The desired system behavior is documented in the SRS by describing two relations, NAT and
REQ, on the monitored and controlled quantities; these relations are part of the Parnas Four
Variable Model [6]. The relation NAT describes the constraints imposed on the environmental
quantities by physical laws and the system environment. REQ describes the relation between the
monitored and controlled quantities that the system must enforce to produce the required
behavior. In developing the SRS, we initially specify REQ in terms of the ideal behavior of the
system; that is, we assume that the system can obtain perfect values of the monitored quantities
and compute perfect values of the controlled quantities. Later, for each controlled variable, we
specify timing constraints (and possibly tolerances).