A bit-oriented, synchronous data communications protocol developed by the International Organization for Standardization (ISO) as a superset of Synchronous Data Link Control (SDLC) and Advanced Data Communications Control Procedures (ADCCP). A version of HDLC is the Link Access Procedure-Balanced (LAPB), which is used in packet-switched networks conforming to the ITU-T X.25 Recommendation. HDLC also was imported into other standards, including ISDN as LAPD and frame relay as LAPF. See also ADCCP, bit-oriented protocol, frame relay, ISDN, ISO, LAPB, LAPD, LAPF, SDLC, synchronous, and X.25.