er. So, for example, the optical signal on the E/W fiber contains the blue, green, and violet wavelengths. The blue and green were added in this office via the add/drop mux and combined with the violet signal which came in from the N/S fiber via the ROADM. At the far end, the splitter replicates the incoming signal to both the add/drop mux and the ROADM. The add/drop mux separates the wavelengths intended to be dropped in this office back into the constituent wavelengths and sends each to the appropriately colored transponder for conversion back to the standard gray wavelength. The ROADM would switch any wavelengths intended to be expressed through this office to the appropriate outgoing fiber. For simplicity, we have shown a ROADM supporting two outgoing fibers but in practice higher degree* ROADM's are deployed. Early dense wave division multiplexing (DWDM) systems literally had a separate stock keeping unit (SKU-the code used to identify an individually orderable part) for each color of transponder. The second generation (early 2000s) brought the advent of tunable lasers, which allowed for the deployment of a common transponder that could then be configured through the management interface to the appropriate color. As explained, ROADMs allow us to add, drop, or express wavelengths in a central office. To illustrate the value of expressing a wavelength, consider a case where a 100 Gb/s optical signal originates in office A, traverses through office B, and terminates in office C. Fixed DWDM optical systems that only have multiplexors must terminate all optical signals in office B, bring the 100 Gb/s optical signal through a transponder back to a standard client signal, and then connect to a second transponder in that office that is part of the line system that connects office B to office C. As shown in Figure 13.5, these back-to-back transponder configurations add extra equipment and cost to the circuit. A ROADM allows us to express the optical signal through office B in the photonic domain without having to pur