A UE/user needs to register with the network to receive services that requires registration.

5GS Registration Management states

Two RM states are used in the UE and the AMF that reflect the registration status of the UE in the selected PLMN.

1. RM-DEREGISTERED.
2. RM-REGISTERED

RM-DEREGISTERED state

• In the RM‑DEREGISTERED state, the UE is not registered with the network. The UE context in AMF holds no valid location or routing information for the UE so the UE is not reachable by the AMF. However, some parts of UE context may still be stored in the UE and the AMF e.g. to avoid running an authentication procedure during every Registration procedure.

In the RM-DEREGISTERED state :

In the RM‑REGISTERED state, the UE is registered with the network. In the RM-REGISTERED state, the UE can receive services that require registration with the network.

In the RM-REGISTERED state, the UE shall

1. Perform Mobility Registration Update procedure if the current TAI of the serving cell (see TS 37.340 [31]) is not in the list of TAIs that the UE has received from the network in order to maintain the registration and enable the AMF to page the UE;
2. Perform Periodic Registration Update procedure triggered by expiration of the periodic update timer to notify the network that the UE is still active.
3. Perform a Mobility Registration Update procedure to update its capability information or to re-negotiate protocol parameters with the network.
4. Perform Deregistration procedure (see TS 23.502 [3] clause 4.2.2.3.1), and enter RM-DEREGISTERED state, when the UE needs to be no longer registered with the PLMN. The UE may decide to deregister from the network at any time.
5. Enter RM-DEREGISTERED state when receiving a Registration Reject message or a Deregistration message. The actions of the UE depend upon the ’cause value’ in the Registration Reject or Deregistration message. See TS 23.502 [3] clause 4.2.2.

When the UE RM state in the AMF is RM-REGISTERED, the AMF shall:

1. Perform Deregistration procedure (see TS 23.502 [3] clauses 4.2.2.3.2, 4.2.2.3.3), and enter RM-DEREGISTERED state for the UE, when the UE needs to be no longer registered with the PLMN. The network may decide to deregister the UE at any time
2. Perform Implicit Deregistration at any time after the Implicit Deregistration timer expires. The AMF shall enter RM-DEREGISTERED state for the UE after Implicit Deregistration;
3. When applicable, accept or reject Registration Requests or Service Requests from the UE.

5GS Registration Management State models