Transferring a user equipment in mobile communication system

FIELD OF THE INVENTION

0001The present invention relates to a technology of managing a user equipment (UE) in a mobile communication system, and more particularly to a method and system for transferring a UE in a mobile communication system.

BACKGROUND

0002Currently, a 3^rd generation (3G) mobile communication system is mainly based on standards of wideband code division multiple access (WCDMA) and code division multiple access (CDMA) communication systems.

0003The universal mobile telecommunication system (UMTS) is a 3G mobile communication system adopting the WCDMA air interface technology, and is thus usually referred to as a WCDMA communication system. The structure of the UMTS is shown in <figref idref="f0001">Figure 1</figref>, which includes a UE, a radio access network (RAN), and a core network (CN). The RAN is a UMTS terrestrial radio access network (UTRAN), and is capable of handling all functions related to radio services. The CN handles all voice calls and data connections in the UMTS, and realizes switching and routing functions with external networks. The CN is logically divided into a circuit switched (CS) domain and a packet switched (PS) domain.

0004The structure of the UTRAN is shown in <figref idref="f0001">Figure 2</figref>, and the UTRAN includes one or more radio network subsystems (RNSs). Each RNS is formed by a radio network controller (RNC) and one or more NodeBs. The RNC is connected to the CN through an Iu interface, and the NodeB is connected to the RNC through an Iub interface. In the UTRAN, the RNCs are interconnected through an Iur interface. The RNCs can be classified into source RNC (SRNC) and target RNC (TRNC) for the UE. The RNC is adapted to allocate and control radio resources of the NodeB connected or related thereto. The NodeB transmits data from the Iub interface to the UE through a Uu interface or transmits data from the Uu interface to the RNC through the Iub interface. The NodeB also participates in some radio resource management.

0005The network architectures shown in <figref idref="f0001">Figures 1 and 2</figref> are old versions before 3GPP Release 6. Considering the network competitive power in the further, a brand-new 3GPP evolved network architecture is developed to meet application requirements of the mobile communication network in the coming decade or even longer time. The evolved network architecture includes system architecture evolution (SAE) and long term evolution (LTE) of the access network, and an evolved access network is called an E-UTRAN. The evolved network aims to provide a low delay, high data rate, high system capacity and coverage, and low cost network completely based on the Internet protocol (IP). As the evolved network is a brand-new network architecture, all the nodes, functions, and flows of the network architectures shown in <figref idref="f0001">Figures 1 and 2</figref> change substantially.

0006Currently, 3GPP includes SAE and LTE. LTE aims to provide a low-cost network capable of reducing the delay, raising the user data rate, and improving the system capacity and coverage. The LTE adopts only PS domain services, and the bearer network is an IP bearer. <figref idref="f0002">Figures 3</figref> is a schematic architectural view of the current evolved network. The LTE RAN is an RAN of the evolved network. In the network, the nodes include, but are not limited to, eNodeBs or control plane servers (CPSs), so logically be treated as LTE-RAN entities. The mobility management entity/user plane entity (MME/UPE) is also a logic entity. The MME is adapted to store the mobility management context of the UE, such as the user ID, mobility status and tracking area (TA) information, and certificate the user. The UPE is adapted to terminate the downstream data of the UE in an idle state, trigger a paging, and store the context of the UE, for example, the IP address and routing information of the UE. The user plane anchor may not change during the session of the UE. Whether the MME/UPE is separated or not makes no difference to the method provided by the present invention.

0007When the UMTS is advancing toward the evolved network, a stage that the UMTS and evolved network coexist appears. As shown in <figref idref="f0002">Figure 4</figref>, interfaces S3 and S4 are added between the UMTS and the evolved network. The interface S3 realizes the exchange of information carried by UEs in an idle and/or active state between 3GPP access systems. The interface S4 provides control and mobility support for user planes between the general packet radio service (GPRS) CN and 3GPP anchors.

0008Currently, in Section 4.5a.1 of 3GPP protocol TS23.236-630, a method of transferring all the UEs of a source CN element in a 3G or 2^nd generation (2G) network to other CN entities is provided. The method is implemented in three stages. In the first stage, UEs initiating a periodic routing area update (RAU) within a preset time limit are transferred according to a method shown in <figref idref="f0003">Figure 5</figref>, and the involved network entities include a UE, an access network, a source CN element, and a target CN element. The method includes the following steps.

0009In Step 501, after entering an idle state, the UE activates a periodic RAU timer.

0010In Step 502, when the periodic RAU timer experiences a time-out, the UE sends an RAU request to the source CN element.

0011In Step 503, when the source CN element receives the RAU request sent by the UE, if the source CN element has already received an operation maintenance notification, all the UE loads have to be transferred to other CN entities. Thus, the source CN element sends an RAU acceptance message to the UE, and the message carries a temporary mobile subscriber identity (TMSI) re-allocated to the UE and a minute periodic RAU timer value (for example, 4 seconds) re-allocated to the UE. The network resource identifier (NRI) field of the TMSI is set as Null.

0012In Step 504, the UE receives the RAU acceptance message, and activates a new periodic RAU timer.

0013In Step 505, when the periodic RAU timer activated in Step 504 experiences a time-out, the UE sends an RAU request carrying the TMSI allocated to the UE in Step 503. After the access network receives the RAU request, as the NRI in the TMSI carried by the request is set as Null, the access network employs a non-access layer node selection function to choose a target CN element for the UE, and forwards the RAU request to the target CN element.

0014In Step 506, the target CN element receives the RAU request, and sends an RAU acceptance message to the UE.

0015However, the method in <figref idref="f0003">Figure 5</figref> has the following defects. The method is only applicable to the circumstance of transferring all the UEs serving in the source CN element. In the method, the source CN element passively waits for the UE to send a periodic RAU, instead of initiating the transferring of the whole or a part of the UEs in service. Moreover, the method is only for transferring UEs within the same pool area. In addition, the method is limited to 2G or 3G networks, and is inapplicable to other networks like evolved networks, networks where both evolved networks and UMTS networks coexist, or post-SAE networks.

0016Currently, in the evolved network, in Section 7.13 of 3GPP protocol TR23.882-130, a reattaching method is provided to transfer a UE from a source MME/UPE to a target MME/UPE. As shown in <figref idref="f0004">Figure 6</figref>, the involved network entities include a UE, an evolved access network (eRAN), a target MME/UPE, a source MME/UPE, an inter AS anchor (IASA), and a home subscriber server (HSS). The method includes the following steps.

0017In Step 601, due to factors such as overload, routes not being optimized, or the reception of an operation and maintenance (O&amp;M) request, the source MME/UPE sends a reattach request to the UE.

0018In Step 602, the UE finds a selectable evolved network access system through the interaction with the eRAN, and selects the access system and network.

0019In Step 603, the UE sends an attach request to the selected target MME/UPE, and the request carries the registration information of the UE such as packet temporary mobile subscriber identity (P-TMSI). If the UE does not store the registration information, the request carries a permanent user ID.

0020As the network is shared, the attach request carries the information of the selected target MME/UPE, and the eRAN selects the target MME/UPE. The attach request may also carry information about default IP access bearer, for example, the IP address and access point name (APN) selected by the UE.

0021In Step 604, on receiving the attach request, if the request carries the registration information of the UE, the target MME/UPE determines the source MME/UPE according to the P-TMSI, and sends a message for obtaining a UE-related user information to the source MME/UPE.

0022In Step 605, the source MME/UPE sends the UE-related user information, for example, permanent user information, to the target MME/UPE.

0023In Step 606, the target MME/UPE authenticates the UE through the HSS.

0024In Step 607, the target MME/UPE sends a registration message to the HSS, indicating that the target MME/UPE is serving the UE.

0025In Step 608, the HSS indicates the source MME/UPE to delete the stored UE-related user information or to identify the absence of the UE.

0026In Step 609, the HSS sends a registration acknowledge message to the target MME/UPE. Meanwhile, user subscription data authorized to the default IP access bearer is sent at the same time, and strategy charging control information of the default IP access bearer is also sent to the target MME/UPE at the same time.

0027In Step 610, the target MME/UPE selects the IASA.

0028In Step 611, the IASA configures an IP layer with the confirmed IP address of the UE, so that a new user plane is established, and a default strategy charging rule is adopted to charge the UE.

0029In Step 612, the target MME/UPE provides the eRAN with an allocation of quality of service (QoS) of the default IP access bearer, for example, an upper limit of the data transmission rate.

0030In Step 613, the target MME/UPE accepts the attach request of the UE, allocates the P-TMSI to the UE, and sends to the UE an attach acknowledge message carrying the confirmed IP address of the UE.

0031In Step 614, the UE sends the attach acknowledge message to the target MME/UPE.

0032The method illustrated in <figref idref="f0004">Figure 6</figref> is only applicable to an evolved network, and does not specify how to transfer a UE between CN entities in a UMTS network or in the case when a UMTS network and an evolved network coexist. <figref idref="f0004">Figure 6</figref> also reveals a defect that, the access network may pick up the source CN element when selecting a CN element if no preventive measure is taken.

0033<patcit id="pcit0001" dnum="WO2004084572A1"><text>WO 2004/084572 A1</text></patcit> discloses a Method for transferring a mobile terminal in e.g. an umts-network from one server node in a pool to another server node in the same pool.

0034<patcit id="pcit0002" dnum="WO2006024307A1"><text>WO 2006/024307 A1</text></patcit> discloses an arrangement and a method in communication networks.

0035<patcit id="pcit0003" dnum="WO2003019958A1"><text>WO 2003/019958 A1</text></patcit> discloses a method and means for redistribution of subscriber information in UMTS networks where the nodes are arranged in pools

0036<patcit id="pcit0004" dnum="EP1684468A1"><text>EP 1684468 A1</text></patcit> discloses A communication system for establishing communication between intelligent radio terminal and a server

00373GPP DRAFT S2-051993 introduces re-distribution of UEs in A/Gb/Iu-flex based pool configuration.

00383GPP TS 23.236 v6.3.0 covers the details for the Intra Domain Connection of RAN Nodes to Multiple CN Nodes for GSM and UMTS systems.

00393GPP TR 23.882 v1.3.0 is to develop a framework for an evolution or migration of the 3GPP system to a higher-data-rate, lower-latency, packet-optimized system that supports, multiple RATs.

00403GPP DRAFT S2-060563_S2-060479 REV2 discusses support for multi-to-multi relationship between MME/UPEs and Node Bs (S1-flex).

SUMMARY

0041Accordingly, the present invention is directed to a method, a UE and system for transferring a UE in a mobile communication system as defined in the independent claims. The method and system are widely applied, and the transferring is initiated by a network side element. Besides, a more preferable CN element is selected as the target CN element according to a certain mechanism, so as to provide a better service for the user of the UE.

0042In order to achieve the above objective, the present invention provides the following solutions.

0043The present invention provides a method for transferring a UE in a mobile communication system including the following steps.

0044A UE receives, from a source core network, CN, element serving the UE, a transfer instruction carrying indication for transferring the UE through a tracking area update, TAU, process; and transmits a transfer request to an access network after reception of the transfer instruction, wherein the transfer request is used for requesting selection of a target CN element that is different from the source CN element.

0045In one possible implementation, the transfer request carries information that makes the access network identify the UE as a new entrant.

0046In one possible implementation, the transfer instruction is a TAU instruction, and the transfer request is a TAU request.

0047The present invention further provides a UE, including: means for receiving, from a source core network, CN, element serving the UE, a transfer instruction carrying indication for transferring the UE through a tracking area update, TAU, process; and means for transmitting a transfer request to an access network after reception of the transfer instruction, wherein the transfer request is used for requesting selection of a target CN element that is different from the source CN element.

0048In one possible implementation, the transfer request carries information that makes the access network identify the UE as a new entrant.

0049In one possible implementation, the transfer instruction is a TAU instruction, and the transfer request is a TAU request.

0050The present invention further provides a system including a UE, a source CN element serving the UE, and an access network. The source CN element is adapted to determine the UE to be transferred and send a transfer instruction carrying indication for transferring the UE through a tracking area update, TAU, process. The UE is adapted to receive the transfer instruction from the source CN, and transmit a transfer request after reception of the transfer instruction, wherein the transfer request is used for requesting selection of a target CN element that is different from the source CN element. The access network, is adapted to receive the transfer request from the UE, and select a target CN element to which the UE is transferred, wherein the target CN element is different from the source CN element.

0051In one possible implementation, the transfer request carries information that makes the access network identify the UE as a new entrant.

0052In one possible implementation, the access network is adapted to select the target CN element based on a load sharing mechanism.

0053In one possible implementation, the transfer instruction is a TAU instruction, and the transfer request is a TAU request.

0054As can be seen from the above solutions, in the the present invention, the network side of the mobile communication system initiates the UE transferring between the CN entities, and several modes are provided to prevent the UE from re-accessing the source CN element.

0055The following subjects, even if named "embodiment(s)", "invention(s)", "aspect(s)" or "example(s)" etc., do not or do not fully correspond to the invention as defined in the claims and are, as well as the whole following description and figures, present for illustration purposes only or to highlight specific aspects or features of the claims.

0056The embodiments are widely applied in the existing networks as well as the future networks, such as the evolved network, universal mobile telecommunication system (UMTS) network, network in which the UMTS network and the evolved network coexist, and even post-SAE network or coexistence network in other forms. Moreover, the method for transferring a user equipment between CN entities according to an embodiment of the present invention is not only applicable to a user equipment transferring process initiated by entities such as a serving GPRS support node (SGSN), MME and UPE, but also applicable to all user equipment transferring processes initiated by a CN element in the above networks. For example, the method is also applicable to the process of transferring a user equipment to another CN element initiated by a CN element in an operation and maintenance centre (OMC), gateway GPRS support node (GGSN), IASA or even post-SAE network. The method provided according to the embodiments of the present invention is initiated by the network side, and a more preferred serving CN element is selected for the user according to a certain mechanism, thus providing a better service for the user of the UE.

BRIEF DESCRIPTION OF THE DRAWINGS

0057<ul id="ul0001" list-style="none"><li><figref idref="f0001">Figure 1</figref> is a schematic structural view of a UMTS in the prior art;</li><li><figref idref="f0001">Figure 2</figref> is a schematic structural view of a UTRAN in the prior art;</li><li><figref idref="f0002">Figure 3</figref> is a schematic architectural view of an evolved network in the prior art;</li><li><figref idref="f0002">Figure 4</figref> is a schematic architectural view of a network where a UMTS network and an evolved network coexist in the prior art;</li><li><figref idref="f0003">Figure 5</figref> is a flow chart of transferring all the UEs of a source CN element to another CN element in a 3G or 2G network in the prior art;</li><li><figref idref="f0004">Figure 6</figref> is a flow chart of transferring a UE from a current MME/UPE to another MME/UPE by a reattach method in an evolved network in the prior art;</li><li><figref idref="f0004">Figure 7</figref> is a flow chart of a method for transferring a UE in a mobile communication system according to an embodiment of the present invention;</li><li><figref idref="f0005">Figure 8</figref> is a flow chart of a first method for transferring a UE triggered by the network side based on an RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0005">Figure 9</figref> is a flow chart of a second method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0006">Figure 10</figref> is a flow chart of a third method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0006">Figure 11</figref> is a flow chart of a fourth method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0007">Figure 12</figref> is a flow chart of a fifth method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0007">Figure 13</figref> is a flow chart of a sixth method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0008">Figure 14</figref> is a flow chart of UE transferring between CN entities in an evolved network triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0009">Figure 15</figref> is a flow chart of UE transferring between CN entities in a UMTS network triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0010">Figure 16</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the RAU technique according to a first embodiment of the present invention;</li><li><figref idref="f0011">Figure 17</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the RAU technique according to a second embodiment of the present invention;</li><li><figref idref="f0012">Figure 18</figref> is a flow chart of replacing an IASA element for a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention;</li><li><figref idref="f0013">Figure 19</figref> is a flow chart of a first method for transferring a UE triggered by the network side based on an attach technique according to an embodiment of the present invention;</li><li><figref idref="f0013">Figure 20</figref> is a flow chart of a second method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention;</li><li><figref idref="f0014">Figure 21</figref> is a flow chart of a third method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention;</li><li><figref idref="f0014">Figure 22</figref> is a flow chart of a fourth method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention;</li><li><figref idref="f0015">Figure 23</figref> is a flow chart of a fifth method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention;</li><li><figref idref="f0015">Figure 24</figref> is a flow chart of a sixth method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention;</li><li><figref idref="f0016">Figure 25</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the attach technique according to a first embodiment of the present invention;</li><li><figref idref="f0017">Figure 26</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the attach technique according to a second embodiment of the present invention;</li><li><figref idref="f0018">Figure 27</figref> is a flow chart of a method for transferring a UE through a detach and reattach process according to an embodiment of the present invention;</li><li><figref idref="f0018">Figure 28</figref> is a schematic view of a system for transferring a UE in a mobile communication system according to an embodiment of the present invention;</li><li><figref idref="f0019">Figure 29</figref> is a schematic view of a network architecture of separated control plane and user plane in the prior art;</li><li><figref idref="f0020">Figure 30</figref> is a flow chart of a method for performing MME user transferring through a tracking area update (TAU) technique in the network architecture of separated control plane and user plane according to an embodiment of the present invention;</li><li><figref idref="f0021">Figure 31</figref> is a flow chart of a method for performing MME user transferring through a reattach technique in the network architecture of separated control plane and user plane according to an embodiment of the present invention;</li><li><figref idref="f0022">Figure 32</figref> is a flow chart of a transferring method when an serving gateway (S-GW) changes in the network architecture of separated control plane and user plane according to an embodiment of the present invention; and</li><li><figref idref="f0023">Figure 33</figref> is a flow chart of a transferring method when a packet data network gateway (PDN GW) changes in the network architecture of separated control plane and user plane according to an embodiment of the present invention.</li></ul>

DETAILED DESCRIPTION

0058In order to make the aforementioned objectives, technical solutions and advantages of the present invention more comprehensible, embodiments accompanied with figures are described in detail below.

0059In order to provide the user of the UE with better services, an embodiment of the present invention provides a method in which a network side element initiates a UE transferring in a mobile communication system. The method is adapted to select more satisfactory service CN entities for the user according to a certain mechanism. The detailed process is shown in <figref idref="f0004">Figure 7</figref>. Referring to <figref idref="f0004">Figure 7</figref>, the involved network entities include a source CN element, a target CN element, an access network and a UE. The method includes the following steps.

0060In Step 701, the source CN element determines to transfer a UE which it serves, and sends a transfer instruction carrying UE transfer restriction information to the UE.

0061In Step 702, on receiving the transfer instruction, the UE sends a transfer request according to the UE transfer restriction information carried in the instruction.

0062In Step 703, the access network selects a target CN element for the UE; and the UE is transferred to the target CN element. The target CN element is a CN element that is different from the source CN element.

0063In Step 703, the access network has many ways to select the target CN element, which will be illustrated in detail below.

0064In the embodiments of the present invention, the UE can be transferred in two modes. One is to implement the transferring through a location area update (LAU) technique, and the other is to implement the transferring through an attach technique. The two modes are illustrated as follows.

0065In the first mode, the UE is transferred through an LAU technique.

0066An LAU process is specified in the UMTS network and the evolved network. The process is called RAU in the UMTS network while TAU in the evolved network. The LAU includes, but is not limited to, the RAU and TAU. For ease of illustration, the RAU is taken as an example below.

0067In the embodiments of the present invention, in order to enable the network side of the mobile communication system to transfer the UE based on the RAU technique, a message is required to indicate the UE to perform the RAU process. In the present invention, a particular message or an information equipment (IE) in an existing message carrying the RAU instruction is sent to the UE to indicate the UE to perform the RAU process. For example, the network side of the mobile communication system indicates the UE to perform the RAU process by sending an RAU instruction thereto, or by carrying an IE having the RAU instruction in a paging message when paging the UE.

0068When sending the RAU instruction to the UE, the source CN element may also set a timer indicating the UE to send an RAU request after the timer experiences a time-out, so that the RAU request is sent after the interface between the element and the CN in the access network is released. Definitely, the timer is optional, and the RAU request may be immediately sent after the UE receives the RAU instruction.

0069The method for transferring a UE based on the RAU technique provided by the embodiment of the present invention is widely applied to various networks, instead of being limited to the UMTS network. That is, the method is also applicable to the evolved network, the network where the evolved network and the UMTS network coexist, and even the post-evolved network as well as coexistence network in other forms. The method of transferring a UE by CN entities provided in the embodiments of the present invention is applicable to not only processes in which the user transferring is initiated by entities such as a serving GPRS support node (SGSN), MME, and UPE, but also processes of the above adaptable networks in which the user transferring is initiated by CN entities. For example, the method is applicable to a process in which a user transferring to other CN entities is initiated by CN entities in an OMC, gateway GPRS support node (GGSN), IASA, or even post-SAE network.

0070Compared with the method for transferring a UE based on the attach technique, the UE transferring based on the RAU technique is advantageous in that the target CN element does not need to reconstruct the context and thus the UE services may not be interrupted. In the UE transferring based on the RAU technique, the target CN element requests the source CN element for the context of the UE instead of reconstructing the UE context, and will not delete the activated context of the packet data protocol (PDP) as in the attach process, so as to ensure the continuity of the UE services. For example, when browsing a web page, the UE performs no operation for quite a long time. Though the mobility management state of the UE is idle, the session management state is active, and at this point, if the network side of the mobile communication system initiates a reattach process to transfer the UE, the CN element will clear the stored context of the UE, thus resulting in a termination of the current session of the UE. However, the UE transferring based on the RAU technique does not have the above problem. In addition, if the UE transferring is implemented by setting certain flag bits or sending other messages instead of using the LAU technique, the context of the source CN element may not be deleted during the user transferring process, and the target CN element obtains the context of the UE from the source CN element, thus ensuring the service continuity. The method also fulfills the purpose of the embodiments of the present invention. Though different in form, the method is substantially the same as the above embodiment of the present invention, thus falling in the protecting range of the present invention.

0071<figref idref="f0005">Figure 8</figref> is a flow chart of a first method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. The method includes the following steps.

0072In Step 801, the source CN element determines to change an Radio Access Technique (RAT) for a UE it serves due to factors such as overload, service requirements, or the reception of an O&amp;M request.

0073In Step 802, the source CN element sends an RAU instruction to the UE, indicating the UE to be transferred to another RAT.

0074The RAT information carried in the transfer instruction may be information for preventing the access of the UE to the original RAT, or one or more RATs selected by the source CN element for the UE. In the latter case, the candidate RATs are arranged in priority order and sent to the UE in the form of an RAT list, so that the UE selects the RAT accordingly.

0075In Step 803, the UE selects an RAT according to the RAT information carried in the received RAU instruction, and sends an RAU request in the RAT. According to a set mechanism, for example, load sharing, the access network selects a target CN element for the UE and sends the RAU request to the target CN element.

0076In Step 804, an RAU process is implemented between the UE and the network.

0077When the UE is transferred to another RAT, the access network equipment needs to be switched. For example, when the UE is transferred from the UMTS network to the evolved network, the access network equipment changes from the RAN of the original 2G/3G network into the eRAN of the evolved network, and both the RAN and eRAN are represented by the access network in <figref idref="f0005">Figure 8</figref>.

0078The method of <figref idref="f0005">Figure 8</figref> is applicable to the circumstance of transferring the RAT for the UE. For example, the first, due to the overload of one CN element in the network where the UMTS network and the evolved network coexist, the UE has to be transferred to another CN element with a lighter load. The second, the RAT is transferred for the UE on receiving an O&amp;M request. The third, due to service requirements, for example, when downstream data with higher QoS is sent to the UE attached to the SGSN, the UMTS network currently serving the UE is unsatisfactory while the evolved network can meet the requirement, so that the UE has to be transferred to the evolved network.

0079<figref idref="f0005">Figure 9</figref> is a flow chart of a second method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. The method includes the following steps.

0080In Step 901, the source CN element determines to transfer the CN element for a UE it serves.

0081In Step 902, the source CN element sends an RAU instruction to the UE, indicating the UE to be transferred to another CN element.

0082In Step 903, on receiving the RAU instruction, the UE sends an RAU request carrying information for preventing the access of the RAU to the source CN element. According to the restriction information carried by the received RAU request, the access network adopts a set mechanism, for example, load sharing, to select the target CN element for the UE, and forwards the RAU request to the target CN element.

0083In Step 904, an RAU process is implemented between the UE and the network.

0084The method of <figref idref="f0005">Figure 9</figref> is generally adopted and widely applied to various circumstances, for example, in the case of an overload of the CN element that serves the UE, an O&amp;M request, or routes not being optimized. In the method, the source CN element and the target CN element can belong to the same network or to different networks.

0085<figref idref="f0006">Figure 10</figref> is a flow chart of a third method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. The method includes the following steps.

0086In Step 1001, the source CN element determines to transfer the CN element for the UE.

0087In Step 1002, as for the UE to be transferred, the source CN element determines it has an associated CN element and the UE can be transferred to the associated CN element.

0088In Step 1003, the source CN element sends an RAU instruction to the UE, indicating the UE to be transferred to the associated CN element.

0089In Step 1004, the UE sends an RAU request carrying an ID related to the associated CN element. According to the ID related to the associated CN element carried by the request, the access network sends the RAU request to the associated CN element. The ID related to the associated CN element may be any identity capable of uniquely determining the associated CN element, for example, the identity of the associated CN element, the NRI of the associated CN element, or the TMSI allocated by the associated CN element to the UE.

0090In Step 1005, an RAU process is implemented between the UE and the network.

0091The method of <figref idref="f0006">Figure 10</figref> is applicable to the circumstance that the UE is simultaneously attached to two CN entities associated with each other, and selects a network to reside in. For example, due to the overload of one CN element, the UE has to be transferred to the other CN element; or due to service requirements, when downstream data with high QoS is sent to the UE residing in the SGSN, the UMTS network is unsatisfactory while the evolved network can meet the requirement, so that the UE needs to be transferred to the MME/UPE of the associated evolved network.

0092<figref idref="f0006">Figure 11</figref> is a flow chart of a fourth method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. The method includes the following steps.

0093In Step 1101, the source CN element determines to transfer the CN element for a UE.

0094In Step 1102, according to a set mechanism, for example, load sharing, the source CN element selects a target CN element with a lighter load from CN entities that share the same interface with the source CN element, and obtains an ID related to the target CN element.

0095The ID related to the target CN element may be any identity capable of uniquely determining the element, for example, the identity of the target CN element, the NRI of the target CN element, or the TMSI allocated by the target CN element to the UE. The TMSI is allocated by the CN to the UE. For example, the TMSI is U-TMSI in the UMTI network and S-TMSI in the evolved network.

0096In Step 1103, the source CN element sends an RAU instruction carrying the ID related to the target CN element to the UE.

0097In Step 1104, on receiving the RAU instruction, the UE sends an RAU request carrying the ID related to the target CN element. The access network receives and forwards the RAU request to the target CN element according to the ID related to the target CN element carried in the request.

0098In Step 1105, an RAU process is implemented between the UE and the network.

0099The method of <figref idref="f0006">Figure 11</figref> is applicable to a circumstance that the source CN element obtains a target CN element for the UE according to a set mechanism. For example, when the source CN element is overloaded, another CN element is selected for the UE within the same pool area. When the route is not optimized, another CN element with a more preferred route is selected for the UE. In case of an O&amp;M operation, for example, an operation and maintenance centre (OMC) determines that part of the UEs on the SGSN element have to be transferred to the MME/UPE element, so that the OMC interacts with the MME/UPE element to obtain a related ID, then issues an O&amp;M command to the SGSN element for transferring some UEs, and notifies the SGSN element of an ID related to the target MME/UPE element, and the SGSN element, on receiving the command, sends an RAU instruction to the UE. Due to service requirements, for example, when downstream data with high QoS is sent to the UE residing in the SGSN element, the UMTS network is unsatisfactory while the SGSN element may obtain an MME/UPE element capable of receiving the UE transferred thereto from the evolved network sharing the same interface with the element, and the MME/UPE element can meet the QoS requirement, the method may be adopted to transfer the UE to the MME/UPE element.

0100In the method of <figref idref="f0006">Figure 11</figref>, the target CN element may be an associated CN element related to the source CN element of the UE, which is a special case of <figref idref="f0006">Figure 10</figref>. In the method, the source CN element and the target CN element can belong to the same network or to different networks.

0101<figref idref="f0007">Figure 12</figref> is a flow chart of a fifth method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. The method includes the following steps.

0102In Step 1201, the source CN element determines to transfer the CN element for a UE.

0103In Step 1202, the source CN element sends an RAU instruction to the UE, indicating the UE to adopt an ID that makes the access network regard the UE as a new entrant to send an RAU request.

0104When the indicated UE accesses again, the ID that makes the access network regard the UE as a new entrant UE has various types, for example, a TMSI which is set as Null, a TMSI with an NRI field set as Null, or IDs of CN entities other than those currently within the area of the UE. In the latter two cases, the ID has to be generated at the source CN element. In the case that the TMSI is set as Null, the source CN element sends a related instruction to the UE.

0105In Step 1203, the UE sends an RAU request carrying the ID indicated in Step 1202. The access network receives the message, and determines the UE as a new entrant according to the ID carried in the message. Therefore, the access network adopts a set mechanism, for example, load sharing, to select the target CN element and then send the RAU request to the target CN element.

0106In Step 1204, an RAU process is implemented between the UE and the network.

0107In Step 1203 of <figref idref="f0007">Figure 12</figref>, the ID indicated in Step 1202 serves as an IE in a radio resource control (RRC) field of the RAU request, but the ID does not need to serve as an IE in a non-access stratum (NAS) field of the RAU request.

0108The method of <figref idref="f0007">Figure 12</figref> is applicable to a circumstance that the source CN element is overloaded and the UE has to be transferred to a CN element with a lighter load. In the method, the source CN element and the target CN element can belong to the same network or to different networks.

0109<figref idref="f0007">Figure 13</figref> is a flow chart of a sixth method for transferring a UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. The method includes the following steps.

0110In Step 1301, the source CN element determines to transfer the CN element for a UE.

0111In Step 1302, the source CN element broadcasts a list of UEs required to be transferred to other CNs based on RAU, and meanwhile broadcasts information that prevents the access of the UEs to the source CN element.

0112In Step 1303, the UE monitors a broadcast message and determines whether the UE itself is arranged in the broadcast list. If the UE itself is arranged in the broadcast list, the UE sends an RAU request carrying the information that prevents its access to the source CN element. According to the information carried in the received request, the access network adopts a set mechanism, for example, load sharing, to select the target CN element for the UE and send the RAU request to the target CN element.

0113In Step 1304, an RAU process is implemented between the UE and the network.

0114The method of <figref idref="f0007">Figure 13</figref> is applicable to circumstances that the source CN element is overloaded, in case of an O&amp;M operation, and the CN element is to be shut down for maintenance or upgrade. In the method, the source CN element and the target CN element can belong to the same network or to different networks.

0115Several embodiments are given below to further illustrate the present invention.

0116<figref idref="f0008">Figure 14</figref> is a flow chart of UE transferring between CN entities in an evolved network triggered by the network side based on the RAU technique according to an embodiment of the present invention. When getting overloaded, the source MME/UPE element detects that another MME/UPE element sharing the same interface bears a lighter load, and thus adopts the method. The method includes the following steps.

0117In Step 1401, the source MME/UPE element determines to replace the CN element for a UE.

0118In Step 1402, according to a load sharing mechanism, the source MME/UPE element selects an MME/UPE element with a lighter load from MME/UPE entities that share the same interface with the source MME/UPE element as a target MME/UPE element, and obtains an ID of the target MME/UPE element or an S-TMSI allocated by the target MME/UPE element to the UE.

0119In Step 1403, the source MME/UPE element sends the UE an RAU instruction carrying the ID of the target MME/UPE element or the S-TMSI allocated by the target MME/UPE element to the UE.

0120In Step 1404, the UE sends an RAU request carrying the ID of the target MME/UPE element or the S-TMSI allocated by the target MME/UPE element to the UE via the access network. The access network determines the target MME/UPE element according to the information carried in the RAU request, and forwards the RAU request to the target MME/UPE element.

0121In Step 1405, the target MME/UPE element requests the source MME/UPE element for a related context of the UE.

0122In Step 1406, the source MME/UPE element returns the related context of the UE to the target MME/UPE element.

0123In Step 1407, the target MME/UPE element authenticates the UE.

0124In Step 1408, the target MME/UPE element replies a context acknowledge message to the source MME/UPE element.

0125In Step 1409, the source MME/UPE element sends related user data of the UE to the target MME/UPE element. This step is optional.

0126In Step 1410, a default IP bearer between the target MME/UPE element and the IASA is updated, and the message adopted may be, but is not limited to, a PDP context update request/response.

0127In Step 1411, the target MME/UPE element sends a location update message to the HSS, and the message may be, but is not limited to, Update Location.

0128In Step 1412, the HSS deletes the related context of the UE from the source MME/UPE element.

0129In Step 1413, the HSS adds UE subscription data into the target MME/UPE element.

0130In Step 1414, the HSS sends a location update acknowledge message to the target MME/UPE element, and the message may be, but is not limited to, Update Location Ack.

0131In Step 1415, if the UE is attached to a UMTS network at the same time, an association relationship is built between the target MME/UPE element and the SGSN element in the UMTS network. This step is optional.

0132In Step 1416, the target MME/UPE element sends an RAU acceptance message to the UE, and if the TMSI/RA changes, this information is also carried in the acceptance message.

0133In Step 1417, the UE sends an RAU complete message to the target MME/UPE element.

0134When the source MME/UPE needs to be shut down for operation maintenance or the route is not optimized due to the motion of the UE, the above process can be employed to perform user transferring in the SAE network. However, the mechanism of selecting the target MME/UPE element may be different. For example, when the source MME/UPE needs to be shut down for maintenance, the OMC determines the UE to be transferred, selects the target MME/UPE element for the UE, and notifies the source MME/UPE to send an RAU instruction to the UE via an O&amp;M command. If the route of the UE is not optimized, the source MME/UPE selects a target MME/UPE with an optimized route for the UE.

0135<figref idref="f0009">Figure 15</figref> is a flow chart of UE transferring between CN entities in a UMTS network triggered by the network side based on the RAU technique according to an embodiment of the present invention. The UE is attached to the SGSN element and the MME/UPE element at the same time, and an association relationship is built between the SGSN element and the MME/UPE element. When the source SGSN element is overloaded, and the associated MME/UPE element is incapable of receiving the UE transferred due to overload or routes not being optimized to the UE, the source SGSN element selects an SGSN element with a lighter load from those sharing the same interface for UE transferring. The method includes the following steps.

0136In Step 1500, the source SGSN element is associated with the target MME/UPE element.

0137In Step 1501, the source SGSN element determines to replace the CN element for a UE.

0138In Step 1502, the source SGSN element obtains an ID of the target SGSN element or a U-TMSI allocated by the target SGSN element to the UE.

0139In Step 1503, the source SGSN element sends the UE an RAU instruction carrying the ID of the target SGSN element or the U-TMSI allocated by the target SGSN element to the UE.

0140In Step 1504, the UE sends an RAU request carrying the ID of the target SGSN element or the U-TMSI allocated by the target SGSN element to the UE via the access network. The access network determines the target SGSN element according to the information carried in the RAU request, and forwards the RAU request to the target SGSN element.

0141In Step 1505, the target SGSN element requests the source SGSN element for a related context of the UE.

0142In Step 1506, the source SGSN element returns the related context of the UE to the target SGSN element.

0143In Step 1507, the target SGSN element authenticates the UE. This step is optional.

0144In Step 1508, the target SGSN element replies a context acknowledge message to the source SGSN element.

0145In Step 1509, the source SGSN element sends related user data of the UE to the target SGSN element. This step is optional.

0146In Step 1510, a default IP bearer between the target SGSN element and the MME/UPE is updated, and the message adopted may be, but is not limited to, a PDP context update request/response.

0147In Step 1511, the target SGSN element sends a location update message to the HSS, and the message may be, but is not limited to, Update Location.

0148In Step 1512, the HSS deletes the related context of the UE from the source SGSN element.

0149In Step 1513, the HSS adds UE subscription data into the target SGSN element.

0150In Step 1514, the HSS sends a location update acknowledge message to the target SGSN element, and the message may be, but is not limited to, Update Location Ack.

0151In Step 1515, an association relationship is built between the target SGSN element and the MME/UPE element in the evolved network.

0152In Step 1516, the target SGSN element sends an RAU acceptance message to the UE, and if the TMSI/RA changes, this information is also carried in the acceptance message.

0153In Step 1517, the UE sends an RAU complete message to the target SGSN element.

0154In <figref idref="f0009">Figure 15</figref>, the UPE is located on a data path of the 2G/3G network, i.e., user plane data is transmitted between the SGSN element, UPE element, and IASA element via the S3 interface in <figref idref="f0002">Figure 3</figref>. Thus, the update of the user plane context in Step 1510 is performed between the target SGSN element and the MME/UPE element. If the user plane data is transmitted between the SGSN element and the IASA element via the S4 interface in <figref idref="f0002">Figure 3</figref>, the update of the user plane context of the UE in Step 1510 is performed between the target SGSN element and the IASA element.

0155When the source SGSN element needs to be shut down for operation maintenance or the route is not optimized due to the motion of the UE, the method of <figref idref="f0009">Figure 15</figref> can be employed to transfer the UE between CN entities in the UMTS network. However, the mechanism of selecting the target SGSN element may be different. For example, when the source SGSN element needs to be shut down for maintenance, the OMC determines whether the UE needs to be transferred, selects the target SGSN element for the UE, and notifies the source SGSN element to send an RAU instruction to the UE via an O&amp;M command. If the route of the UE is not optimized, the source SGSN element selects a target SGSN element with an optimized route for the UE.

0156<figref idref="f0010">Figure 16</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the RAU technique according to a first embodiment of the present invention. In the network where the UMTS network and the evolved network coexist, if the UE residing in the UMTS network is attached to the two networks at the same time, when a downstream service with a high QoS requirement is sent to the UE, the UMTS network unsatisfactory while the evolved network can meet the QoS requirement of the service and is capable of receiving the UE transferred thereto, the UE is thus transferred to an associated CN element, so that the UE resides in the evolved network and communicates via the evolved network. The method includes the following steps.

0157In Step 1600, the source SGSN element is associated with the target MME/UPE element.

0158In Step 1601, the source SGSN element determines to replace the CN element for the UE.

0159In Step 1602, as for a UE to be transferred, the source SGSN element determines that an associated CN element exists, and that the UE can be transferred to the associated CN element.

0160In Step 1603, the source SGSN sends a paging carrying an RAU instruction to the UE, indicating the UE to be transferred to an associated MME/UPE element.

0161In Step 1604, the UE sends an RAU request via the access network. The request carries an ID of the associated MME/UPE element or a T-TMSI allocated by the associated MME/UPE element to the UE. The access network determines the target MME/UPE element according to the information carried in the request, and forwards the RAU request to the target MME/UPE element.

0162In Step 1605, the target MME/UPE element requests the source SGSN element for a related context of the UE.

0163In Step 1606, the source SGSN element returns the related context of the UE to the target MME/UPE element.

0164In Step 1607, the target MME/UPE element authenticates the UE.

0165In Step 1608, the target MME/UPE element replies a context acknowledge message to the source SGSN element.

0166In Step 1609, the source SGSN element sends related user data of the UE to the target MME/UPE element. This step is optional.

0167In Step 1610, a default IP bearer between the target MME/UPE element and the IASA is updated, and the message adopted may be, but is not limited to, a PDP context update request/response.

0168In Step 1611, the target MME/UPE element sends a location update message to the HSS, and the message may be, but is not limited to, Update Location.

0169In Step 1612, the HSS adds UE subscription data into the target MME/UPE element.

0170In Step 1613, the HSS sends a location update acknowledge message to the target MME/UPE element, and the message may be, but is not limited to, Update Location Ack.

0171In Step 1614, an association relationship is built between the target MME/UPE element and the source SGSN element.

0172In Step 1615, the target MME/UPE element sends an RAU acceptance message to the UE, and if the TMSI/RA changes, this information is also carried in the acceptance message.

0173In Step 1616, the UE sends an RAU complete message to the target MME/UPE element.

0174<figref idref="f0011">Figure 17</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the RAU technique according to a second embodiment of the present invention. In the network where the UMTS network and the evolved network coexist, if the UE is attached to and resides in the evolved network, when the MME/UPE pool area in the evolved network is overloaded, this embodiment is adapted to transfer the UE to a UMTS network. The method includes the following steps.

0175In Step 1700, an old SGSN element associated with the source MME/UPE element might exist.

0176In Step 1701, the source MME/UPE element determines to transfer a UE to another RAT.

0177In Step 1702, the source MME/UPE element sends a paging carrying an RAU instruction to the UE, indicating the UE to be transferred to a UMTS network.

0178In Step 1703, the UE sends an RAU request via the UMTS access network. According to a set mechanism, the UMTS access network selects an SGSN element as a target SGSN element, and sends the RAU request to the selected target SGSN element.

0179In Step 1704, the target SGSN element requests the source MME/UPE element for a related context of the UE.

0180In Step 1705, the source MME/UPE element returns the related context of the UE to the target SGSN element.

0181In Step 1706, the target SGSN element authenticates the UE.

0182In Step 1707, the target SGSN element replies a context acknowledge message to the source MME/UPE element.

0183In Step 1708, the source MME/UPE element sends related user data of the UE to the target SGSN element. This step is optional.

0184In Step 1709, a default IP bearer between the target SGSN element and the MME/UPE is updated, and the message adopted may be, but not limited to, a PDP context update request/response.

0185In Step 1710, the target SGSN element sends a location update message to the HSS, and the message may be, but not limited to, Update Location.

0186In Step 1711, the HSS deletes the related context of the UE from the old SGSN element.

0187In Step 1712, the HSS adds UE subscription data into the target SGSN element.

0188In Step 1713, the HSS sends a location update acknowledge message to the target SGSN element, and the message may be, but not limited to, Update Location Ack.

0189In Step 1714, an association relationship is built between the target SGSN element and the source MME/UPE element.

0190In Step 1715, the target MME/UPE sends an RAU acceptance message to the UE, and if the TMSI/RA changes, this information is also carried in the acceptance message.

0191In Step 1716, the UE sends an RAU complete message to the target MME/UPE element.

0192In <figref idref="f0011">Figure 17</figref>, if the old SGSN element exists, the target SGSN element may also trigger a process of obtaining the context of the UE from the old SGSN element, for example, implemented after Step 1705, which is not shown in the <figref idref="f0011">Figure 17</figref>. The method of <figref idref="f0011">Figure 17</figref> assumes that the UPE element is located on a data path of the 2G/3G, i.e., user plane data is transmitted between the SGSN element, UPE element, and IASA element via the S3 interface in <figref idref="f0002">Figure 3</figref>. Thus, the update of the UE context in Step 1709 is performed between the target SGSN element and the source MME/UPE element. If the user plane data is transmitted between the SGSN element and the IASA element via the S4 interface in <figref idref="f0002">Figure 3</figref>, the update of the UE context in Step 1709 is performed between the target SGSN element and the IASA element.

0193<figref idref="f0012">Figure 18</figref> is a flow chart of replacing the IASA element for the UE triggered by the network side based on the RAU technique according to an embodiment of the present invention. Areas A and B both have different CNs. The UE subscribed in the area A moves to the area B after the area A initiates a communication process. During the movement, the change of the RA may cause the UE to initiate an RAU process, thus resulting in the migration of the MME/UPE element. It is specified in the protocol that the migration of the IASA element cannot be performed during the communication process, so after the UE subscribed in the area A during a session moves to the area B, though the current IASA element has already led to a route not being optimized of the UE, the UE cannot be transferred from the IASA of the area A to the IASA element of the area B, the IASA element may only be transferred till the end of the communication. The method includes the following steps.

0194In Step 1801, the source IASA element determines to replace the IASA element for a UE.

0195In Step 1802, the source IASA element notifies the MME/UPE element currently serving the UE to send an RAU instruction to the UE, so as to replace the IASA element for the UE.

0196In Step 1803, the MME/UPE element serving the UE sends an RAU instruction to the UE, indicating to replace the IASA element.

0197In Step 1804, the UE sends an RAU request to the MME/UPE element serving the UE via the access network, requesting to replace the IASA element.

0198In Step 1805, a security authentication is performed between the MME/UPE element serving the UE and the UE. This step is optional.

0199In Step 1806, the MME/UPE element serving the UE releases an IP bearer between itself and the source IASA element.

0200In Step 1807, the MME/UPE element serving the UE establishes an IP bearer to the target IASA element.

0201In Step 1808, the target IASA element is registered in the HSS for the UE. This step is optional.

0202In Step 1809, the MME/UPE element serving the UE sends an RAU acknowledgment to the UE.

0203In Step 1810, the UE sends an RAU complete message to the MME/UPE element serving the UE.

0204In <figref idref="f0012">Figure 18</figref>, the source IASA element determines to replace the IASA element or the MME/UPE element determines to replace the IASA element for the UE, and the RAU is triggered to replace the IASA element. In Step 1806, the release of the IP bearer between the serving MME/UPE and the source IASA element may be performed before the RAU instruction, which is not safe. Once the UE receives no subsequent RAU instruction and does not initiate the RAU process, the UE may not have the IP bearer temporarily. In Step 1808, the registration of a new IASA element in the HSS is optional, and this step may be triggered by the MME/UPE element or by the target IASA element.

0205Seen from the UE transferring through the RAU technique, an embodiment of the present invention provides a method of initiating an RAU process to transfer a UE by the network side in a mobile communication network. Six methods are also provided to prevent the access network from re-selecting the source CN element as the required CN element for the UE, thus effectively solving the problem of user transferring in the mobile communication system. As the UE transferring through the RAU technique does not need to reconstruct the context, the UE service may not be interrupted.

0206In the second mode, the UE is transferred through an attach technique.

0207This mode is widely applied to various networks, instead of being limited to the evolved network. That is, the mode is also applicable to the UMTS network, the network where a UMTS network and an evolved network coexist, and even the post-evolved network as well as coexistence network in other forms.

0208The CN entities for transferring a UE mentioned in this mode may be SGSN entities, MME/UPE entities, or CN entities in the aforementioned networks. For example, the CN entities may be OMC entities, GGSN entities, IASA entities, and even CN entities in the post-evolved network.

0209<figref idref="f0013">Figure 19</figref> is a flow chart of a first method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention. The method includes the following steps.

0210In Step 1901, the source CN element determines to replace an RAT for a UE it serves due to factors such as overload, service requirements, or the reception of an O&amp;M request.

0211In Step 1902, the source CN element sends a reattach request to the UE, indicating the UE to be transferred to another RAT.

0212The RAT information carried in the transfer instruction may be information for preventing the access of the UE to the original RAT, or one or more RATs selected by the source CN element for the UE. In the latter case, the candidate RATs are arranged in priority order and sent to the UE in the form of an RAT list, so that the UE selects the RAT accordingly.

0213In Step 1903, the UE selects an RAT according to the RAT information carried in the received reattach request, and sends an attach request in the RAT. According to a set mechanism, for example, load sharing, the access network selects a target CN element for the UE and sends the attach request to the target CN element.

0214In Step 1904, an attach process is implemented between the UE and the network.

0215When the UE is transferred to another RAT, the access network equipment needs to be switched. For example, when the UE is transferred from the UMTS network to the evolved network, the access network equipment changes from the RAN of the original 2G/3G network into the eRAN of the evolved network, and both the RAN and eRAN are represented by the access network in <figref idref="f0013">Figure 19</figref>.

0216The method of <figref idref="f0013">Figure 19</figref> is applicable to the circumstance of transferring the RAT for the UE. For example, the first, due to the overload of one CN element in the network where the UMTS network and the evolved network coexist, the UE has to be transferred to another CN element with a lighter load. The second, the RAT is transferred for the UE on receiving an O&amp;M request. The third, due to service requirements, for example, when downstream data with high QoS is sent to the UE attached to the SGSN, the UMTS network currently serving the UE is unsatisfactory while the evolved network meets the requirement, so that the UE has to be transferred to the evolved network.

0217<figref idref="f0013">Figure 20</figref> is a flow chart of a second method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention. The method includes the following steps.

0218In Step 2001, the source CN element determines to transfer the CN element for a UE it serves.

0219In Step 2002, the source CN element sends a reattach request to the UE, indicating the UE to be transferred to another CN element.

0220In Step 2003, on receiving the reattach request, the UE sends an attach request carrying information for preventing the access of the UE to the source CN element. According to the restriction information carried by the received attach request, the access network adopts a set mechanism, for example, load sharing, to select the target CN element for the UE, and forwards the attach request to the target CN element.

0221In Step 2004, an attach process is implemented between the UE and the network.

0222The method of <figref idref="f0013">Figure 20</figref> is generally adopted and widely applied to various circumstances, for example, in the case of an overload of the CN element that serves the UE, an O&amp;M request, or routes not being optimized. In the method, the source CN element and the target CN element can belong to the same network or to different networks.

0223<figref idref="f0014">Figure 21</figref> is a flow chart of a third method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention. The method includes the following steps.

0224In Step 2101, the source CN element determines to transfer the CN element for the UE.

0225In Step 2102, as for the UE to be transferred, the source CN element determines it has an associated CN element and the UE can be transferred to the associated CN element.

0226In Step 2103, the source CN element sends a reattach request to the UE, indicating the UE to be transferred to the associated CN element.

0227In Step 2104, the UE sends an attach request carrying an ID related to the associated CN element. According to the ID related to the associated CN element carried by the request, the access network sends the attach request to the associated CN element. The ID related to the associated CN element may be any identity capable of uniquely determining the associated CN element, for example, the identity of the associated CN element or the TMSI allocated by the associated CN element to the UE.

0228In Step 2105, an attach process is implemented between the UE and the network.

0229The method of <figref idref="f0014">Figure 21</figref> is applicable to the circumstance that the UE is simultaneously attached to two CN entities associated with each other, and selects a network to reside in. For example, the first, due to the overload of one CN element, the UE has to be transferred to the other CN element. The second, due to service requirements, when downstream data with high QoS is sent to the UE residing in the SGSN, the UMTS network is unsatisfactory while the evolved network meets the requirement, so that the UE needs to be transferred to the MME/UPE of the associated evolved network.

0230<figref idref="f0014">Figure 22</figref> is a flow chart of a fourth method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention. The method includes the following steps.

0231In Step 2201, the source CN element determines to transfer the CN element for a UE.

0232In Step 2202, according to a set mechanism, for example, load sharing, the source CN element selects a target CN element with a lighter load from CN entities that share the same interface with the source CN element, and obtains an ID related to the target CN element.

0233The ID related to the target CN element may be any identity capable of uniquely determining the element, for example, the identity of the target CN element, the NRI of the target CN element, or the TMSI allocated by the target CN element to the UE. The TMSI is allocated by the CN to the UE. For example, the TMSI is U-TMSI in the UMTI network and S-TMSI in the evolved network.

0234In Step 2203, the source CN element sends a reattach request carrying the ID related to the target CN element to the UE.

0235In Step 2204, on receiving the reattach request, the UE sends an attach request carrying the ID related to the target CN element. The access network receives and forwards the attach request to the target CN element according to the ID related to the target CN element carried in the request.

0236In Step 2205, an attach process is implemented between the UE and the network.

0237The method of <figref idref="f0014">Figure 22</figref> is applicable to a circumstance that the source CN element obtains a target CN element for the UE according to a set mechanism. For example, when the source CN element is overloaded, another CN element is selected for the UE within the same pool area. When the route is not optimized, another CN element with a more preferred route is selected for the UE. In case of an O&amp;M operation, for example, an OMC determines that part of the UEs on the SGSN element have to be transferred to the MME/UPE element, so that the OMC interacts with the MME/UPE element to obtain a related ID, then issues an O&amp;M command to the SGSN element for transferring some UEs, and notifies the SGSN element of an ID related to the target MME/UPE element, and the SGSN element on receiving the command sends a reattach request to the UE. Or due to service requirements, for example, when downstream data with high QoS is sent to the UE residing in the SGSN element, the UMTS network is unsatisfactory while the SGSN element may obtain an MME/UPE element capable of receiving the UE transferred from the evolved network sharing the same interface with the element, and the MME/UPE element meets the QoS requirement, the method may be adopted to transfer the UE to the MME/UPE.

0238In the method of <figref idref="f0014">Figure 22</figref>, the target CN element may be an associated CN element related to the source CN element of the UE, which is a special case of <figref idref="f0014">Figure 21</figref>. In the method, the source CN element and the target CN element belong to the same network or to different networks.

0239<figref idref="f0015">Figure 23</figref> is a flow chart of a fifth method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention. The method includes the following steps.

0240In Step 2301, the source CN element determines to transfer the CN element for a UE.

0241In Step 2302, the source CN element sends a reattach request to the UE, indicating the UE to adopt an ID that makes the access network regard the UE as a new entrant to send an attach request.

0242When the indicated UE accesses again, the ID that makes the access network regard the UE as a entrant UE has various types, for example, a TMSI set as Null, a TMSI with an NRI field set as Null, or IDs of CN entities other than those currently within the area of the UE. Being the latter two, the ID has to be generated at the source CN element. Being the TMSI set as Null, the ID is generated merely by the source CN element sending a related instruction to the UE.

0243In Step 2303, the UE sends an attach request carrying the ID indicated in Step 2302. The access network receives the request, and determines the UE as an entrant UE according to the ID carried in the request. Therefore, the access network adopts a set mechanism, for example, load sharing, to select the target CN element, and sends the RAU request to the target CN element.

0244In Step 2304, an attach process is implemented between the UE and the network.

0245In Step 2303 of <figref idref="f0015">Figure 23</figref>, the ID indicated in Step 2302 serves as an IE in an RRC field of the attach request, but the related ID does not need to serve as an IE in an NAS field of the attach request.

0246The method of <figref idref="f0015">Figure 23</figref> is applicable to a circumstance that the source CN element is overloaded and the UE has to be transferred to a CN element with a lighter load. In the method, the source CN element and the target CN element belong to the same network or to different networks.

0247<figref idref="f0015">Figure 24</figref> is a flow chart of a sixth method for transferring a UE triggered by the network side based on the attach technique according to an embodiment of the present invention. The method includes the following steps.

0248In Step 2401, the source CN element determines to transfer the CN element for a UE.

0249In Step 2402, the source CN element broadcasts a list of UEs required to be reattached to other CNs, and meanwhile broadcasts information that prevents the access of the UEs to the source CN element.

0250In Step 2403, the UE monitors a broadcast message and determines whether the UE itself is arranged in the broadcast list. If the UE itself is arranged in the broadcast list, the UE sends an attach request carrying the information that prevents its access to the source CN element. According to the information carried in the received request, the access network adopts a set mechanism, for example, load sharing, to select the target CN element for the UE, and sends the attach request to the target CN element.

0251In Step 2404, an attach process is implemented between the UE and the network.

0252The method of <figref idref="f0015">Figure 24</figref> is applicable to circumstances that the source CN element is overloaded, in case of an O&amp;M operation, and the CN element is to be shut down for maintenance or upgrade. In the method, the source CN element and the target CN element can belong to the same network or to different networks.

0253Two embodiments are given below to further illustrate the present invention.

0254<figref idref="f0016">Figure 25</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the attach technique according to a first embodiment of the present invention. In the network where the UMTS network and the evolved network coexist, the UE is attached to an SGSN element. When a downstream service with a high QoS requirement is sent to the UE, and the UMTS network is unsatisfactory, the SGSN element selects an MME/UPE element that can meet the QoS requirement from the evolved network sharing the same interface. At this point, the UE has to be transferred to the evolved network. The method includes the following steps.

0255In Step 2501, the source SGSN element determines to reattach a UE to the evolved network, so as to meet the high QoS requirement.

0256In Step 2502, according to a set mechanism, for example, load sharing, the source SGSN element obtains an ID of the target MME/UPE element or an S-TMSI allocated by the target MME/UPE element to the UE.

0257In Step 2503, the source SGSN element sends a reattach request carrying the ID of the target MME/UPE element or the S-TMSI allocated by the target MME/UPE element to the UE, indicating UE to be reattached to the evolved network.

0258In Step 2504, the UE sends an attach request carrying the ID of the target MME/UPE element or the S-TMSI allocated by the target MME/UPE element to the UE via the access network. The access network determines the target MME/UPE element according to the information carried in the attach request, and forwards the attach request to the target MME/UPE element.

0259In Step 2505, if the attach message also includes the U-TMSI allocated by the source SGSN element to the UE, the target MME/UPE element requests the source SGSN element for a user information, for example, a permanent user ID. This step is optional.

0260In Step 2506, if the target MME/UPE element fails to obtain the user information from the source SGSN element, the target MME/UPE element requests the UE for related user information. This step is optional.

0261In Step 2507, the target MME/UPE element authenticates the user.

0262In Step 2508, the target MME/UPE element is registered in the HSS, indicating that the element is currently serving the UE.

0263In Step 2509, the HSS acknowledges the registration of the target MME/UPE element, user subscription data authorized to the default IP access bearer is sent at the same time, and strategy charging control information of the default IP access bearer is sent to the target MME/UPE element at the same time.

0264In Step 2510, an association relationship is built between the source SGSN element and the target MME/UPE element.

0265In Step 2511, the target MME/UPE element selects an IASA.

0266In Step 2512, the IASA configures an IP layer with the confirmed user IP address, so that a user plane is established, and a default strategy charging rule is adapted.

0267In Step 2513, the target MME/UPE element provides the eRAN with an allocation of QoS of the default IP access bearer, for example, an upper limit of the data transmission rate.

0268In Step 2514, the target MME/UPE accepts the network attachment of the UE, temporarily allocates an ID to the UE, and also sends the confirmed user IP address to the UE.

0269In Step 2515, the UE acknowledges the success of the network attachment.

0270In <figref idref="f0016">Figure 25</figref>, Step 2510 may be moved afterward, for example, to be performed after the user plane is established. In the present invention, there are other circumstances that the UE is required to be transferred from the UMTS network to the evolved network. For example, when the SGSN element is overloaded, the UE attached the SGSN element should be transferred to the MME/UPE element. The embodiment of <figref idref="f0016">Figure 25</figref> is also applicable.

0271<figref idref="f0017">Figure 26</figref> is a flow chart of UE transferring between CN entities in a network where a UMTS network and an evolved network coexist triggered by the network side based on the attach technique according to a second embodiment of the present invention. In the network where the UMTS network and the evolved network coexist, if the UE residing in the MME/UPE element is attached to the UMTS and the evolved network at the same time, when the MME/UPE element is overloaded while the associated SGSN element bears a lighter load, part of the UEs on the MME/UPE element are required to be transferred to the associated SGSN element. The method includes the following steps.

0272In Step 2601, the MME/UPE element determines to replace the CN element for a UE.

0273In Step 2602, as for the UE to be transferred, the MME/UPE element determines that an associated SGSN element exists, and that the UE can be transferred to the associated SGSN element.

0274In Step 2603, the MME/UPE element sends a reattach request to the UE, indicating the UE to be reattached to the associated SGSN element.

0275In Step 2604, the UE sends the associated SGSN element an attach request carrying an ID related to the associated SGSN element via the access network. According to the ID related to the associated SGSN element, the access network transfers the UE to the associated SGSN element.

0276In Step 2605, if the attach message also includes the U-TMSI allocated by the source MME/UPE element to the UE, the associated SGSN element requests the source MME/UPE element for a user information, for example, a permanent user ID. This step is optional.

0277In Step 2606, if the associated SGSN element fails to obtain the user information from the source MME/UPE element, the associated SGSN element requests the UE for the permanent user ID. This step is optional.

0278In Step 2607, the associated SGSN authenticates the user. This step is optional.

0279In Step 2608, the associated SGSN element is registered in the HSS, indicating that the element is currently serving the UE.

0280In Step 2609, the HSS acknowledges the registration of the associated SGSN element, user subscription data authorized to the default IP access bearer is sent at the same time, and strategy charging control information of the default IP access bearer is sent to the associated SGSN element at the same time.

0281In Step 2610, an association relationship is rebuilt between the associated SGSN element and the source MME/UPE element. This step is optional.

0282In Step 2611, the associated SGSN element selects an IASA. This step is optional.

0283In Step 2612, the IASA element configures an IP layer with the confirmed user IP address, so that a user plane is established, and a default strategy charging rule is applied.

0284In Step 2613, the associated SGSN element provides the eRAN with an allocation of QoS of the default IP access bearer, for example, an upper limit of the data transmission rate.

0285In Step 2614, the associated SGSN element accepts the network attachment of the UE, temporarily allocates a temporary ID to the UE, and also sends the confirmed user IP address to the UE.

0286In Step 2615, the UE acknowledges the success of the network attachment.

0287In Step 2612 of <figref idref="f0017">Figure 26</figref>, a user plane is established from the associated SGSN element to the source MME/UPE element. That is, the UPE is assumed to be located on a data path of the 2G/3G network, and user plane data is transmitted between the SGSN element, UPE element, and IASA element via the S3 interface in <figref idref="f0002">Figure 3</figref>. If the user plane data is transmitted between the SGSN element and the IASA element via the S4 interface in <figref idref="f0002">Figure 3</figref>, the user plane is established between the SGSN element and the IASA element in Step 2612.

0288As can be seen from the UE transferring through the attach technique, an embodiment of the present invention provides a method for initiating an attach process to transfer a UE by the network side in a mobile communication network. Six methods are also provided to prevent the access network from re-selecting the source CN element as the required CN element for the UE, thus effectively solving the problem of user transferring in the mobile communication system.

0289In an embodiment of the present invention, a method for transferring a UE through a UE detach and reattach process initiated by the network side in a mobile communication network is provided.

0290Compared with the above two modes provided in the embodiment of the present invention, the initiation of the UE detach and reattach process by the network side to transfer the UE has certain limitations, and is mainly applicable to UE transferring between RATs related to the UMTS network. As the UMTS network specifies the process of detaching and reattaching the UE, the method can be adapted to make the UE better compatible in the UMTS network, and thus the UE capable of accessing the UMTS network does not need any change.

0291<figref idref="f0018">Figure 27</figref> is a flow chart of the method for transferring a UE through a detach and reattach process according to an embodiment of the present invention. The method includes the following steps.

0292In Step 2701, the source CN element determines to replace an RAT for a UE.

0293In Step 2702, the source CN element sends a detach request to the UE, indicating the UE to subsequently send a request for being attached to another CN element.

0294In Step 2703, the UE sends an attach request, and the access network selects a target CN element for the UE according to a set mechanism, for example, load sharing.

0295In Step 2704, an attach process is implemented between the UE and the network.

0296In <figref idref="f0018">Figure 27</figref>, in order to reattach the UE to another RAT, a particular ID is carried in the detach request, the UE sends an attach request carrying the particular ID, and the access network determines whether to assign an RAT or not according to the particular ID. If the access network determines to assign an RAT or not according to the particular ID, a normal attach process is performed; otherwise, the access is rejected and the UE initiates a new attach process. For example, the particular ID may be an NRI field in the TMSI, and the length of the NRI field is 10 byte. If the NRI is equal to a binary number 1111111111, it indicates that the UE will access an evolved network, and if the NRI is equal to a binary number 1111111110, it indicates that the UE will access a UMTS network. The access network determines whether to assign an RAT or not according to the NRI field in the TMSI, and performs corresponding operations.

0297The method of <figref idref="f0018">Figure 27</figref> is applicable to the circumstances of replacing the RAT for the UE. For example, due to the overload of one CN element in the network where the UMTS network and the evolved network both coexist, the UE has to be transferred to another CN element with a lighter load. The RAT is replaced for the UE on receiving an O&amp;M request. Due to service requirements, for example, when downstream data with high QoS is sent to the UE attached to the SGSN, the UMTS network currently serving the UE is unsatisfactory while the evolved network meets the requirement, so that the UE has to be transferred to the evolved network.

0298In the embodiments of the present invention, the evolved network may be an SAE.

0299In an embodiment of the present invention, a system for transferring a UE in a mobile communication system is also provided. The system includes a UE, a source CN element, a target CN element and an access network. The source CN element sends a transfer instruction carrying UE transfer restriction information to the UE after determining the UE that the source CN element is serving to be transferred. The UE receives the transfer instruction carrying the UE transfer restriction information from the source CN element, and sends a transfer request according to the restriction information. The access network obtains the transfer request sent by the UE, selects a target CN element that is different from the source CN element according to the received request, and forwards the transfer request sent by the UE to the target CN element. The target CN element receives the transfer request forwarded by the access network, and transfers the UE to the target element.

0300In the embodiments of the present invention, the source CN element further includes a determination module and a transfer instruction constructing module connected to the determination module. The determination module is adapted to determine to transfer the UE that the source CN element serves and send a transfer signaling to the transfer instruction constructing module. The transfer instruction constructing module is adapted to construct a transfer instruction carrying the restriction information to be sent to the UE after receiving the transfer signaling.

0301In the embodiments of the present invention, the UE further includes a transfer instruction receiving &amp; parsing module and a transfer request construction module. The transfer instruction receiving &amp; parsing module is adapted to receive and parse the transfer instruction carrying the restriction information from the source CN element, and send the restriction information to the transfer request construction module. The transfer request construction module is adapted to construct and send a transfer request according to the received restriction information.

0302In practice, the functions of the source CN element and the target CN element may be integrated in one CN element, so as to serve as the source CN element or the target CN element for different UEs in various applications.

0303It should be noted that, the above provided solutions are not only applicable to an architecture of integrated control plane and user plane, but also applicable to an architecture of separated control plane and user plane. <figref idref="f0019">Figure 29</figref> is a schematic view of a network architecture of separated control plane and user plane in TS23.401. The S-GW serves as a UPE and 3GPP anchor in TS23.882, and the PDN GW serves as an SAE anchor.

0304In the architecture of separated control plane and user plane, the user also needs to be transferred, for example, when the MME or GW is overloaded, the route is not optimized, or an O&amp;M request is received. The user transferring in the architecture of separated control plane and user plane may have the following circumstances: (1) the MME changes while the GW stays the same; (2) the GW changes while the MME stays the same; and (3) the MME and the GW change at the same time.

0305As for the circumstance in (3) that the MME and the GW change at the same time, the solution provided in the above embodiment of the present invention may be adopted for user transferring, and the details will not be described herein again. The circumstances in (1) and (2) will be illustrated in detail below.

0306In the circumstance (1), the MME changes while the GW stays the same.

0307This circumstance occurs when the MME is overloaded or an O&amp;M request is received for only replacing the MME. Here, the information telling the UE only to replace the MME but not replace the GW may be selectively carried in the transfer instruction sent to the UE. Further, the target MME obtains a user plane information by requesting the source MME for a user context, so that the original bearer does not need to be released, thus avoiding reconstructing the user context and maintaining the service continuity.

0308In the circumstance that the MME changes while the GW stays the same, a TAU or reattach mode can be adopted for MME user transferring. These two modes will be illustrated in detail below.

0309<figref idref="f0020">Figure 30</figref> is a flow chart of a method for performing MME user transferring through a TAU technique in a network architecture of separated control plane and user plane. The method includes the following steps.

0310In Step 3001, when the network side determines that the MME needs to be replaced while the GW stays the same, the source MME sends a TAU instruction to the UE, indicating the UE to re-initiate a TAU process. Here, the network side may carry the information indicating the UE to be transferred to another MME or carry an ID of the target MME in the TAU instruction to be sent to the UE, so as to inform the UE to replace the MME.

0311In Step 3002, the UE receives the TAU instruction from the source MME, and sends a TAU request to the target MME.

0312In Step 3003, the target MME requests the source MME for a user context.

0313In Step 3004, the source MME returns the user context to the target MME.

0314In Step 3005, the UE is authenticated.

0315In Step 3006, the target MME replies a context acknowledge message to the source MME.

0316In Step 3007, the target MME notifies the S-GW that the serving MME of the UE changes.

0317In Step 3008, the S-GW records the current serving MME of the UE, and returns an acknowledge message to the target MME.

0318Steps 3007 and 3008 may be performed after Step 3014, for notifying the S-GW that the serving MME of the UE changes. When downstream data is sent to the UE subsequently, the S-GW notifies a new serving MME to perform paging.

0319In Step 3009, the target MME sends a location update message to an HSS.

0320In Steps 3010 to 3011, the HSS deletes the related context of the user from the source MME.

0321In Steps 3012 to 3013, the HSS adds user subscription data into the target MME.

0322In Step 3014, the HSS sends a location update acknowledge message to the target MME.

0323In Step 3015, the target MME sends a TAU acceptance message to the UE, and if the SAE-TMSI (S-TMSI)/TA changes, this information is also carried in the acceptance message.

0324In Step 3016, the UE sends a TAU complete message to the target MME.

0325<figref idref="f0021">Figure 31</figref> is a flow chart of a method for performing MME user transferring through a reattach technique in a network architecture of a control plane separated from a user plane. The method includes the following steps.

0326In Step 3101, when the network side determines that the MME needs to be replaced while the GW stays the same, the source MME sends a reattach instruction to the UE, indicating the UE to be reattached. Here, the network side may carry the information indicating the UE to be transferred to another MME or carry an ID of the target MME in the reattach instruction to be sent to the UE, so as to notify the UE to replace the MME.

0327In Step 3102, the UE sends a reattach request to the target MME, indicating the target MME to request the source MME for the context instead of reconstructing the context.

0328In Step 3103, the target MME requests the source MME for the user context.

0329In Step 3104, the source MME returns the user context to the target MME.

0330In Step 3105, the UE is authenticated.

0331In Step 3106, the target MME replies a context acknowledge message to the source MME.

0332In Step 3107, the target MME sends a location update message to the HSS.

0333In Steps 3108 to 3109, the HSS deletes the related context of the user from the source MME.

0334In Steps 3110 to 3111, the HSS adds user subscription data into the target MME.

0335In Step 3112, the HSS sends a location update acknowledge message to the target MME.

0336In Step 3113, the target MME notifies the S-GW that the serving MME of the UE changes.

0337In Step 3114, the S-GW records the current serving MME of the UE, and returns an acknowledge message to the target MME.

0338Steps 3113 and 3114 may be performed after Step 3106, for notifying the S-GW that the serving MME of the UE changes. When downstream data is sent to the UE subsequently, the S-GW notifies a new serving MME to perform paging.

0339In Step 3115, the target MME sends an attach acceptance message to the UE.

0340In Step 3116, the UE sends an attach complete message to the target MME.

0341Further, the MME user transferring may also be realized through a detach mode. The detailed process is as follows. The source MME sends a detach request to the UE, indicating the UE to initiate a reattach process so as to be reattached to another MME. Subsequently, the UE sends an attach request to the network side, and the access network selects a new MME for the UE according to a certain mechanism, for example, load sharing.

0342In the circumstance (2), the GW changes while the MME stays the same.

0343This circumstance occurs when the GW is overloaded and an O&amp;M request is received for only replacing the GW. Here, a target GW needs to allocate a bearer resource to the UE to be transferred, establish a bearer with the neighboring user plane element, and release the bearer resource allocated by a source GW to the UE.

0344The above circumstance can be further divided into two types, i.e., the UE is not notified of the GW replacement and the UE is notified of the GW replacement. These two modes will be illustrated in detail below.

0345When the IP address of the UE is unchanged, for example, the S-GW changes while the PDN GW stays the same, the mode of not notifying the UE about the GW replacement is adopted, and the MME only has to record the change of the GW. <figref idref="f0022">Figure 32</figref> is a flow chart of the transferring when the S-GW changes. The process includes the following steps.

0346In Step 3201, the source S-GW sends an S-GW change request to the MME.

0347In Step 3202, the MME sends a bearer establishment request to the target S-GW, requesting for establishing a bearer between the target S-GW and the PDN GW for the UE to be transferred.

0348A target S-GW address can be carried in the S-GW change request sent by the source S-GW to the MME in Step 3201, and the MME sends a bearer establishment request to the target S-GW according to the received target S-GW address. Or, the MME selects an S-GW which is not the source S-GW as a target S-GW according to a preset mechanism.

0349In Step 3203, the target S-GW allocates a downstream bearer resource to the UE to be transferred, for example, a user plane tunnel endpoint identifier (TEID), and sends a bearer update request carrying the downstream bearer resource information to the PDN GW.

0350In Step 3204, on receiving the bearer update request, the PDN GW performs the bearer update, records the downstream bearer resource information allocated by the target S-GW to the UE, and returns a bearer update response to the target S-GW.

0351Here, it is unnecessary for the PDN GW to re-allocate an upstream bearer resource to the UE. The source S-GW may send to the MME the upstream bearer resource information originally allocated by the PDN GW to the

0352UE in Step 3201, and the MME forwards the upstream bearer resource information to the target S-GW in Step 3202. Or, in Step 3204, the PDN GW notifies the upstream bearer resource information to the target S-GW.

0353In Step 3205, the target S-GW returns to the MME a bearer establishment response carrying the upstream bearer resource information allocated by the target S-GW to the UE, and the MME parses and records the resource information.

0354In Step 3206, the MME sends to the eRAN an S1 interface bearer update request carrying the upstream bearer resource information allocated by the target S-GW to the UE.

0355In Step 3207, the eRAN records the received upstream bearer resource information, and returns an S1 interface bearer update response to the MME.

0356It should be noted that, when the eRAN is unchanged, it is unnecessary for the eRAN to re-allocate a downstream bearer resource to the UE. The source S-GW may send to the MME the downstream bearer resource information originally allocated by the eRAN to the UE in Step 3201, and the MME forwards the downstream bearer resource information to the target S-GW in Step 3202.

0357If the downstream bearer resource information allocated by the eRAN to the UE is not carried in Steps 3201 and 3202 or the eRAN changes, the eRAN has to send to the MME the downstream bearer resource information allocated to the UE in Step 3207, and then perform Step 3208.

0358In Step 3208, the MME sends a bearer update request to the target S-GW. If the downstream bearer resource information allocated by the eRAN to the UE is carried in Step 3207, the downstream bearer resource information is carried in the request message.

0359In Step 3209, the target S-GW records the received downstream bearer resource information, and returns a bearer update response to the MME.

0360In Step 3210, the MME sends an S-GW change response to the source S-GW.

0361In Step 3211, the source S-GW returns an S-GW change acknowledgment to the MME.

0362When to release the original bearer depends on actual requirements. For example, if the original bearer is released in Steps 3210 and 3211, the MME sends a bearer delete request to the source S-GW in Step 3210 so as to delete the user plane context stored in the source S-GW, and the source S-GW returns a bearer delete response to the MME in Step 3211.

0363Steps 3206 to 3209 are optional. If the UE is in an idle state, Steps 3206 to 3209 will not be performed, as the bearer of the S1 interface is released in this state. If the UE is in an active state, Steps 3206 to 3209 are performed to re-establish the bearer on the S 1 interface.

0364It should be noted that, in the process of <figref idref="f0022">Figure 32</figref>, a message is first sent to the MME, and the MME triggers the bearer establishment. However, in practice, it is unnecessary for the MME to trigger the bearer establishment. Instead, after the target S-GW is selected, the source S-GW notifies the PDN GW to replace the S-GW by the target S-GW, and the PDN GW triggers a downward bearer establishment.

0365As for the circumstance that the UE needs to be notified of the change of the GW, one possible reason is that the PDN GW changes. As the PDN GW allocates the IP address, if the new and old PDN GWs do not share the same IP address space, the UE must be notified of the change of the PDN GW.

0366<figref idref="f0012">Figure 18</figref> is a flow chart illustrating an embodiment of the above circumstance based on an architecture specified in TR23.882. <figref idref="f0023">Figure 33</figref> is a flow chart of transferring based on an architecture specified in TS23.401. The two embodiments are slightly different in that the UE is notified after the CN side completes the GW replacement in this embodiment. Referring to <figref idref="f0023">Figure 33</figref>, the process mainly includes the following steps.

0367In Step 3301, the source PDN GW sends a PDN GW change request to the MME.

0368In Step 3302, the MME sends a bearer update request to the S-GW, requesting for establishing a bearer between the S-GW and the target PDN GW for the UE to be transferred.

0369A target PDN GW address can be carried in the PDN GW change request sent by the S-GW to the MME in Step 3301, and the MME sends a bearer update request to the target PDN GW according to the received target PDN GW address. Or, the MME selects a PDN GW which is not the source PDN GW as a target PDN GW according to a preset mechanism.

0370In Step 3303, the S-GW sends to the target PDN GW a bearer establishment request carrying downstream bearer resource information allocated by the S-GW to the UE.

0371Here, it is unnecessary for the S-GW to re-allocate downstream bearer resources to the UE, and the S-GW may directly notify the target PDN GW of the downstream bearer resource information originally allocated to the UE.

0372In Step 3304, the target PDN GW records the received downstream bearer resource information, and returns to the S-GW a bearer establishment response carrying the upstream bearer resource allocated by the target PDN GW to the UE to be transferred. The S-GW parses and records the resource information. In addition, the IP address allocated by the target PDN GW to the UE is also carried in the response message.

0373In Step 3305, the S-GW sends a bearer delete request to the source PDN GW, so as to delete the user plane context of the UE to be transferred stored in the source PDN GW.

0374In Step 3306, after the deletion, the source PDN GW returns a bearer delete response to the S-GW.

0375In Step 3307, the S-GW sends to the MME a bearer update response carrying the IP address allocated by the target PDN GW to the UE.

0376In Step 3308, the MME notifies the UE of the change of the PDN GW and the IP address allocated by the target PDN GW to the UE.

0377In Step 3309, the MME returns a PDN GW change response to the source PDN GW.

0378It should be noted that, in the process of <figref idref="f0023">Figure 33</figref>, a message is first sent to the MME, and the MME triggers the bearer establishment. However, in practice, it is unnecessary for the MME to trigger the bearer establishment. Instead, after the target PDN GW is selected, the target PDN GW directly triggers a downward bearer establishment.

0379Finally, it should be understood that the above embodiments are only used to explain, but not to limit the technical solution of the present invention. In despite of the detailed description of the present invention with reference to above preferred embodiments, it should be understood that various modifications, changes or equivalent replacements can be made by those skilled in the art without departing from scope of the present invention and covered in the claims of the present invention.