A set of communication standards developed to transmit voice, video, data, and other network services over traditional circuit-switched telephone lines. ISDN was introduced by the International Telecommunication Union (ITU) in the 1980s as an upgrade from analog telephone systems, enabling simultaneous digital transmission of multiple types of information over a single line.
Unlike the older analog system, which carried only voice signals, ISDN digitizes the communication path, enabling faster data transfer, improved call quality, and support for additional services such as video conferencing and data sharing.
How ISDN Works
ISDN divides its digital transmission into separate channels, typically known as the B (Bearer) and D (Delta) channels. The B channels carry voice or user data, while the D channel carries signaling and control information. This channelized approach allows for simultaneous use, such as a voice call on one B channel while sending a fax or data on another.
ISDN comes in two main configurations:
- Basic Rate Interface (BRI): Designed for home or small office use, BRI consists of two 64 kbps B channels and one 16 kbps D channel, providing a total bandwidth of 144 kbps.
- Primary Rate Interface (PRI): Used by larger organizations and carriers, PRI varies by region. In North America, it includes 23 B channels and one 64 Kbps D channel (for a total of 1.544 Mbps, matching a T1 line). In Europe and elsewhere, it comprises 30 B channels and one D channel (2.048 Mbps, equivalent to an E1 line).
ISDN Applications and Advantages
Before the rise of broadband technologies, ISDN was widely used for video conferencing, early internet access, corporate WAN connectivity, and high-quality voice services. Its ability to establish connections quickly and carry digital signals with lower noise made it superior to analog modems.
Key advantages included:
- Faster call setup and tear-down times compared to analog systems.
- Clearer, higher-quality voice transmission.
- The ability to combine multiple channels for higher bandwidth (bonding).
- Support for both voice and data on the same line.
ISDN Decline and Legacy
By the early 2000s, ISDN had begun to decline as DSL, cable, and fiber-optic broadband connections became increasingly widespread. However, some telecommunication systems and broadcast networks still rely on ISDN for secure, reliable voice or audio links, especially where IP infrastructure is limited.
ISDN represented an important milestone in the transition from analog to digital telecommunications. It laid the groundwork for modern broadband and digital networking technologies by demonstrating the efficiency and flexibility of digital signal transmission. Though largely obsolete today, ISDN’s concepts continue to influence digital communication standards and the evolution of integrated network services.