S interface
{{Short description|Interface reference point}}
The S interface or S reference point, also known as S0, is a user–network interface reference point in an ISDN BRI environment, characterized by a four-wire circuit using 144{{nbsp}}kbit/s (2 bearer and 1 signaling channel; 2B+D) user rate.
The S interface is the connection between ISDN terminal equipment (TE) or terminal adapters (TAs) and an NT1 (network terminator, type 1.){{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}} 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.){{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}}
Contrast to the T interface, which connects between an NT2 (PBX or other local switching device) and NT1. However, the S interface is electrically equivalent to the T interface, and the two are jointly referred to as the S/T interface.{{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}}
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; i.e., 144{{nbsp}}kbit/s.{{FS1037C}}, [http://www.its.bldrdoc.gov/fs-1037/dir-032/_4668.htm Entry "S interface"]