Basic Frame Structure of HDLC

HDLC stands for High-level Data Link Control which is a collection of the protocol that ensures communication purposes among nodes or network points. It was developed by the International Organization for Standardization (ISO). The data to be transmitted is organized in the form of frames which when transferred to the destination is acknowledged for its appropriate arrival. It can be applied to both point-to-point connections and multipoint connections as it is a bit-oriented protocol.

The system of Automatic Repeat Request is implemented via HDLC and also full-duplex communications could be done with the help of HDLC. Due to its high reliability and high flexibility, it is one of the most used protocols. This data link layer protocol also offers both connection-based and connection-less services for the transmission of data frames.


The communication between the primary and secondary devices can occur through some modes of communication, which are as follows −

  • Normal Response Mode − network configurations like point-to-point or point-to-multipoint can function with this mode.

  • Asynchronous Response Mode − the primary and secondary communication can communicate with each other using ARM.

  • Asynchronous Balanced Mode − the combined stations should also communicate with each other using the point-to-point configuration, for this ABM is required.

Type of frames

The different types of frames of HDLC are −

  • Information frame − it is used to store and transmit user data from the network layer, along with the process flow information and error control measures. The control field has the first bit 0 for this kind of frame.

  • Supervisory frame − it does not carry any information and hence the field of control information is not present. When user information does not need to be sent to the process flow and the error controls could not be attached to them, here S-frame comes into play and helps in transmitting these without attaching them to any other information. The control field has the first 2-bit 10 for this kind of frame.

  • Unnumbered frame − several assorted measures (for example, link management or any system administration task) are taken with the help of U-frame. If the situation demands, it may also contain an information field to establish the link between communication devices. The control field has the first 2-bit 11 for this kind of frame.

Frame structure

The pictorial representation of HDLC frame

The basic structure of the frame of HDLC protocol is as explained below −

  • Flag field − the process of error checking starts and ends with this field as this is used to indicate the initiation and termination of each frame having a pattern like 01111110 comprising of 8 bits, this process is called bit stuffing. The pattern is designed such that it is not replicated anywhere inside the frame and also can be used as a synchronization to the receiver end.

  • Address field − the main responsibility of the address field is to contain the secondary station’s address and also to understand whether it will send or receive the data. The length can vary from 1 byte to several bytes where 1 byte can categorize a maximum of 128 stations, but usually containing 8 bits due to which it is proficient to address 256 addresses, but the length solely depends upon the product requirement. Apart from these requirements, it can also store any specific address, an address of a group, or even the address of a broadcast.

  • Control field − the field which manages and controls the communication process is the control field but has a different setup for all the different kinds of frames. It also manages the flow of the process and manages errors and consists of a minimum of 8 bits to a maximum of 16 bits.

  • Payload field − it is an optional field for HDLC which varies from one network to another. In the case of an I-frame, it stores the user’s information which is the sender broadcasting data to the receiver. In the case of the U-frame, it stores the information related to management or the network layer.

  • Frame check sequence − the process of error detection is done by FCS with the help of 16-bit or 32-bit CRC. CRC stands for Cyclic Redundancy Check which is again carried out at the receiver and then the original value is compared with the data received, if any change is detected and error is considered to occur. Hence the integrity of each data frame is established by this field containing 2 or 4 bytes. The complete 16bit of FCS is used for error identification in all the fields namely: address, control, and information fields.


HDLC protocol can be used to send data or information in both point-to-point and multipoint networks and is very popularly used, which is bit-oriented integrating both controls of errors and management of process flow. The way it can be used to establish information between the nodes is the same way it can also be used to terminate the connection between any two nodes.

Updated on: 05-May-2023


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