H.5 Interworking with networks not supporting IMS and/or ICS
23.2923GPPIP Multimedia Subsystem (IMS) centralized servicesRelease 17Stage 2TS
H.5.1 Overview
In this scenario, one network is fully supporting IMS and SeDoC but the interworking network does not support IMS or ICS.
For inbound roamers, the basic enhancement is to provide VLR services towards the HLR in the HPLMN in an interworking function for inbound roamers, which has the role of a centralized VLR.
The architecture is similar to the normal ICS architecture with the difference that the ICS Interworking Function is shared by all SeDoC -MSCs to support inbound roamers without IMS roaming agreement. The inbound roamers will have a temporary IMS subscription for the VPLMN, all services are executed in the VPLMN.
For outbound roaming UEs, it is the opposite way, i.e. the interworking function provides HLR services to the VLR in the VPLMN and in addition provides interworking of the IMS/CS profile and the service settings. The interworking function provides also signalling interworking towards the MSC in the VPLMN.
H.5.2 Inbound-roaming support
H.5.2.1 Roaming Architecture with legacy home network
For the migration phase the SeDoC enabled network operator is required to provide legacy support for inbound roamers. The architecture proposed in this solution is for the SeDoC enabled visited network to emulate a VLR and allocate to the inbound CS roaming subscriber a temporary IMS identity so that they can also be served by the IMS domain.
CAMEL can be supported by collocating the IP Multimedia Service Switching Function (IM SSF) with the AS. The IM SSF retrieves the CAMEL Subscription Information (CSI) from the ICS IWF at the time of the Attach of the UE. The IM SSF would perform the normal CAMEL requests to the gsmSCF in the HPLMN as specified in TS 23.278 [54].
The detailed architecture diagram is depicted in Figure H.5.2.1-1.
Figure H.5.2.1-1: Architecture for inbound roamer support
H.5.2.2 IMS Centralized Services Interworking Function (ICS-IWF) Features
The ICS-IWF is necessary to support roaming scenarios. The interworking function terminates:
the Cx’ interface towards the I-CSCF to provide the role of HSS for procedures related to Serving CSCF assignment.
the Cx’ interface towards the S-CSCF for procedures related to routing information retrieval from ICS-IWF to CSCF and procedures related to authorisation (e.g. checking of roaming agreement) and procedures related to authentication: transfer of security parameters of the subscriber between ICS-IWF and CSCF
the C interface towards HLR for inbound roamer during migration phase to provide the role as VLR towards HLR to retrieve authentication and subscription data during authentication/registration procedure and to retrieve routing information for the terminating case
the Sh’ interface for the role as HSS towards AS in VPLMN for local IMS registration,
The ICS-IWF provides the several roles:
– role as VLR towards HPLMN HLR, i.e. in course of registration the ICS-IWF retrieves the user profile from HPLMN and creates a temporary user profile. The temporary user profile may be updated due to MAP ISD or deleted due to MAP Cancel or IMS de-registration or IMS registration time out.
– role as HSS towards the S-CSCF in VPLMN for local IMS registration.
– role as AS towards HSS in VPLMN for 3rd party IMS registration to enable terminating session routing to the VPLMN.
– maps supplementary services management commands for inbound roamers to the respective CS MAP command to the HLR
– role as HLR towards VLR for outbound roamers in a legacy network.
H.5.2.3 Call Flows for Inbound roamer
H.5.2.3.1 Authentication/Registration for Inbound roamers
The main difference to the procedure for own users is that for inbound roamers the registration flow goes through an ICS IWF that acts as a HSS towards the I-CSCF and S-CSCF in the VPLMN and as a VLR towards the HPLMN HLR.
Figure H.5.2.3.1-1: Authentication/Registration procedure for inbound roamer
1. -7. Same as steps as in Annex G of this specification with the difference that in step 5, the I-CSCF detects the inbound roaming UE based on a comparison of the MNC/MCC in the IMPI/IMPU. The I-CSCF queries the IWF which may look up a database whether the MNC/MCC operator network (HPLMN of the inbound roaming UE) has an IMS roaming agreement or not. If there is no IMS roaming agreement or any other related Service Level Agreements (SLA) in place with this network, then the IWF selects an S-CSCF which is able to handle the CS authentication procedure and provides the S-CSCF address to the I-CSCF. I-CSCF marks the SIP REGISTER with an inbound roaming indication towards the S-CSCF.
NOTE 1: SeDoC is a homogeneous service and all MSC Servers in the PLMN need to support it.
8. The S-CSCF identifies the REGISTER as being from the MSC Server enhanced for SeDoC for an inbound roamer. The S-CSCF requests the authentication info from the ICS IWF which acts as a HSS towards the S-CSCF.
9.-13.The ICS-IWF acting as VLR retrieves the Authentication Info parameters (authentication quintets required for UMTS AKA) from the HLR. The ICS IWF retrieves the service profile via the D interface, i.e. it behaves like a VLR towards the HPLMN HLR by performing an Update Location Procedure and Insert Subscriber Data Procedure. The ICS IWF creates a temporary record (subscription profile). For invoking other AS(s), the ICS IWF generates the corresponding iFC(s).
NOTE 2: SeDoC does not support 2G authentication.
14-26. Same as steps 10.-22 Annex G of this specification.
H.5.2.4 Originating session for Inbound roamer
The Origination session for inbound roamers is very similar to the Origination using I2 reference point as described in clause 7.3.2.1.2 of this specification with the difference that the S-CSCF and AS is located in the VPLMN.
H.5.2.5 Terminating session for Inbound roamer
UEs which have been successfully registered in IMS by the MSC Server have a registration binding at the S‑CSCF containing the MSC Server as the contact address.
The SCC AS shall be inserted in the IMS session path using the terminating iFC. The SCC AS performs T-ADS for selection of an access and returns information to assist with S‑CSCF selection of a registered contact address. When the T-ADS function selects the MSC Server enhanced for SeDoC, the SCC AS directs the IMS terminating session towards the contact address of the MSC Server.
On receipt of the session initiation message, the MSC Server enhanced for ICS shall perform the necessary interworking between the I2 reference point and CS signalling (e.g. as described in TS 24.008 [6]). The MSC Server shall also control a CS-MGW using the Mc reference point to perform the necessary interworking between RTP bearers on the Mb reference point and CS access bearers and adds the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information to the response to the session initiation.
The SCC AS selects to breakout an incoming session to the CS domain in case UE is registered in IMS as being attached to the CS network at an MSC Server enhanced for SeDoC. In this case, terminating iFC forwards the call to the SCC AS.
Figure H.5.2.5-1: Terminating session via CS Access for inbound roamer
1. The UE A originates a call in the CS domain to party-B according to standard origination procedures.
2. The GMSC sends a Send Routing Information (SRI) request to the HLR.
3.-4. The HLR fetches a MSRN from the ICS-IWF acting as VLR by a Provide Roaming Number (PRN) request/response sequence.
5. The HLR forward the MSRN in the SRI Res. message.
6. The GMSC sets up the call towards the MGCF of the VPLMN with the received MSRN.
7. The MGCF initiates an INVITE towards the I‑CSCF in the visited IMS of the UE B.
8.-9. The I-CSCF retrieves the address assigned to the SeDoC user from the ICS-IWF. For the LIR the I-CSCF uses the called party from the INVITE which is the MSRN. The IWF has assigned this MSRN according to the called user (MSISDN) (step 3,4) and has stored the relation MSISDN/MSRN.
10. The I‑CSCF routes the INVITE to the S‑CSCF.
11. The S‑CSCF performs standard service control execution procedures. Filter criteria direct the S‑CSCF to send the INVITE to the SCC AS.
12. . The SCC AS performs terminating access domain selection. The SCC AS chooses the CS access network and the MSC Server contact address, amongst the registered contact addresses for the UE B, for the setup of the media. The SCC AS establishes a new session by sending an INVITE to the UE B via the S‑CSCF.
13. The S‑CSCF forwards the INVITE to the MSC Server based on the contact address stored during registration, using standard IMS procedures.
14. The MSC Server sends a Setup message to the UE B.
H.5.3 Outbound-roaming support
H.5.3.1 Call flows for outbound-roamers
H.5.3.1.1 Attach/Registration Procedure for outbound roamer
Figure H.5.3.1.1-1: Authentication/Registration for outbound roamer in a SeDoC enhanced network
1.-22. The flow description is the same as in annex XXX of this specification.
H.5.3.1.2 Originating session for own users roaming (outbound roamer)
For session origination for own users please refer to clause 7.3.2.1.2 of this specification.
H.5.3.1.3 Terminating session for own users roaming (outbound roamer)
For session termination for own users please refer to clause 7.4.2.1.2 of this specification.
H.5.3.1.4 Call flow for outbound-roamers to a not enhanced network
The ICS-IWF acts as a HLR towards the not enhanced MSC in the VPLMN, the procedures between MSC and ICS-IWF follow the normal VLR – HLR procedures.
NOTE: The interaction between ICS-IWF and HSS is implementation specific.
Figure H.5.3.2-1: Authentication/Registration for outbound roamer in a not enhanced network
1. The UE sends a Location Update Request message to the MSC-Server in the VPLMN and includes its IMSI and the LAI. In case of a combined attach, or CSFB etc., then the Location Update Request may be sent from the MME or the SGSN of the VPLMN.
2. The MSC detects based on the IMSI that the UE does not belong to the own network and is an inbound roamer. If the request is coming from the MME/SGSN, then the CS authentication may be skipped. The MSC contacts the ICS IWF, acting as a HLR of the HPLMN, indicating that CS authentication is required with a Authentication Parameter Request.
3. The ICS IWF retrieves the CS authentication data from the HSS.
4. The ICS IWF (acting as HLR to the MSC-Server) provides the Authentication Info parameters to the MSC-Server. The MSC-Server stores the parameters and triggers the CS authentication procedure.
5. The MSC-Server sends a Authentication Request with the RAND value to the UE.
6. The UE computes the SRES and provides it back to the MSC-Server in the Authentication Response.
7. The MSC-Server sends an Update Location Request with the IMSI and MSRN to the ICS-IWF, acting as the HLR of the HPLMN.
8. The ICS IWF retrieves the subscription profile and service settings from the HSS and maps them into a CS profile with CS settings.
9. The ICS IWF replies with the Insert Subscriber Data message according to normal VLR-HLR procedures.
10. The MSC Server starts ciphering according to normal procedures.
11. The MSC-Server provides a Location Update Accept message to the UE (or MME,SGSN if originated request comes from here) according to the normal procedures.
12. The MSC-Server exchanges the Subscriber Data Insert Acknowledgement and Location Update Acknowledgement according to normal VLR-HLR procedures.
Annex I (informative):
Change history
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Change history |
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Date |
Meeting |
TDoc |
CR |
Rev |
Cat |
Subject/Comment |
New version |
|
2010-03 |
SP-47 |
SP-100161 |
0142 |
1 |
B |
Support for additional media types in network based ICS |
10.0.0 |
|
2010-06 |
SP-48 |
SP-100350 |
0155 |
3 |
B |
Incorporate CAT supplementary service |
10.1.0 |
|
2010-09 |
SP-49 |
SP-100592 |
0162 |
– |
A |
Clarification of Gm use |
10.2.0 |
|
2010-09 |
SP-49 |
SP-100558 |
0156 |
4 |
F |
Mid-call service continuity for ICS UE handover |
10.2.0 |
|
2010-09 |
SP-49 |
SP-100558 |
0157 |
1 |
F |
Collocation of I1 requirements |
10.2.0 |
|
2010-09 |
SP-49 |
SP-100558 |
0161 |
1 |
B |
Addition of annex on diverting incoming CS calls to IMS for anchoring at SCC AS |
10.2.0 |
|
2011-03 |
SP-51 |
SP-110076 |
0166 |
– |
F |
Correcting the Mobile Station behaviour for Communication Diversion |
10.3.0 |
|
2011-03 |
SP-51 |
SP-110076 |
0169 |
1 |
F |
Clarification of the behaviour of the MSC Server enhanced for ICS |
10.3.0 |
|
2011-03 |
SP-51 |
SP-110064 |
0172 |
1 |
A |
Correction to correlation of charging information collected at the MSC for SRVCC in roaming scenarios |
10.3.0 |
|
2011-06 |
SP-52 |
SP-110343 |
0173 |
4 |
B |
Selection by the ICS UE of the origination procedure with CS media |
11.0.0 |
|
2011-12 |
SP-54 |
SP-110738 |
0177 |
1 |
A |
Correction of roaming procedures for ICS |
11.1.0 |
|
2011-12 |
SP-54 |
SP-110748 |
0178 |
1 |
F |
Support for Completion of Communication Services |
11.1.0 |
|
2011-12 |
SP-54 |
SP-110738 |
0180 |
1 |
A |
Communication Waiting correction for ICS |
11.1.0 |
|
2011-12 |
SP-54 |
SP-110738 |
0182 |
– |
A |
CR for Terminated Case of Video Call Domain Selection |
11.1.0 |
|
2011-12 |
SP-54 |
SP-110738 |
0184 |
– |
A |
ICS and usage of Sh for CSRN fetching |
11.1.0 |
|
2012-03 |
SP-55 |
SP-120081 |
0185 |
1 |
B |
Location information provision during ICS origination/termination procedures |
11.2.0 |
|
2012-03 |
SP-55 |
SP-120075 |
0187 |
– |
A |
Clarifications on supported media |
11.2.0 |
|
2012-06 |
SP-56 |
SP-120243 |
0188 |
– |
F |
Removal of CSG ID as a network provided location information parameter |
11.3.0 |
|
2012-06 |
SP-56 |
SP-120251 |
0189 |
1 |
F |
Correction on CFNRc usage for ICS |
11.3.0 |
|
2012-09 |
SP-57 |
SP-120481 |
0190 |
1 |
F |
RAVEL clarifications for MSC enhanced for ICS |
11.4.0 |
|
2012-12 |
SP-58 |
SP-120728 |
0193 |
2 |
F |
Mobility Management Procedures via Gs Interfaces for Non-ICS UE |
12.0.0 |
|
2013-03 |
SP-59 |
SP-130088 |
0196 |
1 |
A |
Correction on the Call Waiting in the ICS |
12.1.0 |
|
2013-06 |
SP-60 |
SP-130223 |
0198 |
3 |
A |
Support of MTRR feature for T-ADS |
12.2.0 |
|
2013-06 |
SP-60 |
SP-130223 |
0200 |
– |
A |
Clarification regarding conference call participants leaving the conference |
12.2.0 |
|
2014-03 |
SP-63 |
SP-140100 |
0203 |
2 |
A |
ICS Indicator to support MSC not enhanced for ICS |
12.3.0 |
|
2014-03 |
SP-63 |
SP-140100 |
0205 |
1 |
A |
IMS registration via I2 upon Combined Attach via SGs |
12.3.0 |
|
2014-06 |
SP-64 |
SP-140249 |
0211 |
1 |
A |
Access Domain Selection when an ongoing IMS voice over PS session |
12.4.0 |
|
2014-06 |
SP-64 |
SP-140274 |
0213 |
2 |
C |
Synchronizing service settings data management between CS and IMS |
13.0.0 |
|
2014-12 |
SP-66 |
SP-140686 |
0220 |
2 |
A |
Subscription for conference event package |
13.1.0 |
|
2015-06 |
SP-68 |
SP-150239 |
0228 |
1 |
F |
Notify UE conference event package after SRVCC |
13.2.0 |
|
2016-09 |
SP-73 |
SP-160643 |
0230 |
– |
F |
Correcting the Figure Label for Communication Completion Originated at Served User call flow |
13.3.0 |
|
2016-09 |
SP-73 |
SP-160642 |
0229 |
5 |
B |
Extend T-ADS to support WLAN access |
14.0.0 |
|
2016-09 |
SP-73 |
SP-160660 |
0232 |
5 |
F |
Clarification on the T-ADS with the failed resources allocation |
14.0.0 |
|
2016-09 |
SP-73 |
SP-160654 |
0235 |
2 |
B |
Allowing ICS MSC to interact with IMS for IMS emergency call handling. |
14.0.0 |
|
2016-09 |
SP-73 |
SP-160654 |
0236 |
2 |
B |
Registration and Authentication procedure utilizing IMS Authorization |
14.0.0 |
|
2016-09 |
SP-73 |
SP-160654 |
0237 |
2 |
B |
Introduction of Service Domain Centralization feature |
14.0.0 |
|
2016-12 |
SP-74 |
SP-160819 |
0239 |
1 |
F |
Correction of ICS IWF behaviour for inbound roamers |
14.1.0 |
|
2016-12 |
SP-74 |
SP-160809 |
0240 |
3 |
A |
ICS MSC with I2 and T-ADS handling interaction |
14.1.0 |
|
2016-12 |
SP-74 |
SP-160826 |
0243 |
1 |
B |
SCC AS/T-ADS Support for 3GPP PS Data off for SIP-Based Service |
14.1.0 |
|
2017-03 |
SP-75 |
SP-170048 |
0245 |
3 |
F |
Inbound roamer detection at I/S-CSCF |
14.2.0 |
|
2018-06 |
SP-80 |
SP-180475 |
0246 |
1 |
F |
Support of 5GS in T-ADS |
15.0.0 |
|
2020-07 |
SP-88E |
– |
– |
– |
– |
Update to Rel-16 version (MCC) |
16.0.0 |
|
2022-03 |
SP-95E |
– |
– |
– |
– |
Update to Rel-17 version (MCC) |
17.0.0 |