ISDN (Integrated Services Digital Network) is a digital network with integrated services designed to transmit voice, data, and signaling over a single telecommunications line. ISDN technology was developed as an evolution of analog telephony and made it possible to achieve higher transmission quality and stable digital communication channels.
ISDN standardizes methods for establishing connections, encoding information, and controlling calls. Unlike analog lines, ISDN uses fully digital transmission from the subscriber to the switching equipment, which reduces interference and improves communication reliability. The technology was widely used before the mass adoption of broadband access and IP telephony.
ISDN architecture and operating principles
ISDN is based on the use of digital channels with fixed bandwidth. The core elements are the B channel, which is used to transmit user data, and the D channel, which is used for signaling and connection control. This approach made it possible to transmit voice and data simultaneously over a single line without mutual interference.
Connections in ISDN are established using circuit switching. This means that a dedicated channel with guaranteed bandwidth is allocated for the duration of the communication session. This principle ensured stable transmission quality but reduced resource efficiency compared to packet-switched networks.
Types of ISDN
There are two main variants of ISDN, differing in the number and type of channels.
- Basic Rate Interface (BRI) was intended for small businesses and residential users and included two B channels and one D channel.
- Primary Rate Interface (PRI) was used in corporate and carrier networks and provided a larger number of channels for simultaneous connections.
The choice of ISDN type depended on use cases, traffic load, and requirements for the number of concurrent calls.
ISDN use cases
ISDN was widely used for digital telephony, videoconferencing, data transmission, and connecting corporate PBX systems. In the telecom environment, the technology was applied to deliver stable voice channels and services with guaranteed quality of service.
ISDN was also used in early internet connections, where it provided higher speed and reliability compared to analog modems. Over time, ISDN was replaced by DSL, fiber-optic technologies, and IP networks.
The role of ISDN in the development of telecommunications
ISDN played an important role in the transition from analog communications to digital networks. It became a foundation for the development of digital telephony and influenced subsequent data transmission standards. Many signaling and connection control principles implemented in ISDN were later used in more modern telecommunications technologies.
Despite its declining popularity, ISDN may still be found in certain legacy systems and specialized scenarios where compatibility with existing infrastructure is required.