What is HDLC and Understand the Functioning of Each Part of an HDLC Frame

With the increased exchange of multiple information and data units across distinct network models, the need for High-Level Data Link Control (HDLC) protocol arises in the network channel. The HDLC protocols are a part of the data link layer and are applied between multiple endpoints or nodes in a communication channel.

In this article, we will look into the working parts of the HDLC protocol and different forms of HDLC models applied in a network channel.

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What Is HDLC and Its Stations?

The High-Level Data Link Control (HDLC) is part of the data link layer protocol in the OSI Model. HDLC is applied for point-to-point and multipoint link structures based on the bit-oriented data format.

It also applies the Automatic Repeat Request (ARQ) for performing full-duplex communication between network channels due to its high flexibility and reliable network structure.

To efficiently integrate and use the HDLC protocols, different stations are to be effectively applied in the channel:

  • Primary Station - It handles establishing and de-establishing the primary data channel to share frames in the network, known as commands.

  • Secondary Station - They work under the command of the primary station, and the frames transmitted by this station are known as responses.

  • Combined Station - This network station can work as both the primary and secondary stations and handle commands and responses.

Next, let’s look into the types of transfer models in the HDLC structure.

HDLC Transfer Models

The HDLC primarily supports two types of transfer models, which are:

  • Normal Response Model (NRM) - This transfer model combines primary and secondary stations in point-to-point or multipoint network configurations to exchange commands from primary stations and responses from secondary stations.


  • Asynchronous Balanced Model (ABM) - In this transfer model, combined stations are installed in a point-to-point configuration for exchange commands and responses in a balanced format.


Next, in this article, we will look into HDLC frames format, and the types of HDLC frames available.

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HDLC Frames and Types

The data unit for sharing information in HDLC is known as frames. HDLC frame consists of multiple frame fields, which may vary according to the type of frame used, and are as follows:


  • Flag Field - In HDLC, each frame starts and ends with the flag field in the configuration and is defined by an 8-bit octet sequence 01111110 in the flag field.
  • Address Field - It encapsulates the receiver's address in the field. For example, if the frame is sent from the primary station, it contains the secondary station’s address and vice versa.
  • Control Field - This field contains the flow and error control information in byte format.
  • Payload/Information Field - It carries information from the network layer, and the data size may vary depending on the network.
  • FCS Field - This field stands for Frame Check Sequence and acts as an error detection field in the HDLC protocol, which includes a 16-bit CRC check bit.

HDLC frames can be of the following three types, depending on the control field value of the frame:


  • I-Frame - The information frame or I-frame is applied to encapsulate the user information from the upper layer in the model and then transmit it in the network channel and contains 0 in the control field.


  • S-Frame - The supervisory frame or S-frames are used for error and data flow control and do not contain the information field in the frame format. The control field is 1 and 0 for the first two bits.


  • U-Frame - The un-numbered frame or U-frames are used for system management and exchanging control information between the connected network devices.

With this, we have covered all the important headings for understanding the parts and working of the HDLC protocol.

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In this article, we looked into the information related to the parts and functions of the HDLC protocol, which is a part of the data link layer.

You can refer to Simplilearn’s Cyber Security Expert course to further understand the working of the HDLC protocols. By completing this expert course, you’ll become more proficient with terms related to network-related models and protocols.

If you have any questions about this article on ‘What Is HDLC?’. Feel free to mention them in the comment section at the bottom of this page. Our expert team will help you solve your queries at the earliest.

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