Design of any system should always be preceded by a formal determination of the business and corresponding technical requirements that drive the design. Such a formal statement is known as a “functional requirements specification.”
Functional requirements capture the intended behavior of the system. This behavior can be expressed as services, tasks, or functions the system is required to perform.
In the case of SCADA, the specification contains such information as system status points to be monitored, desired control points, and analog quantities to be monitored. It also includes identification of acceptable delays between when an event happens and when it is reported, required precision for analog quantities, and acceptable reliability levels.
The functional-requirements analysis will also include a determination of the number of remote points to be monitored and controlled. It should also include identification of communication stakeholders other than the control center, such as maintenance engineers and system planners who may need communication with the substation for reasons other than real-time operating functionality.
The functional-requirements analysis should also include a formal recognition of the physical, electrical, communications, and security environment in which the communications are expected to operate.
Considerations here include recognizing the possible (likely) existence of electromagnetic interference from nearby power systems, identifying available communications facilities, identifying functionally the locations between which communications are expected to take place, and identifying potential communication security threats to the system.