Increasingly, the use of fiber optic cable as a transport for voice, data, video, and Internet traffic is becoming the norm. Sonet (Synchronous Optical Network) and Sdh (Synchronous Digital Hierarchy) are being used with high-speed data transmission. Although Sonet/Sdh was originally intended for use in broadband Isdn and support leased line services like Ds-1 and Ds-3. Sonet/Sdh expansion is being fuelled by many of today"s biggest industries such as Boeing, Disney and At&T as these companies move their internal global networks to a Sonet/Sdh infrastructure. This work provides coverage of Sonet/Sdh technology written at a level that will be understandable to technicians working in the telecommunications industry. It includes detailed examples of Dwdm (dense wavelength division multiplexing) and Wdm (wavelength division multiplexing).