In this chapter we shall see the message exchanges between UE, RAN and AMF.
1. When a UE is switched on, it will scan the available to perform cell search and cell selection procedure.
2. For that it will decode the CEll ID and PBCH (MIB) information.
3. Once UE will find the suitable cell to latch on, then UE will perform RACH procedure.
4. UE will perform Cell search to achieve downlink synchronization and RACH procedure to achieve Uplink synchronization.
5. RACH procedure include 4 Messages: Msg1, Msg2, Msg3, Msg4.
6. After RACH procedure, UE will send RRC setup complete + Registration Request with dedicatedNAS-Message.
7.gNB will send Initial UE Message along with Registration Request to selected AMF.
8. Then NAS Identity Request message are sent.
9. The Core Network performs Authentication and NAS security to verify the UE..
10. Initial Context Setup Request: Once NAS Security Mode Complete is completed, and UE is verified then AMF will send InitialContextSetupRequest to gNB. “AMF UE NGAP ID” is allocated by AMF, gNB will use this ID to address UE in AMF.
11. gNB can ask UE for its capability through “UECapabilityEnquiry” procedure.
12. After that “ASSecurityModeCommand” procedure. Once this procedure is completed, all the messages after this message are encrypted.
13. After this SRB2 and DRB are established.
14. After this, Registration Complete and PDU session establishment request is sent to AMF.
3GPP TS 23.502