What is the HDLC protocol and how does it work?

In today's post, we will know what is HDLC protocol and how does it work? What are the differences between HDLC and Point to Point protocols What are the features of the HDLC protocol? And learn about different types of frames of HDLC protocol in detail.

What is the HDLC protocol and how does it work?

High-level Data Link Control (HDLC) protocols are a set of communication protocols of the Data Link Layer to transmit 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 devices with the capabilities of Data Flow Control and Error Corrections. HDLC is based on the Synchronous Data Link Control (SDLC) protocol created by IBM. The HDLC protocol is the Default Proprietary Encapsulation used by Cisco Routers on Synchronous Serial Links. Therefore it will not interact with the Implemented HDLC on another vendor Router. It is By Default Configure on all Cisco Routers so there is no need to configure it separately.

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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 Secondary Station responds to the received command. It is used for Point - to - Point and Multi-Point Communications.

What is the HDLC protocol and how does it work?

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

What is the HDLC protocol and how does it work?
Fields of HDLC Frame

HDLC is a Bit-Oriented Protocol where each frame has up to six fields. The structure varies according to the type of frame. Fields of HDLC frames are as follows -

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

Address - It contains the receiver's address. If the frame is sent by the Primary Station, it contains the address of the Secondary station. And if it is sent by Secondary station, then it has the address of Primary Station. The address field can range from 1 byte to several bytes.

Control − It is 1 or 2 bytes containing information of Flow and Error Control.

Payload − It carries data from the network layer. Its length may vary from one network to another.

FCS − This error is a frame check sequence of 2 bytes or 4 bytes.

What is the HDLC protocol and how does it work?

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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 below from Detail.

I-frame: I-frames or Information frames move user data from the network layer. They also include 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 diverse tasks, such as link management. It may contain an information field if necessary.

What is the HDLC protocol and how does it work?

HDLC defines a method for encapsulating or formatting data in a frame for synchronous transmission over the asynchronous serial WAN links to remote sites. HDLC is a bit-stream protocol that uses 32-bit Checksum for Error Correction and supports full-duplex communication. The HDLC frame has a flag byte. A control area is used to establish and terminate the data-link connection at the beginning of the frame.
  • The HDLC link consists of a primary station and a secondary station, issuing commands with the primary station and the secondary station issuing responses.
  • HDLC is mainly used for point-to-point communication, unlike other Layer 2 WAN protocols, such as asynchronous transfer mode (ATM), frame relay, and X.25, which are point-to-point and Points are used for both.
  • HDLC does not need to be implemented at the data-link layer because local and remote stations are directly connected.
  • HDLC can be used in more complex configurations in which one primary link is to multiple secondary links (unbalanced multipoint configuration).
  • HDLC is commonly used in leased-line connections in which the endpoints of the WAN link terminate with the CSU / DSU (Channel Service Unit / Data Service Unit).
 

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

BASIS FOR COMPARISON HDLC PPP
Expands to High-level Data Link Layer Protocol Point-to-Point Protocol
Type of protocols Bit-oriented protocol Byte oriented protocol
Used in Only synchronous media Synchronous as well as asynchronous media
Authentication No provision of authentication Provides authentication
Dynamic addressing It does not offer dynamic addressing. Dynamic addressing is used.
Implemented in Point-to-point and multipoint configurations. Only point-to-point configurations.
Compatibility with other protocols It cannot be operated with non-Cisco devices. Interoperable with non-Cisco devices also.

Conclusion

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

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