Abstract:
A method and apparatus for performing handover on a voice call between a packet network system and a circuit network system is provided. Considering a communication environment where a UE cannot simultaneously use radio resources of both the next generation mobile communication system (LTE) and the legacy mobile communication system (e.g., GSM/WCDMA), the present invention proposes an SCSE for performing signal translation between the packet switched network and the circuit switched network and anchoring a voice call in the application level, and supports a continuous voice call using the SCSE.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention provides an apparatus and method for performing call handover between a packet network system and a circuit network system. 
     2. Description of the Related Art 
     The circuit network system refers to a system that uses a scheme of assigning a fixed call circuit between users intending to enjoy a voice call, such as Global System for Mobile communication/Wideband Code Division Multiple Access Circuit Switch (GSM/WCDMA CS). The users are assumed to use a Circuit Switched (CS) scheme over the GSM/WCDMA system for a voice service. 
     The Long Term Evolution (LTE) system, a packet network system applied in the present invention, is a system that uses a Packet Switched (PS) scheme, which is the next generation mobile communication system evolved based on Universal Mobile Telecommunication Service (UMTS). That is, the LTE system provides a voice call or Voice over IP (VoIP) service using Internet Protocol Multimedia Subsystem (IMS) for a voice service. IMS provides communication through an unfixed path using an Internet Protocol (IP) packet, and a VoIP message is transmitted through an IMS session. 
     An Application Server (AS) of IMS anchors both a VoIP call and a CS call occurring in the LTE system network and the CS system. That is, for the CS call, a Mobile Service switching Center (MSC) acquires a routing address to an IMS network through a Customized Applications for Mobile Network Enhanced Logic (CAMEL) process, and it is delivered to a Media Gateway Control Function (MGCF) of the IMS network using an Integrated Services Digital Network User Part (ISUP) Initial Address Message (IAM) message. The IAM message delivered to MGCF is translated into a Session Initiation Protocol (SIP) INVITE Request and delivered to an Application Server (AS) via a Serving (S)-Call Session Control Server (CSCF). Upon receipt of the SIP INVITE Request, the AS stores information on the corresponding CS call, and delivers a SIP INVITE Request to a receiving side based on the received SIP INVITE Request message to connect a call. An SIP INVITE Request that a User Equipment (UE) has transmitted for a VoIP call is delivered to the AS via a Proxy (P)-CSCF and the S-CSCF. The AS, as in the case of the CS call, anchors a corresponding VoIP call, and delivers an SIP INVITE Request to the receiving side to connect a call. Therefore, both the CS call and the LTE VoIP call are controlled in the IMS network. 
       FIG. 1  is a diagram illustrating the situation where an LTE VoIP call is connected. 
     Referring to  FIG. 1 , a control signal  109  for a VoIP call is anchored from a UE for LTE (UE-LTE)  101   b  to an AS  107  via a P-CSCF  105  and an S-CSCF  106 , and then connected again to a receiving side  108  via the S-CSCF  106 . In this state, voice data  110  is delivered to a System Architecture Evolution (SAE) Anchor  104  via an Enhanced Node B (ENB)  102  and a User Plane Entity (UPE)  103 , and then delivered to the receiving side  108  over the IP network. 
       FIG. 2  is a diagram illustrating the situation where a CS call is connected. 
     Referring to  FIG. 2 , a control signal  209  for a CS call is delivered from a UE for CS (UE-CS)  201   a  to an S-CSCF  206  via an MSC  203  and an MGCF  204  of an integrated Mobile Management Entity (MME), and the S-CSCF  206  sends this signal to an AS  207  to anchor it therein. Thereafter, the AS  207  sends a control signal to a receiving side  208 , completing the call. In this state, voice data  210  is delivered to an SAE Anchor  205  via a Media GateWay (MGW)  202  of the MSC  203  and an MGW  204  of the integrated MME, and then delivered to the receiving side  208  over the IP network. 
     The UE can support both the GSM/WCDMA network and the LTE network in this way, but it has a restriction in simultaneously accessing the two types of networks. In other words, the UE cannot access the GSM/WCDMA network while communicating with the LTE network, and similarly, the UE cannot access the LTE network while communicating with the GSM/WCDMA network. 
     Meanwhile, there is 3GPP TS23.206 as a conventional technology for supporting handover between a VoIP call and a CS call. In TS23.206, a call undergoes call anchoring through a Voice Call Continuity (VCC) application which is an AS. However, it is assumed in TS23.206 that the UE can simultaneously access a CS network and a PS network. Therefore, the UE uses the following method in which when there is a request for handover, the UE forms a radio link to another system while maintaining the call of the old system, to generate a new session, and when the generation of the new session is completed, the UE releases the old session and the old radio link. Therefore, such a method cannot be applied when the UE, as assumed herein, cannot simultaneously access different networks. 
     Therefore, when the UE needs handover to the GSM/WCDMA CS network while receiving the VoIP service over the LTE system, there is a demand for a Radio Access Network (RAN) technique and an IMS session control technique for supporting the handover, and an apparatus for implementing the techniques. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a handover apparatus and method for providing a seamless continual voice call when a UE cannot simultaneously access an LTE network and a GSM/WCDMA network. 
     According to one aspect of the present invention, there is provided a method for performing handover from a packet switched call based on a packet switched network to a circuit switched call based on a circuit switched network. The handover method includes receiving a handover request message from a User Equipment (UE) over the packet switched network; when the handover request message is a GPRS Tunneling Protocol (GTP) message, checking if the handover-requesting UE is an anchored UE; and when the handover-requesting UE is an anchored UE, translating the GTP message into a Mobile Application Part (MAP) message, and then delivering the MAP message to the circuit switched network. 
     According to another aspect of the present invention, there is provided a method for performing handover from a packet switched call based on a packet switched network to a circuit switched call based on a circuit switched network. The handover method includes receiving a handover request message from a User Equipment (UE) over the packet switched network; when the handover request message is a Mobile Application Part (MAP) message, setting up a bearer to the handover-requesting UE, and assigning a circuit switched call routing number for delivering the circuit switched call to the UE; and generating and transmitting a handover response message including the set bearer information and the routing number. 
     According to further another aspect of the present invention, there is provided an apparatus for performing handover from a packet switched call to a circuit switched call. The handover apparatus includes a packet switched network; a circuit switched network; and a network entity, connected between the packet switched network and the circuit switched network, for receiving a handover request message from a User Equipment (UE) over the packet switched network, and when the handover request message is a GPRS Tunneling Protocol (GTP) message, translating the GTP message into a Mobile Application Part (MAP) message, and then delivering the MAP message to the circuit switched network. 
     According to yet another aspect of the present invention, there is provided an apparatus for performing handover from a packet switched call based on a packet switched network to a circuit switched call based on a circuit switched network. The handover apparatus includes a Service Continuity Supporting Entity (SCSE), connected between the packet switched network and the circuit switched network, for receiving a handover request message from a User Equipment (UE) over the packet switched network, and when the handover request message is a GPRS Tunneling Protocol (GTP) message, translating the GTP message into a Mobile Application Part (MAP) message and then delivering the MAP message to the circuit switched network; and a Mobile Service switching Center (MSC) for receiving the MAP message from the SCSE, setting up a bearer to the UE, assigning a circuit switched call routing number for delivering the circuit switched call to the UE, generating a handover response message including the set bearer information and the routing number, and transmitting the handover response message to the SCSE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagram schematically illustrating a system configuration where an LTE VoIP call is connected, to which the present invention is applied; 
         FIG. 2  is a diagram schematically illustrating a system configuration where a CS call is connected, to which the present invention is applied; 
         FIG. 3  is a diagram illustrating a structure of an SCSE according to an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating a process in which a UE supports an LTE VoIP call according to an embodiment of the present invention; 
         FIG. 5A  and  FIG. 5B  are diagrams illustrating a process in which an SCSE switches an LTE VoIP call to a GSM/WCDMA CS call according to an embodiment of the present invention; 
         FIG. 6  is a signaling flowchart illustrating an operation of an SCSE according to an embodiment of the present invention; and 
         FIG. 7  is a signaling flowchart illustrating an operation of an MSC according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness. Terms used herein are defined based on functions in the present invention and may vary according to users, operators&#39; intention or usual practices. Therefore, the definition of the terms should be made based on contents throughout the specification. 
     The present invention provides a network entity for interworking between a packet switched network and a circuit switched network. That is, the present invention proposes to insert a Service Continuity Supporting Entity (SCSE) for handover signal translation between an MME of the LTE system and an MSC of the legacy system. 
       FIG. 3  illustrates a structure of an SCSE according to an embodiment of the present invention. 
     Referring to  FIG. 3 , the key function of the SCSE  310  is to translate a signal for information exchange between two systems, when there is a need for call handover between an LTE system (hereinafter referred to as a network using a packet service domain) and a GSM/WCDMA system (hereinafter referred to as a network using a circuit service domain). For this, the SCSE  310  includes a signaling translation function  311 . The signaling translation function  311  performs translation between a GPRS Tunnel Protocol Control (GTP-C) message used for handover signaling in the LTE system and a Mobile Application Part (MAP) message used for handover signaling in the GSM/WCDMA system. 
     In addition, the SCSE  310  serves as an anchor point for the voice call, and for this, it includes a session anchoring function  312 . According to the present invention, the LTE VoIP call and GSM/WCDMA CS call are both set to undergo the session anchoring function  312  of the SCSE  310  in their generation processes. The session anchoring function  312  operates as an SIP Back-to-Back User Agent (B2BUA) to manage information on the voice call. 
     A detailed description will now be made of a call handover method according to an embodiment of the present invention. 
       FIG. 4  illustrates a process in which a UE generates a VoIP call over an LTE network according to an embodiment of the present invention. According to the present invention, an LTE VoIP call is anchored in an SCSE  450 , and the SCSE  450  serves as an SIP B2BUA for the LTE VoIP call. 
     The term ‘anchoring’ as used herein refers to an operation of managing the basic call information for a generated voice call and information on the UE that has requested the call. 
     Referring to  FIG. 4 , in step  401 , a UE  452  sends INVITE to P-CSCF of an IMS  458  to generate a VoIP call. In step  402 , the P-CSCF delivers the INVITE to S-CSCF. In step  403 , the S-CSCF checks the INVITE according to initial filter criteria, and then delivers the INVITE to the SCSE  450 . 
     Upon receipt of the INVITE, the SCSE  450  anchors the corresponding session in step  404 , and transmits INVITE to the S-CSCF of the IMS  458  to complete the call to a receiving UE  462  in step  405 . The S-CSCF delivers the INVITE to the receiving UE  462  in step  406 . 
     In step  407 , the receiving UE  462  transmits a 200 OK response message to the S-CSCF in response to the INVITE. In step  408 , the S-CSCF delivers the 200 OK to the SCSE  450 . In step  409 , the SCSE  450  transmits the 200 OK to the S-CSCF in order to deliver the received 200 OK to the transmitting UE  452 . The S-CSCF delivers the 200 OK to the P-CSCF in step  410 , and the P-CSCF finally delivers the 200 OK to the transmitting UE  452  in step  411 , completing the generation of the LTE VoIP call. 
       FIG. 5A  and  FIG. 5B  illustrate a process of performing handover from an LTE VoIP call to a GSM/WCDMA CS call using an SCSE according to an embodiment of the present invention. 
     Referring to  FIG. 5A , in step  500 , a UE  552  is now performing a voice call with a receiving UE using an LTE VoIP call. 
     In step  501 , the UE  552  transmits a measurement report for its current radio state to an ENB of an LTE network  556  periodically according to a measurement scheme predetermined in the system, or taking occurrence of a particular event into account. The UE, when it is using a voice call, performs measurement on both on the LTE cell and the GSM/WCDMA cell. 
     The measurement report that the UE transmits in step  501 , includes therein information used for detecting information on a target network. This information includes a Cell Global Identity (CGI) of the cells for which the UE has made a measurement. The CGI is composed of Location Area Identity (LAI) and Mobile Country Code (MCC)+Mobile Network Code (MNC)+Location Area Code (LAC))+Cell Identity (CI)+Routing Area Code (RAC). Herein, when the cell where the UE is now located is a cell servicing only CS, CGI includes no RAC. 
     In step  502 , upon receipt of the measurement report from the UE, the ENB determines if there is a need for handover of the UE, and if it is determined that there is a need for handover, the ENB sends a handover request message to an MME. The handover request message includes therein Message Type, Cause Value, Measured Cell Identifier, and Mobility-related Context and Security-related Context information stored for the UE. 
     In step  503 , upon receipt of the handover request message, the MME translates the received handover request message into a GTP message, and forwards it to an SCSE. 
     In the present invention, the MME analyzes information (e.g., CGI) by which a target cell can be checked, another information by which a target system can be detected, or Indication indicating handover for Signaling Radio Session Continuity, all of which are included in the handover request message, to determine whether it will forward the handover request message to the SCSE. For example, when CGI of a target cell includes no RAC, the MME transmits a handover prepare request message to the SCSE, considering that the requested handover is handover to the cell situated in the GSM/WCDMA network. That is, the MME, after analyzing information included in the handover request message, may determine the request message as a message that should undergo handover to the GSM/WCDMA network, and in the corresponding case, forwards the handover request message to the SCSE. 
     If the CGI information is no longer used in the LTE network, or if the handover request message includes therein another type of information by which information on the service domain in the target system can be acquired, the present invention includes even the operation of determining a target domain based on the newly defined information. 
     In the case where the MME has determined to deliver the handover request message to the SCSE, the MME generates a handover prepare request message using a GTP-C protocol, and then delivers it to the SCSE. The handover prepare request message includes therein Identity of a target cell for handover, International Mobile Subscriber Identity (IMSI) of UE, security information (security context), etc. 
     The SCSE checks if the handover prepare request message is a message scheduled to be delivered to the GSM/WCDMA network, or checks if the corresponding UE is a UE on which it is now performing call anchoring to provide a VCC service. The SCSE can determine if the corresponding message is a message scheduled to be delivered to the GSM/WCDMA network, based on the Cell Identity information in the request message. Meanwhile, when the MME determines if the request message is a message delivered to the GSM/WCDMA network or not, the corresponding of the SCSE can be omitted. 
     Whether the corresponding UE is performing call anchoring for VCC service can be determined by comparing IMSI information and UE&#39;s ID information included in the handover prepare request message with ID information of the UEs that a function unit in the SCSE now in charge of a VCC application function is managing. When the corresponding UE is a UE for the call now anchored by the SCSE, the SCSE translates, in step  504 , the protocol-based request message into a MAP-based handover prepare request message, and delivers the translated message to the target MSC using a target cell ID. 
     In this case, the SCSE checks again if the corresponding UE performing handover is a UE on which call anchoring for VCC is now being performed by the VCC application function of the SCSE. Upon receipt of a handover request for the call that is not undergoing anchoring, the SCSE sends a GTP error response message to the MME. 
     Upon receipt of the handover prepare request message, the target MSC sets up a bearer to a target Base Station System (BSS) in step  505 . The MSC is assigned a handover number for handover in association with a Visitor Location Register (VLR). The handover number, a number in the form of Mobile Subscriber ISDN Number (MSISDN), is a call routing number needed to make a call from the network to the UE. 
     After completing the bearer setup, the MSC transmits a handover prepare complete message to the SCSE in step  506 . In this case, the handover prepare complete message includes the handover number assigned in step  505 . 
     In step  507 , the SCSE transmits an INVITE Request message to the S-CSCF in order to set up a new access leg to the UE using the handover number delivered from the MSC. At this point, REQUEST-Uniform Resource Identifier (URI)/To-header of the INVITE is set as the assigned handover number, and From-header is set as the SCSE. In step  508 , the S-CSCF delivers the INVITE to the MGCF. 
     In step  509 , the MGCF transmits an ISDN Service User Part (ISUP) IAM message including Called Party Number, Calling Party Number and User Service Information (USI) parameter which is information on the type of the service, to the MSC to which the handover number is assigned. 
     In step  510 , the MSC sends an Answer Message (ANM) to the MGCF in response to the ISUP IAM message. In step  511 , the MGCF delivers a 200 OK response to the S-CSCF. In step  512 , the S-CSCF delivers the 200 OK response to the SCSE. 
     In step  530 , the UE moves to the GSM/WCDMA network through steps  504  to  512 , and then completes the necessary resource setup for the circuit network. That is, when a new access leg is set up between BSS and SCSE, a CS bearer passing from the BSS through MSC/MGW and IM-MGW is set up. 
     Referring to  FIG. 5B , after completing the access leg setup as stated in step  530 , the SCSE  550  transmits in step  513  a Re-INVITE to the S-CSCF to update the bearer information of the UE, which is performing handover, with an address of the MGW. In step  514 , the S-CSCF delivers the Re-INVITE to the receiving UE  562 . In step  515 , the receiving UE transmits a 200 OK response to the S-CSCF. In step  516 , the S-CSCF delivers a 200 OK to the SCSE  550 . 
     As a result, the session to the receiving UE is updated from the old session to the LTE domain to the newly set session to the CS domain. 
     In step  517 , the SCSE, after completing the session switching, translates the prepare complete message in the form of MAP handover, received from the MSC, into a GTP protocol handover prepare complete message available in the LTE network, and delivers it to the MME. In step  518 , the MME delivers a handover command to the ENB to notify the UE of the execution of handover. In step  519 , the ENB delivers the handover command to the UE. The handover command includes therein information on the target network to which the UE should move. 
     In step  520 , upon receipt of the handover command, the UE switches the old radio link connected to the LTE network to the GSM/WCDMA network, accesses the GSM/WCDMA network, and performs a process of setting up a radio connection. Therefore, the target BSS perceives the access by the UE. 
     In step  521 , upon detecting the handover of the UE, the BSS transmits a handover complete message to the MSC, thereby resuming the voice service to the UE using the previously set radio bearer and CS access leg information. In step  522 , the MSC delivers a MAP protocol handover complete message to the SCSE. In step  523 , the SCSE translates it into a GTP message, and delivers it to the MME. In step  524 , the MME transmits a resource release command to the ENB to release the old LTE resources. Then the ENB releases the radio resources and sends a resource release complete message in response in step  525 . 
     Now, a description will be made of an operation of the SCSE according to an embodiment of the present invention. First, the SCSE according to an embodiment of the present invention performs translation between a MAP message used for mobility management in the circuit network and a GTP message used for mobility management in the packet network. Second, the SCSE determines whether VCC call anchoring is performed on the handover-requesting UE, and sends an error message to the UE that has not yet undergone VCC call anchoring. Meanwhile, the SCSE performs interworking through a GTP-MAP translation work for the UE that has already undergone VCC call anchoring. Third, the SCSE delivers an INVITE message for generation of an access leg to the UE side using a circuit (CS) call routing number delivered from the MSC. Fourth, the SCSE delivers a handover prepare complete message to the UE at the time the update of the UE side&#39;s access leg and the bearer information of the receiving UE side has been completed. 
       FIG. 6  is a signaling flowchart illustrating an operation of an SCSE according to an embodiment of the present invention. 
     Referring to  FIG. 6 , in step  601 , the SCSE is in an idle state where it is waiting for receipt of a message. In step  602 , the SCSE determines if it has received a GTP handover request message delivered from the LTE network. 
     When the SCSE has received the GTP handover request message, the SCSE checks in step  603  whether the handover-requested target system is the GSM/WCDMA network. If it is determined that the received GTP handover request message is not the message for requesting handover to the GSM/WCDMA network, the SCSE intactly forwards the received GTP message to the network indicated by a cell ID in step  608 , and then returns to step  601 . However, this process can be previously performed by the MME, and when this determination is made by the MME according to implementation, steps  603  and  608  can be omitted. 
     If it is determined that the received GTP handover request message is a message for requesting handover to the GSM/WCDMA network, the SCSE determines in step  604  the presence/absence of VCC call anchoring in the VCC application of the corresponding UE, to check if the corresponding message is a handover prepare request message to the CS domain of the GSM/WCDMA network or a prepare request message to the CS domain of the GSM/WCDMA network. When VCC call anchoring on the handover-requesting UE has already been performed, the SCSE receives a GTP handover request message in step  605 , and translates it into a MAP handover request message. Thereafter, in step  606 , the SCSE delivers the translated MAP handover request message to the MSC of the GSM/WCDMA network, and then returns to step  601 . 
     However, when VCC call anchoring on the handover-requesting UE has not yet been performed even though the SCSE has received the GTP handover request message, the SCSE sends an error message such as a GTP handover reject or a handover failure in step  607 , and then returns to step  601 . 
     When the message received by the SCSE is not a GTP handover request message, the SCSE checks in step  612  if the received message is a MAP handover response message. Upon receipt of the MAP handover response message, the SCSE stores in step  613  the received response message and a CS routing number included in the response message. Thereafter, in step  614 , the SCSE generates an INVITE message used for requesting a network-initiated call to the UE side to previously generate a CS bearer for the UE, and then delivers it to the UE side. In this case, Request-URI in the INVITE message is set as the CS routing number acquired in step  613 , and calling party information of the INVITE message is set as information on a Remote UE. Thereafter, the SCSE returns to step  601 . 
     When the message received by the SCSE is neither the GTP handover request message nor the MAP handover response message, the SCSE checks in step  622  whether the received message is a response message to the INVITE that it delivered to the UE side in step  614 . If the received message is a response message to the INVITE message that it delivered in step  614 , the SCSE generates, in step  623 , an INVITE message used for updating a media bearer to the receiving UE side using the call access leg information of the UE side, included in the response message, i.e., using the address information of the MGW, and delivers it to the receiving UE. In this case, Request-URI of the INVITE is set as a receiving UE, and media contact information of SDP is set as an IP address of the MGW. Thereafter, the SCSE returns to step  601 . 
     When the message received by the SCSE is neither the GTP handover request message, the MAP handover response message, nor the response message to the INVITE generated/delivered in step  614 , the SCSE checks, in step  632 , if the received message is a response message to the INVITE that it delivered to the receiving UE in step  621 . Upon receipt of the response message to the INVITE message in step  621 , the SCSE translates in step  633  the MAP handover response message stored in step  613  into a GTP handover response message, and delivers it to the MME. In this case, when step  633  has already been internally performed by the SCSE at a particular time after step  613 , it can be omitted. In step  634 , the SCSE delivers the GTP handover response message to the LTE MME, and then receives a MAP handover complete message from the MSC of the GSM/WCDMA network within a predetermined time. In step  635 , the SCSE translates the received MAP handover complete message into a GTP handover complete message. In step  636 , the SCSE delivers the GTP handover complete message to the MME. Thereafter, the SCSE returns to step  601 . Even when it is determined in step  632  that the received message is not even a response message to the INVITE that the SCSE delivered to the receiving UE in step  621 , the SCSE returns to step  601 . 
       FIG. 7  illustrates an operation of an MSC according to an embodiment of the present invention. 
     Referring to  FIG. 7 , in step  701 , the MSC is in the idle state where it waits for receipt of a message. In step  702 , the MSC determines if it has received a MAP handover request message. 
     When the MSC has received the MAP handover request message, the MSC sets up a bearer to the handover requested UE through communication with a BSS in step  703 . In step  704 , the MSC assigns a CS call routing number necessary for delivering a CS call to the UE through communication with the VLR. In step  705 , the MSC generates and sends a handover response message indicating the completion of the bearer setup to the handover-requesting UE and the preparation for handover, and then returns to step  701 . In this case, the handover response message includes therein the CS call routing number acquired in step  704 , and information on the set bearer. 
     However, when the received message is not the MAP handover request message, the MSC checks in step  712  whether it has received an ISUP IAM message with a CS call routing number included therein. Upon receipt of the ISUP IAM message with the CS call routing number included, the MSC performs mapping between the bearer information set in step  713  and the call leg requested by ISUP IAM. In this case, the MSC can determine whether it will operate as the legacy MSC or will operate as the MSC proposed by the present invention, by checking if the CS call routing number that it sent in the MAP handover response message in steps  704  and  705 , is included in the IAM message. In step  714 , the MSC generates and sends an ISUP ANM message, and then returns to step  701 . In this case, as the legacy MSC receives a CC CONNECT message from the UE, the MSC internally generates an ISUP ANM message without any message from the UE, unlike in the operation of generating the ISUP ANM message. 
     When the message received by the MSC is neither the MAP handover request message nor the ISUP IAM message with a CS call routing number included therein, the MSC checks in step  722  whether the received message is a MAP handover complete message. Upon receipt of the MAP handover complete message, the MSC delivers the corresponding MAP handover complete message to the SCSE in step  723 , and then returns to step  701 . 
     When the message received by the MSC is neither the MAP handover request message, the ISUP IAM message with a CS call routing number included, nor the MAP handover complete message, the MSC determines in step  725  the types of messages according to the operation of the legacy MSC, performs a process according thereto, and then ends the operation. 
     EFFECT OF THE INVENTION 
     As is apparent from the foregoing description, the present invention inserts the SCSE between the MME and the MSC for handover signal translation, making it possible to translate signals for information exchange between two domains when handover has occurred between the LTE domain and the GSM/WCDMA CS domain. In addition, the present invention can anchor the voice call in the application level.