What is ISDN and How Does It Work?

What is ISDN? A Guide to Integrated Services Digital Network

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In the telecommunication world, traditional landline technology was once the backbone of global communication. However, it had limitations. The primary challenges were clear: limited bandwidth, the inability to handle multiple data types simultaneously, and a general lack of efficiency in data transmission. ISDN emerged as the replacement for these older systems with a digital setup. It increased the capacity and speed of data transmission, providing flexibility of handling diverse communication channels - voice, video, and text, all through a single line.

What is ISDN (Integrated Services Digital Network)?

ISDN stands for Integrated Services Digital Network. It is a circuit-switched telephone network system that sends voice, data, and video over regular telephone lines but in a digital format. ISDN divides the telephone line into different digital channels so that it can perform various functionalities at the same time. For instance, it allows one channel to handle a phone call while another to send a fax or connect to the internet. It makes ISDN a much more versatile telecommunication solution than traditional phone lines.

History of ISDN

ISDN was developed during the 1970s to digitize telecommunications networks. The main idea was to create a unified system using digital transmission to carry both voice data and video.

The 1980s saw a rise in the popularity of ISDN after the International Telecommunication Union (ITU) standardized it, making it a globally recognized technology. ISDN struggled with high costs and complex installation processes during its initial phase. These factors limited its widespread adoption, leading to a decline in popularity by the late 1990s.

Interestingly, the early 2000s witnessed a boost in the use of ISDN networks. This was partly due to technological improvements and the need for reliable digital communication solutions. Despite this revival, ISDN couldn’t keep pace with the rapid advances in other areas of digital communication, particularly with the advent of high-speed internet services. However, the legacy of ISDN remains significant as it laid the groundwork for many of the digital communication technologies we use today.

How Does ISDN Work?

ISDN takes a regular telephone line and turns it into a channel for digital data. It means everything sent over the ISDN connection is in digital format instead of old analog signals, whether a phone call or a video.

The ISDN networking splits the telephone line into separate channels to function. The most common setup uses two types of channels: B (Bearer) channels and D (Delta) channels. The B channels carry the primary data, such as your voice in a phone call or the video in a conference call. Each B channel provides a certain amount of bandwidth for a separate phone call or data transmission.

Meanwhile, the D channel is used for setting up and managing the calls and data transfers on the B channels. It carries information about who’s calling who and sets up the connections.

Types of ISDN

The ISDN is primarily categorized into PRI (Primary Rate Interface) and BRI (Basic Rate Interface). Each type serves different user needs, from home users to large-scale businesses.

Primary Rate Interface

Primary Rate Interface is a high-capacity service that’s primarily used by larger organizations and businesses. A standard PRI line in the US provides 23B channels and one D channel for signaling. The setup allows for many simultaneous transmissions, making it a perfect setup for businesses with high communication demands. Moreover, each B channel in a PRI line operates at 64 kbps. It is the perfect ISDN speed for simultaneously handling multiple phone calls, internet sessions, and data transfers.

Basic Rate Interface

ISDN BRI suits smaller businesses and home users. It typically consists of two B channels and one D channel. Each B channel in a BRI setup also operates at 64 kbps. ISDN BRI provides enough bandwidth for basic communication like standard telephone calls, internet browsing, and fax transmissions. The D channel operates at 16 kbps and is used for control and signaling rather than for carrying data.

Advantages of ISDN

ISDN merges various services into a single line and transforms telecommunication for streamlined business and personal communications. Its digital structure paves the way for faster data transfer and clearer calls, optimizing efficiency and connectivity.

Higher Data Transfer Rate

By transmitting data digitally rather than through analog signals, ISDN aids faster and more reliable data exchange. This results in quicker internet browsing, faster file downloads, and more efficient online transactions. For businesses, it contributes to improving productivity and operational efficiency. For personal use, it means less waiting time and smoother communication.

Integration of Voice and Data Services

ISDN’s integration capability of voice and data services makes it popular for businesses requiring multiple communication services. The integration allows you to use the internet, make phone calls, and send faxes simultaneously without needing separate lines for each service. This simplifies the communication setup, reduces costs, and enhances efficiency.

Higher Call Quality

The digital nature of ISDN also ensures higher call quality. Unlike analog lines that are prone to noise and interference, ISDN's digital transmission provides clearer and more consistent voice quality. It provides crystal clear voice communication channels even for long-distance phone calls.

Multiple Functionality

ISDN connection typically operates by dividing the lines into B and D channels to offer multiple functionality. Each B channel can be independently used for different communication forms, while the D channel manages these connections. The flexibility allows ISDN to cater to various needs, such as conducting voice calls while transferring data.

Call Management Features

ISDN also offers advanced call management features. These include caller ID, call waiting, call forwarding, and conference calls. Such features aid in managing and organizing calls in business settings. They not only streamline the calling process but also add a layer of professionalism to telecommunication efforts.

Disadvantages of ISDN

 While ISDN offers robust digital communication, it has limitations like high costs and inflexibility. Its reliance on specific infrastructure also restricts its availability and adaptability.

Little Flexibility

ISDN phone systems typically require physical wires for connection. It has fixed configurations and capacities. When shifting your network, you must change the entire ISDN setup.


Setting up and maintaining an ISDN line is more expensive than other digital communication methods. The initial installation, equipment, and ongoing service fees can add up. It makes the phone system a less economical choice, especially for small businesses.

Limited Coverage

ISDN’s deployment requires specific infrastructure - ‘Telephone Exchange Equipment’ from the service provider, which is not universally available. This limitation means it is either hard to come by or entirely unavailable in many areas.

How to Set up ISDN?

Before deciding on ISDN implementation, you must check with your local telephone company to see if ISDN service is available in your area. The circuit-controlled telephony network requires a compatible system for telephone exchange equipment, so it’s not available everywhere. After confirming availability, follow these steps to set up an ISDN internet network in your business:


  1. 1. Choose a Service Plan: Telecommunication companies offer various networking plans where the numbers of B and D channels vary. It affects how much data you can send and receive. Choose a service plan that suits your needs.


  2. 2. Installation of ISDN Line: Your telephone company will install an ISDN line at your premises. It involves physical wiring changes to ensure your line can handle digital transmission.


  3. 3. Set Up ISDN Terminal Equipment: Set up an ISDN terminal adapter or an ISDN-compatible phone. It converts the digital signal into a form your devices can use.


  4. 4. Configure Network Settings: Configure your network settings once the hardware is set up. It includes setting up the phone numbers for each B channel and configuring your internet settings.


  5. 5. Test the Connection: Make calls and use the internet to test the ISDN connection. It ensures that everything works as expected and you get the speed and functionality you need.

Is ISDN Switching Off?

ISDN has significantly improved digital communication since its establishment. However, it has struggled to keep up with the dynamic and ever-expanding requirements of the modern digital world. New technologies like broadband internet and VoIP (Voice over Internet Protocol) have outpaced ISDN, offering faster speeds, lower costs, and a wider range of features. As a result, the once-revolutionary ISDN is gradually being phased out.

A notable example would be the announcement from BT (British Telecom) made to shut down the ISDN network by 2025. While BT’s decision primarily impacts the UK, it indicates that the top telecom service providers in the world do not align ISDN as the future of telecommunication.

ISDN Alternatives

As the telecommunications world shifts away from ISDN, the most prominent emerging alternatives are VoIP (Voice over Internet Protocol) and SIP (Session Initiation Protocol). Both offer more advanced, cost-effective, and versatile communication solutions.


VoIP technology allows for voice communication and other telephony services like fax, SMS, and voice messaging over the Internet. Unlike ISDN networking, which uses copper lines and analog signals, VoIP converts voice into digital packets and sends them over an IP network. It offers significant advantages in terms of cost, flexibility, and features.





Transmission Method

Digital network over telephone lines.

Digital packets over the Internet.


Higher due to line rentals and call charges.

Lower, often with free or low-cost internet calls.


Limited, requires physical lines for more channels.

Highly scalable, easy to add lines or features.


Fixed capacity, less adaptable.

Highly adaptable to various communication needs.

Setup and Maintenance

Requires physical infrastructure.

Easier setup with existing internet connection.


SIP (Session Initiation Protocol) is a signaling protocol for initiating, managing, and terminating real-time communication sessions. These sessions can include video, voice, messaging, and other communications. While ISDN primarily focuses on voice transmission over telephone lines, SIP offers more flexibility in managing different forms of communication.






Limited to voice and basic data.

Supports a wide range of communication types.

Device Compatibility

Requires ISDN-compatible devices.

Works with various internet-enabled devices.


Generally higher due to equipment and maintenance.

Lower, due to internet-based infrastructure.


Requires physical installation for expansion.

Easily scalable with minimal physical setup.


Specific ISDN network required.

Utilizes existing internet infrastructure.


ISDN has been a fundamental building block in the evolution of modern telecommunications. It pioneered the concept of multifunctionality using the same telephone line. However, as the digital landscape evolves, it is gradually phasing out. VoIP, in particular, emerges as the more advanced solution. The reasons for the rise of VoIP are clear: it is a comparatively more flexible, cost-effective, and scalable communication tool.

Calilio is a modern VoIP phone system that builds upon the legacy of ISDN. Our VoIP phone services extend the foundational principles of ISDN and offer comprehensive features, including advanced call management, video conferencing, and seamless integration with business applications.

Frequently Asked Questions

What is ISDN used for?

ISDN (Integrated Services Digital Network) is primarily used for transmitting voice, data, and video over traditional telephone lines in digital format. It enables simultaneous telephone calls, faxing, and internet browsing over a single line.

Are ISDN lines still used?

While ISDN lines are still in use, their popularity has significantly declined with the advent of more advanced technologies, such as VoIP (Voice over Internet Protocol) and broadband internet. Many telecommunications companies are gradually phasing out ISDN services.

What is the difference between DSL and ISDN?

DSL (Digital Subscriber Line) and ISDN both provide internet services over telephone lines, but DSL offers higher data transfer rates. ISDN transmits digital data over standard phone lines, whereas DSL uses the same lines to provide a faster, always-on internet connection.


ADSL (Asymmetric Digital Subscriber Line) is not ISDN. ADSL is a type of DSL technology that offers faster download speeds compared to upload speeds, whereas ISDN is a separate technology for the digital transmission of voice and data over telephone lines.

What is the difference between Internet and ISDN?

The Internet is a global network of interconnected computers, while ISDN is a communication standard for transmitting data, voice, and video over traditional phone lines. ISDN can be used for accessing the internet, but it's different from the broader concept of the internet.

Is ISDN the same as Ethernet?

ISDN and Ethernet are different technologies. ISDN is a set of communication standards for digital transmission over traditional telephone lines, whereas Ethernet is a technology for connecting devices in a local area network (LAN) to facilitate data exchange.

Is ISDN copper or Fiber?

ISDN typically uses copper telephone lines for data transmission. It was designed to run over the existing copper wire infrastructure of traditional telephone systems.

What is replacing ISDN?

ISDN is being replaced by newer technologies, such as VoIP and SIP trunking services, which offer more flexible, cost-effective, and scalable solutions for digital communication. These technologies use the internet for transmitting voice, data, and video, providing advanced features and higher efficiency.

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