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Signalling Radio Bearers (SRB) in LTE and 5G NR Networks: Architecture and Functionality

Signalling Radio Bearers (SRB) in LTE and 5G NR Networks: Architecture and Functionality
Signalling Radio Bearers (SRB) in LTE and 5G NR Networks: Architecture and Functionality


Signalling Radio Bearers (SRB) play a critical role in LTE and 5G NR networks by facilitating the exchange of Radio Resource Control (RRC) and Non-Access Stratum (NAS) messages between User Equipment (UE), Base Stations, and network core elements like AMF and SMF. This article provides a detailed technical overview of SRBs, their types, functionalities, and their integration within the network protocol stacks.


In LTE and 5G NR networks, Signalling Radio Bearers (SRB) are essential components that establish logical connections for the transmission of critical control messages between UEs and base stations. This article explores the architecture, types, and operational aspects of SRBs, highlighting their significance in network signaling protocols.

1. SRB Overview and Types in 5G Networks

SRBs serve as dedicated channels for transmitting RRC and encapsulated NAS messages, ensuring reliable and efficient communication between different network entities. 3GPP specifications define four primary types of SRBs in 5G NR:

  1. SRB 0:

  • Purpose: Transfers RRC messages using the Common Control Channel (CCCH) logical channel.

  • Operation: Facilitates initial network access and system information exchange.

  1. SRB 1, 2, and 3:

  • Purpose: Transfer RRC messages using the Dedicated Control Channel (DCCH) logical channel.

  • SRB 1: Used for general RRC signaling.

  • SRB 2: Established after security activation for NAS message encapsulation.

  • SRB 3: Specific to E-UTRAN New Radio Dual Connectivity (EN-DC), managed by NR Secondary Node.

2. Functional Details of SRBs

2.1 SRB Operation and Configuration

  • RRC Messages: Includes setup, reconfiguration, and release of connections.

  • Message Types: Defined for both downlink and uplink transmissions, ensuring bidirectional communication integrity.

  • Security: After activation, all SRB messages are integrity protected and ciphered using Packet Data Convergence Protocol (PDCP).

2.2 SRB Types and Their Use Cases

  • SRB Splitting: Applies to SRB 1 and SRB 2 in Non-Standalone EN-DC configurations.

  • Enhanced Reliability: Utilizes Master and Secondary Node air-interfaces for improved reliability and redundancy.

3. Protocol Stacks and Integration

Integration with RRC and NAS Signaling

  • Logical Connections: SRBs establish logical connections between UEs and base stations.

  • Protocol Stacks: Interfacing between RRC layers, NAS protocols (via NGAP for AMF/SMF), and lower-layer protocols (PDCP, RLC, MAC).

4. Message Mapping and Protocol Efficiency

4.1 Message Mapping to SRBs

  • Table 16: Detailed mapping of RRC messages to respective SRBs (0, 1, 2, 3).

  • Transmission Modes: Transparent Mode (TM) for SRB 0, Acknowledged Mode (AM) for SRB 1 and 2.

4.2 Efficiency Measures

  • Uplink and Downlink Handling: SRB configurations optimize message handling for different network operations.

  • Prioritization: SRB 1 and 2 prioritize critical RRC and NAS messages based on network policies and service requirements.

5. Security and NAS Integration

  • Message Security: NAS messages between UE and AMF/SMF are integrity protected and ciphered, ensuring data confidentiality and authenticity.

  • Integration Points: NAS messages associated with session management terminate at SMF, with AMF acting as a relay for session-related signaling.


Signalling Radio Bearers (SRB) are foundational to the operational integrity and efficiency of LTE and 5G NR networks, enabling robust signaling between UEs, base stations, and core network elements. Understanding SRB types, their functionalities, and integration within protocol stacks is crucial for optimizing network performance and ensuring seamless user experiences.


  • Technical Specifications: TS 38.331, TS 37.340

  • 3GPP Standards: Ensuring compliance and interoperability across network deployments.

This technical article provides a comprehensive exploration of Signalling Radio Bearers (SRB) in LTE and 5G NR networks, emphasizing their critical role in network signaling, protocol integration, and message transmission efficiency. For further insights into specific implementations or detailed technical specifications, readers are encouraged to refer to the referenced standards and documents.

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