HDLC (High-Level Data Link Control) - Introduction, Frames, and Fields

Moving into the blog, let’s have an introductory knowledge of computer networking. To cut a long story short, A computer network is a group of interconnected devices (called nodes), that are linked together to exchange information and data. To understand it better, it’s a way for computers to talk to each other and share resources while being on the same platform. This type of connection allows for communication between multiple devices, enabling users to share media, access the internet, and exchange data in real-time.

Table of Contents:

• Introduction to HDLC
• HDLC Frames and Fields
• HDLC Life Cycle
• Conclusion

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Introduction to HDLC

Let’s answer the question, ‘What is HDLC(High-level Data Link Control)?’. To sum up everything in one section, the HDLC protocol in computer networks is a widely used data link protocol for reliable and efficient data transmission.

• It operates at the data link layer of the OSI (Open Systems Interconnection) model. HDLC is used for point-to-point connections between two devices.
• HDLC is a bit-oriented protocol that uses a frame structure to transmit data.
• Each frame consists of a start flag. It is used to indicate the beginning of a frame. Also, HDLC has an end flag, which indicates the end of a frame.
• HDLC consists of a control field, which contains information about the type of data being transmitted, and a cyclic redundancy check (CRC) code, which detects errors in the transmitted data.
• The control field in HDLC also contains information about the type of control being performed, such as whether the data is a command or a response.
• HDLC supports both unidirectional and bidirectional communication.
• HDLC provides flow control mechanisms during the communication process to ensure that the transmitted data does not overload the receiving node.
• It also supports both asynchronous and synchronous transmission modes in a computer networking channel.
• HDLC can be used with various physical layer technologies, including serial links and point-to-point protocols.
• In case of redundancy, HDLC will retransmit the erroneous frame, ensuring that the data is transmitted correctly.

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HDLC Frames and Fields

Frames are a crucial component of HDLC and play a critical role in modern-day computer networks. Being a networking professional, you should know the frames and types of HDLC. Let’s raise the curtains from it.

To have a count on frames and types of HDLC, there are six types of fields present in HDLC and three types of HDLC frames. Firstly we are going with the types of fields.

• Start Flag: The start flag is a predefined sequence of bits that indicates the beginning of an HDLC frame. The start flag serves as a synchronization mechanism and allows the receiving device to identify the start of the frame.
• Address Field: The address field contains the destination and source addresses of the HDLC frame. This field allows the receiving device to identify the intended recipient of the frame and determine if it should process the frame.
• Control Field: The control field contains information about the type of data being transmitted and the type of control being performed. The control field can indicate whether the frame is an information frame, a supervisory frame, or a control frame.
• Information Field: The information field contains the data being transmitted. This field can be used for unidirectional data transfer or for transmitting data between two devices.
• Frame Check Sequence (FCS): The Frame Check Sequence (FCS) is a cyclic redundancy check (CRC) code used to detect errors in the transmitted data. The FCS allows the receiving device to detect and correct errors that may occur during transmission.
• End Flag: The end flag is a predefined sequence of bits that indicates the end of an HDLC frame. The end flag allows the receiving device to identify the end of the frame and determine if the transmission was successful.

Hoping for the HDLC frames. Each HDLC frame has a specific format, including a start flag, end flag, control field, and cyclic redundancy check (CRC) code. The start flag is used to indicate the beginning, while the end flag indicates the end of a frame. The control field contains information about the type of data being transmitted. The CRC code is used to detect errors in the transmitted data and ensure that it is transmitted accurately.

• Information Frame (I-Frame) – Information frames contain the data to be transmitted and are used for unidirectional data transfer. They include an information field, which holds the data, as well as a control field, which contains information about the type of data being transmitted. The first bit of the control field is 0.
• Supervisory Frame (S-Frame) – It is a type of HDLC frame, used for flow control and error detection and correction. They include a control field, which contains information about the type of control being performed, such as flow control, error detection, and retransmission of lost or damaged frames. The very first bit of the control field is 10.
• Unnumbered Frame (U – Frame) – U-frames, also known as unnumbered frames, are used for system management and communication between interconnected network nodes. U-frame’s first two bits are 11.

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HDLC Life Cycle

The HDLC life cycle is a well-defined process that makes sure that your data is transmitted securely and smoothly from the communication channel. Having an in-depth understanding of the HDLC life cycle is essential for you as an IT professional working with computer networks and communication systems.

• Establishment of a Connection: This is the first phase in the HDLC life cycle, which is the establishment of a connection between two nodes in the same network. This is typically done using a three-way handshake, where one device sends a request to connect, the other device accepts the request, and the first device confirms the connection.
• Data Transmission: Once a connection has been established between the devices. Data can be transmitted between the two devices. During this phase, data is divided into manageable units called frames and transmitted using the HDLC protocol.
• The flow of Control: HDLC provides flow control mechanisms to ensure that the data being transmitted does not overwhelm the receiving device. During this phase, the HDLC protocol regulates the flow of data to ensure that the receiving device is not overwhelmed.
• Error Detection and Correction: HDLC provides error detection and correction mechanisms to ensure that data is transmitted accurately. If an error is detected during transmission, HDLC will retransmit the faulty frame to ensure that the data is transmitted correctly.
• Disconnection: When the data transmission is complete, the HDLC connection can be disconnected. This is typically done using a three-way handshake, where one device sends a request to disconnect, the other device confirms the request, and the first device confirms the disconnection.

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Conclusion

Congratulations on making it to the finals of the blog. In the blog, we briefly discussed HDLC (High-level Data Link Control) and associated knowledge. HDLC is a reliable and efficient data link protocol that is critical in modern-day communication systems. Its support for multiple transmission modes, error detection and correction mechanisms, and flow control capabilities make it an essential protocol for ensuring reliable data transmission in computer networks.

The world of computer networks is constantly evolving, and there is always more to learn. The domain is here to stay. So, keep on exploring, and never stop learning!

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