<s>
The	O
IP	B-Protocol
Multimedia	I-Protocol
Subsystem	I-Protocol
or	O
IP	B-Protocol
Multimedia	I-Protocol
Core	I-Protocol
Network	I-Protocol
Subsystem	I-Protocol
(	O
IMS	O
)	O
is	O
a	O
standardised	O
architectural	B-Library
framework	I-Library
for	O
delivering	O
IP	O
multimedia	O
services	O
.	O
</s>
<s>
Historically	O
,	O
mobile	O
phones	O
have	O
provided	O
voice	O
call	O
services	O
over	O
a	O
circuit-switched-style	O
network	O
,	O
rather	O
than	O
strictly	O
over	O
an	O
IP	O
packet-switched	B-Protocol
network	I-Protocol
.	O
</s>
<s>
Alternative	O
methods	O
of	O
delivering	O
voice	O
(	O
VoIP	B-Application
)	O
or	O
other	O
multimedia	O
services	O
have	O
become	O
available	O
on	O
smartphones	O
,	O
but	O
they	O
have	O
not	O
become	O
standardized	O
across	O
the	O
industry	O
.	O
</s>
<s>
IMS	O
is	O
an	O
architectural	B-Library
framework	I-Library
that	O
provides	O
such	O
standardization	O
.	O
</s>
<s>
Its	O
original	O
formulation	O
(	O
3GPP	O
Rel-5	O
)	O
represented	O
an	O
approach	O
for	O
delivering	O
Internet	O
services	O
over	O
GPRS	B-General_Concept
.	O
</s>
<s>
This	O
vision	O
was	O
later	O
updated	O
by	O
3GPP	O
,	O
3GPP2	O
and	O
ETSI	O
TISPAN	O
by	O
requiring	O
support	O
of	O
networks	O
other	O
than	O
GPRS	B-General_Concept
,	O
such	O
as	O
Wireless	B-Architecture
LAN	I-Architecture
,	O
CDMA2000	O
and	O
fixed	B-Application
lines	I-Application
.	O
</s>
<s>
IMS	O
uses	O
IETF	O
protocols	O
wherever	O
possible	O
,	O
e.g.	O
,	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	B-Application
)	O
.	O
</s>
<s>
According	O
to	O
the	O
3GPP	O
,	O
IMS	O
is	O
not	O
intended	O
to	O
standardize	O
applications	O
,	O
but	O
rather	O
to	O
aid	O
the	O
access	O
of	O
multimedia	O
and	O
voice	O
applications	O
from	O
wireless	O
and	O
wireline	O
terminals	O
,	O
i.e.	O
,	O
to	O
create	O
a	O
form	O
of	O
fixed-mobile	O
convergence	O
(	O
FMC	O
)	O
.	O
</s>
<s>
This	O
is	O
done	O
by	O
having	O
a	O
horizontal	O
control	O
layer	O
that	O
isolates	O
the	O
access	O
network	O
from	O
the	O
service	B-Protocol
layer	I-Protocol
.	O
</s>
<s>
Alternative	O
and	O
overlapping	O
technologies	O
for	O
access	O
and	O
provisioning	O
of	O
services	O
across	O
wired	O
and	O
wireless	O
networks	O
include	O
combinations	O
of	O
Generic	O
Access	O
Network	O
,	O
softswitches	B-Device
and	O
"	O
naked	O
"	O
SIP	B-Application
.	O
</s>
<s>
Since	O
it	O
is	O
becoming	O
increasingly	O
easier	O
to	O
access	O
content	O
and	O
contacts	O
using	O
mechanisms	O
outside	O
the	O
control	O
of	O
traditional	O
wireless/fixed	O
operators	O
,	O
the	O
interest	O
of	O
IMS	O
is	O
being	O
challenged	O
.	O
</s>
<s>
Examples	O
of	O
global	O
standards	O
based	O
on	O
IMS	O
are	O
MMTel	B-Protocol
which	O
is	O
the	O
basis	O
for	O
Voice	O
over	O
LTE	O
(	O
VoLTE	O
)	O
,	O
Wi-Fi	O
Calling	O
(	O
VoWIFI	B-Protocol
)	O
,	O
Video	O
over	O
LTE	O
(	O
ViLTE	O
)	O
,	O
SMS/MMS	O
over	O
WiFi	O
and	O
LTE	O
,	O
USSD	O
over	O
LTE	O
,	O
and	O
Rich	O
Communication	O
Services	O
(	O
RCS	O
)	O
,	O
which	O
is	O
also	O
known	O
as	O
joyn	O
or	O
Advanced	O
Messaging	O
,	O
and	O
now	O
RCS	O
is	O
operator	O
's	O
implementation	O
.	O
</s>
<s>
IMS	O
was	O
defined	O
by	O
an	O
industry	O
forum	O
called	O
3G.IP	O
,	O
formed	O
in	O
1999	O
.	O
</s>
<s>
3G.IP	O
developed	O
the	O
initial	O
IMS	O
architecture	O
,	O
which	O
was	O
brought	O
to	O
the	O
3rd	O
Generation	O
Partnership	O
Project	O
(	O
3GPP	O
)	O
,	O
as	O
part	O
of	O
their	O
standardization	O
work	O
for	O
3G	B-Architecture
mobile	I-Architecture
phone	O
systems	O
in	O
UMTS	B-Protocol
networks	O
.	O
</s>
<s>
It	O
first	O
appeared	O
in	O
Release	O
5	O
(	O
evolution	B-Architecture
from	I-Architecture
2G	I-Architecture
to	I-Architecture
3G	I-Architecture
networks	I-Architecture
)	O
,	O
when	O
SIP-based	O
multimedia	O
was	O
added	O
.	O
</s>
<s>
Support	O
for	O
the	O
older	O
GSM	O
and	O
GPRS	B-General_Concept
networks	O
was	O
also	O
provided	O
.	O
</s>
<s>
3GPP	O
release	O
6	O
added	O
interworking	O
with	O
WLAN	B-Architecture
,	O
inter-operability	O
between	O
IMS	O
using	O
different	O
IP-connectivity	O
networks	O
,	O
routing	O
group	O
identities	O
,	O
multiple	O
registration	O
and	O
forking	O
,	O
presence	O
,	O
speech	O
recognition	O
and	O
speech-enabled	O
services	O
(	O
Push	O
to	O
talk	O
)	O
.	O
</s>
<s>
3GPP	O
release	O
7	O
added	O
support	O
for	O
fixed	B-Application
networks	O
by	O
working	O
together	O
with	O
TISPAN	O
release	O
R1.1	O
,	O
the	O
function	O
of	O
AGCF	O
(	O
access	O
gateway	B-Protocol
control	O
function	O
)	O
and	O
PES	O
(	O
PSTN	O
emulation	O
service	O
)	O
are	O
introduced	O
to	O
the	O
wire-line	O
network	O
for	O
the	O
sake	O
of	O
inheritance	O
of	O
services	O
which	O
can	O
be	O
provided	O
in	O
PSTN	O
network	O
.	O
</s>
<s>
AGCF	O
acts	O
a	O
SIP	B-Application
User	B-Protocol
agent	I-Protocol
towards	O
the	O
IMS	O
and	O
performs	O
the	O
role	O
of	O
P-CSCF	B-Protocol
.	O
</s>
<s>
SIP	B-Application
User	B-Protocol
Agent	I-Protocol
functionality	O
is	O
included	O
in	O
the	O
AGCF	O
,	O
and	O
not	O
on	O
the	O
customer	O
device	O
but	O
in	O
the	O
network	O
itself	O
.	O
</s>
<s>
Also	O
added	O
voice	O
call	O
continuity	O
between	O
circuit	B-Protocol
switching	I-Protocol
and	O
packet	B-Protocol
switching	I-Protocol
domain	O
(	O
VCC	O
)	O
,	O
fixed	B-Application
broadband	O
connection	O
to	O
the	O
IMS	O
,	O
interworking	O
with	O
non-IMS	O
networks	O
,	O
policy	O
and	O
charging	O
control	O
(	O
PCC	O
)	O
,	O
emergency	O
sessions	O
.	O
</s>
<s>
It	O
also	O
added	O
SMS	B-Device
over	O
IP	O
.	O
</s>
<s>
3GPP	O
release	O
8	O
added	O
support	O
for	O
LTE	O
/	O
SAE	O
,	O
multimedia	O
session	O
continuity	O
,	O
enhanced	O
emergency	O
sessions	O
,	O
SMS	B-Device
over	O
SGs	O
and	O
IMS	O
centralized	O
services	O
.	O
</s>
<s>
3GPP	O
release	O
9	O
added	O
support	O
for	O
IMS	O
emergency	O
calls	O
over	O
GPRS	B-General_Concept
and	O
EPS	O
,	O
enhancements	O
to	O
multimedia	B-Protocol
telephony	I-Protocol
,	O
IMS	O
media	O
plane	O
security	O
,	O
enhancements	O
to	O
services	O
centralization	O
and	O
continuity	O
.	O
</s>
<s>
3GPP	O
release	O
11	O
added	O
USSD	O
simulation	O
service	O
,	O
network-provided	O
location	O
information	O
for	O
IMS	O
,	O
SMS	B-Device
submit	O
and	O
delivery	O
without	O
MSISDN	O
in	O
IMS	O
,	O
and	O
overload	O
control	O
.	O
</s>
<s>
In	O
response	O
,	O
a	O
cut-down	O
version	O
of	O
IMS	O
--	O
enough	O
of	O
IMS	O
to	O
support	O
voice	O
and	O
SMS	B-Device
over	O
the	O
LTE	O
network	O
--	O
was	O
defined	O
and	O
standardized	O
in	O
2010	O
as	O
Voice	O
over	O
LTE	O
(	O
VoLTE	O
)	O
.	O
</s>
<s>
The	O
IP	B-Protocol
multimedia	I-Protocol
core	I-Protocol
network	I-Protocol
subsystem	I-Protocol
is	O
a	O
collection	O
of	O
different	O
functions	O
,	O
linked	O
by	O
standardized	O
interfaces	O
,	O
which	O
grouped	O
form	O
one	O
IMS	O
administrative	O
network	O
.	O
</s>
<s>
Each	O
node	O
can	O
also	O
be	O
present	O
multiple	O
times	O
in	O
a	O
single	O
network	O
,	O
for	O
dimensioning	O
,	O
load	B-Application
balancing	I-Application
or	O
organizational	O
issues	O
.	O
</s>
<s>
IMS	O
terminals	O
(	O
such	O
as	O
mobile	O
phones	O
,	O
personal	B-Application
digital	I-Application
assistants	I-Application
(	O
PDAs	B-Application
)	O
and	O
computers	O
)	O
can	O
register	O
directly	O
on	O
IMS	O
,	O
even	O
when	O
they	O
are	O
roaming	O
in	O
another	O
network	O
or	O
country	O
(	O
the	O
visited	O
network	O
)	O
.	O
</s>
<s>
The	O
only	O
requirement	O
is	O
that	O
they	O
can	O
use	O
IP	O
and	O
run	O
SIP	B-Application
user	B-Protocol
agents	I-Protocol
.	O
</s>
<s>
Fixed	B-Application
access	O
(	O
e.g.	O
,	O
digital	O
subscriber	O
line	O
(	O
DSL	O
)	O
,	O
cable	B-Device
modems	I-Device
,	O
Ethernet	O
,	O
FTTx	B-Architecture
)	O
,	O
mobile	O
access	O
(	O
e.g.	O
</s>
<s>
5G	O
NR	O
,	O
LTE	O
,	O
W-CDMA	O
,	O
CDMA2000	O
,	O
GSM	O
,	O
GPRS	B-General_Concept
)	O
and	O
wireless	O
access	O
(	O
e.g.	O
,	O
WLAN	B-Architecture
,	O
WiMAX	O
)	O
are	O
all	O
supported	O
.	O
</s>
<s>
Other	O
phone	O
systems	O
like	O
plain	O
old	O
telephone	O
service	O
(	O
POTS	O
—	O
the	O
old	O
analogue	O
telephones	O
)	O
,	O
H.323	B-Protocol
and	O
non	O
IMS-compatible	O
systems	O
,	O
are	O
supported	O
through	O
gateways	O
.	O
</s>
<s>
Various	O
identities	O
may	O
be	O
associated	O
with	O
IMS	O
:	O
IP	O
multimedia	O
private	O
identity	O
(	O
IMPI	O
)	O
,	O
IP	O
multimedia	O
public	O
identity	O
(	O
IMPU	O
)	O
,	O
globally	B-Protocol
routable	I-Protocol
user	I-Protocol
agent	I-Protocol
URI	I-Protocol
(	O
GRUU	O
)	O
,	O
wildcarded	O
public	O
user	O
identity	O
.	O
</s>
<s>
Both	O
IMPI	O
and	O
IMPU	O
are	O
not	O
phone	O
numbers	O
or	O
other	O
series	O
of	O
digits	O
,	O
but	O
uniform	B-Protocol
resource	I-Protocol
identifier	I-Protocol
(	O
URIs	B-Protocol
)	O
,	O
that	O
can	O
be	O
digits	O
(	O
a	O
Tel	O
URI	B-Protocol
,	O
such	O
as	O
tel	O
:	O
+	O
1-555-123-4567	O
)	O
or	O
alphanumeric	O
identifiers	O
(	O
a	O
SIP	B-Application
URI	B-Protocol
,	O
such	O
as	O
sip:john.doe	O
@example	O
.com	O
"	O
)	O
.	O
</s>
<s>
Several	O
roles	O
of	O
SIP	B-Application
servers	O
or	O
proxies	O
,	O
collectively	O
called	O
Call	O
Session	O
Control	O
Function	O
(	O
CSCF	O
)	O
,	O
are	O
used	O
to	O
process	O
SIP	B-Application
signaling	O
packets	O
in	O
the	O
IMS	O
.	O
</s>
<s>
A	O
Proxy-CSCF	O
(	O
P-CSCF	B-Protocol
)	O
is	O
a	O
SIP	B-Application
proxy	O
that	O
is	O
the	O
first	O
point	O
of	O
contact	O
for	O
the	O
IMS	O
terminal	O
.	O
</s>
<s>
Some	O
networks	O
may	O
use	O
a	O
Session	B-Protocol
Border	I-Protocol
Controller	I-Protocol
(	O
SBC	O
)	O
for	O
this	O
function	O
.	O
</s>
<s>
The	O
P-CSCF	B-Protocol
is	O
at	O
its	O
core	O
a	O
specialized	O
SBC	O
for	O
the	O
User	O
–	O
network	O
interface	O
which	O
not	O
only	O
protects	O
the	O
network	O
,	O
but	O
also	O
the	O
IMS	O
terminal	O
.	O
</s>
<s>
The	O
use	O
of	O
an	O
additional	O
SBC	O
between	O
the	O
IMS	O
terminal	O
and	O
the	O
P-CSCF	B-Protocol
is	O
unnecessary	O
and	O
infeasible	O
due	O
to	O
the	O
signaling	O
being	O
encrypted	O
on	O
this	O
leg	O
.	O
</s>
<s>
The	O
terminal	O
discovers	O
its	O
P-CSCF	B-Protocol
with	O
either	O
DHCP	B-Protocol
,	O
or	O
it	O
may	O
be	O
configured	O
(	O
e.g.	O
</s>
<s>
during	O
initial	O
provisioning	O
or	O
via	O
a	O
3GPP	O
IMS	O
Management	O
Object	O
(	O
MO	O
)	O
)	O
or	O
in	O
the	O
ISIM	O
or	O
assigned	O
in	O
the	O
PDP	O
Context	O
(	O
in	O
General	O
Packet	O
Radio	O
Service	O
(	O
GPRS	B-General_Concept
)	O
)	O
.	O
</s>
<s>
It	O
provides	O
subscriber	O
authentication	O
and	O
may	O
establish	O
an	O
IPsec	B-Protocol
or	O
TLS	B-Protocol
security	O
association	O
with	O
the	O
IMS	O
terminal	O
.	O
</s>
<s>
It	O
can	O
compress	O
and	O
decompress	O
SIP	B-Application
messages	O
using	O
SigComp	B-Protocol
,	O
which	O
reduces	O
the	O
round-trip	O
over	O
slow	O
radio	O
links	O
.	O
</s>
<s>
An	O
Interrogating-CSCF	O
(	O
I-CSCF	B-Protocol
)	O
is	O
another	O
SIP	B-Application
function	O
located	O
at	O
the	O
edge	O
of	O
an	O
administrative	O
domain	O
.	O
</s>
<s>
Its	O
IP	B-Protocol
address	I-Protocol
is	O
published	O
in	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
of	O
the	O
domain	O
(	O
using	O
NAPTR	B-Protocol
and	O
SRV	B-Protocol
type	O
of	O
DNS	B-Protocol
records	O
)	O
,	O
so	O
that	O
remote	O
servers	O
can	O
find	O
it	O
,	O
and	O
use	O
it	O
as	O
a	O
forwarding	O
point	O
(	O
e.g.	O
,	O
registering	O
)	O
for	O
SIP	B-Application
packets	O
to	O
this	O
domain	O
.	O
</s>
<s>
Up	O
to	O
Release	O
6	O
it	O
can	O
also	O
be	O
used	O
to	O
hide	O
the	O
internal	O
network	O
from	O
the	O
outside	O
world	O
(	O
encrypting	O
parts	O
of	O
the	O
SIP	B-Application
message	O
)	O
,	O
in	O
which	O
case	O
it	O
's	O
called	O
a	O
Topology	O
Hiding	O
Inter-network	O
Gateway	B-Protocol
(	O
THIG	O
)	O
.	O
</s>
<s>
From	O
Release	O
7	O
onwards	O
this	O
"	O
entry	O
point	O
"	O
function	O
is	O
removed	O
from	O
the	O
I-CSCF	B-Protocol
and	O
is	O
now	O
part	O
of	O
the	O
Interconnection	O
Border	O
Control	O
Function	O
(	O
IBCF	O
)	O
.	O
</s>
<s>
The	O
IBCF	O
is	O
used	O
as	O
gateway	B-Protocol
to	O
external	O
networks	O
,	O
and	O
provides	O
NAT	B-Protocol
and	O
firewall	B-Application
functions	O
(	O
pinholing	O
)	O
.	O
</s>
<s>
The	O
IBCF	O
is	O
a	O
session	B-Protocol
border	I-Protocol
controller	I-Protocol
specialized	O
for	O
the	O
network-to-network	B-Protocol
interface	I-Protocol
(	O
NNI	O
)	O
.	O
</s>
<s>
A	O
Serving-CSCF	O
(	O
S-CSCF	B-Protocol
)	O
is	O
the	O
central	O
node	O
of	O
the	O
signaling	O
plane	O
.	O
</s>
<s>
It	O
is	O
a	O
SIP	B-Application
server	O
,	O
but	O
performs	O
session	O
control	O
too	O
.	O
</s>
<s>
It	O
uses	O
Diameter	B-Protocol
Cx	O
and	O
Dx	O
interfaces	O
to	O
the	O
HSS	O
to	O
download	O
user	O
profiles	O
and	O
upload	O
user-to-S-CSCF	O
associations	O
(	O
the	O
user	O
profile	O
is	O
only	O
cached	O
locally	O
for	O
processing	O
reasons	O
and	O
is	O
not	O
changed	O
)	O
.	O
</s>
<s>
there	O
can	O
be	O
multiple	O
S-CSCFs	B-Protocol
in	O
the	O
network	O
for	O
load	B-Application
distribution	I-Application
and	O
high	O
availability	O
reasons	O
.	O
</s>
<s>
It	O
's	O
the	O
HSS	O
that	O
assigns	O
the	O
S-CSCF	B-Protocol
to	O
a	O
user	O
,	O
when	O
it	O
's	O
queried	O
by	O
the	O
I-CSCF	B-Protocol
.	O
</s>
<s>
There	O
are	O
multiple	O
options	O
for	O
this	O
purpose	O
,	O
including	O
a	O
mandatory/optional	O
capabilities	O
to	O
be	O
matched	O
between	O
subscribers	O
and	O
S-CSCFs	B-Protocol
.	O
</s>
<s>
SIP	B-Protocol
Application	I-Protocol
servers	I-Protocol
(	O
AS	O
)	O
host	O
and	O
execute	O
services	O
,	O
and	O
interface	O
with	O
the	O
S-CSCF	B-Protocol
using	O
SIP	B-Application
.	O
</s>
<s>
An	O
example	O
of	O
an	O
application	B-Application
server	I-Application
that	O
is	O
being	O
developed	O
in	O
3GPP	O
is	O
the	O
Voice	O
call	O
continuity	O
Function	O
(	O
VCC	O
Server	O
)	O
.	O
</s>
<s>
Depending	O
on	O
the	O
actual	O
service	O
,	O
the	O
AS	O
can	O
operate	O
in	O
SIP	B-Application
proxy	O
mode	O
,	O
SIP	B-Application
UA	O
(	O
user	B-Protocol
agent	I-Protocol
)	O
mode	O
or	O
SIP	B-Application
B2BUA	B-Protocol
mode	O
.	O
</s>
<s>
If	O
located	O
in	O
the	O
home	O
network	O
,	O
it	O
can	O
query	O
the	O
HSS	O
with	O
the	O
Diameter	B-Protocol
Sh	O
or	O
Si	O
interfaces	O
(	O
for	O
a	O
SIP-AS	O
)	O
.	O
</s>
<s>
OSA	O
service	O
capability	O
server	O
(	O
OSA	O
SCS	O
)	O
:	O
Interfaces	O
SIP	B-Application
to	O
the	O
OSA	O
framework	O
;	O
</s>
<s>
The	O
AS-ILCM	O
(	O
Application	B-Application
Server	I-Application
-	O
Incoming	O
Leg	O
Control	O
Model	O
)	O
and	O
AS-OLCM	O
(	O
Application	B-Application
Server	I-Application
-	O
Outgoing	O
Leg	O
Control	O
Model	O
)	O
store	O
transaction	O
state	O
,	O
and	O
may	O
optionally	O
store	O
session	O
state	O
depending	O
on	O
the	O
specific	O
service	O
being	O
executed	O
.	O
</s>
<s>
The	O
AS-ILCM	O
interfaces	O
to	O
the	O
S-CSCF	B-Protocol
(	O
ILCM	O
)	O
for	O
an	O
incoming	O
leg	O
and	O
the	O
AS-OLCM	O
interfaces	O
to	O
the	O
S-CSCF	B-Protocol
(	O
OLCM	O
)	O
for	O
an	O
outgoing	O
leg	O
.	O
</s>
<s>
Public	O
Service	O
Identities	O
(	O
PSI	O
)	O
are	O
identities	O
that	O
identify	O
services	O
,	O
which	O
are	O
hosted	O
by	O
application	B-Application
servers	I-Application
.	O
</s>
<s>
As	O
user	O
identities	O
,	O
PSI	O
takes	O
the	O
form	O
of	O
either	O
a	O
SIP	B-Application
or	O
Tel	O
URI	B-Protocol
.	O
</s>
<s>
In-Line	O
Mode	O
:	O
AS	O
sends	O
a	O
SIP	B-Application
INVITE	O
to	O
the	O
MRB	O
.	O
</s>
<s>
A	O
Breakout	O
Gateway	B-Protocol
Control	O
Function	O
(	O
BGCF	O
)	O
is	O
a	O
SIP	B-Application
proxy	O
which	O
processes	O
requests	O
for	O
routing	O
from	O
an	O
S-CSCF	B-Protocol
when	O
the	O
S-CSCF	B-Protocol
has	O
determined	O
that	O
the	O
session	O
cannot	O
be	O
routed	O
using	O
DNS	B-Protocol
or	O
ENUM/DNS	O
.	O
</s>
<s>
A	O
PSTN/CS	O
gateway	B-Protocol
interfaces	O
with	O
PSTN	O
circuit	B-Protocol
switched	I-Protocol
(	O
CS	O
)	O
networks	O
.	O
</s>
<s>
For	O
signalling	O
,	O
CS	O
networks	O
use	O
ISDN	B-Protocol
User	I-Protocol
Part	I-Protocol
(	O
ISUP	O
)	O
(	O
or	O
BICC	B-Protocol
)	O
over	O
Message	O
Transfer	O
Part	O
(	O
MTP	O
)	O
,	O
while	O
IMS	O
uses	O
SIP	B-Application
over	O
IP	O
.	O
</s>
<s>
For	O
media	O
,	O
CS	O
networks	O
use	O
Pulse-code	B-Algorithm
modulation	I-Algorithm
(	O
PCM	B-Algorithm
)	O
,	O
while	O
IMS	O
uses	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	B-Protocol
)	O
.	O
</s>
<s>
A	O
signalling	O
gateway	B-Protocol
(	O
SGW	O
)	O
interfaces	O
with	O
the	O
signalling	O
plane	O
of	O
the	O
CS	O
.	O
</s>
<s>
It	O
transforms	O
lower	O
layer	O
protocols	O
as	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
,	O
an	O
IP	O
protocol	O
)	O
into	O
Message	O
Transfer	O
Part	O
(	O
MTP	O
,	O
a	O
Signalling	O
System	O
7	O
(	O
SS7	O
)	O
protocol	O
)	O
,	O
to	O
pass	O
ISDN	B-Protocol
User	I-Protocol
Part	I-Protocol
(	O
ISUP	O
)	O
from	O
the	O
MGCF	B-Protocol
to	O
the	O
CS	O
network	O
.	O
</s>
<s>
The	O
SGW	O
does	O
call	O
control	O
protocol	O
conversion	O
between	O
SIP	B-Application
and	O
ISUP/BICC	O
under	O
the	O
control	O
of	O
the	O
MGCF	B-Protocol
.	O
</s>
<s>
A	O
media	O
gateway	B-Protocol
controller	O
function	O
(	O
MGCF	B-Protocol
)	O
is	O
a	O
SIP	B-Application
endpoint	O
that	O
interfaces	O
with	O
the	O
SGW	O
over	O
SCTP	B-Protocol
.	O
</s>
<s>
It	O
also	O
controls	O
the	O
resources	O
in	O
a	O
Media	O
Gateway	B-Protocol
(	O
MGW	O
)	O
across	O
an	O
H.248	O
interface	O
.	O
</s>
<s>
A	O
media	O
gateway	B-Protocol
(	O
MGW	O
)	O
interfaces	O
with	O
the	O
media	O
plane	O
of	O
the	O
CS	O
network	O
,	O
by	O
converting	O
between	O
RTP	B-Protocol
and	O
PCM	B-Algorithm
.	O
</s>
<s>
It	O
can	O
also	O
transcode	O
when	O
the	O
codecs	B-General_Concept
do	O
n't	O
match	O
(	O
e.g.	O
,	O
IMS	O
might	O
use	O
AMR	B-Algorithm
,	O
PSTN	O
might	O
use	O
G.711	B-Algorithm
)	O
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
next-generation	B-Protocol
networking	I-Protocol
interconnection	O
:	O
</s>
<s>
Connectivity-oriented	O
interconnection	O
(	O
CoIx	O
)	O
:	O
The	O
physical	O
and	O
logical	O
linking	O
of	O
carriers	O
and	O
service	O
providers	O
based	O
on	O
simple	O
IP	B-Protocol
connectivity	I-Protocol
irrespective	O
of	O
the	O
levels	O
of	O
interoperability	O
.	O
</s>
<s>
Different	O
interconnection	O
modes	O
may	O
be	O
used	O
for	O
carrying	O
service	B-Protocol
layer	I-Protocol
signalling	O
and	O
media	O
traffic	O
.	O
</s>
<s>
Offline	O
Charging	O
:	O
All	O
the	O
SIP	B-Application
network	O
entities	O
(	O
P-CSCF	B-Protocol
,	O
I-CSCF	B-Protocol
,	O
S-CSCF	B-Protocol
,	O
BGCF	O
,	O
MRFC	O
,	O
MGCF	B-Protocol
,	O
AS	O
)	O
involved	O
in	O
the	O
session	O
use	O
the	O
Diameter	B-Protocol
Rf	O
interface	O
to	O
send	O
accounting	O
information	O
to	O
a	O
CDF	O
located	O
in	O
the	O
same	O
domain	O
.	O
</s>
<s>
The	O
CDF	O
will	O
collect	O
all	O
this	O
information	O
,	O
and	O
build	O
a	O
call	O
detail	O
record	O
(	O
CDR	O
)	O
,	O
which	O
is	O
sent	O
to	O
the	O
billing	O
system	O
(	O
BS	O
)	O
of	O
the	O
domain.Each	O
session	O
carries	O
an	O
IMS	O
Charging	O
Identifier	O
(	O
ICID	O
)	O
as	O
a	O
unique	O
identifier	O
generated	O
by	O
the	O
first	O
IMS	O
entity	O
involved	O
in	O
a	O
SIP	B-Application
transaction	O
and	O
used	O
for	O
the	O
correlation	O
with	O
CDRs	O
.	O
</s>
<s>
Online	O
charging	O
:	O
The	O
S-CSCF	B-Protocol
talks	O
to	O
a	O
IMS	O
gateway	B-Protocol
function	O
(	O
IMS-GWF	O
)	O
which	O
looks	O
like	O
a	O
regular	O
SIP	B-Protocol
application	I-Protocol
server	I-Protocol
.	O
</s>
<s>
The	O
IMS-GWF	O
can	O
signal	O
the	O
S-CSCF	B-Protocol
to	O
terminate	O
the	O
session	O
when	O
the	O
user	O
runs	O
out	O
of	O
credits	O
during	O
a	O
session	O
.	O
</s>
<s>
The	O
AS	O
and	O
MRFC	O
use	O
the	O
Diameter	B-Protocol
Ro	O
interface	O
towards	O
an	O
OCF	O
.	O
</s>
<s>
IMS-based	O
PES	O
allows	O
non-IMS	O
devices	O
to	O
appear	O
to	O
IMS	O
as	O
normal	O
SIP	B-Application
users	O
.	O
</s>
<s>
Via	O
A-MGW	O
(	O
Access	O
Media	O
Gateway	B-Protocol
)	O
that	O
is	O
linked	O
and	O
controlled	O
by	O
AGCF	O
.	O
</s>
<s>
AGCF	O
interprets	O
the	O
H.248	O
signal	O
and	O
other	O
inputs	O
from	O
the	O
A-MGW	O
to	O
format	O
H.248	O
messages	O
into	O
appropriate	O
SIP	B-Application
messages	O
.	O
</s>
<s>
AGCF	O
presents	O
itself	O
as	O
P-CSCF	B-Protocol
to	O
the	O
S-CSCF	B-Protocol
and	O
passes	O
generated	O
SIP	B-Application
messages	O
to	O
S-CSCF	B-Protocol
or	O
to	O
IP	O
border	O
via	O
IBCF	O
(	O
Interconnection	O
Border	O
Control	O
Function	O
)	O
.	O
</s>
<s>
Service	O
presented	O
to	O
S-CSCF	B-Protocol
in	O
SIP	B-Application
messages	O
trigger	O
PES	O
AS	O
.	O
</s>
<s>
Via	O
VGW	O
(	O
VoIP-Gateway	O
)	O
or	O
SIP	B-Application
Gateway/Adapter	O
on	O
customer	O
premises	O
.	O
</s>
<s>
POTS	O
phones	O
via	O
VOIP	B-Protocol
Gateway	I-Protocol
connect	O
to	O
P-CSCF	B-Protocol
directly	O
.	O
</s>
<s>
Operators	O
mostly	O
use	O
session	B-Protocol
border	I-Protocol
controllers	I-Protocol
between	O
VoIP	B-Protocol
gateways	I-Protocol
and	O
P-CSCFs	B-Protocol
for	O
security	O
and	O
to	O
hide	O
network	O
topology	O
.	O
</s>
<s>
VoIP	B-Protocol
gateway	I-Protocol
link	O
to	O
IMS	O
using	O
SIP	B-Application
over	O
Gm	O
reference	O
point	O
.	O
</s>
<s>
The	O
conversion	O
from	O
POTS	O
service	O
over	O
the	O
z	O
interface	O
to	O
SIP	B-Application
occurs	O
in	O
the	O
customer	O
premises	O
VoIP	B-Protocol
gateway	I-Protocol
.	O
</s>
<s>
POTS	O
signaling	O
is	O
converted	O
to	O
SIP	B-Application
and	O
passed	O
on	O
to	O
P-CSCF	B-Protocol
.	O
</s>
<s>
VGW	O
acts	O
as	O
SIP	B-Application
user	B-Protocol
agent	I-Protocol
and	O
appears	O
to	O
P-CSCF	B-Protocol
as	O
SIP	B-Application
terminal	O
.	O
</s>
<s>
Also	O
used	O
for	O
media	O
control	O
related	O
commands.TCP/SCTP	O
channelsCx(I-CSCF, S-CSCF )	O
,	O
HSSUsed	O
to	O
send	O
subscriber	O
data	O
to	O
the	O
S-CSCF	B-Protocol
;	O
including	O
filter	O
criteria	O
and	O
their	O
priority	O
.	O
</s>
<s>
Also	O
used	O
to	O
furnish	O
CDF	O
and/or	O
OCF	O
addresses.DiameterTS29.229	O
,	O
TS29.212DhAS	O
(	O
SIP	B-Application
AS	O
,	O
OSA	O
,	O
IM-SSF	O
)	O
<->	O
SLFUsed	O
by	O
AS	O
to	O
find	O
the	O
HSS	O
holding	O
the	O
user	O
profile	O
information	O
in	O
a	O
multi-HSS	O
environment	O
.	O
</s>
<s>
DH_SLF_QUERY	O
indicates	O
an	O
IMPU	O
and	O
DX_SLF_RESP	O
return	O
the	O
HSS	O
name.DiameterDx(I-CSCF or S-CSCF )	O
<->	O
SLFUsed	O
by	O
I-CSCF	B-Protocol
or	O
S-CSCF	B-Protocol
to	O
find	O
a	O
correct	O
HSS	O
in	O
a	O
multi-HSS	O
environment	O
.	O
</s>
<s>
DX_SLF_QUERY	O
indicates	O
an	O
IMPU	O
and	O
DX_SLF_RESP	O
return	O
the	O
HSS	O
name.DiameterTS29.229	O
,	O
TS29.212GmUE	O
,	O
P-CSCFUsed	O
to	O
exchange	O
messages	O
between	O
SIP	B-Application
user	O
equipment	O
(	O
UE	O
)	O
or	O
Voip	B-Protocol
gateway	I-Protocol
and	O
P-CSCFSIPGoPDF	O
,	O
GGSNAllows	O
operators	O
to	O
control	O
QoS	O
in	O
a	O
user	O
plane	O
and	O
exchange	O
charging	O
correlation	O
information	O
between	O
IMS	O
and	O
GPRS	B-General_Concept
networkCOPS	O
(	O
Rel5	O
)	O
,	O
diameter	B-Protocol
(	O
Rel6+	O
)	O
GqP-CSCF	O
,	O
PDFUsed	O
to	O
exchange	O
policy	O
decisions-related	O
information	O
between	O
P-CSCF	B-Protocol
and	O
PDFDiameterGxPCEF	O
,	O
PCRFUsed	O
to	O
exchange	O
policy	O
decisions-related	O
information	O
between	O
PCEF	O
and	O
PCRFDiameterTS29.211	O
,	O
TS29.212GyPCEF	O
,	O
OCSUsed	O
for	O
online	O
flow-based	O
bearer	O
charging	O
.	O
</s>
<s>
Functionally	O
equivalent	O
to	O
Ro	O
interfaceDiameterTS23.203	O
,	O
TS32.299ISCS-CSCF	O
<->	O
ASReference	O
point	O
between	O
S-CSCF	B-Protocol
and	O
AS	O
.	O
</s>
<s>
Convey	O
charging	O
function	O
addressesSIPMgMGCF	O
->	O
I	O
,	O
S-CSCFISUP	O
signalling	O
to	O
SIP	B-Application
signalling	O
and	O
forwards	O
SIP	B-Application
signalling	O
to	O
I-CSCFSIPMiS-CSCF	O
->	O
BGCFUsed	O
to	O
exchange	O
messages	O
between	O
S-CSCF	B-Protocol
and	O
BGCFSIPMjBGCF	O
->	O
MGCFUsed	O
for	O
the	O
interworking	O
with	O
the	O
PSTN/CS	O
domain	O
,	O
when	O
the	O
BGCF	O
has	O
determined	O
that	O
a	O
breakout	O
should	O
occur	O
in	O
the	O
same	O
IMS	O
network	O
to	O
send	O
SIP	B-Application
message	O
from	O
BGCF	O
to	O
MGCFSIPMkBGCF	O
->	O
BGCFUsed	O
for	O
the	O
interworking	O
with	O
the	O
PSTN/CS	O
domain	O
,	O
when	O
the	O
BGCF	O
has	O
determined	O
that	O
a	O
breakout	O
should	O
occur	O
in	O
another	O
IMS	O
network	O
to	O
send	O
SIP	B-Application
message	O
from	O
BGCF	O
to	O
the	O
BGCF	O
in	O
the	O
other	O
networkSIPMmI-CSCF	O
,	O
S-CSCF	B-Protocol
,	O
external	O
IP	O
networkUsed	O
for	O
exchanging	O
messages	O
between	O
IMS	O
and	O
external	O
IP	O
networksSIPMnMGCF	O
,	O
IM-MGWAllows	O
control	O
of	O
user-plane	O
resourcesH.248MpMRFC	O
,	O
MRFPAllows	O
an	O
MRFC	O
to	O
control	O
media	O
stream	O
resources	O
provided	O
by	O
an	O
MRFP.H.248MrMr	O
'	O
S-CSCF	B-Protocol
,	O
MRFCAS	O
,	O
MRFCUsed	O
to	O
exchange	O
information	O
between	O
S-CSCF	B-Protocol
and	O
MRFCUsed	O
to	O
exchange	O
session	O
controls	O
between	O
AS	O
and	O
MRFCApplication	O
server	O
sends	O
SIP	B-Application
message	O
to	O
MRFC	O
to	O
play	O
tone	O
and	O
announcement	O
.	O
</s>
<s>
This	O
SIP	B-Application
message	O
contains	O
sufficient	O
information	O
to	O
play	O
tone	O
and	O
announcement	O
or	O
provide	O
information	O
to	O
MRFC	O
,	O
so	O
that	O
it	O
can	O
ask	O
more	O
information	O
from	O
application	B-Application
server	I-Application
through	O
Cr	O
Interface.SIPMwP-CSCF	O
,	O
I-CSCF	B-Protocol
,	O
S-CSCF	B-Protocol
,	O
AGCFUsed	O
to	O
exchange	O
messages	O
between	O
CSCFs	O
.	O
</s>
<s>
Replacement	O
for	O
the	O
Gq	O
reference	O
point.DiameterTS29.214ShAS	O
(	O
SIP	B-Application
AS	O
,	O
OSA	O
SCS	O
)	O
,	O
HSSUsed	O
to	O
exchange	O
User	O
Profile	O
information	O
(	O
e.g.	O
,	O
user-related	O
data	O
,	O
group	O
lists	O
,	O
user-service-related	O
information	O
or	O
user	O
location	O
information	O
or	O
charging	O
function	O
addresses	O
(	O
used	O
when	O
the	O
AS	O
has	O
not	O
received	O
the	O
third-party	O
REGISTER	O
for	O
a	O
user	O
)	O
)	O
between	O
an	O
AS	O
(	O
SIP	B-Application
AS	O
or	O
OSA	O
SCS	O
)	O
and	O
HSS	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
important	O
features	O
of	O
IMS	O
,	O
that	O
of	O
allowing	O
for	O
a	O
SIP	B-Application
application	O
to	O
be	O
dynamically	O
and	O
differentially	O
(	O
based	O
on	O
the	O
user	O
's	O
profile	O
)	O
triggered	O
,	O
is	O
implemented	O
as	O
a	O
filter-and-redirect	O
signalling	O
mechanism	O
in	O
the	O
S-CSCF	B-Protocol
.	O
</s>
<s>
The	O
S-CSCF	B-Protocol
might	O
apply	O
filter	O
criteria	O
to	O
determine	O
the	O
need	O
to	O
forward	O
SIP	B-Application
requests	O
to	O
AS	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
services	O
for	O
the	O
originating	O
party	O
will	O
be	O
applied	O
in	O
the	O
originating	O
network	O
,	O
while	O
the	O
services	O
for	O
the	O
terminating	O
party	O
will	O
be	O
applied	O
in	O
the	O
terminating	O
network	O
,	O
all	O
in	O
the	O
respective	O
S-CSCFs	B-Protocol
.	O
</s>
<s>
An	O
initial	O
filter	O
criteria	O
(	O
iFC	O
)	O
is	O
an	O
XML-based	O
format	O
used	O
for	O
describing	O
control	O
logic	O
.	O
</s>
<s>
They	O
are	O
stored	O
in	O
the	O
HSS	O
as	O
part	O
of	O
the	O
IMS	O
Subscription	O
Profile	O
and	O
are	O
downloaded	O
to	O
the	O
S-CSCF	B-Protocol
upon	O
user	O
registration	O
(	O
for	O
registered	O
users	O
)	O
or	O
on	O
processing	O
demand	O
(	O
for	O
services	O
,	O
acting	O
as	O
unregistered	O
users	O
)	O
.	O
</s>
<s>
Trigger	O
point	O
-	O
logical	O
condition(s )	O
which	O
is	O
verified	O
against	O
initial	O
dialog	O
creating	O
SIP	B-Application
requests	O
or	O
stand-alone	O
SIP	B-Application
requests	O
.	O
</s>
<s>
Application	B-Application
server	I-Application
URI	B-Protocol
-	O
specifies	O
the	O
application	B-Application
server	I-Application
to	O
be	O
forwarded	O
to	O
when	O
the	O
trigger	O
point	O
matches	O
.	O
</s>
<s>
During	O
registration	O
,	O
only	O
the	O
number	O
is	O
sent	O
to	O
the	O
CSCF	O
,	O
not	O
the	O
entire	O
XML	B-Protocol
description	O
.	O
</s>
<s>
The	O
complete	O
XML	B-Protocol
will	O
have	O
previously	O
been	O
stored	O
on	O
the	O
CSCF	O
.	O
</s>
<s>
Non-shared	O
-	O
when	O
provisioning	O
,	O
the	O
entire	O
XML	B-Protocol
description	O
of	O
the	O
iFC	O
is	O
assigned	O
to	O
the	O
subscriber	O
.	O
</s>
<s>
During	O
registration	O
,	O
the	O
entire	O
XML	B-Protocol
description	O
is	O
sent	O
to	O
the	O
CSCF	O
.	O
</s>
<s>
It	O
is	O
envisaged	O
that	O
security	O
defined	O
in	O
TS	O
33.203	O
may	O
not	O
be	O
available	O
for	O
a	O
while	O
especially	O
because	O
of	O
the	O
lack	O
of	O
USIM/ISIM	O
interfaces	O
and	O
prevalence	O
of	O
devices	O
that	O
support	O
IPv4	B-Protocol
.	O
</s>
<s>
This	O
mechanism	O
relies	O
on	O
the	O
authentication	O
performed	O
during	O
the	O
network	O
attachment	O
procedures	O
,	O
which	O
binds	O
between	O
the	O
user	O
's	O
profile	O
and	O
its	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Later	O
on	O
,	O
TISPAN	O
also	O
did	O
a	O
similar	O
effort	O
given	O
their	O
fixed	B-Application
networks	O
scopes	O
,	O
although	O
the	O
procedures	O
are	O
different	O
.	O
</s>
<s>
To	O
compensate	O
for	O
the	O
lack	O
of	O
IPsec	B-Protocol
capabilities	O
,	O
TLS	B-Protocol
has	O
been	O
added	O
as	O
an	O
option	O
for	O
securing	O
the	O
Gm	O
interface	O
.	O
</s>
<s>
Although	O
all	O
3	O
variants	O
of	O
Digest-MD5	O
authentication	O
have	O
the	O
same	O
functionality	O
and	O
are	O
the	O
same	O
from	O
the	O
IMS	O
terminal	O
's	O
perspective	O
,	O
the	O
implementations	O
on	O
the	O
Cx	O
interface	O
between	O
the	O
S-CSCF	B-Protocol
and	O
the	O
HSS	O
are	O
different	O
.	O
</s>
