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PSTN: The Definitive Guide to Understand Public Switched Telephone Networks

PTNS Guide

Nearly fifteen decades ago, our way of communication was revolutionized. The world started connecting via telephone lines – a telecommunications system that evolved with underground wires, undersea cables, communication satellites, and into the PSTN of today.

Rather than sending letters that took forever to reach people, through PSTN, we were connected to people in real-time. However, with the disruption of the internet, PSTN is slowly slipping into antiquity. But what exactly is PSTN, and how does it work?

This article will look at PSTN in detail, covering how it works and measures against some modern alternatives.

What is PSTN?

PSTN stands for Public Switched Telephone Network. It is the traditional circuit-switched telephone network, a collection of interconnected voice-oriented public telephone networks all over the world. PSTN consists of all switched telephone networks globally operated either by local, national, or international carriers.

This system that aggregates circuit-switching telephone networks has been in use since the 1800s, evolving from the days of Alexander Graham Bell. Using underground copper wires, these networks provide the services for public telecommunication. For generations, PSTN has provided businesses and households with a trusted means of communication all across the globe.

A PSTN network is a combination of telephone networks including telephone lines, satellites, and cable systems, cellular networks, switching centers, and fiber optic cables. Today, however, calls are almost entirely digital. PSTN is also known by names like Plain Old Telephone Service (POTS), landlines, or fixed-line telephones.

How Does PSTN Work

A PSTN has centralized switching centers that act as nodes to make communication possible between two points on the network. A call is placed on the PSTN service after being routed through multiple switches, and then sound signals are transmitted via the connected phone lines.

In the early days, calls were transmitted over copper wires, and each call has a dedicated wire for the call to connect. PSTN providers would join wires to enable two phones to make a call successfully. However, in this digital age, this switching is automatic and it happens electronically. Voice signals can also share the same wire with other calls, thanks to fiber-optics.

The PSTN line is also used to connect a computer to the internet, with traditional dial-up network modems with an internet speed of up to 56 Kbps. In the early days of the internet, this was the means of home internet access before the broadband internet.

Here’s the breakdown of how PSTN works.

After dialing a number, your phone converts your voice into electrical signals that travel over the terminal via cables. These signals are then sent from the terminal to the central office. Once the central office receives the electrical signals, they are routed to the correct destination.

Depending on where the call is going, the central office will route your call to a CO, tandem office, toll office, or international gateway.


If you’re within the same locality as the recipient, your call would go to the local central office and connect to the recipient.

Tandem Office:

If you and your recipient are in the same city but different parts of town, your call would first be routed to your central office, then to a tandem office, then to the central office of your recipient, and then finally to your recipient.

Toll Office:

The toll office handles national long-distance switching. If you’re both in entirely different cities, then the tandem office would connect to a toll office and the call would be switched from there.

International Gateway:

If you and your recipient are in a different country your call will be routed to the international gateway.

Finally, once the call reaches any of its destinations, the electrical signals are converted by the phone into sound waves. This then makes almost instantaneous communication possible between you and the recipient.

This may sound a bit complex, but it all takes a few seconds for your call to reach its destination with the help of fiber optic cables and a global network of switching centers.

What’s the Difference Between PSTN and ISDN?

ISDN stands for Integrated Services Digital Network. It was developed for the digital transmission of data and voice over ordinary phone lines. One of the special features of the ISDN is that it integrates both speech and data in the same line, which is not available in the features of PSTN. ISDN also provides better voice quality and faster calls than PSTN.

PSTN – Pros & Cons

The telecom industry is experiencing a major shift from PSTN to internet-based networks. And though alternatives like Voice over Internet Protocol (VoIP) have a lot more to offer, it is still not out of place to analyze the pros and cons of PSTN and decide if it is right for your business.


  • Reliability
  • Better Security
  • Easy to use


  • Expensive
  • Fixed Phones
  • Limited Scalability

PSTN Alternatives

The PSTN has been around since the 1800s, and the telecom industry has experienced a lot of growth to improve its functions. Businesses have been making the switch from PSTN to alternatives like VoIP, SIP, and Hosted Phone System.

For businesses, finding the right communication solution is of immense importance. Let’s look at the advantages of these alternatives over PSTN as you may want to consider switching for your business as well.


VoIP stands for Voice Over Internet Protocol. It means voice communication is transmitted over the internet or private wide area network (WAN). VoIP is also known as IP telephony, internet telephony, or broadband telephony.

VoIP eliminates the need for circuit-switched networks for phone calls as it sends your voice communications over the internet rather than through cables. This means you’ll need a reliable internet connection. It’s a cloud-based solution that’s simple to set up, inexpensive, and easy to use.

If you’re considering whether or not PSTN or VoIP is the right communication solution for your business, here’s what you need to know. VoIP has advantages over PSTN, including lower network costs, faster connectivity, scalability, and advanced features, such as unified communications and app integrations.

While PSTN calls aren’t as susceptible to security issues like cyber attacks, VoIP calls today have also become more safe and secure.


SIP means Session Initiation Protocol. It is a signaling protocol for communications ranging from voice and video calls to data transfers. SIP trunking is the direct connection and data routing from one phone to another between a business and an Internet Service Provider (ISP).

The biggest advantage of SIP over PSTN is minimal costs. SIP trunks save you costs compared to a PSTN line as the initial capital expenditure is low. You also get to pay only for the line you use.

Another advantage of SIP is the ease of scalability. If your business is expanding or downsizing, you can easily add new or reduce lines. If you are to use a PSTN phone system, you’d need to get a line for each employee. This becomes a handful if you’ve got a lot of employees and more expensive for every added line.

With SIP, complexity is also reduced. You can start using it right away after set up as it doesn’t require hardware.

PSTN vs. Hosted PBX

Having an on-site PBX system requires a lot to keep it running smoothly – from setup and installation of hardware and software to regular in-house maintenance fees. An alternative however is to use a hosted PBX (or virtual PBX).

A hosted PBX or hosted phone system is a cloud-based solution that connects your phone system to the cloud, giving you all the benefits of a VoIP system while still offering you the use of traditional phone systems and hardware. With this alternative, you no longer worry about set-up, installation, and maintenance fees. 


Public Switched Telephone Network (PSTN) is a laudable phone system model but there are other—more modern—solutions available to businesses. If you love your business and want to scale it, you will need to switch your communications to a cloud-based VoIP phone system. PSTN is great but compared to VoIP or other alternatives, its technology is old-fashioned.

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