ETSI TS 129 171 V18.1.0 (2026-03)

Digital cellular telecommunications system (Phase 2+) (GSM); Universal Mobile Telecommunications System (UMTS); LTE; Location Services (LCS); LCS Application Protocol (LCS-AP) between the Mobile Management Entity (MME) and Evolved Serving Mobile Location Centre (E-SMLC); SLs interface (3GPP TS 29.171 version 18.1.0 Release 18)

RTS/TSGC-0429171vi10

General Information

• Status: Not Published
• Technical Committee: 3GPP CT 4 - MAP/CAMEL/GTP/BCH/SS/TrFO/IMS/GUP/WLAN (formely CN4)
• Current Stage: 8 - Draft receipt by ETSI Secretariat
• Completion Date: 10-Mar-2026

Standards Content (Sample)

TECHNICAL SPECIFICATION
Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS);
LTE;
Location Services (LCS);
LCS Application Protocol (LCS-AP)
between the Mobile Management Entity (MME)
and Evolved Serving Mobile Location Centre (E-SMLC);
SLs interface
(3GPP TS 29.171 version 18.1.0 Release 18)

3GPP TS 29.171 version 18.1.0 Release 18 1 ETSI TS 129 171 V18.1.0 (2026-03)

Reference
RTS/TSGC-0429171vi10
Keywords
GSM,LTE,UMTS
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ETSI
3GPP TS 29.171 version 18.1.0 Release 18 2 ETSI TS 129 171 V18.1.0 (2026-03)
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Legal Notice
This Technical Specification (TS) has been produced by ETSI 3rd Generation Partnership Project (3GPP).
The present document may refer to technical specifications or reports using their 3GPP identities. These shall be
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The cross reference between 3GPP and ETSI identities can be found at 3GPP to ETSI numbering cross-referencing.
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.
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3GPP TS 29.171 version 18.1.0 Release 18 3 ETSI TS 129 171 V18.1.0 (2026-03)
Contents
Intellectual Property Rights . 2
Legal Notice . 2
Modal verbs terminology . 2
Foreword . 6
1 Scope . 8
2 References . 8
3 Definitions and abbreviations . 9
3.1 Definitions . 9
3.2 Abbreviations . 9
4 Functional Overview . 10
4.1 General . 10
5 LCS-AP Message Transport . 10
5.1 General . 10
5.2 Protocol Layering . 10
5.3 Usage of SCTP Associations . 11
6 LCS-AP Procedures . 11
6.1 General . 11
6.2 Procedures Applicable to LCS-AP . 12
6.2.1 Location Service Request . 12
6.2.1.1 General . 12
6.2.1.2 Successful Operation . 12
6.2.1.3 Unsuccessful Operation . 13
6.2.2 Location Information Exchange . 14
6.2.2.1 Connection Oriented Information Transfer . 14
6.2.2.1.1 General . 14
6.2.2.1.2 Successful Operation . 14
6.2.2.1.3 Abnormal Conditions . 15
6.2.2.2 Connectionless Information Transfer . 15
6.2.2.2.1 General . 15
6.2.2.2.2 Successful Operation . 15
6.2.2.2.3 Unsuccessful Operation . 15
6.2.2.2.4 Abnormal Conditions . 16
6.2.3 Ciphering Keys Delivery . 16
6.2.3.1 General . 16
6.2.3.2 Successful Operation . 16
6.2.3.3 Unsuccessful Operation . 16
6.3 Exception Procedures . 17
6.3.1 Location Abort . 17
6.3.1.1 General . 17
6.3.1.2 Normal Operation . 17
6.3.1.3 Abnormal Conditions . 17
6.3.2 Reset . 17
6.3.2.1 General . 17
6.3.2.2 Normal Operation . 18
6.3.2.3 Abnormal Conditions . 18
6.4 Error Handling . 18
6.4.1 Abnormal Conditions . 18
6.4.2 Overload . 18
7 LCS-AP Messages and Message Formats . 19
7.1 General . 19
7.2 Message Formats . 19
7.3 LCS-AP Messages . 19
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7.3.1 LCS-AP Location Request message . 20
7.3.2 LCS-AP Location Response message . 20
7.3.3 LCS-AP Location Abort Request message . 21
7.3.4 LCS-AP Connection Oriented Information message . 21
7.3.5 LCS-AP Connectionless Information message . 21
7.3.6 LCS-AP Reset Request message . 22
7.3.7 LCS-AP Reset Acknowledge message . 22
7.3.8 LCS-AP Ciphering Key Data message . 22
7.3.9 LCS-AP Ciphering Key Data Result message . 22
7.4 Information Elements . 23
7.4.1 General . 23
7.4.2 Message Type . 23
7.4.3 Location Type . 23
7.4.4 E-CGI . 23
7.4.5 LCS Client Type . 24
7.4.6 LCS Priority . 24
7.4.7 LCS QoS . 24
7.4.8 UE Positioning Capability . 25
7.4.9 Include Velocity . 25
7.4.10 IMSI . 25
7.4.11 IMEI. 26
7.4.12 Geographic Area . 27
7.4.13 Positioning Data . 30
7.4.14 Velocity Estimate . 34
7.4.15 Accuracy Fulfilment Indicator . 34
7.4.16 LCS Cause . 35
7.4.17 Payload Type . 36
7.4.18 APDU . 36
7.4.19 Network Element . 36
7.4.20 Return Error Request . 36
7.4.21 Return Error Cause . 36
7.4.22 Altitude and direction . 37
7.4.23 Geographical Coordinates . 37
7.4.24 Uncertainty Ellipse . 38
7.4.25 Horizontal Speed and Bearing . 38
7.4.26 Vertical Velocity . 38
7.4.27 PLMN Identity . 39
7.4.28 Correlation ID . 39
7.4.29 eNB ID . 40
7.4.30 LCS Service Type ID . 40
7.4.31 Cell Portion ID . 40
7.4.32 Civic Address. 40
7.4.33 Barometric Pressure . 41
7.4.34 RAT Type . 41
7.4.35 Coverage Level . 41
7.4.36 Ciphering Data . 41
7.4.37 Ciphering Data Ack . 42
7.4.38 Ciphering Data Error Report . 42
7.4.39 Ciphering Data Set . 43
7.4.40 Ciphering Data Error Report Contents. 48
7.4.41 High Accuracy Geographical Coordinates . 48
7.4.42 High Accuracy Uncertainty Ellipse . 49
7.4.43 High Accuracy Altitude . 49
7.4.44 High Accuracy Extended Uncertainty Ellipse . 50
7.4.45 High Accuracy Scalable Uncertainty Ellipse . 50
7.4.46 High Accuracy Scalable Uncertainty Altitude . 50
7.4.47 UE Country Determination Indication . 51
7.4.48 UE Area Indication . 51
7.5 Message and information element abstract syntax . 51
7.5.1 General . 51
7.5.2 Usage of protocol extension mechanism for non-standard use . 52
7.5.3 Elementary procedure definitions . 52
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7.5.4 PDU definitions . 54
7.5.5 Information element definitions . 59
7.5.6 Common definitions . 68
7.5.7 Constant definitions . 69
7.5.8 Container Definitions . 70
7.5.9 Message transfer syntax . 72
7.6 Handling of unknown, unforeseen and erroneous protocol data . 72
8 LCS-AP Timers . 72
8.1 General . 72
8.2 Timers of LCS-AP. 72
Annex A (informative): Change history . 73
History . 74

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Foreword
This Technical Specification has been produced by the 3rd Generation Partnership Project (3GPP).
The contents of the present document are subject to continuing work within the TSG and may change following formal
TSG approval. Should the TSG modify the contents of the present document, it will be re-released by the TSG with an
identifying change of release date and an increase in version number as follows:
Version x.y.z
where:
x the first digit:
1 presented to TSG for information;
2 presented to TSG for approval;
3 or greater indicates TSG approved document under change control.
y the second digit is incremented for all changes of substance, i.e. technical enhancements, corrections,
updates, etc.
z the third digit is incremented when editorial only changes have been incorporated in the document.
In the present document, modal verbs have the following meanings:
shall indicates a mandatory requirement to do something
shall not indicates an interdiction (prohibition) to do something
The constructions "shall" and "shall not" are confined to the context of normative provisions, and do not appear in
Technical Reports.
The constructions "must" and "must not" are not used as substitutes for "shall" and "shall not". Their use is avoided
insofar as possible, and they are not used in a normative context except in a direct citation from an external, referenced,
non-3GPP document, or so as to maintain continuity of style when extending or modifying the provisions of such a
referenced document.
should indicates a recommendation to do something
should not indicates a recommendation not to do something
may indicates permission to do something
need not indicates permission not to do something
The construction "may not" is ambiguous and is not used in normative elements. The unambiguous constructions
"might not" or "shall not" are used instead, depending upon the meaning intended.
can indicates that something is possible
cannot indicates that something is impossible
The constructions "can" and "cannot" are not substitutes for "may" and "need not".
will indicates that something is certain or expected to happen as a result of action taken by an agency
the behaviour of which is outside the scope of the present document
will not indicates that something is certain or expected not to happen as a result of action taken by an
agency the behaviour of which is outside the scope of the present document
might indicates a likelihood that something will happen as a result of action taken by some agency the
behaviour of which is outside the scope of the present document
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might not indicates a likelihood that something will not happen as a result of action taken by some agency
the behaviour of which is outside the scope of the present document
In addition:
is (or any other verb in the indicative mood) indicates a statement of fact
is not (or any other negative verb in the indicative mood) indicates a statement of fact
The constructions "is" and "is not" do not indicate requirements.

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1 Scope
The present document specifies the procedures and information coding for LCS Application Protocol (LCS-AP) that is
needed to support the location services in E-UTRAN. The LCS-AP message set is applicable to the SLs interface
between the E-SMLC and the MME. LCS-AP is developed in accordance to the general principles stated in
TS 23.271 [3].
2 References
The following documents contain provisions which, through reference in this text, constitute provisions of the present
document.
- References are either specific (identified by date of publication, edition number, version number, etc.) or
non-specific.
- For a specific reference, subsequent revisions do not apply.
- For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including
a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same
Release as the present document.
[1] 3GPP TR 21.905: "Vocabulary for 3GPP Specifications".
[2] 3GPP TS 36.305: "Stage 2 functional specification of User Equipment (UE) positioning in E-
UTRAN".
[3] 3GPP TS 23.271: "Functional stage 2 description of Location Services (LCS)".
[4] IETF RFC 4960: "Stream Control Transmission Protocol".
[5] TIA/EIA/IS-J-STD-036 (2000): "Wireless Enhanced Emergency Services".
[6] 3GPP TS 23.032: "Universal Geographical Area Description (GAD)".
[7] 3GPP TS 36.413: "S1 Application Protocol (S1AP)".
[8] ITU-T Recommendation X.680 (07/2002): "Information Technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation".
[9] ITU-T Recommendation X.681 (07/2002): "Information Technology - Abstract Syntax Notation
One (ASN.1): Information object specification".
[10] 3GPP TS 22.071: "Location Services (LCS); Service Description; Stage1".
[11] 3GPP TS 23.003: "Numbering, addressing and identification".
[12] ITU-T Recommendation X.691 (07/2002): "Information Technology - ASN.1 encoding rules -
Specification of Packed Encoding Rules (PER)".
[13] IETF RFC 4119: "A Presence-based GEOPRIV Location Object Format".
[14] IETF RFC 5139: "Revised Civic Location Format for Presence Information Data Format Location
Object".
[15] IETF RFC 6848: "Specifying Civic Address Extensions in the Presence Information Data Format
Location Object (PIDF-LO)".
[16] IETF RFC 5905: "Network Time Protocol Version 4: Protocol and Algorithms Specification".
[17] 3GPP TS 24.301: "Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage
3".
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[18] IETF RFC 4776: "Dynamic Host Configuration Protocol (DHCPv4 and DHCPv6) Option for
Civic Addresses Configuration Information".
3 Definitions and abbreviations
3.1 Definitions
For the purposes of the present document, the terms and definitions given in TR 21.905 [1] and the following apply. A
term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905 [1].
Elementary Procedure: LCS-AP protocol consists of Elementary Procedures (EPs). An LCS-AP Elementary
Procedure is a unit of interaction between the MME and the E-SMLC. An EP consists of an initiating message and
possibly a response message. Two kinds of EPs are used:
- Class 1: Elementary Procedures with response (success or failure),
- Class 2: Elementary Procedures without response.

3.2 Abbreviations
For the purposes of the present document, the abbreviations given in TR 21.905 [1] and the following apply. An
abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in
TR 21.905 [1].
CID Cell-ID (positioning method)
E-CID Enhanced Cell-ID (positioning method)
E-SMLC Enhanced Serving Mobile Location Centre
E-UTRAN Envolved Universal Terrestrial Radio Access Network
GNSS Global Navigation Satellite System
GPS Global Positioning System
LCS LoCation Services
LCS-AP LCS Application Protocol
LPP LTE Positioning Protocol
LPPa LTE Positioning Protocol Annex
LTE Long Term Evolution
MBS Metropolitan Beacon System
MO-LR Mobile Originated Location Request
MT-LR Mobile Terminated Location Request
NI-LR Network Induced Location Request
MME Mobility Management Entity
OTDOA Observed Time Difference Of Arrival
PDU Protocol Data Unit
SCTP Stream Control Transmission Protocol
SET SUPL Enabled Terminal
SLP SUPL Location Platform
SUPL Secure User Plane Location
TA Timing Advanced
UE User Equipment
U-TDOA Uplink Time Difference Of Arrival

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4 Functional Overview
4.1 General
Figure 4.1-1 below shows the architecture applicable to the positioning of a UE with E-UTRAN access. The SLs
interface is used to convey LCS-AP messages and parameters between the MME to the E-SMLC. It is also used for
tunnelling LTE Positioning Protocols (LPP between the E-SMLC and the target UE, LPPa between the E-SMLC and
the eNB), which are transparent to the MME as described in TS 36.305 [2].

E-SMLC
SLs
S1
UE
Serving
MME
eNB
LTE-Uu
S1
Neighbor
eNB
Figure 4.1-1 Positioning Interfaces in E-UTRAN

5 LCS-AP Message Transport
5.1 General
The LCS-AP is a logical interface between the MME and the E-SMLC. This clause specifies the standards for signaling
transport to be used across LCS-AP.
5.2 Protocol Layering
Figure 5.2-1 below shows the protocol layering used to support the transfer of LCS-AP PDUs between an E-SMLC and
a MME. The LTE Positioning Protocols (LPP and LPPa) can be carried in LCS-AP messages which are transparent to
the MME.
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LPP/
LPPa
Relay
NAS
LCS-AP
LCS-AP
S1-AP
SCTP SCTP
SCTP
IP IP IP
L2 L2 L2
L1 L1 L1
SLs
MME E-SMLC
Figure 5.2-1 Protocol Layering for LCS-AP
5.3 Usage of SCTP Associations
SCTP (see IETF RFC 4960 [4]) shall be supported as the transport layer of LCS-AP messages.
Semi-permanent SCTP associations shall be established between MME and E-SMLC, i.e. the SCTP associations shall
remain up under normal circumstances.
Local multi-homing should be supported. Remote multi-homing shall be supported.
Multiple local SCTP endpoints may be supported. Multiple remote SCTP endpoints shall be supported. When multiple
local or remote SCTP endpoints are configured, several simultaneous SCTP associations shall be supported between
MME and E-SMLC.
The MME shall establish the SCTP association. Since under normal operation there should always be an SCTP
association established between an MME and an E-SMLC, if the E-SMLC needs to initiate a message towards an MME
it shall do so over an existing SCTP association already established with that MME.
When an entity detects that an SCTP association has been lost, all resources for transactions open on that association
shall be released.
The registered port number for LCS-AP is 9082. The registered payload protocol identifier for LCS-AP is 29.
6 LCS-AP Procedures
6.1 General
The LCS-AP interface can be divided into the following procedures:
- Location service request procedure
- Location information exchange procedure
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The E-UTRAN positioning capabilities are intended to be forward compatible to other access types and other position
methods, in an effort to reduce the amount of additional positioning support needed in the future.
6.2 Procedures Applicable to LCS-AP
6.2.1 Location Service Request
6.2.1.1 General
The purpose of the location service request procedure is to obtain the location estimate for a target UE in E-UTRAN.
6.2.1.2 Successful Operation
MME
E-SMLC
LOCATION REQUEST
LOCATION RESPONSE
Figure 6.2.1.2-1 Location Service Request Procedure, Successful Operation
The initiator (MME) of the location service request procedure sends a LCS-AP Location Request message to the E-
SMLC associated with the current or last known serving cell for the target UE and starts the timer T3x01. The message
contains the following mandatory (M), conditional (C) and optional (O) information, where conditional parameters are
required if available.
- Correlation ID (M)
- Location Type (M)
- Cell Identifier (M)
- LCS Client Type (C)
- LCS Priority (C)
- LCS Service Type ID (C)
- LCS QoS (C)
- UE Positioning Capability (O)
- Include Velocity (O)
- IMSI of target UE (O)
- IMEI of target UE (O)
- APDU (O)
- RAT Type (O)
- Coverage Level (O)
- UE Country Determination Indication (O)
The Correlation ID is assigned by the MME and enables association of the location response with the location request
when more than one location service request procedure is ongoing for the UE with the same E-SMLC.
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The Location Type IE indicates the type of Location Information being requested. The following types are supported:
- Current geographic location estimate
- Location assistance data for the target UE
- Last known location estimate
If the location estimate is requested, the E-SMLC performs positioning procedure on the target UE using a particular
position method or a combination of more than one positioning method based on the UE capability. If UE capability is
unknown, the E-SMLC may request UE position capability through LPP as defined in TS 36.305 [2]. If the location
estimate is requested and the UE Country Determination Indication is set to "required", the E-SMLC shall map the
location estimate to the country or the area in the country or international area.
Alternatively, if assistance data was requested, the E-SMLC may provide positioning assistance data to the UE. The E-
SMLC may invoke the following LCS-AP procedures to get assistance data:
- Connection Oriented Information Transfer
- Connectionless Information Transfer
Otherwise, if a last known location estimate is requested, the E-SMLC obtains a geographic location estimate using
only the information provided in the LCS-AP Location Request message. The E-SMLC shall not attempt to obtain
location information for the target UE from either the E-UTRAN or the target UE (e.g. because the UE may not be
currently reachable).
If a location estimate or a last known location estimate was requested and was subsequently obtained, the E-SMLC shall
return a LCS-AP Location Response to the initiator of the location request using the same SCTP association as the
location request. This message contains the following mandatory (M), conditional (C) and optional parameters (O).
- Correlation ID (M)
- Location Estimate (M).
- Accuracy Fulfilment Indicator (O).
- Velocity estimate (C).
- Positioning Data (C).
- E-UTRAN Cell Identifier (O).
- Cell Portion ID (O).
- Civic Address (O).
- Barometric Pressure (O).
- UE Area Indication (O).
If assistance data was instead requested for an UE and the E-SMLC was able successfully to transfer this to the UE, the
E-SMLC shall return a LCS-AP Location Response to the initiator of the location request (MME). This message shall
contain no parameters. The absence of a LCS Cause parameter in this case implies that the transfer was successful.
If the MME receives the LCS-AP Location Response for corresponding request message, the MME shall stop the timer
T3x01.
6.2.1.3 Unsuccessful Operation
If the E-SMLC is unable to obtain any of the location information requested or if requested LCS assistance data could
not be transferred, the E-SMLC shall return a LCS-AP Location Response to the initiator of the Location Request
carrying the following parameters:
- Correlation ID (M)
- LCS Cause (M)
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- Positioning Data (O)
The E-SMLC shall use the same SCTP association for the Location Response as was used for the request.
If the MME receives the LCS-AP Location Response for corresponding request message, the MME shall stop the timer
T3x01.
On the expiry of the timer T3x01, the MME shall abort the procedure, release any resources allocated for this location
request procedure and notify the node that triggered the Location Request about the error.
6.2.2 Location Information Exchange
6.2.2.1 Connection Oriented Information Transfer
6.2.2.1.1 General
The Connection Oriented Information transfer procedure enables two-way transfer of LPP and LPPa messages between
an E-SMLC and a MME. The procedure is only valid while a location request procedure for the target UE is ongoing.
This procedure makes use of the same SCTP association as the location request procedure for the particular target UE.
6.2.2.1.2 Successful Operation

Endpoint A
Endpoint B
CONNECTION ORIENTED INFORMATION

Figure 6.2.2.1.2-1 Connection Oriented Information, Successful Operation
The initiator of the procedure (E-SMLC or MME) with a LPP or LPPa message to transfer concerning a particular
target UE sends a LCS-AP Connection Oriented Information message to a recipient carrying the following parameters:
- Correlation ID (M)
- Payload Type (M)
- APDU (M);
The Correlation ID in this message is the Correlation ID used for the Location Request. It shall be present for a message
transfer from the E-SMLC to the MME and for a message transfer from the MME to the E-SMLC.
The Payload Type shall be present to indicate the type of the APDU. The supported information types are LPP and
LPPa.
The APDU shall contain an LPP APDU when communicating between the E-SMLC and the target UE or an LPPa
APDU when communicating between the E-SMLC and serving eNB. The MME shall forward this to the serving eNB
for the target UE.
If the intended recipient is the E-SMLC for a target UE, the message is terminated in the E-SMLC. The E-SMLC shall
then perform interpretation of the APDU.
ETSI
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