# S interface

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Interface reference point

The **S interface** or **S reference point**, also known as **S0**, is a [user–network interface](/source/User%E2%80%93network_interface) reference point in an [ISDN](/source/Integrated_Services_Digital_Network) [BRI](/source/Basic_Rate_Interface) environment, characterized by a [four-wire circuit](/source/Four-wire_circuit) using 144 kbit/s (2 [bearer](/source/Integrated_Services_Digital_Network#Bearer_channel) and 1 [signaling channel](/source/Integrated_Services_Digital_Network#Signaling_channel); 2B+D) user rate.

The S interface is the connection between ISDN terminal equipment (TE) or [terminal](/source/Terminal_(telecommunication)) 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](/source/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](/source/T_interface), and the two are jointly referred to as the S/T interface.[3]

The S interface operates at 4000 48-[bit](/source/Bit) frames per second; i.e., 192 kbit/s, with a user portion of 36 bits per [frame](/source/Frame_(telecommunications)); i.e., 144 kbit/s.[4]

## See also

- [R interface](/source/R_interface)

- [T interface](/source/T_interface)

- [U interface](/source/U_interface)

## References

1. ^ [***a***](#cite_ref-:0_1-0) [***b***](#cite_ref-:0_1-1) Kessler, Gary C. (1997). [*ISDN : concepts, facilities, and services*](https://archive.org/details/isdnconceptsfaci0003kess). Internet Archive. New York : McGraw-Hill. p. 46. [ISBN](/source/ISBN_(identifier)) [978-0-07-034249-1](https://en.wikipedia.org/wiki/Special:BookSources/978-0-07-034249-1).

1. **[^](#cite_ref-2)** Kessler, Gary C. (1997). [*ISDN : concepts, facilities, and services*](https://archive.org/details/isdnconceptsfaci0003kess). Internet Archive. New York : McGraw-Hill. p. 312. [ISBN](/source/ISBN_(identifier)) [978-0-07-034249-1](https://en.wikipedia.org/wiki/Special:BookSources/978-0-07-034249-1).

1. **[^](#cite_ref-ralphb_3-0)** Becker, Ralph (2006-12-05). ["ISDN Tutorial: Interfaces"](http://www.ralphb.net/ISDN/ifaces.html). *ralphb.net*. Retrieved 2014-01-22.

1. **[^](#cite_ref-FS1037C_4-0)** This article incorporates [public domain material](/source/Copyright_status_of_works_by_the_federal_government_of_the_United_States) from [*Federal Standard 1037C*](https://web.archive.org/web/20220122224547/https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm). [General Services Administration](/source/General_Services_Administration). Archived from [the original](https://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm) on 2022-01-22., [Entry "S interface"](http://www.its.bldrdoc.gov/fs-1037/dir-032/_4668.htm)

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