work used by the Open ROADM team to disaggregate optical net- working functions and define four reference interfaces: G-the gray interface coming from a client. Standards already existed for this interface and no further work was needed. W-the single wavelength output of a transponder. MW-the multiwavelength output of a ROADM. API-the northbound application programming interface (API) to a management and con- trol subsystem. The northbound APIs from the photonic devices will connect into an SDN-based management and control architecture as shown in Figure 13.9. The SDN control will be layered-the lower layer will focus on the management of the L0 optical layer. A higher layer will focus on integrating the management of the packet and optical. Section 13.3 describes the multilayer control in more detail. The initial focus of the Open ROADM effort has been on 100G optical systems where there is broad industry adoption that has driven high volumes and competitive price points. One of the next steps is to investigate whether and how to extend open ROADM to both lower and higher speeds. One of the key design objectives was to remove the distributed proprietary functions that result in today's closed systems. In some cases (e.g., forward error correction [FEC]), this was accomplished by selecting and specifying a single implementation in the specification. In other cases, functions implemented on the hardware elements today were moved into the controller layer by making the management of the function accessible through the API. For example, the programming of opti- cal parameters such as amplifier output levels is done today through proprietary feedback loops built into the equipment. The new architecture provides hooks through the APIs to enable this to Multi-Layer AT&T SDN Controller Federated LO SDN Controller(s) Packet Packet ROADM ROADM ROADM Vendor A Vendor B Vendor C Plugg Plugga optics optics Transponder Transponder FIGURE 13.9 SDN control of packet/optical layer. Network Core happen from the SDN controller. Tod