S interface

S interface: For basic rate access in an Integrated Services Digital Network (ISDN) environment, a user-to-network interface reference point[?] that (a) is characterized by a 4-wire, 144-kb/s (2B+D) user rate, (b) serves as a universal interface between ISDN terminals or terminal adapters and the network channel termination, (c) 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 (d) operates at 4000 48-bit frames per second, i.e., 192 kb/s, with a user portion of 36 bits per frame, i.e., 144 kb/s.

Source: from Federal Standard 1037C