<s>
In	O
the	O
U.S.	O
telephone	O
network	O
,	O
the	O
12-channel	B-Protocol
carrier	I-Protocol
system	I-Protocol
was	O
an	O
early	O
frequency-division	B-Protocol
multiplexing	I-Protocol
system	O
standard	O
,	O
used	O
to	O
carry	O
multiple	O
telephone	O
calls	O
on	O
a	O
single	O
twisted	O
pair	O
of	O
wires	O
,	O
mostly	O
for	O
short	O
to	O
medium	O
distances	O
.	O
</s>
<s>
This	O
became	O
the	O
basic	O
building	O
block	O
,	O
the	O
"	O
channel	O
group	O
"	O
,	O
for	O
all	O
succeeding	O
long	O
haul	O
systems	O
,	O
such	O
as	O
Type	O
K	O
and	O
all	O
the	O
Type	B-Protocol
L	I-Protocol
systems	O
into	O
the	O
late	O
1970s	O
.	O
</s>
<s>
All	O
long	O
haul	O
"	O
channel	O
groups	O
"	O
used	O
the	O
single-sideband/suppressed	O
carrier	O
heterodyne	O
scheme	O
that	O
was	O
produced	O
by	O
a	O
Western	O
Electric	O
Type	O
A-1	O
through	O
A-6	O
channel	O
bank	O
.	O
</s>
<s>
The	O
Bell	O
System	O
vacuum-tube	O
driven	O
N-1	O
Carrier	O
of	O
the	O
early	O
1950s	O
was	O
the	O
most	O
used	O
twelve	O
channel	O
carrier	B-Protocol
system	I-Protocol
,	O
using	O
double	O
sideband/unsuppressed	O
carrier	O
operation	O
which	O
did	O
n't	O
need	O
network	O
timebase	O
synchronization	O
to	O
maintain	O
frequency	O
accuracy	O
.	O
</s>
<s>
N-2	O
was	O
similar	O
in	O
heterodyning	O
scheme	O
,	O
but	O
in	O
discrete	O
transistorized	O
"	O
plug-in	O
unit	O
"	O
architecture	O
,	O
while	O
N3	O
used	O
the	O
same	O
frequency	B-Algorithm
plan	I-Algorithm
but	O
a	O
scheme	O
of	O
using	O
single	B-Device
sideband	I-Device
with	O
a	O
different	O
voice	O
channel	O
on	O
each	O
side	O
of	O
the	O
carrier	O
,	O
a	O
technique	O
first	O
seen	O
on	O
the	O
16	O
channel	O
Type	O
"	O
O	O
"	O
open	O
wire	O
short	O
haul	O
carrier	O
of	O
the	O
1950s	O
.	O
</s>
<s>
BN	O
used	O
the	O
same	O
pairs	O
and	O
repeaters	O
as	O
did	O
the	O
Bell	O
N-CXR	O
,	O
but	O
used	O
four	O
channel	O
"	O
groups	O
,	O
"	O
lower	O
single-sideband	B-Device
heterodyning	O
,	O
and	O
24	O
channels	O
per	O
carrier	O
,	O
as	O
later	O
seen	O
on	O
Western	O
Electric	O
's	O
Type	O
N-3	O
.	O
</s>
<s>
Part	O
of	O
this	O
settlement	O
was	O
for	O
T	O
's	O
Bell	O
Operating	O
Companies	O
to	O
buy	O
and	O
use	O
small	O
amounts	O
of	O
Lenkurt	O
and	O
Collins	O
microwave	O
and	O
carrier	B-Protocol
systems	I-Protocol
.	O
</s>
