HDLC (High Level Data Link Control)

HDLC (High Level Data Link Control) is a group of protocols or rules for transmitting data between network points (sometimes called nodes). In HDLC, data is organized into a into (called a frame) and send across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent.

A bit-oriented, synchronous, link layer, data-framing, flow control and error detection and correction protocol. Uses a header with control information and a trailing cyclic redundancy check character (which is usually 16 or 32 bits in length). Implementations are both standard subsets or vendor-specific (such as that used for the 56,000-bits/s interfaces on a vendor's remote bridge or router). IBM calls HDLC as SDLC.

HDLC uses zero insertion/deletion process (commonly known as bit stuffing) to ensure that the bit pattern of the delimiter flag does not occur in the fields between flags. The HDLC frame is synchronous and therefore relies on the physical layer to provide method of clocking and synchronizing the transmission and reception of frames.

The HDLC protocols is defined by ISO for use on both point-to-point and multipoint (multidrop) data links. It supports full duplex transparent-mode operation and is now extensively used in both multipoint and computer networks.