Intelligent Network Application Protocol

Description

The Intelligent Network Application Protocol (INAP) is a transaction-oriented, application-layer signaling protocol standardized by ITU-T (Q.1218 series) and adopted within 3GPP specifications. It operates over the Signaling System No. 7 (SS7) stack, specifically using the Transaction Capabilities Application Part (TCAP) as its transport. INAP defines the messages and procedures that enable the distributed, interactive dialogue between the Service Switching Function (SSF) at the Service Switching Point (SSP) and the Service Control Function (SCF) at the Service Control Point (SCP). This dialogue is the core mechanism for delivering IN-based services.

INAP works through a series of operations and errors. When a call event at an SSP matches a configured Detection Point (DP), the SSF formulates an Initial Detection Point (IDP) INAP message. This message contains details about the call (calling/called numbers, location, etc.) and the specific trigger that was encountered. The SSP sends this IDP via TCAP/SS7 to the SCP. The service logic on the SCP processes this information, potentially querying a Service Data Point (SDP) for additional data, and then determines how the call should be handled. The SCP responds with one or more INAP operation messages to instruct the SSP. These operations can include "Connect" to route the call to a specific number, "PlayAnnouncement" to provide an audio message to the caller, "RequestReportBCSMEvent" to arm future triggers, or "ApplyCharging" to control billing for prepaid services.

The protocol is defined using Abstract Syntax Notation One (ASN.1), which allows for precise, platform-independent specification of message structures. Different variants or "capability sets" (CS-1, CS-2, etc.) of INAP have been standardized, each adding more complex operations and support for broader service scenarios. Within 3GPP, INAP is particularly relevant for the Customized Applications for Mobile network Enhanced Logic (CAMEL) standard, where a mobile-network-specific variant called CAMEL Application Part (CAP) is used. CAP is derived from INAP but includes extensions tailored for mobile network features like mobility management and GSM Service Control Function (gsmSCF) interactions. INAP/CAP thus forms the critical signaling glue that makes the distributed IN architecture functional.

Purpose & Motivation

INAP was created to provide a standardized, vendor-neutral language for the key communication link in the Intelligent Network architecture: the link between the switch (SSP) and the service controller (SCP). Before INAP, if an operator wanted to deploy an advanced service, the switch vendor would provide a proprietary interface to their own SCP, leading to closed ecosystems and vendor lock-in. This made it impossible for an operator to mix switches from one vendor with service logic servers from another, severely limiting flexibility and increasing costs.

The protocol solves this problem by defining a complete set of abstract operations that cover all necessary interactions for service control, independent of the underlying switch hardware or SCP software implementation. This allows multi-vendor interoperability. The creation of INAP was motivated by the telecommunications industry's push towards open standards in the 1980s and 1990s, driven by deregulation and the desire for increased competition and innovation. It enabled the vision of the IN where services could be created once and deployed across a network comprising equipment from multiple suppliers.

In the 3GPP context, the adoption and reference to INAP (and its derivative, CAP) was essential for bringing IN services to the global mobile network. Mobile networks introduced complexities like roaming and subscriber mobility that fixed-line INAP did not originally address. 3GPP needed a protocol that could support service continuity when a subscriber moved between network areas or different operators' networks. While CAP became the primary protocol for mobile IN, the foundational principles and many operations of INAP remained, ensuring that the core concept of standardized service control signaling was preserved and extended for the mobile era.

Key Features

• Standardized set of operations (e.g., InitialDP, Connect, PlayAnnouncement, ApplyCharging) for IN service control
• Uses TCAP over SS7 as its reliable transport mechanism
• Defined using ASN.1 for unambiguous message encoding and decoding
• Supports multiple capability sets (CS-1, CS-2) for evolving service complexity
• Enables the query-response dialogue between SSP (switch) and SCP (service logic server)
• Foundation for the CAMEL Application Part (CAP) protocol used in mobile networks

Evolution Across Releases

Defining Specifications