Abstract:
A conventional handover method is performed in units of base stations, so that there is a problem in that a large amount of power for handover is consumed and implementation cost is high. 
     Through dedicated protocol message exchanges between a user equipment and a network or between networks, resource availability of candidate networks is checked in advance, a handover target network is selected from among the candidate networks according to a result of checking, and the selected target network is designated as a network to which the user equipment is handed over.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of International Application No. PCT/KR2007/000223, filed Jan. 12, 2007, and claims the benefit of Korean Application No. 10-2006-0109711, filed Nov. 7, 2006, and Korean Application No. 10-2007-0003073, filed Jan. 10, 2007, the disclosures of all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method and system for selecting a handover target network, and more particularly, to a method and system for selecting from among a plurality of networks a network to which handover of a user equipment having multiple external network interfaces is optimal by checking availability of resources of the plurality of networks which can be selected by the user equipment. 
     BACKGROUND ART 
     A method of implementing media irrelevant to handover (U.S. Patent Publication No. 20050249161) is related to the present invention. An object of this method is to implement handover optimization between wireless local area networks (LANs) or a wireless LAN and a global system for mobile communication (GSM) network. This method performs the handover on the basis of status information and link information on networks measured by a system through a handover policy function (HPF) and has an advantage of optimizing the handover between the wireless LANs or the wireless LAN and the GSM network. 
     A method of selecting a network in a wireless terminal is disclosed in U.S. Patent Publication No. 20060025169. An object of this method is to provide a wireless terminal for reducing power consumption of a wireless terminal by using a wireless network selection algorithm. This method has an advantage of reducing the power consumption of the wireless terminal by selecting a network on the basis of power requirements defined in a system and a user profile. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     The present invention provides a method and system used by a user equipment for selecting a handover target network capable of selecting handover candidate networks through predetermined protocol message exchanges, selecting a handover target network from among the candidate networks on a predetermined basis, and optimizing handover of the user equipment to the target network. 
     Technical Solution 
     Through dedicated protocol message exchanges between a user equipment and a network or between networks, resource availability of candidate networks is checked in advance, a handover target network is selected from among the candidate networks according to the result of the checking, and the selected target network is designated as a network to which the user equipment is handed over. 
     ADVANTAGEOUS EFFECTS 
     According to the present invention, in order to execute a handover, availability of resources of all of neighboring candidate networks neighboring a current network to which a user equipment is currently connected is checked. Thus, it is possible to precisely select an optimal target network in a heterogeneous handover circumstance. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a view showing a target network selection mechanism according to an embodiment of the present invention. 
         FIG. 1B  is a view showing message exchanges between elements, which are used for target network selection and handover execution shown in  FIG. 1A . 
         FIG. 1C  is a view showing detailed operations in a handover candidate network selection phase from among operations shown in  FIG. 1A . 
         FIG. 1D  is a view showing detailed operations in a handover target network decision phase from among operations shown in  FIG. 1A . 
         FIG. 2A  is a view showing a target network selection mechanism according to another embodiment of the present invention. 
         FIG. 2B  is a view showing message exchanges between elements, which are used for target network selection and handover execution shown in  FIG. 2A . 
         FIG. 2C  is a view showing detailed operations in a handover candidate network selection phase from among operations shown in  FIG. 2A . 
         FIG. 2D  is a view showing detailed operations in a handover target network decision phase from among operations shown in  FIG. 2A . 
         FIG. 3  is a view showing a system according to an embodiment of the present invention. 
     
    
    
     BEST MODE 
     According to an aspect of the present invention, there is provided a method of selecting a handover target network, comprising: (a) discovering handover object networks by a user equipment which wants to execute a handover; (b) selecting handover candidate networks from the discovered object networks by the user equipment; (c) deciding the only handover target network from among the selected candidate networks by the user equipment; and (d) executing the handover to the selected target network by the user equipment. 
     According to an another aspect of the present invention, there is provided a method of selecting a handover target network, comprising: (a) discovering handover object networks of a user equipment by a PoS (point of service) of a current network to which the user equipment which wants to execute a handover is currently connected; (b) selecting handover candidate networks from among the discovered object networks by the PoS; (c) deciding the only handover target network from among the selected candidate networks by the PoS; and (d) executing the handover of the user equipment to the decided target network by the PoS. 
     According to an aspect of the present invention, there is provided a system of selecting a handover target network, comprising: an object network discovery unit discovering handover object networks of a user equipment which wants to execute a handover; a candidate network selection unit selecting handover candidate networks from among the discovered object networks; and a target network decision unit deciding the only handover target network from among the selected candidate networks. 
     MODE FOR INVENTION 
     For the convenience of description, terms used in the present invention and the core technical solution of the present invention are firstly provided. 
     Terms used in the present invention are as follows. 
     A heterogeneous network environment means an environment including two or more networks having different characteristics. For example, a heterogeneous network is composed of a wireless local area network (LAN), a wireless broadband (Wibro), and a wireless code division multiple access (CDMA) network. 
     A current network means a network to which a user equipment is currently connected for network connection, that is, a network to which the user equipment is currently connected. 
     Neighboring networks mean networks neighboring the current network. 
     Candidate networks mean networks which can be selected as a next destination of the user equipment from among the neighboring networks, that is, networks to which the user equipment can be handed over. 
     A target network means the next destination of the user equipment from among the candidate networks, that is, a network which is selected as a handover target from among the candidate networks. 
     The core technical solution of the present invention is to check resource availability of candidate networks through protocol message exchanges between the user equipment and a network or between networks, select a handover target network from among the candidate networks according to the result of the checking, and determine the selected target network as a network to which the user equipment is handed over. 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings. Like reference numerals in the drawings denote like elements. 
       FIGS. 1A to 1D  are views showing a target network selection mechanism according to an embodiment of the present invention.  FIG. 1A  is a view showing the target network selection mechanism.  FIG. 1B  is a view showing message exchanges between elements, which are used for target network selection and handover execution.  FIGS. 1C and 1D  are views showing the mechanism shown in  FIG. 1A  in detail. 
     Referring to  FIG. 1B , a system for selecting a handover target network by checking resource availability of candidate networks in a heterogeneous network environment according to an embodiment of the present invention includes a user equipment (UE)  10 , a serving point of service (PoS)  11 , candidate PoSs  12 , a target PoS, and a network information server  13 . The target PoS is one of the candidate PoSs  12 . The UE  10  may be a mobile equipment or a fixed equipment which is movable between networks. The serving PoS  11  is a PoS of a current network to which the UE  10  is connected and supports the UE&#39;s selecting a handover target network in the heterogeneous network environment. 
     The candidate PoSs  12  are PoSs in candidate networks which have a possibility of being a handover target network of the UE  10 . The target PoS is a PoS of a network selected as the target network of the UE  10  from among the candidate PoSs  12 . Here, the PoSs may be implemented at a point of attachment (PoA) in a layer  2  where a wired or wireless connection function is performed or at an access router (AR) in a layer  3  where an Internet protocol (IP) connection function is performed. Otherwise, the PoSs may be disposed in a dedicated network device besides the PoA or the AR. 
     The network information server  13  is a server having various information on neighboring networks such as resources associated with the neighboring networks and provides the information on the neighboring networks to the UE  10  or the serving PoS  11  in order to support the UE&#39;s or the serving PoS&#39;s selecting a handover target network. 
     On the basis of the construction of the system, the target network selection by the mechanism shown in  FIG. 1A  is performed as follows. 
     A network discovery phase (operation S 10 ) for discovering handover object networks from among the neighboring networks is performed by the UE  10  which wants to execute the handover. More specifically, in operation S 10 , the UE  10  transmits a neighboring network information query message to the network information server  13  in order to request handover object network information, and the network information server  13  then transmits a handover object network information response message to the UE  10  in response to the request in order to transmit the handover object network information. Here, the neighboring network information acquired by the UE  10  from the network information server  13  includes a network type with respect to each of the neighboring networks (for example, IEEE 802.11, IEEE 802.16, CDMA, universal mobile telecommunication system (UMTS), or the like), a network connection device address (for example, a media access control (MAC) address and an IP address of the PoA or the AR), a network service provider, or the like. 
     After the network discovery phase (operation S 10 ), a handover candidate network selection phase (operation S 11 ) is performed. 
     The UE  10  which acquires the handover object network information from the network information server  13  in the network discovery phase (operation S 10 ) checks whether the handover is needed. In order to check whether the handover is needed, the UE  10  monitors in real-time quality and signal strength of the currently connected network and quality and signal strength of the heterogeneous network to which the UE  10  can be handed over (i.e., be connected). In this case, when the measurements of the monitored current network decrease to less than critical values, or the measurements of the heterogeneous network increase to more than critical values, the UE  10  determines that a handover is needed 
     When the UE  10  determines that a handover is needed, the UE  10  transmits a candidate resource query message to the serving PoS  11  in order to request the serving PoS  11  to check resource availability of the candidate networks for the handover of the UE  10 . Here, the UE  10  requests the serving PoS  11  to check the resource availability of only the candidate networks to which the UE  10  can be handed over from among the neighboring networks. The candidate networks are selected on the basis of the network types to which the UE  10  can be handed over and service subscribing information in the information on all of the neighboring networks acquired from the network information server  13  in the network discovery phase (operation S 10 ). 
     The serving PoS  11  is requested to check the resource availability of the candidate networks from the UE  10 , and transmits candidate resource query messages to the candidate PoSs  12  disposed in the corresponding candidate networks to query the availability of the resources that can support the UE  10 . 
     Each of the candidate PoSs  12  checks the availability of the resources that can support the handover of the UE  10  in its network, and transmits a candidate resource response message to the serving PoS  11  to respond to the query. The serving PoS  11  then collates responses from the candidate PoSs  12  of the candidate networks and transmits a candidate resource response message to the UE  10 . In this manner, the candidate network selection phase (operation S 11 ) is performed. 
     After the candidate network selection phase (S 11 ), a handover target network decision phase (operation S 12 ) is performed. 
     The UE  10  decides a target network on the basis of the resource availability of the candidate networks acquired from the serving PoS  11  in the candidate network selection phase (operation S 11 ). Here, as a specific basis of the target network decision, the target network may be automatically selected on a decision basis set in advance in the UE  10  (for example, quality of service (QoS) and cost etc.), or the target network may be decided by an input of a user of the UE  10  by querying the user. 
     Namely, when the number of the resource available candidate networks is two or more, the only final target network is automatically decided in consideration of the decision basis including the QoS and the cost. When a final target network decided on the basis of the QoS and the cost respectively is different from the decided final target network, a final target network is decided according to decision precedence. Here, the decision precedence is set in advance by the user or a manager. Otherwise, The user may be informed of an information on one or more candidate networks through a user interface in order to decide the final target network by a decision input of the user. 
     When the handover target network is decided in the target network decision phase (operation S 12 ), a handover execution phase (operation S 13 ) for handovering the UE  10  to the target network is performed. 
     After the UE  10  decides the final target network in the target network decision phase (operation S 12 ), the UE  10  attempts to handover (connect) to the corresponding target network. The UE  10  transmits a heterogeneous handover request message to the serving PoS  11  to request the serving PoS  11  to execute the handover to the decided target network. 
     The serving PoS  11  which is requested to execute the handover transmits a heterogeneous handover notification message to a target PoS in the decided target network from among the candidate PoSs  12  to notify the handover request of the UE  10 . The target PoS informs the serving PoS  11  of a response to the handover by transmitting a heterogeneous handover response message to the serving PoS  11 , and the serving PoS  11  informs the UE  10  of the message. In this manner, the handover of the UE  10  is executed. 
     After the handover execution phase (operation S 13 ) for handovering the UE  10  to the target network, a handover completing phase (operation S 14 ) is performed. The handover completing phase is performed by complete message exchanges between the UE  10  and the target PoS and between the serving PoS  11  and the target PoS. 
     More specifically, after the handover of the UE  10  is completed, the UE  10  notifies the target PoS of the handover completion by transmitting a handover complete notification message to the target PoS. The target PoS notifies the serving PoS  11  which is in the network before the handover of the UE  10  of the handover completion by transmitting a handover complete notification message to the serving PoS  11 . 
     Here, when the target PoS can perceive the handover completion without intervention of the UE  10 , the target PoS can arbitrarily notify the serving PoS  11  of the handover completion. The serving PoS  11  responds to the handover completion by transmitting a handover complete response message to the target PoS, and the target PoS transmits the response to the handover completion notification by transmitting a handover complete response message to the UE  10 . 
       FIG. 1C  is a view showing detailed operations in the handover candidate network selection phase (operation S 11 ) from among operations shown in  FIG. 1A . 
     After the UE  10  discovers and collects information on all of the neighboring networks (the handover object network) neighboring the current network (operation S 10 ), the UE  10  determines whether the handover is needed (operation S 111 ) in a preceding operation of the candidate network selection phase (operation S 11 ). A determining basis of the need is described above. When the need is determined, it is determined whether or not information on a specific neighboring network corresponds to an external network interface included in the UE  10  (operation S 112 ). Namely, it is determined whether or not the UE  10  can be connected to the specific neighboring network. 
     When the determination result is negative, the determination (operation S 112 ) is performed on another specific network. When the determination result is positive, it is determined whether or not information on the specific network corresponds to subscribing information on the UE  10  (operation S 113 ). When the determination result is negative, the determinations (operations S 112  and S 113 ) are performed on another specific network. When the determination result is positive, the specific network is designated as one of the candidate networks (operation S 114 ). 
     Namely, only when the specific network corresponds to the external network interface included in the UE  10  and the information on the specific network corresponds to the subscribing information on the UE  10 , the specific network can be selected as the candidate network. Operations S 112  to S 114  are repeatedly performed on all of the individual neighboring network information. Here, the external network interface and the subscribing information above as the basis of the candidate network selection are only an example, and other basis may be used to perform the same operations. 
       FIG. 1D  is a view showing detailed operations in the handover target network decision phase (operation S 12 ) from among the operations shown in  FIG. 1A . 
     It is determined whether or not the number of the resource available candidate networks, that is, the number of networks to which the UE  10  can be handed over, is two or more (operation S 121 ). When the determination result is that the number is two or more, information on all of the resource available candidate networks is checked to select the handover target network on the basis of the QoS and service usage cost set in the UE  10  (operation S 122 ). 
     When the determination result is that the number is not two or more, it means that there is no resource available candidate network or there is one resource available candidate network. When there is no resource available candidate network, the handover of the UE  10  does not occur, and when there is one resource available candidate network, the one resource available candidate network is determined as the only final target network (operation S 124 ). Next, it is determined whether or not the number of selected target networks is two or more (operation S 123 ). When the number is not two or more, it means that the selected target network is the only target network. Therefore, the selected target network is determined as the only final target network (operation S 124 ). When the number of the selected target networks is determined to be two or more, the only final target network is decided according to precedence of the aforementioned basis values. More specifically, when the QoS has precedence over the cost, a network satisfying the basis of the QoS is decided as the only final destination network (operation S 124 ). Here, the QoS and the cost described above as the basis of the target network decision are only an example, and other basis may be used to perform the same operations. 
       FIGS. 2A to 2D  are views showing a target network selection mechanism according to another embodiment of the present invention.  FIG. 2A  is a view showing the target network selection mechanism.  FIG. 2B  is a view showing message exchanges between elements, which are used for target network selection and handover execution.  FIGS. 2C and 2D  are views showing the mechanism shown in  FIG. 2A  in detail. 
     The embodiment shown in  FIGS. 2A to 2D  is different from the embodiment shown in  FIGS. 1A to 1D  in that in the latter, the target network selection mechanism execution is led by the UE  10 , however, in the former, the mechanism execution is led by the serving PoS  11 . 
     A network discovery phase (operation S 20 ) for discovering handover object networks from among neighboring networks is performed by the serving PoS  11  in the current network to which the UE  10  which wants to execute the handover is currently connected. More specifically, in operation S 20 , the serving PoS  11  transmits a neighboring network information query message to the network information server  13  in order to request handover object network (neighboring network) information, and the network information server  13  then transmits a neighboring network information response message to the serving PoS  11  in response to the request in order to transmit the neighboring network information. 
     Here, the neighboring network information acquired by the serving PoS  11  from the network information server  13  includes a network type with respect to each of the neighboring networks (for example, IEEE 802.11, IEEE 802.16, CDMA, UMTS, or the like), a network connection device address (for example, a MAC address and an IP address of the PoA and the AR), a network service provider, or the like. 
     After the network discovery phase (operation S 20 ), a handover candidate network selection phase (operation S 21 ) is performed. 
     The serving PoS  11  which acquires the neighboring network information from the network information server  13  in the network discovery phase (operation S 20 ) checks whether the handover is needed. In order to check whether the handover is needed, the serving PoS  11  monitors in real time quality and signal strength of the current network to which the UE  10  is currently connected and quality and signal strength of the neighboring heterogeneous network to which the UE  10  can be handed over (i.e., be connected). In this case, when the measurements of the monitored current network decrease to less than critical values, or the measurements of the heterogeneous network increase to more than critical values, the serving PoS  11  determines that a handover is needed. 
     When the serving PoS  11  determines that a handover is needed, the serving PoS  11  transmits a handover intention query message to the UE  10  in order to query handover intention of the UE  10 , network interface information included in the UE  10 , and subscribing information on each of the networks. The UE  10  sets a possibility of the handover and heterogeneous network handover intention of the UE  10  in response to the query, and transmits a handover intention response message including the network interface information and the network subscribing information included in the UE  10  to the serving PoS  11 . 
     The serving PoS  11  checks the heterogeneous network handover intention of the UE  10  from the handover intention response message. When the UE  10  has the heterogeneous network handover intention, information on candidate networks to which the UE  10  can be handed over is extracted from the information on all of the neighboring networks acquired from the network information server  13  on the basis of the network interface information and the network subscribing information included in the UE  10 , in order to select the handover candidate networks. 
     For the selection, the serving PoS  11  transmits candidate resource query messages to candidate PoSs  12  disposed in the candidate networks to query availability of resources that can support the UE  10 . Each of the candidate PoSs  12  checks the availability of the resources that can support the UE  10  in its network and transmits a candidate resource response message to the serving PoS  11  in response to the query of the serving PoS  11 . The serving PoS  11  then collates responses from the candidate PoSs  12  of the candidate networks and selects candidate PoSs to which the UE  10  can be handed over. In this manner, the candidate network selection phase (operation S 21 ) is performed. 
     After the candidate network selection phase (S 21 ), a handover target network decision phase (operation S 22 ) is performed. 
     The serving PoS  11  decides a handover target network on the basis of the information on the checked candidate PoSs. When the number of the selected resource available candidate networks is two or more, the only final target network is decided in consideration of service cost and QoS used for connection to a corresponding network. When the number of the final target networks selected on the basis of the QoS and service cost respectively is two or more, the only target network is decided according to a basis having precedence. The precedence is set in advance by a network manager. 
     When the handover target network is decided in the target network decision phase (operation S 22 ), a handover execution phase (operation S 23 ) for handovering the UE  10  to the target network is performed. 
     After the serving PoS  11  decides the only final target network, the serving PoS  11  transmits a heterogeneous handover request message to the UE  10  to request the UE  10  to execute the handover. Simultaneously, the serving PoS  11  transmits a heterogeneous handover notify message to the target PoS in order to notify the target PoS of the handover of the UE  10  to the target network. 
     The UE  10  transmits a heterogeneous handover response message to the serving PoS  11  to notify the serving PoS  11  of the connection (handover) to the final target network in response to the heterogeneous handover request message from the serving PoS  11 . The target PoS transmits a heterogeneous handover response message to the serving PoS  11  to notify the serving PoS  11  of the connection of the UE  10  to the final target network in response to the heterogeneous handover notify message. In this manner, the handover execution phase (operation S 23 ) is performed. 
     After the handover execution phase (operation S 23 ) for handovering the UE  10  to the target network, a handover completing phase (operation S 24 ) is performed. The handover completing phase is performed by complete message exchanges between the serving PoS  11  and the target PoS. 
     More specifically, after the handover of the UE  10  is completed, the target PoS notifies the serving PoS  11  of the handover completion by transmitting a handover compete notification message, and the serving PoS  11  transmits a handover complete response message to the target PoS as a response. In this manner, the handover completing phase is performed. 
       FIG. 2C  is a view showing detailed operations in the handover candidate network selection phase (operation S 21 ) from among operations shown in  FIG. 2A . 
     After the serving PoS  11  discovers and collects information on all of the neighboring networks neighboring the current network (operation S 20 ), the serving PoS  11  perceives an external network interface of the UE  10  and network subscribing information (operation S 211 ). Next, the serving PoS  11  determines whether the handover is needed (operation S 212 ) in a preceding operation of the candidate network selection phase (operation S 21 ). The determining basis of the need is described above. 
     When the need is determined, it is determined whether or not UE  10  intends to handover (operation S 213 ). 
     According to a result of determining, when the UE  10  has the handover intention, it is determined whether or not a specific neighboring network corresponds to the perceived external network interface (operation S 214 ). Namely, it is determined whether or not the UE  10  can be connected to the specific neighboring network. 
     When the determination result is negative, the determination (operation S 214 ) is performed on another specific network. When the determination result is positive, it is determined whether or not information on the specific network corresponds to the perceived subscribing information (operation S 215 ). When the determination result is negative, the determinations (operation S 214  and S 215 ) are performed on another specific network. When the determination result is positive, the specific network is selected as one of the candidate networks (operation S 216 ). 
     Namely, only when the specific network corresponds to the external network interface included in the UE  10  and the information on the specific network corresponds to the subscribing information on the UE  10 , the specific network can be selected as the candidate network. Operations S 214  to S 216  are repeatedly performed on all of the individual neighboring network information. Here, the external network interface and the subscribing information above as the basis of the candidate network selection are only an example, and other basis may be used to perform the same operations. 
       FIG. 2D  is a view showing detailed operations in the handover target network decision phase (operation S 22 ) from among the operations shown in  FIG. 2A . 
     It is determined whether or not the number of the resource available candidate networks, that is, the number of networks to which the UE  10  can be handed over, is two or more (operation S 221 ). When the determination result is that the number is two or more, information on all of the resource available candidate networks is checked to select the handover target network on the basis of the QoS and service usage cost set in the UE  10  (operation S 222 ). 
     When the determination result is that the number is not two or more, it means that there is no resource available candidate network or there is one resource available candidate network. When there is no resource available candidate network, the handover of the UE  10  does not occur, and when there is one resource available candidate network, the one resource available candidate network is determined as the only final target network (operation S 224 ). 
     Next, it is determined whether or not the number of selected target networks is two or more (operation S 223 ). When the number is not two or more, it means that the selected target network is the only target network. Therefore, the selected target network is determined as the only final target network (operation S 224 ). When the number of the selected target networks is determined to be two or more, the only final target network is decided according to precedence of the aforementioned basis values. More specifically, when the QoS has precedence over the cost, a network satisfying the basis of the QoS is decided as the only final destination network (operation S 224 ). Here, the QoS and the cost described above as the basis of the target network decision are only an example, and other basis may be used to perform the same operations. 
       FIG. 3  is a view showing a system according to an embodiment of the present invention. 
     The system according to the embodiment shown in  FIG. 3  can be implemented by including the mechanisms shown in  FIGS. 1A to 2D . 
     An object network discovery unit  31  is a unit for discovering handover object networks of the UE  10  which wants to execute handover from among neighboring networks. The object network discovery unit  31  requests object network information of the network information server  34  having the object network information, and the network information server  34  then transmits the object network information to the object network discovery unit  31  in response to the request so as to enable the object network discovery unit  31  to discover the object networks. 
     A candidate network selection unit  32  is a unit for selecting handover candidate networks from among the object networks discovered by the object network discovery unit  31 . For the candidate network selection, firstly, a handover need determining unit  321  determines a need of handover of the UE  10 . When the need is determined, an information correspondence determining unit  322  determines whether or not the object network information corresponds to information included in the UE  10 . 
     Next, when the object network information corresponds to the information included in the UE  10 , a candidate network designation unit  323  designates the corresponding object network as one of the candidate networks to select the candidate networks. 
     Here, the need determining unit  321  monitors in real time quality and signal strength of the current network to which the UE  10  is currently connected and quality and signal strength of the object networks. In this case, when the quality and the signal strength of the current network decrease to less than critical values, or the quality and the signal strength of the object networks increase to more than critical values, whether or not a handover is needed is determined. 
     The information correspondence determining unit  322  determines whether or not the object network corresponds to an external network interface included in the UE  10  and the object network information corresponds to subscribing information on the UE  10 . A candidate network designation unit  323  designates the object network as the candidate network only when the object network corresponds to the external network interface of the UE  10  and the object network information corresponds to the subscribing information on the UE  10 . 
     A target network decision unit  33  is a unit for deciding the handover target network from among the candidate networks selected by the candidate network selection unit  32  and decides the only final target network from among the selected candidate networks. 
     A candidate network number determining unit  331  determines whether or not the number of the candidate networks selected by the candidate network selection unit  32  is plural. When the determined number is plural, a target network selection unit  332  selects a target network from among the all of the candidate networks with reference to the information on all of the selected candidate networks and the decision basis of the target network set in the UE  10 . 
     Here, the decision basis of the target network includes, as described above with reference to  FIGS. 1A to 2D , the QoS and the service usage cost set in the UE  10 . 
     When the number of the target networks selected by the target network selection unit  332  is two or more, a final decision unit  333  decides the only target network according to precedence of the decision basis. When it is determined that the number of the selected target networks is two or more, the only target network is decided according to precedence of the aforementioned two basis values. More specifically, when the QoS has precedence over the cost, a network satisfying the basis of the QoS is decided as the only final destination network. Here, the QoS and the cost described above as the basis of the target network decision are only example, and other basis may be used to perform the same operations. 
     When the number of the selected candidate networks is one, the target decision unit  33  decides the one candidate network as the only target network. 
     The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments 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. The exemplary embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.