AU2003248465A1

AU2003248465A1 - Satellite communication method, and satellite communication apparatus, earth station, and gateway station for use with the satellite communication method

Info

Publication number: AU2003248465A1
Application number: AU2003248465A
Authority: AU
Inventors: Toshihiro Shikama; Yoshiko Yamada
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 2002-09-30
Filing date: 2003-09-30
Publication date: 2004-04-22
Also published as: KR20040029282A; JP3743413B2; KR100593767B1; JP2004128632A

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): MITSUBISHI DENKI KABUSHIKI KAISHA Invention Title: SATELLITE COMMUNICATION METHOD, AND SATELLITE COMMUNICATION APPARATUS, EARTH STATION, AND GATEWAY STATION FOR USE WITH THE SATELLITE COMMUNICATION

METHOD

The following statement is a full description of this invention, including the best method of performing it known to me/us: SATELLITE COMMUNICATION METHOD, AND SATELLITE COMMUNICATION APPARATUS, EARTH STATION, AND GATEWAY STATION FOR USE WITH THE SATELLITE COMMUNICATION METHOD BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a satellite communication method of carrying out satellite communications between a gateway station and an earth station by way of a plurality of communications satellites that are flying approximately in the zenith direction one after another, and a satellite communication apparatus, an earth station and a gateway station for use with the satellite communication method.

Description of Related Art Systems for carrying out communications by using a group of Earth-orbiting communications satellites include Iridium communication system, Globalstar communication system, and so on. In Iridium system, there are provided 66 low-Earth-orbiting communications satellites each having an exchange function and an intersatellite communication function so that communications between terminals far away from each other can be established via communications among a plurality of adjacent communications satellites. In other words, each communication terminal on the ground can establish a communication link with a communications satellite with which it can communicate so that it can communicate with a faraway terminal that establishes a communication link with another communications satellite through communications between communications satellites.

Because communications satellites orbit around the Earth, a communications satellite that establishes a communication link with each communication terminal on the ground is not always identical, and therefore each H:\Valma\Keep\Specifications\P5088003061-AUdoc 30/09/03 communication terminal goes away from the cover area of one communications satellite and then moves to the cover area of another communications satellite. In this case, each communication terminal on the ground disconnects a communication link with one communications satellite, and then establishes a communication link with another communications satellite. This process is called a handover between communications satellites.

In order to perform a handover between communications satellites in a prior art satellite communication system, each communication terminal on the ground monitors an electric wave transmitted thereto from a communications satellite with which it establishes a communication link, and, when the received electric wave has a level below a predetermined value, transmits a handover request signal to the communications satellite. Disconnecting of the communication link with the communications satellite and establishment of another communication link with another communications satellite are then carried out in response to this request signal.

Fig. 13 is a sequence diagram showing a handover between communications satellites as disclosed in Japanese patent application publication (TOKKAI) No. 2000-315972, for example. Fig. 13 shows a handover from a first Earthorbiting communications satellite to a second Earthorbiting communications satellite, which is intended for a subscriber that is a communication terminal on the ground.

In Fig. 13, an NOC an earth station) transmits the physical location (latitude and longitude) of a point that is located just under each communications satellite, a schedule indicating the time when the physical location is measured, the physical location (latitude and longitude) of a point that is located just under an immediately preceding communications satellite, a schedule indicating the time H:\Valma\Keep\Specifications\P50880.0306 1-AU.doc 30/09/03

I

when the physical location is measured, the physical location (latitude and longitude) of a point that is located just under an immediately following communications satellite, a schedule indicating the time when the physical location is measured, and the physical location (latitude and longitude) of a point that is located just under an adjacent communications satellite, a schedule indicating the time when the physical location is measured to each of all communications satellites. Those pieces of information are registered in each communications satellite (Al, A2) The first communications satellite determines a start time when a handover is started and an end time when the handover is completed based on the information about communications satellites, adds those pieces of time information to a control signal, and transmits the control signal to the second communications satellite that is the target of the handover. When the second communications satellite receives the control signal and the additional information, a handover between the first communications satellite and the second communications satellite is started In addition, a downlink message to the subscriber is transmitted from the first communications satellite to the second communications satellite The first communications satellite transmits the last communication data to the subscriber by way of a communication link that is established with the subscribe After that, the second communications satellite transmits the first communication data to the subscriber by way of a communication link that is newly established with the subscriber Then an upward communication link from the subscriber to the second communications satellite is prepared, and the last uplink (A7) to the first communications satellite and the first uplink (A8) to the second communications satellite are provided.

H:\Valma\Keep\Specifications\P50880.03061-AUdoc 30/09/03 A problem with a prior art satellite communication system is that because each communication terminal on the ground monitors an electric wave transmitted thereto from a communications satellite with which it establishes a communication link, and, when the received electric wave has a level below a predetermined value, transmits a handover request signal to the communications satellite, a communication link between the communication satellite and another communications satellite is established in response to this request signal and a handover between the communications satellites is carried out, and transmission of communication data to each communication terminal is performed, the implementation of the series of handover processes requires transmission of a large amount of data.

Another problem is that because the handover processing is carried out between two communications satellites, and each communications satellite needs a transmitter/receiver intended for establishing a communication link with another communications satellite, a computer intended for performing the handover processing and a storage unit intended for storing processing information, each communications satellite increases in size and decreases in reliability. A problem with a prior art method of performing a handover between communications satellites as disclosed in Japanese patent application publication (TOKKAI) No. 2000-315972 is that though each communication terminal on the ground can omit a procedure for making a request for the handover processing by predicting a start time when a handover is started based on positional information about the positions of communications satellites in question, those communications satellites have to repeatedly determine a start time when the handover processing is started and therefore have a large load of performing the handover processing.

H:\Valma\Keep\Specifications\P50880.03061PAUdoc 30/09/03 SUMMARY OF THE INVENTION The present invention is proposed to solve the abovementioned problems, and it is therefore an object of the present invention to provide a satellite communication method capable of reducing the time required for performing handover processing and reducing the size of handover processing circuitry in a communication system for carrying out communications by way of a plurality of communications satellites that are flying approximately in the zenith direction one after another, and a satellite communication apparatus, an earth station and a gateway station for use with the method.

In accordance with an aspect of the present invention, there is provided a satellite communication method of carrying out communications between a gateway station and an earth station by way of a plurality of communications satellites that are flying approximately in zenith direction one after another, said method comprising the steps of, when switching from communications via a communications satellite to communications via another communications satellite that flies approximately in zenith direction for the next time, said gateway station or said earth station continuously transmitting switching instruction information by way of a transmission line passing through said one communications satellite, and then switching to communications via said other communications satellite.

As a result, the time required for performing the handover processing can be reduced, and the size of circuitry intended for performing the handover processing can be reduced.

In accordance with another aspect of the present invention, there is provided a satellite communication H:\Valma\Keep\Specifications\P50880.0306 I-AUdoc 30109103 apparatus including a plurality of communications satellites that are flying approximately in zenith direction one after another, and a gateway station and an earth station that communicate with each other by way of one of the plurality of communications satellites, when switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, said gateway station continuously transmitting switching instruction information to said earth station and then switching to communications via said other communications satellite, and said earth station receiving the switching instruction information from said gateway station and then switching from communications via said one communications satellite to communications via said other communications satellite.

In accordance with a further aspect of the present invention, there is provided a gateway station including a transmitting unit for communicating with an earth station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another, a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, and a switching instruction packet generating unit for generating a packet including switching instruction information based on the instruction from said satellite switching instructing unit, and for sending the packet to H:\Valma\Keep\Specifications\P50880.0306 I-AUdoc 30/09/03 said transmitting unit, said transmitting unit continuously transmitting the packet including the switching instruction information generated by said switching instruction packet generating unit.

In accordance with another aspect of the present invention, there is provided an earth station including a transmitting unit for communicating with a gateway station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another; a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, and a switching instruction packet generating unit for generating a packet including switching instruction information based on the instruction from said satellite switching instructing unit, and for sending the packet to said transmitting unit, said transmitting unit continuously transmitting the packet including the switching instruction information generated by said switching instruction packet generating unit.

In accordance with a further aspect of the present invention, there is provided a satellite communication apparatus including a plurality of communications satellites that are flying approximately in zenith H:\Valma\Keep\Specifications\P50880.0306 1AU.doc 30109/03 direction one after another, and a gateway station and an earth station that communicate with each other by way of one of the plurality of communications satellites, when switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, said gateway station continuously transmitting switching instruction information to said earth station and then switching to communications via said other communications satellite, and said earth station receiving the switching instruction information from said gateway station, switching from communications via said one communications satellite to communications via said other communications satellite, and then, based on the switching instruction information, transmitting switching instruction information for instructing said gateway station to switch from a receive line passing through said one communications satellite to another transmission passing through said other communications satellite to said gateway station.

As a result, the earth station can omit a process of determining a time when a handover occurs between satellites, thereby reducing the time required for performing the handover processing.

In accordance with another aspect of the present invention, there is provided an earth station including a transmitting unit for communicating with a gateway station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another, a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, H:\Valma\Keep\Specfications\P50880.0306 1-AUdoc 30109I03 and a switching instruction packet generating unit for generating a packet including switching instruction information based on the instruction from said satellite switching instructing unit, and for sending the packet to said transmitting unit, said satellite switching instructing unit instructing said switching instruction packet generating unit to generate the packet including the switching instruction information in response to switching instruction information from said gateway station, and said transmitting unit continuously transmitting the packet including the switching instruction information generated by said switching instruction packet generating unit to said gateway station.

As a result, the earth station can omit a process of determining a time when a handover occurs between satellites, thereby reducing the time required for performing handover processing.

In accordance with a further aspect of the present invention, there is provided an earth station including a receiving unit for receiving a signal transmitted thereto from a gateway station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another, and a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, when said receiving unit receives no switching instruction information from said gateway station after a handover time elapses, said satellite switching instructing unit instructing said receiving unit switch from a communication line passing through said one communications satellite to another communication line passing through said other H:\Valma\Keep\Specifcations\P50880.03061AU.doc 30/09/03 communications satellite.

As a result, even if there is a loss of received switching instruction information due to shielding or the like that occurs over the earth station, a handover between communications satellites can be recovered.

In accordance with another aspect of the present invention, there is provided an earth station including a receiving unit for receiving a signal transmitted thereto from a gateway station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another, a transmitting unit for transmitting a signal to said gateway station, and a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, when said receiving unit receives no switching instruction information from said gateway station after a handover time elapses, said satellite switching instructing unit instructing said transmitting unit to transmit a request signal indicating a request for transmission of switching instruction information to said gateway station.

Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the structure of a H:\Valma\Keep\Specifcations\P50880.0306 1AU~doc 30/09/03 satellite communication system that employs a satellite communication method in accordance with embodiment 1 of the present invention; Fig. 2 is a schematic diagram showing an example of a track that is formed on a ground by a communications satellite for use with the satellite communication method in accordance with embodiment 1 of the present invention; Fig. 3 is a schematic diagram for explaining a handover that is carried out when switching between communications satellites is performed in accordance with the satellite communication method in accordance with embodiment 1 of the present invention; Fig. 4 is a schematic diagram showing contents transmitted and received when a handover between communications satellites is carried out according to the satellite communication method of embodiment 1 of the present invention; Fig. 5 is a schematic diagram showing contents transmitted and received when a handover between communications satellites is carried out according to the satellite communication method of embodiment 1 of the present invention; Fig. 6 is a block diagram showing the structure of a gateway station for use with the satellite communication method in accordance with embodiment 1 of the present invention; Fig. 7 is a block diagram showing the structure of an earth station for use with the satellite communication method in accordance with embodiment 1 of the present invention; Fig. 8 is a schematic diagram showing contents transmitted and received when a handover between communications satellites is carried out according to a satellite communication method in accordance with H:\Valma\Keep\Speifications\P50880.03061AU.doc 30/09/03 embodiment 2 of the present invention; Fig. 9 is a schematic diagram showing contents transmitted and received when a handover between communications satellites is carried out according to a satellite communication method in accordance with embodiment 3 of the present invention; Fig. 10 is a block diagram showing the structure of a switching instruction packet for use in the satellite communication method in accordance with embodiment 3 of the present invention; Fig. 11 is a schematic diagram showing contents transmitted and received when a handover between communications satellites is carried out according to the satellite communication method in accordance with embodiment 3 of the present invention; Fig. 12 is a schematic diagram showing time division frames included in a signal transmitted and received when a handover between communications satellites is carried out according to a satellite communication method in accordance with embodiment 4 of the present invention; and Fig. 13 is a sequence diagram showing a handover process performed by a prior art satellite communication system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1.

A satellite communication method in accordance with embodiment 1 of the present invention, and a satellite communication apparatus, an earth station and a gateway station for use with the satellite communication method will be explained with reference to Figs. 1 to 7. Fig. 1 is a block diagram showing the structure of a satellite communication system that employs the satellite communication method in accordance with embodiment 1. In H:\Valma\Keep\Specifications\P50880.0306 1-AUdoc 30109/03 Fig. i, reference numerals la to Ic denote communications satellites, respectively, reference numeral 2 denotes a communication area formed by the communications satellite la, reference numerals 3a to 3c denote gateway stations, respectively, and reference numerals 4a to 4e denote earth stations, respectively. Each of the earth stations 4a to 4e can be a fixed station fixed to the ground, or a mobile station installed in a vehicle, ship, aircraft, or the like.

Each of the three communications satellites la to ic is a non-stationary communications satellite that can fly approximately in the zenith direction of a specific region, and forms multiple beams on the communication area 2a including this specific region. Fig. 1 shows a case where Japan is the specific region. The three communications satellites la to ic stay one after another in a range having a predetermined elevation angle within the specific region for eight hours in such a manner that the communications satellite la stays in the range having a predetermined elevation angle within the specific region for eight hours, the communications satellite lb then enters the range having a predetermined elevation angle and stays in the range for eight hours after the communications satellite la goes away from the range having a predetermined elevation angle, and the communications satellite ic then enters the range having a predetermined elevation angle and stays in the range for eight hours after the communications satellite Ib goes away from the range having a predetermined elevation angle, and provide communication services one after another. As a result, the group of three communications satellites can ensure communications between gateway stations and earth stations at all times. The above-mentioned range having a predetermined elevation angle can be an elevation angle H:\Valma\Keep\Specifications\P50880.03061-AUdoc 30/09/03 range having elevation angles of 70 to 90 degrees (zenith), for example, and is defined depending upon an individual communication satellite system. Furthermore, each of the plurality of communications satellites la to ic that are flying approximately in the zenith direction can be an elliptic orbit satellite having a periodicity of 24 hours, an orbit tilt angle of 45 degrees, an eccentricity of 0.139, an apogee altitude of 41,650 km, and a perigee altitude of 29,930 km, for example. Fig. 2 shows an example of a track which is formed on the ground by a point located directly under each communications satellite that is flying in such an orbit, and each communications satellite might be called a semi-zenith satellite.

Although the satellite communication system of this embodiment has the three communications satellites, the number of communications satellites included in an individual communication satellite system can be determined independently.

In Fig. 1, each of the gateway stations 3a to 3c establishes a communication link with a client by way of a communication network, and communicates with each of the earth stations 4a to 4e by way of any one of the communications satellites la to ic. As a usage pattern of such a communication system, there are a case in which the client is a broadcaster, each of the plurality of earth stations 4a to 4e is a vehicle-mounted earth station an SNG station: Satellite News Gathering station), and data about video and voice from each vehicle-mounted earth station are transmitted through any one of the plurality of communications satellites la to Ic and are then transmitted to the broadcaster by way of a gateway station, and a case in which the client is a broadcaster, each of the plurality of earth stations 4a to 4e is a receiving antenna station that can be mounted on a general vehicle, and data about H:\Valma\Keep\Specications\P50880.03061-AUdoc 30/09/03 video and voice from the broadcaster are transmitted through any one of the plurality of gateway stations 3a to 3c and are then transmitted to each earth station by way of any one of the plurality of communications satellites la to ic. Furthermore, there can be various usage patterns such as a pattern in which the client is a hospital and each of the plurality of earth stations 4a to 4e is an emergency vehicle, and data about video and voice from each emergency vehicle that is an earth station are transmitted through any one of the plurality of communications satellites la to ic and are then transmitted to the hospital by way of a gateway station.

Fig. 3 is a schematic diagram for explaining a handover that is carried out when switching between communications satellites is performed for a transmission line from a gateway station 3 to an earth station 4. In Fig. 3, reference numerals 5a and 5b denote communication lines used for transmission of data packets from the gateway station 3 to the communications satellites la and ib, respectively, and reference numerals 6a and 6b denote communication lines used for transmission of data packets from the communications satellite la and Ib to the earth station 4, respectively.

The gateway station 3 modulates data packets with a carrier signal having a frequency of fl and transmits them to the communications satellite la by way of the communication line 5a within a time period during which the communications satellite la provides a communication service during which the communications satellite la is staying in a range having a predetermined elevation angle within the specific region). The communications satellite la receives the data packets from the gateway station 3 by way of the communication line 5a, modulates the received data packets with a carrier signal having a H:\Valma\Keep\Specifications\P50880.03061-AUdoc 30/09/03 frequency of f2, and then transmits them to the earth station 4 by way of the communication line 6a. The earth station 4 thus receives the data packets by way of the communication line 6a. When the time period during which the communications satellite la provides a communication service ends and the next time period during which the communications satellite lb provides a communication service comes, the gateway station 3 modulates data packets with a carrier signal having a frequency of fl and transmits them to the other communications satellite Ib by way of the other communication line 5b within the next time period. The other communications satellite Ib receives the data packets from the gateway station 3 by way of the other communication line 5b, modulates the received data packets with a carrier signal having a frequency of f3, and then transmits them to the earth station 4 by way of the other communication line 6b. The earth station 4 thus receives the data packets by way of the other communication line 6b.

Next, handover processing that is carried out when switching from communications via the communications satellite la to communications via the other communications satellite lb will be explained with reference to Fig. 4.

Fig. 4 is a schematic diagram showing contents transmitted and received between communications satellites when a handover is carried out. In Fig. 4, reference numeral 7 denotes a data packet including data about video, voice, and so on. Data packets 7 are transmitted from the gateway station 3 to the earth station in order of P1, P2, P3, and Reference numeral 8 denotes a switching instruction packet for instructing the earth station 4 that is a receiver to perform switching between communication lines.

The gateway station 3 (referred to as GW station in the figure) transmits data packets P1, P2, and P3 to the communications satellite la by way of the communication H:\Valma\Keep\Specifications\P50880.0306 lAU.doc 30/09/03 line 5a (having a frequency of f and the communications satellite la then transmits them to the earth station 4 by way of the communication line 6a (having a frequency of f2). Based on a notification from a communications satellite management station, the gateway station 3 can grasp a time when the communications satellite lb enters the range having a predetermined elevation angle and starts providing a communication service. As an alternative, the gateway station 3 can determine the time when the communications satellite lb enters the range having a predetermined elevation angle and starts providing a communication service based on information about the orbit of the communications satellite Ib or can grasp the time based on an absolute time when the communications satellite Ib comes flying and the periodicity of the communications satellite lb. When switching from the communication line via which communications are being carried out to the other communication line 5b (having a frequency of fl) to the communications satellite ib, the gateway station 3 continuously transmits a switching instruction packet 8 to the earth station 4. This switching instruction packet 8 is continuously transmitted to the communications satellite la by way of the communication line 5a, and is further transmitted from the communications satellite la to the earth station 4 by way of the communication line 6a. The earth station 4 thus receives a series of switching instruction packets 8 by way of the communication line 6a.

When receiving a switching instruction packet 8, the earth station 4 switches to the other communication line (switches to another receive frequency) based on information included in the packet, or switches to a certain communication line that is provided in advance shifts to another channel by one channel, for example). In other words, the earth station 4 switches H:\Valma\Keep\Speifications\P50880.03061-AUdoc 30/09/03 from the communication line 6a to the other communication line 6b (having a frequency of f3) so as to receive data packets from the gateway station 3 by way of the other communication line 6b.

A handover between communications satellites is carried out as previously mentioned. Fig. 4 further shows that the gateway station 3 continuously transmits a switching instruction packet 8 to the earth'station by way of both the communication line 5a and the other communication line 5b so that a series of switching instruction packets 8 is overlappedly transmitted to the earth station when performing switching between transmission lines. In this case, after switching to the other communication line 6b, the earth station 4 can further receive a series of switching instruction packets 8. When receiving a series of switching instruction packets 8 immediately after switching to the other communication line 6b, the earth station 4 can determine that there is no loss of data packets received thereby within a time period during which the earth station 4 performs the switching to the other communication line 6b.

Next, a description will be made as to a handover between communications satellites that is carried out when the earth station 4 is a transmitting side and the gateway station 3 is a receiving side in contrast to the case of Fig. 4. Fig. 5 is a schematic diagram showing contents transmitted and received between communications satellites when a handover is carried out in a case where the gateway station and earth station of Fig. 4 change places. Like the gateway station 3 of Fig. 4, based on a notification from a communications satellite management station, the earth station 4 of Fig. 5 can grasp a time when the communications satellite lb enters the range having a predetermined elevation angle and starts providing a H:\Valma\Keep\Specifications\P50880.03061AU.doc 30/09/03 communication service. As an alternative, the earth station 4 can determine the time when the communications satellite lb enters the range having a predetermined elevation angle and starts providing a communication service based on information about the orbit of the communications satellite lb or can grasp the time based on an absolute time when the communications satellite Ib comes flying and the periodicity of the communications satellite lb. Transmission of data packets P1, P2, and P3 from the earth station 4, a series of switching instruction packets, and data packets P4, P5 and so on, and reception of those packets by the gateway station 3 are carried out in the same way as previously explained with reference to Fig. 4.

In view of bidirectional communications between the earth station 4 and the gateway station 3, as shown in Fig. 5, a communication line 9a (having a frequency of f4) is used between the earth station 4 and the communications satellite la, another communication line 9b (having a frequency of f4) is used between the earth station 4 and the other communications satellite Ib, a communication line (having a frequency of f5) is used between the communications satellite la and the gateway station 4, and another communication line 10b (having a frequency of f6) is used between the other communications satellite lb and the gateway station 4.

Next, the structure of the gateway station 3 will be explained. Fig. 6 is a block diagram showing the structure of the gateway station 3. In Fig. 6, reference numeral 11 denotes a transmitting antenna, reference numeral 12 denotes a receiving antenna, reference numeral 13 denotes a transmitting unit for transmitting packets to the earth station 4 by communicating with a communications satellite by way of the transmitting antenna 11, reference numeral 14 denotes a packet sending unit for sending data packets to H:\Valma\Keep\Specications\P50880.03061-AUdoc 30109/03 the transmitting unit 13, reference numeral 15 denotes a switching instruction packet generating unit for generating a switching instruction packet, reference numeral 16 denotes an interface for interfacing with a communication network, reference numeral 17 denotes a data packet generating unit for generating data packets from communication data transmitted thereto by way of the communication network, reference numeral 18 denotes a data packet storage unit for temporarily storing data packets to be sent out by the packet sending unit, reference numeral 19 denotes a receiving unit for receiving a signal transmitted thereto from the earth station 4 by way of a communications satellite when the earth station 4 transmits the signal thereto, reference numeral 20 denotes a received signal processing unit for converting the received signal from the earth station 4 into a transmission signal according to a protocol used by the communication network, and reference numeral 21 denotes a satellite switching instructing unit for instructing each of the transmitting unit 13 and the receiving unit 19 to switch between communications satellites between communication lines) Next, an operation of the gateway station 3 will be explained with reference to Fig. 6. Communication data from each client are input to the network I/F unit 16 by way of the communication network. The network I/F unit 16 sequentially delivers the received communication data to the data packet generating unit 17. The data packet generating unit 17 generates data packets from the received communication data and delivers them to the packet sending unit 14. When the format of packets transmitted via the communication network is the same as that of packets transmitted via the satellite communication system, the data packet generating unit 17 can make the received H:\Valma\Keep\Specifications\P50880.03061PAUdoc 30/09/03 communication data to pass therethrough and output them just as they are. Furthermore, when the communication data received from the network I/F unit 16 are stream data or the like, the data packet generating unit 17 generates data packets by time-based compressing the communication data when necessary. The data packets generated by the data packet generating unit 17 and a switching instruction packet from the switching instruction packet generating unit 15 are input to the packet sending unit 14. While a switching instruction packet is input, the packet sending unit 14 delivers the packet to the transmitting unit 13 and delivers the data packets generated by the data packet generating unit 17 to the data packet storage unit 18. In contrast, while no switching instruction packet is input, the packet sending unit 14 delivers the data packets generated by the data packet generating unit 17 to the transmitting unit 13. When data packets are stored in the data packet storage unit 18, the packet sending unit 14 gives a higher priority to the data packets and then delivers them to the transmitting unit 13. Though the transmitting unit 13 transmits the generated data packets to the earth station one by one, the transmitting unit 13 continuously transmits the switching instruction packet to the earth station several times. As previously explained with reference to Fig. 4, when performing switching from the communication line 5a to the other communication line the transmitting unit 16 continuously transmits a switching instruction packet to the earth station by way of both the communication line 5a, which has been selected before the switching, and the other communication line which is selected after the switching, so that a series of switching instruction packets are overlappedly transmitted to the earth station. The satellite switching instructing unit 21 determines whether to perform this switching H:\Valma\Keep\Specifcations\P50880.0306 IlAU.doc 30/09/03 between the communication lines, then sends an instruction for switching between the communication lines to the transmitting unit 13, and instructs the switching instruction packet generating unit 15 to generate and output a switching instruction packet. The satellite switching instructing unit 21 grasps a time when the next communications satellite lb enters the range having a predetermined elevation angle and starts providing a communication service based on a notification from a communications satellite management station. As an alternative, the earth station 4 determines the time when the communications satellite lb enters the range having a predetermined elevation angle and starts providing a communication service based on information about the orbit of the communications satellite Ib or grasps the time based on an absolute time when the communications satellite Ib comes flying and the periodicity of the communications satellite lb. The satellite switching instructing unit 21 then provides a sequence of instructions for switching to the next communications satellite lb.

The receiving unit 19 of the gateway station 3 receives a transmission signal transmitted thereto from the earth station 4 by way of the receiving antenna 12. The receiving unit 19 then converts the received RF signal into a signal having a lower frequency and delivers it to the received signal processing unit 20. The received signal processing unit 20 extracts data packets or a series of switching instruction packets from the signal having a lower frequency while distinguishing them from each other.

The received signal processing unit 20 then delivers the data packets to the network I/F unit 16, and the series of switching instruction packets 13 to the satellite switching instructing unit 21. Furthermore, the received signal processing unit 20 time-based expands compressed stream H:\Valma\Keep\Specifications\P50880.03061-AUdoc 30/09/03 data or the like when necessary so as to reproduce stream data, and delivers them to the network I/F unit 16. The network I/F unit 16 then transmits the data packets from the received signal processing unit 20 to a client on the communication network. On the other hand, when receiving a switching instruction packet, the satellite switching instructing unit 21 instructs the receiving unit 19 to perform switching between communications satellites between communication lines) Next, the structure of the earth station 4 will be explained. Fig. 7 is a block diagram showing the structure of the earth station 4. In Fig. 7, reference numeral 22 denotes an input and output I/F unit for inputting or outputting data about voice and video stored in the earth station 4, or control data. In Fig. 7, circuit components designated by the same reference numerals as shown in Fig.

6 are the same as or similar to those of Fig. 6.

The earth station 4 has a structure that is nearly the same as that of the gateway station 3. However, because the gateway station 3 transmits and receives communication data to and from a plurality of clients, and also transmits and receives these communication data to and from a plurality of earth stations 4, the gateway station 3 has a large amount of processed signals and increases in circuit size. In contrast, because data transmitted and received by the earth station 4 are limited to data about voice, video and so on handled by the earth station 4, the earth station 4 has a smaller circuit size than the gateway station 3. When the earth station 4 is a station that carries out only reception of data andso on, the earth station 4 can be comprised of the receiving antenna 12, the receiving unit 19, the received signal processing unit the satellite switching instructing unit 21, and the input and output I/F unit 22.

H:\Valma\Keep\Specifications\P50880.0306 1AU.doc 30/09/03 As previously mentioned, in accordance with embodiment 1, a transmitting station transmits switching instruction information indicating an instruction for switching between communication lines to a receiving station at predetermined time when a handover is carried out, and the receiving station performs switching between communication lines in response to the switching instruction information from the transmitting station. As a result, the time required for performing the handover processing can be reduced, and the size of circuitry intended for performing the handover processing can be reduced.

Embodiment 2.

A satellite communication method in accordance with embodiment 2 of the present invention, and a satellite communication apparatus, an earth station and a gateway station for use with the method will be explained hereafter with reference to Fig. 8. Fig. 8 is a schematic diagram showing contents transmitted and received between communications satellites in accordance with embodiment 2 when a handover is carried out. In Fig. 8, circuit components designated by the same reference numerals as shown in Figs. 4 and 5 are the same as or similar to those of Figs. 4 and In Fig. 8, the gateway station 3 and the earth station 4 transmit and receive data packets and so on bidirectionally to and from each other, and communication lines are used when the gateway station 3 transmits data packets and so on to the earth station and when the earth station 4 receives data packets and so on from the gateway station and contents are transmitted and received by each of the gateway station 3 and the earth station 4, as previously explained with reference to Fig. 4. In Fig. 8, H:\Valma\Keep\Specifications\P50880.03061AU.do 3O93 data packets 7 transmitted from the gateway station 3 are designated as Pal, Pa2, and When receiving a switching instruction packet 8, the earth station 4 determines that a handover occurs between two communications satellites based on this reception, and starts handover processing for transmission. First of all, the earth station 4 transmits data packet Pbl and Pb2 to a communications satellite la by way of a communication line 9a (having a frequency of f4).

The communications satellite la then transmits the data packets to the gateway station 3 by way of a communication line 10a (having a frequency of f5). In a case where the earth station 4 determines that a handover between communications satellites occurs, as previously mentioned, when switching from the communication line 9a via which communications are being carried out to another communication line 9b (having a frequency of f4) associated with another communications satellite ib, the earth station 4 continuously transmits a switching instruction packet 8 to the gateway station 3. This switching instruction packet 8 is continuously transmitted to the communications satellite la by way of the communication line 9a, and is further transmitted from the communications satellite la to the gateway station 3 by way of the communication line (having a frequency of f5). The gateway station 3 thus receives the switching instruction packet 8 by way of the communication line 10a. When receiving the switching instruction packet 8, the gateway station 3 switches between communication lines between receive frequencies) based on information included in the switching instruction packet 8, or switches to a certain communication line that is provided in advance shifts to another channel by one channel). In other words, the gateway station 3 switches from the communication line to the other communication line 10b (having a frequency H:\Valma\Keep\Specifications\P50880.03061-AU.doc 30/09/03 of f6) and then receives subsequent data packets by way of the other communication line Fig. 8 shows a case where a series of switching instruction packets are overlappedly transmitted by way of both the communication line 10a and the other communication line o10b when the earth station 4 performs switching between transmission lines. In this case, after switching to the other communication line o10b, the gateway station 3 can receive the series of switching instruction packets 8.

When receiving a switching instruction packet 8 immediately after switching to the other communication line 10b, the gateway station 3 can determine that there is no loss of received data packets within a time period during which the gateway station 3 performs the switching to the other communication line.

On the other hand, when receiving a switching instruction packet, the earth station 4 determines that a handover occurs between communications satellites based on this reception, and then starts handover processing for transmission. A satellite switching instructing unit 21 as shown in Fig. 7 performs this handover processing. The gateway station 3 in accordance with embodiment 2 has the same structure as shown in Fig. 6, and the earth station 4 in accordance with embodiment 2 has the same structure as shown in Fig. 7.

As previously mentioned, in accordance with embodiment 2, in response to switching instruction information from the gateway station, the earth station performs switching between receive lines, and then transmits switching instruction information for instructing the gateway station to switch between receive lines to the gateway station. As a result, the earth station can omit a process of determining the time when a handover occurs between satellites, thereby reducing the time required for H:\Valma\Keep\Specifications\P50880.03061-AUdoc 30/09/03 performing the handover processing.

Embodiment 3.

A satellite communication method in accordance with embodiment 3 of the present invention, and a satellite communication apparatus, an earth station and a gateway station for use with the method will be explained hereafter with reference to Figs. 9 to 11. In accordance with embodiment 3, the satellite communication method includes the. step of continuously transmitting a switching instruction packet having a data section in which an already transmitted data packet is written and a header section in which switching instruction information is written so as to perform a handover between communications satellites. Fig. 9 is a schematic diagram showing contents transmitted and received between communications satellites in accordance with embodiment 3 when a handover is carried out. In Fig. 9, reference numeral 23 denotes a switching instruction packet having a data section in which an already transmitted data packet is written and a header section in which switching instruction information is written. In Fig. 9, circuit components designated by the same reference numerals as shown in Fig. 4 are the same as or similar to those of Fig. 4.

When switching from communications via a communications satellite la to communications via another communications satellite ib, handover processing is carried out as shown in Fig. 4, with the exception that a series of switching instruction packets 8 shown in Fig. 4 is replaced by a series of switching instruction packets 23 each having a header section in which switching instruction information is written, and the earth station 4 that is a receiver switches between communication lines in. response to a switching instruction packet 23. In addition, as shown in H:\Valma\Keep\Specifications\P50880.0306 1AU.doc 30109/03 Fig. 9, when switching between transmission lines, the gateway station 3 transmits a series of switching instruction packets 23 overlappedly to both a communication line 5a and another communication line 5b. In this case, after switching from a communication line 6a to another communication line 6b, the earth station 4 can receive a series of switching instruction packets 23 from the gateway station 3. When receiving a series of switching instruction packets 23 immediately after switching to the other communication line, the earth station 4 can determine that there is no loss of received data packets within a time period during which the earth station switches to the other communication line. Even when the earth station 4 is a transmitting side and the gateway station 3 is a receiving side, a series of switching instruction packets 8 shown in Fig. 5 is replaced by a series of switching instruction packets 23 and handover processing is carried out between communications satellites as shown in Fig. Fig. 10 shows the structure of a switching instruction packet 23 in accordance with embodiment 3. A switching instruction packet 23 consists of a header section, in which a serial number N assigned to the packet and switching instruction information F are written, and a data section. The serial number N of a switching instruction packet 23 indicates the serial number of a previously transmitted data packet including data written in the data section of the switching instruction packet 23.

In the case of Fig. 10, because each switching instruction packet 23 stores data included in an immediately preceding data packet P3, the serial number of each switching instruction packet 23 is the same as that of the data packet P3. The switching instruction information F has a value that is either 0 indicating non-switching between communication lines or 1 indicating switching between H:\Valma\Keep\Speciflcations\P0880.03061AU.doc 30t09/03 communication lines, for example. The serial number N is the same as that of a preceding packet that has already been transmitted. As a result, the receiving side can determine that each received switching instruction packet 23 is related to retransmission of an already transmitted data packet. In other words, the data section of each received switching instruction packet 23 stores the contents stored in the data section of a packet that has already been transmitted. The switching instruction information F is extracted from each received switching instruction packet 23 by a received signal processing unit as shown in Figs. 6 and 7, and is then delivered to a satellite switching instructing unit 21. When the extracted switching instruction information F indicates an instruction for switching between communication lines F 1 in the above-mentioned example), the satellite switching instructing unit 21 performs a switching process of switching between communication lines. The received signal processing unit 20 then compares the serial number N of each received switching instruction packet 23 with that of each of packets which have already been transmitted thereto, and, when the serial number N is equal to that of a packet which has already been normally transmitted, discards data stored in the data section of each received switching instruction packet 23. In contrast, if the serial number N of each received switching instruction packet 23 is equal to that of a packet that has not been normally received, the received signal processing unit delivers data stored in the data section of each received switching instruction packet 23, as retransmitted data, to a network I/F unit 16 (see Fig. 6) when the received signal processing unit 20 is included in the gateway station 3, or to an input and output I/F unit 22 (see Fig. 7) when the received signal processing unit 20 is included in the earth H:\Valma\Keep\Specifications\P50880.0306 1-At.doc 30/09/03 station 4.

Next, another example of transmission and reception of packets will be explained with reference to Fig. 11. In Fig. 11, reference numeral 24 denotes a switching instruction packet having a data section in which an already transmitted data packet is written and a header section in which switching instruction information is written. As illustrated in Fig. 11, while each switching instruction packet 23 includes the same data as an already transmitted data packet P3, each switching instruction packet 24 includes the same data as another already transmitted data packet P2. In Fig. 11, components or elements designated by the same reference numerals as shown in Fig. 4 are the same as or similar to those of Fig. 4.

It is clear from comparison between Fig. 9 and Fig.

11 that while in the case of Fig. 9 a switching instruction packet 23 is continuously transmitted from the gateway station 3 to the earth station 4, in the case of Fig. 11 a switching instruction packet 24 and a switching instruction packet 23 are mixedly transmitted from the gateway station 3 to the earth station 4. Thus, in the case of Fig. 11, because two switching instruction packets 24 and 23 respectively having data included in the two data packets P2 and P3, serial numbers, and a switching instruction are transmitted, not only data packet P3 but also data packet P2 can be retransmitted to the earth station. Each switching instruction packet 24 has the same structure (as shown in Fig. 10) as each switching instruction packet 23, and has a header section in which a serial number N and switching instruction information F are written, like each switching instruction packet 23.

As previously mentioned, in accordance with embodiment 3, each switching instruction packet has a data section in which already transmitted data are written. As H:\Valma\Keep\Specfications\P50880.03061-AUdoc 30109/03 a result, retransmission of data can be carried out while a series of switching instruction packets for instructing switching between communication lines is transmitted.

Embodiment 4.

A satellite communication method in accordance with embodiment 4 of the present invention, and a satellite communication apparatus, an earth station and a gateway station for use with the method will be explained hereafter with reference to Fig. 12. The satellite communication method in accordance with embodiment 4 includes the step of handling and processing data and switching instruction information in the form of time division frames in each of the gateway station 3 and the earth station 4. Fig. 12 is a schematic diagram showing time division frames included in a signal transmitted and received when a handover occurs between communications satellites in accordance with embodiments 4. In Fig. 12, reference numeral 25 denotes a frame including significant data, each frame of this type being shown by an open box, reference numeral 26 denotes a frame having switching instruction information, each frame of this type being shown by a hatched box, reference numeral 27 denotes a synchronization bit included in each frame, reference numeral 28 denotes a switching instruction bit included in each frame, and reference numeral 29 denotes data included in each frame. Fig. 12 shows both a case where the gateway station is a transmitting station and the earth station is a receiving station and a case where the earth station is a transmitting station and the gateway station is a receiving station. Communication lines being used are the same as those as shown in Figs. 3 and 4.

The transmitting station transmits frames 25 in each of which significant data are stored to the receiving H:\Valma\Keep\Specifications\P50880.03061-AUdoc 30109/03 station by way of a communication line 5a first, and then transmits frames 26 when performing a handover between communications satellites. In response to a frame 26 transmitted thereto by way of the communication line 6a, the receiving station switches to another communication line 6b. After that, the receiving station receives subsequent frames by way of the other communication line 6b. When switching the other communication line, the receiving station performs switching between receive frequencies and resynchronization among frames. The switching instruction bit included in each frame has a value that is either 0 indicating non-switching between communication lines or 1 indicating switching between communication lines, for example. Each frame 26 including switching instruction information has no significant data.

Embodiment A satellite communication method in accordance with embodiment 5 of the present invention has the step of recovering switching information, such as a switching instruction packet, switching instruction information or a switching instruction bit, which is transmitted from a transmitting station, when the switching information cannot be normally received for some reason, in the satellite communication method of either of embodiments 1 to 4.

A receiving station an earth station or gateway station) fails in the reception of the switching information in the following case: when shielding occurs within the range of a predetermined elevation angle in the zenith direction of the receiving station, the receiving station enters a tunnel.

Based on a notification from a communications satellite management station, the receiving station can grasp the time when a communications satellite that comes H:\Valma\Keep\Specifications\P50880.0306 I AU.doc 30t09/03 flying in the zenith direction for the next time enters the range having a predetermined elevation angle and starts providing a communication service. As an alternative, the receiving station can determine the time when the communications satellite enters the range having a predetermined elevation angle and starts providing a communication service based on information about the orbit of the communications satellite or can grasp the time based on an absolute time when the communications satellite comes flying and the periodicity of the communications satellite.

When receiving no switching information from the transmitting station even if the time thus grasped by the receiving station comes, the receiving station can forcedly switch between communication lines. As an alternative, when the receiving station cannot receive the switching information while it is transmitting and receiving bidirectionally to and from the transmitting station and the grasped handover time elapses, the receiving station transmits a request for retransmission of the switching information to the transmitting station and the transmitting station transmits the switching information again to the receiving station in response to the retransmission request. In Figs. 6 and 7, when a satellite switching instructing unit 21cannot receive the switching information from a received signal processing unit 20 after the handover time elapses, the satellite switching instructing unit 21 forcedly instructs a receiving unit 19 to perform switching between communication lines or instructs a transmitting unit 13 to make a request for retransmission of the switching information, so that the process of switching between communication lines is carried out.

As previously mentioned, when the earth station cannot receive any switching instruction information from H:\Valma\Keep\Specifications\P50880.03061AU.doc 30109/03 the gateway station, the earth station forcedly performs switching between communication lines or makes a request of the gateway station for retransmission of switching instruction information. As a result, even if there is a loss of received switching instruction information due to shielding or the like that occurs over the earth station, a handover between communications satellites can be recovered.

Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

H:\Valma\Keep\Spedfications\P50880.03061IAU.doc 30109/03

Claims

1. A satellite communication method of carrying out communications between a gateway station and an earth station by way of a plurality of communications satellites that are flying approximately in zenith direction one after another, said method comprising the steps of: when switching from communications via a communications satellite to communications via another communications satellite that flies approximately in zenith direction for the next time, said gateway station or said earth station continuously transmitting switching instruction information by way of a transmission line passing through said one communications satellite; and then switching to communications via said other communications satellite.

2. The satellite communication method according to Claim 1, wherein either said gateway station or said earth station continuously transmits the switching instruction information by way of the transmission line passing though said one communications satellite, and also continuously transmits the switching instruction information by way of another transmission line passing though said other communications satellite.

3. A satellite communication apparatus comprising: a plurality of communications satellites that are flying approximately in zenith direction one after another; and a gateway station and an earth station that communicate with each other by way of one of the plurality of communications satellites, when switching from communications via one of said plurality of communications H:\Valma\Keep\Spefications\P50880.03061AU.doc 30109/03 satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, said gateway station continuously transmitting switching instruction information to said earth station and then switching to communications via said other communications satellite, and said earth station receiving the switching instruction information from said gateway station and then switching from communications via said one communications satellite to communications via said other communications satellite.

4. A gateway station comprising: a transmitting unit for communicating with an earth station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another; a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time; and a switching instruction packet generating unit for generating a packet including switching instruction information based on the instruction from said satellite switching instructing unit, and for sending the packet to said transmitting unit, said transmitting unit continuously transmitting the packet including the switching instruction information generated by said switching instruction packet generating unit to said earth station.

An earth station comprising: a transmitting unit for communicating with a gateway station by way of one of a plurality of communications H:\Valma\Keep\Specifications\P50880.03061PAUdoc 30/09/03 satellites that are flying approximately in zenith direction one after another; a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time; and a switching instruction packet generating unit for generating a packet including switching instruction information based on the instruction from said satellite switching instructing unit, and for sending the packet to said transmitting unit, said transmitting unit continuously transmitting the packet including the switching instruction information generated by said switching instruction packet generating unit to said gateway station.

6. A satellite communication apparatus comprising: a plurality of communications satellites that are flying approximately in zenith direction one after another; and a gateway station and an earth station that communicate with each other by way of one of the plurality of communications satellites, when switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, said gateway station continuously transmitting switching instruction information to said earth station and then switching to communications via said other communications satellite, and said earth station receiving the switching instruction information from said gateway station, switching from communications via said one communications satellite to communications via said other H:\Valma\Keep\Specifications\P50880.0306 1-AUdoc 30/09/03 communications satellite, and then transmitting switching instruction information for instructing said gateway station to switch from a receive line passing through said one communications satellite to another transmission passing through said other communications satellite to said gateway station.

7. An earth station comprising: a transmitting unit for communicating with a gateway station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another; a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time; and a switching instruction packet generating unit for generating a packet including switching instruction information based on the instruction from said satellite switching instructing unit, and for sending the packet to said transmitting unit, said satellite switching instructing unit instructing said switching instruction packet generating unit to generate the packet including the switching instruction information in response to switching instruction information from said gateway station, and said transmitting unit continuously transmitting the padket including the switching instruction information generated by said switching instruction packet generating unit to said gateway station.

8. The gateway station according to Claim 4, wherein the switching instruction packet generated by said H:\Valma\Keep\Specfications\P5880.0306PAUdoc 30/09/03 switching instruction packet generating unit has a data section for storing data included in a data packet that has been already transmitted before the transmission of the switching instruction packet.

9. The earth station according to Claim 5, wherein the switching instruction packet generated by said switching instruction packet generating unit has a data section for storing data included in a data packet that has been already transmitted before the transmission of the switching instruction packet.

An earth station comprising: a receiving unit for receiving a signal transmitted thereto from a gateway station by way of one of a plurality of communications satellites that are flying approximately in zenith direction one after another; and a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, when said receiving unit receives no switching instruction information from said gateway station after a handover time elapses, said satellite switching instructing unit instructing said receiving unit switch from a communication line passing through said one communications satellite to another communication line passing through said other communications satellite.

11. An earth station comprising: a receiving unit for receiving a signal transmitted thereto from a gateway station by way of one of a plurality of communications satellites that are flying approximately H:\Valma\Keep\Specifications\P50880.0306 lAU.doc 30/09/03 in zenith direction one after another; a transmitting unit for transmitting a signal to said gateway station; and a satellite switching instructing unit for providing an instruction for switching from communications via one of said plurality of communications satellites to communications via another communications satellite that flies approximately in zenith direction for the next time, when said receiving unit receives no switching instruction information from said gateway station after a handover time elapses, said satellite switching instructing unit instructing said transmitting unit to transmit a request signal indicating a request for transmission of switching instruction information to said gateway station. Dated this 30th day of September 2003 MITSUBISHI DENKI KABUSHIKI KAISHA By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia H:\Valma\Keep\Specifications\P50880.03061-AU.doc 30/09/03