In Integrated Services Digital Network (ISDN) service, an S interface is the electrical interface between a network terminating unit 1 (NT1) and up to eight addressable devices such as a computer or a telephone. Like the T interface (which usually connects the signal from the NT1 to a PBX, the S interface has four wires, allowing up to eight devices to be addressed. In Basic Rate Interface ISDN, the bits that flow from the central office through the NT1 are divided into two 64 Kbps channels (known as B, or bearer, channels) and one 16 Kbps channel (the D, for data or delta, channel) for control signals. The control signals allow a specific device to be addressed. The wires to the devices from the NT-1 are sometimes referred to as the S-bus.
The following table summarizes the various ISDN electrical interfaces at different demarcation points or places in the traffic flow:
|Electrical interface||Between what two points|
|U interface||Central office and NT1|
|T interface||NT1 and NT2 devices (such as a PBX)|
|S interface||NT1 or NT2 and ISDN devices (such as a telephone or terminal adapter)|
|R interface||Terminal adapter and non-ISDN devices (such as a computer)|
|V interface||Within the ISDN node at the central office; separates line termination equipment from exchange termination equipment|
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- Asuscom, which sells the NT1-600, provides an illustration showing both the U interface and the S/T (S-bus) interface .