What Is HDLC Protocol And How Does It Work

HDLC Protocol
HDLC Protocol

HDLC Protocol: In today’s post, we will know what is HDLC protocol and how does it work? What is the difference between HDLC and Point to Point protocol What are the features of HDLC protocol. And learn about the different types of frames of HDLC protocol in details.

What is HDLC protocol and how does it work?

High-level Data Link Control (HDLC) protocols are a set of data link layer communication protocols for transmitting data between network points or nodes. Since HDLC is a data link protocol, it is organized in a data frame. The HDLC protocol is designed by ISO to be used on point to point data links. This protocol supports Full Duplex Communication.

The HDLC protocol puts data into a data frame that provides the devices with the capabilities of data flow control and error correction. HDLC is built on the basis of the Synchronous Data Link Control (SDLC) protocol developed by IBM. The HDLC protocol is the Default Proprietary Encapsulation used by Cisco Routers on Synchronous Serial Links. Therefore it will not interact with HDLC implemented on any other vendor’s router. It is configured by default on all Cisco routers so there is no need to configure it separately.

HDLC Operational Modes

HDLC supports two types of transfer modes, Normal Response Mode and Asynchronous Balanced Mode.

Normal Response Mode (NRM): Here, there are two types of stations, a primary station that sends commands and a secondary station that responds to received commands. It is used for Point-to-Point and Multi Point Communications.

Asynchronous Balanced Mode (ABM): The configuration is balanced here, i.e. each station can send commands and respond to commands. It is used only for point-to-point communication.

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Fields of HDLC Frame

HDLC is a Bit-Oriented Protocol where each frame consists of up to six fields. Its structure varies with respect to the type of frame. Some of the fields of HDLC frame are as follows –

Flag: This is an 8-bit sequence that marks the beginning and end of the frame. 01111110 is the bit pattern of the flag.

Address: It contains the receiver’s address. If the frame is sent by the primary station, it includes the address of the secondary station. And if it is sent by the secondary station, then it contains the address of the primary station. The Address field can range from 1 byte to several bytes.

Control: It is 1 or 2 bytes containing information about Flow and Error Control.

Payload: It carries data from the network layer. The length of it may also vary from one network to another network.

FCS: This is a frame check sequence of 2 bytes or 4 bytes to detect the error.

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Types of HDLC Frames

There are three types of HDLC frames. The type of frame is determined by the control area of ​​the frame. These are being given in detail below.

I-frames – I-frames or information frames carry user data from the network layer. They also contain flow and error control information.

S-frame − S-Frames or Supervisory Frames do not have an information field. They are used for flow and error control.

U-frame − U-frame or un-numbered frame is used for a myriad of different functions, such as link management. It can have an information field if needed.

How does HDLC protocol work?

HDLC defines a method for encapsulating or formatting data into frames for synchronous transmission over a synchronous serial WAN link to remote sites. HDLC is a bit stream protocol that uses 32-bit checksum for supports full-duplex communication and error correction. An HDLC frame has a flag byte. A control area at the beginning of the frame is used to establish and end the data link connection.

  • HDLC is primarily used for point-to-point communication, unlike other Layer 2 WAN protocols, such as asynchronous transfer mode (ATM), frame relay, and X.25, which provide point-to-point and Points are used for both.
  • HDLC does not need to be implemented at the data-link layer as the local and remote stations are directly connected.
  • The HDLC link consists of a primary station and a secondary station, with the primary station issuing commands and the secondary station issuing responses.
  • HDLC can be used in more complex configurations in which one primary link leads to several secondary links (unbalanced multipoint configuration).
  • HDLC is commonly used in leased-line connections in which the end points of the WAN link terminate with the CSU/DSU (Channel Service Unit/Data Service Unit).

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What is the difference between HDLC and Point to Point protocols?

Expands toHigh-level Data Link Layer ProtocolPoint-to-Point Protocol
Type of protocolsBit-oriented protocolByte oriented protocol
Used inOnly synchronous mediaSynchronous as well as asynchronous media
AuthenticationNo provision of authenticationProvides authentication
Dynamic addressingDoes not offer dynamic addressing.Dynamic addressing is used.
Implemented inPoint-to-point and multipoint configurations.Only point-to-point configurations.
Compatibility with other protocolsCan not be operated with non-Cisco devices.Interoperable with non-Cisco devices also.


In today’s post, we have discussed what is HDLC protocol and how does it work? What is the difference between HDLC and Point to Point protocol What are the features of HDLC protocol. And know the details about the different types of frames of HDLC protocol. Friends, I hope that you must have liked today’s post.

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