nonHDLC

NonHDLC refers to data-link layer protocols or framing methods that do not use the High-Level Data Link Control (HDLC) framing standard. The term is used mainly in documentation and software implementations to distinguish non-HDLC line disciplines or interfaces from HDLC-based ones. It is not a formal protocol designation.

In practice, nonHDLC implementations are found on serial links or embedded interfaces where simpler or legacy framing schemes are used. Examples include SLIP (Serial Line Internet Protocol), which frames packets with a start/end delimiter rather than HDLC-like fields, and certain proprietary or minimal framing used in early networks. By contrast, HDLC-based methods—such as HDLC itself, PPP’s HDLC-inspired framing, or frame-relay and LAPB derivatives—use HDLC-style fields, bit-stuffing, and a cyclic redundancy check (CRC) or FCS.

In modern networking, nonHDLC approaches are uncommon for mainstream IP transport, having largely been superseded by PPP and Ethernet. They may still appear in embedded systems, legacy equipment, or specialized medical or industrial networks. In Linux and other Unix-like systems, nonHDLC can refer to a line discipline or device driver path that handles non-HDLC framing, providing a parallel to standard HDLC line disciplines for certain serial interfaces.