A high-level data link control (HDLC) is a protocol that is a bit-oriented synchronous data link layer. HDLC ensures the error-free transmission of data to the proper destinations and controls the data transmission speed.
HDLCs can provide both connection-oriented and connectionless services.
A high-level data link control defines rules for transmitting data between network points. Data in an HDLC is organized into units called frames and is sent across networks to specified destinations. HDLC also manages the pace at which data is transmitted. HDLC is commonly used in the open systems interconnection (OSI) model’s layer 2.
HDLC frames are transmitted over synchronous links or asynchronous links, which do not mark the start and end of frames. This is done using a frame delimiter or flag, which contains unique sequence of bits that are not visible inside a frame.
There are three types of HDLC frames:
- Information frames/User data (I-frames)
- Supervisory frames/Control data (S-frames)
- Unnumbered frames (U-frames)
The common fields within an HDLC frame are:
- Control information
- Frame check sequence
The HDLC protocol is used by a variety of standards implemented in the protocol stacks of X.25, V.42 and ISDN and many other protocol stacks.