Data communication method, hub station, and earth station

One earth station transmits an address of a source to another earth station (or HUB station), and the other earth station (or HUB station) stores the address in an address management table in association with the one earth station that has transmitted the address of the source and reads, from destination packet data that is packet data including an address of a destination, the address of the destination, and the other earth station (or HUB station) transmits the destination packet data to the associated earth station with reference to the address management table.

TECHNICAL FIELD

The present disclosure relates to a data communication method, a HUB station, and an earth station that perform packet data communications in the same subnet (layer 2 network) over a communication channel via a satellite.

BACKGROUND ART

In a conventional packet-switching transmission system, packet data communications (packet communications) are performed via a HUB station over a communication channel via a satellite (e.g., see FIG. 6 of PTL 1 or PTL 2). PTL 2 describes a system in which one-to-N connection is performed by a single channel per carrier (SCPC) modem for higher efficiency of communications between earth stations through full mesh connection. In typical packet data communications handled in common LAN communications, destination control is performed by a L2 address, such as a media access control (MAC) address, and accordingly, transfer control is based on similar data (L2 address) performed also in communications over a satellite channel, leading to a high-compatibility implementation system.

Satellite communication systems also include a demand assignment multiple access (DAMA) system (e.g., see PTL 3). In the DAMA satellite communication system, a control station allocates channels of earth stations, and the earth stations (transmitting stations or receiving stations) with the allocated channels perform data communications (IP communications) over a satellite channel. The DAMA system allocates channels as required and can deallocate and reallocate the used channels. The DAMA system thus has a feature of being able to obtain the traffic of the channels. A conventional DAMA satellite communication system having such a feature is independent of land lines, such as cell-phone lines, even in case of emergency or disaster, and thus, is generally used for disaster prevention as a system suitable for emergency communications.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Conventional packet data communications over satellite channels employ a method in which terminals in the same subnet (in the layer 2 network) continuously transfer packet data to all the opposite earth stations, and unnecessary packet data is discarded between the earth stations which have received the packet data. However, when satellite channels have fewer bands or when satellite channels have low speed (e.g., in communications using spread spectrum), the bands of the satellite channels become narrower due to continuous transfer of packet data to opposite earth stations which do not need to receive packet data. As described above, the conventional packet data communications over satellite channels fail to provide a transmission speed sufficient for a communication user or may impede other communications.

The present disclosure has been made to solve the above problem and relates to a data communication method, a HUB station, and an earth station capable of restraining useless transmission of packet data using a L2 address.

Solution to Problem

A data communication method according to the present disclosure is a data communication method of performing packet data communications between a plurality of earth stations in a same subnet over a communication channel via a satellite. The method includes: a L2 address transmission step of transmitting, by a first earth station of the earth stations, a L2 address of a source to a second earth station of the earth stations; a L2 address storage step of storing, by the second earth station, the L2 address in a L2 address management table in association with the first earth station that has transmitted the L2 address of the source; and a data transmission step of reading, by the second earth station, a L2 address of a destination from destination packet data that is packet data including the L2 address of the destination, and transmitting, by the second earth station, the destination packet data to the associated earth station with reference to the L2 address management table.

A HUB station according to the present disclosure is a HUB station for data communications in which a plurality of earth stations in a same subnet perform packet data communications over a communication channel via a satellite. The HUB station includes: a L2 address storage to store a L2 address of a source read from source packet data by a first earth station of the earth stations in a L2 address management table in association with the first earth station that has transmitted the source packet data, the source packet data being packet data including a L2 address of the first earth station; and a HUB station transceiver to receive the source packet data transmitted from the first earth station, read a L2 address of a destination from destination packet data, and with reference to the L2 address management table, transmit the destination packet data to the associated earth station.

An earth station according to the present disclosure is a first earth station of a plurality of earth stations that perform packet data communications in a same subnet over a communication channel via a satellite. The earth station includes: a L2 address storage to store a L2 address of a source read from source packet data by a second earth station of the earth stations in a L2 address management table in association with the second earth station that has transmitted the source packet data, the source packet data being packet data including a L2 address of the second earth station; and a transceiver to receive the source packet data transmitted from the second earth station, read a L2 address of a destination from destination packet data, and with reference to the L2 address management table, transmit the destination packet data to the associated earth station.

An earth station according to the present disclosure is a first earth station of a plurality of earth stations that perform packet data communications in a same subnet over a communication channel via a satellite. The earth station includes: an earth station transceiver to transmit, to a second earth station of the earth stations, source packet data that is packet data including a L2 address of the earth station, which is also a L2 address of a communication terminal connected to the earth station, and receive destination packet data transmitted from the second earth station, the destination packet data being packet data including a L2 address of a destination; and a L2 address validation device to read the L2 address of the destination from the destination packet data, and when the L2 address is not the L2 address of the earth station, discard the destination packet data.

Advantageous Effects of Invention

The present disclosure can thus achieve a data communication method, a HUB station, and an earth station that can use a L2 address management table to reduce transmission (transfer) of unnecessary packet data.

DESCRIPTION OF EMBODIMENTS

A communication system, a communication method (data communication method), a HUB station, and a receiving station according to Embodiment 1 of the present disclosure will now be described with reference toFIGS.1to6. In the drawings, like reference signs designate like or corresponding parts, detailed description of which will not be repeated. The communication system according to Embodiment 1 is suitable for a DAMA satellite communication system. Description will be given to the DAMA satellite communication system by way of example. The communication system according to Embodiment 1 has a plurality of communication stations1, which include a HUB station11(hub station) and earth stations12, a satellite2(satellite station2), and a control station3. Communication terminals4(a HUB station communication terminal4aand communication terminals4b) individually connected to HUB station11and earth stations12may be included in the communication system according to Embodiment 1. Each earth station12performs packet data communications (packet data transmission and reception) via HUB station11over a communication channel5. HUB station11may be also referred to as primary station11or relay station11. Also, earth station12may be a transmitting station12or receiving station12in packet data communications. In the present application, HUB station11may function as earth station12, not as HUB. That is to say, earth stations12may exchange their L2 addresses (L2 addresses of communication terminals4connected thereto). Needless to say, earth station12may be HUB station11. In this case, another earth station12will perform packet data communications via HUB station11in the same subnet over communication channel5.

InFIGS.1to6, in control station3, a DAMA control unit31receives a request for allocation of channel of a satellite communication system via a control channel modem32from communication station1over a control channel33via satellite2. DAMA control unit31notifies, over control channel33, communication station1of an allocated communication channel5allocated to communication station1. Packet data communications are performed via HUB station11between earth stations12of communication stations1to which the respective channels are allocated. HUB station11transfers packet data communications via satellite2to earth stations12among communication stations1. Control channel33is a channel for requesting allocation of a communication channel, and accordingly, is also referred to as CSC channel33. CSC means common signaling channel.

InFIGS.1to6, HUB station11has a HUB station transceiver unit13(transceiver unit13), a L2 address storage unit14(storage unit14), a communication modem15(modem15), a satellite GW16(satellite gateway16), and a HUB station control unit17. HUB station control unit17controls HUB station transceiver unit13(transceiver unit13) and L2 address storage unit14. HUB station control unit17controls an operation as communication station1(requesting allocation of a channel of the satellite communication system) and an operation as HUB station11. Earth station12has an earth station control unit18(earth station transceiver unit18), a L2 address validation unit19(validation unit19), a communication modem20(modem20), a satellite GW21(satellite gateway21), and an earth station control unit22(earth station12side). Earth station control unit22controls earth station control unit18(earth station transceiver unit18) and L2 address validation unit19(validation unit19). Earth station control unit22controls an operation as communication station1(requesting allocation of a channel of the satellite communication system) and an operation as earth station12.

The functional block diagram of the communication system according to Embodiment 1 shown inFIG.2is a variation of the functional block shown inFIG.1.FIG.2illustrates the case where communication station1has the functions of both of HUB station11(hub station) and earth station12. That is to say, in this case, the transceiver unit has the functions of HUB station transceiver unit13and earth station transceiver unit18. The communication modem has functions of communication modem15and communication modem20. Satellite GW (satellite gateway) has the functions of satellite GW16and satellite GW21. The communication station control unit (control unit) has the functions of HUB station control unit17and earth station control unit22. L2 address storage unit14and L2 address validation unit19may be provided integrally as shown inFIG.2or separately. InFIG.2, the functions of L2 address storage unit14and L2 address validation unit19are shown separately from the function of satellite GW. That is to say, L2 address storage unit14may be provided in satellite GW16(satellite GW21) that functions as a gateway into communication channel5. Similarly, L2 address validation unit19may be provided in satellite GW21(satellite GW16) that functions as a gateway into a communication channel.

Similarly, the functional block diagram of the communication system according to Embodiment 1 shown inFIG.3is a variation of the functional block diagram shown inFIG.1.FIG.3illustrates a case where control station3has the function of communication station1(HUB station11). That is to say, it is regarded that in HUB station11in this case, a control station control unit17ahas the function of HUB station control unit17, as well as the function of control station3. Although the case where control station3has the function of earth station12is not illustrated, it is regarded that in earth station12in this case, a control station control unit18ahas the function of earth station control unit18, as well as the function of control station3. The functional block having a combination of the functional block diagram shown inFIG.2and the functional block diagram shown inFIG.3will not be illustrated. Embodiment 1 will mainly describe the functional block diagram shown inFIG.1.

HUB station11performs data communications in which earth stations12in the same subnet (layer 2 network) perform packet communications over communication channel5via satellite2. L2 address storage unit14stores a L2 address of a source, which is read by earth station12from source packet data that is packet data including a L2 address of earth station12, in a L2 address management table141in association with earth station12that has transmitted source packet data. Source packet data is packet data that triggers the storage operation (L2 address storage step) of L2 address storage unit14. It is regarded that HUB station11has learned the L2 address of earth station12(communication terminal4b). The L2 address of earth station12is the L2 address of communication terminal4bconnected to earth station12. Thus, the L2 address of communication terminal4bchanges when communication terminal4bconnected to earth station12changes to another terminal, and accordingly, the L2 address of earth station12also changes. Further, when multiple communication terminals4bare connected to earth station12, multiple L2 addresses are provided.

HUB station transceiver unit13receives the source packet data transmitted from earth station12, reads a L2 address of a destination from destination packet data, and with reference to L2 address management table141, transmits the destination packet data to earth station12that is associated with the L2 address. The destination packet data is packet data that triggers the transmission operation (L2 address transmission step) of HUB station transceiver unit13. The L2 address of the destination is also the L2 address of communication terminal4bconnected to earth station12. The L2 address of communication terminal4bthus changes, and accordingly, the L2 address of earth station12changes as well. Further, when multiple communication terminals4bare connected to earth station12, multiple L2 addresses are also provided.

HUB station transceiver unit13is connected with communication terminal4(HUB station communication terminal4a). Communication terminal4(HUB station communication terminal4a) generates, receives, and transmits packet data. HUB station transceiver unit13has communication modem15(modem15) and satellite GW16(satellite gateway16). Communication modem15(modem15) is connected to an antenna of HUB station11and transmits and receives (transfers) packet data (source packet data, destination packet data) over communication channel5(satellite channel5). Satellite GW16bridges packet data between modem15and HUB station communication terminal4aand sorts packet data to an appropriate communication channel5, that is, serves as a gateway into communication channel5(satellite channel5). InFIGS.1and3, the function of L2 address storage unit14is shown separately from the function of satellite GW16as inFIG.2. That is to say, L2 address storage unit14may be provided in satellite GW16that functions as a gateway into communication channel5.

Earth station12is one of earth stations12that perform packet communications via HUB station11in the same subnet (layer 2 network) over communication channel5via satellite2. Earth stations12(transmitting station12, receiving station12) include n earth stations including earth station121, earth station122, . . . , earth station12n-1, and earth station12n, where n is a positive integer. The case where n=3 will be described in the present application by way of example. Earth station control unit18is connected with communication terminal4(communication terminal4b). Earth station control unit18has communication modem20(modem20) and satellite GW21(satellite gateway21). Communication modem20(modem20) is connected to an antenna of earth station12and transmits and receives packet data (source packet data, destination packet data) via HUB11over communication channel5(satellite channel5). InFIGS.1to4, communication channel5(satellite channel5) indicated by the dot-dash line is a channel allocated between HUB station11and earth station121. Communication channel5(satellite channel5) indicated by the chain double-dashed line is a channel allocated between HUB station11and earth station122. Communication channel5(satellite channel5) indicated by the dashed line is a channel allocated between HUB station11and earth station123.

Earth station transceiver unit18transmits, to HUB station11, source packet data that is packet data including a L2 address of its earth station12, which is also a L2 address of communication terminal4bconnected to its earth station12, and receive destination packet data that is packet data including a L2 address of a destination, which is transmitted from HUB station11. L2 address validation unit19reads the L2 address of the destination from the destination packet data and, when the L2 address is not the L2 address of its earth station12, discards the destination packet data. As described above, multiple L2 addresses are also provided when multiple communication terminals4bare connected to earth station12, and accordingly, L2 address validation unit19also performs validation assuming that multiple L2 addresses of its earth station12are provided.

The channel allocation in the communication system (communication method) according to Embodiment 1 will be described with reference toFIGS.1and3. Communication station1transmits a request for channel allocation over control channel33to control station3. As described above, control station3receives the request for channel allocation of the satellite communication system from communication station1over control channel33. Control station3notifies communication station1of communication channel5that has been allocated (channel allocation step). Next, the communication method (data communication method according to Embodiment 1) in the communication system according to Embodiment 1 after the channel allocation will be described with reference toFIGS.4to6.

HUB station11shown inFIGS.4and5is connected with HUB station communication terminal4athat performs the packet data transfer process (the process of transferring (transmitting) received source packet data as destination packet data). HUB station communication terminal4amay be part of HUB station11. Modem15is provided for each of earth station121, earth station122, and earth station123for transmission and reception of data to and from satellite2. Satellite GW16sorts transmission data to appropriate communication channel5(satellite channel5). Earth station12(earth station121, earth station122, earth station123) shown inFIGS.4and5is connected with communication terminal4bthat performs the transmission process of packet data (source packet data) and the reception process of packet data (destination packet data). Communication terminal4bmay be part of earth station12. Modem20transmits and receives packet data to and from satellite2. Satellite GW21transfers the received data (destination packet data) to communication terminal4b. When communication terminal4bperforms packet data communications over communication channel5(satellite channel5), satellite GW21serves as a gateway of the satellite GW for transmitting packet data to another earth station12via HUB station transceiver unit13. That is to say, satellite GW21bridges packet data between modem20and communication terminal4band sorts packet data to an appropriate communication channel5, that is, functions as a gateway into communication channel5(satellite channel5). The function of L2 address validation unit19is shown separately from the function of satellite GW21inFIGS.1and3as inFIG.2. In other words, L2 address validation unit19may be provided in satellite GW21that functions as a gateway into a communication channel. The dashed lines shown inFIG.5indicate bidirectional flows of communication data (packet data) between HUB station11and three earth stations, namely, earth station121, earth station122, and earth station123.

The example sequence of the data communication method according to Embodiment 1 is as shown inFIG.6, that is, an example sequence in which packet data communications are performed via HUB station11between earth stations12in the same subnet over communication channel5via satellite2. The data communication method according to Embodiment 1 has a L2 address transmission step, a L2 address storage step, and a data transmission step. The L2 address transmission step of the data communication method according to Embodiment 1 is the step of transmitting, by earth station12, a L2 address of a source to HUB station11. The L2 address storage step of the data communication method according to Embodiment 1 is the step of storing, by HUB station11, the L2 address in L2 address management table141in association with earth station12that has transmitted the L2 address of the source.

The data transmission step of the data communication method according to Embodiment 1 is the step of reading, by HUB station11, a L2 address of a destination from destination packet data that is packet data including the L2 address of the destination, and transmitting (transferring), by HUB station11, the destination packet data to the associated earth station12with reference to L2 address management table141. The data transmission step is also the step of transmitting, by HUB station transceiver unit13, destination packet data to all earth stations12connected to communication channel5in the absence of earth station12associated in L2 address management table141. It suffices that the data transmission step transmits destination packet data to all earth stations12connected to communication channel5without referring to L2 address management table141when this step is performed with no earth station12having performed the L2 address transmission step. This is because the L2 address is not stored in L2 address management table141when the data transmission step is performed before the L2 address transmission step.

The L2 address transmission step transmits, by earth station12, source packet data that is the packet data including its L2 address (the L2 address of communication terminal4bconnected to earth station12) to HUB station11, and the L2 address storage step reads the L2 address from the source packet data and associates the L2 address with earth station12that has transmitted the source packet data. The L2 address transmission step may transmit the source packet data only for transmitting the L2 address to HUB station11, or transmit the source packet data together in the communications for transmitting packet data to another earth station12via HUB station11. When the source packet data is transmitted together in the communications for transmitting packet data to another earth station12, it is regarded that the source packet data is destination packet data. In this case, the packet data includes the L2 address of the source and the L2 address of the destination.

Also, the L2 address transmission step is performed at least after the establishment of communication channel5between HUB station11and earth stations12via satellite2. That is to say, earth station control unit18transmits the L2 address (source packet data) of the source at least after the establishment of communication channel5with HUB station11via satellite2. Similarly, HUB station transceiver unit13receives the L2 address (source packet data) of the source at least after the establishment of communication channel5with earth station12via satellite2. In this case, earth station control unit18transmits the L2 address (source packet data) of the source in the initial communications after the establishment of communication channel5. Similarly, HUB station transceiver unit13receives the L2 address (source packet data) of the source in the initial communications after the establishment of communication channel5. Although transmission of unnecessary packet data can be reduced immediately after the establishment of communication channel5by performing the L2 address transmission step in the initial communications after the establishment of communication channel5, for example, in consideration of the number of earth stations12, the L2 address transmission step may be performed on a reduced number of earth stations12in the initial communications. As described above, the L2 address transmission step may be performed together in the communications for transmitting packet data to another earth station12.

The data communication method according to Embodiment 1 may further include a reception step of receiving, by earth station12, destination packet data after the data transmission step. The data reception step is the step of receiving, by earth station transceiver unit18, the destination packet data transmitted from HUB station11and reading, by L2 address validation unit19, the L2 address of the destination and discarding the L2 address when the L2 address is not addressed to its earth station. When multiple communication terminals4bare connected to earth station12, multiple L2 addresses are provided as described above, and thus, the reception step is also performed assuming that multiple L2 addresses of its earth station are provided. To earth station12that has performed the L2 address transmission step among earth stations12, packet data (destination packet data) addressed to any other earth station12is not transmitted (transferred) from HUB station11, and accordingly, the function of L2 address validation unit19may be stopped. In this case, the data reception step is regarded as the step of stopping the determination of whether to discard the destination packet data through the reading of the L2 address of the destination from the destination packet data, after the L2 address transmission step. L2 address validation unit19stops the determination of whether to discard the destination packet data through the reading of the L2 address of the destination from the destination packet data that has been received by earth station transceiver unit18, after earth station transceiver unit18has transmitted source packet data to HUB station11. In this case, stopping the determination includes both the case where reading of the L2 address of the destination is stopped per se and the case where discarding is not determined though reading of the L2 address of the destination is performed. In an alternative configuration, L2 address validation unit19may not be implemented in earth station12, and due to the absence of communication terminal4bof the destination as in a conventional case, destination packet data may be discarded in earth station12. That is to say, the data reception step can be performed even in the absence of L2 address validation unit19.

Although description has been given to the components of the functional block diagram shown inFIG.1, the data communication method, the HUB station, and the earth station according to Embodiment 1 may have components of the functional block diagram shown inFIG.2, as described above. That is to say, communication stations1do not need to be operated separately for each of HUB station11and earth station12. It suffices that one of earth stations12that perform packet data communications in the same subnet over communication channel5has L2 address storage unit14and transceiver unit13. Similarly, the data communication method is a method of performing packet data communications between earth stations12in the same subnet over communication channel5. L2 address storage unit14stores a L2 address of a source, which is read by another earth station12from source packet data that is the packet data including the address of its earth station, in the L2 address management table in association with the other earth station12that has transmitted the source packet data. Transceiver unit13receives the source packet data transmitted from the other earth station12, read the L2 address of the destination from the destination packet data, and with reference to L2 address management table141, transmit the destination packet data to the associated earth station12.

The data communication method according to Embodiment 1 may include the data reception step, as well as the channel allocation step described above. Alternatively, the data communication method according to Embodiment 1 may include no data reception step and include only the channel allocation step. It has been described that the L2 address of communication terminal4bchanges when communication terminal4bconnected to earth station12changes to another terminal, and accordingly, the L2 address of earth station12changes as well. Such a case will be described in Embodiment 2. It is regarded that HUB station11relearns the L2 address of earth station12(communication terminal4b). Note that also in the data communication method according to Embodiment 2, the relation between the channel allocation step and the data reception step is similar to that of the data communication method according to Embodiment 1.

The data communication method according to Embodiment 1 will be described more specifically with reference toFIG.6. As shown inFIG.6, HUB station communication terminal4aconnected to HUB station11and communication terminal4bof each of earth station121, earth station122, and earth station123enable packet data communications in the same subnet over a communication channel.FIG.6illustrates the relation between earth stations12and HUB station11that enables packet data communications, which will not show communication channel5.

The procedure of the data communication method according to Embodiment 1 shown inFIG.6will be described. Satellite GW16of HUB station11manages L2 address management table141and has the function of transferring packet data to modem15in HUB station11in accordance with the content of the table. The correspondence between a L2 address that is the destination of the destination packet data obtained from the source packet data and earth station12(earth station121, earth station122, earth station123) in which communication terminal4bwith the L2 address is installed is written in L2 address management table141. As illustrated in <L2 ADDRESS MANAGEMENT TABLE 1>, nothing is registered in the initial state. Thus, as shown inFIG.6, HUB station11has no information on the L2 address of earth station121in the initial state even when the packet data addressed to earth station121(strictly speaking, addressed to communication terminal4bconnected to earth station121) is transmitted from HUB station11or transmitted via HUB station11, and accordingly, packet data is transmitted to all earth stations12. Earth station122and earth station123excluding earth station121receive the packet data addressed to earth station121and then discard the received packet data.

In earth station12(earth station121, earth station122, earth station123) that is opposite to HUB station11, when packet data (packet data addressed to HUB station11, addressed to another earth station12via HUB station11) is transmitted from communication terminal4b, satellite GW21of earth station12transfers the packet data to the HUB station11side over communication channel5via modem20. The L2 address of communication terminal4bis described as the source L2 address in this packet data.FIG.6shows the state in which packet data is transferred from communication terminal4bconnected to earth station121and satellite GW16on the HUB station11side learns the packet data. Upon receipt of the packet data transferred over communication channel5, satellite GW16on the HUB station11side reads the source L2 address written in the packet data and registers the relevant earth station12in L2 address management table141as illustrated in <L2 ADDRESS MANAGEMENT TABLE 2>.

When HUB station communication terminal4aof HUB station11transmits packet data addressed to earth station121from HUB station11or transmits via HUB station11, earth station121is associated if <L2 ADDRESS MANAGEMENT TABLE 2> is illustrated, and thus, HUB station communication terminal4adoes not need to transmit packet data to all earth stations12. In transmission of packet data, satellite GW16of HUB station11reads a destination L2 address from the packet data and retrieves the destination L2 address from L2 address management table141. When the correspondence between the destination L2 address and earth station12is registered in L2 address management table141, the packet data is transferred only to this earth station12and is not transferred to any other earth station12. When the correspondence between the destination L2 address and earth station12is not registered in L2 address management table141, the packet data is transferred to all earth stations12connected with communication channel5at that time.

As described above, it is regarded that the data communication method, the HUB station, and the earth station according to Embodiment 1 relate to the packet transfer scheme through learning of a L2 address in packet data communications in the same subnet (layer 2 network) over satellite channel5by the satellite communication system. Since the disclosure according to Embodiment 1 restricts a transfer destination of packet data on the HUB station11or earth station12side in accordance with the reception of the packet data from the earth station12side, an amount of packet data transferred to satellite channel5can be reduced, leading to increased use efficiency of satellite channel5. Specifically, transfer or transmission of packet data, transfer of which is not required, to earth station12can be minimized, leading to effective use of the band of satellite channel5. Also, it is regarded that a transmission amount is restricted (a band is effectively used) through learning of a L2 address in the configuration in which earth stations12are connected in the same subnet (layer 2).

A data communication method, a HUB station, and an earth station according to Embodiment 2 of the present disclosure will be described with reference toFIG.7. In the figure, the same references denote the same or corresponding parts, detailed description of which will be omitted. Embodiments 1 and 2 are common in the following respects. Satellite GW16(L2 address storage unit14) of HUB station11manages L2 address management table141. In L2 address management table1411, a L2 address of earth station12that serves as a destination is written, and no earth station12is registered in the initial state. Although Embodiment 2 will also mainly describe the components of the functional block shown inFIG.1, the data communication method, the HUB station, and the earth station according to Embodiment 2 may be the components shown in the functional block diagram shown inFIG.2or the functional block diagram shown inFIG.3.

Differences between Embodiments 1 and 2 will be mainly described. The parts that will not be described are common in Embodiments 1 and 2. In Embodiment 2, HUB station11relearns the L2 address of earth station12a(communication terminal4b). The other parts are essentially common between Embodiments 1 and 2. It is regarded that Embodiment 2 will describe the correspondence when communication terminal4bconnected to earth station12changes to another terminal and the L2 address of communication terminal4bchanges.

The data communication method according to Embodiment 2 will be specifically described with reference toFIG.7.FIG.7differs fromFIG.6used in the description of Embodiment 1 in that communication terminal4bconnected to earth station121moves to earth station122inFIG.7. The operations until this point are common betweenFIGS.6and7, which will not be repeated. In Embodiment 2, when a L2 address of a source read from the source packet data is transmitted from another earth station12different from earth station12associated with the L2 address in L2 address management table141, L2 address storage unit14associates anew the L2 address with the other earth station12to update L2 address management table141. When communication terminal4bconnected to its earth station is changed, L2 address validation unit19determines whether to discard the destination packet data with the changed L2 address as the L2 address of its earth station.

The procedure of the data communication method according to Embodiment 2 shown inFIG.7will be described. When packet data (packet data addressed to HUB station11, addressed to another earth station12via HUB station11) is transmitted from communication terminal4bconnected to earth station122after communication terminal4bconnected to earth station121has moved to earth station122in earth station12(earth station121, earth station122, earth station123) that is opposite to HUB station11, in satellite GW21of earth station12, the packet data is transferred to the HUB station11side over communication channel5via modem20. The L2 address of communication terminal4bis written in this packet data as a source L2 address.FIG.7shows the state in which packet data is transferred from communication terminal4bconnected to earth station122, and satellite GW16on the HUB station11relearns the packet data.

Upon receipt of the packet data transferred over communication channel5, satellite GW16on the HUB station11side reads a source L2 address written in the packet data and registers a relevant earth station12in L2 address management table141as indicated in <L2 ADDRESS MANAGEMENT TABLE 3>. That is to say, earth station121previously registered is deleted, and earth station122is registered. Of the operations in relearning, the operations other than the deletion are essentially similar to the operations of L2 address storage unit14(L2 address storage step) in satellite GW16which have been described in Embodiment 1.

That is to say, it is regarded that the L2 address storage step according to Embodiment 2 is the step of, when an L2 address associated in L2 address management table141is transmitted from another earth station12as a L2 address of a source, associating anew the L2 address with the other earth station12to update L2 address management table141.

When HUB station communication terminal4aof HUB station11transmits packet data addressed to earth station122from HUB station11or transmit via HUB station11, if <L2 ADDRESS MANAGEMENT TABLE 3> is illustrated, earth station122is associated, and accordingly, HUB station communication terminal4adoes not need to transmit packet data to all earth stations12. In transmission of packet data, satellite GW16of HUB station11reads a destination L2 address from packet data and retrieves the destination L2 address from L2 address management table141. When the correspondence between destination L2 address and earth station12is registered in L2 address management table141, packet data is transferred only to the relevant earth station12and is not transferred to any other earth station12. When the correspondence between destination L2 address and earth station12is not registered in L2 address management table141, packet data is transferred to all earth stations12connected with communication channel5at that time.

When communication terminal4bconnected to its earth station is changed, L2 address validation unit19may perform determination of whether to discard destination packet data with the changed L2 address as the L2 address of its earth station, as well as the following operation. After earth station transceiver unit18transmits, to HUB station11, source packet data including the changed L2 address as a L2 address of its earth station, L2 address validation unit19may stop the determination of whether to discard the destination packet data through reading of the L2 address of the destination from destination packet data. Stopping the determination of whether to discard the destination packet data is as described in Embodiment 1.

Although the case where earth station12connected with communication terminal4bis changed has been described, the data communication method, the HUB station, and the earth station according to Embodiment 2 can operate as described below even when communication terminal4bis simply removed. When communication terminal4bconnected to earth station121is removed from earth station121, satellite GW21in earth station12transfers packet data indicating the removed L2 address to the HUB station11side over communication channel5via modem20. The L2 address of communication terminal4bis described as a source L2 address in this packet data. Upon receipt of the packet data transferred through satellite GW16on the HUB station11side and communication channel5, the source L2 address written in the packet data is read and is registered in address management table141so as to indicate that communication terminal4bhas been removed from the relevant earth station12.

In other words, it is registered that communication terminal4bhas been removed from registered earth station121. Even when the packet data addressed to earth station121(strictly speaking, addressed to communication terminal4bconnected to earth station121) is transmitted from HUB station11or transmitted via HUB station11, the information on the L2 address of earth station121indicates that communication terminal4bhas been removed from earth station121, and thus, HUB station11does not transmit packet data to all earth stations12, and HUB station transceiver unit13of HUB station11discards packet data. A notification that communication terminal4bhas been removed from earth station121may be provided to earth station122and earth station123excluding earth station121by transmitting packet data. This notification may be triggered by registration of removal of communication terminal4bfrom earth station121.

As described above, even when earth station12connected with communication terminal4bis changed, the data communication method, the HUB station, and the earth station according to Embodiment 2 can reduce an amount of packet data transferred to satellite channel5to increase the use efficiency of satellite channel5as in Embodiment 1. Also, in Embodiment 2, after L2 address validation unit19(receiving step) transmits source packet data to HUB station11or earth station12, any one of the following operations may be performed: the operation of stopping the determination of whether to discard destination packet data through reading of a L2 address of a destination from destination packet data, and the operation of, when communication terminal4bconnected to its station is changed, determining whether to discard destination packet data with the changed L2 address as the L2 address of its station.

As described above, when communication terminals in the same subnet (layer 2 network) perform packet data communications over a satellite channel, the data communication method, the HUB station, and the earth station according to Embodiment 1, 2 do not need to constantly transfer packet data to all opposite earth stations and discard unnecessary packet data in the earth stations that have received packet data each time. Satellite channels generally have communication bands fewer than those of communication channels within ground equipment, and the bands of the satellite channels become narrower by constantly transferring packet data to an earth station, namely, an opposite station, that does not need to receive packet data. However, the above situation can be restricted. Thus, transfer of packet data to an earth station, namely, an opposite station that does not need to receive packet data, can be minimized, leading to effective use of satellite channel bands.

REFERENCE SIGNS LIST