<s>
TD-2	B-Algorithm
was	O
a	O
microwave	B-Application
relay	I-Application
system	O
developed	O
by	O
Bell	O
Labs	O
and	O
used	O
by	O
AT&T	O
to	O
build	O
a	O
cross-country	O
network	O
of	O
repeaters	O
for	O
telephone	O
and	O
television	O
transmission	O
.	O
</s>
<s>
TD-2	B-Algorithm
was	O
a	O
minor	O
improvement	O
on	O
TDX	O
,	O
moving	O
to	O
the	O
3.7	O
to	O
4.2GHz	O
band	O
set	O
aside	O
in	O
1947	O
for	O
common	O
carrier	O
use	O
.	O
</s>
<s>
The	O
system	O
had	O
six	O
channels	O
,	O
and	O
using	O
frequency-division	B-Protocol
multiplexing	I-Protocol
,	O
each	O
could	O
carry	O
up	O
to	O
480	O
telephone	O
calls	O
or	O
a	O
television	O
signal	O
.	O
</s>
<s>
The	O
first	O
TD-2	B-Algorithm
link	O
between	O
New	O
York	O
and	O
Chicago	O
opened	O
on	O
1	O
September	O
1950	O
,	O
followed	O
by	O
a	O
Los	O
Angeles-San	O
Francisco	O
link	O
on	O
1	O
September	O
.	O
</s>
<s>
After	O
considerable	O
research	O
,	O
Bell	O
developed	O
an	O
antenna	O
that	O
worked	O
for	O
both	O
TD-2	B-Algorithm
and	O
TH	O
,	O
but	O
these	O
improvements	O
also	O
helped	O
TD-2	B-Algorithm
and	O
increased	O
its	O
capacity	O
again	O
to	O
900	O
calls	O
,	O
delaying	O
a	O
widespread	O
rollout	O
of	O
TH	O
which	O
was	O
added	O
only	O
to	O
the	O
busiest	O
links	O
.	O
</s>
<s>
Bell	O
Canada	O
began	O
building	O
a	O
similar	O
TD-2	B-Algorithm
system	O
,	O
the	O
Skyway	O
,	O
which	O
went	O
into	O
service	O
1958	O
.	O
</s>
<s>
By	O
the	O
late	O
1960s	O
,	O
almost	O
all	O
of	O
the	O
population	O
of	O
North	O
America	O
was	O
linked	O
using	O
TD-2	B-Algorithm
and	O
TH	O
.	O
</s>
<s>
During	O
the	O
late	O
1980s	O
and	O
especially	O
1990s	O
,	O
the	O
installation	O
of	O
fibre	B-Architecture
optic	I-Architecture
lines	O
replaced	O
the	O
microwave	O
networks	O
.	O
</s>
<s>
Radio	O
telephone	O
systems	O
had	O
been	O
experimented	O
with	O
as	O
early	O
as	O
1915	O
,	O
the	O
year	O
after	O
AT&T	O
bought	O
Lee	O
de	O
Forest	O
's	O
patents	O
on	O
the	O
audion	B-Application
vacuum	O
tube	O
.	O
</s>
<s>
Research	O
continued	O
on	O
moving	O
to	O
ever-higher	O
frequencies	O
in	O
an	O
effort	O
to	O
avoid	O
interference	O
as	O
well	O
as	O
expand	O
bandwidth	B-Algorithm
.	O
</s>
<s>
This	O
system	O
had	O
enough	O
bandwidth	B-Algorithm
to	O
allow	O
12	O
telephone	O
calls	O
to	O
be	O
sent	O
on	O
the	O
single	O
connection	O
using	O
the	O
same	O
multiplexing	B-Architecture
system	O
used	O
on	O
long-distance	O
calling	O
wires	O
.	O
</s>
<s>
It	O
was	O
already	O
clear	O
that	O
moving	O
to	O
the	O
GHz	O
range	O
would	O
offer	O
far	O
more	O
bandwidth	B-Algorithm
and	O
allow	O
hundreds	O
of	O
calls	O
on	O
a	O
single	O
link	O
.	O
</s>
<s>
The	O
development	O
of	O
the	O
cavity	B-Architecture
magnetron	I-Architecture
and	O
improvements	O
in	O
the	O
power	O
of	O
klystrons	O
along	O
with	O
the	O
associated	O
waveguides	O
,	O
crystal	O
detectors	O
,	O
and	O
microwave	O
switches	O
as	O
part	O
of	O
radar	B-Application
development	O
provided	O
all	O
of	O
the	O
equipment	O
needed	O
to	O
move	O
radiotelephony	O
into	O
the	O
microwave	O
region	O
.	O
</s>
<s>
In	O
the	O
UK	O
,	O
these	O
technologies	O
were	O
used	O
to	O
produce	O
the	O
world	O
's	O
first	O
microwave	B-Application
relay	I-Application
telephone	O
system	O
:	O
Wireless	O
Set	O
No	O
.	O
</s>
<s>
10	O
(	O
WS.10	O
)	O
,	O
which	O
multiplexed	B-Architecture
eight	O
telephone	O
calls	O
into	O
a	O
single	O
microwave	B-Application
link	I-Application
that	O
could	O
be	O
used	O
to	O
the	O
limit	O
of	O
the	O
line	O
of	O
sight	O
.	O
</s>
<s>
This	O
led	O
to	O
a	O
microwave	B-Application
relay	I-Application
group	O
being	O
set	O
up	O
in	O
the	O
Research	O
Department	O
under	O
the	O
direction	O
of	O
Gordon	O
Thayer	O
.	O
</s>
<s>
However	O
,	O
engineering	O
studies	O
demonstrated	O
that	O
a	O
microwave	B-Application
relay	I-Application
would	O
cost	O
less	O
to	O
install	O
for	O
the	O
same	O
network	O
,	O
although	O
there	O
were	O
some	O
questions	O
about	O
the	O
ongoing	O
operational	O
costs	O
.	O
</s>
<s>
The	O
system	O
had	O
four	O
channels	O
of	O
10MHz	O
each	O
spaced	O
over	O
the	O
allocation	O
,	O
and	O
the	O
signals	O
were	O
encoded	O
into	O
the	O
channels	O
using	O
frequency	B-Application
modulation	I-Application
.	O
</s>
<s>
A	O
planning	O
team	O
outlined	O
two	O
plans	O
,	O
one	O
would	O
be	O
completed	O
in	O
June	O
1949	O
and	O
the	O
other	O
in	O
June	O
1950	O
,	O
different	O
mostly	O
in	O
that	O
the	O
former	O
,	O
known	O
as	O
TD1	O
,	O
would	O
use	O
the	O
existing	O
TDX	O
equipment	O
while	O
the	O
later	O
,	O
TD-2	B-Algorithm
,	O
would	O
use	O
improved	O
equipment	O
with	O
six	O
channels	O
instead	O
of	O
four	O
and	O
new	O
receivers	O
that	O
would	O
allow	O
greater	O
distances	O
between	O
the	O
stations	O
.	O
</s>
<s>
Management	O
demanded	O
that	O
they	O
use	O
the	O
more	O
advanced	O
TD-2	B-Algorithm
system	O
but	O
meet	O
the	O
original	O
1949	O
date	O
,	O
as	O
television	O
stations	O
were	O
clamouring	O
for	O
new	O
links	O
.	O
</s>
<s>
Another	O
significant	O
issue	O
with	O
the	O
TD-2	B-Algorithm
system	O
was	O
that	O
only	O
half	O
of	O
the	O
available	O
bandwidth	B-Algorithm
could	O
be	O
used	O
,	O
as	O
microwave	O
frequency	O
filters	O
of	O
the	O
era	O
were	O
not	O
particularly	O
narrow	O
so	O
the	O
channels	O
had	O
to	O
be	O
spaced	O
out	O
significantly	O
.	O
</s>
<s>
A	O
significant	O
feature	O
of	O
TH	O
was	O
that	O
it	O
used	O
polarization	O
to	O
separate	O
the	O
signals	O
,	O
allowing	O
the	O
channels	O
to	O
operate	O
very	O
close	O
to	O
each	O
other	O
in	O
frequency	O
and	O
thereby	O
make	O
much	O
better	O
use	O
of	O
the	O
bandwidth	B-Algorithm
.	O
</s>
<s>
In	O
theory	O
,	O
because	O
they	O
operated	O
on	O
different	O
bands	O
,	O
TH	O
systems	O
could	O
be	O
added	O
to	O
existing	O
TD-2	B-Algorithm
sites	O
to	O
increase	O
the	O
station	O
's	O
capacity	O
.	O
</s>
<s>
Unfortunately	O
,	O
the	O
TD-2	B-Algorithm
antennas	O
could	O
not	O
be	O
used	O
with	O
polarized	O
signals	O
,	O
and	O
TH	O
planned	O
to	O
use	O
horn	O
antennas	O
which	O
preserved	O
polarization	O
.	O
</s>
<s>
That	O
led	O
to	O
the	O
consideration	O
of	O
whether	O
TD-2	B-Algorithm
could	O
also	O
move	O
to	O
a	O
horn	O
design	O
,	O
and	O
whether	O
a	O
single	O
horn	O
could	O
work	O
at	O
both	O
frequencies	O
.	O
</s>
<s>
To	O
do	O
this	O
,	O
the	O
waveguide	O
would	O
have	O
to	O
be	O
circular	O
as	O
far	O
as	O
the	O
point	O
where	O
the	O
TH	O
signal	O
would	O
be	O
tapped	O
off	O
,	O
and	O
large	O
enough	O
to	O
carry	O
the	O
3.7GHz	O
TD-2	B-Algorithm
as	O
opposed	O
to	O
the	O
shorter	O
6GHz	O
TH	O
signals	O
.	O
</s>
<s>
TD-2	B-Algorithm
stations	O
after	O
1955	O
used	O
the	O
new	O
horn	O
design	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
this	O
allowed	O
the	O
existing	O
TD-2	B-Algorithm
stations	O
to	O
be	O
upgraded	O
to	O
also	O
use	O
polarized	O
signals	O
,	O
and	O
new	O
multiplexer	O
designs	O
emerged	O
,	O
which	O
in	O
combination	O
allowed	O
up	O
to	O
600	O
calls	O
per	O
channel	O
.	O
</s>
<s>
Thus	O
,	O
the	O
design	O
effort	O
that	O
considered	O
whether	O
TH	O
could	O
take	O
over	O
existing	O
TD-2	B-Algorithm
sites	O
instead	O
delayed	O
the	O
widespread	O
use	O
of	O
TH	O
as	O
the	O
capacity	O
of	O
the	O
existing	O
TD-2	B-Algorithm
systems	O
improved	O
.	O
</s>
<s>
TH	O
rollout	O
did	O
not	O
begin	O
until	O
1961	O
,	O
and	O
by	O
the	O
mid-1960s	O
,	O
the	O
majority	O
of	O
the	O
network	O
still	O
used	O
TD-2	B-Algorithm
.	O
</s>
<s>
In	O
April	O
1962	O
,	O
it	O
was	O
decided	O
to	O
re-engineer	O
the	O
TD-2	B-Algorithm
system	O
as	O
TD3	O
.	O
</s>
<s>
This	O
was	O
a	O
solid	O
state	O
system	O
with	O
the	O
only	O
remaining	O
tube	O
being	O
the	O
microwave	B-Application
transmitter	I-Application
,	O
which	O
moved	O
from	O
a	O
klystron	O
to	O
a	O
lower	O
noise	O
travelling-wave	O
tube	O
.	O
</s>
<s>
By	O
1968	O
,	O
40%	O
of	O
all	O
the	O
long-distance	O
traffic	O
in	O
the	O
U.S.	O
was	O
being	O
carried	O
by	O
TD-2	B-Algorithm
.	O
</s>
<s>
At	O
Corning	O
Glass	O
,	O
a	O
team	O
led	O
by	O
Robert	O
Maurer	O
developed	O
a	O
new	O
method	O
of	O
making	O
optical	B-Architecture
fibre	I-Architecture
that	O
had	O
much	O
higher	O
quality	O
and	O
lower	O
loss	O
than	O
previous	O
designs	O
.	O
</s>
<s>
This	O
could	O
be	O
switched	O
on	O
and	O
off	O
at	O
very	O
high	O
speed	O
,	O
allowing	O
it	O
to	O
create	O
pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
signals	O
within	O
a	O
fibre	O
.	O
</s>
<s>
In	O
1976	O
,	O
Masaru	O
Horiguchi	O
of	O
NTT	O
introduced	O
a	O
new	O
optical	B-Architecture
fibre	I-Architecture
that	O
was	O
optically	O
clear	O
at	O
1.3	O
micrometers	O
.	O
</s>
<s>
A	O
small	O
number	O
of	O
former	O
TD-2	B-Algorithm
towers	O
have	O
been	O
brought	O
back	O
to	O
use	O
under	O
third-party	O
ownership	O
.	O
</s>
<s>
This	O
corresponded	O
to	O
the	O
opening	O
of	O
the	O
first	O
new	O
microwave	B-Application
link	I-Application
,	O
enough	O
of	O
an	O
improvement	O
for	O
high-frequency	O
trading	O
to	O
pay	O
for	O
itself	O
.	O
</s>
