Abstract:
A system and method are provided for performing voice data handoff from a cellular network to a portable Internet/wireless local area network (WLAN) network by a mobile terminal in a heterogeneous network environment. The system and method are provided wherein whether to perform handoff is determined, and a location of a mobile terminal in the portable Internet/WLAN network is registered, a request for handoff is sent to the cellular network, and in response thereto, a request for voice-over-Internet protocol (VoIP) call setup through a specific upper node of the portable Internet/WLAN network is received, and the VoIP call to the upper node of the portable Internet/WLAN network is set-up, and resources of a circuit voice call to a specific upper node of the cellular network are released.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of application Serial No. 2005-36406, filed in the Korean Intellectual Property Office on Apr. 29, 2005 and application Serial No. 2005-36408, filed in the Korean Intellectual Property Office on Apr. 29, 2005, the entire disclosures of both of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a system and method for voice data handoff between a cellular network and a WiBro/WLAN network in a heterogeneous network environment. More particularly, the present invention relates to a system and method which seamlessly provide a 3rd generation (3G) voice service to a mobile terminal while it moves between a cellular network and a Wireless Broadband (WiBro, also known as “Portable Internet”) network or a wireless local area network (WLAN) in an Internet protocol Multimedia Subsystem (IMS)-based heterogeneous network environment.  
         [0004]     2. Description of the Related Art  
         [0005]     In the following description, the heterogeneous networks include a cellular network (CDMA 2000 1X system) and a WiBro network, by way of example. IMS in the cellular network is a core technology for providing users with ubiquitous services, such as wire/wireless integrated service, voice/data integrated service, and communication/broadcasting convergence service, in a broadband integrated network. IMS users can exchange with each other multimedia contents, such as pictures, video clips, and sound clips, through session-based messages. For example, the session-based message includes a Session Initiation Protocol (SIP) message.  
         [0006]     The major functional elements of a core network subsystem for MBS service include a Call Session Control Function (CSCF), a Home Subscriber Server (HSS), a Media Gateway Control Function (MGCF), and an IMS-Media Gateway (IMS-MGW). The CSCF performs a call and session processing-related function, and is classified into a Proxy CSCF (P-CSCF), an Integrating CSCF (I-CSCF), and a Serving CSCF (S-CSCF) according to its function.  
         [0007]     The P-CSCF is an element for performing a gateway function when a terminal first accesses the network to receive MBS service, and performs a Proxy and User Agent function. Basically, the P-CSCF replays SIP messages between the terminal and the network. The I-CSCF serves as a contact point for all calls incoming to connect with subscribers in the network, and inquires of an HSS for location detection of a called subscriber before routing a call. In addition, the I-CSCF, as it serves as a gateway with another IMS network, also serves as a firewall for hiding a topology in the network for security.  
         [0008]     The S-CSCF performs, during its registration, a Register function and various authentication functions necessary therefor. The S-CSCF performs a series of mechanisms for directly interworking with application servers to provide various multimedia services, routing a call based on triggering information, and providing services.  
         [0009]     The MGCF, located in the contact point between an IMS network and a Public Switch Telephone Network/Plain Land Mobile Network (PSTN/PLMN) network, takes charge of interworking a call, and performs an SIP and ISDN User Part (ISUP) signaling protocol translation function according thereto. The MGCF performs a function of managing and controlling resources in the IMS-MGW for call processing.  
         [0010]     The IMS-MGW, an MGW used in the IMS network, performs a function of converting IP packet media data into the format that can be transmitted on a bearer of a circuit switched network in order to interwork with the PSTN/PLMN network, and performs such functions as transcoding and echo canceling, for that purpose.  
         [0011]     The HSS, an evolved type of the conventional Home Location Register (HLR), is an addition of an Authentication Center (AuC) function to the conventional HLR function. The HSS is a subscriber&#39;s master database for managing user number-related information, location information, and service profile information.  
         [0012]     Meanwhile, the core functional elements of the WiBro network include a Radio Access Station (RAS) and an Access Control Router (ACR). The RAS provides a wireless access function, a wireless resource management and control function, and a handoff support function of Portable Internet (WiBro). The ACR provides IP routing and mobility management functions.  
         [0013]     The issue in the foregoing vertical handoff (or handoff between heterogeneous networks) process is a handoff time and a packet loss caused by the handoff. Therefore, an efficient interworking scheme between the heterogeneous networks should be able to minimize the handoff time and the packet loss. However, there has been no standard defined for the handoff procedure necessary for the case where a user receiving voice service in a heterogeneous network environment moves from the cellular network to the WiBro/WLAN network, making it impossible to perform fast handoff between the heterogeneous networks.  
         [0014]     Accordingly, there is a need for an improved system and method for voice data handoff between a cellular network and a WiBro/WLAN network in a heterogeneous network environment.  
       SUMMARY OF THE INVENTION  
       [0015]     Exemplary embodiments of the present invention address at least the above problems and/or disadvantages and provide at least the advantages described below. It is, therefore, an exemplary object of the present invention to provide a system and method which provide seamless voice service when a mobile terminal moves between cellular coverage and Portable Internet (WiBro) coverage in a heterogeneous network environment.  
         [0016]     According to an exemplary aspect of the present invention, a system and method are provided for performing voice data handoff from a cellular network to a portable Internet/wireless local area network (WLAN) network by a mobile terminal in a heterogeneous network environment. The system and method are provided wherein, whether to perform handoff of the mobile terminal is determined and a location of the terminal in the portable Internet/WLAN network is registered, a request for handoff to the cellular network is sent, and in response thereto a request for voice-over-Internet protocol (VoIP) call setup through a specific upper node of the portable Internet/WLAN network is received, and the VoIP call to the upper node of the portable Internet/WLAN network is set up, and resources of a circuit voice call to a specific upper node of the cellular network are released.  
         [0017]     According to another exemplary aspect of the present invention, there are provided a system and method for performing voice data handoff to a cellular network by a first mobile terminal among the first mobile terminal located in a portable Internet/wireless local area network (WLAN) network and a second mobile terminal located in the portable Internet/WLAN network or the cellular network in a heterogeneous network environment. The system and method are provided in which the first mobile terminal determines whether it has entered the cellular network and its location in the cellular network according to the determination result is registered, the first mobile terminal sends a request for handoff to a specific upper node of the portable Internet/WLAN network, and the upper node of the portable Internet/WLAN network sets up a circuit voice call of the first mobile terminal to the cellular network through a specific upper node of the cellular network. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and other objects, features and advantages of the exemplary embodiments of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:  
         [0019]      FIG. 1  is a diagram illustrating network architecture for providing seamless 3G voice service in a cellular network and a WiBro network according to an exemplary embodiment of the present invention;  
         [0020]      FIG. 2  is a diagram illustrating a scenario in which a first mobile terminal (MT 1 ) located in a cellular network attempts handoff to a WiBro network while performing a voice call with a second mobile terminal (MT 2 ) also located in a cellular network according to a first exemplary embodiment of the present invention;  
         [0021]      FIG. 3  is a call flow diagram in which an MT 1  performs handoff from a cellular network to a WiBro network in the scenario of  FIG. 2  according to the first exemplary embodiment of the present invention;  
         [0022]      FIGS. 4A and 4B  are diagrams illustrating voice data paths before and after an MT 1  performs handoff according to the first exemplary embodiment of the present invention;  
         [0023]      FIG. 5  is a diagram illustrating a scenario in which an MT 1  located in a cellular network attempts handoff to a WiBro network while performing a voice call with an MT 2  located in the WiBro network according to a second exemplary embodiment of the present invention;  
         [0024]      FIG. 6  is a call flow diagram in which an MT 1  performs handoff from a cellular network to a WiBro network in the scenario of  FIG. 5  according to the second exemplary embodiment of the present invention;  
         [0025]      FIGS. 7A and 7B  are diagrams illustrating voice data paths before and after an MT 2  performs handoff according to the second exemplary embodiment of the present invention;  
         [0026]      FIG. 8  is a diagram illustrating a scenario in which an MT 1  located in a WiBro network performs handoff to a cellular network while performing a voice call with an MT 2  also located in the WiBro network according to a third exemplary embodiment of the present invention;  
         [0027]      FIG. 9  is a call flow diagram in which an MT 1  performs handoff from a WiBro network to a cellular network in the scenario of  FIG. 8  according the third exemplary embodiment of the present invention;  
         [0028]      FIGS. 10A and 10B  are diagrams illustrating voice data paths before and after an MT 2  performs handoff according to the third exemplary embodiment of the present invention;  
         [0029]      FIG. 11  is a diagram illustrating a scenario in which an MT 1  located in a WiBro network attempts handoff to a cellular network while performing a voice call with an MT 2  located in the cellular network according to a fourth exemplary embodiment of the present invention;  
         [0030]      FIG. 12  is a call flow diagram in which an MT 1  performs handoff from a WiBro network to a cellular network in the scenario of  FIG. 11  according to the fourth exemplary embodiment of the present invention; and  
         [0031]      FIGS. 13A and 13B  are diagrams illustrating voice data paths before and after an MT 1  performs handoff according to the fourth exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0032]     Several exemplary embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness.  
         [0033]      FIG. 1  is a diagram illustrating network architecture for providing seamless 3G voice service in a cellular network and a WiBro network according to an exemplary embodiment of the present invention.  
         [0034]     Referring to  FIG. 1 , a WiBro (or Portable Internet) network  110  is deployed in a specific area of a cellular network  120 , forming a complementary relationship. Therefore, a dual-band, dual-mode (DBDM) mobile terminal  100 , when it enters a particular hot spot (WiBro coverage) while in 3G service, can receive WiBro service. Cells of the WiBro network  110  and the cellular network  120  may have either an overlapping configuration supporting services of both networks, or a separate configuration supporting only service of any one of the networks. The detailed technical characteristics of the cells are not directly related to the present invention, so a detailed description thereof will be omitted.  
         [0035]     The DBDM mobile terminal and a Base Station System (BSS) of the cellular network according to an exemplary embodiment of the present invention have the following requirements.  
         [0036]     The DBDM mobile terminal, in cellular coverage, receives information indicating the presence or absence of a WiBro cell through an overhead message. The DBDM transmits a Pilot Strength Measurement Message (PSMM) to the BSS of the cellular network if signal strength of a Pseudo Noise (PN) value from a cellular base station overlapping with a WiBro base station exceeds a threshold. Upon recognizing the presence of the WiBro cell, the DBDM terminal activates a WiBro modem. The DBDM terminal, after activating the WiBro modem, registers at the WiBro network and performs location registration (WiBro-CDMA dual-activated state) in an IMS network.  
         [0037]     The BSS of the cellular network manages a cellular-WiBro cell overlapping area with a neighbor list. The BSS inserts an indicator in an overhead message and sends the overhead message to the DBDM mobile terminal in order to indicate the presence of the WiBro cell. If a PN sequence transmitted by the DBDM mobile terminal is a cell identifier (ID) of the cellular-WiBro cell overlapping area, a request for hard handoff is sent to a Mobile Switching Center (MSC).  
       First Exemplary Embodiment  
       [0038]      FIG. 2  is a diagram illustrating a scenario in which a first mobile terminal (MT 1 ) located in a cellular network attempts handoff to a WiBro network while performing a voice call with a second mobile terminal (MT 2 ) also located in a cellular network according to a first exemplary embodiment of the present invention  
         [0039]     In the first exemplary embodiment, described with reference to  FIG. 2 , a first DBDM mobile terminal (MT 1 )  210  and a second DBDM mobile terminal (MT 2 )  220  are performing a 3G circuit call with each other in a cellular network  240  and the MT 1   210  is moving from the cellular network  240  to enter a WiBro network  230 . Also in the first exemplary embodiment, the WiBro network  230  is deployed such that cells of the WiBro network  230  overlap cells of the cellular network  240 . In the cellular-only coverage, if a DBDM mobile terminal monitors both of the cellular wireless section and the WiBro wireless section, its battery consumption increases. In this exemplary embodiment, the mobile terminal does not operate in a WiBro mode to save its battery power. The BSS of the cellular network sends information indicating the presence or absence of a WiBro cell to the DBDM mobile terminal through an overhead message. For example, the BSS may modify the existing field (MSB bits of Packet Zone ID) in a system parameter message, or add a separate field in the message.  
         [0040]      FIG. 3  is a call flow diagram in which an MT 1  performs handoff from a cellular network to a WiBro network in the scenario of  FIG. 2  according to the first exemplary embodiment of the present invention. The messages shown in  FIG. 3  are defined in the 3rd Generation Partnership Project (3GPP) standard, so a detailed description thereof will be omitted.  
         [0041]     Referring to  FIG. 3 , in steps  301  and  302 , an MT 1  enables a WiBro modem upon receiving an overhead message with information indicating the presence of a WiBro coverage (cell) in a cellular coverage (cell). If received strength of a WiBro signal is higher than or equal to a threshold for a time period, the MT 1  determines handoff, considering that it is moving to the WiBro coverage. The MT 1  performs Layer 3 (L3) Attachment to WiBro, and performs an IMS registration procedure defined in the IMS standard. In step  303 , the MT 1  transmits a PSMM with the received signal strength to a Base Station System (BTS/BSC) or a Radio Access Network (RAN).  
         [0042]     In step  304 , the BTS/BSC transmits a Handoff Required message to an MSC. In step  305 , the MSC transmits a FacilitiesDirective (FACDIR) message indicating a start of a handoff procedure to an MGC of an IMS network if a target Cell ID included in the Handoff Required message overlaps with the WiBro cell. Upon receiving the FACDIR message, the MGC selects a channel and determines available media capacity through communication with an IMS-MGW.  
         [0043]     In step  306 , the MGC uses a Mobile Identification Number (MIN) of the MT 1  as an SIP Uniform Resource Locator (URL), and delivers the MIN and an ESN of the MT 1  to an I-CSCF along with an SIP INVITE message. In step  307 , the I-CSCF queries an HSS about an address of an S-CSCF that takes charge of a session of an MT 2 , and receives an address of the S-CSCF from the HSS. In step  308 , the I-CSCF transmits an INVITE message to the S-CSCF. The S-CSCF determines whether this session setup is appropriate. In steps  309  and  310 , the INVITE message is delivered to the MT 1  via a P-CSCF. At this moment, the MT 1  recognizes an IP and a port number of an MGW, included in an SDP.  
         [0044]     In step  311 , the MT 1  transmits an SIP 2000K message indicating connection of a voice call, to the MGC. In step  312 , the MGC delivers a FACDIR message to the MSC because a Voice-over-IP (VoIP) session to the handoff requiring MT 1  is set up. In steps  313  and  314 , the MSC receiving the FACDIR message exchanges Handoff Command/Handoff Commenced messages defined in the International Organization for Standardization (IOS) standard with the source BTS/BSC. However, the 3G BTS/BSC may not transmit a Handoff Direction message to the MT 1  and may include a Proprietary Field in the Handoff Command message in order to simulate as if it transmitted the Handoff Direction message to the MT 1 . In step  315 , the MGC transmits an ACK message to the MT 1 .  
         [0045]     In step  316 , if a channel to the MT 1  and a voice path between the MSC and the trunk are completely set up, the MGC transmits an MSONCH message to the MSC. In step  317 , the MSC transmits a Clear Command message to the BTS/BSC to request release of resources for the corresponding call.  
         [0046]      FIGS. 4A and 4B  are diagrams illustrating voice data paths before and after an MT 1  performs handoff according to the first exemplary embodiment of the present invention.  
         [0047]     Referring to  FIG. 4A , before an MT 1   410  performs handoff to a WiBro network  430 , both the MT 1   410  and an MT 2   420  are located in a cellular network  440 . For example, a voice data path between the MT 1   410  and the MT 2   420  may be set up through a Base station Transceiver Subsystem (BTS)  470 , a Base Station Controller (BSC)  460  and an MSC  450 , located in the cellular network  440 . The set voice data path changes to a voice data path shown in  FIG. 4B  after handoff. Referring to  FIG. 4B , after the MT 1   410  performs handoff to the WiBro network  430 , a voice data path between the MT 1   410  and the MT 2   420  may of pass through the cellular network  440  and the WiBro network  430 . For example, the voice data path between the MT 1   410  and the MT 2   420  may be established such that it sequentially passes through a RAS  490 , an ACR  485  and an MGW  480 , the elements of the WiBro network  430 , starting from the MT 1   410 , and then passes again through the MSC  450 , the BSC  460 , the BTS  470 , and the MT 2   420 , the elements of the cellular network  440 .  
       Second Exemplary Embodiment  
       [0048]      FIG. 5  is a diagram illustrating a scenario in which an MT 1  located in a cellular network attempts handoff to a WiBro network while performing a voice call with an MT 2  located in the WiBro network according to a second exemplary embodiment of the present invention. In this exemplary embodiment, the cellular network and the WiBro network have the same service provider.  
         [0049]     In the exemplary embodiment described with reference to  FIG. 5 , a DBDM MT 1   510  is located in a cellular network  540  and an MT 2   520  is located in a WiBro network  530 , performing a call between heterogeneous networks, and the MT 1   510  is moving from the cellular network  540  to enter the WiBro network  530 . Also in the exemplary embodiment, the WiBro network  530  is deployed such that cells of the WiBro network  530  overlap cells of the cellular network  540 . In the cellular-only coverage, if a DBDM mobile terminal monitors both of the cellular wireless section and the WiBro wireless section, its battery consumption increases. In this exemplary embodiment, the mobile terminal does not operate in a WiBro mode to save its battery power. The BSS of the cellular network sends information indicating the presence or absence of a WiBro cell to the DBDM mobile terminal through an overhead message. For example, the BSS may modify the existing field (MSB bits of Packet Zone ID) in a system parameter message, or add a separate field in the message.  
         [0050]      FIG. 6  is a call flow diagram in which an MT 1  performs handoff from a cellular network to a WiBro network in the scenario of  FIG. 5  according to the second exemplary embodiment of the present invention. In this exemplary embodiment, an MT 1  and an MT 2  set up their session through the same MGC. An MGC and an MSC release the traffic path setup, and voice traffics are delivered through the same MGW. The messages shown in  FIG. 6  are defined in the 3GPP standard, so a detailed description thereof will be omitted.  
         [0051]     Referring to  FIG. 6 , in steps  601  and  602 , an MT 1  enables a WiBro modem upon receiving an overhead message with information indicating the presence of a WiBro coverage (cell) in a cellular coverage (cell). If received strength of a WiBro signal is higher than or equal to a threshold for a time period, the MT 1  determines handoff, considering that it is moving to the WiBro coverage. The MT 1  performs L3 Attachment to WiBro, and performs an IMS registration procedure defined in the IMS standard. In step  603 , the MT 1  transmits a PSMM with the received signal strength to a BTS/BSC, or a Radio Access Network (RAN).  
         [0052]     In step  604 , the BTS/BSC transmits a Handoff Required message to an MSC of the cellular network. In step  605 , the MSC transmits a FacilitiesDirective (FACDIR) message indicating a start of a handoff procedure to an MGC of an IMS network if a target Cell ID included in the Handoff Required message overlaps with the WiBro cell. Upon receiving the FACDIR message, the MGC selects a channel and determines available media capacity through communication with an IMS-MGW.  
         [0053]     In step  606 , the MGC uses a MIN of the MT 1  as an SIP URL, and delivers the MIN and an ESN of the MT 1  to an I-CSCF along with an SIP INVITE message. In step  607 , the I-CSCF queries an HSS about an address of an S-CSCF that takes charge of a session of an MT 2 , and receives an address of the S-CSCF from the HSS. In step  608 , the I-CSCF transmits an INVITE message to the S-CSCF. The S-CSCF determines whether this session setup is appropriate. In steps  609  and  610 , the INVITE message is delivered to the MT 1  via a P-CSCF. At this moment, the MT 1  recognizes an IP and a port number of an MGW, included in an SDP.  
         [0054]     In step  611 , the MT 1  transmits an SIP 2000K message indicating connection of a voice call, to the MGC. In step  612 , the MGC delivers a FACDIR message to the MSC because a VoIP session to the handoff requiring MT 1  is set up. In steps  613  and  614 , the MSC receiving the FACDIR message exchanges Handoff Command/Handoff Commenced messages defined in the IOS standard with the source BTS/BSC. However, the 3G BTS/BSC may not transmit a Handoff Direction message to the MT 1 , and may include a Proprietary Field in the Handoff Command message in order to simulate as if it transmitted the Handoff Direction message to the MT 1 . In step  615 , the MGC transmits an ACK message to the MT 1 .  
         [0055]     In step  616 , the MGC transmits a FacilitiesRelease (FACREL) message to the MSC in order to request release of the trunk to the MSC. In step  617 , upon receiving the FACREL message, the MSC changes the trunk into an idle state through communication with the MGW, and responds to the FACREL message. In step  618 , the MSC transmits a Clear Command message to the BTS/BSC to request release of resources for the corresponding call.  
         [0056]      FIGS. 7A and 7B  are diagrams illustrating voice data paths before and after an MT 2  performs handoff according to the second exemplary embodiment of the present invention.  
         [0057]     Referring to  FIG. 7A , before an MT 2   720  performs handoff to a WiBro network  730 , a voice data path between an MT 1   710  and the MT 2   720  may pass through a cellular network  740  and a WiBro network  730 . For example, the voice data path between the MT 1   710  and the MT 2   720  may be established such that it sequentially passes through a RAS  790 , an ACR  785  and an MGW  780 , the elements of the WiBro network  730 , starting from the MT 1   710 , and then passes again through the MSC  770 , the BSC  760 , the BTS  750 , and the MT 2   720 , the elements of the cellular network  740 . The set voice data path changes to a voice data path shown in  FIG. 7B  after handoff.  
         [0058]     Referring to  FIG. 7B , after the MT 2  performs handoff to the WiBro network  730 , both the MT 1   710  and the MT 2   720  are located in the WiBro network  730 . For example, the voice data path between the MT 1   710  and the MT 2   720  may be established through the ACR  785  and the RASs  790 , using the MGW  780  located in the WiBro network  730  as the vertex.  
       Third Exemplary Embodiment  
       [0059]      FIG. 8  is a diagram illustrating a scenario in which an MT 1  located in a WiBro network performs handoff to a cellular network while performing a voice call with an MT 2  also located in the WiBro network according to a third exemplary embodiment of the present invention.  
         [0060]     In the third exemplary embodiment, described with reference to  FIG. 8 , a first DBDM mobile terminal (MT 1 )  810  and a second DBDM mobile terminal (MT 2 )  820  are performing a VoIP call with each other in a WiBro network  830  and the MT 1   810  is moving from the WiBro network  830  to enter a cellular network  840 . Also in the third exemplary embodiment, the WiBro network  830  is deployed such that cells of the WiBro network  830  overlap cells of the cellular network  840 . If received strength of a WiBro signal is lower than or equal to a threshold, the MT 1  enables a CDMA modem to receive CDMA information. If there is no indicator indicating the presence of a WiBro cell in the received CDMA overhead message, the MT 1  enters the cellular-only coverage.  
         [0061]      FIG. 9  is a call flow diagram in which an MT 1  performs handoff from a WiBro network to a cellular network in the scenario of  FIG. 8  according the third exemplary embodiment of the present invention. The messages shown in  FIG. 9  are defined in the 3GPP standard, so a detailed description thereof will be omitted.  
         [0062]     Referring to  FIG. 9 , in steps  901  to  904 , if received strength of a WLAN/WiBro signal is lower than or equal to a predetermined threshold for a time period, an MT 1  enables a CDMA modem, determining that it is moving to a cellular network. The MT 1  performs location registration in the cellular network in the dual-activated state.  
         [0063]     In step  905 , the MT 1  transmits a re-INVITE message for the current VoIP SIP session. The re-INVITE message has a Session ID of the current session, a Target Cell ID, and an Indicator indicating the ongoing handoff to the cellular network, all of which are included in a predetermined field of its Body. A P-CSCF performs a Network Initiated Session Release procedure for the MT 1  and the MT 2 , upon receiving a re-INVITE Request message with a handoff indicator.  
         [0064]     In steps  906  to  908 , upon receiving the re-INVITE Request message, an MGC sets up a new session to the MT 2 .  
         [0065]     In step  909 , the MGC transmits a FACDIR message indicating a start of a handoff procedure to an MSC of a target cell ID network. In steps  910  to  912 , upon receiving the FACDIR message, the MSC transmits a Handoff Request message to a target BSC. After allocating resources of a target BTS, the target BSC transmits a response message to a target MSC in response to the handoff request. In step  913 , the MSC transmits a FACDIR message to the MGC to notify its completed preparation for the handoff. In this case, the MSC transmits channel information of the target cell together.  
         [0066]     In step  914 , upon receiving the FACDIR message, the MGC transmits a 200 OK message indicating a start of handoff to the MT 1  along with the channel information. In step  915 , the MT 1  transmits an ACK message in response to the 2000K message.  
         [0067]     In step  916 , the MT 1  performs handoff to the cellular network using the channel information, and transmits a message indicating completion of the handoff procedure to the target BTS/BSC. In step  917 , the target BSC delivers a message indicating completion of the handoff procedure to the target MSC. In step  918 , the target MSC transmits a Mobile Station on Channel (MSONCH) message indicating completion of the voice path setup up to the mobile terminal to the MGC.  
         [0068]      FIGS. 10A and 10B  are diagrams illustrating voice data paths before and after an MT 2  performs handoff according to the third exemplary embodiment of the present invention.  
         [0069]     Referring to  FIG. 10A , before an MT 2   1020  performs handoff to a cellular network  1040 , both an MT 1   1010  and the MT 2   1020  are located in a WiBro network  1030 . For example, a voice data path between the MT 1   1010  and the MT 2   1020  may be set up through a RAS  1090  and an ACR  1085  located in the WiBro network  1030 . The set voice data path changes to a voice data path shown in  FIG. 4B  after handoff.  
         [0070]     Referring to  FIG. 10B , after the MT 2   1020  performs handoff to the cellular network  1040 , the voice data path between the MT 1   1010  and the MT 2   1020  assumes the form of passing through the cellular network  1040  and the WiBro network  1030 . For example, the voice data path between the MT 1   010  and the MT 2   1020  may be established such that it sequentially passes through a RAS  1090 , an ACR  1085  and an MGW  1080 , the elements of the WiBro network  1030 , starting from the MT 1   1010  located in the WiBro network  1030 , and then passes again through an MSC  1050 , a BSC  1060 , a BTS  1070 , and the MT 2   1020 , the elements of the cellular network  1040 .  
       Fourth Exemplary Embodiment  
       [0071]      FIG. 11  is a diagram illustrating a scenario in which an MT 1  located in a WiBro network attempts handoff to a cellular network while performing a voice call with an MT 2  located in the cellular network according to a fourth exemplary embodiment of the present invention. In the fourth exemplary embodiment, the cellular network and the WiBro network have the same service provider.  
         [0072]     In the fourth exemplary embodiment, described with reference to  FIG. 11 , a DBDM MT 1   1110  is located in a WiBro network  1130  and an MT 2   1120  is located in a cellular network  1140 , performing a call between heterogeneous networks, and the MT 1   1110  is moving from the WiBro network  1130  to enter the cellular network  1140 . Also in the fourth exemplary embodiment, the WiBro network  1130  is deployed such that cells of the WiBro network  1130  overlap cells of the cellular network  1140 .  
         [0073]      FIG. 12  is a call flow diagram in which an MT 1  performs handoff from a WiBro network to a cellular network in the scenario of  FIG. 11  according to the fourth exemplary embodiment of the present invention. The messages shown in  FIG. 12  are defined in the 3GPP standard, so a detailed description thereof will be omitted.  
         [0074]     Referring to  FIG. 12 , in steps  1201  to  1204 , if received strength of a WLAN/WiBro signal is lower than or equal to a threshold for a time period, an MT 1  enables a CDMA modem, determining that it is moving to a cellular network. The MT 1  performs location registration in the cellular network in the dual-activated state.  
         [0075]     In step  1205 , the MT 1  transmits a re-INVITE message for the current VoIP SIP session. The re-INVITE message has a Session ID of the current session, a Target Cell ID, and an Indicator indicating the ongoing handoff to the cellular network, all of which are included in a predetermined field of its Body. A P-CSCF performs a Network Initiated Session Release procedure for the MT 1  and the MT 2 , upon receiving a re-INVITE Request message with a handoff indicator.  
         [0076]     In step  1209 , an MGC transmits a FACDIR message indicating a start of a handoff procedure to an MSC of a target Cell ID network.  
         [0077]     In steps  1210  to  1212 , upon receiving the FACDIR message, the MSC transmits a Handoff Request message to a target BSC. The target BSC allocates resources of a target BTS, and then transmits a response message to a target MSC in response to the handoff request.  
         [0078]     In step  1213 , the MSC transmits an FACDIR message to the MGC to indicate its completed preparation for the handoff. In this case, the MSC transmits channel information of the target cell together. In step  1214 , upon receiving the FACDIR message, the MGC transmits a 2000K message indicating a start of handoff to the MT 1  along with the channel information.  
         [0079]     In step  1215 , the MT 1  transmits an ACK message in response to the 2000K message. In step  1216 , the MT 1  performs handoff to the cellular network using the channel information, and transmits a message indicating completion of the handoff procedure to the target BTS/BSC. In step  1217 , the target BSC delivers a message indicating completion of the handoff procedure to the target MSC.  
         [0080]     In step  1218 , the target MSC transmits a FACREL message to the MGC to request release of the trunk to the MGC. In step  1219 , upon receiving the FACREL message, the MGC changes the trunk into an idle state through communication with the MGW, and responds to the FACREL message.  
         [0081]      FIGS. 13A and 13B  are diagrams illustrating voice data paths before and after an MT 1  performs handoff according to the fourth exemplary embodiment of the present invention.  
         [0082]     Referring to  FIG. 13A , before an MT 1   1310  performs handoff to a WiBro network  1330 , a voice data path between the MT 1   1310  and an MT 2   1320  may pass through a cellular network  1340  and the WiBro network  1330 . For example, the voice data path between the MT 1   1310  and the MT 2   1320  may be established such that it sequentially passes through a RAS  1390 , an ACR  1385  and an MGW  1380 , the elements of the WiBro network  1330 , starting from the MT 1   1310  located in the WiBro network  1330 , and then passes again through an MSC  1370 , a BSC  1360 , a BTS  1350 , and the MT 2   1320 , the elements of the cellular network  1340 . The set voice data path changes to a voice data path shown in  FIG. 13B  after handoff.  
         [0083]     Referring to  FIG. 13B , after the MT 1   1310  performs handoff to the cellular network  1340 , both the MT 1   1310  and the MT 2   1320  are located in the cellular network  1340 . For example, the voice data path between the MT 1   1310  and the MT 2   1320  may be established through the BSC  1360  and the BTS  1350 , using the MSC  1370  located in the cellular network  1340  as the vertex.  
         [0084]     As can be understood from the foregoing description, the exemplary embodiments of the present invention propose a system and method for fast handoff process between a cellular network and a Portable Internet (WiBro), thereby securing competitive one-phone service.  
         [0085]     In addition, the exemplary embodiments of the present invention allow a circuit network service provider to simply launch wire/wireless integrated voice service.  
         [0086]     Further, the exemplary embodiments of the present invention can serve as a catalytic technology for activating the wire/wireless integrated voice service.  
         [0087]     While the invention has been shown and described with reference to a certain exemplary embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and the full scope of equivalents thereof.