5 Architecture model and reference points

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

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

5.1 General

The SRVCC Architecture for 3GPP2 1xCS reuses many existing elements in 3GPP2 X.S0042 [4] for 3GPP2 1xCS.

The SRVCC and vSRVCC Architectures for 3GPP E-UTRAN / UTRAN(HSPA) to 3GPP UTRAN/GERAN reuses the session transfer function defined for IMS in TS 23.237 [14] for IMS service continuity. The MSC Server that is enhanced for SRVCC and vSRVCC may also be enhanced for ICS as defined in TS 23.292 [13].

The SRVCC Architecture for CS to PS reuses the session transfer function defined for IMS in TS 23.237 [14] for IMS service continuity. The MSC Server that is enhanced for CS to PS SRVCC is also enhanced for ICS as defined in TS 23.292 [13].

The overall model and impacts to the various elements is provided in the following clauses.

5.2 Reference architecture

5.2.1 E-UTRAN to 3GPP2 1xCS SRVCC architecture

This specification introduces an additional functional entity to those defined in the E‑UTRAN architecture TS 23.402 [3], called 1x CS SRVCC interworking solution function (3GPP2 1xCS IWS), see figure 5.2.1‑1.

NOTE: The figure only shows the necessary components related to 3GPP2 1xCS IWS.

Figure 5.2.1-1: SRVCC architecture for E-UTRAN to 3GPP2 1xCS

5.2.2 E-UTRAN to 3GPP UTRAN/GERAN SRVCC architecture

This specification introduces an additional function to those defined in the E‑UTRAN architecture TS 23.401 [2] for SRVCC. This additional function is provided by the MSC Server (i.e., MSC Server enhanced for SRVCC).

NOTE 1: The following figure only shows the necessary components related to MSC Server enhanced for SRVCC.

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

NOTE 3: This architecture also applies to roaming scenario (i.e., S8, S6a 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.

Figure 5.2.2-1: SRVCC architecture for E-UTRAN to 3GPP UTRAN/GERAN

5.2.2a 3GPP E-UTRAN to 3GPP UTRAN vSRVCC architecture

NOTE 1: The following figure only shows the necessary components related to MSC Server enhanced for vSRVCC.

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

NOTE 3: This architecture also applies to roaming scenario (i.e., S8, S6a are not impacted due to vSRVCC).

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

Figure 5.2.2a-1: vSRVCC architecture for E-UTRAN to 3GPP UTRAN

5.2.3 UTRAN (HSPA) to 3GPP UTRAN/GERAN SRVCC architecture

This specification introduces an additional function to those defined in the UTRAN architecture TS 23.060 [10] for SRVCC. This additional function is provided by the MSC Server (i.e., MSC Server enhanced for SRVCC).

Figure 5.2.3-1: SRVCC architecture for UTRAN (HSPA) to 3GPP UTRAN/GERAN with Gn based SGSN

NOTE 1: The above figures only show the necessary components related to MSC Server enhanced for SRVCC.

NOTE 2: MSC Server shown in the above figures are enhanced for SRVCC.

NOTE 3: This architecture also applies to roaming scenario.

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

Figure 5.2.3-2: SRVCC architecture for UTRAN (HSPA) to 3GPP UTRAN/GERAN with S4 based SGSN

5.2.4 3GPP UTRAN/GERAN to 3GPP E-UTRAN or UTRAN (HSPA) SRVCC architecture

NOTE 1: The above figure only shows the necessary components related to MSC Server enhanced for CS to PS SRVCC.

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

NOTE 3: This architecture also applies to roaming scenario (i.e. S8, S6a are not impacted due to CS to PS SRVCC).

Figure 5.2.4: CS to PS SRVCC architecture for UTRAN/GERAN to 3GPP E-UTRAN

5.2.5 3GPP GERAN to 3GPP UTRAN (HSPA) SRVCC architecture

This specification introduces an additional function to those defined in the UTRAN architecture TS 23.060 [10] for CS to PS SRVCC. This additional function is provided by the MSC Server (i.e., MSC Server enhanced for CS to PS SRVCC).

Figure 5.2.5-1: CS to PS SRVCC architecture for 3GPP GERAN to UTRAN (HSPA) with Gn based SGSN

NOTE 1: The above figures only show the necessary components related to MSC Server enhanced with CS to PS SRVCC.

NOTE 2: MSC Server shown in the above figures are enhanced for CS to PS SRVCC.

NOTE 3: This architecture also applies to roaming scenario.

Figure 5.2.5-2: CS to PS SRVCC architecture for 3GPP GERAN to UTRAN (HSPA) with S4 based SGSN

5.2.6 NG-RAN to 3GPP UTRAN 5G-SRVCC architecture

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: The following figure 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.

Figure 5.2.6-1: 5G-SRVCC architecture for NG-RAN to 3GPP UTRAN

5.3 Functional Entities

NOTE 1: 3GPP2 components are not described here. Please refer to 3GPP2 X.S0042 [4].

NOTE 2: IMS components are not described here. Please refer to TS 23.237 [14] and TS 23.292 [13].

5.3.1 3GPP2 1x CS SRVCC interworking solution function (1xCS IWS)

3GPP2 1xCS IWS uses the S102 reference point to communicate with the MME and to transport 3GPP2 1xCS signalling messages to the SRVCC UE. The role of the 3GPP2 1xCS IWS is:

– To be a signalling tunnelling end point towards the MME for receiving/sending encapsulated 3GPP2 1xCS signalling messages to/from the UE; and

– To emulate a 1xRTT BSS towards the 1xRTT MSC (reference point A1 as defined in 3GPP2 A.S0014 [20] between 1xBS and MSC).

5.3.2 MSC Server enhanced for E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN SRVCC

In addition to the standard MSC Server behavior defined in TS 23.002 [15], an MSC Server which has been enhanced for SRVCC provides the following functions as needed for support of SRVCC:

– Handling the Relocation Preparation procedure requested for the voice component from MME/SGSN and with priority if priority indication is received via the Sv reference point;

– Behaving as MSC Server enhanced for ICS as defined in TS 23.292 [13] if supported and when ICS flag is received via the Sv reference point is set to true and optionally if the MSC Server is configured to know that the VPLMN has a suitable roaming agreement with the HPLMN of the UE;

– Invoking the session transfer procedure or emergency session transfer procedure from IMS to CS as according to TS 23.237 [14] and with priority if priority indication is received via the Sv reference point;

– Coordinating the CS Handover and session transfer procedures;

– In the case of a non-emergency session, handling the MAP_Update_Location procedure without it being triggered from the UE;

– In the case of an emergency session, conditionally sending a MAP Subscriber Location Report to a GMLC to support location continuity according to TS 23.271 [29].

5.3.2a MSC Server enhanced for vSRVCC

An MSC Server enhanced for SRVCC may be further enhanced for vSRVCC by providing, in addition to the functions specified in clause 5.3.2, the following functions as needed for the support of vSRVCC:

– Initiating the handover towards the target system for BS30 when it receives the Sv request with a vSRVCC indication from the MME and with priority if priority indication is received via the Sv reference point.

– Negotiating with the SCC AS for the last active session to determine if it should perform SRVCC or vSRVCC procedure.

5.3.2b MSC Server enhanced for CS to PS SRVCC

In addition to the standard MSC Server behaviour defined in TS 23.002 [15] and TS 23.292 [13] for MSC Server enhanced for ICS, an MSC Server which has been enhanced for SRVCC from CS to PS provides the following functions as needed:

– The MSC Server informs the RAN about the possibility to perform CS to PS SRVCC by sending a "CS to PS SRVCC operation possible" to the RNC/BSC only for normal TS 11 call. The RAN uses that information for deciding the cells for which the UE reports measurements that lead to handover request to the core network.

– The MSC Server determines the possibility of performing CS to PS SRVCC based on:

– The CS to PS SRVCC capability indication of the UE received from IMS as defined in TS 23.237 [14].

– The presence of the CS to PS SRVCC allowed indication in the subscription data of the user.

– The IMS registration status of the UE.

– The MSC Server informs RAN that CS to PS SRVCC is not possible by sending a "CS to PS SRVCC operation possible" set to "false" to the RNC/BSC on reception of the UE’s IMS registration expiration notification from SCC AS during an ongoing CS call.

– The target MME/SGSN selection for supporting CS to PS SRVCC is based on Target ID contained in the HO required and can be performed using the SGSN/MME selection procedures specified in TS 29.303 [37] or using local configuration in the MSC Server.

– The call is a normal TS 11 call (i.e. not an emergency call – TS12, TS 22.003 [39]).

NOTE: The IMS emergency session that is transferred from PS to CS is not subjected for this CS to PS SRVCC procedure.

– If roaming restriction to E-UTRAN access needs to be enforced, an MSC server connected to RNCs that may handover or invoke release with redirection to E-UTRAN is configured with a list of HPLMN IDs that are permitted to access E-UTRAN unless restricted by the UE individual access restriction information received from HLR/HSS (see TS 29.002 [41]). Also, the MSC server shall set the E-UTRAN Service Handover IE (in Iu mode) for CS bearers to an appropriate value to inform the RNC about connected-mode mobility restriction to E‑UTRAN.

5.3.3 MME

5.3.3.1 Interworking with 3GPP2 1xCS IWS

5.3.3.1.1 Interworking with 3GPP2 1xCS IWS

If the MME (operator) supports interworking to 3GPP2 1xCS, the MME shall follow the rules and procedures described in TS 23.402 [3] with the following additions and clarifications:

– To be a signalling tunnelling end point towards the 3GPP2 1xCS IWS for sending/receiving encapsulated 3GPP2 1xCS signalling messages to/from the UE, which are encapsulated in S1-MME S1 Information Transfer messages (TS 36.413 [30]).

– release of the E-UTRAN resources after SRVCC to the 3GPP2 1xCS is completed.

– include information to enable 3GPP2 network to determine emergency session.

– insert the equipment identifier during the handover procedure of an emergency session.

If the MME (operator) supports the interworking to 3GPP2 1xCS, and a UE is handed over by an AMF (see TS 23.501 [48]), the MME shall follow the rules and procedures described in TS 23.402 [3] with the following additions and clarifications:

– the MME shall obtain the equipment identifier of the UE.

5.3.3.1.2 Selection of 3GPP2 1xCS IWS

The 3GPP2 1xCS IWS can be selected based on the local configuration in the MME taking into account the Reference CellID received in the Uplink S1 cdma2000 Tunnelling message.

5.3.3.2 Interworking with 3GPP MSC Server enhanced for SRVCC from E-UTRAN / UTRAN (HSPA) to 3GPP UTRAN/GERAN

5.3.3.2.1 Interworking with 3GPP MSC Server enhanced for (v)SRVCC

If the MME (operator) supports the interworking to 3GPP CS, the MME shall follow the rules and procedures described in TS 23.401 [2] with the following additions and clarifications:

– Performing the PS bearer splitting function by separating the voice PS bearer from the non-voice PS bearers.

– Handling the non-voice PS bearers handover with the target cell as according to Inter RAT handover procedure as defined in TS 23.401 [2].

– Initiating the SRVCC handover procedure for handover of the voice component to the target cell via the Sv interface and including an emergency indication if this is an emergency session. If there are multiple voice bearers and one of those is for IMS emergency session then MME shall only execute the SRVCC for emergency.

NOTE: The UE may have 2 voice PS bearers if both emergency and normal IMS voice sessions are ongoing. Only the PS voice bearer associated with IMS emergency session will be executed for SRVCC.

– Coordinating PS handover and (v)SRVCC handover procedures when both procedures are performed.

– Sending the equipment identifier to the MSC Server during the handover procedure of an emergency session.

– In the case of an emergency session, conditionally sending a Subscriber Location Report to a GMLC to support location continuity according to TS 23.271 [29].

– If the MME (operator) supports vSRVCC and if the target is UTRAN, signalling the vSRVCC indication to the source MSC Server and performing bearer splitting of voice and video bearers of the IMS sessions with bi-directional video and voice media e.g. IMS Multimedia Telephony as defined in TS 22.173 [26].

– If (v)SRVCC with priority is supported and (v)SRVCC is performed for IMS-based MPS session, the MME sets the priority indication appropriately toward the MSC Server enhanced for (v)SRVCC via the Sv reference point.

If the MME (operator) supports the interworking to 3GPP CS, and a UE is handed over by an AMF (see TS 23.501 [48]), the MME shall follow the rules and procedures described in TS 23.401 [2] with the following additions and clarifications:

– the MME shall obtain the equipment identifier of the UE.

5.3.3.2.2 PS bearer splitting function

The function identifies the voice PS bearer upon E-UTRAN to UTRAN/GERAN (v)SRVCC and performs different handling of these bearers from the non-voice PS bearers (see detailed procedures in subsequent clauses). For vSRVCC, it also identifies the vSRVCC marked video PS bearer in addition to the voice PS bearer.

5.3.3.2.3 Selection of MSC enhanced for SRVCC

The MSC enhanced for SRVCC can be selected based on DNS procedures (see TS 29.303 [37]) or local configuration in the MME.

5.3.3.3 Interworking of Target MME with 3GPP MSC Server enhanced for SRVCC from CS to PS

If the MME (operator) supports the interworking to 3GPP CS, the target MME shall follow the rules and procedures described in TS 23.401 [2] with the following additions and clarifications:

– Handling the non-voice PS bearers and allocating resources with the target RAN when receiving CS to PS HO request from MSC via the Sv interface.

– Selecting the source MME/SGSN based on information about the MME/SGSN provided by the MSC Server using the SGSN/MME selection procedures specified in TS 29.303 [37] or using local configuration in the target MME.

5.3.3.4 MME_SRVCC

MME_SRVCC supporting the 5G-SRVCC handover from NG-RAN to 3GPP UTRAN shall at minimum support the inter-RAT handover from NG-RAN to E-UTRAN as described in TS 23.502 [45] and SRVCC from E-UTRAN to UTRAN without PS HO procedure as described in clause 6.2.2.1A with the following additions and clarifications:

– Triggers SRVCC without PS HO procedure according to "5G-SRVCC HO Indication" received from AMF.

– Selects a MSC server according to the Target ID received from AMF.

– Converts the message between N26 interface and Sv interface.

– Stores the association between Sv interface and N26 interface for the UE, forwards signalling between AMF and MSC Server/SGSN.

– Releases the association between Sv interface and N26 interface for the UE after PS to CS Complete ACK message.

5.3.3A SGSN

5.3.3A.1 Interworking with 3GPP MSC Server enhanced for SRVCC from E-UTRAN / UTRAN (HSPA) to 3GPP UTRAN/GERAN

5.3.3A.1.1 Interworking with 3GPP MSC Server enhanced for SRVCC

If the SGSN (operator) supports the interworking to 3GPP CS, the SGSN shall follow the rules and procedures described in TS 23.060 [10] with the following additions and clarifications:

– Performing the PS bearer splitting function by separating the voice PS bearer from the non-voice PS bearers. VoIP is detected by traffic class=conversational and Source Statistics Descriptor=’speech’.

– Handling the non-voice PS bearers handover with the target cell as according to Inter/Intra RAT handover procedure as defined in TS 23.060 [10].

NOTE: The UE may have 2 voice PS bearers if both emergency and normal IMS voice sessions are ongoing. Only the PS voice bearer associated with IMS emergency session will be executed for SRVCC.

– Coordinating PS handover and SRVCC handover procedures when both procedures are performed.

– Sending the equipment identifier to the MSC Server during the handover procedure for the case of UEs operating in limited service mode.

– In the case of an emergency session, conditionally sending a MAP Subscriber Location Report to a GMLC to support location continuity according to TS 23.271 [29].

5.3.3A.1.2 PS bearer splitting function

The function identifies the voice PS bearer upon HSPA to UTRAN/GERAN SRVCC and performs different handling on this bearer from the non-voice PS bearers (see detailed procedures in subsequent clauses).

5.3.3A.1.3 Selection of MSC enhanced for SRVCC

The MSC enhanced for SRVCC can be selected based on DNS procedures (see TS 29.303 [37]) or local configuration in the SGSN.

5.3.3A.2 Interworking of Target SGSN with 3GPP MSC Server enhanced for SRVCC from CS to PS

If the SGSN (operator) supports the interworking to 3GPP CS, the target SGSN shall follow the rules and procedures described in TS 23.060 [10] with the following additions and clarifications:

– Handling the non-voice PS bearers and allocating resources with the target RAN when receiving CS to PS HO request from MSC via the Sv interface.

5.3.4 UE enhanced for SRVCC

5.3.4.1 Interworking with 3GPP2 1xCS

If the UE supports 3GPP2 1xCS access, the 1xCS SRVCC UE is a UE that is capable to perform SRVCC to the 3GPP2 1xCS system. The interaction between UE and E-UTRAN is described in TS 36.300 [16]. The interaction with the 3GPP2 1xCS system is described in this specification.

5.3.4.2 Interworking with 3GPP UTRAN/GERAN for SRVCC from E-UTRAN / UTRAN (HSPA) to 3GPP UTRAN/GERAN

3GPP SRVCC UE is needed to perform SRVCC (see clause 3.1 for 3GPP SRVCC UE definition). The interaction between UE and E-UTRAN is described in TS 36.300 [16] and between UE and UTRAN (HSPA) is described in TS 25.331 [19].

The SRVCC UE indicates to the network that the UE is SRVCC capable when being configured for using IMS speech service supported by the home operator, e.g. the IMS Multimedia Telephony Service for bi-directional speech as described in TS 22.173 [26].

For an eCall Over IMS after handover to GERAN/UTRAN is complete, the SRVCC UE shall support in-band transfer of updated MSD according to TS 26.267 [43].

5.3.4.3 Interworking of UE with 3GPP UTRAN/GERAN for SRVCC from CS to PS

The UE supporting CS to PS SRVCC indicates to the network that the UE is CS to PS SRVCC capable when being configured for using IMS speech service supported by the home operator, e.g. the IMS Multimedia Telephony Service for bi-directional speech as described in TS 22.173 [26].

The UE supporting CS to PS SRVCC indicates to the MSC Server the information about the MME / SGSN the UE had last contact with using P‑TMSI+RAI+P‑TMSI signature or GUTI. This information shall be provided by the UE to the MSC server by NAS signalling upon request from the MSC.

5.3.4a UE enhanced for vSRVCC

3GPP vSRVCC UE signals its vSRVCC capability to the network. If the UE supports vSRVCC capability, it shall also support SRVCC capability.

If needed, the vSRVCC UE initiates the multimedia codec negotiation on the CS domain after the vSRVCC handover is completed.

NOTE: It is recommended that the UE and the network support the MONA codec negotiation mechanism (see TR 26.911 [35] and ITU‑T Recommendation H.324 [33], Annex K) to accelerate the call establishment and minimise the interruption time.

5.3.4b UE enhanced for 5G-SRVCC

If the UE supports the 5G-SRVCC from NG-RAN to 3GPP UTRAN, the UE shall follow the rules and procedures described in TS 23.502 [45] with the following additions and clarifications:

– Supports UTRAN cell measurement procedure.

– Supports 5G-SRVCC from 5GS to 3GPP UTRAN in NAS and AS.

– Derives CS security context from EPS security context which are derived from 5GS security context.

5.3.5 Serving/PDN GW

The PDN GW shall support the PS-to-CS handover indicator as notified by the MME/SGSN (see detailed procedures in clause 6.2.2).

NOTE: If SRVCC is deployed and PCC is not used then the PDN GW is not able to enforce the architecture principle to use QCI=1, see clauses 4.1.1 and 4.1.2.

5.3.6 E-UTRAN

5.3.6.1 Interworking with 3GPP2 1xCS

If the E-UTRAN (operator) supports interworking to 3GPP2 1xCS, the E‑UTRAN performs the HO trigger, tunnelling of the 3GPP2 1xCS signalling messages toward the MME, and interacting with the SRVCC UE as described in TS 36.300 [16].

E-UTRAN may be capable of determining the neighbour cell list based on the "SRVCC operation possible" indication and/or presence of an established QCI=1 bearer for a specific UE. An example algorithm is provided in clause A.1.

NOTE: If E-UTRAN does not update the neighbour cell list dynamically, if E-UTRAN triggers handover to 1x when either the "SRVCC operation possible" indication is not set or there is no established QCI=1 bearer for a specific UE, this will result in an error case.

5.3.6.2 Interworking with 3GPP UTRAN/GERAN for SRVCC from E-UTRAN to 3GPP UTRAN/GERAN

Between UE and E-UTRAN, no additional functionality is required for the E‑UTRAN as defined in TS 36.300 [16].

When E‑UTRAN selects a target cell for SRVCC handover, it needs to send an indication to MME that this handover procedure requires SRVCC

E-UTRAN may be capable of determining the neighbour cell list based on the "SRVCC operation possible" indication, the UE’s Radio Access Capabilities for SRVCC from E-UTRAN to UTRAN/GERAN and PS Voice over UTRAN and/or presence of an established QCI=1 bearer for a specific UE. An example algorithm is provided in clause A.2.

The procedure for E-UTRAN Radio Access Network sharing is described in TS 23.251 [40].

NOTE: If E-UTRAN does not update the neighbour cell list dynamically, if E-UTRAN triggers handover to a VoIP-incapable cell when the "SRVCC operation possible" indication is set to "false" and there is an established voice (QCI=1) bearer for a specific UE, the establishment of the voice bearer will be rejected by the target access.

5.3.6.2a Interworking with 3GPP UTRAN for vSRVCC

Between UE and E-UTRAN, no additional functionality is required for the E‑UTRAN as defined in TS 36.300 [16].

E-UTRAN may be capable of determining the neighbour cell list based on simultaneous presence of established QCI=1 (for voice part) and vSRVCC marked bearers for a specific UE.

NOTE 1 The usage of vSRVCC marked bearer by E-UTRAN is outside the scope of this specification.

NOTE 2: If E-UTRAN does not update the neighbour cell list dynamically, if E-UTRAN triggers handover to a VoIP incapable cell when the "SRVCC operation possible" indication is not set and there is an established voice (QCI=1) bearer for a specific UE, the establishment of the voice and video bearers will be rejected by the target access.

5.3.6A UTRAN (HSPA)

When HSPA capable UTRAN selects a target cell for SRVCC handover, it needs to send an indication to SGSN that this handover procedure requires SRVCC.

NOTE 1: UTRAN (HSPA) assumes that SGSN supports SRVCC functionality.

UTRAN may be capable of determining the neighbour cell list based on the "SRVCC operation possible" indication, the UE’s Radio Access Capabilities for SRVCC from UTRAN (HSPA) to UTRAN/GERAN and/or presence of an established voice bearer (i.e. bearer with Traffic Class = Conversational and Source Statistic Descriptor = ‘speech’) for a specific UE. An example algorithm is provided in clause A.3.

The procedure for UTRAN Radio Access Network sharing is described in TS 23.251 [40].

NOTE 2: If UTRAN does not update the neighbour cell list dynamically, if UTRAN triggers handover to a VoIP-incapable cell when the "SRVCC operation possible" indication is not set and there is an established bearer with Traffic Class = Conversational and Source Statistic Descriptor = ‘speech’ for a specific UE, the establishment of the voice bearer will be rejected by the target access.

5.3.6B GERAN/UTRAN CS

When GERAN/UTRAN selects a target cell for CS to PS SRVCC handover, it shall send an indication to MSC Server enhanced for CS to PS SRVCC that this handover procedure requires CS to PS SRVCC.

GERAN/UTRAN may be capable of determining the neighbour cell list based on the "CS to PS SRVCC operation possible" indication received from MSC Server enhanced for CS to PS SRVCC, the UE’s Radio Access Capabilities for CS to PS SRVCC over GERAN/UTRAN, and the existence of CS voice call. An example algorithm is provided in Annex A, clause A.4.

5.3.7 HSS

5.3.7.1 Interworking with 3GPP UTRAN/GERAN for SRVCC or vSRVCC from E-UTRAN / UTRAN (HSPA) to 3GPP UTRAN/GERAN

The STN-SR, vSRVCC flag, C‑MSISDN and optional ICS flag per VPLMN, for the subscriber are downloaded to MME from HSS during E-UTRAN attach procedure. For UTRAN (HSPA), this subscription information is downloaded to SGSN from HSS during GPRS attach procedure. The HSS also informs the MME/SGSN when STN-SR is modified or removed from the subscription.

NOTE 1: When STN-SR is modified or removed from the subscription, the HSS can decide not to inform the MME/SGSN e.g. if the MME/SGSN has previously notified the HSS that it does not support IMS voice over PS in any TA/RA as specified in TS 23.401 [2] clause 4.3.5.8A.

The ICS flag is used by the MSC Server enhanced for (v)SRVCC to behave also as MSC Server enhanced for ICS in TS 23.292 [13] if supported by the network.

NOTE 2: HSS functionality is not impacted when MSC Server enhanced for (v)SRVCC performs the MAP_Update_Location procedure.

5.3.7.2 Interworking with 3GPP UTRAN/GERAN for SRVCC from CS to PS

The "CS to PS SRVCC allowed" indication and optional ICS flag are downloaded to MSC Server from HSS during attach procedure.

5.3.8 PCC

The PCRF enforces the architecture principle to use QCI=1 (and traffic-class conversational with source statistics descriptor ="speech") for voice bearer with IMS sessions anchored in the SCC AS, based on the service the session relates to. This may be achieved by deploying S9 reference point, or configuration and roaming agreements.

The PCEF should enforce to multiplex the media streams from multiple concurrent normal voice sessions.

NOTE: In this Release the UE initiated PDP context for voice bearer is not supported.

For vSRVCC, the PCRF enforces that the video bearer of IMS sessions anchored in the SCC AS, is marked with the "PS to CS session continuity indicator", see TS 23.203 [32], and hence can be identified as vSRVCC marked video PS bearer in clause 5.3.3.2.2.

5.3.9 UDM

UDM/HSS includes STN-SR and C-MSISDN as part of the subscription data sent to the AMF. If the STN-SR is present, this indicates the UE is 5G-SRVCC subscribed. The UDM/HSS also informs the AMF when STN-SR is modified or removed from the subscription.

5.3.10 AMF

If the AMF supports the 5G-SRVCC from NG-RAN to 3GPP UTRAN, the AMF shall follow the rules and procedures described in TS 23.502 [45] with the following additions and clarifications:

– Handles UE 5G-SRVCC capability in NAS message.

– Indicates 5G-SRVCC possible to NG-RAN.

– Selects the MME_SRVCC according to the Target ID and 5G-SRVCC HO Indication.

– Supports N26 interface.

– Receives STN-SR and C-MSISDN as part of the subscription data sent by UDM/HSS.

– Indicates 5G-SRVCC HO Indication in Forward Relocation Request message, with either:

– STN-SR and C-MSISDN during the handover procedure of a non-emergency session; or

– the Emergency Indication and the equipment identifier during the handover procedure of an emergency session.

– Skips Nsmf_PDUSession_Context procedure for all the PDU sessions.

– Shall not include any UE EPS PDN Connection IE into Forward Relocation Request message.

– When 5G-SRVCC is performed for an emergency session, perform "Location continuity for Handover of an Emergency session from NG-RAN" as specified in TS 23.273 [50].

5.3.11 NG-RAN

If the NG-RAN supports 5G-SRVCC from NG-RAN to 3GPP UTRAN, the NG-RAN performs the HO trigger, follow the rules and procedure described in TS 38.300 [49] with the following additions and clarifications:

– Receives the 5G-SRVCC possible indication from AMF.

– Sends the 5G-SRVCC possible indication to Target RAN during Handover procedure.

– Supports the UTRAN cell measurement procedure.

– Determines based on the radio conditions and on UE capability that 5G-SRVCC is to be initiated.

– Indicates 5G-SRVCC HO Indication in HO Required message if the 5G-SRVCC from 5G to 3G is initiated.

5.4 Reference points

5.4.1 MME – 3GPP2 1xCS IWS (S102)

The S102 reference point provides a tunnel between MME and 3GPP2 1xCS IWS to relay 3GPP2 1xCS signalling messages. 1x CS signalling messages are those messages that are defined for A21 interface as described in 3GPP2 A.S0008-C [8].

NOTE. It is up to stage 3 to determine whether the tunnelling protocol for S102 can be defined as exactly as in A21. If so, S102 is then equivalent to A21.

5.4.2 MME/SGSN – MSC Server (Sv)

The Sv reference point provides (v)SRVCC support from 3GPP E-UTRAN/UTRAN (HSPA) to 3GPP UTRAN/GERAN.

MME/SGSN includes the optional ICS flag if received from the HSS.

If (v)SRVCC with priority is supported, MME includes the priority indication if the SRVCC is for IMS-based MPS session.

The Sv reference point provides additionally SRVCC support from CS to PS.

5.4.2A MME_SRVCC – MSC Server (Sv)

The Sv reference point provides 5G-SRVCC support from 3GPP NG-RAN to 3GPP UTRAN.

5.4.3 E-UTRAN – MME (S1-MME)

For 3GPP2 1xCS SRVCC, the S1‑MME reference point provides S1 Information Transfer message (TS 36.413 [30]) between UE and MME to relay the 3GPP2 1xCS signalling messages.

For 3GPP (v)SRVCC, the S1‑MME reference point allows handover signalling between E‑UTRAN and MME. It is defined in TS 36.300 [16]

5.4.3A UTRAN (HSPA) – SGSN (Iu-ps)

For 3GPP HSPA SRVCC, the Iu-ps reference point allows handover signalling between UTRAN and SGSN. It is defined in TS 25.413 [11].

5.4.4 HSS – MME (S6a)

For 3GPP (v)SRVCC, the S6a is used to download SRVCC related information to MME during E‑UTRAN attach procedure or to inform MME that STN-SR information in the HSS has changed.

For 3GPP vSRVCC, the S6a is used to download vSRVCC flag to MME during E‑UTRAN attach procedure or to inform MME that vSRVCC flag in the HSS has changed.

5.4.5 HSS – SGSN (Gr, S6d)

For 3GPP SRVCC, either the Gr or the S6d is used to download SRVCC related information to SGSN during UTRAN (HSPA) attach procedure or to inform SGSN that STN-SR information in the HSS has changed.

5.4.6 HSS – MSC (MAP D)

If the subscriber is allowed to have CS to PS SRVCC, the HSS shall include the "CS to PS SRVCC allowed" indication in the Insert Subscriber Data sent to the MSC Server. The "CS to PS SRVCC allowed" indication may be specific for each VPLMN.

5.4.7 AMF – MME_SRVCC

For 5G-SRVCC, the N26 reference point which is defined in TS 23.501 [48] allows handover signalling between AMF and MME_SRVCC with the following additional parameters:

– For a non-emergency session:

– 5G-SRVCC HO Indication;

– STN-SR;

– C-MSISDN;

– SupportedCodec IE; and

– MS ClassMark 2

– For an emergency session:

– Emergency indication; and

– Equipment identifier.

5.4.8 NG-RAN – AMF

For 5G-SRVCC, the N2 reference point which is defined in TS 38.300 [49] allows handover signalling between NG-RAN and AMF with the following additional parameter:

– 5G-SRVCC HO Indication.

– 5G-SRVCC possible indication.