Also known as Public Switched Telephone Network, it is the dominant telecommunications system of 20th century that uses the circuit-switching technology to make phone calls and transmit voice and data over copper wires or fiber-optic cables. PSTN encompasses an extensive network infrastructure, the equipment and and protocols necessary for landline telephone communication, connecting users through a network of local telephone exchanges, central offices, intermediate switches, trunks and transmission lines. Due to the limitations and inflexibility of circuit switching, it has fallen out of favor in the 21st century, with newer technologies like VoIP, WebRTC and cloud based communication systems gaining popularity. However, PSTN continues to operate and provides reliable voice communication services for many users worldwide.