Abstract:
A method, a system, and a device for network handoff is disclosed. A first evolved Access Network (eAN) sends a session transfer request to the target eAN that corresponds to the network handoff request. The first eAN receives a session transfer response from the target eAN. The first eAN sends a Traffic Channel Assignment (TCA) message to a User Equipment (UE) based on the session transfer response, so that the UE can switch from a Long Term Evolution (LTE) network to an evolved High Rate Packet Data (eHRPD) network based on the TCA message.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Application No. PCT/CN2009/074609, filed on Oct. 26, 2009, which claims priority to Chinese patent application No. 200810172501.2, filed on Oct. 28, 2008, both of which are hereby incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to communications technologies, and more particularly, to a method, a system, and a device for network handoff. 
       BACKGROUND OF THE INVENTION 
       [0003]    The interworking between the 3GPP-based and non-3GPP-based networks, especially the interworking between the Long Term Evolution (LTE) and the evolved High Rate Packet Data (eHRPD) networks, is a hot topic among the standardization organizations.  FIG. 1  illustrates the architecture for the interworking between the LTE and the eHRPD networks. As show in  FIG. 1 , two interfaces are added between the LTE and eHRPD networks: S 101  and S 103 -U. S 101  is an interface between a Mobility Management Entity (MME) and an evolved Access Network (eAN). It is essentially a tunnel for forwarding signaling messages between a terminal and a target network to help the terminal to pre-register with the target network or switch from a source network to a target network. S 103 -U is an interface between a Serving Gateway (S-GW) and a High Rate Packet Data Serving Gateway (HSGW). It is used to forward registration data stored in an LTE network to an eHRPD network, which then forwards the data to a terminal. 
         [0004]    When a UE switches from an LTE network to an eHRPD network, the UE first needs to pre-register with the eHRPD network, namely, to establish a connection with the eHRPD network before accessing it, including negotiating an air interface session with the eAN, performing authentication, establishing a Point to Point Protocol (PPP) connection, and so on. In this way, the switchover delay can be greatly reduced. After determining to switch to the eHRPD network, the UE requests for the establishment of an air interface connection with the eAN. The eAN allocates an air interface resource for the UE, and sends a Traffic Channel Assignment (TCA) message to the UE. After that, the UE can switch from the LTE network to the eHRPD network based on the TCA message. 
         [0005]    During implementation of the present invention, the inventor found that the following defect exists in the prior art: 
         [0006]    An evolved NodeB (eNB) in the LTE network can be statically set with only one HRPD SectorID that corresponds to the eNB. In this way, the UE can establish a connection with the eAN in the eHRPD network that corresponds to the HRPD SectorID, and finally switches to the eHRPD network. However, because one eNB may be covered by a plurality of eHRPD sectors, the eAN in the handoff request sent by the UE may be different from the eAN statically set by the eNB. As a result, the handoff fails. 
       SUMMARY OF THE INVENTION 
       [0007]    A method, a system, and a device for network handoff are provided in the embodiments of the present invention to implement network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB. 
         [0008]    A method for network handoff is provided in an embodiment of the present invention. The method includes: sending, by a first eAN, a session transfer request to the target eAN that corresponds to the network handoff request; receiving, by the first eAN, the network handoff response from the target eAN; and sending, by the first eAN, a Traffic Channel Assignment (TCA) message to a User Equipment (UE) based on the session transfer response, so as to enable the UE to switch from a Long Term Evolution (LTE) network to an evolved High Rate Packet Data (eHRPD) network based on the TCA message. 
         [0009]    A system for network handoff is provided in an embodiment of the present invention. The system includes a first eAN, target eAN, and UE, in which: the first eAN is configured to send a session transfer request to the target eAN that corresponds to a network handoff request, receive a session transfer response from the target eAN, and send a Traffic Channel Assignment (TCA) message to the UE based on the session transfer response, so as to enable the UE to switch from a Long Term Evolution (LTE) network to an evolved High Rate Packet Data (eHRPD) network based on the TCA message. 
         [0010]    An eAN is provided in an embodiment of the present invention. The eAN includes: a session transfer request processing unit, configured to send a network handoff request to the target eAN that corresponds to the network handoff request; a session transfer response receiving unit, configured to receive a session transfer response from the target eAN; and a TCA sending unit, configured to send a TCA message to a UE based on the session transfer response received by the session transfer response receiving unit, so as to enable the UE to switch from an LTE network to an eHRPD network based on the TCA message. 
         [0011]    An eAN is provided in an embodiment of the present invention. The eAN includes: a transfer request receiving unit, configured to receive a session transfer request containing session information of the UE, the address of the Packet Data Network Gateway (PDN-GW), and the Generic Routing Encapsulation (GRE) key of the uplink tunnel between the S-GW and the PDN-GW; a session information processing unit, configured to store the session information received by the transfer request receiving unit; a registration request sending unit, configured to send an HSGW a registration request containing the PDN-GW address and the GRE key of the uplink tunnel received by the transfer response receiving unit, so as to enable the HSGW to send uplink data to the PDN-GW by using the PDN-GW address and the GRE key of the uplink tunnel; a registration response receiving unit, configured to receive from the HSGW a registration response containing the HSGW address and the GRE key of the uni-directional tunnel from the S-GW to the HSGW; an air interface resource allocating unit, configured to allocate air interface resource for the UE; and a transfer response sending unit, configured to send a session transfer response containing information about the air interface resource, the HSGW address, and the GRE key of the uni-directional tunnel. 
         [0012]    In embodiments of the present invention, the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB is implemented through the session transfer between the first eAN and the target eAN. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    To describe the technical solutions in the embodiments of the present invention or in the prior art more clearly, the following describes the accompanying drawings for illustrating such technical solutions. Apparently, the accompanying drawings described below are some embodiments of the present invention, and those skilled in the art can derive other drawings from such accompanying drawings without creative efforts. 
           [0014]      FIG. 1  illustrates architecture for the interworking between LTE and eHRPD networks in the prior art; 
           [0015]      FIG. 2  is a flowchart of a network handoff method provided in an embodiment of the present invention; 
           [0016]      FIG. 3  is a flowchart of a network handoff method provided in an embodiment of the present invention; 
           [0017]      FIG. 4  is a flowchart of a network handoff method provided in an embodiment of the present invention; 
           [0018]      FIG. 5  is a flowchart of a network handoff method provided in an embodiment of the present invention; 
           [0019]      FIG. 6  is a flowchart of a network handoff method provided in an embodiment of the present invention; 
           [0020]      FIG. 7  illustrates architecture of a network handoff system provided in an embodiment of the present invention; 
           [0021]      FIG. 8  illustrates architecture of an eAN provided in an embodiment of the present invention; and 
           [0022]      FIG. 9  illustrates architecture of an eAN provided in an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    The making and using of the presently preferred embodiments are discussed in detail below. It should be The technical solutions in the embodiments of the present invention are described below clearly with reference to the accompanying drawings in the embodiments of the present invention. Apparently, the described embodiments are merely some of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, those skilled in the art can derive other embodiments without creative efforts, which all fall within the scope of the present invention. 
         [0024]    A method for network handoff is provided in an embodiment of the present invention (shown in  FIG. 2 ). The method includes the following steps: 
         [0025]    Step  201 : A first eAN sends a session transfer request to a second eAN that corresponds to a network handoff request. 
         [0026]    Step  202 : The first eAN receives a session transfer response from the second eAN. 
         [0027]    Step  203 : The first eAN sends a TCA message to a UE based on the session transfer response, so as to enable the UE to switch from an LTE network to an eHRPD network based on the TCA message. 
         [0028]    In this embodiment of the present invention, the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB is implemented through the session transfer between the first eAN and the target eAN. 
         [0029]    A method for network handoff is provided in an embodiment of the present invention shown in  FIG. 3 . The method includes the following steps: 
         [0030]    Step  300 : The UE is currently in an LTE network but has pre-registered with the eHRPD network, and the eAN that completes pre-registration for the UE stores session information of the UE. 
         [0031]    Step  301 : The UE sends a network handoff request to the eNB to request access to the eHRPD network. The network handoff request includes HRPD Connection Request and Route Update Request. 
         [0032]    Step  302 : The eNB forwards the network handoff request to the MME, and adds a HRPD SectorID statically set by the eNB to the network handoff request. 
         [0033]    Step  303 : The MME adds a PDN-GW address and the GRE key of the uplink tunnel between the S-GW and the PDN-GW to the network handoff request, and forwards the network handoff request to the eAN that corresponds to the HRPD SectorID. 
         [0034]    Step  304 : When the eAN that corresponds to the HRPD SectorID is the eAN that stores the session information of the pre-registration of the UE, but the eAN detects that it is not the target eAN indicated by the parameter in the Route Update Request which the UE wants to access, the eAN corresponding to the HRPD SectorID sends a session transfer request to the target eAN corresponding to the Route Update Request in the network handoff request. 
         [0035]    Specifically, the eAN that corresponds to the HRPD SectorID sends the target eAN a session transfer request containing the session information of the UE, the PDN-GW address, and the GRE key of the uplink tunnel between the S-GW and the PDN-GW. 
         [0036]    Step  305 : The target eAN sends a registration request containing the PDN-GW address and the GRE key of the uplink tunnel to the HSGW based on the session transfer request. 
         [0037]    Step  306 : Based on the PDN-GW address and the GRE key of the uplink tunnel, the HSGW establishes a temporary uplink tunnel between the HSGW and the PDN-GW, in which the uplink tunnel is used to forward uplink data after the UE switches to the eHRPD network. 
         [0038]    Step  307 : The HSGW sends the target eAN a registration response containing an HSGW address and the GRE key of the uni-directional tunnel from the S-GW to the HSGW. 
         [0039]    Step  308 : The target eAN allocates an air interface resource for the UE, updates session information based on settings of the target eAN, and sends a session transfer response containing updated session information, information about the air interface resource, HSGW address, and the GRE key of the uni-directional tunnel to the eAN that corresponds to the HRPD SectorID. 
         [0040]    In addition, the target eAN can set a reverse initial power used by the UE to access the eHRPD network, allocate a new Unicast Access Terminal Identifier (UATI) for the UE, and add the reverse initial power and/or new UATI to the session transfer response, and then send the session transfer response to the UE through the eAN that corresponds to the HRPD SectorID. In this way, the UE can smoothly access the eHRPD network by using the reverse initial power; the UE can also use the long code mask of the traffic channel which is generated based on the new UATI, so as to enable the UATI allocated by a source eAN to be released and then allocated by the source eAN for another UE. 
         [0041]    Step  309 : After receiving the session transfer response, the eAN that corresponds to the HRPD SectorID stores the session information updated by the target eAN, and sends a session transfer completion message containing the updated session information to the target eAN to complete the session transfer. 
         [0042]    Step  310 : The eAN that corresponds to the HRPD SectorID sends the HSGW address, the GRE key of the uni-directional tunnel, and the TCA message containing information about the air interface resource and the reverse initial power to the MME based on the session transfer response. 
         [0043]    Step  311 : The MME forwards the HSGW address and the GRE key of the uni-directional tunnel to the S-GW, so as to enable the S-GW to establish, based on the HSGW address and the GRE key of the uni-directional tunnel, a uni-directional tunnel from the S-GW to the HSGW, in which the uni-directional tunnel is used to forward the downlink data buffered in the LTE network to the eHRPD network. That is, the S-GW forwards the downlink data sent from the eNB to the HSGW through the uni-directional tunnel from the S-GW to the HSGW; the HSGW buffers downlink data, and sends the downlink data to the UE after the UE switches to the eHRPD network. 
         [0044]    Step  312 : The MME sends a TCA message containing information about the air interface resource and the reverse initial power to the UE. 
         [0045]    Step  313 : The UE sends a Traffic Channel Complete (TCC) message to the target eAN based on the information about the air interface resource allocated by the target eAN and the reverse initial power. 
         [0046]    Step  314 : After receiving the TCC message, the target eAN sends a registration request to the HSGW to indicate that the UE is connected to the HRPD network. If the UE needs to send uplink data, the UE can send the data to the HSGW. The HSGW sends the data to the PDN-GW through the temporary uplink tunnel established in step  306 . 
         [0047]    Step  315 : The HSGW sends a Proactive Mobile IP Binding Update (PMIP BU) request to the PDN-GW to request the establishment of an S2a tunnel with the PDN-GW. 
         [0048]    Step  316 : The PDN-GW sends a Proactive Mobile IP Binding Acknowledgment (PMIP BA) message to the HSGW to indicate that the S2a tunnel is established successfully. After that, the HSGW no longer sends uplink data through the temporary uplink tunnel established at step  306 , but forwards uplink data and downlink data through the S2a tunnel. 
         [0049]    Step  317 : The HSGW returns a registration response to the target eAN through A 11  (shown in  FIG. 1 ), and the UE switches to the eHRPD network. 
         [0050]    In this embodiment of the present invention, the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB is implemented through the session transfer between the first eAN and the target eAN. 
         [0051]    Those skilled in the art should understand the following: In step  310 , if the eAN that corresponds to the HRPD SectorID receives the reverse initial power and/or new UATI from the target eAN in step  308 , it can also send the reverse initial power and/or new UATI to the MME. 
         [0052]    Those skilled in the art should understand the following: In step  312 , if the MME receives the reverse initial power and/or new UATI from the eAN that corresponds to the HRPD SectorID, it can also send the reverse initial power and/or new UATI to the UE. 
         [0053]    Those skilled in the art should understand the following: In step  313 , if the UE obtains the reverse initial power, it can send the TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN and the reverse initial power; if the UE does not obtain the reverse initial power, it can send the TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN. 
         [0054]    A method for network handoff is provided in an embodiment of the present invention shown in  FIG. 4 . The method includes the following steps: 
         [0055]    Step  400 : The UE is currently in an LTE network but has pre-registered with the eHRPD network, and the eAN that completes pre-registration for the UE stores session information of the UE. 
         [0056]    Step  401 : The UE sends a network handoff request to the eNB to request access to the eHRPD network. The network handoff request includes HRPD Connection Request and Route Update Request. 
         [0057]    Step  402 : The eNB forwards the network handoff request to the MME, and adds the HRPD SectorID statically set by the eNB to the network handoff request. 
         [0058]    Step  403 : The MME adds a PDN-GW address and the GRE key of the uplink tunnel between the S-GW and the PDN-GW to the network handoff request, and forwards the network handoff request to the eAN that corresponds to the HRPD SectorID. 
         [0059]    Step  404 : If the eAN that corresponds to the HRPD SectorID is neither the eAN that stores the session information of the UE nor the target eAN indicated by the parameter in the Route Update Request which the UE wants to access, the eAN that corresponds to the HRPD SectorID obtains the session information of the UE from the eAN that stores the session information of the pre-registration of the UE through A 13  (shown in  FIG. 1 ). 
         [0060]    Step  405 : The eAN that corresponds to the HRPD SectorID sends a session transfer request to the target eAN indicated in the Route Update Request. 
         [0061]    Specifically, the eAN that corresponds to the HRPD SectorID sends the target eAN a session transfer request containing the session information of the UE, the PDN-GW address, and the GRE key of the uplink tunnel between the S-GW and the PDN-GW. 
         [0062]    Step  406 : The target eAN sends a registration request containing the PDN-GW address and the GRE key of the uplink tunnel to the HSGW based on the session transfer request. 
         [0063]    Step  407 : Based on the PDN-GW address and the GRE key of the uplink tunnel, the HSGW establishes a temporary uplink tunnel between the HSGW and the PDN-GW, in which the uplink tunnel is used to forward uplink data after the UE switches to the eHRPD network. 
         [0064]    Step  408 : The HSGW sends the target eAN a registration response containing an HSGW address and the GRE key of the uni-directional tunnel from the S-GW to the HSGW. 
         [0065]    Step  409 : The target eAN allocates an air interface resource for the UE, updates session information based on settings of the target eAN, and sends a session transfer response containing updated session information, information about the air interface resource, HSGW address, and the GRE key of the uni-directional tunnel to the eAN that stores the session information. 
         [0066]    In addition, the target eAN can set a reverse initial power used by the UE to access the eHRPD network, allocate a new UATI for the UE, and add the reverse initial power and/or new UATI to the session transfer response, and then send the session transfer response to the UE through the eAN that corresponds to the HRPD SectorID. In this way, the UE can smoothly access the eHRPD network by using the reverse initial power; the UE can also use the long code mask of the traffic channel which is generated based on the new UATI, so as to enable the UATI allocated by a source eAN to be released and then allocated by the source eAN for another UE. 
         [0067]    Step  410 : After receiving the session transfer response, the eAN that corresponds to the HRPD SectorID stores the session information updated by the target eAN, and sends a session transfer completion message containing the updated session information to the target eAN to complete the session transfer. 
         [0068]    Step  411 : The eAN that corresponds to the HRPD SectorID sends the HSGW address, the GRE key of the uni-directional tunnel, and the TCA message containing information about the air interface resource and the reverse initial power to the MME based on the session transfer response. 
         [0069]    Step  412 : The MME forwards the HSGW address and the GRE key of the uni-directional tunnel to the S-GW, so as to enable the S-GW to establish, based on the HSGW address and the GRE key of the uni-directional tunnel, a uni-directional tunnel from the S-GW to the HSGW, in which the uni-directional tunnel is used to forward the downlink data buffered in the LTE network to the eHRPD network. That is, the S-GW forwards the downlink data sent from the eNB to the HSGW through the uni-directional tunnel from the S-GW to the HSGW; the HSGW buffers the downlink data, and sends the downlink data to the UE after the UE switches to the eHRPD network. 
         [0070]    Step  413 : The MME sends a TCA message containing information about the air interface resource and the reverse initial power to the UE. 
         [0071]    Step  414 : The UE sends a TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN and the reverse initial power. 
         [0072]    Step  415 : After receiving the TCC message, the target eAN sends a registration request to the HSGW to indicate that the UE is connected to the HRPD network. If the UE needs to send uplink data, it can send the data to the HSGW. The HSGW sends the data to the PDN-GW through the temporary uplink tunnel established in step  407 . 
         [0073]    Step  416 : The HSGW sends a PMIP BU request to the PDN-GW to request the establishment of an S2a tunnel with the PDN-GW. 
         [0074]    Step  417 : The PDN-GW sends a PMIP BA message to the HSGW to indicate that the S2a tunnel is established successfully. After that, the HSGW no longer sends uplink data through the temporary uplink tunnel established in step  407 , but forwards uplink data and downlink data through the S2a tunnel. 
         [0075]    Step  418 : The HSGW returns a registration response to the target eAN through All (shown in  FIG. 1 ), and the UE switches to the eHRPD network. In this embodiment of the present invention, the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB is implemented through the session transfer between the first eAN and the target eAN. 
         [0076]    Those skilled in the art should understand the following: In step  411 , if the eAN that corresponds to the HRPD SectorID receives the reverse initial power and/or new UATI from the target eAN at step  409 , it can also send the reverse initial power and/or new UATI to the MME. 
         [0077]    Those skilled in the art should understand the following: In step  413 , if the MME receives the reverse initial power and/or new UATI from the eAN that corresponds to the HRPD SectorID, it can also send the reverse initial power and/or new UATI to the UE. 
         [0078]    Those skilled in the art should understand the following: In step  414 , if the UE obtains the reverse initial power, it can send the TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN and the reverse initial power; if the UE does not obtain the reverse initial power, it can send the TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN. 
         [0079]    A method for network handoff is provided in an embodiment of the present invention shown in  FIG. 5 . The method includes the following steps: 
         [0080]    Step  500 : The UE is currently in an LTE network but has pre-registered with the eHRPD network, and the eAN that completes pre-registration for the UE stores session information of the UE. 
         [0081]    Step  501 : The UE sends a network handoff request to the eNB to request access to the eHRPD network. The network handoff request includes HRPD Connection Request and Route Update Request. 
         [0082]    Step  502 : The eNB forwards the network handoff request to the MME, and adds the HRPD SectorID statically set by the eNB to the network handoff request. 
         [0083]    Step  503 : The MME adds a PDN-GW address and the GRE key of the uplink tunnel between the S-GW and the PDN-GW to the network handoff request, and forwards the network handoff request to the eAN that corresponds to the HRPD SectorID. 
         [0084]    Step  504 : If the eAN that corresponds to the HRPD SectorID is neither the eAN that stores the session information of the pre-registration of the UE nor the target eAN indicated by the parameter in the Route Update Request which the UE wants to access, the eAN that corresponds to the HRPD SectorID forwards the network handoff request to the eAN that stores the session information of the UE. 
         [0085]    Step  505 : If the eAN that stores the session information of the UE detects that it is not the target eAN indicated by the parameter in the Route Update Request which the UE wants to access, the eAN sends a session transfer request to the target eAN. 
         [0086]    Specifically, the eAN that stores the session information sends the target eAN a session transfer request containing the session information of the UE, the PDN-GW address, and the GRE key of the uplink tunnel between the S-GW and the PDN-GW. 
         [0087]    Step  506 : The target eAN sends a registration request containing the PDN-GW address and the GRE key of the uplink tunnel to the HSGW based on the session transfer request. 
         [0088]    Step  507 : Based on the PDN-GW address and the GRE key of the uplink tunnel, the HSGW establishes a temporary uplink tunnel between the HSGW and the PDN-GW, in which the uplink tunnel is used to forward uplink data after the UE switches to the eHRPD network. 
         [0089]    Step  508 : The HSGW sends the target eAN a registration response containing an HSGW address and the GRE key of the uni-directional tunnel from the S-GW to the HSGW. 
         [0090]    Step  509 : The target eAN allocates an air interface resource for the UE, updates session information based on settings of the target eAN, and sends a session transfer response containing updated session information, information about the air interface resource, HSGW address, and the GRE key of the uni-directional tunnel to the eAN that stores the session information. 
         [0091]    In addition, the target eAN can set a reverse initial power used by the UE to access the eHRPD network, allocate a new UATI for the UE, and add the reverse initial power and/or new UATI to the session transfer response, and then send the session transfer response to the UE through the eAN that corresponds to the HRPD SectorID. In this way, the UE can smoothly access the eHRPD network by using the reverse initial power; the UE can also use the long code mask of the traffic channel which is generated based on the new UATI, so as to enable the UATI allocated by a source eAN to be released and then allocated by the source eAN for another UE. 
         [0092]    Step  510 : After receiving the session transfer response, the eAN that stores the session information stores the session information updated by the target eAN, and sends a session transfer completion message containing the updated session information to the target eAN to complete the session transfer. 
         [0093]    Step  511 : In response to Step  504 , the eAN that stores the session information sends the HSGW address, the GRE key of the uni-directional tunnel, and the TCA message containing information about the air interface resource and the reverse initial power to the eAN that corresponds to the HRPD SectorID. 
         [0094]    Step  512 : The eAN that corresponds to the HRPD SectorID sends the message received in step  511  to the MME. 
         [0095]    Step  513 : The MME forwards the HSGW address and the GRE key of the uni-directional tunnel to the S-GW, so as to enable the S-GW to establish, based on the HSGW address and the GRE key of the uni-directional tunnel, a uni-directional tunnel from the S-GW to the HSGW, in which the uni-directional tunnel is used to forward the downlink data buffered in the LTE network to the eHRPD network. That is, the S-GW forwards the downlink data sent from the eNB to the HSGW through the uni-directional tunnel from the S-GW to the HSGW; the HSGW buffers the downlink data, and sends the downlink data to the UE after the UE switches to the eHRPD network. 
         [0096]    Step  514 : The MME sends a TCA message containing information about the air interface resource and the reverse initial power to the UE. 
         [0097]    Step  515 : The UE sends a TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN and the reverse initial power. 
         [0098]    Step  516 : After receiving the TCC message, the target eAN sends a registration request to the HSGW to indicate that the UE is connected to the HRPD network. If the UE needs to send uplink data, it can send the data to the HSGW. The HSGW 1 sends the data to the PDN-GW through the temporary uplink tunnel established in step  507 . 
         [0099]    Step  517 : The HSGW sends a PMIP BU request to the PDN-GW to request the establishment of an S2a tunnel with the PDN-GW. 
         [0100]    Step  518 : The PDN-GW sends a PMIP BA message to the HSGW to indicate that the S2a tunnel is established successfully. After that, the HSGW no longer sends uplink data through the temporary uplink tunnel established in step  507 , but forwards uplink data and downlink data through the S2a tunnel. 
         [0101]    Step  519 : The HSGW returns a registration response to the target eAN through All shown in  FIG. 1 , and the UE switches to the eHRPD network. 
         [0102]    In this embodiment of the present invention, the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB is implemented through the session transfer between the first eAN and the target eAN. 
         [0103]    Those skilled in the art should understand the following: In step  511 , if the eAN that stores the session information receives the reverse initial power and/or new UATI from the target eAN in step  509 , it will send the reverse initial power and/or new UATI to the eAN that corresponds to the HRPD SectorID. 
         [0104]    A method for network handoff is provided in an embodiment of the present invention shown in  FIG. 6 . The method includes the following steps: 
         [0105]    Step  600 : The UE is currently in an LTE network but has pre-registered with the eHRPD network, and the eAN that completes pre-registration for the UE stores session information of the UE. 
         [0106]    Step  601 : The UE sends a network handoff request to the eNB to request access to the eHRPD network. The network handoff request includes HRPD Connection Request and Route Update Request. 
         [0107]    Step  602 : The eNB forwards the network handoff request to the MME, and adds the HRPD SectorID statically set by the eNB to the network handoff request. 
         [0108]    Step  603 : The MME adds a PDN-GW address and the GRE key of the uplink tunnel between the S-GW and the PDN-GW to the network handoff request, and forwards the network handoff request to the eAN that corresponds to the HRPD SectorID. 
         [0109]    Step  604 : If the eAN that corresponds to the HRPD SectorID detects that it is neither the eAN that stores the session information for the pre-registration of the UE nor the target eAN indicated by the parameter in the Route Update Request which the UE wants to access, it forwards a response containing the ID or address of the eAN that stores the session information to the MME to instruct the MME to connect to the eAN that stores the session information. 
         [0110]    Step  605 : The MME sends the network handoff request to the eAN that stores the session information of the UE. 
         [0111]    Step  606 : When the eAN that stores the session information of the UE detects that it is not the target eAN indicated by the parameter in the Route Update Request which the UE wants to access, the eAN sends a session transfer request to the target eAN. 
         [0112]    Specifically, the eAN that stores the session information of the UE sends the target eAN a session transfer request containing the session information of the UE, the PDN-GW address, and the GRE key of the uplink tunnel between the S-GW and the PDN-GW. 
         [0113]    Step  607 : The target eAN sends a registration request containing the PDN-GW address and the GRE key of the uplink tunnel to the HSGW based on the session transfer request. 
         [0114]    Step  608 : Based on the PDN-GW address and the GRE key of the uplink tunnel, the HSGW establishes a temporary uplink tunnel between the HSGW and the PDN-GW, in which the uplink tunnel is used to forward uplink data after the UE switches to the eHRPD network. 
         [0115]    Step  609 : The HSGW sends the target eAN a registration response containing an HSGW address and the GRE key of the uni-directional tunnel from the S-GW to the HSGW. 
         [0116]    Step  610 : The target eAN allocates an air interface resource for the UE, updates session information based on settings of the target eAN, and sends a session transfer response containing updated session information, information about the air interface resource, HSGW address, and the GRE key of the uni-directional tunnel to the eAN that stores the session information. 
         [0117]    In addition, the target eAN can set a reverse initial power used by the UE to access the eHRPD network, allocate a new UATI for the UE, and add the reverse initial power and/or new UATI to the session transfer response, and then send the session transfer response to the UE through the eAN that corresponds to the HRPD SectorID. In this way, the UE can smoothly access the eHRPD network by using the reverse initial power; the UE can also use the long code mask of the traffic channel which is generated based on the new UATI, so as to enable the UATI allocated by a source eAN to be released and then allocated by the source eAN for another UE. 
         [0118]    Step  611 : After receiving the session transfer response, the eAN that stores the session information stores the session information updated by the target eAN, and sends a session transfer completion message containing the updated session information to the target eAN to complete the session transfer. 
         [0119]    Step  612 : The eAN that stores the session information sends the HSGW address, the GRE key of the uni-directional tunnel, and the TCA message containing information about the air interface resource and the reverse initial power to the MME based on the session transfer response. 
         [0120]    Step  613 : The MME forwards the HSGW address and the GRE key of the uni-directional tunnel to the S-GW, so as to enable the S-GW to establish, based on the HSGW address and the GRE key of the uni-directional tunnel, a uni-directional tunnel from the S-GW to the HSGW, in which the uni-directional tunnel is used to forward the downlink data buffered in the LTE network to the eHRPD network. That is, the S-GW forwards the downlink data received sent the eNB to the HSGW through the uni-directional tunnel from the S-GW to the HSGW; the HSGW buffers the downlink data, and sends the downlink data to the UE after the UE switches to the eHRPD network. 
         [0121]    Step  614 : The MME sends a TCA message containing information about the air interface resource and the reverse initial power to the UE. 
         [0122]    Step  615 : The UE sends a TCC message to the target eAN based on the information about the air interface resource allocated by the target eAN and the reverse initial power. 
         [0123]    Step  616 : After receiving the TCC message, the target eAN sends a registration request to the HSGW to indicate that the UE is connected to the HRPD network. If the UE needs to send uplink data, it can send the data to the HSGW, which will send the data to the PDN-GW through the temporary uplink tunnel established in step  608 . 
         [0124]    Step  617 : The HSGW sends a PMIP BU request to the PDN-GW to request the establishment of an S2a tunnel with the PDN-GW. 
         [0125]    Step  618 : The PDN-GW sends a PMIP BA message to the HSGW to indicate that the S2a tunnel is established successfully. After that, the HSGW no longer sends uplink data through the temporary uplink tunnel established in step  608 , but forwards uplink data and downlink data through the S2a tunnel. 
         [0126]    Step  619 : The HSGW returns a registration response to the target eAN through All (shown in  FIG. 1 ), and the UE switches to the eHRPD network. 
         [0127]    In this embodiment of the present invention, the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB is implemented through the session transfer between the first eAN and the target eAN. 
         [0128]    Those skilled in the art should understand the following: In step  612 , if the eAN that stores the session information of the UE receives the reverse initial power and/or new UATI from the target eAN in step  610 , it can also send the reverse initial power and/or new UATI to the MME. 
         [0129]    Those skilled in the art should understand the following: In step  614 , if the MME receives the reverse initial power and/or new UATI from the eAN that stores the session information of the UE, it can also send the reverse initial power and/or new UATI to the UE. 
         [0130]    A system for network handoff is provided in an embodiment of the present invention shown in  FIG. 7 . The system includes a first eAN  701 , a target eAN  702 , and a UE  703 , in which: 
         [0131]    the first eAN  701  is configured to send a session transfer request to the target eAN  702  that corresponds to the session transfer request, receive a session transfer response from the target eAN  702 , and send a TCA message to the UE  703  based on the session transfer response, so as to enable the UE  703  to switch from an LTE network to an eHRPD network based on the TCA message. 
         [0132]    In this embodiment of the present invention, a system for network handoff is provided for the session transfer between the first eAN and the target eAN to implement the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB. 
         [0133]    An eAN is provided in an embodiment of the present invention shown in  FIG. 8 . The eAN includes: 
         [0134]    a session transfer request processing unit  801 , configured to send a session transfer request to the target eAN that corresponds to the network handoff request; 
         [0135]    a session transfer response receiving unit  802 , configured to receive a session transfer response from the target eAN; and 
         [0136]    a TCA sending unit  803 , configured to send a TCA message to a UE based on the session transfer response received by the session transfer response receiving unit  802 , so as to enable the UE to switch from an LTE network to an eHRPD network based on the TCA message. 
         [0137]    If the eAN is an eAN that corresponds to the SectorID, the session transfer request processing unit  801  includes: 
         [0138]    a first processing subunit  8011 , configured to send the session transfer request to the target eAN if the eAN that corresponds to the SectorID is the eAN that stores the session information of the UE; or 
         [0139]    a second processing subunit  8012 , configured to obtain the session information of the UE from the eAN that stores the session information of the UE and sends the session transfer request to the target eAN if the eAN that corresponds to the SectorID is not the eAN that stores the session information of the UE. 
         [0140]    If the eAN is an eAN that stores the session information of the UE, the session transfer request processing unit  801  includes: 
         [0141]    a third processing subunit  8013 , configured to receive a network handoff request from the eAN that corresponds to the SectorID and send the session transfer request to the target eAN based on the network handoff request; or 
         [0142]    a fourth processing subunit  8014 , configured to receive a network handoff request from an MME and send a session transfer request to the target eAN based on the network handoff request. 
         [0143]    The session transfer request processing unit  801  is specifically configured to send a session transfer request containing the session information of the UE, the PDN-GW address, and the GRE key of the uplink tunnel between the S-GW and the PDN-GW to the target eAN. 
         [0144]    The session transfer response receiving unit  802  is specifically configured to: 
         [0145]    receive, from the target eAN, a session transfer response containing information about the air interface resource, the HSGW address, and the GRE key of the uni-directional tunnel; or 
         [0146]    receive, from the target eAN, a session transfer response containing information about the air interface resource, the HSGW address, the GRE key of the uni-directional tunnel, and the reverse initial access power; or 
         [0147]    receive, from the target eAN, a session transfer response containing information about the air interface resource, the HSGW address, the GRE key of the uni-directional tunnel, and the new UATI; or 
         [0148]    receive, from the target eAN, a session transfer response containing information about the air interface resource, the HSGW address, the GRE key of the uni-directional tunnel, the reverse initial access power, and the new UATI. 
         [0149]    The TCA sending unit  803  is specifically configured to: 
         [0150]    send a TCA message containing information about the air interface resource to the UE based on the session transfer response; or 
         [0151]    send a TCA message containing information about the air interface resource and the reverse initial power to the UE based on the session transfer response; or 
         [0152]    send a TCA message containing information about the air interface resource and the new UATI to the UE based on the session transfer response; or 
         [0153]    send a TCA message containing information about the air interface resource, the reverse initial power, and the new UATI to the UE based on the session transfer response. 
         [0154]    In this embodiment of the present invention, an eAN is provided for the session transfer between itself and the target eAN to implement the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB. 
         [0155]    An eAN is provided in an embodiment of the present invention shown in  FIG. 9 . The eAN includes: 
         [0156]    a transfer request receiving unit  901 , configured to receive a session transfer request containing session information of the UE, a PDN-GW address, and a GRE key of the uplink tunnel between the S-GW and the PDN-GW; 
         [0157]    a session information processing unit  902 , configured to store the session information received by the transfer request receiving unit  901 ; 
         [0158]    a registration request sending unit  903 , configured to send an HSGW a registration request containing the PDN-GW address and the GRE key of the uplink tunnel received by the transfer response receiving unit  901 , so as to enable the HSGW to send uplink data to the PDN-GW by using the PDN-GW address and the GRE key of the uplink tunnel; 
         [0159]    a registration response receiving unit  904 , configured to receive from the HSGW a registration response containing the HSGW address and the GRE key of the uni-directional tunnel from the S-GW to the HSGW; 
         [0160]    an air interface resource allocating unit  905 , configured to allocate air interface resource for a UE; and 
         [0161]    a transfer response sending unit  906 , configured to send a session transfer response containing information about the air interface resource, the HSGW address, and the GRE key of the uni-directional tunnel. 
         [0162]    The eAN further includes: 
         [0163]    an initial power setting unit  907 , configured to set the reverse initial power used by the UE to access an eHRPD network. 
         [0164]    In this case, the transfer response sending unit  906  can be specifically configured to send a session transfer response containing information about the air interface resource, the HSGW address, the GRE key of the uni-directional tunnel, and the reverse initial access power. 
         [0165]    The eAN further includes: 
         [0166]    a UATI allocating unit  908 , configured to allocate a new UATI for the UE. 
         [0167]    In this case, the transfer response sending unit  906  can be specifically configured to send a session transfer response containing information about the air interface resource, the HSGW address, the GRE key of the uni-directional tunnel, and the new UATI. 
         [0168]    In this embodiment of the present invention, an eAN is provided for the session transfer between itself and the target eAN to implement the network handoff when the target eAN indicated in the handoff request sent by a UE is different from the eAN statically set by the eNB. 
         [0169]    Based on the description of the preceding embodiments, those skilled in the art can clearly understand that the present invention can be implemented through hardware or through software that runs on a necessary universal hardware platform. Therefore, the technical solutions of the present invention can be embodied in form of a software product; the software product can be stored in a non-volatile storage medium (for example, a CD-ROM, a USB disk, a portable hard disk), and incorporates a plurality of instructions to enable a computer (for example, a PC, a server, or a network device) to implement the methods provided in the embodiments of the present invention. 
         [0170]    Described above are merely the exemplary embodiments of the present invention, so the embodiments of the present invention are not limited to these embodiments. Therefore, the equivalent modifications and variations made with reference to the embodiments of the present invention fall within the scope of the present invention.