Publisher Summary This chapter reviews the important aspects of current commercial optical networks in all three segments or layers: access, metro, and core. The lowest layer of the access network consists of copper, coaxial, or fiber media and generally has a tree-like graphical structure. A metro network uses time division multiplexing (TDM), packet, and optical multiplexing technologies at the lowest layers. The core or intercity network uses similar technologies as the metro, but has different traffic clustering and distance criteria and constraints. Network services are provided by network overlays at each layer. Each layer then places requirements on lower layers all the way down to the optical layer. Service requirements can be divided into two basic classes: expectations for provisioning new services or features of existing services, including bandwidth constraints/options, and Quality-of-Service (QoS) parameters, such as Bit Error Rate (BER), packet loss, latency/delay, jitter. While the core segment has an almost ubiquitous penetration of fiber and WDM technologies, the opposite is true for access and metro segments in most areas. Thus, while much of the optics literature and industry focuses on advanced optical technologies for the long-distance network, most of the investment and opportunity for growth resides in the metro/access portion. Beyond network segmentation, since the optical layer is essentially the workhorse for higher network layers, an understanding of network layering is crucial to understanding the requirements and evolution of the optical layer. Commercial networks are continuously upgraded to provide more capacity, new services, and reduced capital and operational costs; seamless network evolution is essential for obvious economic reasons. Even when “disruptive” technologies and platforms are introduced, smooth integration within the existing infrastructure is imperative.
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