Subscriber Controlled Input / Send Charging Information

Description

SCI in 3GPP terminology encompasses two primary definitions, both related to service control but in different domains. The first and historically earlier meaning is 'Subscriber Controlled Input'. This is a service capability within the circuit-switched telephony service architecture. It allows a subscriber to interact with and control network-based supplementary services (like call forwarding, call barring) during an active call. This interaction is performed using in-band Dual-Tone Multi-Frequency (DTMF) tones. The network provides announcements or prompts, and the user responds by pressing keys on their handset, generating DTMF signals that are detected and interpreted by the network's service logic.

The technical implementation involves the Service Switching Function (SSF) in the MSC detecting the DTMF digits during a call. These digits are packaged and relayed to the Service Control Function (SCF), typically residing in a CAMEL or IN service platform. The SCF contains the service logic that interprets the input and executes the corresponding action, such as activating call forwarding or entering a new password. This mechanism provides a user-friendly, real-time interface for managing services without needing to end the call or use separate signaling procedures.

The second major meaning of SCI is 'Send Charging Information'. This is a critical attribute within the Diameter protocol, specifically used in the Ro and Gy reference points for Online Charging System (OCS) interactions. When a network element (like a P-GW or S-CSCF) requests quota for a service session from the OCS, it includes the SCI attribute within a Credit-Control-Request (CCR) message. The value of the SCI attribute instructs the OCS on whether to return charging information, such as the cost of the service unit, in the subsequent Credit-Control-Answer (CCA) message.

This charging-related SCI is a flag that enables the network to provide real-time charging advice to the end-user. For example, in a prepaid data session, the P-GW may set SCI to request that the OCS send the cost per megabyte. The OCS includes this information in its response, and the P-GW or a separate application server can then format and present this cost information to the subscriber via an SMS or USSD message. This functionality is crucial for transparency in prepaid services and is mandated by many regulatory environments.

Purpose & Motivation

The 'Subscriber Controlled Input' feature was created to enhance the usability and flexibility of intelligent network (IN) and CAMEL-based supplementary services. Prior to such features, configuring services often required dialing specific codes before making a call or interacting with a customer service representative. SCI allowed for mid-call control, enabling scenarios like activating call forwarding based on the called party's busy status without hanging up. It addressed the need for dynamic, user-driven service interaction, making advanced telephony features more accessible and practical.

The 'Send Charging Information' attribute was motivated by the rise of prepaid services and regulatory requirements for charging transparency. As mobile operators moved beyond simple postpaid billing, they needed a mechanism for real-time cost control and user notification. The SCI attribute within the Diameter charging protocol standardized how a network element could request cost information from the charging system. This solved the problem of opaque prepaid balances where users could be surprised by deductions. By providing a standardized way to fetch and display cost information, it improved the customer experience and helped operators comply with consumer protection regulations. Its creation was integral to the success of complex prepaid data and content services.