2. Introduction to IMS
What is IMS?
• IMS - IP Multimedia Subsystems
• It is a network architecture established to provide seamless mobility
and service delivery and ensure that subscriber services can be
delivered to them irrespective of their terminal, access and core
n/w connectivity within a home or visited network.
• Also provides service delivery for fixed network.
• In IMS, the signaling sessions are independent of underlying bearer
network which is made possible by providing consistent interface
between terminal and application server (TAS) and also various
functional nodes and the application server.
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4. Introduction to IMS
• IMS is facilitated through the introduction of a service layer that
extends across all functions involved in the delivery of services.
• The service layer encompasses:
AS – Provides service and session control aka. TAS (Telephony Application Server)
CSCF (Call Session Control Function) – Provides SIP Proxy Function
CSCF has three states
S-CSCF (Serving Call Session Control Function)
I-CSCF (Interrogating Call Session Control Function)
P-CSCF(Proxy Call Session Control Function)
HSS (Home Subscriber Server) – Provides subscription, security and mobility
information.
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5. Introduction to IMS
What does the IMS provide that the internet can’t offer the
subscriber, particularly when service is free on the internet
Answer: Provision for QoS is an important factor. Internet is a
collection of IP networks across which end-to-end QoS is very difficult
to provide and maintain.
The richness of service is difficult to offer with the internet. IMS
networks and services are based on IP networks which are managed
and reliable.
Example: An IMS subscriber engaged in Voice or Video call can
seamlessly switch between terminals and access networks whilst still
on call and the service is also providing presence, redirect, voicemail
etc.
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6. Introduction to IMS
IMS Services
• Voice and Video calling including multiparty and multiple session
support.
• Enhanced redirect services and call prioritization
• Ability to split incoming media eg. – Incoming video call has video
split to the TV and audio split to the phone.
• Multiplayer gaming services
• IPTV
• Presence information for all services
• Location based services
• Voice/Video mail
• Prepaid services
• Hi-Definition audio and video services
• Instant messaging
• Push to talk
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7. Introduction to IMS
Some Acronyms
• IMPI - IP Multimedia Private Identity (sip:IMSI@host)
• IMPU - IP Multimedia Public Identity (sip:1MSISDN@host)
• MIME - Multipurpose Internet Mail Extensions
• UAR – User Authorization Request
• UAA – User Authorization Answer
• SAR – Server Assignment Request
• SAA – Server Assignment Answer
• LIR – Location Information Request
• LIA – Location Information Answer
• MAR – Media Authorization Request
• MAA – Media Authorization Answer
• UDR – User Data Request
• UDA – User Data Answer
• PUR – Profile Update Request
• PUA – Profile Update Answer
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Diameter
SIP
8. VoLTE - Registration
(1) REGISTER (IMPU, IMPI)
(2) REGISTER (STN-SR Added)
(3) UAR (IMPU, IMPI)
(4) UAA – SERVER CAPABILITIES
(S-CSCF IP ADDRESS)
(5) REGISTER
(IMPU, IMPI) (6) MAR (IMPU, IMPI)
(7) MAA (IMPU, IMPI, auth vectors)(8) 401
Unauthorized(9) 401
Unauthorized)(10) 401 Unauthorized)
(11) REGISTER (IMPU, IMPI, auth resp) (12)
(13) UAR (IMPU, IMPI)
(14) UAA – S-CSCF NAME (FQDN)
(15) REGISTER (IMPU, IMPI, auth resp)
(16) SAR (IMPU, IMPI, S-CSCF
Name)
(17) SAA (IMPU, User Profile)
VoLTE UE SAEGWeNodeB MME
P-CSCF
/ATCF
I-CSCF S-CSCF
TAS-
SCCAS
HSS
9. VoLTE - Registration
VoLTE UE
(18) 200 OK
(19) 200 OK
(20) 200 OK
(21) REGISTER (IMPU, ServiceInfo)
(22) 200 OK
(23) SUBSCRIBE (Event: Reg)
(24) SUBSCRIBE (Event: Reg)
(25) NOTIFY (Event: Reg, State: Active)
(26) NOTIFY (Event: Reg,
State: Active) (27) SIP INFO MESSAGE WITH ATCF MGMT URI (C-MSISDN)
(28) 200 OK
SAEGWeNodeB MME
P-CSCF
/ATCF
I-CSCF S-CSCF
TAS-
SCCAS
HSS
10. VoLTE – Registration with Sh Interface Enabled
Authentication
REGISTER
SIP: IMPU Authorization: Username: IMPI
REGISTER STN-SR, ATCF-mgmt-URI
UAR
UAA
REGISTER
SAR
SAA
200 OK
200 OK
200 OK
MIME body
(orig. Register including contact)
VoLTE UE SAEGWeNodeB MME
P-CSCF
/ATCF
I-CSCF S-CSCF
TAS-
SCCAS
HSS
11. VoLTE & SRVCC - Registration
Sh UDR (IMPU, IRS)
Sh UDA (IMPU List)
Sh UDR (IMPU , MSISDN)
Sh UDA (C-MSISDN)
Sh PUR (STN-SR)
Sh PUA (IMPU list)
Insert Subscriber Data – Request (STN-SR)
Insert Subscriber Data – Answer
200 OK
MESSAGE (ATU-STI, C-MSISDN)
MESSAGE (ATU-STI, C-MSISDN)
MESSAGE (ATU-STI, C-MSISDN)
200 OK
200 OK
200 OK
VoLTE UE SAEGWeNodeB MME
P-CSCF
/ATCF
I-CSCF S-CSCF
TAS-
SCCAS
HSS
15. SRVCC From LTE to 3GPP UTRAN/GERAN
• SRVCC – Single Radio Voice Call Continuity
• SRVCC 3gpp based architecture is designed to provide seamless voice call
continuity when a VoLTE device with active VoIP call, transitions from LTE to
UTRAN or GSM only coverage.
• The SRVCC nodes on the LTE side are eNodeB, MME.
• The SRVCC nodes on the target CS domain is MSC.
• The SRVCC nodes in the IMS domain are ATCF and SCC AS (TAS)
• The SRVCC nodes on the target CS network and IMS network are enhanced with
additional capabilities to support smooth transition of the UE from LTE to 3GPP
UTRAN/GERAN based networks (e.g. 2G/3G).
16. SRVCC From LTE to 3GPP UTRAN/GERAN
• The UE sends measurement reports to E-UTRAN.
• Based on UE measurement reports, the source E UTRAN decides to trigger an
SRVCC handover to the CS Domain.
• Source E UTRAN sends Handover Required message having Target ID, generic
Source to Target Transparent Container, SRVCC HO indication tothe source
MME. The E UTRAN places the “old BSS to new BSS information IE” for the CS
domain in the generic Source to Target Transparent Container. The SRVCC HO
indication indicates to the MME that the target is only CS capable hence this is
a SRVCC and over operation only toward the CS domain.
• Based on the QCI associated with the voice bearer (QCI 1) andthe SRVCC HO
indication, the source MME splits the voice bearer from the non-voice bearers
and initiates the PS-CS handover procedure for the voice bearer only toward
the MSC Server.
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17. SRVCC From LTE to 3GPP UTRAN/GERAN
17
EPS Reference Architecture for SRVCC to UTRAN as Destination Network
18. SRVCC From LTE to 3GPP UTRAN/GERAN
Some Terminologies
• SCC AS- Session Centralization and Continuity Application Server
• ATCF – Access Transfer Control Function
• STN-SR – Session Transfer Number for SRVCC
• C-MSISDN- Correlation Mobile Subscriber Integrated Services Digital Network-
Number.
• PS – Packet Switched
• CS – Circuit Switched
• PAI – P-Asserted Identity
19. SRVCC From LTE to 3GPP UTRAN/GERAN
Role of STN-SR
• The STN-SR is configured for an SRVCC subscriber at the time of service provisioning (TS 23.237).
• The HSS provides the STN-SR to the MME that is provisioned for a UE during its attach
procedure.
• If the user has an active context at the MME, the HSS also updates the MME if the STN-SR has
been modified for that UE.
• The MME sends the STN-SR in the SRVCC PS to CS request to the MSC over the Sv interface.
• The MSC initiates a request for an access transfer of the active session from the IMS network
and includes the STN-SR in the request.
• Refer to section 6.2.2.1 of TS 23.216
• The SCC AS eventually receives an INVITE with the STN-SR or C-MSISDN which indicates the
need for an access transfer of an active session.
• The SCC AS is the entity in the IMS domain that handles management of the access transfer of
an active session.
• The SCC AS coordinates the proper media and signaling exchange to the access network of
where the UE is present. The STN-SR is the number that is used to facilitate that
• Refer to section 6.3.2.1.4 of TS 23.237
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20. Use of C-MSISDN in SRVCC
• C-MSISDN is one of the variables that is used to identify an anchored voice session in IMS
• Below is a list of events that take place to clarify the use of the C-MSISDN during a PS-to-CS
handover for a VoLTE session:
1) The SCC AS is the anchor of the call in the IMS domain.
2) Per TS 23.216, during the Attach process, the HSS sends the C-MSISDN to the MME as part of
the UE’s subscription data.
3) Per TS 24.237, the SCC AS obtains registration state information from the HSS which includes the
C-MSISDN that is bound to the IMS private user id per TS 23.237.
4) Per TS 23.237, the SCC AS provides the C-MSISDN to the ATCF upon successful IMS registration.
5) As the UE moves out of LTE coverage, the MME requests the MSC to transfer the call to legacy
technologies such as 3G.
6) Per TS 23.216, the MME sends the C-MSISDN to the MSC as part of the SRVCC PS to CS handover.
7) Per TS 23.856, “The MSC Server correlates the incoming PS to CS Handover request with the anchored
session using the C-MSISDN obtained when anchoring the session”.
8) MSC sends an INVITE to the IMS side which includes the C-MSISDN (as a P-Asserted-Identity)
representing the calling party number.
9) Upon receiving the INVITE, the SCC AS determines that an access transfer is being requested. The SCC AS
uses the STN-SR or the C-MSISDN to identify the anchored session.
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SRVCC From LTE to 3GPP UTRAN/GERAN
21. eNodeB MSC P-GW S-CSCF I-CSCF HSS S-CSCF UE-T
MME S-GW
ATCF /
P-CSCF
TAS-O PCRF TAS-T
ATCF /
P-CSCF
UE - O
SRVCC From LTE to 3GPP UTRAN/GERAN
Measurement
Report
HO Req.
SRVCC PS to CS Req.
Ongoing VoIP Session with IMS Established over LTE
MSC
initiates
Bearer
establishme
nt in the CS
domain
(Prep HO
Req. to
Target MSC
SRVCC PS to CS Resp.
22. eNodeB MSC P-GW S-CSCF I-CSCF HSS S-CSCF UE-T
MME S-GW
ATCF /
P-CSCF
TAS-O PCRF TAS-T
ATCF /
P-CSCF
UE - O
SRVCC From LTE to 3GPP UTRAN/GERAN
HO Command
HO from EUTRAN
Command
INVITE tel:+STN-SR
INVITE C-MSISDN in PAI
Session Transfer Update Remote End. Release Resource IMS Access Leg
SRVCC PS to CS Complete
Notification
SRVCC PS to CS Complete
Ack.
24. eNodeB MSC P-GW S-CSCF I-CSCF HSS S-CSCF UE-T
MME S-GW
ATCF /
P-CSCF
TAS-O PCRF TAS-T
ATCF /
P-CSCF
UE - O
SRVCC From LTE to 3GPP UTRAN/GERAN
25. CS to IMS/VoLTE Call Flow
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UE-B ATCF S-CSCF TAS I-CSCF HLR/STP USD HSS
MGCF/
GMSC
SRI [msisdn,
GMSCE164] ADDR
SRI [msisdn,
GMSCE164] ADDR
LDAP: Search Request (IMPU/msisdn,imsi)
LDAP: SearchResEntry, SearchResDone
If subscriber profile is not present,
but the entry is present in the TAS D
SRI Ack [IMRN=<IMRN prefix>MSISDN-B]
INVITE tel:IMRN,c1, SDP1
INVITE sip:MSISDN-B, c2, SDP2
183 Session in Progress, SDP2
PRACK, 200 OK not shown, If preconditions are used, a subsequent UPDATE (SDP offer) and 200 OK (SDP
Answer) will occur end to end here
180 Ringing
200
OK
ACK
