to use network as a platform to expand their service models to include third-party applications. This helps expand the current business mod- els. Rapid development and deployment of not only in-house services but also third-party applications from other content providers is important for the variety and quality of new network services. Third, it can enable flexible and efficient network virtualization. When operating different network services, for example, virtual pri- vate networks (VPNs), video network, service providers usually deploy an overlay network for each type of network. For example, they have one network for wireline application, another one for wireless, and another for business applications. As we move toward converged networks, the goal is to design one network for many services by slicing this physical network into multiple virtual networks, one for each type of application. 4.1.2 Architecture Given the significant interest because of the key drivers, an increas- ing amount of attention has been raised on SDN. However, until now, there has been no consensus on the principles and concepts of SDN. In the following, we take the initiative to summarize the key aspects of SDN from service providers' perspectives and the four main concepts that describe the SDN in service provider networks. Figure 4.1 illus- trates the overview architecture and four concepts. Figure 4.2 shows the components in each layer in an SDN architecture. 4.1.2.1 Separation of Control and Data Plane The separation and centralization of the control plane software from the packet-forwarding data plane is one of the core principles of SDN. This differs from the traditional distributed system architecture in 4 Software-Defined Networks Principles and Applications Northbound open API Figure 4.1 SDN architecture. Centralized control plane Data plane network oftrace oflops openseer Monitoring tools ENVI (GUI) n-Casting Applications Beacon Trema FloodLight Controller FlowVisor console FlowVisor Virtualization Software Broadcom NetFPGA HP