PSTNverkko

PSTNverkko, or Public Switched Telephone Network (PSTN), is the traditional public telecommunications network designed to carry voice calls. It provides a universal reach by connecting subscriber stations through a global hierarchy of exchanges and trunk lines. The core feature is circuit switching: for each call, a dedicated path through multiple switching centers for the duration of the conversation.

The network commonly uses copper pair local loops to connect a customer’s premises to a local exchange (central office), with long-distance and international segments linking exchanges via high-capacity transmission links. Signaling for call setup, routing, and service features is handled by out-of-band signaling, notably Signaling System No. 7 (SS7) in modern networks.

In addition to voice, PSTN historically supported data services such as fax and dial-up modem connections, though these have declined as IP-based services have grown. The PSTN is valued for reliability, universal accessibility, and consistent quality of service, including access to emergency services via national numbers.

Over the last decades, many operators have migrated core networks toward IP-based technology and Next Generation Networks (NGN). VoIP and IP-based trunking can interwork with the PSTN via gateways, allowing continued access to telephone numbers and emergency services while reducing per-call costs and enabling new services.

Despite migration, the PSTN remains in wide use to support traditional telephony, especially where legacy devices or regulatory requirements necessitate a public, non-IP path.