<s>
Stored	B-Device
program	I-Device
control	I-Device
(	O
SPC	O
)	O
is	O
a	O
telecommunications	O
technology	O
for	O
telephone	O
exchanges	O
.	O
</s>
<s>
Its	O
characteristic	O
is	O
that	O
the	O
switching	B-Device
system	I-Device
is	O
controlled	O
by	O
a	O
computer	O
program	O
stored	O
in	O
a	O
memory	B-General_Concept
in	O
the	O
switching	B-Device
system	I-Device
.	O
</s>
<s>
SPC	O
was	O
the	O
enabling	O
technology	O
of	O
electronic	B-Device
switching	I-Device
systems	I-Device
(	O
ESS	B-Device
)	O
developed	O
in	O
the	O
Bell	O
System	O
in	O
the	O
1950s	O
,	O
and	O
may	O
be	O
considered	O
the	O
third	O
generation	O
of	O
switching	O
technology	O
.	O
</s>
<s>
Stored	B-Device
program	I-Device
control	I-Device
was	O
invented	O
in	O
1954	O
by	O
Bell	O
Labs	O
scientist	O
Erna	O
Schneider	O
Hoover	O
,	O
who	O
reasoned	O
that	O
computer	O
software	O
could	O
control	O
the	O
connection	O
of	O
telephone	O
calls	O
.	O
</s>
<s>
Proposed	O
and	O
developed	O
in	O
the	O
1950s	O
,	O
SPC	O
was	O
introduced	O
in	O
production	O
electronic	B-Device
switching	I-Device
systems	I-Device
in	O
the	O
1960s	O
.	O
</s>
<s>
The	O
101ESS	O
private	B-Device
branch	I-Device
exchange	I-Device
(	O
PBX	O
)	O
was	O
a	O
transitional	O
switching	B-Device
system	I-Device
in	O
the	O
Bell	O
System	O
to	O
provide	O
expanded	O
services	O
to	O
business	O
customers	O
that	O
were	O
otherwise	O
still	O
served	O
by	O
an	O
electromechanical	O
central	O
office	O
switch	O
.	O
</s>
<s>
The	O
first	O
central	O
office	O
switch	O
with	O
SPC	O
was	O
installed	O
at	O
Morris	O
,	O
Illinois	O
,	O
in	O
a	O
1960	O
trial	O
of	O
electronic	O
switching	O
,	O
followed	O
by	O
the	O
first	O
Western	O
Electric	O
1ESS	B-Device
switch	I-Device
at	O
Succasunna	O
,	O
NJ	O
in	O
1965	O
.	O
</s>
<s>
Other	O
examples	O
of	O
SPC-based	O
third-generation	O
switching	B-Device
systems	I-Device
include	O
the	O
British	O
GPO	O
TXE	B-Device
(	O
various	O
manufacturers	O
)	O
,	O
Metaconta	O
11	O
(	O
ITT	O
Europe	O
)	O
,	O
and	O
the	O
AKE	O
,	O
ARE	O
.	O
</s>
<s>
Pre-digital	O
(	O
1970s	O
)	O
versions	O
of	O
the	O
AXE	B-Device
telephone	I-Device
exchange	I-Device
by	O
Ericsson	O
and	O
Philips	O
PRX	B-Device
were	O
large-scale	O
systems	O
in	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
.	O
</s>
<s>
Second-generation	O
exchanges	O
such	O
as	O
Strowger	B-Device
,	O
panel	O
,	O
rotary	O
,	O
and	O
crossbar	O
switches	O
were	O
constructed	O
purely	O
from	O
electromechanical	O
switching	O
components	O
with	O
combinational	O
logic	O
control	O
,	O
and	O
had	O
no	O
computer	O
software	O
control	O
.	O
</s>
<s>
The	O
addition	O
of	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
decreased	O
subsystem	O
sizes	O
and	O
dramatically	O
increased	O
the	O
capacity	O
of	O
the	O
telephone	O
network	O
.	O
</s>
<s>
These	O
included	O
Ericsson	O
AXE	O
10	O
,	O
which	O
became	O
the	O
world	O
's	O
most	O
popular	O
switching	O
platform	O
,	O
the	O
Western	O
Electric	O
5ESS	B-Device
used	O
through	O
the	O
US	O
and	O
in	O
many	O
other	O
countries	O
,	O
the	O
German	O
designed	O
Siemens	O
ESWD	O
,	O
the	O
ITT	O
System	O
12	O
(	O
later	O
rebranded	O
Alcatel	O
S12	O
)	O
and	O
NEC	O
NEAX	O
all	O
of	O
which	O
were	O
widely	O
used	O
around	O
the	O
world	O
.	O
</s>
<s>
Some	O
digital	O
switches	O
,	O
notably	O
the	O
5ESS	B-Device
and	O
very	O
early	O
versions	O
of	O
Ericsson	O
AXE	O
10	O
,	O
continued	O
to	O
use	O
analog	O
concentrator	O
stages	O
,	O
using	O
SPC-like	O
technologies	O
,	O
rather	O
than	O
direct	O
connections	O
to	O
the	O
digital	O
line	O
cards	O
containing	O
the	O
CODEC	B-General_Concept
.	O
</s>
<s>
Early	O
in	O
the	O
21st	O
century	O
the	O
industry	O
began	O
using	O
a	O
fifth	O
generation	O
of	O
telephony	O
switching	O
,	O
as	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
and	O
specialist	O
hardware-based	O
digital	O
circuit	O
switching	O
is	O
replaced	O
by	O
softswitches	B-Device
and	O
voice	B-Application
over	I-Application
IP	I-Application
VoIP	B-Application
technologies	O
.	O
</s>
<s>
The	O
principal	O
feature	O
of	O
stored	B-Device
program	I-Device
control	I-Device
is	O
one	O
or	O
multiple	O
digital	O
processing	O
units	O
(	O
stored-program	O
computers	O
)	O
that	O
execute	O
a	O
set	O
of	O
computer	O
instructions	O
(	O
program	O
)	O
stored	O
in	O
the	O
memory	B-General_Concept
of	O
the	O
system	O
by	O
which	O
telephone	O
connections	O
are	O
established	O
,	O
maintained	O
,	O
and	O
terminated	O
in	O
associated	O
electronic	O
circuitry	O
.	O
</s>
<s>
An	O
immediate	O
consequence	O
of	O
stored	B-Device
program	I-Device
control	I-Device
is	O
automation	O
of	O
exchange	O
functions	O
and	O
introduction	O
of	O
a	O
variety	O
of	O
new	O
telephony	O
features	O
to	O
subscribers	O
.	O
</s>
<s>
A	O
telephone	O
exchange	O
must	O
run	O
continuously	O
without	O
interruption	O
at	O
all	O
times	O
;	O
it	O
implements	O
a	O
fault-tolerant	B-General_Concept
design	I-General_Concept
.	O
</s>
<s>
A	O
trial	O
system	O
with	O
stored	B-Device
program	I-Device
control	I-Device
was	O
installed	O
in	O
Morris	O
,	O
Illinois	O
in	O
1960	O
.	O
</s>
<s>
It	O
used	O
a	O
flying-spot	B-General_Concept
store	I-General_Concept
with	O
a	O
word	O
size	O
of	O
18	O
bits	O
for	O
semi-permanent	O
program	O
and	O
parameter	O
storage	O
,	O
and	O
a	O
barrier-grid	O
memory	B-General_Concept
for	O
random	O
access	O
working	O
memory	B-General_Concept
.	O
</s>
<s>
The	O
world	O
’s	O
first	O
electronic	B-Device
switching	I-Device
system	I-Device
for	O
production	O
use	O
,	O
the	O
No.1	B-Device
ESS	I-Device
,	O
was	O
commissioned	O
by	O
AT&T	O
at	O
Succasunna	O
,	O
New	O
Jersey	O
,	O
in	O
May	O
1965	O
.	O
</s>
<s>
1ESS	B-Device
systems	O
.	O
</s>
<s>
Today	O
,	O
SPC	O
is	O
an	O
integral	O
concept	O
in	O
all	O
automatic	O
exchanges	O
,	O
due	O
to	O
the	O
universal	O
application	O
of	O
computers	O
and	O
microprocessor	B-Architecture
technology	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
many	O
space-division	O
switching	B-Device
systems	I-Device
used	O
electromechanical	O
switching	O
networks	O
with	O
SPC	O
,	O
while	O
private	O
automatic	O
branch	O
exchanges	O
(	O
PABX	O
)	O
and	O
smaller	O
public	O
exchanges	O
used	O
electronic	O
switching	O
devices	O
.	O
</s>
<s>
Stored	B-Device
program	I-Device
control	I-Device
implementations	O
may	O
be	O
organized	O
into	O
centralized	O
and	O
distributed	O
approaches	O
.	O
</s>
<s>
Early	O
electronic	B-Device
switching	I-Device
systems	I-Device
(	O
ESS	B-Device
)	O
developed	O
in	O
the	O
1960s	O
and	O
1970s	O
almost	O
invariably	O
used	O
centralized	O
control	O
.	O
</s>
<s>
Although	O
many	O
present	O
day	O
exchange	O
design	O
continue	O
to	O
use	O
centralized	O
SPC	O
,	O
with	O
advent	O
of	O
low	O
cost	O
powerful	O
microprocessors	B-Architecture
and	O
VLSI	O
chips	O
such	O
as	O
programmable	O
logic	O
array	O
(	O
PLA	O
)	O
and	O
programmable	B-Architecture
logic	I-Architecture
controllers	I-Architecture
(	O
PLC	O
)	O
,	O
distributed	O
SPC	O
became	O
widespread	O
by	O
the	O
early	O
21st	O
century	O
.	O
</s>
<s>
When	O
switchover	O
occurs	O
the	O
recent	O
status	O
from	O
the	O
secondary	O
memory	B-General_Concept
is	O
loaded	O
.	O
</s>
<s>
1ESS	B-Device
switch	I-Device
was	O
a	O
prominent	O
example	O
.	O
</s>
<s>
When	O
the	O
faulty	O
processor	O
is	O
repaired	O
and	O
brought	O
in	O
service	O
then	O
memory	B-General_Concept
contents	O
of	O
the	O
active	O
processor	O
are	O
copied	O
into	O
its	O
memory	B-General_Concept
and	O
the	O
two	O
are	O
synchronized	O
and	O
comparator	O
is	O
enabled	O
.	O
</s>
<s>
It	O
uses	O
low	O
cost	O
microprocessors	B-Architecture
.	O
</s>
