<s>
A	O
carrier	B-Protocol
system	I-Protocol
is	O
a	O
telecommunications	O
system	O
that	O
transmits	O
information	O
,	O
such	O
as	O
the	O
voice	O
signals	O
of	O
a	O
telephone	O
call	O
and	O
the	O
video	O
signals	O
of	O
television	O
,	O
by	O
modulation	O
of	O
one	O
or	O
multiple	O
carrier	O
signals	O
above	O
the	O
principal	O
voice	O
frequency	O
or	O
data	O
rate	O
.	O
</s>
<s>
Carrier	B-Protocol
systems	I-Protocol
typically	O
transmit	O
multiple	O
channels	O
of	O
communication	O
simultaneously	O
over	O
the	O
shared	B-Protocol
medium	I-Protocol
using	O
various	O
forms	O
of	O
multiplexing	B-Architecture
.	O
</s>
<s>
Prominent	O
multiplexing	B-Architecture
methods	O
of	O
the	O
carrier	O
signal	O
are	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
and	O
frequency-division	B-Protocol
multiplexing	I-Protocol
(	O
FDM	B-Protocol
)	O
.	O
</s>
<s>
A	O
cable	O
television	O
system	O
is	O
an	O
example	O
of	O
frequency-division	B-Protocol
multiplexing	I-Protocol
.	O
</s>
<s>
Multiple	O
layers	O
of	O
multiplexing	B-Architecture
may	O
ultimately	O
be	O
performed	O
upon	O
a	O
given	O
input	O
signal	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
public	O
switched	O
telephone	O
network	O
,	O
many	O
telephone	O
calls	O
are	O
sent	O
over	O
shared	O
trunk	O
lines	O
by	O
time-division	B-Protocol
multiplexing	I-Protocol
.	O
</s>
<s>
For	O
long-distance	O
calls	O
several	O
of	O
these	O
channels	O
may	O
be	O
sent	O
over	O
a	O
communications	O
satellite	O
link	O
by	O
frequency-division	B-Protocol
multiplexing	I-Protocol
.	O
</s>
<s>
At	O
a	O
given	O
receiving	O
node	B-Protocol
,	O
specific	O
channels	O
may	O
be	O
demultiplexed	O
individually	O
.	O
</s>
<s>
The	O
purpose	O
of	O
carrier	B-Protocol
systems	I-Protocol
is	O
to	O
save	O
money	O
by	O
carrying	O
more	O
traffic	O
on	O
less	O
infrastructure	O
.	O
</s>
<s>
19th	O
century	O
telephone	O
systems	O
,	O
operating	O
at	O
baseband	B-Algorithm
,	O
could	O
only	O
carry	O
one	O
telephone	O
call	O
on	O
each	O
wire	O
,	O
hence	O
routes	O
with	O
heavy	O
traffic	O
needed	O
many	O
wires	O
.	O
</s>
<s>
In	O
the	O
1920s	O
,	O
frequency-division	B-Protocol
multiplexing	I-Protocol
could	O
carry	O
several	O
circuits	O
on	O
the	O
same	O
balanced	O
wires	O
,	O
and	O
by	O
the	O
1930s	O
L-carrier	B-Protocol
and	O
similar	O
systems	O
carried	O
hundreds	O
of	O
calls	O
at	O
a	O
time	O
on	O
coaxial	O
cables	O
.	O
</s>
<s>
Capacity	O
of	O
these	O
systems	O
increased	O
in	O
the	O
middle	O
of	O
the	O
century	O
,	O
while	O
in	O
the	O
1950s	O
researchers	O
began	O
to	O
take	O
seriously	O
the	O
possibility	O
of	O
saving	O
money	O
on	O
the	O
terminal	O
equipment	O
by	O
using	O
time-division	B-Protocol
multiplexing	I-Protocol
.	O
</s>
<s>
Due	O
to	O
the	O
shorter	O
repeater	O
spacings	O
required	O
by	O
digital	O
systems	O
,	O
long-distance	O
still	O
used	O
FDM	B-Protocol
until	O
the	O
late	O
1970s	O
when	O
optical	B-Architecture
fiber	I-Architecture
was	O
improved	O
to	O
the	O
point	O
that	O
digital	O
connections	O
became	O
the	O
cheapest	O
ones	O
for	O
all	O
distances	O
,	O
short	O
and	O
long	O
.	O
</s>
