An umbrella term to describe the use of Internet Protocol (IP) and packet-switching to provide voice, fax, and other services that once were solely accessible through circuit-switched networks such as a PBX or the phone company. ISDN (Integrated Services Digital Network) - An international standard for digital transmissions over standard copper wiring. ISDN is delivered in 2 forms, Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI provides 2 digital lines (called B-channels) of 64 Kbps each, or 128 Kbps total, for a home or small business. PRI consists of 23 B-channel lines. Both BRI and PRI include a smaller D-channel (12 Kbps for BRI, 64 Kbps for PRI) that carries control and signaling information, but does not increase download speeds. ISDN is growing less common and has generally been supplanted by Digital Subscriber Line (DSL) service.