<s>
A	O
class-4	O
,	O
or	O
tandem	B-Application
,	O
telephone	O
switch	O
is	O
a	O
U.S.	O
telephone	O
company	O
central	O
office	O
telephone	O
exchange	O
used	O
to	O
interconnect	O
local	O
exchange	O
carrier	O
offices	O
for	O
long	O
distance	O
communications	O
in	O
the	O
public	O
switched	O
telephone	O
network	O
.	O
</s>
<s>
A	O
class-4	O
switch	O
does	O
not	O
connect	O
directly	O
to	O
telephones	O
;	O
instead	O
,	O
it	O
connects	O
to	O
other	O
class-4	O
switches	O
and	O
to	O
class-5	B-Device
telephone	I-Device
switches	I-Device
.	O
</s>
<s>
Tandem	B-Application
derives	O
from	O
the	O
Latin	O
adverb	O
tandem	B-Application
meaning	O
at	O
length	O
,	O
and	O
is	O
used	O
in	O
English	O
to	O
mean	O
a	O
group	O
of	O
two	O
people	O
or	O
machines	O
working	O
together	O
,	O
usually	O
in	O
series	O
.	O
</s>
<s>
A	O
tandem	B-Device
switch	I-Device
is	O
used	O
to	O
interconnect	O
other	O
switches	O
via	O
trunks	O
.	O
</s>
<s>
Thus	O
,	O
tandem	B-Device
switches	I-Device
are	O
always	O
part	O
of	O
a	O
series	O
of	O
switches	O
and	O
lines	O
that	O
connect	O
telephone	O
callers	O
to	O
each	O
other	O
.	O
</s>
<s>
A	O
sector	O
tandem	B-Device
switch	I-Device
connects	O
local	O
telephone	O
exchanges	O
(	O
class-5	O
switches	O
)	O
and	O
carries	O
traffic	O
within	O
the	O
local	O
access	O
and	O
transport	O
area	O
(	O
LATA	O
)	O
.	O
</s>
<s>
An	O
access	B-Device
tandem	I-Device
switch	O
connects	O
local	O
telephone	O
exchanges	O
to	O
long-distance	O
telephone	O
companies	O
(	O
or	O
interexchange	O
carriers	O
,	O
"	O
IXCs	O
"	O
)	O
.	O
</s>
<s>
The	O
point	O
at	O
which	O
an	O
access	B-Device
tandem	I-Device
connects	O
to	O
the	O
IXC	O
's	O
switch	O
is	O
called	O
the	O
point	O
of	O
presence	O
,	O
or	O
POP	O
.	O
</s>
<s>
Modern	O
tandem	B-Device
switches	I-Device
are	O
often	O
located	O
at	O
the	O
center	O
of	O
the	O
areas	O
they	O
serve	O
,	O
and	O
may	O
act	O
as	O
both	O
sector	O
tandems	B-Application
and	O
access	B-Device
tandems	I-Device
.	O
</s>
<s>
Class-4	O
switches	O
at	O
that	O
time	O
often	O
had	O
an	O
associated	O
Traffic	B-Algorithm
Service	I-Algorithm
Position	I-Algorithm
System	I-Algorithm
(	O
TSPS	B-Algorithm
)	O
to	O
handle	O
operator-assisted	O
calls	O
.	O
</s>
<s>
TSPS	B-Algorithm
automated	O
many	O
functions	O
previously	O
handled	O
by	O
the	O
local	O
operator	O
with	O
a	O
"	O
cordboard	O
"	O
telephone	O
switch	O
,	O
such	O
as	O
certain	O
aspects	O
of	O
coin-operated	O
telephone	O
calls	O
.	O
</s>
<s>
A	O
class-4	O
switch	O
that	O
connects	O
class-5	O
switches	O
to	O
each	O
other	O
,	O
but	O
not	O
to	O
the	O
long-distance	O
network	O
,	O
is	O
called	O
a	O
"	O
local	O
tandem.	O
"	O
</s>
<s>
The	O
majority	O
of	O
class-4	O
switches	O
in	O
the	O
Bell	O
System	O
during	O
the	O
1950s	O
and	O
1960s	O
used	O
crossbar	O
switches	O
,	O
such	O
as	O
the	O
Crossbar	O
Tandem	B-Application
(	O
XBT	O
)	O
variant	O
of	O
the	O
Number	B-Device
One	I-Device
Crossbar	I-Device
Switching	I-Device
System	I-Device
,	O
or	O
1XB	B-Device
switch	I-Device
.	O
</s>
<s>
The	O
Number	O
4	O
Crossbar	O
(	O
"	O
4XB	O
"	O
)	O
tandem	B-Device
switch	I-Device
was	O
used	O
in	O
the	O
North	O
American	O
toll	O
network	O
from	O
1943	O
until	O
the	O
1990s	O
,	O
when	O
it	O
was	O
replaced	O
by	O
more	O
modern	O
digital	O
switching	O
equipment	O
,	O
such	O
as	O
the	O
Lucent	O
4ESS	B-Device
switch	I-Device
or	O
the	O
Nortel	O
DMS-200	B-Protocol
.	O
</s>
<s>
During	O
the	O
1980s	O
,	O
class-4	O
tandem	B-Device
switches	I-Device
were	O
converted	O
to	O
deal	O
only	O
with	O
high-speed	O
digital	O
four-wire	O
circuit	O
connections	O
:	O
T1	O
,	O
T3	O
,	O
OC-3	O
,	O
etc	O
.	O
</s>
<s>
Modern	O
tandem	B-Device
switches	I-Device
,	O
like	O
other	O
classes	O
of	O
telephone	O
switch	O
,	O
are	O
digital	O
,	O
and	O
use	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
to	O
carry	O
circuit-switched	B-Protocol
telephone	O
calls	O
.	O
</s>
<s>
Tandems	B-Application
were	O
more	O
quickly	O
converted	O
to	O
TDM	O
than	O
the	O
class-5	O
end-offices	O
were	O
.	O
</s>
<s>
During	O
the	O
transition	O
to	O
digital	O
switching	O
in	O
the	O
1980s	O
and	O
1990s	O
,	O
when	O
both	O
TDM	O
and	O
traditional	O
"	O
space	O
division	O
"	O
switches	O
were	O
in	O
use	O
,	O
American	O
phone	O
company	O
employees	O
often	O
referred	O
tandems	B-Application
as	O
"	O
TDM	O
switches	O
"	O
as	O
a	O
result	O
.	O
</s>
<s>
In	O
the	O
past	O
,	O
most	O
of	O
the	O
accounting	O
,	O
billing	O
management	O
,	O
and	O
call	B-Application
record-keeping	I-Application
was	O
handled	O
by	O
the	O
tandem	B-Device
switches	I-Device
.	O
</s>
<s>
The	O
Lucent	O
4ESS	B-Device
is	O
a	O
digital	O
switch	O
widely	O
used	O
as	O
a	O
class-4	O
switch	O
in	O
the	O
United	O
States	O
.	O
</s>
<s>
The	O
first	O
4ESS	B-Device
was	O
installed	O
in	O
Chicago	O
in	O
1976	O
.	O
</s>
<s>
The	O
last	O
4ESS	B-Device
in	O
the	O
AT&T	O
Long	O
Lines	O
network	O
was	O
installed	O
in	O
1999	O
.	O
</s>
<s>
In	O
the	O
late	O
2010s	O
and	O
early	O
2020s	O
,	O
the	O
traditional	O
4ESS	B-Device
switch	I-Device
is	O
slowly	O
being	O
replaced	O
by	O
the	O
Nokia	O
N4E	O
(	O
New	O
4ESS	B-Device
)	O
switch	O
in	O
the	O
AT&T	O
Long	O
Distance	O
network	O
.	O
</s>
<s>
The	O
Lucent	O
5ESS	B-Device
,	O
a	O
class-5	O
switching	O
system	O
,	O
is	O
sometimes	O
used	O
as	O
a	O
class-4	O
switch	O
(	O
or	O
as	O
a	O
mixed	O
class-4/5	O
switch	O
)	O
in	O
markets	O
that	O
are	O
too	O
small	O
to	O
justify	O
a	O
4ESS	B-Device
switch	I-Device
.	O
</s>
<s>
The	O
Nortel	O
DMS-250	B-Protocol
,	O
a	O
larger	O
variant	O
of	O
the	O
DMS-100	B-Protocol
,	O
is	O
a	O
popular	O
competitor	O
to	O
Lucent	O
's	O
4ESS	B-Device
,	O
especially	O
among	O
telephone	O
companies	O
that	O
were	O
not	O
previously	O
a	O
part	O
of	O
AT&T	O
.	O
</s>
<s>
Other	O
DMS	O
switches	O
can	O
also	O
be	O
used	O
as	O
tandems	B-Application
.	O
</s>
<s>
The	O
Nortel	O
SP1	B-Device
4-Wire	I-Device
was	O
an	O
early	O
electronic	O
switch	O
used	O
as	O
a	O
class-4	O
switch	O
.	O
</s>
