What is High-Level Data Link Control (HDLC)?

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The HDLC process is regulated by the International Organization for Standardization (ISO). This is possible for high-speed communication of a high amount of information. The necessary control process cannot support this communication.

The HDLC process has been regulated and established on the SDLC. In addition to the characters, bit strings of the acquired length can also be sent through this process. The unit of data communication is known as a frame.

HDLC Frame Structure

To provide support for all options in the mode and configuration, HDLC defines three types of frames.

  • Information Frames (IF)

The I-Frame is as follows −

  User Information


  • Supervisory Frames (SF)

The S-frame is as follows −



  • Un-numbered Frames (UF)

The U-frame is as follows −

  Management Information


HDLC Frame Format

I-Frames can transfer the user data and control the data associated with the user data. S-frames can transfer control data. Generally, it includes the data link layer structure and the error controls. U-frames are constrained for framework administration.

The Flag Field

Each frame on the connection should start and end with a flag sequence Field (F). Stations connected to the data link should continuously listen for a flag sequence is an octet considering like 01111110. Flags are always transmitted onto the connection between frames to keep the connection active.

The Address Field

It can recognize the primary or secondary stations participation in the frame communication or response.

The Control Field

HDLC uses the control field (C) to decide how to control the transmission process. The field includes the commands responses and sequences numbers used to support the link's data flow accountability.

There are three control field format are as follows −

  • Information transfer format: The frame can transmit the user data between two devices.

  • Supervisory Format: The control field executes control functions such as acknowledgement of frames, request for re-transmission, and requests for limited suspension of frames being sent.

  • Unnumbered Format: This control field format can also be used for control purposes. It can implement link initialization, link disconnection and other link control services.

The poll/final bit (p/f)

The control field's fifth-bit position is known as the poll/final bit, or p/f bit. It can only be identified when it is sent to 1. If it is set, too, it is neglected. The secondary station responds to the p bit by transferring a data or status frame to the primary station with the p/f bit set to f = 1.

Information field

This field is not continually in an HDLC frame. It is only existing when the data transfer format is being used in the control field.

Frame Check Sequence Field (FCS)

FCS is a HDLCs error detection field. This field includes a 16 bit or 32-bit cyclic redundancy check bit.

Updated on 05-May-2021 06:51:11