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Table of contents
What is HDLC (High-level Data Link Control) Protocol?
How does HDLC work?
How is HDLC used in IT networks?
Transfer Modes in HDLC
1. Normal Response Mode (NRM)
2. Asynchronous Balanced Mode (ABM)
HDLC Frames
Fields of HDLC Frame
Features of HDLC Protocol
Frequently Asked Questions
Where is HDLC protocol used in which layer?
What is the importance of HDLC protocol?
Which response mode is supported by HDLC protocol?
Is HDLC a reliable protocol?
What type of connection is HDLC?
Last Updated: Mar 27, 2024

High Level Data Link Control (HDLC) Protocol

Author Akshit Mehra
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HDLC (High-level Data Link Control) is a collection of data link layer communication protocols used to transmit data between network nodes. Because it is a data communication protocol, data is organized into frames. A frame is transmitted to the destination over the network, certifying that it arrived securely. The International Organization for Standardisation (ISO) created HDLC, a synchronous, bit-oriented data connection layer protocol. In this article, we will discuss about High Level Data Link Control (HDLC) Protocol.

High Level Data Link Control (HDLC) Protocol

What is HDLC (High-level Data Link Control) Protocol?

HDLC - High-level Data Link Control, a data communications transmission technique used at the data link layer (layer 2) of the OSI seven-layer model. The HDLC protocol embeds data in a data frame, allowing devices to manage data flow and repair errors. HDLC is an ISO standard that evolved from IBM's 1970s Synchronous Data Link Control (SDLC) standard. 

HDLC is a bit-oriented protocol that can communicate in both half-duplex and full-duplex mode via point-to-point and multipoint links.  One station is identified as primary and the other as secondary for any HDLC communications session. A session can use one of the connection modes listed below, which govern how the primary and secondary stations interact.

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How does HDLC work?

Data is arranged into a unit (frame) and delivered across a network to a destination that certifies its successful arrival in HDLC. The HDLC protocol also controls the rate at which data is sent. HDLC is one of the most widely used internet protocols (IP) in Layer 2 of the Open Systems Interconnection industry communication reference architecture (OSI).

  1. The detailed physical level that creates and receives electrical signals is called Layer 1
  2. Layer 3 is the highest level of the network, with access to router tables that show where data should be forwarded or sent

Layer 3 programming constructs a frame with the source and destination network addresses when transmitting. The Layer 3 frames are encapsulated by HDLC (Layer 2), which adds data link control information to a new, bigger frame.

How is HDLC used in IT networks?

HDLC (High-Level Data Link Control) is a basic data link layer protocol that is widely used in computer networks. Its primary function is to ensure data integrity, framing, flow management, and error detection in point-to-point and multipoint communication. Because of its encapsulation and error-checking procedures, HDLC is important in wide area networks (WANs) and legacy systems and as a framing method for many network protocols, and its multiple modes provide versatility in meeting diverse network requirements.

Furthermore, because of its simplicity and dependability, HDLC has functions in IT networks that extend beyond legacy systems. It serves as a solid foundation for numerous network protocols and is still an essential component of many networking systems.

Transfer Modes in HDLC

The HDLC protocol has two transfer modes that can be utilised in a variety of combinations. The following are some of them:

  1. Mode of Response: Normal (NRM)
  2. Mode of Asynchronous Balance (ABM)

Let's take a look at each of these modes separately:

1. Normal Response Mode (NRM)

The station's configuration is imbalanced in this mode. There is one primary station as well as a number of secondary stations. The primary station is in charge of sending orders, whereas the secondary station is merely responsible for responding.

This mode is appropriate for both point-to-point and multipoint communications.

point to point

2. Asynchronous Balanced Mode (ABM)

The station's configuration is balanced in this mode. The link is point-to-point in this mode, and each station can act as a primary and a secondary.

Today, asynchronous balanced mode (ABM) is a widely utilized method.

Asynchronous Balanced Mode

HDLC Frames

In order to give the flexibility required to handle all of the choices available in the modes and Configurations previously discussed. The HDLC distinguishes three types of frames:

  1. Information Frames(I-frames) These frames are used to convey user data as well as control data relevant to the user data. It is identified as an I-frame if the first bit of the control field is 0
  2. Supervision Frames (S-frames) These frames are solely for the transmission of control information. The frame is designated as S-frame if the first two bits of the control field are 1 and 0
  3. Unnumbered Frames (U-Frames) These frames are primarily used for system administration. These frames are utilised by the communicating devices to exchange control information

Each frame is primarily used as an envelope for the delivery of a specific sort of message.         

Types of HDLC Frames

Figure: Types of HDLC Frames


Fields of HDLC Frame

Fields of HDLC Frame are:

  1. Flag: The flag is an 8-bit sequence that indicates the start and end of a frame. The flag's bit pattern is 01111110
  2. Address: The receiver's address is stored in this field. If the primary station sends the frame, it contains the secondary station's address(es) (s). The address field might be anything from a single byte too many bytes long
  3. Control: It consists of one or two bytes that carry flow and error control information
  4. Payload: The data from the network layer is carried by the payload. Its length varies depending on the network
  5. FCS (Federal Civil Service): For error detection and error control, it's a 2-byte or 4-byte frame check sequence. CRC is the standard code (cyclic redundancy code) and is used for error detection and error control

Features of HDLC Protocol

Some of the HDLC protocol's characteristics are listed below:

  1. In this protocol, bits are used to stuff flags in the data
  2. This protocol is used to access both point-to-point and multipoint links
  3. HDLC is one of the most widely used data link layer protocols
  4. HDLC is a protocol that is based on bits
  5. Error control and flow control are both implemented in this protocol

Frequently Asked Questions

Where is HDLC protocol used in which layer?

The HDLC protocol (High-Level Data Link Control) is a data link layer protocol widely used in telecommunications networks such as ISDN and frame relay networks. It is used to ensure reliable and efficient data transfer between network devices.

What is the importance of HDLC protocol?

The HDLC protocol (High-Level Data Link Control) is vital because it provides a dependable and efficient method of delivering data via point-to-point and multipoint communication systems. It is commonly utilized in data transfer applications that are both synchronous and asynchronous.

Which response mode is supported by HDLC protocol?

The HDLC protocol (High-Level Data Link Control) has three response modes: normal response mode (NRM), asynchronous response mode (ARM), and transparent response mode (TRM). The communication application's requirements decide the response mode selected.

Is HDLC a reliable protocol?

HDLC (High-Level Data Link Control) is a highly reliable protocol for transmitting data over communication links. It offers error detection and correction mechanisms to ensure the transmitted data is accurate and complete. Additionally, HDLC provides flow control and retransmission capabilities.

What type of connection is HDLC?

HDLC (High-Level Data Link Control) is a communication protocol in computer networks. It helps two devices transfer data to each other over a direct connection and makes sure the data is sent accurately and without errors. 


In the above blog, we studied the High-Level Data Link Control (HDLC) Protocol. HDLC (High-level Data Link Control) is a data link layer communication protocol for sending data between network points or nodes. Data is arranged into frames since it is a data connection protocol. Next, we saw the working of the HDLC Protocol. Furthermore, we studied various transfer modes of the HDLC Protocol. In the end, we learned about the HDLC Frame structure. Once complete, you may check out our Computer Networks to level your knowledge and get placed at your dream company. 

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