Evolved Packet Core (EPC)

Architektura sieci rdzeniowej All-IP dla system贸w LTE.

1. Introduction: The Brain of the 4G Network

If the radio towers (eNodeBs) are the eyes and ears of the mobile network, then the Evolved Packet Core (EPC) is its brain and central nervous system. It is a sophisticated, high-performance collection of components that acts as the control center and data gateway for the entire LTE system. While the radio network (E-UTRAN) manages the physical connection to your device, the EPC handles everything else: it identifies and authenticates you as a subscriber, manages your data sessions, routes your traffic to the internet, applies quality of service policies, and ensures a seamless connection as you move around.

A Revolution in Core Network Design

The EPC represented a revolutionary step forward from the core networks of previous generations (2G and 3G). Older networks were complex hybrids, built with two separate domains: one for voice calls (circuit-switched) and another bolted on for data (packet-switched). This dual-domain approach was inefficient and not suited for the massive data demands of the smartphone era.

The designers of LTE abandoned this legacy. The Evolved Packet Core was designed with a single, unifying philosophy: it is an All-IP network. This means that every single service, including voice (which is delivered via Voice over LTE, or VoLTE), is treated as data and transported in IP packets, just like traffic on the public internet. This simplifies the network architecture dramatically, reduces costs for operators, and is a key enabler for the low latency and high speeds of 4G.

2. Core Architectural Principles of the EPC

The efficiency and power of the EPC stem from a few key design principles that differentiate it from its predecessors. Understanding these principles is essential to grasping how the entire LTE system functions.

Decoupling of the Control and User Planes

This is perhaps the most important architectural concept within the EPC. The network's functions are cleanly separated into two logical "planes" that operate independently but in coordination.

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 is the network's command and control system. 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). Importantly, no actual user data (like video or web content) ever passes through the Control Plane components.

The User Plane (U-Plane)

The User Plane is the data transport network. It is a streamlined path designed for the sole purpose of moving user IP packets between the radio network and the external internet. It is the data superhighway. 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 to the 3G architecture, which had multiple hierarchical nodes (like the RNC, SGSN, and GGSN), the LTE EPC architecture is "flatter." The number of nodes a data packet must traverse to reach the internet is reduced. The direct connection from the eNodeB to the SGW, bypassing an intermediate controller, is a prime example. This reduction in the number of hops is a major contributor to LTE's lower latency.

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

Let us break down the specific roles of each major component within the Evolved Packet Core.

MME (Mobility Management Entity)

As the primary control node of the EPC, the MME is the "air traffic controller" for mobile devices. It manages everything about a user's connection state without ever touching the user's actual data.

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: It is the first point of contact for security. The MME communicates with the HSS to verify a user's identity using credentials stored on the SIM card and to determine which services the user is authorized to access.
  • 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 orchestrates the creation, modification, and deletion of EPS bearers, which are the logical "pipes" through which user data flows. It handles the signaling to set up the necessary resources in the eNodeB, SGW, and PGW.

SGW (Serving Gateway)

The SGW is the primary node in the User Plane, acting as a router and mobility anchor for devices as they move within the radio access network.

Detailed SGW Functions:

  • Local Mobility Anchor: The SGW's most crucial role is to be the stable point of connection for the User Plane during handovers between eNodeBs. As a user moves from one cell tower to another, their data path switches at the eNodeB level, but the connection to their assigned SGW remains constant. This ensures a smooth transition with no interruption to the data flow.
  • Inter-RAT Mobility Anchor: It serves the same anchor function when a user moves between different radio technologies, for instance, from a 4G LTE area to a 3G UMTS area. The session is seamlessly handed over from the SGW to the older 3G network node (the SGSN).
  • 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 plays a crucial role by temporarily storing (buffering) any downlink packets that arrive for that user. Once the device re-establishes its connection, the SGW releases the buffered data.

PGW (Packet Data Network Gateway)

The PGW is the user's gateway to the world beyond the mobile operator's network. It is the final anchor point for the data session and manages connectivity to external networks like the public internet or a corporate intranet.

Detailed PGW Functions:

  • IP Address Allocation: The PGW is the entity within the EPC that assigns an to the user's device. This IP address remains constant for the duration of the entire session, even if the user travels across the country and connects through multiple different SGWs. This ensures that long-running connections (like a file download or VPN session) are not broken.
  • 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.
  • Policy and Charging Enforcement Function (PCEF): The PGW acts as the enforcement point for the 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, master database of the network. It is the single source of truth for all subscriber information, combining the functions of older 2G/3G nodes like the HLR (Home Location Register) and AuC (Authentication Center).

Data stored in the HSS:

  • Subscriber Identifiers: This includes the IMSI (International Mobile Subscriber Identity) and MSISDN (the user's phone number).
  • Security Information: It stores the secret keys and generates the authentication vectors needed by the MME to verify the user's identity.
  • User Profile: This defines the user's subscription, including subscribed services, allowed APNs, data caps, and QoS profiles.
  • Location Information: The HSS maintains a record of which MME is currently serving the subscriber, allowing the network to route incoming calls and data correctly.

PCRF (Policy and Charging Rules Function)

The PCRF is the policy-making engine of the EPC. It provides the intelligence needed for dynamic control of services and resources based on real-time conditions.

  • Dynamic Policy Decisions: The PCRF takes inputs from multiple sources: the user's subscription profile from the HSS, information about the application being used, and the current state of the network. Based on this, it makes dynamic decisions about how to treat the user's data flows.
  • 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 instructs the PGW on how to charge for specific data flows. This enables sophisticated billing scenarios, such as allowing certain social media apps to be used without consuming the user's main data allowance ("zero-rating").
    Evolved Packet Core (EPC) | Teleinf Edu