Tutorials » LTE » LTE EPC Interfaces Explained: SGi, S1, S1u, S3, S4, S5, S6a, S11, S12

LTE EPC Interfaces Explained: SGi, S1, S1u, S3, S4, S5, S6a, S11, S12

lte epc network interface mobility management

The LTE Evolved Packet Core (EPC) forms the core of the LTE network. It manages data and signaling communications between the LTE radio access network (E-UTRAN) and external networks, like the internet or other service networks.

The EPC comprises several key network elements, including the Mobility Management Entity (MME), Serving Gateway (SGW), Packet Data Network Gateway (PGW), and the Home Subscriber Server (HSS). These elements are interconnected through various interfaces, each with a specific role in handling data traffic and signaling.

This guide explains the LTE EPC interfaces, including S1-MME, S1U, S3, S4, S5, S6a, Gx, S11, S12, and SGi.

Here’s a diagram illustrating the interfaces between the LTE system network elements:

LTE system network interfaces

MME: The control plane entity responsible for EPS mobility management and EPS session management.
Serving Gateway (SGW): Terminates the interface towards the radio network. It handles user plane traffic routing and forwards uplink and downlink packets between the PDN gateway and the radio network.
PDN Gateway (PGW): Provides PDN connectivity to the UE for different IP services offered by operators.

LTE EPC Interfaces Explained

The following network interfaces are defined for the EPC (Evolved Packet Core):

LTE EPC interface	Description
S1-MME interface	Used as a control application protocol between the E-UTRAN (eNodeB or LTE Base Station) and MME. It handles control plane signaling for mobility management, session management, and bearer management between the user equipment (UE) and the core network. It facilitates tasks like user authentication, handovers, and tracking area updates.
S1U interface	Used for S1 user plane data for each bearer between the E-UTRAN or eNodeB and serving gateway (SGW). It enables the serving gateway to anchor inter-eNB handovers. Manages the user plane, responsible for carrying user data packets between the UE and the core network. It ensures that data is routed efficiently from the UE to the internet or other data networks.
S3 interface	Provides a connection between the SGSN (Serving GPRS Support Node) and MME. It enables information exchange for mobility between inter-3GPP access networks. Facilitates the transfer of user and bearer information for inter-RAT (Radio Access Technology) mobility, such as handovers between LTE and 2G/3G networks.
S4 interface	Provides the interface between the SGSN and the serving gateway. It offers user plane support for mobility support between the GPRS core and the serving gateway. It also enables the serving gateway to anchor the inter-3GPP handover. Provides bearer management and user plane transfer for interworking between 2G/3G networks and the EPC, enabling seamless handovers.
S5 interface	Provides user plane tunneling and tunnel management functions between the serving gateway (SGW) and PDN gateway (PGW). It enables the serving gateway to connect to multiple PDN gateways to provide different IP services to the LTE UE. Also used for serving gateway relocation associated with UE mobility. Handles the user plane and provides the mobility anchor point for inter-eNodeB and inter-SGW handovers. It supports the transfer of user data and facilitates seamless mobility as users move across different network areas.
S6a interface	Connects the MME to the HSS. Responsible for exchanging subscription and authentication data between the MME and HSS. This interface is crucial for user authentication, access authorization, and mobility management.
Gx interface	* Function: Connects the PGW to the Policy and Charging Rules Function (PCRF).
* Purpose: Manages policy control and charging rules enforcement. It ensures that user data sessions adhere to the subscribed quality of service (QoS) and charging policies.
S11 interface	* Connects the MME to the SGW.
* Purpose: Handles the control plane signaling for the creation, modification, and deletion of bearers (data sessions). It coordinates the mobility of UEs between eNodeBs and manages handovers within the LTE network.
S12 interface	* Function: Connects the eNodeB directly to the SGW.
* Purpose: Used in some deployments to handle user plane data directly between the eNodeB and SGW without involving the S1-U interface, particularly in situations requiring optimization of user plane data handling.
SGi interface	* Connects the PGW to external networks, such as the internet, IMS (IP Multimedia Subsystem), or corporate intranets.
* Purpose: Acts as the interface through which user data exits the LTE network to reach external networks, enabling internet access, VoIP, and other IP-based services.

Summary

Understanding the various EPC interfaces, including SGi, S1, S1u, S3, S4, S5, S6a, S11, and S12, is crucial for comprehending the architecture and efficient operation of LTE networks. Each interface plays a unique role in connecting different network elements, facilitating smooth data transfer, signaling, and inter-system mobility. By ensuring proper configuration and management of these interfaces, operators can achieve optimal network performance and seamless communication experiences for end-users.