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The X2 Interface in LTE and 5G NR Networks

The X2 Interface in LTE and 5G NR Networks
The X2 Interface in LTE and 5G NR Networks

Introduction

The X2 interface is a crucial component in the architecture of modern cellular networks, enabling communication between Base Stations (BS) in LTE and 5G NR environments. Originally introduced in Release 8 of the 3rd Generation Partnership Project (3GPP) specifications, the X2 interface has evolved to support various advanced network configurations, including Non-Standalone (NSA) architectures which integrate LTE and 5G NR technologies. This article provides a comprehensive overview of the X2 interface, focusing on its protocol stacks, functionalities, and procedural applications.


Protocol Stacks in the X2 Interface In 5G

The X2 interface encompasses two main components: the control plane (X2-C) and the user plane (X2-U). Each component has distinct protocol stacks, which are illustrated in Figure 50 and detailed below:


Control Plane (X2-C)

  • X2 Application Protocol (X2AP): Defined in 3GPP TS 36.423, X2AP handles signaling procedures for mobility, global operations, dual connectivity, and E-UTRAN – NR Dual Connectivity (EN-DC).

  • Stream Control Transmission Protocol (SCTP): SCTP, layered over IP, ensures reliable transport of X2AP messages.

  • Internet Protocol (IP): IP provides the network layer connectivity between Base Stations.

  • Data Link and Physical Layers: These layers are responsible for the actual data transmission over the transport medium.


User Plane (X2-U)

  • GPRS Tunneling Protocol User Plane (GTP-U): According to 3GPP TS 36.425, GTP-U encapsulates and tunnels user data over UDP and IP, facilitating data transfer between Base Stations.

  • User Datagram Protocol (UDP): UDP, layered over IP, allows for the quick and efficient transmission of data packets without the overhead of connection-oriented protocols.

  • Internet Protocol (IP): IP again provides the network layer for data routing.

  • Data Link and Physical Layers: These layers manage the physical transmission of user data packets.


Functional Overview

X2 Interface Deployment

In traditional setups, the X2 interface relies on the transport network to connect Base Stations. However, in configurations where Base Stations, such as eNode B and gNode B, are co-located within a single cabinet (typically in NSA architectures), the X2 interface may be internal, obviating the need for an external transport network.


Control Plane Signaling

X2AP supports various signaling procedures under the EN-DC configuration:

  • Mobility: Manages handover processes between Base Stations.

  • Dual Connectivity: Supports simultaneous connection to LTE (eNode B) and 5G NR (gNode B) nodes, enhancing data throughput and reliability.

  • E-UTRAN – NR Dual Connectivity Procedures: Table 9 lists key procedures such as SgNB Release, SgNB Counter Check, RRC Transfer, and Secondary RAT Data Usage Report.


User Plane Functionality

The user plane (X2-U) in the X2 interface is dedicated to data transfer using GTP-U tunnels. This setup allows the gNode B to report uplink and downlink data volumes, which can be forwarded by the eNode B to the Mobility Management Entity (MME) for charging and monitoring purposes. The X2-U maintains a streamlined protocol stack to ensure efficient data handling and transfer.


E-UTRAN – NR Dual Connectivity (EN-DC) Procedures

EN-DC allows a user equipment (UE) to connect simultaneously to LTE and NR networks, leveraging the X2 interface for communication between eNode B and gNode B. Key EN-DC procedures include:

  • RRC Transfer: Relays Radio Resource Control (RRC) messages between eNode B and gNode B, essential for managing SRB1 and SRB2 in split configurations.

  • Secondary RAT Data Usage Report: Enables reporting of data volumes transferred by each E-RAB, facilitating accurate billing and resource management.

  • SgNB Activity Notification: Indicates UE activity or inactivity, supporting effective Radio Resource Management (RRM).


Conclusion

The X2 interface serves as a vital link between LTE and 5G NR Base Stations, facilitating both control and user plane communications. Its evolution to support NSA architectures and dual connectivity highlights its adaptability in the rapidly advancing landscape of cellular networks. By integrating robust protocol stacks and efficient signaling procedures, the X2 interface ensures seamless operation across multi-generation network deployments.


References


Tables

Procedure

Description

SgNB Initiated SgNB Release

Handles the release of secondary gNode B connections

RRC Transfer

Relays RRC messages between eNode B and gNode B

Secondary RAT Data Usage Report

Reports uplink and downlink data volumes

SgNB Activity Notification

Indicates UE activity/inactivity for RRM

Table 9: X2 Application Protocol (X2AP) Procedures for E-UTRAN – NR Dual Connectivity

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