h operating at 100 Gb/s onto a single fiber pair and transport the composite signal several thousands of kilometers purely in the photonic domain. Higher speeds and longer distances that tradeoff one parameter for another are possible (e.g., higher speed wavelengths can be supported at shorter dis- tances or with lower channel counts). Router B Router A Router C Router B Outside plant fiber Outside plant fiber Optical Optical Optical Router A Transport Transport Transport Router C System System System Office A Up to 96 100Gbps signals Office C on single fiber pair Office B OA's every 80-100 km 1000's of km between offices FIGURE 13.3 Functions of an optical transport system. Network Core Client pluggables Coherent pluggables Optical Transport System xponder Optical Add/Drop xponder E/W fiber carrying three N's combiner Standard client Colored Router Fixed signals wavelengths ROADM splitter xponder Optical splitter Add/Drop xponder combiner Office A N/S fiber carrying three N's FIGURE 13.4 Block diagram of a fixed optical transport system. Figure 13.4 drills down the next level and shows a high-level block diagram of the subsystems within today's fixed optical transport systems. The transponder (xponder) maps a client interface* that uses a standard wavelength (also referred to as "gray" optics) to a unique "colored" wavelength. This is done to condi- tion the optical signal SO that it can be combined with other wavelengths for transport over a single fiber. The wavelengths are combined using a technique known as dense wave division multiplexing-which defines standards for spacing and packing the wave- lengths. For example, the router might transmit a signal with a standard wavelength of 1310 nms on each of its interfaces toward each transponder. The output of the blue tran- sponder might be a wavelength of 1529.55 nms while the green wavelength might be 1530.33 nms. Transponders perform optical-electrical-optical (OEO) conversions as the wavelength shifting function must be performed in the electrical domain. Eac