S interface

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title: "S interface" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["itu-t-recommendations", "integrated-services-digital-network"] description: "Interface reference point" topic_path: "general/itu-t-recommendations" source: "https://en.wikipedia.org/wiki/S_interface" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary 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 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.

References

References

1. Kessler, Gary C.. (1997). "ISDN : concepts, facilities, and services". New York : McGraw-Hill.
2. Kessler, Gary C.. (1997). "ISDN : concepts, facilities, and services". New York : McGraw-Hill.
3. Becker, Ralph. (2006-12-05). "ISDN Tutorial: Interfaces". ralphb.net.
4. {{FS1037C, [http://www.its.bldrdoc.gov/fs-1037/dir-032/_4668.htm Entry "S interface"]

::callout[type=info title="Wikipedia Source"] This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page. ::