ISDN is a set of technologies developed to carry voice, video and data across Public Switched Telephone Network (PSTN) by existing telephone lines. It utilises #Circuit Switching and primarily operates at Physical Layer#, Data Link Layer# and Network Layer#. ISDN offers bandwidth from 128kbps up to 384kbps. It was the first #broadband service deployed in the home but nowadays, it has been outshone by Digital Subscriber Line (DSL) and Cable Modem.
ISDN has 2 types of interfaces: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). Each have an arbitrary number of bearer channels (B) and exactly one delta channel (D). The former are capable of carrying data whereas the latter is responsible for carrying signal (usually for call setup) and call control information, and sometimes low-rate packet data such as alarms. Only after the establishing of a path between switches, passing source and call numbers information by D channel and signalled by destination that it is available, only can the B channels start the data exchange.
BRI offers 2 64kbps B channels (thus its alternative name 2B+D). This means that it has a bandwidth of 128kbps excluding D channel, which is 16kpbs wide. The PRI’s structure in the other hand depends on the region. In Europe, it has 30 B channels (64kpbs each), but in North America and Japan it has only 23 B channels.