The S interface or S reference point, also known as S
0, is a
user–network interface reference point in an
ISDN BRI environment, characterized by a
four-wire circuit In telecommunications, a four-wire circuit is a two-way circuit using two paths so arranged that the respective signals are transmitted in one direction only by one path and in the other direction by the other path. The four-wire circuit gets its ...
using 144kbit/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.)
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.)
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.
The S interface operates at 4000 48-
bit frames per second; i.e., 192kbit/s, with a user portion of 36bits per
frame; i.e., 144kbit/s.
Entry "S interface"
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See also
* R interface
* T interface
* U interface
References
{{Reflist
ITU-T recommendations
Integrated Services Digital Network