5G NR UE RRC States | 5G NR RRC Functions | IDLE,INACTIVE,CONNECTED

This page describes 5G NR UE RRC states. It mentions functions of RRC layer in 5G NR. The 5G NR UE states are RRC_IDLE, RRC_INACTIVE and RRC_CONNECTED mode.

5G NR RRC layer functions

The figure-1 depicts 5G NR protocol stack. The stack is divided into user plane and control plane. RRC layer is part of 5G NR control plane as shown.
Following are the functions performed by RRC layer in 5G NR protocol stack.
➨Broadcast SI (System Information) messages to AS (Access Stratum) and NAS (Non-Access Stratum).
➨Handles paging initiated by 5GC (5G Core Network) or NG-RAN (Radio Access Network).
➨Establishment, maintenance and release of RRC Connection between 5G NR UE and NG-RAN. This includes addition, modification and release of CA(carrier aggregation) and Dual connectivity in NR or between E-UTRA and NR.
➨Security related functions including key management
➨Establishment, configuration, maintenance and release of SRBs (Signaling Radio Bearers) and DRBs (Data Radio Bearers) .
➨Mobility functions such as handover, context transfer, UE cell selection/re-selection, control of cell selection/re-selection, Inter-RAT mobility etc.
➨QoS management
➨UE measurement reporting, control of reporting
➨Detection of radio link failure and recovery from radio link failure
➨NAS message transfer to/from NAS from/to UE

5G NR RRC STATES | RRC_IDLE, RRC_INACTIVE and RRC_CONNECTED

Following are the three RRC states of UE as per 5G NR standard.
• RRC_IDLE : Upon power ON, UE enters into RRC_IDLE mode. UE can move to this mode from either RRC_CONNECTED mode or RRC_INACTIVE mode.
• RRC_INACTIVE: UE moves to this mode from RRC_CONNECTED mode. It is connected but inactive mode of UE. In this mode UE maintains RRC connection and at the same time minimizes signaling and power consumption.
• RRC_CONNECTED: UE remains in connection with the 5G-RAN/5GC in this mode.

RRC_IDLE Mode

Following actions are performed by UE in RRC IDLE mode.
• Selection of PLMN
• Broadcast of SI messages.
• Cell re-selection mobility
• Paging for mobile terminated data is initiated by 5GC
• Paging for mobile terminated data area is managed by 5GC
• DRX for CN paging configured by NAS, Support for UE discontinuous reception (DRX) is carried out to enable power saving in 5G UE.

RRC_INACTIVE Mode

Following actions are performed by UE in RRC INACTIVE mode.
• Broadcast of system information
• Cell re-selection mobility
• Paging is initiated by NG-RAN (RAN paging)
• RAN-based notification area (RNA) is managed by NG-RAN
• DRX for RAN paging configured by NG-RAN
• 5GC to NG-RAN connection (both C/U-planes) is established for UE
• The UE AS context is stored in NG-RAN and the UE
• NG-RAN knows the RNA which the UE belongs to

RRC_CONNECTED Mode

Following actions are performed by UE in RRC CONNECTED mode.
• 5GC to NG-RAN connection (both C/U-planes) is established for UE
• The UE AS context is stored in NG-RAN and the UE
• NG-RAN knows the cell which the UE belongs to
• Transfer of unicast data to/from the UE
• Network controlled mobility including measurements

Reference: 3GPP TS 38.300

Also refer table mentioning links to RRC Information Elements (IEs) >>.