5G NR UE RRC States: IDLE, INACTIVE, CONNECTED Explained

This article explains the 5G NR UE RRC states and the functions of the RRC layer in 5G NR.

The 5G NR UE operates in three RRC states: RRC_IDLE, RRC_INACTIVE, and RRC_CONNECTED.

5G NR RRC Layer Functions

Figure 1: 5G NR Protocol Stack

As shown in Figure 1, the 5G NR protocol stack is divided into the user plane and the control plane. The RRC layer resides within the control plane. Here’s a breakdown of the functions performed by the RRC layer:

• Broadcast System Information (SI): Transmits System Information messages to both the Access Stratum (AS) and Non-Access Stratum (NAS).
• Paging Handling: Manages paging requests initiated by the 5G Core Network (5GC) or NG-RAN (Radio Access Network).
• RRC Connection Management: Establishes, maintains, and releases RRC Connections between the 5G NR UE and the NG-RAN. This includes adding, modifying, and releasing Carrier Aggregation (CA) and Dual Connectivity, both within NR and between E-UTRA and NR.
• Security: Handles security-related functions, including key management.
• Radio Bearer Management: Establishes, configures, maintains, and releases Signaling Radio Bearers (SRBs) and Data Radio Bearers (DRBs).
• Mobility Management: Manages mobility functions such as handover, context transfer, UE cell selection/re-selection, control of cell selection/re-selection, and inter-RAT mobility.
• QoS Management: Handles Quality of Service management.
• UE Measurement Reporting: Controls UE measurement reporting.
• Radio Link Failure Management: Detects radio link failures and manages recovery procedures.
• NAS Message Transfer: Transfers NAS messages to/from the NAS layer from/to the UE.

5G NR RRC States: RRC_IDLE, RRC_INACTIVE, and RRC_CONNECTED

Figure 2: 5G NR UE RRC states

The following describes the three RRC states of the UE as defined by the 5G NR standard:

• RRC_IDLE: Upon power-on, the UE enters RRC_IDLE mode. The UE can transition to this mode from either RRC_CONNECTED or RRC_INACTIVE mode.

• RRC_INACTIVE: The UE transitions to this mode from RRC_CONNECTED mode. It’s essentially a “connected but inactive” state. The UE maintains its RRC connection while minimizing signaling overhead and power consumption.

• RRC_CONNECTED: The UE maintains an active connection with the 5G-RAN/5GC in this mode.

RRC_IDLE Mode Details

In RRC_IDLE mode, the UE performs the following actions:

• PLMN Selection: Selects a Public Land Mobile Network (PLMN).
• SI Broadcasting: Receives broadcasted System Information messages.
• Cell Re-selection Mobility: Performs cell re-selection procedures.
• Mobile Terminated Data Paging (5GC Initiated): Listens for paging initiated by the 5GC for mobile-terminated data.
• Mobile Terminated Data Area Management (5GC): The 5GC manages the paging area for mobile-terminated data.
• CN Paging DRX (NAS Configured): Supports Discontinuous Reception (DRX) for CN paging, configured by the NAS layer, to conserve power.

RRC_INACTIVE Mode Details

In RRC_INACTIVE mode, the UE performs the following actions:

• System Information Broadcasting: Receives broadcasted system information.
• Cell Re-selection Mobility: Performs cell re-selection procedures.
• Paging (NG-RAN Initiated): Listens for paging initiated by the NG-RAN (RAN paging).
• RAN-based Notification Area (RNA) Management: The NG-RAN manages the RAN-based Notification Area (RNA).
• DRX for RAN Paging (NG-RAN Configured): Supports DRX for RAN paging, configured by the NG-RAN.
• 5GC to NG-RAN Connection: A connection (both C/U-planes) is established between the 5GC and NG-RAN for the UE.
• UE AS Context Storage: The UE AS context is stored in both the NG-RAN and the UE.
• RNA Awareness: The NG-RAN knows the RNA to which the UE belongs.

RRC_CONNECTED Mode Details

In RRC_CONNECTED mode, the UE performs the following actions:

• 5GC to NG-RAN Connection: A connection (both C/U-planes) is established between the 5GC and NG-RAN for the UE.
• UE AS Context Storage: The UE AS context is stored in both the NG-RAN and the UE.
• Cell Awareness: The NG-RAN knows the cell to which the UE belongs.
• Unicast Data Transfer: Transfers unicast data to/from the UE.
• Network-Controlled Mobility: Supports network-controlled mobility, including measurements.

Reference: 3GPP TS 38.300

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