This document provides an overview of dense wavelength division multiplexing (DWDM) technology. It discusses the history and components of a DWDM system, including transponders, multiplexers, fiber, amplifiers, and demultiplexers. WDM takes multiple optical signals, maps them to individual wavelengths, and multiplexes the wavelengths over a s ngle fiber. Another fundamental difference between the two technologies is that WDM can carry multiple protocols without a common signal format, while S ET cannot. This technique enables better fiber utilization, as it increases fiber capacity by a factor of 16–96 and enables building effective optical networks.