The mobility management and connection management process in 5GC focuses on Connection Management (CM) and Registration Management (RM).

Registration Management (RM)

The Registration Management state (RM) of a UE can either be RM-Registered or RM-Deregistered. This is akin to the EMM state used in LTE.

RM-Deregistered Mode

From the Core Network’s perspective (Our AMF) a UE that is in RM-Deregistered state has no valid location information in the AMF for that UE.
The AMF can’t page it, it doesn’t know where the UE is or if it’s even turned on.

From the UE’s perspective, being in RM-Deregistered state could mean one of a few things:

• UE is in an area without coverage
• UE is turned off
• SIM Card in the UE is not permitted to access the network

In short, RM-Deregistered means the UE cannot be reached, and cannot get any services.

RM-Registered Mode

From the Core Network’s perspective (the AMF) a UE in RM-Registered state has sucesfully registered onto the network.

The UE can perform tracking area updates, period registration updates and registration updates.

There is a location stored in the AMF for the UE (The AMF knows at least down to a Tracking Area Code/List level where the UE is).

The UE can request services.

Connection Management (CM)

Connection Management (CM) focuses on the NAS signaling connection between the UE and the AMF.

To have a Connection Management state, the Registration Management procedure must have successfully completed (the UE being in RM-Registered) state.

A UE in CM-Connected state has an active signaling connection on the N1 interface between the UE and the AMF.

CM-Idle Mode

In CM-Idle mode the UE has no active NAS connection to the AMF.

UEs typically enter this state when they have no data to send / recieve for a period of time, this conserves battery on the UE and saves network resources.

If the UE wants to send some data, it performs a Service Request procedure to bring itself back into CM-Connected mode.

If the Network wants to send some data to the UE, the AMF sends a paging request for the UE, and upon hearing it’s identifier (5G-S-TMSI) on the paging channel, the UE performs the Service Request procedure to bring itself back into CM-Connected mode.

CM-Connected Mode

In CM-Connected mode the UE has an active NAS connection with the AMF over the N1 interface from the UE to the AMF.

When the access network (The gNodeB) determines this state should change (typically based on the UE being idle for longer than a set period of time) the gNodeB releases the connection and the UE transitions to CM-Idle Mode.