Abstract:
A media gateway device includes a first specific call control unit performing line connection control of one of plural interconnection units and plural second call control units requesting the first specific call control unit to perform the line connection control of one of the plural interconnection units. The first specific call control unit has a management data storage unit storing management data for determining which one of the plural interconnection units controls which line of the public network. Each of the plural second call control units has a control request unit requesting the first specific call control unit to perform line connection control of one of the plural interconnection units.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a U.S. continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT application JP2007/067022, filed on Aug. 31, 2007, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiment discussed herein is related to media gateway devices and, in particular, to a media gateway device that interconnects the Internet and a public network. 
       BACKGROUND 
       [0003]    Up until now, there have been discussed ATM priority control IP gateway devices that realize QoS (quality of service) communication by using a connecting type communication technology on connectionless communication called IP (Internet protocol). 
         [0004]    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-253018 
       SUMMARY 
       [0005]    According to an aspect of the present invention, there is provided a media gateway device that interconnects the Internet and a public network. The media gateway device includes plural call control units that perform call control between the Internet and the public network; and plural interconnection units that perform line connection and media conversion of user data between the Internet and the public network. The plural call control units include a first specific call control unit that performs line connection control of one of the plural interconnection units and plural second call control units, one of the plural second call control units requesting the first specific call control unit to perform line connection control of the one of the plural interconnection units, the first specific call control unit has a management data storage unit that stores management data for determining which one of the plural interconnection units controls which line of the public network, and each of the plural second call control units has a control request unit that requests the first specific call control unit to perform line connection control of the one of the plural interconnection units. 
         [0006]    The object and advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0007]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the present invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  illustrates the sequence of selecting a line in a media gateway device; 
           [0009]      FIG. 2  illustrates the configuration diagram of the media gateway device according to an embodiment of the present invention; 
           [0010]      FIG. 3  illustrates a relationship between VPICs and VXSMs; 
           [0011]      FIG. 4  illustrates the operations explanatory diagram of the media gateway device according to a first embodiment of the present invention; 
           [0012]      FIG. 5A  illustrates distribution data; 
           [0013]      FIG. 5B  illustrates MGCF management data; 
           [0014]      FIG. 6  illustrates the operations sequence of the media gateway device according to the first embodiment of the present invention; 
           [0015]      FIG. 7  illustrates the operations explanatory diagram of the media gateway device according to a second embodiment of the present invention; 
           [0016]      FIG. 8A  illustrates distribution data; 
           [0017]      FIG. 8B  illustrates MGCF management data; 
           [0018]      FIG. 9  illustrates the operations sequence of the media gateway device according to the second embodiment of the present invention; 
           [0019]      FIG. 10  illustrates the operations explanatory diagram of the media gateway device according to a third embodiment of the present invention; 
           [0020]      FIG. 11A  illustrates change destination storage data; 
           [0021]      FIG. 11B  illustrates line allocation information; 
           [0022]      FIG. 12  illustrates the operations sequence of the media gateway device according to a third embodiment of the present invention; 
           [0023]      FIG. 13A  illustrates a transmitting-side and receiving-side separating model; 
           [0024]      FIG. 13B  illustrates the transmitting-side and receiving-side separating model; 
           [0025]      FIG. 14  illustrates the transmitting-side and receiving-side separating model; 
           [0026]      FIG. 15  illustrates the operations explanatory diagram of the media gateway device according to a fourth embodiment of the present invention; and 
           [0027]      FIG. 16  illustrates the operations sequence of the media gateway device according to the fourth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    When connection is established in a direction in which a call is transmitted from an IP network to a CS network in a media gateway device arranged between the CS network (public network), such as an ATM network and a STM network, and the IP (Internet) network, information for selecting a line used for a user plane (U-Plane) is exchanged between the media gateway device and CS network switching equipment. 
         [0029]    If the CS network switching equipment is given a right to select the line at this time, the media gateway device uses the line specified by the CS network switching equipment to establish user-plane line connection. 
         [0030]      FIG. 1  illustrates the sequence of selecting the line. In  FIG. 1 , the media gateway device (MGW) transmits a connection request message (IAM) to the CS network switching equipment. 
         [0031]    Upon receipt of the message, the CS network switching equipment transmits a response message (IAA) to the media gateway device. When the line is specified by the IAA, the media gateway device selects the line specifying the user plane and transfers traffic. 
         [0032]    Note that Patent Document 1 describes gateway devices that are provided at boundaries between plural LANs using an IP as a communication protocol and an ATM communication system and exchange an IP packet with an ATM cell. 
         [0033]    The media gateway device for inter-network connection, which is arranged to establish interconnection between an IP network and a large-scale CS network that handles the large traffic of an existing public network, a mobile communication network, or the like, is demanded to have high processing performance. In order to increase the processing ability of the media gateway device, it is required that the media gateway device be configured to have plural MGCFs (Media Gateway Control Functions) to distribute load, thereby ensuring the processing performance as the whole media gateway device. 
         [0034]    As a method for arranging the MGCFs, they are arranged separately for each destination path. If a large amount of traffic is transferred in the same path, the MGCFs are allocated to distribute load or arranged in combination, thereby increasing the processing ability as the whole media gateway device. In some cases, the MGCFs are arranged to distribute risk in consideration of fault tolerance. 
         [0035]    The line used in the user plane (U-Plane) is negotiated to be selected using an ISUP (ISDN User Part) signal when connection is established between the STM network and the IP network. Furthermore, it is negotiated to be selected using a common line signal system such as B-ISUP (Broadband ISDN User part) when connection is established between the ATM network and the IP network. 
         [0036]    In the media gateway device having the plural MGCFs described above, however, if the CS network switching equipment is given the right to select the line, a line under the control of the MGCF having made a connection request may not be selected. 
         [0037]    Furthermore, when the media gateway device encounters a line disturbance with the CS network on the other end, line congestion, engagement of its resource, or the like even if it is given the right to select the line, the media gateway device is required to perform detour control to select another line again. However, depending on the use status of the line, no detour line may be available under the control of the MGCF having made the connection request. In this case, the media gateway device is required to select a line other than those capable of being controlled by the MGCF having made the connection request from the CS network again to continue connection control. 
         [0038]    Thus, if the media gateway device is requested to select a line other than those capable of being controlled by the MGCS having made the connection request from the CS network or if it encounters a line disturbance with the CS network on the other end, line congestion, the engagement of its resource, or the like as described above, it is required to treat a call as a lost call. 
         [0039]    Next, a description is made of embodiments of the present invention with reference to the accompanying drawings. 
         [0040]    (Configuration of Media Gateway Device) 
         [0041]      FIG. 2  illustrates the configuration diagram of a media gateway device according to an embodiment of the present invention. In  FIG. 2 , the media gateway device  10  establishes connection between an IP network  11  and a CS network  12 . The CS network  12  is provided with CS network switching equipment  13 . 
         [0042]    The media gateway device  10  is composed of a control plane (a C-Plane or a signaling gateway)  20  and a user plane (a U-Plane or a media gateway)  30 . 
         [0043]    In the control plane  20 , a distribution unit  21  distributes a signal from the IP network  11  to plural MGCFs (Media Gateway Control Functions)  22 - 0  through  22 - 2 . At this time, the distribution unit  21  determines the MGCFs by using, for example, an originating station specified by the OPC (Originating Point Code) of the signal from the IP network  11  as a parameter and distributes the signal to the MGCFs. In some cases, the distribution unit  21  determines the MGCFs in order of incoming signals based on a round robin system and distributes the signals to the MGCFs. 
         [0044]    Each of the MGCFs  22 - 0  through  22 - 2  converts a signal between a SIP (Session Initiation Protocol) or a MEGACO (MEdia GAteway COntrol Protocol) serving as a call control protocol used in the IP network  11  and an ISUP or a B-ISUP serving as a call control protocol used in the CS network  12 . Note that the number of the MGCFs is not limited to “three” so long as it is plural. Furthermore, each of the MGCFs  22 - 0  through  22 - 2  selects a line used for establishing the connection between the IP network  11  and the CS network  12 , conducts a negotiation with the CS network switching equipment  13  to determine the control condition of the user plane  30  such as a media conversion pattern, and makes a control request to plural MGXs (Media Gateways) of the user plane  30  in accordance with the determined control condition. 
         [0045]    A distribution unit  23  distributes a signal from the CS network  12  to the plural MGCFs  22 - 0  through  22 - 2 . A SGCS (Signal Gateway Control Server)  24  relays a control plane signal. 
         [0046]    The user plane  30  is provided with the plural MGXs  31 - 1  and  31 - 2 . The MGXs  31 - 1  and  31 - 2  use a line specifying the traffic of the user plane  30  in accordance with the control condition requested by the MGCFs  22 - 0  through  22 - 2  and perform specified media conversion to transfer the traffic. 
         [0047]    The MGXs  31 - 1  and  31 - 2  are provided with plural VXSMs (Voice switch Service Modules)  32 - 1  through  32 - n  and a switch  33 . Each of the VXSMs performs conversion between an IP (Internet Protocol) used in the IP network  11  and a S™ (Synchronous Transfer Mode) and an ATM (Asynchronous Transfer Mode) used in the CS network  12 . The switch  33  is used for switching a line between each of the VXSMs and the CS network switching equipment  13 . 
         [0048]    Note that in the description of the following embodiment, an OC3 (Optical Carrier-3) of an ATM network accommodates lines as represented by VPs (Virtual Paths) and VCs (Virtual Channels) as illustrated in  FIG. 3 . VPCIs (Virtual Path Connection Identifiers) and the VXSMs are linked to each other so that the VXSMs controlling the VPCIs are uniquely determined when the VPICs on the lines of an OC3 interface connected to the ATM network are determined. 
       First Embodiment 
       [0049]    In a first embodiment, the master MGCF (e.g.,  22 - 0 ) is determined in advance, which makes a control request to all the VXSMs  32 - 1  through  32 - n  of all the MGXs  31 - 1  and  31 - 2  of the user plane  30 . The slaves MGCFs  22 - 1  and  22 - 2  other than the master MGCF control the control plane  20  and make a line control request to the master MGCF  22 - 0 . 
         [0050]      FIG. 4  illustrates the operations explanatory diagram of the media gateway device according to the first embodiment of the present invention. 
         [0051]    Step S 1 : A session start message (INVITE) transmitted from the IP network  11  to the CS network  12  is distributed to the MGCF  22 - 1  determined based on, for example, the round robin system by the distribution unit  21 , converted into a connection request message (IAM) by the MGCF  22 - 1 , and transmitted to the SGCS  24 . The above is an example in which the slave MGCF  22 - 1  is initially selected by the distribution unit  21 . 
         [0052]    Step S 2 : A response message (IAA) responding to the IAM and returned from the CS network  12  is routed to the slave MGCF  22 - 1  serving as a requestor via the SGCS  24  and the distribution unit  23 . Note that the distribution unit  23  extracts a MGCF number from the transmission source ID (SAID) included in the response message (IAA) and specifies the IP address of the slave MGCF  22 - 1  serving as the requestor by referring to distribution data as illustrated in  FIG. 5A  based on the extracted MGCF number. The distribution data are stored in a distribution data storage unit  23   a.    
         [0053]    Step S 3 : The slave MGCF  22 - 1  having received the response message (IAA) reads line information (OPC, VPCI, and VCI (Virtual Channel Identifier)) extracted from the response message (IAA) and transmits a line acquisition command (Add) to the master MGCF  22 - 0  to make the line control request. The line control request is executed by the control request function  22   a  of the slave MGCF  22 - 1  ( 22 - 2 ). 
         [0054]    Step S 4 : The master MGCF  22 - 0  transmits the line acquisition command (Add) to the VXSM (e.g.,  32 - 1 ) of the corresponding MGX (e.g.,  31 - 2 ) in accordance with the instruction requested in the previous step to perform connection control between the line specified by the CS network  12  and the IP network  11 . Note that the master MGCF  22 - 0  has MGCF management data as illustrated in  FIG. 5B  in a management data storage unit  22   b  and obtains a VPCI table by referring to the MGCF management data based on the VPCI contained in the line control request from the slave MGCF  22 - 1 . The VXSM number of the obtained VPCI table indicates the VXSM corresponding to the line specified by the CS network  12 . 
         [0055]      FIG. 6  illustrates the operations sequence of the media gateway device according to the first embodiment of the present invention. Step numbers in  FIG. 6  correspond to those in  FIG. 4 . In  FIG. 6 , after the connection control in step S 4 , an address completion message (ACM) is transmitted from the CS network  12  to the slave MGCF  22 - 1  in step S 5 , and then a ring back tone (RBT) is transmitted to the slave MGCF  22 - 1  after the issuance of a call elapse message (CPG) in step S 6 . 
         [0056]    When a session completion request message (BYE) is transmitted from the IP network  11  to the slave MGCF  22 - 1  in step S 7 , the slave MGCF  22 - 1  transmits a path cut command (Subtract) to the VXSM  32 - 1  of the MGX  31 - 2  via the master MGCF  22 - 0  to cut a path, transmits a release message (REL) to the CS network  12 , and reports the success of the request ( 200  OK) to the IP network  11  in step S 8 . 
         [0057]    Therefore, even if the CS network requests the selection of the line other than those capable of being controlled by the MGCF having made the connection request, the master MGCF  22 - 0  performs the connection control of the line requested to be selected by the CS network in accordance with the request from the slave MGCF  22 - 1  having made the connection request. As a result, the occurrence of a lost call can be prevented. 
       Second Embodiment 
       [0058]    In a second embodiment, all the MGCFs  22 - 0  through  22 - 2  have the same MGCF management data. The MGCF management data contain data for determining which one of the MGCFs is configured to control which line on the side of the CS network. Each of the MGCFs specifies the MGCF that controls the line replied with the response message (IAA) based on the MGCF management data and makes the line control request to the MGCF involved. 
         [0059]      FIG. 7  illustrates the operations explanatory diagram of the media gateway device according to the second embodiment of the present invention. 
         [0060]    Step S 11 : The session start message (INVITE) transmitted from the IP network  11  to the CS network  12  is distributed to the MGCF  22 - 0  determined based on, for example, the originating station (OPC) contained in the message by the distribution unit  21 , converted into the connection request message (IAM) by the MGCF  22 - 0 , and transmitted to the SGCS  24 . The above is an example in which the MGCF  22 - 0  is initially selected by the distribution unit  21 . 
         [0061]    Step S 12 : The response message (IAA) returned from the CS network  12  is routed to the MGCF  22 - 0  serving as a requestor via the SGCS  24  and the distribution unit  23 . Note that the distribution unit  23  extracts the MGCF number from the transmission source ID (SAID) included in the response message (IAA) and specifies the IP address of the MGCF  22 - 0  serving as the requestor by referring to distribution data as illustrated in  FIG. 8A  based on the extracted MGCF number. The distribution data are stored in the distribution data storage unit  23   a.    
         [0062]    Step S 13 : The MGCF  22 - 0  having received the response message (IAA) refers to the MGCF management data that the MGCF  22 - 0  itself possesses so as to specify which one of the MGCFs controls the line selected. 
         [0063]    Note that all the MGCFs  22 - 0  through  22 - 2  have the MGCF management data as illustrated in  FIG. 8B  in a management data storage unit  22   b . The MGCF  22 - 0  obtains a TGN (number assigned to a line (trunk) group on the same destination/route) from the originating station (OPC) contained in the response message (IAA), thus obtaining a VXSM number and VPCI corresponding to the TGN. The MGCF number (and the IP address of the MGCF) obtained from the VPCI specify the MGCF that actually makes the control request. Furthermore, the VXSM number indicates the VXSM corresponding to the line specified by the CS network  12 . 
         [0064]    Step S 14 : The MGCF  22 - 0  transmits the line acquisition command (Add) to the MGCF (e.g.,  22 - 1 ) specified in the previous step S 13  to make the line control request. The line control request is executed by the control request function  22   a  of each of the MGCFs  22 - 0  through  22 - 2 . 
         [0065]    Step S 15 : The MGCF  22 - 1  having received the line control request transmits the line acquisition command (Add) to the VXSM (e.g.,  32 - 1 ) of the corresponding MGX (e.g.,  31 - 2 ) in accordance with the instruction requested in the previous step to perform the connection control between the line specified by the CS network  12  and the IP network  11 . 
         [0066]      FIG. 9  illustrates the operations sequence of the media gateway device according to the second embodiment of the present invention. Step numbers in  FIG. 9  correspond to those in  FIG. 7 . In  FIG. 9 , after the connection control in step S 15 , the address completion message (ACM) is transmitted from the CS network  12  to the MGCF  22 - 0  in step S 16 , and then the ring back tone (RBT) is transmitted to the MGCF  22 - 0  after the issuance of the call elapse message (CPG) in step S 17 . 
         [0067]    When the session completion request message (BYE) is transmitted from the IP network  11  to the MGCF  22 - 0  in step S 18 , the MGCF  22 - 0  transmits the path cut command (Subtract) to the VXSM  32 - 1  of the MGX  31 - 2  via the MGCF  22 - 1  to cut a path, transmits the release message (REL) to the CS network  12 , and reports the success of the request ( 200  OK) to the IP network  11  in step S 19 . 
         [0068]    Therefore, even if the CS network requests the selection of the line other than those capable of being controlled by the MGCF having made the connection request, the MGCF makes the connection request to another MGCF capable of controlling the line requested to be selected. 
         [0069]    As a result, the occurrence of a lost call can be prevented. 
       Third Embodiment 
       [0070]    In a third embodiment, the distribution unit  23  returns the response message (IAA) to the MGCF that controls the line specified by the CS network  12 . 
         [0071]      FIG. 10  illustrates the operations explanatory diagram of the media gateway device according to the third embodiment of the present invention. 
         [0072]    Step S 21 : The session start message (INVITE) transmitted from the IP network  11  to the CS network  12  is distributed to the MGCF  22 - 0  determined based on, for example, the originating station (OPC) contained in the message by the distribution unit  21 , converted into the connection request message (IAM) by the MGCF  22 - 0 , and transmitted to the SGCS  24 . The above is an example in which the MGCF  22 - 0  is initially selected by the distribution unit  21 . 
         [0073]    Step S 22 : The response message (IAA) returned from the CS network  12  is received by the distribution unit  23  via the SGCS  24 . 
         [0074]    Step S 23 : The distribution unit  23  analyzes the response message (IAA) to read the line information (OPC, VPCI, and VCI) and specifies the MGCF number (e.g.,  22 - 1 ) that controls the line by referring to line allocation information as illustrated in  FIG. 11B  stored in a line allocation information storage unit  23   b  based on the OPC and the VCI. Note that the line allocation information illustrated in  FIG. 11B  corresponds to the MGCF management data as illustrated in  FIG. 8B . 
         [0075]    Step S 24 : The distribution unit  23  extracts the MGCF number from the transmission source ID (SAID) included in the response message (IAA), specifies the MGCF  22 - 0  that has initially transmitted the connection request message (IAN) by referring to distribution data as illustrated in  FIG. 8A  based on the extracted MGCF number, and acquires control information (IAM information) when the connection request message (IAM) is transmitted. The distribution data are stored in the distribution data storage unit  23   a.    
         [0076]    Step S 25 : The distribution unit  23  transfers the IAM information acquired in the previous step S 24  and the response message (IAA) received in the previous step S 22  to the MGCF  22 - 1  that controls the line specified and selected in the previous step S 23 . 
         [0077]    Step S 26 : At this time, since the MGCF  22 - 1  that controls the line specified and selected in step S 23  is different from the MGCF  22 - 0  specified in step S 24  that has initially transmitted the connection request message, the distribution unit  23  stores in itself the fact that the MGCF to which the IAN information and the response message (IAA) are to be distributed is changed from the MGCF  22 - 0  to the MGCF  22 - 1 . This operation is performed in such a manner that the MGCF number of the MGCF to which the IAM information and the response message (IAA) are to be distributed is stored in change destination storage data as illustrated in  FIG. 11A  stored in a change destination storage unit  23   c  so as to correspond to the transmission source ID (SAID). 
         [0078]    Step S 27 : The MGCF  22 - 1  transmits the line acquisition command (Add) to the VXSM (e.g.,  32 - 1 ) of the corresponding MGX (e.g.,  31 - 2 ) in accordance with the response message (IAA) and the IAM information received in step S 25  to perform the connection control between the line specified by the CS network  12  and the IP network  11 . 
         [0079]    Note that all the MGCFs  22 - 0  through  22 - 2  have the MGCF management data as illustrated in  FIG. 8B . The MGCF  22 - 0  obtains the TGN (number assigned to a line (trunk) group on the same destination/route) from the originating station (OPC) contained in the response message (IAA), thus obtaining the VXSM number and VPCI corresponding to the TGN. The MGCF number (and the IP address of the MGCF) obtained from the VPCI specify the MGCF that actually makes the control request. Furthermore, the VXSM number indicates the VXSM corresponding to the line specified by the CS network  12 . 
         [0080]    After step S 27 , the signal in the control plane  20  between the IP network  11  and the CS network  12  is transferred from the IP network  11  to the CS network  12  through the distribution unit  21 , the MGCF  22 - 1 , and the distribution unit  23  or transferred in the reverse direction. 
         [0081]    Step S 28 : At the end of the call, the MGCF number stored in the change destination storage data in step S 26  is deleted. 
         [0082]      FIG. 12  illustrates the operations sequence of the media gateway device according to the third embodiment of the present invention. Step numbers in  FIG. 12  correspond to those in  FIG. 10 . In  FIG. 12 , after the connection control in step S 27 , the address completion message (ACM) is transmitted from the CS network  12  to the MGCF  22 - 1  in step S 30 , and then the ring back tone (RBT) is transmitted to the MGCF  22 - 1  after the issuance of the call elapse message (CPG) in step S 31 . 
         [0083]    When the session completion request message (BYE) is transmitted from the IP network  11  to the MGCF  22 - 1  in step S 32 , the MGCF  22 - 1  transmits the path cut command (Subtract) to the VXSM  32 - 1  of the MGX  31 - 2  to cut a path, transmits the release message (REL) to the CS network  12 , and reports the success of the request ( 200  OK) to the IP network  11  in step S 33 . Furthermore, the MGCF  22 - 1  transmits the release message (REL) to the distribution unit  23  to delete the MGCF number stored in the change destination storage data in step S 28 . 
         [0084]    Therefore, even if the CS network requests the selection of the line other than those capable of being controlled by the MGCF having made the connection request, the distribution unit  23  makes the connection request to another MGCF capable of controlling the line requested to be selected. As a result, the occurrence of a lost call can be prevented. 
       Fourth Embodiment 
       [0085]    In a fourth embodiment, a transmitting-side and receiving-side separating model is applied to the control according to the second embodiment. In other words, all the MGCFs  22 - 0  through  22 - 2  have the same MGCF management data that contain data for determining which one of the MGCFs is configured to control which line on the side of the CS network. With the application of the transmitting-side and receiving-side separating model, each of the MGCFs specifies the MGCF that controls the line replied with the response message (IAA) based on the MGCF management data and makes the line control request to the MGCF involved. 
         [0086]    A description is now made of the transmitting-side and receiving-side separating model. As illustrated in  FIG. 13A , a general MGCF  40  is composed of the functions of a SIG serving as an object responsible for a signal control function on the side of the IP network, an ISUP on the side of the CS network, a scenario serving as an object responsible for a call connection control scenario function, line control, path control, and MG control. 
         [0087]    Conversely, as illustrated in  FIG. 13B , the MGCF of the transmitting-side and receiving-side separating model is separated into a MGCF  41  that is composed of the functions of the ISUP, the scenario, the line control, the path control, and the MG control on the side of the CS network and performs the line control on the VXSM and separated into a MGCF  42  that is composed of the functions of the SIG serving as an object responsible for the signal control function, the scenario, the line control, and the path control on the side of the IP network. When a call is transmitted from the CS network to the IP network, the MGCF  41  performs transmitting processing while the MGCF  42  performs receiving processing. On the other hand, when a call is transmitted from the IP network to the CS network, the MGCF  42  performs the transmitting processing while the MGCF  41  performs the receiving processing. 
         [0088]    Note that the MG control is the function arranged on the side of the MGCF that performs the receiving processing, and  FIG. 13B  illustrates an example of arranging the functions when a call is transmitted from the IP network to the CS network. Therefore, in  FIG. 13B , the MG control is arranged on the side of the MGCF  41  that performs receiving control. However, when a call is transmitted from the CS network to the IP network, the MG control is arranged on the side of the MGCF  42  that performs receiving-side control. 
         [0089]    Next, a description is specifically made of an example of connection in which a call is transmitted from the IP network to the CS network. 
         [0090]    As illustrated in  FIG. 14 , when the MGCF of the transmitting-side and receiving-side separating model is used, the session start message (INVITE) distributed to the MGCF  22 - 0  is converted into a connection request message (IAN) after passing through the SIG and the scenario of the transmitting-side MGCF of the MGCF  22 - 0  and the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 0 , and then transmitted to the CS network  12  via the distribution unit  23 . 
         [0091]    Subsequently, if the MGCF that controls the line specified by the response message (IAA) returned from the CS network  12  is identical with the MGCF  22 - 0  serving as a requestor that has transmitted the IAN, the response message (IAA) is converted after passing through the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 0  and the SIG and the scenario of the transmitting-side MGCF of the MGCF  22 - 0 , and then transmitted to the distribution unit  21  as illustrated on an upper column in  FIG. 14 . 
         [0092]    On the other hand, if the MGCF (i.e.,  22 - 1 ) that controls the line specified by the response message (IAA) returned from the CS network  12  is different from the MGCF  22 - 0  serving as the requestor that has transmitted the IAM, the response message (IAA) is converted after passing through the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 0  and the SIG and the scenario on the transmitting-side MGCF of the MGCF  22 - 0  via the SIG and the scenario on the receiving-side MGCF of the MGCF- 22 - 1 , and then transmitted to the distribution unit  21 . 
         [0093]      FIG. 15  illustrates the operations explanatory diagram of the media gateway device according to the fourth embodiment of the present invention. 
         [0094]    Step S 41 : The session start message (INVITE) transmitted from the IP network  11  to the CS network  12  is distributed to the MGCF  22 - 0  determined based on, for example, the originating station (OPC) contained in the message by the distribution unit  21 , converted into the connection request message (IAM) by the MGCF  22 - 0 , and transmitted to the SGCS  24 . The above is an example in which the MGCF  22 - 0  is initially selected by the distribution unit  21 . 
         [0095]    Step S 42 : The response message (IAA) returned from the CS network  12  is routed to the MGCF  22 - 0  serving as a requestor via the SGCS  24  and the distribution unit  23 . Note that the distribution unit  23  extracts the MGCF number from the transmission source ID (SAID) included in the response message (IAA) and specifies the IP address of the MGCF  22 - 0  serving as the requestor by referring to the distribution data as illustrated in  FIG. 8A  based on the extracted MGCF number. The distribution data are stored in the distribution data storage unit  23   a.    
         [0096]    Step S 43 : The MGCF  22 - 0  having received the response message (IAA) refers to the MGCF management data that the MGCF  22 - 0  itself possesses so as to specify which one of the MGCFs controls the line selected. 
         [0097]    Note that all the MGCFs  22 - 0  through  22 - 2  have the MGCF management data as illustrated in  FIG. 8B  in the management data storage unit  22   b . The MGCF  22 - 0  obtains the TGN (number assigned to a line (trunk) group on the same destination/route) from the originating station (OPC) contained in the response message (IAA), thus obtaining the VXSM number and VPCI corresponding to the TGN. The MGCF number (and the IP address of the MGCF) obtained from the VPCI specify the MGCF that actually makes the control request. Furthermore, the VXSM number indicates the VXSM corresponding to the line specified by the CS network  12 . 
         [0098]    Step S 44 : The MGCF  22 - 0  transfers the response message (IAA) to the MGCF (e.g.,  22 - 1 ) specified in the previous step S 43  to make a receiving processing request to the MGCF  22 - 1 . The receiving processing request is executed by the control request function  22   a  of each of the MGCFs  22 - 0  through  22 - 2 . 
         [0099]    Step S 45 : The MGCF  22 - 1  having received the receiving processing request transmits the line acquisition command (Add) to the VXSM (e.g.,  32 - 1 ) of the corresponding MGX (e.g.,  31 - 2 ) in accordance with the instruction requested in the previous step to perform the connection control between the line specified by the CS network  12  and the IP network  11 . 
         [0100]      FIG. 16  illustrates the operations sequence of the media gateway device according to the fourth embodiment of the present invention. Step numbers in  FIG. 16  corresponds to those in  FIG. 15 . In  FIG. 16 , after the connection control in step S 45 , when the address completion message (ACM) is transmitted from the CS network  12  to the MGCF  22 - 0  in step S 46 , the address completion message (ACM) is transmitted from the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 0  to the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 1  in step S 47 . In step S 48 , the address completion message (ACM) is next transmitted from the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 1  to the SIG and the scenario of the transmitting-side MGCF of the MGCF  22 - 0 . Then, in step S 49 , a session progress status notification message ( 183  Session Progress) is transmitted to the IP network  11 . 
         [0101]    Furthermore, in step S 51 , after the issuance of the call elapse message (CPG) from the CS network  12  to the MGCF  22 - 0 , the ring back tone (RBT) is transmitted to the MGCF  22 - 0 . In step S 52 , the call elapse message (CPG) is transmitted from the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 0  to the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 1 . In step S 53 , the call elapse message (CPG) is further transmitted from the SIG and the scenario of the receiving-side MGCF of the MGCF  22 - 1  to the SIG and the scenario of the transmitting-side MGCF of the MGCF  22 - 0 . In step S 54 , the session progress status notification message ( 183  Session Progress) is transmitted to the IP network  11 . 
         [0102]    When the session completion request message (BYE) is transmitted from the IP network  11  to the MGCF  22 - 0  in step S 55 , the MGCF  22 - 0  transmits the release message (REL) to the MGCF  22 - 1  in step S 56  while the MGCF  22 - 1  transmits the path cut command (Subtract) to the VXSM  32 - 1  of the MGX  31 - 2  to cut a path. Furthermore, the MGCF  22 - 1  transmits the release message (REL) to the MGCF  22 - 0  in step S 58 , and the MGCF  22 - 0  reports the success of the request ( 200  OK) to the IP network  11  in step S 59  and transmits the release message (REL) to the CS network  12  in step S 60 . 
         [0103]    According to this embodiment, even if the CS network requests the selection of the line other than those capable of being controlled by the MGCF having made the connection request, the MGCF makes the connection request to another MGCF capable of controlling the line requested to be selected. As a result, the occurrence of a lost call can be prevented. Moreover, the configuration of each of the MGCFs  22 - 0  through  22 - 2  can be simplified by separating the transmitting side and the receiving side from each other. 
         [0104]    Note that in this embodiment, the transmitting-side and receiving-side separating model is applied to the control according to the second embodiment. However, it may also be applied to the control according to the first (or the third) embodiment. In this case, the MGCF that initially receives the message is responsible for the transmitting processing, while the master MGCF (or the MGCF that receives the response message from the distribution unit  23 ) is responsible for the receiving processing. 
         [0105]    In the embodiments described above, each of the MGCFs  22 - 0  through  22 - 2  is used as an example of a call control unit; each of the VXSMs  32 - 1  through  32 - n  is used as an example of an interconnection unit; the management data storage unit  22   b  is used as an example of a management data storage unit; the control request function  22   a  is used as an example of a control request unit; the line allocation information storage unit  23   b  is used as an example of a line allocation information storage unit; the distribution unit  23  is used as an example of a distribution unit; the transmitting-side MGCF  42  is used as an example of a transmitting-side processing unit; the receiving-side MGCF  41  is used as an example of a receiving-side control unit; the change destination storage unit  23   c  is used as an example of a change destination storage unit; and the distribution data storage unit  23   a  is used as an example of a distribution data storage unit. 
         [0106]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the present invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the present invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the present invention.