Public Switched Telecommunications Network

Description

The Public Switched Telecommunications Network (PSTN) is the worldwide aggregate of circuit-switched telephone networks, primarily operated by national and regional carriers. It is characterized by its use of dedicated physical circuits (or virtual circuits emulating them) for the duration of a call, employing signaling systems like SS7 (Signaling System No. 7) for call setup, routing, and management. Within the 3GPP architecture, the PSTN is not a 3GPP-defined network but an external network with which 3GPP systems must interwork. The Core Network (CN) elements, specifically the Mobile Switching Center (MSC) in circuit-switched (CS) core and later the IP Multimedia Subsystem (IMS) in packet-switched (PS) core, provide the gateway functionality to connect mobile subscribers to PSTN subscribers.

The interconnection is achieved through defined reference points and protocols. For traditional CS voice, the MSC connects to the PSTN via the TDM-based interface, often using ISUP (ISDN User Part) signaling over SS7. The MSC performs the necessary protocol conversion between mobile-specific signaling (like BSSAP) and PSTN signaling. With the evolution to all-IP networks and Voice over LTE (VoLTE), the PSTN interconnection point shifts to the IMS. Here, the Media Gateway Control Function (MGCF) and Media Gateway (MGW) within the IMS handle the interworking. The MGCF translates between the SIP (Session Initiation Protocol) used in IMS and the ISUP/BICC signaling used toward the PSTN, while the MGW converts the media stream between the packet-based RTP/UDP/IP used in the PS domain and the circuit-switched TDM or packetized voice formats (like G.711) used on the PSTN side.

From a service perspective, the PSTN represents the ultimate destination for many voice calls originating in a mobile network. Ensuring seamless interoperability with the PSTN is a fundamental requirement for any commercial mobile network, as it allows subscribers to call any fixed-line telephone in the world. 3GPP specifications extensively cover this interworking, detailing scenarios for call routing, number translation (using E.164 numbers), supplementary service interworking (like call forwarding, barring), and emergency service routing. The PSTN also serves as a model and fallback for certain telephony services within the mobile network itself, especially before the full deployment of IMS.

Purpose & Motivation

The PSTN existed long before cellular networks. The primary purpose of defining PSTN interworking in 3GPP standards was to ensure that the new digital mobile systems (GSM, UMTS, LTE) could be integrated into the global telephony ecosystem from day one. Without standardized interworking, mobile networks would have been isolated islands. The problem solved was universal connectivity: enabling a mobile subscriber to call any fixed-line phone and vice versa. This was a non-negotiable commercial requirement for the success of 2G GSM.

Historically, the initial 3GPP architectures (GSM, UMTS) were built with a circuit-switched core that mirrored many principles of the PSTN, making interconnection relatively straightforward through standardized TDM interfaces and SS7 signaling. As 3GPP networks evolved toward packet-switched all-IP architectures with LTE, a new challenge arose: how to maintain this seamless PSTN connectivity when the native mobile bearer was packet-based IP and the core network was moving away from circuit-switched elements. This motivated the development of IMS-based solutions like VoLTE and the SRVCC (Single Radio Voice Call Continuity) handover mechanism. IMS, with its MGCF and MGW, provided a standardized, future-proof IP-based gateway to the legacy PSTN, ensuring service continuity while the network infrastructure modernized. Thus, PSTN interworking specifications have evolved from direct TDM trunking to sophisticated IP-based signaling and media translation.