HDLC

HDLC
Name	HDLC
Type	Bit-oriented synchronous data link layer protocol
Developer	International Organization for Standardization, International Telegraph and Telephone Consultative Committee, Bell Laboratories
First published	1979
Layer	OSI model Layer 2
Predecessors	Synchronous Data Link Control, IBM Systems Network Architecture, X.25
Related	Ethernet, Point-to-Point Protocol, Frame Relay, ATM

HDLC

HDLC is a bit-oriented synchronous data link layer protocol developed for reliable, framed, point-to-point and multipoint communication. It provides framing, address/control fields, error detection, and link management used in wide area and local area technologies; it influenced many protocols and standards across telecommunications and computing. Implementations span telephony, mainframes, routers, switches, and embedded systems adopted by standards bodies and vendors.

Overview

High-Level Data Link Control operates as a Layer 2 protocol within the OSI model and was standardized through bodies including the International Telegraph and Telephone Consultative Committee and the International Organization for Standardization. It defines frame formats, sequencing, acknowledgements, and control mechanisms used by systems such as Bell Laboratories equipment, IBM network products, and telecom carriers like AT&T and British Telecom. HDLC influenced protocols like Point-to-Point Protocol, Frame Relay, and X.25, and is conceptually related to link-layer technologies in Cisco Systems routers, Juniper Networks systems, and legacy UNIX network interfaces.

History and Development

Origins trace to research at Bell Laboratories and standards work by the International Telegraph and Telephone Consultative Committee and ISO. Early packet and frame concepts paralleled development of X.25, IBM Systems Network Architecture, and packet switching experiments involving institutions like ARPANET participants and Stanford Research Institute. Subsequent formalization produced ISO standards that influenced vendors such as Nortel Networks, Siemens, Hewlett-Packard, and Cisco Systems. Deployments during the expansion of public data networks involved carriers including Deutsche Telekom and France Télécom, and integration with switching systems from Western Electric.

Protocol Structure and Frame Types

Frames contain a flag sequence, address, control, information, and frame check sequence; implementations in routers and mainframes from IBM and DEC reflect these fields in hardware and firmware. The control field encodes supervisory, information, and unnumbered frames analogous to control concepts in X.25 and signaling in ITU-T recommendations. Typical frame types include Information (I), Supervisory (S), and Unnumbered (U) frames, which parallel mechanisms in Digital Equipment Corporation products and influenced ISO link control standards. Variants used by vendors such as Cisco Systems and Siemens map these frames onto encapsulations used by PPP and link-layer implementations in Microsoft Windows and Linux network stacks.

Framing, Transparency, and Bit/Byte Stuffing

Framing uses a unique flag sequence to delimit frames, a method shared with other framed protocols implemented by vendors like Nortel Networks and Alcatel-Lucent. To ensure transparency, HDLC specifies bit-stuffing rules where a zero is inserted after five consecutive ones; alternative byte-stuffing variants appear in implementations by IBM and in encapsulations used by Cisco Systems. The frame check sequence employs cyclic redundancy checks similar to those recommended by IEEE and ITU-T standards bodies, and vendor implementations in platforms from Hewlett-Packard and Sun Microsystems handle stuffing in hardware or device drivers.

Operation Modes and Link Management

HDLC supports normal response mode, asynchronous balanced mode, and others that govern master-slave and peer-to-peer relationships; these modes influenced designs in AT&T switching and in Nokia telecommunications equipment. Link establishment and teardowns incorporate exchange of unnumbered frames and control functions mirrored in line protocols for Cisco Systems routers, Juniper Networks routers, and legacy DEC hardware. Administrative practices from standards organizations such as ISO and ITU-T guide link management operations used by carriers like Verizon and BT Group.

Error Detection, Flow Control, and Retransmission

Error detection relies on frame check sequences using cyclic redundancy check polynomials standardized by bodies including IEEE and ISO, comparable to error control in X.25 and Frame Relay. Flow control and retransmission are handled with sequence numbering, acknowledgements, and retransmit timers; these mechanisms influenced ARQ implementations in equipment from Bell Laboratories research and products by IBM and Siemens. Selective and go-back-N retransmission strategies are mirrored in protocol stacks implemented in Linux, FreeBSD, and embedded RTOS platforms used by vendors such as Cisco Systems and Motorola.

Implementations, Variants, and Compatibility

Numerous vendors produced HDLC-compatible controllers and firmware: IBM channel hardware, DEC network interfaces, routers by Cisco Systems and Juniper Networks, and integrated circuits from companies like Intel and Texas Instruments. Variants include ISO HDLC, IBM-compatible byte-oriented modes, and derivative encapsulations such as PPP (data link layer), Frame Relay, and proprietary link protocols in Alcatel-Lucent and Siemens equipment. Interoperability is addressed via profile documents from ISO, ITU-T, and vendor implementation notes from Nortel Networks and Hewlett-Packard, and modern virtualized environments in VMware and cloud providers adapt HDLC framing in software gateways.

Category:Telecommunications protocols