ontrol) A data link protocol from ISO for point-to-point communications over serial links. Derived from IBM's SDLC protocol, HDLC has been the basis for numerous protocols including X.25, ISDN, T1, SS7, GSM, CDPD, PPP and others. Various subsets of HDLC have been developed under the name of Link Access Procedure (LAP).
In HDLC, one side is set up as the primary control station and the other as secondary. In Normal mode, the secondary station responds to requests only from the primary station. In Asynchronous mode, the secondary can initiate transmission, and in Asynchronous Balanced mode, both sides can send and receive over full-duplex lines. See LAP