HDLC

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

The current standard for HDLC is ISO 13239, which replaces all of those standards.

HDLC provides both and .

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.

HDLC is based on IBM's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP (Link Access Procedure), while ANSI named their essentially identical version ADCCP.

Derivatives have since appeared in innumerable standards. It was adopted into the X.25 protocol stack as LAPB, into the V.42 protocol as LAPM, into the Frame Relay protocol stack as LAPF and into the ISDN protocol stack as LAPD.

HDLC was the inspiration for the IEEE 802.2 LLC protocol, and it is the basis for the framing mechanism used with the on synchronous lines, as used by many servers to connect to a WAN, most commonly the Internet.

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