<s>
Federated	B-Application
VoIP	I-Application
is	O
a	O
form	O
of	O
packetized	B-Application
voice	I-Application
telephony	O
that	O
uses	O
voice	B-Application
over	I-Application
IP	I-Application
between	O
autonomous	O
domains	O
in	O
the	O
public	O
Internet	O
without	O
the	O
deployment	O
of	O
central	O
virtual	O
exchange	O
points	O
or	O
switching	O
centers	O
for	O
traffic	O
routing	O
.	O
</s>
<s>
Federated	B-Application
VoIP	I-Application
uses	O
decentralized	O
addressing	O
systems	O
,	O
such	O
as	O
ENUM	B-Language
,	O
for	O
location	O
and	O
identity	O
information	O
of	O
participants	O
and	O
implements	O
secure	O
,	O
trusted	O
communications	O
(	O
TLS	O
)	O
for	O
identify	O
verification	O
.	O
</s>
<s>
The	O
traditional	O
telephone	O
networks	O
are	O
based	O
on	O
the	O
principle	O
of	O
point-to-point	O
communications	O
,	O
for	O
example	O
,	O
an	O
office	O
building	O
sends	O
and	O
receives	O
phone	O
calls	O
over	O
an	O
ISDN	B-Protocol
line	I-Protocol
to	O
the	O
telephone	O
exchange	O
.	O
</s>
<s>
Early	O
Voice	B-Application
over	I-Application
IP	I-Application
deployments	O
have	O
copied	O
this	O
model	O
,	O
sending	O
phone	O
calls	O
over	O
SIP	B-Protocol
Trunking	I-Protocol
(	O
a	O
virtual	O
equivalent	O
of	O
ISDN	B-Protocol
)	O
to	O
an	O
Internet	B-Protocol
telephony	I-Protocol
service	I-Protocol
provider	I-Protocol
(	O
ITSP	B-Protocol
)	O
(	O
a	O
virtual	O
telephone	O
exchange	O
)	O
.	O
</s>
<s>
Although	O
the	O
use	O
of	O
virtual	O
exchanges	O
(	O
soft-switches	O
)	O
and	O
ITSPs	B-Protocol
has	O
reduced	O
the	O
cost	O
of	O
telephony	O
,	O
they	O
have	O
retained	O
many	O
constraints	O
of	O
traditional	O
telephone	O
systems	O
.	O
</s>
<s>
The	O
fundamental	O
principle	O
of	O
federated	O
voice	B-Application
over	I-Application
IP	I-Application
is	O
that	O
it	O
removes	O
this	O
concept	O
of	O
a	O
virtual	O
exchange	O
,	O
just	O
as	O
email	O
has	O
eliminated	O
the	O
concept	O
of	O
a	O
centralized	O
system	O
of	O
post	O
offices	O
.	O
</s>
<s>
Although	O
there	O
is	O
no	O
formal	O
specification	O
for	O
federated	B-Application
VoIP	I-Application
as	O
a	O
whole	O
,	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
and	O
Extensible	B-Protocol
Messaging	I-Protocol
and	I-Protocol
Presence	I-Protocol
Protocol	I-Protocol
(	O
XMPP/Jabber	B-Protocol
)	O
both	O
specify	O
similar	O
and	O
compatible	O
ways	O
of	O
implementing	O
federated	B-Application
VoIP	I-Application
.	O
</s>
<s>
Implementation	O
of	O
federated	B-Application
VoIP	I-Application
involves	O
a	O
number	O
of	O
initiatives	O
:	O
</s>
<s>
(	O
optionally	O
)	O
registering	O
existing	O
telephone	O
numbers	O
in	O
a	O
well-known	O
ENUM	B-Language
service	O
,	O
typically	O
the	O
e164.arpa	O
DNS	O
domain	O
.	O
</s>
<s>
To	O
achieve	O
maximum	O
success	O
,	O
most	O
deployments	O
involve	O
both	O
SIP	O
and	O
Jabber	B-Protocol
,	O
to	O
ensure	O
connectivity	O
to	O
any	O
other	O
party	O
that	O
supports	O
only	O
one	O
or	O
the	O
other	O
.	O
</s>
<s>
This	O
is	O
not	O
so	O
difficult	O
in	O
practice	O
,	O
as	O
a	O
network	O
that	O
uses	O
SIP	O
internally	O
can	O
operate	O
a	O
Jabber	B-Protocol
gateway	O
,	O
and	O
a	O
network	O
that	O
uses	O
Jabber	B-Protocol
internally	O
can	O
operate	O
a	O
SIP	O
gateway	O
.	O
</s>
<s>
In	O
either	O
case	O
,	O
the	O
SIP	O
and	O
Jabber	B-Protocol
addresses	O
are	O
in	O
the	O
form	O
of	O
email	O
addresses	O
and	O
are	O
almost	O
always	O
identical	O
,	O
so	O
there	O
is	O
no	O
extra	O
effort	O
required	O
for	O
the	O
user	O
of	O
the	O
service	O
.	O
</s>
<s>
Various	O
open-source	B-Application
VoIP	B-Application
server	O
products	O
provide	O
detailed	O
instructions	O
on	O
how	O
to	O
implement	O
federated	B-Application
VoIP	I-Application
.	O
</s>
