The document discusses the characteristics of critical systems. Critical systems must have high dependability, meaning they must be reliable, available, safe, and secure. They are often systems of systems that rely on external infrastructure and other systems. Critical systems are important to society and can include safety-critical systems, mission-critical systems, business-critical systems, and infrastructure systems. Dependability is the most important attribute of critical systems.

1. Critical systems video, 2013 Slide 1
Critical systems

2. Critical systems video, 2013 Slide 2
Critical system essentials
Reliability
The system must operate without
serious failures
Availability
The system must be available to
deliver services when requested
to
do so
Safety
The system should not harm people or
the system’s environment
Security
The system must be able to protect itself
and its data from malicious use

3. Critical systems video, 2013 Slide 3
Critical system essentials
Safety
The system should not harm
people or the system’s
environment

4. Critical systems video, 2013 Slide 4
Critical system essentials
Reliability
The system must operate
without
serious failures

5. Critical systems video, 2013 Slide 5
Critical system essentials
Availability
The system must be available
to deliver services when
requested to do so

6. Critical systems video, 2013 Slide 6
Critical system essentials
Security
The system must be able to
protect itself and its data from
malicious use

7. Critical systems video, 2013 Slide 7
Dependability
Reliability
The system must operate without
serious failures
Availability
The system must be available to
deliver services when requested
to
do so
Safety
The system should not harm people or
the system’s environment
Security
The system must be able to protect itself
and its data from malicious use

8. Critical systems video, 2013 Slide 8
Classes of critical system
• Safety-critical systems
– Failure results in loss of life, injury or
damage to the environment e.g.
chemical plant protection system;
• Mission-critical systems
– Failure results in failure of some goal-
directed activity e.g. spacecraft
navigation system;

9. Critical systems video, 2013 Slide 9
Classes of critical system
• Business-critical systems
– Failure results in high economic
losses e.g. customer accounting
system in a bank;
• Infrastructure systems
– Failure results in a loss of
infrastructure capability e.g. power
distribution control system, broadband
communications, etc.

10. Critical systems video, 2013 Slide 10
Critical systems stack
Physical infrastructure
Infrastructure systems
System hardware
Operating system and middleware
External systemsCritical system
X

11. Critical systems video, 2013 Slide 11
Independent critical systems
• Infrastructure/hardware is part of
the system
• System operation is not dependent
on external systems
• Embedded control systems such as
those in medical devices

12. Critical systems video, 2013 Slide 12
Critical software systems
• Usually rely on commodity
hardware/OS
• System operation is dependent on
external infrastructure provision
• Hospital appointments system

13. Critical systems video, 2013 Slide 13
Systems of systems
• A critical system is rarely a
single system but is a network
of several software-intensive
systems as well as
infrastructure systems

14. Critical systems video, 2013 Slide 14
Systems of systems
• Systems of systems (SoS) are
complex socio-technical systems
with
– Different owners and management
policies
– Distributed operation
– Heterogeneous hardware and
software

15. Critical systems video, 2013 Slide 15
Systems of systems
• Individual systems may be part of
several SoS so
– Conflicting requirements from different
uses of the system
– Complex negotations may be required
when system changes are to be made

16. Critical systems video, 2013 Slide 16
Systems of systems
• Systems that support organisational
needs (e.g. an inter-bank payments
system) have to be designed to be
robust so that they can cope with
failures and unavailability in the
other systems on which they
depend

17. Critical systems video, 2013 Slide 17
Summary
• Critical systems are usually systems of
systems
• Society depends on a range of critical
systems from control systems to infrastructure
systems
• The more important characteristic of a critical
system is its dependability, where
dependability attributes are safety, reliability,
availability and security

18. Critical systems video, 2013 Slide 18
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