This document provides an introduction to data communications and networks. It discusses the components of a data communication system including transmission medium and data flow. It also defines what a network is and describes different types of network criteria including performance, reliability, and security. Additionally, it covers physical network structures such as connection types, topologies, and categories of networks including local area networks (LANs), wide area networks (WANs), and metropolitan area networks (MANs). The document also provides an overview of the Internet, its organization, and Internet service providers. Finally, it introduces the concept of communication protocols and defines the key elements of a protocol including syntax, semantics, and timing.