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Unformatted text preview: 10 Introduction to Optical Networks 1.3 Optical Networks Optical networks offer the promise to solve many of the problems we have discussed. In addition to providing enormous capacities in the network, an optical network provides a common infrastructure over which a variety of services can be delivered. These networks are also increasingly becoming capable of delivering bandwidth in a exible manner where and when needed. Optical fiber offers much higher bandwidth than copper cables and is less suscep- tible to various kinds of electromagnetic interferences and other undesirable effects. As a result, it is the preferred medium for transmission of data at anything more than a few tens of megabits per second over any distance more than a kilometer. It is also the preferred means of realizing short-distance (a few meters to hundreds of meters), high-speed (gigabits per second and above) interconnections inside large systems. Optical fibers are widely deployed today in all kinds of telecommunications net- works. The amount of deployment of fiber is often measured in sheath miles. Sheath miles is the total length of fiber cables, where each route in a network comprises many fiber cables. For example, a 10-mile-long route using three fiber cables is said to have 10 route miles and 30 sheath (cable) miles. Each cable contains many fibers. If each cable has 20 fibers, the same route is said to have 600 fiber miles. A city or telecommunications company may present its fiber deployment in sheath miles; for example, a metropolitan region may have 10,000 fiber sheath miles. This is one way to promote a location as suitable for businesses that develop or use information technology. When we talk about optical networks, we are really talking about two gener- ations of optical networks. In the first generation, optics was essentially used for transmission and simply to provide capacity. Optical fiber provided lower bit error rates and higher capacities than copper cables. All the switching and other intelligent network functions were handled by electronics. Examples of first-generation optical networks are SONET (synchronous optical network) and the essentially similar SDH (synchronous digital hierarchy) networks, which form the core of the telecommu- nications infrastructure in North America and in Europe and Asia, respectively, as well as a variety of enterprise networks such as Fibre Channel. We will study these first-generation networks in Chapter 6. Second-generation optical networks have routing, switching, and intelligence in the optical layer. Before we discuss this generation of networks, we will first look at the multiplexing techniques that provide the capacity needed to realize these networks. 1.3 Optical Networks 11 1 2 N B b/s NB b/s TDM or OTDM mux 1 1 2 2 N N B b/s B b/s WDM mux l 1 l 1 l 2 l 2 l N l N . . . . . . . . ....
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- Spring '09