Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a U.S. continuation application filed under 35 USC 111(a) claiming benefit under 35 USC 120 and 365(c) of PCT application JP 2005/002033, filed Feb. 10, 2005. The foregoing application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a data transmission method and a terminal used for the data transmission method, and more particularly, a data transmission method and a terminal used for transmitting data from a server to plural terminals via a network.  
         [0004]     2. Description of the Related Art  
         [0005]     In communicating in a conventional network using, for example, Ethernet (Registered Trademark), a server transmits data requested by a terminal in a given network in response to a transmission request from the terminal. In a case where plural terminals exist in a given network, resources such as communication paths are shared by the terminals. For example, the resources may be shared according to time division so that the terminal can communicate without obstructing one another.  
         [0006]     The bandwidth of the network is calculated and determined according to the amount of data exchanged over the network. Furthermore, as for the types of exchanged data, a large part of the data is typically relatively small size data such as data containing documents and still images. Accordingly, the network is configured to match these communication conditions.  
         [0007]     However, in recent years and continuing, performance of terminals is becoming significantly higher and more large size data such as real time image data and audio data are being exchanged. Such improvements can be used for transmitting real time image data/audio data in a network having plural LANs (Local Area Network) connected by a WAN (Wide Area Network). As one example, a company having branches in plural remote areas can transmit data (image data/audio data) containing a salutation of the company&#39;s president throughout the terminals of each branch in real time.  
         [0008]     A technology for providing an efficient network is desired so that terminals in the network can receive large size real time data without experiencing service shutdown or serious delay.  
         [0009]      FIG. 1  is a schematic diagram showing an exemplary configuration for describing a conventional client/server type data broadcasting method. In  FIG. 1 , a router  11  of a LAN of a main office is connected to a server  10  and a WAN  20 . The WAN  20  is also connected to a router  12  of the Osaka branch, a router  13  of the Sapporo branch, and a router  14  of the Fukuoka branch.  
         [0010]     For example, the LAN of the Osaka branch is configured having the router  12  connected to switching hubs  15 ,  16 , and  17 . Each switching hub  15 - 17  is connected to plural terminals. The LANs of the main office and other branches are also configured in the same manner.  
         [0011]     In typical client/server communications, a terminal (client) of each branch sends a request for data transmission by transmitting packets indicating the desire for data transmission to the server  10  assuming the position of transmitting the data. The server  10 , upon receiving the data transmission request, begins to transmit the data to each terminal.  
         [0012]      FIG. 2  is a schematic diagram showing a network configuration for describing a conventional data transmission method using IP multicast. In  FIG. 2 , a router  31  of a LAN of a main office is connected to a server  30  and a WAN  20 . The WAN  20  is also connected to a router  32  of the Osaka branch, a router  33  of the Sapporo branch, and a router  34  of the Fukuoka branch.  
         [0013]     For example, the LAN of the Osaka branch is configured having the router  32  connected to switching hubs  35 ,  36 , and  37 . Each switching hub  35 - 37  is connected to plural terminals. The LANs of the main office and other branches are also configured in the same manner.  
         [0014]     In using IP multicast technology, a destination IP address is designated as a multicast address upon transmitting data from the server  30  to plural specific terminals. A Class D address is used as the multicast address. One multicast address represents a single host group (a group comprising plural terminals). For example, in the Osaka branch shown in  FIG. 2 , all the terminals connected to the switching hubs  35 ,  36 ,  37  form a single host group.  
         [0015]     Multicast packets are basically transmitted to every host in the network. Each host (recipient host) receiving the multicast packets determines whether it (recipient host) is included in a host group(s) designated by the multicast packets and selects packets corresponding to the recipient host.  
         [0016]      FIG. 3  is a schematic diagram showing a network configuration for describing a conventional peer-to-peer type data transmission method. In  FIG. 3 , a router  41  of a LAN of a main office is connected to a server  40  and a WAN  20 . The WAN  20  is also connected to a router  42  of the Osaka branch, a router  43  of the Sapporo branch, and a router  44  of the Fukuoka branch.  
         [0017]     For example, the LAN of the Osaka branch is configured having the router  42  connected to switching hubs  45 ,  46 , and  47 . Each switching hub  45 - 47  is connected to plural terminals. The LANs of the main office and other branches are also configured in the same manner.  
         [0018]     In using a simple peer-to-peer technology as shown in  FIG. 3 , regardless of the physical arrangement of the LANs, packets are transferred among terminals grouped in predetermined units (e.g., segments) in an order beginning from the terminal which first receives the packets, as indicated with broken line arrows ( 1 )-( 5 ).  
         [0019]     As a method of realizing multicast communications, Japanese Laid-Open Patent Application No. 2003-8642 teaches a technology where each terminal performs carrier sensing after a predetermined delay time.  
         [0020]     Furthermore, Japanese Laid-Open Patent Application No. 6-244806 teaches a technology that identifies which slave terminal has malfunctioned by referring to an echo-back status of data flowing in a communication path connecting a host terminal to plural slave terminals.  
         [0021]     When transmitting real time data such as image data and audio data to each terminal by using the conventional client/server type data, the transmission data easily exceed available bandwidth of the network due to the large size of the transmission data. This causes various problems to occur (e.g., shut down of service, serious delay, frequent discontinuation of real time data) and prevents satisfactory communication.  
         [0022]     Although there is a method of solving such problems by increasing the bandwidth of the channels in the LAN or channels of the WAN, such an increase leads to an increase of cost. It is unsuitable to make such an investment in a case where real time technology is used only a few times a year, such as the above-described case of broadcasting a salutation of the company&#39;s president in real time. Furthermore, even in a case where real time data communications are not conducted frequently, there are cases where it is important to transmit the data in real time. Therefore, it is desired to prevent a large increase of cost.  
         [0023]     Furthermore, although it may be beneficial to transmit data by using broadcast packets in a case where the data are to be used by every terminal, the transmission of data using broadcast packets can cause a serious drop of communications efficiency for a terminal not requiring the transmitted data.  
         [0024]     Furthermore, since multicast packets are transmitted to host groups in a case of conducting data transmission by using conventional IP multicast technology, each resource (e.g., the routers  31 - 34 , the server  30 , the terminals, and software thereof in  FIG. 2 ) is to be compatible to the multicast technology. Accordingly, in building the infrastructure of the network, facilities/apparatuses not compatible with multicast technology are to be replaced with those compatible with multicast technology. Therefore, it is unsuitable to make such an investment in a case where real time technology is used only a few times a year, such as the above-described case of broadcasting a salutation of the company&#39;s president in real time.  
         [0025]     Furthermore, it may be possible to reduce access to the channels of the most commonly shared WAN  20  by preparing a streaming server in the LANs of each branch for temporarily storing image/audio data transmitted from the main office and relaying the stored data, so that each terminal of each branch can access the streaming server. However, in a case where there is a large number of client terminals, data cannot be satisfactorily transmitted when the amount of transmission data [(image data+audio data)×number of client terminals] exceeds the bandwidth provided in the LANs.  
         [0026]     In the conventional peer-to-peer type data transmission method, it may be possible to prevent packets from concentrating at the connecting points between the routers  11 - 14  and the WAN  20  as shown in  FIG. 1 , for example. However, in a case of receiving real time data for a long period, the amount of data flowing in the network parts shared between the switching hubs and the routers may increase as transfer of data is repeated.  
       SUMMARY OF THE INVENTION  
       [0027]     The present invention may provide a data transmission method and a terminal used for the data transmission method that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.  
         [0028]     Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by a data transmission method and a terminal used for the data transmission method particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.  
         [0029]     To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an embodiment of the present invention provides a data transmission method for transmitting real-time data from a server to a plurality of terminals via a network having a plurality of local area networks connected to a wide area network, the method including the steps of: grouping a plurality of terminals connected to a same switching hub into a single terminal group; generating a table indicating a rank and an address of each terminal in the terminal group; transmitting the real-time data from the server to a first terminal having the highest rank according to the ranks indicated in the table; and transferring the real-time data from the first terminal to a terminal that is one rank lower than the first terminal according to the ranks indicated in the table.  
         [0030]     Furthermore, another embodiment of the present invention provides a terminal used for a data transmission method for transmitting real-time data from a server to a plurality of terminals via a network having a plurality of local area networks connected to a wide area network, the terminal including: a table indicating a rank and an address of each terminal of a single terminal group, the terminal group including a plurality of terminals connected to a same switching hub; a reception part for receiving the real-time data from the switching hub or another terminal that is one rank higher than the terminal according to the ranks indicated in the table; and a transfer part for transferring the real-time data to another terminal that is one rank lower than the terminal according to the ranks indicated in the table.  
         [0031]     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1  is a schematic diagram showing an exemplary configuration for describing a conventional client/server type data broadcasting method;  
         [0033]      FIG. 2  is a schematic diagram showing a network configuration for describing a conventional data transmission method using IP multicast;  
         [0034]      FIG. 3  is a schematic diagram showing a network configuration for describing a conventional peer-to-peer type data transmission method;  
         [0035]      FIG. 4  is a schematic diagram showing a network configuration for describing a data transmission method according to an embodiment of the present invention;  
         [0036]      FIG. 5  shows a terminal group address table according to an embodiment of the present invention;  
         [0037]      FIG. 6  is an exemplary operation sequence diagram of a data transmission method according to an embodiment of the present invention;  
         [0038]      FIG. 7  is a schematic diagram for describing the functions of a terminal to which a data transmission method according to an embodiment of the present invention is applied;  
         [0039]      FIG. 8  is a flowchart showing a rank defining process conducted by a terminal upon activation of its data transfer system according to an embodiment of the present invention;  
         [0040]      FIG. 9  is a flowchart showing a rank defining process conducted by a terminal upon responding to search data according to an embodiment of the present invention; and  
         [0041]      FIG. 10  is a sequence diagram showing an exemplary operation of defining a rank in a terminal group according to an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]     In the following, embodiments of the present invention will be described with reference to the accompanying drawings.  
         [0000]     [Network Configuration] 
         [0043]      FIG. 4  is a schematic diagram showing a network configuration for describing a data transmission method according to an embodiment of the present invention. In  FIG. 4 , a router  51  of a LAN of a main office is connected to a server  50  and a WAN  60 . The WAN  60  is also connected to a router  52  of the Osaka branch, a router  53  of the Sapporo branch, and a router  54  of the Fukuoka branch.  
         [0044]     For example, the LAN of the Osaka branch is configured having the router  52  connected to switching hubs  55 ,  56 , and  57 . Each switching hub  55 - 57  has a cascade port connected to plural terminals. The LANs of the main office and other branches are also configured in the same manner.  
         [0045]     In each branch, plural terminals connected to each switching hub (e.g., switching hubs  55 - 57  of Osaka branch) form a group (terminal group). The order (rank order) in which a terminal receives image data/audio data (real-time data) is determined beforehand for every terminal in each terminal group. Each terminal has a terminal group address table (see, for example,  FIG. 5 ) indicating the predetermined order (rank order), the MAC addresses, and the IP addresses of the terminals belonging to its corresponding terminal group.  
         [0046]     For example,  FIG. 5  shows a terminal group address table of the terminals  61 - 63 , . . . connected to a switching hub  55  of the Osaka branch. In the row corresponding to order-rank  1 , a MAC address (bbbb) and an IP address (BBBB) corresponding to the terminal  62  are registered. In the row corresponding to order-rank  2 , a MAC address (aaaa) and an IP address (AAAA) corresponding to the terminal  61  are registered.  
         [0047]     Each terminal requests another terminal having a predetermined rank to transfer real-time data at predetermined periods (cycles) based on the terminal group address table. In this example, a terminal makes the transfer request to another terminal that is one rank higher than itself.  
         [0048]     As for the method of generating (setting) the terminal group address tables for each of the terminals, it may be conducted by allowing a system administrator to generate (set) terminal group address tables terminal-by-terminal. Another method is registering (storing) terminal group address tables corresponding to switching hubs of each branch in a database of an existing server connected to a LAN or a WAN beforehand (pre-registration) and setting a corresponding registered group address table upon request from a terminal. Another method is registering (storing) a terminal group address table for each switching hub and generating (setting) a corresponding registered group address table upon request from a terminal. Another method is allowing each terminal to generate (set) a corresponding group address table by using a rank defining process (described below).  
         [0049]      FIG. 6  is an exemplary operation sequence diagram of a data transmission method according to an embodiment of the present invention. In this example, data are transmitted in a terminal group comprising terminals  61 - 64  each connected to a cascade port of the switching hub  55  of the Osaka branch. In this example, each terminal  61 - 64  has a data transfer system which is activated for requesting another terminal that is one rank higher than itself to transfer image data/audio data (real-time data) at a predetermined cycle based on the terminal group address table. Although four terminals are used to describe this operation sequence for the sake of convenience, the number of the terminals may be greater than or less than four.  
         [0050]     In  FIG. 6 , real-time data transmitted from the server  50  of the main office are transmitted to the routers  52 - 54  of the branches via the WAN  60 . The routers  52 - 54  of the branches transmit the real-time data to a predetermined terminal in a terminal group of the switching hubs (switching hubs  55 - 57  in Osaka Branch) connected to the routers  52 - 54 . More specifically, the real-time data are transmitted to a predetermined terminal having the highest rank among the terminals of a terminal group which predetermined terminal has activated its data transfer system and requested transfer of the real-time data to the server  50 .  
         [0051]     In one example, first, a terminal  62  having the highest rank among the terminals of a terminal group receives the real-time data from the server  50  via the switching hub  55 . The terminal  62  buffers the received real-time data and reproduces the received real-time data with upper level software. Subsequently, when the terminal  62  receives a data transfer request from a terminal  61  one rank lower than itself (terminal  62 ), the terminal  62  transfers the buffered real-time data to the terminal  61  that has requested data transfer in the terminal group.  
         [0052]     Then, the terminal  61  buffers the received real-time data and reproduces the received real-time data with upper level software. Subsequently, when the terminal  61  receives a data transfer request from a terminal  63  one rank lower than itself (terminal  61 ), the terminal  61  transfers the buffered real-time data to the terminal  63  that has requested data transfer in the terminal group.  
         [0053]     Likewise, the terminal  63  buffers the received real-time data and reproduces the received real-time data. Subsequently, when the terminal  63  receives a data transfer request from a terminal  64  one rank lower than itself (terminal  63 ), the terminal  63  transfers the buffered real-time data to the terminal  64  that has requested data transfer in the terminal group. Then, the terminal  64  buffers the received real-time data and reproduces the received real-time data.  
         [0054]     Accordingly, real-time data can be transmitted without having all the terminals in a terminal group accessing the server  50  and without causing data traffic concentrating at shared parts of the network.  
         [0055]     Even in a case where a switching hub conducts the above-described transfer operation, this transfer operation does not affect communications with other switching hubs connected to the switching hub or communications of non-transfer ports of the switching hub. This is due to the fact that traffic in one port of the switching hub is physically independent from traffic in the other ports. Here, “non-transfer port” refers to an unused port or a port connected to a terminal not using a data transfer system. Since packets of real-time data are transferred by relaying the packets from one terminal to another, a delay is caused each time the packets are transferred to another terminal. Accordingly, the delay time increases in proportion with the number of times the packets are transferred to another terminal. Nevertheless, since the number of ports of a typical switching hub is 20 at most, the delay time created for the packets to reach the last terminal has little effect in transmitting real-time data.  
         [0056]     With the data transmission method according to an embodiment of the present invention, each terminal belonging to a terminal group connected to a switching hub reproduces real-time data for itself while at the same time sequentially transfers the real-time data to other terminals within the terminal group. Thereby, large size data such as image data and audio data can be provided to all the terminals which desire the data without increasing the data traffic burden.  
         [0057]     Hence, large size data requested by plural terminals can be reproduced without increasing traffic by effectively using existing resources such as routers and switching hubs that are not compatible with multicast. Thereby, the network can be used efficiently.  
         [0000]     [Function of Terminal] 
         [0058]      FIG. 7  is a schematic diagram for describing the functions of a terminal  700  to which a data transmission method according to an embodiment of the present invention is applied. The terminal  700  shown in  FIG. 7  includes a terminal group address table  70  to which an order-rank and an address of each terminal belonging to a terminal group of the same switching hub are registered.  
         [0059]     A reception part  71  is for receiving real-time data transmitted from a server via a corresponding switching hub or a terminal having a rank higher than itself (terminal  700 ) and writing the real-time data to a buffer part  77 .  
         [0060]     A transfer part  72  is for transferring real-time data read out from the buffer part  77  to a terminal one rank lower than itself (terminal  700 ) according to the terminal group address table  70 .  
         [0061]     A search part  73  is for transmitting search data (search packets) to other terminals connected to the same switching hub as itself (terminal  700 ) upon activation.  
         [0062]     In a case where there is no reply from the other terminals in response to the transmitted search data, a table registration part  74  registers itself as the highest rank (rank # 1 ) in the terminal group table  70  together with its own address (address of terminal  700 ). In a case where there is a reply from the other terminals in response to the transmitted search data, the table registration part  74 , based on table data included in the reply from the other terminals, registers itself one rank lower than the lowest ranked terminal in the terminal group table  70  together with its own address.  
         [0063]     A reply &amp; registration part (also referred to as “reply/registration part”)  75  generates a reply including table data indicating the content of the terminal group address table  70  when receiving search data from another terminal, transmits the reply to the origin (original terminal) that transmitted the search data, and registers the original terminal as one rank lower than the lowest ranked terminal in the terminal group address table  70  together with the address of the original terminal.  
         [0064]     A reproduction part  76  is for reproducing real-time data received in the reception part  71 .  
         [0000]     [Rank Defining Process] 
         [0065]      FIG. 8  is a flowchart showing a rank defining process conducted by a terminal (e.g., terminal  700 ) upon activation of its data transfer system according to an embodiment of the present invention.  
         [0066]     In  FIG. 8 , when a data transfer system of terminal  700  is activated, the terminal  700  transmits a search packet (search data) including a MAC address and an IP address of itself to a switching hub connected to the terminal  700  (Step S 10 ). Then, by using a broadcast transmission method, the switching hub transmits the received search packet to all terminals connected to its cascade port except for the port receiving the search packet.  
         [0067]     Then, the terminal  700  determines whether there is a reply from other terminals within a predetermined period in response to the transmitted search packet (Step S 11 ). The predetermined period is a time sufficient for the terminal  700  to receive replies from other terminals belonging to the same terminal group as the terminal  700 . Therefore, even in a case where the search packet is transmitted to terminals belonging to a terminal group different from the terminal group of the terminal  700  via a common part of a switching hub and a router(s), no reply will be received from the terminals of the different terminal group during the predetermined period. Therefore, a terminal belonging to the different terminal group will not be mistaken as a terminal belonging to the same terminal group as the terminal  700 .  
         [0068]     In a case where there is no reply from the other terminals and no other terminal having an activated data transfer system, the terminal  700  defines its rank as number one (Step S 12 ) and registers its rank (# 1 ) and its MAC address and IP address in its terminal group address table  70  (Step S 13 ). It is to be noted that the group address table  70  in the terminal  700  is cleared upon activation of its data transfer system.  
         [0069]     Meanwhile, in a case where there is a reply from the other terminals, the terminal  700  identifies the lowest rank from group address table data included in the reply and defines its rank as being one rank lower than the identified lowest rank (Step S 14 ).  
         [0070]     Then, the terminal  700  registers the group address table data included in the reply, its rank, MAC address, and IP address in its terminal group address table  70  (Step S 15 ).  
         [0071]     It is to be noted that the content of the terminal group address data included in each reply is the same even in a case where the terminal  700  receives plural replies from the other terminals. Therefore, the terminal  700  defines its rank based on the first received reply and registers the rank in its terminal group address table  70 .  
         [0072]      FIG. 9  is a flowchart showing a rank defining process conducted by a terminal (e.g., terminal  700 ) upon responding to search data according to an embodiment of the present invention.  
         [0073]     In  FIG. 9 , when the terminal  700  receives a search packet transmitted from another terminal (original terminal), the terminal  700  generates a reply including terminal group address table data containing a duplicate (copy) of the content of the terminal group address table  70  of the terminal  700  and transmits the generated reply to the original terminal (Step S 20 ). The MAC address and the IP address of the terminal  700  are also included in the reply.  
         [0074]     Then, the terminal  700  identifies the lowest rank from its terminal group address table  70  and defines the rank of the original terminal as being one rank lower than the identified lowest rank (Step S 21 ).  
         [0075]     Then, the terminal  700  registers the defined rank, the MAC address, and the IP address of the original terminal in the terminal group address table  70  (Step S 22 ).  
         [0076]      FIG. 10  is a sequence diagram showing an exemplary operation of defining a rank in a terminal group according to an embodiment of the present invention. This operation is described by using an example of defining the ranks of a terminal group comprising terminals  61 - 64  connected to respective ports (cascade port) of a switching hub  55  of the Osaka branch. Although four terminals are used to describe this operation for the sake of convenience, the number of the terminals may be greater than or less than four. Furthermore, software of a data transfer system configured to execute the processes shown in  FIGS. 8 and 9  is already installed in the terminals  61 - 64  beforehand.  
         [0077]     Among the plural terminals  61 - 64  having the data transfer system built therein, the terminal  62  having the earliest activated data transfer system transmits a search packet for determining whether there is a terminal already having an activated (active) data transfer system among the terminals connected to the switching hub  55  (search for other nodes).  
         [0078]     Then, in a case where there is no reply from the data transfer systems of the other terminals of the terminal group connected to the switching hub  55  in response to the search packet transmitted from the terminal  62 , that is, in a case where there is no other terminal having an activated data transfer system, the terminal  62  defines its rank as having the highest rank (number one) among the data transfer systems along with generating a terminal group address table and registering its rank, the MAC address, and the IP address in the terminal group address table.  
         [0079]     Then, a terminal  61 , having its data transfer system activated, begins to search (transmission of search packet) for other nodes. Then, the terminal  62  transmits a reply to the terminal  61  in response to the search data. The terminal  61  receiving the reply from the terminal  62  defines its own rank as number two and registers the rank of terminal  62  and its rank (rank of terminal  61 ), MAC address, and IP address in its terminal group address table.  
         [0080]     Further, the terminal  62  receiving the search packet from the terminal  61  updates its terminal group address table by adding the rank (number two), the MAC address, and the IP address of the terminal  61  to its terminal group address table.  
         [0081]     Then, a terminal  63 , having its data transfer system activated, begins to search (transmission of search packet) for other nodes. Then, the terminals  61 ,  62  transmit a reply to the terminal  63  in response to the search packet. The terminal  63  receiving the reply from the terminals  61 ,  62  defines its rank as number three and registers the ranks of terminals  61 ,  62  and its rank (rank of terminal  63 ), MAC address, and IP address in its terminal group address table.  
         [0082]     Further, the terminals  61 ,  62  receiving the search packet from the terminal  63  update their terminal group address table by adding the rank (number three), the MAC address, and the IP address of the terminal  63  to their terminal group address tables.  
         [0083]     Then, a terminal  64 , having its data transfer system activated, begins to search (transmission of search packet) for other nodes. After finding other nodes, the terminal  64  registers the ranks, the MAC addresses, and the IP addresses of itself and the other nodes. Further, the terminals  61 - 63  receiving the search packet from the terminal  64  update their terminal group address tables by adding the rank (number four), the MAC address, and the IP address of the terminal  64  to their terminal group address tables.  
         [0084]     In the above-described data transmission method according to an embodiment of the present invention, although real-time data received from the server are reproduced by all of the terminals, the real-time data may also be reproduced by a single terminal having a rank lower than a rank of a predetermined terminal having an activated data transfer system, so that the predetermined terminal serves as a packet relay terminal for simply relaying packets received from the server.  
         [0085]     Furthermore, after the ranks (status) of the data transfer systems of the terminals have been defined, a process of confirming (monitoring) an operating (activation) status of each other&#39;s data transfer systems may be conducted at predetermined intervals. Thereby, in a case where a terminal having an inoperable (inactive) data transfer system is found, the data of the terminal can be deleted from the terminal group address table of each terminal. By periodically conducting such maintenance of data of the original terminal and the destination terminal, reliability of this data transmission method can be enhanced.  
         [0086]     With the above-described data transmission method and terminal according to an embodiment of the present invention, relatively large size data requested by a large number of terminals can be received and reproduced in real time by effectively using existing resources without increasing data traffic, to thereby increase the efficiency of the network.  
         [0087]     Further, the present invention is not limited to these embodiments, but variations and modifications may be made without departing from the scope of the present invention.

Technology Category: 5