LTE S1AP Messages – Complete S1-MME Message List & Procedures
S1AP, or S1 Application Protocol, is the control-plane protocol used on the S1-MME interface between the eNodeB and MME in LTE. Defined in 3GPP TS 36.413, S1AP supports NAS transport, UE context management, E-RAB setup and release, handover, paging, S1 interface management, warning message transmission, trace, location reporting, and reset procedures.
Use this page to search or browse LTE S1AP messages by procedure category. Each message page explains the message direction, trigger, call-flow role, key fields, and practical troubleshooting notes.
Based on 3GPP TS 36.413
81 S1AP messages shown
No S1AP messages found. Try searching by message name, procedure, or acronym.
NAS Transport
S1AP messages used to carry LTE NAS signalling between UE/eNodeB and MME.
Starts UE-associated S1 signalling and carries the first NAS message.
Carries a downlink NAS payload toward the UE.
Carries an uplink NAS payload toward the MME.
Reports that a NAS PDU could not be delivered.
UE Context Management
Messages used to establish, modify, release, and manage UE-associated S1AP context at the eNodeB and MME.
Creates initial UE context and requested bearer resources.
Confirms initial context setup and reports bearer results.
Reports that initial context setup failed.
Requests release of UE-associated S1 context.
Commands release of UE-associated S1 context.
Confirms UE context release completion.
Requests updates to UE context information.
Confirms UE context modification.
Reports failure to modify UE context.
E-RAB Management
Messages used to setup, modify, release, and notify E-RAB resources for EPS bearers.
Requests setup of one or more E-RAB resources.
Reports E-RAB setup success or partial failure.
Requests modification of active E-RAB resources.
Reports the result of E-RAB modification.
Commands release of one or more E-RABs.
Confirms E-RAB release results.
Indicates locally triggered E-RAB release.
Indicates bearer modification information.
Confirms bearer modification indication handling.
Indicates E-RAB modification details.
Confirms E-RAB modify indication handling.
Handover & Mobility
Messages used during S1-based LTE handover preparation, resource allocation, command, notification, path switch, status transfer, and cancellation.
Requests S1 handover preparation.
Delivers the handover command to the source eNB.
Reports failure during handover preparation.
Requests target-side handover resources.
Accepts target-side handover resource preparation.
Reports target-side handover failure.
Notifies that the UE reached the target eNB.
Requests bearer path switch after mobility.
Confirms successful path switch.
Reports path switch failure.
Cancels an ongoing handover procedure.
Acknowledges handover cancellation.
Transfers PDCP/SN status during handover.
Forwards status transfer information.
Paging
Messages used by MME to page UEs through eNodeBs.
Requests paging of a UE in one or more tracking areas.
S1 Interface Management
Non-UE-associated messages used to establish, reset, update, and manage the S1-MME interface.
Starts S1-MME interface setup.
Accepts S1-MME interface setup.
Rejects S1 setup with cause handling.
Resets UE-associated or interface-level S1AP state.
Acknowledges S1AP reset handling.
Updates eNB configuration information.
Acknowledges eNB configuration update.
Rejects eNB configuration update.
Updates MME configuration information.
Acknowledges MME configuration update.
Rejects MME configuration update.
Starts overload control handling.
Stops overload control handling.
Transfers non-UE-associated information from MME.
Transfers non-UE-associated information from eNB.
Warning / PWS
Public warning system messages used to write, replace, cancel, restart, or report warning broadcast status.
Requests warning message broadcast or replacement.
Reports warning write or replacement result.
Requests cancellation of warning broadcast.
Reports warning cancellation result.
Indicates public warning system restart state.
Indicates public warning system failure.
Trace
Messages used to start, stop, or report trace information for UE or cell-level troubleshooting.
Starts trace recording for troubleshooting.
Reports trace activation failure.
Stops an active trace session.
Reports cell traffic trace information.
Location Reporting
Messages used for location reporting control, location report delivery, and location reporting failure handling.
Configures location reporting behavior.
Reports UE location information.
Reports location reporting failure.
Error Handling
Messages used to report protocol errors or unsuccessful procedure handling.
Reports S1AP protocol errors or invalid procedure handling.
Rejects S1 setup with cause handling.
Reports that initial context setup failed.
Reports failure during handover preparation.
Reports target-side handover failure.
Reports path switch failure.
Reports failure to modify UE context.
Advanced / Release-Specific
Additional S1AP messages used for release-specific features, LTE-M/NB-IoT, RIM, relay, or operator-specific behaviours if present.
Carries CDMA2000 tunnelling information uplink.
Carries CDMA2000 tunnelling information downlink.
Carries UE-associated LPPa information uplink.
Carries UE-associated LPPa information downlink.
Carries non-UE-associated LPPa information uplink.
Carries non-UE-associated LPPa information downlink.
Requests rerouting of NAS signalling.
Most Used S1AP Messages
Common S1AP Procedure Flows
S1 Setup
S1 Setup Request -> S1 Setup Response / S1 Setup Failure
LTE Attach / Initial Context
Initial UE Message -> Downlink/Uplink NAS Transport -> Initial Context Setup Request -> Initial Context Setup Response
E-RAB Setup
E-RAB Setup Request -> E-RAB Setup Response
UE Context Release
UE Context Release Request / Command -> UE Context Release Complete
S1 Handover
Handover Required -> Handover Request -> Handover Request Acknowledge -> Handover Command -> Handover Notify -> Path Switch Request
Path Switch
Path Switch Request -> Path Switch Request Acknowledge / Path Switch Request Failure
Paging
Paging -> UE response through Initial UE Message or service request flow
Warning Broadcast
Write Replace Warning Request -> Write Replace Warning Response; Kill Request -> Kill Response
FAQ
What are S1AP messages?
S1AP messages are LTE S1-MME control-plane messages exchanged between eNodeB and MME for NAS transport, UE context, E-RAB management, handover, paging, interface management, warning, trace, and location reporting.
Which interface uses S1AP?
S1AP is used on the S1-MME interface between the LTE eNodeB and MME.
Which specification defines S1AP?
S1AP is defined in 3GPP TS 36.413.
What is the difference between S1AP and NAS?
NAS carries UE-to-core signalling between UE and MME. S1AP carries signalling between eNodeB and MME and can transport NAS messages.
What is the difference between S1AP and RRC?
RRC runs between UE and eNodeB for radio connection control. S1AP runs between eNodeB and MME for core/RAN control-plane signalling.
What are the most important S1AP messages?
Commonly analyzed S1AP messages include Initial UE Message, Downlink NAS Transport, Uplink NAS Transport, Initial Context Setup Request, E-RAB Setup Request, UE Context Release Command, Handover Required, Handover Command, Path Switch Request, Paging, and S1 Setup Request.
What is E-RAB in S1AP?
E-RAB is the radio/core bearer resource associated with an EPS bearer. S1AP E-RAB procedures setup, modify, and release these bearer resources.
References
- 3GPP TS 36.413 - S1 Application Protocol (S1AP)
- 3GPP TS 36.300 - E-UTRAN overall description
- 3GPP TS 24.301 - LTE NAS / EPS NAS
- 3GPP TS 36.331 - LTE RRC