Abstract:
Provided is a technique to provide a handover processing method that allows a mobile terminal to be handed over seamlessly while preventing occurrence of double reservation. The method includes a step where a mobile terminal ( 100 ) decides to connect to a desired access network ( 113 ) and detects the possibility of occurrence of a network-initiated handover processing performed by a predetermined device of the communication network as one of the controlling entities, and a step where the mobile terminal transmits a connection request message requesting a connection to the desired access network, the connection request message including information to maintain the connection with the access network with which the mobile terminal is currently connecting, to a base station disposed in the desired access network based on a detection result of the detection step.

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a handover processing method of a mobile terminal in the case where a plurality of entities exists to control a mobile terminal handover and mobile terminal handovers by the respective control entities occur, and relates to a mobile terminal and a communication management device used in the method. 
       BACKGROUND ART  
       [0002]    In a large-scale network with mobility support for next-generation terminal, Proxy Mobile IP (PMIP) is available that is described in Non-Patent Document 1. According to PMIP, mobility (handover) of a mobile terminal (MN: Mobile Node) at an Internet Protocol (IP) layer is managed by a network side. Meanwhile, System Architecture Evolution (SAE) of 3rd Generation Partnership Project (3GPP) specifies a next-generation Packet Switched (PS) network (see Non-Patent Document 2, Non-Patent Document 3).  FIG. 14  (described in Non-Patent Document 3) illustrates a partial network diagram. 
         [0003]    In a 3GPP access, an eNodeB (not illustrated) as a radio access point for a User Equipment (UE) and a Mobility Management Entity (MME) (not illustrated) managing a handover at a link layer exist. As shown in  FIG. 14 , both of a handover of a UE as a 3GPP mobile terminal between 3GPP accesses and a handover thereof between a 3GPP access and a non-3GPP network are supported in Non-Patent Document 2. In order to manage this handover of a UE at an IP layer, PMIP is mainly used. 
         [0004]      FIG. 15  illustrates an exemplary mobility signaling flow for a UE between 3GPP accesses described in Non-Patent Document 2. As shown in  FIG. 15 , preparation for a handover is performed by a network side (Step S 1501 ), a handover starts (Step S 1502 ), and the handover is completed (Step S 1503 ). The mobility herein is decided by a network side, and the handover is initiated by a network side (a network initiated handover). In this case, the UE cannot notice the handover being implemented. 
         [0005]      FIG. 16  illustrates an exemplary mobility signaling flow for a UE from a 3GPP access to a Trusted non-3GPP IP access (simply called a non-3GPP access as well) described in Non-Patent Document 3. As shown in  FIG. 16 , the handover herein is initiated by a UE side (a UE initiated handover). Assume herein that the UE has already established a communication bearer with the 3GPP access (Step S 1601 ). As a result of checking a measurement result of radio waves receivable from the non-3GPP access network with respect to a handover policy, the UE decides to start a handover to the non-3GPP access (Step S 1602 ), and starts an access authentication processing to connect with the non-3GPP access (Step S 1603 ). When the access authentication is completed so that a connection is authorized (authentication, authorization), a connection processing at a layer 3 (IP layer) starts (L3 (Layer 3) attach trigger: Step S 1604 ). 
         [0006]    As a result, a node (e.g., a not-illustrated Mobile Access Gateway (MAG) in the non-3GPP access network acquires and sets QoS policy and accounting setting information to be set with a core network from a PCRF (gateway control session configuration request (Gateway Control and QoS Policy Rules Request): Step S 1605 , gateway control session configuration response (Ack Gateway Control and QoS Policy Rules Request): Step S 1606 ). Subsequently, the MAG transmits a proxy binding update (proxy BU) to a PDN gateway (PDN-GW) (Step S 1607 ), and the PDN-GW acquires and sets a QoS policy and accounting setting information from the PCRF (policy and accounting information request (Modification of IP-CAN Session): Step S 1608 , policy and accounting information provisioning (Ack IP-CAN Session Modification): Step S 1609 ). 
         [0007]    The PDN-GW further performs AAA authentication (Step S 1610 ). When these processes succeed, the PDN-GW returns a proxy BA (Binding Ack) to the MAG (Step S 1611 ), so that a PMIPv6 tunnel is configured between the MAG and the PDN-GW (Step S 1613 ). Receiving the proxy BA, the MAG transfers an IP address/prefix distributed from the PDN-GW to the UE so as to complete the connection processing at a L3 level (Step S 1612 ). Then, finally, a 3GPP bearer of a handover source access is released to complete the UE initiated handover processing (3GPP EPS bearer resource release: Step S 1614 ). Herein, the 3GPP EPS bearer may be released by the UE or by the PDN-GW. 
         [0008]      FIG. 17  illustrates an exemplary Bearer (link level connection between a UE and a PDN-GW in 3GPP) Release (bearer release) signaling flow described in Non-Patent Document 2. As shown in  FIG. 17 , receiving a radio bearer release request (Step S 1701 ), a UE transmits a radio bearer release response (Step S 1702 ) to implement bearer release. 
       PRIOR ART DOCUMENT  
     Non-Patent Document  
       [0009]    [Non-Patent Document 1] S. Gundavelli et al, “Proxy Mobile IPv6”, draft-ietf-netlmm-proxymip6-11.txt, Feb. 25, 2008. 
         [0010]    [Non-Patent Document 2] “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access (Release 8)”, 3GPP TS 23.401 V8.1.0 March 2008. 
         [0011]    [Non-Patent Document 3] “Architecture enhancements for non-3GPP accesses (Release 8)”, 3GPP TS 23.402 V8.1.1 March 2008. 
         [0012]    Assume herein that a UE currently is connecting with a 3GPP access for communication. Herein, while the UE finds a non-3GPP access and tries to switch a connection therewith, the 3GPP access side starts a network initiated handover. Then, handovers on both sides will fail (interrupted), and further wasted signaling will occur for the bearer release.  FIG. 18  illustrates one example thereof. 
         [0013]    In  FIG. 18 , after signaling relating to a handover from a 3GPP access to a non-3GPP access is completed, a bearer release processing transmitted from a network side (Step S 1806 : Delete Bearer Request) causes a failure in a handover from a 3GPP access to a 3GPP access (interruption occurs in data transmission/reception) and further a handover signal (Step S 1807 : Update Bearer Request) from a 3GPP access to a 3GPP access updates handover information (position registration information of a UE), thus leading to a failure in a handover from a 3GPP access to a non-3GPP access as well. 
         [0014]    Thus, when 3GPP is disconnected and then a connection is carried out with a non-3GPP, a problem (a break before make problem) occurs that a handover is not carried out seamlessly. When 3GPP is not disconnected, a problem of double reservation will occur. 
       SUMMARY OF THE INVENTION  
       [0015]    In view of the above-stated problems, it is an object of the present invention to provide a handover processing method enabling a seamless handover of a mobile terminal and capable of avoiding double reservation, and to provide a mobile terminal and a communication management device used in the method. 
         [0016]    In order to fulfill the above-stated object, the present invention provides a handover processing method for a mobile terminal, the mobile terminal performing communication with a correspondent node via a communication network including a plurality of access networks in a communication system, a plurality of control entities that control a handover of the mobile terminal existing in the communication system. The handover processing method includes the steps of: a step where the mobile terminal decides to connect with a desired access network among the plurality of access networks; a step where the mobile terminal detects a possibility of occurrence of a network initiated handover processing a predetermined device of the communication network performs as one of the control entities; and a step where the mobile terminal transmits, based on the detection, a connection request message to a base station disposed in the desired access network, the connection request message requesting a connection with the desired access network and including predetermined information to maintain a connection with an access network with which the mobile terminal is currently connecting. This configuration allows a mobile terminal to be handed over seamlessly while preventing occurrence of double reservation. The present invention assumes the case where at least one base station exists in an access network. 
         [0017]    In the handover processing method of the present invention, the possibility of occurrence of a network initiated handover processing preferably is detected based on whether or not radio wave intensity the mobile terminal receives being predetermined radio wave intensity or lower, or whether or not the mobile terminal transmitting a report indicating a result of examination on an access network environment surrounding the mobile terminal. This configuration enables appropriate detection of the possibility of occurrence of a network initiated handover processing. 
         [0018]    In the handover processing method of the present invention, the predetermined information preferably includes information indicating a policy of a handover processing when a handover of the mobile terminal is occurred. This configuration allows a release processing of the current connection to be temporarily put on hold. 
         [0019]    In the handover processing method of the present invention, when the mobile terminal detects the network initiated handover processing after the connection request message is transmitted, the mobile terminal preferably transmits, to the base station disposed in the desired access network, a release message to disconnect the connection with the desired access network. This configuration can avoid double reservation. 
         [0020]    In the handover processing method of the present invention, the predetermined information preferably includes information to make a predetermined device of the communication network managing communication with the correspondent node conduct a processing in accordance with information indicating a policy of a handover processing of the mobile terminal, the information being kept by the predetermined device. This configuration allows a release processing of the current connection to be temporarily put on hold. 
         [0021]    The present invention provides a handover processing method for a mobile terminal, the mobile terminal performing communication with a correspondent node via a communication network including a plurality of access networks in a communication system, a plurality of control entities that control a handover of the mobile terminal existing in the communication system. The handover processing method includes the steps of: when a predetermined device of the communication network managing communication of the mobile terminal with the correspondent node receives a first message and a second message, the first message from the mobile terminal requesting connection switching to a base station in a desired access network among the plurality of access networks, and the second message requesting connection switching to a base station of a connection target of the mobile terminal that the communication network decides as an entity, a step where a predetermined device of the communication network determines whether or not a message to release a connection of the mobile terminal with a base station with which the mobile terminal is currently connecting is transmitted to the base station with which the mobile terminal is currently connecting after reception of the first message and before reception of the second message; a step of, when it is determined that the message is transmitted, deciding not to accept a processing of the second message; and a step of, when it is determined that the message is not transmitted, determining that the second message is valid and performing a processing in accordance with the second message. This configuration allows a mobile terminal to be handed over seamlessly while preventing occurrence of double reservation. Herein, the first and the second messages correspond to position registration messages described later. 
         [0022]    In the handover processing method of the present invention, when it is decided not to accept a processing of the second message, a predetermined device of the communication network preferably transmits an error message indicating not acceptance to a base station that transmits the second message. With this configuration, the mobile terminal can notice that a network initiated handover will not be implemented. 
         [0023]    In the handover processing method of the present invention, when a processing is performed in accordance with the second message, a predetermined device of the communication network preferably transmits an error message indicating that the mobile terminal cannot perform a desired connection to the mobile terminal via a base station that transmits the first message. With this configuration, the mobile terminal can tell that a UE initiated handover will not be implemented. 
         [0024]    The present invention provides a mobile terminal used in a handover processing method for the mobile terminal, the mobile terminal performing communication with a correspondent node via a communication network including a plurality of access networks in a communication system, a plurality of control entities that control a handover of the mobile terminal existing in the communication system, there being a possibility of each of the plurality of control entities generating a handover processing of the mobile terminal. The mobile terminal includes: decision unit that decides a connection with a desired access network among the plurality of access networks; detection unit that detects a possibility of occurrence of a network initiated handover processing a predetermined device of the communication network performs as one of the control entities; message generation unit that generates a connection request message based on the detection by the detection unit, the connection request message requesting a connection with the desired access network and including predetermined information to maintain a connection with an access network with which the mobile terminal is currently connecting; and transmission unit that transmits the generated connection request message to a base station disposed in the desired access network. This configuration allows a mobile terminal to be handed over seamlessly while preventing occurrence of double reservation. 
         [0025]    In the mobile terminal of the present invention, the possibility of occurrence of a network initiated handover processing is preferably detected by the detection unit based on whether or not radio wave intensity the mobile terminal receives being predetermined radio wave intensity or lower, or whether or not the mobile terminal transmitting a report indicating a result of examination on an access network environment surrounding the mobile terminal. This configuration enables appropriate detection of the possibility of occurrence of a network initiated handover processing. 
         [0026]    In the mobile terminal of the present invention, the predetermined information preferably includes information indicating a policy of a handover processing when a handover of the mobile terminal is occurred. This configuration allows a release processing of the current connection to be temporarily put on hold. 
         [0027]    In the mobile terminal of the present invention, when detecting the network initiated handover processing after the connection request message is transmitted, preferably the message generation unit generates a release message to disconnect the connection with the desired access network, and the transmission unit transmits the generated release message to the base station disposed in the desired access network. This configuration can avoid double reservation. 
         [0028]    In the mobile terminal of the present invention, the predetermined information preferably includes information to make a predetermined device of the communication network managing communication with the correspondent node conduct a processing in accordance with information indicating a policy of a handover processing of the mobile terminal, the information being kept by the predetermined device. This configuration allows a release processing of the current connection to be temporarily put on hold. 
         [0029]    The present invention provides a communication management device used in a handover processing method for a mobile terminal, the mobile terminal performing communication with a correspondent node via a communication network including a plurality of access networks in a communication system, a plurality of control entities that control a handover of the mobile terminal existing in the communication system. The communication management device managing the communication includes: reception unit that receives a first message and a second message, the first message from the mobile terminal requesting connection switching to a base station in a desired access network among the plurality of access networks, and the second message requesting connection switching to a base station of a connection target of the mobile terminal that the communication network decides as an entity; transmission unit that transmits a release message to release a connection of the mobile terminal with a base station with which the mobile terminal is currently connecting to the base station with which the mobile terminal is currently connecting; determination unit that, after reception of the first message and before reception of the second message, determines whether or not the transmission unit transmits the release message to the base station with which the mobile terminal is currently connecting; and processing unit that, when it is determined that the message is transmitted, decides not to accept a processing of the second message; and when it is determined that the message is not transmitted, determines that the second message is valid and performs a processing in accordance with the second message. This configuration allows a mobile terminal to be handed over seamlessly while preventing occurrence of double reservation. The communication management device herein corresponds to a SG described later. 
         [0030]    The communication management device of the present invention preferably further includes message generation unit that, when the processing unit decides not to accept a processing of the second message, generates an error message indicating not acceptance. The transmission unit preferably transmits the generated error message to a base station that transmits the second message. With this configuration, the mobile terminal can tell that a network initiated handover will not be implemented. 
         [0031]    The communication management device of the present invention preferably further includes message generation unit that, when the processing unit performs a processing in accordance with the second message, generates an error message indicating that the mobile terminal cannot perform a desired connection. The transmission unit preferably transmits the generated error message to the mobile terminal via a base station that transmits the first message. With this configuration, the mobile terminal can notice that a UE initiated handover will not be implemented. 
         [0032]    A handover processing method, and a mobile terminal and a communication management device used in the method of the present invention allow a mobile terminal to be handed over seamlessly while preventing occurrence of double reservation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0033]      FIG. 1  is a configuration diagram illustrating an exemplary configuration of a mobile communication system in Embodiments 1 and 2 of the present invention. 
           [0034]      FIG. 2  is a configuration diagram illustrating an exemplary configuration of a UE according to Embodiment 1 of the present invention. 
           [0035]      FIG. 3  is a flowchart illustrating an exemplary method to detect a possibility of occurrence of a network initiated handover in Embodiment 1 of the present invention. 
           [0036]      FIG. 4  is a flowchart illustrating another exemplary method to detect a possibility of occurrence of a network initiated handover in Embodiment 1 of the present invention. 
           [0037]      FIG. 5  is an exemplary message format in Embodiments 1 and 2 of the present invention. 
           [0038]      FIG. 6  is a sequence chart illustrating an exemplary message sequence in the processing in Embodiment 1 of the present invention. 
           [0039]      FIG. 7  is a sequence chart illustrating another exemplary message sequence in the processing in Embodiment 1 of the present invention. 
           [0040]      FIG. 8  is a sequence chart illustrating still another exemplary message sequence in the processing in Embodiment 1 of the present invention. 
           [0041]      FIG. 9  is a configuration diagram illustrating an exemplary configuration of a SG according to Embodiments 1 and 2 of the present invention. 
           [0042]      FIG. 10  is a flowchart illustrating an exemplary analysis flow to analyze the content of a message by the SG according to Embodiment 1 of the present invention. 
           [0043]      FIG. 11  is a sequence chart illustrating an exemplary message sequence in the processing in Embodiment 2 of the present invention. 
           [0044]      FIG. 12  is a sequence chart illustrating another exemplary message sequence in the processing in Embodiment 2 of the present invention. 
           [0045]      FIG. 13  is a flowchart illustrating an exemplary processing by policy enforcement unit of the SG according to Embodiment 2 of the present invention. 
           [0046]      FIG. 14  illustrates an exemplary conventional network. 
           [0047]      FIG. 15  is a sequence chart illustrating one example of a conventional mobility signaling sequence for a UR between 3GPP accesses. 
           [0048]      FIG. 16  is a sequence chart illustrating one example of a conventional mobility signaling sequence for a UE from a 3GPP access to a Trusted non-3GPP access IP access. 
           [0049]      FIG. 17  is a sequence chart illustrating one example of a conventional signaling sequence for bearer release. 
           [0050]      FIG. 18  is a sequence chart to describe a problem in the conventional UE handover. 
           [0051]      FIG. 19  is a sequence chart illustrating exemplary UE initiated release message transmission. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
     Embodiment 1  
       [0052]    Referring to  FIG. 1 , Embodiment 1 of the present invention is described below. An access domain (AD)  111  and an AD  112  are access networks of a same type, and an AD  113  is an access network of a different type from the AD  111  and the AD  112 . In the AD  111 , the AD  112  and the AD  113 , a Base Station (BS)  105 , a BS  106  and a BS  107  as radio access points for a mobile terminal MN  100  (hereinafter the MN  100  may be described as a UE  100 ) exist respectively, and the BS  105 , the BS  106  and the BS  107  connect to an access gateway (AG)  101 , an AG  102  and an AG  103 , respectively. 
         [0053]    The AG manages the mobility in the AD of the MN  100  connected with the AD. The AG  101 , the AG  102  and the AG  103  each connect with a Service Gateway (SG)  104 . The SG  104  manages mobility of the MN  100  between ADs and mobility when an address of the MN  100  is changed in an AD. The ADs may be 3GPP access networks such as UMTS Terrestrial Radio Access Network (UTRAN), GSM EDGE Radio Access Network (GERAN) and Evolved UTRAN (EUTRAN); non-3GPP access networks such as Wireless LAN (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), High Rate Packet Data (HRPD; standardized at 3GPP2) and Next Generation Network (NGN); or Interworking Wireless LAN (I-WLAN). 
         [0054]    The BSs may be 3GPP base stations (e.g., eNodeB) or access points such as WLAN, WiMAX and I-WLANs, for example. The AGs may be a Serving Gateway described in Non-Patent Document 2, or an ePDG or an Access Gateway (AGW) described in Non-Patent Document 3. The AGs may be a MAG described in Non-Patent Document 1. The Service Gateway may be a PDN GW described in Non-Patent Document 2 or a home agent or a local mobility anchor (LMA) described in Non-Patent Document 1, for example. The MN  100  connects with an Application Service Domain not illustrated via these nodes, and receives an application service. The Application Service Domain may be the Internet, an IP packet network such as an IMS (IP Multimedia Subsystem) network each operator company provides or other Packet Data Networks (PDNs), for example. Hereinafter, the AG  101 , the AG  102  and the AG  103  may be described as the AD  111 , the AD  112  and the AD  113 , respectively. Among them, the MN and the BSs are examples of control entities relating to a decision of a MN handover. 
         [0055]    Now the MN  100  tries a handover from the AD  111  to the AD  113 . At this time, when the MN  100  finds a possible network initiated handover being initiated from the AD  111  to the AD  112 , the MN  100  sends, to the SG  104 , a policy as a handover processing policy so that a handover procedure from the AD  111  to the AD  113  does not interfere with the network initiated handover from the AD  111  to the AD  112 . 
         [0056]    The possibility of occurrence of a network initiated handover is detected (recognized) by the following means, for example. As first means, the weakness of radio waves received from a network (access network) currently connecting is detected, whereby the possibility of handover occurrence can be detected. As second means, transmission of a measurement report determined as inducing a network initiated handover in a network currently connecting is detected, whereby the possibility of handover occurrence can be detected. For instance, the possibility of handover occurrence can be detected by detecting transmission of a report indicating that a wireless environment in the AD  112  is overwhelmingly more preferable than in the AD  111 . 
         [0057]    Further, a history indicating whether a network initiated handover being implemented or not may be checked with respect to the contents of the measurement reports transmitted so far, whereby a condition to initiate a network initiated handover is estimated, or a condition to initiate a network initiated handover may be downloaded and used for the estimation. 
         [0058]    The above-stated policy is included in a connection request message to the AD  113  and is transmitted to the SG  104  (a node through which signaling relating to both of the handover from the AD  111  to the AD  112  and the handover from the AD  111  to the AD  113  transit). Alternatively, the policy may be transmitted beforehand to the SG  104 , and a trigger to make this policy valid may be included in a connection request message to the AD  113  and be transmitted to the SG  104 . 
         [0059]    This policy implements the followings, for example: 1. To put on hold in the SG  104  a release processing (e.g., Delete Bearer Request at Step S 1806  of  FIG. 18 ) to the AD  111  path following the handover from the AD  111  to the AD  113 ; 2. In a state where the UE  100  completes the handover to the AD  113  and is ready to exchange of data with a correspondent node and when it is determined that the connection status maintained in the AD  111  becomes stable again (radio waves status becomes stable, for example), to transmit a release message initiated by the UE (to authorize transmission by the UE  100 ) (e.g., Request Bearer Resource Release of Non-Patent Document 2, see  FIG. 19 ); and 3. When the UE  100  detects that a network initiated handover from the AD  111  to the AD  112  occurs during a handover processing from the AD  111  to the AD  113  or immediately after the handover to the AD  113  is completely performed, the UE  100  makes a path through the AD  113  released. Thereby, a handover of the UE  100  becomes seamless and double reservation of the handover can be avoided. 
         [0060]      FIG. 2  illustrates an exemplary configuration of a UE according to Embodiment 1. Network initiated handover prediction unit  201  is means to detect (understand) a possibility of occurrence of the above-stated network initiated handover.  FIG. 3  and  FIG. 4  illustrate an exemplary detection method.  FIG. 3  is a flowchart illustrating the case where a handover possibility is recognized by detecting weakness of radio waves received from a network (access network) currently connecting. The network initiated handover prediction unit  201  determines whether received radio waves at a 3G interface is weak or not (for example, lower than predetermined radio wave intensity) (Step S 301 ), and if it is determined as weak, determines that a network initiated handover from a 3G to a 3G may occur (Step S 302 ). On the other hand, if it is not determined as weak, the network initiated handover prediction unit  201  determines that a network initiated handover from a 3G to a 3G will not occur for a while (Step S 303 ). 
         [0061]      FIG. 4  is a flowchart illustrating the case where a handover possibility is recognized by detecting transmission of a measurement report determined as inducing a network initiated handover. The network initiated handover prediction unit  201  determines whether a measurement report determined as inducing a network initiated handover is transmitted or not (Step S 401 ). If it is determined that the measurement report is transmitted, the network initiated handover prediction unit  201  determines that a network initiated handover from a 3G to a 3G may occur (Step S 402 ). On the other hand, if it is not determined that the measurement report is transmitted, the network initiated handover prediction unit  201  determines that a network initiated handover from a 3G to a 3G will not occur for a while (Step S 403 ). 
         [0062]    Message generation unit  202  is means to generate a message to transmit the above-stated policy and a trigger to make the policy valid.  FIG. 5  is an exemplary message format that the message generation unit  202  generates. Herein, connection request messages  501 ,  503  to the AD  113  shown in  FIG. 5  may be expansion of a Router Solicitation message described in Non-Patent Document 1 or a totally new message, for example, and position registration messages  502 ,  504  to the SG  104  may be a Proxy Binding Update message described in Non-Patent Document 1 or a message having a function similar thereto, for example. 
         [0063]    Transmission interface selection unit  203  is means to select an interface from which a message is transmitted, i.e., whether a message is to be transmitted from an interface on the AD  111  (and the AD  112 ) side or from an interface from the AD  113  side. Radio reception unit  204  is means to receive a message or the like from the outside, and radio transmission unit  205  is means to transmit a message or the like to the outside. 
         [0064]      FIG. 6  through  FIG. 8  illustrate a message flow in Embodiment 1.  FIG. 6  is a flow where the policy is included in the connection request message to the AD  113  and is transmitted to the SG  104 , and after the UE  100  detects that the connection on the AD  111  side becomes stable, the UE  100  transmits a release request on the AD  111  side. As shown in  FIG. 6 , the UE  100  connects with the AD  111  (Step S 601 ), and the UE  100  decides a handover to the AD  113  (Step S 602 ). 
         [0065]    Thereafter or at the same time, the UE  100  detects (understands) a possibility of a network initiated handover occurring (Step S 603 ). The UE  100  transmits a connection request message including a policy to the AD  113  (Step S 604 ), and the AD  113  transmits a position registration message including the policy to the SG  104  (Step S 605 ). Based on the policy, the SG  104  puts on hold a release processing to the AD  111  (Step S 606 ), and transmits a position registration response message to the AD  113  (Step S 607 ). Based on the position registration response message, the AD  113  transmits a message indicating that a connection with the AD  113  is OK (Step S 608 ). The UE  100  detects that the connection on the AD  111  side becomes stable (Step S 609 ) and determines that a network initiated handover will not occur, and thereafter the UE  100  transmits a release request to the bearer on the AD  111  side (Step S 610 ). 
         [0066]      FIG. 7  illustrates a flow, in the case where the SG  104  keeps a policy of the UE  100  beforehand, to make this policy valid by a trigger. As shown in  FIG. 7 , the UE  100  connects with the AD  111  (Step S 701 ), and the UE  100  decides a handover to the AD  113  (Step S 702 ). At this time, the SG  104  keeps a policy of the UE  100  beforehand. Thereafter, the UE  100  detects (understands) a possibility of a network initiated handover occurring (Step S 703 ). 
         [0067]    The UE  100  transmits, to the AD  113 , a connection request message including a trigger to make the policy valid (Step S 704 ), and the AD  113  transmits a position registration message including the trigger to the SG  104  (Step S 705 ). Based on the trigger, the SG  104  makes the policy valid, puts on hold a release processing to the AD  111  (Step S 706 ), and transmits a position registration response message to the AD  113  (Step S 707 ). Based on the position registration response message, the AD  113  transmits a message indicating that a connection with the AD  113  is OK (Step S 708 ). The UE  100  detects that the connection on the AD  111  side becomes stable (Step S 709 ), and determines that a network initiated handover will not occur, and thereafter the UE  100  transmits a release request on the AD  111  side (Step S 710 ). 
         [0068]      FIG. 8  illustrates a flow in the case where a network initiated handover occurs during a handover processing from the AD  111  to the AD  113 . As shown in  FIG. 8 , the UE  100  connects with the AD  111  (Step S 801 ), and the UE  100  decides a handover to the AD  113  (Step S 802 ). Thereafter, the UE  100  detects (understands) a possibility of a network initiated handover occurring (Step S 803 ). The UE  100  transmits a connection request message including the policy to the AD  113  (Step S 804 ). Thereafter, when detecting a network initiated handover during the handover processing (Step S 805 ), the UE  100  transmits a release request to the AD  113  (Step S 806 ). 
         [0069]    Herein, the above-stated transmission of the release request to the AD  113  (Step S 806 ) may be implemented when it is recognized (or can be determined) that a network initiated handover to the AD  112  succeeds following the handover to the AD  113  or a network initiated handover is superior to a UE initiated handover, in addition to the detection of a network initiated handover during a handover processing (Step S 805 ). Herein, the release processing to the AD  113  may be subject to a further condition that a connection OK message to the AD  113  cannot be received within a predetermined time period. 
         [0070]    The position registration message and the position registration response message of  FIG. 6  to  FIG. 8  are signaling for handover, which may be Proxy Binding Update or Proxy Binding Ack described in Non-Patent Document 1 (and  FIG. 18 ) or Update Bearer Request or Update Bearer Response of  FIG. 18 . 
         [0071]      FIG. 9  illustrates an exemplary configuration of the SG  104  of Embodiment 1, Reception unit  901  is means to receive a message transmitted, and transmission unit  902  is means to transmit a message. Message analysis unit  903  is means to analyze the content of a message transmitted, for example, to analyze whether the position registration message from the AD  113  includes a policy, includes a trigger, or includes nothing. 
         [0072]      FIG. 10  illustrates an exemplary analysis flow. As shown in  FIG. 10 , the message analysis unit  903  analyzes whether a position registration message includes a policy or not (Step S 1001 ). If a policy is included, the message analysis unit  903  instructs policy enforcement unit  905  to perform a predetermined processing (Step S 1002 ), If no policy is included, the message analysis unit  903  analyzes whether the position registration message includes a trigger or not (Step S 1003 ). If a trigger is included, the message analysis unit  903  instructs policy storage unit  904  to make the kept policy valid (Step S 1004 ). If no trigger is included, the message analysis unit  903  instructs message generation unit  906  to generate a predetermined message (Step S 1005 ). 
         [0073]    In the case where a policy of the UE  100  is transmitted beforehand, the policy storage unit  904  keeps the policy, and if the message analysis unit  903  determines that a position registration message includes a trigger, such a policy is made valid. The policy enforcement unit  905  implements a policy included in the position registration message or the policy that the policy storage unit  904  keeps and is made valid. The message generation unit  906  generates a message in accordance with policy implementation by the policy enforcement unit  905  (or in a conventional manner in the absence of a policy). 
       Embodiment 2  
       [0074]    Since a communication network of Embodiment 2 is similar to that of Embodiment 1, the following describes Embodiment 2 with reference to  FIG. 1 . The UE  100  transmits beforehand, to the SG  104 , a possibility of a handover of itself to the AD  113  (different type of network) and a policy relating to a timing of detach from the SG  104 . 
         [0075]    This policy includes the followings, for example: 1. As shown in  FIG. 11 , immediately after the SG  104  transmits (Step S 1106 ) a message relating to a release processing following a handover from the AD  111  to the AD  113 , when the SG  104  receives a position registration message from the AD  112  (Step S 1107 ), to make the SG  104  not accept the position registration message (Step S 1108 ), but to make the SG  104  return an error to the AD  112  (Step S 1109 ); 2. Alternatively, as shown in  FIG. 12 , when the SG  104  receives a position registration message from the AD  112  before transmitting a message relating to a release processing to the AD  111  (Step S 1206 ), to make the SG  104  determine that the position registration message from the AD  112  is valid (Step S 1207 ) and to make the SG  104  not transmit a message relating to a release processing for a path of the AD  111 . In this case, the UE  100  may notice that a handover from the AD  111  to the AD  113  fails or the SG  104  may return an error to the UE  100  (Step S 1208 ). 
         [0076]    Herein, the UE  100  may detect a failure in handover from the AD  111  to the AD  113  based on a status where link establishment with the AD  112  succeeds or where a connection OK message to the AD  113  cannot be received within a predetermined time period. 
         [0077]    Herein, the position registration message and the position registration response message of  FIG. 11  and  FIG. 12  may be Proxy Binding Update and Proxy Binding Ack described in Non-Patent Document 1 and  FIG. 18  or Update Bearer Request or Update Bearer Response of  FIG. 18 , for example. The message of the release processing of  FIG. 11  may be Delete Bearer Request at Step S 1806  of  FIG. 18 . The policy in this case may be transmitted to the SG  104  beforehand (not necessarily at the timing when the UE  100  implements a handover to the AD  113 ). This policy may be transmitted while being added to a message when position registration of the UE  100  is firstly performed with respect to the SG  104 , or may be transmitted with a totally new message. Alternatively, the network side may keep this policy beforehand as a policy of the UE  100 . 
         [0078]    The following describes an exemplary configuration of the SG in Embodiment 2. Since elements thereof are similar to those of Embodiment 1, the following description refers to  FIG. 9 . Reception unit  901  is means to receive a message transmitted, and transmission unit  902  is means to transmit a message. Message analysis unit  903  is means to analyze the order of transmitted messages. Policy storage unit  904  keeps a policy of the UE  100  transmitted beforehand, and policy enforcement unit  905  is means to decide a message to be transmitted in accordance with the order of transmitted messages. Message generation unit  906  generates a message decided by the policy enforcement unit  905 , and passes the same to the transmission unit  902  in the decided order. 
         [0079]    Referring now to  FIG. 13 , the following describes an exemplary processing by the policy enforcement unit  905 . After receiving a position registration message from the AD  113  via the reception unit  901  (Step S 1301 ), the policy enforcement unit  905  determines whether a position registration message is transmitted or not from the AD  112  before transmitting a message of a release processing to the AD  111  (Step S 1302 ). If the position registration message is not transmitted from the AD  112 , the policy enforcement unit  905  determines whether a position registration message is received or not from the AD  112  immediately after transmitting a message of a release processing to the AD  111  (Step S 1303 ). 
         [0080]    In the case where the position registration message is received from the AD  112 , the policy enforcement unit  905  returns an error message (including a message of a handover failure) to the AD  112  (Step S 1304 ). On the other hand, in the case where the position registration message is not received from the AD  112 , the policy enforcement unit  905  performs a normal processing (Step S 1305 ). In the case where it is determined at Step S 1302  that a position registration message is transmitted from the AD  112  before transmitting a message of a release processing to the AD  111 , the policy enforcement unit  905  returns an error message (including a message of a handover failure) to the AD  113  (Step S 1306 ). 
         [0081]    Note that each functional block used in the explanations of each embodiment of the present embodiment may be typically implemented as a LSI that is an integrated circuit. These blocks may be individually configured as one chip, or one chip may include a part or all of the functional blocks. LSIs may be called an IC (Integrated Circuit), a system LSI, a super LSI, and an ultra LSI depending on the degree of integration. 
         [0082]    A technique for integrated circuit is not limited to a LSI, but an integrated circuit may be achieved using a dedicated circuit or a general-purpose processor. A FPGA (Field Programmable Gate Array) capable of programming after manufacturing a LSI and a reconfigurable processor capable of reconfiguring connection and setting of a circuit cell inside a LSI may be used. 
         [0083]    Further, if a technique for integrated circuit that replaces LSIs becomes available by the development of a semiconductor technique or derived techniques, functional blocks may be naturally integrated using such a technique. For instance, biotechnology may be applied thereto. 
       INDUSTRIAL APPLICABILITY  
       [0084]    A handover processing method and a mobile terminal and a communication management device used in the method of the present invention enable a seamless handover of the mobile terminal, and can avoid double reservation. Therefore the handover processing method and the mobile terminal and the communication management device used in the method of the present invention are effective to the case where a plurality of entities exists to control a mobile terminal handover and mobile terminal handovers by the respective control entities occur.