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 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.)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.)2
Contrast to the T interface, which connects between an NT2 (PBX or other local switching device) and NT1.1 However, the S interface is electrically equivalent to the T interface, and the two are jointly referred to as the S/T interface.3
The S interface operates at 4000 48-bit frames per second; i.e., 192 kbit/s, with a user portion of 36 bits per frame; i.e., 144 kbit/s.4
References
References
- Kessler, Gary C. (1997). ISDN : concepts, facilities, and services. Internet Archive. New York : McGraw-Hill. p. 46. ISBN 978-0-07-034249-1.
- Kessler, Gary C. (1997). ISDN : concepts, facilities, and services. Internet Archive. New York : McGraw-Hill. p. 312. ISBN 978-0-07-034249-1.
- Becker, Ralph (2006-12-05). "ISDN Tutorial: Interfaces". ralphb.net. Retrieved 2014-01-22.
- This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22., Entry "S interface"