Base station device, base station device switching method, and program

Abstract

The present invention aims to provide a communication service having high resistance to transmission network failure. A base station device of a mobile communication network includes a context retention unit (174) that retains an MM (Mobility Management) context and an SM (Session Management) context for each terminal connected to the base station device, and a context transfer unit (161) that transfers the MM context and the SM context of the terminal to a base station device that is a handover target when a handover of the terminal is detected.

Description

Technical Field

[0001] This disclosure relates to a base station device, a base station device switching method and procedure, and a base station device switching method and procedure capable of providing communication services with high tolerance to transmission network failures. Background Technology

[0002] With the increasing prevalence of MEC (Multi-access Edge Computing), the opportunities for user terminals (UEs) to receive application services from MEC servers are constantly increasing. By utilizing MEC, for example, the response latency of application services can be reduced.

[0003] Typically, terminals in a mobile communication system communicate with the MEC server via the UPF (User Plane Function), one of the network function units in the core network. Therefore, when the UPF is configured in a specific data center, the advantages of MEC cannot be fully utilized. Thus, in the future, it is likely that multiple UPFs will be configured near each base station (RAN: Radio Access Network).

[0004] Furthermore, a technique for providing seamless streaming even during switching is proposed for the MEC architecture (e.g., see Patent Document 1). This MEC architecture enables the stable provision of low-latency network services.

[0005] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2022-51973 Summary of the Invention

[0006] Technical solutions for solving the problem The base station device disclosed herein is a base station device for a mobile communication network, comprising: a context retention unit that stores MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device; and a context forwarding unit that, when a terminal handover is detected, forwards the MM context and SM context of the terminal to the base station device that is the handover target.

[0007] The base station device handover method disclosed herein is a base station device handover method for mobile communication networks. It stores the MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device. When a terminal handover is detected, the MM context and SM context of the terminal are forwarded to the base station device that serves as the handover target.

[0008] The solutions disclosed herein can also be implemented by a computer. In this case, the program that enables the computer to perform the steps of the above methods, as well as the computer-readable recording medium on which the program is recorded, also fall within the scope of this disclosure. Attached Figure Description

[0009] Figure 1 This is a diagram illustrating the typical structure of a 5G mobile communication system.

[0010] Figure 2 It means Figure 1 A block diagram illustrating the functional configuration of the core network and base station devices.

[0011] Figure 3 This is a block diagram illustrating a functional configuration example of the core network and base station device of the 5G mobile communication system involved in the implementation method.

[0012] Figure 4 This is a block diagram showing a detailed configuration example of the U-plane (user plane) processing unit.

[0013] Figure 5 This diagram illustrates the registration-related processes between the terminal, base station, and core network.

[0014] Figure 6 This is another diagram illustrating the registration-related processes between the terminal, base station, and core network.

[0015] Figure 7 This is another diagram illustrating the registration-related processes between the terminal, base station, and core network.

[0016] Figure 8 This is another diagram illustrating the registration-related processes between the terminal, base station, and core network.

[0017] Figure 9 This diagram illustrates the processes related to PDU Session Establishment between the terminal, base station, and core network.

[0018] Figure 10This diagram illustrates the processes related to PDU Session Establishment between the terminal, base station, and core network.

[0019] Figure 11 This diagram illustrates the communication between the UE and the DN when a transmission network failure occurs.

[0020] Figure 12 This diagram illustrates the UE200 handover between two RANs.

[0021] Figure 13 This is an arrow diagram illustrating the registration sequence.

[0022] Figure 14 This is an arrow diagram illustrating the PDU Session Establishment sequence.

[0023] Figure 15 This is an arrow diagram illustrating the Xn Handover sequence.

[0024] Figure 16 This is a diagram illustrating an example of a computer configuration that executes program instructions; the program is software that performs various functions. Detailed Implementation

[0025] With the increasing prevalence of MEC (Multi-access Edge Computing), the opportunities for user terminals (UEs) to receive application services from MEC servers are constantly increasing. By utilizing MEC, for example, the response latency of application services can be reduced.

[0026] Typically, terminals in a mobile communication system communicate with the MEC server via the UPF (User-Defined Function) – one of the core network functional units. Therefore, when the UPF is configured in a specific data center, the advantages of MEC cannot be fully utilized. Consequently, in the future, it is likely that multiple UPFs will be deployed near each base station (RAN: Radio Access Network).

[0027] On the other hand, current mobile communication systems are relatively vulnerable to interruptions between the RAN and core network caused by transmission network failures. For example, when C-plane (control plane, N2 interface, SCTP, etc.) communication between the RAN and core network is lost, the RAN will shut down the cell.

[0028] In this scenario, for example, if the transmission network between the data center housing the servers corresponding to the various network function units of the core network and the RAN fails, the terminals housed within that RAN cell will be unable to communicate. In this case, even if communication between the RAN and the UPF is possible, the UE will still be unable to receive application services.

[0029] The purpose of this disclosure is to realize a technology that can provide communication services with high tolerance to transmission network failures.

[0030] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Figure 1 This is a diagram illustrating the typical structure of a 5G mobile communication system.

[0031] exist Figure 1 In the example, RAN12a is described as a base station that performs wireless communication with the in-cell terminal (UE: User Equipment) 13. Similarly, RAN12b and RAN12c are also described, and each base station is connected to the core network (CN) 11.

[0032] Furthermore, routers are indicated by elliptical symbols in the diagram. RAN12a and RAN12b are connected to Router #a, and via Router #b, they are connected to core network 11. RAN12c is connected to core network 11 via Router #c. The communication path connecting the base station and core network 11 is called the backhaul or transport network (TN).

[0033] (Functional composition of core network and base station equipment) Figure 2 It means Figure 1 A block diagram illustrating the functional configuration of the core network (CN) 11 and the base station apparatus (RAN) 12. The configuration of RAN12 is represented by summing the configurations of RAN12a, RAN12b, and RAN12c.

[0034] exist Figure 2 In the example, the core network 11 is configured to include Subscriber DB (UDR: User Data Repository) 31, UPF (User Plane Function) 32 and AMF (Access and Mobility Management Function) 33.

[0035] UDR31 is a functional block primarily responsible for saving and retrieving user data and session policies. UPF32 is a functional block primarily responsible for session anchors, mobility anchors, packet forwarding, and access control. AMF33 is a functional block primarily responsible for N2 interface termination, N1 interface termination, registration management, and mobility management.

[0036] In addition to UDR31, UPF32, and AMF33, the actual core network 11 also includes other network function units such as SMF (Session Management Function), UDM (Unified Data Management), and PCF (Policy Control Function). Figure 2 Descriptions of other network functional units have been omitted.

[0037] In addition, Figure 2 In the example, the base station device 12 is configured to include a gNB CU (Central Unit) 61, a gNB DU (Distributed Unit) 62, and a gNB RU (Radio Unit) 63. Furthermore, "gNB" refers to a gNodeB (base station) in a 5G mobile communication network. Hereinafter, gNB CU61, gNB DU62, and gNB RU63 will be abbreviated as CU61, DU62, and RU63, respectively, as appropriate.

[0038] RU63 is a functional block that controls the antenna and enables radio communication with the terminal, including MIMO and beamforming control. DU62 is a functional block that handles signal modulation and demodulation, MAC layer communication control, etc. CU61 is a functional block that processes PDCP (Packet Data Convergence Protocol) and RRC (Radio Resource Control). PDCP controls DU62 and RU63, connects to the core network, and performs packet encryption, while RRC manages the terminal's radio resources.

[0039] In addition, Figure 2In the example, AMF33 includes N2 processing unit 41, N1 processing unit 42, UE N1-MM / SM Contextcache 43, SCTP processing unit 44, and SBI (Service Based Interface) 45.

[0040] The N2 processing unit 41 is a functional block that performs the termination of the N2 interface, the connection interface between the RAN and AMF, and the control or management related to N2 interface communication (e.g., sending and receiving NGAP messages).

[0041] The N1 processing unit 42 is a functional block that performs control or management related to the termination of the N1 interface between the UE and the AMF, and N1 interface communication (e.g., sending and receiving NAS messages). In addition, the N1 processing unit 42 also performs processing related to the generation and updating of the Mobility Management Context and the Session Management Context. The Mobility Management Context is mobility management information used for sending and receiving NAS messages, and the Session Management Context is session management information.

[0042] UE N1-MM / SM Context cache43 is a cache storage device that stores the Mobility Management Context and Session Management Context used for NAS message transmission and reception.

[0043] The SCTP processing unit 44 is a functional block that performs SCTP (Stream Control Transmission Protocol) session termination and management in the communication between the core network 11 and the base station device 12.

[0044] SBI45 is an interface used to call various network function units within the core network that are connected through a service-oriented architecture.

[0045] In addition, Figure 2 In the example, CU61 includes an SCTP processing unit 71 and a UPF 72. The SCTP processing unit 71 is a functional block that performs SCTP session termination and management in communication between the core network 11 and the base station device 12. The UPF 72 is a functional block that enables the base station device 12 to implement UPF-related functions of the core network 11.

[0046] exist Figure 2In the example, for instance, considering the case where a terminal receives application services from an MEC server, base station device 12 includes a UPF 72. A terminal authenticated by the core network can then receive application services from an MEC server or similar entity located outside the core network 11 via the UPF 72 included in the CU 61 of base station device 12, without needing to traverse the transport network. Thus, lower latency communication services can be provided compared to the case via the transport network.

[0047] The SCTP processing unit 71 includes an NGAP processing unit 81 and an Xn processing unit 82. The NGAP processing unit 81 is a functional block that performs processes such as generating, encrypting, and decrypting NGAP messages. The Xn processing unit is a functional block that performs control or management related to Xn interface communication, and the Xn interface is the interface connecting the base station devices 12 to each other.

[0048] For example, when a transmission network failure occurs, the SCTP session between SCTP processing unit 44 and SCTP processing unit 71 is disconnected, and NGAP messages cannot be sent or received between N2 processing unit 41 and NGAP processing unit 81. In this situation, base station device 12 stops transmitting radio waves related to this cell, and the terminal 13 previously housed in this cell will become neither connected to the core network 11 nor to the base station device 12.

[0049] On the other hand, the CU61 of the base station contains a UPF72. After the terminal 13 has been certified by the core network 11, it should be able to receive application services from the MEC server, etc., through the UPF72 of the base station device 12 without going through the transmission network. However, in reality, when the transmission network fails and the SCTP association is lost, the current base station software will execute a process that stops the base station device 12 from transmitting radio waves related to the cell. Therefore, even if the base station device 12 itself does not malfunction, the terminal 13 cannot receive application services.

[0050] To avoid this situation, it is not enough to simply stop transmitting cell-related radio waves; for example, it is necessary to ensure that communication can continue normally even if a handover occurs.

[0051] In other words, traditional mobile communication networks seek to provide low-latency communication services by implementing UPF functions in base station devices, but they also suffer from low tolerance to transmission network failures.

[0052] <First Implementation> Figure 3 This is a block diagram illustrating a functional configuration example of the core network (CN) 110 and base station device (RAN) 120 of the 5G mobile communication system according to this embodiment.

[0053] (Functional composition of the core network) The core network 110 shown in the diagram includes a Subscriber DB (UDR) 131. Network functional units other than UDR 131, such as UPF and AMF, may or may not be included in the core network 110.

[0054] (Functional composition of RAN) Figure 3 The base station apparatus 120 is configured to include a gNB CU (Central Unit) 161, a gNB DU (Distributed Unit) 162, and a gNB RU (Radio Unit) 163. Furthermore, the base station apparatus 120 will be referred to as RAN 120 as appropriate, and the gNB DU 161, gNB DU 162, and gNB RU 163 will be referred to as CU 161, DU 162, and RU 163 as appropriate.

[0055] (CU (Central Unit)) CU161 is a functional block that performs PDCP (Packet Data Convergence Protocol) processing and RRC (Radio Resource Control) processing. PDCP handles the control of DU162 and RU163 (described later), connection to the core network, and packet encryption, while RRC manages the terminal's radio resources. As an example, the functions of CU161 are implemented through software such as programs executed by a computer.

[0056] exist Figure 3 In the example, CU161 includes SBI171, MM / SM processing unit 172, C-plane processing unit 173, context preservation unit 174, SCTP processing unit 175, and U-Plane processing unit 180.

[0057] As an example, SBI171, MM / SM processing unit 172, C-plane processing unit 173, context retention unit 174, SCTP processing unit 175, and U-Plane processing unit 180 can be configured as instances generated by function calls in software that performs processing corresponding to the functions of CU161.

[0058] As described below, the functional blocks included in CU161 perform various processes corresponding to AMF, SMF, UPF, etc. That is, in this embodiment, the base station device 120 performs the processes performed by the network function units of the core network such as AMF, SMF, UPF, etc. in a conventional 5G mobile communication system.

[0059] (SBI) SBI171 is related to Figure 2 The same functional block as SBI45 is used as an interface for receiving service provision from various network functionalities connected via a service-oriented architecture. Figure 3 In the example, SBI171 connects to UDR131 of core network 110 through a service-oriented architecture.

[0060] (MM / SM Processing Department) MM / SM processing unit 172 is responsible for executing and Figure 2 The MM / SM processing unit 172 performs the same processing functions as the N1 processing unit 42. Specifically, it terminates the N1 interface and controls or manages communication with the N1 interface (e.g., sending and receiving NAS messages). Furthermore, the MM / SM processing unit 172 also performs processing related to the generation and updating of the Mobility Management Context and Session Management Context. The Mobility Management Context is mobility management information for sending and receiving NAS messages, and the Session Management Context is session management information.

[0061] The MM / SM processing unit 172 replaces AMF, SMF, etc., and performs the processing that is performed by network function units such as AMF and SMF in traditional 5G mobile communication systems.

[0062] Furthermore, the MM / SM processing unit 172 may be an instance generated by calling a function used to cause the computer to perform the aforementioned processing. Alternatively, it may be part of the computational processing performed in the software that implements the functions of CU161.

[0063] (C-plane processing department) C-plane processing unit 173 is a functional block that performs processes related to procedures such as UE registration and PDU session establishment. As an example, during registration, C-plane processing unit 173 performs processes such as decrypting the SUCI (Subscription Concealed Identifier) ​​and generating Authentication Request messages. Furthermore, during PDU session establishment, C-plane processing unit 173 performs processes such as decrypting PDU Session Establishment Request messages and allocating IP addresses.

[0064] The C-plane processing unit 173 replaces UDM, PCF, etc., and performs the processing that is performed by network function units such as UDM and PCF in traditional 5G mobile communication systems.

[0065] Furthermore, the C-plane processing unit 173 can be an instance generated by calling a function that causes the computer to perform the aforementioned processing. Alternatively, it can be part of a process executed in the software that implements the functions of CU161.

[0066] (Context reserved) Context-preserving part 174 is with Figure 2 It has the same functional block as the UE N1-MM / SM Context cache 43. That is, the context reservation section 174 is a cache memory that stores the Mobility Management Context and Session Management Context for NAS message transmission and reception.

[0067] (SCTP Processing Department) SCTP processing unit 175 is with Figure 2 The functional block corresponding to the SCTP processing unit 71. The Xn processing unit 191 is responsible for executing the functions related to... Figure 2 The Xn processing unit 82 processes the same functional blocks. (Similar to...) Figure 2 The SCTP processing unit 71 is different. Figure 3 The SCTP processing unit 175 does not have a functional block corresponding to the NGAP processing unit 81. That is, in the 5G mobile communication system according to this embodiment, there is no need for communication based on the N2 interface, i.e., communication between the RAN120 and the AMF of the core network 110.

[0068] (U-plane processing department) The U-plane processing unit 180 is a functional block that performs various processes related to data communication with UE application services, etc. For example, it may perform processes related to... Figure 2 The same functional blocks as the UPF72. Furthermore, the U-plane processing unit 180 can be configured to cause the computer to perform operations similar to those in the UPF72. Figure 2 An instance generated by the same processing function as UPF72. Alternatively, it could be part of a process executed in software that implements the CU161 functionality.

[0069] (DU (Distributed Unit) and RU (Radio Unit)) DU162 and RU163 are respectively with Figure 2DU162 and RU63 share the same functional blocks. Specifically, RU163 controls the antenna and enables radio communication with the terminal, including MIMO and beamforming control. DU162 is a functional block that performs signal modulation and demodulation, MAC layer communication control, etc.

[0070] In addition, Figure 3 In the example, only UDR131 is shown in the core network 110, but the core network 110 may also include other network functional units.

[0071] (Detailed structure of the U-plane processing unit) Figure 4 It means Figure 3 A block diagram showing a detailed configuration example of the U-plane processing unit 180. In this example, the U-plane processing unit 180 includes a Session Anchor 181, a Mobility Anchor 182, a Packet Forward 183, and an Access Control 184.

[0072] Session Anchor 181 and Mobility Anchor 182 are functional blocks for terminating PDU sessions. PacketForward 182 is a functional block that performs processing related to U-plane packet forwarding. Access Control 184 is a functional block that performs control related to session rules (described later).

[0073] (Registration) Next, the Registration in the 5G mobile communication system involved in this embodiment will be described. Figures 5-8 This diagram illustrates the registration-related processes between the terminal, base station, and core network. Figures 5-8 The image shows two base stations: RAN120A (referred to as RAN#A in the diagram) and RAN120B (referred to as RAN#B in the diagram). Both RAN#A and RAN#B have the features described above. Figure 3 The aforementioned functional components.

[0074] like Figure 5As shown, a Registration Request message ("reg.req" in the figure) is sent from the terminal connected to RAN120A, namely UE200 (referred to as UE#1 in the figure), to RAN120A. CU161A of RAN#A obtains the SUCI (Subscription Concealed Identifier) ​​contained in the Registration Request message.

[0075] In traditional 5G mobile communication systems, this processing is performed by the AMF, but in the 5G mobile communication system involved in this embodiment, it is performed by the CU161A of the RAN120A.

[0076] Next, as Figure 6 As shown, CU161A decrypts the SUCI ("decrypt SUCI" in the figure) and queries UDR131 to obtain user information ("get subscriber info" in the figure). Based on the obtained user information, CU161A determines whether the user of UE200 is a registered user. If it determines that the user is a registered user, it sends an Authentication Request message to UE200 ("auth req" in the figure). At this time, the Mobility Management context (MM#1 in the figure) is stored in the context reservation section (referred to as MM / SM Ctx) 174A of CU161A.

[0077] Additionally, a MobilityManagement context is generated and saved for each UE that sent a Registration Request message. In this implementation, "#1" is used as an index to indicate the UE#1. In this case, since a Registration Request message was sent from UE#1, MM#1 is retained in the context retention section.

[0078] In addition, such as Figure 7 As shown, UE200 sends an Authentication Response message (“auth res” in the figure) to RAN120A. Upon receiving the Authentication Response message, CU161A generates a key for encrypting the signal path used in communication with UE200 and appends the key information to MM#1.

[0079] CU161A sends a Security Mode Command message (“sec.mod.comm.” in the figure) containing key generation information to UE200. Based on the received Security Mode Command message, UE200 generates a key for encrypting the signal path and sends a Security Mode Complete message (“sec.mod.comp.” in the figure) to CU161A.

[0080] Subsequently, as Figure 8 As shown, CU161A of RAN120A updates the information stored in UDR131 of core network 110. That is, the information stored in UDR131 indicating the status of UE#1 is updated to: network authentication completed ("registered" in the figure).

[0081] Then, CU161A of RAN120A sends a Registration Accept message ("reg.accept" in the diagram) to UE200. Upon receiving the Registration Accept message, UE200 sends a RegistrationComplete message ("reg.comp." in the diagram) to RAN120A. Thus, the registration associated with UE#1 is completed.

[0082] Thus, RAN120A is connected to the core network via the transport network. CU (Central Unit) 161 has SBI (Service Based Interface) 171. When a Registration Request message is received from UE200, the UE200 receives service provision from UDR (User Data Repository) 131 of the core network 110 via SBI 171. Context retention unit 174 stores the MM context of UE200.

[0083] (PDU Session Establishment) Next, the PDU Session Establishment in the 5G mobile communication system involved in this embodiment will be described. Figure 9 and Figure 10 This diagram illustrates the processing related to PDU Session Establishment between the terminal, base station, and core network. Figure 9 and Figure 10 The image shows two base stations, RAN120A and RAN120B.

[0084] like Figure 9 As shown, UE200, having completed registration, sends a PDU SessionEstablishment Request message (“PDU sess.estab.req” in the figure) to RAN120A. The PDU SessionEstablishment Request message contains an MM / SM message. CU161A of RAN#A refers to the SM (Session Management) message in the MM / SM message to obtain the session policy of UE#1 from UDR131 (“get session policy” in the figure).

[0085] In traditional 5G mobile communication systems, this processing is performed by the SMF, but in the 5G mobile communication system involved in this embodiment, it is performed by the CU161A of the RAN120A.

[0086] The CU161A, having obtained the session policy of UE#1, processes it through the U-plane processing unit and generates a session rule for UE#1 based on that session policy ("insert sess.rule" in the diagram). Therefore, UE#1's U-plane data will be transmitted based on this session rule. At this time, the Session Management context (SM#1 in the diagram) is stored in the CU161A's context storage unit, and a PDU session for UE#1 (e.g., PDU#001) is established.

[0087] Thus, when RAN120A receives a PDU session establishment request message from UE200, it accepts the service provision from the UDR of the core network via SBI171, and the context retention unit 174 saves the SM context of UE200.

[0088] After that, as Figure 10 As shown, UE200 can use PDU#001 to send and receive data with DN (Data Network). That is, UE200 can receive application services provided by servers, etc., on the DN.

[0089] As mentioned above, CU161A of RAN120A includes the above reference. Figure 3 and Figure 4The U-plane processing unit 180 allows the UE200 to transmit and receive data with the DN without going through the core network 110. Furthermore, it is assumed that the RAN120A is connected to an MEC server, Internet eXchange, satellite communication system, etc. That is, it is assumed that the RAN120A with the U-plane processing unit 180 is connected to the DN via a connection interface.

[0090] Thus, the CU161A performs the processing corresponding to that performed by the UPF (User Plane Function) as the core network network function unit, and the RAN120A is connected to the DN (Data Network) interface.

[0091] Thus, for example, such as Figure 11 As shown, even if the transmission network fails, the UE200 can still continue to send and receive data with the DN. Figure 11 In the example, the symbol "X" in the diagram indicates that the transmission network connecting RAN120A and RAN120B has failed. Thus, in the 5G mobile communication system described in this embodiment, even if communication between the RAN and the core network is interrupted due to a transmission network failure, the UE that has completed registration can still continue to receive application services.

[0092] (Switch) Next, the handover process in the 5G mobile communication system described in this embodiment will be explained. Figure 12 This diagram illustrates the UE200 handover between RAN120A and RAN120B.

[0093] As shown in the figure, suppose UE200 moves, causing its connection target to switch from RAN#A to RAN#B. In this case, an Xn handover will occur between RAN#A and RAN#B. Furthermore, RAN#A, as the source base station device for the handover, is called the Source base station device, and RAN#B, as the target base station device for the handover, is called the Target base station device.

[0094] As described above, in the 5G mobile communication system according to this embodiment, the CU of the base station device is provided with a context retention unit. In this case, the Mobility Management context (MM#1) and Session Management context (SM#1) of UE#1 are retained in the context retention unit 174A of CU161A of RAN120A.

[0095] Therefore, when UE#1 performs an Xn handover from RAN#A to RAN#B, MM#1 and SM#1 need to be sent from RAN#A to RAN#B. The transmission and reception of MM#1 and SM#1 between RAN#A and RAN#B uses the Xn interface. That is, the transmission and reception of context such as MM#1 and SM#1 are performed directly between RAN120A and RAN120B, without going through the core network 110.

[0096] Since RAN120B has the same configuration as RAN120A, the context sent from RAN120A is stored in the context retention section 174B of RAN120B's CU161B. The contexts MM#1 and SM#1 of UE#1 stored in the context retention section 174A of RAN120A are expired (described as expired in the figure).

[0097] RAN#B continues to use the PDU session established for UE#1, namely PDU#0001, to provide a communication path between UE#1 and DN. That is, based on SM#1 sent from RAN#A, PDU#0001 is used to transmit data related to U-Plane communication with UE#1, and this data is transmitted based on the session rule.

[0098] Thus, the base station device 120A in this embodiment stores the MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device 120. When a handover of a terminal (e.g., UE200) is detected, the MM context and SM context of UE200 are forwarded to the base station device 120B, which is the handover target.

[0099] Thus, for example, such as Figure 12 As shown, switching can still occur even if the transmission network fails. Figure 12 In the example, the symbol "X" in the diagram indicates a failure in the transmission network connecting RAN120A and RAN120B to core network 110. Furthermore, UE200 is able to continue data transmission and reception with DN while switching to the target base station (RAN #B).

[0100] Thus, in the 5G mobile communication system described in this embodiment, even if communication between the RAN and the core network is interrupted due to a transmission network failure, the UE that has completed registration can still continue to receive application services. In this case, even if a handover occurs due to UE movement or other reasons, the UE can continue to receive application services.

[0101] (Registration sequence) Next, the sequence of Registration, one of the system processes executed in the 5G mobile communication system according to this embodiment, will be described. Figure 13 This is an arrow diagram illustrating the registration sequence.

[0102] In this arrow diagram, UE200 (UE#1), CU161A, CN-C170A, context reservation unit 174A, and UDR131 are shown as the entities performing each step. CU161A, CN-C170A, and context reservation unit 174A are included in RAN120A. CN-C170A is the same as the one mentioned above. Figure 3 The entities corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173 may, for example, be instances generated by the CU161A calling a specified function.

[0103] Furthermore, various messages sent from UE200 are actually acquired by CU161A via DU162A of RAN120A. Similarly, various messages sent from CU161A to RAN120A are actually sent via DU162A. For simplicity, the processing performed by DU162 is omitted here.

[0104] In this diagram, a system procedure called RRCSetup (RRC Setup) is first performed between UE200 and RAN120A. After that, the actual Registration (“begin Registration”) begins.

[0105] In step S111, UE200 sends a Registration Request message to RAN120A, and in step S131, CU161A of RAN120A receives the message.

[0106] In step S132, CU161A calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with SUCI#1 as a parameter. In step S161, for example, CN-C170A is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173.

[0107] In step S162, CN-C170A executes the HTTP method "GET Subsc.Info of UE#1". Consequently, in step S191, UDR131 retrieves the user information of UE#1.

[0108] In step S192, UDR131 provides user information (Sub#1) of UE#1 to CN-C170, and in step S163, CN-C170A obtains this information.

[0109] In step S164, CN-C170A performs "Store UE#1 context" using the protocol specified by context retention unit 174A. Therefore, in step S181, the Mobility Management context of UE#1 is retained in context retention unit 174A.

[0110] In step S165, CN-C170A generates an "Authentication Request" as the return value of the function call in step S132, and in step S133, CU161A obtains this return value.

[0111] In step S134, CU161A sends an Authentication Request message to UE200, and in step S112, UE200 receives the message.

[0112] In step S113, UE200 sends an Authentication Response message to RAN120A, and in step S135, CU161A receives the message.

[0113] In step S136, CU161A calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed together with AMF-UE-NGAP-ID#1 as a parameter. In step S166, for example, CN-C170A is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173. Afterwards, a system procedure called Authentication is performed to authenticate UE#1.

[0114] In step S167, CN-C170A generates "Security mode command" as the return value of the function call in step S136, and in step S137, CU161A obtains this return value.

[0115] In step S138, CU161A sends a Security mode command message to UE200, and in step S114, UE200 receives the message.

[0116] In step S115, UE200 sends a Security mode complete message to RAN120A, and in step S139, CU161A receives the message.

[0117] In step S140, CU161A calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with AMF-UE-NGAP-ID#1 as a parameter. In step S168, for example, CN-C170A is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173.

[0118] In step S182, the context retention unit 174A performs "Load UE#1 context" using the specified protocol. As a result, the context related to UE#1 retained by the context retention unit 174 in step S169 is read by CN-C170A.

[0119] In step S170, CN-C170A performs the handle Registration request process. This obtains the location information of the UE#.

[0120] In step S171, CN-C170A performs "Update the state of UE#1" using the protocol specified by context reservation unit 174A. Consequently, in step S193, the user information of UE#1 in UDR131 is updated.

[0121] Furthermore, in step S172, CN-C170A executes "StoreUE#1 context" using the protocol specified by context retention unit 174A. Consequently, in step S183, the updated Mobility Management context of UE#1 is retained by context retention unit 174A.

[0122] In step S173, CN-C170A generates "Registration Accept" as the return value of the function call in step S140, and in step S141, CU161A obtains this return value.

[0123] In step S142, CU161A sends a Registration Accept message to UE200, and in step S116, UE200 receives the message.

[0124] In step S117, UE200 sends a Registration Complete message to RAN120A, and in step S143, CU161A receives the message.

[0125] In step S144, CU161A calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with AMF-UE-NGAP-ID#1 as a parameter. In step S174, for example, CN-C170A is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173.

[0126] In step S175, CN-C170A performs a "handle Registration complete" process. This, for example, stops the timer used during UE#1 registration.

[0127] In step S176, CN-C170A generates the return value of the function call in step S144, and in step S145, CU161A retrieves this return value. Alternatively, in this case, the return value may be, for example, null.

[0128] Then, the registration process ends (“end Registration”).

[0129] In the 5G mobile communication system described in this embodiment, the Registration process is performed as described above.

[0130] (PDU Session Establishment Sequence) Next, the sequence of PDUsession Establishment, one of the system processes executed in the 5G mobile communication system involved in this embodiment, will be described. Figure 14 This is an arrow diagram illustrating the PDU Session Establishment sequence.

[0131] In this arrow diagram, UE200 (UE#1), CU161A, CN-C170A, Context Retention Unit 174A, U-Plane Processing Unit 180A, and UDR131 are shown as the entities performing each step. CU161A, CN-C170A, Context Retention Unit 174A, and U-Plane Processing Unit 180A are included in RAN120A.

[0132] CN-C170A is the same as the one mentioned above. Figure 3The entities corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173, for example, can be instances generated by CU161A calling a specified function. Furthermore, the U-Plane processing unit 180A is the same as described above. Figure 3 The entity corresponding to the U-Plane processing unit 180 may, for example, be an instance generated by CU161A calling a specified function.

[0133] Furthermore, various messages sent from UE200 are actually acquired by CU161A via DU162A of RAN120A. Similarly, various messages sent from CU161A to RAN120A are actually sent via DU162A. For simplicity, the processing performed by DU162 is omitted here.

[0134] Before executing PDU Session Establishment, first execute the above-mentioned reference. Figure 13 The registration process is as described above. After registration is completed, a system procedure called PFCP (Packet Forwarding Control Protocol) Association Setup is executed. After PFCP Association Setup is completed, step S201 is executed.

[0135] In step S201, UE200 performs Gen. PDU Session ID processing. This generates the ID (e.g., ID#1) of the PDU Session it intends to use.

[0136] In step S202, UE200 sends a PDU session establishment request message to RAN120A, and in step S221, CU161A of RAN120A receives the message.

[0137] In step S222, CU161A calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with AMF-UE-NGAP-ID#1 as a parameter. In step S241, for example, CN-C170A is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173.

[0138] In step S271, the context retention unit 174A performs "Load UE#1 context" using the specified protocol. As a result, in step S242, the context related to UE#1 retained by the context retention unit 174A is read by CN-C170A.

[0139] In step S243, CN-C170A executes the handle PDU session establishment request process. In this process, CN-C170A obtains the session policy of UE#1 from UDR131. Based on this, it determines whether an Uplink PDU session can be established. It is assumed that the determination is that an Uplink PDU session can be established.

[0140] Subsequently, a system procedure known as PFCP session establishment is executed. As a result, Uplink's session rule is inserted into the U-Plane processing unit 180A of CU161A.

[0141] In step S244, CN-C170A executes "Store UE#1 context" using the protocol specified by context retention unit 174A. Consequently, in step S181, the Session Management context (SM#1) of UE#1 is retained by context retention unit 174A.

[0142] In step S245, CN-C170A generates "PDU session establishment accept" as the return value of the function call in step S222, and in step S223, CU161A obtains this return value.

[0143] In step S224, CU161A sends a PDU session establishment accept message to UE200, and in step S203, UE200 receives the message.

[0144] In step S204, UE200 sends First Uplink Data to U-plane Processing Unit 180A, and in step S291, U-plane Processing Unit 180A receives the data.

[0145] In step S225, CU161A calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with AMF-UE-NGAP-ID#1 as a parameter. In step S246, for example, CN-C170A is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173.

[0146] In step S273, the context retention unit 174A performs "Load UE#1 context" using the specified protocol. As a result, the context related to UE#1 retained by the context retention unit 174A is read by CN-C170A.

[0147] In step S248, CN-C170A executes the handle PDU session resource setup request. This determines whether a downlink PDU session can be established. Assuming the determination is that a downlink PDU session can be established...

[0148] Subsequently, a system process known as PFCP session modification is executed. As a result, the downlink's session rule is inserted into the U-Plane processing unit 180A of the CU161A.

[0149] In step S249, CN-C170A executes "Store UE#1 context" using the protocol specified by context retention unit 174A. Consequently, in step S274, the Session Management context of UE#1, updated along with the execution of PFCP session modification, is retained by context retention unit 174A.

[0150] In step S250, CN-C170A generates "PDU session resource setup response" as the return value of the function call in step S225, and in step S226, CU161A obtains this return value.

[0151] In step S292, the U-plane processing unit 180A sends First Downlink Data to the UE200, and in step S205, the UE200 receives the data.

[0152] In the 5G mobile communication system described in this embodiment, the PDUSSession Establishment is performed as a system procedure as described above.

[0153] (Switch sequence) Next, the sequence of Xn Handover processes, which are related to base station handover, executed in the 5G mobile communication system involved in this embodiment, will be described. Figure 15 This is an arrow diagram illustrating the Xn Handover sequence.

[0154] In this arrow diagram, UE200 (UE#1), CU161A, U-Plane processing unit 180A, CU161B, CN-C170B, context reservation unit 174B, and U-Plane processing unit 180B are shown as the entities performing each step. CU161A and U-Plane processing unit 180A are included in RAN120A. CU161B, CN-C170B, context reservation unit 174B, and U-Plane processing unit 180B are included in RAN120B.

[0155] CN-C170B is the same as the one mentioned above. Figure 3 The entities corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173, for example, can be instances generated by CU161B calling a specified function. Furthermore, the U-Plane processing unit 180A and U-Plane processing unit 180B are the same as those mentioned above. Figure 3 The entity corresponding to the U-Plane processing unit 180 may be, for example, an instance generated by CU161A and CU161B calling a specified function.

[0156] Furthermore, various messages sent from UE200 are actually acquired by CU161A via DU162A of RAN120A. Similarly, various messages sent from CU161A to RAN120A are actually sent via DU162A. For simplicity, the processing performed by DU162A is omitted here.

[0157] When the UE200 detects a switch from RAN120A to RAN120B between its connected base stations, a system procedure called Handover preparation is performed between RAN120A and RAN120B. Subsequently, the actual Xn Handover (“begin Xn Handover”) begins.

[0158] In step S331, CU161A sends an RRC Reconfiguration message to UE200, and in step S301, UE200 receives the message.

[0159] In step S312, CU161A performs buffer downlink data processing. As a result, the data (downlink data) sent from the DN with which UE#1 is currently communicating to UE#1 is buffered.

[0160] In step S313, CU161A sends the SN (Serial Number) to RAN120B (SNstatus Transfer in the figure), and in step S331, CU161B of RAN120B receives the SN.

[0161] In step S314, CU161A forwards the UE#1 context, namely the SessionManagement context and Mobility Management context, stored in the context reservation unit 174A to CU161B. In step S331, CU161B retrieves this context. At this time, the forwarding of the UE#1 context can be performed using the Xn interface or through other methods. As an example, gRPC (Remote Procedure Calls) can be used for the forwarding of the UE#1 context.

[0162] In step S333, CU161B performs Associate UE context processing. Therefore, the SN status received in step S331 is associated with the context of UE#1 obtained in step S332.

[0163] In step S334, CU161B calls a function to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with the CP UE Context Handover parameter. In step S361, for example, CN-C170B is generated as an instance corresponding to the MM / SM processing unit 172 and the C-plane processing unit 173.

[0164] In step S362, CN-C170B performs "Restore UE context of source CN-C" using the protocol specified by context retention unit 174B. Therefore, in step S381, the context of UE#1 is retained by context retention unit 174B.

[0165] In step S363, CN-C170B generates "CP UE Context Handover response" as the return value of the function call in step S334, and in step S335, CU161B obtains this return value.

[0166] In step S336, CU161B calls functions to perform various processes related to the MM / SM processing unit 172 and the C-plane processing unit 173. Specifically, the function call (func.call) is executed with UP UE Context Handover as a parameter. In step S391, for example, the U-Plane processing unit 180B is generated as an instance.

[0167] In step S392, the U-Plane processing unit 180B performs the Insert session rule process. As a result, the Downlink's session rule is inserted into the U-Plane processing unit 180B of the CU161B.

[0168] In step S393, the U-Plane processing unit 180B generates "UP UE Context Handover response" as the return value of the function call in step S336, and in step S337, the CU161B obtains the return value.

[0169] In step S338, CU161B sends a UE Context Handover response message to RAN120A, and in step S315, CU161A of RAN120A receives the message.

[0170] In step S316, CU161A sends the downlink data buffered by the processing in step S312 to RAN120B, and in step S339, CU161B of RAN120B receives the data.

[0171] In steps S302 and S340, the Random Access Procedure is executed by UE200 and RAN120B’s CU161B.

[0172] Then, Xn Handover ends ("end Xn Handover").

[0173] In the 5G mobile communication system described in this embodiment, Xn Handover is performed in the manner described above.

[0174] (Effects of the first implementation method) As described above, according to this embodiment, since the RAN120 includes a U-plane processing unit 180, the UE200 can directly send and receive data with the DN without going through the core network 110.

[0175] Furthermore, in this embodiment, during registration, communication between RAN120 and core network 110 is unnecessary except for communication with UDR131 to obtain and update user information. Similarly, for processing related to PDU SessionEstablishment, communication between RAN120 and core network 110 is also unnecessary except for communication with UDR131 to obtain session policy.

[0176] Therefore, according to this embodiment, even if communication between the RAN and the core network is interrupted due to a transmission network failure, the UE that has completed registration can still continue to receive application services. Thus, for example, the advantages of MEC can be fully utilized.

[0177] Furthermore, according to this embodiment, all processing related to Xn handover is also performed by RAN120. Therefore, according to this embodiment, even if communication between the RAN and the core network is interrupted due to a transmission network failure, the UE can still be moved freely.

[0178] Furthermore, in this embodiment, as described above, there is no need for communication based on the N2 interface, i.e., communication between RAN120 and the AMF of the core network 110. Therefore, for example, when multiple UEs simultaneously restart communication due to a transmission network failure, it is possible to avoid an increase in the core network processing load caused by N2 interface communication being concentrated in the core network's AMF. That is, it is possible to suppress the problem of increased core network processing load and congestion caused by transmission network failures in the past.

[0179] Thus, according to this embodiment, a technology can be realized that provides a communication service with high tolerance to transmission network failures.

[0180] <Second Implementation> As described above, RAN120 enables CU161 to perform processing tasks that are typically performed by the core network functions such as AMF, SMF, and UPF in traditional 5G mobile communication systems. Therefore, the functional sharing between the base station equipment and the core network can be flexibly determined.

[0181] That is, in the first embodiment described above, only UDR131 in the network function unit of core network 110 is configured to be called from RAN120 via SBI171, but other network function units may also be further configured to be called from RAN120 via SBI171.

[0182] For example, in the network functional units of core network 110, in addition to UDR131, UDM (Unified Data Management) can also be configured to be invoked from RAN120 via SBI171. Alternatively, for example, all processing performed by network functional units other than UDR in the core network can be performed by CU161.

[0183] Alternatively, for example, network function units that process highly confidential information such as user personal information can be configured in the core network 110 and called from the RAN 120 via the SBI 171, while processing related to other network function units is performed by the CU 161.

[0184] Alternatively, the UPF and RAN120 can be configured separately. For example, a server serving as the UPF can be configured within the station where RAN120 is configured, and RAN120 and UPF can be connected via LAN or the like. In this case, the U-Plane processing unit 180 may not be included in RAN120's CU161.

[0185] <Software-based implementation example> The aforementioned base station device 120 can be implemented by a program that enables a computer to function, i.e., a program that enables a computer to function as a base station device 120. In this case, the base station device 120 includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory) as hardware for executing the aforementioned program. Figure 16 An example of such a computer is shown.

[0186] The computer 500 includes at least one processor 501 and at least one memory 502. The memory 502 contains a program 520 for enabling the computer 500 to function as a base station device 120. In the computer 500, the processor 501 reads the program 520 from the memory 502 and executes it, thereby implementing various functions of the base station device 120.

[0187] As a processor 501, for example, a CPU (Central Processing Unit), GPU (Graphics Processing Unit), DSP (Digital Signal Processor), MPU (Micro Processing Unit), FPU (Floating Point Number Processing Unit), PPU (Physics Processing Unit), microcontroller, or a combination thereof can be used.

[0188] As a storage device 502, for example, flash memory, HDD (Hard Disk Drive), SSD (Solid State Drive), or a combination thereof can be used.

[0189] Furthermore, the computer 500 may also include RAM (Random Access Memory) for expanding or temporarily storing various data during program execution 520. Additionally, the computer 500 may further include a communication interface for sending and receiving data with other devices. Moreover, the computer 500 may further include input / output interfaces for connecting input / output devices such as a keyboard, mouse, monitor, and printer.

[0190] Furthermore, the program 520 used to enable the computer 500 to operate as the base station device 120 can be recorded in a non-transitory tangible recording medium 530 readable by the computer 500. Such a recording medium 530 can be, for example, magnetic tape, disk, card, semiconductor memory, or programmable logic circuit. The computer 500 can obtain the program 520 via this recording medium 530.

[0191] Furthermore, the program 520 used to enable the computer 500 to operate as a base station device 120 can be transmitted via a transmission medium. Such a transmission medium could be, for example, a communication network or radio waves. The computer 500 can also acquire the program 520 via this transmission medium.

[0192] Furthermore, some or all of the functions of the base station device 120 can also be implemented using logic circuits. For example, integrated circuits that form the logic circuits that function as the aforementioned control blocks are also included within the scope of this disclosure. Alternatively, the functions of the aforementioned control blocks can also be implemented using a quantum computer, for example.

[0193] Furthermore, although examples of applying this disclosure to 5G communication systems have been described in the above embodiments, this disclosure can also be applied to 6G and later communication systems, as long as they are communication systems that can be constructed in units of NF.

[0194] The schemes described above in this disclosure, by leveraging the aforementioned effects, can contribute to achieving Goal 9 of the Sustainable Development Goals (SDGs), "Industry, Innovation and Infrastructure".

[0195] Furthermore, this disclosure is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of this disclosure.

[0196] 〔Summarize〕 The base station device involved in this disclosure is a base station device for a mobile communication network, comprising: a context retention unit that stores MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device; and a context forwarding unit that, when a terminal handover is detected, forwards the MM context and SM context of the terminal to the base station device that serves as the handover target.

[0197] Regarding the base station device involved in Scheme 2 of this disclosure, in Scheme 1 above, when the terminal requests to start a system process, the CU (Central Unit) of the base station device executes a process corresponding to the process executed by the network function unit of the core network by calling a function for causing the computer to execute the system process.

[0198] Regarding the base station device involved in Scheme 3 of this disclosure, in Scheme 2 above, the CU of the base station device performs processing corresponding to the processing performed by the UPF (User Plane Function) as the core network network function unit, and the base station device is connected to the connection interface of DN (Data Network).

[0199] Regarding the base station device involved in Scheme 4 of this disclosure, in Scheme 2 or 3 above, it is connected to the core network via a transmission network. The CU has an SBI (Service Based Interface). When a Registration Request message is received from the terminal, the CU receives the service provision from the UDR (User Data Repository) of the core network via the SBI. The context retention unit stores the MM context of the terminal.

[0200] Regarding the base station apparatus involved in Scheme 5 of this disclosure, in Scheme 4 above, when a PDUsession establishment request message is received from the terminal, the service is provided from the UDR of the core network via the SBI, and the context retention unit stores the SM context of the terminal.

[0201] Regarding the base station device involved in Scheme 6 of this disclosure, in any of the above schemes 1 to 4, the handover is an Xn handover.

[0202] Regarding the base station apparatus involved in Scheme 7 of this disclosure, in Scheme 6 above, the context forwarding unit uses gRPC to forward the MM context and SM context of the terminal to the base station apparatus that serves as the handover target.

[0203] The base station device handover method disclosed in Scheme 8 is a base station device handover method for mobile communication networks. It stores the MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device. When a terminal handover is detected, the MM context and SM context of the terminal are forwarded to the base station device that serves as the handover target.

[0204] The program involved in this disclosure 9 enables a computer to function as a base station device in a mobile communication network. The base station device includes: a context retention unit that stores MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device; and a context forwarding unit that, when a terminal handover is detected, forwards the MM context and SM context of the terminal to the base station device that is the handover target.

[0205] Symbol Explanation 110 Core Network 120 base station equipment 131 UDR 161 CU 162 DU 163 RU 170 CN-C 171 SBI 172 MM / SM Processing Department 173 C-Plane (Control Plane) Processing Department 174 Context-Preserving Section 175 SCTP Processing Department 180 U-Plane (User Interface) Processing Department 181 Session Anchor 182 Mobility Anchor 183 Packet Forward 184 Access Control

Claims

1. A base station device, which is a base station device for a mobile communication network, comprising: The context retention unit stores the MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device; and When a terminal handover is detected, the context forwarding unit forwards the MM context and SM context of the terminal to the base station device that serves as the handover target.

2. The base station apparatus according to claim 1, wherein, When the terminal requests to start a system process, the CU (Central Unit) of the base station device executes a process corresponding to that executed by the network function unit of the core network by calling a function required for the computer to perform the system process.

3. The base station apparatus according to claim 2, wherein, The CU of the base station device performs processing corresponding to the processing performed by the UPF (User Plane Function), which is a core network network function unit. The base station device is connected to the DN (Data Network) connection interface.

4. The base station device according to claim 2 or 3, wherein it is connected to the core network via a transmission network. The CU has an SBI (Service Based Interface). When a Registration Request message is received from the terminal, the service is provided from the UDR (User Data Repository) of the core network via the SBI, and the context retention unit stores the MM context of the terminal.

5. The base station apparatus according to claim 4, wherein, When a PDU sessionestablishment request message is received from the terminal, the terminal accepts the service provision from the UDR of the core network via the SBI, and the context retention unit stores the SM context of the terminal.

6. The base station apparatus according to any one of claims 1 to 4, wherein, The switching is an Xn switching.

7. The base station apparatus according to claim 6, wherein, The context forwarding unit uses gRPC to forward the terminal's MM context and SM context to the base station device that serves as the handover target.

8. A base station device handover method, which is a base station device handover method for mobile communication networks. The MM (Mobility Management) context and SM (Session Management) context are stored in each terminal connected to the base station device. When a terminal handover is detected, the MM context and SM context of the terminal are forwarded to the base station device that serves as the handover target.

9. A program that enables a computer to function as a base station device for a mobile communication network. The base station device has the following features: The context retention unit stores the MM (Mobility Management) context and SM (Session Management) context in each terminal connected to the base station device; and When a terminal handover is detected, the context forwarding unit forwards the MM context and SM context of the terminal to the base station device that serves as the handover target.

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