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| High-level Data Link Control | |
|---|---|
| Name | High-level Data Link Control |
| Acronym | HDLC |
| Developed | International Organization for Standardization (ISO), International Telecommunication Union (CCITT) |
| First published | 1979 |
| Layer | Data Link Layer |
| Based on | Synchronous Data Link Control |
| Status | Published |
High-level Data Link Control High-level Data Link Control is a bit-oriented data link layer protocol widely used for point-to-point and multipoint telecommunications links and integrated into numerous network protocol suites, enterprise telephony systems, and satellite communications installations. It provides framing, error correction aids, and flow control mechanisms that underpin implementations by major vendors and standards bodies across ITU-T, ISO, IEEE, and national telecom administrations. HDLC's design influenced successors and relatives in standards such as Link Access Procedure, Balanced, Frame Relay, X.25, and various proprietary offerings from Cisco Systems, Nortel, Siemens, and Ericsson.
HDLC is a synchronous, bit-oriented protocol specified by the International Organization for Standardization and the International Telecommunication Union; it organizes transmission into frames and defines roles like primary and secondary stations used in distributed systems and telephony exchanges. The protocol's family ties link to Synchronous Data Link Control, Bisync, and later standards including ISO/IEC 3309 and ITU-T V.24 while impacting implementations in Bell Labs, AT&T, Xerox, and Digital Equipment Corporation. Design principles echo concepts from Claude Shannon's information theory, Donald Davies' packet switching research, and work by Vint Cerf and Robert Kahn on end-to-end communication.
An HDLC frame begins and ends with a unique flag sequence and carries an address field, control field, information field, and a frame check sequence derived from cyclic redundancy checks; this framing model influenced Point-to-Point Protocol and Ethernet framing design choices by Bob Metcalfe and David Boggs. Implementations use bit stuffing to prevent flag emulation, a technique developed alongside error-control research by groups at Bell Labs and IBM. Practical deployments handle serial links, leased lines, and radio links in systems built by Motorola, Hughes Aircraft Company, and Thales Group, and interoperate with standards such as X.21 and V.35.
HDLC defines operation modes including Normal Response Mode (NRM), Asynchronous Response Mode (ARM), and Asynchronous Balanced Mode (ABM), each prescribing primary/secondary station interactions relevant to telegraphy and modern satcom networks. Procedures include link initialization, command/response sequencing, and supervisory frames for acknowledgment and retransmission influenced by research at MIT, Stanford University, and Carnegie Mellon University. Implementers from Sun Microsystems, HPE, IBM, and Nokia tailored mode selection for router-to-router links, switching fabrics, and mobile network backhaul.
Error detection relies on frame check sequences (FCS) computed via CRC polynomials standardized in ISO and ITU documents, and recovery uses retransmission schemes like selective reject and go-back-N that reflect algorithmic developments by Leslie Lamport and earlier reliability work at RAND Corporation. HDLC supervisory frames (RR, RNR, REJ) coordinate acknowledgments and error recovery, a design that informed TCP acknowledgment strategies developed by Vint Cerf and Bob Braden. Equipment vendors such as Alcatel-Lucent, Fujitsu, and Hitachi implemented hardware-assisted CRC engines and retransmission timers to meet carrier-class availability expectations set by organizations like Telia and Deutsche Telekom.
Flow control in HDLC is window-based with configurable receive/transmit window sizes and uses supervisory frames to pause or resume transmission; concepts mirror sliding window techniques from Thomas E. Kurtz and algorithmic work at Bell Labs that also underlie X.25 and TCP/IP. Congestion management at the link layer interacts with upper-layer remedies specified by IETF working groups and standards bodies including IEEE 802 committees, especially where HDLCframes traverse switched networks operated by carriers such as Verizon, BT Group, and Orange S.A..
HDLC spawned variants and derivatives: the balanced LAPB used in X.25 networks, LAPD used in ISDN signaling specified by the CCITT and ITU-T, and numerous proprietary adaptations by Cisco Systems, Juniper Networks, and Huawei. Other relatives include Frame Relay, PPP, and link-layer protocols in Asynchronous Transfer Mode implementations by Alcatel-Lucent and Siemens AG. Embedded systems vendors like ARM Holdings and Texas Instruments produced HDLC controllers, while open-source network stacks in projects associated with FreeBSD, NetBSD, and Linux kernel include HDLC drivers and PPP compatibility layers.
HDLC was formalized during the 1970s and 1980s through collaboration between International Telecommunication Union study groups, national standards agencies, and industry consortia including EIA, CCITT, and ISO, building on research from IBM, Bell Labs, and NPL; subsequent revisions integrated recommendations from ITU-T Study Group 11 and ISO working groups. The protocol's evolution influenced telecommunications regulation and interconnect standards adopted by carriers such as AT&T, British Telecom, and NTT, and it remains referenced in modern interoperability documents from IETF and international standards organizations.
Category:Data link protocols