<s>
A	O
softphone	B-Protocol
is	O
a	O
software	B-Application
program	O
for	O
making	O
telephone	O
calls	O
over	O
the	O
Internet	O
using	O
a	O
general	O
purpose	O
computer	O
rather	O
than	O
dedicated	O
hardware	O
.	O
</s>
<s>
The	O
softphone	B-Protocol
can	O
be	O
installed	O
on	O
a	O
piece	O
of	O
equipment	O
such	O
as	O
a	O
desktop	B-Device
,	O
mobile	B-Application
device	I-Application
,	O
or	O
other	O
computer	O
and	O
allows	O
the	O
user	O
to	O
place	O
and	O
receive	O
calls	O
without	O
requiring	O
an	O
actual	O
telephone	O
set	O
.	O
</s>
<s>
Often	O
,	O
a	O
softphone	B-Protocol
is	O
designed	O
to	O
behave	O
like	O
a	O
traditional	O
telephone	O
,	O
sometimes	O
appearing	O
as	O
an	O
image	O
of	O
a	O
handset	O
,	O
with	O
a	O
display	O
panel	O
and	O
buttons	O
with	O
which	O
the	O
user	O
can	O
interact	O
.	O
</s>
<s>
A	O
softphone	B-Protocol
is	O
usually	O
used	O
with	O
a	O
headset	B-Device
connected	O
to	O
the	O
sound	B-Device
card	I-Device
of	O
the	O
PC	O
or	O
with	O
a	O
USB	O
phone	O
.	O
</s>
<s>
To	O
communicate	O
,	O
both	O
end-points	O
must	O
support	O
the	O
same	O
voice-over-IP	O
protocol	O
,	O
and	O
at	O
least	O
one	O
common	O
audio	O
codec	B-General_Concept
.	O
</s>
<s>
Many	O
service	O
providers	O
use	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
standardized	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
.	O
</s>
<s>
Skype	B-Application
,	O
a	O
popular	O
service	O
,	O
uses	O
proprietary	O
protocols	O
,	O
and	O
Google	B-Application
Talk	I-Application
leveraged	O
the	O
Extensible	B-Protocol
Messaging	I-Protocol
and	I-Protocol
Presence	I-Protocol
Protocol	I-Protocol
(	O
XMPP	B-Protocol
)	O
.	O
</s>
<s>
Some	O
softphones	B-Protocol
also	O
support	O
the	O
Inter-Asterisk	B-Protocol
eXchange	I-Protocol
protocol	O
(	O
IAX	B-Protocol
)	O
,	O
a	O
protocol	O
supported	O
by	O
the	O
open-source	O
software	B-Application
application	O
Asterisk	O
.	O
</s>
<s>
A	O
typical	O
softphone	B-Protocol
has	O
all	O
standard	O
telephony	O
features	O
(	O
DND	O
,	O
Mute	O
,	O
DTMF	O
,	O
Flash	O
,	O
Hold	O
,	O
Transfer	O
etc	O
.	O
)	O
</s>
<s>
Softphones	B-Protocol
provide	O
a	O
variety	O
of	O
audio	O
codecs	B-General_Concept
,	O
a	O
typical	O
minimum	O
set	O
is	O
G.711	O
and	O
G.729	O
.	O
</s>
<s>
A	O
modern	O
PC	O
with	O
a	O
microphone	O
and	O
speaker	O
,	O
or	O
with	O
a	O
headset	B-Device
,	O
or	O
USB	O
phone	O
.	O
</s>
<s>
Account	O
with	O
an	O
Internet	B-Protocol
telephony	I-Protocol
service	I-Protocol
provider	I-Protocol
or	O
IP	O
PBX	O
provider	O
.	O
</s>
