High Quality Content by WIKIPEDIA articles! High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization. HDLC provides both connection-oriented and connectionless service. HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode. The original master-slave modes Normal Response Mode and Asynchronous Response Mode are rarely used.