imported>Bearian: /* See also /==See also== Link access procedure

2025-12-18T01:22:29Z

See also: ==See also== * Link access procedure

← Previous revision		Revision as of 01:22, 18 December 2025
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	==See also==		==See also==
			* [[Link access procedure]]
	* [[Point-to-Point Protocol]]		* [[Point-to-Point Protocol]]
	* [[Serial Line Internet Protocol]]		* [[Serial Line Internet Protocol]]

imported>Amirhusenjihed: /* growthexperiments-addlink-summary-summary:3|0|0 */

2025-10-30T13:01:36Z

growthexperiments-addlink-summary-summary:3|0|0

41.235.89.48: /* Asynchronous framing */

2024-10-25T08:43:04Z

Asynchronous framing

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← Previous revision		Revision as of 13:01, 30 October 2025
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	}}		}}

	'''High-Level Data Link Control''' (HDLC) is a [[communication protocol]] used for transmitting data between devices in [[Telecommunications\|telecommunication]] and [[Computer network\|networking]]. Developed by the International Organization for Standardization (ISO), it is defined in the standard ISO/IEC 13239:2002.		'''High-Level Data Link Control''' (HDLC) is a [[communication protocol]] used for transmitting data between devices in [[Telecommunications\|telecommunication]] and [[Computer network\|networking]]. Developed by the [[International Organization for Standardization]] (ISO), it is defined in the standard ISO/IEC 13239:2002.

	HDLC ensures reliable data transfer, allowing one device to understand data sent by another. It can operate with or without a continuous connection between devices, making it versatile for various network configurations.		HDLC ensures reliable data transfer, allowing one device to understand data sent by another. It can operate with or without a continuous connection between devices, making it versatile for various network configurations.
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	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]].		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 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:		The original ISO standards for HDLC are the following:
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	== HDLC operations and frame types ==		== HDLC operations and frame types ==
	Three fundamental types of HDLC frames may be distinguished:		Three fundamental types of HDLC frames may be distinguished:
	* Information frames, or '''I-frames''', transport user data from the network layer. They can also include flow and error control information piggybacked on data.		* Information frames, or '''I-frames''', transport user data from the [[network layer]]. They can also include flow and error control information piggybacked on data.
	* Supervisory frames, or '''S-frames''', are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames '''do not''' have information fields.		* Supervisory frames, or '''S-frames''', are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames '''do not''' have information fields.
	* Unnumbered frames, or '''U-frames''', are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type.		* Unnumbered frames, or '''U-frames''', are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type.
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	; Disconnected mode (DM) response		; Disconnected mode (DM) response
	: When the secondary is disconnected (the default state on power-up), it sends this generic response to any poll (command frame with the poll flag set) except an acceptable mode setting command. It may alternatively give a FRMR response to an unacceptable mode set command.		: When the secondary is disconnected (the default state on power-up), it sends this generic response to any poll (command frame with the poll flag set) except an acceptable [[mode setting]] command. It may alternatively give a FRMR response to an unacceptable mode set command.
	; Unnumbered acknowledge (UA) response		; Unnumbered acknowledge (UA) response
	: This is the secondary's response to an acceptable mode set command, indicating that it is now in the requested mode.		: This is the secondary's response to an acceptable mode set command, indicating that it is now in the requested mode.
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	: This frame (command or response), a ISO/IEC 13239 addition and rarely used, is like UI but also excludes CRC protection. Only a configurable-length prefix ("header") of the frame is covered by the CRC polynomial; errors in the rest of the frame are not detected.		: This frame (command or response), a ISO/IEC 13239 addition and rarely used, is like UI but also excludes CRC protection. Only a configurable-length prefix ("header") of the frame is covered by the CRC polynomial; errors in the rest of the frame are not detected.
	; Unnumbered poll (UP) command		; Unnumbered poll (UP) command
	: This command solicits a response from the secondary. With the poll bit set, it acts like any other poll frame, without the acknowledgement that must be included in I or S frame. With the poll bit clear, it has a special meaning in normal response mode: the secondary ''may'' respond, even though it has not received the poll bit. This is rarely used in HDLC, but was used in the original IBM SDLC as a substitute for the lack of asynchronous response mode; where the communication channel could handle simultaneous responses, the primary would periodically send UP to the broadcast address to collect any pending responses.		: This command solicits a response from the secondary. With the poll bit set, it acts like any other poll frame, without the acknowledgement that must be included in I or S frame. With the poll bit clear, it has a special meaning in normal response mode: the secondary ''may'' respond, even though it has not received the poll bit. This is rarely used in HDLC, but was used in the original IBM SDLC as a substitute for the lack of asynchronous response mode; where the [[communication channel]] could handle simultaneous responses, the primary would periodically send UP to the [[broadcast address]] to collect any pending responses.

	====Error Recovery====		====Error Recovery====
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	[[Category:Telecommunication protocols]]		[[Category:Telecommunication protocols]]
	[[Category:Networking standards]]		[[Category:Networking standards]]
			[[Category:Encodings]]

High-Level Data Link Control - Revision history

imported>Bearian: /* See also */==See also== * Link access procedure

imported>Amirhusenjihed: /* growthexperiments-addlink-summary-summary:3|0|0 */

41.235.89.48: /* Asynchronous framing */

imported>Bearian: /* See also /==See also== Link access procedure