The High-Level Data Link Control (HDLC) protocol is a link layer protocol that is used to provide reliable data transmission over a variety of physical layers.

HDLC is a bit-oriented protocol, which means that it operates at the level of individual bits rather than at the level of characters or bytes.

In HDLC, data is transmitted in the form of frames, which consist of a series of bits that contain the data being transmitted as well as control information.