Editing High-Level Data Link Control (section)

==History==
HDLC is based on [[IBM]]'s [[Synchronous Data Link Control|SDLC]] protocol, which is the layer 2 protocol for IBM's [[Systems Network Architecture]] (SNA).  It was extended and standardized by the [[International Telecommunication Union|ITU]] as LAP (Link Access Procedure), while [[ANSI]] named their essentially identical version [[ADCCP]].

The HDLC specification does not specify the full semantics of the frame fields. This allows other fully compliant standards to be derived from it, and derivatives have since appeared in innumerable standards.  It was adopted into the [[X.25]] protocol stack as [[LAPB]], into the [[ITU-T V.42|V.42]] protocol as [[LAPM]], into the [[Frame Relay]] protocol stack as [[LAPF]] and into the [[ISDN]] protocol stack as LAPD.

The original ISO standards for HDLC are the following:
* ISO 3309-1979 – Frame Structure<!-- ISO now uses : to delimit the year, but - is what was used at the time -->
* ISO 4335-1979 – Elements of Procedure
* ISO 6159-1980 – Unbalanced Classes of Procedure
* ISO 6256-1981 – Balanced Classes of Procedure

ISO/IEC 13239:2002, the current standard, replaced all of these specifications.

HDLC was the inspiration for the [[IEEE 802.2]] [[Logical Link Control|LLC]] protocol, and it is the basis for the framing mechanism used with the [[Point-to-Point Protocol|PPP]] on synchronous lines, as used by many servers to connect to a [[wide area network|WAN]], most commonly the [[Internet]].

A similar version is used as the control channel for [[E-carrier]] (E1) and [[SONET]] multichannel telephone lines. [[Cisco HDLC]] uses low-level HDLC framing techniques but adds a protocol field to the standard HDLC header.