What is ISDN?
ISDN technology is standardized according to recommendations of the Commité Consultatif International Telegraphique et Telephonique (CCITT), now the International Telecommunications Union (ITU), which describe the protocols and architecture to implement a worldwide digital communications network. These recommendations are observed to varying degrees by all ISDN service providers and equipment vendors.ISDN stands for Integrated Services Digital Network: "Integrated Services" referring to its ability to sustain numerous applications, and "Digital Network" relating to its end-to-end digital connections. Generally, ISDN networks extend from the local telephone exchange to the remote user and include all of the telecommunications and switching equipment in between. ISDN is based on technology developed during the 1970s, designed to address the problem of how to transport digital services across a telephony infrastructure based on copper wiring originally intended to carry analog signals only.As shown in Figure 1, an analog phone line is capable of supporting only one data communications device at a time, in this case, a PC which must use a modem to change its digital signals into analog ones suitable for voice lines. The highest transmission rate reliably obtained through this method is about 28.8 Kbps. In contrast, a PC and other equipment connecting through a basic ISDN link are able to send and receive data in its native digital form, and to enjoy vastly higher transmission rates of up to 128 Kbps.
The high throughput offered by ISDN, rapid call setup, and the high level of accuracy inherent to digital transmission, are the main attractions to ISDN technology. With these attributes, ISDN is able to provide more efficient access to information, enabling many different and novel applications, while keeping costs in the range of regular telephone lines.ISDN provides these advantages without requiring huge new investments by telecommunications carriers, as it is a technology that leverages their existing switching and wiring investments. In fact, it makes better use of carrier resources by doubling the bandwidth potential of existing wiring to subscribers' homes and businesses (the local loop), and reducing the maintenance costs that are associated with analog carrier equipment.For all of its novelty, ISDN is a technology whose viability has already been proven in corporate and academic environments in Europe, Japan, Australia, and North America. With both customers and carriers motivated to see ISDN succeed, one can expect that it will continue to sustain the 40% growth rates it is presently experiencing in many regions. In some parts of the world, the future has already begun, as new homes are typically being installed with ISDN connections rather than standard analog telephone links.Before getting into the details of how ISDN is implemented, the next section provides a brief explanation of some of the concepts related to ISDN technology. These will prove useful when it comes to understanding the essential features of ISDN.