How are CM-Idle and CM-Connected 5G terminals different?

Whenever a terminal (UE) is ready to make a call or transmit data in a mobile communication system, it needs to connect with the core network first, which is due to the fact that the system temporarily removes the connection between the UR and the core network after the first time it is powered on or in an idle state for a period of time; the connection and management of the access connection between the terminal (UE) and the core network (5GC) in 5G (NR) is handled by the AMF unit, whose connection management (CM) is used to establish and release the control plane signaling connection between the UE and the AMF.

I. CM State Describes the signaling connection management (CM) state between the terminal (UE) and the AMF, which is mainly used for transmitting NAS signaling messages; for this purpose 3GPP defines two connection management states for the UE and the AMF respectively:

• CM-Idle (Connection Management in Idle state)
• CM-Connected (Connected state connection management)

CM-Idle and CM-Connected states are maintained by UE and AMF through NAS layer;

II.CM Characteristics Depending on the connection between the UE and the AMF. where:

• CM-Idle state the mobile equipment (UE) has not entered the signaling transmission state (RRC-Idle) with the core node (AMF). when the UE is in CM-Idle state it can move between different cells through mobile control according to the cell reselection principle.
• CM-Connected state the UE establishes a signaling connection (RRC-Connected and RRC-Inactive) with the AMF. the UE and the AMF can establish a connection based on the N1 (logical) interface will enter the CM-Connected state for the following intra interactions:

1. RRC signaling between the UE and the gNB
2. N2-AP signaling between the gNB and the AMF.

III.CM state transition The connected state of UE and AMF can be initiated by UE or AMF respectively, as shown in the following figure:

3.1 UE Initiated State Transition Once the RRC connection is established the UE state will enter CM-Connected; within the AMF once the established N2 context is received the UE state will enter CM-Connected; this can be performed by a registration request and a service request; where:

• When the UE is powered on for the first time, it selects the best gNB according to the cell selection process and sends a registration request to initiate the RRC connection setup signaling to the gNB and sends the N2 signaling to the AMF. The registration request triggers the transition from CM-Idle to CM-Connected.
• When the UE is in CM-Idle state and must send uplink data, the UE triggers a Service Request NAS message to the AMF and changes the CM-Idle to CM-Connected.

3.2 Network initiated state transition When there is downlink data to be transmitted to the CM-Idle UE, the network MUST use paging to initiate the state transition process. Paging triggers the UE to establish an RRC connection and send a Request NAS message to the AMF. The request triggers the N2 signaling connection to move the UE to CM-Connected.

When the signaling connection is released or the signaling connection fails, the UE can move from CM-Connected to CM-Idle.