WO2015182141A1

WO2015182141A1 - Apparatus, system and method for dedicated core network

Info

Publication number: WO2015182141A1
Application number: PCT/JP2015/002695
Authority: WO
Inventors: Xiaowei Zhang; Toshiyuki Tamura; Anand Raghawa Prasad
Original assignee: Nec Corporation
Priority date: 2014-05-30
Filing date: 2015-05-28
Publication date: 2015-12-03

Abstract

　In order for more effectively supporting a Dedicated Core Network, there is provided a network system including a first node (30) that establishes secure connection with a UE (10) initially attempting to attach to a network, through a radio base station (20), and a second node (40) to which the UE (10) is redirected from the first node (30) through the radio base station (20). Upon the redirection, the first node (30) sends information on the first node (30) itself to the second node (40) through the radio base station (20). The second node (40) uses the information to retrieve security context necessary for establishing the connection with the UE (10) from the first node (30).

Description

APPARATUS, SYSTEM AND METHOD FOR DEDICATED CORE NETWORK

　　The present invention relates to an apparatus, a system and a method for a Dedicated Core Network, and particularly to a technique to ensure that a UE (User Equipment) is served by the appropriate core network.

　　Recently, enhancement to support the Dedicated Core Network has been studied by 3GPP (3rd Generation Partnership Project).

　　The Dedicated Core Network intends that for example, a specific type of subscriber (i.e., a specific type of UE) is redirected to an MME (Mobility Management Entity)/SGSN (Serving GPRS (General Packet Radio Service) Support Node) dedicated to serve that UE. The Dedicated Core Network is also sometimes referred to as "Specific Core Network" or "Overlay Core Network". Examples of the specific type of UE include an MTC (Machine Type Communication) device, an M2M (Machine to Machine) terminal and the like, for which a LAPI (Low Access Priority Indicator) is set.

　　NPL 1 discloses the most prompting message flow to realize the Dedicated Core Network. In this message flow, when a UE sends an Attach Request message to new MME through an eNB (evolved Node B) (i.e., in a case of IMSI (International Mobile Subscriber Identity) attach), the one MME obtains information to use another specific MME from an HSS (Home Subscriber Server). Then, the one MME instructs the eNB to redirect the Attach Request message to the specific MME. Upon the redirection, the eNB performs the Attach procedure again to the specific MME.

　　NPL 2 also discloses a message flow similar to that disclosed in NPL 1.

NPL 1: TSG SA WG2 #99 S2-133304, "Addition of Subscription Information for Selecting a Specific Network", 2013-09, pp. 4
NPL 2: SA WG2 #100 S2-133910, "23.401 CR2606R3: Addition of Subscription Information for Selecting a Specific Network", 2013-11, pp. 7 and 9
NPL 3: TSG SA WG2 #100 S2-133909, "Discussion on Core Network Type Selection based on the Subscription Information", 2013-11

　　However, the inventors of this application have found that there is a problem that the message flow disclosed in

NPLs

1 and 2 decreases the efficiency upon supporting the Dedicated Core Network.

　　Specifically, upon the re-performance of Attach procedure, the specific MME redundantly performs AKA (Authentication and Key Agreement) procedure and NAS (Non-Access Stratum) SMC (Security Mode Command) procedure that have been already performed by the different MME to which the UE attempted to attach. Therefore, there are caused signaling overload to devices/nodes involved in the redundant AKA/NAS SMC procedures and all interface therebetween, as well as overload to the specific MME.

　　Moreover,

NPLs

2 and 3 each discloses another message flow for GUTI (Globally Unique Temporary Identity) attach, in which the specific MME contacts an MME referenced by the GUTI to retrieve security context.

　　However, even in this message flow, there are also caused the above-mentioned signaling overload and overload to the specific MME. This is because the GUTI merely indicates the old MME which may has removed the security context after the expiration of timer, after all the specific MME redundantly performs the AKA/NAS SMC procedures.

　　Note that details of these problems will be discussed more fully in the following description.

　　Accordingly, an exemplary object of the present invention is to provide a solution for more effectively supporting a Dedicated Core Network.

　　In order to achieve the above-mentioned object, a network system according to first exemplary aspect of the present invention includes: a first node that establishes secure connection with a UE (User Equipment) initially attempting to attach to a network, through a radio base station; and a second node to which the UE is redirected from the first node through the radio base station. Upon the redirection, the first node sends information on the first node itself to the second node through the radio base station. The second node uses the information to retrieve security context necessary for establishing the connection with the UE from the first node.

　　Further, according to second exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that establishes secure connection with a UE initially attempting to attach to a network, through a radio base station, and a second node to which the UE is redirected from the first node through the radio base station. This method includes: sending, upon the redirection, information on the first node from the first node to the second node through the radio base station; and using, by the second node, the information to retrieve security context necessary for establishing the connection with the UE from the first node.

　　Further, a network system according to third exemplary aspect of the present invention includes: a first node that receives an attach request from a UE initially attempting to attach to a network, through a radio base station; and a second node to which the attach request is redirected from the first node through the radio base station. Upon the reception of the attach request, the first node skips establishment of secure connection with the UE through the radio base station. Upon the redirection, the first node sends, to the second node through the radio base station, subscription information indicating that the UE is one to be redirected to the second node. In response to receiving the subscription information, the second node establishes the secure connection with the UE.

　　Further, according to fourth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that receives an attach request from a UE initially attempting to attach to a network, through a radio base station, and a second node to which the attach request is redirected from the first node through the radio base station. This method includes: skipping, by the first node upon the reception of the attach request, establishment of secure connection with the UE through the radio base station; sending, upon the redirection, from the first node to the second node through the radio base station, subscription information indicating that the UE is one to be redirected to the second node; and establishing, by the second node in response to receiving the subscription information, the secure connection with the UE.

　　Further, a network system according to fifth exemplary aspect of the present invention includes: a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, and that assigned a temporary identity to the UE; a second node that receives an attach request including the temporary identity from the UE through the radio base station; and a third node to which the attach request is redirected from the second node through the radio base station. The second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends information on the second node itself to the third node through the radio base station. The third node uses the information to retrieve the security context from the second node.

　　Further, according to sixth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously and that assigned a temporary identity to the UE, a second node that receives an attach request including the temporary identity from the UE through the radio base station, and a third node to which the attach request is redirected from the second node through the radio base station. This method includes: retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node; sending, upon the redirection, information on the second node from the second node to the third node through the radio base station; and using, by the third node, the information to retrieve the security context from the second node.

　　Further, a network system according to seventh exemplary aspect of the present invention includes: a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, and that assigned a temporary identity to the UE; a second node that receives an attach request including the temporary identity from the UE through the radio base station; and a third node to which the attach request is redirected from the second node through the radio base station. The second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends the security context to the third node through the radio base station.

　　Further, according to eighth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously and that assigned a temporary identity to the UE, a second node that receives an attach request including the temporary identity from the UE through the radio base station, and a third node to which the attach request is redirected from the second node through the radio base station. This method includes: retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node; and sending, upon the redirection, the security context from the second node to the third node through the radio base station.

　　Further, a network system according to ninth exemplary aspect of the present invention includes: a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, and that assigned a temporary identity to the UE; a second node that receives an attach request including the temporary identity from the UE through the radio base station; and a third node to which the attach request is redirected from the second node through the radio base station. The first node maintains security context necessary for establishing the secure connection with the UE till an identification request is received from the third node. Upon the redirection, the third node sends the identification request to the first node to retrieve the security context from the first node.

　　Further, according to tenth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously and that assigned a temporary identity to the UE, a second node that receives an attach request including the temporary identity from the UE through the radio base station, and a third node to which the attach request is redirected from the second node through the radio base station. This method includes: maintaining, by the first node, security context necessary for establishing the secure connection with the UE till an identification request is received from the third node; and sending, by the third node upon the redirection, the identification request to the first node to retrieve the security context from the first node.

　　Further, a network system according to eleventh exemplary aspect of the present invention includes: a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously; a second node that receives a request for TAU (Tracking Area Update) or RAU (Routing Area Update) from the UE through the radio base station; and a third node to which the request is redirected from the second node through the radio base station. The second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends information on the second node itself to the third node through the radio base station. The third node uses the information to retrieve the security context from the second node.

　　Further, according to twelfth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, a second node that receives a request for TAU or RAU from the UE through the radio base station, and a third node to which the request is redirected from the second node through the radio base station. This method includes: retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node; sending, upon the redirection, information on the second node from the second node to the third node through the radio base station; and using, by the third node, the information to retrieve the security context from the second node.

　　Further, a network system according to thirteenth exemplary aspect of the present invention includes: a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously; a second node that receives a request for TAU or RAU from the UE through the radio base station; and a third node to which the request is redirected from the second node through the radio base station. The second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends the security context to the third node through the radio base station.

　　Further, according to fourteenth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, a second node that receives a request for TAU or RAU from the UE through the radio base station, and a third node to which the request is redirected from the second node through the radio base station. This method includes: retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node; and sending, upon the redirection, the security context from the second node to the third node through the radio base station.

　　Further, a network system according to fifteenth exemplary aspect of the present invention includes: a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously; a second node that receives a request for TAU or RAU from the UE through the radio base station; and a third node to which the request is redirected from the second node through the radio base station. The first node maintains security context necessary for establishing the secure connection with the UE till a context request is received from the third node. Upon the redirection, the third node sends the context request to the first node to retrieve the security context from the first node.

　　Further, according to sixteenth exemplary aspect of the present invention, there is provided a method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, a second node that receives a request for TAU or RAU from the UE through the radio base station, and a third node to which the request is redirected from the second node through the radio base station. This method includes: maintaining, by the first node, security context necessary for establishing the secure connection with the UE till a context request is received from the third node; and sending, by the third node upon the redirection, the context request to the first node to retrieve the security context from the first node.

　　Further, a core network system according to sixteenth exemplary aspect of the present invention includes: a plurality of SGSNs (Serving GPRS (General Packet Radio Service) Support Nodes); and a server. In this system, a UE sends a message to a first SGSN through a radio base station. The UE is redirected from the first SGSN to a second SGSN based on subscription information of the UE provided by the server. The first SGSN sends a first message including information on the redirection to the radio base station. The radio base station sends a second message including the information to the second SGSN.

　　Further, according to seventeenth exemplary aspect of the present invention, there is provided a method for a core network system including a plurality of SGSNs and a server. This method includes: a step that a UE sends a message to a first SGSN through a radio base station; a step that the UE is redirected from the first SGSN to a second SGSN based on subscription information of the UE provided by the server; a step that the first SGSN sends a first message including information on the redirection to the radio base station; and a step that the radio base station sends a second message including the information to the second SGSN.

　　Further, a UE according to eighteenth exemplary aspect of the present invention includes: means for sending a message to a first SGSN through a radio base station. The UE is redirected from the first SGSN received the message to a second SGSN based on subscription information of the UE provided by a server. In the redirection, the first SGSN sends a first message including information on the redirection to the radio base station, and the radio base station sends a second message including the information to the second SGSN.

　　Furthermore, according to nineteenth exemplary aspect of the present invention, there is provided a method for a UE. This method includes: a step that the UE sends a message to a first SGSN through a radio base station; and a step that the UE is redirected from the first SGSN received the message to a second SGSN based on subscription information of the UE provided by a server. In the redirection, the first SGSN sends a first message including information on the redirection to the radio base station, and the radio base station sends a second message including the information to the second SGSN.

　　According to the present invention, it is possible to solve at least one of the above-mentioned problems, and thus to provide a solution for more effectively supporting a Dedicated Core Network.

Fig. 1 is a block diagram showing a configuration example of a network system according to a first exemplary embodiment of the present invention. Fig. 2 is a sequence diagram showing a first example of operations in the network system according to the first exemplary embodiment. Fig. 3 is a sequence diagram showing a second example of operations in the network system according to the first exemplary embodiment. Fig. 4 is a sequence diagram showing a first example of operations in a network system according to a second exemplary embodiment of the present invention. Fig. 5 is a sequence diagram showing a second example of operations in the network system according to the second exemplary embodiment. Fig. 6 is a sequence diagram showing a third example of operations in the network system according to the second exemplary embodiment. Fig. 7 is a sequence diagram for explaining problems in typical IMSI attach procedure in E-UTRAN. Fig. 8 is a sequence diagram for explaining problems in typical GUTI attach procedure. Fig. 9 is a block diagram showing a configuration example of a network system according to a third exemplary embodiment of the present invention. Fig. 10 is a sequence diagram showing a first example of operations in the network system according to the third exemplary embodiment. Fig. 11 is a sequence diagram showing a second example of operations in the network system according to the third exemplary embodiment. Fig. 12 is a sequence diagram showing a first example of operations in a network system according to a fourth exemplary embodiment of the present invention. Fig. 13 is a sequence diagram showing a second example of operations in the network system according to the fourth exemplary embodiment. Fig. 14 is a sequence diagram showing a third example of operations in the network system according to the fourth exemplary embodiment. Fig. 15 is a sequence diagram showing a first example of operations in a network system according to a fifth exemplary embodiment of the present invention. Fig. 16 is a sequence diagram showing a second example of operations in the network system according to the fifth exemplary embodiment. Fig. 17 is a sequence diagram showing a third example of operations in the network system according to the fifth exemplary embodiment. Fig. 18 is a sequence diagram showing a first example of operations in a network system according to a sixth exemplary embodiment of the present invention. Fig. 19 is a sequence diagram showing a second example of operations in the network system according to the sixth exemplary embodiment. Fig. 20 is a sequence diagram showing a third example of operations in the network system according to the sixth exemplary embodiment. Fig. 21 is a sequence diagram for explaining problems in typical IMSI attach procedure in UTRAN. Fig. 22 is a sequence diagram for explaining problems in typical P-TMSI attach procedure. Fig. 23 is a sequence diagram for explaining problems in typical TAU procedure. Fig. 24 is a sequence diagram for explaining problems in typical RAU procedure.

　　Hereinafter, several exemplary embodiments according to the present invention will be described with reference to the accompanying drawings.

<First Exemplary Embodiment>
　　As shown in Fig. 1, a network system according to this exemplary embodiment includes

MMEs

30 and 40 by way of example only. Among them, the MME 30 is the one to which a UE 10 initially attaches through an eNB 20 serving as a radio base station, and thus sometimes referred to as "New MME" in the following description. On the other hand, the MME 40 is the one dedicated to serve the UE 10 according to subscription and to which the UE 10 is redirected from the MME 30, and thus referred to as "Dedicated MME" or "DMME". Note that the MME 40 can also be referred to as "Specific MME", in conformity with the Dedicated Core Network being sometimes referred to as "Specific Core Network". If necessary, each of the

MMEs

30 and 40 can obtain, from an HSS 50, subscription information on the UE 10, information necessary for authenticating the UE 10, and the like. The New MME 30 is also connected to an S-GW (Serving Gateway) and/or a P-GW (PDN (Public Data Network) Gateway). Similarly, the Dedicated MME 40 is also connected to a Dedicated S-GW and/or a Dedicated P-GW.

　　In general, this exemplary embodiment deals with IMSI attach in E-UTRAN (Evolved Universal Terrestrial Radio Access Network), particularly with a case where the New MME 30 redirects an Attach Request message received from the eNB 20 to the DMME 40, according to the subscription information on the UE 10 obtained from the HSS 50.

　　Next, prior to describing operation examples of this exemplary embodiment, there are firstly defined problems in typical IMSI attach procedure as disclosed by

NPLs

1 and 2 with reference to Fig. 7. Then, there will be described solutions for addressing these problems as the operation examples. The solutions include Option 1 shown in Fig. 2 and Option 2 shown in Fig. 3.

Problem Defined:
　　As shown in Fig. 7, in the typical IMSI attach procedure, a UE 110 sends an Attach Request message including its IMSI to an eNB 120 (step S101), and the eNB 120 forwards the Attach Request message to an New MME 130 (step S102).

　　Upon receiving the Attach Request message, the New MME 130 sends an Authentication data request message to an HSS 150, and as a response thereto, receives an Authentication data response message including AVs (Authentication Vectors) (step S103). Then, the New MME 130 corporates with the UE 110 to perform AKA procedure and NAS SMC procedure (step S104). As a result, The UE 10 and the New MME 130 each can share Kasme (Key for access security management entity) and NAS keys, so that secure connection is established between them (step S105).

　　After that, the New MME 130 sends an Update Location Request message to the HSS 150, and as a response thereto, receives an Update Location Response message (step S106). The Update Location Response message includes the IMSI and the subscription information/data. Moreover, the subscription information includes information on a Dedicated MME 140 (hereinafter, such information will be sometimes referred to as "DMME information").

　　Then, the New MME 130 selects the Dedicated MME 140 to which the UE 110 should be redirected based on the DMME information (step S107), and sends a Redirect message to the eNB 120 (step S108). Upon receiving the Redirect message, the eNB 120 forwards the Attach Request message to the Dedicated MME 140 (step S109).

　　However, at this time, the following problems arise:
　　(a) Since the Attach Request message is not protected, the Dedicated MME 140 starts AKA procedure to the UE 10;
　　(b) due to (a), the Dedicated MME 140 sends/receives Authentication data request/response messages and Update Location Request/Response messages to/from the HSS 150, and also corporates with the UE 110 to perform AKA procedure and NAS SMC procedure; and
　　(c) due to (a) and (b), there are caused signaling overload to the HSS 150, the Dedicated MME 140, the eNB 120, the UE 110 and all interfaces therebetween, as well as overload to the Dedicated MME 140 for key computation.

Solutions:
(Option 1)
　　Fig. 2 shows a sequence diagram for this option. Processes at steps S1 to S7 are performed in a similar manner to those at the above-mentioned steps S101 to S107 shown in Fig. 7.

　　Meanwhile, upon the redirection, the New MME 30 sends to the eNB 20 a Redirect message including information on the New MME 30 itself (herein after, such information will be sometimes referred to as "MME information") (step S8). For example, the MME information includes an ID (identification, identity), an FQDN (Fully Qualified Domain Name) or an IP (Internet Protocol) address assigned to the New MME 30. Then, the eNB 20 forwards to the Dedicated MME 40 the MME information with being included in an Attach Request message (step S9).

　　The Dedicated MME 40 uses the received MME information to retrieve, from the New MME 30, security context concerning secure connection which has been established between the UE 10 and the New MME 30. Specifically, the Dedicated MME 40 sends an Identification Request message to the New MME 30 (step S10), and as a response thereto, receives an Identification Response message including UE context (step S11). The UE context includes the AVs and all security context. Then, the Dedicated MME 40 extracts the Kasme and the NAS keys from the retrieved UE context (step S12).

　　After that, a message 5b and the subsequent message sequences as disclosed in 3GPP TS 23.401, section 5.3.2.1 follows.

　　According to this option, the Dedicated MME 40 is indicated about the MME information such that the Dedicated MME 40 can retrieve necessary security context from the New MME 30. Therefore, unlike the typical IMSI attach procedure, the Dedicated MME 40 needs not to perform AKA procedure and NAS SMC procedure. Thus, compared with the typical IMSI attach procedure, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated MME for key computation. Moreover, since the existing message sequences can be reused (the Identification Request/Response messages are also the existing ones as will be described later), it is possible to minimize the impact on the mobile communication system.

　　Further, at the above-mentioned step S11, the New MME 30 may include, in the Identification Response message, an Authentication Indication which indicates that the UE 10 has been authenticated as the one to be redirected to the Dedicated MME 40. In this case, the Dedicated MME 40 needs not to contact the HSS 50 for the purpose of authenticating the UE 10, so that it is possible to further reduce signaling overload.

(Option 2)
　　Fig. 3 shows a sequence diagram for this option. Processes at steps S21 and S22 are performed in a similar manner to those at the above-mentioned steps S1 and S2 shown in Fig. 2.

　　Meanwhile, upon receiving the Attach Request message, the New MME 30 firstly retrieves from the HSS 50 the subscription information including the DMME information (step S23a).

　　As described above, the DMME information indicates that the UE 10 should be redirected to the Dedicated MME 40. Therefore, the New MME 30 skips establishment of secure connection with the UE 10. Specifically, the New MME 30 does not perform the AKA procedure, the NAS SMC procedure, the computation of NAS keys, and the like.

　　Then, the New MME 30 selects the Dedicated MME 40 based on the DMME information (step S24), and sends to the eNB 20 a Redirect message including the subscription information (which also includes the DMME information) (step S25). Then, the eNB 20 forwards to the Dedicated MME 40 the subscription information with being included in an Attach Request message (step S26).

　　In response to receiving the subscription information (i.e., DMME information), the Dedicated MME 40 establishes secure connection with the UE 10 through the eNB 20, as a substitute for the New MME 30. Specifically, Dedicated MME 40 sends an Authentication data request message to the HSS 50, and as a response thereto, receives an Authentication data response message including the AVs (step S27). Then, the Dedicated MME 40 corporates with the UE 10 to perform AKA procedure and NAS SMC procedure (step S28). As a result, The UE 10 and the Dedicated MME 40 each can share Kasme and NAS keys, so that secure connection is established therebetween through the eNB 20 (step S29).

　　According to this option, as will be understood by comparing the sequence shown in Fig. 3 with that shown in Fig. 2, it is possible to reduce the amount of signaling compared to the above-mentioned Option 1. Moreover, since the New MME 30 skips the establishment of secure connection, it is also possible to reduce overload to the New MME 30.

　　Further, as a substitute for the above-mentioned step S23a, the DMME information may be preconfigured in the New MME 30 (step S23b). In this case, the New MME 30 can also skip the retrieval of subscription information from the HSS 50 when it receives the initial Attach Request from the UE 10, so that it is possible to further reduce the amount of signaling.

　　Furthermore, at the above-mentioned step S23a, the New MME 30 may further retrieve the AVs from the HSS 50. The retrieved AVs can be transferred to the Dedicated MME 40 through the eNB 20 with the Redirect message shown at the above-mentioned step S25 and the Attach Request message shown at the above-mentioned step S26. In this case, the Dedicated MME 40 can skip the acquisition of AVs from the HSS 50 shown at the above-mentioned step S27, so that it is possible to further reduce the amount of signaling.

<Second Exemplary Embodiment>
　　As shown in Fig. 4, a network system according to this exemplary embodiment further includes an MME 60 in addition to the above-mentioned

MMEs

30 and 40. The MME 60 is the one to which the UE 10 previously attached, and thus sometimes referred to as "Old MME" in the following description. If necessary, the Old MME 60 can also obtain, from the HSS 50, the subscription information, the DMME information, the AVs and the like. Note that there is a case where the UE 10 previously attached to an SGSN (i.e., "Old SGSN"). Even in this case, the following description about the Old MME can be similarly applied to that about the Old SGSN.

　　In general, this exemplary embodiment deals with GUTI attach in E-UTRAN, particularly with a case where the New MME 30 redirects an Attach Request message received from the eNB 20 to the DMME 40, according to the DMME information obtained from the Old MME 60.

　　Next, prior to describing operation examples of this exemplary embodiment, there are firstly defined problems in typical GUTI attach procedure as disclosed by

NPLs

2 and 3 with reference to Fig. 8. Then, there will be described solutions for addressing these problems as the operation examples. The solutions include Option 1 shown in Fig. 4, Option 2 shown in Fig. 5, and Option 3 shown in Fig. 6.

Problem Defined:
　　As shown by dotted lines in Fig. 8, assume that the UE 110 previously attached to an Old MME 160, and the Old MME 160 assigned a GUTI to the UE 110, so that the UE has gotten the GUTI.

　　In the typical GUTI attach procedure, the UE 110 sends an Attach Request message including the GUTI to the eNB 120 (step S201), and the eNB 120 forwards the Attach Request message to the New MME 130 (step S202).

　　Upon receiving the Attach Request message, the New MME 130 sends to the Old MME 160 an Identification Request message including the GUTI and the complete Attach Request message (step S203), and as a response thereto, receives an Identification Response message including the IMSI of the UE 110 and MM (Mobility Management) Context (step S204).

　　Then, the New MME 130 selects the Dedicated MME 140 (step S205), and sends a Redirect message to the eNB 120 (step S206). Upon receiving the Redirect message, the eNB 120 forwards the Attach Request message to the Dedicated MME 140 (step S207).

　　However, the Old MME 160 might remove UE context (i.e., security context) after a predetermined timer expired. Moreover, the GUTI merely indicates the Old MME 160. Therefore, the Dedicated MME 140 fails in retrieving the AVs and the security context from the Old MME 160, and thus starts AKA procedure.

　　Accordingly, as with the typical IMSI attach procedure, the following problems arise:
　　signaling overload to the HSS 150, the Dedicated MME 140, the eNB 120, the UE 110 and all interfaces therebetween; and
　　overload to the Dedicated MME 140 for key computation.

Solutions:
(Option 1)
　　Fig. 4 shows a sequence diagram for this option. Processes at steps S31 to S35 are performed in a similar manner to those at the above-mentioned steps S201 to S205 shown in Fig. 8.

　　Meanwhile, upon the redirection, the New MME 30 sends to the eNB 20 a Redirect message including information on the New MME 30 itself (i.e., MME information) (step S36). Then, the eNB 20 forwards to the Dedicated MME 40 the MME information with being included in an Attach Request message (step S37).

　　The Dedicated MME 40 uses the received MME information to retrieve, from the New MME 30, the security context which has been obtained at the above-mentioned step S34 by the New MME 30 from the Old MME 60. Specifically, the Dedicated MME 40 sends an Identification Request message to the New MME 30 (step S38), and as a response thereto, receives an Identification Response message including UE context (step S39). As described above, the UE context includes the AVs and all security context. Then, the Dedicated MME 40 extracts the Kasme and the NAS keys from the retrieved UE context (step S40).

　　According to this option, the Dedicated MME 40 is indicated about the MME information such that the Dedicated MME 40 can retrieve necessary security context from the New MME 30. Therefore, unlike the typical GUTI attach procedure, the Dedicated MME 40 needs not to contact the Old MME 60 for the purpose of retrieving the AVs and the security context from the Old MME 60, and thus needs not to start AKA procedure if it fails in retrieving the security context from the Old MME 60. Thus, compared with the typical GUTI attach procedure, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated MME for key computation. Moreover, since the existing message sequences can be reused, it is possible to minimize the impact on the mobile communication system.

　　Further, at the above-mentioned step S39, the New MME 30 may include, in the Identification Response message, an Authentication Indication which indicates that the UE 10 has been authenticated as the one to be redirected to the Dedicated MME 40. In this case, the Dedicated MME 40 needs not to contact the HSS 50 for the purpose of authenticating the UE 10, so that it is possible to further reduce signaling overload.

(Option 2)
　　Fig. 5 shows a sequence diagram for this option. Processes at steps S41 to S45 are performed in a similar manner to those at the above-mentioned steps S31 to S35 shown in Fig. 4.

　　Meanwhile, upon the redirection, the New MME 30 sends to the eNB 20 a Redirect message including the UE context (step S46). Then, the eNB 20 forwards to the Dedicated MME 40 the UE context with being included in an Attach Request message (step S47). As described above, the UE context includes the AVs and all security context.

　　Then, the Dedicated MME 40 extracts the Kasme and the NAS keys from the received UE context (step S48).

　　According to this option, the Dedicated MME 40 needs not to contact any MME to retrieve the UE context. Therefore, it is possible to reduce the amount of signaling compared to the above-mentioned Option 1, and it is also possible to reduce to reduce overload to the Dedicated MME 40.

　　Further, this option may be optimized such that the Old MME 60 remembers that integrity check for the Attach Request message is done at the above-mentioned step S43. In this case, the Dedicated MME 40 needs not to do the integrity check again at the above-mentioned step S47, so that it is possible to further reduce the overload to the Dedicated MME 40.

(Option 3)
　　Fig. 6 shows a sequence diagram for this option. Processes at steps S51 to S55 are performed in a similar manner to those at the above-mentioned steps S31 to S35 shown in Fig. 4.

　　Meanwhile, in this option, since the Old MME 60 served as the DMME for subscriber, the Old MME 60 maintains the security context for a while without removing it just till a second Identification Request message is received from a dedicated MME.

　　Upon the redirection, the New MME 30 sends a Redirect message to the eNB 20 (step S56). Then, the eNB 20 forwards the Attach Request message to the Dedicated MME 40 (step S57).

　　Upon receiving the Attach Request message, the Dedicated MME 40 sends to the Old MME 60 an Identification Request message including the GUTI and the complete Attach Request message (step S58).

　　Now, since the second Identification Request message is received, the Old MME 60 sends back to the Dedicated MME 40 an Identification Response message including the maintained MM Context (security context) (step S59).

　　Then, the Dedicated MME 40 extracts the Kasme and the NAS keys from the received MM context (step S60).

　　According to this option, like the typical GUTI attach procedure, the Dedicated MME 40 can retrieve necessary security context from the Old MME 60 by only using the GUTI, and thus needs not to start AKA procedure. Therefore, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated MME for key computation. Moreover, since the Dedicated MME only has to contact the MME indicated by the GUTI, in other words, since the Dedicated MME only performs the existing message sequences, it is possible to minimize the impact on the Dedicated MME.

　　Further, it is preferable that the New MME 30 informs, through the Redirect message and the Attach Request message shown at the above-mentioned steps S56 and S57, the Dedicated MME 40 that no AV is consumed, and the Old MME 60 informs only valid AVs to the Dedicated MME 40 in the second Identification Response message shown at the above-mentioned step S59. This ensures that the Dedicated MME 40 can use the received AVs for conducting communication with the UE 10. Note that as a substitute for the valid AVs or in addition thereto, the Old MME 60 may inform other additional information to the Dedicated MME 40.

　　Furthermore, this option may be optimized such that the Old MME 60 remembers that integrity check for the Attach Request message is done at the above-mentioned step S53. In this case, the Dedicated MME 40 needs not to do the integrity check again at the above-mentioned step S57, and the Old MME 60 also needs not do the integrity check again at the above-mentioned step S58, so that it is possible to reduce the overload to the Dedicated MME 40 and the Old MME 60.

　　Note that although the exemplary embodiments have been described hereinbefore by taking as an example E-UTRAN, the present invention can also be applied to IMSI attach and P-TMSI (Packet-TMSI (Temporary Mobile Subscriber Identity)) attach in UTRAN. The mechanism is basically the same as for attach in E-UTRAN, except that e.g., the message is changed to carry Dedicated MME/SGSN information.

　　If the exemplary embodiments are referred to the UTRAN and GERAN (GSM (Global System for Mobile communications) EDGE (Enhanced Data GSM Environment) RAN), then the following replacements need to be made:
　　replacing MME with SGSN;
　　replacing DMME with DSGSN;
　　replacing eNB with BTS (Base Transceiver Station)/BSC (Base Station Controller);
　　replacing HSS with HLR (Home Location Register)/HSS; and
　　replacing GUTI with P-TMSI.

　　Further, although the illustration is omitted in the first and second exemplary embodiments, the New MME 30, the Dedicated MME 40 and the Old MME 60 each can be configured by, for example, one or more transceivers which conduct communication with the UE 10 through the eNB 20, which conduct communication between the

MMEs

30, 40 and 60 through or not trough the eNB 20 and which conduct communication with the HSS 50, and a controller such as a CPU (Central Processing Unit) which controls these transceivers to execute the processes shown in the accompany sequence diagrams or processes equivalent thereto.

<Third Exemplary Embodiment>
　　As shown in Fig. 9, a network system according to this exemplary embodiment includes

SGSNs

230 and 240 by way of example only. Among them, the SGSN 230 is the one to which a UE 210 initially attaches through an NB (Node B) or eNB 220 serving as a radio base station, and thus sometimes referred to as "New SGSN" in the following description. Note that the NB/eNB 220 will be sometimes referred to as "RAN (Radio Access Network)", and the UE 210 will be sometimes referred to as "MS (Mobile Station)". On the other hand, the SGSN 240 is the one dedicated to serve the UE 210 according to subscription and to which the UE 210 is redirected from the SGSN 240, and thus referred to as "Dedicated SGSN" or "D-SGSN". Note that the SGSN 240 can also be referred to as "Specific SGSN", in conformity with the Dedicated Core Network being sometimes referred to as "Specific Core Network". If necessary, each of the

SGSNs

230 and 240 can obtain, from an HLR or HSS 250, subscription information on the UE 210, information necessary for authenticating the UE 210, and the like. The New SGSN 230 is also connected to an S-GW and/or a P-GW. Similarly, the Dedicated SGSN 240 is also connected to a Dedicated S-GW and/or a Dedicated P-GW.

　　In general, this exemplary embodiment deals with IMSI attach in UTRAN, particularly with a case where the New SGSN 230 redirects an Attach Request message received from the RAN 220 to the D-SGSN 240, according to the subscription information on the UE 210 obtained from the HLR 250. Note that the mechanism is basically the same as for the IMSI attach in E-UTRAN, except that e.g., the message is changed to carry Dedicated MME/SGSN information.

　　Next, prior to describing operation examples of this exemplary embodiment, there are firstly defined problems in typical IMSI attach procedure with reference to Fig. 21. Then, there will be described solutions for addressing these problems as the operation examples. The solutions include Option 1 shown in Fig. 10 and Option 2 shown in Fig. 11.

Problem Defined:
　　As shown in Fig. 21, in the typical IMSI attach procedure, a UE 310 sends an Attach Request message including its IMSI to a RAN 320 (step S501), and the RAN 320 forwards the Attach Request message to a New SGSN 330 (step S502).

　　Upon receiving the Attach Request message, the New SGSN 330 sends a Send Authentication Info message to an HLR 350, and as a response thereto, receives a Send Authentication Info Ack message (step S503). Then, the New SGSN 330 corporates with the UE 310 to perform Authentication procedure (step S504). As a result, The UE 310 and the New SGSN 330 each can share a CK (Ciphering Key) and an IK (Integrity Key), so that secure connection is established between them (step S505).

　　After that, the New SGSN 330 sends an Update Location message to the HLR 350, and as a response thereto, receives an Update Location Ack message (step S506). The Update Location Ack message includes the IMSI and the subscription information/data. Moreover, the subscription information includes information on a Dedicated SGSN 340 (hereinafter, such information will be sometimes referred to as "D-SGSN information").

　　Then, the New SGSN 330 selects the Dedicated SGSN 340 to which the UE 310 should be redirected based on the D-SGSN information (step S507), and sends a Redirect message to the RAN 320 (step S508). Upon receiving the Redirect message, the RAN 320 forwards the Attach Request message to the Dedicated SGSN 340 (step S509).

　　However, at this time, the following problems arise:
　　(a) since the Attach Request message is not protected, the Dedicated SGSN 340 starts AKA procedure to the UE 310;
　　(b) due to (a), the Dedicated SGSN 340 sends/receives Send Authentication Info/Send Authentication Info Ack messages and Update Location/ Update Location Ack messages to/from the HLR 350, and also corporates with the UE 310 to perform AKA procedure; and
　　(c) due to (a) and (b), there are caused signaling overload to the HLR 350, the Dedicated SGSN 340, the NB 320, the UE 310 and all interfaces therebetween, as well as overload to the Dedicated SGSN 340 for key computation.

Solutions:
(Option 1)
　　Fig. 10 shows a sequence diagram for this option. Processes at steps S301 to S307 are performed in a similar manner to those at the above-mentioned steps S501 to S507 shown in Fig. 21.

　　Meanwhile, upon the redirection, the New SGSN 230 sends to the RAN 220 a Redirect message including information on the New SGSN 230 itself (herein after, such information will be sometimes referred to as "SGSN information") (step S308). For example, the SGSN information includes an ID, an FQDN or an IP address assigned to the New SGSN 230. Then, the RAN 220 forwards to the Dedicated SGSN 240 the SGSN information with being included in an Attach Request message (step S309).

　　The Dedicated SGSN 240 uses the received SGSN information to retrieve, from the New SGSN 230, security context concerning secure connection which has been established between the UE 210 and the New SGSN 230. Specifically, the Dedicated SGSN 240 sends an Identification Request message to the New SGSN 230 (step S310), and as a response thereto, receives an Identification Response message including the security context (step S311). The security context includes Authentication Triplets or Authentication Quintets, which includes the CK and the IK. Then, the Dedicated SGSN 240 extracts the CK and the IK from the retrieved security context (step S312).

　　After that, a message 5 and the subsequent message sequences as disclosed in 3GPP TS 23.060, section 6.5.3 follows.

　　According to this option, the Dedicated SGSN 240 is indicated about the SGSN information such that the Dedicated SGSN 240 can retrieve necessary security context from the New SGSN 230. Therefore, unlike the typical IMSI attach procedure, the Dedicated SGSN 240 needs not to perform AKA procedure. Thus, compared with the typical IMSI attach procedure, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated SGSN for key computation. Moreover, since the existing message sequences can be reused, it is possible to minimize the impact on the mobile communication system.

　　Further, at the above-mentioned step S311, the New SGSN 230 may include, in the Identification Response message, an Authentication Indication which indicates that the UE 210 has been authenticated as the one to be redirected to the Dedicated SGSN 240. In this case, the Dedicated SGSN 240 needs not to contact the HLR 250 for the purpose of authenticating the UE 210, so that it is possible to further reduce signaling overload.

(Option 2)
　　Fig. 11 shows a sequence diagram for this option. Processes at steps S321 and S322 are performed in a similar manner to those at the above-mentioned steps S301 and S302 shown in Fig. 10.

　　Meanwhile, upon receiving the Attach Request message, the New SGSN 230 firstly retrieves from the HLR 250 the subscription information including the D-SGSN information (step S323a).

　　As described above, the D-SGSN information indicates that the UE 210 should be redirected to the Dedicated SGSN 240. Therefore, the New SGSN 230 skips establishment of secure connection with the UE 210. Specifically, the New SGSN 230 does not perform the Authentication procedure, the computation of the CK and the IK, and the like.

　　Then, the New SGSN 230 selects the Dedicated SGSN 240 based on the D-SGSN information (step S324), and sends to the RAN 220 a Redirect message including the subscription information (which also includes the D-SGSN information) (step S325). Then, the RAN 220 forwards to the Dedicated SGSN 240 the subscription information with being included in an Attach Request message (step S326).

　　In response to receiving the subscription information (i.e., D-SGSN information), the Dedicated SGSN 240 establishes secure connection with the UE 210 through the RAN 220, as a substitute for the New SGSN 230. Specifically, Dedicated SGSN 240 sends a Send Authentication Info message to the HLR 250, and as a response thereto, receives a Send Authentication Info Ack message including the Authentication Triplets or the Authentication Quintets (step S327). Then, the Dedicated SGSN 240 corporates with the UE 210 to perform Authentication procedure (step S328). As a result, The UE 210 and the Dedicated SGSN 240 each can share the CK and the IK, so that secure connection is established therebetween through the RAN 220 (step S329).

　　According to this option, as will be understood by comparing the sequence shown in Fig. 11 with that shown in Fig. 10, it is possible to reduce the amount of signaling compared to the above-mentioned Option 1. Moreover, since the New SGSN 230 skips the establishment of secure connection, it is also possible to reduce overload to the New SGSN 230.

　　Further, as a substitute for the above-mentioned step S323a, the D-SGSN information may be preconfigured in the New SGSN 230 (step S323b). In this case, the New SGSN 230 can also skip the retrieval of subscription information from the HLR 250 when it receives the initial Attach Request from the UE 210, so that it is possible to further reduce the amount of signaling.

　　Furthermore, at the above-mentioned step S323a, the New SGSN 230 may further retrieve AVs from the HLR 250. The retrieved AVs can be transferred to the Dedicated SGSN 240 through the RAN 220 with the Redirect message shown at the above-mentioned step S325 and the Attach Request message shown at the above-mentioned step S326. In this case, the Dedicated SGSN 240 can skip the acquisition of AVs from the HLR 250 shown at the above-mentioned step S327, so that it is possible to further reduce the amount of signaling.

<Fourth Exemplary Embodiment>
　　As shown in Fig. 12, a network system according to this exemplary embodiment further includes an SGSN 260 in addition to the above-mentioned

SGSNs

230 and 240. The SGSN 260 is the one to which the UE 210 previously attached, and thus sometimes referred to as "Old SGSN" in the following description. If necessary, the Old SGSN 260 can also obtain, from the HLR 250, the subscription information, the D-SGSN information and the like.

　　In general, this exemplary embodiment deals with P-TMSI attach in UTRAN, particularly with a case where the New SGSN 230 redirects an Attach Request message received from the RAN 220 to the D-SGSN 240, according to the D-SGSN information obtained from the Old SGSN 260. Note that the mechanism is basically the same as for the GUTI attach in E-UTRAN, except that e.g., the message is changed to carry Dedicated MME/SGSN information.

　　Next, prior to describing operation examples of this exemplary embodiment, there are firstly defined problems in typical P-TMSI attach procedure with reference to Fig. 22. Then, there will be described solutions for addressing these problems as the operation examples. The solutions include Option 1 shown in Fig. 12, Option 2 shown in Fig. 13, and Option 3 shown in Fig. 14.

Problem Defined:
　　As shown by dotted lines in Fig. 22, assume that the UE 310 previously attached to an Old SGSN 360, and the Old SGSN 360 assigned a P-TMSI to the UE 310, so that the UE has gotten the P-TMSI.

　　In the typical P-TMSI attach procedure, the UE 310 sends an Attach Request message including the P-TMSI to the eNB 320 (step S601), and the eNB 320 forwards the Attach Request message to the New SGSN 330 (step S602).

　　Upon receiving the Attach Request message, the New SGSN 330 sends to the Old SGSN 360 an Identification Request message including the P-TMSI as well as the old RAI (Routing Area Identity) and P-TMSI Signature (step S603), and as a response thereto, receives an Identification Response message including the IMSI of the UE 310, as well as the Authentication Triplets or the Authentication Quintets (step S604).

　　Then, the New SGSN 330 selects the Dedicated SGSN 340 (step S605), and sends a Redirect message to the eNB 320 (step S606). Upon receiving the Redirect message, the eNB 320 forwards the Attach Request message to the Dedicated SGSN 340 (step S607).

　　However, the Old SGSN 360 might remove MM context (i.e., security context) after a predetermined timer expired. Moreover, the P-TMSI merely indicates the Old SGSN 360. Therefore, the Dedicated SGSN 340 fails in retrieving the AVs and the security context from the Old SGSN 360, and thus starts AKA procedure.

　　Accordingly, as with the typical IMSI attach procedure, the following problems arise:
　　signaling overload to the HLR 350, the Dedicated SGSN 340, the NB/eNB 320, the UE 310 and all interfaces therebetween; and
　　overload to the Dedicated SGSN 340 for key computation.

Solutions:
(Option 1)
　　Fig. 12 shows a sequence diagram for this option. Processes at steps S331 to S335 are performed in a similar manner to those at the above-mentioned steps S601 to S605 shown in Fig. 22.

　　Meanwhile, upon the redirection, the New SGSN 230 sends to the eNB 220 a Redirect message including information on the SGSN 230 itself (i.e., SGSN information) (step S336). Then, the eNB 220 forwards to the Dedicated SGSN 240 the SGSN information with being included in an Attach Request message (step S337).

　　The Dedicated SGSN 240 uses the received SGSN information to retrieve, from the New SGSN 230, the security context which has been obtained at the above-mentioned step S334 by the New SGSN 230 from the Old SGSN 260. Specifically, the Dedicated SGSN 240 sends an Identification Request message to the New SGSN 230 (step S338), and as a response thereto, receives an Identification Response message including MM context (step S339). As described above, the MM context includes the AVs and all security context. Then, the Dedicated SGSN 240 extracts the CK and the IK from the retrieved MM context (step S340).

　　According to this option, the Dedicated SGSN 240 is indicated about the SGSN information such that the Dedicated SGSN 240 can retrieve necessary security context from the New SGSN 230. Therefore, unlike the typical P-TMSI attach procedure, the Dedicated SGSN 240 needs not to contact the Old SGSN 260 for the purpose of retrieving the AVs and the security context from the Old SGSN 260, and thus needs not to start AKA procedure if it fails in retrieving the security context from the Old SGSN 260. Thus, compared with the typical P-TMSI attach procedure, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated SGSN for key computation. Moreover, since the existing message sequences can be reused, it is possible to minimize the impact on the mobile communication system.

　　Further, at the above-mentioned step S339, the New SGSN 230 may include, in the Identification Response message, an Authentication Indication which indicates that the UE 210 has been authenticated as the one to be redirected to the Dedicated SGSN 240. In this case, the Dedicated SGSN 240 needs not to contact the HLR 250 for the purpose of authenticating the UE 210, so that it is possible to further reduce signaling overload.

(Option 2)
　　Fig. 13 shows a sequence diagram for this option. Processes at steps S341 to S345 are performed in a similar manner to those at the above-mentioned steps S331 to S335 shown in Fig. 12.

　　Meanwhile, upon the redirection, the New SGSN 230 sends to the eNB 220 a Redirect message including the MM context (step S346). Then, the eNB 220 forwards to the Dedicated SGSN 240 the MM context with being included in an Attach Request message (step S347). As described above, the MM context includes the AVs and all security context.

　　Then, the Dedicated SGSN 240 extracts the CK and the IK from the received MM context (step S348).

　　According to this option, the Dedicated SGSN 240 needs not to contact any SGSN to retrieve the MM context. Therefore, it is possible to reduce the amount of signaling compared to the above-mentioned Option 1, and it is also possible to reduce overload to the Dedicated SGSN 240.

　　Further, this option may be optimized such that the Old SGSN 260 remembers that integrity check for the Attach Request message is done at the above-mentioned step S343. In this case, the Dedicated SGSN 240 needs not to do the integrity check again at the above-mentioned step S347, so that it is possible to further reduce the overload to the Dedicated SGSN 240.

(Option 3)
　　Fig. 14 shows a sequence diagram for this option. Processes at steps S351 to S355 are performed in a similar manner to those at the above-mentioned steps S331 to S335 shown in Fig. 12.

　　Meanwhile, in this option, since the Old SGSN 260 served as the D-SGSN for subscriber, the Old SGSN 260 maintains the security context for a while without removing it just till a second Identification Request message is received from a dedicated SGSN.

　　Upon the redirection, the New SGSN 230 sends a Redirect message to the eNB 220 (step S356). Then, the eNB 220 forwards the Attach Request message to the Dedicated SGSN 240 (step S357).

　　Upon receiving the Attach Request message, the Dedicated SGSN 240 sends to the Old SGSN 260 an Identification Request message including the P-TMSI and the complete Attach Request message (step S358).

　　Now, since the second Identification Request message is received, the Old SGSN 260 sends back to the Dedicated SGSN 240 an Identification Response message including the maintained MM Context (security context) (step S359).

　　Then, the Dedicated SGSN 240 extracts the CK and the IK from the received MM context (step S360).

　　According to this option, like the typical P-TMSI attach procedure, the Dedicated SGSN 240 can retrieve necessary security context from the Old SGSN 260 by only using the P-TMSI, and thus needs not to start AKA procedure. Therefore, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated SGSN for key computation. Moreover, since the Dedicated SGSN only has to contact the SGSN indicated by the P-TMSI, in other words, since the Dedicated SGSN only performs the existing message sequences, it is possible to minimize the impact on the Dedicated SGSN.

　　Further, it is preferable that the New SGSN 230 informs, through the Redirect message and the Attach Request message shown at the above-mentioned steps S356 and S357, the Dedicated SGSN 240 that no AV is consumed, and the Old SGSN 260 informs only valid AVs to the Dedicated SGSN 240 in the second Identification Response message shown at the above-mentioned step S359. This ensures that the Dedicated SGSN 240 can use the received AVs for conducting communication with the UE 210. Note that as a substitute for the valid AVs or in addition thereto, the Old SGSN 260 may inform other additional information to the Dedicated SGSN 240.

　　Furthermore, this option may be optimized such that the Old SGSN 260 remembers that integrity check for the Attach Request message is done at the above-mentioned step S353. In this case, the Dedicated SGSN 240 needs not to do the integrity check again at the above-mentioned step S357, and the Old SGSN 260 also needs not do the integrity check again at the above-mentioned step S358, so that it is possible to reduce the overload to the Dedicated SGSN 240 and the Old SGSN 260.

　　Further, although the illustration is omitted in the third and fourth exemplary embodiments, the New SGSN 230, the Dedicated SGSN 240 and the Old SGSN 260 each can be configured by, for example, one or more transceivers which conduct communication with the UE 210 through the RAN(NB/eNB) 220, which conduct communication between the

SGSNs

230, 240 and 260 through or not trough the RAN 220 and which conduct communication with the HLR/HSS 250, and a controller such as a CPU which controls these transceivers to execute the processes shown in the accompany sequence diagrams or processes equivalent thereto.

<Fifth Exemplary Embodiment>
　　As shown in Fig. 15, a network system according to this exemplary embodiment can be configured as with the above-mentioned second exemplary embodiment. Meanwhile, this exemplary embodiment is different from the above-mentioned second exemplary embodiment, in that this exemplary embodiment deals with TAU (Tracking Area Update) procedure. Note that the mechanism is basically the same as for the GUTI attach procedure, except that e.g., the messages are changed to those in conformity with the TAU procedure.

　　Next, prior to describing operation examples of this exemplary embodiment, there are firstly defined problems in typical TAU procedure with reference to Fig. 23. Then, there will be described solutions for addressing these problems as the operation examples. The solutions include Option 1 shown in Fig. 15, Option 2 shown in Fig. 16 and Option 3 shown in Fig. 17.

Problem Defined:
　　As shown in Fig. 23, assume that there is caused a certain trigger to start TAU procedure (step S701). At this time, the UE 110 sends a TAU Request message to the eNB 120 (step S702), and the eNB 120 forwards the TAU request message to the New MME 130 (step S703). Note that although the illustration is omitted, the TAU Request message includes the GUTI.

　　Upon receiving the TAU Request message, the New MME 130 sends a Context Request message to the Old MME 160 (step S704), and as a response thereto, receives a Context Response message from the Old MME 160 (step S705).

　　Then, the New MME 130 corporates with the UE 110 and the HSS 150 to perform AKA procedure and NAS SMC procedure (step S706). The New MME 130 sends a Context Acknowledgement message to the Old MME 160 (step S707).

　　After that, the New MME 130 selects the Dedicated MME 140 (step S708), and sends a Redirect message to the eNB 120 (step S709). Upon receiving the Redirect message, the eNB 120 forwards the TAU Request message to the Dedicated MME 140 (step S710).

　　Accordingly, as with the typical GUTI attach procedure, the following problems arise:
　　signaling overload to the HSS 150, the Dedicated MME 140, the eNB 120, the UE 110 and all interfaces therebetween; and
　　overload to the Dedicated MME 140 for key computation.

Solutions:
(Option 1)
　　Fig. 15 shows a sequence diagram for this option. Processes at steps S361 to S367 are performed in a similar manner to those at the above-mentioned steps S701 to S708 shown in Fig. 23.

　　Meanwhile, upon the redirection, the New MME 30 sends to the eNB 20 a Redirect message including information on the New MME 30 itself (i.e., MME information) (step S368). Then, the eNB 20 forwards to the Dedicated MME 40 the MME information with being included in a TAU Request message (step S369).

　　The Dedicated MME 40 uses the received MME information to retrieve, from the New MME 30, the security context which has been obtained at the above-mentioned step S365 by the New MME 30 from the Old MME 60. Specifically, the Dedicated MME 40 sends a Context Request message to the New MME 30 (step S370), and as a response thereto, receives a Context Response message including the security context (step S371). The Dedicated MME 40 sends a Context Acknowledgement message to the New MME 30 (step S372). Then, the Dedicated MME 40 extracts the Kasme and the NAS keys from the retrieved security context (step S373).

　　After that, a message 8 and the subsequent message sequences as disclosed in 3GPP TS 23.401, section 5.3.2.1 follows.

　　According to this option, the Dedicated MME 40 is indicated about the MME information such that the Dedicated MME 40 can retrieve necessary security context from the New MME 30. Therefore, unlike the typical TAU procedure, the Dedicated MME 40 needs not to contact the Old MME 60 for the purpose of retrieving the security context from the Old MME 60, and thus needs not to start AKA procedure if it fails in retrieving the security context from the Old MME 60. Thus, compared with the typical TAU procedure, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated MME for key computation. Moreover, since the existing message sequences can be reused, it is possible to minimize the impact on the mobile communication system.

(Option 2)
　　Fig. 16 shows a sequence diagram for this option. Processes at steps S381 to S387 are performed in a similar manner to those at the above-mentioned steps S361 to S367 shown in Fig. 15.

　　Meanwhile, upon the redirection, the New MME 30 sends to the eNB 20 a Redirect message including the security context (step S338). Then, the eNB 20 forwards to the Dedicated MME 40 the security context with being included in a TAU Request message (step S389).

　　Then, the Dedicated MME 40 extracts the Kasme and the NAS keys from the received security context (step S390).

　　According to this option, the Dedicated MME 40 needs not to contact any MME to retrieve the security context. Therefore, it is possible to reduce the amount of signaling compared to the above-mentioned Option 1, and it is also possible to reduce overload to the Dedicated MME 40.

　　(Option 3)
　　Fig. 17 shows a sequence diagram for this option. Processes at steps S391 to S397 are performed in a similar manner to those at the above-mentioned steps S361 to S367 shown in Fig. 15.

　　Meanwhile, in this option, since the Old MME 60 served as the DMME for subscriber, the Old MME 60 maintains the security context for a while without removing it just till a second Context Request message is received from a dedicated MME.

　　Upon the redirection, the New MME 30 sends a Redirect message to the eNB 20 (step S398). Then, the eNB 20 forwards the TAU Request message to the Dedicated MME 40 (step S399).

　　Upon receiving the TAU Request message, the Dedicated MME 40 sends a Context Request message to the Old MME 60 (step S400).

　　Now, since the second Context Request message is received, the Old MME 60 sends back to the Dedicated MME 40 a Context Response message including the maintained security context (step S401).

　　The Dedicated MME 40 sends a Context Acknowledgement message to the Old MME 60 (step S402), and then extracts the Kasme and the NAS keys from the received security context (step S403).

　　According to this option, like the typical TAU procedure, the Dedicated MME 40 can retrieve necessary security context from the Old MME 60 by only using the GUTI, and thus needs not to start AKA procedure. Therefore, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated MME for key computation. Moreover, since the Dedicated MME only has to contact the MME indicated by the GUTI, in other words, since the Dedicated MME only performs the existing message sequences, it is possible to minimize the impact on the Dedicated MME.

<Sixth Exemplary Embodiment>
　　As shown in Fig. 18, a network system according to this exemplary embodiment can be configured as with the above-mentioned fourth exemplary embodiment. Meanwhile, this exemplary embodiment is different from the above-mentioned fourth exemplary embodiment, in that this exemplary embodiment deals with RAU (Routing Area Update) procedure. Note that the mechanism is basically the same as for the P-TMSI attach procedure, except that e.g., the messages are changed to those in conformity with the RAU procedure.

　　Next, prior to describing operation examples of this exemplary embodiment, there are firstly defined problems in typical RAU procedure with reference to Fig. 24. Then, there will be described solutions for addressing these problems as the operation examples. The solutions include Option 1 shown in Fig. 18, Option 2 shown in Fig. 19 and Option 3 shown in Fig. 20.

Problem Defined:
　　As shown by dotted lines in Fig. 24, assume that there is caused a certain trigger to start RAU procedure (step S801). At this time, the MS(UE) 310 sends a RAU Request message to the eNB 320 (step S802), and the eNB 320 forwards the RAU request message to the New SGSN 330 (step S803). Note that although the illustration is omitted, the RAU Request message includes the P-TMSI.

　　Upon receiving the RAU Request message, the New SGSN 330 sends an SGSN Context Request message to the Old SGSN 360 (step S804), and as a response thereto, receives a SGSN Context Response message from the Old SGSN 360 (step S805).

　　Then, the New SGSN 330 corporates with the MS 310 and the HLR 350 to perform AKA procedure (step S806). The New SGSN 330 sends a Context Acknowledgement message to the Old SGSN 360 (step S807).

　　After that, the New SGSN 330 selects the Dedicated SGSN 340 (step S808), and sends a Redirect message to the eNB 320 (step S809). Upon receiving the Redirect message, the eNB 320 forwards the RAU Request message to the Dedicated SGSN 340 (step S810).

　　Accordingly, as with the typical P-TMSI attach procedure, the following problems arise:
　　signaling overload to the HLR 350, the Dedicated SGSN 340, the eNB 320, the MS 310 and all interfaces therebetween; and
　　overload to the Dedicated SGSN 340 for key computation.

Solutions:
(Option 1)
　　Fig. 18 shows a sequence diagram for this option. Processes at steps S411 to S417 are performed in a similar manner to those at the above-mentioned steps S801 to S808 shown in Fig. 24.

　　Meanwhile, upon the redirection, the New SGSN 230 sends to the eNB 220 a Redirect message including information on the New SGSN 230 itself (i.e., SGSN information) (step S418). Then, the eNB 220 forwards to the Dedicated SGSN 240 the SGSN information with being included in a RAU Request message (step S419).

　　The Dedicated SGSN 240 uses the received SGSN information to retrieve, from the New SGSN 230, the security context which has been obtained at the above-mentioned step S415 by the New SGSN 230 from the Old SGSN 260. Specifically, the Dedicated SGSN 240 sends an SGSN Context Request message to the New SGSN 230 (step S420), and as a response thereto, receives an SGSN Context Response message including the security context (step S421). The Dedicated SGSN 240 sends a Context Acknowledgement message to the New SGSN 230 (step S422). Then, the Dedicated SGSN 240 extracts the CK and the IK from the retrieved security context (step S423).

　　After that, a message 6 and the subsequent message sequences as disclosed in 3GPP TS 23.060, section 6.9.2.1 follows.

　　According to this option, the Dedicated SGSN 240 is indicated about the SGSN information such that the Dedicated SGSN 240 can retrieve necessary security context from the New SGSN 230. Therefore, unlike the typical RAU procedure, the Dedicated SGSN 240 needs not to contact the Old SGSN 260 for the purpose of retrieving the security context from the Old SGSN 260, and thus needs not to start AKA procedure if it fails in retrieving the security context from the Old SGSN 260. Thus, compared with the typical RAU procedure, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated SGSN for key computation. Moreover, since the existing message sequences can be reused, it is possible to minimize the impact on the mobile communication system.

(Option 2)
　　Fig. 19 shows a sequence diagram for this option. Processes at steps S431 to S437 are performed in a similar manner to those at the above-mentioned steps S411 to S417 shown in Fig. 18.

　　Meanwhile, upon the redirection, the New SGSN 230 sends to the eNB 220 a Redirect message including the security context (step S438). Then, the eNB 220 forwards to the Dedicated SGSN 240 the security context with being included in a RAU Request message (step S439).

　　Then, the Dedicated SGSN 240 extracts the CK and the IK from the received security context (step S440).

　　According to this option, the Dedicated SGSN 240 needs not to contact any SGSN to retrieve the security context. Therefore, it is possible to reduce the amount of signaling compared to the above-mentioned Option 1, and it is also possible to reduce overload to the Dedicated SGSN 240.

(Option 3)
　　Fig. 20 shows a sequence diagram for this option. Processes at steps S441 to S447 are performed in a similar manner to those at the above-mentioned steps S411 to S417 shown in Fig. 18.

　　Meanwhile, in this option, since the Old SGSN 260 served as the D-SGSN for subscriber, the Old SGSN 260 maintains the security context for a while without removing it just till a second SGSN Context Request message is received from a dedicated SGSN.

　　Upon the redirection, the New SGSN 230 sends a Redirect message to the eNB 220 (step S448). Then, the eNB 220 forwards the RAU Request message to the Dedicated SGSN 240 (step S449).

　　Upon receiving the RAU Request message, the Dedicated SGSN 240 sends a SGSN Context Request message to the Old SGSN 260 (step S450).

　　Now, since the second SGSN Context Request message is received, the Old SGSN 260 sends back to the Dedicated SGSN 40 a SGSN Context Response message including the maintained security context (step S451).

　　The Dedicated SGSN 240 sends a Context Acknowledgement message to the Old SGSN 260 (step S452), and then extracts the CK and the IK keys from the received security context (step S453).

　　According to this option, like the typical RAU procedure, the Dedicated SGSN 240 can retrieve necessary security context from the Old SGSN 260 by only using the P-TMSI, and thus needs not to start AKA procedure. Therefore, it is possible to drastically reduce signaling overload, as well as overload to the Dedicated SGSN for key computation. Moreover, since the Dedicated SGSN only has to contact the SGSN indicated by the P-TMSI, in other words, since the Dedicated SGSN only performs the existing message sequences, it is possible to minimize the impact on the Dedicated SGSN.

　　Note that the present invention is not limited to the above-mentioned exemplary embodiments, and it is obvious that various modifications can be made by those of ordinary skill in the art based on the recitation of the claims.

　　The whole or part of the exemplary embodiments disclosed above can be described as, but not limited to, the following supplementary notes.

(Supplementary note 1)
　　A network system comprising:
　　a first node that establishes secure connection with a UE (User Equipment) initially attempting to attach to a network, through a radio base station; and
　　a second node to which the UE is redirected from the first node through the radio base station,
　　wherein upon the redirection, the first node sends information on the first node itself to the second node through the radio base station, and
　　wherein the second node uses the information to retrieve security context necessary for establishing the connection with the UE from the first node.

(Supplementary note 2)
　　A method of control for a network system including a first node that establishes secure connection with a UE initially attempting to attach to a network, through a radio base station, and a second node to which the UE is redirected from the first node through the radio base station, the method comprising:
　　sending, upon the redirection, information on the first node from the first node to the second node through the radio base station; and
　　using, by the second node, the information to retrieve security context necessary for establishing the connection with the UE from the first node.

(Supplementary note 3)
　　A network system comprising:
　　a first node that receives an attach request from a UE initially attempting to attach to a network, through a radio base station; and
　　a second node to which the attach request is redirected from the first node through the radio base station,
　　wherein upon the reception of the attach request, the first node skips establishment of secure connection with the UE through the radio base station,
　　wherein upon the redirection, the first node sends, to the second node through the radio base station, subscription information indicating that the UE is one to be redirected to the second node, and
　　wherein in response to receiving the subscription information, the second node establishes the secure connection with the UE.

(Supplementary note 4)
　　A method of control for a network system including a first node that receives an attach request from a UE initially attempting to attach to a network, through a radio base station, and a second node to which the attach request is redirected from the first node through the radio base station, the method comprising:
　　skipping, by the first node upon the reception of the attach request, establishment of secure connection with the UE through the radio base station;
　　sending, upon the redirection, from the first node to the second node through the radio base station, subscription information indicating that the UE is one to be redirected to the second node; and
　　establishing, by the second node in response to receiving the subscription information, the secure connection with the UE.

(Supplementary note 5)
　　A network system comprising:
　　a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, and that assigned a temporary identity to the UE;
　　a second node that receives an attach request including the temporary identity from the UE through the radio base station; and
　　a third node to which the attach request is redirected from the second node through the radio base station,
　　wherein the second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends information on the second node itself to the third node through the radio base station, and
　　wherein the third node uses the information to retrieve the security context from the second node.

(Supplementary note 6)
　　A method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously and that assigned a temporary identity to the UE, a second node that receives an attach request including the temporary identity from the UE through the radio base station, and a third node to which the attach request is redirected from the second node through the radio base station, the method comprising:
　　retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node;
　　sending, upon the redirection, information on the second node from the second node to the third node through the radio base station; and
　　using, by the third node, the information to retrieve the security context from the second node.

(Supplementary note 7)
　　A network system comprising:
　　a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, and that assigned a temporary identity to the UE;
　　a second node that receives an attach request including the temporary identity from the UE through the radio base station; and
　　a third node to which the attach request is redirected from the second node through the radio base station,
　　wherein the second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends the security context to the third node through the radio base station.

(Supplementary note 8)
　　A method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously and that assigned a temporary identity to the UE, a second node that receives an attach request including the temporary identity from the UE through the radio base station, and a third node to which the attach request is redirected from the second node through the radio base station, the method comprising:
　　retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node; and
　　sending, upon the redirection, the security context from the second node to the third node through the radio base station.

(Supplementary note 9)
　　A network system comprising:
　　a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, and that assigned a temporary identity to the UE;
　　a second node that receives an attach request including the temporary identity from the UE through the radio base station; and
　　a third node to which the attach request is redirected from the second node through the radio base station,
　　wherein the first node maintains security context necessary for establishing the secure connection with the UE till an identification request is received from the third node, and
　　wherein upon the redirection, the third node sends the identification request to the first node to retrieve the security context from the first node.

(Supplementary note 10)
　　A method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously and that assigned a temporary identity to the UE, a second node that receives an attach request including the temporary identity from the UE through the radio base station, and a third node to which the attach request is redirected from the second node through the radio base station, the method comprising:
　　maintaining, by the first node, security context necessary for establishing the secure connection with the UE till an identification request is received from the third node; and
　　sending, by the third node upon the redirection, the identification request to the first node to retrieve the security context from the first node.

(Supplementary note 11)
　　A network system comprising:
　　a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously;
　　a second node that receives a request for TAU (Tracking Area Update) or RAU (Routing Area Update) from the UE through the radio base station; and
　　a third node to which the request is redirected from the second node through the radio base station,
　　wherein the second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends information on the second node itself to the third node through the radio base station, and
　　wherein the third node uses the information to retrieve the security context from the second node.

(Supplementary note 12)
　　A method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, a second node that receives a request for TAU or RAU from the UE through the radio base station, and a third node to which the request is redirected from the second node through the radio base station, the method comprising:
　　retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node;
　　sending, upon the redirection, information on the second node from the second node to the third node through the radio base station; and
　　using, by the third node, the information to retrieve the security context from the second node.

(Supplementary note 13)
　　A network system comprising:
　　a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously;
　　a second node that receives a request for TAU or RAU from the UE through the radio base station; and
　　a third node to which the request is redirected from the second node through the radio base station,
　　wherein the second node retrieves security context necessary for establishing the connection with the UE from the first node, and upon the redirection, sends the security context to the third node through the radio base station.

(Supplementary note 14)
　　A method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, a second node that receives a request for TAU or RAU from the UE through the radio base station, and a third node to which the request is redirected from the second node through the radio base station, the method comprising:
　　retrieving, by the second node, security context necessary for establishing the connection with the UE from the first node; and
　　sending, upon the redirection, the security context from the second node to the third node through the radio base station.

(Supplementary note 15)
　　A network system comprising:
　　a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously;
　　a second node that receives a request for TAU or RAU from the UE through the radio base station; and
　　a third node to which the request is redirected from the second node through the radio base station,
　　wherein the first node maintains security context necessary for establishing the secure connection with the UE till a context request is received from the third node, and
　　wherein upon the redirection, the third node sends the context request to the first node to retrieve the security context from the first node.

(Supplementary note 16)
　　A method of control for a network system including a first node that established secure connection with a UE through a radio base station when the UE attached to a network previously, a second node that receives a request for TAU or RAU from the UE through the radio base station, and a third node to which the request is redirected from the second node through the radio base station, the method comprising:
　　maintaining, by the first node, security context necessary for establishing the secure connection with the UE till a context request is received from the third node; and
　　sending, by the third node upon the redirection, the context request to the first node to retrieve the security context from the first node.

　　This application is based upon and claims the benefit of priority from Japanese patent application No. 2014-112269 filed on May 30, 2014, and Japanese patent application No. 2014-143176 filed on July 11, 2014, the disclosures of which are incorporated herein in their entireties by reference.

10, 110, 210, 310 　　UE/MS
20, 120 　　eNB
30, 40, 60, 130, 140, 160 　　MME
50, 150 　　HSS
220, 320　　NB/eNB/RAN
230, 240, 260, 330, 340, 360 　　SGSN
250, 350　　HLR/HSS

Claims

　　A core network system comprising:
　　a plurality of SGSNs (Serving GPRS (General Packet Radio Service) Support Nodes); and
　　a server,
　　wherein a UE (User Equipment) sends a message to a first SGSN through a radio base station,
　　wherein the UE is redirected from the first SGSN to a second SGSN based on subscription information of the UE provided by the server,
　　wherein the first SGSN sends a first message including information on the redirection to the radio base station, and
　　wherein the radio base station sends a second message including the information to the second SGSN.
　　The core network system according to Claim 1,
　　wherein the first SGSN does not authenticate the UE before the redirection and the second SGSN authenticates the UE after the redirection.
　　The core network system according to Claim 1,
　　wherein the first SGSN sends the first message without authenticating the UE, and
　　wherein the second SGSN authenticates the UE upon receiving the second message.
　　The core network system according to any one of Claims 1 to 3,
　　wherein the server comprises an HSS (Home Subscriber Server) or an HLR (Home Location Register).
　　The core network system according to any one of Claims 1 to 4,
　　wherein the first SGSN selects the second SGSN that serves the UE by using the subscription information.
　　The core network system according to any one of Claims 1 to5,
　　wherein the second SGSN extracts information for authenticating the UE from the second message.
　　A method for a core network system including a plurality of SGSNs (Serving GPRS (General Packet Radio Service) Support Nodes) and a server, the method comprising:
　　a step that a UE (User Equipment) sends a message to a first SGSN through a radio base station;
　　a step that the UE is redirected from the first SGSN to a second SGSN based on subscription information of the UE provided by the server;
　　a step that the first SGSN sends a first message including information on the redirection to the radio base station; and
　　a step that the radio base station sends a second message including the information to the second SGSN.
　　The method according to Claim 7,
　　wherein the first SGSN does not authenticate the UE before the redirection and the second SGSN authenticates the UE after the redirection.
　　The method according to Claim 7,
　　wherein the first SGSN sends the first message without authenticating the UE, and
　　wherein the second SGSN authenticates the UE upon receiving the second message.
　　The method according to any one of Claims 7 to 9,
　　wherein the server comprises an HSS (Home Subscriber Server) or an HLR (Home Location Register).
　　The method according to any one of Claims 7 to 10,
　　wherein the first SGSN selects the second SGSN that serves the UE by using the subscription information.
　　The method according to any one of Claims 7 to 11,
　　wherein the second SGSN extracts information for authenticating the UE from the second message.
　　A UE (User Equipment) comprising:
　　means for sending a message to a first SGSN (Serving GPRS (General Packet Radio Service) Support Node) through a radio base station,
　　wherein the UE is redirected from the first SGSN received the message to a second SGSN based on subscription information of the UE provided by a server, and
　　wherein in the redirection, the first SGSN sends a first message including information on the redirection to the radio base station, and the radio base station sends a second message including the information to the second SGSN.
　　The UE according to Claim 13, wherein the UE is not authenticated by the first SGSN before the redirection, and the UE is authenticated by the second SGSN after the redirection.
　　A method for a UE (User Equipment) comprising:
　　a step that the UE sends a message to a first SGSN (Serving GPRS (General Packet Radio Service) Support Node) through a radio base station; and
　　a step that the UE is redirected from the first SGSN received the message to a second SGSN based on subscription information of the UE provided by a server,
　　wherein in the redirection, the first SGSN sends a first message including information on the redirection to the radio base station, and the radio base station sends a second message including the information to the second SGSN.
　　The method according to Claim 15, wherein the UE is not authenticated by the first SGSN before the redirection, and the UE is authenticated by the second SGSN after the redirection.