Sign in Subscribe

S1-MME (S1 for the control plane)

Last updated on 


The S1-MME (S1 for the control plane) is a protocol used in the Long-Term Evolution (LTE) and 5G networks for communication between the Evolved Node B (eNodeB) and the Mobility Management Entity (MME). It is part of the S1 interface, which connects the radio access network (RAN) and the core network (CN) in these networks.

The S1-MME protocol is responsible for the control plane signaling between the eNodeB and the MME. It handles the establishment, modification, and release of bearers (communication channels) between the user equipment (UE) and the core network, as well as the management of mobility-related functions.

Let's dive into the details of the S1-MME protocol:

S1-MME Messages:

S1-MME messages are exchanged between the eNodeB and the MME to perform various functions. Some of the key S1-MME messages include:

  • Initial UE Message: Sent by the eNodeB to the MME to initiate the setup of a new UE connection.
  • Paging Message: Sent by the MME to the eNodeB to notify the UE of an incoming call or message.
  • Authentication Request/Response: Messages used for the mutual authentication of the UE and the network.
  • Security Mode Command/Complete: Messages used to establish security algorithms and keys for secure communication.
  • Handover Request/Command/Preparation Failure: Messages exchanged during handover procedures to transfer UE connection from one eNodeB to another.
  • Service Request: Sent by the UE to request a specific service from the network.
  • UE Context Release Request/Command: Messages used to release the UE context between the eNodeB and the MME.

S1-MME Procedures:

The S1-MME protocol supports various procedures to handle different scenarios and events in the network. Some important procedures include:

  • Attach and Default Bearer Setup: When a UE attaches to the network, the eNodeB sends an Initial UE Message to the MME, which initiates the attach procedure. The MME authenticates and authorizes the UE, and if successful, establishes the default bearer for the UE.
  • Handover: When a UE moves from one eNodeB to another, handover procedures are triggered. The source eNodeB sends a Handover Request to the MME, which coordinates with the target eNodeB to perform a smooth handover of the UE.
  • Paging and Service Request: The MME sends a Paging Message to the eNodeB to alert the UE of incoming calls or messages. The UE responds with a Service Request message to request the specific service.
  • Release: When a UE detaches from the network or switches off, the eNodeB sends a UE Context Release Request to the MME, which releases the UE context and resources.

S1-MME Protocol Stack:

The S1-MME protocol uses a layered protocol stack for communication. The key layers in the S1-MME protocol stack are:

  • Stream Control Transmission Protocol (SCTP): It provides reliable transport of S1-MME messages over IP networks. SCTP offers features like message segmentation, retransmission, and congestion control.
  • S1 Application Protocol (S1-AP): S1-AP is the application layer protocol responsible for encoding and decoding the S1-MME messages. It defines the message formats, procedures, and message exchanges between the eNodeB and the MME.
  • IP Transport Layer: S1-MME messages are encapsulated into IP packets for transmission over the network. The IP transport layer handles routing and delivery of these packets.

In summary, the S1-MME protocol is a crucial component of LTE and 5G networks, enabling control plane signaling between the eNodeB and the MME. It facilitates functions such as UE attachment, bearer setup, mobility management, handover, and release. By exchanging S1-MME messages, the eNodeB and the MME coordinate and manage the communication and mobility of user equipment within the network.