High-Level Data Link Control


High-Level Data Link Control (HDLC) is a data communication protocol used to send data over a network. It is a bit-oriented protocol developed by the International Organization for Standardization (ISO) for use on both point-to-point and multipoint data links. HDLC provides error-free communication by verifying data accuracy and managing data flow control.


The phonetic spelling of “High-Level Data Link Control” is /haɪ lɛvəl deɪtə lɪŋk kənˈtroʊl/.

Key Takeaways

  1. Function: High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). It is primarily used to provide error-free communication between systems over a data network.
  2. Versatility: HDLC offers a high degree of flexibility whereby it can be used over a wide range of network types including satellite and radio links, non-dedicated telephone circuits, and leased lines, making it quite versatile.
  3. Frame Structure: All information sent using HDLC is encapsulated within a frame. This frame structure typically includes a frame delimiter (flag), an address field specifying the source and destination, a control field that governs acknowledgment and sequencing, an information field that carries the user data, and a Frame Check Sequence (FCS) for error detection.


High-Level Data Link Control (HDLC) holds substantial importance in the realm of technology because it’s a standard protocol used in digital data transmission, specifically across point-to-point or point-to-multipoint configurations. It operates on the data link layer (Layer 2) of the OSI model, providing reliable and efficient communication between network nodes. HDLC ensures the integrity of data by managing data frames and checks for errors that could occur during transmission. It also supports multiple communication types, including unicast, multicast, and broadcast. Its flexibility, broad compatibility for point-to-point and multipoint applications, and inherent capability to ensure error-free delivery of data make HDLC a vital element in data communication systems.


High-Level Data Link Control (HDLC) is a set of protocols primarily used in telecommunications to transfer information between network points without errors. Specifically, it plays a critical role in the data transmission layer of the Open Systems Interconnection (OSI) model, a structured format for computer network systems. Essentially, the HDLC protocol ensures secure and reliable communication between points in a network by packaging data into frames, verifying the integrity of transmitted data, and ensuring its proper sequencing.One of the main applications of HDLC is in the telecommunications sector, where it is used in both point-to-point and multipoint configurations for the efficient transfer of data. This includes utilization within corporate intranets or extranets, and with systems that require precise, bi-directional and simultaneous transmissions. It is in the implementation of reliable channel communication where HDLC truly shines; its protocol establishes a logical link between two points in a network, synchronizing their conversation and providing a level of error detection and control not typically available in simple communication systems. This contributes to more efficient communication pathways and a more robust networking infrastructure.


1. Network Communications: High-Level Data Link Control (HDLC) is often used in network communications where there is a need for secure and error-free data transmissions. For example, in telecommunication networks, HDLC is utilized to provide reliable data transfer between network nodes. 2. Satellite Communications: Another application of HDLC is in satellite communication systems. Here, it is used to transmit information between a ground station and a satellite. The HDLC ensures smooth data transmission by utilizing protocols for flow and error control, which provide optimal reliability in such critical applications.3. Layman’s Use: HDLC can be found in everyday objects as well, such as in Bluetooth devices. The protocol ensures the integrity and security of data when it’s send and received between paired devices. This is what makes it possible to send files or voice data via Bluetooth without experiencing loss or distortion in the transferred data.

Frequently Asked Questions(FAQ)

Q: What is High-Level Data Link Control (HDLC)?A: High-Level Data Link Control (HDLC) is a protocol used in data transmission that operates on the second layer of the Open Systems Interconnection (OSI) model, known as the data link layer. It ensures a reliable and synchronous data communication path and is capable of both full-duplex and half-duplex communications.Q: How does HDLC work?A: HDLC works by encapsulating the data in information frames before transmission. These frames contain control information such as an address field, control field, and Frame Check Sequence (FCS) to ensure the data is sent and received correctly.Q: What are the primary modes of operation for HDLC?A: HDLC operates mainly in three modes: normal response mode (NRM), asynchronous response mode (ARM), and asynchronous balanced mode (ABM). Q: How does HDLC ensure error-free transmission?A: HDLC utilizes an error-detection mechanism known as Frame Check Sequence (FCS). This technique involves adding a calculated value from the frame data, and the receiver also performs the same calculation and compares the results to identify any errors.Q: What are the advantages of using HDLC?A: HDLC provides synchronous communication and has built-in error detection mechanisms. It can also support multiple types of network configurations, including point-to-point, multipoint, and loop configurations.Q: What is the role of HDLC in networking?A: The role of HDLC in networking is to provide reliable and error-free data transmission. It manages packet framing, sequencing, error detection, and retransmissions to ensure the integrity and reliability of data.Q: Can HDLC operate in a multipoint configuration?A: Yes, HDLC can support multiple network configurations, including multipoint configurations. In this configuration, one station operates in a primary mode while all other stations operate in a secondary mode.Q: Is HDLC used in modern networking protocols?A: Yes, HDLC is used as the basis for several modern networking protocols, including the Point-to-Point Protocol (PPP), which is commonly used to establish direct connections between two nodes in a network.

Related Tech Terms

  • Synchronous Data Link Control (SDLC)
  • Asynchronous Data Link Control (ADLC)
  • Flow Control
  • Error Detection
  • Frame Sequence

Sources for More Information

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