<s>
In	O
network	O
science	O
,	O
the	O
optimization	B-Protocol
mechanism	I-Protocol
is	O
a	O
network	O
growth	O
algorithm	O
,	O
which	O
randomly	O
places	O
new	O
nodes	O
in	O
the	O
system	O
,	O
and	O
connects	O
them	O
to	O
the	O
existing	O
nodes	O
based	O
on	O
a	O
cost-benefit	O
analysis	O
.	O
</s>
<s>
Depending	O
on	O
the	O
parameters	O
used	O
in	O
the	O
optimization	B-Protocol
mechanism	I-Protocol
,	O
the	O
algorithm	O
can	O
build	O
three	O
types	O
of	O
networks	O
:	O
a	O
star	B-Architecture
network	I-Architecture
,	O
a	O
random	O
network	O
,	O
and	O
a	O
scale-free	O
network	O
.	O
</s>
<s>
Optimization	B-Protocol
mechanism	I-Protocol
is	O
thought	O
to	O
be	O
the	O
underlying	O
mechanism	O
in	O
several	O
real	O
networks	O
,	O
such	O
as	O
transportation	O
networks	O
,	O
power	O
grid	O
,	O
router	O
networks	O
,	O
the	O
network	O
of	O
highways	O
,	O
etc	O
.	O
</s>
<s>
The	O
optimization	B-Protocol
mechanism	I-Protocol
is	O
a	O
model	O
with	O
growth	O
,	O
in	O
which	O
preferential	O
attachment	O
is	O
valid	O
under	O
certain	O
assumptions	O
.	O
</s>
<s>
The	O
model	O
's	O
mechanism	O
is	O
based	O
on	O
a	O
cost-benefit	O
comparison	O
,	O
that	O
is	O
for	O
each	O
entering	O
node	B-Protocol
'	O
i	O
 '	O
,	O
the	O
algorithm	O
calculates	O
the	O
net	O
benefit	O
(	O
benefits	O
minus	O
costs	O
)	O
of	O
connecting	O
'	O
i	O
 '	O
to	O
each	O
existing	O
node	B-Protocol
,	O
and	O
connects	O
node	B-Protocol
'	O
i	O
 '	O
to	O
the	O
node	B-Protocol
which	O
gives	O
the	O
highest	O
net	O
benefit	O
.	O
</s>
<s>
The	O
costs	O
and	O
benefits	O
in	O
the	O
optimization	O
models	O
can	O
generally	O
be	O
simplified	O
into	O
two	O
attributes	O
:	O
the	O
distance	O
between	O
the	O
new	O
node	B-Protocol
,	O
and	O
the	O
existing	O
one	O
;	O
and	O
the	O
distance	O
of	O
the	O
existing	O
node	B-Protocol
from	O
the	O
central	O
node	B-Protocol
.	O
</s>
<s>
is	O
a	O
parameter	O
,	O
that	O
determines	O
the	O
weight	O
of	O
the	O
individual	O
distance	O
compared	O
to	O
the	O
distance	O
to	O
the	O
central	O
node	B-Protocol
,	O
and	O
thus	O
it	O
varies	O
across	O
different	O
settings	O
.	O
</s>
<s>
The	O
value	O
of	O
determines	O
the	O
type	O
of	O
the	O
network	O
built	O
by	O
the	O
optimization	B-Protocol
mechanism	I-Protocol
.	O
</s>
<s>
The	O
optimization	B-Protocol
mechanism	I-Protocol
results	O
in	O
a	O
star	B-Architecture
network	I-Architecture
,	O
whenever	O
.	O
</s>
<s>
A	O
unique	O
feature	O
of	O
the	O
star	B-Architecture
network	I-Architecture
is	O
that	O
most	O
of	O
the	O
newly	O
added	O
nodes	O
will	O
connect	O
to	O
the	O
central	O
node	B-Protocol
regardless	O
of	O
the	O
distance	O
.	O
</s>
<s>
One	O
can	O
think	O
of	O
a	O
star	B-Architecture
network	I-Architecture
as	O
a	O
network	O
in	O
which	O
the	O
costs	O
to	O
establish	O
a	O
new	O
link	O
are	O
negligible	O
compared	O
to	O
the	O
benefit	O
of	O
being	O
directly	O
connected	O
to	O
the	O
central	O
node	B-Protocol
.	O
</s>
<s>
Star	B-Architecture
networks	I-Architecture
are	O
rarely	O
observed	O
in	O
reality	O
.	O
</s>
<s>
In	O
case	O
of	O
a	O
high	O
enough	O
,	O
the	O
costs	O
to	O
establish	O
a	O
new	O
link	O
are	O
enormously	O
high	O
,	O
compared	O
to	O
the	O
benefit	O
of	O
being	O
closely	O
connected	O
to	O
the	O
central	O
node	B-Protocol
.	O
</s>
<s>
As	O
a	O
result	O
,	O
most	O
of	O
the	O
new	O
nodes	O
will	O
connect	O
to	O
the	O
closest	O
node	B-Protocol
available	O
.	O
</s>
<s>
Most	O
of	O
the	O
real-life	O
networks	O
characterized	O
by	O
an	O
underlying	O
optimization	B-Protocol
mechanism	I-Protocol
are	O
scale-free	O
networks	O
,	O
such	O
as	O
the	O
router	O
network	O
and	O
the	O
highway	O
network	O
.	O
</s>
