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:
- ISO 3309 – Frame Structure
- ISO 4335 – Elements of Procedure
- ISO 6159 – Unbalanced Classes of Procedure
- ISO 6256 – Balanced Classes of Procedure
The current standard for HDLC is ISO 13239, which replaces all of those standards.
HDLC provides both connection-oriented and connectionless service.
HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode (ABM). The original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM) are rarely used.
Read more about High-Level Data Link Control: History, Framing, Structure, Types of Stations (Computers), and Data Transfer Modes, HDLC Operations, and Frame Types, Link Configurations, HDLC Command and Response Repertoire, HDLC Command/Response Repertoire
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