This course provides an introduction to telecommunication network fundamentals. The lectures start with a broader introduction to connected devices that form a network and the OSI 7-Layer model for standardizing networking functionalities. The course mainly covers Layer-1, Layer-2, Layer-3, and Layer-4 of the OSI model with example networks such as the Internet, Wi-Fi access network, cellular access and core networks (3G/4G/5G network), and the optical fibre transport network. Teletraffic engineering is introduced in the course by covering some basic traffic theories and their design implications on networks for providing the expected quality of service (QoS) and optimizing the network resources.