What are the types of HDLC Frames?

HDLC (High-Level Data Link Control) is a bit-oriented synchronous data link layer protocol that provides error-free transmission of data between network points. It supports both connection-oriented and connectionless services, organizing data into structured units called frames for reliable transmission across networks.

HDLC frames are transmitted over synchronous or asynchronous links using frame delimiters called flags, which contain specific bit sequences that mark the beginning and end of each frame without appearing within the frame content itself.

Types of HDLC Frames

HDLC defines three distinct types of frames, each serving different communication purposes:

• Information Frames (I-Frames) − Transfer user data and control information

• Supervisory Frames (S-Frames) − Handle flow control and error control functions

• Unnumbered Frames (U-Frames) − Manage link setup, disconnection, and maintenance

Frame Structure Components

Flag Field

Every HDLC frame begins and ends with a flag sequence of 01111110. Stations continuously monitor for this 8-bit pattern to detect frame boundaries. Flags are transmitted between frames to maintain link synchronization.

Address Field

The address field identifies which primary or secondary station is involved in the frame transmission or response, enabling proper frame routing in multi-point configurations.

Control Field

The control field determines frame type and manages transmission procedures. It contains commands, responses, and sequence numbers that provide data stream accountability and flow control.

Control Field Formats

Poll/Final Bit (P/F)

The fifth bit in the control field serves as the poll/final bit. When set to 1, it indicates a poll from primary station or final response from secondary station, facilitating proper dialogue control between communicating devices.

Information Field

Present only in I-frames and some U-frames, this variable-length field carries actual user data or management information being transmitted across the link.

Frame Check Sequence (FCS)

The FCS provides error detection using either 16-bit or 32-bit cyclic redundancy check (CRC) calculations, ensuring data integrity during transmission.

Conclusion

HDLC frames are classified into three types: I-frames for data transfer, S-frames for supervisory control, and U-frames for link management. Each frame type uses a standardized structure with flags, addressing, control information, and error detection to ensure reliable data communication.