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IMS IP Multimedia
Subsystem
Rev A
Limited Internal
1
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IMS is…



IP Multimedia Subsystem (IMS) is an architecture for offering
multimedia services on the packet domain.
IMS is access agnostic, hence it is not just for UMTS or GPRS, but
also supports WLAN, fixed line, LTE etc.
IMS is not a service, but a service enabler
 IMS gives Service control by introducing service signalling

All IMS services can be done without IMS but without IMS,
service creation can be complex, and expensive to the operator
and the user
 IMS gives Convergence of all media communications on to the
packet network.
Rev A
Limited Internal
2
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Service interoperability non-IMS
Service 2
Service 2
Service 1
Service 1
Operator 3
Operator 1
Service 2
Service 1
Operator 2
Rev A
Limited Internal
3
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Service interoperability IMS
Service 2
Service 2
Service 1
Service 1
Operator 3
Operator 1
Service 2
Service 1
Operator 2
Rev A
Limited Internal
4
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IMS architecture
SIP Application
Servers
HSS
SIP Application
Servers
IMS
I-CSCF
MRF
P-CSCF
MGCF
CDMA 2000
S-CSCF
MGW
MSC(Server)
RNC
Corporate
SGSN
GGSN
BSC
CN
UMTS/GPRS
MGW
WLAN
Rev A
Limited Internal
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Architecture Elements
Database Elements
• HSS (Home Subscriber Server)
• SLF (Subscription Locator
Function)
IMS Control Elements (Call
Session Control Function)
•
•
•
S-CSCF
P-CSCF
I-CSCF
•
•
Rev A
MGCF - Media Gateway Control
Function
BGCF - Breakout Gateway Control
Function
SGW - Signaling Gateway
Limited Internal
7
SIP Application
Servers
OSA Application
Server
OSA-SCS
IM-SSF
HSS
P-CSCF
Control Plane Interworking
Elements
•
CSE(SCP)
S-CSCF
I-CSCF
MRF
MGCF
BGFC
SGW
MGW
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Other Elements
IMS Service Elements
• AS (Application Server)
CSE(SCP)
SIP Application
Servers
OSA Application
Server
OSA-SCS
IM-SSF
External Service and Service
Interworking Elements
•
•
•
OSA SCS
OSA Framework
OSA Application Server
•
CAMEL elements - IM-SSF (IP Multimedia
HSS
P-CSCF
S-CSCF
I-CSCF
Switching Service Function)
Resource Elements
•
MRF
Media Resources Function (MRF)
MGCF
BGCF
Media Interworking Elements
• MGW (Media Gateway)
Rev A
Limited Internal
SGW
MGW
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Proxy-CSCF (P-CSCF)
Home IMS
Network
CSE(SCP)
• Entry point to IMS from any access
network
• Local outbound stateful proxy for all SIP
requests/responses, ensuring all signalling
is sent via the home network
• Includes a Policy Decision Function (PDF)
that authorizes bearer resources
• The P-CSCF and the GGSN are located in
the same network, that is, either both in the
visited PLMN or both in the home PLMN.
Rev A
Limited Internal
9
IM-SSF
HSS
OSA Application
Server
OSA-SCS
P-CSCF
I-CSCF
S-CSCF
S-CSCF
• Performs compression/decompression for
efficient use of the radio interface
• Performs integrity protection mechanisms
using IPsec
SIP Application
Servers
MRF
MGCF
SGW
MGW
GGSN
Home Access
Network
Visited IMS
Network
P-CSCF
PSTN
PLMN
Internet
IP networks
ETH/RKC Sarolta Dibuz +36-1-4377625
GGSN Visited Access
Network
PDF
• Performs policy control and media
authorisation based on the policy of the
access network and based on the
session parameters carried in the SIP
signalling
Home IMS
Network
CSE(SCP)
SIP Application
Servers
OSA Application
Server
IM-SSF
HSS
OSA-SCS
P-CSCF
S-CSCF
S-CSCF
I-CSCF
MRF
MGCF
SGW
MGW
GGSN
Home Access
Network
P-CSCF
PDF
Rev A
Limited Internal
10
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Interrogating-CSCF (I-CSCF)
• First point of contact in the home network
from a visited NW
• Acts as a SIP proxy located at the edge of a
network.
• Queries the HSS to help in selecting a SCSCF for a user.
Home IMS
Network
CSE(SCP)
SIP Application
Servers
IM-SSF
HSS
OSA Application
Server
OSA-SCS
P-CSCF
I-CSCF
S-CSCF
S-CSCF
MRF
MGCF
SGW
MGW
GGSN
Home Access
Network
Visited IMS
Network
P-CSCF
PSTN
PLMN
Internet
IP networks
Rev A
Limited Internal
11
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GGSN Visited Access
Network
Serving-CSCF (S-CSCF)
• Stateful SIP server providing session
control.
• Acts as a SIP registrar
Home IMS
Network
CSE(SCP)
SIP Application
Servers
IM-SSF
HSS
OSA Application
Server
OSA-SCS
• Always located in the home network.
• Central point for control of operator
provided services.
P-CSCF
I-CSCF
S-CSCF
MRF
• Performs authentication.
MGCF
SGW
MGW
GGSN
Home Access
Network
Visited IMS
Network
P-CSCF
PSTN
PLMN
Internet
IP networks
Rev A
Limited Internal
12
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GGSN Visited Access
Network
HSS and SLF
Home IMS
Network
Home Subscriber Server (HSS)
CSE(SCP)
• An evolution of the HLR
• HLR (subscriber and authentication data for
CS and PS domains) + the new IMS
functionality
• Location Register
• IMS subscriber records
• IMS service profile processing
• IMS authentication data
SIP Application
Servers
IM-SSF
HSS
OSA Application
Server
OSA-SCS
P-CSCF
I-CSCF
S-CSCF
MRF
MGCF
SGW
MGW
GGSN
Home Access
Network
Visited IMS
Network
P-CSCF
PSTN
PLMN
Subscription Locator Function (SLF)
• A look-up function that can be used in networks
where several HSS exist.
Rev A
Limited Internal
13
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Internet
IP networks
GGSN Visited Access
Network
Breakout Gateway Control
Function (BGCF)
• Selects the appropriate PSTN/CS domain
break out point for the session.
• Selects the network in which the
interworking with the PSTN/CS Domain is to
occur. If the interworking is in another
network, then the BGCF will forward the SIP
signalling to the BGCF of that network. If
the interworking is in another network and
network hiding is required by the operator,
the BGCF will forward the SIP signalling via
an I-CSCF toward the BGCF of the other
network.
Home Network
S-CSCF
BGCF
MGCF
Limited Internal
14
BGCF
MGCF
GGSN
MGW
Home
GSM/UMTS
CS Network
• Selects the MGCF in the network in which
the interworking with PSTN/CS domain is to
occur and forwards the SIP signalling to that
MGCF.
Rev A
External Network
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MGW
External
Network
Multimedia Resource
Function (MRF)
Home IMS
Network
CSE(SCP)
The media resource function provides the additional
media resources complementary or fundamental for
services. The functions to be applied will be:
IM-SSF
HSS
• Conference management such as, a floor control
mechanism to allow some members of a conference to
control the conference.
SIP Application
Servers
OSA-SCS
P-CSCF
S-CSCF
• To provide additional media streams to the session,
such as media fade-in for rich calls.
I-CSCF
MRF
• Play announcements and tones
MGCF
MGW
GGSN
The MRF is split in to two functional parts :
Home Access
Network
• Multimedia Resource Function Controller (MRFC)
MRF
• Multimedia Resource Function Processor (MRFP)
MRFC
Mp
MRFP
Rev A
Limited Internal
15
OSA Application
Server
ETH/RKC Sarolta Dibuz +36-1-4377625
SGW
IMS service environment
Home IMS
Network
There are three different types of application
servers, depending on the application hosted:
CSE(SCP)
IM-SSF
HSS
• SIP AS (Application Server): IMS native
applications
• IM-SSF (IP Multimedia Switching Service
Function) Legacy applications
• OSA-SCS (Service Capability Server): Service
frameworks (e.g. OSA)
SIP Application
Servers
OSA Application
Server
OSA-SCS
S-CSCF
P-CSCF
I-CSCF
MRF
MGCF
SGW
MGW
GGSN
Home Access
Network
Visited IMS
Network
P-CSCF
PSTN
PLMN
Internet
IP networks
Rev A
Limited Internal
16
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GGSN Visited Access
Network
SIP Application Server
Home IMS
Network
• Accept requests and responses. It will be able
to control, finish or initiate a new SIP
transaction
• Route the session towards another user or
network
• Interact with other service platforms for the
support of services
• Communicate with the HSS in order to obtain
information about subscriptions and services
CSE(SCP)
IM-SSF
HSS
S-CSCF
17
OSA-SCS
I-CSCF
MRF
MGCF
MGW
Home Access
Network
Limited Internal
OSA Application
Server
P-CSCF
GGSN
Rev A
SIP Application
Servers
ETH/RKC Sarolta Dibuz +36-1-4377625
SGW
Registration to the network
UE
GSN
HLR
1. Bearer Level Registration: GPRS
2. PDP Context Activation
3. CSCF Discovery
4. Application Level Registration
Rev A
Limited Internal
18
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IMS
Session establishment model
Steps:
11
Home A
Home B
AS
HSS
8
10
7
9
I-CSCF
16
12
4
3
S-CSCF
I-CSCF
17
18
15
19
Visited B
P-CSCF
13
5
6
S-CSCF
1) Route
signalling to A’s
home network
AS
Visited A
14
P-CSCF
20
GGSN
SGSN
GGSN
SGSN
Radio Access Network
Radio Access Network
User plane (RTP)
2
1
2) Route
signalling to B’s
home network
3) Route
signalling to B
terminal
Application level Registration (I)
Home A
I-CSCF
S-CSCF
Subscriber
P-CSCF
SIP-AS
HSS
A UES-CSCF
The
I-CSCF
P-CSCF
initiates
finds
finds
a SIP
the
the
contacts
S-CSCF
I-CSCF
downloads
Registration
allocated
the
including
help
toSthe
challenges
HSSwith
to find
the
the
this
of
authentication
his
DNS
public
user
or
user
selects
vector
ID and
a
CSCF
subscriber
serving
this
new
for
his
this
private
S-CSCF
subscriber
user
for ID
this
subscriber.
user. the HSS
from
7
3
4
6
5. REGISTER
I-CSCF
S-CSCF
8. Unauthorized
2. REGISTER
9. Unauthorized
Visited A
P-CSCF
1. REGISTER
10. Unauthorized
GGSN
SGSN
Radio Access Network
A
Rev A
Limited Internal
20
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Application level Registration (II)
Home A
P-CSCF
S-CSCF
I-CSCF
Subscriber
SIP-AS
HSS
17
14
13
18. REGISTER
19. OK
15. REGISTER
I-CSCF
S-CSCF
20. OK
16
The UE
S-CSCF
I-CSCF
P-CSCF
calculates
finds
informs
finds
the
the
the
contacts
the
S-CSCF
I-CSCF
credentials
HSSwith
other
allocated
that
Application
and
the
this
help
StoSauthorizes
to
the
find
the
CSCF
Servers
this
of
reDNS
initiates
user
is
about
serving
the
the
SIP
the
registration
serving
and
this
subscriber.
user's
Registration
HSS
informsregistration
the The
subscriber
sends the S-CSCF the
service profile
12. REGISTER 21. OK
Visited A
P-CSCF
11. REGISTER
22. OK
GGSN
SGSN
Radio Access Network
A
Rev A
Limited Internal
21
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Session Establishment
Home A
Home B
SIP-AS
I-CSCF
6
7. INVITE
4. INVITE
S-CSCF
14. OK
2. INVITE
SIP-AS
HSS
5
3. INVITE
HSS
P-CSCF
I-CSCF
S-CSCF
Subscriber
I-CSCF
13. OK
S-CSCF
12. OK
15. OK
11. OK
Visited A
8. INVITE
Visited B
P-CSCF
1. INVITE
P-CSCF
16. OK
10. OK
GGSN
SGSN
GGSN
SGSN
Radio Access Network
Radio Access Network
9. INVITE
A Calls B
A
Rev A
Subscriber
The
I-CSCF
P-CSCF
S-CSCF
A
routes
isinitiates
in
the
Home
first
the
HSS returns
fetches
the
message
point
B
A
a
SIP
performs
invitation
of contact
tothe
service
the
in the
from
address
the
of
HSS
the
S-CSCF
originating
IMS
invocation
including
for the
for
S-CSCF
subscriber.
SIP
URI
address
serving
subscriber
ofthe
the
S-CSCF
B
It
subscriber
of
forwards
subscriber
A.
B.
theB.
The
ItINVITE
Sthat
is serving
request
forwards
CSCF
finds
tothe
the
INVITE
I-CSCF
entry
subscriber
Bthe
to thein
point
P-CSCF
Home B
and UE
B
Limited Internal
22
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IMS-CS interworking
MG
MGCF
S-CSCF.
S-CSCF/DNS
Subscriber
BGCF
Home A
Subscriber
The
MG performs
S-CSCF
BGCF
home
MGCF
BGCF
selects
performs
Aqueries
detects:
initiates
the
bearer
a
No
selects
the
SIP
SIP
DNS
MGCF
signalling
invitation
interworking
URL
another
forinassigned
the
thetel
between
including
to
URL.
breakout
BGCF,
interworking
theDNS
E.164
perhaps
RTP/IP
network
the
returns
number.
between
E.164
in
and
a
the bearer
number
The
negative
another
SIP
and
call the
network
of
is
match.
used
routedinto
the
subscriber
a
BICC/ISUP
home
PSTN
BGCF.
network
in
B. PSTN
IMS Gateway
BGCF
DNS
4
5
7. INVITE
6. INVITE
15. OK
14. OK
3. INVITE
I-CSCF
S-CSCF
BGCF
16. OK
2. INVITE
11. IAM
8. INVITE
MGCF
12. ANM
13. OK
17. OK
9. ADD
10. RESP
Visited A
P-CSCF
1. INVITE
MG
18. OK
GGSN
SGSN
Radio Access Network
A Calls B (CS Subscriber)
A
Rev A
Limited Internal
23
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Interfaces
of IMS
SIP Application
Servers
CSE(SCP)
Home IMS
Network
OSA Application
Server
ISC
Sh
OSA-SCS
IM-SSF
Sh
ISC
ISC
Cx
HSS
Cx
Mi
Mw
Mw
Mr
P-CSCF
BGCF
S-CSCF
I-CSCF
Mg
Gi
MRF
Mw
MGW
Gi
T-SGW
MGCF
ISUP/BICC
GGSN
Home Access
Network
Visited IMS
Network
P-CSCF
PSTN
PLMN
Internet
IP networks
Rev A
Limited Internal
24
GGSN Visited Access
Network
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SIP
Diameter
IP
TDM
SS7
End-to-End QoS and Policy
Enforcement Architecture
Control
Plane
P-CSCF
SIP UA
Terminal
Bearer
Plane
Rev A
The terminal also
provides the link
between the two
planes.
Bearer
Limited Internal
S-CSCF
PDF
The Go interface provides the
link between the control plane
and the bearer plane in the
network.
GGSN
25
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Policy Control
(4) Generate QoS
parameters
IMS
Control
Plane
(3) Forward token to
terminal
(1) Generate QoS
parameters
(2) Generate
Authorisation token
(0) P-CSCF receives
‘final ‘SDP
P-CSCF
SIP UA
S-CSCF
PDF
NOTE: When the service is
renegotiated, the approval
process as described above is
repeated in its entirety with new
resource allocations.
(8) Authorise
resources.
(5) Request PDP
activation with token
Terminal
UMTS
Control
Plane
(9) Authorise
PDP activation
Session
Manager
GGSN
(10) Commence
policing of traffic
(6) Determine PDF.
(7) Req authorisation
from PDF.
Rev A
Limited Internal
26
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