<s>
4	O
Electronic	B-Device
Switching	I-Device
System	I-Device
(	O
4ESS	B-Device
)	O
is	O
a	O
class	B-Device
4	I-Device
telephone	O
electronic	B-Device
switching	I-Device
system	I-Device
that	O
was	O
the	O
first	O
digital	O
electronic	O
toll	O
switch	O
introduced	O
by	O
Western	O
Electric	O
for	O
long-distance	O
switching	O
.	O
</s>
<s>
At	O
the	O
time	O
of	O
the	O
Bell	O
System	O
divestiture	O
,	O
most	O
of	O
the	O
4ESS	B-Device
switches	I-Device
became	O
assets	O
of	O
AT&T	O
as	O
part	O
of	O
the	O
long-distance	O
network	O
,	O
while	O
others	O
remained	O
in	O
the	O
RBOC	O
networks	O
.	O
</s>
<s>
Over	O
140	O
4ESS	B-Device
switches	I-Device
remained	O
in	O
service	O
in	O
the	O
United	O
States	O
in	O
2007	O
.	O
</s>
<s>
The	O
4ESS	B-Device
Switch	I-Device
is	O
often	O
considered	O
as	O
a	O
switching	O
network	O
and	O
its	O
controlling	O
processor	O
.	O
</s>
<s>
Terminal	O
equipment	O
provides	O
the	O
interface	O
between	O
metallic	O
trunk	O
facilities	O
in	O
the	O
toll	O
network	O
and	O
the	O
4ESS	B-Device
Switch	I-Device
.	O
</s>
<s>
Two-wire	O
and	O
4-wire	O
voice-frequency	O
trunks	O
terminating	O
at	O
metallic	O
terminal	O
facilities	O
and	O
equipment	O
which	O
provides	O
an	O
interface	O
between	O
digital	O
carrier	O
equipment	O
and	O
the	O
4ESS	B-Device
switching	O
network	O
.	O
</s>
<s>
The	O
processor	O
acts	O
as	O
the	O
CPU	B-Device
for	O
the	O
switch	O
.	O
</s>
<s>
Early	O
versions	O
used	O
the	O
same	O
1A	O
processor	O
as	O
the	O
contemporaneous	O
improved	O
1AESS	B-Device
switch	I-Device
.	O
</s>
<s>
The	O
file	O
store	O
provides	O
long	O
term	O
storage	O
(	O
disk	B-Device
storage	I-Device
)	O
of	O
the	O
processor	O
programs	O
(	O
program	O
store	O
)	O
and	O
office	O
data	O
(	O
call	O
store	O
)	O
.	O
</s>
<s>
It	O
was	O
first	O
implemented	O
using	O
disk	B-Device
technology	O
but	O
was	O
replaced	O
by	O
the	O
4E	O
attached	O
processor	O
system	O
(	O
4EAPS	O
)	O
.	O
</s>
<s>
The	O
4EAPS	O
is	O
a	O
3B	B-Device
computer	O
running	O
4EAPS	O
application	O
software	O
on	O
the	O
DMERT	B-Operating_System
operating	O
system	O
.	O
</s>
<s>
The	O
4EAPS	O
interfaces	O
to	O
the	O
4ESS	B-Device
processor	O
via	O
the	O
attached	O
processor	O
interface	O
(	O
API	O
)	O
units	O
.	O
</s>
<s>
The	O
"	O
1A	O
file	O
store	O
"	O
became	O
partitions	O
on	O
the	O
3B	B-Device
computer	B-Device
disks	I-Device
.	O
</s>
<s>
These	O
were	O
all	O
converted	O
to	O
the	O
3B21D	B-Device
around	O
1995	O
.	O
</s>
<s>
This	O
includes	O
the	O
common	O
channel	O
interface	O
signaling	O
(	O
CCIS	O
)	O
terminal	O
,	O
signal	O
processors	O
,	O
time-slot	B-Device
interchanges	I-Device
(	O
TSI	O
)	O
and	O
time	O
multiplexed	O
switches	O
(	O
TMS	O
)	O
.	O
</s>
<s>
4ESS	B-Device
development	O
began	O
circa	O
1970	O
,	O
mainly	O
in	O
Naperville	O
,	O
Illinois	O
under	O
the	O
direction	O
of	O
Henry	O
Earle	O
Vaughan	O
.	O
</s>
<s>
Previous	O
tandem	O
switching	B-Device
systems	I-Device
,	O
primarily	O
the	O
No	O
.	O
</s>
<s>
The	O
last	O
4ESS	B-Device
was	O
installed	O
in	O
suburban	O
Atlanta	O
,	O
GA	O
in	O
1999	O
as	O
a	O
toll	O
tandem	O
for	O
AT&T	O
.	O
</s>
<s>
At	O
the	O
peak	O
of	O
the	O
product	O
's	O
life	O
time	O
in	O
1999	O
,	O
AT&T	O
employed	O
145	O
4ESS	B-Device
switches	I-Device
in	O
its	O
long-haul	O
network	O
,	O
and	O
several	O
were	O
owned	O
by	O
various	O
Regional	O
Bell	O
Operating	O
Companies	O
(	O
RBOCs	O
)	O
.	O
</s>
<s>
AT&T	O
replaced	O
or	O
supplemented	O
the	O
4ESS	B-Device
toll	O
tandem	B-Device
switches	I-Device
with	O
5ESS	B-Device
switches	I-Device
,	O
which	O
featured	O
an	O
advanced	O
design	O
,	O
and	O
are	O
used	O
as	O
edge	O
switches	O
in	O
the	O
network	O
.	O
</s>
<s>
Most	O
RBOCs	O
who	O
used	O
4ESS	B-Device
tandems	O
have	O
replaced	O
them	O
with	O
Class	B-Device
5	I-Device
systems	I-Device
of	O
other	O
manufacturers	O
,	O
e.g.	O
,	O
Nortel	O
.	O
</s>
<s>
As	O
of	O
2014	O
,	O
AT&T	O
operates	O
and	O
maintains	O
approximately	O
one	O
hundred	O
4ESS	B-Device
switches	I-Device
in	O
the	O
public	O
switched	O
telephone	O
network	O
.	O
</s>
<s>
The	O
Nokia	O
N4E-N1B	O
(	O
New	O
4ESS	B-Device
)	O
is	O
the	O
ATCA-based	O
next-generation	O
toll	O
switch	O
for	O
AT&T	O
.	O
</s>
<s>
The	O
N4E-N1B	O
includes	O
the	O
4E	O
APS	O
and	O
4ESS	B-Device
software	O
,	O
but	O
replaces	O
the	O
1B	O
processor	O
and	O
the	O
peripheral	O
units	O
which	O
run	O
in	O
emulated	O
environments	O
on	O
an	O
ATCA	O
blade	O
or	O
commercial	O
off-the-shelf	O
servers	O
.	O
</s>
<s>
Starting	O
in	O
the	O
late	O
2010s	O
and	O
continuing	O
in	O
the	O
early	O
2020s	O
,	O
AT&T	O
is	O
replacing	O
older	O
4ESS	B-Device
switches	I-Device
with	O
N4E-N1B	O
switches	O
,	O
and	O
is	O
also	O
adding	O
new	O
N4E-N1B	O
switches	O
in	O
places	O
where	O
there	O
was	O
no	O
4ESS	B-Device
previously	O
.	O
</s>
<s>
It	O
is	O
assumed	O
that	O
these	O
new	O
N4E-N1B	O
switches	O
are	O
taking	O
over	O
Class-4	O
functions	O
that	O
were	O
previously	O
handled	O
by	O
5ESS	B-Device
switches	I-Device
acting	O
as	O
"	O
edge	O
tandems.	O
"	O
</s>
