ISDN is an international digital telephony standard defined by the International Telecommunications Union, formerly known as the Consultative Committee for International Telephony and Telegraphy (CCITT). ISDN protocols are mapped to Open Systems Interconnectivity (OSI) standards: to OSI layer 2 for call setup, maintenance, and disconnect and to OSI layer 3 for basic call control.
The ISDN standards provide an open architecture for digital telephony, enabling manufacturers to design and produce networking devices that can operate over any ISDN telephone line, regardless of the local telephone network.
ISDN provides end-to-end digital connectivity between a remote computer and another computer by using a local telephone line for network connection. Prior to ISDN, digital data communications were available only on proprietary networks, and Internet connections over telephone networks had to convert from digital to analog transmission. This conversion reduces quality of service, speed of transmission, and available bandwidth.
ISDN is typically supplied by the same company that supplies analog service, also referred to as plain old telephone (POTS) service. However, ISDN differs from analog telephone service in several ways, including:
Data transfer rate
Available channels per call
Availability of service
Cost of service
Quality of connection