School of Specs 24.501v20.0.0

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

5.3.1.1 Establishment of the N1 NAS signalling connection

Taught in 10. Who CT1 writes to, and what the letters count (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).

When the UE is in 5GMM-IDLE mode over 3GPP access and needs to transmit an initial NAS message, the UE shall request the lower layer to establish an RRC connection. Upon indication from the lower layers that the RRC connection has been established, the UE shall consider that the N1 NAS signalling connection over 3GPP access is established and enter 5GMM-CONNECTED mode over 3GPP access.

When the UE is in 5GMM-IDLE mode over non-3GPP access, and the UE receives an indication from the lower layers of non-3GPP access, that the access stratum connection is established between the UE and the network, the UE shall send an initial NAS message, consider the N1 NAS signalling connection is established and enter 5GMM-CONNECTED mode over non-3GPP access.

Initial NAS messages are:

  • REGISTRATION REQUEST message;
  • DEREGISTRATION REQUEST message;
  • SERVICE REQUEST message; and
  • CONTROL PLANE SERVICE REQUEST.

If the UE is capable of both N1 mode and S1 mode and lower layers provide an indication that the current E-UTRA cell is connected to both EPC and 5GCN, for the routing of the REGISTRATION REQUEST message during the initial registration procedure to the appropriate core network (EPC or 5GCN), the UE NAS provides the lower layers with the selected core network type information.

For the routing of the initial NAS message to the appropriate AMF, if the UE holds a 5G-GUTI or 4G-GUTI, the UE NAS provides the lower layers with the UE identity according to the following rules:

  • if the registration procedure for mobility and periodic registration update was triggered due to the last CONFIGURATION UPDATE COMMAND message containing the Configuration update indication IE with the Registration bit set to "registration requested" and including:
    • no other parameters;
    • one or both of the Allowed NSSAI IE and the Configured NSSAI IE; or
    • the Network slicing indication IE with the Network slicing subscription change indication set to "Network slicing subscription changed";

the UE NAS shall not provide the lower layers with the 5G-S-TMSI or the registered GUAMI;

  • if the service request procedure was initiated over non-3GPP access, the UE NAS shall provide the lower layers with the registered GUAMI, but shall not provide the lower layers with the 5G-S-TMSI;
  • if the initial NAS message other than the SERVICE REQUEST or CONTROL PLANE SERVICE REQUEST message was initiated over untrusted non-3GPP access, the UE NAS shall provide the lower layers with the GUAMI of the 5G-GUTI that the UE NAS has selected as specified in the clause 5.5.1.2.2 and 5.5.1.3.2, but shall not provide the lower layers with the 5G-S-TMSI;

if the initial NAS message other than the SERVICE REQUEST or CONTROL PLANE SERVICE REQUEST message was initiated over trusted non-3GPP access, the UE NAS shall provide the lower layers with the 5G-GUTI, if available, otherwise shall provide the lower layers with the SUCI;

if the UE is the 5G-RG and the initial NAS message other than the SERVICE REQUEST or CONTROL PLANE SERVICE REQUEST message was initiated over wireline access, the UE NAS shall provide the lower layers with the GUAMI of the 5G-GUTI that the UE NAS has selected as specified in the clause 5.5.1.2.2 and 5.5.1.3.2, if available, otherwise shall not provide the lower layers with any UE identity;

  • if the UE does not hold a 5G-GUTI that was previously assigned by the same PLMN with which the UE is performing the registration procedure and if:
    • the UE operating in the single-registration mode performs a registration procedure for mobility and periodic registration update indicating "mobility registration updating" following an inter-system change from S1 mode to N1 mode; or
    • the UE which was previously registered in S1 mode before entering state EMM-DEREGISTERED, performs an initial registration procedure, the UE has received the interworking without N26 interface indicator set to "interworking without N26 interface not supported" from the network, and the UE holds a 4G-GUTI;

then the UE NAS provides the lower layers with a GUAMI part of the 5G-GUTI mapped from 4G-GUTI as specified in 3GPP TS 23.003 [4] with an indication that the GUAMI is mapped from EPS; or

  • otherwise:
    • if the tracking area of the current cell is in the registration area, the UE NAS shall provide the lower layers with the 5G-S-TMSI, but shall not provide the registered GUAMI to the lower layers; or
    • if the tracking area of the current cell is not in the registration area, the UE NAS shall provide the lower layers with the GUAMI of the 5G-GUTI that the UE NAS has selected as specified in the clauses 5.5.1.2.2 and 5.5.1.3.2, but shall not provide the lower layers with the 5G-S-TMSI.

For 3GPP access, if:

  • the UE is operating in single-registration mode, the UE does not hold a 5G-GUTI and the UE does not hold a 4G-GUTI; or
  • the UE is operating in dual-registration mode and the UE does not hold a 5G-GUTI;

the UE NAS does not provide the lower layers with the 5G-S-TMSI or the registered GUAMI.

For untrusted non-3GPP access, if the UE does not hold a 5G-GUTI and the UE does not hold a 4G-GUTI, the UE NAS does not provide the lower layers with the 5G-S-TMSI or the registered GUAMI. For trusted non-3GPP access, if the UE does not hold a 5G-GUTI and the UE does not hold a 4G-GUTI, the UE NAS provides the lower layers with the SUCI.

For 3GPP access, if a UE operating as an IAB-node performs a registration procedure or service request procedure (see 3GPP TS 23.501 [8]), the UE NAS shall indicate to the lower layers that the establishment of the NAS signalling connection is for a UE operating as an IAB-node.

The UE NAS also provides the lower layers with the identity of the selected PLMN (see 3GPP TS 38.331 [30]) if the UE is not operating in SNPN access operation mode. If the UE is operating in SNPN access operation mode, the UE NAS provides the lower layers with the SNPN identity of the selected SNPN. In a shared network, the UE shall choose one of the PLMN identity(ies) or SNPN identity(ies) as specified in 3GPP TS 23.122 [5] and 3GPP TS 24.502 [18].

The UE NAS layer may provide the lower layers with an NSSAI as specified in clause 4.6.2.3.

If the UE performs initial registration for onboarding services in SNPN or is registered for onboarding services in SNPN, the UE NAS layer shall provide the lower layers with an indication that the connection is for onboarding.