What is a PSTN Line and What are They Used For?

Published on
October 22, 2021
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Once marked as the standard for telecommunications networks, the PSTN (or Public Switched Telephone Network) is an ever-changing network of infrastructure that is slowly being replaced by next-generation alternatives.

Despite being around since the late 1800s in many countries, there is still a lot of mystery around what is a PSTN line, what is PSTN line used for and what is a PSTN line fault?

This blog will answer your most common questions and take a deep dive into all things PSTN lines.

Introduction: What is a PSTN Line?

What is a PSTN phone line? PSTN lines are an old technology used to transmit phone signals from a landline telephone to its destination. These lines are operated by telephone service providers and allow public communication.

Service providers use copper switching to transmit analogue voice data, typically between homes and small businesses.

When asking what is a PSTN phone line, people are usually referring to what most call "telephone lines" but are called many different names, including landlines, Plain Old Telephone Service (POTS), or fixed-line telephones.

Because PSTN lines are dedicated lines, customers cannot use a PSTN line for anything else while a call signal passes through it, i.e. when you're making a voice calls using your landline through a PSTN line, you cannot make another call alongside it using another phone.

PSTN phone lines are made of copper, meaning there are copper lines that connect most UK homes to street cabinets and then local exchanges to provide voice and data services. As the PSTN is being phased out by 2025, older houses might already be disconnected, or new homes might already have next-generation services such as IP (Internet Protocol).

What is a PSTN Line Used For?

PSTN lines are used to provide services that require the PSTN to function, such as:

  • Connecting subscribers telephone calls between one and other's telephone numbers and telephone systems
  • Providing power to critical infrastructures, such as your landline telephone, house alarms and business's payment terminals
  • Newer PSTN lines can also provide internet connection through the ISDN (Integrated Services Digital Network). This system can transmit data and high-quality voice over a digital line, rather than just voice over an analogue line.

PSTN lines are being used less and less due to their growing maintenance costs and cheaper alternatives. Line faults are often difficult to track and costly to fix - but it is inevitable when some lines are up to 100 years old.

What is a PSTN Line Fault?

A fault is generally when the connection no longer reaches its destination from its origin through the PSTN line or even when the digital signal becomes poor - this can cause problems for customers who might churn away from their service provider.

Due to the age of PSTN lines, it's common for them to become faulty over time. Whether through general degradation of its rubber casing causing copper corrosion, a line cut during construction works, or even theft, as copper has a relatively high scrap value.

To test for these faults, tests on PSTN lines can be completed for the following reasons:

  • A customer has got in touch regarding the loss or drop in quality of service.
  • The line is being checked for suitability for an upgrade from analogue to digital (i.e. just voice to voice and internet).
  • Natural disasters (floods, strong winds etc.)
  • Change of service provider (Local Loop Unbundling)
  • Incorrect removal or repair of equipment in the PSTN (such as in a telephone exchange)

One common method for testing PSTN line faults is automated copper line testing through a Test Access Switch Matrix (TASM), which allows numbers to be tested remotely via a test header. One of the most common TASM providers is UTEL, who provide rack-mounted units catering for 400 traditional circuits through just one port.

Who is Responsible for a PSTN Line Fault?

The party responsible for the PSTN line fault depends on where the fault is located, which may not be known until the above test has been carried out.

The below diagram (Source: Copper Line Testing) highlights that it will either be the customers or the service providers responsibility.

If the fault is located within the home (or past the external wall/termination box), it is the customers' responsibility as it is likely the like has been damaged as a result of someone inside the building.

If the fault is outside of the external wall/termination box (in the telephone exchange or the last mile), it will be the responsibility of telecommunication companies.

How Fast Does Data Travel Through Copper Wire (PSTN Lines)?

The speed of data transfer through PSTN lines depends upon the type of copper wire, distance to travel and some external factors.

Category 6 copper cable has a max bandwidth of 1 Gb/s, and Category 5e copper cables reach up to 100 Mb/s. However, these cables can only be up to 100m long as the high speed and strength of the signal degrade over distance.

Furthermore, due to the electromagnetic properties of copper, any external signals might also impact how fast data travels through copper wire. This is common as copper cables are often placed next to electrical cables, producing electromagnetic fields as they pass electricity.

What is a BT PSTN Line?

BT is a service provider who provides its telephone and internet services over Openreach's PSTN infrastructure, so when someone refers to a BT PSTN Line, this is a line that offers BT's services.

A PSTN line doesn't always have to be BT's - other companies can either add their own equipment and lines or use Openreach's PSTN infrastructure to provide their own services through a Local Loop unbundling process (LLU).

Before 2005, BT owned and managed the phone and broadband network provided over the PSTN. This was until Openreach was created to ensure all service providers could access the copper network fairly.

What type of Phone Line Is PSTN?

The PSTN (also known as POTS or fixed telephone lines) is a dedicated analogue phone line that can only carry a singular phone call signal. They are made from copper and can be upgraded to provide internet connection but is more often than not replaced by next-generation alternatives such as VoIP.

PSTN Replacement with VoIP

PSTN and ISDN lines are slowly being replaced as service providers switch to digital technology such as IP or VoIP (Voice over Internet Protocol). This is a cloud-based solution that moves everything online, meaning your telephone signal and internet will be transferred over next-generation VoIP connections.

With the PSTN and ISDN set to be completely switched off by 2025, all services carried over these lines need to be replaced by then, meaning all PSTN customers will be migrated to IP by 2025.

This is why you may have seen a lot in the news recently about Openreach rolling out fibre optic networks at record speeds, as this is the infrastructure that will make the PSTN redundant and available to be recovered.

To find more about Openreach's Digital Upgrade and PSTN switch-off, check out our previous blog - The Complete Guide to the UK's Digital Upgrade.

What is a PSTN Line Summary

Due to the importance of this infrastructure to society, it must be identified, removed and replaced with next-generation alternatives as fast as possible before it becomes too unreliable and expensive to maintain. Without the PSTN, many of the ways we communicate and interact with each other, keep safe and connect to the internet wouldn't be possible.

With how crucial this infrastructure is to society, it must be identified, removed and replaced with next-generation alternatives as fast as possible before it gets too unreliable and expensive to maintain. Rapid and scalable removal will certainly save customers and network operations significant time and money, while also generating additional revenues through resale.

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