 an interface between external software and each device on the network that is configured for SDN. The API translates between external software on controllers and each device or service on the NFV server. NFV is a key element of WA SDNs. The APIs used in WA SDNs are Open APIs. Open APIs are those which can be used by the public. They enable customers to access features in SDN from enterprises' third-party applications. Private APIs give certain users access to only specified applications. Private APIs are most often deployed in enterprises to give access to particular parts of their network to contractors or employees. For information on NFV see the section "Network Function Virtualization (NFV)- Architecture" in Chapter 4. Companies that supply hardware and software for SDNs include: ADVA, Advantech, Cisco, Ciena, Citrix, Dell, Riverbed Technology, VeloCloud, Versa Networks, and Viptela. www.TechnicalPDF.com Control Plane VS. Data Plane Signals All networks need to transmit both control plane and data plane signals. Control plane signals include addressing, routing, and billing signals. The data plane carries actual applications or user data such as cash register receipts to retailers' headquarters, database look-up requests, applications, and PowerPoint presentations. In newer networks, data and control signals are sent separately SO that the network can route them individually as needed to servers for billing and other functions. In SDN networks addressing, billing, and other overhead bits are transmitted in the control plane and applications and user data on the data plane. Control plane signals and data plane signals are transmitted within both wireline and mobile networks. In SDNs, the controller transmits control plane bits identifying billing and network identification. The actual data and configuration updates and changes are transmitted on the data plane. On SIP trunks, control and data plane signals are also treated separately. Although control and data plane signals are treated individually, both are 