Session Charging with Unit Reservation

Description

Session Charging with Unit Reservation (SCUR) is a core function within the 3GPP Online Charging System (OCS), detailed in specifications such as TS 32.240 and TS 32.251. It operates on the principle of credit reservation before service consumption, enabling real-time charging for prepaid and on-demand services. The architecture involves several key network functions: the Charging Trigger Function (CTF) embedded in network elements like the GGSN, P-GW, or S-CSCF, which detects chargeable events; the Online Charging Function (OCF) within the OCS, which manages credit control; and the Account Balance Management Function (ABMF), which holds subscriber account balances. When a user initiates a service session (e.g., a data session, voice call, or messaging service), the CTF sends a credit control request to the OCF. The OCF then interacts with the ABMF to check the subscriber's balance and, if sufficient, reserves a number of units (monetary, data volume, or time units) for that session. This reservation is granted to the CTF, which allows the service to proceed. As the service is consumed, the CTF reports used units back to the OCF, which deducts them from the reservation and subsequently from the account balance. If the reservation is exhausted, the OCF can grant additional units (re-authorization) or terminate the session if the balance is insufficient.

The SCUR process is governed by the Diameter Credit-Control Application (DCCA) protocol, as defined in RFC 4006 and adopted by 3GPP. This protocol facilitates the exchange of credit control messages between the CTF and OCF. Key message types include Credit-Control-Request (CCR) and Credit-Control-Answer (CCA). The OCF employs rating functions to convert service usage parameters into monetary or unit costs based on tariff information. SCUR supports multiple reservation models: amount reservation (reserving a specific quantity of units), quota management (dynamically adjusting the reservation), and pooled-quota (sharing quota across multiple sessions). This flexibility allows it to cater to various service types, from simple voice calls to complex IMS-based multimedia sessions.

SCUR's role in the network is critical for revenue assurance and service control. It enables operators to offer prepaid services without the risk of bad debt, as usage is strictly limited to available credit. Furthermore, it supports advanced charging scenarios like split charging (where multiple parties are charged), location-based charging, and service-specific tariffs. By integrating with policy control (via the PCRF), SCUR can also influence quality of service (QoS) based on charging decisions, creating a cohesive control plane for monetization and resource management. Its implementation is mandatory for network elements that support online charging, making it a ubiquitous component in modern 3GPP networks.

Purpose & Motivation

SCUR was developed to address the fundamental business need for real-time, prepaid charging in telecommunications networks. Prior to online charging systems, postpaid billing was dominant, where usage was recorded and billed later, posing credit risk for operators. The rise of prepaid mobile services demanded a mechanism to control service usage in real-time based on available credit, preventing subscribers from exceeding their balances. SCUR provided this capability by introducing a reservation-based model, ensuring that network resources are only consumed after securing payment assurance. This solved the problem of revenue leakage and enabled the massive global adoption of prepaid mobile plans.

The motivation for SCUR stemmed from the limitations of offline charging (post-event billing) and simple prepaid solutions that were often vendor-specific and lacked interoperability. As networks evolved to offer diverse services (GPRS, IMS, MMS), a standardized, flexible online charging method became essential. 3GPP introduced SCUR as part of the broader Charging and Billing (CAB) architecture to create a unified framework. It addressed the need for granular charging per session, support for multiple service types, and integration with network policy. This allowed operators to launch innovative tariff plans, such as volume-based data packages, time-limited offers, and content-based charging, thereby enhancing competitiveness and customer satisfaction.

Historically, SCUR enabled the convergence of charging across circuit-switched, packet-switched, and IMS domains. It provided a future-proof foundation for charging emerging services like VoLTE, IoT connectivity, and network slicing. By standardizing the interaction between network elements and the OCS, SCUR reduced operational complexity and facilitated multi-vendor deployments. Its creation was driven by the commercial imperative to monetize network services reliably and flexibly in an increasingly digital and on-demand economy.