<s>
A	O
back-to-back	B-Protocol
user	I-Protocol
agent	I-Protocol
(	O
B2BUA	B-Protocol
)	O
is	O
a	O
logical	O
network	O
element	O
in	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
applications	O
.	O
</s>
<s>
SIP	O
is	O
a	O
signaling	O
protocol	O
for	O
managing	O
multimedia	O
Voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
telephone	O
calls	O
.	O
</s>
<s>
A	O
back-to-back	B-Protocol
user	I-Protocol
agent	I-Protocol
operates	O
between	O
both	O
end	O
points	O
of	O
a	O
communications	O
session	O
and	O
divides	O
the	O
communication	O
channel	O
into	O
two	O
call	O
legs	O
,	O
and	O
mediates	O
all	O
SIP	O
signaling	O
between	O
the	O
endpoints	O
of	O
the	O
session	O
,	O
from	O
establishment	O
to	O
termination	O
.	O
</s>
<s>
As	O
all	O
control	O
messages	O
for	O
each	O
call	O
flow	O
through	O
the	O
B2BUA	B-Protocol
,	O
a	O
service	O
provider	O
may	O
implement	O
value-added	O
features	O
available	O
during	O
the	O
call	O
.	O
</s>
<s>
In	O
the	O
originating	O
call	O
leg	O
,	O
the	O
B2BUA	B-Protocol
acts	O
as	O
a	O
user	B-Application
agent	I-Application
server	I-Application
(	O
UAS	O
)	O
and	O
processes	O
the	O
request	O
as	O
a	O
user	B-Protocol
agent	I-Protocol
client	O
(	O
UAC	O
)	O
to	O
the	O
destination	O
end	O
,	O
handling	O
the	O
signaling	O
between	O
end	O
points	O
back-to-back	O
.	O
</s>
<s>
A	O
B2BUA	B-Protocol
maintains	O
complete	O
state	O
for	O
the	O
calls	O
it	O
handles	O
.	O
</s>
<s>
Each	O
side	O
of	O
a	O
B2BUA	B-Protocol
operates	O
as	O
a	O
standard	O
SIP	O
user	B-Protocol
agent	I-Protocol
network	O
element	O
as	O
specified	O
in	O
.	O
</s>
<s>
In	O
addition	O
to	O
call	O
management	O
,	O
a	O
B2BUA	B-Protocol
may	O
provide	O
billing	O
services	O
,	O
internetworking	O
for	O
protocol	O
conversions	O
,	O
and	O
hiding	O
of	O
network-internal	O
topology	O
and	O
information	O
.	O
</s>
<s>
B2BUAs	B-Protocol
are	O
often	O
implemented	O
in	O
media	O
gateways	O
to	O
bridge	O
the	O
media	O
streams	O
,	O
in	O
addition	O
to	O
the	O
signaling	O
path	O
,	O
for	O
full	O
control	O
over	O
the	O
session	O
.	O
</s>
<s>
A	O
signaling	B-Protocol
gateway	I-Protocol
,	O
part	O
of	O
a	O
session	B-Protocol
border	I-Protocol
controller	I-Protocol
,	O
is	O
an	O
example	O
of	O
a	O
B2BUA	B-Protocol
.	O
</s>
