OLCM

Outgoing Leg Control Model

Core Network
Introduced in R99
A control model used in the CAMEL (Customised Applications for Mobile network Enhanced Logic) architecture to manage the outgoing leg of a call or session. It defines the interaction between the gsmSCF (CAMEL service control function) and the MSC/GMSC for controlling call setup and routing.

Description

The Outgoing Leg Control Model (OLCM) is a fundamental component of the CAMEL (Customised Applications for Mobile network Enhanced Logic) service architecture defined by 3GPP. CAMEL enables operator-specific services (like prepaid, number translation, or VPN) by allowing an external service logic node, the gsmSCF (GSM Service Control Function), to control call handling in the network's switching nodes (MSC - Mobile Switching Centre or GMSC - Gateway MSC). The OLCM specifically governs the control of the 'outgoing leg' of a call—the portion from the point where the call is being routed towards its destination. When a CAMEL-triggered call is detected (based on subscriber data from the HLR), the MSC/GMSC suspends normal call processing and sends an InitialDP (Initial Detection Point) message to the gsmSCF. The gsmSCF, using the OLCM, can then instruct the switch on how to proceed. Key instructions include 'Continue' (to proceed with normal routing), 'Connect' (to route to a specific destination number provided by the gsmSCF), 'ReleaseCall' (to terminate the call), or 'RequestReportBCSMEvent' (to ask for further notifications about call events). The model operates through a sequence of Detection Points (DPs) and Events, allowing the gsmSCF to be notified at specific moments in the call (e.g., answer, busy, no answer) and respond with appropriate control commands. This model is stateful, with the gsmSCF maintaining a dialog with the MSC for the duration of the call leg it is controlling. The OLCM is defined alongside the Incoming Leg Control Model (ILCM) for calls presented to a subscriber, and the T-BCSM (Terminating Basic Call State Model) forms the underlying state machine in the switch with which the OLCM interacts.

Purpose & Motivation

OLCM was created as part of the CAMEL standard to solve the problem of enabling intelligent, real-time control of outbound calls from within the circuit-switched core network. Before CAMEL, advanced call services were hard-coded into switches, making them vendor-specific and slow to deploy. OLCM provides a standardized, open interface (CAP - CAMEL Application Part) that allows a centralized service platform (gsmSCF) to dynamically control call routing and behavior. This was motivated by the need for competitive features like prepaid roaming, freephone numbers, and virtual private networks. It addresses the limitation of traditional IN (Intelligent Network) approaches by being tailored for the mobility aspects of GSM/UMTS networks, such as handling subscriber mobility and leveraging HLR data. The model enables operators to create and deploy new revenue-generating services rapidly without requiring upgrades to every switch in the network.

Key Features

  • Defines the control interface between gsmSCF and MSC/GMSC for the outgoing call leg
  • Utilizes the CAP protocol for signaling and control messages
  • Allows dynamic call routing (e.g., number translation, call forwarding based on service logic)
  • Enables real-time call charging control, essential for prepaid services
  • Supports event reporting, allowing the gsmSCF to monitor call progress (answer, disconnect, etc.)
  • Works in conjunction with the T-BCSM state model in the switch

Evolution Across Releases

R99 Initial

Introduced the Outgoing Leg Control Model as part of CAMEL Phase 3. Initial architecture defined the core interaction between gsmSCF and MSC for controlling outgoing calls, including basic commands like Connect, Continue, and ReleaseCall, and event reporting for call state changes.

TS 23.218
Rel-4

Enhanced CAMEL support for GPRS control, but core OLCM for circuit-switched calls remained largely stable with minor protocol updates and clarifications.

TS 23.218
Rel-5

Introduced CAMEL Phase 4, which expanded OLCM capabilities to support new detection points and events for better service control, such as mid-call triggers and enhanced dialogs.

TS 23.218
Rel-6

Further refinements to CAMEL, including support for IMS service control beginnings. OLCM for CS domain was maintained and optimized.

TS 23.218
Rel-7

Continued support and maintenance of OLCM as part of CAMEL for legacy CS networks, while focus shifted towards IMS and IP-based service control.

TS 23.218
Rel-8

OLCM remained relevant for CS fallback and legacy service support in the emerging LTE/EPC era. Specifications were kept for backward compatibility.

TS 23.218
Rel-9

Maintenance phase for CAMEL and OLCM, ensuring interoperability with new network elements and procedures defined in later releases.

TS 23.218
Rel-10

Minor updates and corrections to the OLCM specifications as part of overall protocol maintenance. No major functional additions.

TS 23.218
Rel-11

Continued specification stability for OLCM, as the industry focus moved decisively towards IMS and VoLTE for advanced services.

TS 23.218
Rel-12

OLCM specifications were maintained for operators still relying on CAMEL-based CS services, but no new features were added.

TS 23.218
Rel-13

Pure maintenance release for CAMEL-related specs, including OLCM, ensuring they remain technically accurate and referenced correctly.

TS 23.218
Rel-14

Stable specification. OLCM is considered a mature technology, with updates only for clarifications or alignment with other stable specs.

TS 23.218
Rel-15

As 5G was introduced, OLCM remained defined for legacy network support but is not part of the native 5G service architecture, which uses HTTP-based services.

TS 23.218
Rel-16

Maintenance continued for legacy support. No evolution expected for the core OLCM model in the context of 5G.

TS 23.218
Rel-17

Specification remains unchanged, preserved for networks still operating 2G/3G circuit-switched domains with CAMEL services.

TS 23.218
Rel-18

Continued existence as a legacy component in 3GPP specifications, with no active development or new features planned.

TS 23.218
Rel-19

Final listed release for OLCM; the model is fully mature and maintained only for backward compatibility purposes in global standards.

TS 23.218

Defining Specifications

SpecificationTitle
TS 23.218 3GPP TS 23.218