very bit interval and if it exceeded a threshold it declared a 1, if not a 0. Nonlinear effects such as chromatic dispersion were corrected by inserting coiled sections of fiber into the data path in the transponder that were very specially engineered to counteract the dispersion of the long haul fiber. This was bulky, costly, and added up to 10% latency. Coherent optics uses sophisticated waveform modulation, such as quadrature amplitude modula- tion (QAM), to increase the rate of information transmitted. At the receiver, a local oscillator mixes the incoming signal into the baseband, then the baseband signal gets digitized with high-speed analog to digital converters (ADC) and all the impairment compensation and signal regeneration is done using purpose-built high-speed digital signal processors (DSPs). There are two types of line side coherent pluggables: digital coherent optics (DCO), where the DSP function is included inside the pluggable module and analog coherent optics (ACO), where the DSP is outside the pluggable optics. As a rule, coherent pluggables are larger than the client plug- gables, because the output power of the transmitter is hotter. While DCO modules usually have a larger footprint than ACO due to the additional power-hungry DSP function inside, ACO modules have an external analog high-frequency interface that requires special engineering and sacrifices performance. 13.1.2 FLEXIBLE SOFTWARE-CONTROLLED OPTICAL NETWORKS Currently deployed ROADMs are referred to as being fixed. With today's technologies, each add/ drop port has a fixed color and direction. Once a transponder is cabled to an add/drop port, the color and direction of that wavelength are determined. Referring to Figure 11.4, to reconfigure the ROADM to allow a wavelength currently going out the E/W interface to instead egress the N/S fiber, cables would need to be physically moved from the upper transponders to connect to the lower add/drop mux ports. Note that the current design also creates process and system complexity as manua