Description
COLI (Connected Line Identity) is a supplementary service defined within the 3GPP specifications that operates within the circuit-switched (CS) telephony domain. Its primary function is to identify and provide the network with the actual telephone number (or identity) of the party who answers a call, which is the 'connected line'. This is distinct from the original dialed number (the 'calling line'), especially in scenarios involving call forwarding, diversion, or connection to a switchboard. The service is integral to the call control signaling, primarily using ISDN User Part (ISUP) or Bearer Independent Call Control (BICC) protocols in the core network.
Architecturally, COLI is implemented within the call session control functions of the network, specifically in the Mobile Switching Center (MSC) for the CS domain. When a call is set up, the MSC involved in the connection is responsible for determining the final connected party's identity. This information is then communicated back through the signaling path to the originating network element and, if subscribed, to the calling party. The process involves analyzing the call routing and any invoked supplementary services (like Call Forwarding Unconditional - CFU) to ascertain the true endpoint.
Key components involved in delivering COLI include the MSC, which performs the call control and service logic, and the Home Location Register (HLR), which stores the subscriber's service profile indicating if COLI presentation is allowed or restricted. The service interacts with other supplementary services like Calling Line Identification Presentation (CLIP) and Connected Line Identification Restriction (COLR). The COLI data itself is carried as a parameter within signaling messages, such as the Initial Address Message (IAM) and Answer Message (ANM) in ISUP, ensuring it is available at the point of call answer for network processing.
Its role in the network extends beyond a simple information service. COLI is fundamental for operational and commercial functions. For the network operator, it enables accurate call detail records (CDRs) for billing, as charges may differ based on the final connected number, especially in inter-operator scenarios. It also supports features like Malicious Call Identification (MCId) and is a critical input for Lawful Interception (LI) systems, ensuring that surveillance is correctly applied to the actual connected party. The service ensures transparency and accuracy in call routing, which is essential for trust in telecommunication services.
Purpose & Motivation
COLI was introduced to address a fundamental gap in telephony networks: the inability to reliably identify who actually answered a call. In early telephony and even in initial digital networks, billing and call logging were based solely on the dialed number. However, with the advent of advanced call handling features like call forwarding, call transfer, and direct dial-in to extensions, the dialed number often did not correspond to the answering party. This created problems for billing accuracy, fraud detection, and customer service.
The historical context for COLI's creation lies in the evolution of intelligent network services and the standardization of ISDN and later 3GPP circuit-switched services. Prior to COLI, operators faced challenges with revenue assurance, as calls forwarded to premium rate numbers or international destinations could not be correctly tariffed if only the original dialed number was known. Furthermore, for security and legal purposes, identifying the actual connected line is essential for tracing malicious calls or executing lawful interception orders accurately.
COLI solves these problems by providing a standardized mechanism within the call control signaling to convey the verified identity of the connected party. It addresses the limitations of previous approaches that relied on inference or post-call analysis, which were error-prone and inefficient. By integrating this identity directly into the real-time signaling fabric, COLI enables precise operational support, fair billing, enhanced security features, and compliance with regulatory requirements for modern telecommunications networks.
Key Features
- Provides the verified telephone number/identity of the party who answers a call
- Essential for accurate call detail record (CDR) generation and billing, especially for forwarded calls
- Supports security features like Malicious Call Identification (MCId)
- Critical input for Lawful Interception (LI) systems to target the correct connection
- Operates via parameters in call control signaling protocols like ISUP and BICC
- Interworks with supplementary services CLIP (Calling Line Identification Presentation) and COLR (Connected Line Identification Restriction)
Evolution Across Releases
COLI was introduced in 3GPP Release 5 as a standardized supplementary service for the Circuit-Switched (CS) core network. The initial architecture defined its integration into call control procedures using ISUP signaling. It established the basic service logic within the MSC to determine and present the connected line identity, particularly for handling call forwarding and diversion scenarios, forming a foundation for accurate billing and call logging.
Defining Specifications
| Specification | Title |
|---|---|
| TS 21.905 | 3GPP TS 21.905 |