Integrated Services Digital Network is a circuit to send voice, video, and data over digital or normal telephone lines at a rate of 64 Kbps.

There are two types of ISDN:

Primary Rate Interface (PRI) -  23 B-channels that carries the voice or data and one D-channel that carries control information

Basic Rate Interface (BRI) - 2 B-channels that carries voice or data and one D-channel that carries control information.