Telephone apparatus used for computer network telephone system

1. Field of the Invention

This invention relates to a telephone system suitable for use as an internet telephone for exchanging audio data through the internet that is a worldwide computer network system.

2. Related Art

The internet is a worldwide computer network system connecting computer networks in corporations or universities beyond countries. Increasingly provided are various services using the internet, such as e-mail service, file transfer service, and information search service.

FIG. 1 schematically shows a general aspect of the internet. In FIG. 1 , each of computer networks NET 101 , NET 102 , NET 103 , . . . has a plurality of terminals T which are connected together by LAN (Local Area Network) in form of Ethernet or a token ring.

These computer networks NET 101 , NET 102 , NET 103 , . . . are connected together through routers R 101 , R 102 , R 103 , . . . that route data from one computer network to another, depending on the destination of the data.

Computer networks NET 101 , NET 102 , NET 103 , . . . connected through the routers R 101 , R 102 , R 103 , . . . form a computer network system. The computer network system is called internet. The internet enables exchanges of data among computer networks NET 101 , NET 102 , NET 103 , . . . .

The internet uses IP (Internet Protocol) as the protocol of its network layer. IP assigns an IP address to each terminal to identify a destination terminal of data. Each IP address is made up of four numerals each of which can be expressed by decimal 8 bits, such as 43.3.25.246.

As the internet is extended, the number of IP addresses possibly becomes insufficient. In some networks in which a large number of terminals are registered but only a small number of terminals are connected simultaneously, for example, it is possible to use a server on the network to allot currently available IP addresses to actually connected terminals alone in order to minimize the number of IP addresses used. In this manner, the network need not prepare IP addresses in the number corresponding to its terminals, but can effectively use a limited number of IP addresses.

The internet uses TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) as protocols of its transport layer. TCP permits communication after establishing a connection-type transmission connection, and deals with packet sequence control, re-transmission, flow control and congestion control. UDP is a connectionless-type protocol that is used in lieu of TCP in networks requiring real-time transmission. In digital audio transmission, for example, re-transmission is not requested even when a part of the packets drops, but audio data is sent successively. In such audio transmission, UDP is used.

Thus, the internet basically uses TCP/IP protocol. That is, IP addresses are assigned to terminals of a computer network to identify individual terminals, and packets are transferred by TCP or UDP.

However, personal computers are not always connected by LAN, and there are some without IP addresses. Therefore, some individuals participating the internet use internet service providers. Through internet service providers, personal computers can be connected to computer networks and can participate the internet by, for example, PPP (Point to Point Protocol) or SLIP (Serial Line IP) through telephone lines.

FIG. 2 shows a construction of an internet service provider. The computer network NET 151 of the internet service provider includes a server S 151 and a router R 151 . The server S 151 is connected to a public telephone line network TEL 151 via modems M 151 , M 152 , M 153 , . . . .

Terminals T 151 , T 152 , T 153 , . . . are those of individuals personally participating the internet. Terminals T 151 , T 152 , T 153 , . . . are connected to the public telephone line network TEL 151 through modems (not shown). Individual terminals T 151 , T 152 , T 153 , . . . may be personal computers having serial ports.

For participation in the internet through an internet service provider, users previously make a contract with an internet service provider in most cases. When a contract is concluded between a user and an internet service provider, an account code and a password are sent to the user.

When an individual participates in the internet from one of the terminals T 151 , T 152 , T 153 , . . . , the user dials into the internet service provider to call up the server S 151 of the computer network NET 151 of the provider. The server S 151 responsively requests entry of the account code and the password for authentication whether the user is a contractor. When the server S 151 authenticates that the entered account code and password are those of a contractor, it searches for an available IP address. If there is any IP address available, it temporarily assigns it to the terminal T 151 , T 152 , T 153 , or any other. Thus, the terminal obtaining the temporary IP address can connect to the internet.

In the above example, terminals are connected by PPP using telephone lines. However, ISDN (Integrated Service Digital Network) may be used alternatively. ISDN 64 includes three channels, namely, two B channels of 64 kbps and one D channel of 16 kbps. When ISDN is used, it can be used as a line of 64 kbps by sending IP packets on the B channels.

Internet telephones for effecting telephone communication using the internet are now being developed. Since the internet is basically free of charge, what is to be paid by the user for internet telephone communication through the internet is the charge based on the contract with the internet service provider and the charge for the call between the user and the internet service provider or the charge for the use of ISDN. Thus, users can enjoy long-distance telephone calls and international telephone calls very economically.

As explained above, individual terminals normally participate the internet by using a telephone circuit by PPP under a contract with a provider. When a mode is used, the maximum data rate is currently 28.8 kbps. As to audio data, however, data transfer rate ranges from 8 kbps to 64 kbps, depending on compression systems, and a data rate around 14.4 kbps is sufficient practically. Therefore, even when a telephone circuit is used, higher data transfer rates are acceptable. If a digital circuit such as ISDN is used, higher transmission rates are acceptable.

Taking it into account, telephone calls of two different routes using a single telephone line will be possible. If telephone calls of two routes using a single line is possible, the apparatus can be used conveniently also for telephone communication by three persons, interrupt calls and delivery of a call from a telephone set to another.

It is therefore an object of the invention to provide a terminal apparatus of a telephone system, which enables a plurality of telephone calls using a telephone line efficiently, and enables conversation by three persons, interrupt calls and delivery of a call from a telephone set to another.

Another object of the invention is to provide a terminal apparatus of a telephone system, which enables a plurality of telephone calls using a single telephone line without assigning a plurality of IP addresses to the single telephone line.

According to the invention, there is provide a telephone apparatus connectable to a computer network through a telephone circuit to transmit data including audio signals via a server of said computer network includes: a first telephone set including a first audio input/output means for converting input voice into a digital audio signal and for converting a digital audio signal into output voice, and a first CPU that executes processing for connecting the first telephone set to the server and processing for compression and expansion of the digital audio signal; a second telephone set including a second audio input/output means for converting input voice into a digital audio signal and for converting a digital audio signal into output voice, and a second CPU that executes processing for connecting the first telephone set to the server and processing for compression or expansion of the digital audio signal; and connection control means connected between the telephone circuit and the first and second CPUs to enable the first and second CPUs to transmit the data.

FIG. 1 is a block diagram for use in explanation of the internet;

FIG. 2 is a block diagram for use in explanation of PPP connection.

FIG. 3 is a block diagram for use in explanation of an internet telephone system to which the invention is applicable;

FIG. 4 is a schematic diagram for use in explanation of an internet telephone system to which the invention is applicable;

FIG. 5 is a flow chart for use in explanation of an internet telephone system to which the invention is applicable;

FIG. 6 is a block diagram for use in explanation of another internet telephone system to which the invention is applicable;

FIG. 7 is a flow chart for use in explanation of another internet telephone system to which the invention is applicable;

FIG. 8 is a perspective view of a telephone set in an internet telephone system to which the invention is applied;

FIG. 9 is a cross-sectional view for use in explanation of a telephone set in an internet telephone system to which the invention is applied;

FIG. 10 is a block diagram of a telephone set in an internet telephone system to which the invention is applied;

FIG. 11 is a schematic diagram of a telephone set in an internet telephone system to which the invention is applied; and

FIG. 12 is a block diagram of another telephone set in an internet telephone system to which the invention is applied.

Embodiments of the invention are explained below with reference to the drawings. The present invention is applied to an internet telephone for transmitting audio data through the internet, and is especially suitable for use of internet telephones connected by PPP through public telephone networks.

FIG. 3 shows an internet telephone system to which the invention is applicable. In FIG. 3 , a computer network NET 1 is, for example, a computer network prepared by an internet service provider. The computer network NET 1 contains a server S 1 and a router R 1 .

The server S 1 is connected to a public telephone network TEL 1 through modems M 1 , M 2 , M 3 , . . . . Currently, data can be transmitted at the rate of 28.8 kbps through the public telephone network TELL by using a high-speed modem.

The computer network NET 1 is connected to other computer networks forming the internet through the router R 1 . The router R 1 routes data on the computer network to another computer network containing a destination terminal.

Terminals T 1 , T 2 , T 3 . . . are those of individuals personally participating in the internet. Individual terminals T 1 , T 2 , T 3 , . . . may be personal computers installed with an internet telephone program or exclusive internet telephone sets. Exclusive internet telephone sets are terminals exclusive to internet telephones facilitating telephone communication using the internet as explained later.

The server S 1 has a data base DB 1 . As shown in FIG. 4 , the data base DB 1 stores “terminal names”, “internet names”, “connection types”, “public phone numbers for PPP”, “users' names”, and other information. The data base DB 1 may be established using information obtained from contents of contracts concluded between the internet service provider and users. When the connection type is PPP, the data base DB 1 contains telephone numbers for PPP of users contracting with the internet service provider by PPP connection.

Although the terminals T 1 , T 2 , T 3 , . . . are connected to the server S 1 by PPP through the public telephone network in the above example, they may be connected through a digital network such as ISDN.

Next explained is a telephone call control in a telephone system to which the invention is applied. Assume here that a telephone call from the terminal T 1 to the terminal T 2 is desired in FIG. 3 . The internet requires an IP address to specify a destination terminal. In this case, it is possible that the destination terminal T 2 to be connected by PPP is not currently connected to the computer network NET 1 . Therefore, if the destination terminal T 2 is out of connection with the computer network NET 1 , it cannot be accessed to by using an IP address. Thus, the data base DB 1 is used.

FIG. 5 is a flow chart showing the accessing process using the data base DB 1 . First, the source terminal T 1 dials the computer network NET 1 of the internet service provider to call up the server S 1 of the computer network NET 1 . Responsively, the server S 1 requests the terminal T 1 to enter its account code and the password to authenticate whether the source terminal T 1 is one of contractors of the internet service provider. The user of the source terminal T 1 answers the request by entering its account code and the password. When the server S 1 authenticates that the entered account code and password are those of a contractor, it assigns a temporary IP address to the terminal T 1 . Thus, PPP connection with the terminal T 1 is started (step ST 1 ).

After that, the terminal T 1 designates a desired destination address (for example, terminal T 2 ) (step ST 2 ).

Responsively, the server S 1 searches into the data base DB 1 to find out information on the terminal T 2 corresponding to the requested destination address. The telephone number of the terminal T 2 for PPP connection can be known from information in the data base DB 1 (step ST 3 ).

The server S 1 subsequently determines an IP address for specifying the destination terminal T 2 within the server to prepare for PPP connection, and gives a notice on the destination terminal's IP address to the source terminal T 1 (step ST 4 ).

Then, the server S 1 dials the telephone number of the terminal T 2 found out from the data base DB 1 to call up the terminal T 2 . When connection of the telephone line to the destination terminal T 2 is confirmed, the server S 1 , after authentication, assigns the IP address to the destination terminal (step ST 5 ).

PPP connection is thus started (step ST 6 ). As a result, audio data is exchanged for communication between the terminals T 1 and T 2 (step ST 7 ). The audio data is transmitted in a compressed form. For exchanging audio data, UDP is used as the protocol of the transport layer.

When the communication ends, all connection including PPP connection and telephone line connection between the terminal T 1 and the server S 1 , those between the terminal T 2 and the server S 1 is disconnected (step ST 8 ).

Although the above example is configured to determine the IP address of the terminal T 2 prior to completing access to the terminal T 2 , the IP address of the terminal T 2 may be determined after the access to the terminal T 2 is completed. It is also possible to inform the source terminal T 1 of the assigned IP address, if necessary. When the source terminal is informed of the IP address at the time when the server determines the IP address of the destination terminal, the source terminal can prepare for communication with the destination terminal such that the terminals can smoothly proceed to communication.

In this manner, the data base DB 1 is provided which stores information on telephone number for PPP connection, and a destination terminal is accessed through the telephone number obtained from the data base DB 1 when the destination terminal is a PPP-connected terminal (T 2 , for example). Then, the destination terminal T 2 is connected to the server S 1 by PPP. Therefore, even when the destination terminal is a PPP-connected terminal, the destination terminal can be called up for communication.

In the above example, a terminal is connected for communication with another terminal in a common computer network. However, a terminal in a computer network can be connected for communication also with a terminal in a different computer network. FIG. 6 shows an example where terminals in different computer networks are connected for communication.

In FIG. 6 , a computer network NET 11 is, for example, a computer network prepared by an internet service provider. The computer network NET 11 contains a server S 11 and a router R 11 . The server S 11 is connected to a public telephone network TEL 11 through modems M 11 , M 12 , M 13 , . . . .

The server S 11 has a data base DB 11 . The data base DB 11 stores information containing telephone numbers of terminals connected by PPP to the computer network NET 11 . The computer network NET 11 is connected to other computer networks forming the internet through the router R 11 . The router R 11 routes data on the computer network to an appropriate computer network containing a destination terminal. Terminals T 11 , T 12 , T 13 . . . are those of individuals personally participating in the internet.

A computer network NET 21 is, for example, a computer network prepared by another internet service provider. The computer network NET 21 contains a server S 21 and a router R 21 . The server S 21 is connected to a public telephone network TEL 21 through modems M 21 , M 22 , M 23 , . . . . The server S 21 has a data base DB 21 . The data base DB 21 stores information containing telephone numbers of terminals connected by PPP to the computer network NET 21 . The computer network NET 21 is connected to other computer networks forming the internet through the router R 21 . The router R 21 routes data on the computer network to an appropriate computer network containing a destination terminal. Terminals T 21 , T 22 , T 23 . . . are those of individuals personally participating the internet.

Assume here that the terminal T 11 desires a telephone call to the terminal T 12 . In this case, a process is progressed as shown in FIG. 7 .

First, the source terminal T 11 dials the computer network NET 11 of the internet service provider to call up the server S 11 of the computer network NET 11 . Responsively, the server S 11 requests the terminal T 11 to enter its account code and the password for authentication whether the source terminal T 11 is one of contractors of the internet service provider.

The user of the source terminal T 11 answers the authentication request by entering its account code and the password. When the server S 11 confirms that the entered account code and password are those of a contractor, it assigns a temporary IP address to the terminal T 11 . Thus, PPP connection of the terminal T 11 is started.

After that, the terminal T 11 sends a call request to the server S 11 , and the server S 11 sends back a call approval. In receipt of the call approval, the terminal T 11 gives a desired destination address (terminal T 21 , for example).

The server S 11 connected to the source terminal in receipt of the destination address sends a call request for communication with the terminal T 21 , for example, to the server S 21 of the computer network (NET 21 , for example) containing the destination terminal. In receipt of the call request for communication with the terminal 21 , the server S 21 sends back a call approval to the server S 11 . In receipt of the call approval, the server S 11 of the network NET 11 containing the source terminal sends the destination address and information on the source terminal.

The server S 21 of the computer network NET 21 containing the destination terminal searches into the data base DB 21 to find out information on the terminal T 21 . The telephone number of the terminal T 21 for PPP connection can be known from information of the data base DB 21 . The server S 21 of the computer network NET 21 dials the telephone number of the terminal T 21 obtained from the data base DB 21 to call up the terminal T 21 .

The destination terminal T 21 accessed by the server S 21 sends back an acknowledgement. The server S 21 in receipt of the acknowledgement requests PPP connection, and the terminal T 21 in receipt of the request for PPP connection gives confirmation of PPP connection.

The server S 21 then request entry of the account code and the password for authentication. In response to the authentication, the user of the destination terminal enters the account code and the password. When the entered account code and password are confirmed to be those of a proper contractor, an IP address is assigned to the terminal T 21 . Thus, PPP connection of the terminal T 21 is started.

When the PPP connection is started, a call request is sent from the server S 21 to the terminal T 21 , and a call approval is sent back from the terminal T 21 to the server S 21 . Then, the server S 21 of the computer network NET 21 sends a call connection completion notice to the server S 11 of the computer network NET 11 , and the server S 11 sends a call connection completion notice to the terminal T 11 . As a result, audio data is exchanged for communication between the terminals T 11 and T 21 .

When a disconnection request is issued from the source terminal T 11 , for example, after the communication ends, the disconnection request is sent to the destination terminal T 21 . In receipt of the disconnection request, the terminal T 21 sends back a disconnection agreement to the terminal T 11 , and all connection is disconnected.

For using an internet telephone by PPP connection, a personal computer is prepared in most cases. Such a personal computer needs connection of a microphone and a speaker and needs installation of appropriate software for connection to the internet. It is difficult for users unfamiliar to computers to set a personal computer ready for connection to the internet. Moreover, a computer connected by PPP for use as an internet telephone is difficult to operate and is expensive.

To avoid this problem, an exclusive internet telephone apparatus is used as a terminal. The internet telephone apparatus does not need installation of software for connection to the internet, and can be easily connected to the internet for audio communication.

FIG. 8 shows such a terminal of an internet telephone apparatus. The telephone apparatus to which the invention is applied uses two main telephone sets 1 A and 1 B and can effect two independent telephone calls of different routes through a single telephone line.

In FIG. 8 , numerals 1 A and 1 B denote main telephone bodies that are connected to each other. The main telephone body 1 A is connected to a public telephone circuit.

The main telephone bodies 1 A and 1 B have display/operators 2 A and 2 B on their upper surfaces. The display/operators 2 A and 2 B are multi-layered panels each including a touch panel 4 stacked on a display panel 3 as shown in FIG. 9 . The display/operators 2 display icons of numerical keys, operational keys, and so forth, which permit a user to enter a desired instruction by pressing the touch panel 4 at the portion of a corresponding icon. The display/operators 2 A and 2 B also display help messages explaining how to operate the keys and the current modes of setting in addition to key icons, etc. Other various information is also displayed on the display/operators 2 A and 2 B. Handsets 5 A and 5 B are connected to the main telephone bodies 1 A and 1 B.

As referred to above, the internet telephone apparatus to which the invention is applied can effect independent telephone calls of two routes using the telephone sets 1 A and 1 B.

More specifically, if data is transmitted via a modem through a normal telephone line, the maximum data transmission rate is currently 28.8 kbps. As to audio data, however, its transmission rate ranges from 8 kbps to 64 kbps, depending on compression systems. To ensure an acceptable quality of sound, audio data must be sent at the rate of 8 kbps to 14 kbps minimum.

Even if a terminal selects a compression system in which the transmission rate of audio data is 12 kbps and transmits data at the rate of 28.8 kbps through a public telephone circuit, the data rate on the circuit increases to 16 kbps, approximately, because asynchronous serial transmission is employed and an overhead for PPP connection is added. However, since the circuit having the rate of 28.8 kbps leaves a margin, it is possible to send data at the transmission rate of 12 kbps approximately. Therefore, taking the overhead attached to the data for communication into account, it is still possible to transmit additional audio data of about 8 kbps. This means that additional compressed audio data can be put onto the circuit in addition to the audio signal of the telephone call. Therefore, when the quality of sound is not of much importance, data of two telephone calls can be put onto the telephone line. If a digital circuit such as ISDN is used, more telephone calls can be made without deteriorating the quality of sound.

As explained above, a plurality of telephone sets 1 A and 1 B can be operated independently through a single line. As its other applications, telephone communication by three persons and interrupt function can be used conveniently. For example a, one-to-two call from two main telephone sets 1 A and 1 B to a single destination is made possible. If desired, the one-to-two talk can be changed to one-to-one call choosing one of the telephone sets 1 A and 1 B. It is also possible to deliver a telephone call from the telephone set 1 A to the telephone set 1 B, or vice versa. When any interrupt occurs to the telephone set 1 A, the new call can be delivered to and continued by the telephone set 1 B.

FIG. 10 shows such an internet telephone set. In this example, telephone sets 11 A and 11 B are operative as independent terminals such that data of two calls can be transmitted through a single public telephone line.

In FIG. 10 , the telephone set 11 A has a multiplexing circuit 13 for transmitting audio data of two routes through a single telephone line, and a model 12 . The modem 12 is connected to a public telephone circuit.

The telephone set 11 A also has a CPU 21 A. Connected to the CPU 21 A are ROM 22 A, EPROM 23 A and RAM 24 A. Input from a touch panel 25 A is given to CPU 21 A, and output from CPU 21 A is displayed on a display panel 26 A. CPU 21 A executes dial connection processing, data transfer processing by IP, and processing for compression and expansion of audio signals. A handset 27 A is connected to the telephone set 11 A. Voice from the handset 27 A is digitalized by an A/D and D/A converter 28 A. Audio data to the handset 27 A is returned to the analog form by the A/D and D/A converter 28 A.

The telephone set 11 B has a CPU 21 B. CPU 21 B of the telephone set 11 B is connected to the multiplexing circuit 13 of the telephone set 11 A. Connected to the CPU 21 B are ROM 22 B, EPROM 23 B and RAM 24 B. Input from a touch panel 25 B is given to CPU 21 B, and output from CPU 21 B is displayed on a display panel 26 B. CPU 21 B executes dial connection processing, data transfer processing by IP, and processing for compression and expansion of audio signals. A handset 27 B is connected to the telephone set 11 B. Voice from the handset 27 B is converted into a digital form by an A/D and D/A converter 28 B. Audio data to the handset 27 B is returned to the analog form by the A/D and D/A converter 28 B.

FIG. 11 shows contents of a data packet for putting audio data onto a circuit. In this example, a preamble and a post-amble are added to opposite ends of an audio data. The preamble contains the IP address of a destination terminal and the IP address of a source terminal. The preamble also contains a port number. The post-amble contains data for detecting and correcting data error.

The IP address contained in data from a server is recognized by the multiplexing circuit 13 . The IP address is a unique code for identifying a terminal on a network as explained before. When a server and a terminal are connected by dial PPP, the IP address is given every time when such connection is started.

In this example, the telephone sets 11 A and 11 B operate as independent terminals. That is, when a telephone call is made using the telephone set 11 A in FIG. 10 , authentication is done between the telephone set 11 A and the server. If the authentication is affirmative, then an IP address is assigned to the telephone set 11 A, and the telephone set 11 A is PPP-connected to the server. As a result, the call from the telephone set 11 A is made possible.

When a telephone call is made using the telephone set 11 B, authentication is done between the telephone set 11 B and the server. If the authentication is affirmative, then an IP address is assigned to the telephone set 11 B, and the telephone set 11 B is PPP-connected to the server. As a result, the call from the telephone set 11 B is made possible.

In the above example, the telephone sets 11 A and 11 B operate as independent terminals. Therefore, two independent telephone calls using a single telephone line can be made. In this example, although the server can communicates with a destination terminal specified by the IP address, data separation and multiplexing by hardware or software are required also on the part of the server because data of two telephone calls are put on a single line. Moreover, when two telephone sets 11 A and 11 B are used simultaneously, two IP address must be assigned to a single line.

FIG. 12 shows another telephone set. This example enables two independent telephone calls using a single IP address.

In FIG. 12 , numerals 31 A and 31 B denote telephone sets. The telephone set 31 A has a communication controller 33 and a modem 32 . The communication controller 33 executes dial connection processing, data transfer processing by IP, and controls communication to permit two telephone sets 31 A and 31 B to send and receive audio data, respectively, by using identifying information. The identifying information may be port numbers or other appropriate materials. The modem 37 is connected to a public telephone line.

The telephone set 31 A also has a CPU 41 A. Connected to the CPU 41 A are ROM 42 A, EPROM 43 A and RAM 44 A. Input from a touch panel 45 A is given to CPU 41 A, and output from CPU 41 A is displayed on a display panel 46 A. CPU 41 A controls the entirety of the telephone set 31 A, and executes processing for compression and expansion of audio signals. An A/D and D/A converter 48 A converts audio signals from a digital form into an analog form, or vice versa. A handset 47 A is connected to the telephone set 41 A.

When a telephone call is done, authentication is made by the communication controller 33 with the server. If the authentication is affirmative, then an IP address is assigned to the communication controller 33 , and the communication controller 33 is PPP-connected to the server. Audio data is sent in form of the above-explained packets, and the communication controller 33 composes or decomposes these packets.

In this example, it is necessary to separate audio data of two routes for telephone sets 31 A and 31 B, which are sent in parallel. To cope with this, identifying information is added to distinguish data for communication through the telephone set 31 A from data for communication through the telephone set 31 B.

Port numbers, for example, may be used as such identifying information. The internet uses the TCP or UDP protocol for data transmission. In these protocols, a number called port number is added to each communication application. Such a port number is added as a part of the preamble of the packet data shown in FIG. 11 . Port numbers make it possible to distinguish audio data for communication with the telephone set 31 A from audio data for communication with the telephone set 31 B.

In receipt of data, the communication controller 33 identifies from its port number whichever telephone set the data should be directed to, and transfers the data to the telephone set 31 A or 31 B. On the other hand, when the communication controller 33 receives data from the telephone set 31 A or 31 B, it writes a corresponding port number in each packet when it processes the data into packets, so as to give information whichever telephone set the data comes from.

Other materials may be used as identifying information in lieu of port numbers used in the above example. For example, identifying information may be added to data adjacent to the preamble.

According to the invention, a plurality of independent telephone calls can be made through a single line. This contributes to efficient use of telephone lines, and permits users to utilize telephone conversation by three persons, interrupt calls, delivery of a call from a telephone set to another, and so forth. Moreover, by distinguishing data for a telephone set from data for another among a plurality of telephone sets by attaching identifying information, the invention enables a plurality of independent calls using a single line without assigning a plurality of IP addresses.