E-UTRA connected to 5G Core network

Description

E-UTRA/5GC is a key deployment configuration introduced in 3GPP Release 15, where the existing Evolved Universal Terrestrial Radio Access (E-UTRA) network—the radio technology for 4G LTE—is connected to the new 5G Core (5GC) network. This architecture decouples the radio access technology from the core network, allowing legacy LTE radios to deliver services over the advanced, cloud-native 5GC. In this mode, the LTE eNodeB (eNB) connects to the 5GC via the NG interface (specifically, the NG-eNB connects to the Access and Mobility Management Function (AMF) and User Plane Function (UPF)). The control plane signaling between the eNB and the AMF uses the NGAP protocol, while the user plane data flows through the N3 interface to the UPF. This setup enables LTE user equipment (UE) to register and establish Packet Data Unit (PDU) sessions with the 5GC, gaining access to services like network slicing, policy control via the Policy Control Function (PCF), and authentication via the Authentication Server Function (AUSF). The UE operates in a mode often referred to as "LTE-only" but with a 5G core, meaning it uses the E-UTRA radio protocols but the NAS signaling is with the 5GC's AMF. This configuration is a cornerstone of non-standalone (NSA) 5G deployment, often used as an intermediate step where LTE provides wide-area coverage anchoring the 5G NR layer for capacity. It allows operators to leverage their extensive LTE investments while introducing the benefits of the 5G core, such as network slicing for different service types, edge computing integration, and a service-based architecture (SBA) with modular network functions.

Purpose & Motivation

E-UTRA/5GC was created to facilitate a smooth and cost-effective migration from 4G to 5G networks. The primary problem it solves is the need to introduce the advanced capabilities of the 5G Core—like network slicing, edge computing, and a service-based architecture—without requiring an immediate and complete overhaul of the existing radio access network (RAN). Before this, LTE was exclusively paired with the Evolved Packet Core (EPC), which lacked the flexibility and cloud-native design needed for 5G's diverse service requirements (eMBB, URLLC, mMTC). The motivation was to allow operators to deploy the new 5GC independently of rolling out new 5G NR radio coverage, enabling early launch of 5G services over LTE coverage areas. This approach reduces initial capital expenditure, accelerates time-to-market for 5G services, and provides a seamless user experience during the transition. It also allows for the introduction of 5G-specific features like enhanced QoS and policy control to LTE users, bridging the gap between generations. Historically, this decoupling represents a significant shift from the tight integration seen in previous generations, offering greater deployment flexibility.