Description
The Charging Trigger Function (CTF) is a fundamental architectural element defined within the 3GPP charging framework. It is not a standalone network node but a logical function embedded within various network entities that provide services, such as the Serving GPRS Support Node (SGSN), Mobility Management Entity (MME), Serving Gateway (S-GW), Packet Data Network Gateway (P-GW), Session Management Function (SMF), Application Functions (AF), and IP Multimedia Subsystem (IMS) nodes like the Call Session Control Function (CSCF). The primary role of the CTF is to monitor user and network activity in real-time, identify events that are subject to charging based on operator policies, and subsequently trigger the charging process.
Operationally, the CTF works by applying charging rules and filters to the data flows and signaling messages it processes. When a predefined chargeable event occurs—such as the start of a data session, an IMS voice call establishment, SMS delivery, or consumption of a specific volume of data—the CTF captures the relevant details. These details include subscriber identifiers (e.g., IMSI, MSISDN), session identifiers, timestamps, service identifiers, data volumes, duration, and location information. For offline charging, the CTF assembles this information into Charging Data Records (CDRs) or Charging Events and forwards them to the Charging Data Function (CDF) via the Rf reference point. For online charging, the CTF interacts directly with the Online Charging Function (OCF) via the Ro reference point to request quota reservations and report quota consumption in real-time, enabling immediate credit control.
The internal logic of the CTF is governed by charging policies provisioned by the operator, which define what constitutes a chargeable event, the applicable tariffs, and the required granularity of information. Its implementation must ensure reliability and integrity, as the generated data directly impacts billing. The CTF's design separates the service logic of the network node from the charging logic, allowing for flexible and scalable billing models. It supports various charging methods, including event-based charging (e.g., per SMS), session-based charging (e.g., per voice call), and volume/duration-based charging (e.g., for data traffic).
In the broader charging architecture, the CTF acts as the source and initiator of all charging information. Its correct function is paramount for the entire billing chain. It interfaces with the core charging systems (OCS/OFCS) using standardized Diameter-based protocols (Ro and Rf interfaces), ensuring interoperability between different vendors' equipment. The CTF's ability to generate detailed, correlated records for complex services, like those in IMS, is essential for modern convergent billing systems that charge for voice, data, messaging, and content in a unified manner.
Purpose & Motivation
The CTF was created to address the fundamental business need for accurate, reliable, and flexible charging mechanisms in telecommunications networks. Prior to standardized charging architectures, billing systems were often proprietary, tightly coupled with switch vendors, and incapable of supporting new, packet-based services like GPRS and later 3G/4G data. This lack of standardization made it difficult for operators to introduce innovative service plans, perform real-time credit control, or implement complex charging scenarios like those required for IP Multimedia Subsystem (IMS) services.
The introduction of the CTF as part of the 3GPP Charging System in Release 5 and its refinement in subsequent releases provided a clear, standardized model for separating the service delivery functions from the charging functions. This separation allowed network equipment vendors to focus on service delivery while ensuring a consistent, interoperable method for generating billing records. The CTF solves the problem of detecting monetizable events across a heterogeneous network comprising nodes from multiple vendors. It enables operators to implement diverse business models, including prepaid (online charging) and postpaid (offline charging), from a single triggering point.
Furthermore, the evolution towards all-IP networks and rich communication services demanded a more granular and real-time charging capability. The CTF, through its defined interactions with the OCS, enabled immediate credit checks and quota management, which is crucial for preventing fraud in prepaid services and for offering spend-control features to customers. Its design supports the charging of any service detectable by the network, making it a future-proof cornerstone for monetizing everything from basic connectivity to 5G network slices and edge computing applications.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (398 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the CTF was enhanced to support 5G within the charging architecture, introducing a new Converged Charging System and the Northbound API (NAPS API) for charging. Specifically for ProSe, the CTF in the ProSe Function was defined to use the VPLMN ID included in MATCH_REPORT messages for charging purposes, as specified in TS 32.277. These changes established the foundation for 5G and converged service charging.
- Introduce the Northbound API charging TS 32.240CR0394
- Introduction of 5G in charging architecture TS 32.240CR0396
- Introduction of 5G converged charging TS 32.240CR0397
- Introduction of 5G in charging mechanisms TS 32.240CR0399
- Introduce the 5G charging in clause 3 TS 32.240CR0401
- Charging mode clarifications in Converged Charging System TS 32.240CR0402
+ 31 more changes
In Release 16, the CTF enhancements focused on expanding offline-only charging support across multiple network functions and scenarios. Key additions included the introduction of offline-only charging for the SMF, for flow-based and QoS flow-based charging, and for CHF selection, along with detailed procedures and message formats for this offline charging service. Furthermore, the release added and clarified charging triggers and information for interworking scenarios with the EPC and introduced charging principles for the I-SMF.
- Introduction of AMF in charging architecture TS 32.240CR0409
- Introduce description of volume based charging for VoLTE in PS TS 32.251CR0514
- Addition of the Triggers for Interworking TS 32.255CR0033
- Add offline only charging TS 32.255CR0035
- Add offline only charging for SMF TS 32.255CR0036
- Add offline only charging CDR generation TS 32.255CR0038
+ 59 more changes
In Release 17, the CTF was enhanced to support charging for new 5G service capabilities, specifically for Ultra-Reliable Low Latency Communication (URLLC) services and 5G LAN services. The updates introduced new charging requirements, triggers, and procedures for URLLC, including quota management and usage reporting flows. Furthermore, the charging architecture was extended to accommodate Edge Computing and the 5G Data Delivery Network Management Function (DDNMF).
- Add PGW in logical ubiquitous charging architecture- service based interface TS 32.240CR0413
- Introduction of 5G DDNMF in charging architecture for 5GS TS 32.240CR0429
- Addition of the 5G LAN service charging TS 32.240CR0434
- Charging architecture for Local Breakout TS 32.240CR0439
- Addition of the architecture for 5G LAN charging TS 32.240CR0442
- Enhance charging architecture for Edge Computing TS 32.240CR0443
+ 88 more changes
In Release 18, the CTF was enhanced with new charging support for several advanced 5G services. Key additions include charging architectures and principles for 5G Multicast/Broadcast Services (5MBS), Time-Sensitive Networking (TSN) services, and 5G satellite access. The release also introduced slice-aware charging for roaming partners and expanded business-to-business (B2B) charging support with updated architectural principles.
- Adding New Consumer for MMS in Charging Architecture TS 32.240CR0446
- Slice-aware charging for Roaming partners TS 32.240CR0467
- Add MB-SMF in charging architecture for 5GS TS 32.240CR0470
- Add charging support for TSN service TS 32.240CR0473
- Add annex for B2B charging TS 32.240CR0474
- Update B2B charging principles TS 32.240CR0475
+ 107 more changes
In Release 19, the CTF (Charging Trigger Function) was enhanced to support new services and architectures, including charging for AIoT services, CAPIF service charging, and disaster roaming. Furthermore, it introduced specific charging principles and triggers for satellite operations, such as store-and-forward satellite operation for SMS and UP CIoT, as well as MVNO charging for satellite service provision. These additions expanded the CTF's role in capturing charging events for emerging network scenarios beyond traditional mobile services.
- Introduction of GMLC in charging architecture for 5GS TS 32.240CR0491
- Rel-19 CR 32.240 Add charging support to AIoT service TS 32.240CR0510
- Rel-19 CR 32.240 CAPIF Logical Charging Architecture TS 32.240CR0511
- Rel-19 CR 32.240 CAPIF Service Charging TS 32.240CR0512
- Add charging principles for UAS TS 32.240CR0517
- Add MVNO charging which provides satellite service TS 32.240CR0518
+ 79 more changes
In Release 20, the CTF was enhanced with new information elements to support charging for multi-modality services and to address the charging impacts from enhanced QoS handling in XRM. Furthermore, corrections and clarifications were made regarding QoS flow-based charging descriptions and the handling of the PDU Session ID for V-SMF charging in EPS–5GS interworking scenarios.
- Add informatin elements to support charging aspects of multi-modality service TS 32.255CR0606
- Rel-20 TS 32.255 Add information elements to address charging impacts from enhanced QoS handling in XRM TS 32.255CR0616
- Rel-20 CR 32.255 Correct some descriptions on QoS flow Based Charging TS 32.255CR0619
- Rel-20 CR 32.255 Correction on PDU Session ID Handling for V-SMF Charging in EPS–5GS Interworking TS 32.255CR0622
Explore further
Broader topics and technologies where CTF plays a role.
Defining Specifications
3GPP specifications that define or reference CTF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.334 vj00 | ProSe Protocols and Procedures | Rel-19 |
| TR 28.816 vh00 | Charging for 5G Cellular IoT | Rel-17 |
| TS 28.849 vj10 | CAPIF Phase2 Charging Study | Rel-19 |
| TS 32.240 vj40 | Charging Management Architecture & Principles | Rel-19 |
| TS 32.250 vj00 | Circuit Switched Offline Charging | Rel-19 |
| TS 32.251 vj00 | PS Domain Charging Management | Rel-19 |
| TS 32.253 vj00 | Charging for Control Plane Data Transfer | Rel-19 |
| TS 32.254 vj21 | Charging for Northbound APIs | Rel-19 |
| TS 32.255 vk10 | Telecom Management; Charging for 5G Data Connectivity | Rel-20 |
| TS 32.256 vj40 | 5G Connection & Mobility Charging Spec | Rel-19 |
| TS 32.270 vj00 | MMS Charging Management Specification | Rel-19 |
| TS 32.271 vj20 | 3GPP LCS Charging Management Spec | Rel-19 |
| TS 32.272 vj00 | Charging for Push-to-Talk over Cellular (PoC) | Rel-19 |
| TS 32.273 vj00 | MBMS Charging Management | Rel-19 |
| TS 32.276 vj00 | VCS Online Charging from Proxy Function | Rel-19 |
| TS 32.277 vj20 | Charging Management for Proximity Services (ProSe) | Rel-19 |
| TS 32.278 vj00 | Monitoring Events Offline Charging Specification | Rel-19 |
| TS 32.279 vj00 | 5G MBS Session Converged Charging | Rel-19 |
| TS 32.291 vj40 | Charging Management: Service-Based Interface Protocol | Rel-19 |
| TS 32.293 vj00 | Proxy Function in Domestic Service Provider | Rel-19 |
| TS 32.297 vj00 | Charging Data Record File Transfer | Rel-19 |
| TS 32.298 vj30 | Charging Data Record (CDR) Parameter Specification | Rel-19 |
| TS 32.808 v1800 | Common User Profile Storage Framework | Rel-8 |
| TS 32.849 vd00 | IMS Roaming Charging Study | Rel-13 |
| TS 32.850 ve00 | IMS Charging Correlation Methods Study | Rel-14 |
| TS 33.303 vj00 | ProSe Security Specification for EPS | Rel-19 |