Terminology » LTE » LTE S1-MME Interface: Position and Function Explained

LTE S1-MME Interface: Position and Function Explained

lte s1 mme interface network architecture mobile communication evolved packet core

This article explains the position of the LTE S1 MME interface within the LTE network architecture and describes its function.

LTE S1 interface

In LTE, the S1-MME interface connects the eNodeB (Evolved NodeB) to the MME (Mobility Management Entity).
The main functions of the S1-MME interface are as follows:

  • Signaling Transport : Carries all control-plane (signaling) messages between eNodeB and MME using SCTP over IP.
  • Bearer Management : Supports bearer setup, modification, and release between eNodeB and MME.
  • Mobility Management : Handles UE (User Equipment) mobility, including handover management (especially S1-based handover).
  • Session Management: Manages UE session states (attach, detach, tracking area update, etc.).
  • Authentication Assistance : Helps in the authentication of UE by exchanging necessary messages with MME.
  • NAS Signaling Transport : Transports NAS (Non-Access Stratum) signaling messages (like Attach Request, TAU Request) between UE and MME transparently.
  • Paging : MME sends paging requests to eNodeB to locate the UE.
  • Overload Control : Supports signaling for overload control if eNodeBs or MMEs are overloaded.
  • Notification Mechanism : Supports notifications from MME to eNodeB (e.g., for UE Reachability changes).

LTE system network interfaces

At protocol level, S1-MME uses following layers.

  1. Application Layer: S1AP (S1 Application Protocol)
  2. Transport Layer: SCTP (Stream Control Transmission Protocol)
  3. Network Layer: IP (Internet Protocol)

Summary

  • S1AP (S1 Application Protocol) is the main protocol handling signaling messages over the S1-MME interface.
  • Only control plane traffic goes over S1-MME. (User plane traffic uses S1-U interface directly between eNodeB and Serving Gateway.)