<s>
The	O
L-carrier	B-Protocol
system	O
was	O
one	O
of	O
a	O
series	O
of	O
carrier	B-Protocol
systems	I-Protocol
developed	O
by	O
AT&T	O
for	O
high-capacity	O
transmission	O
for	O
long-distance	O
communications	O
.	O
</s>
<s>
It	O
was	O
the	O
successor	O
to	O
a	O
series	O
of	O
previous	O
carrier	B-Protocol
systems	I-Protocol
,	O
typically	O
identified	O
by	O
capital	O
letters	O
.	O
</s>
<s>
In	O
the	O
1960s	O
,	O
the	O
system	O
was	O
hardened	O
against	O
the	O
dangers	O
of	O
the	O
Cold	B-Application
War	I-Application
using	O
complete	O
placement	O
of	O
all	O
terminal	O
and	O
repeater	O
equipment	O
in	O
hardened	O
underground	O
vaults	O
.	O
</s>
<s>
The	O
first	O
production	O
installation	O
of	O
the	O
L-1	O
carrier	B-Protocol
system	I-Protocol
went	O
into	O
service	O
between	O
Stevens	O
Point	O
(	O
WI	O
)	O
and	O
Minneapolis	O
(	O
MN	O
)	O
in	O
1941	O
over	O
a	O
distance	O
of	O
almost	O
.	O
</s>
<s>
with	O
a	O
capacity	O
of	O
480	O
channels	O
,	O
far	O
more	O
than	O
could	O
be	O
carried	O
by	O
balanced	O
pair	O
carrier	B-Protocol
systems	I-Protocol
,	O
and	O
cheaper	O
per	O
channel	O
for	O
high-usage	O
routes	O
.	O
</s>
<s>
A	O
small-scale	O
L-type	O
carrier	B-Protocol
system	I-Protocol
between	O
Baltimore	O
(	O
MD	O
)	O
and	O
Washington	O
,	O
D.C.	O
was	O
intended	O
for	O
short-distance	O
low-volume	O
traffic	O
.	O
</s>
<s>
The	O
result	O
of	O
post-war	O
research	O
of	O
this	O
goal	O
was	O
the	O
definition	O
of	O
the	O
L-3	O
carrier	B-Protocol
system	I-Protocol
.	O
</s>
<s>
Starting	O
in	O
1911	O
,	O
telephone	O
networks	O
used	O
frequency-division	B-Protocol
multiplexing	I-Protocol
to	O
carry	O
several	O
voice	O
channels	O
on	O
a	O
single	O
physical	O
circuit	O
,	O
beginning	O
with	O
the	O
first	O
Type	O
C	O
carrier	O
in	O
that	O
year	O
,	O
which	O
heterodyned	O
three	O
voice	O
channels	O
stacked	O
on	O
top	O
of	O
one	O
voice	O
circuit	O
.	O
</s>
<s>
L-carrier	B-Protocol
systems	O
were	O
loaded	O
by	O
multiplexing	O
and	O
supermultiplexing	O
single	B-Device
sideband	I-Device
channels	O
,	O
using	O
the	O
long-standard	O
12	O
channel	O
voice	O
"	O
group	O
"	O
produced	O
by	O
Type	O
A	O
channel	O
banks	O
,	O
occupying	O
a	O
frequency	O
spectrum	O
between	O
60	O
and	O
108kHz	O
.	O
</s>
<s>
This	O
basic	O
"	O
group	O
"	O
was	O
the	O
entire	O
line	O
spectrum	O
on	O
previous	O
long	O
haul	O
carrier	B-Protocol
systems	I-Protocol
,	O
such	O
as	O
Types	O
J	O
and	O
K	O
.	O
The	O
first	O
Type	O
A-1	O
channel	O
banks	O
appeared	O
for	O
use	O
on	O
Type	O
J	O
open	O
wire	O
carrier	O
in	O
1934	O
.	O
</s>
<s>
It	O
was	O
the	O
work	O
of	O
Espenschied	O
and	O
Herman	O
Affel	O
of	O
Bell	O
Labs	O
who	O
patented	O
piezoelectric	O
crystal	O
lattice	O
filters	O
to	O
provide	O
sharp	O
bandpass	O
cutoff	O
that	O
made	O
all	O
single-sideband	B-Device
carrier	O
work	O
.	O
</s>
<s>
Such	O
lattice	O
filters	O
were	O
the	O
heart	O
of	O
all	O
analog	O
multiplex	O
systems	O
using	O
single-sideband/carrier	O
suppressed	O
architecture	O
until	O
active	O
IC-based	O
filtering	O
became	O
available	O
in	O
the	O
mid-1970s	O
.	O
</s>
<s>
In	O
single-sideband	B-Device
modulation	I-Device
schemes	O
,	O
twelve	O
voice	O
channels	O
would	O
be	O
modulated	O
into	O
a	O
channel	O
group	O
.	O
</s>
<s>
L-carrier	B-Protocol
also	O
carried	O
the	O
first	O
television	O
network	O
connections	O
,	O
though	O
the	O
later	O
microwave	O
radio	O
relay	O
system	O
soon	O
became	O
more	O
important	O
for	O
this	O
purpose	O
.	O
</s>
<s>
A	O
variant	O
of	O
the	O
1950s	O
L-3	O
system	O
was	O
designed	O
in	O
the	O
early	O
1960s	O
to	O
provide	O
for	O
land	B-Application
line	I-Application
connections	O
between	O
key	O
military	O
command	O
and	O
control	O
facilities	O
in	O
the	O
United	O
States	O
.	O
</s>
<s>
In	O
the	O
late	O
1970s	O
and	O
early	O
1980s	O
,	O
the	O
L-carrier	B-Protocol
system	O
was	O
determined	O
to	O
be	O
redundant	O
with	O
the	O
advance	O
of	O
satellite	O
and	O
fiber-optic	B-Application
communication	I-Application
.	O
</s>
