Description
Originating Line Information (OLI) is a standardized parameter carried within SIP signaling messages, specifically in the P-Asserted-Identity header or other appropriate fields as defined by 3GPP. It is a numeric or alphanumeric code that indicates the characteristics of the line from which a call or session originates in an IMS network. The OLI value is typically inserted by the originating network's Call Session Control Function (CSCF) or an Application Server (AS) based on subscription data or network configuration.
In the IMS architecture, the OLI parameter is propagated from the originating user's network through the Serving-CSCF (S-CSCF) to the destination network. It forms part of the call detail records (CDRs) generated for billing purposes. Network operators and service providers use OLI to differentiate between various origination types, enabling tailored charging rates, specific routing policies (e.g., emergency call prioritization from payphones), or triggering value-added services.
The parameter's definition and usage are specified in 3GPP TS 24.229 (IP multimedia call control protocol) and TS 29.163 (Interworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networks). While its core function has remained stable, its presence ensures backward compatibility and interworking with legacy circuit-switched networks where similar line information was used in ISUP signaling. OLI is a key element for lawful interception, fraud detection, and ensuring regulatory compliance by accurately identifying the nature of the calling party's access.
Purpose & Motivation
OLI was introduced to bridge the semantic gap between traditional circuit-switched telephony and the new IP-based IMS. In legacy PSTN/ISDN networks, the Calling Party's Category parameter in ISUP signaling conveyed similar information about the originating line (e.g., ordinary subscriber, payphone, test line). This data was essential for billing systems and network operations.
With the migration to IMS and all-IP networks, there was a need to preserve this critical information element to maintain existing business logic, regulatory requirements, and service capabilities. OLI solves the problem of losing origination context when calls transition from TDM to IP. It ensures that service providers can continue to apply differential charging, implement specific routing for certain line types (like free calls from payphones), and meet legal obligations for call tracing and identification.
Its creation was motivated by the requirement for feature parity between networks and seamless interworking during the transition phase. Without OLI, IMS networks would lack a standardized mechanism to convey this class of information, leading to inconsistencies in billing, service treatment, and potential violations of telecommunications regulations.
Key Features
- Standardized parameter within SIP/IMS signaling
- Identifies originating line type (e.g., payphone, cellular, ISDN)
- Used for billing differentiation and charging
- Enables specific service logic and routing decisions
- Supports regulatory compliance and lawful interception
- Facilitates interworking with legacy circuit-switched networks
Evolution Across Releases
Introduced as part of the initial IMS specifications to carry originating line information from CS networks into the IMS domain. Defined in TS 24.229 for inclusion in SIP messages and TS 29.163 for interworking with CS networks, establishing the basic architecture for OLI parameter mapping and propagation.
Defining Specifications
| Specification | Title |
|---|---|
| TS 24.229 | 3GPP TS 24.229 |
| TS 29.163 | 3GPP TS 29.163 |