Public Switched Telephone Network and the Origin of Telecom

Origins of Telecom: The Public Switched Telephone Network

A busy signal used in the North American PSTN prior to the 1980’s.

The Public Switched Telephone Network came with the invention of the telephone in 1876 came the need for infrastructure. Early phone calls took place over a physical wire connecting the two devices. While this direct connection was fine for proof of concept or for voice communications between telegraph offices, imagine the impracticalities of connecting a wire from your house to every other home or business you would need to call. Fortunately, Alexander Graham Bell had other ideas.

Bell had approached Western Union, offering to sell the rights to his PSTN invention for $100,000 — and offer they turned down. In 1877, he founded Bell Telegraph Company and opened the first telephone exchange in Connecticut that same year. Subscribers only needed to connect to one physical location, the telephone exchange. From there, operators manually connected them to the destination phone using another of Bell’s inventions, a switch.

Soon, telephone exchanges were established for other areas with “trunks” being used to connect each exchange. The trunks contained bundles of wires and formed proto-networks that expanded across the nation and ultimately around the world. As the infrastructure developed, various connection methods were used to connect geographically diverse networks including microwave relay and undersea cables.

Automated switches replaced manual ones, allowing callers to place calls without operator assistance. Copper wires eventually gave way to modern, fiber optic cables while analog switches have been replaced by digital switches. Call quality has improved dramatically while costs have fallen.

In general, when you make a phone call, you’re connected to a local exchange which then routes your call to its destination. For local calls, the call stays within the exchange. For long distance calls, your call will be routed through the Toll Office switch. For international calls, your call will be routed through the International Gateway.

Your phone number and the phone number you are calling play a role in how the calls are routed. For example, if your phone number is 1-805–333-1212, the number 333 represents your local exchange while 1212 is your subscriber number. If you were to call a neighbor at 333-1313, your call would remain in the local exchange and be connected to 1313. If you make a call outside of your area code, then your call would be routed through the Toll Office switch. If you need to make an international call, you’d first dial 011, then the country code, area code, and phone number; your call would then be routed through the International Gateway.

Though firmly established in countries like the United States, many developing countries are bypassing traditional telephone systems in favor of mobile networks. Fortunately, these networks play nicely with each other and you can easily call others regardless of the phone type they may be using.

Due to other telecommunications methods such as cellular phone service and VoIP, the public switched telephone network (PSTN) is often referred to as “plain old telephone service.” It’s evolved since the dawn of the telephone and provides a robust, high-quality foundation for newer technologies such as international conference calling and global call forwarding numbers.

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