This document provides an overview of data communication and networking. It discusses the components of data communication, including messages, senders, receivers, transmission medium, and protocols. It then describes the seven-layer OSI model, which standardizes how different computer systems communicate. Each layer is stacked upon the previous one. The document outlines the layers of the OSI model from the physical layer to the application layer. It also discusses the four-layer TCP/IP model and compares it to the OSI model. The document serves to explain the basic concepts and layers involved in data communication and networking.