field of the IGMP part of the received LEAVE packet  901 . The multicast control unit  202  requests the multicast group management unit  204  to check based on the extracted multicast address whether the last responder in the MBS zone is identified with the sender base station or not. 
         [0078]    The multicast control unit  202 , if the sender base station is not identical with the last responder, directly terminates the process on the assumption that there exist other mobile terminals  60  participating in the multicast group. Whereas if the sender base station is the last responder, the multicast control unit  202  sends the GS-query message to the MBS zone where this base station exists in order to check whether other mobile terminals  60  participating in the multicast group exist in this MBS zone or not. The process of sending the GS-query message is the same as described above. Through this process, it is confirmed that there exists no other mobile terminal  60  participating in the LEAVE message target multicast group, and a record related to this multicast group is deleted from the multicast group information table. 
         [0079]    The multicast control unit  202 , when recognizing from the LEAVE message that the record related to the multicast group concerned is deleted from the multicast group information table, in other words, that there exists none of the mobile terminal  60  participating in this multicast group in the MBS zone concerned, extracts the LEAVE packet  901  from the frame, then enters an IP address of the self-device in the SA field of the IP header part of the extracted packet, and transfers the packet to the high-order communication unit  201  so as to forward this packet to the edge router  20 .  FIG. 11  is a diagram illustrating the LEAVE message sent to the edge router  20  from the ASN gateway  30 . As illustrated in  FIG. 11 , the LEAVE packet, in which the ASN gateway  30  is set as Sender, is attached with the L2 header and is then sent to the edge router  20 . 
         [0080]      FIG. 12  is a diagram showing multicast data transmitted from the CSN  10  and distributed to the mobile terminal  60  via the edge router  20 , the ASN gateway  30  and the base station  50 . Multicast data  1201  transmitted from the CSN  10 , of which a multicast address associated with content information contained therein is set, is sent to the ASN gateway  30  via the edge router  20 . The high-order communication unit  201  of the ASN gateway  30 , when receiving the multicast data, transfers this data to the multicast control unit  202 . 
         [0081]    The multicast control unit  202  extracts the multicast address set in the DA field of the multicast data, and requests the multicast group management unit  204  for information about the MBS zone where the mobile terminal  60  participating in the multicast address exists. The multicast control unit  202  sends the information about the MBS zone, of which the multicast group management unit  204  notifies, to the tunneling control unit  208  together with the multicast data  1201 . 
         [0082]    The tunneling control unit  208  copies the multicast data  1201  by a count corresponding to the number of the MBS zones and corresponding to the number of the base stations belonging to each of the MBS zones, attaches predetermined headers  1202  to each of the copies of the multicast data  1201 , and transmits the thus-generated frames to the respective base stations  50  via the GRE tunnels. At this time, the tunneling control unit  208  sets the timing information corresponding to the transmitting destination MBS zone in the GRE header part of the headers  1202 . The respective base stations  50  receiving this frame within the same MBS zone wirelessly transmit the multicast data  1201  of the frame at the same timing specified by the timing information. The MBS system according to the present embodiment attaches the same headers to the multicast data when transmitted to the base station and to the previous multicast group management message also when transmitted to the base station. 
       Base Station  
       [0083]    The base station  50  includes a CPU (Central Processing Unit), a memory, an input/output interface, etc, and the mobile terminals  60  within a communication area covered by the base station  50  are connected to the MBS system through the wireless communications. Further, the base station  50  is connected to the predetermined ASN gateway  30  by a cable, forwards the signals (the multicast packet) transmitted from the mobile terminal  60  to the ASN gateway  30  via the GRE tunnel, and wirelessly transmits the multicast packet sent via the GRE tunnel from the ASN gateway  30  to the mobile terminal  60 . Note that a process of establishing and cancelling the wireless link between the base station  50  and the mobile terminal  60  and a process of assigning the IP address are the same as by the well-known technologies, and hence their explanations are omitted. 
         [0084]    The base station  50  retains information (e.g., an IP address etc) about the ASN gateway  30  that manages the self-device. The base station  50  at the initial operating time generates an uplink GRE tunnel employed for the multicast communications with the ASN gateway  30  by use of the GRE tunneling protocol defined by RFC2784 and RFC2890. 
         [0085]    The base station  50 , when receiving the membership report packet  301  (see  FIG. 6 ) (a response message to the JOIN message and to the GS-query message) and the LEAVE packet  901  (see  FIG. 10 ) transmitted from the mobile terminal  60 , generates the frame by attaching the predetermined headers  302  (the L2 header, the IP header and the GRE header) to these packets, and transmits this frame to the ASN gateway  30 . At this time, the base station  50  sets an IP address of the self-device in the SA field of the IP header and sets an IP address of the ASN gateway  30  in the DA field of the IP header. 
         [0086]    The base station  50 , in the case of receiving the frame transmitted via the GRE tunnel from the ASN gateway  30 , extracts the multicast packet (the GS-query packet  701  (see  FIG. 8 ), the multicast data  1201  (see  FIG. 12 )) from the frame. Subsequently, the base station  50  sends the extracted multicast packet through a predetermined wireless channel at the time specified by the timing information set in the GRE header of the same frame. The timing information set in the multicast packet transmitted via the GRE tunnel from the ASN gateway  30  and in the GRE header, is the same on an every-MBS-zone basis. With this setting of the timing information, the base stations  50  belonging to the same MBS zone transmit the same multicast packet almost simultaneously. 
       Mobile Terminal 
       [0087]    The mobile terminal  60  is a terminal device including a CPU (Central Processing Unit), a memory, an input/output interface, etc and is exemplified such as a mobile phone, a PDA (Personal Digital Assistant) and a personal computer. The mobile terminal  60  has a multicast communication function and a wireless communication function. The mobile terminal  60  performs the wireless communications with the base station  50  that covers a communication area embracing a location of the mobile terminal, corresponding to where the mobile terminal  60  exists. The mobile terminal  60  receives a variety of contents distributed by multicasting from the CSN  10  via the base station  50  with which the wireless link is established. Each mobile terminal  60  participates in the multicast group for a desired content, thereby receiving each content. 
         [0088]    The mobile terminal  60  may download and thus retain a content-to-multicast-address relative table held previously by the CSN  10  in order to participate in a desired multicast group. Further, the mobile terminal  60  may also previously acquire an IP address for connecting to the MBS system. The present invention does not limit the method of knowing the multicast address of the content, the method of acquiring the IP address, etc, and hence their explanations are herein omitted. 
       Operational Example  
       [0089]    An operational example of the MBS system in the present embodiment will hereinafter be described with reference to  FIGS. 13 and 14 . 
         [0090]    The description using  FIG. 13  will start with the operation in the case of transmitting the JOIN message from the mobile terminal  60 .  FIG. 13  is a sequence diagram showing a process when receiving the JOIN message, wherein the system architecture illustrated in  FIG. 1  is exemplified. Exemplified is a case in which as the base stations  50 , the BS 1 , BS 2  and BS 3  are defined to belong to the MBS zone # 1 , the BS 4  and BS 5  are defined to belong to the MBS zone # 2 , the MS 1  as the mobile terminal  60  exists within the communication area of the BS 2 , and the MS 3  and MS 4  as the mobile terminals  60  exist within the communication area of the BS 5 . 
         [0091]    The MS 1  desires to participate in a multicast group (224.22.3.45), and wirelessly transmits the JOIN message in which the multicast address (224.22.3.45) is set (S 1301 ). 
         [0092]    The BS 2 , upon receiving the JOIN message signal, extracts a membership query report packet from this signal. The BS 2  transmits a frame generated by attaching predetermined headers to the membership query report packet to the ASN gateway  30  via the GRE tunnel (S 1302 ). At this time, the BS 2  sets an IP address of the base station itself in the SA field within the predetermined header and sets an IP address of the ASN gateway  30  in the DA field. 
         [0093]    The ASN gateway  30 , when receiving this frame via the GRE tunnel, extracts the membership query report packet within the frame. Furthermore, the ASN gateway  30  specifies the sender BS 2  on the basis of the IP address set in the SA field of the header of this frame, and recognizes based on the MBS zone information table ( FIG. 3 ) that the MBS zone, to which the BS 2  belongs, is the MBS zone # 1 . Subsequently, the ASN gateway  30  checks whether or not an address/zone pair of the multicast address (224.22.3.45) set in the group address field of the IGMP part in the frame and the MBS zone # 1 , is registered in the multicast group information table ( FIG. 4 ) (S 1303 ). 
         [0094]    The ASN gateway  30 , when confirming that the address/zone pair of the multicast address (224.22.3.45) and the MBS zone # 1  is not registered, adds a new record to the multicast group information table and registers these pieces of information (S 1304 ). To be specific, such a new record is added to the multicast group information table that [224.22.3.45] is set in the multicast address field, [MBS zone # 1 ] is set in the MBS zone field, and [BS 2 ] is set in the last responder field. 
         [0095]    The ASN gateway  30 , thereafter, attaches the L2 header to the extracted membership query report packet and thus transmits the packet to the edge router  20  (S 1305 ). Hereat, the ASN gateway  30  sets an IP address of the gateway itself in the SA field of the IP header within this packet. With this address setting, the edge router  20  receiving this packet recognizes from the IP address set in the SA field that the ASN gateway  30  participates in the multicast group (224.22.3.45). 
         [0096]    Next, a process, in which the plurality of mobile terminals MS 3  and MS 4  in the same MBS zone transmits the JOIN message, will be explained. 
         [0097]    At first, an assumption is that the MS 3  desires to participate in a multicast group (224.0.10.15) and wirelessly transmits the JOIN message in which to set the multicast address (224.0.10.15) (S 1310 ). 
         [0098]    The BS 5 , upon receiving this JOIN message signal, extracts the membership query report packet from this signal. The BS 5  transmits a frame generated by attaching predetermined headers to the membership query report packet to the ASN gateway  30  via the GRE tunnel (S 1311 ). At this time, the BS 5  sets an IP address of the base station itself in the SA field of the predetermined header, and sets an IP address of the ASN gateway  30  in the DA field. 
         [0099]    The ASN gateway  30 , when receiving the frame via the GRE tunnel, extracts the membership query report packet within the frame. Further, the ASN gateway  30  specifies the sender BS 5  on the basis of the IP address set in the SA field of the header of this frame, and recognizes from the MBS zone information table that the MBS zone, to which the BS 5  belongs, is the MBS zone # 2 . Subsequently, the ASN gateway  30  checks whether or not an address/zone pair of the multicast address (224.0.10.15) set in the group address field of the IGMP part in the frame and the MBS zone # 2 , is registered in the multicast group information table (S 1312 ). 
         [0100]    The ASN gateway  30 , when confirming that the address/zone pair of the multicast address (224.0.10.15) and the MBS zone # 2  is not registered, adds a new record to the multicast group information table and registers these pieces of information (S 1313 ). Specifically, such a new record is added to the multicast group information table that [224.0.10.15] is set in the multicast address field, [MBS zone # 2 ] is set in the MBS zone field, and [BS 5 ] is set in the last responder field. 
         [0101]    The ASN gateway  30 , thereafter, attaches the L2 header to the extracted membership query report packet and thus transmits the packet to the edge router  20  (S 1314 ). 
         [0102]    Next, it is assumed that the MS 4  desires to participate in the same multicast group (224.0.10.15) as the MS 3  does and wirelessly transmits the JOIN message in which the multicast address (224.0.10.15) is set (S 1320 ). 
         [0103]    The BS 4 , upon receiving the JOIN message signal, extracts the membership query report packet from this signal. The BS 4  sends the frame generated by attaching the predetermined headers to the membership query report packet to the ASN gateway  30  via the GRE tunnel (S 1321 ). 
         [0104]    The ASN gateway  30 , when receiving this frame via the GRE tunnel, extracts the membership query report packet in the frame. Moreover, The ASN gateway  30 , as in the case of the MS 3 , specifies the sender BS 4  of the frame, and recognizes that the MBS zone, to which the BS 4  belongs, is the MBS zone # 2 . Subsequently, the ASN gateway  30  checks whether or not the address/zone pair of the multicast address (224.0.10.15) set in the group address field of the IGMP part in the frame and the MBS zone # 2 , is registered in the multicast group information table (S 1322 ). 
         [0105]    The ASN gateway  30 , when confirming that the address/zone pair of the multicast address (224.0.10.15) and the MBS zone # 2  has already been registered, updates a value in the last responder field in the target record of the multicast group information table into [BS 4 ] (S 1323 ), and terminates the process. With this scheme, the ASN gateway  30 , in the case of receiving the JOIN message with respect to the already-registered multicast group in the same MBS zone, does not send the JOIN message to the edge router  20 . 
         [0106]    Next, an operation, in such a case that the ASN gateway  30  checks existence of the mobile terminal  60  participating in the multicast group set in the multicast group information table, will be described with reference to  FIG. 14 .  FIG. 14  is a sequence diagram showing a process when sending the GS-query message, wherein the system architecture illustrated in  FIG. 1  is exemplified. 
         [0107]    The multicast group information table of the ASN gateway  30  shall be in a status shown in  FIG. 4 . The ASN gateway  30  checks the existence of each of the multicast groups entered in the multicast group information table on a predetermined cycle. 
         [0108]    To be specific, the ASN gateway  30 , for checking the existence of the multicast group (224.0.10.15), generates the GS-query packet by setting the multicast address (224.0.10.15) in the group address field of the IGMP part and in the DA field of the IP header part. The ASN gateway  30  recognizes that the MBS zone registered with respect to the multicast address is the MBS zone # 2 , and specifies the BS 4  and BS 5  as those belonging to the MBS zone # 2 . The ASN gateway  30  specifies the GRE tunnels generated in regard to the multicast address (224.0.10.15) between the specified base stations BS 4 , BS 5  and the ASN gateway  30 . 
         [0109]    The ASN gateway  30  copies the GS-query packet generated earlier, and attaches predetermined headers set for a recipient base station to each GS-query packet. The ASN gateway  30  sends the thus-generated frames to the BS 4  and BS 5  via the respective GRE tunnels (S 1401 ). The timing information set in the GRE header part of each of the frames transmitted contains the same information. 
         [0110]    The BS 4  and BS 5  receiving the frames simultaneously transmit the GS-query packets in these frames through predetermined wireless channels at the timing specified by the timing information set in the GRE header parts (S 1402 ). 
         [0111]    The MS 3  and MS 4  receive the GS-query packet sent from any one of the BS 4  and BS 5 , corresponding to their locations. The MS 3  and MS 4 , because of the same data being transmitted substantially simultaneously from the BS 4  and BS 5  during their movements, receive only the data transmitted from any one of the base stations. Herein, such an example is given that both of the MS 3  and MS 4  continuously participate in the multicast group (224.0.10.15), and the MS 3  sends, ahead of MS 4 , the membership query report packet as a response to the GS-query. 
         [0112]    The MS 3 , for indicating the continuous participation in the multicast group (224.0.10.15), wirelessly transmits a query response message in which to set the multicast address (224.0.10.15) (S 1403 ). 
         [0113]    The BS 5 , when receiving this query response message signal, extracts the membership query report packet from this signal. The BS 5  sends a frame generated by attaching predetermined headers to the membership query report packet to the ASN gateway  30  via the GRE tunnel (S 1404 ). Hereat, the BS 5  sets a self IP address in the SA field of the predetermined header and sets an IP address of the ASN gateway  30  in the DA field. 
         [0114]    The ASN gateway  30 , when receiving the frame via the GRE tunnel, extracts the membership query report packet in the frame. The ASN gateway  30  refers to the multicast address (224.0.10.15) set in the group address field of the IGMP part within the frame. Furthermore, the ASN gateway  30  specifies the sender BS 5  of this frame, and recognizes from the MBS zone information table that the MBS zone, to which the BS 5  belongs, is the MBS zone # 2 . With this scheme, the ASN gateway  30 , as a response to the GS-query message sent earlier, checks that the mobile terminal  60  continuing the participation exists in the MBS zone # 2  with respect to the multicast address (224.0.10.15). 
         [0115]    After checking, the ASN gateway  30  specifies the BS 4  and BS 5  as the base station belonging to the MBS zone # 2 , and specifies the GRE tunnels generated about the multicast address (224.0.10.15) between the specified BS 4 , BS 5  and the ASN gateway  30 . The ASN gateway  30  copies the extracted membership query report packet as it is, and, after attaching predetermined headers to each of the copies of the packet, transmits the thus-generated frames to the BS 4  and BS 5  via the specified GRE tunnels (S 1405 ). The timing information set in the GRE header part of each of the frames transmitted contains the same information. 
         [0116]    The BS 4  and BS 5  receiving the frames simultaneously transmit the membership query report packets in these frames through predetermined wireless channels at the timing specified by the timing information set in the GRE header parts (S 1406 ). 
         [0117]    The MS 3  and MS 4  receive the membership query report packets sent from any one of the BS 4  and BS 5 , corresponding to their locations. The MS 4  thereby recognizes no necessity of making a response to the GS-query packet received earlier, and does not wirelessly transmit the query response message. This is because the ASN gateway  30  checks a status of the participation in each of the multicast groups on every MBS zone basis, and the other mobile terminal MS 3  existing in the same MBS zone has already notified of participation continuing information. 
         [0118]    Given next is a description of a case in which the ASN gateway  30  checks, based on the multicast group information table, the existence of the multicast group (224.22.3.45) about the MBS zone # 1 . Herein, such a case is exemplified that the MS 1  existing in the MBS zone # 1  cancels the participation in the multicast group (224.22.3.45). 
         [0119]    The ASN gateway  30  generates, for checking the existence of the multicast group (224.22.3.45), the GS-query packet in which the multicast address (224.22.3.45) is set in the group address field of the IGMP part and in the DA field of the IP header part. The ASN gateway  30  recognizes that the MBS zone registered with respect to the multicast address is the MBS zone # 1 , and specifies the BS 1 , BS 2  and BS 3  as the base station belonging to the MBS zone # 1 . The ASN gateway  30  specifies the GRE tunnels generated with respect to the multicast address (224.22.3.45) between the specified BS 1 , BS 2 , BS 3  and the ASN gateway  30 . 
         [0120]    The ASN gateway  30  copies the GS-query packet generated earlier, and attaches predetermined headers set for the destination base station to each GS-query packet. The ASN gateway  30  sends the thus-generated frames to the BS 1 , BS 2  and BS 3  via the respective GRE tunnels (S 1410 ). The timing information set in the GRE header part of each of the frames transmitted contains the same information. 
         [0121]    The BS 1 , BS 2  and BS 3  receiving the frames simultaneously transmit the GS-query packets in these frames through predetermined wireless channels at the timing specified by the timing information set in the GRE header parts (S 1411 ). 
         [0122]    The MS 1  receives the GS-query packet sent from any one of the BS 1 , BS 2  and BS 3 , corresponding to its location. The MS 1 , because of cancelling the participation in the multicast group (224.22.3.45), does not make any response to the GS-query message. 
         [0123]    The ASN gateway  30 , if the response to the GS-query message transmitted earlier is not sent for a predetermined timeout period, temporarily determines that any mobile terminal  60  continuing the participation in the multicast group (224.22.3.45) does not exist in the MBS zone # 1  (S 1412 ). The ASN gateway  30  retransmits the same GS-query message as the message transmitted earlier to the MBS zone # 1  (S 1413 ). 
         [0124]    The ASN gateway  30 , if there no response to the retransmitted GS-query message, determines that none of the mobile terminal  60  continuing the participation in the multicast group (224.22.3.45) exists in the MBS zone # 1 , and deletes a target record from the multicast group information table. Subsequently, the ASN gateway  30  checks whether or not the records associated with other MBS zones in regard to the multicast group (224.22.3.45) are registered in the multicast group information table. The ASN gateway  30 , when confirming that none of the records associated with other MBS zones are registered therein, sends the LEAVE message about the multicast group (224.22.3.45) to the edge router  20 . 
         [0125]    The edge router  20  receiving the LEAVE message recognizes that the ASN gateway  30  has left the multicast group (224.22.3.45). This has the same meaning as recognizing that the mobile terminals  60  existing under the ASN gateway  30  include none of the mobile terminals  60  continuing the participation in the multicast group. 
         [0126]    Note that a processing sequence in the case of transmitting the LEAVE message from the mobile terminal  60  is that when the ASN gateway  30  receives the LEAVE message, the GS-query message described above is sent. Subsequent processes are the same as those of the processing sequence shown in  FIG. 14 , and hence the explanation is omitted. 
         [0127]    Next, an operation in such a case that the LEAVE message is sent from the mobile terminal  60  will be described with reference to  FIGS. 15 and 16 .  FIG. 15  is a sequence diagram showing a first processing pattern when receiving the LEAVE message, and  FIG. 16  a sequence diagram showing a second processing pattern when receiving the LEAVE message, wherein the system architecture illustrated in  FIG. 1  is exemplified. 
         [0128]    To begin with, the first processing pattern when receiving the LEAVE message will be explained with reference to  FIG. 15 . Hereat, an assumption is that the multicast group information table of the ASN gateway  30  is in a status illustrated in  FIG. 4 , and only the MS 3 , MS 4  make a request for participating in a multicast group (224.0.10.15) from the MBS zone # 2 . 
         [0129]    The MS 3  wirelessly transmits a LEAVE message signal in which to set the multicast address (224.0.10.15) in order to leave the multicast group (224.0.10.15) (S 1501 ). 
         [0130]    The BS 5 , when receiving the LEAVE message signal, extracts a LEAVE packet from this signal. The BS 5  transmits a frame generated by attaching predetermined headers to this LEAVE packet to the ASN gateway  30  via the GRE tunnel (S 1502 ). At this time, the BS 5  sets an IP address of the base station itself in the SA field in the predetermined header and an IP address of the ASN gateway  30  in the DA field. 
         [0131]    The ASN gateway  30 , upon receiving the frame via the GRE tunnel, extracts the LEAVE packet within the frame. Furthermore, the ASN gateway  30  specifies the sender BS 5  on the basis of the IP address set in the SA field of the header of this frame, and recognizes based on the MBS zone information table ( FIG. 3 ) that the MBS zone, to which the BS 5  belongs, is the MBS zone # 2 . Subsequently, the ASN gateway  30  extracts, from the multicast group information table ( FIG. 4 ), the base station information registered in the last responder field of a record registered with an address/zone pair of the multicast address (224.0.10.15) set in the group address field of the IGMP part within the frame and the MBS zone # 2  (S 1503 ). Herein, the information representing BS 4  is extracted from the last responder field of the record. 
         [0132]    The ASN gateway  30 , when confirming that the sender BS 5  of the LEAVE message is different from the extracted last responder (BS 4 ), determines that the mobile terminal  60  continuing to participate in the multicast address (224.0.10.15) still exists in the MBS zone # 2 , and terminates the process (S 1504 ). 
         [0133]    Thereafter, the MS 3 , for leaving the multicast group (224.0.10.15), wirelessly transmits the LEAVE message in which the multicast address (224.0.10.15) is set (S 1510 ). 
         [0134]    The BS 4 , when receiving the LEAVE message signal, extracts the LEAVE packet from this signal. The BS 4  transmits a frame generated by attaching predetermined headers to the LEAVE packet to the ASN gateway  30  via the GRE tunnel (S 1511 ). Hereat, the BS 4  sets the self IP address in the SA field within the predetermined header and the IP address of the ASN gateway  30  in the DA field. 
         [0135]    The ASN gateway  30 , upon receiving the frame via the GRE tunnel, as in the case of the MS 3 , specifies the sender BS 4  of this frame, and recognizes based on the MBS zone information table ( FIG. 3 ) that the MBS zone to which the BS 4  belongs is the MBS zone # 2 . Subsequently, the ASN gateway  30  extracts, from the multicast group information table ( FIG. 4 ), the base station information (BS 4 ) registered in the last responder field of a record registered with an address/zone pair of the multicast address (224.0.10.15) set in the group address field of the IGMP part within the frame and the MBS zone # 2 . 
         [0136]    The ASN gateway  30 , when confirming that the sender BS 4  of the LEAVE message is coincident with the extracted last responder (BS 4 ), determines that there is a possibility in which none of the mobile terminal  60  continuing to participate in the multicast address (224.0.10.15) might exist in the MBS zone # 2  (S 1512 ). The ASN gateway  30  thereby transmits the GS-query packet via the GRE tunnels respectively to the BS 4  and BS 5  belonging to the MBS zone # 2  (S 1513 ). 
         [0137]    The processing is done in the same way as in the case of checking the existence of the mobile terminal  60  participating in the multicast group shown in  FIG. 14 . At this time, none of the mobile terminal  60  continuing the participation in the multicast group (224.0.10.15) exists in the MBS zone # 2 , and hence any query response message to the GS-query packet is not sent. 
         [0138]    Accordingly, the ASN gateway  30  finally determines that none of the mobile terminal participating in the multicast group (224.0.10.15) exists in the MBS zone # 2 , and deletes a record containing a zone/group pair of the MBS zone # 2  and the multicast group (224.0.10.15) from the multicast group information table (S 1519 ). Subsequently, the ASN gateway  30  checks whether or not records related to other MBS zones with respect to the multicast group (224.0.10.15) are registered in the multicast group information table. The ASN gateway  30 , when confirming that no other MBS zones are registered, sends the LEAVE message about the multicast group (224.0.10.15) to the edge router  20  (S 1520 ). 
         [0139]    Next, the second processing pattern when receiving the LEAVE message will be explained with reference to  FIG. 16 . At this time, an assumption is that the multicast group information table of the ASN gateway  30  is in the status illustrated in  FIG. 4 , and only the MS 3  and MS 4  make a request for participating in the multicast group (224.0.10.15) from the MBS zone # 2 . 
         [0140]    The MS 4 , for leaving the multicast group (224.0.10.15), wirelessly transmits the LEAVE message in which the multicast address (224.0.10.15) is set (S 1610 ). 
         [0141]    The BS 4 , when receiving the LEAVE message signal, extracts the LEAVE packet from this signal. The BS 4  transmits a frame generated by attaching predetermined headers to the LEAVE packet to the ASN gateway  30  via the GRE tunnel (S 1602 ). Hereat, the BS 4  sets the self IP address in the SA field within the predetermined header and the IP address of the ASN gateway  30  in the DA field. 
         [0142]    The ASN gateway  30 , upon receiving the frame via the GRE tunnel, in the same way as above, specifies the sender BS 4  of this frame, and recognizes based on the MBS zone information table ( FIG. 3 ) that the MBS zone to which the BS 4  belongs is the MBS zone # 2 . Subsequently, the ASN gateway  30  extracts, from the multicast group information table ( FIG. 4 ), the base station information (BS 4 ) registered in the last responder field of a record registered with an address/zone pair of the multicast address (224.0.10.15) set in the group address field of the IGMP part within the frame and the MBS zone # 2 . 
         [0143]    The ASN gateway  30 , when confirming that the sender BS 4  of the LEAVE message is coincident with the extracted last responder (BS 4 ), determines that there is a possibility in which none of the mobile terminal  60  continuing to participate in the multicast address (224.0.10.15) might exist in the MBS zone # 2  (S 1603 ). The ASN gateway  30  thereby transmits the GS-query packet via the GRE tunnels respectively to the BS 4  and BS 5  belonging to the MBS zone # 2  (S 1604 ). 
         [0144]    The processing is executed in the same way as in the case of checking the existence of the mobile terminal  60  participating in the multicast group shown in  FIG. 14 . At this time, the mobile terminal (MS 3 ) continuing the participation in the multicast group (224.0.10.15) still exists in the MBS zone # 2 . The MS 3 , upon receiving this GS-query packet, transmits the query response message (S 1606 ). The ASN gateway  30  determines from this response that the mobile terminal  60  continuing the participation in the multicast group (224.0.10.15) still exists in the MBS zone # 2 . The ASN gateway  30  transmits, in the same way as by the process shown in  FIG. 14 , the query response message as it is to the respective BS 4  and BS 5  in the MBS zone # 2  (S 1608 ). 
         [0145]    Finally, the ASN gateway  30  updates the last responder field in the multicast group information table with the information on the base station (BS 5 ) that relayed the query response message (S 1610 ), and terminates the process (S 1611 ). 
       Operations and Effects in the Present Embodiment  
       [0146]    Herein, the operations and effects of the MBS system in the present embodiment will be described. 
         [0147]    In the MBS system according to the present embodiment, when the JOIN message (the request for participating in the predetermined multicast group) sent from the mobile terminal  60  is transmitted to the ASN gateway  30  through the base station  50 , the information about the MBS zone to which the base station relaying the JOIN message belongs and with the information about the base station relaying the JOIN message with respect to the multicast group at which the JOIN message is targeted are stored. 
         [0148]    Then, when the LEAVE message (the request for leaving the predetermined multicast group) sent from the mobile terminal  60  is transmitted to the ASN gateway  30  through the base station  50 , it is checked whether or not the base station relaying the LEAVE message is registered as the last responder in the multicast group information table with respect to the multicast group at which the LEAVE message is targeted. If the registered last responder is coincident with the base station relaying the LEAVE message, the GS-query message (for checking the existence) is sent to the MBS zone to which this via-base-station belongs. From this operation, it is confirmed that no other mobile terminals continuing the participation in the same multicast group exist, and the target record is deleted from the multicast group information table. 
         [0149]    Whereas if the registered last responder is not coincident with the base station relaying the LEAVE message, the process is terminated. This is because it can be recognized that other mobile terminals exist in the MBS zone to which that base station belongs. 
         [0150]    With this scheme, according to the present embodiment, even in the case of receiving the LEAVE message, if there is a high possibility that other mobile terminals continuing their participation might exist in the same MBS zone, the GS-query message for checking the existence may not be transmitted, and hence the unnecessary traffic can be reduced. 
         [0151]    This scheme, according to the present embodiment, enables the multicast communications to be efficiently performed in the MBS system targeted at the mobile terminals  60  on the premise that these terminals are to move. 
       Modified Example  
       [0152]    In the ASN gateway  30  described above, when receiving the JOIN message, the last responder field in the multicast group information table is updated with the information about the base station relaying the JOIN message, however, the information about the base station relaying the JOIN message may be entered in the last responder field only when receiving the JOIN message for the first time with respect to the same MBS zone. 
         [0153]    Even in the case of the operation being thus done, the existence of the mobile terminal participating in the multicast group can be invariably checked on every MBS zone basis by periodically checking the mobile terminal continuing the participation by use of the GS-query message. 
       Others 
       [0154]    The disclosures of Japanese patent application No. JP2006-301594, filed on Nov. 7, 2006 including the specification, drawings and abstract are incorporated herein by reference.