In the realm of 5G technology, efficient communication between user devices (UE), base stations (gNB), and core network components (AMF) is crucial. Here’s a detailed look at the UE’s transition process from RRC_IDLE to RRC_CONNECTED state:
Sequence of Events:
1. RRCSetupRequest (UE ➡️ gNB)
o The UE in RRC_IDLE state initiates the process by sending an RRCSetupRequest message to the gNB to establish an RRC connection.
2. RRCSetup (gNB ➡️ UE)
o The gNB responds with an RRCSetup message, containing the configuration required for the UE to move to the RRC_CONNECTED state.
3. RRCSetup Complete (UE ➡️ gNB)
o The UE acknowledges the setup with an RRCSetup Complete message, indicating it has successfully received and applied the configuration.
4. INITIAL UE MESSAGE (gNB ➡️ AMF)
o The gNB forwards an INITIAL UE MESSAGE to the AMF, signaling the UE’s attempt to connect to the core network.
5. DOWNLINK NAS TRANSPORT (AMF ➡️ gNB)
o The AMF sends a DOWNLINK NAS TRANSPORT message to the gNB, which is then transferred to the UE as DLInformationTransfer.
6. ULInformationTransfer (UE ➡️ gNB)
o The UE responds with ULInformationTransfer, carrying necessary information back to the gNB, which forwards it as UPLINK NAS TRANSPORT to the AMF.
7. INITIAL CONTEXT SETUP REQUEST (AMF ➡️ gNB)
o The AMF sends an INITIAL CONTEXT SETUP REQUEST to the gNB to initiate the context setup for the UE.
8. SecurityModeCommand (gNB ➡️ UE)
o The gNB sends a SecurityModeCommand to the UE to initiate security procedures.
9. SecurityModeComplete (UE ➡️ gNB)
o The UE completes the security setup and responds with a SecurityModeComplete message.
10. RRCReconfiguration (gNB ➡️ UE)
o The gNB sends an RRCReconfiguration message to the UE to finalize the RRC connection setup.
11. RRCReconfigurationComplete (UE ➡️ gNB)
o The UE confirms the reconfiguration with an RRCReconfigurationComplete message.
12. INITIAL CONTEXT SETUP RESPONSE (gNB ➡️ AMF)
o Finally, the gNB sends an INITIAL CONTEXT SETUP RESPONSE to the AMF, completing the transition process.
