In this chapter we shall have a look at the functionalities of different RRC States.
There are 3 RRC states in 5G NR.
1. RRC Connected
2. RRC Inactive
3. RRC Idle
UE will be in RRC_Idle mode when it first camps on 5G cell.
This can be when a device is switched-on. UE will move to RRC Connected when it completes RRC Setup procedure.
When UE in connected mode, SRB will be configured along with one or more DRB.
UE in connected mode, it will register to the network and move from RM-Deregistered to RM-Registered. Registration procedure allows the UE to allocate 5G-S-TMSI.
After that UE will move from CM-IDLE to CM-Connected.
UE will move from RRC Connected to RRC Inactive using RRC Release procedure, “spendConfig” IE will be included within the RRC Release to indicate that the UE is moving to RRC Release instead of RRC Inactive.
UE will move to RRC Idle by completing RRC Connection Release procedure. This will release all the allocated C-RNTI, radio bearers and all radio resources.
RRC Connected State:
1. Initially UE will be in RRC Idle mode and it will move from RRC Idle to RRC Connected state by completing RRC Setup Procedure.
2. In this state UE will be allocated C-RNTI. One C-RNTI will be allocated if it is connected to single BS, 2 C-RNTI will be allocated with UE is connected to Multi-RAT Dual Connectivity.
- CRNTI will be used to address the UE.
3. UE will be in CM-Connected mode.
4. UE stores access stratum context.
5. AMF will have NG signalling connection with BS.
6. UPF will have GTP-U tunnels with BS.
7. Transfer of Uplink and Downlink data.
8. Configuration of Connected Mode DRX.
9. UE will monitor PDCCH DCI using P-RNTI.
10. In this state, UE will transfer both application data and signalling between UE and Network.
RRC Inactive State:
1. UE will move from RRC Connected to RRC Inactive by sending RRC Release Procedure.
2. “suspendConfig” IE is used to indicate that RRC is moving to RRC Inactive instead of RRC Idle state.
3. suspendConfig IE will provide with a Inactive-RNTI to a UE. This is. used to address the UE within RRC paging messages.
4. UE will be moved to RRC Inactive state to reduce the signalling load.
5. UE will be in CM-Connected mode.
6. UE will have access stratum context.
7. UE will do RNA update.
8. UE will apply DRX for paging.
9. UE monitors PDCCH DCI using P-RNTI.
RRC Idle
1. UE will move to RRC Idle by completing the RRC Connection Release Procedure.
2. UE will release C-RNTI along with all radio bearers and radio resources.
3. UE will be in CM-IDLE state.
4. UE will read system information for RRC Setup.
