Abstract:
The migration of telephone services by a telecommunications carrier from a PSTN to an IP network entails that problem that when it is not possible for some telephone subscribers to migrate to the IP network due to the types of their telephone lines or services they subscribe to, other subscribers also cannot migrate to the IP network until the former subscribers migrate to the IP network. 
     The present invention provides a media gateway that enables a gradual transition from a PSTN to an IP network on a subscriber-by-subscriber basis or on a service-by-service basis so that a telecommunications carrier can efficiently migrate its subscribers while continuing to provide the services the subscribers currently use and reducing burdens on the subscribers.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The present application claims priority from Japanese applications JP2008-303416 filed on Nov. 28, 2008, the content of which is hereby incorporated by reference into this application. 
       BACKGROUND OF THE INVENTION  
       [0002]    The present invention relates to media gateways that can be used as means for smoothly migrating telephone subscribers and telephone services from the traditional Public Switched Telephone Network (PSTN) to Internet Protocol (IP) networks. Due to the low profitability and aged components of the PSTN, the transition from the PSTN to IP networks is now being discussed. One advantage of IP networks over the PSTN is that the routers and switches that constitute IP networks are less expensive and lower in maintenance costs than the switches that constitute the PSTN. In addition, IP networks can provide unique services that are not possible with the PSTN. 
         [0003]    To migrate telephone subscribers to an IP network, it is necessary to provide telephone services over the IP network. To achieve this, it is necessary to connect a terminal controller for Internet access to a telephone, convert voice data into IP packets with the terminal controller, and use the Session Initiation Protocol (SIP) for call control. There are two common methods for providing telephone services over an IP network. The first method involves the use of the Asymmetric Digital Subscriber Line (ADSL) technology. The ADSL technology uses copper telephone lines, and an ADSL modem on the subscriber side multiplexes telephone voice data and IP packets. By the ADSL modem having the function of converting voice data into IP packets, telephone services become possible over an IP network. The second method is to replace copper lines between a telecommunications carrier and its subscribers with optical fiber lines used for IP networks. Similar to the first method, by providing a subscriber with a device that converts voice data into IP packets, telephone services and Internet access services become possible over an IP network. 
         [0004]    The first and second methods mentioned above, however, are only effective when a telephone subscriber already uses Internet access services. If those methods are applied to a telephone subscriber without Internet access services, there is a need to install a terminal controller at the subscriber&#39;s home or to replace a copper telephone line between the subscriber and the telecommunication carrier with an optical fiber line, which is considerably time-consuming and costly. Such being the case, there is another method for providing telephone services over an IP network. The method involves the use of a relay apparatus, called a media gateway, which is provided between the PSTN and an IP network (disclosed in Japanese Patent Publication No. 2005-094480). 
         [0005]    While IP networks have advantages over the PSTN, they also have the disadvantage that they cannot provide services that are available over the PSTN. Thus, the ADSL technology mentioned above often employs such means as disclosed in WO No. 03/103259 so that the conventional PSTN is used when a service is not possible over an IP network. 
       SUMMARY OF THE INVENTION 
       [0006]    Upon migration of telephone services from the PSTN to an IP network, replacing a switch to which multiple subscriber lines are connected with a media gateway results in all the subscribers being transferred to the IP network. However, when it is not possible for some telephone subscribers to migrate to the IP network due to the types of their telephone lines or services they subscribe to, other subscribers also cannot migrate to the IP network until the former subscribers migrate to the IP network. Moreover, when a subscriber uses, for example, Service “A” that is available over both of the PSTN and an IP network and Service “B” that is only available over the PSTN, that subscriber cannot be transferred to a media gateway until the subscriber cancels Service “B” or until Service “B” becomes available over an IP network. 
         [0007]    Thus, the migration of telephone services from the PSTN to an IP network with the use of media gateways requires a telecommunications carrier to efficiently migrate its subscribers while continuing to provide the services the subscribers currently use and reducing burdens on the subscribers. 
         [0008]    A media gateway according to the invention is a telecommunications management apparatus with an IP network connection interface and a PSTN connection interface and provides network access for the terminals of telephone subscribers. Upon receipt of a call signal from one of the terminals, the media gateway connects the one of the terminals to the network predetermined by a maintenance person. Thus, the maintenance person can select networks to which to connect subscriber terminals on a subscriber-by-subscriber basis. 
         [0009]    Further, when a subscriber specifies a particular service by a particular dial number, the media gateway connects his or her terminal to the network predetermined by the maintenance person. Thus, the maintenance person can select networks to which to connect subscriber terminals on a service-by-service basis. 
         [0010]    During connection to the PSTN, the media gateway performs PSTN call control. During connection to an IP network, the media gateway performs IP network call control. In this manner, the media gateway can perform path connection processing. 
         [0011]    When the media gateway receives a connection request addressed to a subscriber terminal for which the media gateway provides network access, the media gateway checks to see the connection status of the line of that terminal. If the line is available, the media gateway connects the subscriber terminal to the network through which the connection request was transmitted. Thus, the media gateway can perform path connection processing when it receives a connection request either from an IP network or the PSTN. When the media gateway receives a connection request addressed to a subscriber from the PSTN while the subscriber is connected to the IP network, the media gateway transmits a “busy” response to the PSTN, thereby notifying the caller that the line is busy. Likewise, when the media gateway receives an “INVITE” request addressed to a subscriber from an IP network while the subscriber is connected to the PSTN, the media gateway transmits a “Busy” response to the IP network, thereby notifying the caller that the line is busy. Further, when a call waiting service is available for a subscriber, the media gateway can transmit an incoming call alert to the subscriber. Thus, the media gateway can notify the subscriber of the receipt of a connection request from a caller during a call between the subscriber and another caller. Furthermore, the media gateway can provide a call waiting service across different types of networks (an IP network and the PSTN) by detecting a momentary on-hook signal from a subscriber to which a caller transmitted a connection request while the subscriber was connected to another caller, switching the subscriber&#39;s connection, and transmitting a busy response to the latter caller. 
         [0012]    The media gateway comprises the following components: multiple subscriber interfaces; a switch; a TDM-IP converter; IP network interfaces; a PSTN interface; a subscriber interface controller; PSTN interface controller; a memory; and a processor that performs connection processing between subscribers and the PSTN via the subscriber interface controller and the PSTN interface controller, performs IP network connection processing, and sets up the switch. The memory stores data tables, examples of which include a table of subscriber information and associated network connection information, a table of dial numbers and associated network connection information, and a table indicating the line status information of subscribers. 
         [0013]    In one aspect, a telecommunications management apparatus according to the invention comprises a first interface for exchanging information with a plurality of terminals; a storage device for storing identification information of the plurality of terminals and storing network connection information associated with the identification information; and a processor for receiving information from one of the plurality of terminals, reading from the storage device the network connection information associated with the identification information of the one of the plurality of terminals based on the received information, and connecting the one of the plurality of terminals based on the read network connection information to either a public switched telephone network or an IP network via the telecommunications management apparatus. A telecommunications system according to the invention includes the telecommunications management apparatus described above. 
         [0014]    A telecommunications carrier can migrate telephone services from the PSTN to an IP network while reducing burdens on its telephone subscribers by replacing switches that provide network access for the subscribers with media gateways according to the invention. By a gradual transition from the PSTN to an IP network on a subscriber-by-subscriber basis, toll switches can be geographically consolidated in a planned manner in proportion to the number of subscribers using the PSTN. Further, by a graduation transition from the PSTN to an IP network on a service-by-service basis, all the existing services need not be provided over the IP network, and service migration can be done according to the order of priority. Furthermore, by setting a transition period in which services can be offered over both of an IP network and the PSTN, a service can be switched from the IP network to the PSTN if a problem arises, such as the suspension of that service over the IP network. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  shows an example of networks to which the invention is applied. 
           [0016]      FIG. 2  illustrates the configuration of a media gateway according to the invention. 
           [0017]      FIG. 3  is an example of a subscriber information table. 
           [0018]      FIG. 4  is an example of a service table. 
           [0019]      FIG. 5  is a flowchart illustrating the operation flow of a media gateway when the media gateway receives a call signal from a telephone subscriber. 
           [0020]      FIG. 6  illustrates the operation sequence when the subscriber terminal  2  of  FIG. 1  makes a call to the subscriber terminal  7  of  FIG. 1 . 
           [0021]      FIG. 7  illustrates the operation sequence when the subscriber terminal  1  of  FIG. 1  makes a call to the subscriber terminal  4  of  FIG. 1  with the use of Service 
           [0022]      FIG. 8  illustrates the operation sequence when the subscriber terminal  1  of  FIG. 1  makes a call to the subscriber terminal  8  of  FIG. 1  with the use of Service 
           [0023]      FIG. 9  is an example of a line status table. 
           [0024]      FIG. 10  is a flowchart illustrating the operation flow of a media gateway when the media gateway receives a connection request or an “INVITE” request addressed to a subscriber for which the media gateway provides network access. 
           [0025]      FIG. 11  illustrates the operation sequence when the subscriber terminal  7  of  FIG. 1  makes a call to the subscriber terminal  1  of  FIG. 1 . 
           [0026]      FIG. 12  illustrates the operation sequence when the subscriber terminal  7  of  FIG. 1  makes a call to the subscriber terminal  2  of  FIG. 1 , for which a call waiting service is not available, during a call between the subscriber terminals  2  and  4  of  FIG. 1 . 
           [0027]      FIG. 13  illustrates the operation sequence when the subscriber terminal  7  of  FIG. 1  makes a call to the subscriber terminal  3  of  FIG. 1 , for which a call waiting service is available, during a call between the subscriber terminals  3  and  4  of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0028]    A preferred embodiment of the invention will now be described with reference to the accompanying drawings. The following explanation is based on the assumption that media gateways accommodate analog subscriber lines, but the invention can also be implemented with the use of ISDN lines. The following explanation is also based on the assumption that SIP, the Session Initiation Protocol, is used for call control over an IP network, but other similar protocols can also be used for that purpose. 
         [0029]      FIG. 1  illustrates an example of networks to which the invention is applied. A first media gateway  10  (also called a telecommunications management device) provides network access for subscriber terminals  1 ,  2 , and  3  and a second media gateway  11  for subscriber terminals  4 ,  5 , and  6 . A conventional subscriber switch  20  provides network access for subscriber terminals  7 ,  8 , and  9 . The first and second media gateways  10  and  11  are both connected to an IP network  100  and the PSTN  200 , and the switch  20  is connected to the PSTN  200 . An SIP server  30  is connected to the IP network  100  so that the SIP server  30  connects a caller to a call recipient when the SIP is in use. A gateway  40  is provided between the IP network  100  and the PSTN  200  so that a subscriber connected to the IP network  100  can communicate with a subscriber connected to the PSTN  200 . 
         [0030]      FIG. 2  is a configuration example of the first and second media gateways  10  and  11  (only one of them is shown). Multiple subscriber line interfaces  10 - 1  are connected to the subscriber side of a switch  10 - 2 , and the switch  10 - 2  selectively connects the subscriber line interfaces  10 - 1  to a TDM-IP converter  10 - 3  or to a PSTN interface  10 - 11  in accordance with instructions from a processor  10 - 5 . The network side of the switch  10 - 2  transmits a signal from the TDM-IP converter  10 - 3  or from the PSTN interface  10 - 11  to a particular one of the subscriber line interfaces  10 - 1  in accordance with an instruction from the processor  10 - 5 . A second IP interface  10 - 6  is used for the processor  10 - 5  to exchange call control information with the SIP server  30 . A first IP interface  10 - 4  is used for the transmission/receipt of subscriber data after the completion of connection processing. A memory (storage device)  10 - 8  is connected to the processor  10 - 5  to store various data tables, which are referred to by the processor  10 - 5 . A maintenance interface  10 - 7  is also connected to the processor  10 - 5  for a maintenance person to overwrite or refer to the data tables. A subscriber interface controller  10 - 9  monitors signals from subscriber terminals and notifies the processor  10 - 5  upon receipt of a signal. The subscriber interface controller  10 - 9  also transmits signals to the subscriber terminals in accordance with instructions from the processor  10 - 5 . A PSTN interface controller  10 - 10  monitors signals from the PSTN  200  and notifies the processor  10 - 5  upon receipt of a signal. The PSTN interface controller  10 - 10  also transmits signals to the PSTN  200  in accordance with instructions from the processor  10 - 5 . 
         [0031]      FIG. 3  is a subscriber information table  50  that is stored on the memories  10 - 8  of the first and second media gateways  10  and  11 . The subscriber information table  50  includes subscriber information  50 - 1  and network connection information  50 - 2 . The subscriber information  50 - 1  is used to identify respective subscribers. Examples of the subscriber information  50 - 1  include phone numbers, subscriber interfaces the first media gateway  10  or the second media gateway  11  accommodates, time slots, and services the subscribers use. The network connection information  50 - 2  indicates which network to connect each subscriber to, the IP network  100  or the PSTN  200 . The network connection information  50 - 2  is set by the telecommunications carrier via the maintenance interface  10 - 7 . 
         [0032]      FIG. 4  is a service table  60  that is stored on the memories  10 - 8  of the first and second media gateways  10  and  11 . The service table  60  includes dial number information  60 - 1  and network connection information  60 - 2 . The dial number information  60 - 1  is the dial numbers subscribers use to specify particular services. The network connection information  60 - 2  indicates which network to connect each service to, the IP network  100  or the PSTN  200 . The network connection information  60 - 2  is set by the telecommunications carrier via the maintenance interface  10 - 7 . When a dial number is not applicable to the service table  60 , the subscriber is connected to the network specified by “NA” in the column of the dial number information  60 - 1 . 
         [0033]      FIG. 9  shows a line status table  70  that is stored on the memories  10 - 8  of the first and second media gateways  10  and  11 . The line status table  70  includes subscriber information  70 - 1  and line status information  70 - 2 . The subscriber information  70 - 1  is used to identify respective subscribers as in the subscriber information table  50 . The line status information  70 - 2  is updated when the processor  10 - 5  sets a connection destination for the switch  10 - 2  and changed to “Available” after the connection is lost. 
         [0034]      FIG. 5  is a flowchart illustrating the operation flow of the first media gateway  10  when it receives a call signal from one of the subscriber terminals  1  to  3 . Note that the following explanation also applies to the second media gateway  11  when it receives a call signal from one of the subscriber terminals  4  to  6 . When the first media gateway  10  receives a call signal from one of the subscriber terminals  1  to  3  (Step  300 ), it refers to the subscriber information table  50  (Step  301 ) to see which network is set (“IP network/PSTN” or “PSTN”) for the network connection information  50 - 2  associated with the subscriber information  50 - 1  of that subscriber terminal which transmitted the call signal (Step  302 ). When the network connection information  50 - 2  indicates “PSTN,” the first media gateway  10  sets up a PSTN connection (Step  303 ). When the network connection information  50 - 2  indicates “IP network/PSTN,” the first media gateway  10  waits for the subscriber terminal to transmit a dial signal. After the receipt of a dial signal (Step  304 ), the first media gateway  10  refers to the service table  60  (Step  305 ) to see which network is set (“IP network” or “PSTN”) for the network connection information  60 - 2  associated with the dial number used (Step  306 ). When the network connection information  60 - 2  indicates “PSTN,” the first media gateway  10  sets up a PSTN connection (Step  303 ). When the network connection information  60 - 2  indicates “IP network,” the first media gateway  10  sets up an IP network connection (Step  307 ). 
         [0035]      FIG. 10  illustrates the operation flow of the first media gateway  10  when there is a connection request addressed to one of the subscriber terminals  1  to  3 . Note that the following explanation also applies to the second media gateway  11  when there is a connection request addressed to one of the subscriber terminals  4  to  6 . After the first media gateway  10  receives from the PSTN  200  a connection request addressed to one of the subscriber terminals  1  to  3  (Step  400 ), the first media gateway  10  refers to the line status table  70  (Step  401 ). The first media gateway  10  checks to see whether the subscriber terminal to which the connection request is addressed is available or not by referring to the line status information  70 - 2  associated with the subscriber information  70 - 1  of that subscriber terminal (Step  402 ). If the line status information  70 - 2  indicates the subscriber terminal is “Available,” the first media gateway  10  changes the line status information  70 - 2  of the subscriber terminal from 
         [0036]    “Available” to “Connected to PSTN” (Step  403 ) and then sets up a PSTN connection (Step  404 ). If, on the other hand, the line status information  70 - 2  indicates that the subscriber terminal is being “Connected to IP Network,” the first media gateway  10  refers to the subscriber information  70 - 1  (Step  405 ) to see whether a call waiting service is available for the subscriber terminal or not (Step  406 ). If the service is not available, the first media gateway  10  transmits a “busy” response to the PSTN  200  (Step  407 ). If the service is available, the first media gateway  10  monitors the subscriber terminal for a momentary on-hook signal (Step  408 ). After detecting a momentary on-hook signal, the first media gateway  10  changes the line status information  70 - 2  of the subscriber terminal from “Connected to IP Network” to “Connected to IP Network/PSTN” (Step  409 ) and changes the network connection of the subscriber terminal from the IP network  100  to the PSTN  200  (Step  410 ). Likewise, after the first media gateway  10  receives from the IP network  100  an “INVITE” request addressed to one of the subscriber terminals  1  to  3  (Step  400 ), the first media gateway  10  refers to the line status table  70  (Step  401 ). The first media gateway  10  checks to see whether the subscriber terminal to which the “INVITE” request is addressed is available or not by referring to the line status information  70 - 2  of that subscriber terminal (Step  402 ). If the line status information  70 - 2  indicates the subscriber terminal is “Available,” the first media gateway  10  changes the line status information  70 - 2  of the subscriber terminal from “Available” to “Connected to IP Network” (Step  403 ) and then sets up an IP network connection (Step  404 ). If, on the other hand, the line status information  70 - 2  indicates that the subscriber terminal is being “Connected to PSTN,” the first media gateway  10  refers to the subscriber information  70 - 1  (Step  405 ) to see whether a call waiting service is available for the subscriber terminal or not (Step  406 ). If the service is not available, the first media gateway  10  transmits a “busy” response to the IP network  100  (Step  407 ). If the service is available, the first media gateway  10  monitors the subscriber terminal for a momentary on-hook signal (Step  408 ). After detecting a momentary on-hook signal, the first media gateway  10  changes the line status information  70 - 2  of the subscriber terminal from “Connected to PSTN” to “Connected to IP Network/PSTN” (Step  409 ) and changes the network connection of the subscriber terminal from the PSTN  200  to the IP network  100  (Step  410 ). 
         [0037]    The following explains more in detail the operation of the first media gateway  10  when it receives a call signal from one of the subscriber terminal  1  to  3 . The operation of the first media gateway  10  when there is a connection request addressed to one of the subscriber terminals  1  to  3  will later be described in detail. 
         [0038]    With reference now to  FIG. 6 , how to establish a connection between the subscriber terminal  2 , the caller, and the subscriber terminal  7 , the call recipient, through the first media gateway  10  and the switch  20  will be discussed. Note that the subscriber information  50 - 1  of the subscriber terminal  2  is “B” of  FIG. 3 . After the subscriber terminal  2  transmits a call signal (Step  500 ), the switch  10 - 2  of the first media gateway  10  receives the signal via the subscriber line interface  10 - 1  of the subscriber terminal  2  (Step  300 ). The switch  10 - 2  then transmits the signal to the subscriber interface controller  10 - 9 , and the subscriber interface controller  10 - 9  in turn notifies the processor  10 - 5  of its receipt of the signal with the “B” subscriber information  50 - 1 . The processor  10 - 5  that has received the notification refers to the subscriber information table  50  (Step  301 ) to read the network connection information  50 - 2  associated with the “B” subscriber information  50 - 1 . In this case, the processor  10 - 5  recognizes the network connection information  50 - 2  as being the PSTN  200 , as shown in  FIG. 3  (Step  302 ). The processor  10 - 5  then instructs the switch  10 - 2  to connect the subscriber line with the “B” subscriber information  50 - 1  to the PSTN interface  10 - 11  (Step  303 ). In the meantime, the subscriber interface controller  10 - 9  transmits a dial tone to the subscriber terminal  2  via the switch  10 - 2  at the same time as the receipt of the call signal (Step  501 ). After the subscriber terminal  2  transmits a dial signal to the switch  10 - 2  by dialing the number of the subscriber terminal  7  (Step  502 ), the switch  10 - 2  transmits the dial signal to the processor  10 - 5 , and the processor  10 - 5  transmits a connection request to a given toll switch on the PSTN  200  through the PSTN interface controller  10 - 10  and the PSTN interface  10 - 11  (Step  503 ). The connection request is transmitted from the toll switch on the PSTN  200  to the switch  20  (Step  503 ). The switch  20  that provides network access for the subscriber terminal  7  then transmits a response to the first media gateway  10  in response to the connection request (Step  504 ) and also transmits a ring signal to the subscriber terminal  7  (Step  506 ). After the completion of the connection processing, the switch  20  transmits a ring tone to the subscriber terminal  2  (Step  505 ). After the subscriber terminal  7  transmits a response signal to the switch  20  in response to the ring signal (Step  507 ), the switch  20  transmits a response signal to the first media gateway  10  (Step  508 ). Thereafter, the PSTN interface controller  10 - 10  of the first media gateway  10  notifies the processor  10 - 5  of the receipt of the response signal, and the processor  10 - 5  transmits a response signal to the subscriber terminal  2  through the subscriber interface controller  10 - 9  (Step  509 ). This establishes a connection between the subscriber terminal  2  and the subscriber terminal  7  (Step  510 ). 
         [0039]    With reference now to  FIG. 7 , how to establish a connection between the subscriber terminal  1 , the caller, and the subscriber terminal  4 , the call recipient, through the first media gateway  10  and the second media gateway  11  will be discussed. Note that the subscriber information  50 - 1  of the subscriber terminal  1  is “A” of  FIG. 3  and that the subscriber terminal  1  uses the service specified by the dial number “a” of  FIG. 4 . Similar to  FIG. 6 , after the subscriber terminal  1  transmits a call signal (Step  600 ), the first media gateway  10  receives the signal (Step  300 ). The processor  10 - 5  of the first media gateway  10  then refers to the subscriber information table  50  (Step  301 ) to read the network connection information  50 - 2  associated with the “A” subscriber information  50 - 1 . In this case, the processor  10 - 5  of the first media gateway  10  recognizes the network connection information  50 - 2  as being the IP network  100  or the PSTN  200 , as shown in  FIG. 3  (Step  302 ). At the same time as the receipt of the call signal, the subscriber interface controller  10 - 9  of the first media gateway  10  transmits a dial tone to the subscriber terminal  1  (Step  601 ). After the subscriber terminal  1  transmits a dial signal by dialing the number of the subscriber terminal  4  (Step  602 ), the processor  10 - 5  of the first media gateway  10  receives the dial signal (Step  304 ) and refers to the service table  60  (Step  305 ) to read the network connection information  60 - 2  associated with the dial number “a.” In this case, the processor  10 - 5  of the first media gateway  10  recognizes the network connection information  60 - 2  as being the IP network  100 , as shown in  FIG. 4  (Step  306 ). The processor  10 - 5  of the first media gateway  10  then instructs the switch  10 - 2  of the first media gateway  10  to connect the subscriber line with the “A” subscriber information  50 - 1  to the TDM-IP converter  10 - 3  of the first media gateway  10  (Step  307 ). The processor  10 - 5  of the first media gateway  10  transmits an “INVITE” request to the SIP server  30  via the second IP interface  10 - 6  of the first media gateway  10  (Step  603 ). The SIP server  30  transfers the “INVITE” request to the second media gateway  11  that provides network access for the subscriber terminal  4  (Step  604 ). After the receipt of the “INVITE” request, the second media gateway  11  transmits a “ 180  Ringing” response to the SIP server  30  (Step  605 ) and also transmits a ring signal to the subscriber terminal  4  (Step  606 ). The SIP server  30  then transfers the “ 180  Ringing” response to the first media gateway  10  (Step  607 ). After the receipt of the “ 180  Ringing” response, the first media gateway  10  transmits a ring tone to the subscriber  1  (Step  608 ). After the subscriber terminal  4  transmits a response signal to the second media gateway  11  (Step  609 ), the second media gateway  11  transmits a “ 200  OK” response to the SIP server  30  (Step  610 ). The SIP server  30  transfers the “ 200  OK” response to the first media gateway  10  (Step  611 ). After the receipt of the “ 200  OK” response, the first media gateway  10  transmits a response signal to the subscriber terminal  1  (Step  612 ) and also transmits an “ACK” response to the SIP server  30  (Step  613 ). The SIP server  30  transfers the “ACK” response to the second media gateway  11  (Step  614 ). This establishes a connection between the subscriber terminal  1  and the subscriber terminal  4  (Step  615 ). 
         [0040]    With reference now to  FIG. 8 , how to establish a connection between the subscriber terminal  1 , the caller, and the subscriber terminal  8 , the call recipient, through the first media gateway  10  and the switch  20  will be discussed. Note this time that the subscriber information  50 - 1  of the subscriber terminal  1  is “A” of  FIG. 3  and that the subscriber terminal  1  uses the service specified by the dial number “b” of  FIG. 4 . The operation sequence from the subscriber terminal  1  transmitting a call signal (Step  700 ) up to the processor  10 - 5  of the first media gateway  10  referring to the service table  60  (Step  305 ) is the same as in the above case of  FIG. 7 . Thereafter, the processor  10 - 5  reads from the service table  60  the network connection information  60 - 2  associated the dial number “b” and recognizes the network connection information  60 - 2  as being the PSTN  200 , as shown in  FIG. 4  (Step  306 ). The processor  10 - 5  then sets up a PSTN connection (Step  303 ). Similar to the case of  FIG. 6 , the above process is then followed by the transmission of a connection request (Step  703 ) and finally by the transmission of a response signal to the subscriber  1  (Step  709 ), thereby establishing a connection between the subscriber terminal  1  and the subscriber terminal  8 . 
         [0041]    The operation of the first media gateway  10  when there is a connection request addressed to one of the subscriber terminals  1  to  3  will now be discussed in detail. 
         [0042]    With reference now to  FIG. 11 , how to establish a connection between the subscriber terminal  7 , the caller, and the subscriber terminal  1 , the call recipient, through the switch  20  and the first media gateway  10  will be discussed. Note in this case that the subscriber information  70 - 1  of the subscriber terminal  1  is “A” of  FIG. 9  and that the line of the subscriber terminal  1  is not busy. The process starts with the transmission of a call signal from the subscriber terminal  7  (Step  800 ). The switch  20  then transmits a dial tone to the subscriber terminal  7  (Step  801 ). After the subscriber terminal  7  dials the number of the subscriber terminal  1  (Step  802 ), the switch  20  transmits a connection request to the first media gateway  10  that provides network access for the subscriber terminal  1  (Step  803 ). After the first media gateway  10  receives the connection request (Step  400 ), the PSTN interface controller  10 - 10  notifies the processor  10 - 5  of the receipt of the connection request addressed to the subscriber terminal  1  with the “A” subscriber information  70 - 1 . The processor  10 - 5  then refers to the line status table  70  (Step  401 ) to see the line status information  70 - 2  associated with the “A” subscriber information  70 - 1  of the subscriber terminal  1  (Step  402 ). In this case, the processor  10 - 5  recognizes the line status information  70 - 2  as being “Available,” as shown in  FIG. 9 . Thus, the processor  10 - 5  changes the line status information  70 - 2  from “Available” to “Connected to PSTN” (Step  403 ) and instructs the switch  10 - 2  to connect the subscriber line with the “A” subscriber information  70 - 1  to the PSTN interface  10 - 11  (Step  404 ). The processor  10 - 5  then transmits a response to the switch  20  via the PSTN interface controller  10 - 10  (Step  804 ) and also transmits a ring signal to the subscriber terminal  1  via the subscriber interface controller  10 - 9  (Step  805 ). After the completion of the connection processing, the processor  10 - 5  transmits a ring tone to the subscriber terminal  7  via the PSTN interface controller  10 - 10  (Step  806 ). After the subscriber terminal  1  transmits a response signal to the first media gateway  10  (Step  807 ), the subscriber interface controller  10 - 9  notifies the processor  10 - 5  of the receipt of the response signal from the subscriber terminal  1 , and the processor  10 - 5  transmits a response signal to the switch  20  via the PSTN interface controller  10 - 10  (Step  808 ). After the receipt of the response signal from the processor  10 - 5 , the switch  20  transmits a response signal to the subscriber terminal  7  (Step  809 ). This establishes a connection between the subscriber terminal  7  and the subscriber terminal  1  (Step  810 ). 
         [0043]    With reference next to  FIG. 12 , the process flow when the subscriber terminal  7 , for which the switch  20  provides network access, transmits a connection request addressed to the subscriber terminal  2 , for which the first media gateway  10  provides network access, will be discussed. Assume this time that the subscriber information  70 - 1  of the subscriber terminal  2  is “B” of  FIG. 9 , that a call waiting service is not available for the subscriber terminal  2 , and that the subscriber terminal  7  transmits a connection request addressed to the subscriber terminal  2  while the subscriber terminal  2  is connected via the IP network  100  to the subscriber terminal  4 , for which the second media gateway  11  provides network access. While the subscriber terminals  2  and  4  are connected (Step  900 ), the subscriber terminal  7  transmits a call signal to the switch  20  (Step  901 ). The switch  20  transmits a dial tone to the subscriber terminal  7  (Step  902 ). After the subscriber terminal  7  dials the number of the subscriber terminal  2  (Step  903 ), the switch  20  transmits a connection request addressed to the subscriber terminal  2  to the first media gateway  10  (Step  904 ). Similar to the above case of  FIG. 11 , the processor  10 - 5  of the first media gateway  10  then refers to the line status table  70  (Step  401 ) to see the line status information  70 - 2  associated with the “B” subscriber information  70 - 1  of the subscriber terminal  2  (Step  402 ). In this case, the processor  10 - 5  of the first media gateway  10  recognizes the line status information  70 - 2  as being “Connected to IP Network” as shown in  FIG. 9  and then refers to the subscriber information  70 - 1  (Step  405 ) to see whether a call waiting service is available for the subscriber terminal  2  (Step  406 ). Since the service is not available as stated above, the processor  10 - 5  of the first media gateway  10  transmits a “busy” response to the switch  20  via the PSTN interface controller  10 - 10  of the first media gateway  10  (Step  407  and Step  905 ). After the receipt of the response, the switch  20  transmits a busy tone to the subscriber terminal  7  (Step  906 ). This allows the caller using the subscriber terminal  7  to recognize the line of the subscriber terminal  2  is busy. 
         [0044]    With reference next to  FIG. 13 , the process flow when the subscriber terminal  7 , for which the switch  20  provides network access, transmits a connection request addressed to the subscriber terminal  3 , for which the first media gateway  10  provides network access, will be discussed. Assume this time that the subscriber information  70 - 1  of the subscriber terminal  3  is “C” of  FIG. 9 , that a call waiting service is available for the subscriber terminal  3 , and that the subscriber terminal  7  transmits a connection request addressed to the subscriber terminal  3  while the subscriber terminal  3  is connected via the IP network  100  to the subscriber terminal  4 , for which the second media gateway  11  provides network access. While the subscriber terminals  3  and  4  are connected (Step  1000 ), the subscriber terminal  7  transmits a call signal to the switch  20  (Step  1001 ). The switch  20  transmits a dial tone to the subscriber terminal  7  (Step  1002 ). After the subscriber terminal  7  dials the number of the subscriber terminal  3  (Step  1003 ), the switch  20  transmits a connection request addressed to the subscriber terminal  3  to the first media gateway  10  (Step  1004 ). Similar to the above cases of  FIGS. 11 and 12 , the processor  10 - 5  of the first media gateway  10  then refers to the line status table  70  (Step  401 ) to see the line status information  70 - 2  associated with the “C” subscriber information  70 - 1  of the subscriber terminal  3  (Step  402 ). In this case, the processor  10 - 5  of the first media gateway  10  recognizes the line status information  70 - 2  as being “Connected to IP Network” as shown in  FIG. 9  and then refers to the subscriber information  70 - 1  (Step  405 ) to see whether a call waiting service is available for the subscriber terminal  3  (Step  406 ). Since the service is available as stated above, the processor  10 - 5  of the first media gateway  10  transmits a response to the switch  20  via the PSTN interface controller  10 - 10  of the first media gateway  10  (Step  1005 ) and also transmits an incoming call alert signal to the subscriber terminal  3  via the subscriber interface controller  10 - 9  of the first media gateway  10  (Step  1006 ). After the completion of the connection processing, the processor  10 - 5  of the first media gateway  10  transmits a ring tone to the subscriber terminal  7  via the PSTN interface controller  10 - 10  of the first media gateway  10  (Step  1007 ), and the subscriber interface controller  10 - 9  of the first media gateway  10  monitors the subscriber terminal  3  for a momentary on-hook signal (Step  408 ). After the subscriber terminal  3  transmits a momentary on-hook signal (Step  1008 ), the subscriber interface controller  10 - 9  of the first media gateway  10  receives the signal and notifies the processor  10 - 5  of the first media gateway  10  of the receipt of the signal. Thereafter, the processor  10 - 5  of the first media gateway  10  changes the line status information  70 - 2  of the subscriber terminal  3  from “Connected to IP Network” to “Connected to IP Network/PSTN” (Step  409 ) and instructs the switch  10 - 2  of the first media gateway  10  to connect the line of the subscriber terminal  3  to the PSTN interface  10 - 11  of the first media gateway  10  (Step  410 ). Next, the processor  10 - 5  of the first media gateway  10  transmits a response signal to the switch  20  via the PSTN interface controller  10 - 10  of the first media gateway  10  (Step  1009 ) and also transmits an “on-hold” signal to the SIP server  30  via the second IP interface  10 - 6  of the first media gateway  10  (Step  1011 ). The SIP server  30  then transfers the on-hold signal to the second media gateway  11  (Step  1012 ). After receiving the on-hold signal via the second IP interface  10 - 6  of the second media gateway  11 , the processor  10 - 5  of the second media gateway  11  transmits a holding tone to the subscriber terminal  4  via the subscriber interface controller  10 - 9  of the second media gateway  11  (Step  1013 ). After the receipt of the response signal from the first media gateway  10 , the switch  20  transmits a response signal to the subscriber terminal  7  (Step  1010 ). This establishes a connection between the subscriber terminal  3  and the subscriber terminal  7  (Step  1014 ) while putting the subscriber terminal  4  on hold. 
         [0045]    When a subscriber for which a media gateway provides network access transmits a connection request to a subscriber for which network access is provided by a switch connected to the PSTN, a service that involves the use of an IP network and a gateway that connects the IP network and the PSTN can be realized by the telecommunications carrier setting the network connection information of a subscriber information table to “IP Network/PSTN” and the network connection information of a service table to “IP Network.” On the other hand, when a subscriber for which a media gateway provides network access transmits a connection request to a subscriber connected only to an IP network, a service that involves the use of the PSTN and the above gateway can be realized by the telecommunications carrier setting the network connection information of the subscriber information table to “IP Network/PSTN” and the network connection information of the service table to “PSTN.” 
         [0046]    When a media gateway receives through an IP network an “INVITE” request addressed to a subscriber for which the media gateway provides network access while the subscriber is connected to a caller through the PSTN, a “busy” response can instead be transmitted to an SIP server if a call waiting service is not available for the subscriber. Thus, the media gateway can notify the terminal that sent the “INVITE” request that the line of the subscriber is busy.