Interrogating Network Entity

Description

The Interrogating Network Entity (INE) is a critical functional component within the 3GPP CAMEL (Customised Applications for Mobile network Enhanced Logic) architecture, specified in TS 23.066. It acts as a gateway or interrogating point, typically located within the Visited Public Land Mobile Network (VPLMN). Its primary role is to communicate with the subscriber's Home Public Land Mobile Network (HPLMN) to retrieve the necessary CAMEL Subscription Information (CSI) when the subscriber is roaming. The INE is not necessarily a standalone physical node; its functionality is often integrated into core network elements like the Visited Mobile Switching Center (VMSC) or the Visited Gateway MSC (VGMSC) for circuit-switched services, and the Visited Serving GPRS Support Node (VSGSN) for packet-switched services.

Operationally, the INE works by initiating a dialogue with the Home Location Register (HLR) or Home Subscriber Server (HSS) in the home network. When a roaming subscriber initiates or receives a call, or activates a packet data session, the visited network's serving node (e.g., VMSC) triggers the INE function. The INE sends a standardized MAP (Mobile Application Part) message, such as SendRoutingInfoForSM or ProvideSubscriberInfo, to the HLR. The HLR responds with the subscriber's CSI, which contains the address of the home network's CAMEL Service Environment (CSE) – the gsmSCF (GSM Service Control Function). This CSI is then passed to the visited network's gsmSSF (GSM Service Switching Function), which establishes a direct CAP (CAMEL Application Part) dialogue with the gsmSCF.

This architecture allows the gsmSCF in the home network to control the call or session handling in the visited network in real-time. For example, for a prepaid roaming subscriber, the home gsmSCF can instruct the visited gsmSSF/INE to apply call duration limits or deduct credit. The INE thus enables the visited network to be 'aware' of and able to execute the home operator's custom services without requiring the visited network to host the service logic itself. It is a cornerstone for enabling seamless intelligent network services across international borders, ensuring that a subscriber's service experience is consistent whether at home or roaming.

Purpose & Motivation

The INE was created to solve a fundamental challenge in global mobile roaming: how to allow a home network operator to offer its value-added, operator-specific intelligent network services (like prepaid charging, virtual private networks, or location-based services) to its subscribers when they are outside the home network's coverage area. Before CAMEL and the INE concept, a roaming subscriber would typically only have access to the basic telephony services of the visited network, losing all home-network customizations. This was a significant commercial and technical limitation, especially for prepaid services which were a major driver of mobile adoption.

The development of the INE, starting in Release 4, was part of the broader CAMEL phase 3 and 4 enhancements. It addressed the limitations of earlier CAMEL phases which had more restricted roaming capabilities. The INE formalized the visited network's role as an interrogator and executor. It solved the problem by defining a clear, standardized interface (MAP) between the visited and home networks for the purpose of fetching service logic instructions. This allowed the service control to remain in the home network (with the operator's gsmSCF), maintaining security and control over the business logic, while delegating the service switching and triggering to the visited network's infrastructure.

This architecture motivated the creation of a truly global intelligent network platform for GSM and its successors. It enabled the booming prepaid roaming market, gave operators a competitive differentiation through consistent service offering, and provided a model for how network functions could be distributed across administrative domains—a concept that later influenced the design of the IP Multimedia Subsystem (IMS) and service-based architectures in 5G.

Key Features

• Resides in the Visited Network (VPLMN) and acts as an interrogator towards the HPLMN
• Uses MAP protocols to query the HLR/HSS for CAMEL Subscription Information (CSI)
• Enables the retrieval of the home gsmSCF address to establish CAP connectivity
• Functionally integrated into visited core nodes like VMSC, VGMSC, or VSGSN
• Essential for triggering home-controlled IN services like real-time prepaid charging during roaming
• Provides a standardized mechanism for service interaction between administratively separate networks

Evolution Across Releases

Defining Specifications