High-Level Data Link Control (HDLC)
High-Level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links. It's one of the data link layer protocols defined in the ISO network architecture. HDLC provides both connection-oriented and connectionless service. Here are its key features:
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Frame Structure: HDLC frames include a beginning and ending flag (with a bit pattern of 01111110), an address field, a control field, a data field, and a frame check sequence (FCS) for error checking.
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Types of Frames: HDLC defines three types of frames:
- Information Frames (I-frames): Used for carrying user data and control information.
- Supervisory Frames (S-frames): Provide flow and error control.
- Unnumbered Frames (U-frames): Used for management and control purposes without the need for acknowledgment.
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Modes of Operation: HDLC supports three modes:
- Normal Response Mode (NRM): Used in point-to-point configurations with a primary station controlling the communication.
- Asynchronous Balanced Mode (ABM): Allows both stations in a point-to-point link to transmit data independently.
- Asynchronous Response Mode (ARM): Primarily used in multi-point configurations.
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Error Detection and Correction: HDLC uses a Frame Check Sequence (FCS) at the end of each frame to detect errors. It usually employs a Cyclic Redundancy Check (CRC) for error detection.
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Flow Control: It uses a window-based mechanism to control the flow of data and ensure efficient transmission without overburdening the receiving station.
HDLC is widely used in various communication standards and networks. It has inspired many other data link layer protocols and is known for its reliability and error-handling capabilities.