What is the interface between eNB and MME?

The interface between the eNB (evolved NodeB) and the MME (Mobility Management Entity) in LTE (Long-Term Evolution) networks is known as the S1 interface. This interface is a crucial component of the LTE architecture, facilitating communication and coordination between the eNB, responsible for radio access, and the MME, responsible for mobility management and control plane signaling. Let’s explore the detailed functions, protocols, and interactions on the S1 interface:

1. Definition of S1 Interface:

  • The S1 interface is the interface that connects the eNB to the MME in LTE networks. It serves as the communication link for control plane signaling between the radio access network (eNB) and the core network (MME) elements.

2. Functions of the S1 Interface:

  • Mobility Management: The S1 interface enables the exchange of information related to subscriber mobility. This includes procedures for tracking area updates, handovers, and mobility events as subscribers move within the LTE network.
  • Bearer Management: It supports the establishment, modification, and release of bearers for user data transmission. The bearers represent the logical connections that carry user data between the user equipment (UE) and the Evolved Packet Core (EPC) network.
  • Control Plane Signaling: The S1 interface handles control plane signaling, allowing the eNB and MME to exchange signaling messages for various procedures, including attach and detach procedures, bearer setup and modification, and handover signaling.
  • Initial UE Attach and Authentication: When a UE initially attaches to the LTE network, the S1 interface is involved in the authentication and authorization processes. The MME validates the subscriber’s identity and authorizes access to the network.
  • Handover Execution: During handovers, where a UE moves from one eNB to another, the S1 interface facilitates the signaling and coordination between the source eNB and the target eNB. This ensures a seamless handover experience for the subscriber.
  • Security Key Management: The S1 interface supports the exchange of security keys between the eNB and the MME. These keys are crucial for ensuring the confidentiality and integrity of communication between the UE and the network.

3. Protocols Used on S1 Interface:

  • S1-MME Protocol: The S1-MME (S1 Application Protocol) is used for control plane signaling between the eNB and MME. It includes procedures for initial attach, detach, handover, bearer setup, and other mobility-related events.
  • GTP (GPRS Tunneling Protocol): GTP is used on the S1 interface for the encapsulation and transport of user data bearers between the eNB and the Serving Gateway (SGW) in the EPC. It ensures the reliable and efficient transfer of user data.
  • X2 Protocol: While the X2 interface primarily handles communication between eNBs, it is worth mentioning that the X2 protocol is closely related to the S1 interface. The X2 interface facilitates direct communication between neighboring eNBs for efficient handovers and inter-cell coordination.

4. Key Procedures on S1 Interface:

  • Attach Procedure: The S1 interface is involved in the initial attach procedure when a UE connects to the LTE network. This includes authentication, authorization, and the assignment of temporary identities.
  • Handover Procedure: During a handover, the S1 interface coordinates the transfer of control and user plane connections from the source eNB to the target eNB, ensuring continuity of service.
  • Bearer Setup and Modification: The S1 interface manages the establishment, modification, and release of bearers for user data transmission, adapting to changing network conditions and subscriber requirements.
  • Detach Procedure: When a UE disconnects from the LTE network, the S1 interface is involved in the detach procedure, updating network databases and releasing resources associated with the subscriber.

5. Redundancy and Resilience:

  • To ensure network resilience and reliability, the S1 interface supports mechanisms for redundancy, load balancing, and failover. Redundant paths and backup connections help maintain service continuity in the event of network failures or disruptions.


The S1 interface between the eNB and MME in LTE networks is a vital link for control plane signaling, mobility management, and the establishment of bearers. Its protocols and procedures contribute to the seamless functioning of LTE networks, enabling efficient communication between the radio access network and the core network elements.

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