S interface
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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 kbit/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 kbit/s, with a user portion of 36 bits per frame, i.e., 144 kbit/s.
Source: from Federal Standard 1037C