System Design

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System Design

• CFICSE 2002 SF09
• Michelle L. Crane

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References

• Roger S Pressman, Software Engineering A
  Practitioners Approach, Fifth Edition,
  McGraw-Hill, 2002
• Chapter 10

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Outline

• hierarchy and modeling
• restraining factors
• divide and conquer
• system design
• allocation
• iterative process
• trade-offs
• system modeling template

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Engineering System Hierarchy
Reference Pressman, Figure 10.1
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System Modeling

• system engineering is a modeling process
• create models that
• define the processes
• represent the behaviour of the processes
• explicitly define input to the model
• represent all linkages (including output)

Reference Pressman, Section 10.2.1
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Restraining Factors

• assumptions
• used to reduce the number of possible
  permutations and variations, enabling a model to
  reflect the problem in a reasonable manner
• simplifications
• enable the model to be created in a timely manner
• limitations
• help bound the system

Reference Pressman, Section 10.2.1
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Restraining Factors (contd)

• constraints
• guide the manner in which the model is created
  and the approach taken when the model is
  implemented
• preferences
• indicate the preferred architecture for all data,
  functions and technology
• preferred solution sometimes comes into conflict
  with other restraining factors
• customer satisfaction is often based on the
  degree to which the preferred approach is realized

Reference Pressman, Section 10.2.1
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Divide and Conquer (Decomposition)

• breaking a big problem into little ones
• determine the major computational components of
  the system
• decide which system functions will be
  accomplished by which elements of the system
• human
• hardware
• software
• this is allocation of functions

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Keep in Mind

• control complexity by dividing it into manageable
  units
• traceability of functions to subsystems
• subdivide further to implement functionality

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System Design

• allocation of functionality to type
• analysis of tradeoffs between competing criteria
• very much driven by the current state of
  technologies

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Allocating Functions to Modules

• in an ideal world, done only based on engineering
  criteria
• in reality, reasons for allocation often reflect
  the most serious constraints
• geographic
• environmental
• contractual
• legal
• project control
• criticality

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How to Allocate

• determine areas most likely to change
• high risk
• poor understanding
• new technology
• develop a trial subsystem definition
• high priority requirements first
• minimize external interfaces (coupling)
• group similar functions (cohesion)
• isolate areas of likely change (information
  hiding)
• limit complexity through partitioning
  (testability)
• target subsystems at particular expertise domains

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Breakdown is an Iterative Process

• examine the next level partitioning
• map back to a concept of operations (big picture)
• do scenario analysis
• look at verification and validation strategies
• have domain experts review subsystems
• examine alternatives

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Trade-offs

• trade-offs can be divided into categories
• project
• pre-established cost and schedule bounds
• risks associated with cost and schedule estimates
• business
• most profitable configuration (short and long
  term)
• market for system
• time to market
• competitors

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Trade-offs (contd)

• technical
• present and emerging technologies
• bleeding edge pain vs. market direction
• maintainability
• manufacturing and supply
• resource availability
• quality issues
• human issues
• availability of trained people
• human interfaces

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Trade-offs (contd)

• organizational politics
• security
• environment
• heat, cold, vibration, weathering
• interfaces to the world
• foreign systems, sensors, input/output data
• rates of response
• legal
• liability
• regulations
• intellectual property

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System Modeling
Processing and Control Functions
Input Processing
Output Processing
Reference Pressman, Section 10.6
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System Model Template
User Interface Processing
Processing and Control Functions
Input Processing
Output Processing
Maintenance Self-Test
Reference Pressman, Figure 10.5
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System Context Diagram
Reference Pressman, Figure 10.6
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Different Focuses
Reference Christensen Thayer, Table 1.2
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Different Focuses (contd)
Reference Christensen Thayer, Table 1.2
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Summary

• hierarchy and modeling
• restraining factors
• divide and conquer
• system design
• allocation
• iterative process
• trade-offs
• system modeling template

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