What Is the PSTN and How Does it Work?

June 7, 2023

Our PSTN guide explains what the PSTN is, how it works, and more, with plenty of helpful answers to frequently asked questions on the topic.

What is the PSTN?

The PSTN (Public Switched Telephone Network) is a global telecommunications network that’s been in use since the 19th century. The PSTN uses underground copper wires to provide homes and businesses with a means to communicate with people all over the world. The phones that utilise PSTN have a variety of names, including landlines, Plain Old Telephone Service (POTS), and fixed-line telephones.

Despite providing a reliable means of global communication for generations, PSTN phones are gradually being phased out in favour of more modern technology.

How does the PSTN work?

The PSTN connects a variety of telephone networks used worldwide, including telephone lines, fibre optic cables, switching centres, cellular networks, satellites, and cable systems. This interconnected system allows telephones to communicate with one another.

A PSTN consists of switches at centralised points that function as nodes, enabling two telephones to communicate through the network. Calls are routed through multiple switches, allowing voice signals to travel over the connected phone lines.

What happens when you dial a number through the PSTN?

Every time you dial a number on a POTS, your call passes through the PSTN. Here, the PSTN will connect both phones, allowing the caller and the recipient to communicate. We’ve illustrated how each step of this works below:

1. Your phone converts sound waves into electrical signals, which are then transferred through a cable to a terminal
2. Upon receiving the electrical signals, the terminal sends them to the central office
3. The central office directs phone calls by converting them into electrical signals, which are then transmitted using fibre optic cables. These cables serve as conduits for transmitting the signals as pulses of light to reach their intended destination
4. Next, your call is routed to a tandem office (if it’s a distance call) or a central office (for local calls)
5. Once your call reaches the right office, the signal is converted back to an electrical signal before it’s routed to a terminal
6. The terminal routes the call to the relevant phone number; when receiving the call, the electrical signals are converted back to sound waves.

This whole process plays out over a few seconds and is facilitated by fibre optic cables and a network of switching centres.

Features of a PSTN

Below, we’ve outlined some key features of a PSTN:

Analogue technology

The PSTN relies on analogue technology, where voice signals are converted into analogue electrical signals for transmission over copper wires. These signals are then transmitted through a series of switches and exchanges to connect callers.

Circuit-switching

Circuit switching is another key aspect of the PSTN. When a call is made, a dedicated physical connection is established between the calling and receiving parties for the duration of the call. This connection remains open until the call is terminated.

Basic telephony services

The primary function of the PSTN is to facilitate voice calls. It provides essential telephony services, allowing users to make and receive calls, engage in conversations, and communicate over long distances.

What is switching in telephony?

Switching describes the process of connecting calls from the originating point to the destination point. Essentially, switching refers to the method by which a network routes a telephone call from one telephone line to another.

PSTN and switching

PSTNs rely on switching, but there are various types of switching that take place at different levels. Let’s look at each below:

Local exchanges

Local exchanges, also known as central office or switching exchanges, connect subscribers to a PSTN line. Exchanges can have up to 10,000 lines.

Each telephone in a specific area is connected to the local exchange. If you were to ring someone in the building next door, the call would simply be routed to the recipient’s phone as soon as it reaches the exchange.

The local exchange will recognise the number you’ve dialled and direct it towards its destination. The first three digits of a number relate to the exchange – the local switch – while the last four digits identify the specific subscriber within that exchange. In this way, the local exchange can link a call to a subscriber without requiring further routing.

The tandem office

A tandem office covers a large area that constitutes several local exchanges. Also called a junction network, tandem offices manage switches between local exchanges.

If you called someone who lived in the same city but another area, your call would likely be routed to a tandem office from your local exchange. Next, the tandem office would transfer the signal to the local exchange of your recipient’s location.

The toll office

Toll offices function to switch long-distance calls. All tandem offices are connected to a toll office, allowing them to switch through any calls seamlessly.

The international gateway

International gateways switch international calls, transferring domestic calls to the recipient’s country.

History of PSTN

When telephones were first commercialised in 1876, they were wired in pairs between locations. No networks existed, so it wasn’t possible to call at more than a single location. If you wanted to speak to someone in a different location, you’d have to own a separate telephone paired with the said location. 

As telephone users increased, networks were built to accommodate them. This ultimately led to the growth of the PSTN, and older networks shifted to analogue signalling.

In the 1960s, automatic electronic switching replaced manual switching, and providers began digitising voice calls.

What is the PSTN switch off?

The PSTN switch off describes the gradual phasing out of the public switched telephone network, with the plan for it to be fully closed down by 2025. The aim is to upgrade all old analogue networks to fully digital ones to modernise communications infrastructure. This means that, in the future, every phone line will route calls over Internet Protocol (IP) rather than the current PSTN.   

Learn more about the PSTN switch off in our helpful guide.

Why is the PSTN being switched off?

The PSTN is no longer capable of meeting the demands of a hyper-connected world. Technology is developing rapidly, and the increasing dominance of the internet, smartphones, apps, the cloud, and so on requires a more modern infrastructure to process such high quantities of data. IP is a 21st-century technology to match the demands of modern communications and represents a natural and convenient next step to PSTN.

What are some alternatives to PSTN?

There are a variety of alternatives to PSTN, including VoIP and mobile phones.

What is VoIP?

VoIP services allow you to make calls over the Internet as opposed to traditional phone lines. Typical VoIP services include WhatsApp and FaceTime. We offer a range of VoIP phone packages.

What’s the difference between PSTN and VoIP?

PSTN relies on circuit-switched copper phone lines to transmit analogue voice data. Voice over Internet Protocol (VoIP) turns audio signals into digital transmission data that can be sent through the Internet. VoIP software allows users to make calls over the internet without the need for a traditional phone line, and will soon take the place of physical phone systems.

VoIP has advantages over PSTN, including lower network infrastructure costs, scalability and advanced features, such as unified communications and app integrations.

What is the ISDN?

The ISDN (Integrated Services Digital Network) is telecommunications technology that was developed to provide digital transmission of voice, data, video, and other services over a single network. ISDN was introduced as an upgrade to the traditional analogue PSTN to offer more advanced capabilities and improved quality.

What are the differences between PSTN and ISDN?

While PSTN and ISDN (Integrated Services Digital Network) are both telecommunications systems, there are differences between the two. Some are outlined below:

PSTN

• It’s analogue in nature and was used before ISDN
• Its transfer speed is relatively slow, and only voice transmission is permitted
• It allows simultaneous line connections, so just a single line can be used
• End-to-end connection isn’t possible, and it’s impossible to make fast calls

ISDN

• It’s digital in nature and was first used around 1960
• Data speeds are very quick, and circuit-switching allows both data and voice transmission
• It allows simultaneous line connection and permits up to 30 line connections
• It allows end-to-end connection

The function of ISDN is to enable the transmission of voice and data over PSTN lines. Therefore, the PSTN switch off will also render ISDN obsolete.

Learn more about the ISDN switch off in our helpful blog.

Frequently asked questions

Is PSTN still used?

PSTN is still used globally, although, with the rise of the internet, SIP and VoIP services are becoming far more popular.

What is PSTN being replaced with?

VoIP services will replace the PSTN, as they enable faster, higher quality telephony services at lower costs. Plus, they’re more flexible, scalable, and easier to maintain.

What does PSTN stand for?

PSTN stands for Public Switched Telephone Network.