See Reference Point S.
"S interface." YourDictionary, n.d. Web. 17 August 2018. <http://www.yourdictionary.com/s-interface>.
S interface. (n.d.). Retrieved August 17th, 2018, from http://www.yourdictionary.com/s-interface
The interface used between network terminator 2 (NT2) and an ISDN terminal adapter. This is an internal interface as NT2 is typically built into the terminal adapter. See ISDN.