This International Standard defines the frame structure for data oornmunication systems using bit-oriented high-level data link oontrol (HDLC) procedures. It defines the relative positions of the various oomponents of the basic frame and the bit oombination for the frame delimiting sequence (Flag). The mechanism used to achieve bit pattern independence within the frame is also defined. The document specifies two frame checking sequences (FCS). The rules for address field extension are defined. The addressing oonventions available are outlined. The oontrol field encodings and formats are defined in other International Standard(s).