<s>
The	O
Number	B-Device
One	I-Device
Electronic	I-Device
Switching	I-Device
System	I-Device
(	O
1ESS	B-Device
)	O
was	O
the	O
first	O
large-scale	O
stored	B-Device
program	I-Device
control	I-Device
(	O
SPC	O
)	O
telephone	O
exchange	O
or	O
electronic	B-Device
switching	I-Device
system	I-Device
in	O
the	O
Bell	O
System	O
.	O
</s>
<s>
The	O
switching	B-Architecture
fabric	I-Architecture
was	O
composed	O
of	O
a	O
reed	B-Algorithm
relay	I-Algorithm
matrix	O
controlled	O
by	O
wire	B-Algorithm
spring	I-Algorithm
relays	I-Algorithm
which	O
in	O
turn	O
were	O
controlled	O
by	O
a	O
central	B-Device
processing	I-Device
unit	I-Device
(	O
CPU	B-Device
)	O
.	O
</s>
<s>
The	O
1AESS	B-Device
central	O
office	O
switch	O
was	O
a	O
plug	B-Device
compatible	I-Device
,	O
higher	O
capacity	O
upgrade	O
from	O
1ESS	B-Device
with	O
a	O
faster	O
1A	O
processor	O
that	O
incorporated	O
the	O
existing	O
instruction	B-General_Concept
set	I-General_Concept
for	O
programming	O
compatibility	O
,	O
and	O
used	O
smaller	O
remreed	O
switches	O
,	O
fewer	O
relays	O
,	O
and	O
featured	O
disk	B-Device
storage	I-Device
.	O
</s>
<s>
The	O
voice	O
switching	B-Architecture
fabric	I-Architecture
plan	O
was	O
similar	O
to	O
that	O
of	O
the	O
earlier	O
5XB	B-Device
switch	I-Device
in	O
being	O
bidirectional	O
and	O
in	O
using	O
the	O
call-back	O
principle	O
.	O
</s>
<s>
The	O
traditional	O
implementation	O
of	O
a	O
nonblocking	B-Algorithm
minimal	I-Algorithm
spanning	I-Algorithm
switch	I-Algorithm
able	O
to	O
connect	O
input	O
customers	O
to	O
output	O
customers	O
simultaneously	O
—	O
with	O
the	O
connections	O
initiated	O
in	O
any	O
order	O
—	O
the	O
connection	O
matrix	O
scaled	O
on	O
.	O
</s>
<s>
The	O
B	O
links	O
were	O
often	O
to	O
make	O
a	O
higher	O
LCR	O
,	O
such	O
as	O
3:1	O
or	O
(	O
especially	O
in	O
suburban	O
1ESS	B-Device
)	O
5:1	O
.	O
</s>
<s>
The	O
original	O
1ESS	B-Device
Ferreed	O
switching	B-Architecture
fabric	I-Architecture
was	O
packaged	O
as	O
separate	O
8x8	O
switches	O
or	O
other	O
sizes	O
,	O
tied	O
into	O
the	O
rest	O
of	O
the	O
speech	O
fabric	O
and	O
control	O
circuitry	O
by	O
wire	O
wrap	O
connections	O
.	O
</s>
<s>
The	O
much	O
smaller	O
Remreed	O
crosspoints	O
,	O
introduced	O
at	O
about	O
the	O
same	O
time	O
as	O
1AESS	B-Device
,	O
were	O
packaged	O
as	O
grid	O
boxes	O
of	O
four	O
principal	O
types	O
.	O
</s>
<s>
With	O
a	O
clean	O
FCG	O
test	O
,	O
the	O
call	O
completion	O
software	O
told	O
the	O
"	O
A	O
"	O
relay	O
in	O
the	O
trunk	O
circuit	O
to	O
operate	O
,	O
connecting	O
its	O
transmission	O
and	O
test	O
hardware	O
to	O
the	O
switching	B-Architecture
fabric	I-Architecture
and	O
thus	O
to	O
the	O
line	O
.	O
</s>
<s>
In	O
the	O
late	O
1970s	O
,	O
teleprinter	O
channels	O
were	O
gathered	O
together	O
in	O
Switching	O
Control	O
Centers	O
(	O
SCC	O
)	O
,	O
later	O
Switching	O
Control	O
Center	O
System	O
,	O
each	O
serving	O
a	O
dozen	O
or	O
more	O
1ESS	B-Device
exchanges	O
and	O
using	O
their	O
own	O
computers	O
to	O
analyze	O
these	O
and	O
other	O
kinds	O
of	O
failure	O
reports	O
.	O
</s>
<s>
They	O
generated	O
a	O
so-called	O
histogram	B-Algorithm
(	O
actually	O
a	O
scatterplot	B-Application
)	O
of	O
parts	O
of	O
the	O
fabric	O
where	O
failures	O
were	O
particularly	O
numerous	O
,	O
usually	O
pointing	O
to	O
a	O
particular	O
bad	O
crosspoint	O
,	O
even	O
if	O
it	O
failed	O
sporadically	O
rather	O
than	O
consistently	O
.	O
</s>
<s>
The	O
computer	O
received	O
input	O
from	O
peripherals	O
via	O
magnetic	O
scanners	O
,	O
composed	O
of	O
ferrod	O
sensors	O
,	O
similar	O
in	O
principle	O
to	O
magnetic	O
core	O
memory	B-General_Concept
except	O
that	O
the	O
output	O
was	O
controlled	O
by	O
control	O
windings	O
analogous	O
to	O
the	O
windings	O
of	O
a	O
relay	O
.	O
</s>
<s>
The	O
original	O
1ESS	B-Device
packaging	O
had	O
all	O
the	O
ferrods	O
of	O
an	O
LSF	O
together	O
,	O
and	O
separate	O
from	O
the	O
line	O
switches	O
,	O
while	O
the	O
later	O
1AESS	B-Device
had	O
each	O
ferrod	O
at	O
the	O
front	O
of	O
the	O
steel	O
box	O
containing	O
its	O
line	O
switch	O
.	O
</s>
<s>
SD	O
were	O
originally	O
contact	O
trees	O
of	O
30-contact	O
wire	B-Algorithm
spring	I-Algorithm
relays	I-Algorithm
,	O
each	O
driven	O
by	O
a	O
flipflop	B-General_Concept
.	O
</s>
<s>
In	O
later	O
1AESS	B-Device
versions	O
SD	O
were	O
solid	O
state	O
with	O
several	O
SD	O
points	O
per	O
circuit	O
pack	O
generally	O
on	O
the	O
same	O
shelf	O
or	O
adjacent	O
shelf	O
to	O
the	O
trunk	O
pack	O
.	O
</s>
<s>
The	O
duplicate	O
Harvard	B-Architecture
architecture	I-Architecture
central	O
processor	O
or	O
CC	O
(	O
Central	O
Control	O
)	O
for	O
the	O
1ESS	B-Device
operated	O
at	O
approximately	O
200	O
kHz	O
.	O
</s>
<s>
CPU	B-Device
logic	O
was	O
implemented	O
using	O
discrete	O
diode	B-General_Concept
–	I-General_Concept
transistor	I-General_Concept
logic	I-General_Concept
.	O
</s>
<s>
One	O
hard	O
plastic	O
card	O
commonly	O
held	O
the	O
components	O
necessary	O
to	O
implement	O
,	O
for	O
example	O
,	O
two	O
gates	O
or	O
a	O
flipflop	B-General_Concept
.	O
</s>
<s>
CPU	B-Device
diagnostics	O
could	O
be	O
run	O
that	O
would	O
attempt	O
to	O
identify	O
failing	O
card(s )	O
.	O
</s>
<s>
The	O
CPU	B-Device
design	O
was	O
quite	O
complex	O
-	O
using	O
three	O
way	O
interleaving	O
of	O
instruction	O
execution	O
(	O
later	O
called	O
instruction	B-General_Concept
pipeline	I-General_Concept
)	O
to	O
improve	O
throughput	O
.	O
</s>
<s>
In	O
many	O
instructions	O
of	O
the	O
instruction	B-General_Concept
set	I-General_Concept
,	O
data	O
could	O
be	O
optionally	O
masked	O
and/or	O
rotated	O
.	O
</s>
<s>
Single	O
instructions	O
existed	O
for	O
such	O
esoteric	O
functions	O
as	O
"	O
find	B-Algorithm
first	I-Algorithm
set	I-Algorithm
bit	O
(	O
the	O
rightmost	O
bit	O
that	O
is	O
set	O
)	O
in	O
a	O
data	O
word	O
,	O
optionally	O
reset	O
the	O
bit	O
and	O
tell	O
me	O
the	O
position	O
of	O
the	O
bit	O
"	O
.	O
</s>
<s>
The	O
central	O
processor	O
was	O
implemented	O
as	O
a	O
hierarchical	B-Application
state	I-Application
machine	I-Application
.	O
</s>
<s>
Memory	B-General_Concept
had	O
a	O
44-bit	O
word	O
length	O
for	O
program	O
stores	O
,	O
of	O
which	O
six	O
bits	O
were	O
for	O
Hamming	B-Error_Name
error	B-Error_Name
correction	I-Error_Name
and	O
one	O
was	O
used	O
for	O
an	O
additional	O
parity	B-Error_Name
check	I-Error_Name
.	O
</s>
<s>
Instead	O
,	O
the	O
aluminum	O
memory	B-General_Concept
cards	O
,	O
also	O
called	O
twistor	O
planes	O
,	O
had	O
to	O
be	O
removed	O
in	O
groups	O
of	O
128	O
so	O
their	O
permanent	O
magnets	O
could	O
be	O
written	O
offline	O
by	O
a	O
motorized	O
writer	O
,	O
an	O
improvement	O
over	O
the	O
non	O
motorized	O
single	O
card	O
writer	O
used	O
in	O
Project	O
Nike	O
.	O
</s>
<s>
All	O
memory	B-General_Concept
frames	O
,	O
all	O
busses	O
,	O
and	O
all	O
software	O
and	O
data	O
were	O
fully	O
dual	B-Error_Name
modular	I-Error_Name
redundant	I-Error_Name
.	O
</s>
<s>
The	O
dual	O
CCs	O
operated	O
in	O
lockstep	B-General_Concept
and	O
the	O
detection	O
of	O
a	O
mismatch	O
triggered	O
an	O
automatic	O
sequencer	O
to	O
change	O
the	O
combination	O
of	O
CC	O
,	O
busses	O
and	O
memory	B-General_Concept
modules	O
until	O
a	O
configuration	O
was	O
reached	O
that	O
could	O
pass	O
a	O
sanity	B-Error_Name
check	I-Error_Name
.	O
</s>
<s>
Call	O
Stores	O
were	O
the	O
system	O
's	O
read/write	O
memory	B-General_Concept
,	O
containing	O
the	O
data	O
for	O
calls	O
in	O
progress	O
and	O
other	O
temporary	O
data	O
.	O
</s>
<s>
They	O
had	O
a	O
24-bit	O
word	O
,	O
of	O
which	O
one	O
bit	O
was	O
for	O
parity	B-Error_Name
check	I-Error_Name
.	O
</s>
<s>
They	O
operated	O
similar	O
to	O
magnetic	O
core	O
memory	B-General_Concept
,	O
except	O
that	O
the	O
ferrite	O
was	O
in	O
sheets	O
with	O
a	O
hole	O
for	O
each	O
bit	O
,	O
and	O
the	O
coincident	O
current	O
address	O
and	O
readout	O
wires	O
passed	O
through	O
that	O
hole	O
.	O
</s>
<s>
The	O
separate	O
program	O
memory	B-General_Concept
and	O
data	O
memory	B-General_Concept
were	O
operated	O
in	O
antiphase	O
,	O
with	O
the	O
addressing	O
phase	O
of	O
Program	O
Store	O
coinciding	O
with	O
the	O
data	O
fetch	O
phase	O
of	O
Call	O
Store	O
and	O
vice	O
versa	O
.	O
</s>
<s>
Bugs	O
that	O
previously	O
went	O
unnoticed	O
became	O
prominent	O
when	O
1ESS	B-Device
was	O
brought	O
to	O
big	O
cities	O
with	O
heavy	O
telephone	O
traffic	O
,	O
and	O
delayed	O
the	O
full	O
adoption	O
of	O
the	O
system	O
for	O
a	O
few	O
years	O
.	O
</s>
<s>
Temporary	O
fixes	O
included	O
the	O
Service	O
Link	O
Network	O
(	O
SLN	O
)	O
,	O
which	O
did	O
approximately	O
the	O
job	O
of	O
the	O
Incoming	O
Register	O
Link	O
and	O
Ringing	O
Selection	O
Switch	O
of	O
the	O
5XB	B-Device
switch	I-Device
,	O
thus	O
diminishing	O
CPU	B-Device
load	O
and	O
decreasing	O
response	O
times	O
for	O
incoming	O
calls	O
,	O
and	O
a	O
Signal	O
Processor	O
(	O
SP	O
)	O
or	O
peripheral	O
computer	O
of	O
only	O
one	O
bay	O
,	O
to	O
handle	O
simple	O
but	O
time-consuming	O
tasks	O
such	O
as	O
the	O
timing	O
and	O
counting	O
of	O
Dial	O
Pulses	O
.	O
</s>
<s>
1AESS	B-Device
eliminated	O
the	O
need	O
for	O
SLN	O
and	O
SP	O
.	O
</s>
<s>
The	O
half	O
inch	O
tape	O
drive	O
was	O
write	O
only	O
,	O
being	O
used	O
only	O
for	O
Automatic	B-Application
Message	I-Application
Accounting	I-Application
.	O
</s>
<s>
When	O
a	O
critical	O
hardware	O
failure	O
in	O
the	O
processor	O
or	O
peripheral	O
units	O
occurred	O
,	O
such	O
as	O
both	O
controllers	O
of	O
a	O
line	O
switch	O
frame	O
failing	O
and	O
unable	O
to	O
receive	O
orders	O
,	O
the	O
machine	O
would	O
stop	O
connecting	O
calls	O
and	O
go	O
into	O
a	O
"	O
phase	O
of	O
memory	B-General_Concept
regeneration	O
"	O
,	O
"	O
phase	O
of	O
reinitialization	O
"	O
,	O
or	O
"	O
Phase	O
"	O
for	O
short	O
.	O
</s>
<s>
Most	O
of	O
the	O
thousands	O
of	O
1ESS	B-Device
and	O
1AESS	B-Device
offices	O
in	O
the	O
USA	O
were	O
replaced	O
in	O
the	O
1990s	O
by	O
DMS-100	B-Protocol
,	O
5ESS	B-Device
Switch	I-Device
and	O
other	O
digital	O
switches	O
,	O
and	O
since	O
2010	O
also	O
by	O
packet	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
As	O
of	O
late	O
2014	O
,	O
just	O
over	O
20	O
1AESS	B-Device
installations	O
remained	O
in	O
the	O
North	O
American	O
network	O
,	O
which	O
were	O
located	O
mostly	O
in	O
T	O
's	O
legacy	O
BellSouth	O
and	O
T	O
's	O
legacy	O
Southwestern	O
Bell	O
states	O
,	O
especially	O
in	O
the	O
Atlanta	O
GA	O
metro	O
area	O
,	O
the	O
Saint	O
Louis	O
MO	O
metro	O
area	O
,	O
and	O
in	O
the	O
Dallas/Fort	O
Worth	O
TX	O
metro	O
area	O
.	O
</s>
<s>
In	O
2015	O
,	O
AT&T	O
did	O
not	O
renew	O
a	O
support	O
contract	O
with	O
Alcatel-Lucent	O
(	O
now	O
Nokia	O
)	O
for	O
the	O
1AESS	B-Device
systems	O
still	O
in	O
operation	O
and	O
notified	O
Alcatel-Lucent	O
of	O
its	O
intent	O
to	O
remove	O
them	O
all	O
from	O
service	O
by	O
2017	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
Alcatel-Lucent	O
dismantled	O
the	O
last	O
1AESS	B-Device
lab	O
at	O
the	O
Naperville	O
Bell	O
Labs	O
location	O
in	O
2015	O
,	O
and	O
announced	O
the	O
discontinuation	O
of	O
support	O
for	O
the	O
1AESS	B-Device
.	O
</s>
<s>
In	O
2017	O
,	O
AT&T	O
completed	O
the	O
removal	O
of	O
remaining	O
1AESS	B-Device
systems	O
by	O
moving	O
customers	O
to	O
other	O
newer	O
technology	O
switches	O
,	O
typically	O
with	O
Genband	O
switches	O
with	O
TDM	O
trunking	O
only	O
.	O
</s>
<s>
The	O
last	O
known	O
1AESS	B-Device
switch	I-Device
was	O
in	O
Odessa	O
,	O
TX	O
(	O
Odessa	O
Lincoln	O
Federal	O
wirecenter	O
ODSSTXLI	O
)	O
.	O
</s>
<s>
It	O
was	O
disconnected	O
from	O
service	O
around	O
June	O
3	O
,	O
2017	O
and	O
cut	O
over	O
to	O
a	O
Genband	O
G5/G6	O
packet	B-Protocol
switch	I-Protocol
.	O
</s>
<s>
1	O
Electronic	B-Device
Switching	I-Device
System	I-Device
Arranged	O
with	O
Data	O
Features	O
(	O
No	O
.	O
</s>
<s>
1	O
ESS	B-Device
ADF	O
)	O
was	O
an	O
adaptation	O
of	O
the	O
Number	B-Device
One	I-Device
Electronic	I-Device
Switching	I-Device
System	I-Device
to	O
create	O
a	O
store	B-Protocol
and	I-Protocol
forward	I-Protocol
message	B-Protocol
switching	I-Protocol
system	O
.	O
</s>
<s>
1	O
ESS	B-Device
ADF	O
had	O
a	O
large	O
number	O
of	O
features	O
,	O
including	O
:	O
</s>
