An HDLC (High-Level Data Link Control) interface implements the HDLC protocol, which is a synchronous data link layer protocol used for reliable point-to-point and multipoint communication. According to information available as of August 27, 2024, HDLC is a bit-oriented protocol, meaning it transmits data one bit at a time, and it provides error detection, correction, flow control, and multiplexing capabilities. In essence, the HDLC interface is the hardware and software implementation that enables devices to communicate using the HDLC protocol.
Key Features of HDLC:
- Synchronous Communication: Data transfer relies on a timing signal, ensuring both sender and receiver are synchronized.
- Bit-Oriented Protocol: Data is transmitted as a continuous stream of bits rather than character-based.
- Error Control: HDLC includes mechanisms for detecting and correcting errors that may occur during transmission.
- Flow Control: It manages the rate of data transmission to prevent overwhelming the receiver.
- Multiplexing: Allows multiple logical channels to share a single physical link.
Common Applications of HDLC:
- Wide Area Networks (WANs): Used for communication over long distances.
- Leased Lines: Provides reliable communication over dedicated connections.
- X.25 Networks: An older packet-switching network technology that utilizes HDLC.
HDLC Frame Structure
HDLC frames have a defined structure, typically including the following fields:
| Field | Description |
|---|---|
| Flag | A unique bit pattern (e.g., 01111110) that marks the beginning and end of the frame. |
| Address | Identifies the secondary station involved in the communication (in point-to-multipoint configurations). |
| Control | Contains control information such as frame type, sequence numbers, and flow control commands. |
| Data (Payload) | The actual data being transmitted. |
| FCS | Frame Check Sequence: A checksum used for error detection. Calculated based on the contents of the address, control, and data fields. The receiver recalculates the FCS and compares it to the received FCS. |
