RRC Inactive state in 5G-NR

In the realm of 5G New Radio (NR) connectivity, the operational states of a User Equipment (UE) are categorized into three distinctive RRC states:

  • RRC_IDLE: This state corresponds to the initial mode when the UE is powered up. It signifies a dormant state where the UE is not actively engaged in communication. To initiate data transfer or voice calls, the UE must establish a connection with the network. This connection establishment is executed through the RRC connection establishment procedure.

  • To complete cell selection and cell reselection, the SI is necessary.
  • UE read SI from BCCH.
  • SI also provides the information related to complete the Random Access and RRC Connection Setup procedures.
  • The UE performing mobility triggered Registration Area updates to ensure that the UE is always reachable by the AMF (knows where to forward the paging message).
  • UE monitors the PDCCH (DCI) Format 1 0 using the P-RNTI defined by the Discontinuous Reception (DRX) pattern.
  • 5G-S-TMSI is used to address UE within Paging message.
  • 5G-S-TMSI is allocated by AMF.
  • To improve security 5G doesn’t support IMSI based paging.
  • UE is unable to transfer application data while in RRC Idle.

  • RRC_CONNECTED: Similar in concept to its LTE counterpart, this state is achieved once the UE successfully establishes an RRC connection with the network. In this state, the UE is actively engaged in communication, data transfer, or voice calls.

  • Transfer of both application data and signaling between the UE and network is done in connected mode.
  • Application data transferred by using DRB and signaling can be transferred using SRB.
  • CRNTI is provided by Base station to address UE during RACH.
  • AMF maintains NG signaling connection with Base Station.
  • UPF maintains GTP-U tunnels with Base Station.
  • UE monitors Control Channels for Resource Allocations.
  • The UE reports Channel State information (CSI). It includes CQI, RI, PMI, LI, CRI (CSI-RS Resource Indicator) and SSBRI (SS/PBCH Block Resource Indicator).

  • RRC_INACTIVE: A novel state introduced in 5G NR, the RRC_INACTIVE state is employed to address challenges arising from idle mode transitions in LTE. In LTE, extended inactivity prompts the network to transition the UE to the RRC_IDLE state, leading to power savings. However, resuming activity involves transitioning back to the connected mode (RRC_CONNECTED), necessitating RRC signalling and introducing latency. The modern landscape of frequent small data transmissions from smartphones exacerbates this, causing frequent Idle-Connected-Idle transitions. To mitigate these challenges, 5G NR introduces the RRC_INACTIVE state. This state helps alleviate network signalling load and reduces latency when transitioning to the RRC_CONNECTED state.

  • In the 5G NR landscape, the introduction of the RRC_INACTIVE state tackles the challenges posed by frequent idle-to-connected transitions. By maintaining a suspended yet more responsive connection, the UE can significantly reduce signalling overhead and latency.
  • RRC context and CORE network connection is kept in both the UE and the gNB.
  • Transition to connected state for data transfer is fast.
  • RRC inactive make the device in sleep like an idle state but the mobility is handled through cell reselection without involvement of network.
  • Its acts like mix combo of IDLE and Connected state.

Operational Considerations:

  • Within 5G NR, a UE occupies the RRC_CONNECTED state upon successfully establishing an RRC connection with the network.
  • In instances where an RRC connection is suspended while connected to a 5G Core (5GC) network, the UE shifts to the RRC_INACTIVE state. This state helps balance the need for responsiveness with power-saving measures.
  • If no RRC connection has been established, the UE reverts to the RRC_IDLE state, marking a state of dormancy until communication is required.
  • It’s crucial to recognize that in the Non-Standalone (NSA) mode of operation, where NR cells coexist with LTE infrastructure, only the RRC_CONNECTED state is applicable.

Within the RRC_INACTIVE state, the UE’s uplink transmission capabilities are limited. The only exception is the capability to transmit PRACH as a component of the Random Access (RA) procedure. This procedure is invoked when the UE intends to shift to the RRC_CONNECTED state, facilitating the transmission of an RRCResumeRequest, or when it seeks to request On-demand system information.

On the other hand, it is within the gNB’s purview to transition a UE from the RRC_CONNECTED state to the RRC_INACTIVE state. This transition is executed through the transmission of an RRCRelease message, which is equipped with a suspendConfig parameter. This parameter plays a pivotal role in orchestrating the movement from an active connected state to an inactive yet responsive RRC_INACTIVE state.

suspendConfig holds crucial significance in configuring the RRC_INACTIVE state for a UE. It encompasses several key attributes that collectively shape the behaviour of this state:

Full I-RNTI (40-bits): This identifier, denoted as Inactive-RNTI (I-RNTI), uniquely distinguishes a suspended UE’s context within the RRC_INACTIVE state. It proves especially useful for directing paging messages to UEs in this state, ensuring targeted communication.

Short I-RNTI (24-bits): Offering a more concise identification alternative, the short I-RNTI serves as a shorter but effective identifier for a UE’s suspended context within RRC_INACTIVE. Its reduced bit format aids in streamlined identification.

ran-PagingCycle: Tailored for RAN-initiated paging, the ran-PagingCycle parameter is utilized to determine the UE-specific paging cycle within the RRC_INACTIVE state. This cycle optimizes the paging process, enhancing the responsiveness of communication.

ran-NotificationAreaInfo: Vital for efficient mobility management, this attribute ensures that a UE in RRC_INACTIVE consistently maintains up-to-date and accurate ran-NotificationAreaInfo. This information is integral for smooth mobility management throughout the network.

t380: Operating as a trigger, the t380 timer initiates periodic RNA update procedures within the UE. These updates uphold the relevance and currency of RNA information, bolstering effective mobility management while in the RRC_INACTIVE state.

In the context of RRC states in 5G NR, the process of suspending and resuming the RRC connection involves various actions and outcomes:

  • Suspending RRC Connection: The network initiates the suspension of the RRC connection. Upon suspension, the UE retains the UE Application Server (UE AS) context and network configuration, transitioning to the RRC_INACTIVE state. This transition is safeguarded by integrity protection and encryption, ensuring data security during the suspension.
  • Resuming RRC Connection: The resumption of a suspended RRC connection is instigated either by upper layers when transitioning from RRC_INACTIVE to RRC_CONNECTED, by RRC for RNA (Radio Network Application) updates, or by RAN paging from NG-RAN (Next-Generation Radio Access Network). Upon resumption, the network configures the UE using stored UE AS context and RRC configuration received earlier. This procedure involves reinstating security measures and re-establishing Signalling Radio Bearers (SRBs) and Data Radio Bearers (DRBs).
  • Network Response: When a request to resume the RRC connection is received, the network has several options:
  • The network may resume the suspended RRC connection and shift the UE to the RRC_CONNECTED state.
  • Alternatively, the network might decline the request, directing the UE to RRC_INACTIVE while setting a wait timer.
  • The network could directly re-suspend the RRC connection, moving the UE back to the RRC_INACTIVE state.
  • In some cases, the network might choose to release the RRC connection, transitioning the UE to the RRC_IDLE state.
  • The network also has the option to instruct the UE to initiate NAS (Non-Access Stratum) level recovery, prompting the network to send an RRC setup message.

States and state transitions in RRC_IDLE state and RRC_INACTIVE state

UE state machine and state transitions between NR/5GC, E-UTRA/EPC and EUTRA/5GC

UE state machine and state transitions between NR and UTRAN/GERAN

An alternative depiction of this signaling procedure has been presented by Rahim Navael, and I’m privileged to share this illustration in this context:

In this visual representation, Rahim Navael’s interpretation of the RRC connection suspension and resumption procedure comes to life. This graphic encapsulates the essence of the process, illustrating the transitions, interactions, and key components involved in a concise and visually engaging manner. It provides an insightful perspective on how the network triggers and manages the transitions between RRC_INACTIVE and other states, while reconfiguring the UE based on stored context and network instructions.

This visual aid further enriches the understanding of the complex signalling procedures, enhancing the clarity and depth of comprehension for this critical aspect of 5G NR operation.

Kudos to Rahim Navael for contributing to the visualization of this intricate procedure.

The addition of the RRC_INACTIVE state in 3GPP Release 15 marks a significant advancement in both 5G NR and EUTRA technology. This state offers notable operational improvements by allowing devices to remain connected from the Core Network (CN) perspective, designated as CM-CONNECTED. This connection persists while enabling devices to move within a demarcated region, defined by the 5G Base Station (BS), known as the RAN Notification Area (RNA). Remarkably, this mobility occurs without the need for frequent updates to the device’s location information.

Key Aspects of RAN Notification Area (RNA):

The RAN Notification Area (RNA) is a pivotal concept in this context. It comprises either a single cell or a group of cells, associated with one or multiple 5G BS(s).

RNA boundaries are contained within the registration boundaries of the Core Network (CN).

To ensure timely communication, the last serving 5G BS, upon receiving downlink data from the User Plane Function (UPF) or signalling from the Access and Mobility Management Function (AMF), initiates paging messages across all cells within the RNA.

Furthermore, neighbouring 5G BS(s) interconnected through the Xn interface might also receive these paging messages, enhancing the efficacy of communication.

Enabling RRC_INACTIVE State:

The operationalization of the RRC_INACTIVE state hinges on compatibility between the 5G Base Station and the 5G device.

When a period of inactivity is detected on both uplink and downlink, determined by an inactivity timer, the 5G BS gains the ability to transition the UE to the RRC_INACTIVE state.

This transition deviates from the conventional move to RRC_IDLE state and is achieved through an RRC Release message, which includes suspendConfig parameters.

Maintenance of Device Context and Connections:

Both the 5G device and the last serving gNB node retain the Inactive AS Context of the device. This context encompasses critical elements such as radio protocol information and security parameters.

Additionally, the last serving 5G BS shoulders the responsibility of upholding the UE’s associated Next-Generation (NG) connections, specifically NG-C and NG-U, with the serving AMF and UPF. This sustained connectivity ensures seamless communication.

Role of AMF and Core Network Assistance Information:

The AMF plays a central role in this framework by supplying Core Network Assistance Information to the 5G BS node.

This information empowers the 5G BS to make informed decisions regarding the transition of a device to the RRC_INACTIVE state.

Furthermore, it facilitates UE configuration and paging while the device resides in the RRC_INACTIVE state.

references : Rahim Navael,38.331,38.304,

Leave a Reply