h O-E con- version requires lasers and photodetectors, which are costly components; hence, the use of unnecessary transponders is to be avoided. The add/drop mux combines multiple wavelengths coming from separate transponders into a single photonic signal. In the figure, the top add/drop mux is combining the blue and green wavelengths for transport over the E/W fiber. The add/drop mux on the bottom combines the blue and red wavelengths for transport over the N/S fiber. If an optical spectrum analyzer was used to look at the optical signal on an outgoing fiber, what would be observed is a composite signal with multiple wavelengths all propagating the length of the fiber. Because they operate at different wavelengths, they do not interfere with each other and filters can be used at the far end to separate the constituent signals. A fixed reconfigurable optical add/drop multiplexer (ROADM) provides the ability to selectively switch specific wavelengths coming in on one fiber to exit via another fiber. Note the violet wavelength in the picture that comes in the N/S fiber and gets switched A router was chosen to illustrate the role of the client. The optical network also supports dedicated services in which case the client would be a customer CPE, which could be a router, Ethernet switch, OTN switch, etc. The term "gray" is used to indicate that if a human could observe the light signal it would have the same color on all client interfaces. "Colored" means each would be seen as a unique color. In reality, the wavelengths used for optical com- munication are slightly outside those visible to humans. The term ROADM is sometimes used to refer to the entire optical transport system and other times more narrowly to refer to the photonic switching function. Here, we use the latter definition. Building the Network of the Future to go out the E/W fiber and vice versa. This is referred to as "expressing" the wavelength through the office. The splitters/combiners allow us to combine express and add/drop wavelengths onto the fib