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PDU Sessions in 5G Networks: Evolution from EPS Bearers

PDU Sessions in 5G Networks: Evolution from EPS Bearers
PDU Sessions in 5G Networks: Evolution from EPS Bearers


PDU Sessions are integral to 5G networks, providing flexible and efficient user plane connectivity. This article explores the transition from EPS Bearers in 4G LTE to PDU Sessions in 5G NR, detailing their architecture, functionality, modes of operation, and their role in enabling diverse Quality of Service (QoS) profiles and network slicing.


As 5G networks evolve to meet growing demands for enhanced mobile broadband and massive IoT connectivity, PDU Sessions have emerged as a pivotal component for efficient user plane connectivity. This article delves into the architecture, operational modes, and advanced features of PDU Sessions, highlighting their significant advancements over EPS Bearers used in 4G LTE.

1. Evolution from EPS Bearers to PDU Sessions

In 4G LTE, EPS Bearers were used to establish end-to-end user plane connectivity between the UE and the Packet Gateway (P-GW). Each EPS Bearer corresponded to a single QoS profile, simplifying traffic management but limiting flexibility.

  • EPS Bearer Characteristics:

  • Single QoS profile per bearer.

  • Defined by the Access Point Name (APN) and Quality of Service Class Identifier (QCI).

2. Architecture of PDU Sessions in 5G

5G introduces PDU Sessions, which are more flexible and capable of supporting multiple QoS Flows within a session. Key architectural elements include:

  • PDU Session Components:

  • Data Network Name (DNN): Defines the interface to external data networks.

  • User Plane Function (UPF): Anchors PDU Sessions and manages QoS Flows.

  • Anchor UPF: Responsible for mapping downlink packets to specific QoS Flows.

3. Functional Aspects of PDU Sessions

3.1 QoS Flow Management

  • QoS Flows: Managed within a PDU Session, each supporting a unique 5G Quality of Service Identifier (5QI).

  • SDAP Layer: Responsible for mapping uplink packets to QoS Flows at the UE and Base Station.

3.2 Packet Routing and GTP-U Tunnels

  • GTP-U Tunnels: Establish end-to-end transport connections per PDU Session, optimizing packet delivery.

  • Transport Efficiency: Single tunnel per PDU Session between Base Station and UPF ensures streamlined data transport.

4. Modes of Operation and Flexibility

4.1 Operational Modes

  • Always-On PDU Session: Ensures continuous connectivity during UE transitions.

  • Session Continuity Modes (Mode 1, 2, 3): Define behavior during network handovers and load balancing.

4.2 Multi-Access Edge Computing (MEC) and Multi-Homing

  • MEC Support: Multiple Anchor UPFs can serve a single PDU Session, enhancing service availability and latency.

  • Multi-Homing: Improves reliability by supporting redundancy through multiple IPv6 prefixes.

5. Network Slicing and Service Continuity

  • Single Network Slice Selection Assistance Information (S-NSSAI): Specifies the Network Slice for a PDU Session, ensuring tailored service delivery.

  • SMF Selection: Based on S-NSSAI, enables dynamic selection of Service Management Function (SMF) for optimized network utilization.

6. Advanced Packet Types and Encapsulation

  • Packet Types: Supports IPv4, IPv6, Ethernet, and Unstructured packets.

  • Encapsulation: Layered encapsulation within NAS and NGAP messages for efficient data transfer across network interfaces.


PDU Sessions represent a significant advancement in 5G network architecture, offering enhanced flexibility, scalability, and QoS management compared to their predecessors. Understanding the architecture and operational aspects of PDU Sessions is crucial for deploying efficient and future-proofed 5G networks capable of supporting diverse use cases from enhanced mobile broadband to critical IoT applications.


  • Technical Specifications: TS 38.300, TS 23.501, TS 23.502, TS 24.501

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

This article provides a comprehensive overview of PDU Sessions in 5G networks, highlighting their evolution from EPS Bearers in LTE, their architectural components, operational modes, and advanced features such as network slicing and service continuity. For deeper insights into specific implementations and technical details, readers are encouraged to refer to the referenced 3GPP standards and technical specifications.

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