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Anna Kowalski

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calendar_month2026-02-06

The Pulse of the Past, The Backbone of Today: Understanding PSTN

From copper wires connecting homes to the invisible digital threads of modern data.

Summary: The Public Switched Telephone Network (PSTN) is the world's original, globe-spanning communication system. Often called the "Plain Old Telephone Service" or POTS^[1], it's a massive network of physical lines, switches, and cables that for over a century allowed us to make a call from any phone to any other phone in the world. At its heart is a clever principle called circuit-switching, which establishes a dedicated, temporary pathway for your conversation. This article will explore how this analog^[2] system works, its core components like central offices and subscriber lines, and how it surprisingly adapted to carry the digital data that fuels our internet, forming a crucial bridge between the old and new worlds of communication.

The Foundation: How Circuit-Switching Works

Imagine you want to send a toy car from your house to your friend's house across town. You could put it in a box with the address and send it through a postal network (like a modern data packet). The PSTN works differently. It's more like building a temporary, private toy car track that connects your door directly to your friend's door for the duration of your playtime. Once you're done, the track is taken apart so others can use the pieces.

This is the essence of circuit-switching. When you dial a number, the network's job is to find a continuous physical path of copper wires and electronic switches between your phone and the receiver's phone. It "reserves" this entire path for you until you hang up. During that time, no one else can use that specific circuit, even during moments of silence in your conversation.

Key Formula: The Circuit
In simple terms, a completed PSTN call can be thought of as: $\text{Caller's Phone} \rightarrow \text{Local Loop} \rightarrow \text{Switch}_1 \rightarrow ... \rightarrow \text{Switch}_n \rightarrow \text{Local Loop} \rightarrow \text{Receiver's Phone}$
The Local Loop is the pair of copper wires from your home to the telephone company's first building. Each Switch is a decision point that routes the call along the correct path.

The Anatomy of the PSTN: Key Components

The PSTN is built from specific physical parts, each with a clear job. Think of it as the body of a communication giant.

Component	Nickname / Role	Simple Explanation
Subscriber Line / Local Loop	The "Last Mile"	The pair of copper wires running from your home telephone jack to the nearest telephone company facility.
Central Office (CO)	The "Telephone Exchange"	A building full of switches that connects all local loops in an area. It's your call's first stop.
Switches	The "Traffic Directors"	Electronic devices that connect one telephone line to another, choosing the path for your call.
Trunk Lines	The "Communication Highways"	High-capacity lines (often fiber optic cables today) that carry many calls between central offices and long-distance networks.
Signaling System	The "Network's Nervous System"	The behind-the-scenes network that tells switches a call is coming, what number was dialed, and when to hang up. SS7^[3] is the modern standard.

From Voice to Data: The PSTN's Digital Evolution

The original PSTN carried your voice as a continuously varying electrical signal (analog). But computers speak in binary: 1s and 0s (digital). So how did we get the internet over phone lines? The answer is a brilliant translator called a modem (Modulator-Demodulator).

A modem takes the digital data from your computer and modulates it—turns it into sounds (tones) that can travel over the analog phone line. At the other end, another modem demodulates those sounds back into digital data for the receiving computer. This allowed the PSTN to be used for dial-up internet. The modem would literally "dial" a phone number to connect to an Internet Service Provider^[4] (ISP), establishing a circuit-switched connection used for data.

This adaptation, however, had limits. The circuit was still dedicated, meaning your phone line was busy. Also, data speeds were slow because phone lines were designed for the limited frequency range of the human voice, not high-speed data. This pushed the development of technologies like DSL^[5] (Digital Subscriber Line), which used higher frequencies on the same copper wire to provide "always-on" internet without blocking the voice line, marking a major step in using PSTN infrastructure for modern data.

A Practical Journey: Making a Long-Distance Call in the PSTN Era

Let's follow a call from Maya in New York to Leo in Los Angeles in the 1990s, before cell phones were common. This shows the step-by-step, physical process of circuit-switching.

Step 1: Off-Hook & Dial Tone. Maya picks up her landline phone. This closes an electrical circuit, sending a signal down her local loop to the Central Office (CO). The CO sends back a dial tone, saying "I'm ready."

Step 2: Dialing & Routing. Maya dials Leo's number. The pulses or tones tell her local CO the destination. Her CO, recognizing it's a long-distance number, doesn't have a direct circuit. It routes the call to a toll switch (a specialized switch for long-distance calls).

Step 3: Circuit Building. The toll switch in New York communicates with a toll switch in Los Angeles via the signaling network (SS7). They find an available physical path—perhaps a series of trunk lines through multiple cities. Each switch along the way reserves a piece of the path.

Step 4: Ringing & Conversation. The Los Angeles toll switch connects to Leo's local CO, which sends electricity down Leo's local loop to make his phone ring. When Leo answers, the end-to-end circuit is complete. For the next 20 minutes, that specific chain of wires and switches is dedicated solely to their call.

Step 5: Teardown. When they hang up, a "call finished" signal travels the network. Every switch releases the reserved pieces of the circuit, making them available for the next call. The private "track" is dismantled.

Important Questions

Q1: If PSTN is old, is it still being used today?

Yes, but its role is changing. Traditional copper-line PSTN services are being phased out in many countries in a process called the "PSTN switch-off" or "copper retirement." However, the core concepts and much of the infrastructure (like fiber optic trunk lines and upgraded switches) form the backbone for modern Voice over IP^[6] (VoIP) and cellular networks. The physical wires are often reused for DSL and fiber internet. So, while the classic analog service is fading, the network evolved into today's digital foundation.

Q2: What is the main difference between PSTN (circuit-switching) and the Internet (packet-switching)?

The key difference is in how they handle the connection. PSTN's circuit-switching establishes one dedicated path for the entire call. The Internet's packet-switching breaks data (an email, a video stream) into small chunks called packets. Each packet finds its own, possibly different, route to the destination based on current network traffic. They are reassembled at the end. It's like the toy car track (circuit) versus mailing many pieces of a disassembled car in separate envelopes (packets) that all eventually arrive at your friend's house.

Q3: Why was the dial-up internet connection so slow and why did it make that screeching sound?

The screeching sound was the modems "handshaking"—introducing themselves and agreeing on how to communicate. It was slow because the PSTN's copper lines were engineered for voice frequencies (about 300 Hz to 3400 Hz), which is a very narrow "pipe" for data. A modem had to squeeze digital data into those low-frequency tones. The maximum theoretical speed for a traditional analog modem was around 56 kbps (kilobits per second), which is thousands of times slower than today's average broadband connections.

Conclusion
The PSTN is far more than a relic; it is the foundational layer upon which our modern digital world was built. Its brilliant, physical circuit-switching design connected humanity for generations. More importantly, its vast network of copper lines, central offices, and trunk cables became the essential physical platform that engineers adapted and evolved to carry our digital data. From the first screeching modems to the always-on DSL and fiber connections, the PSTN provided the crucial bridge from the analog age to the internet age. Understanding it helps us appreciate the incredible engineering embedded in everyday life and see how new technologies often grow from the roots of the old.

Footnote

^[1] POTS: Plain Old Telephone Service. The basic, analog telephone service delivered via the PSTN.
^[2] Analog: A signal that is continuously variable, like the sound waves of your voice. Contrast with digital signals which are discrete (on/off, 1/0).
^[3] SS7: Signaling System No. 7. The international standard for the PSTN's separate signaling network that handles call setup, routing, and teardown.
^[4] ISP: Internet Service Provider. A company that provides access to the internet.
^[5] DSL: Digital Subscriber Line. A technology for high-speed internet access over the copper wires of the local loop.
^[6] VoIP: Voice over Internet Protocol. A technology that delivers voice communications and multimedia sessions over Internet Protocol (IP) networks, like the internet.

#Circuit-Switching #Telephone Exchange #Analog vs Digital #Modem #Copper Retirement #AS & A Level

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