What Is PSTN and How It Works: Complete Guide [2025]

PSTN stands for public switched telephone network. It’s a complex communication infrastructure that allows its users to communicate with each other locally or internationally using analog phones, fax machines, and data transmission.

Here’s a handy guide that explains the PSTN and how it works.

Table of Contents

What Is PSTN?

PSTN, or public switched telephone network, is a communication infrastructure that carries voice and data worldwide. It connects landline phones, fiber, fiber optic cables, cellular networks, microwave transmission links, and undersea telephone cables, forming a global communication network operated by telephony operators. Homes and businesses subscribed to PSTN can communicate locally, nationally, or internationally through voice, fax, or data transmission.

In its early days, the PSTN lines relied on twisted pair copper wires to transmit analog voice signals. Over time, the PSTN has undergone a significant modernization. The modern PSTN uses fiber optic cables and Voice Over Internet Protocol (VoIP) technologies. These advancements allow for faster and more reliable communication at a lesser cost. However, traditional PSTN infrastructure still exists in some rural or older areas.

Infrastructure of PSTN: Key Components

PSTN has evolved, as Practical Telecommunications and Wireless Communications shows. From being an analog, voice-only communications network, it now includes a digital network that carries any form of data communications at high speed. Here’s a breakdown of its components:

Telephone lines

Old telephones were connected by copper wires, which allowed voice signals to travel from phone to phone. Today, telephony operators use fiber optic cables, which enable faster and higher-quality communications. The quality of international or long-distance calls is also better when using the modern PSTN network.

Related Read: Are copper phone lines going away?

Local exchange

The local exchange or central office (CO) manages local calls and connects multiple phone lines within a city or region. The service provider manages connections by routing calls between subscribers within the same geographic area. The CO also provides the equipment to handle calls and services such as call forwarding and voicemail.

Switching centers

The switching center connects calls between different local exchanges, regions, or countries. If you call someone in another city or country, the local exchange routes your call to a switching center, which decides on the best route to connect the call. This communication path between two phones remains open during the entire call, even if the caller and receiver don’t talk.

Trunks

Trunks are the main transmission lines between switching centers and local exchanges or between different switching centers. They can simultaneously carry multiple voice signals at longer distances using fiber optics or microwave radio signals.

International gateways and submarine cables

The PSTN relies on undersea cables and satellite systems for international calls. You can view the Submarine Cable Map to appreciate the vastness of this undersea network. Internal gateways, meanwhile, are the major switching centers that route calls between countries.

Signaling systems

PSTN uses signaling protocols or “rules of conversation” that manage the connection between two phones. The network, phones, and exchanges don’t automatically connect when you make a call. They need to signal each other to start and end calls, maintain the call’s connection, and ensure call quality. Different signaling systems handle these processes.

End-user devices

End-user devices are the gadgets you use to make and receive calls. They include old school phones, fax machines, answering machines, public payphones, caller ID services, and modems.

How Does Public Switched Telephone Network (PSTN) Work

PSTN Calls

Your phone sends a signal to the local exchange through fiber optic or copper lines.
The local exchange uses the destination number to route the call.
After passing through switches and trunk lines, the call reaches the recipient’s phone.
The recipient’s phone rings. If they answer their phone, a dedicated circuit establishes a connection throughout the whole duration of the call.

PSTN in Mobile Communication

The PSTN is only involved in mobile communication if the caller or receiver uses a landline. The mobile network interacts with the PSTN only when a mobile phone user calls a landline or vice versa.

The user’s mobile phone sends the call to the Mobile Switching Center (MSC).
The MSC checks the destination number and routes the call to the PSTN via the Interconnection Gateway.
The PSTN receives the call and connects to the local exchange connected to the landline phone.
The call reaches the landline phone.

Is PSTN Still Relevant?

The news about the impending copper network switch off significantly impacts the future and relevance of PSTN (Public Switched Telephone Network). It’s also apparent that more businesses will shift from traditional landline services to digital solutions like VoIP.

Regardless, here’s a breakdown of where PSTN remains relevant:

Mobile coverage may be limited in remote areas. Some people may prefer the reliability of PSTN, which doesn’t depend on battery life or electricity.
In some countries, landline phones are still the most reliable way to connect to emergency services.
PBX systems for businesses may still rely on PSTN for voice communication and faxing.
PSTN is still critical for people who communicate with someone who only uses a landline
Modern business phone systems may use hybrid infrastructure, combining PSTN and VoIP services.

Best PSTN Alternative for Business: VoIP

PSTN remains relevant, but its usage is declining as more users switch to more advanced forms of communication like VoIP. VoIP PBX systems offer many advantages compared to PSTN, especially if you’re running a business:

Lower costs: VoIP has lesser infrastructure requirements and doesn’t require physical phone lines or extensive IT support.
Scalable: VoIP, which is easier to set up than PSTN, allows your business to adapt quickly to changing communication needs.
More features: VoIP offers more features, such as voicemail to email, call forwarding, video conferencing, internet fax, and call recording.
Environmental benefits: VoIP is better for the environment since it has minimal hardware and infrastructure requirements.

VoIP is best for businesses that want to invest in a more flexible, budget-friendly, and high-quality modern phone system.

With analog-based PSTN being replaced by digital systems, the eventual phasing out of landlines could also significantly affect how you send and receive faxes.

Switching to digital faxing will ensure uninterrupted communication with better security and flexibility.

Find out how iFax can help. Request a free demo.