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CHAPTER 12 ANALYSIS MODELING 323

Control
panel
Configure
User commands system
and data
Configuration
data
Interact Configure
with request Configuration information
user
Start Activate/ Configuration
Password stop deactivate data
system
A/d msg.

Process Display Control
password Valid ID msg. messages panel
and status Display display
Configuration information
data
Sensor
information Alarm
Monitor Alarm type
Sensors
Sensor sensors Telephone
status Telephone
number tones line

FIGURE 12.21 Level 1 DFD for SafeHome

sequent DFD. All nouns are either external entities (boxes), data or control objects
(arrows), or data stores (double lines). Note further that nouns and verbs can be
attached to one another (e.g., sensor is assigned number and type). Therefore, by
performing a grammatical parse on the processing narrative for a bubble at any DFD
Be certain that the level, we can generate much useful information about how to proceed with the refine-
processing narrative
you intend to parse is ment to the next level. Using this information, a level 1 DFD is shown in Figure 12.21.
written at the same The context level process shown in Figure 12.20 has been expanded into six processes
level of abstraction derived from an examination of the grammatical parse. Similarly, the information
throughout. flow between processes at level 1 has been derived from the parse.
It should be noted that information flow continuity is maintained between levels
0 and 1. Elaboration of the content of inputs and output at DFD levels 0 and 1 is post-
poned until Section 12.7.
The processes represented at DFD level 1 can be further refined into lower levels.
For example, the process monitor sensors can be refined into a level 2 DFD as shown
in Figure 12.22. Note once again that information flow continuity has been main-
tained between levels.