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  During the initial registration or mobility registration update process, the 5G terminal (UE) will initiate a connection with the network, which is the MICO (Mobile Initiated Connection Only) connection mode; where:   I. The MICO mode allows the AMF to determine whether to allow the UE to use MICO mode and indicate this to the UE during the registration process, based on local configuration, expected UE behavior and/or network configuration parameters (if available from the UDM), UE indicated preferences, UE subscription information, and network policies, or any combination thereof.   If NWDAF is deployed, the AMF may also use UE mobility and/or UE communication analysis data generated by NWDAF (see TS 23.288 [86]) to determine MICO mode parameters. If the UE does not indicate its preference for MICO mode during the registration process, the AMF should not activate MICO mode for that UE. II. The UE and AMF renegotiate the MICO mode during each subsequent registration process; when the UE is in the CM-CONNECTED state, the AMF can deactivate the MICO mode by triggering a mobility registration update process; this process is performed through the UE configuration update process as described in Section 4.2.4 of TS 23.502 [3]; where:   During the registration process, the AMF assigns a registration area to the UE. When the AMF indicates that the UE is in MICO mode, the registration area is not limited by the paging area size. If the AMF's service area covers the entire PLMN, the AMF may decide to provide the UE with a "full PLMN" registration area based on local policies and user information. In this case, re-registration due to mobility within the same PLMN is not applicable. If mobility restrictions are applied to a UE in MICO mode, the AMF needs to assign an allowed area/disallowed area to the UE as specified in Section 5.3.4.1. When the AMF indicates MICO mode to the UE, if the UE's CM state in the AMF is CM-IDLE, the AMF always considers the UE unreachable. For a UE in MICO mode and whose CM state in the AMF is CM-IDLE, the AMF will reject any downlink data transmission requests and provide the corresponding rejection reason. For NAS-based MT-SMS, the AMF will notify the SMSF that the UE is unreachable and then execute the mobile terminal SMS sending failure handling procedure described in TS 23.502 [3, Section 4.13.3.9]. III. Delayed Location Services: The AMF will enable delayed location services, allowing mobile terminal data or signaling communication only for UEs in MICO mode and only when they are in the CM-CONNECTED state.   IV. CM-IDLE State: UEs in CM-IDLE state do not need to listen for paging. UEs in MICO mode can stop any access layer procedures in the CM-IDLE state until the UE initiates a transition from CM-IDLE to CM-CONNECTED due to one of the following trigger conditions: The UE undergoes a change (e.g., configuration change) requiring an update of its registration information in the network. The periodic registration timer expires. MO signaling is pending (e.g., an SM procedure has been initiated). If the registration area assigned to a UE in MICO mode is not the "all PLMNs" registration area, the UE will determine whether it is within that registration area when it has MO data or MO signaling. If the UE is not within the registration area, before initiating MO data or MO signaling,   V. UE and Emergency Services: The UE will perform a mobility registration update; a UE initiating emergency services shall not indicate MICO preference during the registration process. When MICO mode is activated in the UE, the UE and AMF locally disable MICO mode after the successful completion of the emergency service PDU session establishment process. The UE and AMF shall not enable MICO mode until the AMF accepts the use of MICO mode during the next registration process. To enable emergency callback, the UE should wait for a UE implementation-specific duration after the emergency PDU session release before requesting the use of MICO mode.   VI. MT Mode: To achieve energy saving for mobile terminal (UE) MT reachability (e.g., for cellular IoT), enhancements to the MICO mode are specified in the following clauses: MICO mode with extended connection time; MICO mode with active time; MICO mode with periodic registration timer control.

  Reachability management in the 5G (NR) system is responsible for detecting whether a UE is reachable and providing the UE's location (i.e., access node) so that the network can easily access the terminal (UE); this can be achieved through paging the UE and (UE) location tracking; UE location tracking includes: registration area tracking (i.e., UE registration area update) and reachability tracking (i.e., UE periodic registration area update); the reachability management function can be located in the 5GC (CM-IDLE state) or NG-RAN (CM-CONNECTED state).   I. CM-IDLE reachability is the result of negotiation between the UE and the AMF during the registration process. UE reachability in the CM-IDLE state is divided into two types:   1. UE Data Transmission Reachability   The network determines the UE's location based on the tracking area list granularity. Applicable to paging procedures. Applicable to CM-CONNECTED and CM-IDLE states supporting mobile-initiated data and mobile terminal data.   2. MICO (Mobile Initiated Connection Only) mode:   Applicable to CM-CONNECTED and CM-IDLE states supporting mobile-initiated data. Mobile terminal data is only supported when the UE is in the CM-CONNECTED state.   II.When a UE in the RM-REGISTERED state enters the CM-IDLE state, it starts a periodic registration timer based on the periodic registration timer value received from the AMF during the registration process; during this period,   the AMF assigns a periodic registration timer value to the UE based on local policies, subscription information, and information provided by the UE. After the periodic registration timer expires, the UE should perform periodic registration. If the UE moves out of network coverage when its periodic registration timer expires, the UE should perform the registration procedure when it returns to coverage. The AMF runs a mobile reachability timer for the UE. When the CM state of a UE in the RM-REGISTERED state changes to CM-IDLE, this timer starts with a value greater than the UE's periodic registration timer. If the AMF receives elapsed time from the RAN when the RAN initiates UE context release and indicates that the UE is unreachable, the AMF should infer the mobile reachability timer value based on the elapsed time received from the RAN and the normal mobile reachability timer value. If the UE CM state in the AMF changes to CM-CONNECTED state, the AMF stops the mobility reachability timer. If the mobility reachability timer expires, the AMF determines that the UE is reachable. However, the AMF does not know the duration of UE unreachability, so the AMF should not immediately deregister the UE. Instead, after the mobility reachability timer expires, the AMF should clear the PPF (Paging Proceed Flag) and start an implicit deregistration timer, which should have a relatively large value.   III.CM-CONNECTED: If the UE CM state in the AMF changes to CM-CONNECTED state, the AMF should stop the implicit deregistration timer and set the PPF (If the UE CM state in the AMF is CM-IDLE, and the UE is in MICO mode - see Section 5.4.1.3, the AMF considers the UE to be always unreachable).   If the PPF is not set, the AMF will not page the UE and should reject any requests to send downlink signaling or data to that UE. If the implicit deregistration timer expires before the UE contacts the network, the AMF implicitly deregisters the UE.   As part of a specific access (3GPP or non-3GPP) deregistration, the AMF should request the relevant SMF of the UE to release the PDU sessions established on that access.

I. The RRC_INACTIVE state is a fundamental architectural innovation in 5G (NR), designed to address the critical latency and signaling overhead issues that plagued LTE networks. In 4G (LTE), frequent transitions between the RRC_IDLE and RRC_CONNECTED states of the terminal (UE) caused a huge network signaling load and introduced latency penalties during service recovery, which is particularly problematic for modern smartphone usage patterns characterized by frequent small data transmissions. The RRC_INACTIVE state bridges the gap between fully connected and fully disconnected states, enabling fast service recovery while maintaining power efficiency and reducing core network signaling. II. The need for RRC_INACTIVE stems from the limitations of 4G (LTE) and the requirements of 5G: In 4G (LTE) networks, prolonged user inactivity triggers a transition to the RRC_IDLE state to save power. However, restoring to the RRC_CONNECTED state requires re-establishing the RRC connection, which involves a large amount of RRC signaling interaction and introduces significant latency. In modern mobile applications, terminals frequently generate bursts of small data packets (such as social media updates, instant messages, and IoT sensor data), leading to repeated "IDLE-CONNECTED-IDLE" state transitions, burdening both the radio interface and the core network. III. The advantages of RRC_INACTIVE are threefold: Reduced signaling overhead: Both the UE and the gNB store the UE's access stratum (AS) context, so a complete RRC re-establishment process is not required during service recovery. Reduced transition latency: The state transition from INACTIVE to CONNECTED is much faster than from IDLE to CONNECTED because the radio bearer configuration is retained. Maintained core network connectivity: The UE remains in the CM-CONNECTED state relative to the 5G core network (5GC), meaning that the UE's connection on the NG interface between the gNB and the AMF remains active. IV. RRC State Architecture: A 5G (NR) terminal (UE) can be in three different RRC states: RRC_IDLE: The RRC connection does not exist; the UE performs cell selection/reselection and listens for paging. Both the UE and the network's AS context have been released. RRC_INACTIVE: The RRC connection is suspended, and the AS context is retained; the UE monitors paging within the configured RAN Notification Area (RNA), and its behavior is similar to the IDLE state to save power. RRC_CONNECTED: The RRC connection is active and dedicated resources have been allocated; the UE exchanges user plane and control plane data. V. Terminal (UE) Connection Management: In the 5G system, terminal (UE) connection management in the NAS (Non-Access Stratum) interacts with RRC in two states; these are: CM-IDLE: Corresponds to the RRC_IDLE state; there is no NG connection between the gNB and AMF; CM-CONNECTED: Corresponds to the RRC_CONNECTED and RRC_INACTIVE states; the NG signaling connection between the gNB and AMF remains active.