WO2018161804A1

WO2018161804A1 - Method and system for switching from ims call to circuit-switched call

Info

Publication number: WO2018161804A1
Application number: PCT/CN2018/077037
Authority: WO
Inventors: 刘竞翔
Original assignee: 中兴通讯股份有限公司
Priority date: 2017-03-10
Filing date: 2018-02-23
Publication date: 2018-09-13
Also published as: CN108574670A

Abstract

The invention discloses a method and system for switching from an IMS call to a circuit-switched call. The switching method comprises: when a user terminal in an ongoing IMS call moves from a packet-switched domain network to a location where there is only a circuit-switched domain network available, a vMSCS receiving a switching preparation request transmitted by an eMSCS and comprising a STN-SR used to indicate that a destination address is an ATCF; and the vMSCS utilizing the STN-SR in the switching preparation request to establish a multimedia link to the ATCF, so that when the user terminal switches to and accesses the circuit-switched domain network via the vMSCS, the client terminal can continue, via the multimedia link, the IMS call with another user terminal in the IMS call.

Description

Switching method and system for switching IMS call to circuit domain

Technical field

The present invention relates to the field of call handover technology in a communication network, and in particular, to a handover method and system for switching an IP Multimedia Subsystem (IMS) call to a circuit domain.

Background technique

The call service is a basic service provided by the IMS network, and the user who performs the IMS call is generally accessed from a Long Term Evolution (LTE) network. When the user moves to a Circuit Switch (CS) network, a single Radio Voice Call Continuity (SRVCC) handover from the LTE to the CS network is required.

In an implementation manner of the SRVCC handover, the media stream of one user terminal passes through the three nodes of the media gateway-1, the media gateway-2, and the access translation gateway to reach the other user terminal. And multiple forwarding, obviously will extend the transmission path, consume transmission resources, and increase the risk of voice quality degradation.

Summary of the invention

According to an embodiment of the present invention, there is provided a handover method for switching an IMS call to a circuit domain, comprising: visiting a mobile switching center server when a user terminal that is performing an IMS call moves from a packet domain network to a location of only a circuit domain network (vMSCS) receiving a preparation handover request sent by an enhanced mobile switching center server (eMSCS), the preparation handover request including a single frequency session transfer number (STN-SR) for identifying an destination address as an access transition control function (ATCF); The vMSCS establishes a multimedia link to the ATCF by using the STN-SR, so that the one user terminal performs the other user terminal that is in the IMS call via the multimedia link when the vMSCS accesses and switches to the circuit domain network. IMS call.

According to an embodiment of the present invention, there is provided a handover method for switching an IMS call to a circuit domain, comprising: when a user terminal that is performing an IMS call moves from a packet domain network to a location of only a circuit domain network, the eMSCS sends an INVITE to the vMSCS. And receiving a response message including the media gateway 2 (MGW2) media resource returned by the vMSCS; the eMSCS sends an INVITE message including the MGW2 media resource to the ATCF, and receives a response message including the access translation gateway (ATGW) media resource returned by the ATCF eMSCS transmits the ATGW media resource to the vMSCS; and the vMSCS establishes the multimedia link of the MGW2 to the ATGW by using the ATGW media resource, so that the one user terminal accesses and switches to the circuit domain network by using the vMSCS At the time, an IMS call is made via the multimedia link to another user terminal that is on an IMS call.

According to an embodiment of the present invention, there is provided a handover system for switching an IMS call to a circuit domain, including vMSCS, ATCF and eMSCS, wherein when a user terminal that is performing an IMS call moves from a packet domain network to a circuit domain only network The vMSCS receives the preparation handover request sent by the eMSCS, where the preparation handover request includes an STN-SR for identifying the destination address as the ATCF, and the vMSCS establishes a multimedia link to the ATCF by using the STN-SR to make the one user terminal An IMS call is made via the multimedia link to another user terminal that is making an IMS call when the vMSCS accesses and switches to the circuit domain network.

According to an embodiment of the present invention, there is provided a handover system for switching an IMS call to a circuit domain, including eMSCS, vMSCS, and ATCF, wherein a user terminal that is performing an IMS call moves from a packet domain network to a circuit domain only network location The eMSCS sends an INVITE message to the vMSCS, and receives a response message including the MGW2 media resource returned by the vMSCS. The eMSCS sends an INVITE message containing the MGW2 media resource to the ATCF, and receives a response message including the ATGW media resource returned by the ATCF, and the eMSCS will The ATGW media resource is sent to the vMSCS. The vMSCS receives the INVITE message sent by the eMSCS, sends a response message including the MGW2 media resource to the eMSCS, and establishes the multimedia link of the MGW2 to the ATGW by using the ATGW media resource sent by the eMSCS, so that the one The user terminal makes an IMS call with another user terminal that is in the IMS call via the multimedia link by accessing the vMSCS and switching to the circuit domain network. The ATCF receives the INVITE message including the MGW2 media resource sent by the eMSCS, and sends a response message including the ATGW media resource to the eMSCS.

DRAWINGS

1 is a network structure diagram for implementing SRVCC handover according to a manner;

2 is a flow chart for implementing SRVCC switching according to a manner;

3 is a flowchart of a handover method for switching an IMS call to a circuit domain according to an embodiment of the present invention;

4 is a flowchart of a handover method for switching an IMS call to a circuit domain according to another embodiment of the present invention;

5 is a schematic diagram of a handover system for switching an IMS call to a circuit domain according to an embodiment of the present invention;

6 is a flowchart of implementing SRVCC handover according to an embodiment of the present invention;

7 is a flow chart of implementing SRVCC handover in accordance with another embodiment of the present invention.

detailed description

1 is a network structure diagram for implementing Single Radio Voice Call Continuity (SRVCC) handover according to one mode, and FIG. 2 is a flowchart for implementing SRVCC handover according to a manner.

As shown in FIG. 1, when the user equipment (User Equipment, UE) A moves from the packet switched domain network to the location of only the circuit domain network, handover is required. The Enhanced Mobile Switch Center Server (eMSCS) is a mobile switching server that handles handover, provides handover services for UE A, and provides call control functions after handover.

The Mobility Management Entity (MME) is the management network element of UE A in the packet switched domain. Before the handover, UE A resides under the MME and is connected to the Serving-Call Session Control Function (S-CSCF) through the IP connection provided by the MME. When it is required to switch to the circuit domain, the MME sends a handover request to the Mobile Switching Center (MSC) (not shown in FIG. 1), and interacts with the MSC to complete the handover procedure.

The Visiting Mobile Switch Center Server (vMSCS) is the mobile switching server where UE A is located after the handover. The UE A accesses the vMSCS, and the signaling is sent to the eMSCS for processing via the MSC Mobile Application Part (MSCMAP) interface (not shown in FIG. 1).

Service Centralization and Continuity Application Server (SCC AS) is an Application Server (AS) that provides call continuity services in an IMS network. The Access Transfer Control Function (ATCF) assists the SCC AS in providing business continuity. In the handover process, the ATCF can serve as an anchor point to reduce the signaling transmission delay of the handover.

Media Gateway-1 (MGW-1) is a media gateway under eMSCS. Media Gateway-2 (MGW-2) is a media gateway under vMSCS. The Access Transfer Gateway (ATGW) is a media gateway under the ATCF that provides anchor points on the media for the handover function.

Before the handover, the media link is UE A→ATGW→UE B; after the handover, the media link becomes UE A→MGW-2→MGW-1→ATGW→UE B.

As shown in FIG. 2, the flow of implementing SRVCC handover according to one manner may include steps 201 to 219.

At step 201, UE A under LTE (ie, packet switched domain network) calls UE B to make an IMS call. During the call, UE A moves to the circuit domain network, so it is decided to switch by switching the decision.

At step 202, the MME sends a handover request to the eMSCS to request a handover. The routing number to the ATCF is carried in the handover request, that is, the Session Transfer Number-Single Radio (STN-SR).

In step 203, the eMSCS that receives the handover request sends a Prepare Handover Request message to the vMSCS through a Mobile Application Part (MAP) interface.

At step 204, the vMSCS applies for media resources on the MGW 2 for the switched media connection.

In step 205, the vMSCS performs handover preparation work on the radio side to apply for radio resources.

At step 206, the vMSCS sends a Prepare Handover Response message to the eMSCS as a response to the preparation for the handover request. The message carries the assigned handover number for the eMSCS to vMSCS to establish a handover session.

In step 207, after receiving the preparation handover response, the eMSCS applies for the media resource on the MGW1.

At step 208, the eMSCS routes using the received handover number to establish a voice path to the vMSCS and sends an Identity and Access Management (IAM) message to the vMSCS.

In step 209, the vMSCS returns an address complete message (ACM), and both eMSCS and vMSCS establish a voice path.

At step 210, the eMSCS sends an INVITE message to the ATCF to establish a session to the ATCF. The INVITE message takes the STN-SR as the target address and carries the media resource information applied for on the MGW1.

In step 211, the eMSCS returns a handover response to the MME, notifying the MME that the resource preparation is complete.

At step 212, the MME sends a handover command to UE A such that UE A begins to transfer to the circuit domain network.

At step 213, the ATCF that received the INVITE message applies for the media resource on the ATGW.

At step 214, the ATCF returns "200 OK", which carries the media information of the ATGW.

At step 215, UE A receiving the handover command completes the access on the vMSCS.

In step 216, the vMSCS sends a Send End Signal Request to the eMSCS to notify the eMSCS that the handover is complete.

At step 217, the vMSCS sends an Answer Message (ANM) to complete the process of session establishment with the eMSCS. At this time, the media link is from UE A to MGW2, to MGW1, then to ATGW, and finally to UE B to form a connection.

At step 218, the eMSCS that received the response message sends a handover complete message to the MME, notifying the MME that the handover is complete.

At step 219, the ATCF interacts with the SCC AS after returning "200 OK" to update the conversation between the two.

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It is to be understood that the embodiments described below are merely illustrative of the invention and are not intended to limit the invention.

3 is a flow chart of a method of switching an IMS call to a circuit domain in accordance with an embodiment of the present invention.

As shown in FIG. 3, a handover method for switching an IMS call to a circuit domain according to an embodiment of the present invention may include steps S301 and S302.

In step S301, when a user terminal that is performing an IMS call moves from a packet domain network to a location of only the circuit domain network, the vMSCS receives a preparation switching request sent by the eMSCS, where the preparation switching request includes an STN for identifying the destination address as ATCF. -SR.

At step S302, the vMSCS establishes a multimedia link to the ATCF using the STN-SR. Thus, the one user terminal can make an IMS call with another user terminal that is in the IMS call via the multimedia link when the vMSCS accesses and switches to the circuit domain network.

Step S302 can include determining an interface to establish the multimedia link. When the interface establishing the multimedia link is determined to be a Session Initiation Protocol (SIP) interface, the vMSCS sends an INVITE message including the STN-SR to the ATCF. When the interface establishing the multimedia link is determined to be a bearer independent call control (BICC) interface, the vMSCS sends an IAM message including the STN-SR to the ATCF.

According to the embodiment of the present invention, when the interface for establishing the multimedia link is determined to be a SIP interface, the INVITE message sent by the vMSCS to the ATCF further includes an eMSCS address, and the INVITE message is forwarded to the ATCF via the eMSCS.

4 is a flow chart of a method of switching an IMS call to a circuit domain in accordance with another embodiment of the present invention.

As shown in FIG. 4, a handover method for switching an IMS call to a circuit domain according to another embodiment of the present invention may include steps S401 to S404.

In step S401, when a user terminal that is performing an IMS call moves from the packet domain network to the location of only the circuit domain network, the eMSCS transmits an INVITE message to the vMSCS, and receives a response message including the MGW2 media resource returned by the vMSCS.

In step S402, the eMSCS sends an INVITE message containing the MGW2 media resource to the ATCF, and receives a response message including the ATGW media resource returned by the ATCF.

In step S403, the eMSCS sends the ATGW media resource to the vMSCS.

In step S404, the vMSCS establishes the multimedia link of the MGW2 to the ATGW by using the ATGW media resource. Thus, the one user terminal can make an IMS call with another user terminal that is making an IMS call via the multimedia link when the vMSCS accesses and switches to the circuit domain network.

Before the step S401, the method may further include: the eMSCS sends a preparation handover request to the vMSCS; and the vMSCS requests the MGW2 media resource from the MGW2 in response to the received preparation handover request.

5 is a schematic diagram of a handover system for switching an IMS call to a circuit domain in accordance with an embodiment of the present invention.

As shown in FIG. 5, a handover system for switching an IMS call to a circuit domain according to an embodiment of the present invention may include an eMSCS 601, a vMSCS 602, and an ATCF 603.

According to an embodiment of the present invention, when a user terminal that is performing an IMS call moves from a packet domain network to a location of only the circuit domain network, the vMSCS 602 receives a preparation switching request sent by the eMSCS 601, the preparation switching request including the destination address for identifying STN-SR for ATCF. The vMSCS 602 establishes a multimedia link to the ATCF 603 using the STN-SR such that the one user terminal can communicate with another user who is in the IMS via the multimedia link when accessing and switching to the circuit domain network at the vMSCS 602 The terminal makes an IMS call.

In accordance with an embodiment of the present invention, vMSCS 602 can determine an interface to establish the multimedia link. When the interface establishing the multimedia link is determined to be a SIP interface, the vMSCS 602 sends an INVITE message including the STN-SR to the ATCF 603, and when the interface establishing the multimedia link is determined to be the BICC interface, the vMSCS 602 to the ATCF 603 sends an IAM message containing the STN-SR.

According to the embodiment of the present invention, when the interface for establishing the multimedia link is determined to be a SIP interface, the INVITE message sent by the vMSCS 602 to the ATCF 603 may further include an address of the eMSCS 601, so that the INVITE message is forwarded to the eMSCS 601. ATCF 603.

According to another embodiment of the present invention, when a user terminal that is performing an IMS call moves from a packet domain network to a location of only the circuit domain network, the eMSCS 601 sends an INVITE message to the vMSCS 602 and receives the MMSC2 media resource returned by the vMSCS 602. Reply message. The eMSCS 601 sends an INVITE message containing the MGW2 media resource to the ATCF 603, and receives a response message containing the ATGW media resource returned by the ATCF 603. The eMSCS 601 sends the ATGW media resources to the vMSCS 602. The vMSCS 602 receives the INVITE message sent by the eMSCS 601, sends a response message containing the MGW2 media resource to the eMSCS 601, and establishes the multimedia link of the MGW2 to the ATGW by using the ATGW media resource sent by the eMSCS 601. Thus, the one user terminal can make an IMS call with another user terminal that is in the IMS call via the multimedia link when the vMSCS 602 accesses and switches to the circuit domain network. The ATCF 603 receives the INVITE message containing the MGW2 media resource sent by the eMSCS 601, and sends a response message containing the ATGW media resource to the eMSCS 601.

In accordance with an embodiment of the present invention, the eMSCS 601 sends a preparation handover request to the vMSCS 602, and the vMSCS 602 requests the MGW2 for the MGW2 media resource in response to the received preparation handover request.

Two schemes for implementing SRVCC handover are provided in accordance with various embodiments of the present invention. Hereinafter, the two schemes will be described in detail in conjunction with FIGS. 6 and 7, respectively. 6 and 7 are flow diagrams of implementing SRVCC switching in accordance with various embodiments of the present invention.

In the implementation of the first type of SRVCC handover, the eMSCS carries the STN-SR in the preparation handover request sent to the vMSCS. After receiving the preparation handover request, the vMSCS establishes a session to the ATCF with the STN-SR as the target address, and does not establish a session between the eMSCS and the vMSCS.

According to the above scheme, the established voice path from the UE to the MGW2 to the ATGW, the MGW1 under the eMSCS is omitted.

When the vMSCS establishes an ATCF session, if the SIP interface is used, the INVITE message is sent, and the Uniform Resource Identifier (URI) of the INVITE message is STN-SR; if the BICC interface is used, the IAM message is sent, And the called number in the IAM message is STN-SR. Further, if a SIP interface is used, the routing header field of the INVITE message may include the address of the eMSCS, so that the INVITE message passes through the eMSCS and then reaches the ATCF. This allows the eMSCS to be inserted into the session to gain the necessary call control. According to the embodiment of the present invention, the eMSCS may carry its own address in the Prepare Handover Request message. The address can be in the form of a TelURI or SIPURI, or it can be in the form of an E.164 number.

Figure 6 shows a flow chart of the above scheme. As shown in FIG. 6, when a SIP interface is used between the eMSCS and the vMSCS, the method for implementing the SRVCC handover may include steps 701 to 717.

Steps 701 to 705 are substantially the same as steps 201 to 205 shown in FIG. 2. The difference is that, in step 703, the preparation switching request sent by the eMSCS carries the STN-SR.

At step 706, the vMSCS sends an INVITE message with the target address being the STN-SR and inserting the address of the eMSCS in the routing header field. In addition, the International Mobile Subscriber Identification Number (IMSI) identifier of the handover user may be carried in the INVITE message.

In step 707, since the Route header field of the INVITE message is inserted with the address of the eMSCS, the INVITE message is first routed to the eMSCS. The eMSCS associates the conversation and handover of the INVITE according to the IMSI in the INVITE message. In addition, the eMSCS fills in the mobile subscriber international ISDN/PSTN number (MSISDN) of the handover user in the P-Asserted-Identity header field of the INVITE message, and then continues to route the INVITE message to the ATCF.

At step 708, the vMSCS sends a Prepare Handover Reply message to the eMSCS. Since there is no need to establish a media connection between the eMSCS and the vMSCS, the preparation handover response message may not carry the handover number.

Steps 709 to 712 correspond to steps 211 to 214 shown in FIG. 2.

In step 713, the eMSCS forwards the "200 OK" of the media information carrying the ATGW returned by the ATCF to the vMSCS. After receiving the "200 OK" message, the vMSCS completes the media connection establishment process between the vMSCS and the ATCF.

Steps 714 to 717 correspond to step 215, step 216, step 218, and step 219 shown in FIG. 2, respectively.

In the implementation of the second SRVCC handover, a SIP interface may be adopted between the vMSCS and the eMSCS. After receiving the preparation handover response message of the vMSCS, the eMSCS sends an INVITE message that does not carry the media information. Therefore, eMSCS does not need to apply for media resources from MGW1. After receiving the INVITE message, the vMSCS returns a SIP response message. The SIP response message may be "180", "183" or "200 OK", and carries the media information requested by the vMSCS at the MGW2 in the SIP response message. After receiving the SIP response message returned by the vMSCS, the eMSCS sends an INVITE message to the ATCF, and the INVITE message carries the media information of the MGW2. The ATCF returns the media information of the ATGW in response to the eMSCS. The eMSCS returns the media information of the ATGW to the vMSCS in a response message for "180", "183" or "200 OK".

According to the above solution, a direct media connection can be established between the MGW2 and the ATGW under the vMSCS, so that the MGW can no longer be deployed under the eMSCS.

Figure 7 shows a flow chart of the above scheme. As shown in FIG. 7, the method for implementing SRVCC handover may include steps 801 to 820.

Steps 801 to 806 are substantially the same as steps 201 to 206 shown in FIG. 2.

At step 807, the eMSCS sends an INVITE message to the vMSCS. Since there is no media gateway under eMSCS, the INVITE message does not carry Session Description Protocol (SDP) media information.

In step 808, the vMSCS returns a "180" message, and the "180" message carries the SDPOffer to identify the media information on the MGW2 at the vMSCS end.

In step 809, after receiving the "180" message, the eMSCS sends an INVITE message to the ATCF, with the STN-SR as the target address, and carries the media resource information of the vMSCS end in the "180" message.

Steps 810 to 813 correspond to steps 211 to 214 shown in FIG. 2.

At step 814, the eMSCS sends a response message Provisional Response ACKnowledgement (PRACK) for the "180" message and carries therein the media information of the ATGW sent by the ATCF via the "200 OK" message.

At step 815, the vMSCS returns an ACK message for confirmation. According to the PRACK and the media information of the ATGW carried therein, the exchange of media information can be completed between the vMSCS and the ATCF, so that the vMSCS and the ATCF can establish a media connection.

Steps 816 to 820 correspond to steps 215 to 219 shown in FIG. 2. The only difference is that since a SIP interface is employed between the vMSCS and the eMSCS, in step 818, the vMSC sends a "200 OK" message to complete the session establishment process with the eMSCS.

According to the solution provided by the embodiment of the present invention, the media gateway node for implementing the SRVCC handover can be reduced, that is, the media gateway is not required to be deployed under the eMSCS network element, thereby reducing the deployment cost of the system, shortening the transmission path, and improving the SRVCC handover. The efficiency and quality of the voice after switching.

Although the invention has been described in detail above, the invention is not limited thereto, and various modifications may be made by those skilled in the art in accordance with the principles of the invention. Therefore, modifications made in accordance with the principles of the invention are to be understood as falling within the scope of the invention.

Claims

A handover method for switching an IP multimedia subsystem IMS call to a circuit domain, comprising:

When a user terminal that is performing an IMS call moves from a packet domain network to a location of only the circuit domain network, the visited mobile switching center server vMSCS receives a preparation switching request sent by the enhanced mobile switching center server eMSCS, the preparation switching request including for identifying The destination address is a single-frequency session transfer number STN-SR of the access transition control function ATCF;

The vMSCS establishes a multimedia link to the ATCF by using the STN-SR, so that the one user terminal performs the other user terminal that is in the IMS call via the multimedia link when the vMSCS accesses and switches to the circuit domain network. IMS call.
The handover method according to claim 1, wherein the step of the vMSCS establishing the multimedia link to the ATCF by using the STN-SR comprises:

Determining an interface for establishing the multimedia link,

The vMSCS sends an INVITE message including the STN-SR to the ATCF when the interface for establishing the multimedia link is determined as the session initiation protocol SIP interface.

When the interface establishing the multimedia link is determined to be a bearer-independent call control BICC interface, the vMSCS sends an identity recognition and access management IAM message including the STN-SR to the ATCF.
The handover method according to claim 2, wherein when the interface for establishing the multimedia link is determined to be a SIP interface, the INVITE message sent by the vMSCS to the ATCF further includes an eMSCS address, and the INVITE message is sent via the eMSCS. Forward to ATCF.
A handover method for switching an IP multimedia subsystem IMS call to a circuit domain, comprising:

When a user terminal that is performing an IMS call moves from the packet domain network to the location of only the circuit domain network, the enhanced mobile switching center server eMSCS sends an INVITE message to the visited mobile switching center server vMSCS, and receives the media including the media gateway 2MGW2 returned by the vMSCS. Response message of the resource;

The eMSCS sends an INVITE message including the MGW2 media resource to the conversion control function ATCF, and receives a response message that is returned by the ATCF and includes the access translation gateway ATGW media resource;

The eMSCS sends the ATGW media resource to the vMSCS;

The vMSCS establishes the multimedia link of the MGW2 to the ATGW by using the ATGW media resource, so that the one user terminal communicates with the IMS via the multimedia link when accessing and switching to the circuit domain network by the vMSCS Another user terminal makes an IMS call.
The handover method according to claim 4, wherein before the step of the eMSCS transmitting the INVITE message to the vMSCS and receiving the response message including the MGW2 media resource returned by the vMSCS, the method further includes:

The eMSCS sends a preparation handover request to the vMSCS;

The vMSCS applies for the MGW2 media resource to the MGW 2 in response to the received preparation handover request.
A handover system for switching an IP Multimedia Subsystem IMS call to a circuit domain, comprising visiting a mobile switching center server vMSCS, an enhanced mobile switching center server eMSCS, and an access switching control function ATCF,

The vMSCS receives the preparation switching request sent by the eMSCS when the user terminal that is performing the IMS call moves from the packet domain network to the location of only the circuit domain network, where the preparation switching request includes a single frequency session for identifying the destination address as ATCF. Transferring the number STN-SR, and the vMSCS establishes a multimedia link to the ATCF by using the STN-SR, so that the one user terminal passes through the multimedia link and is in the IMS when accessing and switching to the circuit domain network by the vMSCS Another user terminal of the call makes an IMS call.
The switching system of claim 6 wherein said vMSCS is arranged to determine an interface to establish said multimedia link,

When the interface establishing the multimedia link is determined as the Session Initiation Protocol SIP interface, the vMSCS sends an INVITE message including the STN-SR to the ATCF, when the interface establishing the multimedia link is determined to be a bearer-independent call When controlling the BICC interface, the vMSCS sends an Identity and Access Management IAM message containing the STN-SR to the ATCF.
The switching system according to claim 7, wherein when the interface for establishing the multimedia link is determined to be a SIP interface, the INVITE message sent by the vMSCS to the ATCF further includes an eMSCS address, and the INVITE message is sent via the eMSCS. Forward to ATCF.
A handover system for switching an IP Multimedia Subsystem IMS call to a circuit domain, comprising an enhanced mobile switching center server eMSCS, a visited mobile switching center server vMSCS, and an access switching control function ATCF,

Wherein, when a user terminal that is performing an IMS call moves from the packet domain network to the location of only the circuit domain network, the eMSCS sends an INVITE message to the vMSCS, and receives a response message including the media gateway 2MGW2 media resource returned by the vMSCS, the eMSCS Sending an INVITE message including the MGW2 media resource to the ATCF, and receiving a response message that is returned by the ATCF and including the access translation gateway ATGW media resource, and the eMSCS sends the ATGW media resource to the vMSCS,

The vMSCS receives the INVITE message sent by the eMSCS, sends a response message including the MGW2 media resource to the eMSCS, and establishes the multimedia link of the MGW2 to the ATGW by using the ATGW media resource sent by the eMSCS, so that the one The user terminal performs an IMS call with another user terminal that is in the IMS call via the multimedia link when the vMSCS accesses and switches to the circuit domain network.

The ATCF receives the INVITE message including the MGW2 media resource sent by the eMSCS, and sends a response message including the ATGW media resource to the eMSCS.
The handover system according to claim 9, wherein the eMSCS sends a preparation handover request to the vMSCS, and the vMSCS requests the MGW2 media resource from the MGW 2 in response to the received preparation handover request.