<s>
H.323	B-Protocol
is	O
a	O
recommendation	O
from	O
the	O
ITU	O
Telecommunication	O
Standardization	O
Sector	O
(	O
ITU-T	O
)	O
that	O
defines	O
the	O
protocols	O
to	O
provide	O
audio-visual	O
communication	O
sessions	O
on	O
any	O
packet	B-Protocol
network	I-Protocol
.	O
</s>
<s>
The	O
H.323	B-Protocol
standard	O
addresses	O
call	O
signaling	O
and	O
control	O
,	O
multimedia	O
transport	O
and	O
control	O
,	O
and	O
bandwidth	O
control	O
for	O
point-to-point	B-Architecture
and	O
multi-point	O
conferences	O
.	O
</s>
<s>
It	O
is	O
widely	O
implemented	O
by	O
voice	O
and	O
videoconferencing	B-Application
equipment	O
manufacturers	O
,	O
is	O
used	O
within	O
various	O
Internet	O
real-time	O
applications	O
such	O
as	O
GnuGK	B-Language
and	O
NetMeeting	B-Protocol
and	O
is	O
widely	O
deployed	O
worldwide	O
by	O
service	O
providers	O
and	O
enterprises	O
for	O
both	O
voice	O
and	O
video	O
services	O
over	O
IP	B-Protocol
networks	O
.	O
</s>
<s>
It	O
is	O
a	O
part	O
of	O
the	O
ITU-T	O
H.32x	O
series	O
of	O
protocols	O
,	O
which	O
also	O
address	O
multimedia	O
communications	O
over	O
ISDN	B-Protocol
,	O
the	O
PSTN	O
or	O
SS7	O
,	O
and	O
3G	B-Architecture
mobile	I-Architecture
networks	O
.	O
</s>
<s>
H.323	B-Protocol
call	O
signaling	O
is	O
based	O
on	O
the	O
ITU-T	O
Recommendation	O
Q.931	O
protocol	O
and	O
is	O
suited	O
for	O
transmitting	O
calls	O
across	O
networks	O
using	O
a	O
mixture	O
of	O
IP	B-Protocol
,	O
PSTN	O
,	O
ISDN	B-Protocol
,	O
and	O
QSIG	O
over	O
ISDN	B-Protocol
.	O
</s>
<s>
A	O
call	O
model	O
,	O
similar	O
to	O
the	O
ISDN	B-Protocol
call	O
model	O
,	O
eases	O
the	O
introduction	O
of	O
IP	B-Application
telephony	I-Application
into	O
existing	O
networks	O
of	O
ISDN-based	O
PBX	B-Device
systems	O
,	O
including	O
transitions	O
to	O
IP-based	O
PBXs	O
.	O
</s>
<s>
Within	O
the	O
context	O
of	O
H.323	B-Protocol
,	O
an	O
IP-based	O
PBX	B-Device
might	O
be	O
a	O
gatekeeper	B-Application
or	O
other	O
call	O
control	O
element	O
which	O
provides	O
service	O
to	O
telephones	O
or	O
videophones	B-Application
.	O
</s>
<s>
The	O
first	O
version	O
of	O
H.323	B-Protocol
was	O
published	O
by	O
the	O
ITU	O
in	O
November	O
1996	O
with	O
an	O
emphasis	O
of	O
enabling	O
videoconferencing	B-Application
capabilities	O
over	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
,	O
but	O
was	O
quickly	O
adopted	O
by	O
the	O
industry	O
as	O
a	O
means	O
of	O
transmitting	O
voice	O
communication	O
over	O
a	O
variety	O
of	O
IP	B-Protocol
networks	O
,	O
including	O
WANs	O
and	O
the	O
Internet	O
(	O
see	O
VoIP	B-Application
)	O
.	O
</s>
<s>
Over	O
the	O
years	O
,	O
H.323	B-Protocol
has	O
been	O
revised	O
and	O
re-published	O
with	O
enhancements	O
necessary	O
to	O
better	O
enable	O
both	O
voice	O
and	O
video	O
functionality	O
over	O
packet-switched	B-Protocol
networks	I-Protocol
,	O
with	O
each	O
version	O
being	O
backward-compatible	B-General_Concept
with	O
the	O
previous	O
version	O
.	O
</s>
<s>
Recognizing	O
that	O
H.323	B-Protocol
was	O
being	O
used	O
for	O
communication	O
not	O
only	O
on	O
LANs	B-General_Concept
,	O
but	O
over	O
WANs	O
and	O
within	O
large	O
carrier	O
networks	O
,	O
the	O
title	O
of	O
H.323	B-Protocol
was	O
changed	O
when	O
published	O
in	O
1998	O
.	O
</s>
<s>
The	O
current	O
version	O
of	O
H.323	B-Protocol
was	O
approved	O
in	O
2009	O
.	O
</s>
<s>
One	O
strength	O
of	O
H.323	B-Protocol
was	O
the	O
relatively	O
early	O
availability	O
of	O
a	O
set	O
of	O
standards	O
not	O
only	O
defining	O
the	O
basic	O
call	O
model	O
,	O
but	O
also	O
the	O
supplementary	O
services	O
needed	O
to	O
address	O
business	O
communication	O
expectations	O
.	O
</s>
<s>
H.323	B-Protocol
was	O
the	O
first	O
VoIP	B-Application
standard	O
to	O
adopt	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
standard	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
to	O
transport	O
audio	O
and	O
video	O
over	O
IP	B-Protocol
networks	O
.	O
</s>
<s>
H.323	B-Protocol
is	O
a	O
system	O
specification	O
that	O
describes	O
the	O
use	O
of	O
several	O
ITU-T	O
and	O
IETF	O
protocols	O
.	O
</s>
<s>
The	O
protocols	O
that	O
comprise	O
the	O
core	O
of	O
almost	O
any	O
H.323	B-Protocol
system	O
are	O
:	O
</s>
<s>
H.225.0	B-Protocol
Registration	O
,	O
Admission	O
and	O
Status	O
(	O
RAS	O
)	O
,	O
which	O
is	O
used	O
between	O
an	O
H.323	B-Protocol
endpoint	O
and	O
a	O
Gatekeeper	B-Application
to	O
provide	O
address	O
resolution	O
and	O
admission	O
control	O
services	O
.	O
</s>
<s>
H.225.0	B-Protocol
Call	O
Signaling	O
,	O
which	O
is	O
used	O
between	O
any	O
two	O
H.323	B-Protocol
entities	O
in	O
order	O
to	O
establish	O
communication	O
.	O
</s>
<s>
H.245	B-Application
control	O
protocol	O
for	O
multimedia	O
communication	O
,	O
which	O
describes	O
the	O
messages	O
and	O
procedures	O
used	O
for	O
capability	O
exchange	O
,	O
opening	O
and	O
closing	O
logical	O
channels	O
for	O
audio	O
,	O
video	O
and	O
data	O
,	O
control	O
and	O
indications	O
.	O
</s>
<s>
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
,	O
which	O
is	O
used	O
for	O
sending	O
or	O
receiving	O
multimedia	O
information	O
(	O
voice	O
,	O
video	O
,	O
or	O
text	O
)	O
between	O
any	O
two	O
entities	O
.	O
</s>
<s>
Many	O
H.323	B-Protocol
systems	O
also	O
implement	O
other	O
protocols	O
that	O
are	O
defined	O
in	O
various	O
ITU-T	O
Recommendations	O
to	O
provide	O
supplementary	O
services	O
support	O
or	O
deliver	O
other	O
functionality	O
to	O
the	O
user	O
.	O
</s>
<s>
H.235	B-Protocol
series	O
describes	O
security	O
within	O
H.323	B-Protocol
,	O
including	O
security	O
for	O
both	O
signaling	O
and	O
media	O
.	O
</s>
<s>
H.239	B-Application
describes	O
dual	O
stream	O
use	O
in	O
videoconferencing	B-Application
,	O
usually	O
one	O
for	O
live	O
video	O
,	O
the	O
other	O
for	O
still	O
images	O
.	O
</s>
<s>
H.460	B-Protocol
series	O
defines	O
optional	O
extensions	O
that	O
might	O
be	O
implemented	O
by	O
an	O
endpoint	O
or	O
a	O
Gatekeeper	B-Application
,	O
including	O
ITU-T	O
Recommendations	O
H.460.17	O
,	O
H.460.18	O
,	O
and	O
H.460.19	O
for	O
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
/	O
Firewall	B-Application
(	O
FW	O
)	O
traversal	O
.	O
</s>
<s>
In	O
addition	O
to	O
those	O
ITU-T	O
Recommendations	O
,	O
H.323	B-Protocol
implements	O
various	O
IETF	O
Request	O
for	O
Comments	O
(	O
RFCs	O
)	O
for	O
media	O
transport	O
and	O
media	O
packetization	O
,	O
including	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
.	O
</s>
<s>
H.323	B-Protocol
utilizes	O
both	O
ITU-defined	O
codecs	B-General_Concept
and	O
codecs	B-General_Concept
defined	O
outside	O
the	O
ITU	O
.	O
</s>
<s>
Codecs	B-General_Concept
that	O
are	O
widely	O
implemented	O
by	O
H.323	B-Protocol
equipment	O
include	O
:	O
</s>
<s>
All	O
H.323	B-Protocol
terminals	O
providing	O
video	O
communications	O
shall	O
be	O
capable	O
of	O
encoding	O
and	O
decoding	O
video	O
according	O
to	O
H.261	B-Algorithm
QCIF	B-Application
.	O
</s>
<s>
All	O
H.323	B-Protocol
terminals	O
shall	O
have	O
an	O
audio	B-Algorithm
codec	I-Algorithm
and	O
shall	O
be	O
capable	O
of	O
encoding	O
and	O
decoding	O
speech	O
according	O
to	O
ITU-T	O
Rec	O
.	O
</s>
<s>
G.711	B-Algorithm
.	O
</s>
<s>
All	O
terminals	O
shall	O
be	O
capable	O
of	O
transmitting	O
and	O
receiving	O
A-law	B-Algorithm
and	O
μ-law	B-Algorithm
.	O
</s>
<s>
Support	O
for	O
other	O
audio	O
and	O
video	B-Application
codecs	I-Application
is	O
optional	O
.	O
</s>
<s>
The	O
H.323	B-Protocol
system	O
defines	O
several	O
network	O
elements	O
that	O
work	O
together	O
in	O
order	O
to	O
deliver	O
rich	O
multimedia	O
communication	O
capabilities	O
.	O
</s>
<s>
Those	O
elements	O
are	O
Terminals	O
,	O
Multipoint	B-Protocol
Control	I-Protocol
Units	I-Protocol
(	O
MCUs	O
)	O
,	O
Gateways	B-Application
,	O
Gatekeepers	O
,	O
and	O
Border	O
Elements	O
.	O
</s>
<s>
Collectively	O
,	O
terminals	O
,	O
multipoint	B-Protocol
control	I-Protocol
units	I-Protocol
and	O
gateways	B-Application
are	O
often	O
referred	O
to	O
as	O
endpoints	O
.	O
</s>
<s>
H.323	B-Protocol
uses	O
TCP	O
port	O
number	O
1720	O
.	O
</s>
<s>
In	O
most	O
H.323	B-Protocol
deployments	O
,	O
a	O
gatekeeper	B-Application
is	O
employed	O
in	O
order	O
to	O
,	O
among	O
other	O
things	O
,	O
facilitate	O
address	O
resolution	O
.	O
</s>
<s>
Terminals	O
in	O
an	O
H.323	B-Protocol
network	O
are	O
the	O
most	O
fundamental	O
elements	O
in	O
any	O
H.323	B-Protocol
system	O
,	O
as	O
those	O
are	O
the	O
devices	O
that	O
users	O
would	O
normally	O
encounter	O
.	O
</s>
<s>
They	O
might	O
exist	O
in	O
the	O
form	O
of	O
a	O
simple	O
IP	B-Protocol
phone	O
or	O
a	O
powerful	O
high-definition	O
videoconferencing	B-Application
system	O
.	O
</s>
<s>
Inside	O
an	O
H.323	B-Protocol
terminal	O
is	O
something	O
referred	O
to	O
as	O
a	O
Protocol	B-Protocol
stack	I-Protocol
,	O
which	O
implements	O
the	O
functionality	O
defined	O
by	O
the	O
H.323	B-Protocol
system	O
.	O
</s>
<s>
The	O
protocol	B-Protocol
stack	I-Protocol
would	O
include	O
an	O
implementation	O
of	O
the	O
basic	O
protocol	O
defined	O
in	O
ITU-T	O
Recommendation	O
H.225.0	B-Protocol
and	O
H.245	B-Application
,	O
as	O
well	O
as	O
RTP	O
or	O
other	O
protocols	O
described	O
above	O
.	O
</s>
<s>
In	O
reality	O
,	O
most	O
H.323	B-Protocol
systems	O
do	O
not	O
implement	O
such	O
a	O
wide	O
array	O
of	O
capabilities	O
,	O
but	O
the	O
logical	O
arrangement	O
is	O
useful	O
in	O
understanding	O
the	O
relationships	O
.	O
</s>
<s>
A	O
multipoint	B-Protocol
control	I-Protocol
unit	I-Protocol
(	O
MCU	O
)	O
is	O
responsible	O
for	O
managing	O
multipoint	O
conferences	O
and	O
is	O
composed	O
of	O
two	O
logical	O
entities	O
referred	O
to	O
as	O
the	O
Multipoint	O
Controller	O
(	O
MC	O
)	O
and	O
the	O
Multipoint	O
Processor	O
(	O
MP	O
)	O
.	O
</s>
<s>
The	O
most	O
significant	O
difference	O
,	O
however	O
,	O
is	O
that	O
H.323	B-Protocol
MCUs	O
might	O
be	O
capable	O
of	O
mixing	O
or	O
switching	O
video	O
,	O
in	O
addition	O
to	O
the	O
normal	O
audio	O
mixing	O
done	O
by	O
a	O
traditional	O
conference	O
bridge	O
.	O
</s>
<s>
What	O
this	O
means	O
to	O
the	O
end	O
user	O
is	O
that	O
,	O
by	O
placing	O
a	O
video	B-Application
call	I-Application
into	O
an	O
H.323	B-Protocol
MCU	O
,	O
the	O
user	O
might	O
be	O
able	O
to	O
see	O
all	O
of	O
the	O
other	O
participants	O
in	O
the	O
conference	O
,	O
not	O
only	O
hear	O
their	O
voices	O
.	O
</s>
<s>
Gateways	B-Application
are	O
devices	O
that	O
enable	O
communication	O
between	O
H.323	B-Protocol
networks	O
and	O
other	O
networks	O
,	O
such	O
as	O
PSTN	O
or	O
ISDN	B-Protocol
networks	O
.	O
</s>
<s>
If	O
one	O
party	O
in	O
a	O
conversation	O
is	O
utilizing	O
a	O
terminal	O
that	O
is	O
not	O
an	O
H.323	B-Protocol
terminal	O
,	O
then	O
the	O
call	O
must	O
pass	O
through	O
a	O
gateway	B-Application
in	O
order	O
to	O
enable	O
both	O
parties	O
to	O
communicate	O
.	O
</s>
<s>
Gateways	B-Application
are	O
widely	O
used	O
today	O
in	O
order	O
to	O
enable	O
the	O
legacy	O
PSTN	O
phones	O
to	O
interconnect	O
with	O
the	O
large	O
,	O
international	O
H.323	B-Protocol
networks	O
that	O
are	O
presently	O
deployed	O
by	O
services	O
providers	O
.	O
</s>
<s>
Gateways	B-Application
are	O
also	O
used	O
within	O
the	O
enterprise	O
in	O
order	O
to	O
enable	O
enterprise	O
IP	B-Protocol
phones	O
to	O
communicate	O
through	O
the	O
service	O
provider	O
to	O
users	O
on	O
the	O
PSTN	O
.	O
</s>
<s>
Gateways	B-Application
are	O
also	O
used	O
in	O
order	O
to	O
enable	O
videoconferencing	B-Application
devices	O
based	O
on	O
H.320	B-Application
and	O
H.324	O
to	O
communicate	O
with	O
H.323	B-Protocol
systems	O
.	O
</s>
<s>
Most	O
of	O
the	O
third	O
generation	O
(	O
3G	B-Architecture
)	O
mobile	O
networks	O
deployed	O
today	O
utilize	O
the	O
H.324	O
protocol	O
and	O
are	O
able	O
to	O
communicate	O
with	O
H.323-based	O
terminals	O
in	O
corporate	O
networks	O
through	O
such	O
gateway	B-Application
devices	O
.	O
</s>
<s>
A	O
Gatekeeper	B-Application
is	O
an	O
optional	O
component	O
in	O
the	O
H.323	B-Protocol
network	O
that	O
provides	O
a	O
number	O
of	O
services	O
to	O
terminals	O
,	O
gateways	B-Application
,	O
and	O
MCU	O
devices	O
.	O
</s>
<s>
Of	O
the	O
various	O
functions	O
performed	O
by	O
the	O
gatekeeper	B-Application
,	O
address	O
resolution	O
is	O
the	O
most	O
important	O
as	O
it	O
enables	O
two	O
endpoints	O
to	O
contact	O
each	O
other	O
without	O
either	O
endpoint	O
having	O
to	O
know	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
other	O
endpoint	O
.	O
</s>
<s>
Gatekeepers	O
may	O
be	O
designed	O
to	O
operate	O
in	O
one	O
of	O
two	O
signaling	O
modes	O
,	O
namely	O
"	O
direct	O
routed	O
"	O
and	O
"	O
gatekeeper	B-Application
routed	O
"	O
mode	O
.	O
</s>
<s>
In	O
this	O
mode	O
,	O
endpoints	O
utilize	O
the	O
RAS	O
protocol	O
in	O
order	O
to	O
learn	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
remote	O
endpoint	O
and	O
a	O
call	O
is	O
established	O
directly	O
with	O
the	O
remote	O
device	O
.	O
</s>
<s>
In	O
the	O
gatekeeper	B-Application
routed	O
mode	O
,	O
call	O
signaling	O
always	O
passes	O
through	O
the	O
gatekeeper	B-Application
.	O
</s>
<s>
While	O
the	O
latter	O
requires	O
the	O
gatekeeper	B-Application
to	O
have	O
more	O
processing	O
power	O
,	O
it	O
also	O
gives	O
the	O
gatekeeper	B-Application
complete	O
control	O
over	O
the	O
call	O
and	O
the	O
ability	O
to	O
provide	O
supplementary	O
services	O
on	O
behalf	O
of	O
the	O
endpoints	O
.	O
</s>
<s>
H.323	B-Protocol
endpoints	O
use	O
the	O
RAS	O
protocol	O
to	O
communicate	O
with	O
a	O
gatekeeper	B-Application
.	O
</s>
<s>
A	O
collection	O
of	O
endpoints	O
that	O
are	O
registered	O
to	O
a	O
single	O
Gatekeeper	B-Application
in	O
H.323	B-Protocol
is	O
referred	O
to	O
as	O
a	O
“	O
zone	O
”	O
.	O
</s>
<s>
Border	O
Elements	O
and	O
Peer	O
Elements	O
are	O
optional	O
entities	O
similar	O
to	O
a	O
Gatekeeper	B-Application
,	O
but	O
that	O
do	O
not	O
manage	O
endpoints	O
directly	O
and	O
provide	O
some	O
services	O
that	O
are	O
not	O
described	O
in	O
the	O
RAS	O
protocol	O
.	O
</s>
<s>
The	O
role	O
of	O
a	O
border	O
or	O
peer	O
element	O
is	O
understood	O
via	O
the	O
definition	O
of	O
an	O
"	O
administrative	B-Protocol
domain	I-Protocol
"	O
.	O
</s>
<s>
An	O
administrative	B-Protocol
domain	I-Protocol
is	O
the	O
collection	O
of	O
all	O
zones	O
that	O
are	O
under	O
the	O
control	O
of	O
a	O
single	O
person	O
or	O
organization	O
,	O
such	O
as	O
a	O
service	O
provider	O
.	O
</s>
<s>
Within	O
a	O
service	O
provider	O
network	O
there	O
may	O
be	O
hundreds	O
or	O
thousands	O
of	O
gateway	B-Application
devices	O
,	O
telephones	O
,	O
video	O
terminals	O
,	O
or	O
other	O
H.323	B-Protocol
network	O
elements	O
.	O
</s>
<s>
Taken	O
together	O
,	O
all	O
of	O
the	O
zones	O
within	O
the	O
service	O
provider	O
network	O
would	O
appear	O
to	O
another	O
service	O
provider	O
as	O
an	O
"	O
administrative	B-Protocol
domain	I-Protocol
"	O
.	O
</s>
<s>
The	O
border	O
element	O
is	O
a	O
signaling	O
entity	O
that	O
generally	O
sits	O
at	O
the	O
edge	O
of	O
the	O
administrative	B-Protocol
domain	I-Protocol
and	O
communicates	O
with	O
another	O
administrative	B-Protocol
domain	I-Protocol
.	O
</s>
<s>
This	O
communication	O
might	O
include	O
such	O
things	O
as	O
access	O
authorization	O
information	O
;	O
call	O
pricing	O
information	O
;	O
or	O
other	O
important	O
data	O
necessary	O
to	O
enable	O
communication	O
between	O
the	O
two	O
administrative	B-Protocol
domains	I-Protocol
.	O
</s>
<s>
Peer	O
elements	O
are	O
entities	O
within	O
the	O
administrative	B-Protocol
domain	I-Protocol
that	O
,	O
more	O
or	O
less	O
,	O
help	O
to	O
propagate	O
information	O
learned	O
from	O
the	O
border	O
elements	O
throughout	O
the	O
administrative	B-Protocol
domain	I-Protocol
.	O
</s>
<s>
The	O
diagram	O
,	O
figure	O
2	O
,	O
provides	O
an	O
illustration	O
of	O
an	O
administrative	B-Protocol
domain	I-Protocol
with	O
border	O
elements	O
,	O
peer	O
elements	O
,	O
and	O
gatekeepers	O
.	O
</s>
<s>
H.323	B-Protocol
is	O
defined	O
as	O
a	O
binary	O
protocol	O
,	O
which	O
allows	O
for	O
efficient	O
message	O
processing	O
in	O
network	O
elements	O
.	O
</s>
<s>
The	O
syntax	O
of	O
the	O
protocol	O
is	O
defined	O
in	O
ASN.1	B-Protocol
and	O
uses	O
the	O
Packed	O
Encoding	O
Rules	O
(	O
PER	O
)	O
form	O
of	O
message	O
encoding	O
for	O
efficient	O
message	O
encoding	O
on	O
the	O
wire	O
.	O
</s>
<s>
Below	O
is	O
an	O
overview	O
of	O
the	O
various	O
communication	O
flows	O
in	O
H.323	B-Protocol
systems	O
.	O
</s>
<s>
Once	O
the	O
address	O
of	O
the	O
remote	O
endpoint	O
is	O
resolved	O
,	O
the	O
endpoint	O
will	O
use	O
H.225.0	B-Protocol
Call	O
Signaling	O
in	O
order	O
to	O
establish	O
communication	O
with	O
the	O
remote	O
entity	O
.	O
</s>
<s>
H.225.0	B-Protocol
messages	O
are	O
:	O
</s>
<s>
In	O
the	O
simplest	O
form	O
,	O
an	O
H.323	B-Protocol
call	O
may	O
be	O
established	O
as	O
follows	O
(	O
figure	O
3	O
)	O
.	O
</s>
<s>
In	O
this	O
example	O
,	O
the	O
endpoint	O
(	O
EP	O
)	O
on	O
the	O
left	O
initiated	O
communication	O
with	O
the	O
gateway	B-Application
on	O
the	O
right	O
and	O
the	O
gateway	B-Application
connected	O
the	O
call	O
with	O
the	O
called	O
party	O
.	O
</s>
<s>
In	O
reality	O
,	O
call	O
flows	O
are	O
often	O
more	O
complex	O
than	O
the	O
one	O
shown	O
,	O
but	O
most	O
calls	O
that	O
utilize	O
the	O
Fast	O
Connect	O
procedures	O
defined	O
within	O
H.323	B-Protocol
can	O
be	O
established	O
with	O
as	O
few	O
as	O
2	O
or	O
3	O
messages	O
.	O
</s>
<s>
Endpoints	O
must	O
notify	O
their	O
gatekeeper	B-Application
(	O
if	O
gatekeepers	O
are	O
used	O
)	O
that	O
they	O
are	O
in	O
a	O
call	O
.	O
</s>
<s>
Endpoints	O
are	O
then	O
required	O
to	O
notify	O
their	O
gatekeeper	B-Application
(	O
if	O
gatekeepers	O
are	O
used	O
)	O
that	O
the	O
call	O
has	O
ended	O
.	O
</s>
<s>
Endpoints	O
use	O
the	O
RAS	O
protocol	O
in	O
order	O
to	O
communicate	O
with	O
a	O
gatekeeper	B-Application
.	O
</s>
<s>
When	O
an	O
endpoint	O
is	O
powered	O
on	O
,	O
it	O
will	O
generally	O
send	O
a	O
gatekeeper	B-Application
request	O
(	O
GRQ	O
)	O
message	O
to	O
"	O
discover	O
"	O
gatekeepers	O
that	O
are	O
willing	O
to	O
provide	O
service	O
.	O
</s>
<s>
Gatekeepers	O
will	O
then	O
respond	O
with	O
a	O
gatekeeper	B-Application
confirm	O
(	O
GCF	O
)	O
and	O
the	O
endpoint	O
will	O
then	O
select	O
a	O
gatekeeper	B-Application
to	O
work	O
with	O
.	O
</s>
<s>
Alternatively	O
,	O
it	O
is	O
possible	O
that	O
a	O
gatekeeper	B-Application
has	O
been	O
predefined	O
in	O
the	O
system	O
’s	O
administrative	O
setup	O
so	O
there	O
is	O
no	O
need	O
for	O
the	O
endpoint	O
to	O
discover	O
one	O
.	O
</s>
<s>
Once	O
the	O
endpoint	O
determines	O
the	O
gatekeeper	B-Application
to	O
work	O
with	O
,	O
it	O
will	O
try	O
to	O
register	O
with	O
the	O
gatekeeper	B-Application
by	O
sending	O
a	O
registration	O
request	O
(	O
RRQ	O
)	O
,	O
to	O
which	O
the	O
gatekeeper	B-Application
responds	O
with	O
a	O
registration	O
confirm	O
(	O
RCF	O
)	O
.	O
</s>
<s>
When	O
an	O
endpoint	O
wishes	O
to	O
place	O
a	O
call	O
,	O
it	O
will	O
send	O
an	O
admission	O
request	O
(	O
ARQ	O
)	O
to	O
the	O
gatekeeper	B-Application
.	O
</s>
<s>
The	O
gatekeeper	B-Application
will	O
then	O
resolve	O
the	O
address	O
(	O
either	O
locally	O
,	O
by	O
consulting	O
another	O
gatekeeper	B-Application
,	O
or	O
by	O
querying	O
some	O
other	O
network	O
service	O
)	O
and	O
return	O
the	O
address	O
of	O
the	O
remote	O
endpoint	O
in	O
the	O
admission	O
confirm	O
message	O
(	O
ACF	O
)	O
.	O
</s>
<s>
Once	O
a	O
call	O
has	O
initiated	O
(	O
but	O
not	O
necessarily	O
fully	O
connected	O
)	O
endpoints	O
may	O
initiate	O
H.245	B-Application
call	O
control	O
signaling	O
in	O
order	O
to	O
provide	O
more	O
extensive	O
control	O
over	O
the	O
conference	O
.	O
</s>
<s>
H.245	B-Application
is	O
a	O
rather	O
voluminous	O
specification	O
with	O
many	O
procedures	O
that	O
fully	O
enable	O
multipoint	O
communication	O
,	O
though	O
in	O
practice	O
most	O
implementations	O
only	O
implement	O
the	O
minimum	O
necessary	O
in	O
order	O
to	O
enable	O
point-to-point	B-Architecture
voice	O
and	O
video	O
communication	O
.	O
</s>
<s>
H.245	B-Application
provides	O
capabilities	O
such	O
as	O
capability	O
negotiation	O
,	O
master/slave	B-Operating_System
determination	O
,	O
opening	O
and	O
closing	O
of	O
"	O
logical	O
channels	O
"	O
(	O
i.e.	O
,	O
audio	O
and	O
video	O
flows	O
)	O
,	O
flow	O
control	O
,	O
and	O
conference	O
control	O
.	O
</s>
<s>
It	O
has	O
support	O
for	O
both	O
unicast	B-Protocol
and	O
multicast	O
communication	O
,	O
allowing	O
the	O
size	O
of	O
a	O
conference	O
to	O
theoretically	O
grow	O
without	O
bound	O
.	O
</s>
<s>
Of	O
the	O
functionality	O
provided	O
by	O
H.245	B-Application
,	O
capability	O
negotiation	O
is	O
arguably	O
the	O
most	O
important	O
,	O
as	O
it	O
enables	O
devices	O
to	O
communicate	O
without	O
having	O
prior	O
knowledge	O
of	O
the	O
capabilities	O
of	O
the	O
remote	O
entity	O
.	O
</s>
<s>
H.245	B-Application
enables	O
rich	O
multimedia	O
capabilities	O
,	O
including	O
audio	O
,	O
video	O
,	O
text	O
,	O
and	O
data	O
communication	O
.	O
</s>
<s>
For	O
transmission	O
of	O
audio	O
,	O
video	O
,	O
or	O
text	O
,	O
H.323	B-Protocol
devices	O
utilize	O
both	O
ITU-defined	O
codecs	B-General_Concept
and	O
codecs	B-General_Concept
defined	O
outside	O
the	O
ITU	O
.	O
</s>
<s>
Codecs	B-General_Concept
that	O
are	O
widely	O
implemented	O
by	O
H.323	B-Protocol
equipment	O
include	O
:	O
</s>
<s>
H.245	B-Application
also	O
enables	O
real-time	O
data	O
conferencing	O
capability	O
through	O
protocols	O
like	O
T.120	O
.	O
</s>
<s>
T.120-based	O
applications	O
generally	O
operate	O
in	O
parallel	O
with	O
the	O
H.323	B-Protocol
system	O
,	O
but	O
are	O
integrated	O
to	O
provide	O
the	O
user	O
with	O
a	O
seamless	O
multimedia	O
experience	O
.	O
</s>
<s>
When	O
an	O
H.323	B-Protocol
device	O
initiates	O
communication	O
with	O
a	O
remote	O
H.323	B-Protocol
device	O
and	O
when	O
H.245	B-Application
communication	O
is	O
established	O
between	O
the	O
two	O
entities	O
,	O
the	O
Terminal	O
Capability	O
Set	O
(	O
TCS	O
)	O
message	O
is	O
the	O
first	O
message	O
transmitted	O
to	O
the	O
other	O
side	O
.	O
</s>
<s>
After	O
sending	O
a	O
TCS	O
message	O
,	O
H.323	B-Protocol
entities	O
(	O
through	O
H.245	B-Application
exchanges	O
)	O
will	O
attempt	O
to	O
determine	O
which	O
device	O
is	O
the	O
"	O
master	O
"	O
and	O
which	O
is	O
the	O
"	O
slave.	O
"	O
</s>
<s>
This	O
process	O
,	O
referred	O
to	O
as	O
Master/Slave	B-Operating_System
Determination	O
(	O
MSD	O
)	O
,	O
is	O
important	O
,	O
as	O
the	O
master	O
in	O
a	O
call	O
settles	O
all	O
"	O
disputes	O
"	O
between	O
the	O
two	O
devices	O
.	O
</s>
<s>
Once	O
capabilities	O
are	O
exchanged	O
and	O
master/slave	B-Operating_System
determination	O
steps	O
have	O
completed	O
,	O
devices	O
may	O
then	O
open	O
"	O
logical	O
channels	O
"	O
or	O
media	O
flows	O
.	O
</s>
<s>
A	O
typical	O
H.245	B-Application
exchange	O
looks	O
similar	O
to	O
figure	O
5	O
:	O
</s>
<s>
For	O
this	O
reason	O
,	O
H.323	B-Protocol
version	O
2	O
(	O
published	O
in	O
1998	O
)	O
introduced	O
a	O
concept	O
called	O
Fast	O
Connect	O
,	O
which	O
enables	O
a	O
device	O
to	O
establish	O
bi-directional	O
media	O
flows	O
as	O
part	O
of	O
the	O
H.225.0	B-Protocol
call	O
establishment	O
procedures	O
.	O
</s>
<s>
Even	O
so	O
,	O
most	O
devices	O
still	O
implement	O
the	O
complete	O
H.245	B-Application
exchange	O
as	O
shown	O
above	O
and	O
perform	O
that	O
message	O
exchange	O
in	O
parallel	O
to	O
other	O
activities	O
,	O
so	O
there	O
is	O
no	O
noticeable	O
delay	O
to	O
the	O
calling	O
or	O
called	O
party	O
.	O
</s>
<s>
Voice	B-Application
over	I-Application
Internet	I-Application
Protocol	I-Application
(	O
VoIP	B-Application
)	O
describes	O
the	O
transmission	O
of	O
voice	O
using	O
the	O
Internet	O
or	O
other	O
packet	B-Protocol
switched	I-Protocol
networks	I-Protocol
.	O
</s>
<s>
ITU-T	O
Recommendation	O
H.323	B-Protocol
is	O
one	O
of	O
the	O
standards	O
used	O
in	O
VoIP	B-Application
.	O
</s>
<s>
VoIP	B-Application
requires	O
a	O
connection	O
to	O
the	O
Internet	O
or	O
another	O
packet	B-Protocol
switched	I-Protocol
network	I-Protocol
,	O
a	O
subscription	O
to	O
a	O
VoIP	B-Application
service	O
provider	O
and	O
a	O
client	O
(	O
an	O
analogue	B-Protocol
telephone	I-Protocol
adapter	I-Protocol
(	O
ATA	O
)	O
,	O
VoIP	B-Application
Phone	O
or	O
"	O
soft	B-Protocol
phone	I-Protocol
"	O
)	O
.	O
</s>
<s>
The	O
service	O
provider	O
offers	O
the	O
connection	O
to	O
other	O
VoIP	B-Application
services	O
or	O
to	O
the	O
PSTN	O
.	O
</s>
<s>
Using	O
VoIP	B-Application
between	O
two	O
enterprise	O
locations	O
would	O
not	O
necessarily	O
require	O
a	O
VoIP	B-Application
service	O
provider	O
,	O
for	O
example	O
.	O
</s>
<s>
H.323	B-Protocol
has	O
been	O
widely	O
deployed	O
by	O
companies	O
who	O
wish	O
to	O
interconnect	O
remote	O
locations	O
over	O
IP	B-Protocol
using	O
a	O
number	O
of	O
various	O
wired	O
and	O
wireless	O
technologies	O
.	O
</s>
<s>
A	O
videoconference	B-Application
,	O
or	O
videoteleconference	O
(	O
VTC	O
)	O
,	O
is	O
a	O
set	O
of	O
telecommunication	O
technologies	O
allowing	O
two	O
or	O
more	O
locations	O
to	O
interact	O
via	O
two-way	O
video	O
and	O
audio	O
transmissions	O
simultaneously	O
.	O
</s>
<s>
There	O
are	O
basically	O
two	O
types	O
of	O
videoconferencing	B-Application
;	O
dedicated	O
VTC	O
systems	O
have	O
all	O
required	O
components	O
packaged	O
into	O
a	O
single	O
piece	O
of	O
equipment	O
while	O
desktop	O
VTC	O
systems	O
are	O
add-ons	O
to	O
normal	O
PC	B-Device
's	I-Device
,	O
transforming	O
them	O
into	O
VTC	O
devices	O
.	O
</s>
<s>
Simultaneous	O
videoconferencing	B-Application
among	O
three	O
or	O
more	O
remote	O
points	O
is	O
possible	O
by	O
means	O
of	O
a	O
Multipoint	B-Protocol
Control	I-Protocol
Unit	I-Protocol
(	O
MCU	O
)	O
.	O
</s>
<s>
There	O
are	O
MCU	O
bridges	O
for	O
IP	B-Protocol
and	O
ISDN-based	O
videoconferencing	B-Application
.	O
</s>
<s>
Due	O
to	O
the	O
price	O
point	O
and	O
proliferation	O
of	O
the	O
Internet	O
,	O
and	O
broadband	O
in	O
particular	O
,	O
there	O
has	O
been	O
a	O
strong	O
spurt	O
of	O
growth	O
and	O
use	O
of	O
H.323-based	O
IP	B-Protocol
videoconferencing	B-Application
.	O
</s>
<s>
H.323	B-Protocol
is	O
accessible	O
to	O
anyone	O
with	O
a	O
high	O
speed	O
Internet	O
connection	O
,	O
such	O
as	O
DSL	O
.	O
</s>
<s>
Videoconferencing	B-Application
is	O
utilized	O
in	O
various	O
situations	O
,	O
for	O
example	O
;	O
distance	O
education	O
,	O
telemedicine	B-Application
,	O
Video	O
Relay	O
Service	O
,	O
and	O
business	O
.	O
</s>
<s>
The	O
IETF	O
produced	O
a	O
standard	O
called	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
that	O
also	O
enables	O
voice	O
and	O
video	O
communication	O
over	O
IP	B-Protocol
.	O
</s>
<s>
There	O
are	O
also	O
other	O
ITU-T	O
recommendations	O
used	O
for	O
videoconferencing	B-Application
and	O
videophone	B-Application
services	O
–	O
H.320	B-Application
(	O
using	O
ISDN	B-Protocol
)	O
and	O
H.324	O
(	O
using	O
regular	O
analog	O
phone	O
lines	O
and	O
3G	B-Architecture
mobile	I-Architecture
phones	O
)	O
.	O
</s>
<s>
Jingle	O
(	O
Jabber/XMPP	O
extension	O
)	O
also	O
enables	O
video	O
and	O
voice	B-Application
over	I-Application
IP	I-Application
.	O
</s>
<s>
Access	B-Application
Grid	I-Application
provides	O
broadly	O
similar	O
functionality	O
,	O
with	O
more	O
emphasis	O
on	O
open-source	O
and	O
utilizing	O
multicast	O
.	O
</s>
<s>
EVO	O
also	O
provides	O
relatively	O
open	O
functionality	O
via	O
Java	O
,	O
and	O
includes	O
H.323	B-Protocol
support	O
.	O
</s>
