Understanding the Combination of 5G Core and LTE Core Networks: Architecture, Interfaces, and Deployment Options

Combining 5G Core with LTE Core Network: A Detailed Technical Overview

As operators make the shift from 4G LTE to full 5G Standalone (SA), it’s important to note that the network core, which has usually been the EPC (Evolved Packet Core), now works alongside the Next-Generation 5G Core (5GC). The image provided gives a comprehensive look at how the LTE and 5G core networks interact, ensuring support for various devices—think 4G UEs, NSA Option 3 5G devices, and fully standalone 5G UEs.

This blended architecture is crucial for facilitating a smooth transition, maintaining backward compatibility, and ensuring effective spectrum use throughout the multi-year rollout of 5G.

In this article, we’ll explore the architecture, interfaces, functions, and operational dynamics depicted in the diagram. Whether you’re in telecom or just interested in 5G, this post aims to clarify how we can bring EPC and 5GC together.

Device Categories in the Integrated LTE + 5G Core Network

The image sorts devices into two main categories:

1. Devices Not Compatible with 4G

These connect solely through traditional EPC components:

SGW (Serving Gateway)

PGW (Packet Gateway)

PCRF (Policy and Charging Rules Function)

2. Devices That Support 4G and 5G

This group includes:

4G LTE UEs

NSA Option 3 “dual connectivity” UEs (LTE + NR)

SA 5G UEs

These devices can interface with both the EPC and 5GC depending on their capabilities and how the operator has set things up.

Overview of LTE EPC Architecture

The left side of the image showcases the EPC, which includes:

Key Components of EPC

MME (Mobility Management Entity): Manages control signaling for LTE.

SGW (Serving Gateway): Handles user-plane packets during user mobility.

PGW (Packet Gateway): Allocates IP addresses and connects to external networks.

PCRF (Policy and Charging Rules Function): Controls policy for Quality of Service (QoS) and charging.

Important Interfaces of EPC

InterfacePurposeS1-CMME ↔ LTE eNodeB control planeS1-UeNodeB ↔ SGW user planeS11MME ↔ SGW control planeS5/S8SGW ↔ PGW user/control plane

These interfaces are key to supporting 4G UEs and remain vital in NSA setups.

NR (New Radio) in NSA Option 3 Mode

The diagram emphasizes NR (Option 3)—the most broadly used 5G configuration worldwide. This option allows for 5G radios to be integrated into an existing LTE-EPC framework with minimal adjustments.

How Option 3 Works

LTE acts as the Master Node (MN)

NR gNB serves as the Secondary Node (SN)

The control plane is anchored to the MME and EPC

User-plane data might be split or directed through both LTE and NR

This combination enables operators to offer faster data rates using NR even before the full 5G Core is in place.

Benefits of Option 3

Lower deployment costs

Quick time-to-market

Full compatibility with existing LTE devices

Gradual rollout of 5G radio components

Overview of 5G Core (5GC) Architecture

On the right side, you’ll see the service-oriented, cloud-native structure of the 5G Core.

Main Functions of 5GC

AMF (Access and Mobility Management Function): Handles registration, mobility, and access control.

SMF (Session Management Function): Setups PDU sessions and oversees policies and session rules.

UPF (User Plane Function): Responsible for packet routing, quality of service enforcement, and edge breakout.

PCF (Policy Control Function): Unifies policy models, taking over from PCRF.

UDM (Unified Data Management): Manages subscriber data and authentication.

HSS (for interworking with EPC): Manages legacy data relevant to 4G identity.

Service-Based Interfaces (SBI)

The 5GC operates using HTTP/2 + JSON for signaling through REST-like APIs. The interfaces mentioned include:

N2: AMF ↔ RAN

N3: RAN ↔ UPF

N4: SMF ↔ UPF

N7: SMF ↔ PCF

N8: AMF ↔ UDM

N10: AMF ↔ UDM (HTTP2/JSON)

N11: AMF ↔ SMF

N15: PCF ↔ AMF

This layout provides flexible scaling, quicker updates, and network slicing—key parts of the 5G framework.

Link Between EPC and 5GC

A significant feature in the diagram illustrates the connection between EPC and 5GC, making for a seamless transition.

S-BASED EPC ↔ 5GC CONNECTIVITY

N26 Interface links the MME (from EPC) with the AMF (from 5GC), enabling:

Inter-system mobility (like transitioning from SA 5G to LTE)

Smooth session continuity

Shared authentication and mobility contexts

This ensures that users on SA 5G can revert to LTE without losing their PDU sessions.

Evolved LTE (eLTE)

This term refers to an upgraded LTE that supports:

Integration with 5GC via N2/N3

Improved QoS mapping

RAN features similar to 5G

Network slicing

It’s seen as a stepping stone before achieving full 5G NR coverage.

UE Scenarios Illustrated in the Diagram

The bottom section depicts three types of devices:

1. 4G UE

Connects only to LTE EPC

Does not use NR or 5GC

Functions through MME, SGW, PGW

Policies delivered via PCRF

2. Option 3 (NSA) 5G UE

Anchored to LTE EPC

Achieves higher speeds via NR layer

Control plane remains on EPC

Does not utilize 5GC functions

3. 5G SA UE

Connects directly to 5GC

Uses AMF, SMF, UPF for signaling and user plane tasks

Offers slicing, URLLC, and edge computing

The diagram clearly shows how all three types of UEs can coexist during the initial and mid-phase 5G deployments.

Comparative Overview: EPC vs. 5GC

FeatureEPC5G Core (5GC)Control PlaneMME-drivenService-based AMF/SMFUser PlaneSGW + PGWUPFPolicyPCRFPCFSignalingDiameterHTTP2/JSONArchitectureMonolithicCloud-native, containerizedSlicingNot supportedFully supportedMEC IntegrationLimitedDeep integration with UPF

This table reflects the capabilities illustrated in the accompanying image.

Reasons Operators Opt for a Combined EPC + 5GC Model

1. Gradual Migration Approach

No operator can implement nationwide SA all at once. The hybrid Evolved Packet Core + 5GC enables:

Phased implementation of NR

Device compatibility

Reuse of current infrastructure

2. Cost Efficiency

Connecting EPC and 5GC helps prevent costly full-scale migrations.

3. Performance Improvements

NSA Option 3 increases data speeds using NR without altering the EPC.

4. Coverage Continuity

While 5G SA relies on 5GC, coverage gaps call for EPC fallback via N26.

5. Support for All Device Types

The network can cater to:

Legacy 4G devices

5G NSA devices

5G SA devices
No one gets left behind.

Conclusion

The merged LTE EPC and 5G Core framework allows operators to roll out 5G services while keeping backward compatibility in check and ensuring a smooth migration path. The diagram illustrates how EPC elements (like MME, SGW, PGW, PCRF) collaborate with 5G Core functions (like AMF, SMF, UPF, PCF, UDM) across interfaces such as N26, N2, N3, and N4.

This hybrid model supports a range of user equipment—from older 4G models to advanced 5G SA devices—while giving operators the chance to gradually upgrade their networks to a fully cloud-native 5G setup.

As the adoption of 5G Standalone grows, we can expect UPFs to move closer to the network edge, slicing to become more standard, and for the EPC to eventually phase out. But for now, during this lengthy shift, the combined core model is critical, effective, and technically sound.