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Welcome back! In the previous lecture we discussed
the importance of standardization in systems-of-systems.

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Now, we'll focus on the interconnections.

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In tightly coupled interconnected systems,
a disturbance happening in some part of the

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system may have far reaching consequences.

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If the disturbance cannot be contained and
remediated at the local level,

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it will cascade through the network and affect
the service level in a wide region.

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This is what happens in a power blackout,
and if such a complete blackout occurs,

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it is hard to restart the system again because
of the tight couplings between subsystems.

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In countries where the supply of electricity
cannot keep up with increasing demand,

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system operators will try to prevent blackouts
by planned demand curtailment in parts of the network.

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This is what is called a brownout.

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The system operator may inform you that your
neighborhood will be out of power for certain

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hours on certain days,
and during the hours that your neighborhood

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is supplied, other neighborhoods will be curtailed.
And you know from experience that

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being out of power severely limits you in
doing the things you want to do

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and I think all of us understand
how detrimental it is to the economy.

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This is evident when you think about all the
economic activities that depend on electricity.

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The electricity infrastructure is a so-called
critical infrastructure.

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If malfunctioning, 
society and the economy will be disrupted.

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Electrified trains will come to a stand-still.

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At this point,
let me take the notion of systems-of-systems

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to another level,
that is, to the level of interconnections

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and interactions between infrastructures in
different sectors.

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The definition of critical infrastructure
varies between countries,

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but in all cases includes electricity and
telecommunications as "most" critical infrastructures,

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which all other critical infrastructures depend upon.

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Emergency services,
financial services,

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public health services,
agriculture and food supply and many other

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sectors,
including government,

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cannot be maintained for any sustained period
of time without electricity and telecommunication.

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However, electricity and telecom infrastructures
themselves use resources that are,

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in part, provided by other infrastructures.

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To supply telecom services,
the telecommunication system uses electricity,

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water and natural gas,
to name just a few of the required infrastructure

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related services.

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At the same time,
the electricity infrastructure uses telecom services

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but it also uses water, to name few of the resources
that are vital to the electricity infrastructure.

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In other words,
there is a high level of interdependency between

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infrastructure systems across different sectors.

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These interdependencies come in various types:

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Physical (or functional) interdependencies
exist when resources are exchanged: when the

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resources produced by some infrastructure
are used by another infrastructure to produce

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its own resources and the other way around.

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Geographic interdependencies occur as a consequence
of geographic proximity between infrastructures,

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so that a problem (for example a fire or a
pipeline burst) in one infrastructure can

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adversely affect other infrastructure.

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Digital interdependencies are related to
the exchange of data on computer systems,

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or to sharing data on control systems.

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And then we have logical interdependencies
and organizational dependencies,

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which are related to organizational context
or to market realities.

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The web of interdependencies between critical
and not-so-critical infrastructures creates

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a highly complex system-of-systems,
in which the failure of a single element or

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a single resource may cause a domino effect
that affects all critical infrastructures

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in a region and could bring the entire region
to a stand-still.

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Many governments have been working and are
still working on strategies to reduce their

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vulnerability to outages of critical infrastructures.

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Critical functions such as hospitals are very
well aware of their dependence on electricity

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and have installed backup generation capacity,
such as batteries or diesel fueled generators,

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to keep going in the event of a power blackout.

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However, in the case of a prolonged outage,
the supply of diesel will become a bottleneck.

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You can imagine that a study of those interdependencies,
from first order to second order interdependencies

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and so on,
is a hell of a job,

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and that these interdependencies vary widely
between different locations.

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For example, whereas in some places,
such as in the Netherlands, the entire drinking

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water system needs electrical pumps,
it is gravity based in other places,

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and such places would therefore be
less vulnerable to power outages.

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at least as far as drinking water supply is concerned.

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The vulnerability of the system-of-infrastructure
systems can be reduced in many ways,

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for example by keeping strategic reserves
of critical resources,

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by building redundancy in critical conversion
units (for example by installing

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parallel units,
with one always in the stand-by mode),

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or by building redundancy in the transportation
network in such a way that,

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if one path gets blocked, the service can still
be delivered through an alternative pathway.

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The latter strategy refers to the network topology,
about which you will learn more next weeks.

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Please note that all these measures
require substantial investment,

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which makes it unlikely that such measures
will be taken just for the sake of a very

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low probability event,
even if the consequences may be disastrous.

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The system-of-systems character
of infrastructure systems,

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both within and across infrastructure sectors,
implies that strategies to avoid or recover

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from a breakdown require a
multitude of actors to interact,

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including infrastructure owners and operators,
producers of infrastructure resources,

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service providers and government.

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Some of these actors will probably be based
in the private sector,

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others in the public sector,
implying that they are subject to different

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incentive systems and possibly different laws.

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Especially since most infrastructure providers
are incentivized to pursue their own economic interest,

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the interdependencies across infrastructure
sectors are unlikely to be properly managed

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without government intervention.

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I hope this gave you something to think about.
Thank You!