Evolved Packet Core

Description

The Evolved Packet Core (EPC) is the core network component of the System Architecture Evolution (SAE) that was introduced in 3GPP Release 8. It represents a fundamental shift from the circuit-switched core network of 2G and 3G to an all-IP, packet-switched architecture designed to support high-speed data services, low latency, and multiple radio access technologies (RATs). The EPC is the heart of the LTE network and continues to play a role in 5G through interworking with the 5G Core (5GC).

The EPC architecture consists of several key network elements: the Mobility Management Entity (MME), Serving Gateway (SGW), Packet Data Network Gateway (PGW), Policy and Charging Rules Function (PCRF), and Home Subscriber Server (HSS). The MME handles control plane functions such as authentication, authorization, mobility management, and bearer management. The SGW routes and forwards user data packets and acts as a mobility anchor for handovers between eNodeBs. The PGW provides connectivity to external packet data networks (PDNs) and performs functions such as IP address allocation, policy enforcement, and charging. The PCRF manages policy and charging rules, while the HSS stores subscriber profiles and authentication vectors.

The EPC supports multiple access technologies, including LTE, 5G NR, and non-3GPP accesses such as Wi-Fi and CDMA2000. This is achieved through the concept of the Access Network Discovery and Selection Function (ANDSF) and the S2a/S2b interfaces for trusted and untrusted non-3GPP access. The EPC also introduces the concept of bearers, which are virtual connections that provide a specific quality of service (QoS) for user traffic. Bearers can be dedicated or default, and they are established and modified through signaling procedures involving the MME, SGW, and PGW.

One of the key innovations of the EPC is the separation of control plane and user plane (CUPS), which allows for independent scaling and optimization of signaling and data forwarding. This architecture also enables network functions to be virtualized and deployed in cloud environments, leading to the concept of Network Function Virtualization (NFV) and Software-Defined Networking (SDN). The EPC also supports advanced features such as carrier aggregation, dual connectivity, and interworking with 5G, ensuring a smooth evolution path.

The EPC is specified in a series of 3GPP Technical Specifications, including 23.401 (General Packet Radio Service enhancements for E-UTRAN), 23.402 (Architecture enhancements for non-3GPP accesses), and 29.274 (Evolved GPRS Tunnelling Protocol). The specifications 21.905 and 29.279 provide the vocabulary and the Mobility Management Entity (MME) and Serving GPRS Support Node (SGSN) interfaces, respectively. The EPC has been continuously evolved through releases, adding features such as Machine-Type Communication (MTC), Voice over LTE (VoLTE), and enhanced support for IoT devices.

Purpose & Motivation

The Evolved Packet Core was created to address the limitations of the previous generation core networks, which were primarily designed for circuit-switched voice and low-speed data. The rapid growth of mobile broadband and the need for higher data rates, lower latency, and support for multiple access technologies drove the development of a new, flat, all-IP architecture. The EPC provides a framework that enables operators to deliver a wide range of services, from basic internet access to real-time applications like VoLTE and video streaming, with guaranteed quality of service.

Before the EPC, core networks were hierarchical and complex, with separate domains for circuit-switched and packet-switched traffic. This made it difficult to scale and optimize for data services. The EPC simplifies the architecture by eliminating the circuit-switched domain and introducing a flat, IP-based core. It also introduces the concept of always-on connectivity, where users are always connected to the network and can be reached for services like push notifications. The EPC's support for multiple RATs allows operators to leverage existing investments in 2G and 3G networks while migrating to LTE and 5G.

Historically, the EPC was introduced in Release 8 as part of the SAE initiative, which aimed to create a new core network that could support the high data rates and low latency requirements of LTE. It has since been enhanced in subsequent releases to support new features such as carrier aggregation, dual connectivity, and network slicing. The EPC continues to be relevant as part of the 5G system, where it interworks with the 5G Core to provide seamless mobility and service continuity.