Session Based Charging Function

Description

The Session Based Charging Function (SBCF) is a fundamental logical function within the 3GPP Online Charging System (OCS) architecture, as defined in the Telecommunication Management (TS 32.2xx) series. It operates as the central engine for real-time, session-based credit control. The SBCF interacts with network elements known as Charging Trigger Functions (CTFs)—such as the P-GW, S-CSCF, or MME—via the standardized Ro reference point using the Diameter Credit Control Application (DCCA) protocol. When a subscriber initiates a service session (e.g., a voice call, a data session, an IMS session), the CTF sends a credit control request to the SBCF. The SBCF then performs rating based on the subscriber's profile, service details, and tariff plans, and reserves an appropriate amount of credit from the subscriber's account.

Upon successful credit reservation, the SBCF grants a quota (e.g., data volume, time duration) to the network element, authorizing the service to proceed. The network element then monitors the resource usage against this quota. As the quota is consumed, the CTF may send intermediate update requests to the SBCF to report usage and request additional credit. The SBCF manages these updates, potentially applying re-rating, and grants new quotas. This process continues until the session ends or the subscriber's credit is exhausted. If credit runs out, the SBCF instructs the network element to terminate or redirect the service session. Throughout this cycle, the SBCF generates charging events that are forwarded to the Charging Gateway Function (CGF) or directly to the billing domain for final bill calculation.

Architecturally, the SBCF is often implemented as part of a larger OCS node alongside other functions like the Account Balance Management Function (ABMF) and the Rating Function (RF). It plays a critical role in enabling prepaid services, spending limit controls, and real-time policy enforcement. By managing sessions dynamically, it allows operators to offer complex tariff plans, such as tiered data bundles, time-of-day rates, and service-specific charging, while guaranteeing that no revenue is lost due to unpaid usage. Its operation is crucial for the convergence of charging across access technologies (2G/3G/4G/5G) and service domains (voice, data, messaging, IMS).

Purpose & Motivation

The SBCF was created to solve the limitations of traditional offline (postpaid) charging, which could not prevent service usage when a subscriber had insufficient funds or credit. This was a significant business risk for operators, especially with the rise of prepaid mobile services. Early prepaid systems were often proprietary, siloed, and could not easily support new services or convergent charging across multiple service types. The standardization of the OCS and the SBCF in 3GPP Release 5 (for IMS) and its solidification in Release 8 provided a unified, real-time charging framework.

The primary motivation was to enable operators to offer innovative prepaid and hybrid payment models for all services, including emerging IP-based services like IMS and mobile data, in a scalable and interoperable way. The SBCF addresses this by providing a standardized, session-aware credit control mechanism. It allows for immediate authorization and continuous control of resource consumption, protecting operator revenue. Furthermore, it supports the convergence of charging for voice, data, and multimedia services onto a single platform, simplifying operations and enabling personalized, real-time tariff plans that enhance customer experience and loyalty.