National User Part

Description

The National User Part (NUP) is a component of the Signaling System No. 7 (SS7/C7) protocol stack, operating at the User Part layer, which is analogous to the application layer in the OSI model. SS7 is the foundational signaling system for circuit-switched telecommunications networks, enabling the setup, management, and teardown of calls and services. The User Part defines the procedures and messages for specific services. While the ISDN User Part (ISUP) is the internationally standardized variant for call control between countries, the NUP is a nationally defined adaptation of this protocol. It is specified by individual national telecommunications administrations or standardization bodies to meet domestic requirements.

Architecturally, NUP operates between Service Switching Points (SSPs) and between SSPs and Transit Exchanges within a national network. It uses the services of the Message Transfer Part (MTP) Levels 1-3 for reliable, connectionless routing of signaling messages. The NUP protocol defines national-specific message types, parameters, and procedures for basic call control, such as Initial Address Message (IAM), Address Complete Message (ACM), Answer Message (ANM), and Release (REL). These messages carry dialed digits, calling party number, circuit identification code, and other call-related information. The key difference from ISUP lies in the encoding of parameters, the definition of national-specific parameters (e.g., for carrier selection, billing information, or emergency service routing), and sometimes in the state machines governing call setup and release.

How it works involves the exchange of NUP messages over dedicated signaling links to reserve and manage trunks (voice circuits). For instance, when a call is placed, the originating exchange formulates an NUP IAM containing the called number and sends it via the SS7 network to the next exchange. Each subsequent exchange processes the message, selects an outgoing trunk, and forwards the IAM until the destination is reached. Successful call progression triggers ACM and ANM messages back to the originator. NUP ensures that national numbering plans, carrier codes, and specific charging or interception regulations are correctly handled. Its role is critical for the seamless operation of the national PSTN/PLMN backbone, interfacing at national borders with the international ISUP protocol.

Purpose & Motivation

NUP exists to bridge the gap between the need for international interoperability and the reality of national regulatory and technical differences in telecommunications. The ISUP protocol, standardized by ITU-T, provides a common language for cross-border call signaling. However, individual countries often have unique requirements for their domestic networks that are not covered by the international standard. These can include specific numbering plan interpretations, carrier identification codes for liberalized markets, legal interception mandates, emergency service routing (like 911, 112), and national billing or charging formats. Using pure ISUP domestically could be inefficient or non-compliant.

Before widespread standardization, many countries developed proprietary signaling systems, leading to fragmentation. NUP was motivated by the desire to leverage the robust, reliable SS7 infrastructure while allowing national customization. It solves the problem of adapting a global standard to local needs without breaking international connectivity. By defining a national variant of the User Part, a country can ensure seamless call handling within its borders while using standard ISUP at international gateways. This approach addressed limitations of earlier national systems by providing a structured, message-oriented signaling method that improved call setup speed, reliability, and support for advanced services like freephone or credit-card calling within the national context.

Historically, NUP implementations were common in many national networks during the evolution from analog to digital signaling. Its specification in 3GPP documents (like vocabulary spec 21.905) acknowledges its role in the ecosystem, particularly for legacy GSM core networks interfacing with national PSTNs. While modern networks increasingly use IP-based signaling (SIP, Diameter), NUP remains operational in many legacy circuit-switched cores, ensuring backward compatibility and service continuity for traditional telephony services.

Key Features

• Nationally defined variant of the SS7 User Part protocol for domestic call control.
• Operates over the Message Transfer Part (MTP) for reliable signaling message delivery.
• Defines national-specific message types, parameters, and call state procedures.
• Handles national numbering plans, carrier selection, and regulatory requirements.
• Interworks with the international ISUP protocol at national gateway exchanges.
• Manages the establishment, supervision, and release of voice circuits (trunks) within a country.

Evolution Across Releases

Defining Specifications