<s>
In	O
systems	O
engineering	O
,	O
dependability	B-Application
is	O
a	O
measure	O
of	O
a	O
system	O
's	O
availability	O
,	O
reliability	O
,	O
maintainability	O
,	O
and	O
in	O
some	O
cases	O
,	O
other	O
characteristics	O
such	O
as	O
durability	O
,	O
safety	O
and	O
security	O
.	O
</s>
<s>
In	O
real-time	B-General_Concept
computing	I-General_Concept
,	O
dependability	B-Application
is	O
the	O
ability	O
to	O
provide	O
services	O
that	O
can	O
be	O
trusted	O
within	O
a	O
time-period	O
.	O
</s>
<s>
The	O
International	O
Electrotechnical	O
Commission	O
(	O
IEC	O
)	O
,	O
via	O
its	O
Technical	O
Committee	O
TC	O
56	O
develops	O
and	O
maintains	O
international	O
standards	O
that	O
provide	O
systematic	O
methods	O
and	O
tools	O
for	O
dependability	B-Application
assessment	O
and	O
management	O
of	O
equipment	O
,	O
services	O
,	O
and	O
systems	O
throughout	O
their	O
life	O
cycles	O
.	O
</s>
<s>
The	O
IFIP	O
Working	O
Group	O
10.4	O
on	O
"	O
Dependable	O
Computing	O
and	O
Fault	B-General_Concept
Tolerance	I-General_Concept
"	O
plays	O
a	O
role	O
in	O
synthesizing	O
the	O
technical	O
community	O
's	O
progress	O
in	O
the	O
field	O
and	O
organizes	O
two	O
workshops	O
each	O
year	O
to	O
disseminate	O
the	O
results	O
.	O
</s>
<s>
Dependability	B-Application
can	O
be	O
broken	O
down	O
into	O
three	O
elements	O
:	O
</s>
<s>
But	O
the	O
word	O
predates	O
that	O
period	O
,	O
with	O
the	B-Application
Oxford	I-Application
English	I-Application
Dictionary	I-Application
finding	O
its	O
first	O
use	O
in	O
1901	O
.	O
</s>
<s>
As	O
interest	O
in	O
fault	B-General_Concept
tolerance	I-General_Concept
and	O
system	O
reliability	O
increased	O
in	O
the	O
1960s	O
and	O
1970s	O
,	O
dependability	B-Application
came	O
to	O
be	O
a	O
measure	O
of	O
 [ x ] 	O
as	O
measures	O
of	O
reliability	O
came	O
to	O
encompass	O
additional	O
measures	O
like	O
safety	O
and	O
integrity	O
.	O
</s>
<s>
In	O
the	O
early	O
1980s	O
,	O
Jean-Claude	O
Laprie	O
thus	O
chose	O
dependability	B-Application
as	O
the	O
term	O
to	O
encompass	O
studies	O
of	O
fault	B-General_Concept
tolerance	I-General_Concept
and	O
system	O
reliability	O
without	O
the	O
extension	O
of	O
meaning	O
inherent	O
in	O
reliability	O
.	O
</s>
<s>
The	O
field	O
of	O
dependability	B-Application
has	O
evolved	O
from	O
these	O
beginnings	O
to	O
be	O
an	O
internationally	O
active	O
field	O
of	O
research	O
fostered	O
by	O
a	O
number	O
of	O
prominent	O
international	O
conferences	O
,	O
notably	O
the	O
International	O
Conference	O
on	O
Dependable	B-Application
Systems	I-Application
and	O
Networks	O
,	O
the	O
International	B-Protocol
Symposium	I-Protocol
on	I-Protocol
Reliable	I-Protocol
Distributed	I-Protocol
Systems	I-Protocol
and	O
the	O
International	B-Application
Symposium	I-Application
on	I-Application
Software	I-Application
Reliability	I-Application
Engineering	I-Application
.	O
</s>
<s>
Traditionally	O
,	O
dependability	B-Application
for	O
a	O
system	O
incorporates	O
availability	O
,	O
reliability	O
,	O
maintainability	O
but	O
since	O
the	O
1980s	O
,	O
safety	O
and	O
security	O
have	O
been	O
added	O
to	O
measures	O
of	O
dependability	B-Application
.	O
</s>
<s>
These	O
can	O
be	O
assessed	O
to	O
determine	O
its	O
overall	O
dependability	B-Application
using	O
Qualitative	O
or	O
Quantitative	O
measures	O
.	O
</s>
<s>
define	O
the	O
following	O
Dependability	B-Application
Attributes	O
:	O
</s>
<s>
Security	O
is	O
sometimes	O
classed	O
as	O
an	O
attribute	O
but	O
the	O
current	O
view	O
is	O
to	O
aggregate	O
it	O
together	O
with	O
dependability	B-Application
and	O
treat	O
Dependability	B-Application
as	O
a	O
composite	O
term	O
called	O
Dependability	B-Application
and	O
Security	O
.	O
</s>
<s>
Practically	O
,	O
applying	O
security	O
measures	O
to	O
the	O
appliances	O
of	O
a	O
system	O
generally	O
improves	O
the	O
dependability	B-Application
by	O
limiting	O
the	O
number	O
of	O
externally	O
originated	O
errors	O
.	O
</s>
<s>
Threats	O
are	O
things	O
that	O
can	O
affect	O
a	O
system	O
and	O
cause	O
a	O
drop	O
in	O
Dependability	B-Application
.	O
</s>
<s>
An	O
error	O
may	O
not	O
necessarily	O
cause	O
a	O
failure	O
,	O
for	O
instance	O
an	O
exception	O
may	O
be	O
thrown	O
by	O
a	O
system	O
but	O
this	O
may	O
be	O
caught	O
and	O
handled	O
using	O
fault	B-General_Concept
tolerance	I-General_Concept
techniques	O
so	O
the	O
overall	O
operation	O
of	O
the	O
system	O
will	O
conform	O
to	O
the	O
specification	O
.	O
</s>
<s>
Since	O
the	O
mechanism	O
of	O
a	O
Fault-Error-Chain	O
is	O
understood	O
it	O
is	O
possible	O
to	O
construct	O
means	O
to	O
break	O
these	O
chains	O
and	O
thereby	O
increase	O
the	O
dependability	B-Application
of	O
a	O
system	O
.	O
</s>
<s>
Fault	B-General_Concept
Tolerance	I-General_Concept
deals	O
with	O
putting	O
mechanisms	O
in	O
place	O
that	O
will	O
allow	O
a	O
system	O
to	O
still	O
deliver	O
the	O
required	O
service	O
in	O
the	O
presence	O
of	O
faults	O
,	O
although	O
that	O
service	O
may	O
be	O
at	O
a	O
degraded	O
level	O
.	O
</s>
<s>
Dependability	B-Application
means	O
are	O
intended	O
to	O
reduce	O
the	O
number	O
of	O
failures	O
made	O
visible	O
to	O
the	O
end	O
users	O
of	O
a	O
system	O
.	O
</s>
<s>
Some	O
works	O
on	O
dependability	B-Application
use	O
structured	O
information	O
systems	O
,	O
e.g.	O
</s>
