3GPP 24.501 v20.0.0 — the document's own text

5.5.1.1 General

Taught in 6. Getting on the network, and getting a connection through it (CT1 — the group that writes what your phone says, in depth), 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview), 2. What the group produces, and what one of its documents looks like (CT1 — the group that writes what your phone says, quick start).

The registration procedure is always initiated by the UE and used for initial registration as specified in clause 5.5.1.2.2 or mobility and periodic registration update as specified in clause 5.5.1.3.2.

When the UE needs to initiate registration over both 3GPP access and non-3GPP access in the same PLMN (e.g. the 3GPP access and the selected N3IWF are located in the same PLMN), the UE:

in 5GMM-REGISTERED-INITIATED over 3GPP access shall not initiate registration over non-3GPP access; or
in 5GMM-REGISTERED-INITIATED over non-3GPP access shall not initiate registration over 3GPP access.

NOTE 1: To which access (i.e. 3GPP access or non-3GPP access) the UE initiates registration first is up to UE implementation.

When the UE is registered with a PLMN over a non-3GPP access, the AMF and the UE maintain:

registration state and state machine over non-3GPP access;
5G NAS security context;
5G-GUTI;
registration area for non-3GPP access, which is associated with a single TAI; and
non-3GPP de-registration timer in the UE and non-3GPP implicit de-registration timer in the AMF.

A registration attempt counter is used to limit the number of subsequently rejected registration attempts. The registration attempt counter shall be incremented as specified in clause 5.5.1.2.7 or clause 5.5.1.3.7. Depending on the value of the registration attempt counter, specific actions shall be performed. The registration attempt counter shall be reset when:

the UE is powered on;
a USIM is inserted;
a registration procedure is successfully completed;
an EPS attach, combined EPS attach procedure, a normal or periodic tracking area updating or a combined tracking area updating procedure is successfully completed in S1 mode and the UE is operating in single-registration mode. In this case, the UE shall reset the registration attempt counter for 3GPP access;

NOTE 2: The registration attempt counter for non-3GPP access is not impacted by the EPS attach and the combined EPS attach procedure.

a registration procedure is rejected with cause #11, #12, #13, #15, #27, #31, #62, #72, #73, #74, #75, #76, #77 or #78;
a registration procedure is rejected with cause #3, #6 or #7, the REGISTRATION REJECT message is received without integrity protection and the counter for "SIM/USIM considered invalid for GPRS services" events has a value less than a UE implementation-specific maximum value.
a network initiated de-registration procedure is completed with cause #11, #12, #13, #15, #27; #62, #72, #74, #75, #76, #77 or #78; or
a new PLMN or SNPN is selected.

Additionally, the registration attempt counter shall be reset when the UE is in substate 5GMM-DEREGISTERED.ATTEMPTING-REGISTRATION or 5GMM-REGISTERED.ATTEMPTING-REGISTRATION-UPDATE, and:

the current TAI is changed;
timer T3502 expires; or
timer T3346 is started.

When the registration attempt counter is reset, the UE shall stop timer T3519 if running, and delete any stored SUCI.

The UE implementation specific attempt counter for lower layer failure defined in clause 5.5.1.2.7 case d1) clause 5.5.1.3.7 case d1) and shall be reset when:

the UE is powered on;
a USIM is inserted;
the UE enters 5GMM-CONNECTED mode;
a registration procedure is successfully completed;
timer T3502 expires;
an EPS attach, combined EPS attach procedure, a normal or periodic tracking area updating or a combined tracking area updating procedure is successfully completed in S1 mode and the UE is operating in single-registration mode. In this case, the UE shall reset the registration attempt counter for 3GPP access; or
a new suitable cell of the selected PLMN or SNPN or a new PLMN or a new SNPN is selected in N1 mode.

The lower layers indicate to NAS whether the network supports emergency services for the UE in limited service state (see 3GPP TS 38.331 [30]). This information is taken into account when deciding whether to initiate an initial registration for emergency services.

The counter of attempts to stop T3511 and the counter of attempts to register while T3502 running shall be reset when the timer T3502 is started or when the registration attempt counter is reset.