Core Network: Architecture and Signaling Procedures v2.2.0 PDF

5.1. S1AP (S1 Application Protocol)

5.1.1. Technical Overview

The S1 Application Protocol (S1AP) is the radio network layer signaling protocol of the S1-MME interface between the E-UTRAN and the MME, a scope that TS 36.413 clause 1 defines. S1AP carries UE mobility and session control, and it relays Non-Access Stratum (NAS) messages between the UE and the MME without inspecting their content.

S1AP consists of Elementary Procedures (EPs), where an EP is a unit of interaction between an eNodeB and the MME that comprises an initiating message and, for a Class 1 EP, a response message. TS 36.413 clause 3.1 defines two EP classes:

Class 1

An EP with a response, which reports a successful outcome, an unsuccessful outcome, or both outcomes for different requests within the same message.

Class 2

An EP without a response; the network always considers a Class 2 EP successful.

An eNodeB and the MME can run several EPs independently of each other, in parallel, unless a specific EP description restricts this independence.

S1AP provides two groups of services, a grouping that TS 36.413 clause 5 defines:

Non-UE-associated services

These are services related to the whole S1 interface instance between an eNodeB and the MME, for example S1 Setup or Reset.

UE-associated services

These are services related to one UE, over a UE-associated logical S1-connection that TS 36.413 clause 3.1 defines and that can exist before the eNodeB establishes the S1 UE context. This connection uses the MME UE S1AP ID and the eNB UE S1AP ID to identify the UE at the MME and at the eNodeB.

S1AP runs over the Stream Control Transmission Protocol (SCTP), which provides in-sequence delivery of S1AP messages and notifies S1AP if the signaling connection breaks. TS 36.413 clause 6 defines this transport requirement; TS 36.410 defines the S1 transport network layer in detail.

Unlike NAS, PFCP, and the GPRS Tunnelling Protocol (GTP), S1AP does not encode its messages with a fixed-position byte header. S1AP messages use the Packed Encoding Rules (PER) of Abstract Syntax Notation One (ASN.1), as defined in ITU-T Recommendation X.691, so a message’s on-the-wire layout depends on the abstract syntax of its information elements rather than on fixed octet positions. The Message Format section below describes the message structure at the ASN.1 level instead of a byte-position diagram.

5.1.2. Message Format

5.1.2.1. Elementary Procedure Structure

TS 36.413 clause 9.3.2 defines the S1AP-ELEMENTARY-PROCEDURE ASN.1 class, which every S1AP EP conforms to:

S1AP-ELEMENTARY-PROCEDURE ::= CLASS {
    &InitiatingMessage             ,
    &SuccessfulOutcome              OPTIONAL,
    &UnsuccessfulOutcome            OPTIONAL,
    &procedureCode      ProcedureCode  UNIQUE,
    &criticality        Criticality    DEFAULT ignore
}
InitiatingMessage

This is the message that starts the EP, and every EP defines it.

SuccessfulOutcome

This is the response message for a successful outcome, and only a Class 1 EP defines it.

UnsuccessfulOutcome

This is the response message for an unsuccessful outcome, and only a Class 1 EP that supports an explicit failure response defines it.

procedureCode

This is a value that uniquely identifies the EP, for example the value assigned to the Handover Preparation procedure or the E-RAB Setup procedure.

criticality

This is the default criticality of the EP itself, used when an S1AP node does not comprehend the procedure code.

5.1.2.2. S1AP-PDU

TS 36.413 clause 9.3.3 defines the top-level S1AP-PDU as a CHOICE between three message classes:

S1AP-PDU ::= CHOICE {
    initiatingMessage   InitiatingMessage,
    successfulOutcome   SuccessfulOutcome,
    unsuccessfulOutcome UnsuccessfulOutcome,
    ...
}

InitiatingMessage ::= SEQUENCE {
    procedureCode   S1AP-ELEMENTARY-PROCEDURE.&procedureCode,
    criticality     S1AP-ELEMENTARY-PROCEDURE.&criticality,
    value           S1AP-ELEMENTARY-PROCEDURE.&InitiatingMessage
}

SuccessfulOutcome and UnsuccessfulOutcome follow the same procedureCode/criticality/value structure as InitiatingMessage.

procedureCode

This field identifies which EP the message belongs to, for example E-RAB Setup or Path Switch Request.

criticality

This field states how a receiving node handles the message if it does not comprehend the EP, and the Criticality Values table below lists the possible values.

value

This field carries the message-specific information elements (IEs) for this EP and outcome, for example the IEs of an E-RAB SETUP REQUEST message.

5.1.2.3. Criticality Values

Each IE or IE group within a message can carry its own criticality information, independent of the criticality of the EP itself. TS 36.413 clause 10.3.2 defines these values.

Value Meaning

reject

The receiving node rejects the entire procedure if it does not comprehend the IE or IE group.

notify

The receiving node ignores the IE or IE group if it does not comprehend it, and notifies the sender.

ignore

The receiving node ignores the IE or IE group if it does not comprehend it, without notifying the sender.

5.1.3. Message Categories

5.1.3.1. E-RAB Management Messages

These messages assign, modify, and release the E-UTRAN Radio Access Bearers (E-RABs) and their Data Radio Bearers for a UE.

Message Functionality Direction

E-RAB Setup Request

Asks the eNodeB to assign Uu and S1 resources and set up Data Radio Bearers for one or more E-RABs.

MME → eNodeB

E-RAB Setup Response

Reports the E-RABs successfully set up and the E-RABs that failed to be set up.

eNodeB → MME

E-RAB Modify Request

Asks the eNodeB to modify one or more already established E-RABs for a UE.

MME → eNodeB

E-RAB Modify Response

Reports the E-RABs successfully modified and the E-RABs that failed to be modified.

eNodeB → MME

E-RAB Release Command

Asks the eNodeB to release one or more established E-RABs for a UE.

MME → eNodeB

E-RAB Release Response

Reports the E-RABs successfully released and the E-RABs that failed to be released.

eNodeB → MME

E-RAB Release Indication

Tells the MME that the eNodeB released one or more E-RABs on its own initiative.

eNodeB → MME

E-RAB Modification Indication

Asks the MME to accept a modification of one or more established E-RABs that the eNodeB initiated, for example a new downlink transport address.

eNodeB → MME

E-RAB Modification Confirm

Reports the E-RABs successfully modified, the E-RABs that failed to be modified, and the E-RABs to be released, for an E-RAB Modification Indication message.

MME → eNodeB
5.1.3.2. Context Management Messages

These messages establish, modify, suspend, resume, and release a UE’s context at the eNodeB and the MME, and transfer UE information between them.

Message Functionality Direction

Initial Context Setup Request

Establishes the initial UE context in the eNodeB, sets up the default IP connectivity, and optionally sets up one or more E-RABs.

MME → eNodeB

Initial Context Setup Response

Reports that the eNodeB established the initial UE context.

eNodeB → MME

Initial Context Setup Failure

Reports that the eNodeB could not establish the initial UE context.

eNodeB → MME

UE Context Release Request

Asks the MME to release the UE-associated logical S1-connection for an E-UTRAN generated reason, for example user inactivity.

eNodeB → MME

UE Context Release Command

Orders the release of the UE-associated logical S1-connection, for example after a completed handover.

MME → eNodeB

UE Context Release Complete

Confirms that the eNodeB released all signaling and user-plane transport resources for the UE.

eNodeB → MME

UE Context Modification Request

Asks the eNodeB to partly modify the established UE context, for example the Security Key or the UE Aggregate Maximum Bit Rate.

MME → eNodeB

UE Context Modification Response

Confirms that the eNodeB applied the requested UE context modification.

eNodeB → MME

UE Context Modification Failure

Reports that the eNodeB could not apply the requested UE context modification.

eNodeB → MME

UE Radio Capability Match Request

Asks the eNodeB to derive whether the UE radio capabilities are compatible with the network configuration for voice continuity.

MME → eNodeB

UE Radio Capability Match Response

Returns the Voice Support Match Indicator that a UE Radio Capability Match Request message asked for.

eNodeB → MME

UE Context Modification Indication

Asks the MME to verify Closed Subscriber Group (CSG) membership for the established UE context.

eNodeB → MME

UE Context Modification Confirm

Returns the CSG membership verification result for a UE Context Modification Indication message.

MME → eNodeB

UE Context Suspend Request

Asks the MME to suspend the UE context, the UE-associated logical S1-connection, and the related bearer contexts.

eNodeB → MME

UE Context Suspend Response

Confirms that the MME suspended the UE context.

MME → eNodeB

UE Context Resume Request

Tells the MME that the UE resumed its suspended Radio Resource Control (RRC) connection, and asks the MME to resume the UE context.

eNodeB → MME

UE Context Resume Response

Confirms that the MME resumed the UE context.

MME → eNodeB

UE Context Resume Failure

Reports that the MME could not resume at least one suspended E-RAB, and releases the UE-associated logical S1-connection.

MME → eNodeB

Connection Establishment Indication

Completes the establishment of the UE-associated logical S1-connection, and can trigger the eNodeB to obtain and report the UE radio capability.

MME → eNodeB

Retrieve UE Information

Asks the MME for the UE information of a Narrowband Internet of Things (NB-IoT) UE that uses Control Plane Cellular Internet of Things (CIoT) EPS Optimisation.

eNodeB → MME

UE Information Transfer

Sends the requested UE information, for example QoS parameters and UE radio capability, to the eNodeB.

MME → eNodeB

eNB CP Relocation Indication

Asks the MME to authenticate a UE’s re-establishment request for an NB-IoT UE that uses Control Plane CIoT EPS Optimisation.

eNodeB → MME

MME CP Relocation Indication

Tells the eNodeB that the UE’s connection is to be relocated to another eNodeB.

MME → eNodeB
5.1.3.3. Handover Signalling Messages

These messages prepare, execute, confirm, and cancel a handover of a UE between eNodeBs or between radio access technologies, and transfer Packet Data Convergence Protocol (PDCP) status between the source and the target eNodeB.

Message Functionality Direction

Handover Required

Starts the Handover Preparation procedure. The source eNodeB requests preparation of resources at the target side via the EPC.

eNodeB → MME

Handover Command

Reports that the MME accepted the handover, and carries the target-side transparent container for the UE.

MME → eNodeB

Handover Preparation Failure

Reports that the handover preparation failed.

MME → eNodeB

Handover Request

Asks the target eNodeB to reserve resources for the handover of a UE.

MME → eNodeB

Handover Request Acknowledge

Reports that the target eNodeB reserved the requested resources.

eNodeB → MME

Handover Failure

Reports that the target eNodeB could not reserve the requested resources.

eNodeB → MME

Handover Notify

Tells the MME that the UE arrived at the target cell and the S1 handover completed successfully.

eNodeB → MME

Path Switch Request

Establishes a UE-associated signaling connection to the EPC, and requests the switch of a downlink GTP tunnel to a new endpoint.

eNodeB → MME

Path Switch Request Acknowledge

Reports that the EPC switched the downlink path for at least one requested E-RAB.

MME → eNodeB

Path Switch Request Failure

Reports that the EPC failed to switch the downlink path for every requested E-RAB.

MME → eNodeB

Handover Cancel

Asks the EPC to cancel an ongoing handover preparation or an already prepared handover.

eNodeB → MME

Handover Cancel Acknowledge

Confirms that the EPC released the resources of the cancelled handover.

MME → eNodeB

eNB Status Transfer

Transfers the uplink and downlink PDCP Sequence Number (PDCP-SN) and Hyper Frame Number (HFN) status from the source eNodeB to the target eNodeB via the MME.

eNodeB → MME

MME Status Transfer

Forwards the PDCP-SN and HFN status that an eNB Status Transfer message carried to the target eNodeB.

MME → eNodeB

Handover Success

Tells the source eNodeB, during a Dual Active Protocol Stack (DAPS) handover, that the UE successfully accessed the target eNodeB.

MME → eNodeB

eNB Early Status Transfer

Transfers, during a DAPS handover, the COUNT of the first downlink Service Data Unit (SDU) that the source eNodeB forwards to the target eNodeB.

eNodeB → MME

MME Early Status Transfer

Forwards the COUNT that an eNB Early Status Transfer message carried to the target eNodeB.

MME → eNodeB
5.1.3.4. Paging Messages
Message Functionality Direction

Paging

Asks the eNodeB to page a UE in the cells of the tracking areas listed in the message.

MME → eNodeB
5.1.3.5. NAS Transport Messages

These messages carry NAS signaling, opaque to the eNodeB, between the UE and the MME over the S1 interface. TS 36.413 clause 8.6.1 defines this scope.

Message Functionality Direction

Initial UE Message

Forwards the UE’s first uplink NAS message to the MME, and can establish the UE-associated logical S1-connection.

eNodeB → MME

Downlink NAS Transport

Forwards a NAS message from the MME to the UE.

MME → eNodeB

Uplink NAS Transport

Forwards a NAS message from the UE to the MME, over an existing UE-associated logical S1-connection.

eNodeB → MME

NAS Non Delivery Indication

Reports that the eNodeB could not deliver, or decided not to start delivering, a NAS message to the UE.

eNodeB → MME

NAS Delivery Indication

Reports that the eNodeB successfully delivered a downlink NAS message to the UE, when the MME requested this indication.

eNodeB → MME

Reroute NAS Request

Asks the eNodeB to reroute an Initial UE Message message to the MME of the indicated Dedicated Core Network (DCN).

MME → eNodeB
5.1.3.6. Management Messages

These messages initialize the S1 interface, exchange interoperability data, report unrecoverable errors, and manage MME overload.

Message Functionality Direction

Reset

Initializes or re-initializes the E-UTRAN, or part of the E-UTRAN S1AP UE-related contexts, after a failure at the sending node.

Both

Reset Acknowledge

Confirms that the receiving node released the S1 resources and UE associations indicated in a Reset message.

Both

Error Indication

Reports a detected error in an incoming message, when no dedicated failure message exists to report it.

Both

S1 Setup Request

Starts the S1 Setup procedure, the first S1AP procedure after the transport network layer association becomes operational.

eNodeB → MME

S1 Setup Response

Reports that the MME accepted the S1 Setup Request, and exchanges application-level configuration data.

MME → eNodeB

S1 Setup Failure

Reports that the MME rejected the S1 Setup Request.

MME → eNodeB

eNB Configuration Update

Updates application-level configuration data that the eNodeB and the MME need to interoperate on the S1 interface.

eNodeB → MME

eNB Configuration Update Acknowledge

Confirms that the MME updated the configuration data.

MME → eNodeB

eNB Configuration Update Failure

Reports that the MME rejected the configuration update.

MME → eNodeB

MME Configuration Update

Updates application-level configuration data that the eNodeB needs to interoperate with the MME.

MME → eNodeB

MME Configuration Update Acknowledge

Confirms that the eNodeB updated the configuration data.

eNodeB → MME

MME Configuration Update Failure

Reports that the eNodeB rejected the configuration update.

eNodeB → MME

Overload Start

Tells the eNodeB to reduce the signaling load it sends towards the concerned MME.

MME → eNodeB

Overload Stop

Tells the eNodeB that the overload situation at the MME ended, and normal operation can resume.

MME → eNodeB
5.1.3.7. S1 CDMA2000 Tunnelling Messages

These messages tunnel CDMA2000 signaling, opaque to the eNodeB, between the UE and the CDMA2000 radio access technology over the S1 interface.

Message Functionality Direction

Downlink S1 CDMA2000 Tunnelling

Forwards a CDMA2000 message from the MME to the UE, through the eNodeB.

MME → eNodeB

Uplink S1 CDMA2000 Tunnelling

Forwards a CDMA2000 message from the UE to the MME, through the eNodeB.

eNodeB → MME
5.1.3.8. UE Capability Info Indication Messages
Message Functionality Direction

UE Capability Info Indication

Provides UE capability-related information, received from the UE, to the MME.

eNodeB → MME
5.1.3.9. Trace Messages

These messages control a trace session or a Minimization of Drive Tests (MDT) session for a UE, as specified in TS 32.422.

Message Functionality Direction

Trace Start

Asks the eNodeB to start a trace function, and optionally an MDT session, for a UE.

MME → eNodeB

Trace Failure Indication

Reports that a Trace Start or Deactivate Trace procedure failed because of an interaction with a handover.

eNodeB → MME

Deactivate Trace

Asks the eNodeB to stop the trace session identified by the given trace reference.

MME → eNodeB

Cell Traffic Trace

Sends the allocated Trace Recording Session Reference and Trace Reference to the MME.

eNodeB → MME
5.1.3.10. Location Reporting Messages
Message Functionality Direction

Location Reporting Control

Asks the eNodeB to report the UE’s current location, for example on every serving-cell change.

MME → eNodeB

Location Report Failure Indication

Reports that a Location Reporting Control procedure failed because of an interaction with a handover.

eNodeB → MME

Location Report

Provides the UE’s current location to the MME.

eNodeB → MME
5.1.3.11. Warning Message Transmission Messages

These messages start, replace, and cancel the broadcast of Public Warning System (PWS) messages, and report the eNodeB’s PWS status to the MME.

Message Functionality Direction

Write-Replace Warning Request

Starts, or replaces, the broadcast of a warning message in the indicated warning area.

MME → eNodeB

Write-Replace Warning Response

Reports the area(s) where the eNodeB is broadcasting the warning message.

eNodeB → MME

Kill Request

Asks the eNodeB to cancel an already ongoing broadcast of a warning message.

MME → eNodeB

Kill Response

Reports the area(s) where the eNodeB stopped the warning message broadcast.

eNodeB → MME

PWS Restart Indication

Tells the MME that PWS information for some or all cells of the eNodeB is available for reloading from the Cell Broadcast Centre (CBC).

eNodeB → MME

PWS Failure Indication

Tells the MME that an ongoing PWS operation for one or more cells of the eNodeB has failed.

eNodeB → MME
5.1.3.12. Direct Information Transfer Messages

These messages transfer RAN Information Management (RIM) information, in unacknowledged mode, between an eNodeB and a peer RAN node via the core network. The MME does not interpret the transferred information.

Message Functionality Direction

eNB Direct Information Transfer

Transfers RAN information from the eNodeB to the MME.

eNodeB → MME

MME Direct Information Transfer

Transfers RAN information from the MME to the eNodeB.

MME → eNodeB
5.1.3.13. Configuration Transfer Messages

These messages transfer Self-Organizing Network (SON) configuration information, in unacknowledged mode, between an eNodeB and a peer RAN node via the core network.

Message Functionality Direction

eNB Configuration Transfer

Transfers RAN configuration information from the eNodeB to the MME.

eNodeB → MME

MME Configuration Transfer

Transfers RAN configuration information from the MME to the eNodeB.

MME → eNodeB
5.1.3.14. LPPa Transport Messages

These messages carry LTE Positioning Protocol Annex (LPPa) signaling between an eNodeB and the Evolved Serving Mobile Location Centre (E-SMLC), relayed transparently by the MME, as specified in TS 36.455.

Message Functionality Direction

Downlink UE Associated LPPa Transport

Forwards a UE-associated LPPa message from the MME to the eNodeB, for example to support Enhanced Cell-ID (E-CID) or Uplink Time Difference Of Arrival (UTDOA) positioning of that UE.

MME → eNodeB

Uplink UE Associated LPPa Transport

Forwards a UE-associated LPPa message from the eNodeB to the MME.

eNodeB → MME

Downlink Non UE Associated LPPa Transport

Forwards a non-UE-associated LPPa message from the MME to the eNodeB, for example to obtain Observed Time Difference Of Arrival (OTDOA) assistance data.

MME → eNodeB

Uplink Non UE Associated LPPa Transport

Forwards a non-UE-associated LPPa message from the eNodeB to the MME.

eNodeB → MME
5.1.3.15. Secondary RAT Data Usage Report Messages
Message Functionality Direction

Secondary RAT Data Usage Report

Reports the resources used on a secondary Radio Access Technology (RAT), for example New Radio (NR) resources during E-UTRA-NR Dual Connectivity (EN-DC) operation.

eNodeB → MME
5.1.3.16. UE Radio Capability ID Mapping Messages
Message Functionality Direction

UE Radio Capability ID Mapping Request

Asks the MME for the UE radio capability information that maps to a given UE Radio Capability ID.

eNodeB → MME

UE Radio Capability ID Mapping Response

Returns the UE radio capability information that a UE Radio Capability ID Mapping Request message asked for.

MME → eNodeB

5.1.4. References

Specification Clauses and coverage

TS 36.413

  • Clause 1 — Scope

  • Clause 3.1 — Definitions, covering Elementary Procedures and EP classes, and the UE-associated logical S1-connection

  • Clause 5 — S1AP services

  • Clause 6 — Services expected from the signaling transport

  • Clause 8.1 — List of S1AP Elementary procedures (Table 1: Class 1 procedures, Table 2: Class 2 procedures)

  • Clause 8.2 — E-RAB Management procedures

  • Clause 8.3 — Context Management procedures

  • Clause 8.4 — Handover Signalling

  • Clause 8.5 — Paging

  • Clause 8.6 — NAS transport

  • Clause 8.7 — Management procedures

  • Clause 8.8 — S1 CDMA2000 Tunnelling Procedures

  • Clause 8.9 — UE Capability Info Indication

  • Clause 8.10 — Trace Procedures

  • Clause 8.11 — Location Reporting Procedures

  • Clause 8.12 — Warning Message Transmission Procedures

  • Clause 8.13 — eNB Direct Information Transfer

  • Clause 8.14 — MME Direct Information Transfer

  • Clause 8.15 — eNB Configuration Transfer

  • Clause 8.16 — MME Configuration Transfer

  • Clause 8.17 — LPPa transport

  • Clause 8.18 — Secondary RAT Data Usage Report

  • Clause 8.19 — UE Radio Capability ID Mapping

  • Clause 9.3.2 — Elementary Procedure definitions, covering the S1AP-ELEMENTARY-PROCEDURE class

  • Clause 9.3.3 — PDU Definitions, covering the S1AP-PDU choice and the InitiatingMessage, SuccessfulOutcome, and UnsuccessfulOutcome types

  • Clause 10.3.2 — Criticality information

ITU-T X.691

ASN.1 encoding rules: Specification of PER, used to encode S1AP messages

TS 36.410

E-UTRAN; S1 general aspects and principles: defines the S1 transport network layer

TS 32.422

Subscriber and equipment trace; Trace control and configuration management: defines the trace and MDT sessions that the Trace Messages control

TS 36.455

LPPa, the protocol relayed transparently between the eNodeB and the E-SMLC by the LPPa Transport messages

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