Figure 1 is the basis for a four-step process for
constructing a requirements specification. The first
step creates the System Requirements Specification
(SRS), which describes the required external
behavior of the system in terms of monitored and
controlled quantities in the system environment. We
refer to the behavior described by the SRS as the
"ideal" system behavior because it omits any
mention of I/O devices and hardware failures. The
remaining steps refine and extend the SRS. Step 2
creates the System Design Specification (SDS),
which identifies the system's input and output
devices (e.g., sensors and actuators). Step 3 creates
the Software Requirements Specification (SoRS),
which refines the SRS by adding modules which
use values read from input devices to calculate
values of the monitored quantities and which use
the computed values of controlled quantities to
drive output devices. Step 4 extends the SRS by
adding behavior that reports hardware malfunctions,
e.g., sensor failures.
1 .