What is PSTN? Your Global Telephone Network

All you need to know about the Public Switched Telephone Network

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Unified CommunicationsLatest News

Published: November 21, 2022

Ryan Smith

Technology Journalist

PSTN stands for “Public Switched Telephone Network”, and it refers to the world’s oldest collection of interconnected communication solutions. 

Some people also refer to this communications option as the “Plain Old Telephone Service” or POTS. 

Today, we know the PSTN as the evolution of the circuit-switching networks that have evolved progressively since the days of Alexander Graham Bell.  

What started as single lines connected from one phone to another is now almost entirely digital in structure.  

Moreover, the PSTN also furnishes much of the long-distance infrastructure of the internet. 

Internet service providers (ISPs) pay other providers with connections across long distances to access their infrastructure.  

Internet users can avoid paying excess usage tolls to anyone but their ISPs. 

The History of PSTN 

The Public Switched Telephone Network is the accumulation of global circuit-switched networks for telephone communication. 

It is operated on various levels, such as local, regional, and national. 

PSTN provides the infrastructure for public communication, consisting of fibre optic cables, telephone lines, microwave transmissions, communication satellites, cellular transmissions, and under-sea cables. 

The very first telephones had no network connections. Instead, they were private-use solutions for communication wired together in pairs.  

This meant that it was impossible to call more than one location.  

Users who wanted to talk to a range of people had to have several different telephones – each with its own purpose.  

By the 1960s, voice calls started to be digitised, with manual solutions for switching replaced by automated electronic switching.  

Digital voice signals could share the same wire with other phone calls, and the communication abilities of the masses grew. 

Over time, the PSTN transformed from fixed-line telephone analogue systems to an almost entirely digital solution.  

The development of fibre optic cables now means thousands of calls can share the same line. 

Despite technological developments, high-bandwidth and fibre optic cables have not changed the fundamental nature of circuit switching. 

This method of communication still requires a circuit or connection to remain open throughout a phone call. 

The technical operation of the PSTN adheres to standards that were initially created by the ITU-T or the International Telegraph Union Telecommunication sector.  

These standards allow for different networks to connect seamlessly across countries.  

The E.164 and E.163 standards of the ITU-T allow for a single address space for telephone numbers across the globe. 

The nature of PSTN today means that we have never been freer to connect with people worldwide. 

The Digital World of PSTN 

Most automated solutions for telephone exchanges now utilise digital methods for switching rather than analogue or mechanical switching.  

When someone enters a number into their phone, they’re automatically connected to the right person.  

The trunks that connect these exchanges are also digital – known as channels or circuits.  

For a phone call to move from one party to another, an analogue audio signal must be made digital at an 8 kHz sample rate, using a unique pulse-code modulation (PCM) frequently referred to as G.711.  

After the transformation is complete, the call can transmit from one end of the PSTN to another via a range of telephone exchanges. 

In most countries, the central PSTN solution has a regulator which monitors the PSTN services in that country.  

The tasks of this regulator will include ensuring that end customers aren’t overcharged for services. 

Using the PSTN Today 

In the modern world, the PSTN is one of the primary solutions for connecting people worldwide through voice communications.  

Several of the largest private telephone networks aren’t linked to the PSTN – typically for military reasons.  

There is also a range of private networks run by large companies that link to the PSTN through limited gateways, such as a private branch exchange. 

Today, it is even possible for digital applications to connect to PSTN networks.  

For instance, Microsoft Teams users can utilise plugins that allow them to call mobile and landlines as well as other computers.  

Alternatively, there are cloud connector solutions that allow businesses to use their office applications normally while experiencing the benefits of a third-party PSTN breakout. 

UK Switch Off 

In 2017, it was announced by Openreach that the PSTN in the UK would be entirely switched off by 2025. 

The entire country will be moving to a digital network, and work is underway to make the switch. 

The switch off doesn’t only affect telephone services but other means of communication and operating devices as well. 

Things such as alarms, EPOS machines, CCTV, fax machines, door entry systems, and more are all connected to the PSTN infrastructure. 

In 2021, Dominic Martini, Senior Manager of Hosted IP Voice and Unified Comms at BT Wholesale, told UC Today that channel partners need to outline the effects of the PSTN switch-off on end-users.

Martini said channel partners have a role to play by “helping businesses think more strategically about their needs to understand the benefits offered by hosted voice and collaboration tools”. 

Countries such as Germany, Japan, and Sweden are currently ahead of the UK in switching to a digital network. 

The Netherlands and Estonia have already switched off their PSTN networks in favour of a digital model. 

 

 

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