Patent Publication Number: US-2007116011-A1

Title: Method and apparatus for communications of user equipment using internet protocol address in a mobile communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit under 35 U.S.C. § 119(a) of Korean Patent Application No. 2005-94611 filed Oct. 7, 2005, in the Korean Intellectual Property Office, the entire disclosure of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates generally to a mobile communication system. More particularly, the present invention relates to a method and apparatus for communications using an Internet Protocol (IP) address in a User Equipment (UE).  
      2. Description of the Related Art  
      The traditional Internet which provides connectivity to fixed hosts in a wired network has evolved to support UEs roaming from one network to another in a wired-wireless network interworking environment. A protocol developed to support the mobility of such UEs over the Internet is IP. Mobile IP enables a UE to continue on-going communications with a Correspondent Node (CN) even after the UE changes its point of access to an Internet link. In Mobile IP, however, the UE has to support a mobile IP stack and a network has to support new entities such as a Home Agent (HA) and a Foreign Agent (FA).  
      A mobile communication network is typically configured to include a Node B wirelessly connectable to the UE and a GateWay (G/W) for connecting the Node B to an IP network. The G/W functions to connect the UE connected to the Node B over a wireless network to the IP network inside or outside the wireless network. The coverage area of the G/W is called a subnet. The G/W allocates an IP address to a UE within the subnet, and manages and maintains the mobility of the UE. The Node B provides radio resources to the UE so that the UE can connect to the IP network via the G/W. The IP network is defined as a network for providing an IP service to the UE, like the Internet.  
       FIG. 1  is a diagram illustrating a signal flow for a conventional operation in the G/W for managing communications of UEs. In the following procedure, a UE  100  notifies a G/W  102  to which it belongs of a change of its routing area. The UE  100  is aware of entry into a new routing area by a Routing Area Identification (RAI) included in information broadcast from a Node B.  
      Referring to  FIG. 1 , when the UE  100  moves to a new routing area, it sends a Routing Area Update (RAU) request message to the G/W  102  in step  104 . The RAU request message contains an old RAI, a new RAI, and an old Primary Temporary Mobile Station/Subscriber Identity (P-TMSI).  
      The old RAI is the ID of an old routing area before the movement of the UE  100 , and the new RAI is the ID of the new routing area acquired from the broadcast information. A P-TMSI is a unique ID identifying the UE  100  within a routing area and thus the old P-TMSI identifies the UE  100  within the old routing area.  
      After the G/W  102  receives the RAU request message, security functions are performed between the UE  100  and the G/W  102 , when needed in step  106 . Then the G/W  102  allocates a new P-TMSI to the UE  100  and notifies the UE  100  of the new P-TMSI by an RAU accept message in step  108 . The UE  100  replies with an RAU complete message in step  110 , thus completing the routing area update.  
      The development of communication technology and a variety of demands from users and service provides have driven the use of a plurality of G/Ws in communication networks. The G/Ws manage the IP addresses of UEs within their subnets and a UE moving between subnets is allocated a new IP address.  
      According to the above-described conventional technology, when the UE moves to the service area of a new G/W, it cannot reuse an old IP address used in the subnet of an old G/W. In a system with a plurality of G/Ws, the change of the G/W managing the UE leads to the change of the IP address, thus causing service interruptions. Therefore, it is impossible to provide a seamless service to the UE.  
      Accordingly, there is a need for an improved method and apparatus for controlling communications of a user equipment using internet protocol.  
     SUMMARY OF THE INVENTION  
      Exemplary embodiments of the present invention address at least the above problems and/or disadvantages and provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method and apparatus for allocating a new IP address to a UE or allowing the UE to reuse an old IP address according to the radio resource and service status of the UE.  
      According to one exemplary aspect of the present invention, in a method of communicating using an IP address in a UE in a mobile communication system, when the UE moves from an old subnet managed by an old gateway to a new subnet managed by a new gateway, it determines whether there are its allocated radio resources. In the absence of the allocated radio resources, the UE determines whether an IP-based application service is in progress. If the IP-based application service is in progress, the UE determines to reuse an old IP address in the new subnet. The old IP address is an IP address used in the old subnet. If the IP-based application service is not in progress, the UE determines to use a new IP address in the new subnet. Then the UE sends to the new gateway an RAU request message including an indication indicating whether the old IP address or the new IP address will be used in the new subnet.  
      According to another exemplary aspect of the present invention, in a method of supporting communications of a UE using an IP address in a new gateway in a mobile communication system, the new gateway receives an RAU request message from the UE, when the UE in an idle state moves from an old subnet managed by an old gateway to a new subnet managed by the new gateway. The new gateway determines whether the UE will reuse an old IP address in the new subnet, referring to an indication included in the RAU request message. The old IP address is an IP address used in the old subnet. If the indication indicates that the old IP address will be reused, the new gateway registers the old IP address in a routing table for the UE so that the old IP address can be used in the new subnet. If the indication indicates that the old IP address will not be reused, the new gateway acquires a new IP address and stores the new IP address for the UE. Here, the indication is decided according to whether the UE has allocated radio resources and an IP-based application service is in progress for the UE, when the UE moves to the new subnet.  
      According to a third exemplary aspect of the present invention, in a method of supporting communications of a UE using an IP address in an old gateway in a mobile communication system, the old gateway receives a gateway context request message from a new gateway, when the UE in an idle state moves from an old subnet managed by the old gateway to a new subnet managed by the new gateway. The old gateway determines whether the UE will reuse an old IP address in the new subnet, the old IP address being an IP address used in the old subnet, referring to an indication included in the gateway context request message. The old gateway registers a new RAI being an ID of a new RA to which the UE moves, and a new P-TMSI allocated to the UE by the new gateway in a routing table for the UE. Here, the indication is decided according to whether the UE has allocated radio resources and an IP-based application service is in progress for the UE, when the UE moves to the new subnet.  
      According to a fourth exemplary aspect of the present invention, in an apparatus for communicating using an IP address in a UE in a mobile communication system, a controller determines whether an IP-based application service is in progress for the UE, when the UE in an idle state moves from an old subnet managed by an old gateway to a new subnet managed by a new gateway, determines to reuse an old IP address in the new subnet, the old IP address being an IP address used in the old subnet, if the IP-based application service is in progress, determines to use a new IP address in the new subnet, if the IP-based application service is not in progress, and generates an indication indicating whether the old IP address or the new IP address will be used in the new subnet. A transmitter sends an RAU request message including the indication to the new gateway.  
      According to a fifth exemplary aspect of the present invention, in a mobile communication system, a plurality of gateways are connected to an IP network. A UE capable of communicating using radio resources and supporting an IP-based application service determines whether an IP-based application service is in progress for the UE, when the UE in an idle state moves from an old subnet managed by an old gateway to a new subnet managed by a new gateway. If the IP-based application service is in progress, the UE determines to reuse an old IP address in the new subnet. The old IP address is an IP address used in the old subnet. If the IP-based application service is not in progress, the UE determines to use a new IP address in the new subnet. Then the UE sends to the new gateway an RAU request message including an indication indicating whether the old IP address or the new IP address will be used in the new subnet.  
      According to a sixth exemplary aspect of the present invention, in a method of communicating using an IP address in a UE in a mobile communication system, the UE moves from an old subnet managed by an old gateway to a new subnet managed by a new gateway. It sends a request message including an old IP address used in the old subnet to the new gateway in order to request reuse of the old IP address in the new subnet. The UE receives from the new gateway a response message indicating the old IP address can be used in the new subnet.  
      According to a seventh exemplary aspect of the present invention, in a method of supporting communications of a UE using an IP address in a new gateway in a mobile communication system, the new gateway receives from the UE a request message with an old IP address used in an old subnet, requesting reuse of the old IP address in a new subnet, when the UE moves from the old subnet managed by an old gateway to the new subnet managed by the new gateway. The new gateway registers the old IP address in a routing table for the UE so that the old IP address can be used in the new subnet, and sends to the UE a response message indicating that the old IP address can be used in the new subnet.  
      According to an eighth exemplary aspect of the present invention, in a mobile communication system, a plurality of gateways are connected to an IP network. When a UE capable of communicating using radio resources and supporting an IP-based application service moves from an old subnet managed by an old gateway to a new subnet managed by a new gateway, it sends a request message including an old IP address used in the old subnet to the new gateway in order to request reuse of the old IP address in the new subnet, and receives from the new gateway a response message indicating the old IP address can be used in the new subnet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, 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:  
       FIG. 1  is a diagram illustrating a signal flow for a conventional operation in the G/W for managing communications of UEs;  
       FIG. 2  illustrates the configuration of a mobile communication network to which an exemplary embodiment of the present invention is applied;  
       FIG. 3  is a flowchart illustrating a UE&#39;s operation when the UE moves from an old subnet to a new subnet according to an exemplary embodiment of the present invention;  
       FIG. 4  is a flowchart illustrating a new G/W&#39;s operation when the UE moves from the old subnet to the new subnet according to an exemplary embodiment of the present invention;  
       FIG. 5  is a flowchart illustrating an old G/W&#39;s operation when the UE moves from the old subnet to the new subnet according to an exemplary embodiment of the present invention;  
       FIG. 6  is a diagram illustrating a signal flow for changing the IP address of the UE when the UE moves from the old subnet to the new subnet in an IP_RESET mode according to an exemplary embodiment of the present invention;  
       FIG. 7  is a diagram illustrating a signal flow for reusing the IP address of the UE when the UE moves from the old subnet to the new subnet in an IP_PRESERVE mode according to an exemplary embodiment of the present invention;  
       FIG. 8  illustrates packet transmission to the UE via the old G/W when the UE moves from the old subnet to the new subnet according to an exemplary embodiment of the present invention;  
       FIG. 9  illustrates packet transmission to the UE via the old G/W and the new G/W when the UE moves from the old subnet to the new subnet according to an exemplary embodiment of the present invention; and  
       FIG. 10  is a block diagram of the UE according to an exemplary embodiment of the present invention. 
    
    
      Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention and are merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. Exemplary embodiments of the present invention will be described herein below with reference to the accompanying drawings.  
      Exemplary embodiments of the present invention are intended to provide a technique for allocating a new IP address to a UE or allowing the UE to reuse an old IP address by identifying the state of the UE defined according to radio resource allocation and service provisioning to the UE, when a UE enters into a new subnet. If the UE reuses the old IP address in the new subnet, it sends the old IP address directly to a new G/W managing the new subnet so that the new G/W can manage the old IP address.  
      While exemplary embodiments of the present invention will be described in detail in the context of nodes and messages used in a particular system, it is clearly understood to those skilled in the art that a UE&#39;s IP address management according to exemplary embodiments of the present invention can be carried out in any other mobile communication system with a similar technological background and channel structure with a slight modification which is within the scope of the present invention.  
       FIG. 2  illustrates the configuration of a mobile communication network to which exemplary embodiments of the present invention are applied. In the illustrated case of  FIG. 2 , a UE moves between subnets.  
      Referring to  FIG. 2 , an old G/W (O-G/W)  222  manages a first subnet  218  (subnet  1 ), and first and second Node Bs  210  and  212  (Node B  1  and Node B  2 ) reside within subnet  1 . The O-G/W, as does a new G/W, provides a connection between the Node Bs and the IP Network  226 . Node B  1  is located in a first Routing Area (RA) (RA  1 ), and Node B  2  is located in a second RA (RA  2 ). An RA is a logical area within which an incoming message from a source entity is routed to a UE and an outgoing message from the UE is routed to a destination entity. The ID of RA  1  is a first RAI (RAI  1 ), and the ID of RA  2  is a second RAI (RAI  2 ).  
      Initially, a UE  200  communicates with Node B  1  in subnet  1  under management of the O-G/W  222 . During the communications, the UE  200  is connected to Node B  2  belonging to a new G/W (N-G/W)  224  by inter-RA movement  202  (movement A). The UE  200  (in other words  204 ) accesses a third Node B  214  (Node B  3 ) by inter-subnet movement  206  (movement B). The N-G/W  224  manages a second subnet  220  (subnet  2 ) and a third Node B  214  (Node B  3 ) and a fourth Node B  216  (Node B  4 ) are located in subnet  2 . Node B  3  and Node B  4  are within a third RA (RA  3 ) and thus have the same RAI, RAI  3 .  
      As described above, the RA change may cause the change of the subnet or not. The movement  202  is made from RA  1  to RA  2 . Despite the movement  202 , the UE  204  maintains its IP address unchanged because it is still in the same subnet  218 . On the other hand, the movement  206  is made from RA  2  to RA  3 , and the UE  204  (in other words  208 ) is in subnet  2  after the movement  206 . With the inter-subnet movement  206 , the UE  208  is allocated a new IP address by the N-G/W  224 .  
      As a result, an on-going service for the UE  208  is interrupted. Accordingly, there exists a need for deciding as to whether an IP-based application service is in progress for the UE  208 .  
      A plurality of LE modes are defined depending on whether radio resources are in use or not for the UE. When the LE is allocated radio resources and a call is in progress, it is in a CONNECTED state. If the radio resources are not used, the LE is in an IDLE state. For example, the LE conducts a voice call with the other party or receives data streams continuously from a streaming server in the CONNECTED state. According to the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, an LTE Active state and an LTE IDLE state are equivalent to the CONNECTED state and the IDLE state, respectively.  
      It may occur that the UE is receiving a particular service without occupying radio resources. For example, a File Transfer Protocol (FTP) service or a HyperText Transfer Protocol (HTTP) service is bursty rather than continuous. In this case, the UE is neither fully in the CONNECTED state nor in the IDLE state. The state where a call is connected (in other words a service application is in progress) but no file transmission using radio resources takes place is defined as an IP_PRESERVE mode, and the rest IDLE state is defined as an IP_RESET mode.  
      In other words, the UE in the IDLE state can be in the IP_PRESERVE mode or the IP_RESET mode depending on whether the LE is receiving an application service. The IP_PRESERVE mode is an IDLE+ state in which no data transmission is carried out using radio resources with an application service in progress. When the UE is in this IP_PRESERVE mode, not the Node B but the G/W manages a context required for the communications of the LE (hereinafter, referred to as a LE context). The IP_RESET mode is an IDLE-state where the UE is not receiving the application service, in other words the call is not connected. If the UE is in the IP_RESET mode, the UE context is maintained in neither the Node B nor the G/W. In case of inter-subnet movement, the UE is allocated a new IP address or reuses an old IP address according to its state.  
       FIG. 3  is a flowchart illustrating an operation of the UE according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 3 , the UE detects its movement into a new subnet by system information broadcast from the Node B in step  300  and determines whether available radio resources have been allocated to the UE in step  302 . In the presence of allocated radio resources, the UE considers that it is in the CONNECTED state in step  304  and continues communications in a general procedure in step  306 .  
      In the absence of the allocated radio resources, the UE determines whether an IP-based application service is in progress, considering that it is in the IDLE state in step  308 . If the service is in progress, the UE determines to reuse an old IP address used in an old subnet in the new subnet considering that it is in the IP_PRESERVE mode in step  310 . On the contrary, if the service is not on-going, the UE determines to be allocated a new IP address in the new subnet, considering that it is in the IP_RESET mode in step  312 .  
      In step  314 , the UE sends an indication indicating reuse of the old IP address or allocation of the new IP address to the N-G/W of the new subnet. Then the UE communicates using the old IP address or the new IP address in the new subnet. The indication may be included in an RAU request message that the UE sends to the N-GW in the IDLE state in order to notify the entry into the new subnet.  
      That is, the UE adds the indication indicating whether its IP address will be maintained or changed to the RAU request message delivered to notify the N-G/W of the entry into the new subnet. The new G/W determines from the indication set in the received RAU request message whether to operate in the IP_PRESERVE mode or the IP_RESET mode.  
      The indication includes at least one of a 1-bit con-indicator, indicating whether the IP address is maintained or changed, and the old IP address. If the con-indicator is “1”, this implies that the UE&#39;s IP address will be changed. If the con-indicator is “0”, this implies that the UE&#39;s IP address will be kept unchanged. While it is described herein that the con-indicator is 1 bit, it can be one or more bits depending on system implementation. The con-indicator is determined at the application level of the UE, that is, at a layer above an IP level. When the UE is receiving an application/service (for example TCP/IP service) requiring no IP change for its seamless provisioning, it sets the con-indicator to “0”. In another case, the UE sends the old IP address in the RAU request message. The N-G/W uses the old IP address for the UE in the new subnet when the received RAU request message includes the old IP address of the UE.  
       FIG. 4  is a flowchart illustrating an operation of the N-G/W according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 4 , upon receipt of the RAU request message from the UE in step  400 , the N-G/W checks whether an indication included in the RAU request message indicates maintenance of the IP address of the UE in step  402  and operates in the IP_PRESERVE mode (step  404 ) or in the IP_RESET mode (step  406 ) according to the value of the indication.  
      The IP_PRESERVE mode operation is that the N-G/W registers the old IP address of the UE in a routing table in order to reuse the old IP address for communications of the UE in the new subnet. The N-G/W may acquire the old IP address from the UE or query the O-G/W for the old IP address.  
      The IP_RESET mode operation is that the N-G/W uses a new IP address for the UE&#39;s communications in the new subnet. The new IP address is allocated to the UE by the N-G/W or generated from the UE itself.  
      If a con-indicator is used as the indication and it is set to 0, the N-G/W operates in the IP_PRESERVE mode in step  404 . If the con-indicator is 1, the N-G/W operates in the IP_RESET mode in step  406 .  
      If the old IP address is used as the indication and included in the RAU request message, the N-G/W operates in the IP_PRESERVE mode in step  404 . In the absence of the old IP address, the N-G/W operates in the IP_RESET mode in step  406 .  
      After receiving the RAU request message from the UE, the N-G/W notifies the O-G/W whether it will operate in the IP_PRESERVE mode or in the IP_RESET mode. The O-G/W decides as to whether to operate in the IP_PRESERVE mode or in the IP_RESET mode according to the information received from the N-G/W.  
       FIG. 5  is a flowchart illustrating an operation of the O-G/W according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 5 , the O-G/W receives from the N-G/W a Gateway Context Request message indicating that the UE has moves from an old subnet of the O-G/W to the new subnet of the N-G/W in step  500  and checks an indication indicating whether the IP address of the UE is to be changed in the Gateway Context Request message in step  502 . The indication can be an afore-mentioned con-indicator, for example. In this case, the O-G/W determines whether the con-indicator is 0. If the con-indicator is 0, which implies that the IP address will be maintained, the O-G/W operates in the IP_PRESERVE mode in step  504 . On the contrary, if the con-indicator is 1, which implies that the IP address will be changed, the O-G/W operates in the IP_RESET mode in step  506 .  
       FIG. 6  is a diagram illustrating a signal flow for changing the IP address of the UE when the UE moves from the old subnet to the new subnet in the IP_RESET mode according to a preferred embodiment of the present invention.  
      Referring to  FIG. 6 , a UE  600  detects its movement into a new RA and sends an RAU request message to an N-G/W  608  in step  612 . The RAU request message contains an old RAI, a new RAI, and old P-TMSI, and a con-indicator. In the illustrated case of  FIG. 2 , the old RAI is RAI  2  and the new RAI is RAI  3 . The UE  600  moves from subnet  1  to subnet  2  and the con-indicator is set to 1 to indicate that the IP address of the UE  600  is to be changed. It can be further contemplated as another exemplary embodiment that if the change of the IP address is requested by excluding the old IP address from the RAU request message, the con-indicator is omitted.  
      Upon receipt of the RAU request message, the N-G/W  608  sends to an O-G/W  606  a Gateway Context Request message to acquire a UE context from the O-G/W  606  in step  614 . The Gateway Context Request message contains the old RAI, the old P-TMSI, and the con-indicator. The old P-TMSI is used to detect the context of the UE  600  among UE contexts stored in the O-G/W, and the con-indicator is set to 1 as received from the UE  600 . It can be further contemplated as another exemplary embodiment that if the change of the IP address is requested by excluding the old IP address from the RAU request message, the N-G/W  608  generates a con-indicator to be included in the Gateway Context Request message.  
      In step  616 , the O-G/W  606  sends the UE context for the UE  600  to the N-G/W  608  by a Gateway Context Response message. The N-G/W  608  stores the UE context included in the received Gateway Context Response message. Thereafter, unless otherwise specified by the network, the O-G/W  606  does not need to maintain UE information including the old IP address of the UE  600 . The UE context contains authorization information and Quality of Service (QoS) information. It may further include other information. In step  618 , a security-related operation can be performed between the UE  600  and the N-G/W  608  and between the N-G/W  608  and a Home Subscriber Server (HSS)  610 . The HSS  610  stores and manages subscription information and location information of the UE  600 . The O-G/W  606 , the N-G/W  608  and the HSS  610  update the location information of the UE  600  and the N-G/W  608  stores subscriber data of the UE  600  received from the HSS  610  in step  620 . In this way, when the HSS  610  recognizes the movement of the UE  600  to the N-G/W  608 , it can command deletion of the UE information of the UE  600  to the O-G/N  606 .  
      The following operation after step  620  can be embodied in at least two ways. One exemplary method, shown in step  636 , is that the UE  600  itself generates an IP address. Specifically, the N-G/W  608  allocates a new P-TMSI to the UE  600  in step  622 . The new P-TMSI together with the address prefix of the N-G/W  608  is sent to the UE  600  by an RAU accept message. The UE  600  generates a new IP address for use in the new subnet, referring to the address prefix of the new N-G/W  608  in step  624  and notifies the N-G/W  608  of the new IP address by an RAU complete message in step  626 .  
      Another exemplary operation, shown in step  638 , is that the N-G/W  608  generates a new IP address and allocates it to the UE  600 . Specifically, the N-G/W  608  generates and stores the new IP address for the UE  600  to use in the new subnet in step  628  and sends the new IP address together with a new P-TMSI to the UE  600  by an RAU accept message in step  630 . In step  632 , the UE  600  replies with an RAU complete message.  
      After the IP address allocation by the operation  636  or  638 , the N-G/W  608  stores the new IP address in a routing table for the UE  600  so that the UE  600  can use the new IP address for communications in the subnet of the N-G/W  608 .  
      After acquiring the new IP address, the UE  600  sends an IP Multimedia Subsystem (IMS) registration message including the new IP address to a server for managing packet communications over an IP network, for example, an IMS server in step  634 . Thus, the UE  600  is able to send packet data by paging.  
      The IMS server, which is defined in the 3GPP, functions to provide an IP service to subscribers. The IMS registration message is used to register the UE  600  to the IMS server.  
       FIG. 7  is a diagram illustrating a signal flow for reusing the IP address of the UE when the UE moves from the old subnet to the new subnet in the IP_PRESERVE mode according to an exemplary embodiment of the present invention.  
      Referring to  FIG. 7 , a UE  700  detects its movement into a new RA and sends an RAU request message to an N-G/W  708  in step  712 . The RAU request message contains an old RAI, a new RAI, an old P-TMSI, a con-indicator, and the old IP address of the UE  700 . The old RAI, the new RAI, and the old P-TMSI have been described earlier. The con-indicator is set to 0 to indicate that the IP address of the UE  700  will not be changed. The old IP address is the IP address of the UE  700  used in an old subnet and will be reused in the new subnet. It can be further contemplated as another exemplary embodiment that if the maintenance of the IP address is requested by including the old IP address in the RAU request message, the con-indicator can be omitted. In a further exemplary embodiment of the present invention, the RAU request message includes both the con-indicator and the old IP address.  
      Upon receipt of the RAU request message, the N-G/W  708  updates a routing table for the UE  700  by registering the old IP address, the old RAI, and the old P-TMSI in the routing table in step  714 . Thus, the old IP address can be used in the new subnet of the N-G/W  708 . After the routing table update, the N-G/W  708  sends to an O-G/W  706  a Gateway Context Request message to acquire a UE context from the O-G/W  706  in step  716 . The Gateway Context Request message contains the old RAI, the new RAI, the old P-TMSI, a new P-TMSI, and the con-indicator. It can be further contemplated as another exemplary embodiment that if the reuse of the IP address is requested by including the old IP address in the RAU request message, the old IP address is included in the Gateway Context Request message, instead of the con-indicator.  
      In step  718 , the O-G/W  706  updates its routing table for the UE  700  by registering information about the N-G/W  706 , the new P-TMSI, and the new RAI based on the Gateway Context Request message. Then the O-G/W  706  replies with a Gateway Context Response message including the UE context in step  720 . Since the routing table already has the old IP address of the UE  700 , the old IP address of the UE  700  is managed by both the O-G/W  706  and the N-G/W  708  in effect. In step  722 , a security-related operation can be performed between the UE  700  and the N-G/W  708  and between the N-G/W  708  and an HSS  710 . The O-G/W  706 , the N-G/W  708  and the HSS  710  update the location information of the UE  700  and the N-G/W  708  stores subscriber data of the UE  700  received from the HSS  710  in step  724 .  
      After the registration of the old IP address of the UE  700  for use in the new subnet, the N-G/W  708  sends an RAU accept message including the new P-TMSI to the UE  700  in step  726 . In step  728 , the UE  700  replies with an RAU complete message. Thus, the UE  700  is able to send packet data by paging.  
      When the UE moves to the new subnet in the above procedures, incoming packets for the UE in the IP_PRESERVE mode are delivered to the O-G/W because their destination address is set to the subnet address of the O-G/W: Hereinbelow, an exemplary method of sending the packets to the UE which has moved to the new subnet through paging will be described. The path of the incoming packets to the UE after the inter-subnet movement depends on whether a data tunnel is established between the O-G/W and a new Node B ( FIG. 8 ) or between the O-G/W and the N-G/W ( FIG. 9 ).  
       FIG. 8  illustrates packet transmission to the UE via the O-G/W according to an exemplary embodiment of the present invention. A UE  800  moves from an old subnet to a new subnet during communicating with an O-G/W  806  via an old Node B  802 , and after the inter-subnet movement, communicates with an N-G/W  808  through a new Node B  804 .  
      Referring to  FIG. 8 , incoming packets for the UE  800  are delivered to the O-G/W  806  in step  810 . The O-G/W  806  sends a paging request message to the N-G/W  808  to which the UE  800  now belongs, referring to the location information of the UE  800  in step  812 .  
      In step  814 , the N-G/W  808  broadcasts the paging request message to Node Bs within its subnet and the Node Bs page the UE  800  according to the paging request message. The new Node B  804  covering the UE  800  receives a paging response message from the UE  800 .  
      The new Node B  804  sends the paging response message to the N-G/W  808  in step  816  and the N-G/W  808  sends the paging response message with information about the new Node B  804  to the O-G/W  806  in step  818 . The O-G/W  806  establishes a data tunnel with the new Node B  804  based on the information about the new Node B  804  in step  820  and forwards the packets to the UE  800  via the data tunnel in step  822 . In this way, packets from the IP network are delivered to the UE  800  using the old IP address.  
       FIG. 9  illustrates packet transmission to the UE via the old G/W and the new G/W according to an exemplary embodiment of the present invention. A UE  900  moves from an old subnet to a new subnet during communicating with an O-G/W  906  via an old Node B  902 , and after the inter-subnet movement, communicates with an N-G/W  908  through a new Node B  904 .  
      Referring to  FIG. 9 , incoming packets for the UE  900  are delivered to the O-G/W  906  in step  910 . The O-G/W  906  establishes a data tunnel with the N-G/W  908  in step  912  and forwards the packets to the N-G/W  908  via the data tunnel in step  914 .  
      The N-G/W  908  broadcasts a paging request message for the received packets to Node Bs within its subnet and the Node Bs page the UE  900  according to the paging request message in step  916 . The new Node B  904  covering the UE  900  receives a paging response message from the UE  900 .  
      The new Node B  904  sends the paging response message to the N-G/W  908  in step  918 . In step  920 , the N-G/W  908  establishes a data tunnel with the new Node B  904 . Then, the N-G/W  908  forwards the packets to the LE  900  via the data tunnel between the N-G/W  908  and the new Node B  904  in step  922 .  
       FIG. 10  is a block diagram of the UE according to an exemplary embodiment of the present invention. Only components related to the operation of the present invention are shown.  
      Referring to  FIG. 10 , the UE includes a radio resource manager  1000 , a controller  1002 , a transceiver  1004 , and an application controller  1006 . The controller  1002  determines whether the LE has moved from the subnet of the O-G/W to the subnet of the N-G/W based on information broadcast by the Node B. If the UE has moved to the new subnet, the controller  1002  determines whether the UE is in the CONNECTED state or in the IDLE state by querying the radio resource manager  1000  whether radio resources were allocated to the UE.  
      In the presence of allocated radio resources for the UE, the controller  1002  queries the application controller whether the UE is using an IP-based application service. If the IP-based application service is in progress, the controller  1002  determines that the LE is in the IP_PRESERVE mode and otherwise, the controller  1002  determines that the LE is in the IP_RESET mode. The controller  1002  generates an RAU request message according to the mode of the LE, including at least one of a con-indicator and the old IP address of the UE as an indication indicating whether the old IP address will be reused or changed in the new subnet. The RAU request message is sent to the N-G/W through the transceiver  1004 . Then the controller  1002  completes an RAU by exchanging messages illustrated in  FIGS. 6 and 7  through the transceiver  1004 . Thus, the UE is able to exchange packets through the N-G/W.  
      In accordance with exemplary embodiments of the present invention as described above, since it is determined whether to change or reuse the IP address of a UE according to the state of the UE, a seamless service can be provided to the UE even when the UE moves to another subnet.  
      While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.