<s>
WebRTC	B-Protocol
Gateway	I-Protocol
connects	O
between	O
WebRTC	B-Protocol
and	O
an	O
established	O
VoIP	B-Application
technology	O
such	O
as	O
SIP	B-Application
.	O
</s>
<s>
WebRTC	B-Protocol
(	O
Web	O
Real-Time	O
Communication	O
)	O
is	O
an	O
API	B-Application
definition	O
drafted	O
by	O
the	O
World	O
Wide	O
Web	O
Consortium	O
(	O
W3C	O
)	O
that	O
supports	O
browser-to-browser	O
applications	O
for	O
voice	O
calling	O
,	O
video	B-Application
chat	I-Application
,	O
and	O
messaging	O
without	O
the	O
need	O
of	O
either	O
internal	O
or	O
external	O
plugins	B-Application
.	O
</s>
<s>
To	O
enable	O
browsers	B-Application
using	O
different	O
application	O
providers	O
to	O
communicate	O
with	O
each	O
other	O
(	O
e.g.	O
</s>
<s>
a	O
user	O
logged	O
into	O
application	O
providers	O
X	O
wants	O
to	O
call	O
someone	O
that	O
is	O
logged	O
into	O
application	O
provider	O
Y	O
)	O
a	O
so-called	O
WebRTC	B-Protocol
trapezoid	O
can	O
be	O
used	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
two	O
providers	O
use	O
a	O
widely	O
used	O
VoIP	B-Application
signalling	O
protocol	O
such	O
as	O
SIP	B-Application
to	O
federate	O
between	O
them	O
.	O
</s>
<s>
However	O
,	O
each	O
of	O
their	O
respective	O
browser-based	O
clients	O
signals	O
to	O
its	O
server	O
using	O
proprietary	O
application	O
protocols	O
built	O
on	O
top	O
of	O
HTTP	B-Protocol
and	O
WebSocket	B-Protocol
.	O
</s>
<s>
This	O
component	O
that	O
mediates	O
between	O
WebRTC	B-Protocol
and	O
SIP	B-Application
is	O
referred	O
to	O
as	O
a	O
WebRTC	B-Protocol
Gateway	I-Protocol
.	O
</s>
<s>
Beside	O
connecting	O
different	O
WebRTC	B-Protocol
applications	O
,	O
a	O
WebRTC	B-Protocol
gateway	I-Protocol
also	O
enables	O
the	O
communication	O
between	O
a	O
WebRTC	B-Protocol
phone	O
and	O
a	O
VoIP	B-Application
or	O
even	O
a	O
PSTN	O
phone	O
.	O
</s>
<s>
Thereby	O
,	O
a	O
WebRTC	B-Protocol
gateway	I-Protocol
extends	O
the	O
scope	O
of	O
WebRTC	B-Protocol
applications	O
and	O
enables	O
much	O
wider	O
reach	O
and	O
usage	O
scenarios	O
.	O
</s>
<s>
The	O
usual	O
process	O
with	O
WebRTC	B-Protocol
is	O
that	O
a	O
user	O
downloads	O
a	O
WebRTC	B-Protocol
JavaScript	B-Language
application	O
.	O
</s>
<s>
A	O
WebRTC	B-Protocol
gateway	I-Protocol
would	O
usually	O
contain	O
the	O
server	O
from	O
where	O
a	O
user	O
would	O
download	O
the	O
WebRTC	B-Protocol
JavaScript	B-Language
application	O
.	O
</s>
<s>
When	O
receiving	O
a	O
call	O
from	O
the	O
user	O
,	O
the	O
WebRTC	B-Protocol
gateway	I-Protocol
needs	O
to	O
decide	O
whether	O
the	O
callee	O
is	O
reachable	O
over	O
WebRTC	B-Protocol
.	O
</s>
<s>
If	O
not	O
,	O
then	O
the	O
call	O
will	O
have	O
to	O
be	O
translated	O
into	O
SIP	B-Application
for	O
example	O
.	O
</s>
<s>
To	O
translate	O
a	O
call	O
into	O
SIP	B-Application
,	O
the	O
gateway	O
will	O
have	O
to	O
map	O
different	O
layers	O
:	O
</s>
<s>
Signalling	O
:	O
There	O
is	O
no	O
standardised	O
signalling	O
protocol	O
for	O
WebRTC	B-Protocol
applications	O
.	O
</s>
<s>
However	O
,	O
SIP	B-Application
over	O
WebSockets	B-Protocol
(	O
)	O
is	O
often	O
used	O
partially	O
due	O
to	O
the	O
applicability	O
of	O
SIP	B-Application
to	O
most	O
of	O
the	O
envisaged	O
communication	O
scenarios	O
as	O
well	O
as	O
the	O
availability	O
of	O
open	O
source	O
software	O
such	O
as	O
JsSIP	B-Language
.	O
</s>
<s>
In	O
such	O
a	O
case	O
,	O
the	O
gateway	O
would	O
only	O
need	O
to	O
repackage	O
the	O
SIP	B-Application
packets	O
from	O
the	O
WebSocket	B-Protocol
layer	O
into	O
UDP	B-Protocol
,	O
TCP	B-Protocol
or	O
TLS	B-Protocol
.	O
</s>
<s>
Media	O
transport	O
:	O
The	O
WebRTC	B-Protocol
specifications	O
indicate	O
that	O
for	O
security	O
reasons	O
WebRTC	B-Protocol
applications	O
must	O
use	O
SRTP	B-Protocol
for	O
transporting	O
media	O
content	O
.	O
</s>
<s>
While	O
some	O
VoIP	B-Application
applications	O
support	O
SRTP	B-Protocol
as	O
well	O
,	O
this	O
is	O
optional	O
and	O
hence	O
not	O
always	O
the	O
case	O
.	O
</s>
<s>
If	O
the	O
callee	O
does	O
not	O
support	O
SRTP	B-Protocol
then	O
the	O
WebRTC	B-Protocol
gateway	I-Protocol
will	O
have	O
to	O
map	O
between	O
SRTP	B-Protocol
and	O
RTP	B-Protocol
.	O
</s>
<s>
Media	O
content	O
:	O
The	O
WebRTC	B-Protocol
specifications	O
indicate	O
that	O
WebRTC	B-Protocol
applications	O
must	O
use	O
for	O
audio	O
communication	O
either	O
G.711	B-Algorithm
or	O
OPUS	B-Algorithm
as	O
the	O
Audio	B-Algorithm
codec	I-Algorithm
.	O
</s>
<s>
Applications	O
using	O
SIP	B-Application
for	O
establishing	O
audio	O
session	O
are	O
free	O
to	O
choose	O
any	O
type	O
of	O
codec	B-General_Concept
.	O
</s>
<s>
If	O
the	O
callee	O
does	O
not	O
support	O
OPUS	B-Algorithm
or	O
G.711	B-Algorithm
then	O
the	O
WebRTC	B-Protocol
gateway	I-Protocol
will	O
have	O
to	O
transcode	O
between	O
the	O
WebRTC	B-Protocol
and	O
SIP	B-Application
sides	O
of	O
the	O
communication	O
.	O
</s>
<s>
Media	O
address	O
negotiation	O
:	O
In	O
order	O
to	O
be	O
able	O
to	O
traverse	O
all	O
kinds	O
of	O
NAT	B-Protocol
,	O
the	O
WebRTC	B-Protocol
specifications	O
indicate	O
that	O
WebRTC	B-Protocol
applications	O
must	O
use	O
STUN	B-Protocol
and	O
ICE	O
in	O
order	O
to	O
detect	O
the	O
addresses	O
under	O
which	O
two	O
end	O
points	O
can	O
exchange	O
media	O
packets	O
.	O
</s>
<s>
While	O
these	O
technologies	O
are	O
also	O
implemented	O
by	O
some	O
SIP	B-Application
user	O
agents	O
,	O
this	O
is	O
not	O
mandatory	O
.	O
</s>
<s>
If	O
the	O
callee	O
does	O
not	O
support	O
ICE	O
or	O
in	O
case	O
media	O
transport	O
layer	O
needs	O
mapping	O
or	O
media	O
transcoding	O
is	O
required	O
then	O
the	O
WebRTC	B-Protocol
gateway	I-Protocol
will	O
have	O
to	O
act	O
as	O
an	O
ICE	O
end	O
point	O
and	O
route	O
the	O
media	O
packets	O
between	O
the	O
caller	O
and	O
callee	O
.	O
</s>
<s>
There	O
are	O
already	O
a	O
number	O
of	O
open	O
source	O
and	O
commercial	O
solutions	O
available	O
for	O
providing	O
the	O
WebRTC	B-Protocol
gateway	I-Protocol
functionality	O
.	O
</s>
<s>
As	O
a	O
lot	O
of	O
required	O
functionality	O
of	O
a	O
WebRTC	B-Protocol
gateway	I-Protocol
such	O
as	O
media	O
handling	O
,	O
signalling	O
mapping	O
is	O
supported	O
by	O
SBC	B-Protocol
the	O
function	O
of	O
WebRTC	B-Protocol
gateway	I-Protocol
is	O
often	O
integrated	O
into	O
SBCs	O
or	O
provided	O
by	O
SBC	B-Protocol
vendors	O
.	O
</s>
