4 High level Principles and Concepts

23.2163GPPRelease 16Single Radio Voice Call Continuity (SRVCC)Stage 2TS

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

4.1 High level Principles

4.1.1 Architectural Principles for 3GPP2 1xCS SRVCC

The solution for SRVCC fulfils the requirements of TS 22.278 [9] and the following architectural principles:

1. The solution shall allow coexistence and be compatible with the 1xCS procedures specified in the 3GPP2 VCC specification, X.S0042 [4].

2. The solution shall not require UE with multiple RATs capability to simultaneously signal on two different RATs.

3. The solution shall be transparent to E-UTRA only terminal or network.

4. The solution shall minimize the coupling between the E-UTRAN and the 3GPP2 access. In particular, the solution shall allow the cdma2000 1xRTT specification to evolve without necessitating a modification to the E-UTRAN specifications.

5. RAT change and domain selection should be under network control.

6. In roaming cases, the Visited PLMN should control the RAT change and/or domain selection while taking into account any related HPLMN policies.

7. The solution shall not impact cdma2000 RAT.

8. The solution shall not impact cdma2000 CS CN.

9 All IMS sessions that may be subject to SRVCC shall be anchored in the IMS (VCC Application).

10. When SRVCC is deployed, QCI=1:

– shall not be used for IMS sessions that are not anchored in the IMS (VCC Application); and

– shall only be used for the voice bearer.

4.1.2 Architectural Principles for SRVCC and vSRVCC to 3GPP UTRAN/GERAN

The solution for (v)SRVCC fulfils the requirements of TS 22.278 [9] and the following architectural principles:

1. The solution shall allow coexistence and be compatible with TS 23.292 [13] and TS 23.237 [14].

2. The solution shall not require UE with multiple RATs capability to simultaneously signal on two different RATs.

3. RAT change and domain selection should be under network control.

4. E-UTRAN/UTRAN (HSPA) to UTRAN/GERAN CS handover for SRVCC is triggered by the same radio handover conditions and mechanisms as for an E-UTRAN/UTRAN (HSPA) to UTRAN/GERAN PS handover.

5. The Video Call by IMS over E-UTRAN is the IMS session with bi-directional video and voice media e.g. IMS Multimedia Telephony as defined in TS 22.173 [26] which uses separate EPS bearers for video and voice components, respectively.

6. In roaming cases, the VPLMN shall be able to control the RAT/domain selection change while taking into account any related HPLMN policies for IMS sessions with bi-directional video and voice media e.g. IMS Multimedia Telephony as defined in TS 22.173 [26].

7 All IMS sessions that may be subject to (v)SRVCC shall be anchored in the IMS (SCC AS).

8. When SRVCC is deployed, QCI=1 / traffic-class ‘Conversational’ with Source Statistics Descriptor =’speech’:

– shall not be used for IMS sessions that are not anchored in the IMS (SCC AS); and

– shall only be used for the voice bearer.

NOTE 1: The UE may have multiple voice media streams that are multiplexed over a single voice (e.g. QCI=1) bearer. Selection of the voice streams for SRVCC by the SCC AS is as specified in TS 23.237 [14].

NOTE 2: The UE may have multiple voice and video media streams that are carried over a single voice but multiple video (QCI=1 and the vSRVCC marked bearer) bearers or are multiplexed each over a single media bearer. Only one of these voice or voice and video streams is selected for SRVCC or vSRVCC by the SCC AS (see TS 23.237 [14]).

4.1.3 Architectural Principles for SRVCC to 3GPP E-UTRAN/UTRAN (HSPA)

The solution for CS to PS SRVCC fulfils the following architectural principles in addition to the ones defined in clause 4.1.2:

– A CS to PS SRVCC procedure shall be possible for CS call that is originated from UTRAN/GERAN.

– After transfer from CS to PS SRVCC, it shall support moving the session back to UTRAN/GERAN CS domain if SRVCC from E-UTRAN/HSPA PS-domain is supported.

– A CS to PS SRVCC procedure shall be possible after SRVCC from E-UTRAN/HSPA PS-domain to CS domain has occurred.

– Emergency session is not subjected to CS to PS SRVCC procedure.

A prerequisite for the CS to PS SRVCC procedure to take place is that the UE is registered in IMS and has at least one PS bearer (usable for SIP signalling).

4.1.4 Architectural Principles for 5G-SRVCC to UTRAN

The solution for 5G-SRVCC fulfils the requirements of TS 22.278 [9] and the following architectural principles:

1. The solution shall not require UE with multiple RATs capability to simultaneously signal on two different RATs.

2. RAT change and domain selection should be under network control.

3. NG-RAN to UTRAN CS handover for 5G-SRVCC is triggered by radio handover conditions.

4. All IMS sessions that may be subject to 5G-SRVCC shall be anchored in the IMS (SCC AS).

5. In roaming cases, the VPLMN shall be able to control the RAT/domain selection and change while taking into account any related HPLMN policies for IMS sessions with voice media.

4.2 Concepts

4.2.1 E-UTRAN to 3GPP2 1xCS SRVCC

For SRVCC-capable UEs, the call is always anchored at the VCC AS in the 3GPP2’s IMS. The 3GPP2 1xCS IWS enables a single radio UE to communicate in parallel both with the source system and the target system. From VCC perspective, this mechanism minimizes the voice gap by supporting the transport of signalling for establishment of the target CS access leg while the terminal is connected to the source PS access network.

Figure 4.2.1-1: Transport of 3GPP2 1xCS signalling messages for preparation of the CS access leg in the target system

The S102 reference point is used to convey 3GPP2 1xCS signalling messages between the MME and 3GPP2 1xCS IWS. These 1x CS signalling messages are actually exchanged between the UE and the 3GPP2 1xCS IWS, and S102 is only one link in the overall UE‑1xCS IWS tunnelling path. On the remaining portion of the tunnelling path, the 3GPP2 1xCS signalling messages are encapsulated in E‑UTRAN/EPS tunnelling messages (UE‑MME).

4.2.2 E-UTRAN to 3GPP UTRAN/GERAN SRVCC

For facilitating session transfer (SRVCC) of the voice component to the CS domain, the IMS multimedia telephony sessions needs to be anchored in the IMS.

For SRVCC from E‑UTRAN to UTRAN/GERAN, MME first receives the handover request from E‑UTRAN with the indication that this is for SRVCC handling, and then triggers the SRVCC procedure with the MSC Server enhanced for SRVCC via the Sv reference point if MME has STN-SR information for this UE. If SRVCC with priority is supported, based on the ARP associated with the EPS bearer used for IMS signalling, the MME sets the priority indication appropriately toward the MSC Server. MME aware of which EPS bearer is used for IMS signalling based on local configuration. MSC Server enhanced for SRVCC then initiates the session transfer procedure to IMS and coordinates it with the CS handover procedure to the target cell. If SRVCC with priority is supported, IMS session transfer procedure and the CS handover procedure are performed with priority handling per the priority indication received from MME. MSC Server enhanced for SRVCC then sends PS-CS handover Response to MME, which includes the necessary CS HO command information for the UE to access the UTRAN/GERAN.

Handling of any non‑voice PS bearer is done by the PS bearer splitting function in the MME. MME starts the handover of non‑voice PS bearer during SRVCC procedure based on the information received from E‑UTRAN. The handover of non‑voice PS bearer(s), if performed, is done as according to Inter RAT handover procedure as defined in TS 23.401 [2]. The MME is responsible to coordinate the Forward Relocation Response from PS‑PS handover procedure and the SRVCC PS to CS Response.

NOTE: Depending on operator’s policy, when 5GS is deployed, the eNB can switch the PS HO off when it initiates SRVCC procedure, i.e. SRVCC only for CS voice.

Figure 4.2.2-1: Overall high level concepts for SRVCC from E-UTRAN to UTRAN/GERAN

4.2.2a E-UTRAN to 3GPP UTRAN vSRVCC

For facilitating session transfer of the voice and video components to the CS domain, the IMS multimedia telephony sessions needs to be anchored in the IMS.

For vSRVCC, the UE uses one voice and one video media component over the associated QCI=1 and vSRVCC marked PS bearers for bearer identification reasons. The MME first receives the handover request from E‑UTRAN. It then triggers the vSRVCC procedure with the MSC Server enhanced for vSRVCC via the Sv reference point with vSRVCC related information. MSC Server enhanced for vSRVCC then interacts with IMS and initiates the session transfer procedure to IMS and coordinates it with the CS handover procedure to the target cell. If SRVCC with priority is supported, IMS session transfer procedure and the CS handover procedure are performed with priority handling according to the priority indication received from MME.

MSC Server performs SRVCC procedure if the current active session is voice only. MSC Server enhanced for vSRVCC then sends PS-CS Handover Response to MME, which includes the necessary CS HO command information for the UE to access the UTRAN. If the target cell is GERAN, MME only triggers SRVCC (i.e. only the voice component of the Video Call is transferred using the SRVCC procedure).

If SCC AS indicates current active session is voice and video, the MSC Server requests UTRAN radio resources for BS30 bearer and continues with the vSRVCC procedure. The BS30 bearer is a 64 kbps bearer for multimedia as defined in clause 3.1.2.13 of TS 22.002 [38].If the BS30 bearer reservation was unsuccessful, then vSRVCC procedure is considered failed and appropriate rejection cause is given back to E-UTRAN.

Handling of any non QCI=1 and vSRVCC marked PS bearer is done by the PS bearer splitting function in the MME. MME starts the handover of non QCI=1 and vSRVCC marked PS bearer during vSRVCC procedure based on the information received from E‑UTRAN. The handover of non QCI=1 and vSRVCC marked PS bearer(s), if performed, is done as according to Inter RAT handover procedure, as defined in TS 23.401 [2]. The MME is responsible to coordinate the Forward Relocation Response from PS‑PS handover procedure and the vSRVCC PS to CS Response.

When the UE receives the HO Command indicating a TS 11 or BS30 bearer, it knows whether it should start the CS 3G-324M video codec negotiation or SRVCC.

Figure 4.2.2a-1: Overall high level concepts for vSRVCC from E-UTRAN to UTRAN

4.2.3 UTRAN (HSPA) to 3GPP UTRAN/GERAN SRVCC

For facilitating session transfer (SRVCC) of the voice component to the CS domain, the IMS multimedia telephony sessions needs to be anchored in the IMS.

For SRVCC from UTRAN (HSPA) to UTRAN/GERAN, SGSN first receives the handover request from UTRAN (HSPA) with the indication that this is for SRVCC handling, and then triggers the SRVCC procedure with the MSC Server enhanced for SRVCC via the Sv if SGSN has STN-SR information for this UE. MSC Server enhanced for SRVCC then initiates the session transfer procedure to IMS and coordinates it with the CS handover procedure to the target cell. MSC Server enhanced for SRVCC then sends PS-CS handover Response to SGSN, which includes the necessary CS HO command information for the UE to access the UTRAN/GERAN.

Handling of any non-voice PS bearer is done by the PS bearer splitting function in the SGSN. SGSN starts the handover of non-voice PS bearer during SRVCC procedure based on the information received from UTRAN (HSPA). The handover of non-voice PS bearer(s), if performed, is done as according to Inter/Intra RAT handover procedure as defined in TS 23.060 [10] and TS 25.413 [11]. The SGSN is responsible to coordinate the Forward Relocation Response from PS-PS handover procedure and the SRVCC PS to CS Response.

Figure 4.2.3-1: Overall high level concepts for SRVCC from UTRAN (HSPA) to UTRAN/GERAN

4.2.4 SRVCC for IMS emergency sessions

4.2.4.1 E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN

UE initiates the IMS emergency session as specified in TS 23.167 [28], TS 23.401 [2] for E-UTRAN or TS 23.060 [10] for UTRAN (HSPA). For facilitating session transfer (SRVCC) of the IMS emergency session to the CS domain, the IMS emergency session needs to be anchored in the serving IMS (i.e., in visited PLMN when roaming) as specified in TS 23.237 [14].

The E-UTRAN initiates the SRVCC procedure as specified for regular Voice over IMS session.

SRVCC for IMS emergency session can be supported regardless of the subscription data from the HPLMN. If the MME detects IMS emergency session event (e.g., emergency APN and associated QCI-1 bearer setup), MME shall update the E-UTRAN that SRVCC is possible by including "SRVCC operation possible" indication via S1 AP UE CONTEXT MODIFICATION REQUEST, if needed.

After IMS emergency session is released, MME shall restore the SRVCC indication to the same status as prior to the emergency session was established. If needed, reverting SRVCC indication to "not possible" is performed by including "SRVCC operation not possible" indication via S1 AP UE CONTEXT MODIFICATION REQUEST. This allows the MME to update E-UTRAN to invoke SRVCC for emergency session while the SRVCC is not allowed for normal IMS voice session due to subscription data from the HPLMN (i.e., No STN-SR and C-MSISDN received from the subscription data are not considered by the serving MME (e.g. HSS).

The MME is aware that this is an emergency session and sends an indication to the MSC Server enhanced for SRVCC. MSC Server then initiates the IMS service continuity procedure with the locally configured E-STN-SR to the serving IMS. When handover of the emergency session has been completed, the MME/SGSN or the MSC Server may initiate location continuity procedures for the UE as defined in TS 23.271 [29].

Figure 4.2.4.1-1: Overall high level concepts for SRVCC IMS emergency session with E-STN-SR

4.2.4.2 E-UTRAN to 3GPP2 1xCS

The UE initiates emergency session over E-UTRAN as specified in TS 23.167 [28], TS 23.401 [2], upon detecting handover is required from E-UTRAN to CDMA 1x, the SRVCC emergency procedure apply. To support handover of emergency session the network is aware that the UE and core network support SRVCC and has information to identify Emergency session. When handover of the emergency session has been completed, the MME or the 1xRTT side may initiate location continuity procedures for the UE as defined in TS 23.271 [29].

Figure 4.2.4.2-1: E-UTRAN to 3GPP2 1xCS

4.2.4.3 SRVCC in Limited Service Mode

4.2.4.3.1 E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN

In order to support SRVCC emergency session domain transfer for UEs in Limited Service Mode (e.g. UICC-less), the MME/SGSN shall support Limited Service Mode UE emergency attach defined in TS 23.401 [2] and TS 23.060 [10] using unauthenticated IMSI or equipment identifier.

When E-UTRAN/UTRAN determines that SRVCC is needed, the MME/SGSN invokes SRVCC procedures to the MSC Server including the UE’s equipment identifier. The MSC Server will setup the call leg towards the EATF with the UE’s equipment identifier. This procedure is defined in TS 23.237 [14].

4.2.4.3.2 E-UTRAN to 3GPP2 1xCS

In order to support SRVCC emergency session domain transfer for UEs in Limited Service Mode (e.g. UICC-less), the MME shall support Limited Service Mode UE emergency attach defined in TS 23.401 [2] using unauthenticated IMSI or equipment identifier.

When E-UTRAN determines that SRVCC is needed, the MME invokes SRVCC procedures to the 1xCS IWS including the UE’s equipment identifier.

4.2.4.3.3 NG-RAN to 3GPP UTRAN

Access to 5GS in Limited Service Mode is specified in TS 23.501 [48].

The AMF invokes SRVCC procedures to the MSC Server enhanced for SRVCC via the MME_SRVCC, including the UE’s equipment identifier. The MSC Server will setup the call leg towards the EATF with the UE’s equipment identifier. This procedure is specified in TS 23.237 [14].

4.2.4.4 eCall over IMS

SRVCC is supported for handover of the voice channel of an eCall Over IMS from E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN. The support is exactly the same as for handover of the voice channel of an IMS emergency call from E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN with the addition of continuing support for transfer of updated MSD from the UE to an emergency centre/PSAP by in-band means.

NOTE: An eCall over IMS can only be originated on E-UTRAN but for a UE not in eCall Only mode, handover from E-UTRAN to UTRAN (HSPA) is possible, thence leading to a possible handover to GERAN/UTRAN in the CS domain.

4.2.5 Void

4.2.6 CS to PS SRVCC

For facilitating session transfer (SRVCC) of the voice component from the CS domain, the voice session shall be anchored in the IMS.

Figure 4.2.6-1: UTRAN/GERAN to E-UTRAN/UTRAN(HSPA)

The UE informs the MSC Server about the MME/SGSN the UE had last contact with using P‑TMSI+RAI+P‑TMSI signature or GUTI upon MSC request. For CS to PS SRVCC, MSC Server first receives the handover request from UTRAN/GERAN with the indication that this is for CS to PS SRVCC handling and then triggers the CS to PS SRVCC procedure with the target MME/SGSN enhanced with CS to PS SRVCC via the enhanced Sv reference point. The MME enhanced with CS to PS SRVCC triggers PS HO in target E-UTRAN/UTRAN (HSPA). If the target MME/SGSN does not have the UE context then the target MME/SGSN retrieves the UE context from the source SGSN/MME.

MSC Server then initiates the session transfer procedure to IMS and coordinates it with the PS handover procedure to the target cell. MME then sends CS-PS handover Response to MSC Server, which includes the necessary CS to PS HO command information for the UE to access the E-UTRAN/UTRAN (HSPA).

4.2.7 NG-RAN to 3GPP UTRAN 5G-SRVCC

4.2.7.1 General

Service continuity to UTRAN is supported only for the voice component of an IMS Session or IMS Emergency Session and only when both the UE and the network support 5G-SRVCC.

For facilitating session transfer (SRVCC) of the voice component to the CS domain, the IMS multimedia telephony sessions needs to be anchored in the IMS.

For 5G-SRVCC from NG-RAN to UTRAN, AMF first receives the handover request from NG-RAN with the indication that this is for 5G-SRVCC handling, and then AMF sends the forward relocation request with 5G-SRVCC HO Indication to MME_SRVCC together with STN-SR and C-MSISDN. This MME_SRVCC then triggers the SRVCC procedure with the MSC Server enhanced for SRVCC via the Sv reference point. AMF is aware of which PDU session is used for IMS session based on the DNN. The MSC Server enhanced for SRVCC then initiates the session transfer procedure to IMS and coordinates it with the CS handover procedure to the target cell (as in other SRVCC cases). MSC Server enhanced for SRVCC then sends PS-CS handover Response to MME_SRVCC, which includes the necessary CS HO command information for the UE to access the UTRAN. MME_SRVCC sends the forward relocation response message including this information to AMF. AMF sends the Handover Command to NG-RAN and NG-RAN sends the Handover Command to UE.

After UE moves to UTRAN and AMF receives the forward relocation complete, AMF requests PDU session release for all the PDU session.

Figure 4.2.7-1: Overall high level concepts for 5G-SRVCC from NG-RAN to UTRAN

This specification introduces an additional function to those defined in the 5GC architecture in TS 23.501 [48] for 5G-SRVCC. This additional function is provided by the MSC Server (i.e., MSC Server enhanced for SRVCC) and the MME_SRVCC.

NOTE 1: Figure 4.2.7-1 only shows the necessary components related to 5G-SRVCC.

NOTE 2: MSC Server shown in the figure is enhanced for SRVCC.

NOTE 3: This architecture also applies to roaming scenario (i.e. N16, N9 are not impacted due to SRVCC).

NOTE 4: The MSC Server enhanced for SRVCC may not be the final target MSC which connects to the target cell.

4.2.7.2 5G-SRVCC for IMS emergency sessions

UE initiates the IMS emergency session as specified in TS 23.167 [28] and TS 23.501 [48]. For facilitating session transfer (SRVCC) of the IMS emergency session to the CS domain, the IMS emergency session needs to be anchored in the serving IMS (i.e., in visited PLMN when roaming) as specified in TS 23.237 [14].

The NG-RAN initiates the SRVCC procedure as specified for regular Voice over IMS session.

If the UE has an Emergency PDU Session established, the AMF shall send the Emergency Indication and Equipment Identifier to the MSC Server enhanced for SRVCC via the MME_SRVCC. MSC Server then initiates the IMS service continuity procedure with the locally configured E-STN-SR to the serving IMS. When handover of the emergency session has been completed, the MSC Server may initiate location continuity procedures for the UE as defined in TS 23.271 [29].

NOTE: If 5G-SRVCC HO is triggered by NG-RAN while the UE has an Emergency PDU Session without voice (i.e. because the UE simultaneously has a non-emergency voice session), the 5G SRVCC procedure will be attempted for an emergency session and will fail.

Figure 4.2.7.2-1: Overall high level concepts for 5G-SRVCC IMS emergency session with E-STN-SR