Integrated services digital network (ISDN) is a WAN technology that offers increments of 64-Kbps connections most often used by SOHO (small office/home office) users.
Definition 2.
Integrated Services Digital Network (ISDN) is a digital network which uses the commercial telephone system to provide users with a wide array of telecommunication services. The ISDN relies on a set of international-standard interfaces that allow users to transmit data and voice, in digital form, over telephone lines.
ISDN revolves around five key principles: openness, modularity, communications-based intelligence, network management and control, and integrated products and services.
Definiton 3.
ISDN: Integrated Services Digital Network– a network designed to replace the standard telephone system in which optical fibres are used in place of coaxial cables. This type of network is capable of carrying digital information in the form of voice, text, and voice simultaneously. This makes the following services available to users: interactive information services, fax, electronic mail, and wideband digital video. An ISDN therefore constitutes a faster alternative to existing analogue services.