5G Core Network
Service-Based Architecture
& Beyond

1.1 The Evolution to Service-Based Architecture

The 5G Core Network represents a fundamental paradigm shift from the point-to-point reference point architecture of 4G EPC to a Service-Based Architecture (SBA). In SBA, Network Functions (NFs) communicate through well-defined service-based interfaces using HTTP/2 and RESTful APIs.

This isn't just a cosmetic change — it's a complete rethinking of how a mobile core network operates:

• Decomposition: Monolithic network elements are broken into microservices
• Service Discovery: NFs register with NRF and discover peers dynamically
• Stateless Design: State is separated from processing (UDSF, UDR)
• Cloud-Native: Designed from day one for containerized deployment

1.2 From EPC to 5GC — What Changed?

In 4G EPC, the MME, SGW, and PGW communicate using GTP-C/GTP-U tunnels over well-defined reference points (S1, S5, S11). Each interface is fixed and protocol-specific.

5G Core replaces this with a service bus model. Instead of N dedicated interfaces between N entities, each NF exposes services that any authorized NF can consume. The AMF doesn't "talk to the SMF over N11" — it consumes the Nsmf_PDUSession service.

Key Network Functions

The 5G Core introduces a rich set of Network Functions, each with a focused responsibility:

• AMF — Access and Mobility Management: Registration, connection, mobility
• SMF — Session Management: PDU session lifecycle, QoS, UPF selection
• UPF — User Plane: Packet routing, inspection, QoS enforcement
• NRF — NF Repository: Service registration and discovery
• AUSF — Authentication Server: 5G-AKA, EAP-AKA′
• UDM — Unified Data Management: Subscription data, authentication vectors
• PCF — Policy Control: QoS policies, charging rules, slicing policies
• NSSF — Network Slice Selection: Slice selection during registration