Evolved Packet Core (EPC)

1. Introduction: The Role of EPC in 4G

The Evolved Packet Core (EPC) is the packet core of LTE. Its functions handle signaling, subscriber authentication, data sessions, routing toward external networks, and quality-of-service policy. E-UTRAN manages the radio connection to the device, while EPC coordinates core resources and packet transport as the user moves.

A change in core network design

EPC changed the core model used in comparison with typical 2G and 3G networks. Those generations commonly contained a circuit-switched voice domain alongside a packet-switched data domain. LTE was designed as a packet core that can serve growing data traffic without duplicating the complete transport path.

The Evolved Packet Core is an All-IP network. EPC carries packet services as IP. VoLTE uses IMS over the IP transport provided by EPC. EPC supplies bearers, routing, and policy, while IMS supplies the voice-service semantics. This model simplifies the packet path and supports the low latency and high speeds of 4G.

2. Core Architectural Principles of the EPC

The EPC architecture follows several design principles that distinguish it from earlier cores. These principles organize how the LTE system operates.

Decoupling of the Control and User Planes

The network's functions are separated into two logical "planes" that operate independently while remaining coordinated.

The Control Plane (C-Plane)

The Control Plane is the signaling network. It handles all the management and control messages required to set up and maintain a connection. It carries NAS, mobility, bearer, and policy signaling. Key tasks managed by the C-Plane include:

Authenticating the user
Establishing and tearing down data bearers
Managing mobility (tracking user location and orchestrating handovers)
Applying policy and charging rules

The main EPC node in the Control Plane is the MME (Mobility Management Entity). User IP payload such as video or web content travels through the User Plane, while the Control Plane carries signaling and control information.

The User Plane (U-Plane)

The User Plane is the data transport network. It is a path designed to move user IP packets between the radio network and external networks. The U-Plane is optimized for:

High-speed packet forwarding
Low latency
Routing user data
Acting as the mobility anchor during handovers

The main EPC nodes in the User Plane are the SGW (Serving Gateway) and the PGW (Packet Data Network Gateway).

This separation allows network operators to scale the resources for signaling and data traffic independently. For instance, an operator could add more User Plane capacity (more SGWs/PGWs) in an area with heavy video streaming without needing to scale the Control Plane (MME) in the same proportion.

Flattened Architecture

Compared with the 3G architecture, which had hierarchical nodes such as the RNC, SGSN, and GGSN, the LTE EPC architecture is "flatter." A typical packet path contains fewer intermediate functions. The eNodeB connection to the SGW over S1-U bypasses the RNC and illustrates this change. Fewer hops can reduce latency, although the result depends on the transport topology, routing policy, and deployment of the external network.

3. The Functional Nodes of the EPC: A Detailed Look

The following functions describe the roles of the main components within the Evolved Packet Core.

MME (Mobility Management Entity)

The MME is the primary control node of the EPC. It manages connection state, NAS procedures, and mobility while user data remains in the User Plane. It exchanges signaling with E-UTRAN over S1-MME and controls procedures toward the SGW over S11. It selects serving gateways, maintains tracking-area information, and coordinates bearer establishment with the radio and User Plane nodes.

Detailed MME Functions:

Attachment and Detachment: The MME manages the entire process when a device first connects to (attaches) or disconnects from (detaches) the LTE network.
Authentication and Authorization: The MME participates in EPS security procedures. Through the HSS, it receives data needed to verify the UE identity using USIM credentials and to check service authorization.
Idle Mode Mobility Management: To conserve battery, a mobile device enters an idle mode when it is not actively transferring data. The MME is responsible for knowing the device's general location, defined by a group of cells called a . When new data arrives for the device, the MME initiates a "paging" procedure, instructing all eNodeBs in that TA to broadcast a message to wake the device up.
Bearer Management: The MME coordinates creation, modification, and deletion of EPS bearers, the logical paths for user data. It handles the signaling needed to establish resources in the eNodeB, SGW, and PGW.

SGW (Serving Gateway)

The SGW is a User Plane node that forwards packets and acts as a mobility anchor as a device moves within the radio access network.

Detailed SGW Functions:

Local Mobility Anchor: The SGW remains the logical User Plane anchor during handovers between eNodeBs. As the user changes cells, the access-side data path is switched while the SGW context can remain the same. The procedure aims to preserve session continuity, while the result depends on handover and transport behavior.
Inter-RAT Mobility Anchor: The SGW participates in mobility between radio technologies, for example between a 4G LTE area and a 3G UMTS area. Interworking procedures use the SGW and SGSN to transfer the packet context and path.
Packet Routing and Forwarding: At its simplest, the SGW is a high-performance router. It receives user data packets from the eNodeB (via the S1-U interface) and forwards them towards the PGW, and vice versa for downlink traffic.
Downlink Data Buffering: When the MME pages an idle device, the SGW temporarily stores downlink packets that arrive for that user. Once the device re-establishes its connection, the SGW releases the buffered data according to the procedure. The SGW uses S1-U toward E-UTRAN and S5 or S8 toward the PGW, depending on the architecture.

PGW (Packet Data Network Gateway)

The PGW is the user's gateway to networks beyond the mobile operator. It anchors the PDN connection and manages connectivity to external networks such as the public internet or a corporate intranet.

Detailed PGW Functions:

IP Address Allocation: The PGW assigns an to the user's device for a PDN connection. The address can remain constant during that connection, including when the device moves through different SGWs. Releasing or re-establishing the connection can result in a different allocation. A file download or VPN session illustrates the benefit within an active connection.
Connectivity to Packet Data Networks (PDNs): The PGW provides the actual connection to external networks via the SGi interface. Each PDN is identified by an . For most users, the APN connects to the internet. For corporate users, a special APN might connect to their company's private network. SGi separates the EPC side from the external PDN.
Policy and Charging Enforcement Function (PCEF): In the usual EPC architecture, the PGW hosts the PCEF and acts as the enforcement point for rules decided by the PCRF. It inspects user data packets and applies the appropriate Quality of Service (QoS) markings, filters traffic, and ensures that data usage aligns with the subscriber's plan.

HSS (Home Subscriber Server)

The HSS is the central subscriber database. It combines functions of older 2G/3G nodes such as the HLR (Home Location Register) and AuC (Authentication Center), and stores information needed for subscription and security procedures.

Data stored in the HSS:

Subscriber Identifiers: This includes the IMSI (International Mobile Subscriber Identity) and MSISDN (the user's phone number).
Security Information: The AuC/HSS security function protects subscriber secret material and generates authentication vectors used by the MME to verify identity.
User Profile: This defines the subscription, including services, allowed APNs, and QoS or AMBR parameters associated with the profile.
Location Information: The HSS stores serving-node information, including the MME, so the network can direct procedures and data according to registration state.

PCRF (Policy and Charging Rules Function)

The PCRF is the EPC decision function for policy and charging. It sends rules to the PCEF over Gx and can receive application information from an AF over Rx. It uses these inputs to control services and resources according to session and charging state.

Dynamic Policy Decisions: The PCRF uses service information from the AF, subscription information provided by the SPR, and session or network state. It decides how to treat user data flows and sends the rules to the PCEF in the PGW.
Quality of Service (QoS) Control: It is the PCRF that decides that a VoLTE call requires a bearer with guaranteed low latency, while a background cloud sync can use a best-effort bearer. It sends these policy rules to the PGW (the PCEF) for enforcement.
Charging Control: The PCRF provides policy and charging rules to the PCEF. An operator can configure a scenario in which selected social media applications do not consume the user's main data allowance, known as "zero-rating." This example depends on operator policy and the commercial plan.