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Integrated Services Digital Network (ISDN) is a set of communication standards for digital telephone connection and the transmission of voice and data over a digital line. These digital lines are commonly telephone lines and exchanges established by the government. Before ISDN, it was not possible for ordinary telephone lines to provide fast transportation over a single line.
ISDN was designed to run on digital telephone systems that were already in place. As such, it meets telecom's digital voice network specifications. However, it took so long for ISDN to be standardized that it was never fully deployed in the telecommunications networks it was intended for.
ISDN takes all kinds of data over a single telephone line at the same time. As such, voice and data are no longer separated as they were in earlier technologies, which used separate lines for different services. ISDN is a circuit-switched telephone network system, but it also allows access to packet-switched networks.
ISDN is also used with specific protocols, such as Q.931, where it acts as the network, data link and physical layers in the OSI model. Therefore, in broad terms, ISDN is actually a suite of transmission services on the first, second and third layers of the OSI model.