In this chapter we shall learn about 5G NR initial access process in brief.
1. UE sends RRC connection request message to gNB-DU.
2. gNB-DU will assign a C-RNTI to the UE and will include corresponding low level configuration for UE, will be included in F1AP INITIAL UL RRC MESSAGE TRANSFER message.
3. gNB-CU assigns F1AP ID to UE and generates RRC CONNECTION SETUP messages to UE. This will be encapsulated in F1AP DL RRC MESSAGE TRANSFER message.
4. gNB-DU sends RRC CONNECTION SETUP message to UE.
5. UE sends RRC CONNECTION SETUP COMPLETE message to gNB-DU.
6. gNB-DU encapsulates RRC messages in F1AP UL RRC MESSAGE TRANSFER messages and sends them to gNB-CU.
7. gNB-CU sends the INITIAL UE MESSAGE message to AMF.
8. AMF sends the initial UE context creation request message to gNB-CU.
9. gNB-CU sends a UE context creation request message to establish a UE context in gNB-DU. RRC SECURITY MODE COMMAND message will be encapsulated inside this.
10. gNB-DU sends RRC SECURITY MODE COMMAND message to UE.
11. gNB-DU sends the UE CONTEXT SETUP RESPONSE message to gNB-CU.
12. UE sends RRC SECURITY MODE COMPLETE message.
13. gNB-DU encapsulates RRC messages in F1AP UL RRC MESSAGE TRANSFER message and sends them to gNB-CU.
14. gNB-CU generates RRC CONNECTION RECONFIGURATION message and encapsulates them in F1AP DL RRC MESSAGE TRANSFER message
15. gNB-DU sends RRC CONNECTION RECONFIGURATION message to UE.
16. UE sends RRC CONNECTION RECONFIGURATION COMPLETE message to gNB-DU.
17. gNB-DU encapsulates RRC messages in F1AP UL RRC MESSAGE TRANSFER messages and sends them to gNB-CU.
18. gNB-CU sends INITIAL UE CONTEXT SETUP RESPONSE message to AMF.
3GPP TS 38.331: “NR; Radio Resource Control (RRC) protocol specification”;
3GPP TS 38.413: “NG-RAN; NG Application Protocol (NGAP)”;
3GPP TS 38.473: “NG-RAN; F1 application protocol (F1AP)”;
