Call Segment Identifier

Description

The Call Segment Identifier (CSID) is a fundamental construct within 3GPP charging and control architectures, specifically defined in the context of the IP Multimedia Subsystem (IMS) and legacy Circuit-Switched (CS) domains. It serves as a unique label assigned to a distinct segment of a call or session. A 'call segment' represents a logical portion of an end-to-end communication, which could be an initial call attempt, a forwarded leg, a transferred segment, or a participant's stream within a conference. The CSID allows network entities, particularly the Charging Data Function (CDF) and Charging Trigger Function (CTF), to correlate charging events and apply policies to these individual segments rather than treating the entire call as a monolithic entity.

Architecturally, the CSID is generated and managed by network nodes involved in session control, such as the Serving Call Session Control Function (S-CSCF) in IMS or the Mobile Switching Center (MSC) in CS networks. When a call setup, modification, or forwarding event occurs, these control nodes assign a new CSID to the newly created segment. This identifier is then included in charging messages, such as Accounting Request (ACR) records in the Diameter-based Offline Charging System (OFCS) or charging events in the Online Charging System (OCS). The Charging Data Function uses the CSID, along with other identifiers like the IMS Charging Identifier (ICID), to assemble a complete charging data record (CDR) that accurately reflects the complex topology of a call involving multiple parties or services.

The role of CSID extends beyond simple charging correlation. It enables advanced service logic and policy enforcement. For instance, in a call forwarding scenario, the network can apply different tariff rates to the initial calling leg and the forwarded leg based on their respective CSIDs. In a prepaid scenario, the Online Charging System can monitor credit consumption per segment, allowing for more granular control. Furthermore, CSID facilitates error handling and troubleshooting by providing a clear traceable identifier for each segment within a call's signaling path, aiding in fault isolation and service assurance.

Purpose & Motivation

CSID was introduced to address the limitations of earlier charging systems that struggled with complex call scenarios prevalent in modern telecommunications. Prior to its standardization, charging mechanisms often treated a multi-segment call (e.g., one involving call forwarding, transfer, or conferencing) as a single event. This made it difficult to accurately attribute costs, apply differentiated tariffs for different legs of a call, or implement fair usage policies for specific service components. The lack of segment granularity also complicated billing disputes and service logic execution.

The primary motivation for creating CSID was to support the sophisticated service capabilities of the IP Multimedia Subsystem (IMS) and evolving circuit-switched networks. IMS enabled rich communication services like call forwarding on no reply, call transfer, and multimedia conferences, which inherently consist of multiple interconnected segments. CSID provides the necessary technical foundation to track these segments independently. It solves the problem of charging correlation in a distributed, multi-operator environment where different network nodes may handle different segments of the same end-to-end call.

By enabling per-segment identification, CSID allows operators to implement detailed, accurate, and flexible charging models. It supports business models where different rates apply to different portions of a service (e.g., a premium rate for a conference bridge leg). Furthermore, it provides the granularity needed for lawful interception and service monitoring on a per-segment basis, meeting regulatory requirements. Its creation was driven by the industry's need for a standardized, robust mechanism to handle the financial and operational complexities introduced by advanced telephony services.

Key Features

• Unique identification of individual call/session segments
• Enables correlation of charging events across complex call topologies
• Supports differentiated charging policies per call leg
• Facilitates accurate CDR generation in offline and online charging systems
• Integrates with IMS and legacy CS network charging architectures
• Aids in service logic execution and troubleshooting by segment

Evolution Across Releases

Defining Specifications