S interface

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]

See also

• R interface
• T interface
• U interface