LTE Architecture Explained (E-UTRAN & EPC Overview)

LTE architecture diagram

Quick facts

Contents

1. LTE architecture diagram
2. What this diagram shows
3. Why LTE architecture matters
4. The two main parts of LTE architecture
5. E-UTRAN at a glance
6. EPC at a glance
7. Key LTE interfaces
8. Control plane and user plane
9. Bearer-based LTE connectivity
10. How LTE works end to end
11. Example: LTE attach in the architecture
12. LTE architecture and mobility
13. LTE architecture and IMS
14. Start here: explore LTE architecture by topic
15. Related message libraries
16. Related procedures
17. Key takeaways
18. FAQ

What this diagram shows

• The UE reaches LTE service through the eNodeB.
• The eNodeB is the LTE access node in E-UTRAN.
• The MME is the main EPC control-plane node for LTE access.
• The Serving Gateway anchors user-plane transport during mobility.
• The PDN Gateway connects LTE users to packet data networks and operator services.
• The HSS provides subscriber and authentication-related data, while the PCRF supplies policy and charging control functions.
• X2 provides inter-eNB coordination, especially for mobility scenarios.

Why LTE architecture matters

Without the architecture view, attach, service request, paging, bearer setup, and handover all look like isolated message sequences. With the architecture view, they become understandable as paths through real network entities and interfaces. That is why this page should be the starting point before deeper pages on LTE RAN, EPC, interfaces, protocol stack, and VoLTE.

• radio access and RRC handling in the E-UTRAN
• NAS signaling and mobility control in the EPC
• user-plane packet transport through S1-U, S5/S8, and SGi
• policy, authentication, and subscription handling through nodes such as PCRF and HSS

The two main parts of LTE architecture

The E-UTRAN side is organized around eNBs and their S1 and X2 connectivity, while the EPC side defines the core-network control, bearer, subscriber, and external-connectivity model.

E-UTRAN at a glance

The E-UTRAN consists of eNBs. Each eNB terminates the LTE radio stack toward the UE, including PDCP, RLC, MAC, PHY, and the RRC control plane. eNBs are interconnected by X2 and connected to the EPC by S1.

• radio resource management
• radio bearer control
• radio admission control
• connection mobility control
• dynamic resource allocation in uplink and downlink
• routing user-plane data toward the Serving Gateway

EPC at a glance

The core side contains the main mobility, session, policy, and external-connectivity functions. The Serving Gateway acts as the local mobility anchor point for inter-eNodeB handover, while the PDN Gateway sits on the path toward operator IP services and external packet data networks.

Key LTE interfaces

Control plane and user plane

On the LTE access side, the eNB terminates RRC toward the UE. On the core side, signaling flows through S1-MME toward the MME, with NAS handled in the EPC control path.

The radio user plane includes PDCP, RLC, MAC, and PHY. Traffic continues from the eNB over S1-U to the Serving Gateway, then across S5/S8 to the PDN Gateway, and finally over SGi to operator services or external packet data networks.

This split is the foundation for understanding why some LTE issues are purely signaling problems while others are user-plane transport problems.

Bearer-based LTE connectivity

LTE uses a bearer model. A default bearer provides the UE with connectivity throughout the lifetime of the PDN connection, which is why it is restricted to Non-GBR. For the hub page, the key idea is that a UE gets packet connectivity through an EPS bearer realized across the radio side and the core side, with different services later using dedicated QoS treatment when needed.

How LTE works end to end

Example: LTE attach in the architecture

• The UE enters through LTE-Uu.
• The eNB handles access-side signaling and forwards toward the EPC.
• The MME takes the control-plane role for mobility and NAS procedures.
• The HSS supports authentication and subscription handling over S6a.
• The S-GW and P-GW establish the packet path.
• The default bearer provides ongoing PDN connectivity for the UE.

LTE architecture and mobility

That is why LTE handover analysis always needs both a RAN view and a core-anchoring view.

• X2 supports inter-eNB coordination.
• S1 connects eNBs into the EPC.
• The Serving Gateway acts as a local mobility anchor for inter-eNB handover.

LTE architecture and IMS

Operator IP services such as IMS sit on the packet-network side of the architecture. That means IMS is not outside LTE architecture; it is reached through the LTE core and packet-data connectivity path.

Start here: explore LTE architecture by topic

Related message libraries

• LTE RRC
• LTE NAS
• LTE S1AP

Related procedures

• Attach procedure
• Service request
• Tracking Area Update procedure
• Paging procedure

Key takeaways

• LTE architecture is built from E-UTRAN + EPC.
• The eNodeB is the main LTE access node and terminates both the LTE radio user plane and RRC toward the UE.
• The EPC architecture includes MME, Serving Gateway, PDN Gateway, HSS, and PCRF.
• S1-MME, S1-U, X2, S11, S5/S8, S6a, and SGi are the key interfaces that make the LTE system work end to end.
• The default bearer provides connectivity for the lifetime of the PDN connection.

FAQ