Which is used for communication directly to the SDN devices in the network?

The southbound interface (SBI) directly communicates with the SDN devices. This control is done via several different types of SBI protocols, such as OpenFlow, OpFlex, and CLI (Telnet/SSH). The northbound interface (NBI) is responsible for allowing communication between applications and the core of the controller. The core of the controller is the mechanism that connects the NBI to the SBI. Applications hosted on the controller interface with the NBI.