Event Charging with Unit Reservation

Description

Event Charging with Unit Reservation (ECUR) is a critical component of the 3GPP Online Charging System (OCS), defined within the Charging and Billing domain. It operates by handling charging events where service usage requires prior reservation of units (e.g., monetary credit, data volume, time) before the service is delivered. The process involves the OCS interacting with network elements like the Policy and Charging Rules Function (PCRF) or application servers to authorize and monitor resource consumption in real-time.

Architecturally, ECUR functions through a series of standardized interfaces, primarily the Sy reference point between the OCS and the PCRF, and the Ro reference point for Diameter-based communication with network elements. When a service request is initiated, the network element sends a Credit Control Request (CCR) to the OCS, specifying the required units. The OCS then performs balance checks, reserves the appropriate units from the user's account, and responds with a Credit Control Answer (CCA) granting a quota. During service delivery, usage reports are sent periodically, and the OCS deducts from the reserved units, potentially issuing new reservations as needed until the session ends or units are exhausted.

Key components include the OCS's Account Balance Management Function (ABMF) for tracking balances, the Rating Function (RF) for determining cost, and the Charging Trigger Function (CTF) in network elements that detect chargeable events. ECUR supports various service types, such as data access, voice calls, SMS, and multimedia services, allowing for flexible tariff models like prepaid and hybrid plans. Its operation ensures that charging is tightly coupled with service control, preventing revenue leakage and enabling immediate service termination upon insufficient funds. This mechanism is fundamental for real-time charging scenarios in LTE and 5G networks, where high-speed data usage demands instantaneous billing updates.

Purpose & Motivation

ECUR was developed to address the limitations of offline charging and simple event-based charging in pre-3GPP systems, which could not prevent service usage beyond a user's credit limit in real-time. As mobile services evolved to include data-intensive applications and prepaid models became widespread, there was a need for a mechanism that could reserve units upfront, ensuring that operators could control financial risk while providing seamless user experiences. ECUR enables operators to offer complex, usage-sensitive tariffs and prevent fraud by integrating charging directly with service authorization.

Its introduction in Release 8 coincided with the rollout of LTE and the enhanced OCS architecture, supporting the shift towards all-IP networks and richer service offerings. By combining event charging with reservation, it solves the problem of accurate, immediate billing for on-demand services, which is essential for customer satisfaction and operational efficiency in competitive telecom markets. Historical context includes earlier mechanisms like Advice of Charge (AoC), which were less dynamic and not integrated with policy control.