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A Data Flow Diagram (DFD) is a graphical representation of the “flow” of data and is the important preliminary step used to create an overview of the system and is elaborated by another, more detailed DFDs, each more granular, to visualize the complete view of the data flow. Lowest level DFD is the communications protocol diagram (CPD) which shows the protocol to be used and the media. Once all the data sources, destinations, and data flow attributes are understood, protocols are considered to support each flow. Protocol selection considers human to machine H2M or machine to machine M2M, source (standard development organization SDO, standard related organization SRO), proprietary/non-proprietary nature, and cyber defense properties. This fourth installment explains how protocols are often chosen for other reasons and may not support all the attributes identified in the DFDs and so it is necessary to revisit them and the design choices until the requirements are modified to match the protocols.