HDLC

High-Level Datalink Control

High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The original ISO standards for HDLC are as follows:

ISO 3309-1979 -- Frame Structure\ ISO 4335-1979 -- Elements of Procedure\ ISO 6159-1980 -- Unbalanced Classes of Procedure\ ISO 6256-1981 -- Balanced Classes of Procedure\ The current standard for HDLC is ISO/IEC 13239:2002, which replaces all of those standards.

HDLC provides both connection-oriented and connectionless service.

HDLC can be used for point-to-multipoint connections via the original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM), but they are now rarely used; is now used almost exclusively to connect one device to another, using Asynchronous Balanced Mode (ABM).