<s>
In	O
telecommunications	O
,	O
frequency-division	B-Protocol
multiplexing	I-Protocol
(	O
FDM	O
)	O
is	O
a	O
technique	O
by	O
which	O
the	O
total	O
bandwidth	B-Algorithm
available	O
in	O
a	O
communication	O
medium	O
is	O
divided	O
into	O
a	O
series	O
of	O
non-overlapping	O
frequency	B-Algorithm
bands	I-Algorithm
,	O
each	O
of	O
which	O
is	O
used	O
to	O
carry	O
a	O
separate	O
signal	O
.	O
</s>
<s>
This	O
allows	O
a	O
single	O
transmission	O
medium	O
such	O
as	O
a	O
microwave	O
radio	O
link	O
,	O
cable	O
or	O
optical	B-Architecture
fiber	I-Architecture
to	O
be	O
shared	O
by	O
multiple	O
independent	O
signals	O
.	O
</s>
<s>
The	O
most	O
common	O
example	O
of	O
frequency-division	B-Protocol
multiplexing	I-Protocol
is	O
radio	O
and	O
television	O
broadcasting	O
,	O
in	O
which	O
multiple	O
radio	O
signals	O
at	O
different	O
frequencies	O
pass	O
through	O
the	O
air	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
FDM	O
is	O
also	O
used	O
by	O
telephone	O
systems	O
to	O
transmit	O
multiple	O
telephone	O
calls	O
through	O
high	O
capacity	O
trunklines	O
,	O
communications	O
satellites	O
to	O
transmit	O
multiple	O
channels	O
of	O
data	O
on	O
uplink	O
and	O
downlink	O
radio	O
beams	O
,	O
and	O
broadband	O
DSL	B-Device
modems	I-Device
to	O
transmit	O
large	O
amounts	O
of	O
computer	O
data	O
through	O
twisted	O
pair	O
telephone	O
lines	O
,	O
among	O
many	O
other	O
uses	O
.	O
</s>
<s>
An	O
analogous	O
technique	O
called	O
wavelength	B-Architecture
division	I-Architecture
multiplexing	I-Architecture
is	O
used	O
in	O
fiber-optic	B-Application
communication	I-Application
,	O
in	O
which	O
multiple	O
channels	O
of	O
data	O
are	O
transmitted	O
over	O
a	O
single	O
optical	B-Architecture
fiber	I-Architecture
using	O
different	O
wavelengths	O
(	O
frequencies	O
)	O
.	O
</s>
<s>
The	O
multiple	O
separate	O
information	O
(	O
modulation	B-Application
)	O
signals	O
that	O
are	O
sent	O
over	O
an	O
FDM	O
system	O
,	O
such	O
as	O
the	O
video	O
signals	O
of	O
the	O
television	O
channels	O
that	O
are	O
sent	O
over	O
a	O
cable	O
TV	O
system	O
,	O
are	O
called	O
baseband	B-Algorithm
signals	I-Algorithm
.	O
</s>
<s>
The	O
carrier	O
is	O
much	O
higher	O
in	O
frequency	O
than	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
.	O
</s>
<s>
The	O
carrier	O
signal	O
and	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
are	O
combined	O
in	O
a	O
modulator	B-Application
circuit	O
.	O
</s>
<s>
The	O
modulator	B-Application
alters	O
some	O
aspect	O
of	O
the	O
carrier	O
signal	O
,	O
such	O
as	O
its	O
amplitude	B-Application
,	O
frequency	O
,	O
or	O
phase	O
,	O
with	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
,	O
"	O
piggybacking	O
"	O
the	O
data	O
onto	O
the	O
carrier	O
.	O
</s>
<s>
The	O
result	O
of	O
modulating	B-Application
(	O
mixing	O
)	O
the	O
carrier	O
with	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
is	O
to	O
generate	O
sub-frequencies	O
near	O
the	O
carrier	O
frequency	O
,	O
at	O
the	O
sum	O
(	O
fC	O
+	O
fB	O
)	O
and	O
difference	O
(	O
fC	O
−	O
fB	O
)	O
of	O
the	O
frequencies	O
.	O
</s>
<s>
The	O
information	O
from	O
the	O
modulated	B-Application
signal	O
is	O
carried	O
in	O
sidebands	O
on	O
each	O
side	O
of	O
the	O
carrier	O
frequency	O
.	O
</s>
<s>
Therefore	O
,	O
all	O
the	O
information	O
carried	O
by	O
the	O
channel	O
is	O
in	O
a	O
narrow	O
band	O
of	O
frequencies	O
clustered	O
around	O
the	O
carrier	O
frequency	O
,	O
this	O
is	O
called	O
the	O
passband	B-Algorithm
of	O
the	O
channel	O
.	O
</s>
<s>
Similarly	O
,	O
additional	O
baseband	B-Algorithm
signals	I-Algorithm
are	O
used	O
to	O
modulate	O
carriers	O
at	O
other	O
frequencies	O
,	O
creating	O
other	O
channels	O
of	O
information	O
.	O
</s>
<s>
The	O
carriers	O
are	O
spaced	O
far	O
enough	O
apart	O
in	O
frequency	O
that	O
the	O
band	O
of	O
frequencies	O
occupied	O
by	O
each	O
channel	O
,	O
the	O
passbands	B-Algorithm
of	O
the	O
separate	O
channels	O
,	O
do	O
not	O
overlap	O
.	O
</s>
<s>
All	O
the	O
channels	O
are	O
sent	O
through	O
the	O
transmission	O
medium	O
,	O
such	O
as	O
a	O
coaxial	O
cable	O
,	O
optical	B-Architecture
fiber	I-Architecture
,	O
or	O
through	O
the	O
air	O
using	O
a	O
radio	O
transmitter	O
.	O
</s>
<s>
As	O
long	O
as	O
the	O
channel	O
frequencies	O
are	O
spaced	O
far	O
enough	O
apart	O
that	O
none	O
of	O
the	O
passbands	B-Algorithm
overlap	O
,	O
the	O
separate	O
channels	O
will	O
not	O
interfere	O
with	O
each	O
other	O
.	O
</s>
<s>
Thus	O
the	O
available	O
bandwidth	B-Algorithm
is	O
divided	O
into	O
"	O
slots	O
"	O
or	O
channels	O
,	O
each	O
of	O
which	O
can	O
carry	O
a	O
separate	O
modulated	B-Application
signal	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
coaxial	O
cable	O
used	O
by	O
cable	O
television	O
systems	O
has	O
a	O
bandwidth	B-Algorithm
of	O
about	O
1000	O
MHz	O
,	O
but	O
the	O
passband	B-Algorithm
of	O
each	O
television	O
channel	O
is	O
only	O
6MHz	O
wide	O
,	O
so	O
there	O
is	O
room	O
for	O
many	O
channels	O
on	O
the	O
cable	O
(	O
in	O
modern	O
digital	B-Device
cable	I-Device
systems	O
each	O
channel	O
in	O
turn	O
is	O
subdivided	O
into	O
subchannels	O
and	O
can	O
carry	O
up	O
to	O
10	O
digital	O
television	O
channels	O
)	O
.	O
</s>
<s>
At	O
the	O
destination	O
end	O
of	O
the	O
cable	O
or	O
fiber	O
,	O
or	O
the	O
radio	O
receiver	O
,	O
for	O
each	O
channel	O
a	O
local	O
oscillator	O
produces	O
a	O
signal	O
at	O
the	O
carrier	O
frequency	O
of	O
that	O
channel	O
,	O
that	O
is	O
mixed	O
with	O
the	O
incoming	O
modulated	B-Application
signal	O
.	O
</s>
<s>
The	O
frequencies	O
subtract	O
,	O
producing	O
the	O
baseband	B-Algorithm
signal	I-Algorithm
for	O
that	O
channel	O
again	O
.	O
</s>
<s>
This	O
is	O
called	O
demodulation	B-Device
.	O
</s>
<s>
The	O
resulting	O
baseband	B-Algorithm
signal	I-Algorithm
is	O
filtered	O
out	O
of	O
the	O
other	O
frequencies	O
and	O
output	O
to	O
the	O
user	O
.	O
</s>
<s>
For	O
long	O
distance	O
telephone	O
connections	O
,	O
20th	O
century	O
telephone	O
companies	O
used	O
L-carrier	B-Protocol
and	O
similar	O
coaxial	O
cable	O
systems	O
carrying	O
thousands	O
of	O
voice	O
circuits	O
multiplexed	O
in	O
multiple	O
stages	O
by	O
channel	O
banks	O
.	O
</s>
<s>
Those	O
cables	O
did	O
not	O
allow	O
such	O
large	O
bandwidths	B-Algorithm
,	O
so	O
only	O
12	O
voice	O
channels	O
(	O
double	O
sideband	O
)	O
and	O
later	O
24	O
(	O
single	B-Device
sideband	I-Device
)	O
were	O
multiplexed	O
into	O
four	O
wires	O
,	O
one	O
pair	O
for	O
each	O
direction	O
with	O
repeaters	O
every	O
several	O
miles	O
,	O
approximately	O
10km	O
.	O
</s>
<s>
See	O
12-channel	B-Protocol
carrier	I-Protocol
system	I-Protocol
.	O
</s>
<s>
Modern	O
telephone	O
systems	O
employ	O
digital	O
transmission	O
,	O
in	O
which	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
is	O
used	O
instead	O
of	O
FDM	O
.	O
</s>
<s>
Since	O
the	O
late	O
20th	O
century	O
,	O
digital	O
subscriber	O
lines	O
(	O
DSL	O
)	O
have	O
used	O
a	O
Discrete	B-Protocol
multitone	I-Protocol
(	O
DMT	O
)	O
system	O
to	O
divide	O
their	O
spectrum	O
into	O
frequency	O
channels	O
.	O
</s>
<s>
A	O
once	O
commonplace	O
FDM	O
system	O
,	O
used	O
for	O
example	O
in	O
L-carrier	B-Protocol
,	O
uses	O
crystal	O
filters	O
which	O
operate	O
at	O
the	O
8MHz	O
range	O
to	O
form	O
a	O
Channel	O
Group	O
of	O
12	O
channels	O
,	O
48kHz	O
bandwidth	B-Algorithm
in	O
the	O
range	O
8140	O
to	O
8188kHz	O
by	O
selecting	O
carriers	O
in	O
the	O
range	O
8140	O
to	O
8184kHz	O
selecting	O
upper	O
sideband	O
this	O
group	O
can	O
then	O
be	O
translated	O
to	O
the	O
standard	O
range	O
60	O
to	O
108kHz	O
by	O
a	O
carrier	O
of	O
8248kHz	O
.	O
</s>
<s>
132	O
voice	O
channels	O
(	O
2SG	O
+	O
1G	O
)	O
can	O
be	O
formed	O
using	O
DTL	O
plane	O
the	O
modulation	B-Application
and	O
frequency	B-Algorithm
plan	I-Algorithm
are	O
given	O
in	O
FIG1	O
and	O
FIG2	O
use	O
of	O
DTL	O
technique	O
allows	O
the	O
formation	O
of	O
a	O
maximum	O
of	O
132	O
voice	O
channels	O
that	O
can	O
be	O
placed	O
direct	O
to	O
line	O
.	O
</s>
<s>
FDM	O
can	O
also	O
be	O
used	O
to	O
combine	O
signals	O
before	O
final	O
modulation	B-Application
onto	O
a	O
carrier	O
wave	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
carrier	O
signals	O
are	O
referred	O
to	O
as	O
subcarriers	O
:	O
an	O
example	O
is	O
stereo	O
FM	O
transmission	O
,	O
where	O
a	O
38kHz	O
subcarrier	O
is	O
used	O
to	O
separate	O
the	O
left-right	O
difference	O
signal	O
from	O
the	O
central	O
left-right	O
sum	O
channel	O
,	O
prior	O
to	O
the	O
frequency	O
modulation	B-Application
of	O
the	O
composite	O
signal	O
.	O
</s>
<s>
Where	O
frequency-division	B-Protocol
multiplexing	I-Protocol
is	O
used	O
as	O
to	O
allow	O
multiple	O
users	O
to	O
share	O
a	O
physical	O
communications	O
channel	O
,	O
it	O
is	O
called	O
frequency-division	O
multiple	O
access	O
(	O
FDMA	O
)	O
.	O
</s>
