Digital Network (ISDN) is a set of communication standards for
simultaneous digital transmission of voice, video, data, and
other network services over the digitalised circuits of the
public switched telephone network. ISDN has largely been
replaced with digital subscriber line (DSL) systems of much
Prior to ISDN, the telephone system consisted of digital links
like T1/E1 on the long-distance lines between telephone company
offices and analog signals on copper telephone wires to the
customers, the "last mile". At the time, the network was viewed
as a way to transport voice, with some special services
available for data using additional equipment like modems or by
providing a T1 on the customer's location.
With ISDN allowing
digital-quality voice, two separate lines and all-the-time data,
the telephony world was convinced there would be high customer
demand for such systems in both the home and office. This proved
not to be the case.
ISDN becomes ADSL
ADSL quickly replaced
ISDN as the customer-facing solution for last-mile connectivity.
ISDN has largely disappeared on the customer side, remaining in
use only in niche roles like dedicated teleconferencing systems
and similar legacy systems.