RRC-Radio Resource Control in LTE

The Radio Resource Control (RRC) protocol is a fundamental layer in the LTE (Long-Term Evolution) control plane that is defined in 3GPP TS 36.331. It plays a crucial role in the configuration, management, and release of radio resources between the User Equipment (UE) and the Evolved Node B (eNodeB). The RRC layer sits at the top of the LTE Layer 2 stack and is responsible for maintaining the signalling connection that enables mobility management, session management, QoS provisioning, and other critical network functionalities.

In LTE, the RRC layer governs the transitions between idle and connected modes, handles system information broadcast, and manages security parameters and mobility procedures. Given its essential role in network operation and user experience, understanding RRC in-depth is necessary for telecom engineers, network designers, researchers, and testing professionals alike.

This article, based primarily on 3GPP TS 36.331, delves into the RRC protocol’s comprehensive structure, functionality, and procedures. It explains the message types, information elements, state transitions, and critical operations like connection setup, security handling, measurement reporting, mobility support, and capability exchange.

What is RRC?

RRC stands for Radio Resource Control. It’s a protocol that resides in the control plane of the LTE architecture, acting as the brain behind managing radio resources and connections between your phone (User Equipment or UE) and the cell tower (evolved NodeB or eNB).

The RRC layer is a Layer 3 protocol in the LTE control plane architecture. It operates between the NAS (Non-Access Stratum) and the PDCP (Packet Data Convergence Protocol) layer.

In the UE, the RRC is responsible for interpreting messages from the NAS layer (which handles mobility and session management) and implementing them via interaction with the PDCP and lower layers.

In the eNodeB, RRC interfaces with the S1-AP layer towards the EPC (Evolved Packet Core) and controls radio bearers and signalling towards UEs.

Key Functions of RRC:

• Connection Management: RRC establishes, maintains, and releases radio connections between your phone and the cell tower. It’s like setting up and closing communication channels.

• Resource Allocation: RRC efficiently allocates precious radio resources like frequencies and time slots to your phone based on its needs and network conditions. Think of it as assigning optimal lanes on the highway.

• Mobility Management: As you move between cell towers, RRC seamlessly hands you over to the next eNB without interrupting your calls or data sessions. It’s like ensuring a smooth lane change without hitting the brakes.

• Security Control: RRC sets up encryption and authentication mechanisms to protect your communication from eavesdropping and tampering. It’s like putting up toll booths with secure access.

• Power Control: RRC adjusts the transmission power of your phone to optimize battery life and network performance. Imagine it as regulating the engine power for fuel efficiency.

• Measurements and Reporting: RRC constantly gathers information about signal quality, interference, and network load. This data is used to optimize resource allocation and network performance. It’s like having traffic sensors to monitor the highway flow.

Signaling Radio Bearers (SRBs)

RRC messages are transported using Signalling Radio Bearers:

• SRB0: For RRC Connection Request (common channel)
• SRB1: For most RRC signalling post-connection
• SRB2: For carrying NAS messages

RRC States in LTE

In LTE, the User Equipment (UE) operates in one of two primary Radio Resource Control (RRC) states:

• RRC_IDLE
• RRC_CONNECTED.

The UE is considered to be in the RRC_CONNECTED state when an RRC connection has been successfully established with the eNodeB. If no such connection exists, the UE remains in the RRC_IDLE state.

Each state serves distinct purposes and dictates how the UE interacts with the network in terms of mobility, signaling, and resource usage.

RRC_IDLE State

In the RRC_IDLE state, the UE is not actively connected to the eNodeB but remains registered with the network. It performs autonomous cell reselection and listens for system information and paging messages.

Key Characteristics:

• Mobility: Controlled by the UE through cell selection and reselection; periodic Tracking Area Update (TAU) if required.
• Paging Monitoring: The UE monitors the paging channel to detect:
• Incoming calls or data
• System information changes
• Emergency alerts such as ETWS (Earthquake and Tsunami Warning System) or CMAS (Commercial Mobile Alert System)
• System Information: Regular acquisition of broadcast MIB/SIB messages from the serving cell.
• Neighbour Cell Monitoring: UE performs measurements and reselection between cells.
• DRX Support: A UE-specific DRX (Discontinuous Reception) cycle may be configured to reduce power consumption.

RRC_CONNECTED State

Once the RRC connection is established, the UE enters the RRC_CONNECTED state. In this state, the UE is actively managed by the network and can send/receive user and control plane data.

Key Characteristics:

• Data Transfer: Supports unicast data transmission between UE and the network.
• Mobility: Controlled by the network through handover procedures; supports inter-RAT transitions using NACC (Network Assisted Cell Change) for GERAN.
• Paging and System Info Monitoring:
• UE monitors the Paging channel and System Information Block Type 1 (SIB1) for updates.
• Also listens for ETWS and CMAS notifications.
• Control Channel Monitoring: Continuously monitors PDCCH to check for scheduled downlink transmissions.
• Measurement Reporting: UE performs measurements and provides reporting for handover and optimization.
• Channel Feedback: Reports channel quality indicators such as CQI, PMI, and RI.
• System Information Acquisition: Continues to acquire and update system information as needed.
• DRX Configuration: UE may be configured with a specific DRX cycle to conserve power during periods of inactivity.

System Information Broadcasting

The RRC layer is responsible for managing and broadcasting system information essential for UE operation. These messages are sent on BCCH using either BCH or DL-SCH channels.

• MIB (Master Information Block): Contains PHICH config, system frame number, bandwidth
• SIB1: Includes PLMN identity, tracking area code, cell access parameters
• SIB2–SIB13/SIB26: Provide configurations for RACH, timers, mobility, neighbouring cells, etc.

UEs decode the MIB and SIBs during initial access and periodically monitor them for changes. The eNodeB schedules SIBs and notifies changes using paging.

RRC Connection Management

This is a core RRC function that controls the establishment, maintenance, and release of RRC connections between UE and eNodeB.

RRC Connection Establishment

The process of establishing an RRC connection involves several steps:

• RRC Connection Request: Sent by the UE to the eNodeB to request an RRC connection.
• RRC Connection Setup: eNodeB responds with an RRC Connection Setup message.
• RRC Connection Setup Complete: UE completes the setup by sending an RRC Connection Setup Complete message.

RRC Connection Release

RRC connection release can be initiated by either the eNodeB or the UE.

The primary reasons for releasing an RRC connection include:

1. Inactivity timeout
2. UE request
3. Network optimization

Procedure:

• eNodeB sends RRC Connection Release message.
• UE acknowledges and transitions to RRC_IDLE state.
• UE performs cell reselection if needed.

RRC Connection Reconfiguration

RRC Connection Reconfiguration is used to modify the parameters of an existing RRC connection, such as handover, measurement configuration, or radio bearer modification.

Procedure:

• RRC Connection Reconfiguration Message: eNodeB sends an RRC Connection Reconfiguration message to the UE with the required parameters.
• UE Processes the Reconfiguration: UE applies the reconfiguration parameters as instructed by the eNodeB.
• RRC Connection Reconfiguration Complete: UE sends an RRC Connection Reconfiguration Complete message to the eNodeB.

RRC Connection Re-establishment

RRC Connection Re-establishment is triggered when the UE detects a radio link failure or upon certain mobility scenarios to quickly restore the connection.

Procedure:

• Radio Link Failure Detected: UE detects a failure in the existing radio link.
• UE Sends RRC Connection Re-establishment Request: UE sends an RRC Connection Re-establishment Request to the eNodeB.
• eNodeB Responds with Re-establishment: eNodeB processes the request and sends an RRC Connection Re-establishment message to the UE.
• UE Reconfigures and Responds: UE applies the re-establishment parameters and sends an RRC Connection Re-establishment Complete message.

Paging

Paging enables the network to notify a UE in RRC_IDLE or RRC_INACTIVE state of incoming services (e.g., calls, data) or system info updates.

• UE receives Paging Messages via PCCH.
• Paging includes UE ID (IMSI/S-TMSI), DRX cycle, and causes (MT call, ETWS, CMAS).
• Triggered by MME via S1 interface and relayed by eNodeB.

The RRC layer is responsible for initiating and configuring paging mechanisms in both the network and UE.

Security Functions

Security procedures are triggered immediately after RRC connection establishment:

• Key Derivation: From NAS keys using KDF to derive AS keys (K_RRCint, K_RRCenc).
• Security Mode Command/Complete: Negotiation and activation of encryption and integrity protection.
• Integrity Protection: Prevents tampering of signalling messages.
• Ciphering: Protects data confidentiality on air interface.

References

1. 3GPP TS 36.331 – Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol Specification, Version 15.3.0, Release 15, 3rd Generation Partnership Project (3GPP), September 2018.
   https://www.3gpp.org/DynaReport/36331.htm
2. 3GPP TS 36.300 – Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description, Version 15.9.0, Release 15.
   https://www.3gpp.org/DynaReport/36300.htm
3. 3GPP TS 36.304 – E-UTRA; User Equipment (UE) Procedures in Idle Mode, Version 15.3.0, Release 15.
   https://www.3gpp.org/DynaReport/36304.htm
4. 3GPP TS 36.331 ASN.1 – Radio Resource Control ASN.1 Definitions, included in 3GPP TS 36.331 specification package. Used for RRC message encoding/decoding.
5. ETSI – ETSI Portal and 3GPP specifications, European Telecommunications Standards Institute (ETSI).
   https://www.etsi.org/deliver/etsi_ts/136300_136399/
6. Sesia, S., Toufik, I., & Baker, M. (2011). LTE – The UMTS Long Term Evolution: From Theory to Practice (2nd ed.). Wiley.
7. Dahlman, E., Parkvall, S., & Skold, J. (2016). 4G: LTE/LTE-Advanced for Mobile Broadband (2nd ed.). Academic Press.