Chapter 2 Socio-technical Systems _Computer-based System Engineering__1_ by hcj


									                Chapter 2
     Socio-technical Systems
    (Computer-based System Engineering)

System Engineering
Is the activity of specifying, designing,
implementing, deploying, maintaining systems,
which include hardware, software , people and
interaction of the system with users and its

• To introduce the concept of emergent
  system properties such as reliability and

• To explain system engineering and system

           What is a system?
• A collection of inter-related components working
  together towards some common objective.

• A system may include software, mechanical,
  electrical and electronic hardware and be operated
  by people.

• System components are dependent on other
  system components

    What is a system? Cont…

• System is more than simply the sum of its

• It has a properties of the system as a whole.
  (emergent properties)

             System categories
• Socio-technical systems
    – Systems that include technical systems
      but also operational processes and people
      (knowledge) who use and interact with the
      technical system.

    – Socio-technical systems are governed by
      organizational policies and rules.

      Socio-technical system
• They have emergent properties
   – Properties of the system of a whole that
     depend on the system components and
     their relationships.

• They are often Non-deterministic
   – They do not always produce the same
     output when presented with the same
     input because the system’s behaviour is
     partially dependent on human operators.

    System categories , cont…
• Technical computer-based systems
    – Systems that include hardware and
      software but where the operators are not
      normally considered to be part of the

    – The system is not self-aware (Power point
      is not aware that is it being used to
      present Software Engineering course) .

      Problems of systems engineering
• Large systems are usually designed to solve
  'wicked' problems

• Systems engineering requires a great deal of
  coordination across disciplines
   – distrust and lack of understanding across
     engineering disciplines

• Systems must be designed to last many
  years in a changing environment
       Emergent System properties
• Properties of the system as a whole rather than
  properties that can be derived from the properties
  of components of a system.

• Emergent properties are a consequence (result) of
  the relationships between system components

• They can therefore only be assessed and
  measured ONCE the components have been
  integrated into a system
         Examples of emergent

• The reliability of the system
  – This depends on the reliability of system
    components and the relationships between the
• The usability of a system
   – This is a complex property which depends on
     the system operators and the environment
     where it is used.
       Types of emergent properties
• Functional properties
  – These appear when all the parts of a system
    work together to achieve some objective.

  – For example, a bicycle has the functional
    property of being a transportation device once it
    has been assembled from its components.

     Types of emergent properties
• Non-functional emergent properties
  – Examples are reliability, performance,
    safety, and security.

     – These relate to the behaviour of the
       system in its operational environment.

     – They are often critical for computer-based
       systems as failure that may make the
       system unusable.
       Complexity of emergent system properties-

• Because of component inter-dependencies, faults
  can be propagated through the system, so failure
  in one component can affect the operation of
  other components.

• System failures often occur because of
  unforeseen inter-relationships between

           Influences on a system
• Hardware reliability
   – What is the probability of a hardware component failing
     and how long does it take to repair that component?
• Software reliability
   – How likely is it that a software component will produce
     an incorrect output.
   – Software failure is usually distinct from hardware failure
     in that software does not wear out.
• Operator reliability
   – How likely is it that the operator of a system will make
      an error?
               Reliability relationships
• Hardware failure can generate a non genuine

• Software errors can cause alarms to be activated
  which cause operator stress and lead to operator
• The environment in which a system is installed can
  affect its reliability( a system design to operate in temp
  between 0 and 10 degrees will feel if the air condition
  broke, components will behave in unpredictable way)

• Properties such as performance, usability, and reliability
  are difficult to assess but can be measured after the
  system is operational.

• However, some properties such as safety and security
  pose a different problem, because it is very hard to predict
  all possible modes of access and explicitly forbid them.

   – Safety - the system that reflects the system’s ability to
     operate without danger
   – Security - the system should not permit unauthorised

• Measuring or assessing these properties is very hard

  The system engineering process
• Usually follows a ‘waterfall’ model because of the
  need for development of different parts of the

• There are important distinctions between the
  system engineering process and the software
  development process:

     The system engineering process
• Little scope for rework during system
  development because hardware changes are
  very expensive, reworking the system design
  to solve these problems. Software becomes
  so important in systems because of its
     • Example: siting of Base station in mobile cell

• Always involves engineers from different disciplines
  - interdisciplinary involvement- many engineering
  disciplines who must work together, much scope for
  misunderstanding here. Different disciplines use a
  different vocabulary and much negotiation is
  required. Engineers may have personal agendas to
         The system engineering
Requirements                                                                   System
 definition                                                                decommissioning

               System                                               System
               design                                              evolution

                        Sub-system                    System
                        development                 installation

     Inter-disciplinary involvement
            Air traffic control
  Software       Electronic    Mechanical
 engineering    engineering    engineering

  Structural    ATC systems    User interface
 engineering     engineering      design

    Civil        Electrical
 engineering    engineering
Before a software can be engineered, the system
in which it resides must be understood.

To accomplish this, the overall objective of the
system must be determined;
the hardware, software, people, databases,
procedures, and other system elements must be

system requirements must be elicited, analyzed,
specified, modeled, validated, and managed. All
the above activities are the foundation of system
Quote: the hardest part of building a
software system is deciding what to build

Advice: be sure you have assessed overall
feasibility before you expend effort and time
eliciting detailed requirements.

Advice: if different customers cannot agree
on requirements, the risk of failure is very
     System requirements definition
• System requirements
   – A structured document setting out detailed descriptions of
      the system services. Written as a contract between client
      and contractor
• Three types of requirement defined at this stage
   – Abstract functional requirements.
   The basic functions that the system must provide are defined
      in an abstract way
   – System properties.
     Non-functional requirements for the system in general
      are defined (availability, performance)
   – Desirable and Undesirable characteristics.
   24Unacceptable system behaviour is specified (what the
      system must not do)
Also, the system requirement must define overall
organizational objectives which the system should
Consider a system for an office building to provide for
fire protection and intruder detection.
A statement of objective might be:
• Functional objectives
   – To provide a fire and intruder alarm system for the
     building which will provide internal and external
     warning of fire or unauthorized intrusion

   System requirements problems:
• Complex systems are usually developed to
  address wicked problems
   – Problems that are not fully understood;
   Example : earthquake

  Can tackle the problem after it has happened

     The system design process, 1

  Partition                                                     Define sub-system
requirements                                                        interfaces

                 Identify                       Specify sub-system
               sub-system s                        functionality

                              Assign requirements
                                to sub-systems

    The system design process
• Partition requirements, organize
  requirements into related groups , HW Req.
  , SW Req.
• Identify sub-systems, identify a set of sub-
  systems which collectively can meet the
  system requirements

• Assign requirements to sub-systems,
  causes particular problems when are
  integrated , happen when purchase any
The system design process, cont…

• Specify sub-system functionality
  and relationships between them

• Define sub-system interfaces, if the
  interface has been agreed then a
  parallel sub-system development
  becomes possible.

           System design 1 problems:
• As a problems appear you often have to redo work
  done in earlier stages.

• Solution : Using spiral process , each round on
  the spiral may add more detail to design.

     Spiral model of requirements and design
                Spiral process 2


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