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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 user rate.
The S interface is the connection between ISDN terminal equipment or terminal adapters and an NT1 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
Contrast to the T interface, which connects between an NT2 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., 192 kbit/s, with a user portion of 36 bits per frame; i.e., 144 kbit/s.