What Is the ISDN and How Does it Work? | NBC

What Is the ISDN and How Does it Work?

June 7, 2023

what is isdn

Our ISDN guide explains what the ISDN is, how it works, and more. Read on for helpful answers to frequently asked questions on the topic.

What is the ISDN?

The ISDN (Integrated Services Digital Network) is a telecommunications technology that was developed to provide digital transmission of voice, data, video, and other services over a single network. It’s also a set of communication standards for transmitting data, voice, and signalling. ISDN was introduced as an upgrade to the traditional analogue PSTN to offer more advanced capabilities and improved quality. Compared with traditional lines, ISDN provides faster speeds and more reliable connections.

BT stopped the purchase of ISDN lines in 2020 with the plan to switch off all lines in 2025. Modern technology means that legacy landlines are becoming outdated, while the costs involved with maintaining PSTN/ISDN are inefficient in the digital age, particularly considering the convenience of VoIP (Voice over Internet Protocol) systems. Learn more about the ISDN switch off in our helpful blog.

History of ISDN

The unreliability of analogue phones, particularly over long distances, bred a need for a more dependable network. To remedy this, the 1960s saw the first transition towards a more packet-based digital system. Work on the ISDN communication standards started in 1980, and by 1988, the International Telecommunications Union began recommending ISDN as a new system.

However, by then, more modern systems with faster speeds had been released, and the ISDN didn’t reach the mass uptake that was expected. Today, digital subscriber line systems, internet connections, and wide area networks have largely replaced the network to accommodate the hyperconnectivity of the modern world. The ISDN is still available as a backup when these lines fail.

How does the ISDN work?

ISDN splits a traditional copper line into a variety of digital channels which function simultaneously on a single line. In this way, multiple phones can make and receive calls over one line. There are two main configurations of ISDN: Basic Rate Interface (BRI) and Primary Rate Interface (PRI).


BRI is primarily used for voice-grade telephone services. It consists of two bearer channels (B-channels) at 64 kbit/s and one delta channel (D-channel) at 16 kbit/s. B-channels are utilised for voice and user data, while D-channels combine data, packet networking and control and signalling. The combination of these channels allows BRI to successfully transmit information.


PRI transmits multiple Digital Signal Zero services and data between the ISDN network and a user.


The main difference between BRI and PRI is speed. BRI can reach maximum speeds of around 128 kbps, while the much faster PRI can reach up to around 2.94 Mbps.

What are the advantages of ISDN?

Today, one key benefit of ISDN is that it’s available when other lines fail, or where broadband internet isn’t accessible. We’ve included some benefits of ISDN below. However, it’s worth noting that these should be considered in comparison to PSTN lines, as more modern systems with further benefits are currently replacing ISDN. 

Benefits of ISDN include:

What are the disadvantages of ISDN?

As ISDN is being phased out, the disadvantages of the network compared to more modern systems like VoIP are becoming clearer. 

We’ve included some examples below:

ISDN vs DSL: What are the differences?

The main difference between ISDN and DSL (Digital Subscriber Lines) is that DSL transmits data a lot faster than ISDN lines. As ISDN is a dial-up service, it must go through a singular line. In comparison, DSL connections don’t need to dial. Thus, DSL can reach speeds of 100 Mbps, compared to max speeds of around 128 Kbps with ISDN.

What are the differences between PSTN and ISDN?

ISDN and PSTN (Public Switched Telephone Network) are both telecommunications systems; however, there are some key differences between the two. We’ve included some below.



You can learn more about the PSTN switch off in our helpful article.

What are some alternatives to ISDN?

Modern alternatives to ISDN include VoIP, SIP (Session Initiation Protocol), mobile networks, cloud-based communication services, and more.

We offer a range of VoIP phone packages, so find one that suits you today.

What’s the difference between ISDN and VoIP?

There are several differences between ISDN and VoIP, including:

Frequently asked questions

Is ISDN still used?

ISDN is still used, although its general function is to provide network services when other systems are unavailable. BT plans to fully phase out ISDN by the end of 2025.

What will replace ISDN?

More modern technologies like VoIP, SIP and mobile networks will replace ISDN when it’s phased out in 2025.

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