<s>
A	O
VoIP	B-Application
VPN	I-Application
combines	O
voice	B-Application
over	I-Application
IP	I-Application
and	O
virtual	B-Application
private	I-Application
network	I-Application
technologies	O
to	O
offer	O
a	O
method	O
for	O
delivering	O
secure	O
voice	O
.	O
</s>
<s>
Because	O
VoIP	B-Application
transmits	O
digitized	O
voice	O
as	O
a	O
stream	O
of	O
data	O
,	O
the	O
VoIP	B-Application
VPN	I-Application
solution	O
accomplishes	O
voice	O
encryption	O
quite	O
simply	O
,	O
applying	O
standard	O
data-encryption	O
mechanisms	O
inherently	O
available	O
in	O
the	O
collection	O
of	O
protocols	O
used	O
to	O
implement	O
a	O
VPN	B-Application
.	O
</s>
<s>
The	O
VoIP	O
gateway-router	O
first	O
converts	O
the	O
analog	O
voice	O
signal	O
to	O
digital	O
form	O
,	O
encapsulates	O
the	O
digitized	O
voice	O
within	O
IP	B-Protocol
packets	O
,	O
then	O
encrypts	O
the	O
digitized	O
voice	O
using	O
IPsec	B-Protocol
,	O
and	O
finally	O
routes	O
the	O
encrypted	O
voice	O
packets	O
securely	O
through	O
a	O
VPN	B-Application
tunnel	O
.	O
</s>
<s>
At	O
the	O
remote	O
site	O
,	O
another	O
VoIP	B-Application
router	O
decodes	O
the	O
voice	O
and	O
converts	O
the	O
digital	O
voice	O
to	O
an	O
analog	O
signal	O
for	O
delivery	O
to	O
the	O
phone	O
.	O
</s>
<s>
A	O
VoIP	B-Application
VPN	I-Application
can	O
also	O
run	O
within	O
an	O
IP	B-Protocol
in	I-Protocol
IP	I-Protocol
tunnel	O
or	O
using	O
SSL-based	O
OpenVPN	B-Language
.	O
</s>
<s>
There	O
is	O
no	O
encryption	O
in	O
former	O
case	O
,	O
but	O
traffic	O
overhead	O
is	O
significantly	O
lower	O
in	O
comparison	O
with	O
IPsec	B-Protocol
tunnel	O
.	O
</s>
<s>
The	O
advantage	O
of	O
OpenVPN	B-Language
tunneling	O
is	O
that	O
it	O
can	O
run	O
on	O
a	O
dynamic	O
IP	B-Protocol
and	O
may	O
provide	O
up	O
to	O
512	O
bits	O
SSL	O
encryption	O
.	O
</s>
<s>
Security	O
is	O
not	O
the	O
only	O
reason	O
to	O
pass	O
Voice	B-Application
over	I-Application
IP	I-Application
through	O
a	O
virtual	B-Application
private	I-Application
network	I-Application
,	O
however	O
.	O
</s>
<s>
Session	B-Application
Initiation	I-Application
Protocol	I-Application
,	O
a	O
commonly	O
used	O
VoIP	B-Application
protocol	O
is	O
notoriously	O
difficult	O
to	O
pass	O
through	O
a	O
firewall	B-Application
because	O
it	O
uses	O
random	O
port	O
numbers	O
to	O
establish	O
connections	O
.	O
</s>
<s>
A	O
VPN	B-Application
is	O
also	O
a	O
workaround	O
to	O
avoid	O
a	O
firewall	B-Application
issue	O
when	O
configuring	O
remote	O
VoIP	B-Application
clients	O
.	O
</s>
<s>
However	O
,	O
latest	O
VoIP	B-Application
standard	O
STUN	B-Protocol
,	O
ICE	O
and	O
TURN	B-Protocol
eliminate	O
natively	O
some	O
NAT	B-Protocol
problems	O
of	O
VoIP	B-Application
.	O
</s>
<s>
Installing	O
an	O
extension	B-Application
on	O
a	O
VPN	B-Application
is	O
a	O
simple	O
means	O
to	O
obtain	O
an	O
off-premises	O
extension	B-Application
(	O
OPX	O
)	O
,	O
a	O
function	O
which	O
in	O
conventional	O
landline	O
telephony	O
required	O
a	O
leased	B-Architecture
line	I-Architecture
from	O
the	O
private	B-Device
branch	I-Device
exchange	I-Device
to	O
the	O
remote	O
site	O
.	O
</s>
<s>
The	O
protocol	O
overhead	O
caused	O
by	O
the	O
encapsulation	O
of	O
VoIP	B-Application
protocol	O
within	O
IPSec	B-Protocol
dramatically	O
increases	O
the	O
bandwidth	O
requirements	O
for	O
VoIP	B-Application
calls	I-Application
,	O
thus	O
making	O
the	O
VoIP	B-Application
over	O
VPN	B-Application
protocols	O
too	O
"	O
fat	O
"	O
to	O
be	O
used	O
over	O
a	O
mobile	O
data	O
connections	O
like	O
GPRS	B-General_Concept
,	O
EDGE	O
or	O
UMTS	B-Protocol
.	O
</s>
<s>
Although	O
VoIP	B-Application
over	O
VPN	B-Application
is	O
not	O
as	O
usable	O
in	O
mobile	O
environments	O
,	O
it	O
is	O
sometimes	O
used	O
to	O
create	O
"	O
encrypted	O
VoIP	B-Application
trunk	O
"	O
between	O
different	O
sites	O
of	O
a	O
corporations	O
,	O
running	O
VoIP	B-Application
PBX	B-Device
interconnections	O
over	O
a	O
VPN	B-Application
connection	O
.	O
</s>
<s>
The	O
recent	O
publication	O
of	O
new	O
VoIP	B-Application
encryption	O
standards	O
built	O
into	O
the	O
protocol	O
,	O
such	O
as	O
ZRTP	B-Protocol
and	O
SRTP	B-Protocol
,	O
allow	O
the	O
VoIP	B-Application
client	O
to	O
run	O
without	O
the	O
VPN	B-Application
overhead	O
,	O
integrating	O
with	O
standard	O
features	O
of	O
VoIP	B-Application
PBX	B-Device
without	O
having	O
to	O
manage	O
both	O
the	O
VPN	B-Application
gateway	O
and	O
the	O
PBX	B-Device
.	O
</s>
<s>
VoIP	B-Application
VPN	I-Application
solution	O
may	O
be	O
accomplished	O
with	O
free	O
open	B-Application
source	I-Application
software	I-Application
by	O
using	O
a	O
Linux	B-Application
distribution	I-Application
or	O
BSD	B-Operating_System
as	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
a	O
VoIP	B-Application
server	O
,	O
and	O
an	O
IPsec	B-Protocol
server	O
.	O
</s>
