Fundamentals of DNP3

This chapter focuses on the fundamentals of distributed network protocol (DNP3). DNP3 is based on enhanced performance architecture (EPA), a three-layer model. The layers used in EPA model are the two hardware layers and the top software layer, the application layer. DNP3 uses these three layers, but adds some transport functions. These are often referred to as the pseudo-transport layer, and are sometimes depicted as corresponding to the transport and network layers in a limited manner. The pseudo-transport layer is included in DNP3 to allow for the transmission of larger blocks of data than could otherwise be handled. The application layer is the level where the data is generated for sending, or requested to be sent. The layer interfaces to the lower levels in order to transmit the required information. In turn, the DNP3 application layer provides its services to the user application programs, such as an HMI system, an RTU, or other system. DNP3 supports either master-slave or peer-to-peer communications, with either one-on-one or multi-drop topologies. Point-to-point or direct topology refers to the case of two DNP3 devices connected together, either directly with a cable, or via modems and a communications path. This could be via a leased line, via radios, or via the public switched telephone network (PSTN). A serial bus topology is the alternative to a direct topology. This may also be referred to as multi-drop. In this case, multiple devices are connected to the same communications path.