Infrastructure dependability

Infrastructure dependability, 2013 Slide 1

Critical National Infrastructure 2:

Infrastructure dependabilityIan Sommerville


Influences on dependability

• Infrastructure ownership

• Infrastructure funding

• Infrastructure dependencies


Infrastructure ownership

• No single owner or authority

• Mixture of public and private ownership

• Increasing privatisation


Infrastructure internationalisation

• Owners may be international companies located in different countries from the infrastructure that they own


UK rail privatisation

British rail


Regulation

• Government regulators are responsible for safety and have powers to enforce safety-related changes on companies


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



• 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


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)


Infrastructure replacement

• Across the developed world, ageing infrastructure built in the 1950s and 1960s is in need of replacement.

© Dennis Bratland



• 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


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


(Source: US DOE)


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


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


Cascade failure

• A failure in one area cascades to another infrastructure sector, causing failures and disruption, sometimes in quite unrelated areas


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.


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


www.geography.org.uk


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)


Single point of failure

• Transport, communication and power cables use the same bridge to cross a river or sea channel


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


Shared components

• Failure of a reused component can lead to failures in apparently unrelated parts of the infrastructure



• 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


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

Business

Infrastructure dependability