U-CSI

USSD CAMEL Subscription Information

Services
Introduced in Rel-4
A CAMEL feature enabling USSD (Unstructured Supplementary Service Data) interactions with the network. It allows operators to offer interactive services like balance checks or menu-driven applications via short codes, extending CAMEL's service control to USSD sessions.

Description

U-CSI (USSD CAMEL Subscription Information) is a CAMEL (Customized Applications for Mobile network Enhanced Logic) subscription data element defined in 3GPP specifications. It resides within the HLR (Home Location Register) or HSS (Home Subscriber Server) as part of a subscriber's profile. When a mobile user initiates a USSD session—typically by dialing a short code like *123#—the network checks for the presence of U-CSI data. If present, this triggers CAMEL procedures.

The core mechanism involves the MSC (Mobile Switching Centre) or SGSN (Serving GPRS Support Node) detecting the USSD request. It queries the HLR/HSS for subscriber data, retrieves the U-CSI, and based on its contents, establishes a dialogue with a designated gsmSCF (CAMEL Service Control Function). The gsmSCF, an external application server, then controls the USSD session. It can send USSD strings to the user's handset, receive responses, and orchestrate complex service logic, such as querying a database or initiating a transaction, before returning a final USSD result to the user.

Architecturally, U-CSI points to a specific gsmSCF address and includes service key, triggers, and other CAMEL parameters. It integrates USSD, a simple text-based bearer, into the intelligent, event-driven CAMEL framework. This allows USSD sessions to become programmable and interactive beyond basic network commands, enabling value-added services. Its role is to bridge the USSD transport mechanism with CAMEL's service control capabilities, making USSD a tool for operator-defined applications.

Purpose & Motivation

U-CSI was introduced to leverage the widespread availability of USSD as a simple, reliable communication channel between handsets and the network. Prior to CAMEL integration, USSD was primarily used for static, network-internal commands (like checking voicemail). The creation of U-CSI addressed the need to make USSD dynamic and application-driven.

It solved the problem of offering interactive, menu-based services without requiring complex client software on the handset or relying on SMS, which could be slower and more costly. By tying USSD to CAMEL, operators could deploy subscriber-specific services like prepaid balance inquiries, menu-driven information services, or even simple mobile banking interactions. This extended the CAMEL paradigm—originally focused on call and SMS control—to include the USSD domain, providing a uniform mechanism for service control across multiple bearer types.

Historically, it emerged in Release 4 as part of the broader CAMEL Phase 3 enhancements, which aimed to expand CAMEL's reach to new services and sessions. It allowed operators to create standardized, network-controlled interactive applications, enhancing service offerings without depending on handset capabilities.

Key Features

  • Triggers CAMEL procedures upon USSD session initiation
  • Enables external gsmSCF control over USSD dialogue
  • Supports dynamic, interactive menu-based services via USSD
  • Integrates USSD bearer into CAMEL service control framework
  • Allows operator-defined application logic for USSD interactions
  • Uses standard CAMEL parameters like Service Key and gsmSCF address

Evolution Across Releases

Rel-4 Initial

Initial introduction of U-CSI as part of CAMEL Phase 3. Defined the subscription information structure in the HLR/HSS to trigger CAMEL involvement for USSD sessions, enabling basic control by a gsmSCF.

TS 23.078

Defining Specifications

SpecificationTitle
TS 23.078 3GPP TS 23.078