In brief
The EPC is the core network behind LTE radio access. It registers subscribers, manages their connections and forwards data to other IP networks. The two diagrams show the role of each network element.
When you open a website over LTE, the network checks your SIM, establishes a data connection and forwards packets to their destination. The Evolved Packet Core, or EPC, performs these tasks.
Why LTE needed a new core network
LTE was intended to carry more data while reducing the cost per transmitted bit. This involved changes to both radio access and the core network. The radio access network is called E-UTRAN, and its base stations are known as eNodeBs. They provide the radio connection to the end device. The EPC handles the functions beyond them.
The redesign was standardised as System Architecture Evolution (SAE). The resulting core network is the EPC, standardised by 3GPP. A simpler network structure was intended to improve transmission efficiency and reduce delay between sender and receiver.
LTE technology explains the radio-access foundations, from OFDMA and MIMO to FDD and TDD. LTE Advanced describes the developments introduced through Carrier Aggregation and enhanced antenna techniques.
EPC, EPS and SAE: what is the difference?
The abbreviations describe different things. EPC refers to the core network. EPS means Evolved Packet System and covers both LTE radio access and the core network. SAE describes the evolution of the system architecture. The EPC is therefore part of the EPS.
The EPC is packet-switched and IP-based. It has no circuit-switched transport of its own. Voice can be carried as an IP service. Connections to other networks also need to be considered; their implementation depends on the access and network architecture.
MME and HSS: who can join the network?
The Mobility Management Entity (MME) controls the connection. Its responsibilities include registration, authentication-related tasks and mobility management. It holds information about the area in which a device can be reached and exchanges signalling messages to perform these tasks.
The Home Subscriber Server (HSS) provides subscriber and authorisation data used for registration and other functions. The HSS predates LTE. In older mobile networks, the Home Location Register (HLR) manages subscriber data.
The distinction between signalling and user data is central: the MME controls the connection, while gateways forward the website's data. Earlier mobile systems also made this distinction. The EPC introduced its particular architecture and division of responsibilities.
S-GW and P-GW: where does the data go?
The Serving Gateway (S-GW) forwards user data between the base station and the PDN Gateway. PDN stands for Packet Data Network. The PDN Gateway is also commonly abbreviated to P-GW.
The P-GW connects to external IP networks. It also participates in providing the device's IP address. In the basic architecture, the user-data path runs from the device through the eNodeB and S-GW to the P-GW, then to the relevant IP network.
S-GW and P-GW are separate logical functions. An operator can implement them in one physical unit. Older descriptions also call this combination an SAE Gateway. Keeping their responsibilities distinct helps explain how data is forwarded.
PCRF: which rules apply to a data flow?
The Policy and Charging Rules Function (PCRF) supplies rules for data flows. These cover, for example, quality of service and the recording of usage relevant to charging. A voice service can have different requirements from a file download.
In a conventional EPC, the Policy and Charging Enforcement Function (PCEF) in the P-GW applies the PCRF's rules. The PCRF supplies usage-recording rules; other functions handle billing.
Key abbreviations at a glance
| Term | Full name | Role |
|---|---|---|
| E-UTRAN | Evolved UMTS Terrestrial Radio Access Network | LTE radio access with eNodeB base stations |
| EPC | Evolved Packet Core | Core network behind radio access |
| EPS | Evolved Packet System | Radio access and core network together |
| MME | Mobility Management Entity | Control, registration and mobility |
| HSS | Home Subscriber Server | Subscriber and authorisation data |
| S-GW | Serving Gateway | Forwarding between eNodeB and P-GW |
| P-GW | PDN Gateway | Gateway to the IP network |
| PCRF | Policy and Charging Rules Function | Rules for quality of service and charging |
Sources and context
This article explains the conventional LTE/EPC architecture. The diagrams show logical functions rather than particular devices or complete protocol sequences. Roaming, additional access networks and later architectural variants are omitted.
- LTEmobile: EPC – the LTE core network. Existing article, reorganised and technically clarified in this test version.
- 3GPP TS 23.401 / ETSI TS 123 401 V18.5.0, particularly sections 4.2, 4.4 and 4.7.5.
- 3GPP: LTE Advanced – System Architecture Overview, section 2.
Editorially revised test version dated 26 October 2026. The original article on ltemobile.de remains unchanged.