3GPP System Core Network

Description

The term '3GPP System core network' is defined in specification 21.905 as 'an evolved GSM core network infrastructure.' This definition, though concise, encompasses the core network elements that have evolved from the original GSM core (which included MSC, VLR, HLR, etc.) to support packet-switched services, and later to the Evolved Packet Core (EPC) and 5G Core (5GC). It represents the centralized part of the 3GPP system responsible for managing user sessions, mobility, security, and connectivity to external networks.

Architecturally, the core network comprises several functional entities. In GSM/UMTS, the core includes the Circuit Switched (CS) domain (MSC, GMSC) and Packet Switched (PS) domain (SGSN, GGSN). With LTE, the EPC introduced a flat, all-IP architecture with MME (mobility management), SGW and PGW (user plane), PCRF (policy), and HSS (subscription data). In 5G, the 5GC adopts a service-based architecture with AMF, SMF, UPF, PCF, UDM, etc., separating control and user planes and enabling network slicing.

The core network interfaces with access networks via standardized reference points (e.g., S1, N2, N3) and with external networks (e.g., Internet, IMS) via Gi/SGi/N6. It enforces QoS policies, manages bearer/session establishment, handles handovers between access technologies, and performs authentication and authorization using SIM-based credentials. It also supports roaming through inter-operator interfaces.

Key components include subscriber databases (HLR/HSS/UDM), gateways (GGSN/PGW/UPF) for data routing, and control nodes (MSC/SGSN/MME/AMF) for signaling. The core network is critical for enabling services like voice (VoLTE, VoNR), data, messaging, and IoT. It also provides lawful interception, charging, and policy control.

Over releases, the core network has evolved from a hierarchical, circuit-centric design to a flat, IP-based, and now service-based architecture. The definition in 21.905 has remained unchanged since Rel-5, reflecting that the term broadly covers all these evolutions as an 'evolved GSM core network infrastructure.' This stability underscores the continuity from GSM to 5G, where the core network remains the central control and gateway hub.

Purpose & Motivation

The term '3GPP System core network' exists to provide a generic reference to the core network infrastructure within the 3GPP system, without tying it to a specific technology generation. It acknowledges that the core network has evolved from its GSM origins but retains fundamental roles: managing subscribers, routing traffic, and enforcing policies.

Historically, the GSM core network was designed for circuit-switched voice and later enhanced with GPRS for packet data. As 3GPP developed UMTS, LTE, and 5G, the core network underwent significant transformations, but the term 'evolved GSM core network infrastructure' was retained to emphasize backward compatibility and continuity. This is important for specifications that need to refer to the core network generically, regardless of the release.

The purpose is also to distinguish the core network from the access network and user equipment. By defining it as an 'evolved GSM core network infrastructure,' 3GPP signals that the core network is not a static entity but a continuously evolving framework that supports new services and technologies while building on established principles.