High-Level Data Link Control

What Does High-Level Data Link Control Mean?

A high-level data link control (HDLC) is a protocol that is a bit-oriented synchronous data link layer. HDLC ensures the error-free transmission of data to the proper destinations and controls the data transmission speed.

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HDLCs can provide both connection-oriented and connectionless services.

Techopedia Explains High-Level Data Link Control

A high-level data link control defines rules for transmitting data between network points. Data in an HDLC is organized into units called frames and is sent across networks to specified destinations. HDLC also manages the pace at which data is transmitted. HDLC is commonly used in the open systems interconnection (OSI) model’s layer 2.

HDLC frames are transmitted over synchronous links or asynchronous links, which do not mark the start and end of frames. This is done using a frame delimiter or flag, which contains unique sequence of bits that are not visible inside a frame.

There are three types of HDLC frames:

  • Information frames/User data (I-frames)
  • Supervisory frames/Control data (S-frames)
  • Unnumbered frames (U-frames)

The common fields within an HDLC frame are:

  • Flag
  • Address
  • Control information
  • Frame check sequence

The HDLC protocol is used by a variety of standards implemented in the protocol stacks of X.25, V.42 and ISDN and many other protocol stacks.

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Margaret Rouse

Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…