Patent Publication Number: US-2019182761-A1

Title: Base station device, terminal device, location management device, and control method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of copending application Ser. No. 15/517,145, filed on Apr. 5, 2017, which is the National Phase under 35 U.S.C. § 371 of International Application No. PCT/JP2015/074483, filed on Aug. 28, 2015, which claims the benefit under 35 U.S.C. § 119 (a) to Patent Application No. 2014-205334, filed in Japan on Oct. 6, 2014, all of which are hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a base station device, a terminal device, a location management device, and a control method. 
     BACKGROUND ART 
     The 3rd Generation Partnership Project (3GPP), which undertakes activities for standardizing recent mobile communication systems, is in the process of creating specifications for the Evolved Packet System (EPS), which realizes an all-IP-based architecture (see NPL 1). EPS is a mobile communication system through which mobile operators and the like provide mobile telephone services, and is constituted of a core network called the Evolved Packet Core (EPC), an access network based on the Long Term Evolution (LTE) radio communication standard, and the like. 
     In developing the specifications for EPS, the 3GPP is further considering dedicated core networks (DECOR) for the purpose of overlaying core networks. According to DECOR, a communication system has multiple networks overlaid therein, and terminal devices that connect to different networks in a manner that depends on, for example, their respective terminal types. This configuration makes it possible to distribute terminal traffic in a manner that depends on the terminal type, and manage traffic in a manner that depends on the terminal type. 
     For example, the configuration can accommodate machine to machine (M2M) terminals by connecting to an M2M-dedicated core network, which is more prevalent recently, whereas normal mobile phone terminals such as smartphones connect to a main core network. 
     CITATION LIST 
     Non Patent Literature 
     NPL 1: 3GPP TS23.401 Technical Specification Group Services and System Aspects, General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access (Release 12) 
     SUMMARY OF INVENTION 
     Technical Problem 
     DECOR asserts that the main core network is separated from overlaid core networks in a manner that depends on the characteristics of terminal devices. These core networks are configured to include different mobile management devices. 
     As such, it is necessary to appropriately select the core network to which the terminal device will connect to based on the type or characteristics of the terminal. To do so, it is necessary to appropriately select a location management device based on the type or characteristics of the terminal. 
     However, at present, there is no clear method for appropriately selecting a core network based on the type or characteristics of the terminal, and connecting the terminal to the selected core network. 
     In light of such circumstances, an object of the present invention is to provide a favorable method for a terminal, in a mobile communication system constituted of multiple overlaid core networks, to connect to an appropriate core network and perform mobile communication. 
     Solution to Problem 
     A base station device according to the present invention is a base station device connected to a first core network including a first location management device and a second core network including a second location management device. The base station device is configured to: receive, from the first location management device, a context release message or a route reconfiguration request message including at least first identification information, the first identification information being identification information that gives an instruction or a request for connection to the second core network; receive an attach request message or a tracking area update request message from a terminal device; select the second location management device based on the first identification information; and transmit the attach request message or the tracking area update request message to the selected second location management device. 
     In the base station device according to the present invention, the first identification information is a core network type associated with the second core network. 
     In the base station device according to the present invention, the first identification information is group identification information on a location management device included in the second core network. 
     The base station device according to the present invention is configured to: start a timer upon reception of the context release request message; select the second location management device based on the first identification information when the timer is in operation; and transmit the attach request message or the tracking area update request message to the first location management device when the timer is not in operation. 
     The base station device according to the present invention is configured to: transmit an RRC connection release request message to the terminal device on the basis of reception of the context release request message; and include, in the RRC connection release request message, at least identification information and second identification information that gives an instruction or a request for connection to the second core network. 
     A terminal device according to the present invention is configured to: receive a detach request message including at least first identification information and information indicating that attach is required, from a first location management device included in a first core network, the first identification information being identification information that gives an instruction or a request for connection to a second core network; transmit, to a base station device, an attach request message with at least second identification information included in the attach request message to start an attach procedure triggered by the reception of the detach message, the second identification information being identification information that gives an instruction or a request for connection to the second core network; and establish, upon completion of the attach procedure, a packet data network (PDN) connection with a gateway device included in the second core network. 
     A terminal device according to the present invention is configured to: receive a paging message including at least first identification information from a first location management device included in a first core network, the first identification information being identification information that gives an instruction or a request for connection to a second core network; transmit a service request message to the first location management device triggered by the reception of the paging message; receive a service reject message from the first location management device, the service reject message being a response to the service request message and including at least identification information indicating a detached state; transmit, to a base station device, an attach request message with at least second identification information included in the attach request message to start an attach procedure triggered by the reception of the service reject message, the second identification information being identification information that gives an instruction or a request for connection to the second core network; and establish, upon completion of the attach procedure, a packet data network (PDN) connection with a gateway device included in the second core network. 
     In the terminal device according to the present invention, the first identification information and/or the second identification information is a core network type associated with the second core network. 
     In the terminal device according to the present invention, the first identification information and/or the second identification information is comprised of group identification information on a location management device included in the second core network. 
     A location management device according to the present invention is a location management device included in a first core network. The location management device is configured to: transmit a context release request message or a route reconfiguration request message including at least first identification information to a base station device, the first identification information being identification information that gives an instruction or a request for connection to a second core network; and receive, from the base station device, a response message to the context release request message or the route reconfiguration request message. 
     The location management device according to the present invention is configured to transmit a detach request message or a paging message including at least the first identification information to a terminal device; and transmit the context release request message or the route reconfiguration request message after the transmission of the detach request message or the paging message. 
     In the location management device according to the present invention, the first identification information is a core network type associated with the second core network. 
     In the location management device according to the present invention, the first identification information is comprised of group identification information on a location management device included in the second core network. 
     A method for controlling a base station device according to the present invention involves the controlling of a base station device connected to a first core network including a first location management device and a second core network including a second location management device. The method includes the steps of: receiving, from the first location management device, a context release request message or a route reconfiguration request message including at least first identification information, the first identification information being identification information that gives an instruction or a request for connection to the second core network; receiving an attach request message or a tracking area update request message from a terminal device; and selecting the second location management device based on the first identification information and transmitting the attach request message or the tracking area update request message to the selected location management device. 
     A method for controlling a base station device according to the present invention is a method for controlling a base station device connected to a first core network including a first location management device and a second core network including a second location management device. The method includes the steps of: receiving, from the first location management device, a context release request message or a route reconfiguration request message including at least first identification information, the first identification information being identification information that gives an instruction or a request for connection to the second core network; starting a timer upon reception of the context release request message; receiving an attach request message or a tracking area update request message from a terminal device; selecting the second location management device based on the first identification information when the timer is in operation and transmitting the attach request message or the tracking area update request message to the selected location management device; and transmitting the attach request message or the tracking area update request message to the first location management device when the timer is not in operation. 
     With regards to the method for controlling a base station device according to the present invention, the first identification information is a core network type associated with the second core network. 
     With regards to the method for controlling a base station device according to the present invention, the first identification information is a group identification information on a location management device included in the second core network. 
     The method for controlling a base station device according to the present invention includes the steps of: transmitting an RRC connection release request message to the terminal device triggered by the reception of the context release request message; and including, in the RRC connection release request message, at least identification information and second identification information that gives an instruction or a request for connection to the second core network. 
     A method for controlling a terminal device according to the present invention includes the steps of: receiving a detach request message including at least first identification information and information indicating that attach is required, from a first location management device included in a first core network, the first identification information being identification information that gives an instruction or a request for connection to a second core network; transmitting, to a base station device, an attach request message with at least second identification information included in the attach request message to start an attach procedure on the basis of reception of the detach message, the second identification information being identification information that gives an instruction or a request for connection to the second core network; and establishing, upon completion of the attach procedure, a packet data network (PDN) connection with a gateway device included in the second core network. 
     A method for controlling a terminal device according to the present invention includes the steps of: receiving a paging message including at least first identification information from a first location management device included in a first core network, the first identification information being identification information that gives an instruction or a request for connection to a second core network; transmitting a service request message to the first location management device triggered by reception of the paging message; receiving a service reject message from the first location management device, the service reject message being a response to the service request message and including at least identification information indicating a detached state; transmitting, to a base station device, an attach request message with at least second identification information included in the attach request message to start an attach procedure triggered by the reception of the service reject message, the second identification information being identification information that gives an instruction or a request for connection to the second core network; and establishing, upon completion of the attach procedure, a packet data network (PDN) connection with a gateway device included in the second core network. 
     With regards to the method for controlling the terminal device according to the present invention, the first identification information and/or the second identification information is a core network type associated with the second core network. 
     With regards to the method for controlling the terminal device according to the present invention, the first identification information and/or the second identification information is a group identification information on a location management device included in the second core network. 
     A method for controlling a location management device according to the present invention is a method for controlling a location management device included in a first core network, which includes the steps of: transmitting a context release request message or a route reconfiguration request message including at least first identification information to a base station device, the first identification information being identification information that gives an instruction or a request for connection to a second core network; and receiving, from the base station device, a response message to the context release request message or the route reconfiguration request message. 
     The method for controlling a location management device according to the present invention includes the steps of: transmitting a detach request message or a paging message including at least the first identification information to a terminal device; and transmitting the context release request message or the route reconfiguration request message after the transmission of the detach request message or the paging message. 
     With regards to the method for controlling a location management device according to the present invention, the first identification information is a core network type associated with the second core network. 
     With regards to the method for controlling a location management device according to the present invention, the first identification information is a group identification information on a location management device included in the second core network. 
     Advantageous Effects of Invention 
     According to the present invention, procedures whereby a terminal device switches core networks and connects to one of the core networks in a communication network constituted of overlaid core networks can be implemented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an overview of a mobile communication system. 
         FIGS. 2A and 2B  are diagrams illustrating the configuration of an IP mobile communication network and the like. 
         FIGS. 3A to 3C  are diagrams illustrating the functional configuration of an eNB. 
         FIGS. 4A to 4C  are diagrams illustrating the functional configuration of an MME. 
         FIG. 5  is a diagram illustrating a first procedure according to an embodiment. 
         FIG. 6  is a diagram illustrating a configuration example of the mobile communication system. 
         FIG. 7  is a diagram illustrating a second procedure according to the embodiment. 
         FIG. 8  is a diagram illustrating a third procedure according to the embodiment. 
         FIG. 9  is a diagram illustrating a fourth procedure according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, a preferred embodiment for carrying out the present invention will be described with reference to the drawings. Note that the present embodiment describes an example of a mobile communication system to which the present invention is applied. 
     1. First Embodiment 
     Hereinafter, a radio communication technology according to an embodiment of the present invention will be described in detail with reference to the drawings. 
     1.1 System Overview 
       FIG. 1  is a diagram illustrating an overview of a mobile communication system according to the present embodiment. As illustrated in  FIG. 1 , a mobile communication system  1  is constituted of a mobile terminal device UE  10 , an access network  80 , a core network (type  1 )  90 , a core network (type  2 )  92 , a packet data network (PDN)_A  100 , and a PDN_B  102 . 
     Here, “core network” refers to an IP mobile communication network run by a mobile operator. Note that the core network (type  1 )  90  and the core network (type  2 )  92  may be networks that are overlaid in the mobile communication system  1 . 
     The core network (type  1 )  90  and the core network (type  2 )  92  may be core networks run and managed by a single mobile operator. Alternatively, these core networks may be core networks run and managed by respective mobile operators. For example, the core network (type  1 )  90  may be a core network for the mobile operator that runs and manages the mobile communication system  1 , whereas the core network (type  2 )  92  may be a core network for a virtual mobile operator such as a mobile virtual network operator (MVNO). Thus, the core network (type  2 )  92  may be configured as a virtual mobile operator network. Here, “type”, such as type  1  and type  2 , may refer to information identifying a core network. The information identifying a core network is not limited to “type” and may be any information capable of identifying a core network. For example, identification numbers assigned to core networks by network operators may be used to identify multiple overlaid core networks. Alternatively, when multiple mobile network operators including an MVNO have respective core networks, the core networks may be identified by information identifying the mobile operators. 
     Furthermore, the UE  10  may have a core network type assigned thereto in advance. For example, the UE  10  may be managed as a terminal that can be connected to a type  2  core network. The UE  10  may be managed as a terminal that can be connected to a type  1  core network in a place where the type  2  core network is not provided. 
     Such management information may be stored in a control device, such as a home subscriber server (HSS) or a mobility management entity (MME), in the core networks as subscriber information. The UE  10  may also store the management information. 
     A UE to which a core network type has been assigned and a UE to which a core network type has not been assigned may coexist. The UE to which a core network type has not been assigned may connect to a default core network. The core network which serves as the default core network may be managed by the mobile operator or the like in advance, and the default network may be determined in accordance with such management information. For example, a PDN connection may be established with the core network (type  1 )  90 , which is a main core network, serving as the default network. On the other hand, the UE to which a core network type has been assigned may connect to a dedicated core network corresponding to the specified type to establish a PDN connection. 
     Furthermore, such types may be set to each UE before terminal shipment. Additionally, multiple types may be set before shipment, and a user or the UE itself may then change the type in accordance with the purpose of communication. 
     The core network (type  1 )  90  includes a mobility management entity (MME)_A  40 . 
     Likewise, the core network (type  2 )  92  includes an MME_B  42 . 
     Each core network is connected to a corresponding packet data network (PDN). The PDN is a packet data service network providing communication services to the UE  10 , and may be constituted for each of the services. The PDN has a communication terminal connected thereto, and the UE  10  can transmit/receive user data to/from the communication terminal deployed in the PDN. 
     For example, as illustrated in  FIG. 1 , the core network (type  1 )  90  may be connected to the PDN_A  100 , and the core network (type  2 )  92  may be connected to the PDN_B  102 . As described above, the core networks may be connected to different PDNs. 
     Furthermore, the PDN_A  100  and the PDN_B  102  may be packet data networks, outside the core networks, corresponding to the purposes of the UE. For example, when the core network (type  1 )  90  is a main core network, the PDN_A  100  may be an IP multimedia subsystem (IMS) or the like. Furthermore, when the core network (type  2 )  92  is a core network dedicated to M 2 M terminals, the PDN_B  102  may be an M2M service network. 
     Next, some examples of core network constitutions will be described. Each core network is constituted of a home subscriber server (HSS), an authentication, authorizing, accounting (AAA), a policy and charging rules function (PCRF), a packet data network gateway (PGW), an enhanced packet data gateway (ePDG), a serving gateway (SGW), and an MME. 
     Such devices may be deployed in each core network. Alternatively, overlaying the core networks is intended to distribute control message traffic or user data traffic, which allows the management devices such as the HSS, the AAA and the PCRF to be shared by multiple core networks instead of being deployed in each core network. 
     Furthermore, when the overlaying of the core networks is only intended to distribute control message traffic, devices that forward user data such as the PGW and the SGW may also be shared by the multiple core networks instead of being deployed in each core network. 
     Note that the present embodiment will be mainly described with reference to an example in which the core network (type  1 )  90  is constituted of the HSS  50 , the AAA  55 , the PCRF  60 , the PGW_A  30 , the ePDG  65 , the SGW_A  35 , and the MME_A  40  as illustrated in  FIGS. 2A and 2B . 
     In addition, the present embodiment will be mainly described with reference to an example in which the core network (type  2 )  92  is constituted of the HSS  50 , the AAA  55 , the PCRF  60 , the PGW_B  32 , the ePDG  65 , the SGW_B  37 , and the MME_B  42 . 
     In other words, the HSS  50 , the AAA  55 , the PCRF  60 , and the ePDG  65  are shared by the core networks. Moreover, the present embodiment will be mainly described with reference to an example in which the MME, the PGW, and the SGW are deployed in each core network. 
     Additionally, each core network can connect to multiple access networks (an LTE AN  80 , a WLAN ANb  75 , and a WLAN ANa  70 ). 
     Such a radio access network may be configured with connections to multiple different access networks, or may be configured with a connection to any one of the access networks. Furthermore, the UE  10  can wirelessly connect to the radio access network. 
     Furthermore, the WLAN access network b (WLAN ANb  75 ) that connects to the core network via the ePDG  65  and the WLAN access network a (WLAN ANa  75 ) that connects to the PGW (PGW_A  30 , PGW_B  32 ), the PCRF  60 , and the AAA  55  can be configured as access networks that can connect in a WLAN access system. 
     Note that each device has the same configuration as conventional devices in a mobile communication system using EPS, and thus detailed descriptions will be omitted. Each device will be described briefly hereinafter. 
     The PGW_A  30  is connected to the PDN_A  100 , the SGW_A  35 , the ePDG  65 , the WLAN ANa  70 , the PCRF  60 , and the AAA  55  and is a relay device that forwards user data by functioning as a gateway device between the PDN_A  100  and the core network  90 . 
     The SGW_A  35  is connected to the PGW_A  30 , the MME_A  40 , and the LTE AN  80 , and is a relay device that forwards user data by functioning as a gateway device between the core network (type  1 )  90  and the LTE AN  80 . 
     The MME_A  40  is connected to the SGW_A  35 , the LTE AN  80 , and the HSS  50  and is an access control device that is responsible for location information management and access control for the UE  15  via the LTE AN  80 . The core network (type  1 )  90  may be configured to include multiple location management devices. For example, an MME_C may be configured as a location management device different from the MME_A  40 . Like the MME_A  40 , the MME_C may be connected to the SGW_A  35 , the LTE AN  80 , and the HSS  50 . 
     Additionally, the MME_C and the MME_A  40  may be connected to each other. This configuration allows the MME_C and the MME_A  40  to transmit/receive the context of the UE  10  to/from each other. 
     The HSS  50  is connected to the MME_A  40  and the AAA  55  and is a managing node that manages subscriber information. The subscriber information in the HSS  50  is referred to during MME_A  40  access control, for example. 
     The HSS  50  may also be connected to the MME_B  42 . As illustrated in  FIG. 1 , the MME_B  42  is included in a network overlaid on the core network (type  1 )  90 , namely the core network (type  2 )  92 . 
     In other words, the HSS  50  may be connected to an MME, such as the MME_B  42 , included in a core network different from the core network in which the HSS  50  is included. Furthermore, the subscriber information in the HSS  50  is also referred to during MME_B  42  access control. 
     The AAA  55  is connected to the PGW_A  30 , the HSS  50 , the PCRF  60 , and the WLAN ANa  70 , and is responsible for access control for the UE  10  connected via the WLAN ANa  70 . 
     The PCRF  60  is connected to the PGW_A  30 , the WLAN ANa  75 , the AAA  55 , and the PDN  100 , and manages QoS for data delivery. For example, the PCRF  60  manages QoS of a communication path between the UE  10  and the PDN_A  100 . 
     The ePDG  65  is connected to the PGW_A  30  and the WLAN ANb  75 , and delivers user data by functioning as a gateway device between the core network (type  1 )  90  and the WLAN ANb  75 . 
     As illustrated in  FIG. 2B , each radio access network includes devices to which the UE  10  is actually connected (such as a base station device and an access point device), and the like. The devices used in these connections can be thought of as devices adapted to the radio access networks. 
     In the present embodiment, the LTE AN  80  is configured to include an eNB  45 . The eNB  45  is a radio base station to which the UE  10  connects in an LTE access system, and the LTE AN  80  may be configured to include one or multiple radio base stations. 
     The WLAN ANa  70  is configured to include a WLAN APa  72  and a gateway (GW)  74 . The WLAN APa  72  is a radio base station to which the UE  10  connects in a WLAN access system trusted by the operator running the core network (type  1 )  90 , and the WLAN ANa  70  may be configured to include one or multiple radio base stations. The GW  74  is a gateway device between the core network (type  1 )  90  and the WLAN ANa  70 . The WLAN APa  72  and the GW  74  may be configured as a single device. 
     Even when the operator that runs the core network (type  1 )  90  is different from the operator that runs the WLAN ANa  70 , contracts or agreements between the operators may allow such a configuration. 
     Additionally, the WLAN ANb  75  is configured to include a WLAN APb  76 . The WLAN APb  76  is a radio base station to which the UE  10  connects in a WLAN access system when a trust relationship cannot be established with the operator that runs the core network (type  1 )  90 , and the WLAN ANb  75  may be configured to include one or multiple radio base stations. 
     As described above, the WLAN ANb  75  is connected to the core network (type  1 )  90  via the ePDG  65 , which is a device included in the core network (type  1 )  90 , serving as a gateway. The ePDG  65  has a security function for ensuring security. 
     Next, each device included in the core network (type  2 )  92  will be described briefly. 
     The PGW_B  32  is connected to the PDN_B  102 , the SGW B_ 37 , the ePDG  65 , the WLAN ANa  70 , the PCRF  60 , and the AAA  55 , and is a relay device that forwards user data by functioning as a gateway device between the PDN  102  and the core network  92 . 
     The SGW_B  37  is connected to the PGW_B  32 , the MME_B  42 , and the LTE AN  80 , and is a relay device that forwards user data by functioning as a gateway device between the core network (type  2 )  92  and the LTE AN  80 . 
     The MME_B  42  is connected to the SGW_B  37 , the LTE AN  80 , and the HSS  50 , and is an access control device that is responsible for location information management and access control for the UE  10  via the LTE AN  80 . The core network (type  2 )  92  may be configured to include multiple location management devices. For example, an MME_D may be configured as a location management device different from the MME_B  42 . Like the MME_B  42 , the MME_D may be connected to the SGW_B  37 , the LTE AN  80 , and the HSS  50 . 
     Additionally, the MME_D and the MME_B  42  may be connected to each other. This configuration allows the MME_D and the MME_B  42  to transmit/receive the context of the UE  10  to/from each other. 
     The HSS  50  is connected to the MME_B  42  and the AAA  55  and is a managing node that manages subscriber information. The subscriber information in the HSS  50  is referred to during MME_B  42  access control, for example. 
     The AAA  55 , the PCRF  60  and the ePDG  65  has been already described; thus, the description thereof will be omitted. 
     Note that in the present specification, the UE  10  connected to each radio access network refers to the UE  10  connected to a base station device, an access point, or the like included in each radio access network, causing data, signals, and the like, which are to be transmitted/received, to go through those base station devices, access points, or the like. 
     1.2. Device Configuration 
     Next, the configuration of each device will be described briefly with reference to the drawings. 
     1.2.1. eNB Block Diagram 
       FIG. 3A  is a block diagram illustrating the eNB  45 . As illustrated in  FIG. 3A , the eNB  45  is constituted of an IP mobile communication network interface unit  220 , a control unit  200 , and a storage  240 . The IP mobile communication network interface unit  220  and the storage  240  are connected to the control unit  200  via a bus. 
     The control unit  200  is a function unit for controlling the eNB  45 . The control unit  200  reads out and executes various programs stored in the storage  240  to perform various processes. 
     The IP mobile communication network interface unit  220  is a function unit for the eNB  45  to connect with the UE  10  and the UE_B  15 , and with the core network (type  1 )  90  and the core network (type  2 )  92 . 
     The storage  240  stores MME information  242 . The MME information  242  contains MMEs to which the eNB  45  can connect. For example, as illustrated in  FIG. 3B , the MME information  242  may contain identification information identifying an MME to which the eNB  45  can connect and area information identifying the service area of each MME with the identification information and the area information associated with each other. The example  FIG. 3B  indicates that different MMEs are stored for each service area and a base station device deployed in each service area can connect to the MMEs associated with that service area. Note that MMEs included in different types of core networks may be stored for the same service area. For example, an MME belonging to a type  1  core network and an MME belonging to a type  2  core network may be stored with both of the MMEs associated with a service area 
     Here, the MME identification information may be any information capable of identifying an MME, and thus may be a globally unique MME identifier (GUMMEI), for example. The GUMMEI is MME identification information, which includes a public land mobile network (PLMN) that identifies a mobile operator network, an MME group number identifying multiple MMEs, and an MME code identifying an individual MME. 
     Note that the MME group identity may be an MME group identifier (MMEGI). Note that different MMEGIs for each core network type may be stored. In this case, the core network can be identified by the MME group number. Furthermore, with a core network associated with the MME group number, MMEs belonging to the associated core network may be managed as a group. Thus, in accordance with the MME identification information and the MME group number, the type of core network including the corresponding MMEs may be identified. 
     Alternatively, type information on the core network may be expressed by an information element independent from the MME identification information. For example, as illustrated in  FIG. 3C , the MME information  242  may contain type information that is core network identification information, area information, and MME identification information with all the pieces of information associated with each other. This makes it possible to configure areas based on the core network type. Moreover, an MME deployed in each area can be stored. Furthermore, an MME can be stored for each type of core network. Furthermore, an MME deployed in a specific area of a specific core network can be stored. 
     Note that the MME identification information may be the GUMMEI, as described above. 
     With regards to the method for managing the MME identification information, the area information does not necessarily have to be stored, and the core network type and MME identification information may be stored with the core network type and the MME identification information associated with each other. 
     Note that the area information described thus far may also be tracking area information or routing area information. 
     The eNB  45  may store UE context information when the UE  10  has attached to a network and is in an active state (connected state) in which a radio resource has been allocated to the UE  10 . Note that the UE context information may contain identification information identifying the UE  10 . Here, the identification information identifying the UE  10  may be a globally unique temporary identity (GUTI) assigned globally and uniquely to the UE  10 . Alternatively, the identification information may be subscriber information such as an international mobile subscriber identity (IMGI). Alternatively, the identification information may be identification information such as a packet temporary mobile subscriber identity (P-TMSI). Alternatively, two or more pieces of the identification information may be combined and be used as the identification information identifying the UE  10 . 
     1.2.2. MME Block Diagram 
     Next, the configuration of the MME_A  40  will be described. Note that the MME_B  42  has the same configuration and thus detailed descriptions thereof will be omitted. 
       FIG. 4A  is a block diagram illustrating the MME_A  40 . As illustrated in  FIG. 4A , the MME_A  40  is constituted of an IP mobile communication network interface unit  320 , a control unit  300 , and a storage  340 . The IP mobile communication network interface unit  320  and the storage  340  are connected to the control unit  300  via a bus. 
     The control unit  300  is a function unit for controlling the MME_A  40 . The control unit  300  reads out and executes various programs stored in the storage  340  to perform various processes. 
     The IP mobile communication network interface unit  320  is a function unit for the MME_A  40  to connect with the eNB  45 , the SGW_A  35 , the HSS  50 , and the like. 
     As illustrated in  FIG. 4A , the storage  340  stores MME information  342  and context information  344 . 
     As illustrated in  FIG. 4B , the MME information  342  may contain MME identification information, MME service area information, and core network identification information with all the pieces of information associated with each other. 
     Here, the MME identification information may be any information capable of identifying the MME, and thus may be a globally unique MME identifier (GUMMEI), for example. The GUMMEI is MME identification information, which includes a public land mobile network (PLMN) that is identification information identifying a mobile operator network, an MME group number identifying multiple MMEs, and an MME code identifying an individual MME. 
     Note that the MME group number may be an MME group identifier (MMEGI). Note that different MMEGIs for each core network type may be stored. In this case, the core network can be identified by the MME group number. Furthermore, using a core network associated with the MME group number, MMEs belonging to the associated core network may be managed as a group. Thus, on the basis of the MME identification information and the MME group number, the type of core network including the corresponding MMEs may be identified. 
     In this case, it is only required that the MME information  342  contain the MME identification information and the service area information with both the pieces of information associated with each other. Thus, the MME information  342  do not necessarily have to contain the core network type information. 
     With regards to the method for managing the MME identification information, the area information does not necessarily have to be stored, and the core network type and MME identification information may be stored with the core network type and the MME identification information associated with each other. 
     Note that the area information described thus far may also be tracking area information or routing area information. 
     As illustrated in  FIG. 4C , information on UEs stored in the MME, on a UE-by-UE basis, is stored as the context information  344 . The UE context information may be one or both of a mobility management (MM) context, which is conventionally stored in an MME, and an evolved packet system (EPS) bearer context. 
     Note that information identifying a core network, such as type information, may be identification information assigned in accordance with distinction made in order for the core network to be divided from the viewpoint of operation and management by the mobile operator. 
     Alternatively, the information identifying a core network, such as type information, may be information associated with a type or attribute of a communication terminal. For example, the information may be identification information identifying the core network to which an M2M terminal connects, identification information identifying the core network to which a normal call terminal such as a smartphone connects, or the like. 
     Alternatively, the information identifying a core network, such as type information, may be identification information associated with a service provided by the mobile operator. For example, information identifying a specific service provided to an M2M terminal and identification information for providing a call service such as IMS may be associated with the information identifying a core network. In this case, the core networks can be configured to be separated by service. Note that managing terminals to which services are provided on a service-by-service basis makes it possible to classify the core networks for each terminal to which a corresponding service is provided. 
     The context information  344  may contain the identification information identifying the UE. Here, the identification information identifying the UE  10  may be a globally unique temporary identity (GUTI) assigned globally and uniquely to the UE  10 . Alternatively, the identification information may be subscriber information such as an international mobile subscriber identity (IMGI). Alternatively, the identification information may be identification information such as a packet temporary mobile subscriber identity (P-TMSI). Alternatively, two or more pieces of the above described identification information may be combined to be the identification information identifying the UE  10 . 
     Note that the MME_A  40  may store the context information when the UE  10  has attached to a network. In other words, the context information may be generated upon the attach. 
     1.3. Description of Processing 
     Next, a procedure for changing a core network will be described. 
     1.3.1 First Procedure Example 
     First, a first procedure example for changing a core network to which the UE  10  is connected will be described. In the initial state of the procedure, the UE  10  has a connection to the core network (type  1 )  90 . More specifically, the UE  10  has a connection to the eNB  45 . In addition, the UE  10  has a connection to the MME_A  40  via the eNB  45 . Moreover, the UE  10  has a PDN connection established with the PGW_A  30 . Here, the PDN connection refers to a communication path for delivering user data. 
     Note that the core network (type  1 )  90  and the core network (type  2 )  92  may both have different PGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the PGW_A  30  and the core network (type  2 )  92  may be configured to include the PGW_B  32 . 
     Furthermore, the core network (type  1 )  90  and the core network (type  2 )  92  may both have different SGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the SGW_A  35  and the core network (type  2 )  92  may be configured to include the SGW_B  37 . 
     The first procedure example will be described on the assumption that the UE  10  has a PDN connection established with the PGW_A  30  in the initial state. Specifically, the PDN connection between the UE  10  and the PGW_A  30  is established via the SGW_A  35 . 
     Note that, in the first procedure example, the identification information identifying the UE  10  may be a globally unique temporary identity (GUTI) assigned globally and uniquely to the UE  10 . Alternatively, the identification information may be subscriber information such as an international mobile subscriber identity (IMGI). Alternatively, the identification information may be identification information such as a packet temporary mobile subscriber identity (P-TMSI). Alternatively, two or more pieces of the identification information may be combined to be the identification information identifying the UE  10 . 
     Furthermore, in the first procedure example, the identification information identifying an MME may be a globally unique MME identifier (GUMMEI) that corresponds to each MME. Note that the GUMMEI is globally unique identification information including information such as a PLMN, an MME group, and an MME code. Alternatively, the identification information identifying an MME may be an MME group identifier (MMEGI) identifying an MME group. 
     Note that the eNB  45  retains in advance an MMEGI and a GUMMEI that belongs to an MME group identified by the MMEGI with the MMEGI and the GUMMEI associated with each other. Thus, upon receiving an MMEGI, the eNB  45  can select a GUMMEI that belongs to the received MMEGI in accordance with the MMEGI. 
     For example, the eNB  45  may store in advance, in MME information, an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ) and GUMMEIs corresponding to MMEs such as the MME_B  42  that belong to the group with the MMEGI and the MMEGI associated with each other. 
     Specific steps of the first procedure example will be described below with reference to  FIG. 5 . 
     First, the MME_A  40  detects a trigger for starting the procedure for changing a core network to which the UE  10  is connected (S 502 ). 
     The MME_A  40  may detect the trigger in response to an update of the subscriber information. More specifically, upon a change in subscriber information stored in the HSS  50  that correspond to the UE  10 , the HSS  50  transmits, to the MME_A  40 , a control message including identification information indicating that the change has been made. The MME_A  40  may detect that the core network to which the UE  10  is connected needs to be changed, upon receiving the control message transmitted by the HSS  50  or based on the identification information included in the control message. 
     Furthermore, the identification information included in the control message may be information on a core network indicating the core network type  2 , or may be information such as a flag indicating a request to change a core network type. In addition, the control message may be a control message for requesting to switch a core network. Moreover, the control message may contain identification information identifying the UE  10 . 
     The eNB  45  may receive an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ), and perform an MME selecting process. Note that the eNB  45  may select the MME_B  42  based on the received MMEGI. 
     The MME_A  40  may detect a trigger in response to an event that the core network (type  2 )  92  has been configured to. More specifically, the procedure may be triggered by installation of a control device, such as the MME_B  42  or the PGW_B  32 , included in the core network (type  2 )  92 . Note that the operator of the core network may install such a control device, and manually operate the MME_A  40  after the installation. The MME_A  40  may detect a trigger in response to such manual operation by the operator. 
     Next, the MME_A  40  transmits a detach request message to the UE  10  upon the detection of the trigger (S 504 ). 
     Here, the MME_A  40  may transmit the detach request message with at least first indication information (Indicator  1 ) included in the detach request message. 
     Note that the first indication information may be information giving a request or an instruction for switching core networks. 
     Alternatively, the first indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the first indication information may be information indicating that a core network type will be switched. More specifically, the information indicating that the core network type will be switched may be a cause value indicating that a core network needs to be switched. 
     Alternatively, the first indication information may be information indicating that the UE  10  needs to be re-attached. More specifically, the information indicating that the UE  10  needs to be re-attached may be a cause value indicating Attach Required. 
     Alternatively, the first indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the first indication information may be information obtained by combining two or more of: the information giving a request or an instruction for switching core networks; the core network type; the information indicating that a core network type will be switched; the information indicating that re-attach is necessary; and the identification information identifying an MME, each of which has been described above. In addition, the first indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type is to be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     Furthermore, the MME_A  40  may transmit a UE context release message to the eNB  45 , in parallel with transmission of the detach request message or after transmission of the detach request message (S 506 ). When a radio resource has been allocated between the UE  10  and the eNB  45 , the MME_A  40  may use the UE context release message to request the release of the radio resource. 
     Here, the MME_A  40  may transmit the UE context release message with at least second indication information (Indicator  2 ) included in the UE context release message. 
     The second indication information may be a request or an instruction for switching core networks. 
     Alternatively, the second indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information indicating that a core network type will be switched. More specifically, the information indicating that the core network type will be switched may be a cause value indicating that a core network needs to be switched. 
     Alternatively, the second indication information may be information indicating that the UE  10  needs to be re-attached. More specifically, the information indicating that the UE  10  needs to be re-attached may be a cause value indicating Attach Required. 
     Alternatively, the second indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information obtained by combining two or more of: the information giving a request or an instruction for switching core networks; the core network type; the information indicating that a core network type will be switched; the information indicating that re-attach is necessary; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type will be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     The MME_A  40  may transmit a UE context release message with at least the identification information identifying the UE  10  and a timer value included in the UE context release message. Note that the timer value may be a value of a timer used by the eNB  45  to select an MME. 
     Next, the eNB  45  receives the UE context release message from the MME_A  40 . The eNB  45  may perform an MME-information updating process based on the information included in the UE context release message (S 508 ). More specifically, the eNB  45  may perform the MME-information updating process based on the second indication information. 
     Note that the MME-information updating process may be a process in which MME information is updated in a manner such that the eNB  45  selects an MME included in the core network (type  2 )  92 , at the time of MME selection performed upon reception of an attach request message transmitted by the UE  10 . 
     More specifically, in the MME-information updating process, the eNB  45  may store the identification information identifying the UE  10  and the second indication information in association with each other. 
     Furthermore, having acquired the second indication information, the eNB  45  may retain the UE context information without deleting it. On the other hand, when the second indication information has not been acquired, the eNB  45  may delete the UE context information. 
     Furthermore, the eNB  45  may start a timer based on the reception of the second indication information. While the timer is in operation, the eNB  45  may retain the updated MME information or the UE context. In addition, when the timer expires or the timer stops for some reason, the eNB  45  may delete the updated MME information or the UE context. 
     Here, a value retained in advance by the eNB  45  may be used as a value for the timer, or a timer value included in the UE context release message may be used. 
     Note that the eNB  45  may still retain information on an MME that the eNB  45  has already retained before the MME-information updating process is performed. For example, the eNB  45  may still retain the GUMMEI corresponding to the MME_A  40  included in the core network (type  1 )  90 , the MMEGI indicating a group of MMEs to which the MME_A  40  belongs, and the like. The MME-information updating process is completed through above-described steps. 
     The eNB  45  may transmit an RRC connection release message to the UE  10  upon reception of the UE context release message or completion of the MME-information updating process (S 510 ). The eNB  45  may release a radio resource allocated to the UE  10  upon transmission of the UE context release message. 
     Alternatively, the eNB  45  may transmit the UE context release message with information, such as Acknowledge mode set to 1 that requires a response included in the UE context release message. In this case, the UE  10  transmits, to the eNB  45 , a response to the UE context release message as a response to the UE context release message. Upon reception of the response message, the eNB  45  may release a radio resource allocated to the UE  10 . 
     Note that when the timer is in operation at the time when the UE context release message is transmitted or the response message is received, the eNB  45  may retain the context of the UE  10  including, for example, the identification information identifying the UE  10 . Alternatively, although the eNB  45  deletes the UE context, the eNB  45  may store the identification information identifying the UE  10  and the second indication information with the identification information and the second indication information associated with each other while the timer is in operation. 
     Note that the eNB  45  may transmit the RRC connection release message with at least third indication information (Indicator  3 ) included in the RRC connection release message. 
     The third indication information may be information triggering a request or an instruction for switching core networks. 
     Alternatively, the third indication information may be identification information indicating that the UE  10  is requested or instructed to be re-attached. Specifically, the identification information requesting reattach may be a cause value indicating Attach Requeired. 
     Alternatively, the third indication information may be identification information indicating that a tracking area update procedure is necessary for the UE  10 . Specifically, the identification information indicating that the tracking area update procedure is necessary may be a cause value indicating TAU Require. 
     Alternatively, the third indication information may be identification information indicating that a core network needs to be switched. Specifically, the identification information indicating that a core network needs to be switched may be a core network type identifying the core network (type  2 )  92 . The core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . Alternatively, the identification information may be information that does not indicate a specific MMEGI or NRI by using MMEGI or NRI having zero or other specific value, such as NULL-MMEGI or NULL-NRI. 
     Alternatively, the third indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the third indication information may be information obtained by combining two or more of: the information triggering a request or an instruction for switching core networks; the identification information indicating that the UE  10  is requested to be re-attached; the identification information indicating that a tracking area update procedure is necessary for the UE  10 ; the identification information indicating that a core network needs to be switched; and the identification information identifying an MME, each of which has been described above. The third indication information obtained by combining two or more pieces of information as described above may be used as the identification information indicating that the UE  10  is requested to be reattached, the identification information indicating that a tracking area update procedure is necessary for the UE  10 , the identification information indicating that a core network needs to be switched, or the identification information identifying an MME. 
     Next, the eNB  45  transmits, to the MME_A  40 , a context release complete message as a response to the UE context release message (S 512 ). The context release complete message is a response message to the UE context release message. When the eNB  45  has performed the release of a radio resource of the UE  10 , deletion of the context of a UE, or the like, the context release complete message serves as a message that notifies that these processes are completed. 
     Furthermore, the eNB  45  may perform the MME-information updating process upon acquisition of the second indication information, and when the eNB  45  retains the MME information and the like through the MME-information updating process, the eNB  45  may transmit the UE context release complete message with information indicating that the UE context or the MME information on the core network (type  2 )  92  is retained in the UE context release complete message. 
     Furthermore, the MME_A  40  may receive the UE context release complete message, and delete the UE context that the MME_A  40  retains. 
     Furthermore, upon receiving the detach request message, the UE  10  may start an attach procedure. Note that the UE  10  may start the attach procedure based on the first indication information. In this case, the UE  10  may start the attach procedure even if the UE  10  has not received the RRC connection release message. 
     Alternatively, the UE  10  may start the attach procedure after receiving the RRC connection release message. 
     For example, the UE  10  may start the attach procedure upon reception of the first indication information and the RRC connection release message. Alternatively, the UE  10  may start the attach procedure based on the third indication information. Alternatively, the UE  10  may start the attach procedure based on the first indication information and the third indication information. 
     Furthermore, the UE  10  may delete the retained information on an MME, upon reception of the first indication information or the second indication information, or reception of both the first indication information and the second indication information. The information on an MME to be deleted may be a GUMMEI for the MME_A  40 . Alternatively, the information on an MME to be deleted may be an MMEGI indicating a group to which the MME_A  40  belongs. As described above, the first indication information or the second indication information may be used as information indicating that old information on an MME has been deleted. 
     Next, the attach procedure will be described. The UE  10  transmits an attach request message to the eNB  45  to start the attach procedure (S 514 ). The UE  10  transmits the attach request message to make a request for connection to a core network again. Note that the UE  10  may transmit the attach request message with the identification information identifying the UE included in the attach request message. 
     Here, the UE  10  may transmit the attach request message with at least fourth indication information (Indicator  4 ) included in the attach request message. Note that the UE  10  may decide to include the fourth indication information on the basis of reception of one or both of the first indication information and the third indication information. 
     The fourth indication information may be information giving a request or an instruction for switching core networks. 
     Alternatively, the fourth indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the fourth indication information may be identification information indicating that a core network needs to be switched. Specifically, the identification information indicating that a core network needs to be switched may be a core network type identifying the core network (type  2 )  92 . The identification information may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . Alternatively, the identification information may be information that does not indicate a specific MMEGI or NRI by using MMEGI or NRI having zero or other specific value, such as NULL-MMEGI or NULL-NRI. 
     Alternatively, the fourth indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the fourth indication information may be information obtained by combining two or more of: the information giving a request or an instruction for switching core networks; the core network type; the identification information indicating that the core network needs to be switched; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type is to be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     Furthermore, the UE  10  does not necessarily have to transmit the fourth indication information with the fourth indication information included in the attach request message. The UE  10  may transmit the fourth indication information with the fourth indication information included in the control message to be transmitted by the UE  10  in the attach procedure. 
     The eNB  45  receives the attach request message from the UE  10 . The eNB  45  performs the MME selecting process upon reception of the attach request message (S 516 ). 
     In the MME selecting process, the eNB  45  selects the MME_B  42  on the basis of the MME-information updating process. 
     Alternatively, the eNB  45  may select the MME_B  42  based on the fourth indication information. As described above, when not having received the fourth indication information, the eNB  45  may select the MME_A  40 . 
     Alternatively, the eNB  45  may select the MME_B  42  on the basis of the MME-information updating process and reception of the fourth indication information. As described above, even when the MME selection on the basis of the MME-information updating process results in MME_B  42 , the eNB  45  may select the MME_A  40  when not having received the fourth indication information. In addition, even when the fourth indication information has already been received, the eNB  45  may select the MME_A  40  when having not performed the MME-information updating process. 
     As described above, the eNB  45  can select either the MME_A  40  included in the core network (type  1 )  90  or the MME_B  42  included in the core network (type  2 )  92  based on one or both of the second indication information and the fourth indication information. 
     Note that the eNB  45  may select an MME included in the core network (type  2 )  92  when a timer is in operation, and may select an MME included in the core network (type  1 ) based on information transmitted by the UE  10  when the timer is not in operation or a core network need not be switched. 
     As for the selection method in such a case, selection may be made in accordance with information transmitted by the UE  10 . For example, the UE  10  transmits the attach request message with information identifying an MME such as a GUMMEI included in the attach request message. When the received MME is available, the eNB may select the MME. Here, the eNB  45  may decide whether the MME is available on the basis of whether the MME has connectivity. In the present embodiment, the information transmitted by the UE  10  may be information identifying the MME_A  40  that has performed location management of the UE  10 . 
     When the MME received from the UE  10  is not available, the UE  10  may transmit the attach request message with identification information identifying a group of MMEs such as an MMEI, identification information identifying the UE  10  such as P-TMSI, location information such as tracking area ID (TAI) and tracking area code (TAC), or any information obtained by combining two or more pieces of the information described above included in the attach request message. The eNB  45  may receive these pieces of information, and select an MME based on these pieces of information. 
     Thus, the eNB  45  completes the MME selecting process. 
     Note that the eNB  45  may decide whether to perform the MME selecting process, on the basis of a timer initiated in unison with the MME-information updating process. For example, when the timer is counting, the MME selecting process may be performed, and after the timer has stopped or when the timer is not counting, the MME selecting process need not be performed. 
     More specifically, the eNB  45  may select the MME_B  42  when the timer is in operation, and may select the MME_A  40  when the timer is not in operation. Note that, when the timer is not in operation, the eNB  45  may select an MME based on the identification information identifying an MME included in the attach request. For example, the eNB  45  may select an MME identified by the GUMMEI included in the attach request message, or may select an MME based on the MMEGI included in the attach request message, or may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     Note that the eNB  45  may stop the timer upon receiving the attach request message transmitted by the UE  10 . 
     Furthermore, when a terminal device that has not performed the MME-information updating process transmits the attach request message, the eNB  45  may select an MME based on the identification information identifying an MME included in the attach request. For example, the eNB  45  may select an MME identified by the GUMMEI included in the attach request message, or may select an MME based on the MMEGI included in the attach request message, or may select an MME based on the MMEGI included in the attach request message, or may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     Next, when the eNB  45  has selected the MME_B  42 , the eNB  45  transmits the attach request message to the MME_B  42  (S 518 ). Note that the attach procedure after transmission of the attach request message may be transmitting and/or receiving of control messages and processes similar to those in conventional attach procedures. Thus, detailed description thereof will be omitted. 
     Furthermore, when the eNB  45  selects the MME_A  40 , the eNB  45  may transmit the attach request message to the MME_A  40 . In addition, the eNB  45  may receive a route reconfiguration request from the MME_A  40 . Moreover, the eNB  45  may select the MME_B  42  in accordance with the route reconfiguration request, and transmit the attach request message to the MME_B  42 . Note that the attach procedure after transmission of the attach request message may involve transmitting and/or receiving of control messages and processes similar to those in conventional attach procedures. Thus, detailed description thereof will be omitted. 
     As descried above, when the eNB  45  has selected the MME_B  42  through the MME selecting process, it is possible, for example, to perform the attach procedure without receiving a reroute reconfiguration request, and hence, transmitting and/or receiving of the control messages or other processes can be reduced. 
     Note that the MME_B  42  receives the attach request message from the eNB  45 , and selects an SGW and a PGW. Here, the MME_B  42  may select the SGW_B  37  and the PGW_B  32  for the core network (type  2 )  92 , and establish a PDN connection between the UE  10  and the PGW_B  32 . 
     Through the attach procedure, the UE  10  establishes a PDN connection with the PGW_B  32 . In addition, the PDN connection between the UE  10  and the PGW_B  32  is established via the SGW_B  37 . Moreover, with the established PDN connection, the UE  10  can transmit/receive user data to/from a communication device included in the PDN_B  102 . 
     Note that it has been described that the core network (type  1 )  90  and the core network (type  2 )  92  are connected to the PDN_A  100  and PDN_B  102 , respectively, in the present procedure. However, as illustrated in  FIG. 6 , the PDN_A  100  and the PDN_B  102  may be the same network (PDN  100 ). In this case, the PGW_A  30  and the PGW_B  32  may be the same device (PGW_A  30 ). 
     In other words, with the present procedure, the UE  10  changes an MME that performs movement control from the MME_A  40  to the MME_B  42 . In addition, a new PDN connection may be established without changing a PGW serving as an end point of a PDN connection. Moreover, in the attach procedure described through the present procedure, the MME_B  42  may select the PGW_A  30  serving as an end point of a PDN connection that the UE  10  established in the past. With this selection, the UE  10  may establish a PDN connection with the PGW_A  30 . 
     1.3.2 Second Procedure Example 
     Next, a second procedure example for changing a core network to which the UE  10  is connected will be described. The initial state of the procedure may be the same as the initial state described in the first procedure example. 
     In the initial state, the UE  10  has established a connection to the core network (type  1 )  90 . More specifically, the UE  10  has established a connection to the eNB  45 . In addition, the UE  10  has a connection to the MME_A  40  via the eNB  45 . Moreover, the UE  10  has a PDN connection established with the PGW_A  30 . Here, the PDN connection refers to a communication path for delivering user data. 
     Note that the core network (type  1 )  90  and the core network (type  2 )  92  may have mutually different PGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the PGW_A  30 , and the core network (type  2 )  92  may be configured to include the PGW_B  32 . 
     Furthermore, SGWs different from each other may be deployed in the core network (type  1 )  90  and in the core network (type  2 )  92 . For example, the core network (type  1 )  90  may be configured to include the SGW_A  35 , and the core network (type  2 )  92  may be configured to include the SGW_B  37 . 
     The second procedure example will be described on the assumption that the UE  10  has a PDN connection established with the PGW_A  30  in the initial state. In addition, the PDN connection between the UE  10  and the PGW_A  30  is established via the SGW_A  35 . 
     Note that, in the second procedure example, the identification information identifying the UE  10  may be a globally unique temporary identity (GUTI) assigned globally and uniquely to the UE  10 . Alternatively, the identification information may be subscriber information such as an international mobile subscriber identity (IMGI). Alternatively, the identification information may be identification information such as a packet temporary mobile subscriber identity (P-TMSI). Alternatively, two or more pieces of the identification information may be combined to comprise the identification information identifying the UE  10 . 
     Furthermore, in the second procedure example, the identification information identifying an MME may be a globally unique MME identifier (GUMMEI) that corresponds to each MME. Note that the GUMMEI is globally unique identification information including information such as a PLMN, an MME group, and an MME code. Alternatively, the identification information identifying an MME may be an MME group identifier (MMEGI) identifying an MME group. 
     Note that the eNB  45  retains in advance an MMEGI and a GUMMEI that belongs to an MME group identified by the MMEGI with the MMEGI and the GUMMEI associated with each other. Thus, upon receiving an MMEGI, the eNB  45  can select a GUMMEI that belongs to the received MMEGI based on the MMEGI. 
     For example, the eNB  45  may store in advance, in the MME information, an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ) and GUMMEIs corresponding to MMEs such as the MME_B  42  that belong to the group of MMEGI with the MMEGI and the GUMMEI associated with each other. 
     Specific steps of the second procedure example will be described below with reference to  FIG. 7 . 
     First, the MME_A  40  detects a trigger for starting the procedure of changing the core network to which the UE  10  is connected (S 702 ). 
     Note that the trigger detection process may be the same as the trigger detection process (S 502 ) described in the first procedure example. The trigger detection process will be described below. 
     The MME_A  40  may detect the trigger in response to an update of the subscriber information. More specifically, upon a change in subscriber information stored in the HSS  50  that also corresponds to the UE  10 , the HSS  50  transmits, to the MME_A  40 , a control message including identification information indicating that the change has been made. The MME_A  40  may detect that the core network to which the UE  10  is connected needs to be changed, upon reception of the control message transmitted by the HSS  50  or based on the identification information included in the control message. 
     Furthermore, the identification information included in the control message may be information on a core network indicating the core network type  2 , or may be information such as a flag indicating a request to change a core network type. In addition, the control message may be a request to switch a core network. Moreover, the control message may include identification information identifying the UE  10 . 
     The eNB  45  may receive an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ), and perform an MME selecting process. Note that the eNB  45  may select the MME_B  42  based on the received MMEGI. 
     The MME_A  40  may detect a trigger in response to an event in which the core network (type  2 )  92  has been configured. More specifically, the procedure may be triggered by installation of a control device, such as the MME_B  42  or the PGW_B  32 , included in the core network (type  2 )  92 . Note that the operator of the core network may install such a control device, and manually operate the MME_A  40  after installation. The MME_A  40  may detect a trigger in response to such manual operation by the operator. 
     Next, the MME_A  40  may transmit a UE context release message to the eNB  45  upon the detection of the trigger (S 704 ). When a radio resource has been allocated between the UE  10  and the eNB  45 , the MME_A  40  may use the UE context release message to request the release of the radio resource. 
     Here, the MME_A  40  may transmit the UE context release message with at least second indication information (Indicator  2 ) included in the UE context release message. 
     Note that the second indication information may be information giving a request or an instruction for switching core networks. 
     Alternatively, the second indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). More specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information indicating that a core network type will be switched. More specifically, the information indicating that the core network type will be switched may be a cause value indicating that a core network needs to be switched. 
     Alternatively, the second indication information may be information indicating that the UE  10  needs to be re-attached. More specifically, the information indicating that the UE  10  needs to be re-attached may be a cause value indicating Attach Required. 
     Alternatively, the second indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information obtained by combining two or more of: the information giving a request or an instruction for switching core networks; the core network type; the information indicating that a core network type will be switched; the information indicating that re-attach is necessary; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type will be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     The MME_A  40  may transmit a transmission UE context release message with at least the identification information identifying the UE  10  and a timer value included in the transmission UE context release message. Note that the timer value may be a value of a timer used by the eNB  45  to select an MME. 
     Next, the eNB  45  receives the UE context release message from the MME_A  40 . The eNB  45  may perform an MME-information updating process based on the information included in the UE context release message (S 706 ). More specifically, the eNB  45  may perform the MME-information updating process based on the second indication information. 
     Note that the MME-information updating process may be a process in which MME information is updated in a manner such that the eNB  45  selects an MME included in the core network (type  2 )  92 , at the time of MME selection performed upon reception of a tracking area update request message transmitted by the UE  10 . 
     More specifically, in the MME-information updating process, the eNB  45  may store the identification information identifying the UE  10  and the second indication information in association with each other. 
     Furthermore, after having acquired the second indication information, the eNB  45  may still retain the UE context information without deleting the UE context information. On the other hand, when not having acquired the second indication information, the eNB  45  may delete the UE context information. 
     Furthermore, the eNB  45  may start a timer to measure time progression based on the reception of the second indication information. While the timer is in operation, the eNB  45  may retain the updated MME information or the UE context. In addition, when the timer expires or the timer stops for some reason reason, the eNB  45  may delete the updated MME information or the UE context. 
     Here, a value retained in advance by the eNB  45  may be used as the value for the timer, or a timer value included in the UE context release message may be used. 
     Note that the eNB  45  may still retain information on an MME that the eNB  45  has already retained before the MME-information updating process is performed. For example, the eNB  45  may still retain the GUMMEI corresponding to the MME_A  40  included in the core network (type  1 )  90 , the MMEGI indicating a group of MMEs to which the MME_A  40  belongs, and the like. 
     The MME-information updating process is completed through the above-described steps. 
     The NB  45  may transmit an RRC connection release message to the UE  10  upon reception of the UE context release message or completion of the MME-information updating process (S 708 ). The eNB  45  may release a radio resource allocated to the UE  10  upon transmission of the UE context release message. 
     Alternatively, the eNB  45  may transmit the UE context release message with information, corresponding to, for example, Acknowledge mode set to 1, that requires a response included in the UE context release message. In this case, the UE  10  transmits, to the eNB  45 , a response to the UE context release message as a response to the UE context release message. Upon reception of the response message, the eNB  45  may release a radio resource allocated to the UE  10 . 
     Note that when the timer is in operation at the time when the UE context release message is transmitted or the response message is received, the eNB  45  may retain the context of the UE  10  including, for example, the identification information identifying the UE  10 . Alternatively, although the eNB  45  deletes the UE context, the eNB  45  may store the identification information identifying the UE  10  and the second indication information in association with each other while the timer is in operation. 
     Note that the eNB  45  may transmit the RRC connection release message with at least third indication information (Indicator  3 ) included in the RRC connection release message. 
     The third indication information may be information triggering a request or an instruction for switching core networks. 
     Alternatively, the third indication information may be identification information indicating that the UE  10  is requested to be re-attached. Specifically, the identification information requesting re-attach may be a cause value indicating Attach Requeired. 
     Alternatively, the third indication information may be identification information indicating that a tracking area update procedure is necessary for the UE  10 . Specifically, the identification information indicating that the tracking area update procedure is necessary may be a cause value indicating TAU Required. 
     Alternatively, the third indication information may be identification information indicating that a core network needs to be switched. Specifically, the identification information indicating that a core network needs to be switched may be a core network type identifying the core network (type  2 )  92 . The identification information may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . Alternatively, the identification information may be information that does not indicate a specific MMEGI or NRI by using MMEGI or NRI having zero or other specific value, such as NULL-MMEGI or NULL-NRI. 
     Alternatively, the third indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the third indication information may be information obtained by combining two or more of: the information triggering a request or an instruction for switching core networks; the identification information indicating that the UE  10  is requested to be re-attached; the identification information indicating that a tracking area update procedure is necessary for the UE  10 ; the identification information indicating that a core network needs to be switched; and the identification information identifying an MME, each of which has been described above. The third indication information obtained by combining two or more pieces of information as described above may be used as the identification information indicating that the UE  10  is requested to be re-attached, the identification information indicating that a tracking area update procedure is necessary for the UE  10 , the identification information indicating that a core network needs to be switched, or the identification information identifying an MME. 
     Next, the eNB  45  transmits, to the MME_A  40 , a context release complete message as a response to the UE context release message (S 710 ). The context release complete message is a response message to the UE context release message. When the eNB  45  has performed release of a radio resource for the UE  10 , deletion of a context of a UE, or the like, the context release complete message serves as a message that notifies that these processes are completed. 
     Furthermore, the eNB  45  may perform the MME-information updating process upon acquisition of the second indication information, and when the eNB  45  retains the MME information and the like through the MME-information updating process, the eNB  45  may transmit the UE context release complete message with information indicating that the UE context or the MME information on the core network (type  2 )  92  is retained included in the UE context release complete message. 
     Furthermore, the MME_A  40  may receive the UE context release complete message, and delete the UE context that the MME_A  40  retains. 
     The UE  10  may receive the RRC connection release, and start the tracking area update procedure. Note that the UE  10  may start the tracking area update procedure based on the third indication information. Alternatively, the UE  10  may start the tracking area update procedure upon expiration of a tracking area update timer counting transmission interval of tracking area update requests. 
     Furthermore, the UE  10  may delete the retained information on an MME, upon reception of the first indication information. The information on an MME to be deleted may be a GUMMEI for the MME_A  40 . Alternatively, the information on an MME to be deleted may be an MMEGI indicating a group to which the MME_A  40  belongs. As described above, the first indication information or the second indication information may be used as information indicating that old information on an MME has been deleted. 
     Next, the tracking area update procedure will be described. The UE  10  transmits a tracking area update request message to the eNB  45  to start the tracking area update procedure (S 712 ). The UE  10  transmits the tracking area update message to request confirmation or update of a tracking area in which the UE  10  is located. 
     Here, the UE  10  may transmit the tracking area update message with at least the identification information identifying the UE included in the tracking area update message. 
     In addition, the UE  10  may transmit the attach request message with at least fourth indication information (Indicator  4 ) included in the attach request message. Note that the UE  10  may decide to include the fourth indication information based on the third indication information. 
     The fourth indication information may be information giving a request or an instruction for switching core networks. 
     Alternatively, the fourth indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the fourth indication information may be identification information indicating that a core network needs to be switched. Specifically, the identification information indicating that a core network needs to be switched may be a core network type identifying the core network (type  2 )  92 . The identification information may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . Alternatively, the identification information may be information that does not indicate a specific MMEGI or NRI by using MMEGI or NRI having zero or other specific value, such as NULL-MMEGI or NULL-NRI. 
     Alternatively, the fourth indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the fourth indication information may be information obtained by combining two or more of: the information triggering a request or an instruction for switching core networks; the core network type; the identification information indicating that the core network needs to be switched; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type is to be switched, or the identification information identifying an MME. 
     The eNB  45  receives the tracking update request message from the UE  10 . The eNB  45  may perform the MME selecting process upon reception of the tracking area request message (S 714 ). 
     In the MME selecting process, the eNB  45  selects the MME_B  42  on the basis of the MME-information updating process. 
     Alternatively, the eNB  45  may select the MME_B  42  based on the fourth indication information. As described above, when the fourth indication information has not been received, the eNB  45  may select the MME_A  40 . 
     Alternatively, the eNB  45  may select the MME_B  42  on the basis of the MME-information updating process and reception of the fourth indication information. As described above, even when the MME selection on the basis of the MME-information updating process results in MME_B  42 , the eNB  45  may select the MME_A  40  when the fourth indication information has not been received. In addition, even the fourth indication information has already been received, the eNB  45  may select the MME_A  40  when having not performed the MME-information updating process. 
     As described above, the eNB  45  can select either the MME_A  40  included in the core network (type  1 )  90  or the MME_B  42  included in the core network (type  2 )  92  based on one or both of the second indication information and the fourth indication information. 
     Note that the eNB  45  may select an MME included in the core network (type  2 )  92  when a timer is in operation, and may select an MME included in the core network (type  1 ) based on information transmitted by the UE  10  when the timer is not in operation or a core network does not need to be switched. 
     As for the selection method in such a case, selection may be made based on information transmitted by the UE  10 . For example, the UE  10  transmits the tracking area update request message with information identifying an MME such as a GUMMEI included in the tracking area update request message. When the received MME is available, the eNB may select the MME. Here, the eNB  45  may decide whether the MME is available on the basis of whether the MME has connectivity. In the present embodiment, the information transmitted by the UE  10  may be information identifying the MME_A  40  that has performed location management of the UE  10 . 
     When the MME received from the UE  10  is not available, the UE  10  may transmit the tracking area update request message with identification information identifying a group of MMEs such as an MMEI, identification information identifying the UE  10  such as P-TMSI, location information such as tracking area ID (TAI) and tracking area code (TAC), or any information obtained by combining two or more pieces of the information described above included in the tracking area update request message. The eNB  45  may receive these pieces of information, and select an MME based on these pieces of information. 
     Thus, the eNB  45  completes the MME selecting process. 
     Note that the eNB  45  may decide whether to perform the MME selecting process, on the basis of a timer initiated in unison with the MME-information updating process. For example, when the timer is counting, the MME selecting process may be performed, and after the timer already ends or when the timer is not counting, the MME selecting process does not have to be performed. 
     More specifically, the eNB  45  may select the MME_B  42  when the timer is in operation, and may select the MME_A  40  when the timer is not in operation. Note that, when the timer is not in operation, the eNB  45  may select an MME based on the identification information identifying an MME included in the tracking area update request message. For example, the eNB  45  may select an MME identified by the tracking area update request message, or may select an MME based on the MMEGI included in the tracking area update request message, or may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     Note that the eNB  45  may stop the timer upon receiving the tracking area update request message transmitted by the UE  10 . 
     Furthermore, when a terminal device that has not performed the MME-information updating process transmits the tracking area update request message, the eNB  45  may select an MME based on the identification information identifying an MME included in the tracking area update request message. For example, the eNB  45  may select an MME identified by the GUMMEI included in the tracking area update request message, or may select an MME based on the MMEGI included in the tracking area update request message, or may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     When the eNB  45  has selected the MME_B  42 , the eNB  45  transmits the tracking area request message to the MME_B  42  (S 716 ). Note that the tracking area update procedure after transmission of the tracking area request message may involve the transmitting and/or receiving of control messages and processes similar to those used in conventional tracking update procedure. Thus, a detailed description thereof will be omitted. 
     Furthermore, when the eNB  45  has selected the MME_A  40 , the eNB  45  may transmit the tracking area update request message to the MME_A  40 . In addition, the eNB  45  may receive a route reconfiguration request from the MME_A  40 . Moreover, the eNB  45  may select the MME_B  42  based on the route reconfiguration request, and transmit the tracking area update request message to the MME_B  42 . Note that the attach procedure after transmission of the tracking update request message may involve transmitting and/or receiving of control messages and processes similar to those use in conventional tracking update request messages. Thus, a detailed description thereof will be omitted. 
     As descried above, when the eNB  45  has selected the MME_B  42  through the MME selecting process, it is possible, for example, to transmit the tracking update request message without receiving a reroute reconfiguration request, and hence, transmitting and/or receiving of the control messages or other processes can be reduced. 
     Once completing the tracking area update procedure, the UE  10  can communicate using the PDN connection. 
     Note that that the core network (type  1 )  90  and the core network (type  2 )  92  are connected to the PDN_A  100  and PDN_B  102 , respectively, according to the present procedure. However, as illustrated in  FIG. 6 , the PDN_A  100  and the PDN_B  102  may be the same network (PDN  100 ). In this case, the PGW_A  30  and the PGW_B  32  may be the same device (PGW_A  30 ). 
     In the present procedure, the UE  10  changes an MME that performs movement control from the MME_A  40  into the MME_B  42 , without the reestablishment of a PDN connection. The UE  10  can continue communication using the PDN connection established with the PGW_A  30 . 
     1.3.3 Third Procedure Example 
     Next, a third procedure example for changing a core network to which the UE  10  is connected will be described. The initial state in the procedure may be the same as the initial state described in the first procedure example. 
     In the initial state, the UE  10  has a connection to the core network (type  1 )  90 . More specifically, the UE  10  has a connection to the eNB  45 . In addition, the UE  10  has a connection to the MME_A  40  via the eNB  45 . Moreover, the UE  10  has a PDN connection established with the PGW_A  30 . Here, the PDN connection refers to a communication path for delivering user data. 
     Note that the core network (type  1 )  90  and the core network (type  2 )  92  may both have different PGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the PGW_A  30 , and the core network (type  2 )  92  may be configured to include the PGW_B  32 . 
     Furthermore, the core network (type  1 )  90  and the core network (type  2 )  92  may have mutually different SGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the SGW_A  35 , and the core network (type  2 )  92  may be configured to include the SGW_B  37 . 
     The third procedure example will be described based on the assumption that the UE  10  has a PDN connection established with the PGW_A  30  in the initial state. In addition, the PDN connection between the UE  10  and the PGW_A  30  is established via the SGW_A  35 . 
     Note that, in the third procedure example, the identification information identifying the UE  10  may be a globally unique temporary identity (GUTI) assigned globally and uniquely to the UE  10 . Alternatively, the identification information may be subscriber information such as an international mobile subscriber identity (IMGI). Alternatively, the identification information may be identification information such as a packet temporary mobile subscriber identity (P-TMSI). Alternatively, two or more pieces of the identification information may be combined to be the identification information identifying the UE  10 . 
     Furthermore, in the third procedure example, the identification information identifying an MME may be a globally unique MME identifier (GUMMEI) that corresponds to each MME. Note that the GUMMEI is globally unique identification information including information such as a PLMN, an MME group, and an MME code. Alternatively, the identification information identifying an MME may be an MME group identifier (MMEGI) identifying an MME group. 
     Note that the eNB  45  retains in advance an MMEGI and a GUMMEI that belongs to an MME group identified by the MMEGI with the MMEGI and the GUMMEI associated with each other. Thus, upon receiving an MMEGI, the eNB  45  can select a GUMMEI that belongs to the received MMEGI based on the MMEGI. 
     For example, the eNB  45  may store in advance, in the MME information, an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ) and GUMMEIs corresponding to MMEs, such as the MME_B  42 , that belong to the group of MMEGI with the MMEGI and the GUMMEI associated with each other. 
     Specific steps of the third procedure example will be described below with reference to  FIG. 8 . 
     First, the MME_A  40  detects a trigger for starting the procedure of changing a core network to which the UE  10  is connected (S 802 ). 
     Note that the trigger detection process may be the same as the trigger detection process (S 502 ) described in the first procedure example. The trigger detection process will be described below. 
     The MME_A  40  may detect the trigger in response to an update of the subscriber information. More specifically, upon a change in subscriber information stored in the HSS  50  that also corresponds to the UE  10 , the HSS  50  transmits, to the MME_A  40 , a control message including identification information indicating that the change has been made. The MME_A  40  may detect that the core network to which the UE  10  is connected needs to be changed, upon reception of the control message transmitted by the HSS  50  or based on the identification information included in the control message. 
     Furthermore, the identification information included in the control message may be information on a core network indicating the core network type  2 , or may be information such as a flag indicating a request to change a core network type. In addition, the control message may be a request to switch a core network. Moreover, the control message may include identification information identifying the UE  10 . 
     The eNB  45  may receive an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ), and perform an MME selecting process. Note that the eNB  45  may select the MME_B  42  based on the received MMEGI. 
     The MME_A  40  may detect a trigger in response to an event that the core network (type  2 )  92  has been configured. More specifically, the procedure may be triggered by installation of a control device, such as the MMEB  42  or the PGW_B  32 , included in the core network (type  2 )  92 . Note that the operator of the core network may install such a control device, and manually operate the MME_A  40  after the installation. The MME_A  40  may detect a trigger in response to such a manual operation by the operator. 
     Next, the MME_A  40  transmits a paging message to the UE  10  upon the detection of the trigger (S 804 ). 
     Here, the MME_A  40  may transmit the paging message with at least first indication information (Indicator  1 ) included in the paging message. 
     Note that the first indication information may be information triggering a request or an instruction for switching core networks. 
     Alternatively, the first indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the first indication information may be information indicating that a core network type will be switched. More specifically, the information indicating that the core network type will be switched may be a cause value indicating that a core network needs to be switched. 
     Alternatively, the first indication information may be information indicating that the UE  10  needs to be re-attached. More specifically, the information indicating that the UE  10  needs to be re-attached may be a cause value indicating Attach Required. 
     Alternatively, the first indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the first indication information may be information obtained by combining two or more of: the information triggering a request or an instruction for switching core networks; the core network type; the information indicating that a core network type will be switched; the information indicating that re-attach is necessary; and the identification information identifying an MME, each of which has been described above. In addition, the first indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type will be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     Furthermore, the MME_A  40  may transmit a route reconfiguration request message to the eNB  45 , in parallel with transmission of the paging message or after transmission of the paging message (S 806 ). The MME_A may transmit the route reconfiguration request message to request an update of information on an MME. 
     Here, the MME_A  40  may transmit the route reconfiguration request message with at least second indication information (Indicator  2 ) included in the route reconfiguration request message. 
     The second indication information may be information triggering a request or an instruction for switching core networks. 
     Alternatively, the second indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information indicating that a core network type will be switched. More specifically, the information indicating that the core network type will be switched may be a cause value indicating that a core network needs to be switched. 
     Alternatively, the second indication information may be information indicating that the UE  10  needs to be re-attached. More specifically, the information indicating that the UE  10  needs to be re-attached may be a cause value indicating Attach Required. 
     Alternatively, the second indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information obtained by combining two or more of: the information giving a request or an instruction for switching core networks; the core network type; the information indicating that a core network type will be switched; the information indicating that re-attach is necessary; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type is to be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     The MME_A  40  may transmit a transmission UE context release message with at least the identification information identifying the UE  10  and a timer value included in the transmission UE context release message. Note that the timer value may be a value of a timer used by the eNB  45  to select an MME. 
     Note that the route reconfiguration request message may be a control message for giving a request or an instruction for updating information on an MME, and may be any control message. Therefore, the MME_A  40  may request or instruct the eNB  45  to update information on an MME with another control message. In this case, the description of the route reconfiguration request message in the present embodiment can be applied to the control message used for giving a request or an instruction for updating information on an MME. Next, the eNB  45  receives the route reconfiguration request message from the MME_A  40 . The eNB  45  may perform an MME-information updating process based on the information included in the route reconfiguration request message (S 808 ). More specifically, the eNB  45  may perform the MME-information updating process based on the second indication information. 
     Note that the MME-information updating process may be a process in which MME information is updated in a manner such that the eNB  45  selects an MME included in the core network (type  2 )  92 , at the time as MME selection, which is performed upon reception of an attach request message transmitted by the UE  10 . 
     More specifically, in the MME-information updating process, the eNB  45  may store the identification information identifying the UE  10  and the second indication information in association with each other. 
     Furthermore, when the second indication information has been acquired, the eNB  45  may retain the UE context information without deleting the UE context information. On the other hand, when having not acquired the second indication information, the eNB  45  may delete the UE context information. 
     Furthermore, the eNB  45  may start a timer to measure time based on the reception of the second indication information. While the timer is in operation, the eNB  45  may retain the updated MME information or the UE context. In addition, when the timer expires or the timer stops for some reason, the eNB  45  may delete the updated MME information or the UE context. 
     Here, a value retained in advance by the eNB  45  may be used as the value of a timer, or a timer value included in the UE context release message may be used. 
     Note that the eNB  45  may still retain information on an MME that the eNB  45  has already retained before the MME-information updating process is performed. For example, the eNB  45  may still retain the GUMMEI corresponding to the MME_A  40  included in the core network (type  1 )  90 , the MMEGI indicating a group of MMEs to which the MME_A  40  belongs, and the like. 
     The MME-information updating process is completed through the above-described steps. Furthermore, the eNB  45  may transmit the route reconfiguration response message to the MME_A  40  as a response to the route reconfiguration request message, on the basis of completion of the MME-information updating process (S 810 ). The eNB  45  may transmit the route reconfiguration response message to notify completion of the MME-information updating process. 
     Furthermore, the eNB  45  may perform the MME-information updating process upon acquisition of the second indication information, and when the eNB  45  retains the MME information or the like through the MME-information updating process, the eNB  45  may transmit the route reconfiguration response message with information indicating that the UE context or the MME information on the core network (type  2 )  92  is included in the route reconfiguration response message. 
     Note that the route reconfiguration response message may be any response to a control message for triggering a request or an instruction for updating information on an MME, and may also be any control message. Therefore, the MME_A  40  may transmit to the MME_A  40 , the response with another control message. In this case, the description for the route reconfiguration response message in the present embodiment can be applied to the control message used for the response. 
     The MME_A  40  may receive the route reconfiguration response message, and perform a detach process (Implicitly detach) without starting a control procedure on the basis of MME_A  40  initiation (S 812 ). Note that, in the detach process, the UE context retained by the MME_A  40  may be deleted. 
     Furthermore, upon receiving the paging message, the UE  10  may start a service request procedure. Note that the UE  10  may start the service request procedure based on the first indication information. 
     Specifically, the UE  10  may transmit a service request message to the MME_A  40  (S 814 ). The UE  10  may transmit the service request message to request the initiation of a service or allocation of a radio resource between the UE  10  and the eNB  45 . 
     Upon receiving the service request message, the MME_A  40  may transmit a service rejection message as a response to the service request message (S 816 ). 
     The transmission of the service rejection message may be made on the basis of the completion of the detach process in the MME_A  40 . 
     Here, the MME_A  40  may transmit the service reject message with at least information indicating that the detach process is completed included in the service reject message. More specifically, the information indicating that the detach process is completed may be a cause value indicating Implicitly Detach. 
     Note that the first indication information included in the service reject message instead of the paging message may be transmitted to the UE  10 . 
     The UE  10  receives the service reject message. The UE  10  may start the attach procedure upon the reception of the service rejection or based on information indicating that the detach process has been completed. Alternatively, upon receiving the service rejection message, the UE  10  may start the attach procedure on the basis of the acquisition of the first indication information. 
     Furthermore, the UE  10  may delete the information retained in an MME, upon receiving the first indication information. The information in an MME to be deleted may be a GUMMEI for the MME_A  40 . Alternatively, the information in an MME to be deleted may be an MMEGI indicating a group to which the MME_A  40  belongs. As described above, the first indication information may be used as information indicating that the old information on the MME has been deleted. The information retained in an MME may be deleted after the reception of the service reject message. 
     Next, the attach procedure will be described. The UE  10  transmits an attach request message to the eNB  45  to start the attach procedure (S 818 ). The UE  10  transmits the attach request message to request a connection to a core network again. Note that the UE  10  may transmit the attach request message with the identification information identifying the UE included in the attach request message. 
     Here, the UE  10  may transmit the attach request message with at least fourth indication information (Indicator  4 ) included in the attach request message. Note that the UE  10  may decide to include the fourth indication information on the basis of the reception of one or both of the first indication information and the third indication information. 
     The fourth indication information may be information triggering a request or an instruction for switching core networks. 
     The fourth indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the fourth indication information may be identification information indicating that a core network needs to be switched. Specifically, the identification information indicating that a core network needs to be switched may be a core network type identifying the core network (type  2 )  92 . The identification information may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . Alternatively, the identification information may be information that does not indicate a specific MMEGI or NRI by using MMEGI or NRI that has zero or another specific value, such as NULL-MMEGI or NULL-NRI. 
     Alternatively, the fourth indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the fourth indication information may be information obtained by combining two or more of: the information triggering a request or an instruction for switching core networks; the core network type; the identification information indicating that the core network needs to be switched; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type will be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     Furthermore, the UE  10  does not necessarily have to transmit the fourth indication information with the fourth indication information included in the attach request message. The UE  10  may transmit the fourth indication information with the fourth indication information included in the control message to be transmitted by the UE  10  in the attach procedure. 
     The eNB  45  receives the attach request message from the UE  10 . The eNB  45  performs the MME selecting process upon reception of the attach request message (S 820 ). 
     In the MME selecting process, the eNB  45  selects the MME_B  42  on the basis of the MME-information updating process. 
     Alternatively, the eNB  45  may select the MME_B  42  based on the fourth indication information. As described above, when the fourth indication information has not been received, the eNB  45  may select the MME_A  40 . 
     Alternatively, the eNB  45  may select the MME_B  42  on the basis of the MME-information updating process and reception of the fourth indication information. As described above, even when the MME selection on the basis of the MME-information updating process results in MME_B  42 , the eNB  45  may select the MME_A  40  when the fourth indication information has not been received. In addition, even when the fourth indication information has been received, the eNB  45  may select the MME_A  40  when the MME-information updating process has not been performed. 
     As described above, the eNB  45  can select either the MME_A  40  included in the core network (type  1 )  90  or the MME_B  42  included in the core network (type  2 )  92  based on one or both of the second indication information and the fourth indication information. 
     Note that the eNB  45  may select an MME included in the core network (type  2 )  92  when the timer is in operation, and may select an MME included in the core network (type  1 ) based on information transmitted by the UE  10  when the timer is not in operation or a core network does not have to be switched. 
     As for the selection method in such a case, selection may be made based on information transmitted by the UE  10 . For example, the UE  10  transmits the attach request message with information identifying an MME such as a GUMMEI included in the attach request message. When the received MME is available, the eNB may select the MME. Here, the eNB  45  may decide whether the MME is available on the basis of whether the MME has connectivity. In the present embodiment, the information transmitted by the UE  10  may be information identifying the MME_A  40  that has performed location management of the UE  10 . 
     When the MME received from the UE  10  is not available, the UE  10  may transmit the attach request message with identification information identifying a group of MMEs such as an MMEI, identification information identifying the UE  10  such as P-TMSI, location information such as tracking area ID (TAI) and tracking area code (TAC), or any information obtained by combining two or more pieces of the information described above that are included in the attach request message. The eNB  45  may receive these pieces of information, and select an MME based on these pieces of information. 
     Thus, the eNB  45  completes the MME selecting process. 
     Note that the eNB  45  may decide whether to perform the MME selecting process on the basis of a timer initiated in unison with the MME-information updating process. For example, when the timer is counting, the MME selecting process may be performed, and after the timer already ends or when the timer is not counting, the MME selecting process does not have to be performed. 
     More specifically, the eNB  45  may select the MME_B  42  when the timer is in operation, and may select the MME_A  40  when the timer is not in operation. Note that when the timer is not in operation, the eNB  45  may select an MME based on the identification information identifying an MME included in the attach request. For example, the eNB  45  may select an MME identified by the GUMMEI included in the attach request message, or may select an MME based on the MMEGI included in the attach request message, or may select a default MME. Note that in this case, the eNB  45  may select the MME_A  40 . 
     Note that the eNB  45  may stop the timer upon receiving the attach request message transmitted by the UE  10 . 
     Furthermore, when a terminal device that has not performed the MME-information updating process transmits the attach request message, the eNB  45  may select an MME based on the identification information identifying an MME included in the attach request. For example, the eNB  45  may select an MME identified by the GUMMEI included in the attach request message, the MMEGI included in the attach request message, or the MMEGI included in the attach request message, or may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     When the eNB  45  has selected the MME_B  42 , the eNB  45  transmits the attach request message to the MME_B  42  (S 822 ). Note that the attach procedure after transmission of the attach request message may involve transmitting and/or receiving of control messages and processes similar to those used in conventional attach procedures. Thus, a detailed description thereof will be omitted. 
     Furthermore, when the eNB  45  has selected the MME_A  40 , the eNB  45  may transmit the attach request message to the MME_A  40 . In addition, the eNB  45  may receive a route reconfiguration request from the MME_A  40 . Moreover, the eNB  45  may select the MME_B  42  in accordance with the route reconfiguration request, and transmit the attach request message to the MME_B  42 . Note that the attach procedure after transmission of the attach request message may involve transmitting and/or receiving of control messages and processes similar to those used in conventional attach procedures. Thus, a detailed description thereof will be omitted. 
     As described above, when the eNB  45  has selected the MME_B  42  through the MME selecting process, it is possible, for example, to perform the attach procedure without receiving a reroute reconfiguration request, and hence, transmitting and/or receiving of the control messages or other processes can be reduced. 
     Note that the MME_B  42  receives the attach request message from the eNB  45 , and selects an SGW and a PGW. Here, the MME_B  42  may select the SGW_B  37  and the PGW_B  32  for the core network (type  2 )  92 , and establish a PDN connection between the UE  10  and the PGW_B  32 . 
     Through the attach procedure, the UE  10  establishes a PDN connection with the PGW_B  32 . In addition, the PDN connection between the UE  10  and the PGW_B  32  is established via the SGW_B  37 . Moreover, with the established PDN connection, the UE  10  can transmit/receive user data to/from a communication device included in the PDN_B  102 . 
     Note that the core network (type  1 )  90  and the core network (type  2 )  92  are connected to the PDN_A  100  and PDN_B  102 , respectively, in the present procedure. However, as illustrated in  FIG. 6 , the PDN_A  100  and the PDN_B  102  may be the same network (PDN  100 ). In this case, the PGW_A  30  and the PGW_B  32  may be the same device (PGW_A  30 ). 
     In other words, with the present procedure, the UE  10  changes an MME that performs movement control from the MME_A  40  to the MME_B  42 . In addition, a new PDN connection may be established without changing a PGW serving as an end point of a PDN connection. Moreover, in the attach procedure described in the present procedure, the MME_B  42  may select the PGW_A  30  that serves as an end point of a PDN connection that the UE  10  established in the past. With this selection, the UE  10  may establish a PDN connection with the PGW_A  30 . 
     1.3.4 Fourth Procedure Example 
     Next, a fourth procedure example for changing a core network to which the UE  10  is connected will be described. The initial state of the procedure may be the same as the initial state described in the first procedure example. 
     In the initial state, the UE  10  has a connection established with the core network (type  1 )  90 . More specifically, the UE  10  has a connection established with the eNB  45 . In addition, the UE  10  has a connection established with the MME_A  40  via the eNB  45 . Moreover, the UE  10  has a PDN connection established with the PGW_A  30 . Here, the PDN connection refers to a communication path for delivering user data. 
     Note that the core network (type  1 )  90  and the core network (type  2 )  92  may both have different PGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the PGW_A  30 , and the core network (type  2 )  92  may be configured to include the PGW_B  32 . 
     Furthermore, the core network (type  1 )  90  and the core network (type  2 )  92  may both have different SGWs deployed therein. For example, the core network (type  1 )  90  may be configured to include the SGW_A  35 , and the core network (type  2 )  92  may be configured to include the SGW_B  37 . 
     The fourth procedure example will be described on the assumption that the UE  10  has a PDN connection established with the PGW_A  30  in the initial state. In addition, the PDN connection between the UE  10  and the PGW_A  30  is established via the SGW_A  35 . 
     Note that, in the fourth procedure example, the identification information identifying the UE  10  may be a globally unique temporary identity (GUTI) assigned globally and uniquely to the UE  10 . Alternatively, the identification information may be subscriber information such as an international mobile subscriber identity (IMGI). Alternatively, the identification information may be identification information such as a packet temporary mobile subscriber identity (P-TMSI). Alternatively, two or more pieces of the identification information may be combined to be the identification information identifying the UE  10 . 
     Furthermore, in the fourth procedure example, the identification information identifying an MME may be a globally unique MME identifier (GUMMEI) that corresponds to each MME. Note that the GUMMEI is globally unique identification information including information such as a PLMN, an MME group, and an MME code. Alternatively, the identification information identifying an MME may be an MME group identifier (MMEGI) identifying an MME group. 
     Note that the eNB  45  retains in advance an MMEGI and a GUMMEI that belongs to an MME group identified by the MMEGI with the MMEGI and the GUMMEI associated with each other. Thus, upon receiving an MMEGI, the eNB  45  can select a GUMMEI that belongs to the received MMEGI in based on the MMEGI. 
     For example, the eNB  45  may store in advance, in the MME information, an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ) and GUMMEIs corresponding to MMEs, such as the MME_B  42 , that belong to the group of MMEGI with the MMEGI and the GUMMEI associated with each other. 
     Specific steps of the fourth procedure example will be described below with reference to  FIG. 9 . 
     First, the MME_A  40  detects a trigger for starting a procedure of changing a core network to which the UE  10  is connected (S 902 ). 
     Note that the trigger detecting process may be the same as the trigger detecting process (S 502 ) described in the first procedure example. The trigger detecting process will be described below. 
     The MME_A  40  may detect the trigger on the basis of the updating of the subscriber information. More specifically, on the basis of a change in subscriber information corresponding to the UE  10  that is stored in the HSS  50 . The HSS  50  transmits, to the MME_A  40 , a control message including identification information indicating that the change has been made. The MME_A  40  may detect that the core network to which the UE  10  is connected needs to be changed, upon receiving the control message transmitted by the HSS  50  or based on the identification information included in the control message. 
     Furthermore, the identification information may be information on a core network indicating the core network type  2 , or information such as a flag indicating a request to change a core network type. In addition, the control message may be a control message for requesting the switching of a core network. Moreover, the control message may include identification information identifying the UE  10 . 
     The eNB  45  may receive an MMEGI corresponding to a group of MMEs that belong to the core network (type  2 ), and perform an MME selecting process. Note that the eNB  45  may select the MME_B  42  based on the received MMEGI. 
     The MME_A  40  may detect a trigger on the basis of an event that the core network (type  2 )  92  has been configured. More specifically, the procedure may be triggered by installation of a control device, such as the MME_B  42  or the PGW_B  32 , included in the core network (type  2 )  92 . Note that the operator of the core network may install such a control device, and manually operate the MME_A  40  after the installation. The MME_A  40  may detect a trigger on the basis of such manual operation by the operator. 
     Next, the MME_A  40  may transmit a route reconfiguration request message to the eNB  45  upon the detection of the trigger. The MME_A may transmit the route reconfiguration request message to request an update of information on an MME (S 906 ). The MME_A may transmit the route reconfiguration request message to request an update of information on an MME. 
     Here, the MME_A  40  may transmit the route reconfiguration request message with at least second indication information (Indicator  2 ) included in the route reconfiguration request message. 
     The second indication information may be information giving a request or an instruction for switching core networks. 
     Alternatively, the second indication information may be a core network type. Note that the core network type may be information identifying the core network (type  2 ). Specifically, the core network type may be network resource identifiers (NRIs) identifying the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information indicating that a core network type will be switched. More specifically, the information indicating that the core network type will be switched may be a cause value indicating that a core network needs to be switched. 
     Alternatively, the second indication information may be information indicating that the UE  10  needs to be re-attached. More specifically, the information indicating that the UE  10  needs to be re-attached may be a cause value indicating Attach Required. 
     Alternatively, the second indication information may be identification information identifying an MME. Here, the identification information identifying an MME may be information identifying an MME included in the core network (type  2 )  92 . In addition, the identification information identifying an MME may be a GUMMEI identifying the MME_B  42 . Moreover, the identification information identifying an MME may be an MMEGI identifying a group of MMEs included in the core network (type  2 )  92 . 
     Alternatively, the second indication information may be information obtained by combining two or more of: the information giving a request or an instruction for switching core networks; the core network type; the information indicating that a core network type is to be switched; the information indicating that re-attach is necessary; and the identification information identifying an MME, each of which has been described above. In addition, the second indication information obtained by combining two or more pieces of information as described above may be used as the core network type, the information indicating that a core network type is to be switched, the information indicating that re-attach is necessary, or the identification information identifying an MME. 
     The MME_A  40  may transmit a UE context release message with at least the identification information identifying the UE  10  and a timer value included in the transmission UE context release message. Note that the timer value may be the value of a timer used by the eNB  45  to select an MME. 
     The timer value may be configured to be a value equal to or greater than the time it takes for the UE  10  to perform the tracking area update procedure. Alternatively, the timer value may be configured to a value equal to or greater than a time interval up to when the UE  10  performs the next tracking area update procedure. Alternatively, the timer value may be configured to be a value equal to or greater than the remaining time of the tracking area update timer of the UE  10 . Note that the tracking area update timer is a timer initiated to count by the UE  10  and the MME_A  40  upon execution of the tracking area update procedure, and the UE  10  starts the tracking area update procedure upon expiration of the timer. Therefore, the remaining time of the tracking area update timer corresponds to the time up to when the next tracking area update procedure is started. 
     Next, the eNB  45  receives the route reconfiguration request message from the MME_A  40 . The eNB  45  may perform an MME-information updating process based on the information included in the route reconfiguration request message (S 908 ). More specifically, the eNB  45  may perform the MME-information updating process based on the second indication information. 
     Note that the MME-information updating process may be a process in which MME information is updated in a manner such that the eNB  45  selects an MME included in the core network (type  2 )  92 , at the time of MME selection, which is performed upon reception of a tracking area request message transmitted by the UE  10 . 
     More specifically, in the MME-information updating process, the eNB  45  may store the identification information identifying the UE  10  and the second indication information in association with each other. 
     Furthermore, when the second indication information has been acquired, the eNB  45  may retain the UE context information without deleting the UE context information. On the other hand, when the second indication information has not been acquired, the eNB  45  may delete the UE context information. 
     Furthermore, the eNB  45  may start the timer based on reception of the second indication information. While the timer is in operation, the eNB  45  may retain the updated MME information or the UE context. In addition, when the timer expires or the timer stops for some reason reason, the eNB  45  may delete the updated MME information or the UE context. 
     Here, a value retained in advance by the eNB  45  may be used as the value for the timer, or a timer value included in the UE context release message may be used. 
     Note that the timer value may be configured to a timer value equal to or greater than the duration of the tracking area update procedure performed by the UE  10 . Alternatively, the timer value may be configured to be a value equal to or greater than the remaining time of the tracking area update timer of the UE  10 . 
     Note that the eNB  45  may still retain information on an MME that the eNB  45  has already retained before the MME-information updating process is performed. For example, the eNB  45  may still retain the GUMMEI corresponding to the MME_A  40  included in the core network (type  1 )  90 , the MMEGI indicating a group of MMEs to which the MME_A  40  belongs, and the like. 
     The MME-information updating process is completed through the above-described steps. Furthermore, the eNB  45  may transmit the route reconfiguration response message to the MME_A  40  as a response to the route reconfiguration request message, on the basis of completion of the MME-information updating process (S 910 ). The eNB  45  may transmit the route reconfiguration response message to notify the completion of the MME-information updating process. 
     Furthermore, the eNB  45  may perform the MME-information updating process upon acquisition of the second indication information, and when the eNB  45  retains the MME information or the like through the MME-information updating process, the eNB  45  may transmit the route reconfiguration response message with information indicating that the UE context or the MME information on the core network (type  2 )  92  is included in the route reconfiguration response message. 
     The UE  10  may start the tracking area update procedure. Note that the UE  10  may start the tracking area update procedure upon expiration of the tracking area update timer, which counts the transmission duration of tracking area update requests. 
     Next, the tracking area update procedure will be described. The UE  10  transmits a tracking area update request message to the eNB  45  to start the tracking area update procedure (S 912 ). 
     The eNB  45  receives the tracking update request message from the UE  10 . The eNB  45  may perform the MME selecting process on the basis of reception of the tracking area request message (S 914 ). 
     In the MME selecting process, the eNB  45  selects the MME_B  42  on the basis the MME-information updating process. In addition, the eNB  45  may select the MME_A  40  when having not performed the MME-information updating process. 
     As described above, the eNB  45  can select either the MME_A  40  included in the core network (type  1 )  90  or the MME_B  42  included in the core network (type  2 )  92  in accordance with the second indication information. 
     Note that the eNB  45  may select an MME included in the core network (type  2 )  92  when the timer is in operation, and may select an MME included in the core network (type  1 ) based on information transmitted by the UE  10  when the timer is not in operation or a core network does not have to be switched. 
     The eNB  45  may select an MME included in the core network (type  1 ) in accordance with information transmitted by the UE  10  when the timer is not in operation or when a core network does not have to be switched. 
     As for the selection method in such a case, selection may be made based on information transmitted by the UE  10 . For example, the UE  10  transmits the tracking area update request message with information identifying an MME such as a GUMMEI included in the tracking area update request message. When the received MME is available, the eNB may select the received MME. Here, the eNB  45  may decide whether the MME is available on the basis of whether the MME has connectivity. In the present embodiment, the information transmitted by the UE  10  may be information identifying the MME_A  40  that has performed location management for the UE  10 . 
     When the MME received from the UE  10  is not available, the UE  10  may transmit the tracking area update request message with identification information identifying a group of MMEs such as an MMEI, UE  10  such as P-TMSI, location information such as tracking area ID (TAI) and tracking area code (TAC), or any information obtained by combining two or more pieces of the information described above included in the tracking area update request message. The eNB  45  may receive these pieces of information, and select an MME based on these pieces of information. 
     Thus, the eNB  45  completes the MME selecting process. 
     Note that the eNB  45  may decide whether to perform the MME selection process, on the basis of a timer initiated in unison with the MME-information updating process. For example, the MME selecting process may be performed when the timer is counting, and does not have to be performed after the timer has stopped or when the timer is not counting. 
     More specifically, the eNB  45  may select the MME_B  42  when the timer is in operation, and may select the MME_A  40  when the timer is not in operation. Note that when the timer is not in operation, the eNB  45  may select an MME based on the identification information identifying an MME included in the tracking area update request message. For example, the eNB  45  may select an MME identified by the tracking area update request message, an MME based on the MMEGI included in the tracking area update request message, or it may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     Note that the eNB  45  may stop the timer upon receiving the tracking area update request message transmitted by the UE  10 . 
     Furthermore, when a terminal device that has not performed the MME-information updating process transmits the tracking area update request message, the eNB  45  may select an MME based on the identification information identifying an MME included in the tracking area update request message. For example, the eNB  45  may select an MME identified by the GUMMEI included in the tracking area update request message, an MME based on the MMEGI included in the tracking area update request message, or it may select a default MME. Note that, in this case, the eNB  45  may select the MME_A  40 . 
     When the eNB  45  has selected the MME_B  42 , the eNB  45  transmits the tracking area request message to the MME_B  42  (S 916 ). Note that the tracking area update procedure after transmission of the tracking area request message may involve the transmitting and/or receiving of control messages and processes similar to those used in conventional tracking update procedures. Thus, a detailed description thereof will be omitted. 
     Furthermore, when the eNB  45  has selected the MME_A  40 , the eNB  45  may transmit the tracking area update request message to the MME_A  40 . In addition, the eNB  45  may receive a route reconfiguration request from the MME_A  40 . Moreover, the eNB  45  may select the MME_B  42  based on the route reconfiguration request, and transmit the tracking area update request message to the MME_B  42 . Note that the attach procedure after transmission of the tracking update request message may involve the transmitting and/or receiving of control messages and processes similar to conventional tracking update request messages. Thus, a detailed description thereof will be omitted. 
     As described above, when the eNB  45  has selected the MME_B  42  through the MME selection process, it is possible, for example, to execute the tracking update request message without receiving a reroute reconfiguration request, and hence, transmitting and/or receiving of the control messages or other processes can be reduced. 
     Once completing the tracking area update procedure, the UE  10  can communicate using the PDN connection. 
     Note that the core network (type  1 )  90  and the core network (type  2 )  92  are connected to the PDN_A  100  and PDN_B  102 , respectively, in the present procedure. However, as illustrated in  FIG. 6 , the PDN_A  100  and the PDN_B  102  may be the same network (PDN  100 ). In this case, the PGW_A  30  and the PGW_B  32  may be the same device (PGW_A  30 ). 
     In the present procedure, the UE  10  changes an MME that performs movement control from the MME_A  40  into the MME_B, but the PDN connection does not have to be reestablished. The UE  10  can continue communication using the PDN connection established with the PGW_A  30 . 
     1.3.5. Procedure Selection Process 
     The first procedure example to the fourth procedure example have been described in detail above, as procedures for changing a core network. Note that the MME_A  40  may decide the appropriate procedures from the first procedure example to the fourth procedure example based on the conditions. 
     For example, the MME_A  40  may select a procedure based on on the state of an attached UE  10 . More specifically, the MME _A  40  manages the state of an attached UE  10 . For example, the state of the UE  10  may be managed based on whether the UE is in an idle state in which a radio resource has been released or in an active state in which a radio resource has been allocated and data transmission and reception is available. 
     Moreover, when changing a core network, the MME_A  40  may select a procedure on the basis of whether or not the PGW needs be changed. Note that the decision of whether or not the PGW will to be changed may be made in accordance with the policy of a network operator. 
     Furthermore, the decision may be based on both the state of the UE  10  and the necessity for change regarding the PGW. For example, the MME_A  40  may select the first procedure when the UE  10  is in the active state and the PGW needs be changed. More specifically, when the UE  10  is in an active state and the PGW needs be changed, the UE  10  may transmit a detach request message (S 504 ) and subsequently perform the first procedure. 
     The MME_A  40  may select the second procedure when the UE  10  is in an active state and the PGW does not have to be changed. More specifically, when the UE  10  is in the active state and the PGW does not have to be changed, the UE  10  may transmit a UE context release message (S 704 ) and subsequently perform the second procedure. 
     The MME_A  40  may select the third procedure when the UE  10  is in an idle state and the PGW does not have to be changed. More specifically, when the UE  10  is in an idle state and the PGW does not have to be changed, the UE  10  may transmit a paging message (S 804 ) and subsequently perform the third procedure. 
     The MME_A  40  may select the fourth procedure when the UE  10  is in an idle state and the PGW does not have to be changed. More specifically, when the UE  10  is an idle state and the PGW does not have to be changed, the UE  10  may transmit a route reconfiguration request message (S 904 ) and subsequently perform the fourth procedure. 
     Note that the MME_A  40  may select any of the procedures after detecting a trigger to start the corresponding procedure. 
     The procedures described above make it possible to change a core network to which the UE  10  is connected on the basis of MME_A  40  initiation. 
     2. Variations 
     An embodiment of the invention and variations thereof have been described in detail thus far with reference to the drawings, but the specific configuration is not limited to the embodiment. Other designs and the like that do not depart from the essential spirit of the invention also fall within the scope of the patent claims. 
     Additionally, the program run on the devices in the embodiments are programs that control a CPU (programs that cause a computer to function) so as to realize the functions of the above-described embodiments. The information handled by these devices is temporarily held in a transitory storage device (RAM, for example) at the time of processing, and is then stored in various storage devices such as a ROM and an HDD, read out by the CPU as necessary, and edited and written. 
     Here, a semiconductor medium (a ROM, a non-volatile memory card, or the like, for example), an optical recording medium/magneto-optical recording medium (a digital versatile disc (DVD), a magneto optical disc (MO), a mini disc (MD), a compact disc (CD), a BD, or the like, for example), a magnetic recording medium (magnetic tape, a flexible disk, or the like, for example), and the like can be given as examples of recording media for storing the programs. In addition to realizing the functions of the above-described embodiments by executing programs that have been loaded, there are also cases where the functions of the present invention are realized by the programs running cooperatively with an operating system, other application programs, or the like on the basis of instructions included in those programs. 
     When delivering these programs to market, the programs can be stored in a portable recording medium, or transferred to a server computer connected via a network such as the Internet. In this case, the storage device serving as the server computer is of course also included in the present invention. 
     Additionally, each device in the above-described embodiment may be partially or completely realized as a large scale integration (LSI) circuit, which is a typical integrated circuit. The functional blocks of each device may be individually realized as chips, or may be partially or completely integrated into a chip. The circuit integration technique is not limited to LSI, and the integrated circuits for the functional blocks may be realized as dedicated circuits or a general purpose processor. Furthermore, if advances in semiconductor technology produce circuit integration technology capable of replacing LSI, it is of course possible to use integrated circuits based on the technology. 
     Additionally, although the above-described embodiment mentions LTE and a WLAN (IEEE 802.11a/b/n, for example) as examples of the wireless access network, the connections may be made with WiMAX instead of a WLAN. 
     REFERENCE SIGNS LIST 
       1  Communication system 
       10  UE 
       30  PGW_A 
       32  PGW_B 
       35  SGW_A 
       37  SGW_B 
       40  MME_A 
       42  MME_B 
       45  eNB 
       50  HSS 
       55  AAA 
       60  PCRF 
       65  ePDG 
       70  WLAN ANa 
       75  WLAN ANb 
       80  LTE AN 
       90  Core network (type  1 ) 
       92  Core network (type  2 ) 
       100  PDN_A 
       102  PDN_B