public switched telephone network

public switched telephone network
Name	Public Switched Telephone Network
Caption	A manual switchboard operator, circa 1890.
Industry	Telecommunications
Foundation	1876
Founder	Based on inventions by Alexander Graham Bell
Defunct	Phased out in the 21st century
Fate	Largely supplanted by Voice over IP and mobile networks
Successor	Next Generation Network

public switched telephone network. The Public Switched Telephone Network is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telephony operators. It provided the infrastructure for traditional plain old telephone service, enabling voice communications between subscribers globally. The development of this network was a foundational achievement of the Industrial Revolution, fundamentally reshaping business and social interaction.

Overview

The network's core principle was establishing a dedicated, or switched, electrical circuit between two parties for the duration of a call. This interconnected system evolved from manual switchboard operations to highly automated electronic exchanges. Pioneering work by Alexander Graham Bell and Thomas Edison led to its commercialization by entities like the Bell Telephone Company and the General Post Office in the United Kingdom. For over a century, it was considered a natural monopoly and critical infrastructure, often administered by state-owned entities such as AT&T in the United States or Nippon Telegraph and Telephone in Japan.

Technical details

The network utilized analog signals for local subscriber loops, with voice signals typically transmitted within a 300–3400 Hz frequency band. Call setup and teardown were managed by a separate signaling system, initially using dial pulses from a rotary dial and later Dual-tone multi-frequency tones. For long-distance connections, analog signals were converted to digital via Pulse-code modulation and multiplexed using Time-division multiplexing standards like the T-carrier system in North America or the E-carrier in Europe. Critical transmission mediums included twisted pair copper, coaxial cable, and microwave radio links.

Network architecture

The architecture was a hierarchical, tree-like structure of switching centers. A subscriber's telephone was connected to a local central office, often called a Class 5 switch. These offices were linked to tandem switches (Class 4) for intra-city traffic, which then connected to toll centers (Class 3, 2, and 1) for national and international routing. Major international gateways handled traffic between countries, governed by agreements from the International Telecommunication Union. The Signaling System No. 7 protocol became the global standard for out-of-band call signaling between these exchanges.

Services and features

Beyond basic voice calls, the network supported a range of services. These included Direct distance dialing, toll-free service via 800 numbers, and operator assistance. Enhanced features like call waiting, call forwarding, and conference calling were introduced. It also provided the access infrastructure for dial-up Internet connections and facsimile transmission. Specialized circuits, known as leased lines, were offered for permanent point-to-point connections for businesses or between Internet service provider facilities.

Evolution and decline

The network reached its zenith in the late 20th century before facing disruptive competition. The rise of cellular networks, championed by companies like Vodafone and NTT Docomo, offered mobile alternatives. The development of the Internet Protocol suite enabled Voice over IP services from providers such as Skype and Vonage, which bypassed traditional circuits entirely. This technological shift led to the policy of IP Multimedia Subsystem and the decommissioning of Class 4 and Class 5 switch hardware, a process known as the IP transition.

Standards and regulation

Technical standards were developed by bodies like the International Telecommunication Union's ITU-T sector and the European Telecommunications Standards Institute. In the United States, the Federal Communications Commission regulated interstate aspects, while state commissions like the California Public Utilities Commission oversaw local service. Landmark antitrust litigation, such as United States v. AT&T, led to the Bell System divestiture in 1984. Globally, deregulation and privatization in the 1990s, influenced by policies in the United Kingdom under Margaret Thatcher, transformed the industry landscape.

Category:Telecommunications Category:Telephone exchanges Category:History of telecommunications