# 5G UE RRC States

Radio Resource Control (RRC) is a layer within the 5G NR protocol stack. It exists only in the control plane, in the UE and in the gNB. The behaviour and functions of RRC are governed by the current state of RRC. In 5G NR, RRC has three distinct states: RRC_IDLE, RRC_CONNECTED and RRC_INACTIVE.

For each UE RRC state, applicable functions are clearly defined in the standard. Moreover, it's also defined how state transitions will happen, not only within 5G but also to 2G/3G/4G systems via handover or PLMN/cell reselection.

RRC_INACTIVE is not applicable for Non-Standalone (NSA) mode of operation.

## Discussion

• Why was RRC_INACTIVE state introduced in 5G?

Before 5G, RRC had only two states, idle and connected. In connected state, radio resources are allocated to the UE and typically active communication (user plane or control plane) is taking place between the UE and the network. Otherwise, UE is in idle state.

While releasing an RRC connection is good for capacity utilization and power saving, it's not ideal from a latency perspective. For Machine Type Communications (MTC) and IoT applications, it's typical for devices to send small amounts of data. The overhead in establishing an RRC connection to do this is bad from a power perspective. The extra signalling also introduces delay that's not ideal for URLLC use cases.

Since 5G caters to new cases such as mMTC and URLLC, RRC_INACTIVE has been introduced. While entering this state, both UE and NG-RAN save radio and security configurations. This saved UE Inactive Access Stratum (AS) context can be quickly restored with minimal signalling when moving to connected state.

Essentially, RRC_INACTIVE is UE RRC's way of implementing an "always on" radio connection with the network.

• What are the functions in each of the 5G UE RRC states?

In RRC_INACTIVE and RRC_CONNECTED, UE and NG-RAN store AS Inactive context and AS context respectively. In RRC_IDLE, UE may be registered with the Core Network (CN) but no AS context is stored.

In both RRC_IDLE and RRC_INACTIVE, UE does measurements of neighbouring cells and can do reselection. In RRC_CONNECTED, UE mobility is controlled by the CN and handovers can be initiated. Data transfer, CA, DC and measurement reporting are supported.

In RRC_IDLE, UE paging is initiated by the CN. In RRC_INACTIVE, UE paging is initiated by NG-RAN. To page a UE, it's location must be known. In RRC_IDLE, this is the Tracking Area (RA). In RRC_INACTIVE, this is the RAN-based Notification Area (RNA) and UE may initiate RNA updates.

In RRC_IDLE, Discontinuous Reception (DRX) is configured by higher layers. In RRC_INACTIVE, configuration comes from higher layers and RRC. In RRC_CONNECTED, DRX configuration is for discontinuously monitoring DL PDCCH.

• How does UE RRC transition from one state to another in 5G NR?

RRC_IDLE to RRC_CONNECTED happens via the RRC Connection Setup procedure. This consists of three messages: RRCSetupRequest (UE initiated), RRCSetup, and RRCSetupComplete.

RRC_CONNECTED to RRC_IDLE is via RRC Connection Release procedure with network-initiated RRCRelease message. Upper layers in the UE may also request a release. RRC connection is also released due to radio link failure, handover failure or cell not meeting cell selection criteria.

RRC_CONNECTED to RRC_INACTIVE is network initiated. It's entered via RRCRelease message with suspendConfig information element (IE). When UE is using a Dual Active Protocol Stack (DAPS) bearer or is redirected to an inter-RAT carrier frequency, this IE is not configured.

RRC_INACTIVE to RRC_CONNECTED is triggered by the network via RAN paging. A paged UE will start with RRC Connection Resume procedure consisting of three messages: RRCResumeRequest, RRCResume (or RRCSetup), RRCResumeComplete (or RRCSetupComplete). This procedure can also be initiated by UE for uplink transfer, including RNA update.

RRC_INACTIVE to RRC_IDLE happens when network responds to RRCResumeRequest with RRCRelease. Alternatively, the UE may be asked to remain in RRC_INACTIVE for some more time.

• How do UE RRC states map to states in other layers of the protocol stack?

When RRC is in either RRC_INACTIVE or RRC_CONNECTED states, there's a Non-Access Stratum (NAS) connection between the UE and the CN. Thus, Connection Management (CM) is in the Connected state.

In RRC_CONNECTED, Mobility Management (MM) may be in either Deregistered or Registered state. If the UE is in the process of registering to the network (attach procedure), then MM is in Deregistered state. Otherwise, it's in Registered state.

When a UE is just powered up, RRC is in RRC_IDLE and MM is in Deregistered state. After MM registration, RRC can move from RRC_CONNECTED to RRC_INACTIVE, and return to RRC_IDLE only as part of deregistration (detach procedure). It's also possible for a UE to move to RRC_IDLE and still be available for CN-initiated paging. For simplicity, this case in not shown in the figure.

• Could you describe RRC states and transitions across different cellular generations?

Reselection can happen in idle states. In 2G, GSM_Idle and GPRS Packet_Idle are the idle states. In 3G, reselection is also possible from connected states Cell_FACH, Cell_PCH and URA_PCH.

Reselection is possible from GPRS Packet Transfer Mode. Cell Change Order (CCO) is possible from 2G idle state, GPRS Packet Transfer Mode and E-UTRA RRC Connected. For reselection from UTRAN to GERAN, CCO can be assisted with Network Assisted Cell Change (NACC).

In connected states, handovers are possible. For handovers, relevant states are GSM_Connected, GPRS Packet Transfer Mode (2G); Cell_DCH (3G); E-UTRA RRC Connected (4G); and NR RRC_CONNECTED (5G).

## Milestones

2015

In the 5G-PPP European project METIS-II, the main 5G pre-standards project, there's discussion and effort towards supporting an RRC inactive state. In June 2016, a draft titled Draft Asynchronous Control Functions and Overall Control Plane Design is published with mention of RRC Connected Inactive state.

Aug
2016

LTE RRC specification TS 36.331, Release 13, version 13.2.0 is published. In this release, LTE RRC has only two states, RRC_IDLE and RRC_CONNECTED. This version includes suspend/resume of RRC connection. When suspended, RRC goes to RRC_IDLE. However, the connection can be resumed with minimal signalling due to stored UE AS context. This feature caters to NB-IoT requirements.

Dec
2017

3GPP publishes Release 15 "early drop". In 5G RRC specification TS 38.331, version 15.0.0, there are three states: RRC_IDLE, RRC_CONNECTED and RRC_INACTIVE. RRC_INACTIVE state applies only in SA mode, that is, it's not applicable in EN-DC (E-UTRA-NR Dual Connectivity).

Mar
2018

3GPP publishes version 1.0.0 of specification TS 38.304 titled User Equipment (UE) procedures in idle mode and in RRC Inactive state. In June, this is re-versioned to 15.0.0 for Release 15. Version 16.3.0 comes out in January 2021. It's mentioned that, "The UE initiates RRC Connection Resume procedure upon receiving RAN initiated paging. If the UE receives a CN initiated paging in RRC_INACTIVE state, the UE moves to RRC_IDLE and informs NAS."

Oct
2018

In LTE RRC specification TS 36.331, version 15.3.0, RRC_INACTIVE state is introduced. At the same time, 5G RRC specification TS 38.331, version 15.3.0 includes ASN.1 definition for RRCRelease message. This include suspendConfig IE that indicates transition to RRC_INACTIVE. Periodic RNA Update timer can range from 5-720 minutes.

Dec
2018

Hailu et al. publish performance analysis showing improvements due to RRC_INACTIVE state. They use the term "RRC Connected Inactive" for this and refer to LTE Release 13 suspend/resume procedure as "RRC Suspended". Compared to LTE idle state, RRC_INACTIVE brings 8x latency improvement, 5x power efficiency and 3.5x less signalling.

Jul
2020

3GPP publishes Release 16 specifications. In 5G RRC specification TS 38.331, version 16.1.0, preferredRRC-State IE is introduced as part of UE's release preference. UE can indicate preference towards RRC_IDLE, RRC_INACTIVE, leave RRC_CONNECTED without any preference for the next state or revert to an earlier indicated preference.

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## Cite As

Devopedia. 2021. "5G UE RRC States." Version 6, March 6. Accessed 2022-10-09. https://devopedia.org/5g-ue-rrc-states
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Last updated on
2021-03-06 08:47:34
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