HDLC

HDLC, or High-Level Data Link Control, is a synchronous, bit-oriented data link layer protocol developed by ISO. It provides framing, error detection via a frame check sequence, and optional flow control to support reliable data transfer over serial links. It is used on point-to-point and multipoint connections and serves as the basis for several other WAN protocols.

A HDLC frame consists of a flag sequence (0x7E) that marks the frame boundaries, an address field, a control field, an information field (present only for information frames), and a frame check sequence (FCS). Frames are transmitted with bit stuffing: after five consecutive 1 bits in the data, a 0 bit is inserted to prevent accidental flag recognition. The FCS is typically a 16-bit CRC (CRC-CCITT), though some implementations may use a 32-bit CRC.

HDLC defines three frame types. Information frames (I-frames) carry payload data and use sequence numbers to track transmission and acknowledgment. Supervisory frames (S-frames) handle control information such as acknowledgments and flow control (for example, RR, RNR, REJ, SREJ). Unnumbered frames (U-frames) perform control functions such as link establishment and termination (for example, SABM, DISC, UA). The protocol supports a sliding window mechanism to allow multiple outstanding I-frames, improving efficiency on higher-latency links.

HDLC can operate in point-to-point or multipoint configurations, with the address field differentiating among stations in multipoint setups. It forms the basis for other protocols and framing schemes, including SDLC, and influences modern implementations such as Frame Relay and PPP, which employ HDLC-like framing.

HDLC originated in ISO standards during the 1970s and remains a foundational data link protocol due to its simplicity, robust framing, and broad hardware support.