1. Infrastructure dependability, 2013 Slide 1
Critical National Infrastructure 2:
Infrastructure dependability
Ian Sommerville

2. Infrastructure dependability, 2013 Slide 2
Influences on dependability
• Infrastructure ownership
• Infrastructure funding
• Infrastructure dependencies

3. Infrastructure dependability, 2013 Slide 3
Infrastructure ownership
• No single owner or authority
• Mixture of public and private ownership
• Increasing privatisation

4. Infrastructure dependability, 2013 Slide 4
Infrastructure
internationalisation
• Owners may be
international
companies located
in different
countries from the
infrastructure that
they own

5. Infrastructure dependability, 2013 Slide 5
UK rail privatisation
British rail

6. Infrastructure dependability, 2013 Slide 6
Regulation
• Government
regulators are
responsible for
safety and have
powers to enforce
safety-related
changes on
companies

7. Infrastructure dependability, 2013 Slide 7
Dependability consequences
• No single organisation is responsible
for overall system dependability and for
liaison with other infrastructure
providers
• Cost and budgets rather than safety are
the key drivers as company
shareholders have to be satisfied

8. Infrastructure dependability, 2013 Slide 8
Dependability consequences
• Companies try to avoid responsibility for
incidents and accidents as these mean
that compensation may have to be paid
to other companies or individuals
• There is a tendency to try to blame
other companies when things go wrong

9. Infrastructure dependability, 2013 Slide 9
Infrastructure funding
• Infrastructure
replacement is very
expensive
• Governments and
companies are often
reluctant to pay for
infrastructure
©Stefan Kuhn (flickr)
© UK Highways Agency (via Jimmy Guano)

10. Infrastructure dependability, 2013 Slide 10
Infrastructure replacement
• Across the developed world, ageing infrastructure
built in the 1950s and 1960s is in need of
replacement.
© Dennis Bratland

11. Infrastructure dependability, 2013 Slide 11
Dependability consequences
• Because of the high costs, there is a
tendency to delay infrastructure
investment and maintenance but to wait
until failures occur and then repair the
infrastructure
• This leads to longer periods of
unavailability and disruption for users of
the infrastructure

12. Infrastructure dependability, 2013 Slide 12
Infrastructure dependencies
• Infrastructure elements are often
dependent on each other so that
failure of one element may be caused
by failure of some other element
elsewhere

13. Infrastructure dependability, 2013 Slide 13
(Source: US DOE)

14. Infrastructure dependability, 2013 Slide 14
Types of dependency
• Functional
– Use of common functions by different
elements of the infrastructure
• Informational
– Decision making may depend on data from
the same source
• Shared Control
– Control is from the same system/location

15. Infrastructure dependability, 2013 Slide 15
Types of dependency
• Geospatial
– Components are located physically close to each
other
• Purpose
– Elements of the infrastructure may have a shared
function or purpose so components may be reused
• Policy
– Different elements of the infrastructure may be
governed by the same policies

16. Infrastructure dependability, 2013 Slide 16
Cascade failure
• A failure in one area cascades to
another infrastructure sector, causing
failures and disruption, sometimes in
quite unrelated areas

17. Infrastructure dependability, 2013 Slide 17
Cascade failure
• Failure of flood defences results in an electricity
distribution centre being threatened by flooding
• Shut down for safety reasons
• Leads to a loss of power in the local area
• An ISP in that area goes offline, leading to a loss of
internet service to customers who may be anywhere
in the country.

18. Infrastructure dependability, 2013 Slide 18
Mutual dependencies
• Mutual dependencies cause major
problems in the event of failure
– Communications systems depend on
power
– Power recovery depends on
communications
– Power failure that leads to loss of
communications is difficult to recover from

19. Infrastructure dependability, 2013 Slide 19
www.geography.org.uk

20. Infrastructure dependability, 2013 Slide 20
Single points of failure
• Several services
share some facility
so that failure at
that facility leads to
a loss of all of these
services.
• Power and
communication
cables in the same
duct
© Martin Pettitt (2006)

21. Infrastructure dependability, 2013 Slide 21
Single point of failure
• Transport, communication and
power cables use the same
bridge to cross a river or sea
channel

22. Infrastructure dependability, 2013 Slide 22
Shared system components
• Control systems for different
infrastructure elements may be supplied
by the same company
• Software components may be reused
across systems for different types of
infrastructure control

23. Infrastructure dependability, 2013 Slide 23
Shared components
• Failure of a reused component can lead
to failures in apparently unrelated parts
of the infrastructure

24. Infrastructure dependability, 2013 Slide 24
Dependability consequences
• Infrastructure systems are not
independent
• Assessment of the dependability of one
part of the infrastructure has to consider
the dependability of other infrastructure
elements
• Dependencies may be unknown
• Contingencies for the failure of these
other elements may have to be made

25. Infrastructure dependability, 2013 Slide 25
Key points
• The fragmented ownership of national
infrastructure has overall effects on its
dependability
• Infrastructure elements are rarely
independent but depend on other elements of
the national infrastructure
• When considering the dependability of part of
the infrastructure, you have to take into
account its possible dependencies with other
parts of the infrastructure and their
dependability