The S interface or S reference point, also known as S0, is a User–network interface reference point for basic rate access in an Integrated Services Digital Network (ISDN) environment, that
is characterized by a Four-wire circuit, 144 kbit/s (2B+D) user rate,
serves as a universal interface between ISDN terminals or terminal adapters and the network channel termination,
allows a variety of terminal types and subscriber networks, such as PBXs, local area networks (LANs), and controllers, to be connected to the network, and
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.
The S interface is electrically equivalent to the T interface, and the two are jointly referred to as the S/T interface.
