Abstract:
A seamless handoff system and method reduces delay and packet loss during handoff of user equipment (UE) from a Wireless Local Area Network (WLAN) network to a Universal Mobile Telecommunication System (UMTS) network. The handoff system and method performs pre-registration and pre-authentication processes before the occurrence of a handoff, buffers packets being delivered to a UE during a handoff period, and forwards the buffered packets to the UE after completion of the handoff, thereby reducing packet loss that can occur during handoff.

Description:
PRIORITY  
       [0001]     This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 2004-98232, filed Nov. 26, 2004 in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to handoff between heterogeneous networks. More particularly, the present invention relates to a handoff method for improving delay and packet loss which may frequently occur when a mobile node (MN) performs handoff from a Wireless Local Area Network (WLAN) network to a Universal Mobile Telecommunication System (UMTS) network.  
         [0004]     2. Description of the Related Art  
         [0005]     Mobile (Internet Protocol (IP)) Working Group of Internet Engineering Task Force (IETF), an international standardization organization, proposes a pre-registration handoff method as one of the methods capable of supporting a low delay during handoff. The pre-registration handoff method refers to a method in which a Mobile Node (MN) performs L 3  (Network Layer) handoff before completion of L 2  (Link Layer) handoff with the help of a network. As referenced to herein, the L 2  handoff refers to an operation of switching a Foreign Agent (FA) that the MN accesses, and the L 3  handoff refers to an operation of registering an IP address of the MN in a Home Agent (HA).  
         [0006]      FIG. 1  depicts a signaling diagram illustrating a conventional pre-registration handoff process. Referring to  FIG. 1 , an MN  110  desires to perform pre-registration handoff from an old FA (oFA)  120  to a new FA (nFA)  130 .  
         [0007]     Before the pre-registration handoff is performed, the oFA  120  transmits an Agent Solicitation message for requesting information about the nFA  130 , step  101 . Upon receiving the Agent Solicitation message from the oFA  120 , the nFA  130  transmits an Agent Advertisement message including information on the nFA  130  back to the oFA  120 , step  102 . The MN  110  acquires information on the nFA  130  through the Agent Advertisement message.  
         [0008]     Thereafter, if the MN  110  or the oFA  120  receives an L 2  trigger, step  107 , handoff is triggered (initiated) at the MN  110  or the oFA  120 . Specifically, if the MN  110  receives the L 2  trigger in step  107 , the handoff is triggered by the MN  110 . Upon receiving the L 2  trigger, the MN  110  transmits a Proxy Agent Solicitation message to the oFA  120  in step  103 . Upon receiving the Proxy Agent Solicitation message, the oFA  120  transmits a Proxy Agent Advertisement message to the MN  110  in response thereto, step  104 . If the oFA  120  receives the L 2  trigger in step  107   a,  however, the handoff is triggered by the oFA  120 . That is, upon receiving the L 2  trigger, the oFA  120  transmits a Proxy Agent Advertisement message to the MN  110  in step  104 .  
         [0009]     Upon receiving the Proxy Agent Advertisement message, the MN  110  transmits a Registration Request message to the nFA  130  via the oFA  120  in step  105  because it is not yet connected to the nFA  130 . Upon receiving the Registration Request message, the nFA  130  registers the MN  110  in an HA  140  through a Mobile IP registration process, in step  106 , which is the L 3  handoff. Thereafter, if the HA  140  delivers a Registration Reply message to the MN  110  via the nFA  130  and the oFA  120  in step  108 , the L 2  handoff is initiated in step  109 . The L 2  handoff allows the MN  110  to access the network via the nFA  130  instead of the oFA  120 .  
         [0010]     The foregoing pre-registration handoff scheme separates the L 2  handoff and the L 3  handoff, performing first the L 3  handoff (that is, the Mobile IP registration process), which may cause a long handoff delay and thereby contribute to a reduction in the total handoff delay.  
         [0011]     The pre-registration handoff scheme, which operates based on Mobile IP, is not flexible when applied to a WLAN-UMTS interworking configuration, and must be adapted to perform an actual handoff process in WLAN and UMTS networks, as well as meet the requirements of the networks. That is, during WLAN-UMTS interworking the Mobile IP registration process includes an authentication process, whereas the conventional handoff scheme presents only the mobility-related registration signaling but suggests nothing about the authentication process. This causes a considerable delay and packet loss problem during actual handoff between the WLAN and UMTS networks.  
         [0012]     Accordingly, what is needed is a system and method capable of providing fast, seamless handoff during WLAN-UMTS interworking.  
       SUMMARY OF THE INVENTION  
       [0013]     To substantially solve at least the above problems and/or disadvantages, aspects of the present invention provide a system and method capable of providing fast, seamless handoff during WLAN-UMTS interworking.  
         [0014]     Another aspect of the present invention provides a system and method for performing pre-registration and pre-authentication processes before occurrence of handoff in order to solve the delay problem occurring during handoff from a WLAN network to a UMTS network.  
         [0015]     Another aspect of the present invention provides a system and method for reducing packet loss during handoff, using a scheme that buffers packets being delivered to an MN during a handoff period, then delivering the buffered packet to the MN upon completion of the handoff.  
         [0016]     According to an aspect of the present invention, there is provided a method for performing handoff of user equipment (UE) connected to a wireless local area network (WLAN) to a mobile communication network in an interworking system between the WLAN and the mobile communication network including a gateway General Packet Radio Service (GPRS) support node (GGSN) and a serving GPRS support node (SGSN). The method comprises receiving, by the SGSN, a pre-registration request message including a home IP address and an international mobile station identifier (IMSI) of the UE from the UE via the WLAN when the UE determines handoff in response to a link layer trigger while it communicates with the WLAN using a first temporary Internet protocol (IP) address. The method further comprises performing, between the UE and the SGSN, authentication vector generation, authentication for the UE, and encryption key distribution processes based on the IMSI. The method further comprises generating, by the GGSN, a second temporary IP address to be used by the UE in the mobile communication network, and transmitting a first message for requesting registration of the second temporary IP address to a home agent (HA) of the UE. The method further comprises receiving, by the GGSN, a second message indicating registration of the second temporary IP address from the HA of the UE, and transmitting a third message indicating completion of pre-authentication for the UE and registration of the second temporary IP address, to the UE via the WLAN. The method further comprises transmitting, by the UE, a fourth message indicating completion of pre-registration handoff for the UE to the HA of the UE when the handoff of the UE to the mobile communication network is completed.  
         [0017]     According to another aspect of the present invention, there is provided a method for performing handoff of user equipment (UE) connected to a wireless local area network (WLAN) to a mobile communication network by a gateway General Packet Radio Service (GPRS) support node (GGSN) in an interworking system between the WLAN and the mobile communication network including the GGSN and a serving GPRS support node (SGSN). The method comprises performing authentication for the UE and encryption key distribution processes between the UE and the SGSN based on an international mobile station identifier (IMSI) of the UE after the UE determines handoff in response to a link layer trigger while it communicates with the WLAN using a first temporary Internet protocol (IP) address, and authentication vector generation. The method further comprises generating, by the UE, a second temporary IP address to be used in the mobile communication network, and transmitting a first message for requesting registration of the second temporary IP address to a home agent (HA) of the UE. The method further comprises transmitting a third message indicating completion of pre-authentication for the UE and registration of the second temporary IP address to the UE via the WLAN upon receiving a second message indicating registration of the second temporary IP address from the HA of the UE so that the UE performs handoff to the mobile communication network.  
         [0018]     According to yet another aspect of the present invention, there is provided a mobile communication system comprising user equipment (UE) connected to a wireless local area network (WLAN) through interworking between a mobile communication network and the WLAN. The system further comprises a serving General Packet Radio Service (GPRS) support node (SGSN) for receiving a pre-registration request message including a home IP address and an international mobile station identifier (IMSI) of the UE from the UE via the WLAN when the UE determines handoff in response to a link layer trigger while it communicates with the WLAN using a first temporary Internet protocol (IP) address, and performing authentication vector generation, authentication for the UE, and encryption key distribution processes with the UE based on the IMSI. The system further comprises a gateway GPRS support node (GGSN) for generating a second temporary IP address to be used by the UE in the mobile communication network, transmitting a first message for requesting registration of the second temporary IP address, receiving a second message indicating registration of the second temporary IP address, and transmitting a third message indicating completion of pre-authentication for the UE and registration of the second temporary IP address to the UE via the WLAN The system further comprises a home agent (HA) for registering the second temporary IP address upon receiving the first message from the GGSN, and transmitting the second message to the GGSN in response to the first message. The UE performs handoff to the mobile communication network upon receiving the third message, and transmits a fourth message indicating completion of pre-registration handoff for the UE to the HA.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The above and other objects, exemplary features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
         [0020]      FIG. 1  depicts a signaling diagram illustrating a conventional pre-registration handoff process;  
         [0021]      FIG. 2  is a block diagram illustrating a network interworking configuration according to an exemplary embodiment of the present invention;  
         [0022]      FIG. 3  depicts a signaling diagram illustrating the overall operation in which a UE performs pre-registration handoff from a WLAN network to a UMTS network, according to a first exemplary embodiment of the present invention; and  
         [0023]      FIG. 4  depicts a signaling diagram illustrating the overall operation in which a UE performs pre-registration handoff from a WLAN network to a UMTS network, according to a second exemplary embodiment of the present invention. 
     
    
       [0024]     Throughout the drawings, like reference numbers should be understood to refer to like elements, features, and structures.  
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0025]     The matters exemplified in this description are provided to assist in a comprehensive understanding of certain exemplary embodiments of the present invention disclosed with reference to the accompanying figures. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the exemplary embodiments described herein can be made without departing from the scope and spirit of the claimed invention. Descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
         [0026]      FIG. 2  is a block diagram illustrating a network interworking configuration according to an exemplary embodiment of the present invention. Referring to  FIG. 2 , a WLAN Access Network (AN)  210 , connected to the Intranet/Internet  220 , includes an Access Point (AP)  202  wirelessly connectable to a mobile terminal  201  (hereinafter referred to as a User Equipment (UE) connectable to a UMTS network) and a WLAN agent  203  for managing the AP  202 . The WLAN AN  210  is connected to a UMTS core network  230  via a WLAN Access Gateway (WAG)  204 .  
         [0027]     A UMTS network  250  includes an Ultimate Radio Access Network (UTRAN)  207  wirelessly connectable to the UE  201 , a Serving GPRS Support Node (SGSN)  208 , a Gateway GPRS Support Node (GGSN)/Foreign Agent (FA)  209 , a Packet Data Gateway (PDG)/FA  212  and an Authorization, Authentication and Accounting (AAA) server  211 . The UTRAN  207  includes a Node B directly wirelessly connectable to the UE  201 , and a Radio Network Controller (RNC) for controlling radio resources of the UE  201 .  
         [0028]     The PDG/FA  212  has a WLAN-related FA function, and the GGSN/FA  209  has a UMTS-related FA function. A Home Agent (HA)  215  is connected to the FA functions of the PDG/FA  212  and the GGSN/FA  209  via an external Packet Data Network (PDN)/Internet  240 .  
         [0029]     The UE  201  can communicate with the WLAN AN  210  or the UMTS network  250  via the AP  202  or the UTRAN  207 . The UE  201  can communicate with any one of the WLAN AN  210  and the UMTS network  250  at a time.  
         [0030]     In an interworking configuration between the WLAN AN  210  and the UMTS network  250 , if the UE  201  currently connected to the WLAN AN  210  performs handoff to the UMTS network  250 , an exemplary embodiment of the present invention performs UMTS authentication and Packet Data Protocol (PDP) context generation through the WLAN AN  210  before the handoff, thereby enabling seamless handoff. That is, in order to address the handoff delay problem, an exemplary embodiment of the present invention performs authentication and registration processes before completion of L 2  handoff, the authentication and registration process being responsible for long handoff delay.  
         [0031]     With reference to Table 1 below, a description will now be made of a method for managing an address management table in the HA  215  according to an exemplary embodiment of the present invention.  
         [0032]     According to an exemplary embodiment of the present invention, in order to support mobility of the UE  201  during handoff, the HA  215  further manages a ‘Next IP address’ field for storing the next IP address values in an address management table shown in Table 1, while managing a ‘Current IP address’ field mapped to a ‘Home IP address’ field of the UE  201  using Mobile IP. That is, the HA  215  previously registers, in the ‘Next IP address’ field, an IP address to be allocated from a new FA to which the UE  201  will move.  
                               TABLE 1                                   Home IP address   Current IP address   Next IP address                           IP MN1     IP C1     IP N1             IP MN2     IP C2     IP N2             . . .   . . .   . . .                      
 
         [0033]     Referring to Table 1, the ‘Next IP address’ field in the address management table is unfilled in the general case; the ‘Next IP address’ field is filled only when the UE  201  is in a handoff situation. If the ‘Next IP address’ field has a particular address value, the HA  215  buffers packets being delivered to the UE  201 , determining that the UE  201  is in the handoff situation. If pre-registration handoff for the UE  201  is completed, an address value in the ‘Next IP address’ field is replaced with an address value in the ‘Current IP address’ field, and the ‘Next IP address’ field is cleared. The buffered packets are then forwarded to the UE  201  using the address value in the ‘Current IP address’ field.  
         [0034]     For example, if a UE  201  with a Home IP address of ‘IP MN1 ’ generates a handoff request, the HA  215  searches the address management table to read an address value ‘IP N1 ’ in the ‘Next IP address’ field for the UE  201  with ‘IP MN1 ’. Thereafter, the HA  215  buffers packets being delivered to an address ‘IP C1 ’ in the ‘Current IP address’ field, determining that a new IP address that the UE  201  will use after handoff is ‘IP N1 . In other words, HA  215  determines that the ‘Next IP address’ field is not clear. If the pre-registration handoff for the UE  201  is completed, an address value in the ‘Current IP address’ field is replaced with an address value ‘IP N1 ’ in the ‘Next IP address’ field, and the ‘Next IP address’ field remains ‘Null.’ The buffered packets are then forwarded to the address ‘IP N1 ’, the handoff target.  
         [0035]     A detailed description will now be made of UE  201  IP addresses and signaling messages used when the UE  201  performs handoff from the WLAN AN  210  to the UMTS network  250 , according to an exemplary embodiment of the present invention.  
         [0036]     The possible IP addresses of the UE  201  include a home IP address ‘IP UE ’, a temporary IP address ‘IP UMTS ’ used by the UE  201  in the UMTS network  250  after handoff from the WLAN AN  210  to the UMTS network  250 , and a temporary IP address ‘IP WLAN ’ used by the UE  201  in the WLAN AN  210  before the handoff.  
         [0037]     Next, newly added signaling messages will be described in detail.  
         [0038]     A Pre-Registration Request message is delivered by a terminal to an FA (new FA) of a new network to which it will move for pre-registration handoff. In an exemplary embodiment of the present invention, the new FA corresponds to the GGSN/FA  209  of the UMTS network  250 . Upon receiving the Pre-Registration Request message, the new GGSN/FA  209  delivers an Authentication Request message to the SGSN  208  to request for pre-authentication. The Authentication Request message includes an IP UE , and an International Mobile Station Identifier (IMSI) of the UE  201 . A Pre-Registration Response message indicates completion of the pre-registration handoff from the new GGSN/FA  209  to the UE  201 , and includes an IP UMTS .  
         [0039]     A Pre-Registration Completion message includes an IP UE  or an IP UMTS  of the UE  201 , and provides the HA  215  with information indicating that the UE  201  has moved to the UMTS network  250  so that the HA  215  may update the ‘Current IP address’ field corresponding to the IP UE  with the IP UMTS  in its address management table.  
         [0040]     When pre-authentication for the UE  201  is completed, a PDP Address Request message is used by the SGSN  208  to deliver the IMSI of the UE  201  or the IP UE  to the GGSN/FA  209  to request for an address value IP UMTS  to be used as a temporary address of the UE  201  in the UMTS network  250 . A PDP Address Response message is used by the GGSN/FA  209  to deliver a PDP address to the SGSN  208 , the PDP address being generated for the UE  201  from which a request for the IP UMTS  was received.  
         [0041]     A Pre-Registration Update Request message is used by a new GGSN/FA  209  to deliver the IP UE  and the IP UMTS  to its HA  215  upon receiving the Pre-Registration Response message from the SGSN  208  to request updating the ‘Next IP address’ field corresponding to the IP UE  in the address management table with the IP UMTS . Further, a Pre-Registration Update Response message is used by the HA  215  to indicate completion of the update upon receiving the Pre-Registration Response message and is delivered to the GGSN/FA  209 .  
         [0042]     If the UE  201  currently connected to the WLAN AN  210  moves to the UMTS network  250 , the UMTS network  250  has no node capable of serving as the WLAN agent  203 . Therefore, the SGSN  208  of the UMTS network  250  substitutes for the WLAN agent  203 . That is, the UE  201  performs pre-authentication and pre-registration processes through the SGSN  208 , and performs handoff based on information acquired from the SGSN  208  through the processes.  
         [0043]     A detailed description will now be made of exemplary pre-authentication and pre-registration processes in which a mobile terminal performs handoff from the WLAN AN  210  to the UMTS network  250 .  
       First Exemplary Embodiment  
       [0044]     In a first exemplary embodiment of the present invention, a UE  201  performs a pre-registration handoff process of collectively performing authentication, IP address allocation and registration through a Pre-Registration Request message.  
         [0045]      FIG. 3  depicts a signaling diagram illustrating the overall operation in which a UE  201  performs pre-registration handoff from a WLAN AN  210  to a UMTS network  250 , according to a first exemplary embodiment of the present invention. Referring to  FIG. 3 , packet exchange between a WLAN AN  210  and a HA  215  is performed in step  300 . If a UE  201  determines that the handoff is in response to an L 2  trigger, step  302 , the UE  201  delivers a Pre-Registration Request message for requesting pre-authentication and pre-registration to an SGSN  208  via a GGSN/FA  209  of a UMTS network  250  through a WLAN AN  210 , step  304 . Herein, the Pre-Registration Request message includes such information as an IP UE  and an IMSI of the UE  201 , and the WLAN AN  210  holds address information of the GGSN/FA  209  that manages cells of the UMTS network  250  to which the UE  201  intends to move.  
         [0046]     In step  306 , the SGSN  208  performs authentication vector generation, terminal authentication, and encryption key distribution processes through UMTS Authentication and Key Agreement (AKA) based on the IMSI provided from the Pre-Registration Request message. The UMTS AKA process may be classified into two possible processes: one process that does not include a UTRAN  207  and another process that does include the UTRAN  207 .  
         [0047]     In the UMTS AKA process that does not include the UTRAN  207 , the UE  201  does not establish a radio connection before handoff, thus the encryption key values generated after completion of mutual authentication between the UE  201  and the SGSN  208  are not delivered to the UTRAN (that is, the RNC)  207 . However, in the UMTS AKA process that does include the UTRAN  207 , a radio connection between the SGSN  208  and the UTRAN  207  is established before start of the UMTS AKA process, thus the encryption key values generated after completion of mutual authentication between the UE  201  and the SGSN  208  are delivered to the UTRAN (that is, the RNC)  207 .  
         [0048]     After completion of the terminal authentication process through the pre-authentication process, the SGSN  208  delivers a PDP Address Request message including an IP UE  or an IMSI of the UE  201  to the GGSN/FA  209  to request a PDP address IP UMTS  to be used after the handoff, step  308 . The process of requesting a new PDP address for the UE  201  is automatically performed by the SGSN  208  after completion of the UMTS AKA process.  
         [0049]     Upon receiving the PDP Address Request message, the GGSN/FA  209  generates an IP UMTS  for the UE  201  and delivers the IP UMTS  to the SGSN  208  using a PDP Address Response message, step  310 . The SGSN  208  delivers the PDP Address Response message to the GGSN/FA  209 .  
         [0050]     In step  312 , the GGSN/FA  209  delivers the IP UMTS  and the IP UE  to the HA  215  using a Pre-Registration Update Request message.  
         [0051]     Upon receiving the Pre-Registration Update Request message, the HA  215  updates its address management table by registering the IP UMTS  in a ‘Next IP address’ field for the UE  201 , and transmits the update result to the GGSN/FA  209  using a Pre-Registration Update Response message, step  314 . That is, a ‘Current IP address’ field and the ‘Next IP address’ field for the IP UE  in the address management table are set to IP WLAN  and IP UMTS , respectively. In step  316 , the HA  215  buffers packets targeted to a UE  201  with the IP WLAN .  
         [0052]     Upon receiving the Pre-Registration Update Response message, the GGSN/FA  209  transmits a Pre-Registration Response message to the UE  201  via the WLAN AN  210  to inform the UE  201  of completion of the pre-authentication and pre-registration processes, step  318 . The Pre-Registration Response message includes the IP UMTS . In step  320 , the UE  201  performs detachment from the WLAN AN  210  through an AP  202 , and then performs L 2  handoff to the UMTS network  250  (UE movement).  
         [0053]     After movement to the UMTS network  250 , the UE  201  performs a UMTS attachment process in step  322 . If encryption key values for the previously established radio connection between the SGSN  208  and the UTRAN  207  are delivered to the UTRAN  207 , the AKA process may be completely omitted from the UMTS attachment process. However, when only the authentication vector generation/distribution and mutual authentication between the UE  201  and the SGSN  208  are performed without a previously established radio connection between the SGSN  208  and the UTRAN  207 , the AKA process of delivering encryption keys from the SGSN  208  to the UTRAN  207  may be included in the UMTS attachment process.  
         [0054]     In step  324 , the UE  201  generates, that is activates, a PDP context for data transmission. Because the UE  201  was previously allocated the PDP address IP UMTS  through the pre-authentication and pre-registration processes, it uses the intact IP UMTS  in the PDP context activation process of step  324 .  
         [0055]     In step  326 , the UE  201  delivers a Pre-Registration Completion message including the IP UE  or the IP UMTS  to the HA  215 . Upon receiving the Pre-Registration Completion message, the HA  215  updates its address management table using the IP UE  or the IP UMTS . That is, the HA  215  sets the IP UMTS  previously registered in the ‘Next IP address’ field as an address value in the ‘Current IP address’ field, and performs ‘Null’ processing on the ‘Next IP address’ field, thereby completing a Mobile IP registration process. In step  328 , the HA  215  stops the packet buffering that has been performed during handoff. In step  330 , the HA  215  forwards the buffered packets to the UE  201 . In step  332 , packets are exchanged between the UE  201 , with the IP UMTS , and the HA  215 .  
         [0056]     As described above, according to the first exemplary embodiment of the present invention, the packets generated during handoff of the UE  201  are all buffered in the HA  215 , and then transmitted to the UE  201  with the IP UMTS  after completion of the handoff of the UE  201 , thereby reducing a packet loss in the handoff process.  
       Second Exemplary Embodiment  
       [0057]     In a second exemplary embodiment of the present invention, a UE  201  first performs a pre-authentication process through Pre-Authentication Request/Response messages, then performs a pre-registration process through Pre-Registration Request/Response messages after completion of the pre-authentication process.  
         [0058]      FIG. 4  depicts a signaling diagram illustrating the overall operation in which a UE  201  performs pre-registration handoff from a WLAN AN  210  to a UMTS network  250 , according to a second exemplary embodiment of the present invention. Referring to  FIG. 4 , packet exchange between a UE  201 , with an IP WLAN  located in a WLAN AN  210 , and a HA  215  is performed in step  400 . If the UE  201  determines that the handoff is in response to an L 2  trigger, step  402 , the UE  201  delivers a Pre-Authentication Request message for requesting pre-authentication to an SGSN  208  via the WLAN AN  210  and a GGSN/FA  209 , step  409 . The Pre-Authentication Request message includes an IP UE  and an IMSI of the UE  201 , and the WLAN AN  210  holds address information of the GGSN/FA  209  that manages cells of the UMTS network  250  to which the UE  201  intends to move.  
         [0059]     In step  406 , the SGSN  208  performs authentication vector generation, terminal authentication, and encryption key distribution processes through UMTS AKA based on the IMSI provided from the Pre-Authentication Request message. The UMTS AKA process may be classified into two possible processes: one process that does not include a UTRAN  207  and another process that does include the UTRAN  207 .  
         [0060]     In the UMTS AKA process that does not include the UTRAN  207 , the UE  201  does not establish a radio connection before handoff, thus the encryption key values generated after completion of mutual authentication between the UE  201  and the SGSN  208  are not delivered to the UTRAN (that is, the RNC)  207 . However, in the UMTS AKA process that does include the UTRAN  207 , a radio connection between the SGSN  208  and the UTRAN  207  is established before start of the UMTS AKA process, thus the encryption key values generated after completion of mutual authentication between the UE  201  and the SGSN  208  are delivered to the UTRAN (that is, the RNC)  207 .  
         [0061]     After completion of the terminal authentication process for the UE  201  through the pre-authentication process, the SGSN  208  delivers a Pre-Authentication Response message indicating the completion of the pre-authentication process to the UE  201  via the GGSN/FA  209  and the WLAN AN  210 , step  408 . Unlike the first exemplary embodiment, the second exemplary embodiment first performs the authentication process for the UE, thereby preventing a possible handoff delay due to an authentication failure.  
         [0062]     If mutual authentication and key distribution processes between the UE  201  and the SGSN  208  are completed, the UE  201  delivers a Pre-Registration Request message for requesting pre-registration for the UMTS network  250  to the GGSN/FA  209 , step  410 . The Pre-Registration Request message includes a home address IP UE  of the UE  201 . Upon receiving the Pre-Registration Request message, the GGSN/FA  209  generates a PDP address IP UMTS  to be used for Mobile IP handoff of the UE  201 , and transmits the IP UMTS  and the IP UE  to the HA  215  using a Pre-Registration Update Request message for requesting update of the address management table, step  412 . Because the second exemplary embodiment is performed through a separate pre-registration request by a UE after completion of the UMTS AKA process, the Pre-Registration Request message is directly delivered to the GGSN/FA  209  that will generate a new PDP address IP UMTS  for the UE  201 .  
         [0063]     Upon receiving the Pre-Registration Update Request message, the HA  215  searches its address management table for a ‘Next IP address’ field for the IP UE  and registers the IP UMTS  in the searched ‘Next IP address’ field, accomplishing the pre-registration. As a result, a ‘Current IP address’ field and the ‘Next IP address’ field for the IP UE  in the address management table are set to IP WLAN  and IP UMTS , respectively.  
         [0064]     Thereafter, in step  414 , the HA  215  buffers all packets targeted to the IP WLAN  beginning at the time when the ‘Next IP address’ field in the address management table was updated. In step  416 , the HA  215  transmits a Pre-Registration Update Response message to the GGSN/FA  209 , indicating the completed update of the address management table and the start of buffering.  
         [0065]     Upon receiving the Pre-Registration Update Response message, the GGSN/FA  209  transmits a Pre-Registration Response message to the UE  201  via the WLAN AN  210  to inform the UE  201  of the completion of the pre-registration process, step  418 . The Pre-Registration Response message includes the IP UMTS . The UE  201  performs detachment from the WLAN AN  210  through an AP  202 , step  420 , and then performs L 2  handoff to the UMTS network  250 , step  422 .  
         [0066]     Next, the UE  201  performs a UMTS attachment process in step  424 . If encryption key values for the previously established radio connection between the SGSN  208  and the UTRAN  207  are delivered to the UTRAN  207 , the AKA process may be completely omitted from the UMTS attachment process. However, when only the authentication vector generation/distribution and mutual authentication between the UE  201  and the SGSN  208  are performed without a previously established radio connection between the SGSN  208  and the UTRAN  207 , the AKA process of delivering encryption keys from the SGSN  208  to the UTRAN  207  may be included in the UMTS attachment process.  
         [0067]     In step  426 , the UE  201  generates, that is, activates, a PDP context for data transmission. Because the UE  201  was previously allocated the PDP address IP UMTS  through the pre-authentication and pre-registration processes, it uses the intact IP UMTS  in the PDP context activation process.  
         [0068]     In step  428 , the UE  201  delivers a Pre-Registration Completion message including the IP UE  or the IP UMTS  to the HA  215 . Upon receiving the Pre-Registration Completion message, the HA  215  searches its address management table for a ‘Next IP address’ field for the IP UMTS  or the IP UE , sets the IP UMTS  previously registered in the searched ‘Next IP address’ field as an address value in the ‘Current IP address’ field, and performs ‘Null’ processing on the ‘Next IP address’ field, thereby completing a Mobile IP registration process. Next, the HA  215  stops the packet buffering that has been performed during handoff of the UE  201 , and forwards the buffered packets to the UE  201  in step  430 . In step  432 , packets are exchanged between the UE  201 , with the IP UMTS , and the HA  215 .  
         [0069]     As can be understood from the foregoing description, exemplary embodiments of the present invention can perform pre-authentication and pre-registration processes during handoff between heterogeneous networks of WLAN and UMTS networks, thereby maximizing a reduction in the delay which may be caused by authentication and registration processes after a UE moves. In addition, the exemplary embodiments can buffer received packets targeted to a UE during a handoff period, and then forward the buffered packets to the UE after completion of the handoff, thereby contributing to a reduction in packet loss which may occur during handoff. Moreover, the exemplary embodiments of the present invention can enable fast, seamless handoff during interworking between a WLAN network and a UMTS network, thereby guaranteeing service continuity.  
         [0070]     While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and equivalents thereof.