CM (Connection Management) Status of 5G Terminals

Two CM (Connection Management) statuses are used in the 5G (UE) system to reflect the NAS signaling connection between the terminal (UE) and the AMF. They are:

• CM-IDLE
• CM-CONNECTED

I. 5G Terminal (UE) Connection Status When the terminal accesses 3GPP and non-3GPP systems, its CM status is independent of each other. That is, one CM status can be in CM-IDLE state, while the other CM status can be in CM-CONNECTED state.

II. CM-IDLE State When in CM-IDLE:

2.1 The 5G terminal (UE)

• has not established a NAS signaling connection with the AMF through N1; at this time, the UE performs cell selection/cell reselection according to TS 38.304[50] and PLMN selection according to TS 23.122[17].
• The UE has no AN signaling connection, N2 connection, or N3 connection.
• If the UE is simultaneously in CM-IDLE and RM-REGISTERED states (unless otherwise specified in Clause 5.3.4.1), the UE shall:
• Respond to paging by executing the service request procedure (see Clause 4.2.3.2 of TS 23.502 [3]), unless the UE is in MICO mode (see Clause 5.4.1.3);
• Execute the service request procedure when the UE has uplink signaling or user data to send (see Clause 4.2.3.2 of TS 23.502 [3]). LADN has specific conditions (see Clause 5.6.5).

2.2 When the UE state in the AMF is RM-REGISTERED, the terminal information required to initiate communication with the UE shall be stored. The AMF shall be able to retrieve the stored information required to initiate communication with the UE using 5G-GUTI. ---- In 5GS, paging is not required using the UE's SUPI/SUCI.

2.3 During AN signaling connection establishment, the UE shall provide 5G-S-TMSI as part of the AN parameters in accordance with TS 38.331[28] and TS 36.331[51]. When the UE establishes an AN signaling connection with the AN (entering the RRC_CONNECTED state via 3GPP access, establishing a UE-N3IWF connection via untrusted non-3GPP access, or establishing a UE-TNGF connection via trusted non-3GPP access), the UE shall enter the CM-CONNECTED state. Sending an initial NA message (registration request, service request, or deregistration request) initiates the transition from CM-IDLE to CM-CONNECTED state.

2.4 When the AMF is in the CM-IDLE or RM-REGISTERED state, the AMF should execute a network-triggered service request procedure when it needs to send signaling or mobile terminal data to the UE. This is done by sending a paging request to the UE (see Section 4.2.3.3 of TS 23.502[3]), provided that the UE is not unable to respond due to MICO mode or mobility restrictions. Among them:

• When the AN and AMF establish an N2 connection for the UE, the AMF should enter the CM-CONNECTED state.
• Receiving an initial N2 message (e.g., N2 INITIAL UE MESSAGE) will trigger the AMF to transition from the CM-IDLE state to the CM-CONNECTED state.
• When the UE is in the CM-IDLE state, the UE and AMF can optimize the UE's power efficiency and signaling efficiency, for example, by activating MICO mode (see Section 5.4.1.3).

III. CM-CONNECTED State The UE in the CM-CONNECTED state establishes a NAS signaling connection with the AMF through N1. NAS signaling connections utilize the RRC connection between the UE and the NG-RAN, and the NGAP UE association between the AN and the AMF, to achieve 3GPP access. The UE can be in the CM-CONNECTED state, but its NGAP UE association is not bound to any TNLA between the AN and the AMF.

For a UE in the CM-CONNECTED state, the AMF can decide to release the NAS signaling connection with the UE after the NAS signaling procedure is completed.

3.1 In the CM-CONNECTED state, the UE should:

• Enter the CM-IDLE state when the AN signaling connection is released (e.g., entering the RRC_IDLE state via 3GPP access, or when the UE detects the release of the UE-N3IWF connection via an untrusted non-3GPP access, or the release of the UE-TNGF connection via a trusted non-3GPP access).

3.2 When the UE's CM state in the AMF is CM-CONNECTED, the AMF shall:

--When the UE's logical NGAP signaling connection and N3 user plane connection are released after the AN release procedure specified in TS 23.502[3] is completed, the UE shall enter the CM-IDLE state.

--The AMF may maintain the UE's CM state in the CM-CONNECTED state until the UE is deregistered from the core network.

3.3 A UE in the CM-CONNECTED state may be in the RRC_INACTIVE state, see TS 38.300[27]. When the UE is in the RRC_INACTIVE state, the following rules apply:

• - UE reachability is managed by the RAN and auxiliary information is provided by the core network;
• - UE paging is managed by the RAN;
• - The UE listens for paging using its CN (5G S-TMSI) and RAN identifier.