What is High-Level Data Link Control (HDLC)?

High-Level Data Link Control (HDLC) is a bit-oriented data link layer protocol standardized by the International Organization for Standardization (ISO). It enables reliable, high-speed communication by providing error detection, flow control, and frame sequencing for data transmission between network devices.

HDLC was developed based on IBM's Synchronous Data Link Control (SDLC) protocol. Unlike character-oriented protocols, HDLC can transmit bit strings of variable length, making it more flexible and efficient. The basic unit of data communication in HDLC is called a frame.

HDLC Frame Types

HDLC defines three types of frames to support different communication modes and configurations:

• Information Frames (I-frames) − Used to transfer user data along with control information for flow control and error recovery.

• Supervisory Frames (S-frames) − Used for control functions such as acknowledgments, retransmission requests, and flow control without carrying user data.

• Unnumbered Frames (U-frames) − Used for link management functions like connection establishment, disconnection, and mode setting.

HDLC Frame Format

Flag Field

Each HDLC frame begins and ends with a flag sequence of 8 bits (01111110). This field marks the beginning and end of each frame, allowing receiving stations to identify frame boundaries. Flags are continuously transmitted between frames to maintain link synchronization.

Address Field

The address field identifies the secondary station involved in the communication. In point-to-point links, this field may be omitted since there is only one possible destination. In multipoint configurations, it distinguishes between different secondary stations.

Control Field

The control field determines the frame type and contains sequence numbers for flow control and error recovery. It has three formats:

• Information format − Used in I-frames for data transfer with sequence numbering

• Supervisory format − Used in S-frames for acknowledgments and flow control

• Unnumbered format − Used in U-frames for link management commands

Poll/Final Bit

The poll/final (P/F) bit is the fifth bit in the control field. When set to 1 by a primary station, it acts as a poll bit requesting a response. When set to 1 by a secondary station, it acts as a final bit indicating the end of its transmission.

Information Field

The information field contains the actual user data and is present only in I-frames and some U-frames. Its length is variable and depends on the specific HDLC implementation and configuration.

Frame Check Sequence (FCS)

The FCS field provides error detection using a 16-bit or 32-bit Cyclic Redundancy Check (CRC). The transmitter calculates the CRC based on the frame contents, and the receiver verifies frame integrity by recalculating and comparing the CRC value.

Advantages

• Bit transparency − Can transmit any bit pattern without confusion with control sequences

• Error detection − Built-in CRC provides reliable error detection

• Flow control − Supports both stop-and-wait and sliding window protocols

• Flexibility − Works in various network topologies and configurations

Conclusion

HDLC is a robust data link control protocol that provides reliable frame transmission through its structured frame format and error detection mechanisms. Its three frame types?Information, Supervisory, and Unnumbered?enable efficient data transfer, flow control, and link management in various network environments.