Page 199 - Software and Systems Requirements Engineering in Practice

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C C h a p t e r 5 : Q Q u a l i t y A t t r i b u t e R e q u i r e m e n t s 165
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Operator
Standard
Operating Executed by
Procedure
*
* *
Facilities
Acknowledges 1
Handles 1 Person
1
Defines
* * *
Alarm Generated by Rule
* 1 Issues
0.1 1
Generated by Issues
* *
Field Contains Field Configured by Command
System Device
1 * 1 * *
FIGURE 5.8 Domain model
scenario just described. Once the model is created, the computational
needs of the software and hardware resources are determined. Finally,
this model is evaluated against the specified performance objectives.
The purpose of this exercise is to ensure the proposed architecture meets
the stipulated performance objectives and explore alternatives if any
serious design flaws are discovered. In some cases, a simulation of the
system performance is created in addition to or instead of a performance
model; e.g., for a nuclear reactor, simulation may be the only way to
verify that the design meets the requirements prior to construction.
Figure 5.9 shows the key end-to-end scenario or workflow for the
building automation system. Many field systems concurrently
transmit data to the virtual field system. The virtual field system
processes the raw data and persists it to a database after gaining
secure access through the access control component. This data is then
made available for analysis by the alarm subsystem, and when alarms
are detected, they are reported to the monitoring clients for the
facilities manager and the public safety system for the public safety
officials. This execution snapshot can be used as a basis for creating a
performance model when sufficient information is available on data
volumes, data arrival rates, and processing requirements of the
individual software elements shown in this figure.