Integrated Services Digital Network (ISDN)

A legacy technology, Integrated Services Digital Network (ISDN), was a complete game-changer in digital communication when it first came onto the scene. It allowed voice and data to be transmitted together over traditional phone lines, laying the groundwork for modern telecoms. Although it’s now mostly being replaced, ISDN’s still play a role in global communication and are worth looking into.

What is ISDN?

ISDN is a digital communication standard that replaced analog systems for the transmission of voice, video, and data when the conversion was first taking place. A big benefit of ISDN is the variety of styles of data that could travel simultaneously. This allowed for more streamlined communication and improved efficiency, creating significant advantages over analog systems of the era.

A Brief History of ISDN

The International Telecommunication Union (ITU) first developed ISDN in the 1980s to standardize digital communication globally. This helped to enable digital communication to travel much more effectively across regions. Although there were competing standards, ISDN won out as it surged in popularity in the 1990s, especially for its benefits in providing internet access and video conferencing. However, the rise of broadband technologies like DSL and fibre in the last decade gradually overshadowed ISDN, and today it’s being phased out.

Key Features of ISDN

ISDN introduced several key features that set it apart from both analog and its competitors:

Digital Transmission: A big leap from analog, offering better reliability and speed.
Circuit-Switching: Guaranteed a stable connection for voice and data.
Multiple Channels: Supported voice and data simultaneously using B (bearer) and D (data) channels.
Faster Speeds: Each B channel provided 64 kbps, which was fast for its time.
Global Standardization: Once it started getting used, it became a global standard that helped ease transmissions across the globe.

Types of ISDN

Basic Rate Interface (BRI): BRI was designed specifically to work well enough for small businesses and residential users. It was limited to just two B channels of 64 kbps each for data and then one D channel of 16 kbps for signalling. It was practical for most everyday needs like video conferencing and internet access.
Primary Rate Interface (PRI): Designed for larger organizations, PRI’s main difference from BRI is its larger capacity. In North America and Japan, it provided 23 B channels and one D channel for a total of 1.5 Mbps. Europe had a different standard for PRI, featuring 30 B channels and one D channel, delivering 2.0 Mbps. The main uses of PRI were for high-traffic operations like call centers and corporate networks.

Components of ISDN

Terminal Equipment (TE): This includes devices like ISDN phones, routers, and fax machines. These are the pieces of hardware that are serviced by the ISDN network.
Network Termination (NT): NT devices acted as the bridge between the user’s equipment and the carrier’s network. For modern day internet, this is equivalent to Wifi routers and similar network hubs. Common examples of these for ISDN are NT1 and NT2 devices, which handle connections and data flow.
Reference Points and Channels: Reference points are the “connection points” in ISDN. The U interface connects the service provider’s network to your ISDN equipment, while the T/S interface connects your ISDN devices (like phones or routers) to the network. The R interface bridges non-ISDN devices (like analog phones) to the ISDN system. ISDN uses two types of channels: B channels carry the actual data (like voice or internet traffic) at 64 kbps each, while the D channel handles control signals (like call setup) at 16 kbps or 64 kbps.

How ISDN Works

ISDN operations can be broken into a few steps:

Call Setup: First, the D channel uses signals to establish the right connection.
Data and Voice Transmission: Then, the B channels handle the actual communication or data transmission for the internet.
Multiplexing: Channels can be combined for higher speeds, like achieving 128 kbps by using two B channels together. This was common in industry uses.

Applications of ISDN

During its peak, ISDN was used for:

Video Conferencing: Providing reliable, high-quality video calls.
LAN Interconnectivity: Connecting local area networks.
Internet Access: Offering faster speeds than dial-up connections.
Backup Lines: Serving as reliable failover solutions in critical systems.

Even today, some industries still use ISDN for niche applications, like broadcasting or connecting to remote locations.

Advantages of ISDN

Speed and Reliability: A significant upgrade from analog.
Clear Voice Quality: Gave near crystal-clear communication.
Simultaneous Services: Allowed voice and data to be transmitted simultaneously.
Global Reach: Standardized use across different countries.

Limitations of ISDN

Despite this, ISDN had enough downsides that it is now out of date:

High Costs: Installation and equipment were expensive.
Limited Bandwidth: The biggest problem is bandwidth, which is very limited with ISDN and broader with DSL and fibre.
Complex Setup: It required specialized knowledge to install and configure.
Phased-Out Support: As others have taken hold, support for ISDN has fallen, making it harder to use.

ISDN vs. Modern Technologies

There are a few main reasons ISDNs died out. But mainly, it came down to the speed of transmission.

Comparison

Speed: ISDN maxed out at 128 kbps, while DSL and fibre deliver speeds in the Mbps or Gbps range.
Cost: ISDN’s hardware was pricey compared to cost-effective VoIP systems.
Flexibility: Circuit-switched ISDN wasn’t as flexible nor robust as packet-switched IP networks.

Transition

For these reasons, VoIP, SIP trunking, and broadband have largely replaced ISDN, as they offer faster, cheaper, and more adaptable solutions.

The Future of ISDN

Phase-Out: ISDN is being retired globally, with countries like the UK aiming to complete the transition by the end of 2025 and others within a similar timeframe. Its functions are currently handled by IP-based systems.
Niche Roles: ISDN still holds value in legacy systems and remote areas where modern infrastructure isn’t available, so there are times when knowing how it works is valuable.

Conclusion

ISDN was a crucial step in the evolution of communications technologies, but now its acclaim has concluded. With the advent of DSL, Broadband, and fibre, there are just too many great alternatives. Still, its influence on modern telecom systems remains a significant chapter in the history of communication that’s worth knowing.