S interface

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 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 The S interface is electrically equivalent to the T interface, and the two are jointly referred to as the S/T interface.