<s>
A	O
VoIP	B-Device
phone	I-Device
or	O
IP	B-Device
phone	I-Device
uses	O
voice	B-Application
over	I-Application
IP	I-Application
technologies	O
for	O
placing	O
and	O
transmitting	O
telephone	O
calls	O
over	O
an	O
IP	B-Protocol
network	I-Protocol
,	O
such	O
as	O
the	O
Internet	O
.	O
</s>
<s>
Digital	O
IP-based	O
telephone	O
service	O
uses	O
control	O
protocols	O
such	O
as	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
,	O
Skinny	B-Protocol
Client	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
SCCP	O
)	O
or	O
various	O
other	O
proprietary	O
protocols	O
.	O
</s>
<s>
VoIP	B-Device
phones	I-Device
can	O
be	O
simple	O
software-based	O
softphones	B-Protocol
or	O
purpose-built	O
hardware	O
devices	O
that	O
appear	O
much	O
like	O
an	O
ordinary	O
telephone	O
or	O
a	O
cordless	O
phone	O
.	O
</s>
<s>
Traditional	O
PSTN	O
phones	O
can	O
be	O
used	O
as	O
VoIP	B-Device
phones	I-Device
with	O
analog	B-Protocol
telephone	I-Protocol
adapters	I-Protocol
(	O
ATA	O
)	O
.	O
</s>
<s>
A	O
VoIP	B-Device
phone	I-Device
or	O
application	O
may	O
have	O
many	O
features	O
an	O
analog	O
phone	O
does	O
n't	O
support	O
,	O
such	O
as	O
e-mail-like	O
IDs	O
for	O
contacts	O
that	O
may	O
be	O
easier	O
to	O
remember	O
than	O
names	O
or	O
phone	O
numbers	O
,	O
or	O
easy	O
sharing	O
of	O
contact	O
lists	O
among	O
multiple	O
accounts	O
.	O
</s>
<s>
Generally	O
the	O
features	O
of	O
VoIP	B-Device
phones	I-Device
follow	O
those	O
of	O
Skype	B-Application
and	O
other	O
PC-based	O
phone	O
services	O
,	O
which	O
have	O
richer	O
feature	O
sets	O
but	O
may	O
experience	O
latency-related	O
problems	O
,	O
because	O
they	O
rely	O
on	O
mainstream	O
operating	O
systems	O
 '	O
IP	O
and	O
audio	O
support	O
.	O
</s>
<s>
becomes	O
available	O
from	O
wireless	O
carriers	O
,	O
tablets	B-Device
and	O
smartphones	B-Application
became	O
the	O
dominant	O
interfaces	O
.	O
</s>
<s>
iPhone	B-Device
,	O
Android	B-Application
and	O
the	O
QNX	B-Operating_System
OS	O
used	O
in	O
2012-and-later	O
BlackBerry	B-Operating_System
phones	I-Operating_System
are	O
widely	O
capable	O
of	O
VoIP	B-Application
performance	O
.	O
</s>
<s>
Besides	O
wireless	O
,	O
they	O
also	O
typically	O
support	O
USB	O
,	O
but	O
not	O
Ethernet	O
or	O
Power	B-Device
over	I-Device
Ethernet	I-Device
interfaces	O
.	O
</s>
<s>
The	O
smartphone	B-Application
became	O
the	O
dominant	O
VoIP	B-Device
phone	I-Device
because	O
it	O
works	O
both	O
indoors	O
and	O
outdoors	O
,	O
and	O
shifts	O
base	O
stations/protocols	O
easily	O
.	O
</s>
<s>
The	O
PoE/USB	O
VoIP	B-Device
phone	I-Device
was	O
thus	O
relegated	O
to	O
the	O
role	O
of	O
a	O
transitional	O
device	O
,	O
except	O
in	O
traditional	O
business	O
office	O
,	O
where	O
it	O
is	O
still	O
widely	O
used	O
as	O
a	O
desk	O
phone	O
.	O
</s>
<s>
A	O
VoIP	B-Application
telephone	O
consist	O
of	O
the	O
hardware	O
and	O
software	O
components	O
.	O
</s>
<s>
The	O
software	O
requires	O
standard	O
networking	O
components	O
such	O
as	O
a	O
TCP/IP	B-Protocol
network	I-Protocol
stack	O
,	O
client	O
implementation	O
for	O
DHCP	B-Protocol
,	O
and	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
.	O
</s>
<s>
In	O
addition	O
,	O
a	O
VoIP	B-Application
signalling	O
protocol	O
stack	O
,	O
such	O
as	O
for	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
,	O
H.323	B-Protocol
,	O
Skinny	B-Protocol
Client	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
Cisco	O
)	O
,	O
and/or	O
Skype	B-Application
,	O
is	O
needed	O
.	O
</s>
<s>
For	O
media	O
streams	O
,	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	B-Protocol
)	O
is	O
used	O
in	O
most	O
VoIP	B-Application
systems	O
.	O
</s>
<s>
For	O
voice	O
and	O
media	O
encoding	O
,	O
a	O
variety	O
of	O
codecs	O
are	O
available	O
,	O
such	O
as	O
for	O
audio	O
:	O
G.711	B-Algorithm
,	O
GSM	O
,	O
iLBC	B-Algorithm
,	O
Speex	B-Application
,	O
G.729	B-Algorithm
,	O
G.722	B-Algorithm
,	O
G.722.2	B-Algorithm
(	O
AMR-WB	B-Algorithm
)	O
,	O
other	B-Algorithm
audio	I-Algorithm
codecs	I-Algorithm
,	O
and	O
for	O
video	O
H.263	B-Algorithm
,	O
H.263	B-Algorithm
+	O
,	O
H.264	B-Application
.	O
</s>
<s>
User	O
interface	O
software	O
controls	O
the	O
operation	O
of	O
the	O
hardware	O
components	O
,	O
and	O
may	O
respond	O
to	O
user	O
actions	O
with	O
messages	O
to	O
a	O
display	B-Device
screen	O
.	O
</s>
<s>
To	O
enable	O
the	O
VoIP	B-Application
communications	O
,	O
the	O
SIP/RTP	O
packets	O
should	O
be	O
utilised	O
and	O
STUN	B-Protocol
client	O
would	O
be	O
the	O
key	O
component	O
for	O
VoIP	B-Application
communications	O
with	O
management	O
of	O
the	O
SIP/RTP	O
packets	O
.	O
</s>
<s>
A	O
Session	B-Protocol
Traversal	I-Protocol
Utilities	I-Protocol
for	I-Protocol
NAT	I-Protocol
(	O
STUN	B-Protocol
)	O
client	O
is	O
used	O
on	O
some	O
SIP-based	O
VoIP	B-Device
phones	I-Device
as	O
firewalls	O
on	O
network	O
interface	O
sometimes	O
block	O
SIP/RTP	O
packets	O
.	O
</s>
<s>
Some	O
special	O
mechanism	O
is	O
required	O
in	O
this	O
case	O
to	O
enable	O
routing	O
of	O
SIP	B-Application
packets	O
from	O
one	O
network	O
to	O
other	O
.	O
</s>
<s>
STUN	B-Protocol
is	O
used	O
in	O
some	O
of	O
the	O
sip	B-Device
phones	I-Device
to	O
enable	O
the	O
SIP/RTP	O
packets	O
to	O
cross	O
boundaries	O
of	O
two	O
different	O
IP	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
A	O
packet	O
becomes	O
unroutable	O
between	O
two	O
sip	B-Application
elements	O
if	O
one	O
of	O
the	O
networks	O
uses	O
private	O
IP	O
address	O
range	O
and	O
other	O
is	O
in	O
public	O
IP	O
address	O
range	O
.	O
</s>
<s>
Stun	B-Protocol
is	O
a	O
mechanism	O
to	O
enable	O
this	O
border	O
traversal	O
.	O
</s>
<s>
There	O
are	O
alternate	O
mechanisms	O
for	O
traversal	O
of	O
NAT	O
,	O
STUN	B-Protocol
is	O
just	O
one	O
of	O
them	O
.	O
</s>
<s>
STUN	B-Protocol
or	O
any	O
other	O
NAT	O
traversal	O
mechanism	O
is	O
not	O
required	O
when	O
the	O
two	O
SIP	B-Device
phones	I-Device
connecting	O
are	O
routable	O
from	O
each	O
other	O
and	O
no	O
firewall	O
exists	O
in	O
between	O
.	O
</s>
<s>
DHCP	B-Protocol
client	O
software	O
simplifies	O
connection	O
of	O
a	O
device	O
to	O
an	O
IP	B-Protocol
network	I-Protocol
.	O
</s>
<s>
The	O
software	O
automatically	O
configures	O
the	O
network	O
and	O
VoIP	B-Application
service	O
parameters	O
.	O
</s>
<s>
A	O
voice	B-Protocol
engine	I-Protocol
or	O
a	O
digital	B-Architecture
signal	I-Architecture
processor	I-Architecture
(	O
DSP	O
)	O
to	O
process	O
RTP	B-Protocol
messages	O
.	O
</s>
<s>
There	O
are	O
several	O
Wi-Fi	O
enabled	O
mobile	O
phones	O
and	O
PDAs	B-Application
that	O
have	O
pre-installed	O
SIP	B-Application
client	O
software	O
,	O
or	O
are	O
capable	O
of	O
running	O
IP	B-Application
telephony	I-Application
clients	O
,	O
including	O
most	O
smartphones	B-Application
.	O
</s>
<s>
Analog	B-Protocol
telephone	I-Protocol
adapters	I-Protocol
provide	O
an	O
interface	O
for	O
traditional	O
analog	O
telephones	O
to	O
a	O
voice-over-IP	B-Application
network	O
.	O
</s>
<s>
They	O
connect	O
to	O
the	O
Internet	O
or	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
using	O
an	O
Ethernet	O
port	O
and	O
have	O
jacks	O
that	O
provide	O
a	O
standard	O
RJ11	O
interface	O
that	O
can	O
accommodate	O
a	O
standard	O
analog	O
telephone	O
.	O
</s>
<s>
Regular	O
mobile	O
phones	O
can	O
connect	O
to	O
this	O
device	O
,	O
and	O
make	O
VoIP	B-Application
calls	I-Application
.	O
</s>
<s>
Some	O
VoIP	B-Device
phones	I-Device
and	O
ATAs	O
also	O
support	O
PSTN	O
phone	O
lines	O
directly	O
.	O
</s>
<s>
Support	O
for	O
multiple	O
VoIP	B-Application
accounts	O
–	O
the	O
phone	O
may	O
register	O
with	O
more	O
than	O
one	O
VoIP	B-Application
server/provider	O
.	O
</s>
<s>
A	O
more	O
sophisticated	O
feature	O
is	O
dynamic	O
download	O
of	O
account	O
settings	O
,	O
also	O
known	O
as	O
"	O
extension	B-Protocol
mobility	I-Protocol
"	O
.	O
</s>
<s>
This	O
feature	O
requires	O
both	O
a	O
client	O
(	O
phone	O
)	O
and	O
a	O
server	O
,	O
usually	O
in	O
the	O
context	O
of	O
unified	B-Protocol
communications	I-Protocol
systems	O
.	O
</s>
