SDN is a new approach to designing, building and managing networks that decouples the network's data plane from the control plane.
SDN centralizes the network control by separating control logic to off-device computers, requiring you to have a method of communication between the two planes, data and control. With SDN you have a high level of flexibility in terms of dictating switches and routers, and how the forwarding plane handles network traffic and data flow through the network.
Network TAPs play a critical role in the design of a virtual network such as SDN. Visibility is required for both the data plane and control plane. A Network TAP is placed between the ends of the network to pass complete packet copies of traffic flowing between the control and forwarding devices/software.
As bandwidth has increased and applications have become more complex, the reality of SDN has become apparent. In large enterprises with physical connections spread out over a number of remote locations, SDN permits the manipulation of traffic and data flow from a single dashboard.