``Integrated Services Digital Networks'' (ISDN) is a term coined by the International Telecommunication Union (ITU, formerly CCITT) to describe a collection of telecommunications protocols that specify how digital information can be passed over telephone lines in high-speed, end-to-end channels. ISDN transmission and switching services are provided by local telephone companies (``telcos'').
UnixWare 7 supports Basic Rate Interface (BRI) ISDN service. It consists of two 64Kbps B (``bearer'') channels, used to convey user information (voice or data) and one 16Kbps D (``delta'') channel, used for signalling between the user and the ISDN network. (The D-channel can also be used as a bearer channel for X.25 packet service.) Future releases will support Primary Rate Interface (PRI) service, consisting of 23 64Kbps B-channels (30 B-channels in Europe) and one 64Kbps D-channel.
Before BRI or PRI data can be sent to an ISDN network, the multiple channels must be multiplexed or de-multiplexed with a Network Terminating device (NT-1). NTs also provide B-channel access to multiple ISDN terminal devices (for example, computers, telephones, fax machines), and serve to isolate the user from the transmission technology of the ISDN network. If a Private Branch Exchange (PBX) is implemented, an additional Network Terminating device (NT-2) is required on the customer side of the (NT-1). NT-2s and PBXs are usually necessary only for PRI service. In North America, it is the customer's responsibility to provide network terminating equipment; elsewhere, they are supplied by the ISDN provider and included with the tariff.
ISDN service is implemented in three ``layers'' that correspond to the first three layers of the OSI (Open Systems Interconnection) model:
In an effort to coordinate the diverse international ISDN protocol specifications, ISDN equipment vendors have promoted standards to improve interoperability. Because of the variety of telephone service providers, these standards are often specific to a geographic region. UnixWare 7 supports terminal adapters that conform to these ISDN standards:
For more information, see the ISDN FAQ: