<s>
A	O
computer	B-Architecture
network	I-Architecture
is	O
a	O
set	O
of	O
computers	O
sharing	O
resources	O
located	O
on	O
or	O
provided	O
by	O
network	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
Computers	O
use	O
common	O
communication	O
protocols	O
over	O
digital	O
interconnections	B-Architecture
to	O
communicate	O
with	O
each	O
other	O
.	O
</s>
<s>
These	O
interconnections	B-Architecture
are	O
made	O
up	O
of	O
telecommunication	O
network	O
technologies	O
based	O
on	O
physically	O
wired	O
,	O
optical	O
,	O
and	O
wireless	O
radio-frequency	O
methods	O
that	O
may	O
be	O
arranged	O
in	O
a	O
variety	O
of	O
network	B-Architecture
topologies	I-Architecture
.	O
</s>
<s>
The	O
nodes	O
of	O
a	O
computer	B-Architecture
network	I-Architecture
can	O
include	O
personal	B-Device
computers	I-Device
,	O
servers	B-Application
,	O
networking	B-Device
hardware	I-Device
,	O
or	O
other	O
specialized	O
or	O
general-purpose	O
hosts	B-Device
.	O
</s>
<s>
They	O
are	O
identified	O
by	O
network	O
addresses	O
and	O
may	O
have	O
hostnames	B-Protocol
.	O
</s>
<s>
Hostnames	B-Protocol
serve	O
as	O
memorable	O
labels	O
for	O
the	O
nodes	O
and	O
are	O
rarely	O
changed	O
after	O
initial	O
assignment	O
.	O
</s>
<s>
Network	O
addresses	O
serve	O
for	O
locating	O
and	O
identifying	O
the	O
nodes	O
by	O
communication	O
protocols	O
such	O
as	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
Computer	B-Architecture
networks	I-Architecture
may	O
be	O
classified	O
by	O
many	O
criteria	O
,	O
including	O
the	O
transmission	O
medium	O
used	O
to	O
carry	O
signals	O
,	O
bandwidth	O
,	O
communications	O
protocols	O
to	O
organize	O
network	O
traffic	O
,	O
the	O
network	O
size	O
,	O
the	O
topology	B-Architecture
,	O
traffic	O
control	O
mechanisms	O
,	O
and	O
organizational	O
intent	O
.	O
</s>
<s>
Computer	B-Architecture
networks	I-Architecture
support	O
many	O
applications	B-Application
and	O
services	B-General_Concept
,	O
such	O
as	O
access	O
to	O
the	O
World	O
Wide	O
Web	O
,	O
digital	O
video	O
and	O
audio	O
,	O
shared	O
use	O
of	O
application	B-Application
and	I-Application
storage	I-Application
servers	I-Application
,	O
printers	O
and	O
fax	O
machines	O
,	O
and	O
use	O
of	O
email	O
and	O
instant	B-Application
messaging	I-Application
applications	B-Application
.	O
</s>
<s>
Computer	B-Architecture
networking	I-Architecture
may	O
be	O
considered	O
a	O
branch	O
of	O
computer	B-General_Concept
science	I-General_Concept
,	O
computer	O
engineering	O
,	O
and	O
telecommunications	O
,	O
since	O
it	O
relies	O
on	O
the	O
theoretical	O
and	O
practical	O
application	O
of	O
the	O
related	O
disciplines	O
.	O
</s>
<s>
Computer	B-Architecture
networking	I-Architecture
was	O
influenced	O
by	O
a	O
wide	O
array	O
of	O
technology	O
developments	O
and	O
historical	O
milestones	O
.	O
</s>
<s>
In	O
the	O
late	O
1950s	O
,	O
a	O
network	O
of	O
computers	O
was	O
built	O
for	O
the	O
U.S.	O
military	O
Semi-Automatic	B-Device
Ground	I-Device
Environment	I-Device
(	O
SAGE	O
)	O
radar	B-Application
system	I-Application
using	O
the	O
Bell	O
101	O
modem	B-Application
.	O
</s>
<s>
It	O
was	O
the	O
first	O
commercial	O
modem	B-Application
for	O
computers	O
,	O
released	O
by	O
AT&T	O
Corporation	O
in	O
1958	O
.	O
</s>
<s>
The	O
modem	B-Application
allowed	O
digital	B-General_Concept
data	I-General_Concept
to	O
be	O
transmitted	O
over	O
regular	O
unconditioned	O
telephone	O
lines	O
at	O
a	O
speed	O
of	O
110	O
bits	O
per	O
second	O
(	O
bit/s	O
)	O
.	O
</s>
<s>
In	O
1959	O
,	O
Christopher	O
Strachey	O
filed	O
a	O
patent	O
application	O
for	O
time-sharing	B-General_Concept
and	O
John	O
McCarthy	O
initiated	O
the	O
first	O
project	O
to	O
implement	O
time-sharing	B-General_Concept
of	O
user	O
programs	O
at	O
MIT	O
.	O
</s>
<s>
McCarthy	O
was	O
instrumental	O
in	O
the	O
creation	O
of	O
three	O
of	O
the	O
earliest	O
time-sharing	B-General_Concept
systems	I-General_Concept
(	O
the	O
Compatible	B-Operating_System
Time-Sharing	I-Operating_System
System	I-Operating_System
in	O
1961	O
,	O
the	O
BBN	B-Operating_System
Time-Sharing	I-Operating_System
System	I-Operating_System
in	O
1962	O
,	O
and	O
the	O
Dartmouth	B-Operating_System
Time	I-Operating_System
Sharing	I-Operating_System
System	I-Operating_System
in	O
1963	O
)	O
.	O
</s>
<s>
In	O
1960	O
,	O
the	O
commercial	O
airline	O
reservation	O
system	O
semi-automatic	B-Application
business	I-Application
research	I-Application
environment	I-Application
(	O
SABRE	B-Application
)	O
went	O
online	O
with	O
two	O
connected	O
mainframes	B-Architecture
.	O
</s>
<s>
In	O
1963	O
,	O
J	O
.	O
C	O
.	O
R	O
.	O
Licklider	O
sent	O
a	O
memorandum	O
to	O
office	O
colleagues	O
discussing	O
the	O
concept	O
of	O
the	O
"	O
Intergalactic	B-General_Concept
Computer	I-General_Concept
Network	I-General_Concept
"	O
,	O
a	O
computer	B-Architecture
network	I-Architecture
intended	O
to	O
allow	O
general	O
communications	O
among	O
computer	O
users	O
.	O
</s>
<s>
Throughout	O
the	O
1960s	O
,	O
Paul	O
Baran	O
and	O
Donald	O
Davies	O
independently	O
developed	O
the	O
concept	O
of	O
packet	B-Protocol
switching	I-Protocol
to	O
transfer	O
information	O
between	O
computers	O
over	O
a	O
network	O
.	O
</s>
<s>
The	O
NPL	B-Protocol
network	I-Protocol
,	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
at	O
the	O
National	O
Physical	O
Laboratory	O
(	O
United	O
Kingdom	O
)	O
used	O
a	O
line	O
speed	O
of	O
768	O
kbit/s	O
and	O
later	O
high-speed	O
T1	O
links	O
(	O
1.544	O
Mbit/s	O
line	O
rate	O
)	O
.	O
</s>
<s>
In	O
the	O
early	O
1970s	O
,	O
Leonard	O
Kleinrock	O
carried	O
out	O
mathematical	O
work	O
to	O
model	O
the	O
performance	O
of	O
packet-switched	B-Protocol
networks	I-Protocol
,	O
which	O
underpinned	O
the	O
development	O
of	O
the	O
ARPANET	O
.	O
</s>
<s>
His	O
theoretical	O
work	O
on	O
hierarchical	B-Protocol
routing	I-Protocol
in	O
the	O
late	O
1970s	O
with	O
student	O
Farouk	O
Kamoun	O
remains	O
critical	O
to	O
the	O
operation	O
of	O
the	O
Internet	O
today	O
.	O
</s>
<s>
In	O
1972	O
,	O
commercial	O
services	B-General_Concept
were	O
first	O
deployed	O
on	O
public	O
data	B-Architecture
networks	I-Architecture
in	O
Europe	O
,	O
which	O
began	O
using	O
X.25	B-Protocol
in	O
the	O
late	O
1970s	O
and	O
spread	O
across	O
the	O
globe	O
.	O
</s>
<s>
The	O
underlying	O
infrastructure	O
was	O
used	O
for	O
expanding	O
TCP/IP	B-Protocol
networks	I-Protocol
in	O
the	O
1980s	O
.	O
</s>
<s>
In	O
1973	O
,	O
the	O
French	O
CYCLADES	O
network	O
was	O
the	O
first	O
to	O
make	O
the	O
hosts	B-Device
responsible	O
for	O
the	O
reliable	O
delivery	O
of	O
data	O
,	O
rather	O
than	O
this	O
being	O
a	O
centralized	O
service	O
of	O
the	O
network	O
itself	O
.	O
</s>
<s>
In	O
July	O
1976	O
,	O
Robert	O
Metcalfe	O
and	O
David	O
Boggs	O
published	O
their	O
paper	O
"	O
Ethernet	O
:	O
Distributed	O
Packet	B-Protocol
Switching	I-Protocol
for	O
Local	O
Computer	B-Architecture
Networks	I-Architecture
"	O
and	O
collaborated	O
on	O
several	O
patents	O
received	O
in	O
1977	O
and	O
1978	O
.	O
</s>
<s>
In	O
1974	O
,	O
Vint	O
Cerf	O
,	O
Yogen	O
Dalal	O
,	O
and	O
Carl	O
Sunshine	O
published	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
specification	O
,	O
,	O
coining	O
the	O
term	O
Internet	O
as	O
a	O
shorthand	O
for	O
internetworking	B-Architecture
.	O
</s>
<s>
In	O
1976	O
,	O
John	O
Murphy	O
of	O
Datapoint	O
Corporation	O
created	O
ARCNET	B-Protocol
,	O
a	O
token-passing	O
network	O
first	O
used	O
to	O
share	O
storage	O
devices	O
.	O
</s>
<s>
In	O
1977	O
,	O
Xerox	B-Protocol
Network	I-Protocol
Systems	I-Protocol
(	O
XNS	O
)	O
was	O
developed	O
by	O
Robert	O
Metcalfe	O
and	O
Yogen	O
Dalal	O
at	O
Xerox	O
.	O
</s>
<s>
A	O
computer	B-Architecture
network	I-Architecture
extends	O
interpersonal	O
communications	O
by	O
electronic	O
means	O
with	O
various	O
technologies	O
,	O
such	O
as	O
email	O
,	O
instant	B-Application
messaging	I-Application
,	O
online	O
chat	O
,	O
voice	O
and	O
video	O
telephone	O
calls	O
,	O
and	O
video	O
conferencing	O
.	O
</s>
<s>
Distributed	B-Architecture
computing	I-Architecture
uses	O
computing	O
resources	O
across	O
a	O
network	O
to	O
accomplish	O
tasks	O
.	O
</s>
<s>
Most	O
modern	O
computer	B-Architecture
networks	I-Architecture
use	O
protocols	O
based	O
on	O
packet-mode	O
transmission	O
.	O
</s>
<s>
A	O
network	B-Protocol
packet	I-Protocol
is	O
a	O
formatted	O
unit	O
of	O
data	O
carried	O
by	O
a	O
packet-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Packets	B-Protocol
consist	O
of	O
two	O
types	O
of	O
data	O
:	O
control	O
information	O
and	O
user	O
data	O
(	O
payload	O
)	O
.	O
</s>
<s>
The	O
control	O
information	O
provides	O
data	O
the	O
network	O
needs	O
to	O
deliver	O
the	O
user	O
data	O
,	O
for	O
example	O
,	O
source	O
and	O
destination	O
network	O
addresses	O
,	O
error	B-Error_Name
detection	I-Error_Name
codes	I-Error_Name
,	O
and	O
sequencing	O
information	O
.	O
</s>
<s>
Typically	O
,	O
control	O
information	O
is	O
found	O
in	O
packet	B-General_Concept
headers	I-General_Concept
and	O
trailers	B-Device
,	O
with	O
payload	O
data	O
in	O
between	O
.	O
</s>
<s>
With	O
packets	B-Protocol
,	O
the	O
bandwidth	O
of	O
the	O
transmission	O
medium	O
can	O
be	O
better	O
shared	O
among	O
users	O
than	O
if	O
the	O
network	O
were	O
circuit	B-Protocol
switched	I-Protocol
.	O
</s>
<s>
When	O
one	O
user	O
is	O
not	O
sending	O
packets	B-Protocol
,	O
the	O
link	O
can	O
be	O
filled	O
with	O
packets	B-Protocol
from	O
other	O
users	O
,	O
and	O
so	O
the	O
cost	O
can	O
be	O
shared	O
,	O
with	O
relatively	O
little	O
interference	O
,	O
provided	O
the	O
link	O
is	O
n't	O
overused	O
.	O
</s>
<s>
Often	O
the	O
route	O
a	O
packet	B-Protocol
needs	O
to	O
take	O
through	O
a	O
network	O
is	O
not	O
immediately	O
available	O
.	O
</s>
<s>
In	O
that	O
case	O
,	O
the	O
packet	B-Protocol
is	O
queued	B-Operating_System
and	O
waits	O
until	O
a	O
link	O
is	O
free	O
.	O
</s>
<s>
The	O
physical	B-Application
link	I-Application
technologies	O
of	O
packet	B-Protocol
networks	I-Protocol
typically	O
limit	O
the	O
size	O
of	O
packets	B-Protocol
to	O
a	O
certain	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
.	O
</s>
<s>
A	O
longer	O
message	O
may	O
be	O
fragmented	O
before	O
it	O
is	O
transferred	O
and	O
once	O
the	O
packets	B-Protocol
arrive	O
,	O
they	O
are	O
reassembled	O
to	O
construct	O
the	O
original	O
message	O
.	O
</s>
<s>
The	O
physical	O
or	O
geographic	O
locations	O
of	O
network	B-Protocol
nodes	I-Protocol
and	O
links	O
generally	O
have	O
relatively	O
little	O
effect	O
on	O
a	O
network	O
,	O
but	O
the	O
topology	B-Architecture
of	O
interconnections	B-Architecture
of	O
a	O
network	O
can	O
significantly	O
affect	O
its	O
throughput	O
and	O
reliability	O
.	O
</s>
<s>
With	O
many	O
technologies	O
,	O
such	O
as	O
bus	O
or	O
star	B-Architecture
networks	I-Architecture
,	O
a	O
single	O
failure	O
can	O
cause	O
the	O
network	O
to	O
fail	O
entirely	O
.	O
</s>
<s>
In	O
general	O
,	O
the	O
more	O
interconnections	B-Architecture
there	O
are	O
,	O
the	O
more	O
robust	O
the	O
network	O
is	O
;	O
but	O
the	O
more	O
expensive	O
it	O
is	O
to	O
install	O
.	O
</s>
<s>
Therefore	O
,	O
most	O
network	O
diagrams	O
are	O
arranged	O
by	O
their	O
network	B-Architecture
topology	I-Architecture
which	O
is	O
the	O
map	B-Application
of	O
logical	O
interconnections	B-Architecture
of	O
network	B-Device
hosts	I-Device
.	O
</s>
<s>
Bus	B-Architecture
network	I-Architecture
:	O
all	O
nodes	O
are	O
connected	O
to	O
a	O
common	O
medium	O
along	O
this	O
medium	O
.	O
</s>
<s>
This	O
is	O
still	O
a	O
common	O
topology	B-Architecture
on	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
although	O
modern	O
physical	B-Application
layer	I-Application
variants	O
use	O
point-to-point	B-Architecture
links	O
instead	O
,	O
forming	O
a	O
star	O
or	O
a	O
tree	O
.	O
</s>
<s>
Star	B-Architecture
network	I-Architecture
:	O
all	O
nodes	O
are	O
connected	O
to	O
a	O
special	O
central	O
node	O
.	O
</s>
<s>
This	O
is	O
the	O
typical	O
layout	O
found	O
in	O
a	O
small	O
switched	B-Protocol
Ethernet	I-Protocol
LAN	B-General_Concept
,	O
where	O
each	O
client	O
connects	O
to	O
a	O
central	O
network	B-Protocol
switch	I-Protocol
,	O
and	O
logically	O
in	O
a	O
wireless	B-Architecture
LAN	I-Architecture
,	O
where	O
each	O
wireless	O
client	O
associates	O
with	O
the	O
central	O
wireless	B-Device
access	I-Device
point	I-Device
.	O
</s>
<s>
Ring	B-Architecture
network	I-Architecture
:	O
each	O
node	O
is	O
connected	O
to	O
its	O
left	O
and	O
right	O
neighbor	O
node	O
,	O
such	O
that	O
all	O
nodes	O
are	O
connected	O
and	O
that	O
each	O
node	O
can	O
reach	O
each	O
other	O
node	O
by	O
traversing	O
nodes	O
left	O
-	O
or	O
rightwards	O
.	O
</s>
<s>
Token	B-Device
ring	I-Device
networks	I-Device
,	O
and	O
the	O
Fiber	B-Protocol
Distributed	I-Protocol
Data	I-Protocol
Interface	I-Protocol
(	O
FDDI	B-Protocol
)	O
,	O
made	O
use	O
of	O
such	O
a	O
topology	B-Architecture
.	O
</s>
<s>
Mesh	B-Architecture
network	I-Architecture
:	O
each	O
node	O
is	O
connected	O
to	O
an	O
arbitrary	O
number	O
of	O
neighbors	O
in	O
such	O
a	O
way	O
that	O
there	O
is	O
at	O
least	O
one	O
traversal	O
from	O
any	O
node	O
to	O
any	O
other	O
.	O
</s>
<s>
Tree	B-Data_Structure
network	I-Data_Structure
:	O
nodes	O
are	O
arranged	O
hierarchically	O
.	O
</s>
<s>
This	O
is	O
the	O
natural	O
topology	B-Architecture
for	O
a	O
larger	O
Ethernet	O
network	O
with	O
multiple	O
switches	B-Protocol
and	O
without	O
redundant	O
meshing	O
.	O
</s>
<s>
The	O
physical	O
layout	O
of	O
the	O
nodes	O
in	O
a	O
network	O
may	O
not	O
necessarily	O
reflect	O
the	O
network	B-Architecture
topology	I-Architecture
.	O
</s>
<s>
As	O
an	O
example	O
,	O
with	O
FDDI	B-Protocol
,	O
the	O
network	B-Architecture
topology	I-Architecture
is	O
a	O
ring	O
,	O
but	O
the	O
physical	B-Architecture
topology	I-Architecture
is	O
often	O
a	O
star	O
,	O
because	O
all	O
neighboring	O
connections	O
can	O
be	O
routed	B-Protocol
via	O
a	O
central	O
physical	O
location	O
.	O
</s>
<s>
An	O
overlay	B-Architecture
network	I-Architecture
is	O
a	O
virtual	O
network	O
that	O
is	O
built	O
on	O
top	O
of	O
another	O
network	O
.	O
</s>
<s>
Nodes	O
in	O
the	O
overlay	B-Architecture
network	I-Architecture
are	O
connected	O
by	O
virtual	O
or	O
logical	O
links	O
.	O
</s>
<s>
Each	O
link	O
corresponds	O
to	O
a	O
path	O
,	O
perhaps	O
through	O
many	O
physical	B-Application
links	I-Application
,	O
in	O
the	O
underlying	O
network	O
.	O
</s>
<s>
The	O
topology	B-Architecture
of	O
the	O
overlay	B-Architecture
network	I-Architecture
may	O
(	O
and	O
often	O
does	O
)	O
differ	O
from	O
that	O
of	O
the	O
underlying	O
one	O
.	O
</s>
<s>
For	O
example	O
,	O
many	O
peer-to-peer	O
networks	O
are	O
overlay	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Overlay	B-Architecture
networks	I-Architecture
have	O
been	O
around	O
since	O
the	O
invention	O
of	O
networking	O
when	O
computer	O
systems	O
were	O
connected	O
over	O
telephone	O
lines	O
using	O
modems	B-Application
before	O
any	O
data	B-Architecture
network	I-Architecture
existed	O
.	O
</s>
<s>
The	O
most	O
striking	O
example	O
of	O
an	O
overlay	B-Architecture
network	I-Architecture
is	O
the	O
Internet	O
itself	O
.	O
</s>
<s>
Even	O
today	O
,	O
each	O
Internet	O
node	O
can	O
communicate	O
with	O
virtually	O
any	O
other	O
through	O
an	O
underlying	O
mesh	O
of	O
sub-networks	B-Protocol
of	O
wildly	O
different	O
topologies	B-Architecture
and	O
technologies	O
.	O
</s>
<s>
Address	O
resolution	O
and	O
routing	B-Protocol
are	O
the	O
means	O
that	O
allow	O
mapping	O
of	O
a	O
fully	O
connected	O
IP	B-Protocol
overlay	B-Architecture
network	I-Architecture
to	O
its	O
underlying	O
network	O
.	O
</s>
<s>
Another	O
example	O
of	O
an	O
overlay	B-Architecture
network	I-Architecture
is	O
a	O
distributed	B-Operating_System
hash	I-Operating_System
table	I-Operating_System
,	O
which	O
maps	O
keys	O
to	O
nodes	O
in	O
the	O
network	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
the	O
underlying	O
network	O
is	O
an	O
IP	B-Protocol
network	I-Protocol
,	O
and	O
the	O
overlay	B-Architecture
network	I-Architecture
is	O
a	O
table	O
(	O
actually	O
a	O
map	B-Application
)	O
indexed	O
by	O
keys	O
.	O
</s>
<s>
Overlay	B-Architecture
networks	I-Architecture
have	O
also	O
been	O
proposed	O
as	O
a	O
way	O
to	O
improve	O
Internet	O
routing	B-Protocol
,	O
such	O
as	O
through	O
quality	O
of	O
service	O
guarantees	O
achieve	O
higher-quality	O
streaming	O
media	O
.	O
</s>
<s>
Previous	O
proposals	O
such	O
as	O
IntServ	O
,	O
DiffServ	O
,	O
and	O
IP	B-Protocol
multicast	I-Protocol
have	O
not	O
seen	O
wide	O
acceptance	O
largely	O
because	O
they	O
require	O
modification	O
of	O
all	O
routers	B-Protocol
in	O
the	O
network	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
an	O
overlay	B-Architecture
network	I-Architecture
can	O
be	O
incrementally	O
deployed	O
on	O
end-hosts	O
running	O
the	O
overlay	O
protocol	O
software	O
,	O
without	O
cooperation	O
from	O
Internet	O
service	O
providers	O
.	O
</s>
<s>
The	O
overlay	B-Architecture
network	I-Architecture
has	O
no	O
control	O
over	O
how	O
packets	B-Protocol
are	O
routed	B-Protocol
in	O
the	O
underlying	O
network	O
between	O
two	O
overlay	O
nodes	O
,	O
but	O
it	O
can	O
control	O
,	O
for	O
example	O
,	O
the	O
sequence	O
of	O
overlay	O
nodes	O
that	O
a	O
message	O
traverses	O
before	O
it	O
reaches	O
its	O
destination	O
.	O
</s>
<s>
For	O
example	O
,	O
Akamai	B-Application
Technologies	I-Application
manages	O
an	O
overlay	B-Architecture
network	I-Architecture
that	O
provides	O
reliable	O
,	O
efficient	O
content	O
delivery	O
(	O
a	O
kind	O
of	O
multicast	O
)	O
.	O
</s>
<s>
Academic	O
research	O
includes	O
end	O
system	O
multicast	O
,	O
resilient	O
routing	B-Protocol
and	O
quality	O
of	O
service	O
studies	O
,	O
among	O
others	O
.	O
</s>
<s>
The	O
transmission	O
media	O
(	O
often	O
referred	O
to	O
in	O
the	O
literature	O
as	O
the	O
physical	O
medium	O
)	O
used	O
to	O
link	O
devices	O
to	O
form	O
a	O
computer	B-Architecture
network	I-Architecture
include	O
electrical	O
cable	O
,	O
optical	B-Architecture
fiber	I-Architecture
,	O
and	O
free	O
space	O
.	O
</s>
<s>
In	O
the	O
OSI	O
model	O
,	O
the	O
software	O
to	O
handle	O
the	O
media	O
is	O
defined	O
at	O
layers	O
1	O
and	O
2	O
—	O
the	O
physical	B-Application
layer	I-Application
and	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
.	O
</s>
<s>
A	O
widely	O
adopted	O
family	O
that	O
uses	O
copper	O
and	O
fiber	O
media	O
in	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
technology	O
are	O
collectively	O
known	O
as	O
Ethernet	O
.	O
</s>
<s>
Wireless	B-Architecture
LAN	I-Architecture
standards	O
use	O
radio	O
waves	O
,	O
others	O
use	O
infrared	O
signals	O
as	O
a	O
transmission	O
medium	O
.	O
</s>
<s>
Power	B-Device
line	I-Device
communication	I-Device
uses	O
a	O
building	O
's	O
power	O
cabling	O
to	O
transmit	O
data	O
.	O
</s>
<s>
The	O
following	O
classes	O
of	O
wired	O
technologies	O
are	O
used	O
in	O
computer	B-Architecture
networking	I-Architecture
.	O
</s>
<s>
Coaxial	B-Protocol
cable	I-Protocol
is	O
widely	O
used	O
for	O
cable	O
television	O
systems	O
,	O
office	O
buildings	O
,	O
and	O
other	O
work-sites	O
for	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
ITU-T	B-General_Concept
G.hn	I-General_Concept
technology	O
uses	O
existing	O
home	O
wiring	O
(	O
coaxial	B-Protocol
cable	I-Protocol
,	O
phone	O
lines	O
and	O
power	B-Device
lines	I-Device
)	O
to	O
create	O
a	O
high-speed	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
.	O
</s>
<s>
An	O
optical	B-Architecture
fiber	I-Architecture
is	O
a	O
glass	O
fiber	O
.	O
</s>
<s>
Some	O
advantages	O
of	O
optical	B-Architecture
fibers	I-Architecture
over	O
metal	O
wires	O
are	O
very	O
low	O
transmission	O
loss	O
and	O
immunity	O
to	O
electrical	O
interference	O
.	O
</s>
<s>
Using	O
dense	B-Architecture
wave	I-Architecture
division	I-Architecture
multiplexing	I-Architecture
,	O
optical	B-Architecture
fibers	I-Architecture
can	O
simultaneously	O
carry	O
multiple	O
streams	O
of	O
data	O
on	O
different	O
wavelengths	O
of	O
light	O
,	O
which	O
greatly	O
increases	O
the	O
rate	O
that	O
data	O
can	O
be	O
sent	O
to	O
up	O
to	O
trillions	O
of	O
bits	O
per	O
second	O
.	O
</s>
<s>
Optic	B-Architecture
fibers	I-Architecture
can	O
be	O
used	O
for	O
long	O
runs	O
of	O
cable	O
carrying	O
very	O
high	O
data	O
rates	O
,	O
and	O
are	O
used	O
for	O
undersea	O
communications	O
cables	O
to	O
interconnect	B-Architecture
continents	O
.	O
</s>
<s>
There	O
are	O
two	O
basic	O
types	O
of	O
fiber	B-Architecture
optics	I-Architecture
,	O
single-mode	B-Device
optical	I-Device
fiber	I-Device
(	O
SMF	O
)	O
and	O
multi-mode	B-Device
optical	I-Device
fiber	I-Device
(	O
MMF	O
)	O
.	O
</s>
<s>
Single-mode	B-Device
fiber	I-Device
has	O
the	O
advantage	O
of	O
being	O
able	O
to	O
sustain	O
a	O
coherent	O
signal	O
for	O
dozens	O
or	O
even	O
a	O
hundred	O
kilometers	O
.	O
</s>
<s>
Multimode	B-Device
fiber	I-Device
is	O
cheaper	O
to	O
terminate	O
but	O
is	O
limited	O
to	O
a	O
few	O
hundred	O
or	O
even	O
only	O
a	O
few	O
dozens	O
of	O
meters	O
,	O
depending	O
on	O
the	O
data	O
rate	O
and	O
cable	O
grade	O
.	O
</s>
<s>
Radio	O
and	O
spread	B-Architecture
spectrum	I-Architecture
technologies	O
–	O
Wireless	B-Architecture
LANs	I-Architecture
use	O
a	O
high-frequency	O
radio	O
technology	O
similar	O
to	O
digital	O
cellular	O
.	O
</s>
<s>
Wireless	B-Architecture
LANs	I-Architecture
use	O
spread	B-Architecture
spectrum	I-Architecture
technology	O
to	O
enable	O
communication	O
between	O
multiple	O
devices	O
in	O
a	O
limited	O
area	O
.	O
</s>
<s>
IP	B-Protocol
over	I-Protocol
Avian	I-Protocol
Carriers	I-Protocol
was	O
a	O
humorous	O
April	O
fool	O
's	O
Request	O
for	O
Comments	O
,	O
issued	O
as	O
.	O
</s>
<s>
Apart	O
from	O
any	O
physical	O
transmission	O
media	O
,	O
networks	O
are	O
built	O
from	O
additional	O
basic	O
system	O
building	O
blocks	O
,	O
such	O
as	O
network	B-Protocol
interface	I-Protocol
controllers	I-Protocol
,	O
repeaters	O
,	O
hubs	B-Protocol
,	O
bridges	B-Protocol
,	O
switches	B-Protocol
,	O
routers	B-Protocol
,	O
modems	B-Application
,	O
and	O
firewalls	B-Application
.	O
</s>
<s>
A	O
network	B-Protocol
interface	I-Protocol
controller	I-Protocol
(	O
NIC	O
)	O
is	O
computer	B-Architecture
hardware	I-Architecture
that	O
connects	O
the	O
computer	O
to	O
the	O
network	B-Protocol
media	I-Protocol
and	O
has	O
the	O
ability	O
to	O
process	O
low-level	O
network	O
information	O
.	O
</s>
<s>
To	O
avoid	O
address	O
conflicts	O
between	O
network	B-Device
devices	I-Device
,	O
the	O
Institute	O
of	O
Electrical	O
and	O
Electronics	O
Engineers	O
(	O
IEEE	O
)	O
maintains	O
and	O
administers	O
MAC	O
address	O
uniqueness	O
.	O
</s>
<s>
The	O
signal	O
is	O
retransmitted	B-Protocol
at	O
a	O
higher	O
power	O
level	O
,	O
or	O
to	O
the	O
other	O
side	O
of	O
obstruction	O
so	O
that	O
the	O
signal	O
can	O
cover	O
longer	O
distances	O
without	O
degradation	O
.	O
</s>
<s>
With	O
fiber	B-Architecture
optics	I-Architecture
,	O
repeaters	O
can	O
be	O
tens	O
or	O
even	O
hundreds	O
of	O
kilometers	O
apart	O
.	O
</s>
<s>
Repeaters	O
work	O
on	O
the	O
physical	B-Application
layer	I-Application
of	O
the	O
OSI	O
model	O
but	O
still	O
require	O
a	O
small	O
amount	O
of	O
time	O
to	O
regenerate	O
the	O
signal	O
.	O
</s>
<s>
This	O
can	O
cause	O
a	O
propagation	O
delay	O
that	O
affects	O
network	B-General_Concept
performance	I-General_Concept
and	O
may	O
affect	O
proper	O
function	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
many	O
network	O
architectures	O
limit	O
the	O
number	O
of	O
repeaters	O
used	O
in	O
a	O
network	O
,	O
e.g.	O
,	O
the	O
Ethernet	O
5-4-3	B-Protocol
rule	I-Protocol
.	O
</s>
<s>
An	O
Ethernet	B-Protocol
repeater	I-Protocol
with	O
multiple	O
ports	B-Protocol
is	O
known	O
as	O
an	O
Ethernet	B-Protocol
hub	I-Protocol
.	O
</s>
<s>
In	O
addition	O
to	O
reconditioning	O
and	O
distributing	O
network	O
signals	O
,	O
a	O
repeater	B-Protocol
hub	I-Protocol
assists	O
with	O
collision	O
detection	O
and	O
fault	O
isolation	O
for	O
the	O
network	O
.	O
</s>
<s>
Hubs	B-Protocol
and	O
repeaters	O
in	O
LANs	B-General_Concept
have	O
been	O
largely	O
obsoleted	O
by	O
modern	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
Network	B-Protocol
bridges	I-Protocol
and	O
network	B-Protocol
switches	I-Protocol
are	O
distinct	O
from	O
a	O
hub	B-Protocol
in	O
that	O
they	O
only	O
forward	O
frames	B-Protocol
to	O
the	O
ports	B-Protocol
involved	O
in	O
the	O
communication	O
whereas	O
a	O
hub	B-Protocol
forwards	O
to	O
all	O
ports	B-Protocol
.	O
</s>
<s>
Bridges	B-Protocol
only	O
have	O
two	O
ports	B-Protocol
but	O
a	O
switch	O
can	O
be	O
thought	O
of	O
as	O
a	O
multi-port	O
bridge	O
.	O
</s>
<s>
Switches	B-Protocol
normally	O
have	O
numerous	O
ports	B-Protocol
,	O
facilitating	O
a	O
star	B-Architecture
topology	I-Architecture
for	O
devices	O
,	O
and	O
for	O
cascading	O
additional	O
switches	B-Protocol
.	O
</s>
<s>
Bridges	B-Protocol
and	O
switches	B-Protocol
operate	O
at	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
(	O
layer	B-Protocol
2	I-Protocol
)	O
of	O
the	O
OSI	O
model	O
and	O
bridge	O
traffic	O
between	O
two	O
or	O
more	O
network	B-Protocol
segments	I-Protocol
to	O
form	O
a	O
single	O
local	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
Both	O
are	O
devices	O
that	O
forward	O
frames	B-Protocol
of	O
data	O
between	O
ports	B-Protocol
based	O
on	O
the	O
destination	O
MAC	O
address	O
in	O
each	O
frame	O
.	O
</s>
<s>
They	O
learn	O
the	O
association	O
of	O
physical	O
ports	B-Protocol
to	O
MAC	O
addresses	O
by	O
examining	O
the	O
source	O
addresses	O
of	O
received	O
frames	B-Protocol
and	O
only	O
forward	O
the	O
frame	O
when	O
necessary	O
.	O
</s>
<s>
If	O
an	O
unknown	O
destination	O
MAC	O
is	O
targeted	O
,	O
the	O
device	O
broadcasts	O
the	O
request	O
to	O
all	O
ports	B-Protocol
except	O
the	O
source	O
,	O
and	O
discovers	O
the	O
location	O
from	O
the	O
reply	O
.	O
</s>
<s>
Bridges	B-Protocol
and	O
switches	B-Protocol
divide	O
the	O
network	O
's	O
collision	O
domain	O
but	O
maintain	O
a	O
single	O
broadcast	O
domain	O
.	O
</s>
<s>
Network	O
segmentation	O
through	O
bridging	B-Protocol
and	O
switching	O
helps	O
break	O
down	O
a	O
large	O
,	O
congested	O
network	O
into	O
an	O
aggregation	O
of	O
smaller	O
,	O
more	O
efficient	O
networks	O
.	O
</s>
<s>
A	O
router	B-Protocol
is	O
an	O
internetworking	B-Architecture
device	O
that	O
forwards	O
packets	B-Protocol
between	O
networks	O
by	O
processing	O
the	O
addressing	O
or	O
routing	B-Protocol
information	I-Protocol
included	O
in	O
the	O
packet	B-Protocol
.	O
</s>
<s>
The	O
routing	B-Protocol
information	I-Protocol
is	O
often	O
processed	O
in	O
conjunction	O
with	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
A	O
router	B-Protocol
uses	O
its	O
routing	B-Protocol
table	I-Protocol
to	O
determine	O
where	O
to	O
forward	O
packets	B-Protocol
and	O
does	O
not	O
require	O
broadcasting	O
packets	B-Protocol
which	O
is	O
inefficient	O
for	O
very	O
big	O
networks	O
.	O
</s>
<s>
Modems	B-Application
(	O
modulator-demodulator	O
)	O
are	O
used	O
to	O
connect	O
network	B-Protocol
nodes	I-Protocol
via	O
wire	O
not	O
originally	O
designed	O
for	O
digital	B-Architecture
network	I-Architecture
traffic	O
,	O
or	O
for	O
wireless	O
.	O
</s>
<s>
To	O
do	O
this	O
one	O
or	O
more	O
carrier	O
signals	O
are	O
modulated	B-Application
by	O
the	O
digital	O
signal	O
to	O
produce	O
an	O
analog	O
signal	O
that	O
can	O
be	O
tailored	O
to	O
give	O
the	O
required	O
properties	O
for	O
transmission	O
.	O
</s>
<s>
Early	O
modems	B-Application
modulated	B-Application
audio	O
signals	O
sent	O
over	O
a	O
standard	O
voice	O
telephone	O
line	O
.	O
</s>
<s>
Modems	B-Application
are	O
still	O
commonly	O
used	O
for	O
telephone	O
lines	O
,	O
using	O
a	O
digital	O
subscriber	O
line	O
technology	O
and	O
cable	O
television	O
systems	O
using	O
DOCSIS	B-Protocol
technology	O
.	O
</s>
<s>
A	O
firewall	B-Application
is	O
a	O
network	B-Device
device	I-Device
or	O
software	O
for	O
controlling	O
network	O
security	O
and	O
access	O
rules	O
.	O
</s>
<s>
Firewalls	B-Application
are	O
inserted	O
in	O
connections	O
between	O
secure	O
internal	B-General_Concept
networks	I-General_Concept
and	O
potentially	O
insecure	O
external	O
networks	O
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
Firewalls	B-Application
are	O
typically	O
configured	O
to	O
reject	O
access	O
requests	O
from	O
unrecognized	O
sources	O
while	O
allowing	O
actions	O
from	O
recognized	O
ones	O
.	O
</s>
<s>
The	O
vital	O
role	O
firewalls	B-Application
play	O
in	O
network	O
security	O
grows	O
in	O
parallel	O
with	O
the	O
constant	O
increase	O
in	O
cyber	O
attacks	O
.	O
</s>
<s>
They	O
may	O
be	O
connection-oriented	B-Protocol
or	O
connectionless	B-Protocol
,	O
they	O
may	O
use	O
circuit	B-Protocol
mode	I-Protocol
or	O
packet	B-Protocol
switching	I-Protocol
,	O
and	O
they	O
may	O
use	O
hierarchical	O
addressing	O
or	O
flat	O
addressing	O
.	O
</s>
<s>
In	O
a	O
protocol	B-Protocol
stack	I-Protocol
,	O
often	O
constructed	O
per	O
the	O
OSI	O
model	O
,	O
communications	O
functions	O
are	O
divided	O
up	O
into	O
protocol	O
layers	O
,	O
where	O
each	O
layer	O
leverages	O
the	O
services	B-General_Concept
of	O
the	O
layer	O
below	O
it	O
until	O
the	O
lowest	O
layer	O
controls	O
the	O
hardware	B-Architecture
that	O
sends	O
information	O
across	O
the	O
media	O
.	O
</s>
<s>
The	O
use	O
of	O
protocol	B-Protocol
layering	I-Protocol
is	O
ubiquitous	O
across	O
the	O
field	O
of	O
computer	B-Architecture
networking	I-Architecture
.	O
</s>
<s>
An	O
important	O
example	O
of	O
a	O
protocol	B-Protocol
stack	I-Protocol
is	O
HTTP	B-Protocol
(	O
the	O
World	O
Wide	O
Web	O
protocol	O
)	O
running	O
over	O
TCP	B-Protocol
over	O
IP	B-Protocol
(	O
the	O
Internet	B-Protocol
protocols	I-Protocol
)	O
over	O
IEEE	O
802.11	O
(	O
the	O
Wi-Fi	O
protocol	O
)	O
.	O
</s>
<s>
This	O
stack	O
is	O
used	O
between	O
the	O
wireless	B-Device
router	I-Device
and	O
the	O
home	O
user	O
's	O
personal	B-Device
computer	I-Device
when	O
the	O
user	O
is	O
surfing	O
the	O
web	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
,	O
also	O
called	O
TCP/IP	B-Protocol
,	O
is	O
the	O
foundation	O
of	O
all	O
modern	O
networking	O
.	O
</s>
<s>
It	O
offers	O
connection-less	B-Protocol
and	O
connection-oriented	B-Protocol
services	I-Protocol
over	O
an	O
inherently	O
unreliable	O
network	O
traversed	O
by	O
datagram	O
transmission	O
using	O
Internet	B-Protocol
protocol	I-Protocol
(	O
IP	B-Protocol
)	O
.	O
</s>
<s>
At	O
its	O
core	O
,	O
the	O
protocol	B-Protocol
suite	I-Protocol
defines	O
the	O
addressing	O
,	O
identification	O
,	O
and	O
routing	B-Protocol
specifications	O
for	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
and	O
for	O
IPv6	B-Protocol
,	O
the	O
next	O
generation	O
of	O
the	O
protocol	O
with	O
a	O
much	O
enlarged	O
addressing	O
capability	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
is	O
the	O
defining	O
set	O
of	O
protocols	O
for	O
the	O
Internet	O
.	O
</s>
<s>
IEEE	O
802	O
is	O
a	O
family	O
of	O
IEEE	O
standards	O
dealing	O
with	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
and	O
metropolitan	O
area	O
networks	O
.	O
</s>
<s>
The	O
complete	O
IEEE	O
802	O
protocol	B-Protocol
suite	I-Protocol
provides	O
a	O
diverse	O
set	O
of	O
networking	O
capabilities	O
.	O
</s>
<s>
For	O
example	O
,	O
MAC	B-Protocol
bridging	I-Protocol
(	O
IEEE	O
802.1D	O
)	O
deals	O
with	O
the	O
routing	B-Protocol
of	O
Ethernet	O
packets	B-Protocol
using	O
a	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
IEEE	O
802.1Q	O
describes	O
VLANs	B-General_Concept
,	O
and	O
IEEE	O
802.1X	O
defines	O
a	O
port-based	O
Network	O
Access	O
Control	O
protocol	O
,	O
which	O
forms	O
the	O
basis	O
for	O
the	O
authentication	O
mechanisms	O
used	O
in	O
VLANs	B-General_Concept
(	O
but	O
it	O
is	O
also	O
found	O
in	O
WLANs	B-Architecture
)	O
–	O
it	O
is	O
what	O
the	O
home	O
user	O
sees	O
when	O
the	O
user	O
has	O
to	O
enter	O
a	O
"	O
wireless	O
access	O
key	O
"	O
.	O
</s>
<s>
Ethernet	O
is	O
a	O
family	O
of	O
technologies	O
used	O
in	O
wired	O
LANs	B-General_Concept
.	O
</s>
<s>
Wireless	B-Architecture
LAN	I-Architecture
based	O
on	O
the	O
IEEE	O
802.11	O
standards	O
,	O
also	O
widely	O
known	O
as	O
WLAN	B-Architecture
or	O
WiFi	O
,	O
is	O
probably	O
the	O
most	O
well-known	O
member	O
of	O
the	O
IEEE	O
802	O
protocol	B-Protocol
family	I-Protocol
for	O
home	O
users	O
today	O
.	O
</s>
<s>
Synchronous	B-Protocol
optical	I-Protocol
networking	I-Protocol
(	O
SONET	B-Protocol
)	O
and	O
Synchronous	B-Protocol
Digital	I-Protocol
Hierarchy	I-Protocol
(	O
SDH	O
)	O
are	O
standardized	O
multiplexing	B-Architecture
protocols	O
that	O
transfer	O
multiple	O
digital	O
bit	O
streams	O
over	O
optical	B-Architecture
fiber	I-Architecture
using	O
lasers	O
.	O
</s>
<s>
They	O
were	O
originally	O
designed	O
to	O
transport	O
circuit	B-Protocol
mode	I-Protocol
communications	O
from	O
a	O
variety	O
of	O
different	O
sources	O
,	O
primarily	O
to	O
support	O
circuit-switched	B-Protocol
digital	B-General_Concept
telephony	I-General_Concept
.	O
</s>
<s>
However	O
,	O
due	O
to	O
its	O
protocol	O
neutrality	O
and	O
transport-oriented	O
features	O
,	O
SONET/SDH	B-Protocol
also	O
was	O
the	O
obvious	O
choice	O
for	O
transporting	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
frames	B-Protocol
.	O
</s>
<s>
It	O
uses	O
asynchronous	O
time-division	B-Protocol
multiplexing	I-Protocol
and	O
encodes	O
data	O
into	O
small	O
,	O
fixed-sized	O
cells	B-Protocol
.	O
</s>
<s>
This	O
differs	O
from	O
other	O
protocols	O
such	O
as	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
or	O
Ethernet	O
that	O
use	O
variable-sized	O
packets	B-Protocol
or	O
frames	B-Protocol
.	O
</s>
<s>
ATM	O
has	O
similarities	O
with	O
both	O
circuit	O
and	O
packet	B-Protocol
switched	I-Protocol
networking	O
.	O
</s>
<s>
This	O
makes	O
it	O
a	O
good	O
choice	O
for	O
a	O
network	O
that	O
must	O
handle	O
both	O
traditional	O
high-throughput	O
data	O
traffic	O
,	O
and	O
real-time	O
,	O
low-latency	B-Protocol
content	O
such	O
as	O
voice	O
and	O
video	O
.	O
</s>
<s>
ATM	O
uses	O
a	O
connection-oriented	B-Protocol
model	I-Protocol
in	O
which	O
a	O
virtual	B-Protocol
circuit	I-Protocol
must	O
be	O
established	O
between	O
two	O
endpoints	O
before	O
the	O
actual	O
data	O
exchange	O
begins	O
.	O
</s>
<s>
ATM	O
still	O
plays	O
a	O
role	O
in	O
the	O
last	B-Protocol
mile	I-Protocol
,	O
which	O
is	O
the	O
connection	O
between	O
an	B-Architecture
Internet	I-Architecture
service	O
provider	O
and	O
the	O
home	O
user	O
.	O
</s>
<s>
There	O
are	O
a	O
number	O
of	O
different	O
digital	O
cellular	O
standards	O
,	O
including	O
:	O
Global	O
System	O
for	O
Mobile	O
Communications	O
(	O
GSM	O
)	O
,	O
General	O
Packet	B-Protocol
Radio	O
Service	O
(	O
GPRS	O
)	O
,	O
cdmaOne	O
,	O
CDMA2000	O
,	O
Evolution-Data	O
Optimized	O
(	O
EV-DO	O
)	O
,	O
Enhanced	O
Data	O
Rates	O
for	O
GSM	O
Evolution	O
(	O
EDGE	O
)	O
,	O
Universal	B-Protocol
Mobile	I-Protocol
Telecommunications	I-Protocol
System	I-Protocol
(	O
UMTS	B-Protocol
)	O
,	O
Digital	B-Protocol
Enhanced	I-Protocol
Cordless	I-Protocol
Telecommunications	I-Protocol
(	O
DECT	B-Protocol
)	O
,	O
Digital	O
AMPS	O
(	O
IS-136/TDMA	O
)	O
,	O
and	O
Integrated	O
Digital	O
Enhanced	O
Network	O
(	O
iDEN	O
)	O
.	O
</s>
<s>
Routing	B-Protocol
is	O
the	O
process	O
of	O
selecting	O
network	O
paths	O
to	O
carry	O
network	O
traffic	O
.	O
</s>
<s>
Routing	B-Protocol
is	O
performed	O
for	O
many	O
kinds	O
of	O
networks	O
,	O
including	O
circuit	B-Protocol
switching	I-Protocol
networks	O
and	O
packet	B-Protocol
switched	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
In	O
packet-switched	B-Protocol
networks	I-Protocol
,	O
routing	B-Protocol
protocols	I-Protocol
direct	O
packet	B-Protocol
forwarding	I-Protocol
through	O
intermediate	B-Protocol
nodes	I-Protocol
.	O
</s>
<s>
Intermediate	B-Protocol
nodes	I-Protocol
are	O
typically	O
network	B-Device
hardware	I-Device
devices	O
such	O
as	O
routers	B-Protocol
,	O
bridges	B-Protocol
,	O
gateways	O
,	O
firewalls	B-Application
,	O
or	O
switches	B-Protocol
.	O
</s>
<s>
General-purpose	O
computers	O
can	O
also	O
forward	O
packets	B-Protocol
and	O
perform	O
routing	B-Protocol
,	O
though	O
because	O
they	O
lack	O
specialized	O
hardware	B-Architecture
,	O
may	O
offer	O
limited	O
performance	O
.	O
</s>
<s>
The	O
routing	B-Protocol
process	O
directs	O
forwarding	O
on	O
the	O
basis	O
of	O
routing	B-Protocol
tables	I-Protocol
,	O
which	O
maintain	O
a	O
record	O
of	O
the	O
routes	O
to	O
various	O
network	O
destinations	O
.	O
</s>
<s>
Most	O
routing	B-Protocol
algorithms	I-Protocol
use	O
only	O
one	O
network	O
path	O
at	O
a	O
time	O
.	O
</s>
<s>
Multipath	B-Protocol
routing	I-Protocol
techniques	O
enable	O
the	O
use	O
of	O
multiple	O
alternative	O
paths	O
.	O
</s>
<s>
Routing	B-Protocol
can	O
be	O
contrasted	O
with	O
bridging	B-Protocol
in	O
its	O
assumption	O
that	O
network	O
addresses	O
are	O
structured	O
and	O
that	O
similar	O
addresses	O
imply	O
proximity	O
within	O
the	O
network	O
.	O
</s>
<s>
Structured	O
addresses	O
allow	O
a	O
single	O
routing	B-Protocol
table	I-Protocol
entry	O
to	O
represent	O
the	O
route	O
to	O
a	O
group	O
of	O
devices	O
.	O
</s>
<s>
In	O
large	O
networks	O
,	O
the	O
structured	O
addressing	O
used	O
by	O
routers	B-Protocol
outperforms	O
unstructured	O
addressing	O
used	O
by	O
bridging	B-Protocol
.	O
</s>
<s>
Structured	O
IP	B-Protocol
addresses	I-Protocol
are	O
used	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Unstructured	O
MAC	O
addresses	O
are	O
used	O
for	O
bridging	B-Protocol
on	O
Ethernet	O
and	O
similar	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
A	O
nanoscale	B-General_Concept
network	I-General_Concept
has	O
key	O
components	O
implemented	O
at	O
the	O
nanoscale	O
,	O
including	O
message	O
carriers	O
,	O
and	O
leverages	O
physical	O
principles	O
that	O
differ	O
from	O
macroscale	O
communication	O
mechanisms	O
.	O
</s>
<s>
A	O
personal	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
PAN	O
)	O
is	O
a	O
computer	B-Architecture
network	I-Architecture
used	O
for	O
communication	O
among	O
computers	O
and	O
different	O
information	O
technological	O
devices	O
close	O
to	O
one	O
person	O
.	O
</s>
<s>
Some	O
examples	O
of	O
devices	O
that	O
are	O
used	O
in	O
a	O
PAN	O
are	O
personal	B-Device
computers	I-Device
,	O
printers	O
,	O
fax	O
machines	O
,	O
telephones	O
,	O
PDAs	O
,	O
scanners	O
,	O
and	O
video	O
game	O
consoles	O
.	O
</s>
<s>
A	O
wired	O
PAN	O
is	O
usually	O
constructed	O
with	O
USB	B-Protocol
and	O
FireWire	B-Protocol
connections	O
while	O
technologies	O
such	O
as	O
Bluetooth	B-Protocol
and	O
infrared	O
communication	O
typically	O
form	O
a	O
wireless	B-General_Concept
PAN	I-General_Concept
.	O
</s>
<s>
A	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
is	O
a	O
network	O
that	O
connects	O
computers	O
and	O
devices	O
in	O
a	O
limited	O
geographical	O
area	O
such	O
as	O
a	O
home	O
,	O
school	O
,	O
office	O
building	O
,	O
or	O
closely	O
positioned	O
group	O
of	O
buildings	O
.	O
</s>
<s>
Wired	O
LANs	B-General_Concept
are	O
most	O
commonly	O
based	O
on	O
Ethernet	O
technology	O
.	O
</s>
<s>
Other	O
networking	O
technologies	O
such	O
as	O
ITU-T	B-General_Concept
G.hn	I-General_Concept
also	O
provide	O
a	O
way	O
to	O
create	O
a	O
wired	O
LAN	B-General_Concept
using	O
existing	O
wiring	O
,	O
such	O
as	O
coaxial	O
cables	O
,	O
telephone	O
lines	O
,	O
and	O
power	B-Device
lines	I-Device
.	O
</s>
<s>
A	O
LAN	B-General_Concept
can	O
be	O
connected	O
to	O
a	O
wide	O
area	O
network	O
(	O
WAN	O
)	O
using	O
a	O
router	B-Protocol
.	O
</s>
<s>
The	O
defining	O
characteristics	O
of	O
a	O
LAN	B-General_Concept
,	O
in	O
contrast	O
to	O
a	O
WAN	O
,	O
include	O
higher	O
data	O
transfer	O
rates	O
,	O
limited	O
geographic	O
range	O
,	O
and	O
lack	O
of	O
reliance	O
on	O
leased	B-Architecture
lines	I-Architecture
to	O
provide	O
connectivity	O
.	O
</s>
<s>
Current	O
Ethernet	O
or	O
other	O
IEEE	O
802.3	O
LAN	B-General_Concept
technologies	O
operate	O
at	O
data	O
transfer	O
rates	O
up	O
to	O
and	O
in	O
excess	O
of	O
100	O
Gbit/s	O
,	O
standardized	O
by	O
IEEE	O
in	O
2010	O
.	O
</s>
<s>
A	O
home	O
area	O
network	O
(	O
HAN	O
)	O
is	O
a	O
residential	O
LAN	B-General_Concept
used	O
for	O
communication	O
between	O
digital	O
devices	O
typically	O
deployed	O
in	O
the	O
home	O
,	O
usually	O
a	O
small	O
number	O
of	O
personal	B-Device
computers	I-Device
and	O
accessories	O
,	O
such	O
as	O
printers	O
and	O
mobile	O
computing	O
devices	O
.	O
</s>
<s>
An	O
important	O
function	O
is	O
the	O
sharing	O
of	O
Internet	O
access	O
,	O
often	O
a	O
broadband	O
service	O
through	O
a	O
cable	B-Device
Internet	I-Device
access	I-Device
or	O
digital	O
subscriber	O
line	O
(	O
DSL	O
)	O
provider	O
.	O
</s>
<s>
SANs	O
are	O
primarily	O
used	O
to	O
make	O
storage	O
devices	O
,	O
such	O
as	O
disk	O
arrays	O
,	O
tape	O
libraries	O
,	O
and	O
optical	O
jukeboxes	O
,	O
accessible	O
to	O
servers	B-Application
so	O
that	O
the	O
storage	O
appears	O
as	O
locally	O
attached	O
devices	O
to	O
the	O
operating	O
system	O
.	O
</s>
<s>
A	O
SAN	O
typically	O
has	O
its	O
own	O
network	O
of	O
storage	O
devices	O
that	O
are	O
generally	O
not	O
accessible	O
through	O
the	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
by	O
other	O
devices	O
.	O
</s>
<s>
A	O
campus	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
CAN	O
)	O
is	O
made	O
up	O
of	O
an	O
interconnection	B-Architecture
of	O
LANs	B-General_Concept
within	O
a	O
limited	O
geographical	O
area	O
.	O
</s>
<s>
The	O
networking	B-Device
equipment	I-Device
(	O
switches	B-Protocol
,	O
routers	B-Protocol
)	O
and	O
transmission	O
media	O
(	O
optical	B-Architecture
fiber	I-Architecture
,	O
Cat5	B-Protocol
cabling	O
,	O
etc	O
.	O
)	O
</s>
<s>
For	O
example	O
,	O
a	O
university	O
campus	B-General_Concept
network	I-General_Concept
is	O
likely	O
to	O
link	O
a	O
variety	O
of	O
campus	O
buildings	O
to	O
connect	O
academic	O
colleges	O
or	O
departments	O
,	O
the	O
library	O
,	O
and	O
student	O
residence	O
halls	O
.	O
</s>
<s>
A	O
backbone	B-General_Concept
network	I-General_Concept
is	O
part	O
of	O
a	O
computer	B-Architecture
network	I-Architecture
infrastructure	O
that	O
provides	O
a	O
path	O
for	O
the	O
exchange	O
of	O
information	O
between	O
different	O
LANs	B-General_Concept
or	O
subnetworks	B-Protocol
.	O
</s>
<s>
A	O
backbone	B-General_Concept
can	O
tie	O
together	O
diverse	O
networks	O
within	O
the	O
same	O
building	O
,	O
across	O
different	O
buildings	O
,	O
or	O
over	O
a	O
wide	O
area	O
.	O
</s>
<s>
When	O
designing	O
a	O
network	B-General_Concept
backbone	I-General_Concept
,	O
network	B-General_Concept
performance	I-General_Concept
and	O
network	B-Protocol
congestion	I-Protocol
are	O
critical	O
factors	O
to	O
take	O
into	O
account	O
.	O
</s>
<s>
Normally	O
,	O
the	O
backbone	B-General_Concept
network	I-General_Concept
's	O
capacity	O
is	O
greater	O
than	O
that	O
of	O
the	O
individual	O
networks	O
connected	O
to	O
it	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
large	O
company	O
might	O
implement	O
a	O
backbone	B-General_Concept
network	I-General_Concept
to	O
connect	O
departments	O
that	O
are	O
located	O
around	O
the	O
world	O
.	O
</s>
<s>
The	O
equipment	O
that	O
ties	O
together	O
the	O
departmental	O
networks	O
constitutes	O
the	O
network	B-General_Concept
backbone	I-General_Concept
.	O
</s>
<s>
Another	O
example	O
of	O
a	O
backbone	B-General_Concept
network	I-General_Concept
is	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
,	O
which	O
is	O
a	O
massive	O
,	O
global	O
system	O
of	O
fiber-optic	B-Architecture
cable	I-Architecture
and	O
optical	O
networking	O
that	O
carry	O
the	O
bulk	O
of	O
data	O
between	O
wide	O
area	O
networks	O
(	O
WANs	O
)	O
,	O
metro	O
,	O
regional	O
,	O
national	O
and	O
transoceanic	O
networks	O
.	O
</s>
<s>
A	O
metropolitan	O
area	O
network	O
(	O
MAN	O
)	O
is	O
a	O
large	O
computer	B-Architecture
network	I-Architecture
that	O
interconnects	B-Architecture
users	O
with	O
computer	O
resources	O
in	O
a	O
geographic	O
region	O
of	O
the	O
size	O
of	O
a	O
metropolitan	O
area	O
.	O
</s>
<s>
A	O
wide	O
area	O
network	O
(	O
WAN	O
)	O
is	O
a	O
computer	B-Architecture
network	I-Architecture
that	O
covers	O
a	O
large	O
geographic	O
area	O
such	O
as	O
a	O
city	O
,	O
country	O
,	O
or	O
spans	O
even	O
intercontinental	O
distances	O
.	O
</s>
<s>
WAN	O
technologies	O
generally	O
function	O
at	O
the	O
lower	O
three	O
layers	O
of	O
the	O
OSI	O
model	O
:	O
the	O
physical	B-Application
layer	I-Application
,	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
and	O
the	O
network	B-Application
layer	I-Application
.	O
</s>
<s>
An	O
enterprise	O
private	O
network	O
is	O
a	O
network	O
that	O
a	O
single	O
organization	O
builds	O
to	O
interconnect	B-Architecture
its	O
office	O
locations	O
(	O
e.g.	O
,	O
production	O
sites	O
,	O
head	O
offices	O
,	O
remote	O
offices	O
,	O
shops	O
)	O
so	O
they	O
can	O
share	O
computer	O
resources	O
.	O
</s>
<s>
A	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
is	O
an	O
overlay	B-Architecture
network	I-Architecture
in	O
which	O
some	O
of	O
the	O
links	O
between	O
nodes	O
are	O
carried	O
by	O
open	O
connections	O
or	O
virtual	B-Protocol
circuits	I-Protocol
in	O
some	O
larger	O
network	O
(	O
e.g.	O
,	O
the	O
Internet	O
)	O
instead	O
of	O
by	O
physical	O
wires	O
.	O
</s>
<s>
The	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocols	O
of	O
the	O
virtual	O
network	O
are	O
said	O
to	O
be	O
tunneled	O
through	O
the	O
larger	O
network	O
.	O
</s>
<s>
One	O
common	O
application	O
is	O
secure	O
communications	O
through	O
the	O
public	O
Internet	O
,	O
but	O
a	O
VPN	B-Application
need	O
not	O
have	O
explicit	O
security	O
features	O
,	O
such	O
as	O
authentication	O
or	O
content	O
encryption	O
.	O
</s>
<s>
VPNs	B-Application
,	O
for	O
example	O
,	O
can	O
be	O
used	O
to	O
separate	O
the	O
traffic	O
of	O
different	O
user	O
communities	O
over	O
an	O
underlying	O
network	O
with	O
strong	O
security	O
features	O
.	O
</s>
<s>
VPN	B-Application
may	O
have	O
best-effort	O
performance	O
or	O
may	O
have	O
a	O
defined	O
service	O
level	O
agreement	O
(	O
SLA	O
)	O
between	O
the	O
VPN	B-Application
customer	O
and	O
the	O
VPN	B-Application
service	I-Application
provider	I-Application
.	O
</s>
<s>
A	O
global	O
area	O
network	O
(	O
GAN	O
)	O
is	O
a	O
network	O
used	O
for	O
supporting	O
mobile	O
users	O
across	O
an	O
arbitrary	O
number	O
of	O
wireless	B-Architecture
LANs	I-Architecture
,	O
satellite	O
coverage	O
areas	O
,	O
etc	O
.	O
</s>
<s>
In	O
IEEE	O
Project	O
802	O
,	O
this	O
involves	O
a	O
succession	O
of	O
terrestrial	O
wireless	B-Architecture
LANs	I-Architecture
.	O
</s>
<s>
Private	O
enterprise	B-General_Concept
networks	I-General_Concept
may	O
use	O
a	O
combination	O
of	O
intranets	B-General_Concept
and	O
extranets	B-Architecture
.	O
</s>
<s>
An	O
intranet	B-General_Concept
is	O
a	O
set	O
of	O
networks	O
that	O
are	O
under	O
the	O
control	O
of	O
a	O
single	O
administrative	O
entity	O
.	O
</s>
<s>
An	O
intranet	B-General_Concept
typically	O
uses	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
and	O
IP-based	O
tools	O
such	O
as	O
web	O
browsers	O
and	O
file	O
transfer	O
applications	B-Application
.	O
</s>
<s>
The	O
administrative	O
entity	O
limits	O
the	O
use	O
of	O
the	O
intranet	B-General_Concept
to	O
its	O
authorized	O
users	O
.	O
</s>
<s>
Most	O
commonly	O
,	O
an	O
intranet	B-General_Concept
is	O
the	O
internal	O
LAN	B-General_Concept
of	O
an	O
organization	O
.	O
</s>
<s>
A	O
large	O
intranet	B-General_Concept
typically	O
has	O
at	O
least	O
one	O
web	O
server	B-Application
to	O
provide	O
users	O
with	O
organizational	O
information	O
.	O
</s>
<s>
An	O
extranet	B-Architecture
is	O
a	O
network	O
that	O
is	O
also	O
under	O
the	O
administrative	O
control	O
of	O
a	O
single	O
organization	O
but	O
supports	O
a	O
limited	O
connection	O
to	O
a	O
specific	O
external	O
network	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
organization	O
may	O
provide	O
access	O
to	O
some	O
aspects	O
of	O
its	O
intranet	B-General_Concept
to	O
share	O
data	O
with	O
its	O
business	O
partners	O
or	O
customers	O
.	O
</s>
<s>
Network	O
connection	O
to	O
an	O
extranet	B-Architecture
is	O
often	O
,	O
but	O
not	O
always	O
,	O
implemented	O
via	O
WAN	O
technology	O
.	O
</s>
<s>
An	O
internetwork	B-Architecture
is	O
the	O
connection	O
of	O
multiple	O
different	O
types	O
of	O
computer	B-Architecture
networks	I-Architecture
to	O
form	O
a	O
single	O
computer	B-Architecture
network	I-Architecture
by	O
layering	O
on	O
top	O
of	O
the	O
different	O
networking	B-Architecture
software	I-Architecture
and	O
connecting	O
them	O
together	O
using	O
routers	B-Protocol
.	O
</s>
<s>
The	O
Internet	O
is	O
the	O
largest	O
example	O
of	O
internetwork	B-Architecture
.	O
</s>
<s>
It	O
is	O
a	O
global	O
system	O
of	O
interconnected	O
governmental	O
,	O
academic	O
,	O
corporate	O
,	O
public	O
,	O
and	O
private	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
It	O
is	O
based	O
on	O
the	O
networking	O
technologies	O
of	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
.	O
</s>
<s>
The	O
Internet	O
utilizes	O
copper	O
communications	O
and	O
the	O
optical	O
networking	O
backbone	B-General_Concept
to	O
enable	O
the	O
World	O
Wide	O
Web	O
(	O
WWW	O
)	O
,	O
the	B-Operating_System
Internet	I-Operating_System
of	I-Operating_System
things	I-Operating_System
,	O
video	O
transfer	O
,	O
and	O
a	O
broad	O
range	O
of	O
information	O
services	B-General_Concept
.	O
</s>
<s>
Participants	O
on	O
the	O
Internet	O
use	O
a	O
diverse	O
array	O
of	O
methods	O
of	O
several	O
hundred	O
documented	O
,	O
and	O
often	O
standardized	O
,	O
protocols	O
compatible	O
with	O
the	O
Internet	B-Protocol
protocol	I-Protocol
suite	I-Protocol
and	O
an	O
addressing	O
system	O
(	O
IP	B-Protocol
addresses	I-Protocol
)	O
administered	O
by	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
and	O
address	O
registries	O
.	O
</s>
<s>
Service	O
providers	O
and	O
large	O
enterprises	O
exchange	O
information	O
about	O
the	O
reachability	O
of	O
their	O
address	O
spaces	O
through	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
,	O
forming	O
a	O
redundant	O
worldwide	O
mesh	O
of	O
transmission	O
paths	O
.	O
</s>
<s>
A	O
darknet	B-Operating_System
is	O
an	O
overlay	B-Architecture
network	I-Architecture
,	O
typically	O
running	O
on	O
the	O
Internet	O
,	O
that	O
is	O
only	O
accessible	O
through	O
specialized	O
software	O
.	O
</s>
<s>
A	O
darknet	B-Operating_System
is	O
an	O
anonymizing	O
network	O
where	O
connections	O
are	O
made	O
only	O
between	O
trusted	O
peers	O
—	O
sometimes	O
called	O
"	O
friends	O
"	O
(	O
F2F	O
)	O
—	O
using	O
non-standard	O
protocols	O
and	O
ports	B-Protocol
.	O
</s>
<s>
Darknets	B-Operating_System
are	O
distinct	O
from	O
other	O
distributed	O
peer-to-peer	O
networks	O
as	O
sharing	O
is	O
anonymous	O
(	O
that	O
is	O
,	O
IP	B-Protocol
addresses	I-Protocol
are	O
not	O
publicly	O
shared	O
)	O
,	O
and	O
therefore	O
users	O
can	O
communicate	O
with	O
little	O
fear	O
of	O
governmental	O
or	O
corporate	O
interference	O
.	O
</s>
<s>
Network	B-General_Concept
services	I-General_Concept
are	O
applications	B-Application
hosted	O
by	O
servers	B-Application
on	O
a	O
computer	B-Architecture
network	I-Architecture
,	O
to	O
provide	O
some	O
functionality	O
for	O
members	O
or	O
users	O
of	O
the	O
network	O
,	O
or	O
to	O
help	O
the	O
network	O
itself	O
to	O
operate	O
.	O
</s>
<s>
The	O
World	O
Wide	O
Web	O
,	O
E-mail	O
,	O
printing	O
and	O
network	B-Protocol
file	I-Protocol
sharing	I-Protocol
are	O
examples	O
of	O
well-known	O
network	B-General_Concept
services	I-General_Concept
.	O
</s>
<s>
Network	B-General_Concept
services	I-General_Concept
such	O
as	O
DNS	B-Protocol
(	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
)	O
give	O
names	O
for	O
IP	B-Protocol
and	O
MAC	O
addresses	O
(	O
people	O
remember	O
names	O
like	O
"	O
nm.lan	O
"	O
better	O
than	O
numbers	O
like	O
"	O
210.121.67.18	O
"	O
)	O
,	O
and	O
DHCP	B-Protocol
to	O
ensure	O
that	O
the	O
equipment	O
on	O
the	O
network	O
has	O
a	O
valid	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Services	B-General_Concept
are	O
usually	O
based	O
on	O
a	O
service	O
protocol	O
that	O
defines	O
the	O
format	O
and	O
sequencing	O
of	O
messages	O
between	O
clients	B-Protocol
and	O
servers	B-Application
of	O
that	O
network	B-General_Concept
service	I-General_Concept
.	O
</s>
<s>
The	O
throughput	O
is	O
affected	O
by	O
technologies	O
such	O
as	O
bandwidth	B-Application
shaping	I-Application
,	O
bandwidth	O
management	O
,	O
bandwidth	O
throttling	O
,	O
bandwidth	O
cap	O
,	O
bandwidth	B-Algorithm
allocation	I-Algorithm
(	O
for	O
example	O
bandwidth	B-Protocol
allocation	I-Protocol
protocol	I-Protocol
and	O
dynamic	B-Protocol
bandwidth	I-Protocol
allocation	I-Protocol
)	O
,	O
etc	O
.	O
</s>
<s>
Network	B-Protocol
delay	I-Protocol
is	O
a	O
design	O
and	O
performance	O
characteristic	O
of	O
a	O
telecommunications	O
network	O
.	O
</s>
<s>
It	O
specifies	O
the	O
latency	B-Protocol
for	O
a	O
bit	O
of	O
data	O
to	O
travel	O
across	O
the	O
network	O
from	O
one	O
communication	B-Protocol
endpoint	I-Protocol
to	O
another	O
.	O
</s>
<s>
A	O
certain	O
minimum	O
level	O
of	O
delay	O
is	O
experienced	O
by	O
signals	O
due	O
to	O
the	O
time	O
it	O
takes	O
to	O
transmit	O
a	O
packet	B-Protocol
serially	O
through	O
a	O
link	O
.	O
</s>
<s>
This	O
delay	O
is	O
extended	O
by	O
more	O
variable	O
levels	O
of	O
delay	O
due	O
to	O
network	B-Protocol
congestion	I-Protocol
.	O
</s>
<s>
IP	B-Protocol
network	I-Protocol
delays	O
can	O
range	O
from	O
a	O
few	O
milliseconds	O
to	O
several	O
hundred	O
milliseconds	O
.	O
</s>
<s>
Depending	O
on	O
the	O
installation	O
requirements	O
,	O
network	B-General_Concept
performance	I-General_Concept
is	O
usually	O
measured	O
by	O
the	O
quality	O
of	O
service	O
of	O
a	O
telecommunications	O
product	O
.	O
</s>
<s>
The	O
parameters	O
that	O
affect	O
this	O
typically	O
can	O
include	O
throughput	O
,	O
jitter	O
,	O
bit	O
error	O
rate	O
and	O
latency	B-Protocol
.	O
</s>
<s>
The	O
following	O
list	O
gives	O
examples	O
of	O
network	B-General_Concept
performance	I-General_Concept
measures	O
for	O
a	O
circuit-switched	B-Protocol
network	I-Protocol
and	O
one	O
type	O
of	O
packet-switched	B-Protocol
network	I-Protocol
,	O
viz	O
.	O
</s>
<s>
Circuit-switched	B-Protocol
networks	I-Protocol
:	O
In	O
circuit	B-Protocol
switched	I-Protocol
networks	I-Protocol
,	O
network	B-General_Concept
performance	I-General_Concept
is	O
synonymous	O
with	O
the	O
grade	O
of	O
service	O
.	O
</s>
<s>
ATM	O
:	O
In	O
an	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
network	O
,	O
performance	O
can	O
be	O
measured	O
by	O
line	O
rate	O
,	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
,	O
data	O
throughput	O
,	O
connect	O
time	O
,	O
stability	O
,	O
technology	O
,	O
modulation	B-Application
technique	O
,	O
and	O
modem	B-Application
enhancements	O
.	O
</s>
<s>
For	O
example	O
,	O
state	B-Application
transition	I-Application
diagrams	I-Application
are	O
often	O
used	O
to	O
model	O
queuing	O
performance	O
in	O
a	O
circuit-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Network	B-Protocol
congestion	I-Protocol
occurs	O
when	O
a	O
link	O
or	O
node	O
is	O
subjected	O
to	O
a	O
greater	O
data	O
load	O
than	O
it	O
is	O
rated	O
for	O
,	O
resulting	O
in	O
a	O
deterioration	O
of	O
its	O
quality	O
of	O
service	O
.	O
</s>
<s>
When	O
networks	O
are	O
congested	O
and	O
queues	O
become	O
too	O
full	O
,	O
packets	B-Protocol
have	O
to	O
be	O
discarded	O
,	O
and	O
so	O
networks	O
rely	O
on	O
re-transmission	B-Protocol
.	O
</s>
<s>
Typical	O
effects	O
of	O
congestion	O
include	O
queueing	B-Protocol
delay	I-Protocol
,	O
packet	B-Protocol
loss	I-Protocol
or	O
the	O
blocking	O
of	O
new	O
connections	O
.	O
</s>
<s>
Network	O
protocols	O
that	O
use	O
aggressive	O
retransmissions	B-Protocol
to	O
compensate	O
for	O
packet	B-Protocol
loss	I-Protocol
tend	O
to	O
keep	O
systems	O
in	O
a	O
state	O
of	O
network	B-Protocol
congestion	I-Protocol
—	O
even	O
after	O
the	O
initial	O
load	O
is	O
reduced	O
to	O
a	O
level	O
that	O
would	O
not	O
normally	O
induce	O
network	B-Protocol
congestion	I-Protocol
.	O
</s>
<s>
These	O
techniques	O
include	O
:	O
exponential	B-Algorithm
backoff	I-Algorithm
in	O
protocols	O
such	O
as	O
802.11	O
'	O
s	O
CSMA/CA	O
and	O
the	O
original	O
Ethernet	O
,	O
window	O
reduction	O
in	O
TCP	B-Protocol
,	O
and	O
fair	B-Algorithm
queueing	I-Algorithm
in	O
devices	O
such	O
as	O
routers	B-Protocol
.	O
</s>
<s>
Another	O
method	O
to	O
avoid	O
the	O
negative	O
effects	O
of	O
network	B-Protocol
congestion	I-Protocol
is	O
implementing	O
priority	O
schemes	O
so	O
that	O
some	O
packets	B-Protocol
are	O
transmitted	O
with	O
higher	O
priority	O
than	O
others	O
.	O
</s>
<s>
Priority	O
schemes	O
do	O
not	O
solve	O
network	B-Protocol
congestion	I-Protocol
by	O
themselves	O
,	O
but	O
they	O
help	O
to	O
alleviate	O
the	O
effects	O
of	O
congestion	O
for	O
some	O
services	B-General_Concept
.	O
</s>
<s>
A	O
third	O
method	O
to	O
avoid	O
network	B-Protocol
congestion	I-Protocol
is	O
the	O
explicit	O
allocation	O
of	O
network	O
resources	O
to	O
specific	O
flows	O
.	O
</s>
<s>
One	O
example	O
of	O
this	O
is	O
the	O
use	O
of	O
Contention-Free	O
Transmission	O
Opportunities	O
(	O
CFTXOPs	O
)	O
in	O
the	O
ITU-T	B-General_Concept
G.hn	I-General_Concept
standard	O
,	O
which	O
provides	O
high-speed	O
(	O
up	O
to	O
1Gbit/s	O
)	O
Local	B-General_Concept
area	I-General_Concept
networking	I-General_Concept
over	O
existing	O
home	O
wires	O
(	O
power	B-Device
lines	I-Device
,	O
phone	O
lines	O
and	O
coaxial	O
cables	O
)	O
.	O
</s>
<s>
Network	B-General_Concept
resilience	I-General_Concept
is	O
"	O
the	O
ability	O
to	O
provide	O
and	O
maintain	O
an	O
acceptable	O
level	O
of	O
service	O
in	O
the	O
face	O
of	O
faults	B-General_Concept
and	O
challenges	O
to	O
normal	O
operation.	O
"	O
</s>
<s>
Computer	B-Architecture
networks	I-Architecture
are	O
also	O
used	O
by	O
security	O
hackers	O
to	O
deploy	O
computer	O
viruses	O
or	O
computer	O
worms	O
on	O
devices	O
connected	O
to	O
the	O
network	O
,	O
or	O
to	O
prevent	O
these	O
devices	O
from	O
accessing	O
the	O
network	O
via	O
a	O
denial-of-service	O
attack	O
.	O
</s>
<s>
Network	O
Security	O
consists	O
of	O
provisions	O
and	O
policies	O
adopted	O
by	O
the	O
network	O
administrator	O
to	O
prevent	O
and	O
monitor	O
unauthorized	O
access	O
,	O
misuse	O
,	O
modification	O
,	O
or	O
denial	O
of	O
the	O
computer	B-Architecture
network	I-Architecture
and	O
its	O
network-accessible	O
resources	O
.	O
</s>
<s>
Network	O
security	O
is	O
used	O
on	O
a	O
variety	O
of	O
computer	B-Architecture
networks	I-Architecture
,	O
both	O
public	O
and	O
private	O
,	O
to	O
secure	O
daily	O
transactions	O
and	O
communications	O
among	O
businesses	O
,	O
government	O
agencies	O
,	O
and	O
individuals	O
.	O
</s>
<s>
Network	O
surveillance	O
is	O
the	O
monitoring	O
of	O
data	O
being	O
transferred	O
over	O
computer	B-Architecture
networks	I-Architecture
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
Examples	O
of	O
end-to-end	O
encryption	O
include	O
HTTPS	B-Protocol
for	O
web	O
traffic	O
,	O
PGP	B-Application
for	O
email	O
,	O
OTR	B-Protocol
for	O
instant	B-Application
messaging	I-Application
,	O
ZRTP	B-Protocol
for	O
telephony	O
,	O
and	O
TETRA	O
for	O
radio	O
.	O
</s>
<s>
Typical	O
server-based	O
communications	O
systems	O
do	O
not	O
include	O
end-to-end	O
encryption	O
.	O
</s>
<s>
These	O
systems	O
can	O
only	O
guarantee	O
the	O
protection	O
of	O
communications	O
between	O
clients	B-Protocol
and	O
servers	B-Application
,	O
not	O
between	O
the	O
communicating	O
parties	O
themselves	O
.	O
</s>
<s>
Examples	O
of	O
non-E2EE	O
systems	O
are	O
Google	B-Application
Talk	I-Application
,	O
Yahoo	B-Application
Messenger	I-Application
,	O
Facebook	B-Application
,	O
and	O
Dropbox	B-Application
.	O
</s>
<s>
Some	O
systems	O
that	O
normally	O
offer	O
end-to-end	O
encryption	O
have	O
turned	O
out	O
to	O
contain	O
a	O
back	O
door	O
that	O
subverts	O
negotiation	O
of	O
the	O
encryption	O
key	O
between	O
the	O
communicating	O
parties	O
,	O
for	O
example	O
Skype	B-Application
or	O
Hushmail	B-Application
.	O
</s>
<s>
The	O
end-to-end	O
encryption	O
paradigm	O
does	O
not	O
directly	O
address	O
risks	O
at	O
the	O
endpoints	O
of	O
the	O
communication	O
themselves	O
,	O
such	O
as	O
the	O
technical	O
exploitation	O
of	O
clients	B-Protocol
,	O
poor	O
quality	O
random	O
number	O
generators	O
,	O
or	O
key	O
escrow	O
.	O
</s>
<s>
At	O
the	O
time	O
,	O
the	O
dominant	O
web	O
browser	O
was	O
Netscape	B-Protocol
Navigator	I-Protocol
.	O
</s>
<s>
SSL	O
requires	O
a	O
server	B-Application
with	O
a	O
certificate	O
.	O
</s>
<s>
When	O
a	O
client	O
requests	O
access	O
to	O
an	O
SSL-secured	O
server	B-Application
,	O
the	O
server	B-Application
sends	O
a	O
copy	O
of	O
the	O
certificate	O
to	O
the	O
client	O
.	O
</s>
<s>
The	O
SSL	O
client	O
checks	O
this	O
certificate	O
(	O
all	O
web	O
browsers	O
come	O
with	O
an	O
exhaustive	O
list	O
of	O
CA	O
root	O
certificates	O
preloaded	O
)	O
,	O
and	O
if	O
the	O
certificate	O
checks	O
out	O
,	O
the	O
server	B-Application
is	O
authenticated	O
and	O
the	O
client	O
negotiates	O
a	O
symmetric-key	O
cipher	O
for	O
use	O
in	O
the	O
session	O
.	O
</s>
<s>
The	O
session	O
is	O
now	O
in	O
a	O
very	O
secure	O
encrypted	O
tunnel	O
between	O
the	O
SSL	O
server	B-Application
and	O
the	O
SSL	O
client	O
.	O
</s>
<s>
Users	O
can	O
share	O
printers	O
and	O
some	O
servers	B-Application
from	O
a	O
workgroup	B-Device
,	O
which	O
usually	O
means	O
they	O
are	O
in	O
the	O
same	O
geographic	O
location	O
and	O
are	O
on	O
the	O
same	O
LAN	B-General_Concept
,	O
whereas	O
a	O
Network	O
Administrator	O
is	O
responsible	O
to	O
keep	O
that	O
network	O
up	O
and	O
running	O
.	O
</s>
<s>
A	O
community	O
of	O
interest	O
has	O
less	O
of	O
a	O
connection	O
of	O
being	O
in	O
a	O
local	O
area	O
and	O
should	O
be	O
thought	O
of	O
as	O
a	O
set	O
of	O
arbitrarily	O
located	O
users	O
who	O
share	O
a	O
set	O
of	O
servers	B-Application
,	O
and	O
possibly	O
also	O
communicate	O
via	O
peer-to-peer	O
technologies	O
.	O
</s>
<s>
The	O
physical	O
perspective	O
involves	O
geographic	O
locations	O
,	O
physical	O
cabling	O
,	O
and	O
the	O
network	O
elements	O
(	O
e.g.	O
,	O
routers	B-Protocol
,	O
bridges	B-Protocol
and	O
application	O
layer	O
gateways	O
)	O
that	O
interconnect	B-Architecture
via	O
the	O
transmission	O
media	O
.	O
</s>
<s>
Logical	O
networks	O
,	O
called	O
,	O
in	O
the	O
TCP/IP	B-Protocol
architecture	O
,	O
subnets	B-Protocol
,	O
map	B-Application
onto	O
one	O
or	O
more	O
transmission	O
media	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
common	O
practice	O
in	O
a	O
campus	O
of	O
buildings	O
is	O
to	O
make	O
a	O
set	O
of	O
LAN	B-General_Concept
cables	O
in	O
each	O
building	O
appear	O
to	O
be	O
a	O
common	O
subnet	B-Protocol
,	O
using	O
VLAN	B-General_Concept
technology	O
.	O
</s>
<s>
Again	O
using	O
TCP/IP	B-Protocol
architectural	O
terminology	O
,	O
an	O
intranet	B-General_Concept
is	O
a	O
community	O
of	O
interest	O
under	O
private	O
administration	O
usually	O
by	O
an	O
enterprise	O
,	O
and	O
is	O
only	O
accessible	O
by	O
authorized	O
users	O
(	O
e.g.	O
</s>
<s>
Intranets	B-General_Concept
do	O
not	O
have	O
to	O
be	O
connected	O
to	O
the	O
Internet	O
,	O
but	O
generally	O
have	O
a	O
limited	O
connection	O
.	O
</s>
<s>
An	O
extranet	B-Architecture
is	O
an	O
extension	O
of	O
an	O
intranet	B-General_Concept
that	O
allows	O
secure	O
communications	O
to	O
users	O
outside	O
of	O
the	O
intranet	B-General_Concept
(	O
e.g.	O
</s>
<s>
From	O
an	O
engineering	O
viewpoint	O
,	O
the	O
Internet	O
is	O
the	O
set	O
of	O
subnets	B-Protocol
,	O
and	O
aggregates	O
of	O
subnets	B-Protocol
,	O
that	O
share	O
the	O
registered	O
IP	B-Protocol
address	I-Protocol
space	O
and	O
exchange	O
information	O
about	O
the	O
reachability	O
of	O
those	O
IP	B-Protocol
addresses	I-Protocol
using	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
Typically	O
,	O
the	O
human-readable	B-General_Concept
names	O
of	O
servers	B-Application
are	O
translated	O
to	O
IP	B-Protocol
addresses	I-Protocol
,	O
transparently	O
to	O
users	O
,	O
via	O
the	O
directory	O
function	O
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
.	O
</s>
<s>
Intranets	B-General_Concept
and	O
extranets	B-Architecture
can	O
be	O
securely	O
superimposed	O
onto	O
the	O
Internet	O
,	O
without	O
any	O
access	O
by	O
general	O
Internet	O
users	O
and	O
administrators	O
,	O
using	O
secure	O
Virtual	B-Application
Private	I-Application
Network	I-Application
(	O
VPN	B-Application
)	O
technology	O
.	O
</s>
