Section: 5.3.1.1 Establishment of the NAS signalling connection
Content: When the UE is in EMM-IDLE mode without suspend indication and needs to transmit an initial NAS message, the UE shall request the lower layer to establish a RRC connection. In this request to the lower layer the NAS shall provide to the lower layer the RRC establishment cause and the call type as specified in annex D of this specification and, for the case specified in clause 5.6.1.2.2, shall also provide the initial NAS message, otherwise NAS may also provide the initial NAS message. Initial NAS messages are: - ATTACH REQUEST; - DETACH REQUEST; - TRACKING AREA UPDATE REQUEST; - SERVICE REQUEST; - EXTENDED SERVICE REQUEST; and - CONTROL PLANE SERVICE REQUEST. When the UE is in EMM-IDLE mode with suspend indication, the UE shall proceed the behaviour as specified in clauses 5.3.1.3. For the routing of the initial NAS message to the appropriate MME, the UE NAS provides the lower layers with either the S-TMSI, the registered globally unique MME identifier (GUMMEI) that consists of the PLMN ID, the MME group ID, and the MME code (see 3GPP TS 23.003[ Numbering, addressing and identification ]  [2]), or none of them according to the following rules: - If the UE has received the interworking without N26 interface indicator set to "interworking without N26 interface not supported" from the network, the UE holds a valid 5G-GUTI and: a) the UE performs an initial EPS attach procedure or tracking area updating procedure following an inter-system change from N1 mode to S1 mode; or b) the UE which was previously registered in N1 mode before entering state 5GMM-DEREGISTERED, performs an initial EPS attach procedure, then the UE NAS shall provide the lower layers with the MME identifier part of the mapped GUTI, which is generated from the 5G-GUTI as specified in 3GPP TS 23.003[ Numbering, addressing and identification ]  [4], an indication that the identifier is a native GUMMEI and an indication that the identifier is mapped from 5GS; - If the TIN indicates "GUTI" or "RAT-related TMSI", or the TIN is not available, and the UE holds a valid GUTI: a) When the UE in EMM-IDLE mode initiates a tracking area updating or combined tracking area updating procedure for load balancing purposes, the UE NAS shall provide the lower layers with neither S-TMSI nor registered MME identifier; b) When the tracking area of the current cell is in the list of tracking areas that the UE previously registered in the MME during the NAS signalling connection establishment, the UE NAS shall provide the lower layers with the S-TMSI, but shall not provide the registered MME identifier to the lower layers; or c) When the tracking area of the current cell is not in the list of tracking areas that the UE previously registered in the MME during the NAS signalling connection establishment, the UE NAS shall provide the lower layers with the MME identifier part of the valid GUTI with an indication that the identifier is a native GUMMEI. - If the TIN indicates "P-TMSI", or the TIN is not available, and the UE holds a valid P-TMSI and RAI, the UE NAS shall provide the lower layers with the MME identifier part of the mapped GUTI, which is generated from the P-TMSI and RAI with an indication that the identifier is a mapped GUMMEI; or - Otherwise, the UE NAS does not provide the lower layers with the S-TMSI, the registered GUMMEI and the mapped GUMMEI. The UE NAS also provides the lower layers with the identity of the selected PLMN (see 3GPP TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [22]). In a shared network, the UE shall choose one of the PLMN identities as specified in 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [6]. When an ATTACH REQUEST message, or a TRACKING AREA UPDATE REQUEST message when the current TAI of the current cell is not included in the TAI list, is sent to establish a signalling connection, the UE NAS also provides the lower layers with the DCN-ID according to the following rules: a) if a DCN-ID for the PLMN code of the selected PLMN is available in the UE, the UE NAS shall provide this DCN-ID to the lower layers; or b) if no DCN-ID for the PLMN code of the selected PLMN is available but a Default_DCN_ID value is available in the UE, as specified in 3GPP TS 24.368[ Non-Access Stratum (NAS) configuration Management Object (MO) ]  [15A] or in USIM file NASCONFIG as specified in 3GPP TS 31.102[ Characteristics of the Universal Subscriber Identity Module (USIM) application ]  [17], the UE NAS shall provide this DCN-ID to the lower layers. If a relay node is attaching for relay node operation (see 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [10]), the NAS in the relay node shall indicate to the lower layers that the establishment of the NAS signalling connection is for a relay node. If a UE operating as an IAB-node performs an attach procedure, tracking area updating procedure, or service request procedure (see 3GPP TS 23.401[ General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access ]  [10]), 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. In S1 mode, when the RRC connection has been established successfully, the UE shall enter EMM-CONNECTED mode and consider the NAS signalling connection established. In S101 mode, when the cdma2000® HRPD access network resources are available for tunnelled NAS signalling, the UE shall enter EMM-CONNECTED mode and consider the S101 mode NAS signalling connection established.