he UE then sends the RRC Setup Request message as a CCCH message, also referred to as RACH message 3. It contains the NAS UE identifier if available, or a random number if not. 4. The UE ID contained in the RACH message 3 is then echoed back to the UE in the UE Contention Resolution Identity MAC CE (3GPP TS 38.321) for contention resolution in RACH message 4. The RRC Setup message, also a CCCH message, could be included in 3 5G System Overview 1. RACH 2. RACH response (RAR) 3. RRCSetupRequest 4. RRCSetup 5. RRCSetupComplete [NAS: Registration Request] 6. AMF selection 7. NG-AP:INITIAL UE MESSAGE [NAS: Registration Request] 8. Authentication/NAS security/other NAS procedures 9. NG-AP: INITIAL CONTEXT SETUP [NAS: Registration Accept] 10. SecurityModeCommand 11. SecurityModeComplete 12. UECapabilityEnquiry 13. UECapabilityInformation 15. DLInformation Transfer [NAS: Registration Accept] 15. ULInformation Transfer [NAS: Registration Complete] 16. NGAP: INITIAL CONTEXT SETUP RESPONSE 17. NG-AP:INITIAL UE MESSAGE [NAS: Registration Complete] 18. PDU session establishment procedures 19. NG-AP: PDU SESSION RESOURCE SETUP REQUEST 20. RRCReconfiguration [NAS: PDU Session Establishment Accept] [NAS PDU] 21. RRCReconfigurationComplete 22. NG-AP: PDU SESSION RESOURCE SETUP RESPONSE Figure 3.4.10 An example message flow for initial NAS registration. 3.4 NR Protocol Stack the RACH message 4 or sent separately to the UE. It contains the initial configuration to set up the DCCH logical channel and SRB1. Subsequent RRC messages are sent over DCCH on SRB1. 5. After the set up of SRB1, the UE can send NAS messages to the core network that are encapsulated in RRC messages and subsequently sent to the core network by the gNB. The NAS registration request is encapsulated in the RRC Setup Complete message. Additional NAS messages can be exchanged from the UE to the core network encap- sulated in the RRC ULInformationTransfer message over SRB1 and SRB2 after SRB2 is established. 6. The gNB selects the AMF based on the UE ID and other NAS