Integrated Services Digital Network (ISDN) is a digital communications system using a set of communication standards for digital telephone connections and the transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. ISDN can combine both analog/voice data and digital data on the same network link.
There are two common types of ISDN networks:
Basic Rate Interface (BRI) which is an entry level interface capable of generating transmission speeds up to 128Kbps over standard twisted pair lines and is intended for home and small business use.
Primary Rate Interface (PRI) which is intended for larger enterprise users, and is carried over an E1 (E-carrier system) at 2048 kbit/s.
The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system.