ISDN (Integrated Series Digital Network) is a multifunctional concept, conventionally dividing the system of international standards into digital signals format and digital devices that meet these standards, designed to connect end users to existing analog phone lines.
Digital ISDN adapters allow to connect up to 8 terminal devices (phones, fax machines, modems, PCs, videophones, etc.) to a single telephone line in any combinations. Each terminal device connected to a line through such an adapter, can have its own number, which allows direct calls to a particular subscriber unit.
While ordinary analog phone line is focused on audio information exchange between subscribers with data rates up to 56 kbit/sec, the expansion based on digital ISDN network devices allows multiple subscribers to use this line for simultaneous speech, text, data, and motion pictures exchangeat higher transmission rates. The rate of 144 kbit/s isprovided by BRI (Basic Rate Interface) devices while PRI (Primary Rate Interface) provides speeds up to 1984 kbit/sec. Synchronization of communication for different types ofsubscribers' terminal devices that are physically connected one phone line, is reached by passing the converted digital signals at different frequencies through this line.
Thus, the ISDN network provides a wide range of service functions, including the confidentiality of communications, rapid connection of subscribers, the possibility of combining telephone conversations with data transmission through a single leased telephone line and allows subscribers to save their resources by significantly increasing the speed of information transmission.
See http://en.wikipedia.org/wiki/Integrated_Services_Digital_Network for more information on ISDN.