Purpose: The purpose of this document is to provide the reader with material to enhance the understanding of High Frequency Data Link. This document provides the reader with a snapshot of guidance material used in the course of development of ARINC Characteristic 753, "HF Data Link System," and ARINC Specification 635, "HF Data Link Protocols. This document also provides a baseline for further system development and documentation effort within RTCA and ICAO. The information should be considered in several areas within the HF Data Link system, including the airborne avionics, the propagation media, and the terrestrial components.