7.4.2 Terminating sessions that use CS media

23.2923GPPIP Multimedia Subsystem (IMS) centralized servicesRelease 17Stage 2TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

7.4.2.1 Non ICS UE terminating sessions that use CS media

7.4.2.1.1 Overview

All ICS User incoming sessions are directed to IMS for delivery to the ICS User.

Non ICS UEs which have been successfully registered in IMS by the MSC Server will 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 ICS, 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 may select to breakout an incoming session to the CS domain in following cases:

– A non ICS UE is not registered in IMS but can be attached to the CS network at an MSC Server. In this case, terminating iFC forwards the call to the SCC AS.

– A non ICS UE is registered in IMS, the access network does not support the media type (as defined in clause 22.4 of TS 22.101 [9]), but the UE still can be attached to the CS network at an MSC Server. In this case, terminating iFC forwards the call to the SCC AS.

For both scenarios the SCC AS shall retrieve from the HSS the C-MSISDN(s) associated with all the IMPIs that are associated with the IMPU that is the target of the session. The SCC AS shall select a single C‑MSISDN and shall use it to fetch a CSRN for routing the call to the CS domain. The INVITE shall be sent to the S‑CSCF which then performs CS breakout according to standard IMS procedures. The INVITE may include an indication that this is an ICS session, which can be used by the MGCF to handle this session in a specific manner, for example to make a different cause mapping between ISUP/BICC and SIP than a normal session breakout would do. The SCC AS may also retrieve the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information from the HSS at this time or at the reception of the response, to include it into the response to the session initiation.

NOTE: The algorithm used by the SCC AS to select a C‑MSISDN is implementation dependent.

For video call terminating sessions that use CS media, the MSC Server enhanced for ICS shall also, after the multimedia connection is established, perform the video codec negotiation for the non ICS UE and set up the video media bearer based upon the procedures defined in TS 29.163 [11] for 3GPP systems and based on procedures defined in 3GPP2 C.S0042 [38] for 3GPP2 systems. If the UE does not support video, or the access network is not capable of video, then the video component of the session shall not be accepted by the MSC Server enhanced for ICS.

For voice call terminating sessions that use CS media via I2,, re-routing from the terminating MSC Server enhanced for ICS to another MSC (ICS enhanced or not) may be triggered (i.e, due to Mobile Terminating Roaming Retry Call procedure as defined in TS 23.018 [49], Roaming Retry for CS fallback or Roaming Forwarding for CS fallback as defined in TS 23.272 [44] is triggered).

If the SCC AS receives from the new MSC (enhanced for ISC- see clause 7.2.1.2) a new IMS registration for the UE for which the SCC AS just delivered a session, then:

– S-CSCF releases the current terminating session setup toward SCC AS/T-ADS as defined in TS 24.229 [31]; and

– T-ADS redirects the session to the new MSC.

If the old MSC (terminating MSC) performs IMS deregistration over I2 (see clause 7.2.1.3), then:

– S-CSCF releases the current terminating session setup toward SCC AS/T-ADS as defined in TS 24.229 [31];

– Based on operator policy, T-ADS may start a timer. The timer is stopped when new registration is received for the UE;

– If the T-ADS stopped the timer itself, T-ADS redirects the session to the new MSC;

– If the timer expires, T-ADS may invoke other procedure to redeliver the call (e.g. routing the call to the CS domain).

7.4.2.1.2 Termination using I2 reference point

Figure 7.4.2.1.2-1 describes how IMS terminations are performed via CS access for non ICS UE registered in IMS. This call flow also applies for an ICS UE attached to an MSC Server enhanced for ICS where the incoming session is delivered to the ICS UE using standard CS terminating procedures, as specified in clause 7.4.2.2.5.

Figure 7.4.2.1.2-1: IMS Termination via CS Access for non ICS UE registered in IMS

1. An incoming INVITE is received at the S‑CSCF of the B-party via the I‑CSCF.

2. The S‑CSCF performs standard service control execution procedures. Filter criteria direct the S‑CSCF to send the INVITE to the SCC AS.

3. 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.

4. The SCC AS establishes a new session by sending an INVITE to the UE B via the S‑CSCF.

5. The S‑CSCF forwards the INVITE to the MSC Server based on the contact address stored during registration, using standard IMS procedures.

6. The MSC Server sends a Setup message to the UE B.

7. Completion of the session and bearer control setup procedures. For video call, to complete the session and bearer setup, the specific handling as described in clause 7.4.2.1.1 applies. The MSC Server adds the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information of the terminating user to the session initiation response message.

7.4.2.1.3 Termination to non ICS UE not registered in IMS

Figure 7.4.2.1.3-1 describes how IMS terminations are performed via CS access for non ICS UE not registered in IMS. This call flow also applies for an ICS UE attached to an MSC Server where the incoming session is delivered to the ICS UE using standard CS terminating procedures, as specified in clause 7.4.2.2.5.

Figure 7.4.2.1.3-1: IMS Termination via CS Access for non ICS UE not registered in IMS

1. An incoming INVITE is received at the S‑CSCF of the B-party.

2. The S‑CSCF performs standard unregistered service control execution procedures. The terminating iFC direct the INVITE to the SCC AS for terminating access domain selection.

3. The T-ADS function chooses breakout to the CS domain. A CSRN is fetched for routing to the CS domain. If the SCC AS interacts with the HSS to fetch the CSRN, it may also indicate to the HSS the support of MTRR as specified in TS 23.018 [49].

NOTE 1: At this point, the SCC AS can, as an implementation option, interact with the HSS to obtain location information.

NOTE 2: Fetching the CSRN is implementation specific.

NOTE 3: If Sh is used for interaction with the HSS and SCC AS indicates support of MTRR, the SCC AS also provides its MAP address to the HSS.

4. The SCC AS establishes a new session by sending an INVITE containing the CSRN and ICS indicator towards the S‑CSCF.

5. The S‑CSCF then performs CS breakout according to existing IMS procedures. The call is routed to the CS domain via the BGCF/MGCF.

6. The MGCF sends an IAM to the MSC Server.

7. The MSC Server performs paging procedures and sends the CS call setup message to the UE B. If the UE does not respond to the paging request as meanwhile it has performed registration with a new MSC Server, the network may either use the MTRF or MTRR feature as specified in TS 23.018 [49] in order to re-direct the call towards the new MSC Server. If MTRR is used the SCC AS emulates GMSC behavior and may request to to re-restart call handling from step 3 onwards.

8. Completion of the session and bearer control setup procedures. To complete the session and bearer setup for a video call the specific handling as described in clause 7.4.2.1.1 applies. The MGCF may map cause codes received over the CS leg differently compared to normal CS breakout. If the SCC AS has User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information available (e.g. from step 3, or by interacting with the HSS at this time), it adds it to the SIP response.

7.4.2.2 ICS UE Terminating sessions that use CS media

7.4.2.2.1 Overview

IMS service control is used for ICS UE terminating sessions that use CS media.

All ICS User incoming sessions are directed to IMS for delivery to the ICS User. The SCC AS is inserted in the IMS session path using the terminating iFCs. The SCC AS performs T-ADS for selection of a contact address amongst the registered contact addresses for the ICS User and subsequently selects an access network for delivery of the session to the selected contact address.

When using the Gm reference point for delivery of the incoming session to the ICS UE, the SCC AS directs the IMS terminating session toward the ICS User’s selected contact indicating use of CS bearer for the session. On receipt of the session initiating message, the ICS UE sets up a standard CS originated session addressing a PSI DN associated with the SCC AS to establish the CS bearer if a CS bearer is not available for the ICS UE. The CS bearer shall be reused if already established using the Gm reference point. The service control signalling is combined with the description of the CS bearer at the SCC AS for presentation of IMS terminating session to the S‑CSCF over the Remote Leg.

When a subsequent session termination with different CS bearer requirement occurs (i.e. CS audio bearer is changed to CS video bearer, or vice versa), the CS bearer shall be updated if possible through SCUDIF or redial (refer to clause 7.9.2).

The following clauses show pairs of flows, one for an ICS UE when using an MSC Server and the other for an MSC Server enhanced for ICS. They are for the most part identical except that in the case of an MSC Server enhanced for ICS the INVITE towards the SCC AS is sent directly from the MSC Server otherwise an MSC sends an ISUP IAM and the MGCF interworks this to an INVITE towards the SCC AS.

For video call terminating sessions that use CS media, the following apply:

– When the ICS UE is attached to the MSC Server enhanced for ICS, after the multimedia connection is established, the MSC Server enhanced for ICS shall perform the video codec negotiation for the ICS UE and set up the video media bearer based upon the procedures defined in TS 29.163 [11] for 3GPP systems and based on the procedures defined in 3GPP2 C.S0042 [38] for 3GPP2 systems;

– When the ICS UE is attached to the MSC Server not enhanced for ICS, the MGCF/IMS-MGW shall perform the video codec negotiation for the ICS UE and set up the video media bearer as specified in TS 29.163 [11] for 3GPP systems and as specified in 3GPP2 C.S0042 [38] for 3GPP2 systems.

7.4.2.2.2 Terminations with CS media using the Gm reference point

Figure 7.4.2.2.2-1 provides an example flow for a call destined to an ICS UE attached to an MSC server enhanced for ICS, where the incoming session is delivered over the Gm reference point and the media is established via the CS network.

Figure 7.4.2.2.2-1: ICS UE Terminations with CS media information flows using Gm reference point when using an MSC Server enhanced for ICS

NOTE 1: Steps 11, 12, 13 and 14 are related to the setup of the CS Bearer Control Signalling Path. The other steps are related to the setup of the Service Control Signalling Path.

1. An incoming SIP INVITE is received at the S‑CSCF of the B party from UE A.

2. The S‑CSCF executes terminating initial filter criteria and forwards the INVITE to the SCC AS.

3. The SCC AS sends a Session Progress response to the S‑CSCF.

4. The S‑CSCF forwards the Session Progress response to UE A.

Steps 5a – 7a are for the case of T-ADS performed by the SCC AS.

5a. The SCC AS performs Terminating Access Domain Selection and chooses the CS domain for the setup of the media.

6a. The SCC AS terminates the session from the A-party and establishes a new session by sending an INVITE to the B-party via the I/S‑CSCF. This INVITE contains an indication to inform the UE to initiate the CS bearer establishment procedure. The INVITE also contains a dynamic SCC AS PSI to enable the SCC AS to later on correlate the outgoing service control signalling with the incoming CS bearer control signalling. When the T-ADS function selects the Gm reference point for service control the SCC AS prevents the S‑CSCF from selecting the contact address of the MSC Server, and the S‑CSCF selects the IP-CAN

7a. The I/S‑CSCF sends the INVITE to B-party.

Steps 5b – 10 are for the case of UE T-ADS.

5b. Alternatively, in the case of UE T-ADS as defined in clause 5.3.1, the SCC AS performs initial T‑ADS selecting IMS for the service control signalling when UE-B is registered in the IMS.

6b The SCC AS terminates the session from the A-party and establishes a new session by sending an INVITE to the B-party via the I/S‑CSCF. The INVITE contains options that enable UE-B to choose a CS bearer for bidirectional speech media transport if it determines that this is not supported by serving PS Access Network. The INVITE also contains a dynamic SCC AS PSI to enable the SCC AS to later on correlate the outgoing service control signalling with the incoming CS bearer control signalling. When the T-ADS function selects the Gm reference point for service control, the SCC AS prevents the S‑CSCF from selecting the contact address of the MSC Server, and the S‑CSCF selects the IP-CAN.

7b. The I/S‑CSCF sends the INVITE to B-party.

8-10 UE-B responds with a Session Progress message. In the case that the PS access network does not support bidirectional speech media and in the event the "IMS voice over PS session supported indication" indicates voice is not supported as defined in clause 4.3.5.8 of TS 23.401 [34], UE-B shall indicate in the Session Progress message that a CS bearer is required for the bidirectional speech component of the session and that the Gm reference point is used for service control. The S‑CSCF forwards the Session Progress message to the SCC AS.

11. If the establishment mechanism is for UE originated, the ICS UE sends a CS call setup message to the MSC Server using the SCC AS PSI DN to establish the CS Bearer Control Signalling Path. This will establish the circuit bearer between the UE and IMS.

12. The MSC Server responds with a call proceeding message and begins to set up the CS Bearer Control Signalling Path.

13. The MSC Server sends an INVITE towards the I/S‑CSCF containing the SCC AS PSI and an SDP Offer with the media description from the MGW. The MSC Server adds the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information to the INVITE.

14. The I/S‑CSCF forwards the INVITE to the SCC AS.

15. The SCC AS continues the session setup towards the A-party for the original INVITE in Step 1. The response contains an SDP answer with the media description from the SDP offer received in Step 13.

16. The SCC AS completes the set-up of the CS Bearer Signalling Path towards ICS UE B which involves sending a 200 OK in response to the INVITE in Step 14. The ICS UE B, the MSC Server and the SCC AS complete the setup of the CS Bearer Control Signalling Path and the Service Control Signalling Path with UE A.

17. The ICS UE B continues with the set-up of the Service Control Signalling Path towards the UE A which includes sending a Ringing response to the SCC AS via the S‑CSCF. The ICS UE B then completes the set-up of the Service Control Signalling Path towards the Remote End which includes sending a final 200 OK message in response to the INVITE received at steps 7a or 7b.

For video call, in Steps 16 and 17, to complete the session and bearer setup, the specific handling as described in clause 7.4.2.2.1 applies.

Figure 7.4.2.2.2-2 provides an example flow for a call destined to an ICS UE attached to an MSC server, where the incoming session is delivered over the Gm reference point and the media is established via the CS network.

Figure 7.4.2.2.2-2: ICS UE Terminations with CS media information flows using Gm reference point when using an MSC Server

NOTE 2: Steps 11, 12,13,14 and 15 are related to the setup of the CS Bearer Control Signalling Path. The other steps are related to the setup of the Service Control Signalling Path.

Steps 1-12 in Figure 7.4.2.2.2-2 are identical to steps 1-12 in Figure 7.4.2.2.2-1.

At Steps 13 and 14, the MSC server sends the IAM to an MGCF using the SCC AS PSI DN which is subsequently inter-worked to an INVITE containing the SCC AS PSI and an SDP Offer from the MGW.

Steps 15-18 in Figure 7.4.2.2.2-2 are identical to steps 14-17 in Figure 7.4.2.2.2-1.

7.4.2.2.3 Terminations with CS media using the I1 reference point

Figure 7.4.2.2.3-1 provides an example flow for a call destined to an ICS UE attached to an MSC server enhanced for ICS, where the incoming session is delivered over the I1 reference point and the media is established via the CS network.

Figure 7.4.2.2.3-1: ICS UE Terminations with CS media information flows using I1 reference point when using an MSC Server enhanced for ICS

NOTE 1: Steps 1-10, 15 and 17-22 are related to the setup of the Service Control Signalling Path. Steps 11-14 and 16 are related to the setup of the CS Bearer Control Signalling Path.

1. An incoming SIP INVITE is received at the S‑CSCF of the B party from UE A.

2. The S‑CSCF executes terminating initial filter criteria and forwards the INVITE to the SCC AS.

3. The SCC AS sends a Session Progress response to the S‑CSCF.

4. The S‑CSCF forwards the Session Progress response to UE A.

Steps 5a – 6a are for the case of T-ADS performed by the SCC AS.

5a. The SCC AS performs Terminating Access Domain Selection and if informed of the UE capability for I1 during IMS registration, chooses the CS domain for the setup of the media.

NOTE 2: Conveyance of UE capability of support for I1 does not imply that I1 can be established (e.g. the visited network does not support I1).

6a. The SCC AS terminates the session from the A-party and establishes a new session over I1 by sending an Incoming Call Request to the B-party via the HSS and MSC Server. The Incoming Call Request contains an indication to inform the UE to initiate the CS bearer establishment procedure. The Incoming Call Request also contains a dynamic SCC AS PSI (DN) to enable the SCC AS to later on correlate the outgoing service control signalling with the incoming CS bearer control signalling.

Steps 5b – 10 are for the case of UE assisted T-ADS.

5b. Alternatively, in the case of UE assisted T-ADS the SCC AS performs initial T-ADS selecting IMS for the service control signalling when UE-B is registered in the IMS.

6b The SCC AS terminates the session from the A-party and establishes a new session by sending an INVITE to the B-party via the I/S‑CSCF. The INVITE contains options that enable UE-B to choose a CS bearer for bidirectional speech media transport if it determines that this is not supported by serving PS Access Network. The INVITE also contains a dynamic SCC AS PSI (DN) to enable the SCC AS to later on correlate the outgoing service control signalling with the incoming CS bearer control signalling. When the T-ADS function selects the Gm reference point for service control, the SCC AS prevents the S‑CSCF from selecting the contact address of the MSC server, and the S‑CSCF selects the IP-CAN.

7b. The I/S‑CSCF sends the INVITE to B-party.

8-10. In the case where the access network does not support simultaneous PS and CS, UE-B responds with a Session Progress message and in the event the "IMS voice over PS session supported indication" indicates voice is not supported as defined in TS 23.401 [34]. UE-B shall indicate in the Session Progress message that a CS bearer is required for the session and that further signalling for the session will be via I1. The S‑CSCF forwards the Session Progress message to the SCC AS.

11. The ICS UE sends a CS Setup message to the MSC Server using the SCC AS PSI DN to establish the CS Bearer Control Signalling Path. This will establish the circuit bearer between the UE and IMS.

12. The MSC Server responds with a call proceeding message and begins to set up the CS Bearer Control Signalling Path.

14. The I/S‑CSCF forwards the INVITE to the SCC AS.

17. UE-B sends a Ringing indication to the SCC AS via I1.

18-19. The SCC AS sends a Ringing message to the remote end.

20. UE-B sends a Connecting message to the SCC AS via I1.

21-22. The SCC AS sends a 200 OK message to the remote end in response to the INVITE received at step 2. For video call, to complete the session and bearer setup, the specific handling as described in clause 7.4.2.2.1 applies.

Figure 7.4.2.2.3-2 provides an example flow for a call destined to an ICS UE attached to an MSC server where the incoming session is delivered over the I1 reference point and the media is established via the CS network.

Figure 7.4.2.2.3-2: ICS UE Terminations with CS media information flows using I1 reference point when using an MSC Server

NOTE 3: Steps 1-10, 16 and 18-23 are related to the setup of the Service Control Signalling Path. Steps 11-15 and 17 are related to the setup of the CS Bearer Control Signalling Path.

Steps 1-12 in Figure 7.4.2.2.3-2 are identical to steps 1-12 in Figure 7.4.2.2.3-1.

At Steps 13 and 14, the MSC server sends the IAM to an MGCF using the SCC AS PSI DN which is subsequently inter-worked to an INVITE containing the SCC AS PSI and an SDP Offer from the MGW.

Steps 15-23 in Figure 7.4.2.2.3-2 are identical to steps 14-22 in Figure 7.4.2.2.3-1.

7.4.2.2.4 Terminations with CS media using CS control with I1 augmentation

Standard CS terminating procedures are used to deliver the incoming session to the ICS UE as described in Figure 7.4.2.2.3-1 when the ICS UE is attached to an MSC server enhanced for ICS, and in Figure 7.4.2.2.3-2 when the ICS UE is attached to an MSC server. Additional IMS parameters can be optionally communicated to the ICS UE using I1 after the incoming session has been delivered.

7.4.2.2.5 Terminations with CS media when not using Gm or I1

Figure 7.4.2.1.2-1 provides an example flow for a call destined to an ICS UE attached to an MSC server enhanced for ICS, where the incoming session is delivered using standard CS terminating procedures.

If an ICS UE is attached to an MSC server where the incoming session is delivered to it using standard CS terminating procedures, an example call flow is shown in Figure 7.4.2.1.3-1.

7.4.2.2.6 Void

7.4.2.2.7 Terminating access network reselection – fallback to alternate access

When using the Gm reference point for delivery of the incoming session to the ICS UE, if it fails to set up the bearer in the selected terminating access network and other access network(s) is available for the service, the UE may send a message to the SCC AS indicating the failure of bear setup with the information required for access network reselection (e.g. the information of available access network(s)), or indicate a newly selected access network. The SCC AS can then, based on the message and operator policy, update the relevant decision information for T-ADS if necessary and reselect a new access network for bearer setup to complete the terminating session establishment.

Figure 7.4.2.2.7-1 gives an example flow of terminating access reselection for ICS UE terminating session establishment using Gm reference point.

Figure 7.4.2.2.7-1: Access reselection for ICS UE termination over Gm reference point

1. An incoming call arrives at UE B from UE A. T-ADS is executed by the SCC AS and an access network is chosen for bearer establishment (refer to Steps 1-7a in Figure 7.4.2.2.2-1).

2. Bearer setup fails in the selected access network.

3. UE B sends a message to the I/S‑CSCF indicating the failure of bearer setup. Information required for the SCC AS to re-execute T-ADS and to select a new access network is provided (e.g. info of access network(s) available for the service), or a new access network selected by the UE for bearer setup is indicated.

4. The I/S‑CSCF forwards the message to the SCC AS based on originating initial filter criteria.

5. The SCC AS updates the relevant decision information for T-ADS and re-executes T-ADS to select a new access network for bearer setup of the session if no new access network is selected by the UE.

6-7. The SCC AS sends an Invite message to UE B via the S‑CSCF to establish the session, with the newly selected access network for bearer setup indicated or confirmed (if selected by the UE).

8. Bearer setup in the newly selected terminating access network for the session.

9. Completion of session setup procedures between UE A and UE B.