General Switched Telephone Network

Description

The General Switched Telephone Network (GSTN) refers to the global, interconnected aggregate of public circuit-switched telephone networks, commonly known as the Public Switched Telephone Network (PSTN). In 3GPP specifications, particularly those related to core network architecture and interworking (such as TS 24.229 for IP Multimedia Subsystem protocols), GSTN is the term used to denote this legacy telephony infrastructure. It is a circuit-switched network where a dedicated physical or logical connection (a "circuit") is established between two parties for the duration of a voice call. The network is composed of switches (like Class 4 and Class 5 switches), transmission lines (copper, fiber), and signaling systems (primarily Signaling System No. 7 - SS7).

Architecturally, the GSTN operates on the principle of time-division multiplexing (TDM) for voice channels. Each voice call is allocated a specific time slot within a digital stream, providing predictable, constant bit-rate service. The key components include local exchanges (connecting subscriber lines), tandem/transit exchanges (for routing calls between exchanges), and international gateways. The control plane is handled by the SS7 network, which uses separate signaling channels to set up, manage, and tear down calls, and to provide services like caller ID and call forwarding. This separation of voice and signaling paths is a hallmark of the GSTN architecture.

In the context of 3GPP systems, the GSTN is an external network with which the mobile core network must interwork. This interworking is crucial for providing voice call services between mobile subscribers and fixed-line PSTN subscribers. In 2G/3G circuit-switched cores, this was achieved through the Mobile Switching Center (MSC). In 4G and 5G, where the core is packet-switched (IP-based), the interworking function is performed by specific nodes. For IMS-based voice (VoLTE, VoNR), the Media Gateway Control Function (MGCF) and IMS Media Gateway (IM-MGW) interact with the GSTN. The MGCF translates SIP signaling from the IMS into ISUP messages for the SS7 network, while the IM-MGW converts between IP-based media (RTP) and TDM-based media streams. This allows seamless voice service continuity between the packet-switched mobile world and the legacy circuit-switched world.

Purpose & Motivation

The GSTN exists as the historical foundation of global telephony, providing ubiquitous, reliable, and high-quality voice communication for over a century. Its purpose within the 3GPP ecosystem is to ensure backward compatibility and service continuity. When cellular networks were first deployed, their primary service was mobile telephony, and they needed to connect to the vast existing base of fixed-line subscribers and services on the PSTN. The GSTN represented the established standard to which mobile networks had to connect.

The creation and evolution of 3GPP standards had to account for this legacy infrastructure. The problem it solves is interconnection: without defined interworking procedures, mobile networks would be isolated islands. The motivation for specifying GSTN interworking in standards like TS 24.229 was to guarantee that a call from a 5G smartphone can reach a landline phone, and vice versa, with proper signaling translation and media conversion. This addresses the limitation of new network technologies operating in a vacuum; they must integrate with the installed base. As networks evolve towards all-IP, the GSTN represents the legacy domain that is gradually being replaced but remains critically important during the transition period.

Key Features

• Circuit-switched architecture with dedicated connections per call
• Uses Time-Division Multiplexing (TDM) for voice channels
• Relies on Signaling System No. 7 (SS7) for out-of-band call control
• Provides high reliability and voice quality (POTS)
• Globally interconnected and standardized for international calling
• Supports basic supplementary services (call waiting, forwarding, etc.)

Evolution Across Releases

Defining Specifications