ed networks. Consider a 16-channel DWDM system. This system has 16 lasers, one for each channel, which means that the service provider must maintain 16 spare lasers in case of a laser failure. The lat- est effort underway is the deployment of tunable lasers, which allow the laser to be tuned to any output wavelength, thus reducing the volume of spares that must be maintained and, by extension, the cost. So what do we find in a typical WDM system? A variety of compo- nents, including multiplexers, which combine multiple optical signals for transport across a single fiber; demultiplexers, which disassemble the aggregate signal so that each signal component can be delivered to the appropriate optical receiver (PIN or APD); active or passive switches or routers, which direct each signal component in a variety of directions; fil- ters, which serve to provide wavelength selection; and finally, optical add- drop multiplexers, which give the service provider the ability to pick up and drop off individual wavelength components at intermediate loca- tions throughout the network. Together, these components make up the heart of the typical high-bandwidth optical network. And why is DWDM so important? Because of the cost differential that exists between a DWDM-enhanced network and a traditional network. To expand network capacity today by putting more fiber in the ground costs, on average, about $70,000 per mile. To add the same bandwidth using DWDM by changing out the endpoint electronics costs roughly one-sixth that amount. There is clearly a financial incentive to go with the WDM solution. Optical Switching and Routing DWDM facilitates the transport of massive volumes of data from a source to a destination. Once the data arrives at the destination, how- ever, it must be terminated and redirected to its final destination on a lambda-by-lambda basis. This is done with switching and routing tech- nologies. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw