Editing ISDN (section)

=== Telephone industry ===

ISDN is a core technology in the telephone industry. A [[telephone network]] can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires is [[Digital Signal 1|T1]] or [[E-carrier|E1]]. Between telephone company switches, the signaling is performed via [[Signaling System No 7|SS7]]. Normally, a PBX is connected via a T1 with [[robbed bit signaling]] to indicate on-hook or off-hook conditions and [[Multi-frequency|MF]] and [[Dual-tone multi-frequency|DTMF]] tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode numbers as long (100 [[Millisecond|ms]] per digit) tone sequences. This results in faster call setup times. Also, a greater number of features are available and fraud is reduced.

In common use, ISDN is often limited to usage to [[Q.931]] and related protocols, which are a set of [[Signaling (telecommunications)|signaling protocol]]s establishing and breaking circuit-switched connections, and for advanced [[calling feature]]s for the user.<ref>{{cite journal |last1 =Aaron |first1 =R | last2 = Wyndrum | first2 = R |title= Future trends |journal= IEEE Communications Magazine |date=March 1986 | volume = 24 | issue = 3 | pages = 38–43 | publisher = [[AT&T Bell Laboratories]] |doi= 10.1109/MCOM.1986.1093028}}</ref> Another usage was the deployment of [[videoconference]] systems, where a direct end-to-end connection is desirable. ISDN uses the [[H.320]] standard for [[audio coding]] and [[video coding]].

ISDN is also used as a smart-network technology intended to add new services to the [[public switched telephone network]] (PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or failsafe circuit solution for critical use data circuits.