Also known as the S interface.The ISDN point of reference for the interface between an ISDN terminal and the NT2, which is the user side of the local loop.The S interface serves to distinguish between terminal equipment and network-related functions. The S interface is defined as a passive bus on Cat 5 cable for up to 8 NT2 devices, which are intelligent and ISDN-compatible. See also Cat 5, ISDN, and NT2.