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Integrated Services Digital Network (ISDN)

ISDN provides an all-digital replacement for the PSTN. ISDN is widely available in Australia from Telstra Corporation (although not from other telcos), and most Telstra customers now have the option of choosing it instead of PSTN.
 
The basic-rate ISDN service provides, conceptually:
ISDN conceptual diagram
An ISDN service is integrated in the sense that each B channel can be used to carry either a digitised voice (or video, FAX, etc) call, or a 64 kbps data call. The D channel is normally used for signalling (call setup, etc), but can be used as a 16 kbps data link under some circumstances. Although....
 
Note that in Australia, the ISDN interface is owned by the telecommunications provider (or Telco), eg Telstra. This is not the case in some other countries.
Lecture 14: Point-to-Point Data Links Copyright © 2002 P.Scott, La Trobe University Bendigo.

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