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).