x Foreword What at the time was far less obvious to most, even if a few could foresee it, was that in enabling DWDM transmission, the optical amplifier was also preparing the way to a fundamentally new network architecture using wavelengths as the net- worked parameterdthe common unit of "currency" for enabling and managing a network. These WDM networks, while they also depended on an array of other new technologies, including, most importantly, optical switching elements to build the wavelength add/drop multiplexers and wavelength cross-connect networking elements, depended on optical amplifiers not only to enable WDM transmission but also to compensate for the losses in these switching elements. WDM networks offered the potential to provision, manage, and protect capacity based on wavelength "chunks" via fully flexible, switched wavelength networks. It is these WDM networks that are the focus of this book. While to many this vision appeared far-out, it was actually a very natural consequence of adopting WDM for transmission systems. Nevertheless, a tremen- dous world-wide research effort was required to provide the knowledge base needed to answer key questions, invent and develop new technologies, and refine and demonstrate the value proposition of WDM networks to convince service providers around the globe to deploy these networks for both long-haul and metro networks. The editors of Optically Amplified WDM Networks, John Zyskind and Atul Srivastava, who played key roles in taking optical networks from a vision to reality,