<s>
A	O
virtual	B-Protocol
circuit	I-Protocol
(	O
VC	O
)	O
is	O
a	O
means	O
of	O
transporting	O
data	O
over	O
a	O
data	O
network	O
,	O
based	O
on	O
packet	B-Protocol
switching	I-Protocol
and	O
in	O
which	O
a	O
connection	O
is	O
established	O
within	O
the	O
network	O
between	O
two	O
endpoints	O
.	O
</s>
<s>
The	O
network	O
,	O
rather	O
than	O
having	O
a	O
fixed	O
data	O
rate	O
reservation	O
per	O
connection	O
,	O
like	O
in	O
circuit	B-Protocol
switching	I-Protocol
,	O
takes	O
advantage	O
of	O
the	O
statistical	B-Algorithm
multiplexing	I-Algorithm
on	O
its	O
transmission	O
links	O
(	O
an	O
intrinsic	O
feature	O
of	O
packet	B-Protocol
switching	I-Protocol
,	O
well	O
suited	O
to	O
data	O
traffic	O
)	O
.	O
</s>
<s>
They	O
thus	O
eliminate	O
the	O
need	O
for	O
the	O
network	O
to	O
lose	O
user	O
packets	O
in	O
heavily	O
loaded	O
network	O
zones	O
,	O
an	O
intrinsic	O
feature	O
of	O
datagram	B-Protocol
networks	I-Protocol
for	O
their	O
congestion	O
control	O
.	O
</s>
<s>
Before	O
a	O
connection	O
or	O
virtual	B-Protocol
circuit	I-Protocol
may	O
be	O
used	O
,	O
it	O
must	O
be	O
established	O
between	O
two	O
or	O
more	O
nodes	B-Protocol
or	O
software	B-Application
applications	I-Application
by	O
means	O
of	O
call	O
setup	O
.	O
</s>
<s>
After	O
that	O
,	O
a	O
bit	O
stream	O
or	O
byte	O
stream	O
may	O
be	O
delivered	O
between	O
the	O
nodes	B-Protocol
;	O
hence	O
,	O
a	O
virtual	B-Protocol
circuit	I-Protocol
protocol	O
allows	O
higher-level	O
protocols	O
to	O
avoid	O
dealing	O
with	O
the	O
division	O
of	O
data	O
into	O
Protocol	B-Protocol
data	I-Protocol
units	I-Protocol
.	O
</s>
<s>
Many	O
virtual	B-Protocol
circuit	I-Protocol
protocols	O
,	O
but	O
not	O
all	O
,	O
provide	O
reliable	B-Protocol
communication	O
service	O
through	O
the	O
use	O
of	O
data	O
retransmissions	O
invoked	O
by	O
error	B-Error_Name
detection	I-Error_Name
and	O
automatic	B-Error_Name
repeat	I-Error_Name
request	I-Error_Name
(	O
ARQ	O
)	O
.	O
</s>
<s>
An	O
alternative	O
to	O
virtual-circuit	O
networks	O
are	O
datagram	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
Virtual	B-Protocol
circuit	I-Protocol
communication	O
resembles	O
circuit	B-Protocol
switching	I-Protocol
,	O
since	O
both	O
are	O
connection	B-Protocol
oriented	I-Protocol
,	O
meaning	O
that	O
in	O
both	O
cases	O
data	O
is	O
delivered	O
in	O
correct	O
order	O
,	O
and	O
signalling	O
overhead	O
is	O
required	O
during	O
a	O
connection	O
establishment	O
phase	O
.	O
</s>
<s>
However	O
,	O
circuit	B-Protocol
switching	I-Protocol
provides	O
a	O
constant	B-General_Concept
bit	I-General_Concept
rate	I-General_Concept
and	O
latency	O
,	O
while	O
these	O
may	O
vary	O
in	O
a	O
virtual	B-Protocol
circuit	I-Protocol
service	O
due	O
to	O
factors	O
such	O
as	O
:	O
</s>
<s>
varying	O
packet	O
queue	O
lengths	O
in	O
the	O
network	B-Protocol
nodes	I-Protocol
,	O
</s>
<s>
varying	O
load	O
from	O
other	O
users	O
sharing	O
the	O
same	O
network	O
resources	O
by	O
means	O
of	O
statistical	B-Algorithm
multiplexing	I-Algorithm
,	O
etc	O
.	O
</s>
<s>
An	O
alternative	O
approach	O
to	O
virtual	O
calls	O
is	O
connectionless	B-Protocol
communication	I-Protocol
using	O
datagrams	B-Protocol
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
the	O
"	O
virtual	O
call	O
"	O
concept	O
was	O
used	O
in	O
the	O
British	O
EPSS	O
and	O
enhanced	O
by	O
Rémi	O
Després	O
as	O
"	O
virtual	B-Protocol
circuits	I-Protocol
"	O
in	O
the	O
French	O
RCP	O
.	O
</s>
<s>
Connection	B-Protocol
oriented	I-Protocol
transport	B-Architecture
layer	I-Architecture
protocols	O
such	O
as	O
TCP	B-Protocol
may	O
rely	O
on	O
a	O
connectionless	B-Protocol
packet	B-Protocol
switching	I-Protocol
network	B-Application
layer	I-Application
protocol	O
such	O
as	O
IP	B-Protocol
,	O
where	O
different	O
packets	O
may	O
be	O
routed	O
over	O
different	O
paths	O
,	O
and	O
thus	O
be	O
delivered	O
out	O
of	O
order	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
possible	O
to	O
use	O
TCP	B-Protocol
as	O
a	O
virtual	B-Protocol
circuit	I-Protocol
,	O
since	O
TCP	B-Protocol
includes	O
segment	O
numbering	O
that	O
allows	O
reordering	O
on	O
the	O
receiver	O
side	O
to	O
accommodate	O
out-of-order	O
delivery	O
.	O
</s>
<s>
Data	B-Protocol
link	I-Protocol
layer	I-Protocol
and	O
network	B-Application
layer	I-Application
virtual	B-Protocol
circuit	I-Protocol
protocols	O
are	O
based	O
on	O
connection-oriented	B-Protocol
packet	B-Protocol
switching	I-Protocol
,	O
meaning	O
that	O
data	O
is	O
always	O
delivered	O
along	O
the	O
same	O
network	O
path	O
,	O
i.e.	O
,	O
through	O
the	O
same	O
nodes	B-Protocol
.	O
</s>
<s>
Advantages	O
with	O
this	O
over	O
connectionless	B-Protocol
packet	B-Protocol
switching	I-Protocol
are	O
:	O
</s>
<s>
For	O
example	O
,	O
a	O
constant	B-General_Concept
bit	I-General_Concept
rate	I-General_Concept
QoS	O
class	O
may	O
be	O
provided	O
,	O
resulting	O
in	O
emulation	O
of	O
circuit	B-Protocol
switching	I-Protocol
.	O
</s>
<s>
Only	O
a	O
small	O
virtual	B-Protocol
channel	I-Protocol
identifier	O
(	O
VCI	O
)	O
is	O
required	O
in	O
each	O
packet	O
.	O
</s>
<s>
Routing	O
information	O
is	O
only	O
transferred	O
to	O
the	O
network	B-Protocol
nodes	I-Protocol
during	O
the	O
connection	O
establishment	O
phase	O
.	O
</s>
<s>
The	O
network	B-Protocol
nodes	I-Protocol
are	O
faster	O
and	O
have	O
higher	O
capacity	O
in	O
theory	O
since	O
they	O
are	O
switches	O
that	O
only	O
perform	O
routing	O
during	O
the	O
connection	O
establishment	O
phase	O
,	O
while	O
connectionless	B-Protocol
network	B-Protocol
nodes	I-Protocol
are	O
routers	O
that	O
perform	O
routing	O
for	O
each	O
packet	O
individually	O
.	O
</s>
<s>
Switching	O
only	O
involves	O
looking	O
up	O
the	O
virtual	B-Protocol
channel	I-Protocol
identifier	O
in	O
a	O
table	O
rather	O
than	O
analyzing	O
a	O
complete	O
address	O
.	O
</s>
<s>
However	O
,	O
because	O
of	O
the	O
large	O
market	O
of	O
IP	B-Protocol
routers	O
,	O
and	O
because	O
advanced	O
IP	B-Protocol
routers	O
support	O
layer	B-Application
3	I-Application
switching	O
,	O
modern	O
IP	B-Protocol
routers	O
may	O
today	O
be	O
faster	O
than	O
switches	O
for	O
connection-oriented	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Examples	O
of	O
transport	B-Architecture
layer	I-Architecture
protocols	O
that	O
provide	O
a	O
virtual	B-Protocol
circuit	I-Protocol
:	O
</s>
<s>
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
,	O
where	O
a	O
reliable	B-Protocol
virtual	B-Protocol
circuit	I-Protocol
is	O
established	O
on	O
top	O
of	O
the	O
underlying	O
unreliable	O
and	O
connectionless	B-Protocol
IP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
The	O
virtual	B-Protocol
circuit	I-Protocol
is	O
identified	O
by	O
the	O
source	O
and	O
destination	O
network	B-Protocol
socket	I-Protocol
address	O
pair	O
,	O
i.e.	O
</s>
<s>
the	O
sender	O
and	O
receiver	O
IP	B-Protocol
address	O
and	O
port	O
number	O
.	O
</s>
<s>
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
,	O
where	O
a	O
virtual	B-Protocol
circuit	I-Protocol
is	O
established	O
on	O
top	O
of	O
the	O
IP	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Examples	O
of	O
network-layer	B-Application
and	O
data-link-layer	B-Protocol
virtual	B-Protocol
circuit	I-Protocol
protocols	O
,	O
where	O
data	O
always	O
is	O
delivered	O
over	O
the	O
same	O
path	O
:	O
</s>
<s>
X.25	B-Protocol
,	O
where	O
the	O
VC	O
is	O
identified	O
by	O
a	O
virtual	B-Protocol
channel	I-Protocol
identifier	O
(	O
VCI	O
)	O
.	O
</s>
<s>
X.25	B-Protocol
provides	O
reliable	B-Protocol
node-to-node	O
communication	O
and	O
guaranteed	O
QoS	O
.	O
</s>
<s>
Frame	B-Protocol
Relay	I-Protocol
,	O
where	O
the	O
VC	O
is	O
identified	O
by	O
a	O
DLCI	B-Protocol
.	O
</s>
<s>
Frame	B-Protocol
Relay	I-Protocol
is	O
unreliable	O
,	O
but	O
may	O
provide	O
guaranteed	O
QoS	O
.	O
</s>
<s>
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
,	O
where	O
the	O
circuit	O
is	O
identified	O
by	O
a	O
virtual	O
path	O
identifier	O
(	O
VPI	O
)	O
and	O
virtual	B-Protocol
channel	I-Protocol
identifier	O
(	O
VCI	O
)	O
pair	O
.	O
</s>
<s>
The	O
ATM	O
layer	O
provides	O
unreliable	O
virtual	B-Protocol
circuits	I-Protocol
,	O
but	O
the	O
ATM	O
protocol	O
provides	O
for	O
reliability	O
through	O
the	O
ATM	B-Protocol
adaptation	I-Protocol
layer	I-Protocol
(	O
AAL	B-Protocol
)	O
Service	O
Specific	O
Convergence	O
Sublayer	O
(	O
SSCS	O
)	O
(	O
though	O
it	O
uses	O
the	O
terms	O
"	O
assured	O
"	O
and	O
"	O
non-assured	O
"	O
rather	O
than	O
"	O
reliable	B-Protocol
"	O
and	O
"	O
unreliable	O
"	O
)	O
.	O
</s>
<s>
Multiprotocol	B-Protocol
label	I-Protocol
switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
,	O
which	O
can	O
be	O
used	O
for	O
IP	B-Protocol
over	O
virtual	B-Protocol
circuits	I-Protocol
.	O
</s>
<s>
MPLS	B-Protocol
is	O
unreliable	O
but	O
provides	O
eight	O
different	O
QoS	O
classes	O
.	O
</s>
<s>
Switched	O
virtual	B-Protocol
circuits	I-Protocol
(	O
SVCs	O
)	O
are	O
generally	O
set	O
up	O
on	O
a	O
per-call	O
basis	O
and	O
are	O
disconnected	O
when	O
the	O
call	O
is	O
terminated	O
;	O
however	O
,	O
a	O
permanent	O
virtual	B-Protocol
circuit	I-Protocol
(	O
PVC	O
)	O
can	O
be	O
established	O
as	O
an	O
option	O
to	O
provide	O
a	O
dedicated	B-Architecture
circuit	I-Architecture
link	O
between	O
two	O
facilities	O
.	O
</s>
<s>
A	O
switched	O
virtual	B-Protocol
circuit	I-Protocol
(	O
SVC	O
)	O
is	O
a	O
virtual	B-Protocol
circuit	I-Protocol
that	O
is	O
dynamically	O
established	O
on	O
demand	O
and	O
is	O
torn	O
down	O
when	O
transmission	O
is	O
complete	O
,	O
for	O
example	O
after	O
a	O
phone	O
call	O
or	O
a	O
file	O
download	O
.	O
</s>
<s>
A	O
permanent	O
virtual	B-Protocol
circuit	I-Protocol
(	O
PVC	O
)	O
is	O
a	O
virtual	B-Protocol
circuit	I-Protocol
established	O
for	O
repeated/continuous	O
use	O
between	O
the	O
same	O
DTE	O
.	O
</s>
<s>
Permanent	O
virtual	B-Protocol
circuits	I-Protocol
eliminate	O
the	O
need	O
for	O
repeated	O
call	O
set-up	O
and	O
clearing	O
.	O
</s>
<s>
Frame	B-Protocol
Relay	I-Protocol
is	O
typically	O
used	O
to	O
provide	O
PVCs	O
.	O
</s>
<s>
ATM	O
provides	O
both	O
switched	O
virtual	B-Protocol
connections	I-Protocol
and	O
permanent	O
virtual	B-Protocol
connections	I-Protocol
,	O
as	O
they	O
are	O
called	O
in	O
ATM	O
terminology	O
.	O
</s>
