volve independently of hardware and to benefit from a declining cost curve of commodity computing infrastructure. In the new architecture, PNF are supplanted by VNF that run on NFVI, which is the heart of Network Cloud. Key drivers for the platform are the ability to utilize open source software, create sharable resources, and provide for the use of modern implementation practices such as continuous Building the Network of the Future integration/continuous deployment (CI/CD). The Open Stack environment is an obvious choice for NFVI. Open Stack provides the functionality to schedule compute tasks on servers and administrate the environment. In addition, it provides a fairly extensive set of features; this is discussed in more detail in Chapter 4. 2.5.3 SOFTWARE-DEFINED NETWORKING While the general perception is that SDN for networks is a relatively new phenomenon with roots in data centers, the reality is that telecom networks have been using these techniques for several decades. An example is the use of techniques to control the data plane in circuit-switched networks, via the control plane from a controller, to provide 800 number services (for long-distance automatic routing of calls and reverse billing). In the Network Cloud, SDN is further advanced by applying these learnings and those from cloud data centers, to create a layered distributed control framework. A "master" (or global) con- troller that has a network-wide view of connectivity, controls subsidiary controllers associated with different functional parts of the network. Each controller may also be replicated for scale. For the IP/Multiple Protocol Label Switching (MPLS) network fabric, the Network Cloud uses a hybrid approach retaining distributed control planes and protocols for responsiveness and using the cen- tralized control plane for optimization and complex control. One very unique aspect of Network Cloud SDN is the shift to model-driven software and net- work configuration. In the past, these tasks were done by creating detailed requirements 