Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalized circuits of the PSTN. There are several types of ISDN including:
- Basic Rate Interface (BRI) which consists of two 64 Kbps B channels (bearer channels carrying digital voice ) and one 16Kbps D (data) channel for signaling.
- Primary Rate Interface (PRI) (North America) which consists of 23 64Kbps B channels and one 64Kbps D channel. This PRI circuit corresponds in capacity with the typical T1 digital link available in the USA and Canada.
- Primary Rate Interface (PRI) (Europe and the rest of the world) which consists of 30 B channels and two D channels, corresponding with the typical E1 digital link available in Europe and the rest of the world.
ISDN was typically used in the 1990s and 2000s to connect digital Private Branch Exchanges (PBXs) to connect internal telephony network with the PSTN. ISDN has been largely superseded by VoIP technologies typically using SIP trunks.