<s>
A	O
Class-5	B-Device
telephone	I-Device
switch	I-Device
is	O
a	O
telephone	O
exchange	O
in	O
the	O
public	O
switched	O
telephone	O
network	O
(	O
PSTN	O
)	O
that	O
directly	O
serves	O
subscribers	O
and	O
manages	O
subscriber	O
calling	O
features	O
.	O
</s>
<s>
Class-5	O
services	O
include	O
basic	O
dial-tone	O
,	O
calling	O
features	O
,	O
and	O
additional	O
digital	O
and	O
data	O
services	O
to	O
subscribers	O
connected	O
to	O
a	O
local	B-Architecture
loop	I-Architecture
.	O
</s>
<s>
Class-4	O
switches	O
do	O
not	O
provide	O
subscriber	B-Architecture
lines	I-Architecture
,	O
their	O
role	O
is	O
to	O
route	O
calls	O
between	O
other	O
switches	O
.	O
</s>
<s>
Since	O
the	O
replacement	O
of	O
electromechanical	B-Device
exchanges	O
by	O
modern	O
digital	O
ones	O
,	O
the	O
function	O
of	O
a	O
class-5	O
switch	O
in	O
rural	O
areas	O
is	O
often	O
performed	O
by	O
a	O
remote	B-Device
switch	I-Device
or	O
Remote	B-Application
Digital	I-Application
Terminal	I-Application
installed	O
at	O
the	O
original	O
switch	O
site	O
to	O
handle	O
local	O
switching	O
or	O
concentration	O
,	O
respectively	O
.	O
</s>
<s>
Before	O
the	O
office	O
classification	O
system	O
for	O
Direct	O
Distance	O
Dialing	O
(	O
DDD	O
)	O
was	O
established	O
,	O
the	O
principal	O
designs	O
in	O
use	O
for	O
Class	O
5	O
in	O
the	O
US	O
were	O
crossbar	B-Device
systems	I-Device
,	O
Panel	O
switches	O
,	O
and	O
Strowger-type	B-Device
step-by-step	O
systems	O
.	O
</s>
<s>
The	O
DDD	O
program	O
involved	O
installations	O
of	O
large	O
numbers	O
of	O
new	O
5XB	B-Device
crossbar	I-Device
switches	I-Device
in	O
the	O
1950s	O
and	O
1960s	O
.	O
</s>
<s>
In	O
addition	O
,	O
1ESS	B-Device
switches	I-Device
and	O
their	O
variants	O
appeared	O
in	O
the	O
1960s	O
.	O
</s>
<s>
Most	O
of	O
these	O
systems	O
were	O
removed	O
in	O
the	O
late	O
20th	O
century	O
,	O
primarily	O
replaced	O
in	O
North	O
America	O
by	O
DMS-10	B-Protocol
,	I-Protocol
DMS-100	I-Protocol
and	O
5ESS	B-Device
switches	I-Device
in	O
the	O
Bell	O
operating	O
territories	O
and	O
the	O
GTD-5	B-Device
EAX	I-Device
in	O
the	O
GTE	O
operating	O
areas	O
.	O
</s>
<s>
Principal	O
European	O
products	O
include	O
Ericsson	O
AXE	B-Device
telephone	I-Device
exchange	I-Device
,	O
Siemens	O
EWSD	B-Device
and	O
Alcatel-Lucent	O
S12	O
and	O
E10	O
.	O
</s>
<s>
In	O
the	O
21st	O
century	O
,	O
US	O
and	O
European	O
service	O
providers	O
continued	O
to	O
upgrade	O
their	O
networks	O
,	O
replacing	O
older	O
DMS-10	B-Protocol
,	I-Protocol
DMS-100	I-Protocol
,	O
5ESS	B-Device
,	O
GTD-5	O
and	O
EWSD	B-Device
switches	O
with	O
Voice	B-Application
over	I-Application
IP	I-Application
technology	O
.	O
</s>
