Integrated Services Digital Network (ISDN) is
|“||[a] public-switched network providing digital connections for the concurrent transmission of voice, video, data, and images.||”|
|“||[a] unified end-to-end digital network, in which data originating from all types of communication (e.g., voice, text, data, still and moving pictures) are transmitted from one port (terminal) in the exchange (switch) over one access line to and from the subscriber.||”|
|“||[a] hierarchy of digital switching and transmission systems that provides voice, data, and image in a unified manner. ISDN is synchronized so that all digital elements communicate in the same protocol at the same speed.||”|
|“||a set of international standards that specify the manner in which different types of information (e.g., voice, data, video) can be transmitted in the same communications system.||”|
A basic ISDN line provides a 16 kbps signaling channel and two 64 kbps digital transmission channels used to provide switched circuit connections. It allows digitized voice and data to be sent over existing telephone lines at high speed. ISDN is considered to be the basis for a "universal network" that can support almost any type of communications device or service.