Telephone User Part

Description

The Telephone User Part (TUP) is a key protocol within the Signalling System No. 7 (SS7) suite, specifically designed for managing call control in circuit-switched telephone networks. It operates at the application layer of the SS7 protocol stack, interacting with the Message Transfer Part (MTP) for reliable message delivery. TUP's primary function is to establish, maintain, and release voice circuits between telephone exchanges by exchanging standardized signaling messages. These messages include Initial Address Messages (IAM) to initiate a call, Address Complete Messages (ACM) to indicate the called party is being alerted, and Answer Messages (ANM) to confirm call connection, along with Release (REL) and Release Complete (RLC) messages for call teardown.

Architecturally, TUP is implemented in telephone exchanges or switches, such as those in the Public Switched Telephone Network (PSTN) or early mobile switching centers (MSCs) in GSM networks. It handles essential call-related tasks like digit transmission (using en-bloc or overlap signaling), call supervision to monitor the state of a connection, and basic charging indications. TUP does not support advanced services like caller ID or call forwarding natively; these were later introduced in the ISDN User Part (ISUP). The protocol relies on point codes for addressing within the SS7 network and uses circuit identification codes (CICs) to associate signaling messages with specific physical or logical trunk circuits.

In operation, TUP enables end-to-end call control by exchanging messages over dedicated signaling links, separate from the voice bearer channels. When a call is initiated, the originating switch sends an IAM containing the dialed digits and circuit information. Subsequent switches use this to route the call and reserve a trunk, with ACM and ANM messages completing the connection. TUP also manages error conditions, such as sending unsuccessful backward setup messages for failed calls. Its role was critical in automating telephony, replacing earlier in-band signaling methods like pulse dialing, and providing a foundation for digital network evolution. While largely superseded by ISUP in modern networks, TUP remains relevant in legacy systems and some regions.

Purpose & Motivation

TUP was created to provide a standardized, reliable signaling protocol for call control in digital telephone networks, addressing the limitations of earlier analog signaling methods. Prior to SS7 and TUP, telephony relied on in-band signaling, where control tones and pulses were sent over the same channel as voice, leading to inefficiencies, vulnerability to fraud (e.g., phreaking), and limited functionality. The move to out-of-band signaling with SS7 separated control and bearer paths, improving security, speed, and network capacity. TUP specifically solved the problem of automating call setup and teardown between exchanges in national and international PSTN networks, enabling features like direct dialing and faster connection times.

The historical context stems from the 1970s and 1980s, as telecom operators transitioned to digital switching and needed a robust signaling system to support growing traffic. TUP, as part of SS7 standardized by ITU-T, provided a globally interoperable framework for basic telephony services. It addressed key issues such as efficient use of trunk circuits through precise call supervision, reduced post-dial delay by speeding up signaling, and support for incremental network upgrades. However, TUP's limitations in handling data services or advanced call features motivated the development of ISUP, which integrated ISDN capabilities. In 3GPP, TUP is referenced for legacy interworking, particularly in early GSM releases where mobile networks interfaced with PSTN infrastructure.

Key Features

• Basic call setup and teardown using IAM, ACM, ANM, REL, and RLC messages
• Support for en-bloc and overlap signaling methods for digit transmission
• Call supervision to monitor connection states and detect answer/disconnect events
• Circuit identification using CICs to associate signaling with trunk resources
• Backward setup messages for handling unsuccessful call attempts (e.g., congestion)
• Compatibility with SS7 MTP for reliable, sequenced message delivery over signaling links

Evolution Across Releases

Defining Specifications