This document provides an overview of key concepts in data communication including: 1. The five main components of a data communication system are the sender, receiver, message, transmission medium, and protocol. 2. Data flow can be simplex, half-duplex, or full-duplex depending on whether transmission is unidirectional or bidirectional. 3. There are two main types of signals used in data transfer - analog signals which are continuous and digital signals which can only have discrete values like 0 and 1.