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{{Short description|Interface reference point}}
The '''S interface''' or '''S reference point''', also known as '''S<sub>0</sub>''', is a [[user–network interface]] reference point in an [[Integrated Services Digital Network|ISDN]] [[Basic Rate Interface|BRI]] environment, characterized by a [[four-wire circuit]] using 144{{nbsp}}kbit/s (2 [[Integrated Services Digital Network#Bearer channel|bearer]] and 1 [[Integrated Services Digital Network#Signaling channel|signaling channel]]; 2B+D) user rate.

The S interface is the connection between ISDN terminal equipment (TE) or [[terminal (telecommunication)|terminal]] adapters (TAs) and an NT1 (network terminator, type 1.)<ref name=":0">{{Cite book|last=Kessler|first=Gary C.|url=https://archive.org/details/isdnconceptsfaci0003kess|title=ISDN : concepts, facilities, and services|date=1997|publisher=New York : McGraw-Hill|others=Internet Archive|isbn=978-0-07-034249-1|pages=46}}</ref> Not all TE or TAs connect externally to an S interface, but instead integrate an NT1 so they can connect directly to a U interface (local loop from central office.)<ref>{{Cite book|last=Kessler|first=Gary C.|url=https://archive.org/details/isdnconceptsfaci0003kess|title=ISDN : concepts, facilities, and services|date=1997|publisher=New York : McGraw-Hill|others=Internet Archive|isbn=978-0-07-034249-1|pages=312}}</ref>

Contrast to the [[T interface]], which connects between an NT2 (PBX or other local switching device) and NT1.<ref name=":0" /> However, the S interface is electrically equivalent to the [[T interface]], and the two are jointly referred to as the S/T interface.<ref name="ralphb">{{cite web|last=Becker|first=Ralph|date=2006-12-05|title=ISDN Tutorial: Interfaces|url=http://www.ralphb.net/ISDN/ifaces.html|access-date=2014-01-22|work=ralphb.net}}</ref>

The S interface operates at 4000 48-[[bit]] frames per second; i.e., 192{{nbsp}}kbit/s, with a user portion of 36{{nbsp}}bits per [[frame (telecommunications)|frame]]; i.e., 144{{nbsp}}kbit/s.<ref name="FS1037C">{{FS1037C}}, [http://www.its.bldrdoc.gov/fs-1037/dir-032/_4668.htm Entry "S interface"]</ref>

==See also==
* [[R interface]]
* [[T interface]]
* [[U interface]]

==References==
{{Reflist}}

[[Category:ITU-T recommendations]]
[[Category:Integrated Services Digital Network]]

S interface - Revision history

imported>Adam MLIS: #suggestededit-add 1.0