atibility with older applications and equipment. This is particularly true in large conglomerates often made up of a variety of subsidiaries and departments, many of which have different applications and requirements. Features of Software-Defined Networks As the name implies, software-defined networks are controlled by software rather than hardware. The software is located in a central controller in off-the-shelf, general-purpose computers. The controller has policies in software that define how packets are forwarded. The control plane distributes policies that the controller transmits to it. The data plane is made up of tunnels that carry data to endpoints. Endpoints are also referred to as nodes, devices connected to the LAN. The key advantage of distributing policies to endpoints is that endpoints do not need to continuously communicate with a central controller because the policies are forwarded as they change with instructions on how to treat data that is transmitted by the endpoints. These policies have instructions on how to treat traffic during peak times. The endpoints do not need to send data back to a central point for instructions because they are automatically sent to the endpoints whenever there are changes in policies. Thus, LANs can respond dynamically in real time, to changing traffic patterns, congestion, and outages. For example, if one route is congested or out of service, data is automatically transmitted on other routes. www.TechnicalPDF.com Dependence on a Central Controller-The Need for Redundancy Controller redundancy is important in SDNs because no new policies can be transmitted to endpoints if the links to the central controller or the central controller itself is out of service. Once service is reestablished, controllers automatically transmit policies that were programmed during the outage to endpoints. Flexibility Changes can be implemented rapidly because many changes in software-defined networks do not require additional hardware. Changes are made by programming new policies in ce