GMLC & Le Interface Operations Guide

OmniLCS includes a Gateway Mobile Location Centre (GMLC) function that exposes the Le interface to external LCS clients. This allows Public Safety Answering Points (PSAPs), fleet management systems, lawful intercept platforms, and other external parties to request subscriber locations via the standard Diameter-based Le reference point defined in 3GPP TS 29.172.

The GMLC supports both immediate and deferred location requests. Deferred requests include periodic location (position reported at regular intervals) and triggered location (position reported when a subscriber enters, leaves, or remains inside a geographic area).

To fulfil a request the GMLC follows the standard 3GPP procedure: it discovers the subscriber's serving MME over the SLh interface (LCS-Routing-Info-Request to the HSS, per TS 29.173), then sends a Provide-Location-Request (PLR) to that MME over the SLg interface. The MME performs the positioning (directly, or by delegating to an E-SMLC over SLs/LCS-AP) and returns the Location-Estimate in the Provide-Location-Answer (PLA). In this GMLC role OmniLCS does not position the UE itself — it queries the serving MME and relays the result.

This is one of several location options OmniLCS offers. For active, higher-accuracy positioning it uses its E-SMLC role over SLs instead, driving LPP/LPPa to the UE and eNB — see Two LCS roles, one location request. For subscribers on 2G/3G the GMLC reaches the serving MSC/SGSN over MAP/Lg instead of Diameter SLg — see 2G/3G location over MAP/Lg. The same POST /api/location call reaches every path; the GMLC/SLg path described here is selected for 4G cell/last-known requests.

Architecture

Request Flow

Immediate Location Request

An external LCS client sends a Provide-Location-Request (PLR) via Diameter. OmniLCS authorizes the client, discovers the serving MME over SLh, sends a PLR to that MME over SLg, and returns the MME's result in a Provide-Location-Answer (PLA).

Periodic Location Request

When the PLR contains a Deferred-Location-Type with the periodic bit set and a Periodic-LDR-Information grouped AVP, OmniLCS creates a periodic session that runs location fixes at the specified interval and delivers each result via a Location-Report-Request (LRR).

Triggered (Geo-fence) Location Request

When the PLR contains area event information, OmniLCS creates a triggered session that polls the UE position and fires when the trigger condition is met.

Emergency Call Routing with OmniCall (IMS)

The primary consumer of the GMLC is OmniCall, the IMS core. When a subscriber dials an emergency number, OmniCall's E-CSCF (Emergency Call Session Control Function) must route the call to the PSAP that serves the caller's physical location — routing to the wrong PSAP costs critical time. OmniCall gets that location from OmniLCS, which fills the LRF / RDF (Location Retrieval Function / Routing Determination Function) role of 3GPP TS 23.167.

The flow:

1. The UE places an IMS emergency call (urn:service:sos); it reaches OmniCall's E-CSCF.
2. OmniCall asks OmniLCS for the caller's location — over the Le interface (Diameter PLR, using the :emergency_services client type) or the REST API. OmniLCS answers through whichever role fits — GMLC/SLg for the serving cell, or E-SMLC/SLs for an active GNSS/OTDOA fix (see Two LCS roles, one location request).
3. OmniLCS returns the position (coordinates + serving cell ECGI).
4. OmniCall maps that location to the serving PSAP (routing determination) and forwards the INVITE there.
5. OmniCall enriches the SIP signalling so the PSAP receives the location with the call.

SIP location header enrichment

Using the position returned by OmniLCS, OmniCall adds the standard IMS-emergency location elements to the outbound INVITE, so the PSAP (and any intermediary) receives the caller's location with the call rather than having to query for it separately:

SIP element	Standard	Carries
Geolocation header + PIDF-LO body	RFC 6442 / RFC 4119	Geodetic (lat/lon + uncertainty) and/or civic location, by value or by reference
P-Access-Network-Info	RFC 7315	Serving access network and cell identity (ECGI)
Geolocation-Routing	RFC 6442	Whether downstream proxies may route on the conveyed location

The PSAP call-taker therefore sees the caller's location on call arrival, derived from the same OmniLCS positioning used everywhere else — the serving cell for a fast fix, or an active GNSS/OTDOA position when the UE and radio support it.

Configuration

GMLC Configuration

config :omnilcs, :gmlc,
  # Enable/disable the GMLC Le interface
  enabled: true,

  # Allow requests from clients not in the authorized list
  allow_unknown_clients: false,

  # Authorized external LCS clients
  authorized_clients: [
    %{
      name: "psap-01",
      type: :emergency_services,
      allowed_methods: [:cell, :ecid, :gnss, :otdoa],
      rate_limit: 100,
      description: "Primary PSAP"
    },
    %{
      name: "fleet-mgmt",
      type: :value_added_services,
      allowed_methods: [:cell, :ecid],
      rate_limit: 50,
      description: "Fleet management system"
    }
  ],

  # Allow deferred (periodic/triggered) location requests
  allow_deferred: true,

  # Maximum concurrent periodic sessions
  max_periodic_sessions: 100,

  # Maximum concurrent triggered (geo-fence) sessions
  max_triggered_sessions: 50,

  # Default interval for periodic sessions (ms)
  default_periodic_poll_interval_ms: 60_000,

  # Default polling interval for triggered sessions (ms)
  default_triggered_poll_interval_ms: 30_000,

  # Log all location fixes to InfluxDB
  influx_logging: true

GMLC Parameters

Parameter	Type	Required	Default	Description
enabled	Boolean	No	false	Enable the GMLC Le interface handler. When disabled, all PLRs are handled as standard SLg requests.
allow_unknown_clients	Boolean	No	false	Accept requests from clients not listed in authorized_clients. Set to true for permissive mode during integration testing.
authorized_clients	List	No	[]	List of authorized external LCS client configurations. See Client Parameters below.
allow_deferred	Boolean	No	true	Accept deferred (periodic and triggered) location requests. When false, only immediate requests are handled.
max_periodic_sessions	Integer	No	100	Maximum number of concurrent periodic location sessions. Returns Diameter 3004 (TOO_BUSY) when exceeded.
max_triggered_sessions	Integer	No	50	Maximum number of concurrent triggered location sessions.
default_periodic_poll_interval_ms	Integer	No	60000	Default interval between periodic location fixes when not specified in the PLR.
default_triggered_poll_interval_ms	Integer	No	30000	Default polling interval for triggered sessions when not specified in the Area-Event-Info AVP.
influx_logging	Boolean	No	true	Write all location fixes from GMLC sessions to InfluxDB.

Client Parameters

Each entry in authorized_clients:

Parameter	Type	Required	Default	Description
name	String	Yes	--	Client identity. Matched against the LCS-EPS-Client-Name or LCS-Requestor-ID-String AVP in the PLR.
type	Atom	No	:any	Expected LCS-Client-Type: :emergency_services, :value_added_services, :plmn_operator_services, :lawful_intercept_services, or :any.
allowed_methods	List	No	[:cell, :ecid, :gnss, :otdoa]	Positioning methods this client may request.
rate_limit	Integer	No	0	Maximum requests per minute. 0 means unlimited.
description	String	No	""	Human-readable description for the control panel.

LCS Client Types

Per 3GPP TS 29.172 section 7.4.4:

Value	Type	Description
0	Emergency Services	PSAP, E-CSCF emergency call routing
1	Value Added Services	Fleet management, asset tracking, location-based services
2	PLMN Operator Services	Operator internal services (O&M, network optimization)
3	Lawful Intercept Services	Law enforcement location requests

Deferred Location Types

Periodic Location

A periodic location session performs a configurable number of position fixes at a regular interval. Each fix is:

1. Performed by sending an SLg PLR to the subscriber's serving MME (resolved over SLh)
2. Written to InfluxDB as a location_fix measurement with session metadata
3. Logged via the standard location logger (CSV + ETS)
4. Delivered to the originating LCS client via Diameter LRR

The session terminates when the reporting count is exhausted.

Diameter AVPs used:

AVP	Code	Description
Deferred-Location-Type	1480	Bit 4 set indicates periodic LDR
Periodic-LDR-Information	2025	Grouped AVP containing reporting parameters
Reporting-Amount	2026	Number of position fixes to perform
Reporting-Interval	2027	Interval between fixes in seconds

Triggered (Geo-fence) Location

A triggered location session polls the UE position and evaluates it against one or more geographic areas. When the trigger condition is met, the position is reported.

Event types:

Event	Deferred-Location-Type Bit	Trigger Condition
Entering	Bit 1	UE transitions from outside to inside the area
Leaving	Bit 2	UE transitions from inside to outside the area
Being Inside	Bit 3	UE is inside the area on each poll

Supported area definitions:

• Circle -- Center point (latitude, longitude) and radius in meters
• Polygon -- List of vertices defining a closed polygon

The geo-fence evaluator reuses the ray-casting algorithm from the CAP alert polygon resolver for polygon containment checks, and haversine distance for circular area checks.

InfluxDB Location Logging

All GMLC session fixes are written to InfluxDB using the location_fix measurement:

Tags:

Tag	Description
imsi	Subscriber IMSI
method	Positioning method used (cell, ecid, gnss, otdoa)
source	Position source from the engine result
session_type	periodic, triggered, or immediate
client_name	Name of the requesting LCS client

Fields:

Field	Type	Description
latitude	Float	WGS84 latitude in degrees
longitude	Float	WGS84 longitude in degrees
altitude	Float	Altitude in meters (when available)
uncertainty	Float	Position uncertainty in meters
confidence	Integer	Confidence level (0-100)
duration_ms	Integer	Time taken to perform the fix

Example InfluxQL queries:

-- Latest fixes for a subscriber
SELECT * FROM location_fix WHERE imsi = '001010000000001' ORDER BY time DESC LIMIT 10

-- Periodic session track over the last hour
SELECT latitude, longitude FROM location_fix
WHERE session_type = 'periodic' AND imsi = '001010000000001' AND time > now() - 1h

-- Fix success rate by method
SELECT COUNT(*) FROM location_fix WHERE time > now() - 24h GROUP BY method

-- Geo-fence trigger events
SELECT * FROM location_fix WHERE session_type = 'triggered' AND time > now() - 24h

REST API

The deferred location API is available at https://<host>:8443/api/deferred_location.

List Active Sessions

GET /api/deferred_location

Response:

{
  "status": "ok",
  "count": 2,
  "data": [
    {
      "session_id": "a1b2c3d4-...",
      "type": "periodic",
      "imsi": "001010000000001",
      "method": "cell",
      "client_name": "rest-api",
      "status": "active",
      "interval_ms": 60000,
      "remaining_reports": 7,
      "total_reports": 10,
      "started_at": "2026-04-09T10:00:00Z",
      "last_fix_at": "2026-04-09T10:03:00Z"
    }
  ]
}

Create Periodic Session

POST /api/deferred_location
Content-Type: application/json

{
  "type": "periodic",
  "imsi": "001010000000001",
  "method": "cell",
  "interval_seconds": 60,
  "count": 10
}

Create Triggered Session

POST /api/deferred_location
Content-Type: application/json

{
  "type": "triggered",
  "imsi": "001010000000001",
  "method": "cell",
  "event_type": "entering",
  "poll_interval_seconds": 30,
  "max_reports": 0,
  "areas": [
    {
      "type": "circle",
      "center": {"lat": -33.8688, "lon": 151.2093},
      "radius_meters": 500
    }
  ]
}

Cancel Session

DELETE /api/deferred_location/:session_id

Metrics

Le Interface Requests

Metric: omnilcs_gmlc_le_request_total Type: Counter Description: Total Le interface requests from external LCS clients Labels:

• client_type -- LCS client type (emergency_services, value_added_services, etc.)
• result -- Request outcome: received, success, error, unauthorized

Metric: omnilcs_gmlc_le_request_duration Type: Histogram Description: Le request processing time in milliseconds Labels:

• client_type -- LCS client type

MAP / Lg Requests (2G/3G)

Metric: omnilcs_map_request_total Type: Counter Description: Total MAP/Lg location requests for 2G/3G subscribers (SendRoutingInfoForLCS, ProvideSubscriberLocation) Labels:

• type -- MAP operation (MAP-PSL)
• result -- sent, success

Location Report Requests

Metric: omnilcs_gmlc_lrr_total Type: Counter Description: Total Location Report Requests sent to external clients Labels:

• session_type -- periodic or triggered
• result -- sent or error

Session Tracking

Metric: omnilcs_gmlc_session_periodic_active Type: Gauge Description: Number of active periodic location sessions

Metric: omnilcs_gmlc_session_triggered_active Type: Gauge Description: Number of active triggered/geo-fence location sessions

Metric: omnilcs_gmlc_session_total Type: Counter Description: Total deferred sessions created Labels:

• type -- periodic or triggered

Geo-fence Events

Metric: omnilcs_gmlc_geofence_trigger_total Type: Counter Description: Total geo-fence trigger events fired Labels:

• event_type -- entering, leaving, or being_inside

InfluxDB Writes

Metric: omnilcs_gmlc_influx_write_total Type: Counter Description: Total InfluxDB location fix writes from GMLC sessions Labels:

• result -- success or error

Example Prometheus queries:

# Le request rate
rate(omnilcs_gmlc_le_request_total[5m])

# Deferred session creation rate by type
rate(omnilcs_gmlc_session_total[5m])

# Geo-fence trigger rate
rate(omnilcs_gmlc_geofence_trigger_total[5m])

# LRR delivery error ratio
sum(rate(omnilcs_gmlc_lrr_total{result="error"}[5m]))
  / sum(rate(omnilcs_gmlc_lrr_total[5m]))

Troubleshooting

Le Requests Returning 5012 (UNABLE_TO_COMPLY)

Symptoms: External LCS client receives result code 5012 for all PLRs.

Possible causes:

• GMLC not enabled in config (enabled: false)
• Client not in authorized_clients and allow_unknown_clients is false
• SLh resolution failed or there is no SLg route to the serving MME (check DRA/HSS/MME connectivity)

Resolution:

1. Verify config :omnilcs, :gmlc, enabled: true is set
2. Check the client name matches the LCS-EPS-Client-Name AVP in the PLR
3. Verify the SLh/SLg Diameter peers (DRA, HSS, MME) are connected on the Dashboard

Periodic Sessions Not Starting

Symptoms: PLR with periodic deferred type returns 3004 (TOO_BUSY).

Possible causes:

• max_periodic_sessions limit reached
• Session supervisor not running

Resolution:

1. Check active session count on the GMLC control panel page
2. Increase max_periodic_sessions if needed
3. Verify OmniLcs.Gmlc.SessionSupervisor is running in the Application page

Location Fixes Not Appearing in InfluxDB

Symptoms: Periodic/triggered sessions are running but no data in InfluxDB.

Possible causes:

• influx_logging set to false
• InfluxDB connection down
• Database does not exist

Resolution:

1. Verify influx_logging: true in GMLC config
2. Check InfluxDB connectivity (cell sync status is a good proxy)
3. Query InfluxDB directly: SHOW MEASUREMENTS should include location_fix

Geo-fence Not Triggering

Symptoms: Triggered session is active but never fires.

Possible causes:

• Area definition too small for the positioning method's accuracy
• Wrong event type (e.g., :leaving when subscriber is already outside)
• Positioning method returning nil coordinates

Resolution:

1. Check the positioning method accuracy -- Cell ID has kilometre-scale uncertainty, which may be too coarse for small geo-fences
2. Use a more accurate method (:ecid or :gnss) for smaller areas
3. Verify the area coordinates are correct (latitude/longitude order)