Abstract:
A monitor terminal transmits a monitor request command to monitor a repeater station designated by an operator to a monitor server via the Internet. The monitor server detects a monitor station located in the communication range of the designated repeater station from the content of the received monitor request command, and transmits a monitor instruction command to the detected monitor station via the Internet. The monitor station intercepts a radio signal during transmission from the repeater station to a radio communication device or vice versa. Upon reception of the monitor instruction command, the monitor station transmits the intercepted radio signal as monitor data to the monitor server. The monitor server transfers the monitor data to the monitor terminal. Upon reception of the monitor data transferred from the monitor server, the monitor terminal radio-transmits the received monitor data using a radio wave format receivable by the radio communication device.

Description:
RELATED APPLICATION 
     This application claims the priority of Japanese Patent Application No. 2008-294503 filed on Nov. 18, 2008, which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a communication technology using both radio communication and the Internet, and, particularly, to a technique for monitoring the use conditions of a remote repeater. 
     2. Description of the Related Art 
       FIG. 11  is a structural diagram showing the outline of a D-STAR (Digital Smart Technologies for Amateur Radio) system which is one example of a communication system using both radio communication and the Internet. 
     This D-STAR system uses an IP (Internet Protocol) network (hereinafter referred to as Internet). The D-STAR system has a plurality of repeater stations  2  for each of a plurality of zones  7  to relay radio waves from a radio device or radio  1 , and a gateway  3  which manages the operations of the repeater stations  2  and relays communication between the repeater stations  2  and an Internet  5 . In each zone  7 , all the repeater stations  2  are connected by a trunk line  4  which is a high-speed dedicated line. The Internet  5  is connected with a management server  6  which manages communication information or the like of the individual radios  1 . 
     Each radio  1  has a user ID (e.g., call sign of the amateur radio) or a unique identification code to specify a user. Each repeater station  2  has an area ID (e.g., call sign of the amateur radio) or an address to specify the radio communication range. Each gateway  3  and the management server  6  are given IP addresses for specifying their positions on the 
     Internet. 
     In such a D-STAR system, each gateway  3  monitors the state of the radio  1  present in each area via the repeater station  2  in the same zone  7 , and informs the management server  6  of the communication information or the like of each radio  1  via the Internet  5 . The management server  6  saves the communication information or the like of the radio  1  informed by each gateway  3  in a database, and informs the communication information or the like of each radio  1  in response to an inquiry from each gateway  3 . 
     In a case where, for example, a radio  1   a  in a zone  7   a  communicates with a radio  1   b  in a zone  7   b  in the D-STAR system, the sender radio  1   a  transmits, to a repeater station  2   a , a message of voice, data or the like, by designating the user ID of the destination radio  1   b . The message transmitted from the radio  1   a  is received by a repeater station  2   a  present in the area where the radio  1   a  is located, and is transferred to a gateway  3   a  directly or via the trunk line  4 . 
     The gateway  3   a  informs the management server  6  of the user ID of the destination radio  1   b , and inquires about the communication information or the like of the radio  1   b . Then, the gateway  3   a  acquires the area ID of a repeater station  2   b  communicatable with the destination radio  1   b  and the IP address of a gateway  3   b  which controls the repeater station  2   b  from the management server  6 . The gateway  3   a  adds the acquired IP address and area ID to the message transmitted from the radio  1  a, and transmits the resultant message onto the Internet  5 . 
     Upon reception of the message, transmitted from the gateway  3   a , via the Internet  5 , the destination gateway  3   b  transfers the received message to the corresponding repeater station  2   b  according to the area ID added to the message. The repeater station  2   b  converts the message transferred from the gateway  3   b  to a radio signal, and transmits the radio signal to the radio  1   b . This allows the radio  1   b  located in the area of the repeater station  2   b  to receive the message from the sender radio  1   a.    
     As apparent from the above, the D-STAR system uses the Internet  5  as a relay circuit to enable communication between remote radios  1  which cannot be covered by radio communication. 
     However, D-STAR system cannot know the status of a repeater station  2  provided at a remote site in an attempt to communicate with the repeater station  2 . Problematically, this will cause a failure to connect to such a remote repeater station  2  when the repeater station  2  being busy, or will be the cause of interruption to the communication being performed by the repeater station  2 . 
     As an art relating to this problem, Patent Document 1 (Unexamined Japanese Patent Application KOKAI Publication No. 2001-136206) describes a communication system which monitors what is talked over a transceiver and transmits the talking contents to a transceiver or the like located out of the communication range via the Internet. Patent Document 2 (Unexamined Japanese Patent Application KOKAI Publication No. 2002-330108) describes a radio communication monitor system in which radio transmitting/receiving devices that are sited zone by zone and perform transmission/reception communication with a plurality of radios are connected by an intranet, and the contents of communication between a radio transmitting/receiving device and a radio in another zone are monitored. 
     However, the Patent Documents 1 and 2 merely disclose techniques intended to monitor the contents of communication with another transceiver or the like, and have no description on monitoring the use conditions or the like of each repeater station which relays a message. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a repeater monitor system and repeater monitor method which can monitor contents of communication of a remote repeater station. 
     To achieve the object, according to a first aspect of the invention, there is provided a repeater monitor system in a communication system including plural sets each having a single repeater station or a plurality of repeater stations which relay communication between radio communication devices, and a gateway which manages an operation of the single repeater station or the plurality of repeater stations and relays communication between the repeater station to be managed and an Internet, the repeater monitor system monitoring communication contents of the repeater station and comprising: 
     a monitor terminal; and 
     a plurality of monitor stations, 
     wherein the monitor terminal transmits via the Internet a monitor request command for requesting monitor data on the communication content of the repeater station designated by an operator, to a monitor station that corresponds to the repeater station, receives via the Internet the monitor data transmitted from the monitor station, and radio-transmits the received monitor data, and 
     the monitor station is located in a communication range of the repeater station to intercept a radio signal during transmission from the repeater station to the radio communication device or vice versa, and transmit via the Internet the intercepted radio signal as the monitor data to the monitor terminal in response to the monitor request command received via the Internet. 
     In the repeater monitor system according to the first aspect of the present invention, 
     the monitor terminal may detect a monitor station that corresponds to a repeater station designated by an operator, transmit via the Internet the monitor request command to the detected monitor station, and receive via the Internet the monitor data transmitted from the monitor station. 
     According to a second aspect of the invention, there is provided a repeater monitor system in a communication system including plural sets each having a single repeater station or a plurality of repeater stations which relay communication between radio communication devices, and a gateway which manages an operation of the single repeater station or the plurality of repeater stations and relays communication between the repeater station to be managed and an Internet, the repeater monitor system monitoring communication contents of the repeater station and comprising: 
     a monitor terminal; 
     a monitor server; and 
     a plurality of monitor stations, 
     wherein the monitor terminal transmits via the Internet a monitor request command for requesting monitor data on the communication content of a repeater station designated by an operator to the monitor server, receives via the Internet the monitor data transmitted from the monitor server, and radio-transmits the received monitor data, 
     the monitor server detects a monitor station corresponding to the designated repeater station from a content of the monitor request command received from the monitor terminal, transmits via the Internet a monitor instruction command generated based on the monitor request command to the detected monitor station, receives via the Internet the monitor data transmitted from the monitor station, and transmits via the Internet the received monitor data to the monitor terminal, and 
     the monitor station is located in a communication range of the repeater station to intercept a radio signal during transmission from the repeater station to the radio communication device or vice versa, and transmit via the Internet the intercepted radio signal as the monitor data to the monitor server in response to the monitor instruction command received via the Internet. 
     According to a third aspect of the invention, there is provided a repeater monitor system in a communication system including plural sets each having a single repeater station or a plurality of repeater stations which relay communication between radio communication devices, and a gateway which manages an operation of the single repeater station or the plurality of repeater stations and relays communication between the repeater station to be managed and an Internet, the repeater monitor system monitoring communication contents of the repeater station and comprising: 
     a monitor terminal that transmits via the Internet a monitor request command for requesting monitor data on the communication content of the repeater station to a gateway that corresponds to the repeater station, receives via the Internet the monitor data transmitted from the gateway, and radio-transmits the received monitor data, 
     wherein upon reception of the monitor request command via the Internet, the gateway transmits, via the Internet, data to be transmitted to the radio communication device, or data received from the radio communication device, by the repeater station, to the monitor terminal as the monitor data. 
     In the repeater monitor system according to a third aspect of the present invention, 
     the monitor terminal may detect a gateway that corresponds to a repeater station designated by an operator, transmit via the Internet the monitor request command to the detected gateway, and receive via the Internet the monitor data transmitted from the gateway. 
     According to a fourth aspect of the invention, there is provided a repeater monitor system in a communication system including plural sets each having a single repeater station or a plurality of repeater stations which relay communication between radio communication devices, and a gateway which manages an operation of the single repeater station or the plurality of repeater stations and relays communication between the repeater station to be managed and an Internet, the repeater monitor system monitoring communication contents of the repeater station and comprising: 
     a monitor terminal; and 
     a monitor server, 
     wherein the monitor terminal transmits via the Internet a monitor request command for requesting monitor data on the communication content of a repeater station designated by an operator to the monitor server, receives via the Internet the monitor data transmitted from the monitor server, and radio-transmits the received monitor data, 
     the monitor server detects a gateway corresponding to the designated repeater station from a content of the monitor request command received from the monitor terminal, transmits via the Internet a monitor instruction command generated based on the monitor request command to the detected gateway, receives via the Internet the monitor data transmitted from the gateway, and transmits via the Internet the received monitor data to the monitor terminal, and 
     wherein upon reception of the monitor instruction command via the Internet, the gateway transmits, via the Internet, data to be transmitted to the radio communication device, or data received from the radio communication device, by the repeater station, to the monitor server as the monitor data. 
     In the repeater monitor system according to the first to the fourth aspects of the present invention, the monitor terminal may radio-transmit the monitor data using a radio wave format receivable by the radio communication device. 
     In the repeater monitor systems according to the second and fourth aspects of the present invention, upon reception of monitor request commands designating a same repeater station from a plurality of monitor terminals, the monitor server may transmit the monitor instruction command to a corresponding monitor station (or gateway) only once, and upon reception of monitor data corresponding to the monitor instruction command transmitted from the monitor station (or gateway), the monitor server may transmit the received monitor data to the plurality of monitor terminals. 
     According to a fifth aspect of the invention, there is provided a repeater monitor method in a communication system including plural sets each having a single repeater station or a plurality of repeater stations which relay communication between radio communication devices, and a gateway which manages an operation of the single repeater station or the plurality of repeater stations and relays communication between the repeater station to be managed and an Internet, the repeater monitor method monitoring communication contents of the repeater station and comprising: 
     causing a monitor terminal to transmit via the Internet a monitor request command for requesting monitor data on the communication content of the repeater station designated by an operator, to a monitor station that corresponds to the repeater station; 
     causing the monitor station to intercept a radio signal during transmission from the repeater station to the radio communication device or vice versa and transmit via the Internet the intercepted radio signal as the monitor data to the monitor terminal after receiving the monitor request command; and 
     causing the monitor terminal to receive the monitor data transmitted from the monitor station via the Internet and radio-transmit the received monitor data using a radio wave format receivable by the radio communication device. 
     In the repeater monitor systems according to the fifth aspect of the present invention, the monitor terminal may detect a monitor station located in a communication range of the repeater station designated by an operator, transmit via the Internet the monitor request command to the detected monitor station, and receive via the Internet the monitor data transmitted from the monitor station. 
     According to a sixth aspect of the invention, there is provided a repeater monitor method in a communication system including plural sets each having a single repeater station or a plurality of repeater stations which relay communication between radio communication devices, and a gateway which manages an operation of the single repeater station or the plurality of repeater stations and relays communication between the repeater station to be managed and an Internet, the repeater monitor method monitoring communication contents of the repeater station and comprising: 
     causing a monitor terminal to transmit via the Internet a monitor request command for requesting monitor data on the communication content of a repeater station designated by an operator to the monitor server; 
     causing the monitor server to detect a monitor station located in a communication range of the designated repeater station from a content of the monitor request command received from the monitor terminal, and transmit via the Internet a monitor instruction command generated based on the monitor request command to the detected monitor station, receives via the Internet the monitor data transmitted from the monitor station; 
     causing the monitor station to intercept a radio signal during transmission from the repeater station to the radio communication device or vice versa and transmit via the Internet the intercepted radio signal as the monitor data to the monitor server after receiving the monitor instruction command; 
     causing the monitor server to receive via the Internet the monitor data transmitted from the monitor station and transmit via the Internet the received monitor data to the monitor terminal; and 
     causing the monitor terminal to receive via the Internet the monitor data transmitted from the monitor server and radio-transmit the received monitor data using a radio wave format receivable by the radio communication device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of this application can be obtained when the following detailed description is considered in conjunction with the following drawings, in which: 
         FIG. 1  is a diagram showing the general configuration of a data communication system according to a first embodiment of the invention; 
         FIG. 2  is a diagram showing the configuration of a monitor terminal in  FIG. 1 ; 
         FIG. 3  is a diagram showing the configuration of a monitor server in  FIG. 1 ; 
         FIG. 4  is a diagram showing the configuration of a monitor station in  FIG. 1 ; 
         FIG. 5  is a flowchart illustrating operation procedures of a monitor process according to the first embodiment; 
         FIG. 6  is a diagram showing the frame structure of a command packet to be transmitted on the Internet; 
         FIG. 7  is a diagram showing the frame structure of a radio packet to be used in a D-STAR system; 
         FIG. 8  is a diagram showing the frame structure of a data packet to be transmitted on the Internet; 
         FIG. 9  is a diagram showing the general configuration of a data communication system according to a second embodiment of the invention; 
         FIG. 10  is a diagram showing the configuration of a gateway/monitor station in  FIG. 9 ; and 
         FIG. 11  is a diagram showing the schematic configuration of a conventional D-STAR system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [First Embodiment] 
       FIG. 1  is a diagram showing the general configuration of a data communication system according to the first embodiment of the invention. The data communication system according to the embodiment has the configuration of a D-STAR system as its basic configuration, so that same reference numerals are given to those components in  FIG. 1  which are common to the components of the D-STAR system (see  FIG. 11 ). 
     The data communication system according to the embodiment is the D-STAR system added with a function of monitoring the communication conditions of a designated repeater station  2 . 
     The data communication system according to the embodiment has a plurality of repeater stations  2  for each of a plurality of zones  7  to relay radio waves from a radio device or radio  1 , and a gateway  3  which manages the operations of the repeater stations  2  and relays communication between the repeater stations  2  and an Internet  5 . In each zone  7 , all the repeater stations  2  are connected by a trunk line  4  which is a high-speed dedicated line. To the Internet  5  is connected a management server  6  which manages communication information or the like of the individual radios  1 . 
     To monitor the communication conditions of the repeater stations  2 , the data communication system further includes monitor terminals  10 , a monitor server  20  and monitor stations  30 . The monitor terminals  10 , the monitor server  20  and the monitor stations  30  are all connected to the Internet  5 , and are given unique IP addresses to specify their positions on the Internet  5 . 
     The monitor terminal  10  transmits a monitor request command to the monitor server  20  via the Internet  5  to request initiation or termination of monitoring of a designated repeater station  2 . Using a weak radio wave, the monitor terminal  10  sends the radios  1  monitor data for the repeater station  2  of interest transmitted from the monitor server  20  in response to the monitor request command. 
     As shown in  FIG. 2 , the monitor terminal  10  has an operation section  11 , a command transmitting section  12 , an Internet connecting section  13 , a monitor data receiving section  14 , a radio transmitting section  15 , and a transmission antenna  16 . 
     The operation section  11  is used to accept an input made by an operator to transmit the monitor request command and monitor a response to the transmitted monitor request command, for example. 
     The command transmitting section  12 , connected to the operation section  11 , generates the monitor request command which is a message of a predetermined format according to the operator&#39;s input operation, and outputs the monitor request command to the Internet connecting section  13 . 
     The Internet connecting section  13  adds a header according to the Internet protocol to the message (i.e., monitor request command) output from the command transmitting section  12 , and transmits the resultant message to the monitor server  20  via the Internet  5 . The 
     Internet connecting section  13  also has a function of outputting a message received from the monitor server  20  via the Internet  5  (monitor data, another response message or the like sent from the repeater station  2 ) to the monitor data receiving section  14 . 
     The monitor data receiving section  14  converts the monitor data transmitted from the repeater station  2  to a predetermined format, and outputs the monitor data to the radio transmitting section  15 . The monitor data receiving section  14  also has a function of outputting a response message or the like from the monitor server  20  to the operation section  11 . The contents of the response message or the like output to the operation section  11  may be displayed on, for example, a monitor or the like (not shown) so that the operator is easier to confirm it. 
     The radio transmitting section  15  generates data obtained by adding a radio part header such as a sync code to the monitor data output from the monitor data receiving section  14 , and transmits the data from the transmission antenna  16  on a weak radio wave. This weak radio wave has a weak level which is not restricted by the modulation method or the occupying band according to the radio law (e.g., transmission output of 245 pW or so). To simplify the radio circuit using higher harmonics, the transmission frequency in use is 432.00 MHz for the 430 MHz band, and 1296.00 MHz for the 1.2 GHz band. 
     The radio signal transmitted from the transmission antenna  16  is received by a radio  1  located nearby. 
     The monitor server  20  relays a message which is exchanged between the monitor terminal  10  and the monitor station  30 . 
     In response to the monitor request command transmitted from the monitor terminal  10 , the monitor server  20  transmits a monitor instruction command instructing the initiation or termination of transmission of monitor data to the monitor station  30  of interest via the Internet  5 . Then, the monitor server  20  transfers monitor data or a response message transmitted from the monitor station  30  to which the monitor instruction command has been transmitted, to the requesting monitor terminal  10  via the Internet  5 . 
     As shown in  FIG. 3 , the monitor server  20  includes an Internet connecting section  21 , a command transmitting/receiving section  22 , a monitor data transmitting/receiving section  23 , an address information storage section  24 , and a monitor data buffer  25 . 
     The Internet connecting section  21  transmits/receives a message (command, monitor data) to/from the monitor terminal  10  and the monitor station  30  via the Internet  5  according to the Internet protocol. 
     The command transmitting/receiving section  22 , connected to the Internet connecting section  21 , transmits a monitor instruction command instructing the initiation or termination of monitoring to the monitor station  30  corresponding to the repeater station  2  to be monitored, in response to the monitor request command received from the monitor terminal  10 . 
     The address information storage section  24 , connected to the command transmitting/receiving section  22 , stores a table where the area IDs of the individual repeater stations  2  are associated with the IP addresses of the corresponding monitor stations  30 . 
     The monitor data transmitting/receiving section  23 , connected to the Internet connecting section  21 , transfers monitor data received from the monitor station  30  to the requesting monitor terminal  10 . 
     The monitor transmitting/receiving section  23  temporarily saves the received monitor data in the monitor data buffer  25 . As the monitor data is temporarily saved in the monitor data buffer  25  this way, it is possible to delay (adjust the time of) the transfer of the monitor data as needed. 
     The monitor station  30  intercepts a radio signal transmitted from the repeater station  2  located within the same area, and transmits the intercepted communication contents (or communication conditions) to the monitor server  20  as monitor data via the Internet  5  in response to the monitor instruction command from the monitor server  20 . 
     As shown in  FIG. 4 , the monitor station  30  includes a reception antenna  31 , a radio receiving section  32 , a monitor data transmitting section  33 , an Internet connecting section  34 , and a command receiving section  35 . 
     The radio receiving section  32  receives a radio signal (radio packet), transmitted from the repeater station  2 , via the reception antenna  31 , and outputs the received radio packet to the monitor data transmitting section  33 . 
     Upon reception of a monitor start instruction command from the monitor server  20  via the Internet  5 , the command receiving section  35  instructs the monitor data transmitting section  33  to transmit the radio packet received by the radio receiving section  32 . In response to the instruction, the monitor data transmitting section  33  outputs monitor data generated based on the radio packet received by the radio receiving section  32  to the Internet connecting section  34 . The Internet connecting section  34  transmits this monitor data to the monitor server  20  via the Internet  5 . Upon reception of a monitor end instruction command from the monitor server  20  via the Internet  5 , the command receiving section  35  instructs the monitor data transmitting section  33  to terminate the transmission of the monitor data. In response to the instruction, the monitor data transmitting section  33  terminates the outputting of the monitor data to the Internet connecting section  34 . 
     Next, the operation of the monitor process according to the data communication system shown in  FIG. 1  will be described referring to  FIG. 5 . 
     First, when the operator uses the operation section  11  in the monitor terminal  10  to perform an operation to request the initiation of monitoring (YES in step S 1 ), the command transmitting section  12  of the monitor terminal  10  generates a monitor start request command (step S 2 ). This operation includes, for example, designation of the area ID (e.g., call sign of the amateur radio) of a desired repeater station  2  to be monitored, and depression of a “Start button” or the like. The monitor start request command which is generated by the command transmitting section  12  includes the call sign of the repeater station  2  to be monitored and information indicating a request to start monitoring (type of a command indicating a monitor start request). 
     The Internet connecting section  13  adds a header according to the Internet protocol to the monitor start request command generated by the command transmitting section  12  to form a command packet as shown in  FIG. 6 , and transmits the command packet onto the Internet  5  (step S 3 ). 
     At this time, the IP address of the monitor terminal  10  is stored at a sender address in the IP (Internet Protocol) header of the command packet, and the IP address of the monitor server  20  is stored at a destination address. 
     The monitor start request command transmitted onto the Internet  5  from the monitor terminal  10  is received by the Internet connecting section  21  of the monitor server  20  (step S 4 ), and is output to the command transmitting/receiving section  22 . 
     Referring to the address information storage section  24 , the command transmitting/receiving section  22  searches for the IP address of the monitor station  30  corresponding to the area ID (call sign) of the repeater station  2  to be monitored. Then, the command transmitting/receiving section  22  generates a message (monitor start instruction command) to instruct the searched monitor station  30  to start monitoring. 
     The Internet connecting section  21  forms the monitor start instruction command, generated by the command transmitting/receiving section  22 , into a command packet, and transmits the command packet onto the Internet  5  (step S 5 ). The format of the command packet at this time is substantially similar to the one shown in  FIG. 6 ; the IP address of the monitor server  20  is stored at the sender address while the IP address of the searched monitor station  30  is stored at the destination address. 
     The Internet connecting section  34  of the destination monitor station  30  receives the message (monitor start instruction command) transmitted from the monitor server  20  (step S 6 ), and outputs the received message to the command receiving section  35 . When confirming that the received message is the monitor start instruction command instructing the initiation of monitoring, the command receiving section  35  instructs the monitor data transmitting section  33  to output the radio packet received from the repeater station  2  as monitor data. 
     In response to the output instruction from the command receiving section  35 , the monitor data transmitting section  33  starts a process of transmitting monitor data. Specifically, the monitor data transmitting section  33  converts a signal in the radio packet of the D-STAR format received from the repeater station  2  to a data packet signal according to the Internet protocol. 
     The frame structure of a radio packet of the D-STAR format will be described below. A radio packet of the D-STAR format has a radio part header and a data part as shown in  FIG. 7 . The radio part header includes a sync pattern including a bit sync and frame sync, an ID area for the call signs or the like of the sender and the destination, and an error correction code (FCS) and the like. The data part alternately includes audio frames and data frames. The audio frame is a compressed code digitized by a codec complying with the AMBE (Advanced Multi-Band Excitation) standard developed by DVS, Inc. in America. 
     As shown in  FIG. 8 , a data part includes a header part, such as an MAC header and IP header, and a data part having an audio frame and a data frame. 
     The monitor data transmitting section  33  divides the data part of the received radio packet of the D-STAR format to data segments of a predetermined size, adds a header part converted according to the Internet protocol to each data segment, thereby generating monitor data. Then, the monitor data transmitting section  33  transmits the generated monitor data onto the Internet  5  via the Internet connecting section  34  (step S 7 ). Here, the IP address of the monitor station  30  is stored at the sender address of the IP header while the IP address of the monitor server  20  which has transmitted the monitor start instruction command is stored at the destination address. 
     The monitor transmitting/receiving section  23  of the monitor server  20  temporarily saves the monitor data, received via the Internet connecting section  21 , in the monitor data buffer  25 . The monitor transmitting/receiving section  23  reads the saved monitor data from the monitor data buffer  25  at a given timing, and transfers the monitor data to the monitor terminal  10  (step S 8 ). 
     The monitor data receiving section  14  of the monitor terminal  10  receives the monitor data, transferred from the monitor server  20 , via the Internet connecting section  13  (step S 9 ). Then, the monitor data receiving section  14  converts the received monitor data to a radio packet of the D-STAR format, and outputs the radio packet to the radio transmitting section  15 . The format of the radio packet at this time is similar to the one shown in  FIG. 7 ; “receiver relay station call sign” and “destination station call sign” in the ID of the radio part header are written over with predetermined text data (e.g., “MONITOR), and “sender relay station call sign” and “local station call sign  1 ,  2 ” are filled with space. 
     The radio transmitting section  15  of the monitor terminal  10  transmits the radio packet, output from the monitor data receiving section  14 , from the transmission antenna  16  using a weak radio wave (step S 10 ). A radio  1  located near the monitor terminal  10  can receive the radio packet transmitted from the transmission antenna  16 , convert the audio frame in its data part to an analog signal, and output the analog signal from a speaker. 
     The processes in the steps S 7  to S 10  are repeatedly executed until the operator uses the operation section  11  to perform an operation to request the termination of monitoring (YES in step S 11 ) every time the monitor station  30  receives a radio signal from a repeater station  2  to be monitored. Hearing the audio signal output from the radio  1  located near the monitor terminal  10 , the operator can grasp the communication conditions of the repeater station  2 . 
     When the operator performs an operation to request the termination of monitoring (e.g., depression of the “End button”) (YES in step S 11 ), the command transmitting section  12  of the monitor terminal  10  generates a monitor end request command including the call sign of the repeater station  2  to be monitored and information indicating the termination of monitoring being requested (command type indicating the monitor end request). The generated monitor end request command is transmitted onto the Internet  5  by the Internet connecting section  13  in the same process as that of the step S 3  (step S 12 ). 
     The monitor end request command transmitted onto the Internet  5  from the monitor terminal  10  is received by the Internet connecting section  21  of the monitor server  20  (step S 13 ), and is output to the command transmitting/receiving section  22 . The command transmitting/receiving section  22  generates a monitor end instruction command for the monitor station  30 , and transmits the monitor end instruction command onto the Internet  5  via the Internet connecting section  21  in the same process as that of the step S 5  (step S 14 ). 
     The Internet connecting section  34  of the monitor station  30  receives a message (monitor end instruction command) transmitted from the monitor server  20 , and outputs the received message to the command receiving section  35 . When confirming that the received message is the monitor end instruction command instructing the termination of monitoring, the command receiving section  35  instructs the monitor data transmitting section  33  to terminate the operation of transmitting monitor data. 
     As described above, to enable monitoring the communication conditions of a remote repeater station  2 , the data communication system according to the embodiment is provided with the monitor terminals  10 , the monitor server  20  and the monitor stations  30  in addition to the basic configuration of the D-STAR system. This can allow the communication conditions of a desired repeater station  2  to be monitored without affecting the existing D-STAR system at all. It is therefore possible to adequately use a remote repeater station  2  and call a destination radio  1  at an adequate timing. 
     Because the monitor terminal  10  transmits monitor data on a weak radio wave using a radio packet of the same format as that of the D-STAR system, the radio transmitting section  15  does not need particularly high specifications. Therefore, the monitor terminal  10  can be manufactured at a low cost. 
     Further, the data communication system does not require any dedicated monitor receiver having the codec for the D-STAR system to grasp communication conditions of a desired repeater station  2 . Advantageously, the system can grasp the concerned communication conditions by using a general-purpose radio terminal (radio  1 ) used in the D-STAR system. 
     [Second Embodiment] 
       FIG. 9  is a diagram showing the general configuration of a data communication system according to the second embodiment of the invention. According to the first embodiment, to monitor a repeater station  2 , the monitor station  30  which intercepts a radio wave transmitted from the repeater station  2  is provided. The data communication system according to the second embodiment differs from the data communication system according to the first embodiment in that a gateway/monitor station  40  which provides a gateway with the function of a monitor station is provided in place of the gateway  3  and the monitor station  30  (see  FIG. 1 ). 
     That is, in addition to the function of the conventional gateway, the gateway/monitor station  40  has a function of transmitting, as the monitor data, data to be exchanged between a repeater station  2  under control and a radio  1  to the monitor server  20  via the Internet  5  in response to a monitor instruction command from the monitor server  20 . 
     The gateway/monitor station  40 , as shown in  FIG. 10 , has a command receiving section  43  and a monitor data transmitting section  44  which are needed for the gateway/monitor station  40  to serve as a monitor station, in addition to an Internet connecting section  41  and a data transfer section  42  which are the original functions of the gateway. 
     Upon reception of a monitor start instruction command from the monitor server  20  via the Internet connecting section  41 , the command receiving section  43  instructs the monitor data transmitting section  44  to start transmitting monitor data. 
     Upon reception of a monitor end instruction command from the monitor server  20 , the command receiving section  43  instructs the monitor data transmitting section  44  to terminate the transmission of monitor data. 
     Upon reception of an instruction to start transmission of monitor data from the command receiving section  43 , the monitor data transmitting section  44  acquires, from the repeater station  2 , data transmitted to the radio  1  from the repeater station  2  and data received by the repeater station  2  from the radio  1 . Then, the monitor data transmitting section  44  outputs the data acquired from the repeater station  2  to the Internet connecting section  41  as monitor data. The Internet connecting section  41  transmits the monitor data to the monitor server  20  via the Internet  5 . 
     Upon reception of an instruction to terminate the transmission of monitor data from the command receiving section  43 , the monitor data transmitting section  44  stops outputting the monitor data to the Internet connecting section  41 . 
     The operation of the monitor process in the data communication system according to the second embodiment is substantially the same as that of the first embodiment excluding the operation of the gateway/monitor station  40 . 
     That is, upon reception of the monitor start instruction command from the monitor server  20 , the gateway/monitor station  40  does not receive a radio signal to be transmitted from a repeater station  2 , but transmits the same data as data which the repeater station  2  transmits/receives to/from the radio  1  to the monitor server  20  as monitor data via the Internet  5 . 
     The operation of the monitor terminal  10  according to the second embodiment is the same as that of the first embodiment. In addition, the monitor server  20  performs the operation as described in the foregoing description of the first embodiment, except that the destinations of the monitor start instruction command and the monitor end instruction command are set to the IP address of the gateway/monitor station  40 . 
     As apparent from the above, the data communication system according to the second embodiment has the configuration of the D-STAR system as its basic configuration and is provided with the gateway/monitor station  40  which has the gateway function and the monitor function in order to monitor the communication conditions of a remote repeater station  2 . 
     Accordingly, unlike the first embodiment, the second embodiment does not require the monitor station  30 , and adds slight functions to the gateway and the repeater station  2  of the existing D-STAR system to enable monitoring the communication conditions of a desired repeater station  2 , thus making it possible to adequately use a remote repeater station  2  and call a destination radio  1  at an adequate timing. 
     Because the monitor terminal  10  transmits monitor data on a weak radio wave using a radio packet of the same format as that of the D-STAR system, the radio transmitting section  15  does not need particularly high specifications. Therefore, the monitor terminal  10  can be manufactured at a low cost. 
     Further, a dedicated monitor receiver having the codec for the D-STAR system is unnecessary, which brings about an advantage such that a general-purpose radio terminal (radio  1 ) to be used in the D-STAR system can be used to grasp the communication conditions of a desired repeater station  2 . 
     The present invention is not limited to the foregoing embodiments, and can be modified in various other forms.
     (a) Although the foregoing descriptions of the embodiments have been given of the system that monitors repeater stations  2  in the amateur radio D-STAR system, the invention can also be adapted to various data communication systems which use the Internet or other communication networks as a relay for radio communication.   (b) According to the embodiments, the monitor server  20  relays a message (command, monitor data or the like) between the monitor terminal  10  and the monitor station  30  (or gateway/monitor station  40 ). However, the invention may be configured so that the monitor terminal  10  and the monitor station  30  exchange a message directly without intervening the monitor server  20 .   

     Such a configuration can be achieved by, for example, providing the monitor terminal  10  with information indicating the association of the monitor station  30  (or gateway/monitor station  40 ) with the repeater station  2 , or by allowing the monitor terminal  10  to inquire the management server  6  about such information.
     (c) Although the foregoing descriptions of the embodiments have been given of the case where the individual components of the monitor terminal  10  (see  FIG. 2 ), the monitor server  20  (see  FIG. 3 ), the monitor station  30  (see  FIG. 4 ) and the gateway/monitor station  40  (see  FIG. 10 ) are hardware components which have independent functions, the functions of the individual components may be realized by software or running a program.   (d) The frame structure of a packet at the time of data communication is not limited to those exemplified in  FIGS. 6 to 8 , and a frame structure matching a system to which the invention is adapted can be selected and used adequately.   (e) Although the monitor station  30  intercepts a radio signal transmitted from the repeater station  2 , and transmits the intercepted radio signal to the monitor server  20  as monitor data according to the first embodiment, the monitor station  30  may intercept a radio signal transmitted from a radio  1  (i.e., radio signal received by the repeater station  2 ), and may transmit the intercepted radio signal as monitor data. Further, the monitor station  30  may intercept a radio signal transmitted/received from/by the repeater station  2 , and may transmit the intercepted radio signal to the monitor server  20  as monitor data.   (f) Although the foregoing descriptions of the embodiments have been given of the operation in the case where a single monitor terminal  10  outputs a monitor request command to a single monitor station  30  or a single gateway/monitor station  40  (hereinafter referred to as “monitor station  30  or the like”), it is possible to cope with a case where a plurality of monitor terminals  10  output monitor request commands to the same monitor station  30  or the like.   

     The operation of the monitor server  20  in this case will be described. Upon reception of monitor request commands targeted for the same repeater station  2  from a plurality of monitor terminals  10 , the monitor server  20  transmits only a single monitor instruction command to the associated monitor station  30  or the like. 
     Then, upon reception of monitor data from the monitor station  30  or the like, the monitor server  20  transmits the received monitor data to all the monitor terminals  10  that have transmitted the monitor request commands. 
     More specifically, upon reception of a monitor start request command targeted for a repeater station  2  from a first monitor terminal  10 , the monitor server  20  transmits a monitor start instruction command to the associated monitor station  30  or the like. Thereafter, upon reception of a monitor start request command targeted for the same repeater station  2  as the one designated by the previous monitor start request command from a second monitor terminal  10 , the monitor server  20  does not transmit the monitor start instruction command to the associated monitor station  30  or the like again. 
     Upon reception of monitor data from the monitor station  30  or the like, the monitor server  20  transfers the received monitor data to the first and second monitor terminals  10 . 
     When monitor end request commands are received from all the monitor terminals  10  (the first and second monitor terminals  10  herein), the monitor server  20  transmits a monitor end instruction command to the monitor station  30  or the like. In other words, when the monitor end request command is received from either the first monitor terminal  10  or the second monitor terminal  10 , the monitor server  20  does not transmit the monitor end instruction command to the monitor station  30  or the like. In this case, monitor data may not be transferred to the monitor terminal  10  that has transmitted the monitor end request command. 
     As apparent from the above, the operation of the monitor server  20  can reduce the load of the monitor station  30  or the like even when a plurality of monitor terminals  10  output monitor request commands targeted for the same monitor station  30  or the like.
     (g) Although the monitor server  20  has the monitor data buffer  25  to temporarily store monitor data according to the embodiments, monitor data may not be stored in the monitor data buffer  25  and may be transferred at once. In this case, the monitor server  20  can be configured not to have the monitor data buffer  25 .   

     Various embodiments and changes may be made thereunto without departing from the broad spirit and scope of the invention. The above-described embodiments are intended to illustrate the present invention, not to limit the scope of the present invention. The scope of the present invention is shown by the attached claims rather than the embodiments. Various modifications made within the meaning of an equivalent of the claims of the invention and within the claims are to be regarded to be in the scope of the present invention.