LTE had only 2 states i.e RRC_IDLE, RRC_CONNECTED.
5G NR has 3 states, the new state is RRC_INACTIVE state.
When UE is in RRC_INACTIVE state, it will be in CM-CONNECTED state.
UE can move in RNA (RAN-Based Notification Area) without notifying NG_RAN.
So when there is a downlink data from UPF or downlink signalling from AMF, gNB will page the UE in all the cells of RNA.
The difference between the 3 states:
RRC_CONNECTED: UE to NG-RAN -> Connected. NG-RAN to 5GC -> Connected
RRC_IDLE: UE to NG-RAN -> Released. NG-RAN to 5GC -> Released
RRC_INACTIVE: UE to NG-RAN -> Suspended. NG-RAN to 5GC -> connected
What is the use of the new RRC_INACTIVE state?
This state has been introduced to reduce the signalling and power consumption.
As you can see from the above points, RRC and NAS context are retained in the BS and CN.
For UE can quickly change from Inactive to Connected state reducing the signalling and saving the power.
This is suitable for millions of large-scale IoT devices.