HDLC
High-level Data Link Control: A synchronous data-link protocol. A bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO).
Each piece of data is encapsulated in an HDLC frame by adding a trailer and a header. The header contains an HDLC address and an HDLC control field. The trailer is found at the end of the frame, and contains a Cyclic Redundancy Check (CRC) which detects any errors which may occur during transmission.
The frames are separated by HDLC flag sequences which are transmitted between each frame and whenever there is no data to be transmitted.