<s>
A	O
softswitch	B-Device
(	O
software	O
switch	O
)	O
is	O
a	O
call-switching	O
node	O
in	O
a	O
telecommunications	O
network	O
,	O
based	O
not	O
on	O
the	O
specialized	O
switching	O
hardware	O
of	O
the	O
traditional	O
telephone	O
exchange	O
,	O
but	O
implemented	O
in	O
software	O
running	O
on	O
a	O
general-purpose	O
computing	O
platform	O
.	O
</s>
<s>
Often	O
a	O
softswitch	B-Device
is	O
implemented	O
to	O
switch	O
calls	O
using	O
voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
technologies	O
,	O
but	O
hybrid	O
systems	O
exist	O
.	O
</s>
<s>
Although	O
the	O
term	O
softswitch	B-Device
technically	O
refers	O
to	O
any	O
such	O
device	O
,	O
it	O
is	O
conventionally	O
applied	O
to	O
a	O
device	O
that	O
handles	O
IP-to-IP	O
phone	O
calls	O
,	O
while	O
the	O
phrase	O
access	O
server	O
or	O
"	O
media	O
gateway	O
"	O
is	O
used	O
to	O
refer	O
to	O
devices	O
that	O
either	O
originate	O
or	O
terminate	O
traditional	O
land	B-Application
line	I-Application
phone	O
calls	O
.	O
</s>
<s>
An	O
access	O
server	O
might	O
take	O
a	O
mobile	O
call	O
or	O
a	O
call	O
originating	O
from	O
a	O
traditional	O
telephone	O
line	O
,	O
convert	O
it	O
to	O
IP	O
traffic	O
,	O
then	O
send	O
it	O
over	O
an	O
IP	O
network	O
to	O
another	O
such	O
device	O
,	O
which	O
terminates	O
the	O
call	O
by	O
reversing	O
the	O
process	O
and	O
converting	O
the	O
voice	B-Application
over	I-Application
IP	I-Application
call	O
to	O
circuit-switched	O
digital	O
systems	O
using	O
traditional	O
digital	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
TDM	O
)	O
or	O
analog	O
POTS	O
protocols	O
.	O
</s>
<s>
The	O
call	O
agents	O
are	O
the	O
software	O
switching	O
elements	O
of	O
the	O
softswitch	B-Device
.	O
</s>
<s>
The	O
network	O
elements	O
that	O
convert	O
voice	O
streams	O
between	O
VoIP	B-Application
links	O
and	O
traditional	O
media	O
technologies	O
,	O
such	O
as	O
analog	O
telephone	O
lines	O
,	O
pair-gain	O
devices	O
,	O
carrier	O
systems	O
,	O
are	O
called	O
media	O
gateways	O
.	O
</s>
<s>
The	O
softswitch	B-Device
generally	O
resides	O
in	O
a	O
building	O
owned	O
by	O
a	O
telephone	O
company	O
,	O
called	O
a	O
telephone	O
exchange	O
or	O
central	O
office	O
,	O
or	O
in	O
a	O
data	B-Operating_System
center	I-Operating_System
.	O
</s>
<s>
Access	O
devices	O
to	O
the	O
services	O
of	O
a	O
softswitch	B-Device
range	O
from	O
large	O
media	O
gateways	O
with	O
high	O
port	O
density	O
to	O
integrated	O
access	O
devices	O
(	O
IAD	O
)	O
at	O
office	O
locations	O
,	O
to	O
small	O
analog	O
telephone	O
adaptors	O
(	O
ATA	O
)	O
which	O
provide	O
just	O
one	O
RJ11	O
telephone	O
jack	O
to	O
a	O
residence	O
.	O
</s>
<s>
A	O
softswitch	B-Device
routes	O
telephone	O
calls	O
using	O
the	O
Signalling	O
System	O
No	O
.	O
</s>
<s>
SS7	O
modules	O
may	O
be	O
implemented	O
directly	O
in	O
the	O
softswitch	B-Device
,	O
or	O
accessed	O
from	O
standalone	O
signaling	O
servers	O
.	O
</s>
<s>
At	O
the	O
turn	O
of	O
the	O
21st	O
century	O
with	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
(	O
IMS	O
)	O
,	O
the	O
softswitch	B-Device
element	O
is	O
represented	O
by	O
the	O
media	B-Protocol
gateway	I-Protocol
controller	I-Protocol
(	O
MGC	O
)	O
element	O
,	O
while	O
the	O
term	O
softswitch	B-Device
is	O
rarely	O
used	O
in	O
the	O
IMS	O
context	O
,	O
where	O
it	O
is	O
called	O
an	O
access	O
gateway	O
control	O
function	O
(	O
AGCF	O
)	O
.	O
</s>
<s>
VoIP	B-Application
softswitches	B-Device
are	O
subdivided	O
into	O
Class	O
4	O
and	O
Class	O
5	O
systems	O
,	O
in	O
analogy	O
to	O
the	O
traditional	O
functions	O
in	O
the	O
public	O
switched	O
telephone	O
network	O
.	O
</s>
<s>
Softswitches	B-Device
used	O
for	O
transit	O
VoIP	B-Application
traffic	O
between	O
carriers	O
are	O
usually	O
called	O
Class	O
4	O
softswitches	B-Device
.	O
</s>
<s>
Analogous	O
with	O
other	O
Class	B-Device
4	I-Device
telephone	I-Device
switches	I-Device
,	O
the	O
main	O
function	O
of	O
the	O
Class	O
4	O
softswitch	B-Device
is	O
the	O
routing	O
of	O
large	O
volumes	O
of	O
long-distance	O
VoIP	B-Application
calls	I-Application
.	O
</s>
<s>
The	O
most	O
important	O
characteristics	O
of	O
Class	O
4	O
softswitch	B-Device
are	O
protocol	O
support	O
and	O
conversion	O
,	O
transcoding	B-Algorithm
,	O
calls	O
per	O
second	O
rate	O
,	O
average	O
time	O
of	O
one	O
call	O
routing	O
,	O
number	O
of	O
concurrent	O
calls	O
.	O
</s>
<s>
Class	O
5	O
softswitches	B-Device
are	O
intended	O
to	O
serve	O
subscribers	O
.	O
</s>
<s>
Class	O
5	O
softswitches	B-Device
are	O
characterized	O
by	O
additional	O
services	O
for	O
end-users	O
and	O
corporate	O
clients	O
such	O
as	O
IP	O
PBX	O
features	O
,	O
call	O
center	O
services	O
,	O
calling	O
card	O
platform	O
,	O
types	O
of	O
authorization	O
,	O
QoS	O
,	O
Business	O
Groups	O
and	O
other	O
features	O
similar	O
to	O
other	O
Class	B-Device
5	I-Device
telephone	I-Device
switches	I-Device
.	O
</s>
